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Geophysical Research Letters     Full-text available via subscription   (Followers: 104, SJR: 3.323, h-index: 185)
Global Biogeochemical Cycles     Full-text available via subscription   (Followers: 15, SJR: 3.22, h-index: 136)
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J. of Geophysical Research : Solid Earth     Full-text available via subscription   (Followers: 44)
J. of Geophysical Research : Space Physics     Full-text available via subscription   (Followers: 119)
Paleoceanography     Full-text available via subscription   (Followers: 5, SJR: 3.067, h-index: 100)
Radio Science     Full-text available via subscription   (Followers: 37, SJR: 1.072, h-index: 59)
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Tectonics     Full-text available via subscription   (Followers: 14, SJR: 2.628, h-index: 96)
Water Resources Research     Full-text available via subscription   (Followers: 79, SJR: 2.661, h-index: 144)
Journal Cover Geochemistry, Geophysics, Geosystems
  [SJR: 2.439]   [H-I: 91]   [26 followers]  Follow
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   ISSN (Online) 1525-2027
   Published by AGU Homepage  [17 journals]
  • The formation of gold-rich seafloor sulfide deposits: Evidence from the
           Beebe Hydrothermal Vent Field, Cayman Trough
    • Authors: Alexander P. Webber; Stephen Roberts, Bramley J. Murton, Rachel A. Mills, Matthew R. S. Hodgkinson
      Abstract: The Beebe vent field (BVF) in the Cayman Trough has built an auriferous massive sulfide deposit on the ultra-slow spreading mid-Cayman spreading centre. The genesis of auriferous sulfide deposits at mid-ocean ridges is not fully understood, although there is a growing recognition that slow and ultra-slow spreading centres are conducive to gold mineralization. Analysis of hydrothermal precipitates from the BVF indicates that the highest gold contents are present within “beehive diffusers”, which have developed a highly porous pyrrhotite framework. The beehive structure allows vent fluids to effuse slowly, while allowing ingress of seawater to cool the fluid. The prevalence of pyrrhotite in the beehive samples, lack of sulfates, association between pyrrhotite and gold grains, and results of thermodynamic modelling, suggests gold precipitation occurred under highly reduced conditions even during mixing with seawater. In contrast, high temperature chimneys, with a single orifice, maintain high temperatures to the primary vent orifice and much of the gold is lost to seawater. Despite this, both chimney types are relatively gold enriched, which points to a further underlying cause for high gold at the BVF such as interaction of hydrothermal fluids with ultramafic lithologies in the basement. The final gold composition of the deposit is partially controlled by loss of gold during mass-wasting of the material, with gold depletion most prevalent in blocks formed at beehive-type chimneys. The BVF demonstrates that the overall gold content of a massive sulfide deposit is the sum of basement, precipitation, and surface processes.
      PubDate: 2017-05-08T04:50:44.982809-05:
      DOI: 10.1002/2017GC006922
  • Superweak asthenosphere in light of upper mantle seismic anisotropy
    • Authors: Thorsten W. Becker
      Abstract: Earth's upper mantle includes a ∼ 200 km thick asthenosphere underneath the plates where viscosity and seismic velocities are reduced compared to the background. This zone of weakness matters for plate dynamics and may be required for the generation of plate tectonics itself. However, recent seismological and electro-magnetic studies indicate strong heterogeneity in thinner layers underneath the plates which, if related to more extreme, global viscosity reductions, may require a revision of our understanding of mantle convection. Here, I use dynamically-consistent mantle flow modeling and the constraints provided by azimuthal seismic anisotropy as well as plate motions to explore the effect of a range of global and local viscosity reductions. The fit between mantle flow model predictions and observations of seismic anisotropy is highly sensitive to radial and lateral viscosity variations. I show that moderate sub-oceanic viscosity reductions, to ∼ 0.01 to 0.1 times the upper mantle viscosity, are preferred by the fit to anisotropy and global plate motions, depending on layer thickness. Lower viscosities degrade the fit to azimuthal anisotropy. Localized patches of viscosity reduction, or layers of subducted asthenosphere, however, have only limited additional effects on anisotropy or plate velocities. This indicates that it is unlikely that regional observations of sub-plate anomalies are both continuous and indicative of dramatic viscosity reduction. Locally, such weak patches may exist and would be detectable by regional anisotropy analysis, for example. However, large-scale plate dynamics are most likely governed by broad continent-ocean asthenospheric viscosity contrasts rather than a thin, possibly high melt fraction layer.
      PubDate: 2017-04-27T15:07:22.512707-05:
      DOI: 10.1002/2017GC006886
  • Understanding volcanic hazard at the most populated caldera in the world:
           Campi Flegrei, Southern Italy
    • Authors: Giuseppe De Natale; Claudia Troise, Christopher R.J. Kilburn, Renato Somma, Roberto Moretti
      Abstract: Naples and its hinterland in Southern Italy are one of the most urbanized areas in the world under threat from volcanic activity. The region lies within range of three active volcanic centers: Vesuvius, Campi Flegrei, and Ischia. The Campi Flegrei caldera, in particular, has been in unrest for six decades. The unrest followed four centuries of quiescence and has heightened concern about an increased potential for eruption. Innovative modelling and scientific drilling are being used to investigate Campi Flegrei, and the results highlight key directions for better understanding the mechanisms of caldera formation and the roles of magma intrusion and geothermal activity in determining the volcano's behavior. They also provide a framework for evaluating and mitigating the risk from this caldera and other large ones worldwide.
      PubDate: 2017-04-20T09:00:07.038048-05:
      DOI: 10.1002/2017GC006972
  • Seafloor age dependence of Rayleigh wave phase velocities in the Indian
    • Authors: Karen E. Godfrey; Colleen A. Dalton, Jeroen Ritsema
      Abstract: Variations in the phase velocity of fundamental-mode Rayleigh waves across the Indian Ocean are determined using two inversion approaches. First, variations in phase velocity as a function of seafloor age are estimated using a pure-path age-dependent inversion method. Second, a two-dimensional parameterization is used to solve for phase velocity within 1.25° × 1.25° grid cells. Rayleigh wave travel-time delays have been measured between periods of 38 and 200 s. The number of measurements in the study area ranges between 4139 paths at a period of 200 s and 22,272 paths at a period of 40 s. At periods 
      PubDate: 2017-04-18T15:46:04.860724-05:
      DOI: 10.1002/2017GC006824
  • Possible sources of hydrothermal activity and mud volcanism in southern
           Sakhalin inferred from local earthquake seismic tomography
    • Authors: Ivan Koulakov; Aleksander S. Serdyukov, Alexey V. Konovalov, Valentin I. Mikhailov, Dmitry A. Safonov, Anton A. Duchkov, Nassir Al-Arifi, Sami El Khrepy
      Abstract: We present the first seismic model of the crust beneath Sakhalin based on P- and S-wave arrival time data from local earthquakes. Based on the results of numerous synthetic tests, we conclude that this model has fair horizontal and vertical resolution to 20–25 km depth. At shallow depths, seismic anomalies are clearly associated with known geological structures, such as the high-velocity Paleozoic Susunai block and the low-velocity Cenozoic fold belts along the West Sakhalin Mountains. In vertical sections, we observe westward underthrusting of the Susunai block to a distance of at least 70 km, which may represent the regional compression and considerable crustal shortening in this area. Based on the tomography results, we hypothesize about the origin of the mud volcanism in southern Sakhalin. We propose that because of the general westward underthrusting regime in Sakhalin, hydrocarbon-rich shelf sediments may be entrained to considerable depths under the rigid Susunai block, which serves as a non-permeable cover. The released gases find the weakest zones around the Susunai block and along the Tym-Poronay Fault and escape to the surface to form the South Sakhalin and Lesnovsky mud volcano fields.
      PubDate: 2017-04-18T15:45:34.823377-05:
      DOI: 10.1002/2017GC006820
  • Can high-temperature, high-heat flux hydrothermal vent fields be explained
           by thermal convection in the lower crust along fast spreading mid-ocean
    • Authors: Fabrice J. Fontaine; M. Rabinowicz, M. Cannat
      Abstract: We present numerical models to explore possible couplings along the axis of fast spreading ridges, between hydrothermal convection in the upper crust, and magmatic flow in the lower crust. In an end-member category of models corresponding to effective viscosities μM lower than 1013 Pa.s in a melt-rich lower crustal along-axis corridor and permeability k not exceeding ∼10−16 m2 in the upper crust, the hot, melt-rich, gabbroic lower crust convects as a viscous fluid, with convection rolls parallel to the ridge axis. In these models, we show that the magmatic-hydrothermal interface settles at realistic depths for fast ridges, i.e. 1-2 km below seafloor. Convection cells in both horizons are strongly coupled and km-wide hydrothermal upflows/plumes, spaced by 8-10 km, arise on top of the magmatic upflows. Such magmatic-hydrothermal convective couplings may explain the distribution of vent fields along the East (EPR) and South-East Pacific Rise (SEPR). The lower crustal plumes deliver melt locally at the top of the magmatic horizon possibly explaining the observed distribution of melt-rich regions/pockets in the axial melt lenses of EPR and SEPR. Crystallization of this melt provides the necessary latent heat to sustain permanent ∼100 MW vents fields. Our models also contribute to current discussions on how the lower crust forms at fast ridges: they provide a possible mechanism for focused transport of melt-rich crystal mushes from moho level to the axial melt lens where they further crystallize, feed eruptions and are transported both along and off-axis to produce the lower crust.
      PubDate: 2017-04-18T15:45:31.656303-05:
      DOI: 10.1002/2016GC006737
  • Rock record and magnetic response to large earthquakes within Wenchuan
           Earthquake Fault Scientific Drilling cores
    • Authors: Lei Zhang; Zhiming Sun, Haibing Li, Laishi Zhao, Sheng-Rong Song, Yu-Min Chou, Yong Cao, Xiaozhou Ye, Huan Wang, Xiangli He
      Abstract: Fault-related pseudotachylytes are often considered to be produced by large seismic events. To investigate the rock record and magnetic response to large earthquakes within cores from the Wenchuan Earthquake Fault Scientific Drilling borehole 2 (WFSD-2), we carried out microstructural, geochemical and rock magnetic analyses of representative cores. Based on microstructural observations and powder X-ray diffraction analyses, we found 21 layers of melt-origin pseudotachylytes from 579.62–599.31 m-depth in the cores. The presence of early-formed pseudotachylyte fragments in the new layer suggests that seismic faulting processes exploited the same fault strand more than once. Pseudotachylyte veins have higher values of magnetic susceptibility relative to wall rocks. Rock magnetic results indicate that the magnetic minerals within the pseudotachylyte veins are magnetite with varying amounts of paramagnetic minerals. Magnetic hysteresis loops show that a reduction of the grain-size of ferromagnetic minerals is not a plausible explanation for the higher magnetic susceptibility values in pseudotachylyte veins. Rock magnetic analyses indicate that frictional heating (>500°C) occurred in the pseudotachylyte veins during large earthquakes. The resulting high temperatures induced thermal decomposition of paramagnetic minerals, forming magnetite and contributing to the higher magnetic susceptibility values. Different generations of pseudotachylytes and numerous high magnetic susceptibility zones together demonstrate that ancient powerful earthquakes may have occurred repeatedly in the Longmen Shan thrust belt.
      PubDate: 2017-04-12T14:17:43.058504-05:
      DOI: 10.1002/2017GC006822
  • 3-D basin-scale reconstruction of natural gas hydrate system of the Green
           Canyon, Gulf of Mexico
    • Authors: Ewa Burwicz; Thomas Reichel, Klaus Wallmann, Wolf Rottke, Matthias Haeckel, Christian Hensen
      Abstract: Our study presents a basin-scale 3D modeling solution, quantifying and exploring gas hydrate accumulations in the marine environment around the Green Canyon (GC955) area, Gulf of Mexico. It is the first modeling study that considers the full complexity of gas hydrate formation in a natural geological system. Overall, it comprises a comprehensive basin re-construction, accounting for depositional and transient thermal history of the basin, source rock maturation, petroleum components generation, expulsion and migration, salt tectonics and associated multi-stage fault development. The resulting 3D gas hydrate distribution in the Green Canyon area is consistent with independent borehole observations. An important mechanism identified in this study and leading to high gas hydrate saturation (> 80 vol. %) at the base of the gas hydrate stability zone (GHSZ), is the recycling of gas hydrate and free gas enhanced by high Neogene sedimentation rates in the region. Our model predicts the rapid development of secondary intra-salt mini-basins situated on top of the allochthonous salt deposits which leads to significant sediment subsidence and an ensuing dislocation of the lower GHSZ boundary. Consequently, large amounts of gas hydrates located in the deepest parts of the basin dissociate and the released free methane gas migrates upwards to recharge the GHSZ. In total, we have predicted the gas hydrate budget for the Green Canyon area that amounts to ∼3,256 Mt of gas hydrate which is equivalent to ∼340 Mt of carbon (∼7 x 1011 m3 of CH4 at STP conditions), and consists mostly of biogenic hydrates.
      PubDate: 2017-04-12T14:14:51.098131-05:
      DOI: 10.1002/2017GC006876
  • Plume-ridge interaction via melt channelization at Galápagos and other
           near-ridge hotspot provinces
    • Authors: Tushar Mittal; Mark A. Richards
      Abstract: The interaction of mantle plume driven flow with upwelling flow due to a nearby mid-ocean ridge occurs for many mantle plumes including Galápagos and Iceland. This interaction is typified by trace element and isotopic signatures demonstrating the “contamination” of normal ridge composition by relatively enriched plume material. However, another common signature of plume-ridge interaction is volcanic lineaments linking ridges and nearby plumes, perhaps most conspicuously the Wolf-Darwin lineament (WDL) at Galápagos and the Rodrigues Ridge (RR) at La Réunion. These enigmatic features remain unexplained. Plume-ridge interaction is commonly modeled in terms of interaction between solid-state plume flow and divergent ridge flow, but such models do not likely lead to the kind of solid-state flow-channelization that might explain narrow features such as the WDL and RR. Likewise, models involving tapping of anomalously hot and/or fertile asthenosphere between the plume and ridge due to lithospheric faulting appear to be inconsistent with a variety of evidence. We propose an alternative model in which the lineaments are the surface expressions of localized melt channels in the asthenosphere formed due to instabilities in a two-phase partially-molten system. A thermodynamic analysis shows that given the magma fluxes inferred to be associated with structures such as WDL and RR, these melt channels can be maintained over plume-ridge distances up to $\sim$1000 km. These results suggest that plume-ridge interaction in general, possibly including transport of plume-derived material along ridge axes (e.g., Iceland), may involve transport in high-melt-fraction channels, as opposed to just solid-state mantle flow.
      PubDate: 2017-04-12T14:14:21.866048-05:
      DOI: 10.1002/2016GC006454
  • Explosive processes during the 2015 eruption of Axial Seamount, as
           recorded by seafloor hydrophones
    • Authors: J. Caplan-Auerbach; R. P. Dziak, J. Haxel, D. R. Bohnenstiehl, C. Garcia
      Abstract: Following the installation of the Ocean Observatories Initiative cabled array, the 2015 eruption of Axial Seamount, Juan de Fuca ridge, became the first submarine eruption to be captured in real time by seafloor seismic and acoustic instruments. This eruption also marked the first instance where the entire eruption cycle of a submarine volcano, from the previous eruption in 2011 to the end of the month-long 2015 event, was monitored continuously using autonomous ocean bottom hydrophones. Impulsive sounds associated with explosive lava-water interactions are identified within hydrophone records during both eruptions. Explosions within the caldera are acoustically distinguishable from those occurring in association with north rift lava flows erupting in 2015. Acoustic data also record a series of broadband diffuse events, occurring in the waning phase of the eruption, and are interpreted as submarine Hawaiian explosions. This transition from gas-poor to gas-rich eruptive activity coincides with an increase in water temperature within the caldera and with a decrease in the rate of deflation. The last recorded diffuse events coincide with the end of the eruption, represented by the onset of inflation. All the observed explosion signals couple strongly into the water-column, and only weakly into the solid Earth, demonstrating the important of hydroacoustic observations as a compliment to seismic and geodetic studies of submarine eruptions.
      PubDate: 2017-04-12T14:13:14.057134-05:
      DOI: 10.1002/2016GC006734
  • Paleoenvironment change and its impact on carbon and nitrogen accumulation
           in the Zoige wetland, northeastern Qinghai–Tibetan Plateau over the past
           14,000 years
    • Authors: Mengxiu Zeng; Cheng Zhu, Yougui Song, Chunmei Ma, Zhenjing Yang
      Abstract: As the largest alpine wetland and peat deposition area in China, the Zoige wetland is climatically sensitive. The organic matter (OM) in peat stores copious environmental information. Here, we report new data on the organic geochemistry of a 4.5 m peat profile HY2014 from southern Zoige wetland. Based on closely spaced accelerator mass spectrometry (AMS) 14C dating, we established a high-resolution geochronological framework beginning at 14057 a BP. Moreover, we estimated the sedimentation flux of TOC and TN (SFs) and their influencing factors. Before 10916 a BP, the lake shrunk and peat began to develop under cold and dry conditions, and SFs were at their lowest values due to low productivity. More OM originated from hydrophyte and marsh plants. From 10916 to 3050 a BP, peat was widely and well developed, and the climate was warm and humid, despite a cooling and drying trend. The HY2014 profile experienced an optimum climate during 10916 − 6000 a BP, when SFs had the highest values that benefited from high productivity, and OM mainly originated from terrestrial plants. After 3050 a BP, the climate was the coldest and driest. The higher SFs over the past 2000 a BP was mainly resulted from the low decomposition rate. The plant community, primary productivity and decomposition rate were closely linked with the temporal variation of SFs. The environment change was mainly controlled by summer solar insolation, and the Zoige wetland was significantly influenced by the Indian summer monsoon.
      PubDate: 2017-04-12T14:12:42.793095-05:
      DOI: 10.1002/2016GC006718
  • Evolution of the South Pacific helium plume over the past 3 decades
    • Authors: J.E. Lupton; W.J. Jenkins
      Abstract: The recent GEOTRACES Eastern Pacific Zonal Transect in 2013 crossed the East Pacific Rise at 15°S following the same track as the 1987 Helios Expedition along the core of the mid-depth helium plume that spreads westward from the East Pacific Rise (EPR) axis. The fact that several stations were co-located with the earlier Helios stations has allowed a detailed comparison of the changes in the helium plume over the intervening 26 years. While the plume in many areas is unchanged, there is a marked decrease in plume intensity at longitude 120°W in the 2013 data which was not present in 1987. Recent radioisotope measurements along the plume track suggest that this decrease is due to the intrusion of a different water mass into the plume, rather than a modulation of hydrothermal input on the EPR axis. Analysis of GEOTRACES hydrographic data shows excess heat present in the plume up to 0.04°C, corresponding to a 3He/heat ratio of ∼2.5 × 10−18 mol J−1, similar to that found in mature hydrothermal vents. RAFOS floats deployed in 1987 indicate an average westward transport of ∼0.3 cm s−1 at 2500 m depth in the off-axis plume, in agreement with recent estimates of ∼0.4 cm s−1 based on “aging” of the plume from 227Ac/3He ratios.
      PubDate: 2017-04-12T14:12:32.435656-05:
      DOI: 10.1002/2017GC006848
  • The dynamical control of subduction parameters on surface topography
    • Authors: F. Crameri; C.R. Lithgow-Bertelloni, P.J. Tackley
      Abstract: The long-wavelength surface deflection of Earth's outermost rocky shell is mainly controlled by large-scale dynamic processes like isostasy or mantle flow. The largest topographic amplitudes are therefore observed at plate boundaries due to the presence of large thermal heterogeneities and strong tectonic forces. Distinct vertical surface deflections are particularly apparent at convergent plate boundaries mostly due to the convergence and asymmetric sinking of the plates. Having a mantle convection model with a free surface that is able to reproduce both realistic single-sided subduction and long-wavelength surface topography self-consistently, we are now able to better investigate this interaction. We separate the topographic signal into distinct features and quantify the individual topographic contribution of several controlling subduction parameters. Results are diagnosed by splitting the topographic signal into isostatic and residual components, and by considering various physical aspects like viscous dissipation during plate bending. Performing several systematic suites of experiments, we are then able to quantify the topographic impact of the buoyancy, rheology and geometry of the subduction-zone system to each and every topographic feature at a subduction zone and to provide corresponding scaling laws. We identify slab dip and, slightly less importantly, slab buoyancy as the major agents controlling surface topography at subduction zones on Earth. Only the island-arc high and the back-arc depression extent are mainly controlled by plate strength. Overall, his modelling study sets the basis to better constrain deep-seated mantle structures and their physical properties via the observed surface topography on present-day Earth and back through time.
      PubDate: 2017-04-12T14:12:10.14624-05:0
      DOI: 10.1002/2017GC006821
  • Large-volume lateral magma transport from the Mull volcano: An insight to
           magma chamber processes
    • Authors: Osamu Ishizuka; Rex N. Taylor, Nobuo Geshi, Nobutatsu Mochizuki
      Abstract: Long-distance lateral magma transport within the crust has been inferred for various magmatic systems including oceanic island volcanoes, mid oceanic ridges and large igneous provinces. However, studying the physical and chemical properties of active fissure systems is difficult. Hence this study investigates the movement of magma away from the Mull Volcano in the North Atlantic Igneous Province, where erosion has exposed its upper crustal dike networks. Magmatic lineations within dikes indicate that the magma flow in the Mull dike suite changed from near vertical to horizontal within 30 km of the volcanic center. This implies that distal dikes were fed by lateral magma transport from Mull. Geochemical characteristics indicate that many
      PubDate: 2017-04-12T14:11:42.366896-05:
      DOI: 10.1002/2016GC006712
  • Utilizing 210Po deficit to constrain particle dynamics in mesopelagic
           water, western South China Sea
    • Authors: Haoyang Ma; Weifeng Yang, Lihao Zhang, Run Zhang, Min Chen, Yusheng Qiu, Minfang Zheng
      Abstract: The 210Po-210Pb pair is increasingly used as a proxy of quantifying organic carbon export from the euphotic zone. However, disequilibria between 210Po and 210Pb in mesopelagic water have been poorly studied. Here we present unusual deficiencies of 210Po with respect to 210Pb in mesopelagic water (200-1000 m) in the South China Sea (SCS). The total particulate matter (TPM) increased by up to 32% in the mesopelagic layer comparing with the euphotic zone. The total 210Po/210Pb ratio varied from 0.41 to 0.98 with an average of 0.72±0.19, showing an enhanced removal of 210Po in mesopelagic water. On average, particulate 210Po and 210Pb increased by 23% and 32% at the slope stations respectively. These results indicated that the 210Po deficits result from lateral transport, probably via benthic nepheloid layer. Based on the deficiency of 210Po, the residence times of particulate 210Po were estimated to range from 0.11 to 0.25 yr (avg. 0.17±0.07 yr), allowing re-suspended sediment to disperse over a long-range. The export fluxes of 210Po varied from 68 to 121 dpm m−2 d−1 with an average of 96±27 dpm m−2 d−1, which was 6 times that out of the euphotic zone. Using the 210Po deficits, the export fluxes of TPM out of the mesopelagic layer were quantified to vary from 4.19 to 10.20 g m−2 d−1, revealing a large amount of particles from the shelf to the SCS basin. This study suggests that 210Po-210Pb could be an effective tracer of tracking particle cycling in mesopelagic water.
      PubDate: 2017-04-12T14:11:11.300684-05:
      DOI: 10.1002/2017GC006899
  • Miocene-Recent sediment flux in the south-central Alaskan forearc basin
           governed by flat-slab subduction
    • Authors: Emily S. Finzel; Eva Enkelmann
      Abstract: The Cook Inlet in south-central Alaska contains the early Oligocene to Recent stratigraphic record of a forearc basin adjacent to a shallowly subducting oceanic plateau. Our new measured stratigraphic sections and detrital zircon U-Pb geochronology and Hf isotopes from Neogene strata and modern rivers illustrate the effects of flat-slab subduction on the depositional environments, provenance, and subsidence in forearc sedimentary systems. During the middle Miocene, fluvial systems emerged from the eastern, western, and northern margins of the basin. The axis of maximum subsidence was near the center of the basin, suggesting equal contributions from subsidence drivers on both margins. By the late Miocene, the axis of maximum subsidence had shifted westward and fluvial systems originating on the eastern margin of the basin above the flat slab traversed the entire width of the basin. These mud-dominated systems reflect increased sediment flux from recycling of accretionary prism strata. Fluvial systems with headwaters above the flat-slab region continued to cross the basin during Pliocene time, but a change to sandstone-dominated strata with abundant volcanogenic grains signals a reactivation of the volcanic arc. The axis of maximum basin subsidence during late Miocene to Pliocene time is parallel to the strike of the subducting slab. Our data suggest that the character and strike-orientation of the down-going slab may provide a fundamental control on the nature of depositional systems, location of dominant provenance regions, and areas of maximum subsidence in forearc basins.
      PubDate: 2017-04-03T02:46:13.694783-05:
      DOI: 10.1002/2016GC006783
  • Natural H2 in Kansas: Deep or shallow origin?
    • Authors: J. Guélard; V. Beaumont, V. Rouchon, F. Guyot, D. Pillot, D. Jézéquel, M. Ader, K. D. Newell, E. Deville
      Abstract: A geochemical study of gas coming from three wells in northeastern Kansas supplements previous studies from the 1980s and points to a persistent regional phenomenon of H2 production. In 2008, a new well showed, just after drilling, a free gas phase with more than 80 mole % of H2, followed by water production associated with gas. This gas is mainly composed of N2, He, H2 and occasionally CH4, with changing proportions through time. A drastic decrease in H2 at the well was observed since the aquifer is produced, along with occasional recharges in H2 evidenced notably in the early phases of gas sampling. We demonstrate that this evolution of gas composition is closely associated to the well completion story. Accordingly, two distinct origins of H2 are proposed: (1) deep crustal H2: water reduction associated to iron oxidation in the Precambrian basement; (2) reactions occurring in the tubing, primarily attributed to high contents of reduced iron and/or dissolved organic carbon (DOC=4.1 mg.L−1) in the water. The low δD values averaging -760 ‰ are attributed to a low temperature process, possibly a re-equilibration with water. Furthermore, the suggested origins are supported by the observed gas associations: (a) deep crustal H2 with radiogenic gases (4He and 40Ar) and metamorphic N2 (δ15N averaging +2.5‰); (b) surficial H2 with methane produced in the sedimentary aquifer and the tubing by methanogenic organisms.
      PubDate: 2017-04-03T02:41:08.147781-05:
      DOI: 10.1002/2016GC006544
  • Near-vent chemical processes in a hydrothermal plume: Insights from an
           integrated study of the Endeavour segment
    • Authors: L. A. Coogan; A. Attar, S. F. Mihaly, M Jeffries, M. Pope
      Abstract: The Endeavour segment of the Juan de Fuca mid-ocean ridge is one of the best-studied ridge segments and has recently been instrumented as part of Ocean Networks Canada's NEPTUNE cabled observatory. Here we investigate the interaction between high-temperature vent fluids and the overlying water column. A new tow-yo survey found the average temperature anomaly in the neutrally buoyant plume was ∼0.043°C. The water column temperature and light attenuation anomalies correlate linearly in some areas of the plume but in other areas there is a low light attenuation anomaly relative to the temperature anomaly. This temperature excess is interpreted to reflect heat input through (particle poor) diffuse flow. If this is correct, about half of the heat flux along the Endeavour segment comes from diffuse flow. Sediment trap and push core data show the mass accumulation rate of the hydrothermal component of the sediments decreases rapidly with distance from the major vent fields. Large changes in the composition of the hydrothermal component of the sediments also occur with distance from the vent fields. The composition of the sediments indicates: (i) sulfides precipitate early and accumulate most rapidly close to the vents with a preferential order of element removal from the plume of Cd>Ag>Cu>Co∼Fe; (ii) barite is deposited somewhat further from the vents. Strontium and Pb appear to be strongly incorporated in barite and/or other sulfate minerals; and (iii) at most a few percent of the mass of these “insoluble” elements that is vented gets deposited within 1.5 km of the vents.
      PubDate: 2017-04-03T02:40:37.744061-05:
      DOI: 10.1002/2016GC006747
  • Aluminum in zircon as evidence for peraluminous and metaluminous melts
           from the Hadean to present
    • Authors: Dustin Trail; Nicholas Tailby, Yanling Wang, T. Mark Harrison, Patrick Boehnke
      Abstract: Zircon structurally accommodates a range of trace impurities into its lattice, a feature which is used extensively to investigate the evolution of silicate magmas. One key compositional boundary of magmas is defined by whether the molar ratio of Al2O3/(CaO+Na2O+K2O) is larger or smaller than unity. Here, we report ∼800 Al in zircon concentrations from 19 different rocks from the Lachlan Fold Belt (southeastern Australia), New England (USA), and Arunachal leucogranites (eastern Himalaya) with Al2O3/(CaO+Na2O+K2O) whole rock values that range from 0.88 to 1.6. Zircons from peraluminous rocks yield an average Al concentration of ∼10 ppm, which distinguishes them from crystals found in metaluminous rocks (∼1.3 ppm). This difference is related to the materials involved in the melting, assimilation, and/or magma differentiation processes; for example, magmas that assimilate Al-rich material such as metapelites are expected to produce melts with elevated alumina activities, and thus zircons with high Al concentrations. These observations are applied to the Archean and Hadean Jack Hills detrital zircon record. Detrital Archean zircons, with ages from about 3.30 to 3.75 Ga, yield Al in zircon concentrations consistent with origins in peraluminous rocks in ∼8% of the cases (n =236). A single zircon from the pre-3.9 Ga age group (n = 39) contains elevated Al contents, which suggests that metaluminous crustal rocks were more common than peraluminous rocks in the Hadean. Weathered material assimilated into these Hadean source melts was not dominated by Al-rich source material.
      PubDate: 2017-04-03T02:36:01.241732-05:
      DOI: 10.1002/2016GC006794
  • Shallow melting of MORB-like mantle under hot continental lithosphere,
           Central Anatolia
    • Authors: Mary R. Reid; W. Kirk Schleiffarth, Michael A. Cosca, Jonathan R. Delph, Janne Blichert-Toft, Kari M. Cooper
      Abstract: Widespread mafic volcanism, elevated crustal temperatures, and plateau-type topography in Central Anatolia, Turkey, could collectively be the result of lithospheric delamination, mantle upwelling, and tectonic escape. We use results from 40Ar/39Ar geochronology, basalt geochemistry, and a passive-source broadband seismic experiment obtained in a collaborative international effort (Continental Dynamics – Central Anatolia Tectonics) to investigate the upper mantle structure and evolution of melting conditions over an ∼2400 km2 area south and west of Hasan volcano. New 40Ar/39Ar dates for the basalts mostly cluster between 0.2 and 0.6 Ma, but some scoria cones are as old as 2.5 Ma. Basalts are dominantly Mg-rich (Mg#=62 to 71), moderately alkaline (normative Ne
      PubDate: 2017-04-03T02:35:57.642899-05:
      DOI: 10.1002/2016GC006772
  • P and S wave attenuation tomography of the Japan subduction zone
    • Authors: Zewei Wang; Dapeng Zhao, Xin Liu, Chuanxu Chen, Xibing Li
      Abstract: We determine the first high-resolution P and S wave attenuation (Q) tomography beneath the entire Japan Islands using a large number of high-quality t* data collected from P and S wave velocity spectra of 4222 local shallow and intermediate-depth earthquakes. The suboceanic earthquakes used in this study are relocated precisely using sP depth phases. Significant landward-dipping high-Q zones are revealed clearly, which reflect the subducting Pacific slab beneath Hokkaido and Tohoku, and the subducting Philippine Sea (PHS) slab beneath SW Japan. Prominent low-Q zones are visible in the crust and mantle wedge beneath the active arc volcanoes in Hokkaido, Tohoku and Kyushu, which reflect source zones of arc magmatism caused by fluids from the slab dehydration and corner flow in the mantle wedge. Our results also show that non-volcanic low-frequency earthquakes (LFEs) in SW Japan mainly occur in the transition zone between a narrow low-Q belt and its adjacent high-Q zones right above the flat segment of the PHS slab. This feature suggests that the non-volcanic LFEs are caused by not only fluid-affected slab interface but also specific conditions such as high pore pressure which is influenced by the overriding plate.
      PubDate: 2017-04-03T02:35:46.936791-05:
      DOI: 10.1002/2017GC006800
  • Multidecadal oceanographic changes in the western Pacific detected through
           high-resolution bomb-derived radiocarbon measurements on corals
    • Authors: S. Hirabayashi; Y. Yokoyama, A. Suzuki, Y. Miyairi, T. Aze
      Abstract: High-resolution measurements of radiocarbon (14C) in corals can be used to reconstruct past variability in ocean conditions. Here, we report seasonal Δ14C changes in coral from Ishigaki Island, Japan, and compare with previously reported data from Palau and Guam. Our data clearly indicate a significant increase in Δ14C from 1947 to 1998 related to atmospheric nuclear bomb testing. The three early Δ14C spikes related to the atmospheric nuclear bomb tests in the US Proving Grounds at Bikini and Enewerak atoll conducted in 1954, 1956 and 1958 were detected from the Ishigaki coral. After 1976, variability in the Mindanao Dome region related to North Equatorial Current (NEC) bifurcation latitude migration affected the Δ14C difference between Palau and Guam, whereas the difference between Ishigaki and Guam was not correlated with the bifurcation latitude. The Δ14C difference between Ishigaki and Guam may be due to mesoscale eddies in the Kuroshio area. On the decadal scale, the northward shift of NEC bifurcation latitude after 1976, the year as known as Pacific Decadal Oscillation regime shift from negative to positive, was concurrent with the abundant westward-propagating mesoscale eddies in the Subtropical Counter current region and stronger Kuroshio transport off the east Taiwan, which may be represented by a smaller Δ14C difference between Ishigaki and Guam after 1976.
      PubDate: 2017-04-02T03:05:28.144077-05:
      DOI: 10.1002/2017GC006854
  • Lithospheric structure of Iberia and Morocco using finite-frequency
           Rayleigh wave tomography from earthquakes and seismic ambient noise
    • Authors: I. Palomeras; A. Villaseñor, S. Thurner, A. Levander, J. Gallart, M. Harnafi
      Abstract: We present a new 3D shear velocity model of the western Mediterranean from the Pyrenees, Spain, to the Atlas Mountains, Morocco, and the estimated crustal and lithospheric thickness. The velocity model shows different crustal and lithospheric velocities for the Variscan provinces, those which have been affected by Alpine deformation, and those which are actively deforming. The Iberian Massif has detectable differences in crustal thickness that can be related to the evolution of the Variscan orogen in Iberia. Areas affected by Alpine deformation have generally lower velocities in the upper and lower crust than the Iberian Massif. Beneath the Gibraltar Strait and surrounding areas the crustal thickness is greater than 50 km, below which a high velocity anomaly (>4.5 km/s) is mapped to depths greater than 200 km. We identify this as a subducted remnant of the NeoTethys plate referred to as the Alboran and western Mediterranean slab. Beneath the adjacent Betic and Rif Mountains, the Alboran slab is still attached to the base of the crust, depressing it, and ultimately delaminating the lower crust and mantle lithosphere as the slab sinks. Under the adjacent continents the Alboran slab is surrounded by low upper mantle shear wave velocities (Vs
      PubDate: 2017-03-28T11:00:29.795201-05:
      DOI: 10.1002/2016GC006657
  • Investigations of the spatial and temporal variations of Sr and Nd
           isotopes in sediments from two Indian Rivers: Implications to source
    • Authors: Sonali Pradhan; Jing Zhang, Mark Baskaran, Prabhaker Vasant Shirodkar, Ying Wu, Umesh Kumar Pradhan
      Abstract: Radiogenic isotopes of Sr and Nd provide crucial information on chemical and physical erosion processes, therefore used as tracers for fingerprinting the sources of fluvial sediments. Moreover, elemental geochemistry, 87Sr/86Sr and 143Nd/144Nd signatures in silicate fractions of sediments primarily reflect intricate controls of characteristics and provenance of sediment. A suite of sediment samples collected from two rivers of India, Narmada (large) and Netravati (small) were analyzed for elemental concentrations, 87Sr/86Sr and 143Nd/144Nd in silicate as well as acid leachable fractions. 87Sr/86Sr in acid leachable sediment fractions from both the rivers (0.7094 ± 0.0002 for Narmada and 0.7158 ± 0.003 for Netravati) showed more radiogenic 87Sr/86Sr in Netravati compared to Narmada indicating influence of source rocks in their watersheds. Weak correlation of sedimentary 87Sr/86Sr and 143Nd/144Nd with Al and Chemical Index of Alteration (CIA) in silicate fraction indicate that isotopic compositions are primarily controlled by their sources and not by chemical weathering in both the rivers. Provenance of sediments in Narmada is dominated by their supply from Deccan deposits with secondary supply from mid–late Proterozoic Vindhyan along with Mesoproterozoic to Mesoarchean TDMNd ages. Sediments from Netravati reflected signatures of peninsular gneisses dominant with gneisses schists and granodiorite along with Paleoarchean TDMNd ages. Based on variability of 87Sr/86Sr and 143Nd/144Nd, present study highlight that sediment supplies from tributaries to the mainstream in Narmada is inadequate as compared to that in Netravati. 87Sr/86Sr and 143Nd/144Nd signatures in Narmada and Netravati are comparable with Global Rivers. This article is protected by copyright. All rights reserved.
      PubDate: 2017-03-28T10:55:25.693058-05:
      DOI: 10.1002/2016GC006669
  • Thermal state of the Explorer segment of the Cascadia subduction zone:
           Implications for seismic and tsunami hazards
    • Authors: Dawei Gao; Kelin Wang, Earl E. Davis, Yan Jiang, Tania L. Insua, Jiangheng He
      Abstract: The Explorer segment of northernmost Cascadia is an end-member “warm” subduction zone with very young incoming plate and slow convergence rate. Understanding the megathrust earthquake potential of this type of subduction zone is of both geodynamic and societal importance. Available geodetic observations indicate that the subduction megathrust of the Explorer segment is currently locked to some degree, but the downdip extent of the fault area that is potentially seismogenic is not known. Here we construct finite element models to estimate the thermally allowed megathrust seismogenic zone, using available knowledge of regional plate kinematics, structural data, and heat flow observations as constraints. Despite ambiguities in plate interface geometry constrained by hypocenter locations of low-frequency earthquakes beneath Vancouver Island, the thermal models suggest a potential rupture zone of ∼60 km downdip width located fully offshore. Using dislocation modeling, we further illustrate that a rupture zone of this size, even with a conservative assumption of ∼100 km strike length, can cause significant tsunami-genic deformation. Future seismic and tsunami hazard assessment in northern Cascadia must take the Explorer segment into account. This article is protected by copyright. All rights reserved.
      PubDate: 2017-03-20T11:15:27.693403-05:
      DOI: 10.1002/2017GC006838
  • The potential of sedimentary foraminiferal rare earth element patterns to
           trace water masses in the past
    • Authors: Anne H. Osborne; Ed C. Hathorne, Johan Schijf, Yves Plancherel, Philipp Böning, Martin Frank
      Abstract: Dissolved rare earth element (REE) concentration data from intermediate and deep seawater form an array characterized by higher middle REE enrichments (MREE/MREE*) in the North Atlantic and a progressive increase in heavy-to-light REE ratios (HREE/LREE) as water masses age. The REEs in foraminifera are fractionated towards higher MREE/MREE* and lower HREE/LREE relative to seawater. Calculations based on a scavenging model show that the REE patterns in uncleaned core-top foraminifera resemble those adsorbed onto calcite, particulate organic material, and hydrous ferric oxides but the full extent of the REE fractionation measured in foraminifera was not reproduced by the model. However, differences in the HREE/LREE, MREE/MREE* ratios and the cerium anomaly between ocean basins are preserved and are in agreement with the seawater REE distribution. Under oxic conditions, the HREE/LREE and MREE/MREE* compositions of uncleaned foraminifera at the sediment/seawater boundary are preserved during burial but the cerium anomaly is sensitive to burial depth. In suboxic sedimentary environments, all uncleaned foraminiferal REE concentrations are elevated relative to core-top values indicating addition of REEs from pore waters. The HREE/LREE ratio is highest when sedimentation rates were greatest [Lippold et al., 2009] and when high Fe/Ca ratios in the uncleaned foraminifera indicate that Fe was mobile. In sediments that have not experienced suboxic conditions during burial, uncleaned foraminifera preserve the seawater signal taken up at the sediment/seawater interface and are therefore suggested to be a suitable archive of changes in the REE signal of past bottom waters. This article is protected by copyright. All rights reserved.
      PubDate: 2017-03-17T19:20:31.094052-05:
      DOI: 10.1002/2016GC006782
  • Differential response of corals to regional mass-warming events as evident
           from skeletal Sr/Ca and Mg/Ca ratios
    • Authors: Harry Clarke; Juan Pablo D'Olivo, James Falter, Jens Zinke, Ryan Lowe, Malcolm McCulloch
      Abstract: During the summer of 2010/11, a regional marine heat wave resulted in coral bleaching of variable severity along much of the western coastline of Australia. At Ningaloo Reef, a 300km long fringing reef system and World Heritage site, highly contrasting coral bleaching was observed between two morphologically distinct nearshore reef communities located on either side of the Ningaloo Peninsula: Tantabiddi (∼20% bleaching) and Bundegi (∼90% bleaching). For this study, we collected coral cores (Porites sp.) from Tantabiddi and Bundegi reef sites to assess the response of the Sr/Ca temperature proxy and Mg/Ca ratios to the variable levels of thermal stress imposed at these two sites during the 2010/11 warming event. We found that there was an anomalous increase in Sr/Ca and decrease in Mg/Ca ratios in the Bundegi record that was coincident with the timing of severe coral bleaching at the site, while no significant changes were observed in the Tantabiddi record. We show that the change in the relationship of Sr/Ca and Mg/Ca ratios with temperature at Bundegi during the 2010/11 event reflects changes in related coral ‘vital' processes during periods of environmental stress. These changes were found to be consistent with a reduction in active transport of Ca2+ to the site of calcification leading to a reduction in calcification rates and reduced Rayleigh fractionation of incorporated trace elements. This article is protected by copyright. All rights reserved.
      PubDate: 2017-03-17T19:20:26.74265-05:0
      DOI: 10.1002/2016GC006788
  • Yttrium and rare earth element partitioning in seawaters from the Bay of
    • Authors: Zhaojie Yu; Christophe Colin, Eric Douville, Laure Meynadier, Stéphanie Duchamp-Alphonse, Sophie Sepulcre, Shiming Wan, Lina Song, Qiong Wu, Zhaokai Xu, Frank Bassinot
      Abstract: The dissolved Yttrium (Y) and Rare Earth Element (REE) concentrations of seawater samples collected along a north-south hydrological transect within the Bay of Bengal (BoB) have been analyzed to estimate contributions of the Ganges and Brahmaputra (G-B) river inputs to the dissolved REE distribution of the Northern Indian Ocean. Surface water masses of the BoB are characterized by Y/Ho ratios (84) intermediate between the G-B river suspended sediment (41) and water mass from the South Indian Ocean (93). Co-variation of MREE (Middle REE, Sm) and LREE (Light REE, La) concentrations suggests that the dissolved REEs in surface waters (upper 100 m depth) of the BoB (Sm/La = 0.21) appear to derive mainly from the freshwater discharge of the G-B river system. In contrast, values obtained in the intermediate and deep-waters (Sm/La = 0.14) suggest a mixing of dissolved REEs deriving from the release of G-B river suspended particles (Sm/La = 0.16) and the contribution of Antarctic Bottom Water (AABW) (Sm/La = 0.12). Consequently, we propose that MREE/MREE* ratios in the BoB waters could be an accurate proxy to trace lithogenic inputs from the G-B river system. The dissolved and particle re-mineralization Nd fluxes from G-B river system are calculated to constitute about 9% and 4% of the global dissolved river discharge and ‘Boundary inputs' flux. Our estimation indicates that the massive G-B river system inputs could greatly alter the dissolved REEs distribution in the BoB and contribute to the dissolved REEs budget in the ocean. This article is protected by copyright. All rights reserved.
      PubDate: 2017-03-17T19:15:33.493806-05:
      DOI: 10.1002/2016GC006749
  • Mantle heterogeneity in the source region of mid-ocean ridge basalts along
           the northern central Indian Ridge (8°–17°S)
    • Authors: Jonguk Kim; Sang-Joon Pak, Jai-Woon Moon, Sang-Mook Lee, Jihye Oh, Finlay M. Stuart
      Abstract: The northern Central Indian Ridge (CIR) between 8°S and 17°S is composed of seven segments whose spreading rates increase southward from ∼35 to ∼40 mm/yr. During expeditions of R/V Onnuri to study hydrothermal activity on the northern CIR in 2009 to 2011, high-resolution multibeam mapping was conducted and ridge axis basalts were dredged. The major and trace element, and Sr–Nd–Pb-He isotopic compositions of basaltic glasses dredged from the spreading axis require three mantle sources; depleted mantle and two distinct enriched mantle sources. The southern segments have Sr, Nd, and Pb that are a mix of depleted mantle and an enriched component as recorded in southern CIR MORB. This enrichment is indistinguishable from Rèunion plume mantle, except for He isotopes. This suggests that the southern segments have incorporated a contribution of the fossil Rèunion plume mantle, as the CIR migrated over hotspot-modified mantle. The low 3He/4He (7.5 to 9.2 RA) of this enriched component may result from radiogenic 4He ingrowth in the fossil Rèunion mantle component. Basalts from the northern segments have high 206Pb/204Pb (18.53-19.15) and low 87Sr/86Sr (0.70286-0.70296) that are distinct from the Rèunion plume but consistent with derivation from mantle with FOZO signature, albeit with 3He/4He (9.2 to 11.8 RA) that are higher than typical. The FOZO-like enriched mantle cannot be attributed to the track of a nearby mantle plume. Instead, this enrichment may have resulted from recycling oceanic crust, possibly accompanied by small plume activity. This article is protected by copyright. All rights reserved.
      PubDate: 2017-03-17T11:40:40.124914-05:
      DOI: 10.1002/2016GC006673
  • A pure dipole analysis of the Gondwana apparent polar wander path:
           Paleogeographic implications in the evolution of Pangea
    • Authors: L. C. Gallo; R. N. Tomezzoli, E. O. Cristallini
      Abstract: The paleogeography of pre break-up Pangea at the beginning of the Atlantic Spreading has been a subject of debate for the past 50 years. Reconciling this debate involves theoretical corrections that cast doubt on available data and paleomagnetism as an effective tool for performing paleoreconstructions. This 50-year-old debate focuses specifically on magnetic remanence and its ability to correctly record the inclination of the paleomagnetic field. In this paper, a selection of paleopoles was made to find the great circles containing the paleomagnetic pole and the respective sampling site. The true dipole pole (TDP) was then calculated by intersecting these great circles, effectively avoiding non-dipolar contributions and inclination shallowing, in an innovative method. The great circle distance between each of these TDPs and the paleomagnetic means show the accuracy of paleomagnetic determinations in the context of a dominantly geocentric, axial and dipolar geomagnetic field. The TDPs calculated allowed a bootstrap analysis to be performed to further consider the flattening factor that should be applied to the sedimentary derived paleopoles. It is argued that the application of a single theoretical correction factor for clastic sedimentary-derived records could lead to a bias in the paleolatitude calculation and therefore to incorrect paleogeographic reconstructions. The unbiased APWP makes it necessary to slide Laurentia to the West in relation to Gondwana in a B-type Pangea during the Upper Carboniferous, later evolving, during the Early Permian, to reach the final A-type Pangea configuration of the Upper Permian. This article is protected by copyright. All rights reserved.
      PubDate: 2017-03-17T11:40:36.100697-05:
      DOI: 10.1002/2016GC006692
  • Remanence acquisition efficiency in biogenic and detrital magnetite and
           recording of geomagnetic paleointensity
    • Authors: Liang Chen; David Heslop, Andrew P. Roberts, Liao Chang, Xiang Zhao, Helen V. McGregor, Gianluca Marino, Laura Rodriguez-Sanz, Eelco J. Rohling, Heiko Pälike
      Abstract: Relative paleointensity (RPI) variations of Earth's magnetic field are widely used to understand geomagnetic field behavior and to develop age models for sedimentary sequences. RPI estimation is based on a series of assumptions. One key assumption that is rarely considered is that all magnetic particles in the sediment acquired a magnetization in an identical manner. In this paper, we test this assumption for sediments from the eastern equatorial Pacific Ocean that record well-documented global RPI variations over the last ∼780 kyr. The magnetization is carried by two stable single domain magnetic components, which we identify as magnetite magnetofossils and titanomagnetite nanoparticle inclusions within larger silicate particles. By analyzing signals carried by the two components separately, we determine for the first time that magnetic nanoparticle inclusions can cause their host particles to record reliable but inefficient sedimentary paleomagnetic signals. The magnetization carried by biogenic magnetite is acquired more efficiently than that carried by the nanoparticle inclusions. Variations in the concentration of both components are modulated climatically so that they record nearly identical RPI signals. In many sediment types, there is no correlation between the concentrations of different magnetic components so that variable remanence acquisition efficiency will complicate RPI recording. Our work demonstrates that detailed assessment of paleomagnetic recording by each constituent magnetic component needs to become a routine part of sedimentary RPI analysis. This article is protected by copyright. All rights reserved.
      PubDate: 2017-03-17T11:35:33.94083-05:0
      DOI: 10.1002/2016GC006753
  • Impact of silica diagenesis on the porosity of fine-grained strata: An
           analysis of Cenozoic mudstones from the North Sea
    • Authors: Thilo Wrona; Kevin G. Taylor, Christopher A-L. Jackson, Mads Huuse, Jens Najorka, Indranil Pan
      Abstract: Silica diagenesis has the potential to drastically change the physical and fluid flow properties of its host strata and therefore plays a key role in the development of sedimentary basins. The specific processes involved in silica diagenesis are, however, still poorly explained by existing models. This knowledge gap is addressed by investigating the effect of silica diagenesis on the porosity of Cenozoic mudstones of the North Viking Graben, northern North Sea through a multiple linear regression analysis. First, we identify and quantify the mineralogy of these rocks by scanning electron microscopy and X-ray diffraction, respectively. Mineral contents and host rock porosity data inferred from wireline data of two exploration wells are then analyzed by multiple linear regressions. This robust statistical analysis reveals that biogenic opal-A is a significant control and authigenic opal-CT is a minor influence on the porosity of these rocks. These results suggest that the initial porosity of siliceous mudstones increases with biogenic opal-A production during deposition and that the porosity reduction during opal-A/CT transformation results from opal-A dissolution. These findings advance our understanding of compaction, dewatering and lithification of siliceous sediments and rocks. Moreover, this study provides a recipe for the derivation of the key controls (e.g. composition) on a rock property (e.g. porosity) that can be applied to a variety of problems in rock physics. This article is protected by copyright. All rights reserved.
      PubDate: 2017-03-14T11:31:05.615078-05:
      DOI: 10.1002/2016GC006482
  • Along-strike variations in the Himalayan orogenic wedge structure in
           Bhutan from ambient seismic noise tomography
    • Authors: Julia Singer; Anne Obermann, Eduard Kissling, Hongjian Fang, György Hetényi, Djordje Grujic
      Abstract: The geological units and tectonic structure exposed in the Bhutan Himalaya document significant regional variations, expressed primarily as tectonic windows and klippen. The along-strike variations of these structures and their metamorphic grade is usually associated with the formation of local duplexes in the underlying tectonic units. To investigate these variations and their extent in depth, we image the isotropic shear-wave velocity structure of the orogenic wedge by ambient noise tomography. Group velocities are extracted from cross-correlations of ambient seismic noise, recorded by the temporary GANSSER network in Bhutan. The upper crustal structure beneath Bhutan is mapped down to 18 km depth by directly inverting Rayleigh-wave group velocity measurements in the period range between 2-20 s with a ray-tracing based inversion approach. Our results reveal several distinct high shear-wave velocity anomalies (≥3.6 km/s) and reflect the along-strike variations in the upper crustal structure in relation to the alternating tectonic windows and klippen at the surface. In correlation with the surface geology in the northern part of Bhutan, we interpret shallow high shear-wave velocity anomalies as quarzite-dominated rocks or felsic migmatites with large intrusions of leucogranites. High velocity anomalies in the orogenic wedge in eastern and western Bhutan correlate with the local geometry of the Main Himalayan Thrust and provide evidence for the formation and depth extent of localized duplexes of quartzite dominated lithology in association with the formation of tectonic windows in the Bhutan Himalaya. This article is protected by copyright. All rights reserved.
      PubDate: 2017-03-12T19:45:39.033311-05:
      DOI: 10.1002/2016GC006742
  • Sub-ice shelf sediment geochronology utilizing novel radiocarbon
           methodology for detrital-rich sediments
    • Authors: C. Subt; H. I. Yoon, K. C. Yoo, J. I. Lee, A. Leventer, E.W. Domack, B. E. Rosenheim
      Abstract: Sub-ice shelf sediments near Larsen C ice shelf (LIS-C) show fine-scale rhythmic laminations that could provide a near-continuous seasonal-resolution record of regional ice mass changes. Despite the great potential of these sediments, a dependable Late Quaternary chronology is difficult to generate. As with many marginal Antarctic sediments, in the absence of preserved carbonate microfossils, the reliability of radiocarbon chronologies depends on presence of high proportions of autochthonous organic carbon with minimized detrital organic carbon. Consequently, acid insoluble organic (AIO) 14C dating works best where high productivity drives high sediment accumulation rates, but can be problematic in condensed sequences with high proportions of detrital organic carbon. Ramped PyrOx 14C dating has progressively been shown to improve upon AIO 14C dates, to the point of matching foraminiferal carbonate 14C dates, through differential thermochemical degradation of organic components within samples. But in highly detrital sediments, proportions of contemporaneously-deposited material are too low to fully separate autochthonous organic carbon from detrital carbon in samples large enough to 14C date. We introduce two modifications of the Ramped PyrOx 14C approach applied to highly-detrital sediments near LIS-C to maximize accuracy by utilizing ultra-small fractions of the highly-detrital AIO material. With minimization of the uncertainty cost, these techniques allow us to generate chronologies for cores that would otherwise go undated, pushing the limits of radiocarbon dating to regions and facies with high proportions of pre-aged detritus. Wider use of these techniques will enable more coordinated a priori coring efforts to constrain regional glacial responses to rapid warming where sediments had previously been thought too difficult to date. This article is protected by copyright. All rights reserved.
      PubDate: 2017-03-07T19:30:35.726316-05:
      DOI: 10.1002/2016GC006578
  • Lithospheric thickness estimation beneath Northwestern South America from
           an S-wave Receiver Function analysis
    • Authors: J. Faustino Blanco; Carlos A. Vargas, Gaspar Monsalve
      Abstract: We make use of the S-to-P receiver function technique beneath Colombia and neighboring regions to make a first-order approximation of the depth of the Lithosphere-Asthenosphere Boundary (LAB) and therefore, of lithospheric thickness. A deconvolution technique was used to calculate the receiver functions, and after a move-out correction and a time-depth conversion, LAB depths for different tectonic regions of northwestern South America were obtained. Results are typically between 65 and 110 km, consistent with previous estimates around the world and other regions in South America. Lithospheric thickness beneath an oceanic island in the Caribbean is ∼80 km, whereas for the Ecuador-Colombia Trench it is ∼65 km, and around 100 km for the Panama Arc. The transition to the continent is associated with an increase in LAB depth, where it can reach ∼110 km, with no significant differences among terranes and/or tectonic blocks. This article is protected by copyright. All rights reserved.
      PubDate: 2017-03-07T19:25:33.440473-05:
      DOI: 10.1002/2016GC006785
  • Rock magnetic investigation and its geological significance for vein-type
           uranium deposits in Southern China
    • Authors: Kunpeng Ge; Qingsong Liu, Juzhi Deng, David Nobes, Yang Wang, Yanguo Wang, Xiao Chen
      Abstract: To characterize the metallogenic environment of a typical vein-type uranium deposit, samples from diabase dykes, alteration zones including metamorphic diabase and uranium ore, and granites were systematically investigated for 6 boreholes from southeastern China. Rock magnetic results indicate that coarse-grained magnetites (pseudo-single domain, PSD, and multi-domain, MD) are dominant magnetic carriers in diabase. In contrast, the uranium ore is dominated by fine-grained magnetites (superparamagnetic, SP, and single-domain, SD). The concentration of magnetic particles in fresh granites is low. Magnetic properties of metamorphic diabases exhibit much greater variability of magnetic properties and higher degrees of sulfuration than unaltered diabase and granite, due to contact metasomatism and reduction effects close to the vein. Compared with diabase, magnetic remanence of the uranium ore is much lower, but displays much higher stability. The Koenigsberger ratio Q peaks in the uranium ore with a value of ∼1.00. Using the systematic rock magnetic results to constrain the interpretation, the contribution of the intersection zone of diabase dyke and silicified fault to magnetic anomalies was further modeled, and the effects of the ore body are significant for magnetic exploration. Overall, rock magnetic investigations of vein-type uranium deposit provide a better understanding of the interactions between different rock types, and further facilitate regional magnetic surveys on the ground. This article is protected by copyright. All rights reserved.
      PubDate: 2017-03-07T19:25:30.287372-05:
      DOI: 10.1002/2016GC006756
  • Two-layered oceanic lithospheric mantle in a Tibetan ophiolite produced by
           episodic subduction of Tethyan slabs
    • Authors: Qing Xiong; William L. Griffin, Jian-Ping Zheng, Norman J. Pearson, Suzanne Y. O'Reilly
      Abstract: The origin and evolution of the Yarlung Zangbo ophiolites (South Tibet, China) is the key to the tectonics of the Neo-Tethyan Ocean between Greater India and Asia and the underlying upper-mantle dynamics. This study presents a detailed investigation of the Zedang ultramafic body (comprising a harzburgitic and a lherzolitic domain) in the eastern Yarlung Zangbo Suture. Major-element compositions and Ti, Y and HREE concentrations in peridotites and their minerals indicate that the harzburgites experienced higher degrees of melting than the lherzolites (∼13-19% vs ∼7-12%). The overall enrichment of LREE, Zr and Sr in harzburgites and their clinopyroxenes suggest that the harzburgites were pervasively metasomatized (cryptically) by silicate melts. The harzburgites also record local strong metasomatism close to melt channels. Nd isotopes indicate that both metasomatic agents were derived from forearc basaltic magmas that intruded the harzburgites at ∼130-120 Ma. The lherzolites did not experience such metasomatism. Thermometry shows that the harzburgites experienced a thorough, lower-temperature re-equilibration process in lithosphere, while the lherzolites rapidly accreted from the asthenosphere and preserved high equilibration temperatures (up to ∼1320°C). Comparable enrichment in fluid-mobile elements and radiogenic Sr-isotope compositions in both harzburgitic and lherzolitic pyroxenes reflect slab-fluid infiltration into both mantle domains. All the evidence and the presence of subduction-related chromitites in the harzburgites suggest that the Zedang harzburgites formed in a possibly Jurassic mature subduction system, while the lherzolites accreted later in an early Cretaceous forearc during subduction initiation. The two-layered lithospheric mantle reflects the episodic subduction of the Tethyan slabs. This article is protected by copyright. All rights reserved.
      PubDate: 2017-03-06T11:10:36.283844-05:
      DOI: 10.1002/2016GC006681
  • Pressure evolution in shallow magma chambers upon buoyancy-driven
    • Authors: P. Papale; C.P. Montagna, A. Longo
      Abstract: The invasion of active magma chambers by primitive magma of deeper provenance is a frequent occurrence in volcanic systems, and it is commonly associated with pressurization. Chamber replenishment is driven by pressure and buoyancy forces that cause magma ascent towards shallow depths. We examine the end-member case of pure buoyancy-driven (natural) convection in crustal reservoirs deriving from the presence of degassed, dense magma at shallow level, that can originate a gravitational instability. Space-time-dependent numerical simulations of magma dynamics in composite underground systems reveal highly non-linear pressure evolution dominated by decompression at shallow depths. This counter-intuitive result originates from the compressible nature of multiphase magmas and their complex convection and mixing dynamics. Shallow magma chamber decompression upon replenishment is favored by large volatile contents of the uprising magma, resulting in large density contrasts among the resident and the incoming components. These results show that the intuitive concept of magma chamber pressurization upon replenishment may not always hold in real situations dominated by buoyancy, and provide new perspectives for the interpretation of geophysical records at active volcanoes. This article is protected by copyright. All rights reserved.
      PubDate: 2017-03-06T11:10:30.949284-05:
      DOI: 10.1002/2016GC006731
  • The role of mantle-derived magmas in the isotopic evolution of
           Yellowstone's magmatic system
    • Authors: Mark E. Stelten; Kari M. Cooper, Josh B. Wimpenny, Jorge A. Vazquez, Qing-Zhu Yin
      Abstract: Injection of mantle-derived magmas into the Earth's crust provides the heat necessary to develop and maintain large silicic magmatic systems. However, the role of mantle-derived magmas in controlling the compositional evolution of large silicic systems remains poorly understood. Here we examine the role of mantle-derived magmas in the post-caldera magmatic system at Yellowstone Plateau, the youngest magmatism associated with the Yellowstone hotspot. Using microbeam techniques we characterize the age and Hf isotope composition of single zircon crystals hosted in rhyolites from the most recent eruptive episode at Yellowstone Plateau, which produced the Central Plateau Member rhyolites. We place these zircon data into context by comparing them to new solution Hf isotope data for the Central Plateau Member glasses, Yellowstone basalts, and potential local crustal sources. Zircons in the Central Plateau Member rhyolites record a wide range of Hf isotope compositions relative to their host melts and extend from values similar to previously erupted Yellowstone rhyolites to values similar to Yellowstone basalts. Most zircons (∼90%) are in isotopic equilibrium with their host melt, but a significant proportion show εHf values higher than their host melt, thus providing the direct evidence that silicic derivatives of mantle-derived basalts have recharged Yellowstone's magmatic system. Mixing models confirm that the isotopic characteristics of the youngest Yellowstone rhyolites can be explained by recharge of Yellowstone's magma reservoir with silicic derivatives of underplating, mantle-derived basalts (∼5% to 10% material added by mass). This process helps drive the long-term isotopic evolution of Yellowstone's magmatic system. This article is protected by copyright. All rights reserved.
      PubDate: 2017-03-02T10:35:33.198874-05:
      DOI: 10.1002/2016GC006664
  • Origins of water content variations in the sub-oceanic upper mantle:
           Insight from Southwest Indian Ridge abyssal peridotites
    • Authors: Wei Li; Vincent Soustelle, Zhenmin Jin, Huaiming Li, Tao Chen, Chunhui Tao
      Abstract: To investigate the origin of heterogeneous water distribution in the sub-oceanic lithospheric mantle, we performed a detailed petrological and geochemical analysis of abyssal peridotites collected from two localities (53°E and 63.5°E) on the Southwest Indian Ridge. These serpentinized peridotites display primary olivine-orthopyroxene-clinopyroxene-spinel assemblage and record equilibrium temperature of 1150-1200°C and around 1000°C for the 53°E and 63.5°E locations, respectively. The rocks were thus equilibrated in the spinel stability field prior to their exhumation to the seafloor. Our FTIR analyses show variable water contents in orthopyroxene ranging from 24 to 262 wt. ppm H2O. Orthopyroxene in the 63.5°E peridotites is characterized by homogeneous and high water content (>200 ppm), whereas orthopyroxene in the 53°E peridotites displays a wider range of water contents (24-246 ppm). We first demonstrate that differences in equilibrium conditions (i.e. pressure and temperature) and mineral chemistry and serpentinization cannot explain the water content variations. Melting modeling show that a fractional melting in both garnet and spinel stability fields is needed to explain the MREE and HREE concentrations in clinopyroxene. Enrichment in LREE, high water contents and high H2O/Ce, however, require a post-melting rehydration event such as metasomatism. Based on petrographic evidence and investigation of chemical heterogeneities at segment/dredge scale, we suggest that this event involves a metasomatic agent enriched in water and incompatible elements. We infer that the small-scale heterogeneities in water and trace elements may result either from successive infiltration of various amounts of melt/fluids as described in the formation of oceanic core complex or from spatial heterogeneities of melt infiltration as observed in peridotite massifs. This article is protected by copyright. All rights reserved.
      PubDate: 2017-03-02T10:35:31.552898-05:
      DOI: 10.1002/2016GC006767
  • Removal of deep lithosphere in ancient continental collisional orogens: A
           case study from central Tibet, China
    • Authors: Sheng-Sheng Chen; Wei-Ming Fan, Ren-Deng Shi, Xiao-Han Gong, Kang Wu
      Abstract: Widespread but small-volume Late Cretaceous volcanic rocks in central Tibet contain important information on the Lhasa–Qiangtang collision process. In this contribution, we focus on Late Cretaceous volcanics in the southern Qiangtang subterrane, and present zircon LA–ICP–MS U–Pb ages, whole-rock major and trace element compositions, and Sr–Nd isotopic data. Zircon LA–ICP–MS U–Pb dating yielded a concordant age of 80 Ma, which postdates the Early Cretaceous collision of the Qiangtang and Lhasa terranes. The volcanic rocks are potassium-rich alkaline andesites with high contents of K2O (3.45–5.11 wt.%) and Th (13.39–25.02 ppm), as well as high K2O/Na2O ratios (0.6–0.9). They have higher REE and HFSE contents than coeval Mg-rich and adakite-like magmatic rocks that can be related to partial melting of a thickened lower crust. Moreover, they have higher values of Mg# and lower contents of SiO2 than lower continental crust-derived rocks in central Tibet and experimental data of mafic rocks. We argue that the andesites were generated after the removal of thickened lithospheric mantle and subsequent to the final Lhasa–Qiangtang amalgamation in a post-collisional setting. The high-K characteristics can be explained by producing the primitive andesite magmas from partial melting of the residual and shallow metasomatized lithospheric mantle (the K-rich layer) during heating by upwelling asthenosphere; subsequently, these primitive andesite magmas were subjected to fractional crystallization to generate the Amdo andesites. The way in which these andesites were formed provides evidence for the lithospheric thickening and uplifting of central Tibet during the Late Cretaceous prior to India–Asia collision. This article is protected by copyright. All rights reserved.
      PubDate: 2017-03-02T10:35:26.269102-05:
      DOI: 10.1002/2016GC006678
  • Comparison of thermal modeling, microstructural analysis, and Ti-in-quartz
           thermobarometry to constrain the thermal history of a cooling pluton
           during deformation in the Mount Abbot Quadrangle, CA
    • Authors: Johanna M. Nevitt; Jessica M. Warren, Steven Kidder, David D. Pollard
      Abstract: Granitic plutons commonly preserve evidence for jointing, faulting, and ductile fabric development during cooling. Constraining the spatial variation and temporal evolution of temperature during this deformation could facilitate an integrated analysis of heterogeneous deformation over multiple length-scales through time. Here, we constrain the evolving temperature of the Lake Edison granodiorite within the Mount Abbot Quadrangle (central Sierra Nevada, CA) during late Cretaceous deformation by combining microstructural analysis, titanium-in-quartz thermobarometry (TitaniQ), and thermal modeling. Microstructural and TitaniQ analyses were applied to 12 samples collected throughout the pluton, representative of either the penetrative “regional” fabric or the locally strong “fault-related” fabric. Overprinting textures and mineral assemblages indicate the temperature decreased from 400-500°C to
      PubDate: 2017-03-01T06:41:17.543138-05:
      DOI: 10.1002/2016GC006655
  • An activity index for geomagnetic paleosecular variation, excursions, and
    • Authors: S. Panovska; C.G. Constable
      Abstract: Magnetic indices provide quantitative measures of space weather phenomena that are widely used by researchers in geomagnetism. We introduce an index focused on the internally generated field that can be used to evaluate long term variations or climatology of modern and paleomagnetic secular variation, including geomagnetic excursions, polarity reversals, and changes in reversal rate. The paleosecular variation index, Pi, represents instantaneous or average deviation from a geocentric axial dipole field using normalized ratios of virtual geomagnetic pole colatitude and virtual dipole moment. The activity level of the index, σPi, provides a measure of field stability through the temporal standard deviation of Pi. Pi can be calculated on a global grid from geomagnetic field models to reveal large scale geographic variations in field structure. It can be determined for individual time series, or averaged at local, regional, and global scales to detect long term changes in geomagnetic activity, identify excursions, and transitional field behavior. For recent field models Pi ranges from less than 0.05 to 0.30. Conventional definitions for geomagnetic excursions are characterized by Pi exceeding 0.5. Strong field intensities are associated with low Pi unless they are accompanied by large deviations from axial dipole field directions. σPi provides a measure of geomagnetic stability that is modulated by the level of PSV or frequency of excursional activity and reversal rate. We demonstrate uses of Pi for paleomagnetic observations and field models and show how it could be used to assess whether numerical simulations of the geodynamo exhibit Earth-like properties. This article is protected by copyright. All rights reserved.
      PubDate: 2017-02-28T03:40:47.643151-05:
      DOI: 10.1002/2016GC006668
  • Across-arc versus along-arc Sr-Nd-Pb isotope variations in the Ecuadorian
           volcanic arc
    • Authors: Marie-Anne Ancellin; Pablo Samaniego, Ivan Vlastélic, François Nauret, Mouhcine Gannoun, Silvana Hidalgo
      Abstract: Previous studies of the Ecuadorian arc (1°N - 2°S) have revealed across-arc geochemical trends that are consistent with a decrease in mantle melting and slab dehydration away from the trench. The aim of this work is to evaluate how these processes vary along the arc in response to small-scale changes in the age of the subducted plate, subduction angle, and continental crustal basement. We use an extensive database of 1524 samples containing 71 new analyses, of major and trace elements as well as Sr-Nd-Pb isotopes from Ecuadorian and South Colombian volcanic centers. Large geochemical variations are found to occur along the Ecuadorian arc, in particular along the front arc, which encompasses 99% and 71% of the total variations in 206Pb/204Pb and 87Sr/86Sr ratios of Quaternary Ecuadorian volcanics, respectively. The front arc volcanoes also show two major latitudinal trends: (1) the southward increase of 207Pb/204Pb and decrease of 143Nd/144Nd reflect more extensive crustal contamination of magma in the southern part (up to 14%); and (2) the increase of 206Pb/204Pb and decrease of Ba/Th away from ∼0.5°S result from the changing nature of metasomatism in the sub-arc mantle wedge with the aqueous fluid/siliceous slab melt ratio decreasing away from 0.5°S. Subduction of a younger and warmer oceanic crust in the Northern part of the arc might promote slab melting. Conversely, the subduction of a colder oceanic crust south of the Grijalva Fracture Zone and higher crustal assimilation lead to the reduction of slab contribution in southern part of the arc. This article is protected by copyright. All rights reserved.
      PubDate: 2017-02-27T18:45:42.056952-05:
      DOI: 10.1002/2016GC006679
  • Modelling the impact of melt on seismic properties during mountain
    • Authors: Amicia L. Lee; Andrew M. Walker, Geoffrey E. Lloyd, Taija Torvela
      Abstract: Initiation of partial melting in the mid/lower crust causes a decrease in P-wave and S-wave velocities; recent studies imply that the relationship between these velocities and melt is not simple. We have developed a modelling approach to assess the combined impact of various melt and solid phase properties on seismic velocities and anisotropy. The modelling is based on crystallographic preferred orientation (CPO) data measured from migmatite samples, allowing quantification of the variation of seismic velocities with varying melt volumes, shapes, orientations, and matrix anisotropy. The results show non-linear behaviour of seismic properties as a result of the interaction of all of these physical properties, which in turn depend on lithology, stress regime, strain rate, pre-existing rock fabrics, and pressure-temperature conditions. This non-linear behaviour is evident when applied to a suite of samples from a traverse across a migmatitic shear zone in the Seiland Igneous Province, Northern Norway. Critically, changes in solid phase composition and CPO, and melt shape and orientation with respect to the wave propagation direction can result in huge variations in the same seismic property even if the melt fraction remains the same. A comparison with surface wave interpretations from tectonically active regions highlights the issues in current models used to predict melt percentages or partially molten regions. Interpretation of seismic data to infer melt percentages or extent of melting should, therefore, always be underpinned by robust modelling of the underlying geological parameters combined with examination of multiple seismic properties in order to reduce uncertainty of the interpretation. This article is protected by copyright. All rights reserved.
      PubDate: 2017-02-27T18:30:32.839069-05:
      DOI: 10.1002/2016GC006705
  • Tectonic structure, evolution, and the nature of oceanic core complexes
           and their detachment fault zones (13°20'N and 13°30'N, Mid-Atlantic
    • Authors: J. Escartín; C. Mével, S. Petersen, D. Bonnemains, M. Cannat, M. Andreani, N. Augustin, A. Bezos, V. Chavagnac, Y. Choi, M. Godard, K. Haaga, C. Hamelin, B. Ildefonse, J. Jamieson, B. John, T. Leleu, C. J. MacLeod, M. Massot-Campos, P. Nomikou, J. A. Olive, M. Paquet, C. Rommevaux, M. Rothenbeck, A. Steinfuhrer, M. Tominaga, L. Triebe, R. Campos, N. Gracias, R. Garcia
      Abstract: Microbathymetry data, in-situ observations, and sampling along the 13°20'N and 13°20'N oceanic core complexes (OCCs) reveal mechanisms of detachment fault denudation at the seafloor, links between tectonic extension and mass wasting, and expose the nature of corrugations, ubiquitous at OCCs. In the initial stages of detachment faulting and high angle fault scarps show extensive mass-wasting that reduces their slope. Flexural rotation further lowers scarp slope, hinders mass wasting, resulting in morphologically complex chaotic terrain between the breakaway and the denuded corrugated surface. Extension and drag along the fault plane uplifts a wedge of hangingwall material (apron). The detachment surface emerges along a continuous moat that sheds rocks and covers it with unconsolidated rubble, while local slumping emplaces rubble ridges overlying corrugations. The detachment fault zone is a set of anostomosed slip planes, elongated in the along-extension direction. Slip planes bind fault rock bodies defining the corrugations observed in microbathymetry and sonar. Fault planes with extension-parallel stria are exposed along corrugation flanks, where the rubble cover is shed. Detachment fault rocks are primarily basalt fault breccia at 13°20‘N OCC, and gabbro and peridotite at 13°30'N, demonstrating that brittle strain localization in shallow lithosphere form corrugations, regardless of lithologies in the detachment zone. Finally, faulting and volcanism dismember the 13°30'N OCC, with widespread present and past hydrothermal activity (Semenov fields), while the Irinovskoe hydrothermal field at the 13°20'N core complex suggests a magmatic source within the footwall. These results confirm the ubiquitous relationship between hydrothermal activity and oceanic detachment formation and evolution. This article is protected by copyright. All rights reserved.
      PubDate: 2017-02-23T07:31:48.636698-05:
      DOI: 10.1002/2016GC006775
  • Revisit of rare earth element fractionation during chemical weathering and
           river sediment transport
    • Authors: Ni Su; Shouye Yang, Yulong Guo, Wei Yue, Xiaodan Wang, Ping Yin, Xiangtong Huang
      Abstract: Although rare earth element (REE) has been widely applied for provenance study and paleoenvironmental reconstruction, its mobility and fractionation during earth surface processes from weathering to sediment deposition remain more clarification. We investigated the REE fractionations during chemical weathering and river sediment transport based on the systematic observations from a granodiorite-weathering profile and Mulanxi River sediments in southeast China. Two chemical phases (leachates and residues) were separated by 1 N HCl leaching and the leachates account for 20∼70% of the bulk REE concentration. REEs in the weathering profile have been mobilized and fractionated to different extents during chemical weathering and pedogenesis. Remarkable cerium anomalies (Ce/Ce*=0.1∼10.6) occur during weathering as a result of co-precipitation with Mn (hydro)oxides in the profile, while poor or no Ce anomalies in the river sediments were observed. This contrasting feature sheds new light on the indication of Ce anomaly for redox change. The hydraulic sorting-induced mineral redistribution can further homogenize the weathering and pedogenic alterations and thus weaken the REE fractionations in river sediments. The mineral assemblage is the ultimate control on REE composition, and the Mn–Fe (hydro)oxides and secondary phosphate minerals are the main hosts of acid-leachable REEs, while the clay minerals could be important reservoirs for residual REEs. We thus suggest that the widely-used REE proxies such as (LREE/HREE)UCC ratio in the residues is reliable for the indication of sediment provenance, while the ratio in the leachates can indicate the total weathering process to some extent. This article is protected by copyright. All rights reserved.
      PubDate: 2017-02-23T07:10:54.201866-05:
      DOI: 10.1002/2016GC006659
  • Incipient mantle plume evolution: Constraints from ancient landscapes
           buried beneath the North Sea
    • Authors: G. Stucky de Quay; G. G. Roberts, J. S. Watson, C. A.-L. Jackson
      Abstract: Geological observations that constrain the history of mantle convection are sparse despite its importance in determining vertical and horizontal plate motions, plate rheology, and magmatism. We use a suite of geological and geophysical observations from the northern North Sea to constrain evolution of the incipient Paleocene-Eocene Icelandic plume. Well data and a three-dimensional seismic survey are used to reconstruct a 58–55 Ma landscape now buried ∼1.5 km beneath the seabed in the Bressay region. Geochemical analyses of cuttings from wells that intersect the landscape indicate the presence of angiosperm debris. These observations, combined with presence of coarse clastic material, interpreted beach ridges, and a large dendritic drainage network, indicate that this landscape formed subaerially. Longitudinal profiles of palaeo-rivers were extracted and inverted for an uplift rate history, indicating three distinct phases of uplift and total cumulative uplift of ∼350 m. Dinoflagellate cysts in the surrounding marine stratigraphy indicate that this terrestrial landscape formed in ∼150 km/Ma. This article is protected by copyright. All rights reserved.
      PubDate: 2017-02-23T07:10:44.459644-05:
      DOI: 10.1002/2016GC006769
  • 40Ar/39Ar geochronology reveals rapid change from plume-assisted to
           stress-dependent volcanism in the Newer Volcanic Province, SE Australia
    • Authors: K. F. Oostingh; F. Jourdan, E. L. Matchan, D. Phillips
      Abstract: Here, we present 40Ar/39Ar ages of volcanic features in the Cenozoic intraplate Newer Volcanic Province in southeast Australia. The 
      PubDate: 2017-02-21T03:30:38.933184-05:
      DOI: 10.1002/2016GC006601
  • Quantifying K, U, and Th contents of marine sediments using shipboard
           natural gamma radiation spectra measured on DV JOIDES Resolution
    • Authors: David De Vleeschouwer; Ann G. Dunlea, Gerald Auer, Chloe H. Anderson, Hans Brumsack, Aaron de Loach, Michael C. Gurnis, Youngsook Huh, Takeshige Ishiwa, Kwangchul Jang, Michelle A. Kominz, Christian März, Bernhard Schnetger, Richard W. Murray, Heiko Pälike,
      Abstract: During International Ocean Discovery Program (IODP) expeditions, shipboard-generated data provide the first insights into the cored sequences. The natural gamma radiation (NGR) of the recovered material, for example, is routinely measured on the ocean drilling research vessel DV JOIDES Resolution. At present, only total NGR counts are readily available as shipboard data, although full NGR spectra (counts as a function of gamma-ray energy level) are produced and archived. These spectra contain unexploited information, as one can estimate the sedimentary contents of potassium (K), thorium (Th), and uranium (U) from the characteristic gamma-ray energies of isotopes in the 40K, 232Th, and 238U radioactive decay series.Dunlea et al. [2013] quantified K, Th and U contents in sediment from the South Pacific Gyre by integrating counts over specific energy levels of the NGR spectrum. However, the algorithm used in their study is unavailable to the wider scientific community due to commercial proprietary reasons. Here, we present a new MATLAB algorithm for the quantification of NGR spectra that is transparent and accessible to future NGR users. We demonstrate the algorithm's performance by comparing its results to shore-based inductively coupled plasma-mass spectrometry (ICP-MS), inductively coupled plasma-emission spectrometry (ICP-ES), and quantitative wavelength-dispersive X-ray fluorescence (XRF) analyses. Samples for these comparisons come from eleven sites (U1341, U1343, U1366-U1369, U1414, U1428-U1430, U1463) cored in two oceans during five expeditions. In short, our algorithm rapidly produces detailed high-quality information on sediment properties during IODP expeditions at no extra cost. This article is protected by copyright. All rights reserved.
      PubDate: 2017-02-20T07:30:25.598961-05:
      DOI: 10.1002/2016GC006715
  • Olivine fabrics and tectonic evolution of forearc mantles: A natural
           perspective from the Songshugou dunite and harzburgite in the Qinling
           orogenic belt, central China
    • Authors: Yi Cao; Haemyeong Jung, Shuguang Song
      Abstract: To advance our understanding of deformation characteristics, rheological behaviors and tectonic evolution of the forearc lithospheric mantle, we analyzed mineral fabrics for a large spinel-bearing ultramafic massif in the Songshugou area in the Qinling orogenic belt, central China. In the spinel-poor coarse-grained dunite, stronger A-/D-type and weaker C-type-like fabrics were found, whereas the spinel-rich coarse-grained dunite displayed a comparatively stronger B-type-like fabric. These olivine fabrics are high-T fabrics influenced by the presence of melt, in which B- and C-type-like fabrics are inferred to be produced by melt-assisted grain boundary sliding during synkinematic high-T melt‒rock reactions. In contrast, the spinel-poor porphyroclastic and fine-grained dunites present weak AG- and B-type-like fabrics, respectively. Their olivine fabrics (low-T fabrics) are inferred to transform from A-/D-type fabric in their coarse-grained counterparts possibly through mylonitization process assisted by low-T fluid‒rock reactions, during which strain was accommodated by the fluid-enhanced dislocation slip and/or fluid-assisted grain boundary sliding processes. Combined with the tectonic results of our previous work [Cao et al., 2016], the high-T olivine fabrics are probably related to a young and warm forearc mantle where intense partial melting and high-T boninitic melt‒rock reactions prevalently occurred, whereas the low-T olivine fabrics likely reflect the evolving tectonic settings through the cooling forearc mantle to a continental lower crust in a collisional orogeny where low-T fluid‒rock reactions were pervasively activated. These low-T olivine fabrics imply that, though cold, the forearc lithospheric mantle may be locally weak (∼20‒30 MPa), allowing ductile deformation to occur at a geologically significant strain rate. This article is protected by copyright. All rights reserved.
      PubDate: 2017-02-17T03:32:17.450918-05:
      DOI: 10.1002/2016GC006614
  • Iron isotopic evolution during fractional crystallization of the uppermost
           Bushveld Complex layered mafic intrusion
    • Authors: Laura D. Bilenker; Jill A. VanTongeren, Craig C. Lundstrom, Adam C. Simon
      Abstract: We present δ56Fe (56Fe/54Fe relative to standard IRMM-014) data from whole rock and magnetite of the Upper and Upper Main Zones (UUMZ) of the Bushveld Complex. With it, we assess the role of fractional crystallization in controlling the Fe isotopic evolution of a mafic magma. The UUMZ evolved by fractional crystallization of a dry tholeiitic magma to produce gabbros and diorites with cumulus magnetite and fayalitic olivine. Despite previous experimental work indicating a potential for magnetite crystallization to drastically change magma δ56Fe, we observe no change in whole rock δ56Fe above and below magnetite saturation. We also observe no systematic change in whole rock δ56Fe with increasing stratigraphic height, and only a small variation in δ56Fe in magnetite separates above magnetite saturation. Whole rock δ56Fe (errors twice standard deviation, ±2σ) throughout the UUMZ ranges from -0.01 ±0.03‰ to 0.21 ±0.09‰ (δ56FeaverageWR = 0.10 ±0.09‰; n=21, isotopically light outlier: δ56FeWR = -0.15‰), and magnetites range from 0.28 ±0.04‰ to 0.86 ±0.07‰ (δ56FeaverageMgt = 0.50 ±0.15‰; n=20), similar to values previously reported for other layered intrusions. We compare our measured δ56FeWR to a model that incorporates the changing normative mineralogy, calculated temperatures, and published fractionation factors of Fe-bearing phases throughout the UUMZ and produces δ56FeWR values that evolve only in response to fractional crystallization. Our results show that the Fe isotopic composition of a multiply-saturated (multiple phases on the liquidus) magma is unlikely to change significantly during fractional crystallization of magnetite due to the competing fractionation of other Fe-bearing cumulus phases. This article is protected by copyright. All rights reserved.
      PubDate: 2017-02-16T01:05:56.84891-05:0
      DOI: 10.1002/2016GC006660
  • Composition law of oblique anhysteretic remanent magnetization and its
           relation to the magnetostatic interaction
    • Authors: Masahiko Sato; Nobutatsu Mochizuki, Minako Watanabe, Hideo Tsunakawa
      Abstract: The basic properties of oblique anhysteretic remanent magnetization (OARM) acquired in a weak and steady magnetic field with an arbitrary angle to the alternating field direction were studied. OARM and rock-magnetic experiments were conducted on samples of basalt, granite, and sediment containing non-interacting single-domain (SD), interacting SD, pseudo-single-domain, and multidomain low-Ti titanomagnetites. The intensity of OARM (MOARM) systematically increased or decreased with increasing angle between alternating and steady field directions (θSF), while the angle between alternating field and OARM directions (θOARM) increased with increasing θSF for all samples. During stepwise alternating field demagnetization, the OARM vector shows a single component parallel to the steady field direction for θSF = 0° (ARMǁ) and 90° (ARM⊥). The median destructive field of ARM⊥ is larger than that of ARMǁ. For intermediate angles (θSF = 30°, 45°, and 60°), the OARM vector was not parallel to the applied steady field; instead, it gradually increased with coercivity. These experiments indicate that the OARM vector is approximately given by the sum of two orthogonal magnetizations coinciding with ARMǁ and ARM⊥, respectively. Thus, the OARM vector can be determined by acquisition efficiencies of ARMǁ and ARM⊥ in an individual sample. Based on these experiments and associated rock-magnetic measurements, non-interacting SD samples show lower ARM⊥/ARMǁ ratios, compared to other samples. This result suggests that OARM can be used as a useful tool to detect non-interacting SD particles in the paleomagnetic samples. This article is protected by copyright. All rights reserved.
      PubDate: 2017-02-14T18:05:31.630523-05:
      DOI: 10.1002/2016GC006671
  • Geodynamic implications for zonal and meridional isotopic patterns across
           the northern Lau and North Fiji Basins
    • Authors: Allison A. Price; Matthew G. Jackson, Janne Blichert-Toft, Mark D. Kurz, Jim Gill, Jerzy Blusztajn, Frances Jenner, Raul Brens, Richard Arculus
      Abstract: We present new Sr-Nd-Pb-Hf-He isotopic data for sixty-five volcanic samples from the northern Lau and North Fiji Basin. This includes forty-seven lavas obtained from forty dredge sites spanning an east-west transect across the Lau and North Fiji basins, ten ocean island basalt (OIB)-type lavas collected from seven Fijian islands, and eight OIB lavas sampled on Rotuma. For the first time we are able to map clear north-south and east-west geochemical gradients in 87Sr/86Sr across the northern Lau and North Fiji Basins: lavas with the most geochemically enriched radiogenic isotopic signatures are located in the northeast Lau Basin, while signatures of geochemical enrichment are diminished to the south and west away from the Samoan hotspot. Based on these geochemical patterns and plate reconstructions of the region, these observations are best explained by the addition of Samoa, Rurutu, and Rarotonga hotspot material over the past 4 Ma. We suggest that underplated Samoan material has been advected into the Lau Basin over the past ∼4 Ma. As the slab migrated west (and toward the Samoan plume) via rollback over time, younger and hotter (and therefore less viscous) underplated Samoan plume material was entrained. Thus, entrainment efficiency of underplated plume material was enhanced, and Samoan plume signatures in the Lau Basin became stronger as the trench approached the Samoan hotspot. The addition of subducted volcanoes to the Cook-Austral Volcanic Lineament material, first from the Rarotonga hotspot, then followed by the Rurutu hotspot, contributes to the extreme geochemical signatures observed in the northeast Lau Basin. This article is protected by copyright. All rights reserved.
      PubDate: 2017-02-11T03:45:32.295224-05:
      DOI: 10.1002/2016GC006651
  • Neogene paleoceanography of the eastern equatorial Pacific based on the
           radiolarian record of IODP drill sites off Costa Rica
    • Authors: María I. Sandoval; Demetrio Boltovskoy, Alan T. Baxter, Peter O. Baumgartner
      Abstract: The Integrated Ocean Drilling Program (IODP) Expedition 344 drilled cores following a transect across the convergent margin off Costa Rica. Two of the five sites (U1381 and U1414) are the subject of the present study. Major radiolarian faunal breaks and characteristic species groups were defined with the aid of cluster analysis, nodal analysis and discriminant analysis of principal components. A middle-late Miocene to Pleistocene age (radiolarian zones RN5 to RN16) was determined for the sites, which agrees with the nannofossil zonations and 40Ar/39Ar and tephra layers. Considering the northward movement of the Cocos plate (∼7.3 cm/year), and a paleolatitude calculator, it is assumed that during the Miocene the two sites were located ∼1000 km to the southwest of their current position, slightly south of the equator. The radiolarian faunas retrieved were thus seemingly formed under the influence of different oceanic currents and sources of nutrients. Changes in the radiolarian assemblages at Site U1414 point at dissimilar environmental settings associated with the colder South Equatorial Current and the warmer Equatorial Countercurrent, as well as to coastal upwelling. These differences are best reflected by changes in the abundance of the morphotype Spongurus spp., with noticeably higher values during the Miocene, than in the Pliocene and the Pleistocene. Because Spongurus spp. is generally associated with cooler waters, these abundance variations (as well as those of several other species) suggest that during the Miocene the area had a stronger influence of colder waters than during younger periods. During the Pliocene and the lowermost Pleistocene, biogenic remains are scarce, presumably due to the terrigenous input, which could have diluted and affected the preservation of pelagic fossils, as well as to the displacement of the site to warmer waters. A typically tropical fauna characterized the Pleistocene, yet with widespread presence of colder water species, most probably indicative of the influence of coastal upwelling processes. This article is protected by copyright. All rights reserved.
      PubDate: 2017-02-11T03:40:58.811024-05:
      DOI: 10.1002/2016GC006623
  • Boron isotope variations in Tonga-Kermadec-New Zealand arc lavas:
           Implications for origin of subduction components and mantle influences
    • Authors: William P. Leeman; Sonia Tonarini, Simon Turner
      Abstract: The Tonga-Kermadec-New Zealand volcanic arc is an end-member of arc systems with fast subduction suggesting that the Tonga sector should have the coolest modern slab thermal structure on Earth. New data for boron concentration and isotopic composition are used to evaluate the contrasting roles of postulated subduction components (sediments and oceanic slab lithologies) in magma genesis. Major observations include: (a) Tonga-Kermadec volcanic front lavas are enriched in B (as recorded by B/Nb and similar ratios) and most have relatively high δ11B (> +4‰), whereas basaltic lavas from New Zealand have relatively low B/Nb and δ11B (< -3.5‰); (b) both δ11B and B/Nb generally increase northward from New Zealand along with convergence rate and overall slab flux; (c) δ11B and B/Nb decrease toward the back-arc, as observed elsewhere; and (d) low δ11B is observed in volcanic front samples from Ata, an anomalous sector where the back-arc Valu Fa Spreading Center impinges on the arc and the Louisville Seamount Chain is presently subducting. Otherwise, volcanic front lavas exhibit positive correlations for both B/Nb and δ11B with other plausible indicators of slab-derived fluid contributions (e.g., Ba/Nb, U/Th, (230Th/232Th) and 10Be/9Be), and with estimated degree of melting to produce the mafic lavas. Inferred B-enrichments in the arc magma sources are likely dominated by serpentinite domains deeper within the subducting slab (± altered oceanic crust), and B systematics are consistent with dominant transport by slab-derived aqueous fluids. Effects of this process are amplified by mantle wedge source depletion due to prior melt extraction. . This article is protected by copyright. All rights reserved.
      PubDate: 2017-02-11T03:40:50.41082-05:0
      DOI: 10.1002/2016GC006523
  • Variations in pockmark composition at the Vestnesa Ridge: Insights from
           marine controlled source electromagnetic and seismic data
    • Authors: Bedanta K. Goswami; Karen A. Weitemeyer, Stefan Bünz, Timothy A. Minshull, Graham K. Westbrook, Stephan Ker, Martin C. Sinha
      Abstract: The Vestnesa Ridge marks the northern boundary of a known submarine gas hydrate province in the west Svalbard margin. Several seafloor pockmarks at the eastern segment of the ridge are sites of active methane venting. Until recently, seismic reflection data was the main tool for imaging beneath the ridge. Coincident controlled source electromagnetic (CSEM), high-resolution two-dimensional (2D) airgun, sweep frequency SYSIF and three-dimensional (3D) p-cable seismic reflection data were acquired at the south-eastern part of the ridge between 2011 and 2013. The CSEM and seismic data contains profiles across and along the ridge, passing several active and inactive pockmarks. Joint interpretation of resistivity models obtained from CSEM and seismic reflection data provides new information regarding the fluid composition beneath the pockmarks. There is considerable variation in transverse resistance and seismic reflection characteristics of the gas hydrate stability zone (GHSZ) between the ridge flanks and chimneys beneath pockmarks. Layered seismic reflectors on the flanks are associated with around 300Ωm2 transverse resistance, whereas the seismic reflectors within the chimneys exhibit amplitude blanking and chaotic patterns. The transverse resistance of the GHSZ within the chimneys vary between 400 and 1200 Ωm2. Variance attributes obtained from the 3D p-cable data also highlight faults and chimneys, which coincide with the resistivity anomalies. Based on the joint data interpretation, widespread gas hydrate presence is likely at the ridge, with both hydrates and free gas contained within the faults and chimneys. However, at the active chimneys the effect of gas likely dominate the resistive anomalies. This article is protected by copyright. All rights reserved.
      PubDate: 2017-02-10T17:35:40.546073-05:
      DOI: 10.1002/2016GC006700
  • Primary magmas and mantle temperatures through time
    • Authors: Jérôme Ganne; Xiaojun Feng
      Abstract: Chemical composition of mafic magmas is a critical indicator of physico-chemical conditions, such as pressure, temperature and fluid availability, accompanying melt production in the mantle and its evolution in the continental or oceanic lithosphere. Recovering this information has fundamental implications in constraining the thermal state of the mantle and the physics of mantle convection throughout the Earth's history. Here, a statistical approach is applied to a geochemical database of about 22,000 samples from the mafic magma record. Potential temperatures (Tps) of the mantle derived from this database, assuming melting by adiabatic decompression and a Ti-dependent (Fe2O3/TiO2=0.5) or constant redox condition (Fe2+/∑Fe = 0.9 or 0.8) in the magmatic source, are thought to be representative of different thermal “horizons” (or thermal heterogeneities) in the ambient mantle, ranging in depth from a shallow sublithospheric mantle (Tp minima) to a lower thermal boundary layer (Tp maxima). The difference of temperature (Δ Tp) observed between Tp maxima and minima did not change significantly with time (∼170°C). Conversely, a progressive but limited cooling of ∼150°C is proposed since ∼2.5 Gyr for the Earth's ambient mantle, which falls in the lower limit proposed by Herzberg et al [2010] (∼ 150 to 250°C hotter than today). Cooling of the ambient mantle after 2.5 Ga is preceded by a high-temperature plateau evolution and a transition from dominant plumes to a plate tectonics geodynamic regime, suggesting that subductions stabilized temperatures in the Archaean mantle that was in warming mode at that time. This article is protected by copyright. All rights reserved.
      PubDate: 2017-02-09T10:45:30.478804-05:
      DOI: 10.1002/2016GC006787
  • The undatables: Quantifying uncertainty in a highly expanded Late
           Glacial-Holocene sediment sequence recovered from the deepest Baltic Sea
           basin: IODP Site M0063
    • Authors: S.P. Obrochta; T. Andrén, S.Z. Fazekas, B.C. Lougheed, I. Snowball, Y. Yokoyama, Y. Miyairi, R. Kondo, A.T. Kotilainen, O. Hyttinen, A Fehr
      Abstract: Laminated, organic-rich silts and clays with high dissolved gas content characterize sediments at IODP Site M0063 in the Landsort Deep, which at 459 m is the deepest basin in the Baltic Sea. Cores recovered from Hole M0063A experienced significant expansion as gas was released during the recovery process, resulting in high sediment loss. Therefore during operations at subsequent holes, penetration was reduced to 2 m per 3.3 m core, permitting expansion into 1.3 m of initially empty liner. Fully filled liners were recovered from Holes B through E, indicating that the length of recovered intervals exceeded the penetrated distance by a factor of >1.5. A typical down-core logarithmic trend in gamma density profiles, with anomalously low density values within the upper ∼1 m of each core, suggests that expansion primarily occurred in this upper interval. Thus, we suggest that a simple linear correction is inappropriate. This interpretation is supported by anisotropy of magnetic susceptibility data that indicate vertical stretching in the upper ∼1.5 m of expanded cores. Based on the mean gamma density profiles of cores from Holes M0063C and D, we obtain an expansion function that is used to adjust the depth of each core to conform to its known penetration. The variance in these profiles allows for quantification of uncertainty in the adjusted depth scale. Using a number of bulk 14C dates, we explore how the presence of multiple carbon source pathways leads to poorly constrained radiocarbon reservoir age variability that significantly affects age and sedimentation rate calculations. This article is protected by copyright. All rights reserved.
      PubDate: 2017-02-07T03:20:46.276395-05:
      DOI: 10.1002/2016GC006697
  • Geochemistry of Zr, Hf, and REE in a wide spectrum of Eh and water
           composition: The case of Dead Sea fault system (Israel)
    • Authors: P. Censi; M. Raso, Y. Yechieli, H. Ginat, F. Saiano, P. Zuddas, L. Brusca, W. D'Alessandro, C. Inguaggiato
      Abstract: Along the Jordan Valley-Dead Sea fault area several natural waters in springs, wells and catchments occur. The chemical-physical characters of the studied waters allowed for the first time the investigation of the Zr and Hf geochemical behaviour, apart from REE, extended to a wide range of Eh, temperature, salinity and pH conditions. The results of this study indicate that the dissolved Zr and Hf distribution in natural waters is strongly influenced by redox conditions since these in turn drive the deposition of Fe-oxyhydroxides or pyrite. In oxidizing waters saturated or oversaturated in Fe-oxyhydroxides [Group-1], superchondritic Zr/Hf values are measured. On the contrary, in waters where Eh
      PubDate: 2017-02-07T03:20:40.473874-05:
      DOI: 10.1002/2016GC006704
  • A geochemical and geophysical reappraisal to the significance of the
           recent unrest at Campi Flegrei caldera (Southern Italy)
    • Authors: Roberto Moretti; Giuseppe De Natale, Claudia Troise
      Abstract: Volcanic unrest at calderas involve complex interaction between magma, hydrothermal fluids and crustal stress and strain. Campi Flegrei caldera (CFc), located in the Naples (Italy) area and characterised by the highest volcanic risk on Earth for the extreme urbanisation, undergoes unrest phenomena involving several meters of uplift and intense shallow micro-seismicity since several decades. Despite unrest episodes display in the last decade only moderate ground deformation and seismicity, current interpretations of geochemical data point to a highly pressurized hydrothermal system. We show that at CFc, the usual assumption of vapour-liquid coexistence in the fumarole plumes leads to largely overestimated hydrothermal pressures and, accordingly, interpretations of elevated unrest. By relaxing unconstrained geochemical assumptions, we infer an alternative model yielding better agreement between geophysical and geochemical observations. The model reconciles discrepancies between what observed 1) for two decades since the 1982-84 large unrest, when shallow magma was supplying heat and fluids to the hydrothermal system, and 2) in the last decade. Compared to the 1980's unrest, the post-2005 phenomena are characterized by much lower aquifers overpressure and magmatic involvement, as indicated by geophysical data and despite large changes in geochemical indicators. Our interpretation points out a model in which shallow sills, intruded during 1969-1984, have completely cooled, so that fumarole emissions are affected now by deeper, CO2-richer, magmatic gases producing the modest heating and overpressure of the hydrothermal system. Our results have important implications on the short-term eruption hazard assessment and on the best strategies for monitoring and interpreting geochemical data. This article is protected by copyright. All rights reserved.
      PubDate: 2017-02-03T20:00:41.742925-05:
      DOI: 10.1002/2016GC006569
  • Classification of geochemical data based on multivariate statistical
           analyses: Complementary roles of cluster, principal component, and
           independent component analyses
    • Authors: Hikaru Iwamori; Kenta Yoshida, Hitomi Nakamura, Tatsu Kuwatani, Morihisa Hamada, Satoru Haraguchi, Kenta Ueki
      Abstract: Identifying the data structure including trends and groups/clusters in geochemical problems is essential to discuss the origin of sources and processes from the observed variability of data. An increasing number and high dimensionality of recent geochemical data require efficient and accurate multivariate statistical analysis methods. In this paper, we show the relationship and complementary roles of k-means cluster analysis (KCA), principal component analysis (PCA), and independent component analysis (ICA) to capture the true data structure. When the data are preprocessed by primary standardization (i.e., with the zero mean and normalized by the standard deviation), KCA and PCA provide essentially the same results, although the former returns the solution in a discretized space. When the data are preprocessed by whitening (i.e., normalized by eigenvalues along the principal components), KCA and ICA may identify a set of independent trends and groups, irrespective of the amplitude (power) of variance. As an example, basalt isotopic compositions have been analyzed with KCA on the whitened data, demonstrating clear rock‒tectonic occurrence‒mantle end-member discrimination. Therefore, the combination of these methods, particularly KCA on whitened data, is useful to capture and discuss the data structure of various geochemical systems, for which an Excel program is provided. This article is protected by copyright. All rights reserved.
      PubDate: 2017-01-28T03:21:49.561039-05:
      DOI: 10.1002/2016GC006663
  • Issue Information
    • Pages: 825 - 826
      PubDate: 2017-04-18T23:13:08.234613-05:
      DOI: 10.1002/ggge.21119
  • The biokarst system and its carbon sinks in response to pH changes: A
           simulation experiment with microalgae
    • Authors: Tengxiang Xie; Yanyou Wu
      Abstract: This study aims to explore the changes in a microalgal biokarst system as a potential carbon sink system in response to pH changes. The bidirectional isotope labeling method and mass balance calculation were adopted in a simulated biokarst environment with a series of set pH conditions and three microalgal species. Three key processes of the microalgal biokarst system, including calcite dissolution, CaCO3 reprecipitation, and inorganic carbon assimilation by microalgae, were completely quantitatively described. The combined effects of chemical dissolution and species-specific bio-dissolution caused a decrease in overall dissolution rate when the pH increased from 7 to 9. CaCO3 reprecipitation and the utilization of dissolved inorganic carbon originating from calcite dissolution decreased when the pH increased from 7 to 9. The three processes exhibited different effects in changing the CO2 atmosphere. The amount of photosynthetic carbon sink was larger at high pH values than at low pH values. However, the CO2 sequestration related to the biokarst process (biokarst carbon sink) increased with decreasing pH. Overall, the total amount of sequestered CO2 produced by the biokarst system (CaCO3-CO2-microalgae) shows a minimum at a specific pH then increases with decreasing pH. Therefore, various processes and carbon sinks in the biokarst system are sensitive to pH changes, and biokarst processes play an important negative feedback role in the release of CO2 by acidification. The results also suggest that the carbon sink associated with carbonate weathering cannot be neglected when considering the global carbon cycle on the scale of thousands of years (
      PubDate: 2016-12-20T13:21:08.2333-05:00
      DOI: 10.1002/2016GC006628
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