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Publisher: AGU   (Total: 17 journals)   [Sort by number of followers]

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Geochemistry, Geophysics, Geosystems     Full-text available via subscription   (Followers: 25, SJR: 2.439, h-index: 91)
Geophysical Research Letters     Full-text available via subscription   (Followers: 100, SJR: 3.323, h-index: 185)
Global Biogeochemical Cycles     Full-text available via subscription   (Followers: 12, SJR: 3.22, h-index: 136)
J. of Advances in Modeling Earth Systems     Open Access   (Followers: 4, SJR: 4.444, h-index: 18)
J. of Geophysical Research : Atmospheres     Partially Free   (Followers: 109)
J. of Geophysical Research : Biogeosciences     Full-text available via subscription   (Followers: 24)
J. of Geophysical Research : Earth Surface     Partially Free   (Followers: 48)
J. of Geophysical Research : Oceans     Partially Free   (Followers: 45)
J. of Geophysical Research : Planets     Full-text available via subscription   (Followers: 95)
J. of Geophysical Research : Solid Earth     Full-text available via subscription   (Followers: 42)
J. of Geophysical Research : Space Physics     Full-text available via subscription   (Followers: 114)
Paleoceanography     Full-text available via subscription   (Followers: 7, SJR: 3.067, h-index: 100)
Radio Science     Full-text available via subscription   (Followers: 36, SJR: 1.072, h-index: 59)
Reviews of Geophysics     Full-text available via subscription   (Followers: 33, SJR: 8.833, h-index: 107)
Space Weather     Full-text available via subscription   (Followers: 16, SJR: 1.341, h-index: 26)
Tectonics     Full-text available via subscription   (Followers: 13, SJR: 2.628, h-index: 96)
Water Resources Research     Full-text available via subscription   (Followers: 75, SJR: 2.661, h-index: 144)
Journal Cover Geochemistry, Geophysics, Geosystems
  [SJR: 2.439]   [H-I: 91]   [25 followers]  Follow
    
   Full-text available via subscription Subscription journal  (Not entitled to full-text)
   ISSN (Online) 1525-2027
   Published by AGU Homepage  [17 journals]
  • Biomarkers in Lake Van sediments reveal dry conditions in Eastern Anatolia
           during 110.000-10.000 years B.P.
    • Authors: Marie-Eve Randlett; Achim Bechtel, Marcel T.J. van der Meer, Francien Peterse, Thomas Litt, Nadine Pickarski, Ola Kwiecien, Mona Stockhecke, Bernhard Wehrli, Carsten J. Schubert
      Abstract: Lipid biomarkers were analyzed in Lake Van sediments covering the last 600 ka, with a focus on the period between 110 and 10 ka, when a broad maximum in pore water salinity as a relict from the past suggests dry conditions. The occurrence and distribution of biomarkers indicative for terrestrial plants (long-chain n-alkane C29), haptophyte algae (methyl alkenones C37) and halophilic archaea (archaeol) all point towards a dry climate in Lake Van region during this time interval. The hydrogen isotopic composition of C29 n-alkanes (δDC29) and C37 alkenones (δDC37) is enriched between MIS 4 and MIS 2, which is interpreted as a decrease in the regional ratio of precipitation to evaporation. Similarly, the low abundance of the acyclic glycerol dialkyl glycerol tetraether GDGT-0 relative to archaeol, quantified by the Archaeol and Caldarchaeol Ecometric (ACE) is assumed to reflect the presence of halophilic euryarchaeota adapted to high salinity water. The climate around Lake Van appears in phase with the Yammouneh basin 800 km southwest and Lake Urmia 250 km southeast of Lake Van over the last two glacial periods. The results highlight the potential of combining ACE, δDC29, and δDC37 for reconstructing salinity changes and regional precipitation to evaporation ratio from lake sediments. This article is protected by copyright. All rights reserved.
      PubDate: 2017-01-16T04:15:54.883525-05:
      DOI: 10.1002/2016GC006621
       
  • Mid-Brunhes magnetic excursions in marine isotope stages 9, 13, 14, and 15
           (286, 495, 540, and 590 ka) at North Atlantic IODP Sites U1302/3, U1305
           and U1306
    • Authors: J.E.T. Channell
      Abstract: Integrated Ocean Drilling Program (IODP) Site U1302/3 (Orphan Knoll, off Newfoundland) recorded magnetic excursions in marine isotope stages (MIS) 9a (at 286 ka) and 13a (at 495 ka). Sites U1306 and U1305 (Eirik Drift, off SE Greenland) record excursions in MIS 14a/b (at 540 ka) and 15b/c (at 590 ka). In the excursion intervals, magnetic measurements of continuous “u-channel” samples from multiple holes within site are augmented by measurements of cubic (8-cm3) discrete samples. The excursions lie in relative paleointensity (RPI) minima at each site and in RPI reference stacks, and correspond to dated intervals of 10Be overproduction in other deep-sea sediment records. Although observed at multiple holes at each site, and from u-channel and discrete samples, the excursions are not observed at all three sites, and often at only one of the three sites. Sporadic recording of these magnetic excursions, and excursions in general, is attributed to a combination of filtering by the process of acquisition of detrital remanent magnetization (DRM), post-depositional overprint of weak excursion magnetizations, the millennial or even centennial duration of directional excursions, and non-uniform sedimentation rates at these timescales in North Atlantic sediment drifts. This article is protected by copyright. All rights reserved.
      PubDate: 2017-01-13T05:06:23.809131-05:
      DOI: 10.1002/2016GC006626
       
  • Anisotropy in the lowermost mantle beneath the Indian Ocean Geoid Low from
           ScS splitting measurements
    • Authors: B. Padma Rao; M. Ravi Kumar, Arun Singh
      Abstract: The Indian Ocean Geoid Low (IOGL) to the south of Indian sub-continent is the world's largest geoid anomaly. In this study, we investigate the seismic anisotropy of the lowermost mantle beneath the IOGL by analyzing splitting of high quality ScS phases corrected for source and receiver side upper mantle anisotropy. Results reveal significant anisotropy (1.01 in the $D^{\prime\prime}$layer. The observed fast axis polarization azimuths in the ray coordinate system indicate a TTI (transverse isotropy with a tilted axis of symmetry) style of anisotropy. Lattice Preferred Orientation (LPO) deformation of the palaeo-subducted slabs experiencing high shear strain is a plausible explanation for the observed anisotropy beneath the IOGL. This article is protected by copyright. All rights reserved.
      PubDate: 2017-01-13T03:40:25.071435-05:
      DOI: 10.1002/2016GC006604
       
  • Shear localization in a mature mylonitic rock analogue during fast slip
    • Authors: M. Takahashi; M. van den Ende, A. R. Niemeijer, C. J. Spiers
      Abstract: Highly localized slip zones developed within ductile shear zones, such as pseudotachylite bands occurring within mylonitic fabric rocks, are frequently interpreted as evidence for earthquake nucleation and/or propagation within the ductile regime. To understand brittle/frictional shear localization processes in ductile shear zones and to relate these to earthquake nucleation and propagation, we performed tests with large change in velocity on a brine-saturated, 80:20 (wt.%) mixture of halite and muscovite gouge after forming a mature mylonitic structure through frictional-viscous flow. The direct effect a on shear strength that occurs in response to an instantaneous upward velocity-step is an important parameter in determining the nature of seismic rupture nucleation and propagation. We obtained reproducible results regarding low velocity mechanical behavior compared with previous work, but also obtained new insights into effects of sudden increases in slip velocity on localization and strength evolution, at velocities above a critical velocity Vc (∼20 μm/s). We found that once a ductile, mylonitic structure has developed in a shear zone, subsequent cataclastic deformation is consistently localized in a narrow zone. This switch to localized deformation is controlled by the imposed velocity, and becomes most apparent at velocities above Vc. In addition, the direct effect drops rapidly when the velocity exceeds Vc. This implies that slip can accelerate towards seismic velocities almost instantly and without much loss of fracture energy, once Vc is exceeded. Obtaining a measure for Vc in natural faults is therefore of key importance for understanding earthquake nucleation and propagation in the brittle-ductile transitional regime.
      PubDate: 2017-01-10T18:00:33.237004-05:
      DOI: 10.1002/2016GC006687
       
  • A new high-resolution seafloor age grid for the South Atlantic
    • Authors: L. Pérez-Díaz; G. Eagles
      Abstract: Digital grids of basement age of the world's oceans are essential for modern geodynamic and paleoceanographic studies. Any such grid is built using a plate kinematic model, whose accuracy and reliability directly influence the accuracy and reliability of the grid. We present a seafloor age grid for the South Atlantic based on a recent high-resolution plate kinematic model. The grid is built from a dataset of points whose ages are defined in or for the plate kinematic model, incorporating breaks at tectonic boundaries like fracture zones where the age function is discontinuous. We compare predictions of the new grid and of a previously published one, which is based on an older plate kinematic model, to magnetic isochron pick datasets. The comparison shows the new grid to provide a more reliable depiction of seafloor age in the South Atlantic. Numerical estimates of the new grid's uncertainty are determined by interpolation between (1) misfits at grid cells coinciding with magnetic isochron ages, (2) misfits implied by locational uncertainties in predicted isochrons propagated from uncertainties in the plate kinematic model and (3) by the proximities of cells to fracture zone traces or ridge-jump scars. Estimated total uncertainty is
      PubDate: 2017-01-10T17:20:28.416999-05:
      DOI: 10.1002/2016GC006750
       
  • Reaction kinetics of alkenone and n-alkane thermal alteration at seismic
           timescales
    • Authors: H. S. Rabinowitz; P. J. Polissar, H. M. Savage
      Abstract: Recent experiments and field observations have indicated that biomarker molecules can react over short timescales relevant to seismic slip, thereby making these compounds a useful tool in studying temperature rise in fault zones. However, short-timescale biomarker reaction kinetics studies have previously focused on compounds that have already experienced burial heating. Here, we present a set of hydrous pyrolysis experiments on Pleistocene-aged shallow marine sediment to develop the reaction kinetics of long-chain alkenone destruction, change in the alkenone unsaturation ratio (Uk'37), and change in the n-alkane chain length distribution. Our results show that biomarker thermal maturity provides a useful method for detecting temperature rise in the shallow reaches of faults, such as subduction zone trench environments. Through the course of our work, we also noted the alteration of total alkenone concentrations and Uk'37 values in crushed sediments stored dry at room temperature for durations of months to years but not in the solvent extracts of these materials. This result, though parenthetical for our work in fault zones, has important implications for proper storage of sedimentary samples to be used for alkenone paleotemperature and productivity analysis. This article is protected by copyright. All rights reserved.
      PubDate: 2017-01-07T03:50:24.15317-05:0
      DOI: 10.1002/2016GC006553
       
  • MeltMigrator: A MATLAB-based software for modeling three-dimensional melt
           migration and crustal thickness variations at mid-ocean ridges following a
           rules-based approach
    • Authors: Hailong Bai; Laurent G. J. Montési, Mark D. Behn
      Abstract: MeltMigrator is a MATLAB®-based melt migration software developed to process three-dimensional mantle temperature and velocity data from user-supplied numerical models of mid-ocean ridges, calculate melt production and melt migration trajectories in the mantle, estimate melt flux along plate boundaries and predict crustal thickness distribution on the seafloor. MeltMigrator is also capable of calculating compositional evolution depending on the choice of petrologic melting model. Programmed in modules, MeltMigrator is highly customizable and can be expanded to a wide range of applications. We have applied it to complex mid-ocean ridge model settings, including transform faults, oblique segments, ridge migration, asymmetrical spreading, background mantle flow and ridge-plume interaction. In this technical report, we include an example application to a segmented mid-ocean ridge. MeltMigrator is available as a supplement to this paper, and it's also available from GitHub and the University of Maryland Geodynamics Group website. This article is protected by copyright. All rights reserved.
      PubDate: 2017-01-03T03:25:25.96655-05:0
      DOI: 10.1002/2016GC006686
       
  • Influence of mantle flow on the drainage of eastern Australia since the
           Jurassic Period
    • Authors: T. Salles; N. Flament, D. Müller
      Abstract: Recent studies of the past eastern Australian landscape from present-day longitudinal river profiles and from mantle flow models suggest that the interaction of plate motion with mantle convection accounts for the two phases of large scale uplift of the region since 120 Ma. We coupled the dynamic topography predicted from one of these mantle flow models to a surface process model to study the evolution of the eastern Australian landscape since the Jurassic Period. We varied the rainfall regime, erodibility, sea level variations, dynamic topography magnitude and elastic thickness accross a series of experiments. The approach accounts for erosion and sedimentation and simulates catchment dynamics. Despite the relative simplicity of our model, the results provide insights on the fundamental links between dynamic topography and continental-scale drainage evolution. Based on temporal and spatial changes in longitudinal river profiles as well as erosion and deposition maps, we show that the motion of the Australian plate over the convecting mantle has resulted in significant reorganization of the eastern Australian drainage. The model predicts that the Murray river drained eastward between 150 and ∼120 Ma, and switched to westward draining due to the tilting of the Australian plate from ∼120 Ma. First order comparisons of eight modelled river profiles and of the catchment shape of modelled Murray-Darling Basin are in agreement with present-day observations. The predicted denudation of the eastern highlands is compatible with thermochronology data and sedimentation rates along the southern Australian margin are consistent with cumulative sediment thickness. This article is protected by copyright. All rights reserved.
      PubDate: 2017-01-03T03:05:59.551976-05:
      DOI: 10.1002/2016GC006617
       
  • The impact of rapid sediment accumulation on pore pressure development and
           dehydration reactions during shallow subduction in the Gulf of Alaska
    • Authors: L.N. Meridth; E. J. Screaton, J.M. Jaeger, S. R. James, T. Villaseñor
      Abstract: In the Gulf of Alaska region, sediment has rapidly accumulated (>1 km/my) in the trench sourced from intensified glaciation in the past ∼1.2 million years. This rapid sediment accumulation increases overburden and should accelerate dehydration of hydrous minerals by insulating the underlying sediment column. These processes have the potential to generate fluid overpressures in the low permeability sediments entering the subduction zone. A 1-D model was developed to simulate dehydration reaction progress and investigate excess pore pressures as sediments approach the trench and are subducted. At the deformation front, simulated temperatures increase by ∼30°C due to the insulating effect of trench sediments. As a result, opal-A begins to react to form quartz while smectite remains mostly unreacted. Loading due to the trench sediments elevates excess pore pressures to ∼30% of lithostatic pressure at the deformation front; however, deformation front excess pore pressures are sensitive to assumptions about the permeability of outer wedge sediments. If the outer wedge sediments are coarse-grained and high-permeability rather than mud-dominated, excess pore pressures are lower but still have an insulating effect. During early subduction, simulated pore pressures continue to rise and reach ∼70% of lithostatic by 60 km landward. The 1-D modeling results suggest that the elevated pore pressures are primarily due to loading and that dehydration reactions are not a significant component of excess pore pressure generation at this margin. This article is protected by copyright. All rights reserved.
      PubDate: 2017-01-03T03:01:08.70781-05:0
      DOI: 10.1002/2016GC006693
       
  • Mid-crustal shearing and doming in a Cenozoic compressive setting along
           the Ailao Shan-Red River shear zone
    • Authors: B. Zhang; C. Y. Yin, J. J. Zhang, J. M. Wang, D. L. Zhong, Y. Wang, Q. Z. Lai, Y. H. Yue, Q. Y. Zhou
      Abstract: The Cenozoic Xuelong Shan antiformal dome is located along the northern segment of the Ailao Shan-Red River shear zone in Yunnan, China. Subhorizontal foliation in the gneiss core is recognized, representing a broad top-to-NE shear initiated under amphibolite facies conditions and propagating into greenschist facies in the mantling schist and strike-slip shear zone. Microfabrics of crystallographic preferred orientations (CPOs) in quartz suggest that the deformation temperatures increased with increasing structural depth from the upper crust (300-500°C) in the mantling schist to the mid-crust (15 km or more, ≥650°C) in the gneissic core. This trend is mirrored by variations in the metamorphic grade of the syn-kinematic mineral assemblages and microstructures, which range from garnet + amphibole + biotite + sillimanite + rutile + feldspar in the core to garnet + staurolite + biotite + epidote + muscovite within the limb units. The dome experienced the following deformation history: (1) a broad top-to-NE shear in the subhorizontal foliation of the gneiss core during the first stage of deformation (D1); (2) opposing reverse-sense shear along the two schist limbs of the dome during contraction-related doming (D2-D3); (3) sinistral strike-slip shearing within the eastern limb (D4); and (4) extensional deformation (D5). The structural-thermal patterns suggest the antiformal dome formation was roughly coeval with top-to-NE ductile shearing in the mid-crust of Tibet at 32 Ma or earlier. A major implication is that there was a phase of contractional ductile deformation in the region prior to the initiation of strike-slip deformation. This article is protected by copyright. All rights reserved.
      PubDate: 2017-01-03T03:01:07.464885-05:
      DOI: 10.1002/2016GC006520
       
  • Mineralogical, geochemical and magnetic signatures of surface sediments
           from the Canadian Beaufort Shelf and Amundsen Gulf (Canadian Arctic)
    • Authors: Adriana Gamboa; Jean-Carlos Montero-Serrano, Guillaume St-Onge, André Rochon, Pierre-Arnaud Desiage
      Abstract: Mineralogical, geochemical, magnetic, and siliciclastic grain-size signatures of 34 surface sediment samples from the Mackenzie-Beaufort Sea Slope and Amundsen Gulf were studied in order to better constrain the redox status, detrital particle provenance, and sediment dynamics in the western Canadian Arctic. Redox-sensitive elements (Mn, Fe, V, Cr, Zn) indicate that modern sedimentary deposition within the Mackenzie-Beaufort Sea Slope and Amundsen Gulf took place under oxic bottom-water conditions, with more turbulent mixing conditions and thus a well-oxygenated water column prevailing within the Amundsen Gulf. The analytical data obtained, combined with multivariate statistical (notably, principal component and fuzzy c-means clustering analyses) and spatial analyses, allowed the division of the study area into four provinces with distinct sedimentary compositions: (1) the Mackenzie Trough-Canadian Beaufort Shelf with high phyllosilicate-Fe oxide-magnetite and Al-K-Ti-Fe-Cr-V-Zn-P contents; (2) Southwestern Banks Island, characterized by high dolomite-K-feldspar and Ca-Mg-LOI contents; (3) the Central Amundsen Gulf, a transitional zone typified by intermediate phyllosilicate-magnetite-K-feldspar-dolomite and Al-K-Ti-Fe-Mn-V-Zn-Sr-Ca-Mg-LOI contents; and (4) mud volcanoes on the Canadian Beaufort Shelf distinguished by poorly sorted coarse-silt with high quartz-plagioclase-authigenic carbonate and Si-Zr contents, as well as high magnetic susceptibility. Our results also confirm that the present-day sedimentary dynamics on the Canadian Beaufort Shelf is mainly controlled by sediment supply from the Mackenzie River. Overall, these insights provide a basis for future studies using mineralogical, geochemical, and magnetic signatures of Canadian Arctic sediments in order to reconstruct past variations in sediment inputs and transport pathways related to late Quaternary climate and oceanographic changes. This article is protected by copyright. All rights reserved.
      PubDate: 2017-01-03T03:01:02.726976-05:
      DOI: 10.1002/2016GC006477
       
  • TerraFERMA: The Transparent Finite Element Rapid Model Assembler for
           multiphysics problems in Earth sciences
    • Authors: Cian R. Wilson; Marc Spiegelman, Peter E. van Keken
      Abstract: We introduce and describe a new software infrastructure TerraFERMA, the\emph{Transparent Finite Element Rapid Model Assembler}, for the rapid and reproducible description and solution of coupled multiphysics problems. The design of TerraFERMA is driven by two computational needs in Earth sciences. The first is the need for increased flexibility in both problem description and solution strategies for coupled problems where small changes in model assumptions can lead to dramatic changes in physical behavior. The second is the need for software and models that are more transparent so that results can be verified, reproduced, and modified in a manner such that the best ideas in computation and Earth science can be more easily shared and reused. TerraFERMA leverages three advanced open-source libraries for scientific computation that provide high-level problem description (FEniCS), composable solvers for coupled multiphysics problems (PETSc), and an options handling system (SPuD) that allows the hierarchical management of all model options. TerraFERMA integrates these libraries into an interface that organizes the scientific and computational choices required in a model into a single options file from which a custom compiled application is generated and run. Because all models share the same infrastructure, models become more reusable and reproducible, while still permitting the individual researcher considerable latitude in model construction. TerraFERMA solves partial differential equations using the finite element method. It is particularly well suited for nonlinear problems with complex coupling between components. TerraFERMA is open-source and available at http://terraferma.github.io, which includes links to documentation and example input files. This article is protected by copyright. All rights reserved.
      PubDate: 2017-01-03T03:00:41.43675-05:0
      DOI: 10.1002/2016GC006702
       
  • Fidelity of the Sr/Ca proxy in recording ocean temperature in the western
           Atlantic coral Siderastrea siderea
    • Authors: Ilsa B. Kuffner; Kelsey E. Roberts, Jennifer A. Flannery, Jennifer M. Morrison, Julie N. Richey
      Abstract: Massive corals provide a useful archive of environmental variability, but careful testing of geochemical proxies in corals is necessary to validate the relationship between each proxy and environmental parameter throughout the full range of conditions experienced by the recording organisms. Here we use samples from a coral-growth study to test the hypothesis that Sr/Ca in the coral Siderastrea siderea accurately records sea-surface temperature (SST) in the subtropics (Florida, USA) along 350 km of reef tract. We test calcification rate, measured via buoyant weight, and linear extension (LE) rate, estimated with Alizarin Red-S staining, as predictors of variance in the Sr/Ca records of 39 individual S. siderea corals grown at four outer-reef locations next to in-situ temperature loggers during two, year-long periods. We found that corals with calcification rates 
      PubDate: 2017-01-03T03:00:27.388429-05:
      DOI: 10.1002/2016GC006640
       
  • Metamorphic records for subduction erosion and subsequent underplating
           processes revealed by garnet-staurolite-muscovite schists in central
           Qiangtang, Tibet
    • Authors: Xiu-Zheng Zhang; Yong-Sheng Dong, Qiang Wang, Wei Dan, Chunfu Zhang, Wang Xu, Ming-Liang Huang
      Abstract: Subduction erosion is confirmed as a crucial geodynamic process of crustal recycling based on geological, geochemical, and geophysical observations at modern convergent plate margins. So far, not a single metamorphic record has been used for constraining a general tectonic evolution for subduction erosion. Here we first revealed metamorphic records for a subduction erosion process based on our study of the Late Paleozoic garnet-staurolite-muscovite schists in the central Qiangtang block, Tibet. Provenance analyses suggest that the protoliths of garnet-staurolite-muscovite schists have the Northern Qiangtang-affinity and were deposited in an active continental margin setting. Mineral inclusion data show that the early metamorphic stage (M1) recorded blueschist facies pressure–temperature (P–T) conditions of 0.8–1.1 GPa and 402–441°C, indicating that a part of the material from the overriding plate had been abraded into the subduction channel and undergone high-pressure/low-temperature metamorphism. The peak metamorphic stage (M2) recorded amphibolite facies P–T conditions of 0.3–0.5 GPa and 470–520°C. The 40Ar/39Ar cooling ages (263–259 Ma) yielded from muscovite suggest the amphibolite facies metamorphism (> 263 Ma) occurred at oceanic subduction stage. The distinctly-staged metamorphism defines a clockwise and warming decompression P–T–t path which reveals an underplating process following the early subduction erosion. During the tectonic process, the eroded low-density material escaped from the cold subduction channel and rise upwards into the warm middle-lower crust of the upper plate, undergoing amphibolite facies metamorphism. Our new results revealed a complete evolutional process from the early subduction erosion to the subsequent underplating during the northward subduction of the Paleo-Tethys Ocean. This article is protected by copyright. All rights reserved.
      PubDate: 2017-01-03T02:55:52.333201-05:
      DOI: 10.1002/2016GC006576
       
  • Seismic structure of the lithosphere beneath NW Namibia: Impact of the
           Tristan da Cunha mantle plume
    • Authors: Xiaohui Yuan; Benjamin Heit, Sascha Brune, Bernhard Steinberger, Wolfram H. Geissler, Wilfried Jokat, Michael Weber
      Abstract: Northwestern Namibia, at the landfall of the Walvis Ridge, was affected by the Tristan da Cunha mantle plume during continental rupture between Africa and South America, as evidenced by the presence of the Etendeka continental flood basalts. Here we use data from a passive-source seismological network to investigate the upper mantle structure and to elucidate the Cretaceous mantle plume-lithosphere interaction. Receiver functions reveal an interface associated with a negative velocity contrast within the lithosphere at an average depth of 80 km. We interpret this interface as the relic of the lithosphere-asthenosphere boundary (LAB) formed during the Mesozoic by interaction of the Tristan da Cunha plume head with the pre-existing lithosphere. The velocity contrast might be explained by stagnated and “frozen” melts beneath an intensively depleted and dehydrated peridotitic mantle. The present-day LAB is poorly visible with converted waves, indicating a gradual impedance contrast. Beneath much of the study area, converted phases of the 410 and 660 km mantle transition zone discontinuities arrive 1.5 s earlier than in the landward plume-unaffected continental interior, suggesting high velocities in the upper mantle caused by a thick lithosphere. This indicates that after lithospheric thinning during continental breakup, the lithosphere has increased in thickness during the last 132 Myr. Thermal cooling of the continental lithosphere alone cannot produce the lithospheric thickness required here. We propose that the remnant plume material, which has a higher seismic velocity than the ambient mantle due to melt depletion and dehydration, significantly contributed to the thickening of the mantle lithosphere. This article is protected by copyright. All rights reserved.
      PubDate: 2017-01-03T02:55:38.265192-05:
      DOI: 10.1002/2016GC006645
       
  • Formation of hydrothermal pits and the role of seamounts in the Guatemala
           Basin (Equatorial East Pacific) from heat flow, seismic and core studies
    • Authors: H. W. Villinger; T. Pichler, N. Kaul, S. Stephan, H. Pälike, F. Stephan
      Abstract: We acquired seismic and heat flow data and collected sediment cores in three areas in the Guatemala Basin (Cocos Plate, Eastern Pacific) to investigate the process by which depressions (pits) in the sedimentary cover on young oceanic crust were formed. Median heat flow of 55 mW/m2 for the three areas is about half of the expected conductive cooling value. The heat deficit is caused by massive recharge of cold seawater into the upper crust through seamounts which is inferred from depressed heat flow in the vicinity of seamounts. Heat flow inside of pits is always elevated, in some cases up to three times (max. 300 mW/m2) relative to background. None of the geochemical pore water profiles from cores inside and outside of the pits show any evidence of active fluid flow inside the pits. All three areas originated within the high productivity equatorial zone and moved northwest over the past 15 to 18 Ma. Pits found in the working areas are likely dissolution structures formed by diffuse hydrothermal venting in a zone of high biogenic carbonate production which were sealed when they moved north. It is likely that these pits were discharge sites of 'hydrothermal siphons' where recharging seamounts could feed cold seawater via the upper crust to several discharging pits. Probably pit density on the whole Cocos Plate is similar to the three working areas and which may explain the huge heat deficit of the Cocos Plate. This article is protected by copyright. All rights reserved.
      PubDate: 2016-12-29T18:26:17.034835-05:
      DOI: 10.1002/2016GC006665
       
  • Trade winds drive pronounced seasonality in carbonate chemistry in a
           tropical Western Pacific island cave - implications for speleothem
           paleoclimatology
    • Authors: Alexandra L. Noronha; Benjamin F. Hardt, Jay L. Banner, John W. Jenson, Judson W. Partin, Eric W. James, Mark A. Lander, Kaylyn K. Bautista
      Abstract: Carbon dioxide concentrations in caves are a primary driver of rates of carbonate dissolution and precipitation, exerting strong control on speleothem growth rate and geochemistry. Long-term cave monitoring studies in mid-latitude caves have observed seasonal variability in cave pCO2, whereby airflow is driven by temperature contrasts between the surface and subsurface. In tropical settings, where diurnal temperature cycles are larger than seasonal temperature cycles, it is has been proposed caves will ventilate on daily timescales, preventing cave pCO2 from increasing substantially above atmospheric pCO2. By contrast, the relatively small temperature difference between the surface and subsurface may be insufficient to drive complete ventilation of tropical caves. Here we present results of an 8-year cave monitoring study, including observations of cave pCO2 and carbonate chemistry, at Jinapsan Cave, Guam (13.4°N, 144.5°E). We find that cave pCO2 in Jinapsan Cave is both relatively high and strongly seasonal, with cave pCO2 ranging from 500 - 5000 ppm. The seasonality of cave pCO2 cannot be explained by temperature contrasts, instead we find evidence that seasonal trade winds drive cave ventilation and modulate cave pCO2. Calcite deposition rates at seven drip sites in Jinapsan Cave are shown to be seasonally variable, demonstrating that speleothem growth rates in Jinapsan Cave are strongly affected by seasonal variations in cave pCO2. These results highlight the importance that advection can have on cave ventilation processes and carbonate chemistry. Seasonality in carbonate chemistry and calcite deposition in this cave effect the interpretation of speleothem-based paleoclimate records. This article is protected by copyright. All rights reserved.
      PubDate: 2016-12-29T18:10:26.996805-05:
      DOI: 10.1002/2016GC006644
       
  • Geochemical and microstructural evidence for interseismic changes in fault
           zone permeability and strength, Alpine Fault, New Zealand
    • Authors: Carolyn Boulton; Catriona D. Menzies, Virginia G. Toy, John Townend, Rupert Sutherland
      Abstract: Oblique dextral motion on the central Alpine Fault in the last c. 5 Myr has exhumed garnet-oligoclase facies mylonitic fault rocks from c. 35 km depth. During exhumation, deformation, accompanied by fluid infiltration, has generated complex lithological variations in fault-related rocks retrieved during Deep Fault Drilling Project (DFDP-1) drilling at Gaunt Creek, South Island, New Zealand. Lithological, geochemical, and mineralogical results reveal that the fault comprises a core of highly comminuted cataclasites and fault gouges bounded by a damage zone containing cataclasites, protocataclasites, and fractured mylonites. The fault core-alteration zone extends c. 20-30 m from the principal slip zone (PSZ) and is characterized by alteration of primary phases to phyllosilicate minerals. Alteration associated with distinct mineral phases occurred proximal the brittle-to-plastic transition (T≤300-400°C, 6-10 km depth) and at shallow depths (T=20-150°C, 0-3 km depth). Within the fault core-alteration zone, fractures have been sealed by authigenic precipitation of calcite and phyllosilicates. This sealing has decreased fault normal permeability and increased rock mass competency, potentially promoting interseismic strain buildup. This article is protected by copyright. All rights reserved.
      PubDate: 2016-12-29T05:36:59.690503-05:
      DOI: 10.1002/2016GC006588
       
  • Influence of diatom microfossils on sediment shear strength and slope
           stability
    • Authors: G. Wiemer; A. Kopf
      Abstract: Diatom microfossils have been detected in many natural marine sediment deposits around the globe and are held responsible for the disobedience to well-established geotechnical relationships between index-properties and shear strength. We revisit the static shear strength and present the first cyclic undrained shear strength experiments on diatom microfossil – clayey-silt mixtures to study the role of diatoms on submarine slope stability. It is attested that the angle of internal friction (Φ) increases with diatom content, however, we provide evidence for a significant overestimation of Φ in previous studies. Based on direct shear tests at varying normal stresses ≤ 600 kPa we find Φ = 32° in contrast to 43° in pure diatom. Similarly, to static shear strength, cyclic shear strength increases with diatom content, however, in contrast to static shear strength the most drastic increase does not occur from 0% to 25% diatoms but from 75% to 100%. Interestingly, diatomaceous sediments tend to fail by liquefaction although well-established relations between index properties and liquefaction susceptibility predict the opposite. Liquefaction failure is observed solely in samples containing ≥ 50% diatoms whereas samples with lower diatom content fail by cyclic softening. We conclude diatom microfossils in marine sediments significantly contribute to an increased slope stability under static and cyclic loading conditions since diatoms lead to higher resistance independently of the loading mode. The strength increase is interpreted as a result of particle interlocking and surface roughness, which is very efficient given the highly variable habitus of diatom species. This article is protected by copyright. All rights reserved.
      PubDate: 2016-12-29T05:30:36.67717-05:0
      DOI: 10.1002/2016GC006568
       
  • Focused hydrocarbon-migration in shallow sediments of a pockmark cluster
           in the Niger Delta (Off Nigeria)
    • Authors: Alexis de Prunelé; Livio Ruffine, Vincent Riboulot, Carl A. Peters, Claire Croguennec, Vivien Guyader, Thomas Pape, Claire Bollinger, Germain Bayon, Jean-Claude Caprais, Yoan Germain, Jean-Pierre Donval, Tania Marsset, Gerhard Bohrmann, Louis Géli, Abdulkarim Rabiu, Marc Lescanne, Eric Cauquil, Nabil Sultan
      Abstract: The Niger Delta is one of the largest hydrocarbon basin offshore Africa and it is well known for the presence of active pockmarks on the seabed. During the Guineco-MeBo cruise in 2011, long cores were taken from a pockmark cluster in order to investigate the state of its current activity. Gas hydrates, oil and pore-water were sampled for geochemical studies.The resulting dataset combined with seismic data reveal that shallow hydrocarbon migration in the upper sedimentary section was focused exclusively within the pockmarks. There is a clear tendency for gas migration within the hydrate-bearing pockmarks, and oil migration within the carbonate-rich one. This trend is interpreted as a consequence of hydrate dissolution followed by carbonate precipitation in the course of the evolution of these pockmarks. We also demonstrate that Anaerobic Oxidation of Methane (AOM) is the main process responsible for the depletion of pore-water sulfate, with depths of the Sulfate-Methane Transition Zone (SMTZ) ranging between 1.8 and 33.4 m. In addition, a numerical transport-reaction model was used to estimate the age of hydrate-layer formation from the present-day sulfate profiles. The results show that the sampled hydrate-layers were formed between 21 and 3750 years before present.Overall, this work shows the importance of fluid flow on the dynamics of pockmarks, and the investigated cluster offers new opportunities for future cross-site comparison studies. Our results imply that sudden discharges of gas can create hydrate layers within the upper sedimentary column which can affect the seafloor morphology over few decades. This article is protected by copyright. All rights reserved.
      PubDate: 2016-12-29T05:30:33.492413-05:
      DOI: 10.1002/2016GC006554
       
  • Dawsonite occurrences related to the age and origin of CO2 influx in
           sandstone reservoirs: A case study in the Songliao Basin, NE China
    • Authors: Fulai Li; Wenshuai Li, Na Liu, Zhichao Yu, Huidong Yang, Li Liu
      Abstract: Dawsonite is often associated with CO2-rich gas reservoirs, and it is regarded as a “trace mineral” for recording migration and accumulation of CO2. Following accepted petrological, mineralogical and geochemical principles, we used several methods (described herein) to study the Cretaceous dawsonite-bearing sandstone reservoirs in the Songliao Basin, China. We used the ideas of “sequencing” and “timing” to verify the influx stages of CO2 and hydrocarbons, dividing their influx sequence pattern and building a CO2-influx timeframe. First, we determined the stable isotopic ratios of dawsonite and CO2 in gas and oil reservoirs, and found that the CO2 in? dawsonite is of a mantle-derived magma origin. Second, we differentiated an early/late-stage oil and gas influx and a mid-mantle source influx through the study of diagenetic paragenetic sequences, formation water, and fluid inclusions in the dawsonite-bearing sandstones. Combining burial/thermal-history curves and illite K-Ar dates from the study area, we determined that the early-stage oil and gas influx, late oil and gas influx and medium CO2 influx occurred at 85 - 58.8 Ma, 41 - 20 Ma and 58.8 - 41 Ma (Paleocene and Eocene), respectively. Finally, we observed a coupling relationship between CO2 influx and Shuangliao volcanic activities and material compositions, as constrained by volcanic activity history in the basin since the Late Cretaceous. This article is protected by copyright. All rights reserved.
      PubDate: 2016-12-29T05:30:30.256954-05:
      DOI: 10.1002/2016GC006555
       
  • Additive effects of acidification and mineralogy on calcium isotopes in
           Triassic/Jurassic boundary limestones
    • Authors: Adam B. Jost; Aviv Bachan, Bas van de Schootbrugge, Shaun T. Brown, Donald J. DePaolo, Jonathan L. Payne
      Abstract: The end-Triassic mass extinction coincided with a negative δ13C excursion, consistent with release of 13C-depleted CO2 from the Central Atlantic Magmatic Province. However, the amount of carbon released and its effects on ocean chemistry are poorly constrained. The coupled nature of the carbon and calcium cycles allows calcium isotopes to be used for constraining carbon cycle dynamics and vice versa. We present a high-resolution calcium isotope (δ44/40Ca) record from 100 m of marine limestone spanning the Triassic/Jurassic boundary in two stratigraphic sections from northern Italy. Immediately above the extinction horizon and the associated negative excursion in δ13C, δ44/40Ca decreases by ca. 0.8‰ in 20 m of section and then recovers to pre-excursion values. Coupled numerical models of the geological carbon and calcium cycles demonstrate that this δ44/40Ca excursion is too large to be explained by changes to seawater δ44/40Ca alone, regardless of CO2 injection volume and duration. Less than 20% of the δ44/40Ca excursion can be attributed to acidification. The remaining 80% likely reflects a higher proportion of aragonite in the original sediment, based largely on high concentrations of Sr in the samples. Our study demonstrates that coupled models of the carbon and calcium cycles have the potential to help distinguish contributions of primary seawater isotopic changes from local or diagenetic effects on the δ44/40Ca of carbonate sediments. Differentiating between these effects is critical for constraining the impact of ocean acidification during the end-Triassic mass extinction, as well as for interpreting other environmental events in the geologic past. This article is protected by copyright. All rights reserved.
      PubDate: 2016-12-29T05:30:26.875021-05:
      DOI: 10.1002/2016GC006724
       
  • High-sensitivity multifunctional spinner magnetometer using a
           magnetoimpedance sensor
    • Authors: Kazuto Kodama
      Abstract: A novel spinner magnetometer was developed with a wide dynamic range from 10−10−10−4 Am2 and a resolution of 10−11 Am2. High sensitivity was achieved with the use of a magneto-impedance (MI) sensor, which is a compact, sensitive magnetic sensor used industrially. Its slow spinning rate (5 Hz) and the incorporation of a unique mechanism for adjusting the spacing between the sensing unit and the spinning axis allows the measurement of fragile samples sized 10–50 mm. The sensor configuration, in which a pair of MI sensors is connected in opposite serial, along with an amplification circuit with a programmable low-pass filter, reduces the problems of external noise and sensor drift. The signal, with reference to the spinning frequency, is detected with a lock-in amplifier. The MI spinner has two selectable measurement modes: the fundamental mode (F mode) and the harmonic mode (H mode). Measurements in the F mode detect signals of the fundamental frequency (5 Hz), in the same way as conventional spinner magnetometers. In the H mode, the second (10 Hz) and the third (15 Hz) harmonic components are measured, in addition to the fundamental component. Tests in the H mode were performed using a small coil and a natural sample to simulate dipoles with various degrees of offset. The results revealed that the magnitude of the fundamental component of the offset dipole was systematically larger (by several percent) than that of the non-offset dipole. These findings suggest that this novel MI spinner will be useful in estimating the inhomogeneity of the magnetization of a sample that can equivalently be described by an offset dipole. This article is protected by copyright. All rights reserved.
      PubDate: 2016-12-29T05:30:23.050893-05:
      DOI: 10.1002/2016GC006615
       
  • Rapid variations in fluid chemistry constrain hydrothermal phase
           separation at the Main Endeavour Field
    • Authors: Brooke Love; Marvin Lilley, David Butterfield, Eric Olson, Benjamin Larson
      Abstract: Previous work at the Main Endeavour Field (MEF) has shown that chloride concentration in high-temperature vent fluids has not exceeded 510 mmol/kg (94% of seawater), which is consistent with brine condensation and loss at depth, followed by upward flow of a vapor phase toward the seafloor. Magmatic and seismic events have been shown to affect fluid temperature and composition and these effects help narrow the possibilities for sub-surface processes. However, chloride-temperature data alone are insufficient to determine details of phase separation in the upflow zone. Here we use variation in chloride and gas content in a set of fluid samples collected over several days from one sulfide chimney structure in the MEF to constrain processes of mixing and phase separation. The combination of gas (primarily magmatic CO2 and seawater-derived Ar) and chloride data, indicate that neither variation in the amount of brine lost, nor mixing of the vapor phase produced at depth with variable quantities of (i) brine or (ii) altered gas rich seawater that has not undergone phase separation, can explain the co-variation of gas and chloride content. The gas-chloride data require additional phase separation of the ascending vapor-like fluid. Mixing and gas partitioning calculations show that near-critical temperature and pressure conditions can produce the fluid compositions observed at Sully vent as a vapor-liquid conjugate pair or as vapor-liquid pair with some remixing, and that the gas partition coefficients implied agree with theoretically predicted values. This article is protected by copyright. All rights reserved.
      PubDate: 2016-12-29T05:28:27.915162-05:
      DOI: 10.1002/2016GC006550
       
  • A preliminary 1-D model investigation of tidal variations of temperature
           and chlorinity at the Grotto mound, Endeavour segment, Juan de Fuca ridge7
           
    • Authors: G. Xu; B. I. Larson, K. G. Bemis, Marvin D. Lilley
      Abstract: Tidal oscillations of venting temperature and chlorinity have been observed in the long-term times-series data recorded by the Benthic and Resistivity Sensors (BARS) at the Grotto mound on the Juan de Fuca Ridge. In this study, we use a one-dimensional two-layer poroelastic model to conduct a preliminary investigation of three hypothetical scenarios in which seafloor tidal loading can modulate the venting temperature and chlorinity at Grotto through the mechanisms of subsurface tidal mixing and/or subsurface tidal pumping. For the first scenario, our results demonstrate that it is unlikely for subsurface tidal mixing to cause coupled tidal oscillations in venting temperature and chlorinity of the observed amplitudes. For the second scenario, the model results suggest it is plausible that the tidal oscillations in venting temperature and chlorinity are decoupled with the former caused by subsurface tidal pumping and the latter caused by subsurface tidal mixing, although the mixing depth is not well constrained. For the third scenario, out results suggest it is plausible for subsurface tidal pumping to cause coupled tidal oscillations in venting temperature and chlorinity. In this case, the observed tidal phase lag between venting temperature and chlorinity is close to the poroelastic model prediction if brine storage occurs throughout the upflow zone under the premise that layer 2A and 2B have similar crustal permeabilities. However, the predicted phase lag is poorly constrained if brine storage is limited to layer 2B as would be expected when its crustal permeability is much smaller than that of layer 2A. This article is protected by copyright. All rights reserved.
      PubDate: 2016-12-29T05:25:34.269619-05:
      DOI: 10.1002/2016GC006537
       
  • The biokarst system and its carbon sinks in response to pH changes: A
           simulation experiment with microalgae
    • Authors: Tengxiang Xie; Yanyou Wu
      Abstract: This study aims to explore the changes in a microalgal biokarst system as a potential carbon sink system in response to pH changes. The bidirectional isotope labeling method and mass balance calculation were adopted in a simulated biokarst environment with a series of set pH conditions and three microalgal species. Three key processes of the microalgal biokarst system, including calcite dissolution, CaCO3 reprecipitation, and inorganic carbon assimilation by microalgae, were completely quantitatively described. The combined effects of chemical dissolution and species-specific bio-dissolution caused a decrease in overall dissolution rate when the pH increased from 7 to 9. CaCO3 reprecipitation and the utilization of dissolved inorganic carbon originating from calcite dissolution decreased when the pH increased from 7 to 9. The three processes exhibited different effects in changing the CO2 atmosphere. The amount of photosynthetic carbon sink was larger at high pH values than at low pH values. However, the CO2 sequestration related to the biokarst process (biokarst carbon sink) increased with decreasing pH. Overall, the total amount of sequestered CO2 produced by the biokarst system (CaCO3-CO2-microalgae) shows a minimum at a specific pH then increases with decreasing pH. Therefore, various processes and carbon sinks in the biokarst system are sensitive to pH changes, and biokarst processes play an important negative feedback role in the release of CO2 by acidification. The results also suggest that the carbon sink associated with carbonate weathering cannot be neglected when considering the global carbon cycle on the scale of thousands of years (
      PubDate: 2016-12-20T13:21:08.2333-05:00
      DOI: 10.1002/2016GC006628
       
  • Fitful and protracted magma assembly leading to a Giant eruption, Youngest
           Toba Tuff, Indonesia
    • Authors: Mary R. Reid; Jorge A. Vazquez
      Abstract: The paroxysmal eruption of the 74 ka Youngest Toba Tuff (YTT) of northern Sumatra produced an extraordinary 2800 km3 of non-welded to densely welded ignimbrite and co-ignimbrite ash-fall. We report insights into the duration of YTT magma assembly obtained from ion microprobe U-Th and U-Pb dates, including continuous age spectra over >50% of final zircon growth, for pumices and a welded tuff spanning the compositional range of the YTT. A relatively large subpopulation of zircon crystals nucleated before the penultimate caldera-related eruption at 501 ka, but most zircons yielded interior dates 100-300 ka thereafter. Zircon nucleation and growth was likely episodic and from diverse conditions over protracted time intervals of >100 to >500 ka. Final zircon growth is evident as thin rim plateaus that are in Th/U chemical equilibrium with hosts, and that give crystallization ages within tens of ka of eruption. The longevity and chemical characteristics of the YTT zircons, as well as evidence for intermittent zircon isolation and remobilization associated with magma recharge, is especially favored at the cool and wet eutectoid conditions that characterize at least half of the YTT, wherein heat fluxes could dissolve major phases but have only a minor effect on larger zircon crystals. Repeated magma recharge may have contributed to the development of compositional zoning in the YTT but, considered together with limited allanite, quartz, and other mineral dating and geospeedometry, regular perturbations to the magma reservoir over >400 ka did not lead to eruption until 74 ka ago. This article is protected by copyright. All rights reserved.
      PubDate: 2016-12-20T13:21:04.763827-05:
      DOI: 10.1002/2016GC006641
       
  • Controls on evolution of gas-hydrate system in the Krishna-Godavari Basin,
           offshore India
    • Authors: F. Badesab; P. Dewangan, A. Usapkar, M. Kocherla, A. Peketi, K. Mohite, S.J. Sangode, K. Deenadayalan
      Abstract: In this study, we integrate environmental magnetic, sedimentological and geochemical records of sediment core of Hole NGHP-01-10D overlying methane hydrate deposits to decipher the controls on the evolution of fracture-filled gas hydrate system in the Krishna-Godavari (K-G) basin. Four distinct sedimentary units have been identified, based on the sediment magnetic signatures. An anomalous zone of enhanced magnetic susceptibility (Unit – III: 51.9 - 160.4 mbsf) coinciding with the gas hydrate bearing intervals is due to the presence of magnetite-rich detrital minerals brought-in by the river systems as a result of higher sedimentation events in K-G basin and has no influence over hydrate formation. A strong to moderate correlation between magnetite concentration and chromium reducible sulphur (CRS) content indicates significant influence of sulfidization on the magnetic record and could be further exploited as a proxy to decipher paleo-H2S seepage events. Analysis of high-resolution seismic, bathymetry and sub-bottom profiler data reveals the existence of a regional fault system in K-G basin. The opening and closing dynamics of the faults facilitated the migration and trapping of required gas concentrations resulting in accumulation of gas hydrates at the studied site. The seismic data provides support to the rock-magnetic interpretations. The observed variations in magnetic and geochemical properties have resulted from the episodic flow of methane and sulphide-enriched fluids through the fracture-filled network formed as a result of shale-tectonism. Our study demonstrated the potential of using an enviro-magnetic approach in combination with other proxies to constrain the evolution of gas hydrate system in marine environments. This article is protected by copyright. All rights reserved.
      PubDate: 2016-12-20T13:20:55.91911-05:0
      DOI: 10.1002/2016GC006606
       
  • Short organic carbon turnover time and narrow 14C Age spectra in early
           Holocene wetland paleosols
    • Authors: Lael Vetter; Brad E. Rosenheim, Alvaro Fernandez, Torbjörn E. Törnqvist
      Abstract: Paleosols contain information about the rates of soil organic carbon turnover when the soil was actively forming. However, this temporal information is often difficult to interpret without tight stratigraphic control on the age of the paleosol. Here we apply ramped pyrolysis/oxidation (Ramped PyrOx) 14C analyses to evaluate age spectra of transgressive early Holocene paleosols from the Mississippi Delta in southeastern Louisiana, U.S.A. We find 14C age spectra from soil organic matter (SOM) in both paleosols and overlying basal peats that represent variability in age that is close to, or only slightly greater than, analytical uncertainty of 14C measurements. Such age spectra have not previously been observed in the sedimentary record. Here, they indicate vigorous soil carbon turnover prior to burial, which homogenized 14C ages within SOM across the entire thermochemical spectrum. The weighted bulk 14C ages from Ramped PyrOx of paleosols and overlying peats are identical within analytical and process-associated uncertainty, and corroborate 14C ages from charcoal fragments and plant macrofossils from the overlying peat. The youngest ages from Ramped PyrOx age spectra may also potentially be applied as chronometers for stratigraphic burial ages. Our results suggest rapid turnover (≪300 yr) of carbon in these soils relative to input of allochthonous carbon, indicating that the 14C age of different soil components is decoupled from thermochemical stability and instead reflects vigorous turnover processes. The concurrence of paleosol and peat 14C ages also suggests that pedogenic processes were linked with the development of coastal marshes, and that the priming effect potentially masked the signal of allochthonous carbon inputs during sea-level rise. This article is protected by copyright. All rights reserved.
      PubDate: 2016-12-20T13:20:52.913745-05:
      DOI: 10.1002/2016GC006526
       
  • Variation in magnetic properties of serpentinized peridotites exposed on
           the Yokoniwa Rise, Central Indian Ridge: Insights into the role of
           magnetite in serpentinization
    • Authors: Masakazu Fujii; Kyoko Okino, Hiroshi Sato, Kentaro Nakamura, Taichi Sato, Toshitsugu Yamazaki
      Abstract: Magnetic properties in serpentinized peridotites are of increasing interest in seafloor mapping and petrologic studies because such data can promote the understanding of serpentinization reactions and hydrogen creation in ultramafic rocks. In order to reveal the magnetic properties and magnetite growth in serpentinized peridotites, we analyzed 30 serpentinized peridotite samples from a non-transform offset massif called the Yokoniwa Rise in the Central Indian Ridge. The results from multiple rock magnetic analyses and petrological observations illustrate the details of the creation and growth of magnetite in serpentinized peridotites that have undergone 17 − 100% serpentinzation. The magnetic carrier of these samples is pure magnetite, which did not suffer from maghemitization (low-temperature oxidation). The magnetic susceptibility ranged from 0.002 − 0.087 SI and increased nonlinearly with the progression of the serpentinization reaction. The natural remanent magnetization intensities of 0.2 − 8.4 A/m are comparable to those of basalts, which suggests that the remanence as well as induced magnetization of highly serpentinized peridotite can contribute to magnetization of the oceanic lithosphere. The amount of magnetite estimated from saturation magnetization increased nonlinearly from 0.1 wt% to 5.5 wt% with the progression of the serpentinization. Highly serpentinized peridotites have a well-developed serpentine mesh texture. Pseudo-single-domain (PSD) and multi-domain (MD) grains were formed during igneous processes in the mantle and/or during the initial stages of serpentinization. Super-paramagnetic (SP) particles were formed during the initial stages of serpentinization. Single-domain (SD) magnetite was formed during the later stage of serpentinization, and it is assembled inside of mesh structures with strong magnetostatic interactions. This article is protected by copyright. All rights reserved.
      PubDate: 2016-12-20T13:20:51.120621-05:
      DOI: 10.1002/2016GC006511
       
  • Subduction, high–P metamorphism, and collision fingerprints in SW Iran:
           Constraints from zircon U–Pb and mica Rb–Sr geochronology
    • Authors: H. Shafaii Moghadam; M. Bröcker, W. L. Griffin, X. H– Li, R. X. Chen, S. Y. O'Reilly
      Abstract: The Esfandagheh region of the Zagros Orogenic Belt is an ideal area to address many aspects of continental convergence between Arabia and Eurasia, including incorporation of Late Neoproterozoic (Ediacaran) basement, subduction–related magmatism and the formation of HP/LT rocks. The rock units exposed here represent a presumably Jurassic magmatic arc within the Sanandaj–Sirjan Zone (SSNZ), remnants of oceanic lithosphere, blueschist– and greenschist–facies rocks, and a distinct group of poorly characterized rocks.U–Pb ages define four populations, related to Paleoproterozoic, Ediacaran, Carboniferous and Jurassic magmatic events. U–Pb ages of ca. 1.8–1.7 Ga for a pegmatite represent the first report of Paleoproterozoic rocks in Iran. Zircon U–Pb ages from the SSNZ provide evidence for Ediacaran (547 Ma), Carboniferous (326–312 Ma) and Jurassic (194–186 Ma) magmatic activity. Zircons from the Haji–Abad ophiolites yielded Jurassic ages.The new Rb–Sr results from white micas provide indications of a poorly–constrained >85 Ma high–pressure metamorphic history. Rb–Sr ages of two chlorite–epidote–actinolite schists indicate that greenschist–facies P–T conditions had already been attained around 130–125 Ma.The new results are consistent with a model in which the closure of the Esfandagheh Ocean and the subsequent collision between Arabia and Iran led to incorporation of Paleoproterozoic and Cadomian rock units and tectonic juxtaposition of all lithotectonic elements, from oceanic lithosphere to continental crust, along the Main Zagros Suture Zone. This article is protected by copyright. All rights reserved.
      PubDate: 2016-12-20T13:20:48.757196-05:
      DOI: 10.1002/2016GC006585
       
  • Madagascar's escape from Africa: A high-resolution plate reconstruction
           for the Western Somali Basin and implications for supercontinent dispersal
           
    • Authors: Jordan J.J. Phethean; Lara M. Kalnins, Jeroen van Hunen, Paolo G. Biffi, Richard J. Davies, Ken J.W. McCaffrey
      Abstract: Accurate reconstructions of the dispersal of supercontinent blocks are essential for testing continental breakup models. Here, we provide a new plate tectonic reconstruction of the opening of the Western Somali Basin during the breakup of East and West Gondwana. The model is constrained by a new comprehensive set of spreading lineaments, detected in this heavily sedimented basin using a novel technique based on directional derivatives of free-air gravity anomalies. Vertical gravity gradient and free-air gravity anomaly maps also enable the detection of extinct mid-ocean ridge segments which can be directly compared to several previous ocean magnetic anomaly interpretations of the Western Somali Basin. The best-matching interpretations have basin symmetry around the M0 anomaly; these are then used to temporally constrain our plate tectonic reconstruction. The reconstruction supports a tight fit for Gondwana fragments prior to breakup, and predicts that the continent-ocean transform margin lies along the Rovuma Basin, not along the Davie Fracture Zone (DFZ) as commonly thought. According to our reconstruction, the DFZ represents a major ocean-ocean fracture zone formed by the coalescence of several smaller fracture zones during evolving plate motions as Madagascar drifted southwards, and offshore Tanzania is an obliquely rifted, rather than transform, margin. New seismic reflection evidence for oceanic crust inboard of the DFZ strongly supports these conclusions. Our results provide important new constraints on the still enigmatic driving mechanism of continental rifting, the nature of the lithosphere in the Western Somali Basin, and its resource potential. This article is protected by copyright. All rights reserved.
      PubDate: 2016-12-20T13:20:30.107079-05:
      DOI: 10.1002/2016GC006624
       
  • Paleoweathering and paleoenvironmental change recorded in lacustrine
           sediments of the early to middle Eocene in Fushun Basin, Northeast China
    • Authors: Zuoling Chen; Zhongli Ding, Zihua Tang, Shiling Yang, Xu Wang, Linlin Cui
      Abstract: Deciphering the long-term interaction between continental silicate weathering, global climate, and atmospheric CO2 concentrations is helpful in understanding the mechanisms of the Cenozoic climate change and accessing the future climatic and environmental response to anthropogenic carbon emissions. The Eocene, which is characterized by the Early Eocene Climatic Optimum (EECO) and the following global cooling, represents an ideal test case. Here we generate geochemical data of the Eocene lacustrine sediments from the Fushun Basin, northeast China, to explore the regional climatic response to the global climate change. The chemical index of alteration (CIA) and plagioclase index of alteration (PIA) consistently show a gradual, long-term decrease, indicating a climatic transition from warm and humid to relatively cold and arid during the Eocene in the Fushun Basin. This climatic trend is broadly coincident with the global cooling and decreasing CO2 concentration, implying that the regional climate is closely correlated with the global climate change over geological time scales. Additionally, the extreme silicate weathering and high lake productivity as reflected by relatively positive δ13C values of lacustrine organic matter are associated with the EECO. This consistency may demonstrate that enhanced continental weathering and lake productivity had served as effective sinks to lower atmospheric CO2 across the EECO. Collectively, our new geochemical data add supporting evidence for a long-term, close coupling between continental silicate weathering, climate and global carbon cycle during the Eocene. This article is protected by copyright. All rights reserved.
      PubDate: 2016-12-20T12:47:06.240895-05:
      DOI: 10.1002/2016GC006573
       
  • Effect of graphite on the electrical conductivity of the lithospheric
           mantle
    • Authors: Baohua Zhang; Takashi Yoshino
      Abstract: Graphite is considered as one of candidate to explain the high-conductivity anomalies revealed through magnetotelluric (MT) observations. To investigate the effect of interfacial energy on the interconnection of graphite in olivine matrix, we measured the electrical conductivity of polycrystalline San Carlos olivine mixed with 0.8 vol.% graphite on the grain boundaries via impedance spectroscopy at 1 GPa and 300–1700 K in a cubic multi-anvil apparatus. The olivine–graphite dihedral angle of the recovered sample was also measured to determine interfacial energy between graphite and olivine. The bulk electrical conductivities and large activation enthalpy (∼1.32 eV) of the carbon-bearing sample were consistent with those of dry polycrystalline olivine. This behavior implies that graphite cannot be interconnected on olivine grain boundaries, which is also supported by the large dihedral angle (98°) of the olivine/graphite system. Impedance spectroscopy measurements were performed at 3 GPa and a temperature of up to 1700 K for carbon-coated olivine bicrystal samples to investigate the stability of graphite films on the grain boundaries of silicate minerals under upper-mantle conditions. The conductivities rapidly or slowly dropped as a function of time and graphite film thickness during annealing at the target temperature. This phenomenon exhibits that graphite film on the olivine grain boundary is readily destroyed under upper-mantle conditions as supported by microstructural observations on the recovered carbon-coated olivine bicrystal samples. Higher interfacial energy and larger dihedral angle (∼ 98°) between graphite and olivine would not allow the maintenance of graphite film on olivine grain boundaries. The activation enthalpy for the apparent disconnection rate of a graphite film on olivine grain boundaries is close to that of carbon diffusion in olivine grain boundaries, which implies suggests that the disconnection of the graphite film is likely to be controlled by carbon grain boundary diffusion. Therefore, graphite is an unlikely candidate to explain the high-conductivity anomalies revealed by MT surveys in the upper mantle. This article is protected by copyright. All rights reserved.
      PubDate: 2016-12-20T12:40:40.05325-05:0
      DOI: 10.1002/2016GC006530
       
  • Splay fault branching from the Hikurangi subduction shear zone:
           Implications for slow slip and fluid flow
    • Authors: A. Plaza-Faverola; S. Henrys, I. Pecher, L. Wallace, D. Klaeschen
      Abstract: Pre-stack depth migration data across the Hikurangi margin, East Coast of the North Island, New Zealand, are used to derive subducting slab geometry, upper crustal structure and seismic velocities resolved to ∼14 km depth. We investigate the potential relationship between the crustal architecture, fluid migration and short-term geodetically determined slow-slip events. The subduction interface is a shallow dipping thrust at 
      PubDate: 2016-12-01T08:47:43.122429-05:
      DOI: 10.1002/2016GC006563
       
  • Effects of titanomagnetite reordering processes on thermal demagnetization
           and paleointensity experiments
    • Authors: Julie A. Bowles; Mike J. Jackson
      Abstract: Titanomagnetite (Fe3-xTixO4, 0 ≤ x ≤ 1) is a common, naturally-occurring magnetic mineral critical to many paleomagnetic studies. Underlying most interpretations is the assumption that, lacking chemical alteration, Curie temperature (Tc) remains constant. However, recent work has demonstrated that Tc of many natural titanomagnetites varies strongly as a function of thermal history, independent of chemical alteration. This is inferred to arise from reordering of cations and/or vacancies in the crystal structure, and changes occur at temperatures and times relevant to standard paleomagnetic thermal treatments. Because changes take place at T  Tclose will have their original Tb spectrum truncated at T ≈ Tclose. Predicted behavior during Thellier-type paleointensity experiments results in only modest deviations in NRM-lost or pTRM*-gained from the non-reordering case. Much larger deviations are predicted for pTRM checks. Compared to paleointensity results from titanomagnetite-bearing pyroclastic deposits, modeled non-ideal behavior occurs in the same temperature intervals, but is much more systematic. Reordering is likely one contributing factor to failure of paleointensity experiments. This article is protected by copyright. All rights reserved.
      PubDate: 2016-12-01T02:25:22.951611-05:
      DOI: 10.1002/2016GC006607
       
  • Postrift magmatic evolution of the eastern North American
           “passive-aggressive” margin
    • Authors: Sarah E. Mazza; Esteban Gazel, Elizabeth A. Johnson, Michael Bizimis, Ryan McAleer, C. Berk Biryol
      Abstract: Understanding the evolution of passive margins requires knowledge of temporal and chemical constraints on magmatism following the transition from super-continent to rifting, to post-rifting evolution. The Eastern North American Margin (ENAM) is an ideal study location as several magmatic pulses occurred in the 200 My following rifting. In particular, the Virginia-West Virginia region of the ENAM has experienced two post-rift magmatic pulses at ∼152 Ma and 47 Ma, and thus provides a unique opportunity to study the long-term magmatic evolution of passive margins. Here we present a comprehensive set of geochemical data that includes new 40Ar/39Ar ages, major and trace-element compositions, and analysis of radiogenic isotopes to further constrain their magmatic history. The Late Jurassic volcanics are bi-modal, from basanites to phonolites, while the Eocene volcanics range from picrobasalt to rhyolite. Modeling suggests that the felsic volcanics from both the Late Jurassic and Eocene events are consistent with fractional crystallization. Sr-Nd-Pb systematics for the Late Jurassic event suggests HIMU and EMII components in the magma source that we interpret as upper mantle components rather than crustal interaction. Lithospheric delamination is the best hypothesis for magmatism in Virginia/West Virginia, due to tectonic instabilities that are remnant from the long-term evolution of this margin, resulting in a “passive-aggressive” margin that records multiple magmatic events long after rifting ended. This article is protected by copyright. All rights reserved.
      PubDate: 2016-11-26T04:08:07.788298-05:
      DOI: 10.1002/2016GC006646
       
  • Boron desorption and fractionation in Subduction Zone Forearcs:
           Implications for the sources and transport of deep fluids
    • Authors: Demian M. Saffer; Achim J. Kopf
      Abstract: At many subduction zones, pore water geochemical anomalies at seafloor seeps and in shallow boreholes indicate fluid flow and chemical transport from depths of several km. Identifying the source regions for these fluids is essential toward quantifying flow pathways and volatile fluxes through forearcs, and in understanding their connection to the loci of excess pore pressure at depth. Here, we develop a model to track the coupled effects of boron desorption, smectite dehydration, and progressive consolidation within sediment at the top of the subducting slab, where such deep fluid signals likely originate. Our analysis demonstrates that the relative timing of heating and consolidation is a dominant control on pore water composition. For cold slabs, pore water freshening is maximized because dehydration releases bound water into low porosity sediment, whereas boron concentrations and isotopic signatures are modest because desorption is strongly sensitive to temperature and is only partially complete. For warmer slabs, freshening is smaller, because dehydration occurs earlier and into larger porosities, but the boron signatures are larger. The former scenario is typical of non-accretionary margins where insulating sediment on the subducting plate is commonly thin. This result provides a quantitative explanation for the global observation that signatures of deeply-sourced fluids are generally strongest at non-accretionary margins. Application of our multi-tracer approach to the Costa Rica, N. Japan, N. Barbados, and Mediterranean Ridge subduction zones illustrates that desorption and dehydration are viable explanations for observed geochemical signals, and suggest up-dip fluid migration from these source regions over tens of km. This article is protected by copyright. All rights reserved.
      PubDate: 2016-11-23T03:40:39.676815-05:
      DOI: 10.1002/2016GC006635
       
  • Physical Properties and Seismic Structure of Izu-Bonin-Mariana Fore Arc
           
    • Authors: G.L. Christeson; S. Morgan, S. Kodaira, M. Yamashita, R.R. Almeev, K. Michibayashi, T. Sakuyama, E.C. Ferré, W. Kurz
      Abstract: Most of the well-preserved ophiolite complexes are believed to form in supra-subduction zone (SSZ) settings. We compare physical properties and seismic structure of SSZ crust at the Izu-Bonin-Mariana (IBM) fore arc with oceanic crust drilled at Holes 504B and 1256D to evaluate the similarities of SSZ and oceanic crust. Expedition 352 basement consists of fore arc basalt (FAB) and boninite lavas and dikes. P-wave sonic log velocities are substantially lower for the IBM fore arc (mean values 3.1-3.4 km/s) compared to Holes 504B and 1256D (mean values 5.0-5.2 km/s) at depths of 0-300 m below the sediment-basement interface. For similar porosities, lower P-wave sonic log velocities are observed at the IBM fore arc than at Holes 504B and 1256D. We use a theoretical asperity compression model to calculate the fractional area of asperity contact Af across cracks. Af values are 0.021-0.025 at the IBM fore arc and 0.074-0.080 at Holes 504B and 1256D for similar depth intervals (0-300 m within basement). The Af values indicate more open (but not necessarily wider) cracks in the IBM fore arc than for the oceanic crust at Holes 504B and 1256D, which is consistent with observations of fracturing and alteration at the Expedition 352 sites. Seismic refraction data constrains a crustal thickness of 10-15 km along the IBM fore arc. Implications and inferences are that crust composing ophiolites formed at SSZ settings could be thick and modified after accretion, and these processes should be considered when using ophiolites as an analog for oceanic crust. This article is protected by copyright. All rights reserved.
      PubDate: 2016-11-22T03:35:40.415991-05:
      DOI: 10.1002/2016GC006638
       
  • Dynamics of primary productivity in the northern South China Sea over the
           past 24,000 years
    • Authors: Hongrui Zhang; Chuanlian Liu, Xiaobo Jin, Jiangnan Shi, Shaohua Zhao, Zhimin Jian
      Abstract: In this study, paleoproductivity on millennial scales was precisely reconstructed from core MD12-3428cq in the northern South China Sea (SCS) over the past 24 kyr, based on a transfer function derived from the strong exponential negative correlation between relative abundance of Florisphaera profunda (%FP) in core top sediments and basin-wide satellite-based primary productivity (PP) in the SCS. To detect the potential driving mechanisms of PP, correlation analyses were carried out among our PP records and other paleoenvironment parameters. PP peaked during 18–15 ka in parallel with the strong East Asian Winter Monsoon (EAWM). From 15 ka to the early Holocene, a decrease in PP coincided with sea level progradation and weakening of EAWM, which ultimately reduced fluvial nutrient levels and wind-driven upper water column mixing. Since the middle Holocene, gradually more frequent El Niño-Southern Oscillation (ENSO) events have taken place, further decreasing PP by injecting oligotrophic Kuroshio water masses into the northern SCS. Associated findings conclusively indicated that the main controlling factors of PP in the northern SCS have shifted from EAWM (glacial) to ENSO (interglacial) over the past 24 kyr. This article is protected by copyright. All rights reserved.
      PubDate: 2016-11-18T09:40:21.994586-05:
      DOI: 10.1002/2016GC006602
       
  • Log-ratio transformed major element based multidimensional classification
           for altered High-Mg igneous rocks
    • Authors: Surendra P. Verma; M. Abdelaly Rivera-Gómez, Lorena Díaz-González, Alfredo Quiroz-Ruiz
      Abstract: A new multidimensional classification scheme consistent with the chemical classification of the International Union of Geological Sciences (IUGS) is proposed for the nomenclature of High-Mg altered rocks. Our procedure is based on an extensive database of major element (SiO2, TiO2, Al2O3, Fe2O3t, MnO, MgO, CaO, Na2O, K2O, and P2O5) compositions of a total of 33868 (920 High-Mg and 32948 “Common”) relatively fresh igneous rock samples. The database consisting of these multinormally distributed samples in terms of their isometric log-ratios was used to propose a set of 11 discriminant functions and 6 diagrams to facilitate High-Mg rock classification. The multinormality required by linear discriminant and canonical analysis was ascertained by a new computer program DOMuDaF. One multidimensional function can distinguish the High-Mg and Common igneous rocks with high percent success values of about 86.4% and 98.9%, respectively. Similarly, from 10 discriminant functions the High-Mg rocks can also be classified as one of the four rock types (komatiite, meimechite, picrite, and boninite), with high success values of about 88% to 100%. Satisfactory functioning of this new classification scheme was confirmed by seven independent tests. Five further case studies involving application to highly altered rocks illustrate the usefulness of our proposal. A computer program HMgClaMSys was written to efficiently apply the proposed classification scheme, which will be available for online processing of High-Mg igneous rock compositional data. Monte Carlo simulation modelling and mass-balance computations confirmed the robustness of our classification with respect to analytical errors and post-emplacement compositional changes. This article is protected by copyright. All rights reserved.
      PubDate: 2016-11-17T18:25:33.009641-05:
      DOI: 10.1002/2016GC006652
       
  • Trace element and Sr isotope records of multi-episode carbonatite
           metasomatism on the eastern margin of the North China Craton
    • Authors: Lixu Deng; Yongsheng Liu, Keqing Zong, Lüyun Zhu, Rong Xu, Zhaochu Hu, Shan Gao
      Abstract: Lherzolite xenoliths entrained in the Changle Cenozoic basalts were analyzed to infer mantle process beneath the eastern block of the North China Craton. These xenoliths were classified into two types. Clinopyroxenes (Cpx) in the type-1 xenoliths are strongly enriched in large ion lithophile elements and light rare earth elements (LREE) but depleted in high field-strength elements and heavy rare earth elements, and show high Ca/Al, Zr/Hf and (La/Yb)N ratios but low Ti/Eu ratios. These features indicate that they were crystallized from a carbonatitic melt. Cpx in the type-2 xenoliths are mostly characterized by a chemical zonation, i.e., LREE and Sr contents and (La/Yb)N and Eu/Ti ratios gradually increase from the cores to the rims. Some fresh cores preserve the original signatures of the depleted mantle. These observations indicate partial modification of pre-existing Cpx by carbonatite metasomatism.Two episodes of metasomatism were identified based on Sr isotopic compositions of Cpx and carbonate inclusions within olivines. Both the carbonate inclusions and Cpx cores in the type-2 xenoliths have relatively high 87Sr/86Sr ratios (>0.7033), suggesting metasomatism due to CO2-rich silicate melt derived from the recycled oceanic crust. However, low 87Sr/86Sr ratios of Cpx rims in the type-2 xenoliths suggest a late stage of metasomatism by a low-87Sr/86Sr carbonatitic melt (
      PubDate: 2016-11-17T18:25:29.985081-05:
      DOI: 10.1002/2016GC006618
       
  • A practical tool for examining paleoerosion rates from sedimentary
           deposits using cosmogenic radionuclides: Examples from hypothetical
           scenarios and data
    • Authors: Pedro Val; Greg Hoke
      Abstract: We provide a MatlabTM-based algorithm that calculates paleo-erosion rates based on measured Cosmogenic Radionuclide (CRN) concentrations from sedimentary deposits and additional geological constraints provided by the user. Based on the input data, the algorithm models CRN concentrations accumulated due to sediment burial and the subsequent exhumation of these deposits. Description of the methods and applications of the algorithm is provided, which includes single or coupled CRNs (i.e. 10Be or 10Be + 26Al). Results from the literature are reproduced. Further applicability of this algorithm is demonstrated with hypothetical scenarios of paleo-erosion rates. The tool can be used towards the exploration of necessary assumptions, feasibility of case-studies, and method limits. This contribution accompanies an overview of the paleo-erosion rate method, its applications, and necessary assumptions. This article is protected by copyright. All rights reserved.
      PubDate: 2016-11-17T18:25:24.107505-05:
      DOI: 10.1002/2016GC006608
       
  • Cenozoic epeirogeny of the Indian Peninsula
    • Authors: F. D. Richards; M. J. Hoggard, N. J. White
      Abstract: Peninsular India is a cratonic region with asymmetric relief manifest by eastward tilting from the 1.5 km high Western Ghats escarpment toward the floodplains of eastward-draining rivers. Oceanic residual depth measurements on either side of India show that this west-east asymmetry is broader scale, occurring over distances of >2,000 km. Admittance analysis of free-air gravity and topography shows that the elastic thickness is 10 ±3 km, suggesting that regional uplift is not solely caused by flexural loading. To investigate how Indian physiography is generated, we have jointly inverted 530 river profiles to determine rock uplift rate as a function of space and time. Key erosional parameters are calibrated using independent geologic constraints (e.g. emergent marine deposits, elevated paleosurfaces, uplifted lignite deposits). Our results suggest that regional tilt grew at rates of up to 0.1 mm a– 1 between 25 Ma and the present day. Neogene uplift initiated in the south and propagated northward along the western margin. This calculated history is corroborated by low-temperature thermochronologic observations, by sedimentary flux of clastic deposits into the Krishna-Godavari delta, and by sequence stratigraphic architecture along adjacent rifted margins. Onset of regional uplift predates intensification of the Indian monsoon at 8 Ma, suggesting that rock uplift rather than climatic change is responsible for modern-day relief. A positive correlation between residual depth measurements and shear wave velocities beneath the lithosphere suggests that regional uplift is generated and maintained by temperature anomalies of ±100°C within a 200 ±25 km thick asthenospheric channel. This article is protected by copyright. All rights reserved.
      PubDate: 2016-11-17T11:00:42.57124-05:0
      DOI: 10.1002/2016GC006545
       
  • Estimating the duration of geologic intervals from a small number of age
           determinations: A challenge common to petrology and paleobiology
    • Authors: Allen F. Glazner; Peter M. Sadler
      Abstract: The duration of a geologic interval, such as the time over which a given volume of magma accumulated to form a pluton, or the lifespan of a large igneous province, is commonly determined from a relatively small number of geochronologic determinations (e.g., 4-10) within that interval. Such sample sets can underestimate the true length of the interval by a significant amount. For example, the average interval determined from a sample of size n = 5, drawn from a uniform random distribution, will underestimate the true interval by 50%. Even for n = 10 the average sample only captures ∼80% of the interval. If the underlying distribution is known then a correction factor can be determined from theory or Monte Carlo analysis; for a uniform random distribution this factor is . Systematic undersampling of interval lengths can have a large effect on calculated magma fluxes in plutonic systems. The problem is analogous to determining the duration of an extinct species from its fossil occurrences. Confidence interval statistics developed for species origination and extinction times are applicable to the onset and cessation of magmatic events. This article is protected by copyright. All rights reserved.
      PubDate: 2016-11-17T10:34:35.380231-05:
      DOI: 10.1002/2016GC006542
       
  • Sediment melting during subduction initiation: Geochronological and
           geochemical evidence from the Darutso high-Mg andesites within ophiolite
           melange, central Tibet
    • Authors: Zeng Yunchuan; Chen Jianlin, Xu Jifeng, Wang Baodi, Huang Feng
      Abstract: In addition to fluids, the concept of sediment-derived melts infiltrating the fore-arc mantle during subduction initiation has been proposed based on studies of modern subduction zones and ophiolite mélange. However, outcrops that contain the products of such melts are rare, especially in conjunction with boninite. New U–Pb zircon dating reveals that the Darutso volcanic rocks (DVRs) within ophiolitic mélange in the Beila area, central Tibet, crystallized at ∼164–162 Ma. This is the first recognition of Jurassic volcanic rocks in the middle section of the Bangong–Nujiang Suture Zone. Geochemically, the DVRs are high-Mg andesites with moderate SiO2 (59.03–63.62 wt%) and high MgO (3.74–6.53 wt%), Cr (up to 395 ppm), and Mg# (50.3–67.9). They also have high Th contents, (La/Sm)N ratios, and (87Sr/86Sr)i values (0.7085–0.7147); low Ba/Th, U/Th, and Sr/Y ratios; and negative values of εNd(t) (−8.7 to −9.8) and zircon εHf(t) (−7.4 to −9.9). The εNd(t) values of the DVRs overlap those of regional sediments. Detailed analyses of these geochemical characteristics indicate that the DVRs were derived from partial melting of subducted sediments and subsequent interaction with overlying mantle peridotite in a shallow and hot setting. In combination with the regional geology, in particular adjacent ophiolites that contain MORB-like and boninite mafic lavas, these rocks collectively recorded the evolution of a fore-arc setting during the initiation of the northward subduction of the south branch of the Bangong–Nujiang Ocean. Therefore, the results provide direct evidence for sediment melting during subduction initiation and constrain the Jurassic tectonic evolution of the Lhasa terrane. This article is protected by copyright. All rights reserved.
      PubDate: 2016-11-17T10:34:32.471811-05:
      DOI: 10.1002/2016GC006456
       
  • Fault zone controlled seafloor methane seepage in the rupture area of the
           2010 Maule earthquake, Central Chile
    • Authors: Jacob Geersen; Florian Scholz, Peter Linke, Mark Schmidt, Dietrich Lange, Jan H. Behrmann, David Völker, Christian Hensen
      Abstract: Seafloor seepage of hydrocarbon-bearing fluids has been identified in a number of marine forearcs. However, temporal variations in seep activity and the structural and tectonic parameters that control the seepage often remain poorly constrained. Subduction-zone earthquakes for example, are often discussed to trigger seafloor seepage but causal links that go beyond theoretical considerations have not yet been fully established. This is mainly due to the inaccessibility of offshore epicentral areas, the infrequent occurrence of large earthquakes, and challenges associated with offshore monitoring of seepage over large areas and sufficient time periods. Here, we report visual, geochemical, geophysical, and modelling results and observations from the Concepción Methane Seep Area (offshore Central Chile) located in the rupture area of the 2010 Mw. 8.8 Maule earthquake. High methane concentrations in the oceanic water column and a shallow sub-bottom depth of sulfate penetration indicate active methane seepage. The stable carbon isotope signature of the methane and hydrocarbon composition of the released gas indicate a mixture of shallow-sourced biogenic gas and a deeper sourced thermogenic component. Pristine fissures and fractures observed at the seafloor together with seismically imaged large faults in the marine forearc may represent effective pathways for methane migration. Upper-plate fault activity with hydraulic fracturing and dilation is in line with increased normal Coulomb stress during large plate-boundary earthquakes, as exemplarily modelled for the 2010 earthquake. On a global perspective our results point out the possible role of recurring large subduction-zone earthquakes in driving hydrocarbon seepage from marine forearcs over long timescales. This article is protected by copyright. All rights reserved.
      PubDate: 2016-11-14T19:01:26.751583-05:
      DOI: 10.1002/2016GC006498
       
  • Variations in slow slip moment rate associated with rapid tremor reversals
           in Cascadia
    • Authors: Jessica C. Hawthorne; Michael G. Bostock, Alexandra A. Royer, Amanda M. Thomas
      Abstract: During large slow slip events, tremor sometimes propagates in the reverse along-strike direction for a few hours, at speeds 10 to 40 times faster than the forward propagation. We examine the aseismic slip that underlies this rapidly propagating tremor. We use PBO (Plate Boundary Observatory) borehole strainmeter data to search for variations in the slow slip moment rate during 35 rapid tremor reversals (RTRs) that occurred beneath Vancouver Island. and were identified via low frequency earthquake (LFE) analysis of tremor. The strain records reveal that, on average, the strain rate increases by about 100% (±30%) during RTRs. Given the Green's functions expected for slip in the RTR locations, these strain rate increases imply 50 to 130% increases in the aseismic moment rate. The median moment released per RTR is between 8 and 21% of the daily slow slip moment, equivalent to that of a MW 5.0 to 5.1 earthquake. By combining the RTR moments with the spatial extents suggested by tremor, we estimate that a typical RTR has peak slip of roughly one-sixth of the peak slip in the main slow slip event, near-front slip rate of a few to ten times the main front slip rate, stress drop around half the main event stress drop, and strain energy release rate around one-tenth that of the main front. Our observations support a picture of RTRs as aseismic subevents with high slip rates but modest strain energy release. RTRs appear to contribute to but not dominate the overall slow slip moment, though they may accommodate most of the slip in certain locations. This article is protected by copyright. All rights reserved.
      PubDate: 2016-11-14T03:50:56.590098-05:
      DOI: 10.1002/2016GC006489
       
  • Compositional layering within the large low shear-wave velocity provinces
           in the lower mantle
    • Authors: Maxim D. Ballmer; Lina Schumacher, Vedran Lekic, Christine Thomas, Garrett Ito
      Abstract: The large low shear-wave velocity provinces (LLSVP) are thermochemical anomalies in the deep Earth's mantle, thousands of km wide and ∼1,800 km high. This study explores the hypothesis that the LLSVPs are compositionally subdivided into two domains: a primordial bottom domain near the core-mantle boundary and a basaltic shallow domain extending from 1,100∼2,300 km depth. This hypothesis reconciles published observations in that it predicts that the two domains have different physical properties (bulk-sound vs. shear-wave speed vs. density anomalies), the transition in seismic velocities separating them is abrupt, and both domains remain seismically distinct from the ambient mantle. We here report underside reflections from the top of the LLSVP shallow domain, supporting a compositional origin. By exploring a suite of two-dimensional geodynamic models, we constrain the conditions under which well-separated “double-layered” piles with realistic geometry can persist for billions of years. Results show that long-term separation requires density differences of ∼100 kg/m3 between LLSVP materials, providing a constraint for origin and composition. The models further predict short-lived “secondary” plumelets to rise from LLSVP roofs and to entrain basaltic material that has evolved in the lower mantle. Long-lived, vigorous “primary” plumes instead rise from LLSVP margins and entrain a mix of materials, including small fractions of primordial material. These predictions are consistent with the locations of hotspots relative to LLSVPs, and address the geochemical and geochronological record of (oceanic) hotspot volcanism. The study of large-scale heterogeneity within LLSVPs has important implications for our understanding of the evolution and composition of the mantle. This article is protected by copyright. All rights reserved.
      PubDate: 2016-11-12T03:50:41.169931-05:
      DOI: 10.1002/2016GC006605
       
  • Upper mantle structure of the Tonga-Lau-Fiji region from Rayleigh wave
           tomography
    • Authors: S. Shawn Wei; Yang Zha, Weisen Shen, Douglas A. Wiens, James A. Conder, Spahr C. Webb
      Abstract: We investigate the upper mantle seismic structure in the Tonga-Lau-Fiji region by jointly fitting the phase velocities of Rayleigh waves from ambient-noise and two-plane-wave tomography. The results suggest a wide low-velocity zone beneath the Lau Basin, with a minimum SV-velocity of about 3.7 ± 0.1 km/s, indicating upwelling hot asthenosphere with extensive partial melting. The variations of velocity anomalies along the Central and Eastern Lau Spreading Centers suggest varying mantle porosity filled with melt. In the north where the spreading centers are distant from the Tonga slab, the inferred melting commences at about 70 km depth, and forms an inclined zone in the mantle, dipping to the west away from the arc. This pattern suggests a passive decompression melting process supplied by the Australian plate mantle from the west. In the south, as the supply from the Australian mantle is impeded by the Lau Ridge lithosphere, flux melting controlled by water from the nearby slab dominates in the back-arc. This source change results in the rapid transition in geochemistry and axial morphology along the spreading centers. The remnant Lau Ridge and the Fiji Plateau are characterized by 60–80 km thick lithosphere underlain by a low-velocity asthenosphere. Our results suggest the removal of the lithosphere of the northeastern Fiji Plateau-Lau Ridge beneath the active Taveuni Volcano. Azimuthal anisotropy shows that the mantle flow direction rotates from trench-perpendicular beneath Fiji to spreading-perpendicular beneath the Lau Basin, which provides evidence for the southward flow of the mantle wedge and the Samoan plume. This article is protected by copyright. All rights reserved.
      PubDate: 2016-11-07T10:33:07.425762-05:
      DOI: 10.1002/2016GC006656
       
  • The effects of 10 to >160 GPa shock on the magnetic properties of
           basalt and diabase
    • Authors: N. S. Bezaeva; N. L. Swanson-Hysell, S. M. Tikoo, D. D. Badyukov, M. Kars, R. Egli, D. A. Chareev, L. M. Fairchild, E. Khakhalova, B. E. Strauss, A. K. Lindquist
      Abstract: Hypervelocity impacts within the solar system affect both the magnetic remanence and bulk magnetic properties of planetary materials. Spherical shock experiments are a novel way to simulate shock events that enable materials to reach high shock pressures with a variable pressure profile across a single sample (ranging between ∼10 and >160 GPa). Here we present spherical shock experiments on basaltic lava flow and diabase dike samples from the Osler Volcanic Group whose ferromagnetic mineralogy is dominated by pseudo-single-domain (titano)magnetite. Our experiments reveal shock-induced changes in rock magnetic properties including a significant increase in remanent coercivity. Electron and magnetic force microscopy support the interpretation that this coercivity increase is the result of grain fracturing and associated domain wall pinning in multidomain grains. We introduce a method to discriminate between mechanical and thermal effects of shock on magnetic properties. Our approach involves conducting vacuum-heating experiments on untreated specimens and comparing the hysteresis properties of heated and shocked specimens. First order reversal curve (FORC) experiments on untreated, heated and shocked specimens demonstrate that shock and heating effects are fundamentally different for these samples: shock has a magnetic hardening effect that does not alter the intrinsic shape of FORC distributions, while heating alters the magnetic mineralogy as evident from significant changes in the shape of FORC contours. These experiments contextualize paleomagnetic and rock magnetic data of naturally shocked materials from terrestrial and extraterrestrial impact craters. This article is protected by copyright. All rights reserved.
      PubDate: 2016-11-07T09:30:40.686061-05:
      DOI: 10.1002/2016GC006583
       
  • The temperature of the Icelandic mantle from olivine-spinel aluminum
           exchange thermometry
    • Authors: S. Matthews; O. Shorttle, J. Maclennan
      Abstract: New crystallisation temperatures for four eruptions from the Northern Volcanic Zone of Iceland are determined using olivine-spinel aluminum exchange thermometry. Differences in the olivine crystallisation temperatures between these eruptions are consistent with variable extents of cooling during fractional crystallisation. However, the crystallisation temperatures for Iceland are systematically offset to higher temperatures than equivalent olivine-spinel aluminum exchange crystallisation temperatures published for MORB, an effect that cannot be explained by fractional crystallisation. The highest observed crystallisation temperature in Iceland is 1399±20°C.In order to convert crystallisation temperatures to mantle potential temperature, we developed a model of multi-lithology mantle melting that tracks the thermal evolution of the mantle during isentropic decompression melting. With this model, we explore the controls on the temperature at which primary melts begin to crystallise, as a function of source composition and the depth from which the magmas are derived. Large differences (C) in crystallisation temperature can be generated by variations in mantle lithology, a magma's inferred depth of origin, and its thermal history. Combining this model with independent constraints on the magma volume flux and the effect of lithological heterogeneity on melt production, restricted regions of potential temperature-lithology space can be identified as consistent with the observed crystallisation temperatures. Mantle potential temperature is constrained to be 1480−30+37 ° C for Iceland and 1318−32+44 °C for MORB. This article is protected by copyright. All rights reserved.
      PubDate: 2016-11-07T09:21:45.063567-05:
      DOI: 10.1002/2016GC006497
       
  • A quantitative assessment of methane cycling in Hikurangi Margin sediments
           (New Zealand) using geophysical imaging and biogeochemical modeling
    • Authors: Min Luo; Andrew W. Dale, Laura Haffert, Matthias Haeckel, Stephanie Koch, Gareth Crutchley, Henko De Stigter, Duofu Chen, Jens Greinert
      Abstract: Takahe seep, located on the Opouawe Bank, Hikurangi Margin, is characterized by a well-defined subsurface seismic chimney structure ca. 80,500 m2 in area. Sub-seafloor geophysical data based on acoustic anomaly layers indicated the presence of gas hydrate and free gas layers within the chimney structure. Reaction-transport modeling was applied to porewater data from 11 gravity cores to constrain methane turnover rates and benthic methane fluxes in the upper 10 m. Model results show that methane dynamics were highly variable due to transport and dissolution of ascending gas. The dissolution of gas (up to 3761 mmol m−2 yr−1) dwarfed the rate of methanogenesis within the simulated sediment column (2.6 mmol m−2 yr−1). Dissolved methane is mainly consumed by anaerobic oxidation of methane (AOM) at the base of the sulfate reduction zone and trapped by methane hydrate formation below it, with maximum rates in the central part of the chimney (946 and 2420 mmol m−2 yr−1, respectively). A seep-wide methane budget was constrained by combining the biogeochemical model results with geophysical data and led to estimates of AOM rates, gas hydrate formation and benthic dissolved methane fluxes of 3.68 × 104 mol yr−1, 73.85 × 104 mol yr−1and 1.19 × 104 mol yr−1, respectively. A much larger flux of methane probably escapes in gaseous form through focused bubble vents. The approach of linking geochemical model results with spatial geophysical data put forward here can be applied elsewhere to improve benthic methane turnover rates from limited single spot measurements to larger spatial scales. This article is protected by copyright. All rights reserved.
      PubDate: 2016-11-07T09:21:27.390467-05:
      DOI: 10.1002/2016GC006643
       
  • Elemental changes and alteration recorded by basaltic drill core samples
           recovered from in-situ temperatures up to 345°C in the active,
           seawater-recharged Reykjanes geothermal system, Iceland
    • Authors: Andrew P. G. Fowler; Robert A. Zierenberg
      Abstract: Hydrothermal activity results in element exchanges between seawater and oceanic crust that contribute to many aspects of ocean chemistry; therefore, improving knowledge of the associated chemical processes is of global significance. Hydrothermally altered basaltic drill core samples from the seawater recharged Reykjanes geothermal system in Iceland record elemental gains and losses similar to those observed in samples of hydrothermally altered oceanic crust. At Reykjanes, rocks originally emplaced on the seafloor were buried by continued volcanism and subsided to the current depths (>2250 m below surface). These rocks integrate temperature-dependent elemental gains and losses from multiple stages of hydrothermal alteration that correspond to chemical exchanges observed in rocks from different crustal levels of submarine hydrothermal systems. Specifically, these lithologies have gained U, Mg, Zn and Pb, and have lost K, Rb, Ba, Cu, and light rare earth elements (La through Eu). Alteration and elemental gains and losses in lithologies emplaced on the seafloor can only be explained by a complex multi-stage hydrothermal alteration history. Reykjanes dolerite intrusions record alteration similar to that reported for the sheeted dike section of several examples of oceanic crust. Specifically, Reykjanes dolerites have lost K, Rb, Ba and Pb, and gained Cu. The Reykjanes drill core samples provide a unique analog for seawater-oceanic crust reactions actively occurring at high-temperatures (275°C to 345°C) beneath a seafloor hydrothermal system. This article is protected by copyright. All rights reserved.
      PubDate: 2016-11-07T09:21:21.297306-05:
      DOI: 10.1002/2016GC006595
       
  • Benthic foraminiferal growth seasons implied from Mg/Ca-temperature
           correlations for three Arctic species
    • Authors: Kari Skirbekk; Morten Hald, Thomas Marchitto, Juho Junttila, Dorthe Klitgaard Kristensen, Steffen Aagaard Sørensen
      Abstract: Core-top sediment samples from Kongsfjorden, Svalbard and adjacent fjord and shelf areas were collected in order to investigate a potential relationship between Mg/Ca-ratios of Arctic benthic foraminifera and the ambient bottom water temperatures (BWT). The area is influenced by large seasonal variation in factors such as light and temperature, which is further strengthened by oceanographic shifts, including inflow of relatively warm Atlantic water. Four hydrological seasons have been defined. The studied samples were collected during the years 2005-2010 and comprise data from three hydrological seasons: spring, summer and autumn. Five common species of cold-water benthic foraminifera were investigated: Islandiella helenae/norcrossi, Buccella frigida, Nonionellina labradorica, Elphidium clavatum and Cassidulina reniforme. For E. clavatum and C. reniforme, the investigations failed. For the remaining three species, the Mg/Ca-temperature correlations initially appeared stochastic holding correlation coefficients between 0.01 and 0.15. However, grouping the data based on seasons gave stronger Mg/Ca-temperature correlations, indicating specific growing seasons for the three species. The equations represent a starting point for a discussion on seasonality rather than robust, “ready-to-use” equations. I. helenae/norcrossi seem to reproduce and grow during summer (July/August) in outer Kongsfjorden. For B. frigida, a Mg/Ca-temperature correlation is seen both in summer (July/August) and autumn (October/November) samples, indicative of a continuous reproduction/growth-season lasting from July-November. N. labradorica appears to reproduce and grow during autumn (October/November). The results indicate that temperature reconstructions based on these benthic foraminifera reproduce seasonal temperatures rather than annual average temperatures. This article is protected by copyright. All rights reserved.
      PubDate: 2016-11-03T04:04:53.306914-05:
      DOI: 10.1002/2016GC006505
       
  • Seismic imaging of the metamorphism of young sediment into new crystalline
           crust in the actively rifting Imperial Valley, California
    • Authors: Liang Han; John A. Hole, Joann M. Stock, Gary S. Fuis, Colin F. Williams, Jonathan R. Delph, Kathy K. Davenport, Amanda J. Livers
      Abstract: Plate-boundary rifting between transform faults is opening the Imperial Valley of southern California and the rift is rapidly filling with sediment from the Colorado River. Three 65-90 km long seismic refraction profiles across and along the valley, acquired as part of the 2011 Salton Seismic Imaging Project, were analyzed to constrain upper crustal structure and the transition from sediment to underlying crystalline rock. Both first arrival travel-time tomography and frequency-domain full-waveform inversion were applied to provide P-wave velocity models down to ∼7 km depth. The valley margins are fault-bounded, beyond which thinner sediment has been deposited on pre-existing crystalline rocks. Within the central basin, seismic velocity increases continuously from ∼1.8 km/s sediment at the surface to >6 km/s crystalline rock with no sharp discontinuity. Borehole data show young sediment is progressively metamorphosed into crystalline rock. The seismic velocity gradient with depth decreases approximately at the 4 km/s contour, which coincides with changes in the porosity and density gradient in borehole core samples. This change occurs at ∼3 km depth in most of the valley, but at only ∼1.5 km depth in the Salton Sea geothermal field. We interpret progressive metamorphism caused by high heat flow to be creating new crystalline crust throughout the valley at a rate comparable to the ≥2 km/Myr sedimentation rate. The newly formed crystalline crust extends to at least 7-8 km depth, and it is shallower and faster where heat flow is higher. Most of the active seismicity occurs within this new crust. This article is protected by copyright. All rights reserved.
      PubDate: 2016-11-03T04:04:47.756497-05:
      DOI: 10.1002/2016GC006610
       
  • Physical interpretation of isothermal remanent magnetization endmembers:
           New insights into the environmental history of Lake Hovsgul, Mongolia
    • Authors: Karl Fabian; Valeriy P. Shcherbakov, Lina Kosareva, Danis Nourgaliev
      Abstract: Acquisition curves of isothermal remanent magnetization for 1057 samples of core KDP-01 from Lake Hovsgul (Mongolia) are decomposed into three endmembers using non-negative matrix factorization. The obtained mixing coefficients also decompose hysteresis loops, back-field, and strong-field thermomagnetic curves into their related endmember components. This proves that the endmembers represent different mineralogical fractions of the Lake Hovsgul sedimentary environment. The method used for unmixing offers a new possibility to apply rock magnetism in paleoecological and paleoclimatic studies. For Lake Hovsgul it indicates that a low-coercivity component with a co-varying paramagnetic phase represents a coarse-grained magnetite fraction from terrigenous influx probably via fluvial transport. A second component with coercivities close to 50∼mT is identified as a magnetite fraction related to magnetosomes of magnetotactic bacteria. The third component has coercivities near 85∼mT and is identified as greigite of biotic or abiotic origin common in suboxic/anoxic sediments. Significant positive correlations between variations of intensity of all three mineralogical components along the core are found. A rapid drop in all endmember concentrations by more than one order of magnitude at about 20∼m depth testifies to a major change of the environmental or geological conditions of Lake Hovsgul. It possibly is related to the onset of MIS∼10 marking the termination of arid climate conditions. Short intervals of high productivity are characterized by an abundance of magnetite magnetosomes and may highlight glacial-interglacial transition intervals. For the rest of the core greigite magnetization substantially exceeds that of magnetite, indicating a predominantly anoxic environment. This article is protected by copyright. All rights reserved.
      PubDate: 2016-11-03T04:04:39.721302-05:
      DOI: 10.1002/2016GC006506
       
  • Late Cenozoic tephrostratigraphy offshore the southern Central American
           Volcanic Arc: 1. Tephra ages and provenance
    • Authors: J.C. Schindlbeck; S. Kutterolf, A. Freundt, G.E. Alvarado, K.-L. Wang, S.M. Straub, S.R. Hemming, M. Frische, J.D. Woodhead
      Abstract: We studied the tephra inventory of 18 deep sea drill sites from six DSDP/ODP legs (Legs 84, 138, 170, 202, 205, 206) and two IODP legs (Legs 334 and 344) offshore the southern Central American Volcanic Arc (CAVA). Eight drill sites are located on the incoming Cocos plate and ten drill sites on the continental slope of the Caribbean plate. In total we examined ∼840 ash-bearing horizons and identified ∼650 of these as primary ash beds of which 430 originated from the CAVA. Correlations of ash beds were established between marine cores and with terrestrial tephra deposits, using major and trace element glass compositions with respect to relative stratigraphic order. As a prerequisite for marine-terrestrial correlations we present a new geochemical data set for significant Neogene and Quaternary Costa Rican tephras. Moreover, new Ar/Ar ages for marine tephras have been determined and marine ash beds are also dated using the pelagic sedimentation rates. The resulting correlations and provenance analyses build a tephrochronostratigraphic framework for Costa Rica and Nicaragua that covers the last >8 Myr. We define 39 correlations of marine ash beds to specific tephra formations in Costa Rica and Nicaragua; from the 4.15 Ma Lower Sandillal Ignimbrite to the 3.5 ka Rincón de la Vieja Tephra from Costa Rica, as well as another 32 widely distributed tephra layers for which their specific region of origin along Costa Rica and Nicaragua can be constrained. This article is protected by copyright. All rights reserved.
      PubDate: 2016-11-01T18:35:51.31487-05:0
      DOI: 10.1002/2016GC006503
       
  • Magnetic anomalies associated with abundant production of pyrrhotite in a
           sulfide deposit in the Okinawa Trough, Japan
    • Authors: Chie Honsho; Toshitsugu Yamazaki, Tamaki Ura, Kyoko Okino, Haruhisa Morozumi, Satoshi Ueda
      Abstract: We report here results from a deep-sea magnetic survey using an autonomous underwater vehicle over the Hakurei hydrothermal site, in the middle Okinawa Trough. Magnetic inversion revealed that the Hakurei site is associated with well-defined high-magnetization zones distributed within a broad low-magnetization zone. Results from rock magnetic measurements, performed on sulfide ore samples obtained by drilling, showed that some samples possessed extremely high natural remanent magnetization (NRM) (as much as 6.8–953.0 A/m), although most of the measured samples had much lower NRM. These high-NRM samples were characterized by high Königsberger ratios (101−103), indicating much larger NRM than induced magnetization, and contained pyrrhotite as the only magnetic mineral. This suggests that NRM carried by pyrrhotite is the source of the observed magnetic anomalies. The wide range of NRM intensity was considered to be due to a highly heterogeneous distribution of pyrrhotite, because pyrrhotite was commonly identified in both the high- and low-NRM samples. Pyrrhotite production may have been occasionally drastically increased, with highly magnetic ores formed as a result. Rapid burial of active vents may result in the creation of an extensive reducing environment under the seafloor, which is favorable to pyrrhotite production, and may also prevent oxidation of pyrrhotite by isolating it from seawater. Because the magnetization intensity of sulfide ores was highly variable, it would not be straightforward to estimate the quantity of ore deposits from the magnetic anomalies. Nevertheless, this study demonstrates the usefulness of magnetic surveys in detecting hydrothermal deposits. This article is protected by copyright. All rights reserved.
      PubDate: 2016-11-01T18:35:38.207522-05:
      DOI: 10.1002/2016GC006480
       
  • Holocene glacial activity in Barilari Bay, west Antarctic Peninsula,
           tracked by magnetic mineral assemblages: Linking ice, ocean, and
           atmosphere
    • Authors: Brendan T. Reilly; Carl J. Natter, Stefanie A. Brachfeld
      Abstract: We investigate the origin and fate of lithogenic sediments using magnetic mineral assemblages in Barilari Bay, west Antarctic Peninsula (AP) from sediment cores recovered during the Larsen Ice Shelf System, Antarctica (LARISSA) NBP10-01 cruise. To quantify and reconstruct Holocene changes in co-varying magnetic mineral assemblages, we adopt an unsupervised mathematical un-mixing strategy and apply it to measurements of magnetic susceptibility as a function of increasing temperature. Comparisons of the unmixed end-members with magnetic observations of northwestern AP bedrock and the spatial distribution of magnetic mineral assemblages within the fjord, allow us to identify source regions, including signatures for ‘inner bay', ‘outer bay', and ‘northwestern AP' sources. We find strong evidence that supports the establishment of a late Holocene ice shelf in the fjord coeval with the Little Ice Age. Additionally, we present new evidence for late Holocene sensitivity to conditions akin to positive mean Southern Annual Mode states for western AP glaciers at their advanced Neoglacial positions. This article is protected by copyright. All rights reserved.
      PubDate: 2016-11-01T04:10:42.677663-05:
      DOI: 10.1002/2016GC006627
       
  • Reconstruction of rocks petrophysical properties as input data for
           reservoir modeling
    • Authors: B. Cantucci; G. Montegrossi, F. Lucci, F. Quattrocchi
      Abstract: The worldwide increasing energy demand triggered studies focused on defining the underground energy potential even in areas previously discharged or neglected. Nowadays, geological gas storage (CO2 and/or CH4) and geothermal energy are considered strategic for low-carbon energy development.A widespread and safe application of these technologies needs an accurate characterization of the underground, in terms of geology, hydrogeology, geochemistry and geomechanics. However, during pre-feasibility study-stage, the limited number of available direct measurements of reservoirs, and the high costs of reopening closed deep wells must be taken into account.The aim of this work is try to overcome these limits, proposing a new methodology to reconstruct vertical profiles, from surface to reservoir base, of: i) thermal capacity, ii) thermal conductivity, iii) porosity and iv) permeability, through integration of well-log information, petrographic observations on inland outcropping samples and, flow and heat transport modelling.As case study to test our procedure we selected a deep-structure, located in the medium Tyrrhenian Sea (Italy). Obtained results are consistent with measured data, confirming the validity of the proposed model.Notwithstanding intrinsic limitations due to manual calibration of the model with measured data, this methodology represents a useful tool for reservoir and geochemical modellers that need to define petrophysical input data for underground modelling before the well reopening. This article is protected by copyright. All rights reserved.
      PubDate: 2016-11-01T04:01:09.670565-05:
      DOI: 10.1002/2016GC006548
       
  • Porosity and fluid budget of a water-rich megathrust revealed with
           electromagnetic data at the Middle America Trench
    • Authors: Samer Naif; Kerry Key, Steven Constable, Rob L. Evans
      Abstract: At convergent margins, the distribution of fluids released from the downgoing slab modulates the state of stress and seismic coupling at the megathrust plate interface. However, existing geophysical data are unable to quantify the porosity along this interface. Here, we use controlled-source electromagnetic data collected across the Middle America Trench offshore Nicaragua to image the electrical conductivity structure of the outer forearc. Our results detect a highly conductive channel, inferred to be the region around the décollement, showing the entire section of water-rich seafloor sediments underthrust with the subducting lithosphere. We use an empirical model of the electrical conductivity of porous media to quantify the channel porosity. Our estimates are consistent with sediment compaction studies, showing a rapid decay of 65% to 10% porosity from the trench to 25 km landward. We constrain the channel thickness and use the porosity estimates to determine the water budget, which represents the fraction taken up by fluid. The porosity and water budget estimates show significant lateral variations that we attribute to changes in subducted sediment thickness caused by outer rise bending faults. Between 18-23 km from the trench the conductive channel broadens greatly to 1.5-2 km thick, possibly due to concentrated blind faults or sediment underplating, which suggests a sudden change in hydrogeologic structure at the plate interface. The impact of the anomalous conductor on the seismic coupling and mechanical properties of the megathrust are potentially related to the discrepancy in estimated fault slip between seismic and tsunami source inversions for the 1992 Nicaragua tsunami earthquake. This article is protected by copyright. All rights reserved.
      PubDate: 2016-10-27T03:35:33.95596-05:0
      DOI: 10.1002/2016GC006556
       
  • The Campi Flegrei Deep Drilling Project (CFDDP): New insight on caldera
           structure, evolution and hazard implications for the Naples area (Southern
           Italy)
    • Authors: Giuseppe De Natale; Claudia Troise, Darren Mark, Angela Mormone, Monica Piochi, Mauro Antonio Di Vito, Roberto Isaia, Stefano Carlino, Diana Barra, Renato Somma
      Abstract: The 501m-deep hole of the Campi Flegrei Deep Drilling Project, located west of the Naples metropolitan area and inside the Campi Flegrei caldera, gives new insight to reconstruct the volcano-tectonic evolution of this highly populated volcano. It is one of the highest risk volcanic areas in the world, but its tectonic structure, eruptive history and size of the largest eruptions are intensely debated in literature. New stratigraphic and 40Ar/39Ar geochronological dating allow us to determine, for the first time, the age of intra-caldera deposits belonging to the two highest magnitude caldera-forming eruptions (i.e. Campanian Ignimbrite, CI, 39 ka, and Neapolitan Yellow Tuff, NYT, 14.9 ka) and to estimate the amount of collapse. Tuffs from 439 m of depth yield the first 40Ar/39Ar age of c. 39 ka within the caldera, consistent with the CI. Volcanic rocks from the NYT were, moreover, detected between 250 m and 160 m. Our findings highlight: i) a reduction of the area affected by caldera collapse, which appears to not include the city of Naples; ii) a small volume of the infilling caldera deposits, particularly for the CI and iii) the need for reassessment of the collapse amounts and mechanisms related to larger eruptions. Our results also imply a revaluation of volcanic risk for the eastern caldera area, including the city of Naples. The results of this study point out that large calderas are characterized by complex collapse mechanisms and dynamics, whose understanding needs more robust constraints, which can be obtained from scientific drilling. This article is protected by copyright. All rights reserved.
      PubDate: 2016-10-27T03:30:29.546041-05:
      DOI: 10.1002/2015GC006183
       
  • Dating kimberlite emplacement with zircon and perovskite (U-Th)/He
           geochronology
    • Authors: Jessica R. Stanley; Rebecca M. Flowers
      Abstract: Kimberlites provide rich information about the composition and evolution of cratonic lithosphere. Accurate geochronology of these eruptions is key for discerning spatiotemporal trends in lithospheric evolution, but kimberlites can sometimes be difficult to date with available methods. We explored whether (U-Th)/He dating of zircon and perovskite can serve as reliable techniques for determining kimberlite emplacement ages. We obtained zircon and/or perovskite (U-Th)/He (ZHe, PHe) dates from sixteen southern African kimberlites. Most samples with abundant zircon yielded reproducible ZHe dates (≤15% dispersion) that are in good agreement with published eruption ages. The majority of dated zircon were xenocrystic. Zircons with reproducible dates were fully reset during eruption or resided at temperatures above the ZHe closure temperature prior to entrainment in the kimberlite magma. Not dating hazy and radiation damaged grains can help avoid anomalous results for more shallowly sourced zircons that underwent incomplete damage annealing and/or partial He loss during the eruptive process. All seven kimberlites dated with PHe yielded reproducible (≤15% dispersion) and reasonable results. We conducted two preliminary perovskite 4He diffusion experiments, which suggest a PHe closure temperature of >300°C. Perovskite in kimberlites is unlikely to be xenocrystic and its relatively high temperature sensitivity suggests that PHe dates will typically record emplacement rather than post-emplacement processes. ZHe and PHe geochronology can effectively date kimberlite emplacement and provide useful complements to existing techniques. This article is protected by copyright. All rights reserved.
      PubDate: 2016-10-27T03:30:22.639386-05:
      DOI: 10.1002/2016GC006519
       
  • Precessional control on ocean productivity in the Western Pacific Warm
           Pool for the last 400 kyr: Insight from biogenic magnetite
    • Authors: Toshitsugu Yamazaki; Kazuho Horiuchi
      Abstract: The Western Pacific Warm Pool plays a significant role in large-scale atmospheric circulation and global hydrology. We conducted an environmental magnetic study of two late Pleistocene sediment cores from the western equatorial Pacific Ocean offshore of New Guinea in order to better constrain climatic and oceanographic variability, particularly spatiotemporal ocean productivity variations. Magnetic property measurements and transmission electron microscopy reveal that the magnetic mineral assemblages in the studied sediments are a mixture of biogenic and terrigenous magnetite. Variations in the acid soluble sediment component, interpreted as carbonate content, and the proportion of biogenic to terrigenous magnetite estimated from the ratio of anhysteretic to saturation remanent magnetizations are in-phase with northern hemisphere summer insolation variations. We interpret that ocean productivity increased during insolation maxima, which induced higher populations of magnetotactic bacteria through a larger nutrient supply to the seafloor. This interpretation assumes that magnetotactic bacterial populations are greatest in sediments just below the seafloor. Precessional frequencies in magnetic mineral concentration variations are suppressed after correction for carbonate dilution, whereas cyclic changes with a ∼100 kyr periodicity remain in carbonate-free magnetic concentration variations. Glacial/interglacial changes in bottom water currents may have influenced transportation and deposition of magnetic minerals. We demonstrate the usefulness of magnetic proxies for paleoceanographic studies, particularly of biogenic magnetite proxies for estimating paleoproductivity variations. This article is protected by copyright. All rights reserved.
      PubDate: 2016-10-22T10:35:27.811126-05:
      DOI: 10.1002/2016GC006446
       
  • Influence of continental growth on mid-ocean ridge depth
    • Authors: Shi Sim; Dave R. Stegman, Nicolas Coltice
      Abstract: The interconnectedness of life, water, and plate tectonics is strikingly apparent along mid-ocean ridges (MOR) where communities of organisms flourish off the disequilibrium of chemical potentials created by circulation of hydrothermal fluids driven by Earth's heat [Nisbet and Sleep, 2001; Staudigel et al., 2004]. Moreover, submarine hydrothermal environments may be critical for the emergence of life on Earth [Nisbet and Sleep, 2001]. Oceans were likely present in the Hadean [Valley et al., 2002; Harrison, 2009] but questions remain about early Earth's global tectonics [Van Hunen and Moyen, 2012], including when seafloor spreading began and whether mid-oceanic ridges were deep enough for maximum hydrothermal activities [Kasting et al., 2006]. For example, plate tectonics influences global sea level by driving secular variations in the volume of ocean basins due to continental growth [Flament et al., 2008]. Similarly, variations in the distribution of seafloor age and associated subsidence [Flament et al., 2008], due to assembly and dispersal of supercontinents [Coltice et al., 2012], explains the largest sea level variation over the past 140 Myr [Müller et al., 2008]. Using synthetic plate configurations derived from numerical models of mantle convection [Coltice et al., 2012, 2014] appropriate for early Earth, we show that MOR has remained submerged and its depths potentially constant over geologic time. Thus, conditions in the early Earth existed for hydrothermal vents at similar depths as today, providing environments conducive for the development of life and allowing for processes such as hydrothermal alteration of oceanic crust to influence the mantle's geochemical evolution This article is protected by copyright. All rights reserved.
      PubDate: 2016-10-20T20:55:21.238348-05:
      DOI: 10.1002/2016GC006629
       
  • Application of the probabilistic model BET_UNREST during a volcanic unrest
           simulation exercise in Dominica, Lesser Antilles
    • Authors: Robert Constantinescu; Richard Robertson, Jan M. Lindsay, Roberto Tonini, Laura Sandri, Dmitri Rouwet, Patrick Smith, Roderick Stewart
      Abstract: We report on the first ‘real-time' application of the BET_UNREST (Bayesian Event Tree for Volcanic Unrest) probabilistic model, during a VUELCO Simulation Exercise carried out on the island of Dominica, Lesser Antilles, in May 2015. Dominica has a concentration of nine potentially active volcanic centers and frequent volcanic earthquake swarms at shallow depths, intense geothermal activity and recent phreatic explosions (1997) indicate the region is still active. The exercise scenario was developed in secret by a team of scientists from University of West Indies (Trinidad and Tobago) and University of Auckland (New Zealand). The simulated unrest activity was provided to the exercise's Scientific Team in three ‘phases' through exercise injects comprising processed monitoring data. We applied the newly created BET_UNREST model through its software implementation PyBetUnrest, to estimate the probabilities of having i) unrest of ii) magmatic, hydrothermal or tectonic origin, which may or may not lead to iii) an eruption. The probabilities obtained for each simulated phase raised controversy and intense deliberations among the members of the scientific team. The results were often considered to be ‘too high', and were not included in any of the reports presented to ODM (Office for Disaster Management) revealing interesting crisis communication challenges. We concluded that the PyBetUnrest application itself was successful and brought the tool one step closer to a full implementation. However, as with any newly proposed method it needs more testing, and in order to be able to use it in the future we make a series of recommendations for future applications. This article is protected by copyright. All rights reserved.
      PubDate: 2016-10-19T10:20:25.392151-05:
      DOI: 10.1002/2016GC006485
       
  • 3-D density model of the upper mantle of Asia based on inversion of
           gravity and seismic tomography data
    • Authors: Mikhail K. Kaban; Ward Stolk, Magdala Tesauro, Sami El Khrepy, Nassir Al-Arifi, Fred Beekman, Sierd A.P.L. Cloetingh
      Abstract: We construct a new-generation 3D density model of the upper mantle of Asia and its surrounding areas based on a joint interpretation of several datasets. A recent model of the crust combining nearly all available seismic data is employed to calculate the impact of the crust on the gravity anomalies and observed topography and to estimate the residual mantle anomalies and residual topography. These fields are jointly inverted to calculate the density variations in the lithosphere and upper mantle down to 325 km. As an initial approximation, we estimate density variations using a seismic tomography model. Seismic velocity variations are converted into temperatures and then to density variations based on mineral physics constraints. In the Occam-type inversion, we fit both the residual mantle gravity anomalies and residual topography by finding deviations to the initial model. The obtained corrections improve the resolution of the initial model and reflect important features of the mantle structure that are not well resolved by the seismic tomography. The most significant negative corrections of the upper mantle density, found in the Siberian and East European cratons, can be associated with depleted mantle material. The most pronounced positive density anomalies are found beneath the Tarim and South Caspian basins, Barents Sea, and Bay of Bengal. We attribute these anomalies to eclogites in the uppermost mantle, which have substantially affected the evolution of the basins. Furthermore, the obtained results provide evidence for the presence of eclogites in the oceanic subducting mantle lithosphere. This article is protected by copyright. All rights reserved.
      PubDate: 2016-10-17T10:40:30.199619-05:
      DOI: 10.1002/2016GC006458
       
  • Present-day stress states underneath the Kumano basin to 2 km below
           seafloor based on borehole wall failures at IODP Site C0002, Nankai
           accretionary wedge
    • Authors: Chandong Chang; Insun Song
      Abstract: We constrain the state of stress to 2 km below seafloor in the Nankai accretionary prism at the Integrated Ocean Drilling Program (IODP) site C0002F, southwest Japan, based on borehole wall failures and rock strengths. The logging-while-drilling resistivity images from 872.5 to 2005.5 meters below seafloor show that drilling-mud control in riser drilling worked properly to minimize borehole wall failures. Available breakouts indicate a consistent maximum compression orientation subparallel to the subducting plate margin. Breakout analysis with drill logs suggests that breakouts occurred only when borehole pressure was slightly lowered and time lag between hole cutting and image logging was several hours. This indicates that the observed breakouts are not immediate stress-induced failure, but brought up into shape gradually with time due to other mechanisms. Laboratory investigations on deformation and failure of the cores suggest that the time-delayed breakout might be a result of progressive rock spall-out in borehole wall damage zones that occur at a stress level close to failure condition. We constrain stress magnitudes assuming that the stress state is sufficient to bring about the damage zones at the borehole wall. An integrated method utilizing breakouts, drilling-induced tensile fractures, and a leak-off test suggests that the stress states are on the boundary between strike-slip faulting and normal faulting stress regimes, and somewhat variable depending on depth. The stress magnitudes in the accretionary wedge appear to be controlled by frictional strength of the rock, such that the differential stresses are constrained by the laboratory determined frictional coefficients. This article is protected by copyright. All rights reserved.
      PubDate: 2016-10-17T10:40:26.835074-05:
      DOI: 10.1002/2016GC006562
       
  • Submarine groundwater discharge into typical tropical lagoons: A case
           study in Eastern Hainan Island, China
    • Authors: Xilong Wang; Jinzhou Du
      Abstract: Assessing submarine groundwater discharge (SGD) into lagoons and bays can be helpful to understand biogeochemical processes, especially nutrient dynamics. In the present paper, radium (Ra) isotopes were used to quantify SGD in two typical tropical lagoons (Laoye Lagoon (LY Lagoon) and Xiaohai Lagoon (XH Lagoon)) of Eastern Hainan Island, China. The Ra mass balance model provided evidence that SGD plays an important role in the hydrology of the LY Lagoon and the XH Lagoon, delivering average SGD fluxes of 1.7 × 106 (94 L m−2 d−1) and 1.8 × 106 (41 L m−2 d−1) m3 d−1, respectively. Tidal pumping was one of the important driving forces for SGD fluxes in the LY and the XH Lagoons. Tidal-driven SGD into the tidal channels of both lagoons can account for approximately 10% of the total SGD flux into the lagoons. In addition, the dissolved inorganic nutrient budgets were reassessed in the LY Lagoon and the XH Lagoon, which showed that SGD was the major source of nutrients entering the LY Lagoon and that the LY Lagoon behaved as a source for dissolved inorganic nitrogen (DIN) and dissolved inorganic phosphorus (DIP) and as a sink for dissolved silicate (DSi). Nutrient loads in the XH Lagoon were mainly derived from riverine inputs and SGD, and the XH Lagoon behaved as a source for DIP, but a sink for DIN and DSi. This article is protected by copyright. All rights reserved.
      PubDate: 2016-10-17T10:40:24.126333-05:
      DOI: 10.1002/2016GC006502
       
  • Asian monsoon modulation of nonsteady state diagenesis in hemipelagic
           marine sediments offshore of Japan
    • Authors: Liao Chang; Clara T. Bolton, Mark J. Dekkers, Akira Hayashida, David Heslop, Wout Krijgsman, Kazuto Kodama, Greig A. Paterson, Andrew P. Roberts, Eelco J. Rohling, Yuhji Yamamoto, Xiang Zhao
      Abstract: We have identified millennial-scale variations in magnetic mineral diagenesis from Pacific Ocean sediments offshore of Japan that we correlate with changes in organic carbon burial that were likely driven by Asian monsoon fluctuations. The correlation was determined by identifying offsets between the positions of fossil diagenetic fronts and climatically induced variations in organic carbon burial inferred from magnetic and geochemical analyses. Episodes of intense monsoon activity and attendant sediment magnetic mineral diagenesis also appear to correlate with Heinrich events, which supports the existence of climatic telecommunications between Asia and the North Atlantic region. Several lines of evidence support our conclusions: (1) fluctuations in down-core magnetic properties and diagenetic pyrite precipitation are approximately coeval; (2) localized stratigraphic intervals with relatively stronger magnetic mineral dissolution are linked to enhanced sedimentary organic carbon contents that gave rise to non-steady state diagenesis; (3) down-core variations in elemental S content provide a proxy for non-steady state diagenesis that correlate with key records of Asian monsoon variations; and (4) relict titanomagnetite that is preserved as inclusions within silicate particles, rather than secondary authigenic phases (e.g., greigite), dominates the strongly diagenetically altered sediment intervals and are protected against sulfidic dissolution. We suggest that such millennial-scale environmental modulation of non-steady state diagenesis (that creates a temporal diagenetic filter and relict magnetic mineral signatures) is likely to be common in organic-rich hemipelagic sedimentary settings with rapidly varying depositional conditions. Our work also demonstrates the usefulness of magnetic mineral inclusions for recording important environmental magnetic signals. This article is protected by copyright. All rights reserved.
      PubDate: 2016-10-17T10:35:22.606372-05:
      DOI: 10.1002/2016GC006344
       
  • Eruptive activity at Turrialba volcano (Costa Rica): Inferences from
           3He/4He in fumarole gases and chemistry of the products ejected during
           2014 and 2015
    • Authors: Andrea Luca Rizzo; Andrea Di Piazza, J. Maarten de Moor, Guillermo E. Alvarado, Geoffroy Avard, Maria Luisa Carapezza, Mauricio M. Mora
      Abstract: A new period of eruptive activity started at Turrialba volcano, Costa Rica, in 2010 after almost 150 years of quiescence. This activity has been characterized by sporadic explosions whose frequency clearly increased since October 2014. This study aimed to identify the mechanisms that triggered the resumption of this eruptive activity and characterize the evolution of the phenomena over the past 2 years. We integrate 3He/4He data available on fumarole gases collected in the summit area of Turrialba between 1999 and 2011 with new measurements made on samples collected between September 2014 and February 2016. The results of a petrological investigation of the products that erupted between October 2014 and May 2015 are also presented. We infer that the resumption of eruptive activity in 2010 was triggered by a replenishment of the plumbing system of Turrialba by a new batch of magma. This is supported by the increase in 3He/4He values observed since 2005 at the crater fumaroles and by comparable high values in September 2014, just before the onset of the new eruptive phase. The presence of a number of fresh and juvenile glassy shards in the erupted products increased between October 2014 and May 2015, suggesting the involvement of new magma with a composition similar to that erupted in 1864–1866. We conclude that the increase in 3He/4He at the summit fumaroles since October 2015 represents strong evidence of a new phase of magma replenishment, which implies that the level of activity remains high at the volcano. This article is protected by copyright. All rights reserved.
      PubDate: 2016-10-17T10:30:23.510383-05:
      DOI: 10.1002/2016GC006525
       
  • Subsea ice-bearing permafrost on the U.S. Beaufort Margin: 2. Borehole
           constraints
    • Authors: C. Ruppel; B. Herman, L. Brothers, P. Hart
      Abstract: Borehole logging data from legacy wells directly constrain the contemporary distribution of subsea permafrost in the sedimentary section at discrete locations on the US Beaufort Margin and complement recent regional analyses of exploration seismic data to delineate the permafrost's offshore extent. Most usable borehole data were acquired on a ∼500 km stretch of the margin and within 30 km of the contemporary coastline from north of Lake Teshekpuk to nearly the US-Canada border. Relying primarily on deep resistivity logs that should be largely unaffected by drilling fluids and hole conditions, the analysis reveals the persistence of several hundred vertical meters of ice-bonded permafrost in nearshore wells near Prudhoe Bay and Foggy Island Bay, with less permafrost detected to the east and west. Permafrost is inferred beneath many barrier islands and in some nearshore and lagoonal (back-barrier) wells. The analysis of borehole logs confirms the offshore pattern of ice-bearing subsea permafrost distribution determined based on regional seismic analyses and reveals that ice content generally diminishes with distance from the coastline. Lacking better well distribution, it is not possible to determine the absolute seaward extent of ice-bearing permafrost, nor the distribution of permafrost beneath the present-day continental shelf at the end of the Pleistocene. However, the recovery of gas hydrate from an outer shelf well (Belcher) and previous delineation of a log signature possibly indicating gas hydrate in an inner shelf well (Hammerhead 2) imply that permafrost may once have extended across much of the shelf offshore Camden Bay. (248 words) This article is protected by copyright. All rights reserved.
      PubDate: 2016-10-11T08:36:07.168739-05:
      DOI: 10.1002/2016GC006582
       
  • Subsea ice-bearing permafrost on the U.S. Beaufort Margin: 1. Minimum
           seaward extent defined from multichannel seismic reflection data
    • Authors: Laura L. Brothers; Bruce M. Herman, Patrick E. Hart, Carolyn D. Ruppel
      Abstract: Subsea ice-bearing permafrost (IBPF) and associated gas hydrate in the Arctic have been subject to a warming climate and saline intrusion since the last transgression at the end of the Pleistocene. The consequent degradation of IBPF is potentially associated with significant degassing of dissociating gas hydrate deposits. Previous studies interpreted the distribution of subsea permafrost on the US Beaufort continental shelf based on geographically sparse datasets and modeling of expected thermal history. The most cited work projects subsea permafrost to the shelf edge (∼100-m isobath). This study uses a compilation of stacking velocity analyses from ∼100,000 line-km of industry-collected multichannel seismic reflection data acquired over 57,000 km2 of the US Beaufort shelf to delineate continuous subsea IBPF. Gridded average velocities of the uppermost 750 ms two-way travel time range from 1475 to 3110 m s−1. The monotonic, cross-shore pattern in velocity distribution suggests that the seaward extent of continuous IBPF is within 37 km of the modern shoreline at water depths 
      PubDate: 2016-10-11T08:30:51.0019-05:00
      DOI: 10.1002/2016GC006584
       
  • Effect of melt/mantle interactions on MORB chemistry at the easternmost
           Southwest Indian Ridge (61 to 67°E)
    • Authors: M. Paquet; M. Cannat, D. Brunelli, C. Hamelin, E. Humler
      Abstract: The easternmost part of the Southwest Indian Ridge (61°-67°E) is an end-member of the global ridge system in terms of very low magma supply. As such, it is a good laboratory to investigate the effect of melt/mantle interactions on the composition of erupted basalts: for a given volume of erupted basaltic melt, the volume of reacted mantle is potentially greater than at more magmatically robust ridges. We analyzed major, trace element and isotopic compositions in three groups of rocks: plagioclase-bearing ultramafic and gabbroic rocks dredged in nearly amagmatic spreading corridors; basalts from the sparse volcanic cover of these corridors (“ultramafic seafloor basalts”); and basalts dredged from the intervening, more volcanically active domains (“volcanic seafloor basalts”). Ultramafic seafloor basalts have significantly lower CaO and Al2O3 contents at a given MgO than most volcanic seafloor basalts. We propose that both types of basalts are derived from similar parental melts, but that the ultramafic seafloor basalts are more affected by reactions between these parent melts and the mantle rocks in the lithosphere below the ridge. We infer that these reactions occur in the walls of conduits that allow the aggregated melts extracted from the melting mantle to rise through the axial lithosphere and to the eruption sites. The principal effect of these reactions is to enrich the asthenospheric melts in MgO through olivine dissolution. This effect is not expected to be as noticeable, but could still play a role in basalt petrogenesis at more magmatic regions of the global slow-spreading MOR system. This article is protected by copyright. All rights reserved.
      PubDate: 2016-10-03T04:07:14.459134-05:
      DOI: 10.1002/2016GC006385
       
  • Late Cenozoic tephrostratigraphy offshore the southern Central American
           Volcanic Arc: 2. Implications for magma production rates and subduction
           erosion
    • Authors: J.C. Schindlbeck; S. Kutterolf, A. Freundt, S.M. Straub, P. Vannucchi, G.E. Alvarado
      Abstract: Pacific drill sites offshore Central America provide the unique opportunity to study the evolution of large explosive volcanism and the geotectonic evolution of the continental margin back into the Neogene. The temporal distribution of tephra layers established by tephrochonostratigraphy in Part 1 indicates a nearly continuous highly explosive eruption record for the Costa Rican and the Nicaraguan volcanic arc within the last 8 M.y.The widely distributed marine tephra layers comprise the major fraction of the respective erupted tephra volumes and masses thus providing insights into regional and temporal variations of large‐magnitude explosive eruptions along the southern Central American Volcanic Arc (CAVA). We observe three pulses of enhanced explosive magmatism between 0‐1 Ma at the Cordillera Central, between 1‐2 Ma at the Guanacaste and at >3 Ma at the Western Nicaragua segments. Averaged over the long‐term the minimum erupted magma flux (per unit arc length) is ∼0.017 g/ms.Tephra ages, constrained by Ar‐Ar dating and by correlation with dated terrestrial tephras, yield time‐variable accumulation rates of the intercalated pelagic sediments with four prominent phases of peak sedimentation rates that relate to tectonic processes of subduction erosion. The peak rate at >2.3 Ma near Osa particularly relates to initial Cocos Ridge subduction which began at 2.91±0.23 Ma as inferred by the 1.5 M.y. delayed appearance of the OIB geochemical signal in tephras from Barva volcano at 1.42 Ma. Subsequent tectonic re‐arrangements probably involved crustal extension on the Guanacaste segment that favored the 2‐1 Ma period of unusually massive rhyolite production. This article is protected by copyright. All rights reserved.
      PubDate: 2016-09-27T01:32:10.005754-05:
      DOI: 10.1002/2016GC006504
       
  • The crustal structure of the Arizona Transition Zone and southern Colorado
           Plateau from multiobservable probabilistic inversion
    • Authors: Mehdi Tork Qashqai; Juan Carlos Afonso, Yingjie Yang
      Abstract: The Arizona Transition Zone is a narrow band that separates two of the main and most contrasting tectonic provinces in western US, namely the southern Colorado Plateau and the southern Basin and Range provinces. As such, the internal crustal structure and physical state of this transitional zone hold clues for understanding i) the amalgamation of these provinces, ii) the partitioning of deformation due to both past and present‐day stress fields and iii) the role of thermal versus compositional effects in controlling surface observables. Here we employ and expand a novel multi‐observable probabilistic inversion method [Afonso et al., 2013a,b] and jointly invert fundamental mode Rayleigh phase velocities, receiver functions, surface heat flow, geoid height and absolute elevation to obtain an internally‐consistent 3D model of the temperature, density, Vs and Vp of the Arizona Transition Zone and the southern portions of the Colorado Plateau and Basin and Range. Our results confirm a significant crustal thickening from ∼ 28 km in the SW of the Arizona Transition Zone and southern Basin and Range to ∼ 48 km beneath the southern Colorado Plateau. Inverted temperatures agree well with the location of recent volcanism and indicate that the lithosphere‐asthenosphere boundary is not deeper than ∼ 70 km in most of the region. We find that major pre‐Cambrian surface structures and/or shear zones separate crustal domains with distinct bulk properties, suggesting that the juxtaposed crustal blocks still retain, at least in part, their original characteristics. However, widespread intrusions of significant volumes of mafic magmas have affected these blocks at different depths, locally overprinting their original compositions and creating highly heterogeneous crustal sections. A dominant and large‐scale internal crustal pattern of SW dipping planes/structures is evident in our models, coinciding with the orientation of deep faults previously inferred from earthquake focal mechanisms. While we cannot categorically corroborate the presence of melt or aqueous fluids within the crust, our results are compatible with these scenarios beneath some parts of the Basin and Range, the Mogollon‐Datil and Springerville volcanic fields. This article is protected by copyright. All rights reserved.
      PubDate: 2016-09-27T01:27:08.103307-05:
      DOI: 10.1002/2016GC006463
       
  • Issue Information
    • Pages: 4287 - 4288
      PubDate: 2016-12-20T22:14:12.691168-05:
      DOI: 10.1002/ggge.20838
       
 
 
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