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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: 102, SJR: 3.323, h-index: 185)
Global Biogeochemical Cycles     Full-text available via subscription   (Followers: 12, SJR: 3.22, h-index: 136)
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J. of Geophysical Research : Planets     Full-text available via subscription   (Followers: 101)
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: 112)
Paleoceanography     Full-text available via subscription   (Followers: 6, 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)
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Tectonics     Full-text available via subscription   (Followers: 12, SJR: 2.628, h-index: 96)
Water Resources Research     Full-text available via subscription   (Followers: 74, SJR: 2.661, h-index: 144)
Journal Cover Geochemistry, Geophysics, Geosystems
  [SJR: 2.439]   [H-I: 91]   [25 followers]  Follow
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   ISSN (Online) 1525-2027
   Published by AGU Homepage  [17 journals]
  • 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
    • 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
    • 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
    • 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
    • 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
  • High-precision U-Pb geochronology of the Jurassic Yanliao Biota from
           Jianchang (western Liaoning Province, China): Age constraints on the rise
           of feathered dinosaurs and eutherian mammals
    • Authors: Zhuyin Chu; Huaiyu He, Jahandar Ramezani, Samuel A. Bowring, Dongyu Hu, Lijun Zhang, Shaolin Zheng, Xiaolin Wang, Zhonghe Zhou, Chenglong Deng, Jinghui Guo
      Abstract: The Yanliao Biota of northeastern China comprises the oldest feathered dinosaurs, transitional pterosaurs, as well as the earliest eutherian mammals, multituberculate mammals, and new euharamiyidan species that are key elements of the Mesozoic biotic record. Recent discovery of the Yanliao Biota in the Daxishan section near the town of Linglongta, Jianchang County in western Liaoning Province have greatly enhanced our knowledge of the transition from dinosaurs to birds, primitive to derived pterosaurs, and the early evolution of mammals. Nevertheless, fundamental questions regarding the correlation of fossil-bearing strata, rates of dinosaur and mammalian evolution, and their relationship to environmental change in deep time remain unresolved due to the paucity of precise and accurate temporal constraints. These limitations underscore the importance of placing the rich fossil record of Jianchang within a high-resolution chronostratigraphic framework that has thus far been hampered by the relatively low precision of in situ radioisotopic dating techniques. Here we present high-precision U-Pb zircon geochronology by the chemical abrasion isotope dilution thermal ionization mass spectrometry (CA-ID-TIMS) from three interstratified ash beds previously dated by secondary-ion mass spectrometry (SIMS) technique. The results constrain the key fossil horizons of the Daxishan section to an interval spanning 160.89 to 160.25 Ma with 2σ analytical uncertainties that range from ±46 to ±69 kyr. These data place the Yanliao Biota from Jianchang in the Oxfordian Stage of the Late Jurassic, and mark the Daxishan section as the site of Earth's oldest precisely dated feathered dinosaurs and eutherian mammals.
      PubDate: 2016-10-22T19:05:50.289375-05:
      DOI: 10.1002/2016GC006529
  • 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
  • 3-D high-speed imaging of volcanic bomb trajectory in basaltic explosive
    • Authors: D. Gaudin; J. Taddeucci, B. F. Houghton, T. R. Orr, D. Andronico, E. Del Bello, U. Kueppers, T. Ricci, P. Scarlato
      Abstract: Imaging, in general, and high speed imaging in particular are important emerging tools for the study of explosive volcanic eruptions. However, traditional 2-D video observations cannot measure volcanic ejecta motion toward and away from the camera, strongly hindering our capability to fully determine crucial hazard-related parameters such as explosion directionality and pyroclasts' absolute velocity. In this paper, we use up to three synchronized high-speed cameras to reconstruct pyroclasts trajectories in three dimensions. Classical stereographic techniques are adapted to overcome the difficult observation conditions of active volcanic vents, including the large number of overlapping pyroclasts which may change shape in flight, variable lighting and clouding conditions, and lack of direct access to the target. In particular, we use a laser rangefinder to measure the geometry of the filming setup and manually track pyroclasts on the videos. This method reduces uncertainties to 10° in azimuth and dip angle of the pyroclasts, and down to 20% in the absolute velocity estimation. We demonstrate the potential of this approach by three examples: the development of an explosion at Stromboli, a bubble burst at Halema'uma'u lava lake, and an in-flight collision between two bombs at Stromboli.
      PubDate: 2016-10-20T02:05:27.918099-05:
      DOI: 10.1002/2016GC006560
  • 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
  • Miocene high-elevation landscape of the eastern Tibetan Plateau
    • Authors: Qiang Xu; Xiaohui Liu, Lin Ding
      Abstract: The high topography of central Asia is the most distinctive expression of the India-Asia collision, yet a broad understanding of the timing and processes involved in the development of the Tibetan Plateau remains elusive. Here, we investigate the Neogene Songpan Basin located on the eastern margin of the plateau using oxygen isotope paleoaltimetry to determine when the steep Longmen Shan margin obtained its present elevations. Hydrologically open lacustrine and fluvial-alluvial authigenic carbonates from the basin record the paleoelevations of the eastern Tibetan Plateau and suggest that the area had attained near-present elevations of ∼3000 m by the late Miocene. This reconstruction is consistent with the results from the comparison of pollen fossil to their nearest living relatives in this area (2750-3050 m). We propose that the eastern Tibetan Plateau may have attained significant elevations prior to the late Miocene through an uplift scenario involving eastward growth of the plateau between the Eocene and the Miocene. Two tectonic processes, either crustal shortening in the Oligocene-Miocene or eastward propagation of weakened lower crust starting in the Eocene, most likely thickened the crust and raised the surface of the eastern Tibetan Plateau to its present elevation by the late Miocene. This article is protected by copyright. All rights reserved.
      PubDate: 2016-10-13T03:26:07.102905-05:
      DOI: 10.1002/2016GC006437
  • Subsea ice-bearing permafrost on the U.S. Beaufort Margin: 2. Borehole
    • 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
  • Magnetic and geochemical signatures of flood layers in a lake system
    • Authors: Eivind W. N. Støren; Øyvind Paasche, Ann M. Hirt, Monika Kumari
      Abstract: River floods holds the capasity to erode and transport sediments that are deposited whenever the discharge is redused. In catchments that are subjected to repeated flooding, downstream lakes can therefore contain a record of past events across multiple timescales. High-resolution core scanning analyses such as X-ray fluorescence (XRF) scanning and magnetic susceptibility (MS) provide data that are frequently used to detect flood layers in soft sediment archives such as lakes, fjords and ocean basins. Deposits of past floods also can potentially reveal information about the evolution of flood events as well as source area. Here we explore ways in which subtle variability in high-resolution data can be utilized and subsequently vetted by high-precision measurements in order to delineate the copious information that can be extracted from soft sediment records. By combining magnetic hysteresis measurements and first-order reversal curves (FORCs) with inductively coupled plasma optical emission spectrometer (ICP-OES) measurements of chemical elements on 36 samples, questions about flood dynamics and variability are raised, and also sources of noise in high-resolution scanning techniques are discussed. Specifically, we show that a lake flood record from Southern Norway containing 92 floods distributed over 10,000 years can be sub-divided into two groups of floods that were generated either by spring snow melting, intense summer rainstorms, or a combination of both. The temporal evolution of this pattern shows a marked shift towards spring floods around 2000 years ago compared to the earlier part of the record. This article is protected by copyright. All rights reserved.
      PubDate: 2016-10-11T08:30:45.466588-05:
      DOI: 10.1002/2016GC006540
  • The change in contact angle at unsaturated CO2-water conditions:
           Implication on geological carbon dioxide sequestration
    • Authors: Mohammad Jafari; Jongwon Jung
      Abstract: The performance of a geologic carbon storage site strongly depends on the capillary pressure of sealing rock and formations. While wettability of minerals is a key factor in capillary pressure, published contact angles are inconsistent. This study explores the discrepancy of published contact angles in order to reduce the uncertainty of measured laboratory contact angles, and understand the variation of contact angles at unsaturated CO2-water conditions. A ratio of droplet dimension and triple line (or contact line) are used to explain the observed wide range of contact angles and the variation of contact angle at unsaturated conditions. Results show that the shape factor has a good agreement with contact angle change during CO2 dissolution in water. Silica substrate has clear two pinned and slip stages of triple line during CO2 droplet dissolution, which cause contact angle on silica substrate to increase from 34.5º to 42.1º. However, mica substrate has the repeated pinned and slip stages due to the heterogeneity of mica surface, which cause contact angle to increase dramatically from 25.4º to 68.1º. Thus, both the impact of the unsaturated CO2-water conditions on the wide range of contact angle and the heterogeneity of mineral surface should be considered when one estimates capillary pressure based on contact angle in geological CO2 sequestration. This article is protected by copyright. All rights reserved.
      PubDate: 2016-10-04T10:21:34.43699-05:0
      DOI: 10.1002/2016GC006510
  • Tectonic structure of the mid-Atlantic Ridge near 16°30'N
    • Authors: Ross Parnell-Turner; Hans Schouten, Deborah K. Smith
      Abstract: The 16°30'N area of the Mid-Atlantic Ridge represents an area of present-day detachment faulting. Here we present shipboard bathymetric, magnetic and gravity data acquired up to 65 km from the ridge axis that reveal a varied tectonic history of this region. Magnetic data are used to calculate spreading rates and examine spreading rate variability along and across the axis. Bathymetric and gravity data are used to infer the crustal structure. A central magnetic anomaly 40% narrower than expected is observed along much of the study area. Misalignment between modern-day spreading center and magnetic anomalies indicates tectonic reorganization of the axis within the past 780 ka. Observed magnetic anomalies show a pattern of anomalous skewness consistent with rotation of magnetic vectors probably associated with detachment faulting. Relatively thin crust north of a small (∼7 km) non-transform offset coincides with a weakly magmatic spreading axis. In contrast, to the south a robust axial volcanic ridge is underlain by thicker crust. Variations in crustal structure perpendicular to the axis occur over tens of kilometers, indicating processes which occur over timescales of 1-2 Ma. This article is protected by copyright. All rights reserved.
      PubDate: 2016-10-04T10:21:33.434126-05:
      DOI: 10.1002/2016GC006514
  • Hydrothermal fluid venting in the offshore sector of Campi Flegrei
           caldera: A geochemical, geophysical, and volcanological study
    • Authors: R. Di Napoli; A. Aiuppa, A. Sulli, S. Caliro, G. Chiodini, V. Acocella, G. Ciraolo, M.A. Di Vito, F. Interbartolo, C. Nasello, M. Valenza
      Abstract: The ongoing unrest at the Campi Flegrei caldera (CFc) in southern Italy is prompting exploration of its poorly studied offshore sector. We report on a multidisciplinary investigation of the Secca delle Fumose (SdF), a submarine relief known since antiquity as the largest degassing structure of the offshore sector of CFc. We combined high-resolution morpho-bathymetric and seismo-stratigraphic data with onshore geological information to propose that the present-day SdF morphology and structure developed during the initial stages of the last CFc eruption at Monte Nuovo in AD 1538. We suggest that the SdF relief stands on the eastern uplifted border of a N-S-trending graben-like structure formed during the shallow emplacement of the Monte Nuovo feeding dike. We also infer that the high-angle bordering faults that generated the SdF relief now preferentially allow the ascent of hot brines (with an equilibrium temperature of 179°C), thereby sustaining hydrothermal degassing on the seafloor. Systematic vertical seawater profiling shows that hydrothermal seafloor venting generates a sizeable CO2, pH, and temperature anomaly in the overlying seawater column. Data for the seawater vertical profile can be used to estimate the CO2 and energy (heat) outputs from the SdF area at ∼50 tons/day (∼0.53 kg/s) and ∼80MW, respectively. In view of the cause-effect relationship with the Monte Nuovo eruption, and the substantial gas and energy outputs, we consider that the SdF hydrothermal system needs to be included in monitoring programs of the ongoing CFc unrest. This article is protected by copyright. All rights reserved.
      PubDate: 2016-10-04T10:21:31.573388-05:
      DOI: 10.1002/2016GC006494
  • Origin of ultrarear-arc magmatism at Rishiri Volcano, Kuril Arc
    • Authors: Takeshi Kuritani; Mitsuhiro Nakagawa
      Abstract: The Rishiri Volcano is located at the very rear of the Kuril Arc at its junction with the NE Japan Arc, and its 300 km depth to the slab surface is one of the deepest among the active arc volcanoes in the world. In this study, the origin of this ultra rear-arc magmatism was investigated by analyzing the basaltic lavas from the volcano. The lavas consist of low-K and high-K groups, with the low-K lavas predating the high-K lavas. Since it is unlikely that the high-K magmas are derivatives of the low-K magmas, the two magmas are thought to be derived from different source mantle materials. Analyses using multicomponent thermodynamics suggest that these magmas were both generated through the ∼2% melting of a source mantle with 0.04–0.11 wt.% H2O at 1280–1330°C and ∼2.3 GPa. The temperatures at the surface of the subducting Pacific slab, from which the slab fluids were released, were estimated to be 860–960°C for the low-K magmas and 930–1040°C for the high-K magmas. These temperatures of the slab surface are remarkably higher than those predicted by thermal models. The estimated high temperatures of the slab surface and the latest detailed seismic tomography results suggest that the low-K and high-K magmatism resulted from the progressive production of fluids at the slab surface due to heating by the injection of hot mantle materials into a relatively large-scale fracture in the distorted portion of the subducting Pacific plate. This article is protected by copyright. All rights reserved.
      PubDate: 2016-10-04T10:21:27.39338-05:0
      DOI: 10.1002/2016GC006594
  • Spawning superplumes from the midmantle: The impact of spin transitions in
           the mantle
    • Authors: M. H. Shahnas; R.N. Pysklywec, D. A. Yuen
      Abstract: The formation of large scale upwellings with lateral extents of several hundreds of kilometers, reaching up to ∼10000 km or more, still remains a hotly debated topic. Some seismic imaging studies based on high resolution data suggest that the main superplumes underneath Africa and South-central Pacific are clusters, composed of several individual plumes rather than being a single large mantle upwelling. The iron spin transition in the lower mantle minerals may present a new idea on the origin and the formation of such superplumes, notably sourcing such features in the mid-mantle. Stagnation of both cold sinking slabs and hot rising plumes can be caused by density and viscosity variation due to the spin transition in iron in ferropericlase (Fp) and a possible spin-dependent bulk modulus hardening in bridgmanite silicate perovskite (Pv). This process produces intermittent downward spin transition-induced mid-mantle avalanches (SIMMA) of the cold sinking flow as well as upward spin transition-induced mid-mantle superplume avalanches (SIMMSA) of the rising hot plumes, triggered at the spin transition-induced thermal boundary layer at around 1600 km depth. Our high resolution axi-symmetric models reveal that the hot upwellings, trapped below ∼1600 km depth, can suddenly penetrate into the upper levels in the mantle and spread laterally for hundreds of kilometres. Owing to the upward penetration of the mid-mantle rooted superplumes, as broad as ∼1500 km across, a large amount of heat can be delivered to the upper mantle and base of the lithosphere with implications for large volcanic episodes. This article is protected by copyright. All rights reserved.
      PubDate: 2016-10-04T10:21:01.575037-05:
      DOI: 10.1002/2016GC006509
  • Reconciling mantle wedge thermal structure with arc lava thermobarometric
           determinations in oceanic subduction zones
    • Authors: Alexander Perrin; Saskia Goes, Julie Prytulak, D Rhodri Davies, Cian Wilson, Stephan Kramer
      Abstract: Subduction zone mantle wedge temperatures impact plate interaction, melt generation, and chemical recycling. However, it has been challenging to reconcile geophysical and geochemical constraints on wedge thermal structure. Here we chemically determine the equilibration pressures and temperatures of primitive arc lavas from worldwide intra-oceanic subduction zones and compare them to kinematically driven thermal wedge models. We find that equilibration pressures are typically located in the lithosphere, starting just below the Moho, and spanning a wide depth range of ∼25 km. Equilibration temperatures are high for these depths, averaging ∼1300°C. We test for correlations with subduction parameters and find that equilibration pressures correlate with upper plate age, indicating overriding lithosphere thickness plays a role in magma equilibration. We suggest that most, if not all, thermobarometric pressure and temperature conditions reflect magmatic re-equilibration at a mechanical boundary, rather than reflecting the conditions of major melt generation. The magma re-equilibration conditions are difficult to reconcile, to a first order, with any of the conditions predicted by our dynamic models, with the exception of subduction zones with very young, thin upper plates. For most zones, a mechanism for substantially thinning the overriding plate is required. Most likely thinning is localised below the arc, as kinematic thinning above the wedge corner would lead to a hot forearc, incompatible with forearc surface heat flow and seismic properties. Localised sub-arc thermal erosion is consistent with seismic imaging and exhumed arc structures. Furthermore, such thermal erosion can serve as a weakness zone and affect subsequent plate evolution. This article is protected by copyright. All rights reserved.
      PubDate: 2016-10-04T10:20:55.837532-05:
      DOI: 10.1002/2016GC006527
  • Time-scales of foam stability in shallow conduits: Insights from analogue
    • Authors: L. Spina; B. Scheu, C. Cimarelli, A. Arciniega-Ceballos, D.B. Dingwell
      Abstract: Volcanic systems can exhibit periodical trends in degassing activity, characterized by a wide range of time-scales. Understanding the dynamics that control such periodic behavior can provide a picture of the processes occurring in the feeding system. Towards this end, we analyzed the periodicity of outgassing in a series of decompression experiments performed on analogue material (argon-saturated silicone oil plus glass beads/fibers) scaled to serve as models of basaltic magma. To define the effects of liquid viscosity and crystal content on the time-scale of outgassing we investigated both: 1) pure liquid systems, at differing viscosities (100 and 1000 Pa s), 2) particle-bearing suspensions (diluted and semi-diluted). The results indicate that under dynamic conditions (e.g. decompressive bubble growth, fluid ascent within the conduit), the periodicity of foam disruption may be up to several orders of magnitude less than estimates based on the analysis of static conditions. This difference in foam disruption time-scale is inferred to result from the contribution of bubble shear and bubble growth to inter-bubble film thinning. The presence of particles in the semi-diluted regime is further linked to shorter bubble bursting times, likely resulting from contributions of the presence of a solid network and coalescence processes to the relative increase in bubble break-up rates. Finally, it is argued that these experiments represent a good analogue of gas-piston activity (i.e., the periodical rise-and-fall of a basaltic lava lake surface), implying a dominant role for shallow foam accumulation as a source process for these phenomena. This article is protected by copyright. All rights reserved.
      PubDate: 2016-10-04T10:15:42.639335-05:
      DOI: 10.1002/2016GC006455
  • Sources and turnover of organic carbon and methane in fjord and shelf
           sediments off Northern Norway
    • Authors: Simone Sauer; Wei-Li Hong, Jochen Knies, Aivo Lepland, Matthias Forwick, Martin Klug, Florian Eichinger, Soma Baranwal, Antoine Crémière, Shyam Chand, Carsten J. Schubert
      Abstract: To better understand the present and past carbon cycling and transformation processes in methane influenced fjord and shelf areas of Northern Norway we compared two sediment cores from the Hola trough and from Ullsfjorden. We investigated (1) the organic matter composition and sedimentological characteristics to study the sources of organic carbon (Corg) and the factors influencing Corg burial, (2) pore water geochemistry to determine the contribution of organoclastic sulfate reduction and methanogenesis to total organic carbon turnover, and (3) the carbon isotopic signature of hydrocarbons to identify the carbon transformation processes and gas sources.High sedimentation and Corg accumulation rates in Ullsfjorden support the notion that fjords are important Corg sinks. The depth of the sulfate-methane-transition (SMT) in the fjord is controlled by the supply of predominantly marine organic matter to the sediment. Organoclastic sulfate reduction accounts for 60% of the total depth-integrated sulfate reduction in the fjord. In spite of the presence of ethane, propane and butane, we suggest a purely microbial origin of light hydrocarbons in the sediments based on their low δ13C values.In the Hola trough, sedimentation and Corg accumulation rates changed during the deglacial-to-post-glacial transition from approximately 80 cm ka−1 to erosion at present. Thus, Corg burial in this part of the shelf is presently absent. Low organic matter content in the sediment and low rates of organoclastic sulfate reduction (only 3% of total depth-integrated sulfate reduction) entail that the shallow depth of the SMT is controlled mostly by ascending thermogenic methane from deeper sources. This article is protected by copyright. All rights reserved.
      PubDate: 2016-10-04T10:11:36.496836-05:
      DOI: 10.1002/2016GC006296
  • Structure of magma reservoirs beneath Merapi and surrounding volcanic
           centers of Central Java modeled from ambient noise tomography
    • Authors: Ivan Koulakov; Gulzhamal Maksotova, Kayrly Jaxybulatov, Ekaterina Kasatkina, Nikolai M. Shapiro, Birger-G. Luehr, Sami El Khrepy, Nassir Al-Arifi
      Abstract: We present a three-dimensional model of the distribution of S-wave velocity in the upper crust to a depth of 20 km beneath Central Java based on the analysis of seismic ambient noise data recorded by more than 100 seismic stations in 2004 associated with the MERAMEX project. To invert the Rayleigh wave dispersion curves to construct 2D group-velocity maps and 3D distributions of S-wave velocity, we have used a new tomographic algorithm based on iterative linearized inversion. We have performed a series of synthetic tests that demonstrate significantly higher resolution in the upper crust with this model compared to the local earthquake travel-time tomography (LET) model previously applied for the same station network. Beneath the southern flank of Merapi, we identify a large low-velocity anomaly that can be split into two layers. The upper layer reflects the ∼ 1 km thick sedimentary cover of volcanoclastic deposits. The deeper anomaly at depths of ∼ 4–8 km may represent a magma reservoir with partially molten rock that feeds several volcanoes in Central Java. Beneath the Merapi summit, we observe another low-velocity anomaly as deep as 8 km that may be associated with the active magma reservoir that feeds the eruptive activity of Merapi. In the southern portion of the study area, in the lower crust, we identify a low-velocity anomaly that may represent the top of the pathways of volatiles and melts ascending from the slab that was previously inferred from the LET model results. We observe that this anomaly is clearly separate from the felsic magma reservoirs in the upper crust. This article is protected by copyright. All rights reserved.
      PubDate: 2016-10-04T10:11:29.017074-05:
      DOI: 10.1002/2016GC006442
  • Preglacial to glacial sediment thickness grids for the southern Pacific
           Margin of West Antarctica
    • Authors: Ansa Lindeque; Karsten Gohl, Florian Wobbe, Gabriele Uenzelmann-Neben
      Abstract: Circum-Antarctic sediment thickness grids provide constraints for basin evolution and paleotopographic reconstructions, which are important for paleo-ice sheet formation histories. By compiling old and new seismic data, we identify sequences representing pre-glacial, transitional and full glacial deposition processes along the Pacific margin of West Antarctica. The pre-glacial sediment grid depicts 1.3 to 4.0 km thick depocenters, relatively evenly distributed along the margin. The depocenters change markedly in the transitional phase at, or after, the Eocene/Oligocene boundary, when the first major ice sheets reached the shelf. Full glacial sequences, starting in the middle Miocene, indicate new depocenter formation North of the Amundsen Sea Embayment and localized eastward shifts in the Bellingshausen Sea and Antarctic Peninsula basins. Using present-day drainage paths and source areas on the continent, our calculations indicate an estimated observed total sedimentary volume of ∼10 x 106 km3 was eroded from West Antarctica since the separation of New Zealand in the Late Cretaceous. Of this 4.9 x 106 km3 predates the onset of glaciation and need to be considered for a paleotopography reconstruction of 34 Ma. Whereas 5.1 x 106 km3 postdate the onset of glaciation, of which 2.5 x 106 km3 were deposited in post mid-Miocene full glacial conditions. This article is protected by copyright. All rights reserved.
      PubDate: 2016-10-03T04:11:49.208516-05:
      DOI: 10.1002/2016GC006401
  • 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
  • Exploring the potential of clumped isotope thermometry on coccolith‐rich
           sediments as a sea surface temperature proxy
    • Authors: Anna Joy Drury; Cédric M. John
      Abstract: Understanding past changes in sea surface temperatures (SSTs) is crucial; however, existing proxies for reconstructing past SSTs are hindered by unknown ancient seawater composition (foraminiferal Mg/Ca and δ18O), or reflect subsurface temperatures (TEX86) or have a limited applicable temperature range (Uk'37). We examine clumped isotope (Δ47) thermometry to fossil coccolith‐rich material as an SST proxy, as clumped isotopes are independent of original seawater composition and applicable to a wide temperature range and coccolithophores are widespread and dissolution resistant. The Δ47‐derived temperatures from
      PubDate: 2016-09-29T04:11:02.355559-05:
      DOI: 10.1002/2016GC006459
  • The Fina Nagu Volcanic Complex: Unusual submarine arc volcanism in the
           rapidly deforming southern Mariana margin
    • Authors: Maryjo N. Brounce; Katherine A. Kelley, Robert Stern, Fernando Martinez, Elizabeth Cottrell
      Abstract: In the Mariana convergent margin, large arc volcanoes disappear south of Guam even though the Pacific plate continues to subduct and instead, small cones scatter on the seafloor. These small cones could form either due to decompression melting accompanying back‐arc extension or flux melting, as expected for arc volcanoes, or as a result of both processes. Here, we report the major, trace, and volatile element compositions, as well as the oxidation state of Fe, in recently‐dredged, fresh pillow lavas from the Fina Nagu volcanic chain, an unusual alignment of small, closely‐spaced submarine calderas and cones southwest of Guam. We show that Fina Nagu magmas are the consequence of mantle melting due to infiltrating aqueous fluids and sediment melts sourced from the subducting Pacific plate into a depleted mantle wedge, similar in extent of melting to accepted models for arc melts. Fina Nagu magmas are not as oxidized as magmas elsewhere along the Mariana arc, suggesting that the subduction component responsible for producing arc magmas is either different or not present in the zone of melt generation for Fina Nagu, and that amphibole or serpentine mineral destabilization reactions are key in producing oxidized arc magmas. Individual Fina Nagu volcanic structures are smaller in volume than Mariana arc volcanoes, although the estimated cumulative volume of the volcanic chain is similar to nearby submarine arc volcanoes. We conclude that melt generation under the Fina Nagu chain occurs by similar mechanisms as under Mariana arc volcanoes, but that complex lithospheric deformation in the region distributes the melts among several small edifices that get younger to the northeast. This article is protected by copyright. All rights reserved.
      PubDate: 2016-09-29T03:47:32.643149-05:
      DOI: 10.1002/2016GC006457
  • Influence of microscale weak zones on bulk strength
    • Authors: Christopher Gerbi; Scott E. Johnson, Deborah Shulman, Keith Klepeis
      Abstract: Shear zones have different rheological properties than the surrounding rocks, indicating that the bulk strength of regions containing shear zone networks cannot be determined by considering the the host rock rheology alone. We demonstrate the value of this concept at the microscale. We first consider the phase arrangements in naturally deformed rocks and document that weak phases exhibit little interconnection within a microstructure. Rather, three‐dimensional weak zones, analogous to viscous shear zones, can interconnect or bridge weak phases. These zones typically form at high stress sites, comprise multiple minerals, and deform by mechanisms independent of those in the surrounding minerals. The presence of weak zones strongly affects the bulk strength of the rock, disproportionate to the mode of the weak zones. For example, the development of 1% mode of a weak zone at a high stress site can reduce the bulk strength of the rock nearly an order of magnitude. Calculation of the bulk strength of the rock by some averaging algorithm of the deformation mechanisms operating outside the weak zones will overestimate strength. Instead, accurate calculations and predictions of bulk strength require accounting for the presence and geometry of weak zones. For this reason, we advocate use of the scale‐independent conceptual rheological model of interconnected weak zones or layers rather than that of interconnected weak phases. More generally, the way forward in improving quantification of the mechanical properties of the lithosphere requires recognizing and explicitly accounting for the spatial and temporal distribution of deformation mechanisms operating throughout a rock. This article is protected by copyright. All rights reserved.
      PubDate: 2016-09-29T03:47:11.596491-05:
      DOI: 10.1002/2016GC006551
  • Late Cenozoic tephrostratigraphy offshore the southern Central American
           Volcanic Arc: 2. Implications for magma production rates and subduction
    • 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
  • High‐resolution carbon cycle and seawater temperature evolution during
           the Early Jurassic (Sinemurian–Early Pliensbachian)
    • Authors: Gregory D. Price; Sarah J. Baker, Justin VanDeVelde, Marie‐Emilie Clémence
      Abstract: The Early Jurassic was marked by a progressive recovery from the end‐Triassic mass extinction and punctuated by recurring episodes of anoxia. These changes, associated with fluctuations in carbon isotope composition of marine carbonates, remain incompletely understood. Here we present a high‐resolution carbon and oxygen isotope record for the Early Jurassic based on well‐preserved marine mollusks (belemnites) from Dorset, UK. Our new data show a number of δ13C excursions, starting with a negative excursion at the Sinemurian–Pliensbachian boundary Event followed by lesser negative excursions showing in the Polymorphous, Jamesoni and Masseanum‐Valdani Subzones. The recognition of the Sinemurian–Pliensbachian boundary Event in this study and elsewhere suggests that observed carbon‐isotope trends are likely to represent a supra regional perturbation of the carbon cycle. A prominent positive carbon isotope event is also seen within the Pliensbachian Ibex Zone. This event is also clearly evident in the data from belemnites from Spain. This carbon isotope excursion is not, however, coincident with inferred peak temperatures. The oxygen isotope and Mg/Ca data allows the determination of a number of pronounced Pliensbachian cool events. From the low point in the Brevispina Subzone, oxygen isotopes become more negative coupled with an increase in Mg/Ca values culminating in an Early Pliensbachian thermal maximum during the Davoei Zone. Taken with existing data it appears that the Pliensbachian is characterized by 2 major warmings, firstly within the Davoei Zone followed by warming beginning in the latest Pliensbachian and peaking in the Early Toarcian. This article is protected by copyright. All rights reserved.
      PubDate: 2016-09-27T01:27:03.48118-05:0
      DOI: 10.1002/2016GC006541
  • REEBOX PRO: A forward model simulating melting of thermally and
           lithologically variable upwelling mantle
    • Authors: Eric L. Brown; Charles E. Lesher
      Abstract: The compositions and volumes of basalts erupted in divergent margin environments provide a record of the thermal, chemical, and dynamical state of their mantle source regions. To relate basalt compositions and volumes to the underlying thermochemical and dynamical state of their mantle source regions, we have developed REEBOX PRO, a compiled stand‐alone application that simulates adiabatic decompression melting of passively or actively upwelling mantle containing up to five distinct lithologies. The model calculates melt compositions using thermodynamic and experimental constraints on the melting behaviors and mineral‐melt partitioning behavior of homogeneous and lithologically heterogeneous mantle sources containing anhydrous peridotite, hydrous peridotite, harzburgite, and/or silica‐saturated/–undersaturated pyroxenite. Key model outputs include the mean composition and crustal thickness for the bulk basaltic crust, calculated for passive and active upwelling scenarios. Here, we present the mathematical formulations underlying the model and benchmark it against existing hydrous melting models and models for mid‐ocean ridge basalt formation. We show that the hydrous and anhydrous peridotite melting models incorporated in REEBOX PRO capture the essential differences in basalt composition and volume demonstrated by previous models, and constrain the ambient mantle beneath the global spreading ridge system to be between 1319 and 1366°C, depending on the relative fertility and/or water content of the mid‐ocean ridge mantle source. We also show how model outputs may be manipulated outside of the modeling program to calculate non‐traditional melt mixing scenarios. These examples highlight the flexibility of REEBOX PRO for simulating melt generation within a range of geodynamical contexts. This article is protected by copyright. All rights reserved.
      PubDate: 2016-09-26T02:58:07.808111-05:
      DOI: 10.1002/2016GC006579
  • Thermochronological evidence of a low‐angle, midcrustal detachment plane
           beneath the central South Island, New Zealand
    • Authors: Emily Warren‐Smith; Simon Lamb, Diane Seward, Euan Smith, Frédéric Herman, Tim Stern
      Abstract: Oblique continental convergence and uplift in the Southern Alps, New Zealand is largely accommodated by dextral transpression on the Alpine Fault. However, towards the south of the orogen the Alpine Fault becomes increasingly strike‐slip, despite evidence for high exhumation rates in the Pacific plate. Here, we present 41 new apatite and zircon fission‐track ages to investigate the role of the southern Alpine Fault in Pacific plate exhumation since the Miocene. Through development of a new, maximum likelihood fission‐track age calculation method (to overcome extremely low (< 0.1 ppm) 238U concentrations in apatites) we estimate the width of the fully reset apatite zone (ages 
      PubDate: 2016-09-21T11:10:35.52932-05:0
      DOI: 10.1002/2016GC006402
  • Effect of melt composition on crustal carbonate assimilation: Implications
           for the transition from calcite consumption to skarnification and
           associated CO2 degassing
    • Authors: L. B. Carter; R. Dasgupta
      Abstract: Skarns are residue of relatively low‐temperature magma‐induced decarbonation in the crust largely associated with silicic plutons. Mafic magmatic intrusions are also capable of releasing excess CO2 due to carbonate assimilation. However, the effect of mafic to silicic melt evolution on the decarbonation processes, in addition to temperature controls on carbonate‐intrusive magmatic systems, particularly at continental arcs, remains unclear. In this study, experiments performed in a piston cylinder apparatus at mid‐crustal depth (0.5 GPa) at supersolidus temperatures (900 to 1200°C) document calcite interaction with andesite and dacite melts at equilibrium under closed‐system conditions at calcite saturation in a 1:1 melt‐calcite ratio by weight. With increasing silica content in the starting melt, at similar melt fractions and identical pressure, assimilation decreases drastically (≤65% andesite‐calcite to ≤18% dacite‐calcite). In conjunction, the CaO/SiO2 ratio in melts resulting from calcite assimilation in andesitic starting material is >1, but ≤0.3 in those formed from dacite‐calcite interaction. With increasing silica‐content in the starting melt skarn mineralogy, particularly wollastonite, increases in modal abundance while diopsidic clinopyroxene decreases slightly. More CO2 is released with andesite‐calcite reaction (≤2.9 × 1011 g/y) than with more skarn‐like dacite‐calcite interaction (≤8.1 × 1010 g/y, at one volcano assuming respective calcite‐free‐superliquidus conditions and a magma flux of 1012 g/y). Our experimental results thus suggest that calcite assimilation in more mafic magmas may have first degassed a significant amount of crustal carbon before the melt evolves to more silicic compositions, producing skarn. Crustal decarbonation in long‐lived magmatic systems may hence deliver significant albeit diminishing amounts of carbon to the atmosphere and contribute to long‐term climate change. This article is protected by copyright. All rights reserved.
      PubDate: 2016-09-20T18:50:21.881601-05:
      DOI: 10.1002/2016GC006444
  • Experimental constraints on the relationship between clay abundance, clay
           fabric, and frictional behavior for the Central Deforming Zone of the San
           Andreas Fault
    • Authors: Jasmaria Wojatschke; Marco M. Scuderi, Laurence N. Warr, Brett M. Carpenter, Demian Saffer, Chris Marone
      Abstract: The presence of smectite (saponite) in fault gouge from the Central Deforming Zone of the San Andreas Fault at Parkfield, CA has been linked to low mechanical strength and aseismic slip. However, the precise relationship between clay mineral structure, fabric development, fault strength, and the stability of frictional sliding is not well understood. We address these questions through the integration of laboratory friction tests and FIB‐SEM analysis of fault rock recovered from the San Andreas Fault Observatory at Depth (SAFOD) borehole. Intact fault rock was compared with experimentally sheared fault gouge and different proportions of either quartz clasts or SAFOD clasts extracted from the sample. Nano‐textural measurements show the development of localized clay particle alignment along shear folia developed within synthetic gouges; such slip planes have multiples of random distribution (MRD) values of 3.0‐4.9. The MRD values measured are higher than previous estimates (MRD 1.5) that show lower degrees of shear localization and clay alignment averaged over larger volumes. The intact fault rock exhibits less well‐developed nano‐clay fabrics than the experimentally sheared materials, and MRD values decrease with smectite content. We show that the abundance, strength, and shape of clasts all influence fabric evolution via strain localization: quartz clasts yield more strongly developed clay fabrics than serpentine‐dominated SAFOD clasts. Our results suggest that: 1) both clay abundance and the development of nano‐scale fabrics play a role in fault zone weakening and 2) aseismic creep is promoted by slip along clay shears with >20 wt% smectite content and MRD values ≥2.7. This article is protected by copyright. All rights reserved.
      PubDate: 2016-09-19T18:55:33.539017-05:
      DOI: 10.1002/2016GC006500
  • Large‐scale tectonic cycles in Europe revealed by distinct Pb
           isotope provinces
    • Authors: Janne Blichert‐Toft; Hugo Delile, Cin‐Ty Lee, Zofia Stos‐Gale, Kjell Billström, Tom Andersen, Huhma Hannu, Francis Albarède
      Abstract: Lead isotopic systematics of U‐poor minerals, such as sulfides and feldspars, can provide unique insights into the origin and evolution of continents because these minerals ‘freeze in' the Pb isotopic composition of the crust during major tectonothermal events, allowing the history of a continent to be told through Pb isotopes. Lead model ages constrain the timing of crust formation while time‐integrated U/Pb, Th/Pb, and Th/U ratios shed light onto key geochemical processes associated with continent formation. Using ∼6800 Pb isotope measurements of primarily lead ores and minor K‐feldspar, we mapped out the Pb isotope systematics across Europe and the Mediterranean. Lead model ages define spatially distinct age provinces, consistent with major tectonic events ranging from the Paleozoic to the Proterozoic and latest Archean. However, the regions defined by time‐integrated U/Pb and Th/Pb ratios cut across the boundaries of age provinces, with high U/Pb systematics characterizing most of southern Europe. Magmatic influx, followed by segregation of dense sulfide‐rich mafic cumulates, resulted in foundering of U‐ and Th‐poor lower crust, thereby changing the bulk composition of the continental crust and leading to distinct time‐integrated U‐Th/Pb provinces. We show that the tectonic assembly of small crustal fragments leaves the crust largely undifferentiated, whereas the formation of supercontinents results in fundamental changes in the composition of the crust, identifiable in time and space by means of Pb isotope systematics. Observations based on Pb isotopes open up a new perspective on possible relationships between crustal thickness and geodynamic processes, in particular the role of crustal foundering into the mantle and the mechanisms responsible for the existence of cratons. This article is protected by copyright. All rights reserved.
      PubDate: 2016-09-15T08:25:33.51387-05:0
      DOI: 10.1002/2016GC006524
  • Dependence of seismic coupling on normal fault style along the Northern
           Mid‐Atlantic Ridge
    • Authors: Jean‐Arthur Olive; Javier Escartín
      Abstract: While normal faults are essential in shaping the seafloor formed at slow‐spreading mid‐ocean ridges, information on their behavior on short (seismic cycle) time scales is limited. Here we combine catalogs of hydro‐acoustically and teleseismically recorded earthquakes to characterize the state of seismic coupling along the Northern Mid‐Atlantic Ridge (MAR) between 12 and 35ºN. Along this portion of the MAR axis, tectonic extension is either taken up by steep conjugate faults that outline well‐defined ridge‐parallel abyssal hills, or dominantly by a large‐offset detachment fault on one side of the axis.We investigate variations in seismicity and seismic moment release rates across thirty ridge sections that can be clearly characterized either as abyssal hill or detachment bearing. We find that detachment‐bearing sections are associated with significantly greater seismicity and moment release rates than abyssal hill bearing sections, but show variability that may reflect the along‐axis extent of individual detachment faults. Overall, the measured seismic moment release rates fail to account for the long‐term fault slip rates. This apparent seismic deficit could indicate a mixed‐mode of fault slip where earthquakes only account for ∼10–30% of offset build‐up at abyssal hill faults, while the rest is accommodated by some form of transient aseismic creep. We find this seismic coupling fraction to be significantly greater (∼40–60%) at individual detachment systems, which is somewhat at odds with the common inference that detachment faults can sustain long‐lived localized strain because they are weak. We therefore propose alternative interpretations for seismic coupling based on dynamic friction theory. This article is protected by copyright. All rights reserved.
      PubDate: 2016-09-10T10:05:32.020298-05:
      DOI: 10.1002/2016GC006460
  • Lithium isotopic systematics of submarine vent fluids from arc and
           back‐arc hydrothermal systems in the western Pacific
    • Authors: Daisuke Araoka; Yoshiro Nishio, Toshitaka Gamo, Kyoko Yamaoka, Hodaka Kawahata
      Abstract: The Li concentration and isotopic composition (δ7Li) in submarine vent fluids are important for oceanic Li budget and potentially useful for investigating hydrothermal systems deep under the seafloor because hydrothermal vent fluids are highly enriched in Li relative to seawater. Although Li isotopic geochemistry has been studied at mid‐ocean‐ridge (MOR) hydrothermal sites, in arc and back‐arc settings Li isotopic composition has not been systematically investigated. Here, we determined the δ7Li and 87Sr/86Sr values of 11 end‐member fluids from 5 arc and back‐arc hydrothermal systems in the western Pacific and examined Li behavior during high‐temperature water–rock interactions in different geological settings. In sediment‐starved hydrothermal systems (Manus Basin, Izu‐Bonin Arc, Mariana Trough, and North Fiji Basin), the Li concentrations (0.23–1.30 mmol/kg) and δ7Li values (+4.3‰ to +7.2‰) of the end‐member fluids are explained mainly by dissolution‐precipitation model during high‐temperature seawater–rock interactions at steady state. Low Li concentrations are attributable to temperature‐related apportioning of Li in rock into the fluid phase and phase separation process. Small variation in Li among MOR sites is probably caused by low‐temperature alteration process by diffusive hydrothermal fluids under the seafloor. In contrast, the highest Li concentrations (3.40 − 5.98 mmol/kg) and lowest δ7Li values (+1.6‰ to +2.4‰) of end‐member fluids from the Okinawa Trough demonstrate that the Li is predominantly derived from marine sediments. The variation of Li in sediment‐hosted sites can be explained by the differences in degree of hydrothermal fluid–sediment interactions associated with the thickness of the marine sediment overlying these hydrothermal sites. This article is protected by copyright. All rights reserved.
      PubDate: 2016-09-10T10:05:23.387838-05:
      DOI: 10.1002/2016GC006355
  • Determining the flux of methane into Hudson Canyon at the edge of methane
           clathrate hydrate stability
    • Authors: Alexander Weinstein; Luis Navarrete, Carolyn Ruppel, Thomas C. Weber, Mihai Leonte, Matthias Kellermann, Eleanor Arrington, David L. Valentine, Mary I. Scranton, John D. Kessler
      Abstract: Methane seeps were investigated in Hudson Canyon, the largest shelf‐break canyon on the northern US Atlantic Margin. The seeps investigated are located at or updip of the nominal limit of methane clathrate hydrate stability. The acoustic identification of bubble streams was used to guide water column sampling in a 32 km2 region within the canyon's thalweg. By incorporating measurements of dissolved methane concentration with methane oxidation rates and current velocity into a steady‐state box model, the total emission of methane to the water column in this region was estimated to be 12 kmol methane per day (range: 6 – 24 kmol methane per day). These analyses suggest this methane is largely retained inside the canyon walls below 300 m water depth, and that it is aerobically oxidized to near completion within the larger extent of Hudson Canyon. Based on estimated methane emissions and measured oxidation rates, the oxidation of this methane to dissolved CO2 is expected to have minimal influences on seawater pH. This article is protected by copyright. All rights reserved.
      PubDate: 2016-08-29T09:20:23.123763-05:
      DOI: 10.1002/2016GC006421
  • Issue Information
    • Pages: 3833 - 3834
      PubDate: 2016-11-25T04:02:07.676368-05:
      DOI: 10.1002/ggge.20837
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