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Geochemistry, Geophysics, Geosystems     Full-text available via subscription   (Followers: 30, SJR: 2.439, h-index: 91)
Geophysical Research Letters     Full-text available via subscription   (Followers: 130, 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 : Planets     Full-text available via subscription   (Followers: 111)
J. of Geophysical Research : Solid Earth     Full-text available via subscription   (Followers: 46)
J. of Geophysical Research : Space Physics     Full-text available via subscription   (Followers: 129)
Paleoceanography     Full-text available via subscription   (Followers: 5, SJR: 3.067, h-index: 100)
Radio Science     Full-text available via subscription   (Followers: 38, SJR: 1.072, h-index: 59)
Reviews of Geophysics     Full-text available via subscription   (Followers: 36, SJR: 8.833, h-index: 107)
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Tectonics     Full-text available via subscription   (Followers: 16, SJR: 2.628, h-index: 96)
Water Resources Research     Full-text available via subscription   (Followers: 80, SJR: 2.661, h-index: 144)
Journal Cover Geochemistry, Geophysics, Geosystems
  [SJR: 2.439]   [H-I: 91]   [30 followers]  Follow
   Full-text available via subscription Subscription journal
   ISSN (Online) 1525-2027
   Published by AGU Homepage  [17 journals]
  • 3-D Seismic Imaging of Ancient Submarine Lava Flows: An Example From the
           Southern Australian Margin
    • Authors: P. Reynolds; S. Holford, N. Schofield, A. Ross
      Abstract: Submarine lava flows are the most common surficial igneous rock on the Earth. However, they are inherently more difficult to study than their subaerial counterparts due to their inaccessibility. In this study we use newly-acquired 3D (three-dimensional) seismic reflection data to document the distribution and morphology of 26 ancient, buried lava flows within the Middle Eocene-aged Bight Basin Igneous Complex, offshore southern Australia. Many of these lava flows are associated with volcanoes that vary from 60 − 625 m in height and 0.3 − 10 km in diameter. Well data and seismic-stratigraphic relationships suggest that the lava flows and volcanoes were emplaced offshore in water depths of
      PubDate: 2017-10-03T10:30:57.403555-05:
      DOI: 10.1002/2017GC007178
  • A Hybrid Approach to Data Assimilation for Reconstructing the Evolution of
           Mantle Dynamics
    • Authors: Quan Zhou; Lijun Liu
      Abstract: Quantifying past mantle dynamic processes represents a major challenge in understanding the temporal evolution of the solid earth. Mantle convection modeling with data assimilation is one of the most powerful tools to investigate the dynamics of plate subduction and mantle convection. Although various data assimilation methods, both forward and inverse, have been created, these methods all have limitations in their capabilities to represent the real earth. Pure forward models tend to miss important mantle structures due to the incorrect initial condition and thus may lead to incorrect mantle evolution. In contrast, pure tomography-based models cannot effectively resolve the fine slab structure and would fail to predict important subduction-zone dynamic processes. Here we propose a hybrid data assimilation approach that combines the unique power of the sequential and adjoint algorithms, which can properly capture the detailed evolution of the downgoing slab and the tomographically constrained mantle structures, respectively. We apply this new method to reconstructing mantle dynamics below the western U.S. while considering large lateral viscosity variations. By comparing this result with those from several existing data assimilation methods, we demonstrate that the hybrid modeling approach recovers the realistic 4-D mantle dynamics the best.
      PubDate: 2017-10-03T10:30:42.067151-05:
      DOI: 10.1002/2017GC007116
  • Paleoproterozoic Geomagnetic Field Strength From the Avanavero Mafic
           Sills, Amazonian Craton, Brazil
    • Authors: A. Di Chiara; A. R. Muxworthy, R.I.F. Trindade, F. Bispo-Santos
      Abstract: A recent hypothesis has suggested that Earth's inner core nucleated during the Mesoproterozoic, as evidenced by a rapid increase in the paleointensity (ancient geomagnetic field intensity) record; however, paleointensity data during the Paleoproterozoic and Mesoproterozoic period are limited. To address this problem, we have determined paleointensity from samples from three Paleoproterozoic Avanavero mafic sills (Amazonian Craton, Brazil): Cotingo, 1782 Ma, Puiuà 1788, and Pedra Preta, 1795 Ma. We adopted a multi-protocol approach for paleointensity estimates combining Thellier-type IZZI and LTD-IZZI methods, and the non-heating Preisach protocol. We obtained an average VDM value of 1.3 ± 0.7 × 1022Am2 (Cotingo) of 2.0 ± 0.4 × 1022Am2 (Puiuà) and 6 ± 4 × 1022Am2 (Pedra Preta); it is argued that the Cotingo estimate is the most robust. Our results are the first data from the upper Paleoproterozoic for South America and are comparable to data available from other regions and similar periods. The new data do not invalidate the hypothesis of that Earth's inner core nucleated during the Mesoproterozoic.
      PubDate: 2017-09-29T11:46:51.426935-05:
      DOI: 10.1002/2017GC007175
  • Holocene Paleointensity of the Island of Hawai'i From Glassy Volcanics
    • Authors: G. Cromwell; F. Trusdell, L. Tauxe, H. Staudigel, H. Ron
      Abstract: This study presents new high-quality paleointensity records and 14C radiocarbon age determinations from the Island of Hawai'i during the Holocene. Previous studies on Hawai'i use experimental methods and statistical selection criteria that may produce inaccurate geomagnetic field strength estimates. Additional high-quality paleointensity results can be used to evaluate the existing Hawaiian dataset and investigate Holocene geomagnetic field behavior. New paleointensity sites from 22 lava flows were calculated using the IZZI-Thellier laboratory technique and a strict set of selection criteria. Rapidly cooled, glassy volcanic material was collected for all sites. Isotopic age determinations range from 270- > 10,000 years before present (nine new 14C ages are also presented as part of this study). The median intensity for the 22 flows is 47.5 μT, with a median absolute deviation uncertainty of 5.6 μT; substantially greater than the present day field strength at Hawai'i (∼36 μT). These new results are comparable to previously published data from this location and are consistent with global paleointensity models. There is no evidence of an intensity “spike” at 3,000 years before present, as seen in the Levant and elsewhere. Previously published data vary in intensity by experimental technique relative to data using glassy material and strict selection criteria. Non-Thellier-type data are biased low, a result of these techniques estimating intensity from possibly non-single domain magnetic carriers. Thellier-Thellier data are biased high, the reasons for which remain unclear as no cooling rate effect was demonstrated and we were unable to reproduce the high bias with different selection criteria.
      PubDate: 2017-09-29T11:46:16.303262-05:
      DOI: 10.1002/2017GC006927
  • Extracting a Detailed Magnetostratigraphy From Weakly Magnetized,
           Oligocene to Early Miocene Sediment Drifts Recovered at IODP Site U1406
           (Newfoundland Margin, Northwest Atlantic Ocean)
    • Authors: Tim E. van Peer; Chuang Xuan, Peter C. Lippert, Diederik Liebrand, Claudia Agnini, Paul A. Wilson
      Abstract: Fine-grained magnetic particles in deep-sea sediments often statistically align with the ambient magnetic field during (and shortly after) deposition and can therefore record geomagnetic reversals. Correlation of these reversals to a geomagnetic polarity time scale is an important geochronological tool that facilitates precise stratigraphic correlation and dating of geological records globally. Sediments often carry a remanence strong enough for confident identification of polarity reversals, but in some cases a low signal-to-noise ratio prevents the construction of a reliable and robust magnetostratigraphy. Here we implement a data-filtering protocol, which can be integrated with the UPmag software package, to automatically reduce the maximum angular deviation and statistically mask noisy data and outliers deemed unsuitable for magnetostratigraphic interpretation. This protocol thus extracts a clearer signal from weakly magnetized sediments recovered at Integrated Ocean Drilling Program (IODP) Expedition 342 Site U1406 (Newfoundland margin, northwest Atlantic Ocean). The resulting magnetostratigraphy, in combination with shipboard and shore-based biostratigraphy, provides an age model for the study interval from IODP Site U1406 between Chrons C6Ar and C9n (∼21-27 Ma). We identify rarely observed geomagnetic directional changes within Chrons C6Br, C7r, and C7Ar, and perhaps within Subchron C8n.1n. Our magnetostratigraphy dates three intervals of unusual stratigraphic behavior within the sediment drifts at IODP Site U1406 on the Newfoundland margin. These lithostratigraphic changes are broadly concurrent with the coldest climatic phases of the middle Oligocene to early Miocene and we hypothesize that they reflect changes in bottom-water circulation.
      PubDate: 2017-09-29T11:45:34.767422-05:
      DOI: 10.1002/2017GC007185
  • Tibetan Magmatism Database
    • Authors: James B. Chapman; Paul Kapp
      Abstract: A database containing previously published geochronologic, geochemical, and isotopic data on Mesozoic to Quaternary igneous rocks in the Himalayan-Tibetan orogenic system is presented. The database is intended to serve as a repository for new and existing igneous rock data and is publicly accessible through a web-based platform that includes an interactive map and data table interface with search, filtering, and download options. To illustrate the utility of the database, the age, location, and εHft composition of magmatism from the central Gangdese batholith in the southern Lhasa terrane are compared. The data identify three high-flux events, which peak at 93 Ma, 50 Ma, and 15 Ma. They are characterized by inboard arc migration and a temporal and spatial shift to more evolved isotopic compositions.
      PubDate: 2017-09-29T11:44:57.880911-05:
      DOI: 10.1002/2017GC007217
  • Redistribution of Iron and Titanium in High-Pressure Ultramafic Rocks
    • Authors: Rosalind J. Crossley; Katy A. Evans, Steven M. Reddy, Gregory W. Lester
      Abstract: The redox state of iron in high-pressure serpentinites, which host a significant proportion of Fe3+ in subduction zones, can be used to provide an insight into iron cycling and constrain the composition of subduction zone fluids. In this study, we use oxide and silicate mineral textures, interpretation of mineral parageneses, mineral composition data, and whole rock geochemistry of high-pressure retrogressed ultramafic rocks from the Zermatt-Saas Zone to constrain the distribution of iron and titanium, and iron oxidation state. These data provide an insight on the oxidation state and composition of fluids at depth in subduction zones. Oxide minerals host the bulk of iron, particularly Fe3+. The increase in mode of magnetite and observation of magnetite within antigorite veins in the investigated ultramafic samples during initial retrogression is most consistent with oxidation of existing iron within the samples during the infiltration of an oxidizing fluid since it is difficult to reconcile addition of Fe3+ with the known limited solubility of this species. However, high Ti contents are not typical of serpentinites and also cannot be accounted for by simple mixing of a depleted mantle protolith with the nearby Allalin gabbro. Titanium-rich phases coincide with prograde metamorphism and initial exhumation, implying the early seafloor and/or prograde addition and late mobilization of Ti. If Ti addition has occurred, then the introduction of Fe3+, also generally considered to be immobile, cannot be disregarded. We explore possible transport vectors for Ti and Fe through mineral texture analysis.
      PubDate: 2017-09-28T10:36:55.195733-05:
      DOI: 10.1002/2017GC007145
  • Spatially Variable CO2 Degassing in The Main Ethiopian Rift: Implications
           For Magma Storage, Volatile Transport And Rift-Related Emissions
    • Authors: Jonathan A. Hunt; Amdemichael Zafu, Tamsin A. Mather, David M. Pyle, Peter H. Barry
      Abstract: Deep carbon emissions from historically inactive volcanoes, hydrothermal and tectonic structures are among the greatest unknowns in the long-term (∼Myr) carbon cycle. Recent estimates of diffuse CO2 flux from the Eastern Rift of the East African Rift System (EARS) suggest this could equal emissions from the entire mid-ocean ridge system. We report new CO2 surveys from the Main Ethiopian Rift (MER, northernmost EARS), and reassess the rift-related CO2 flux. Since degassing in the MER is concentrated in discrete areas of volcanic and off-edifice activity, characterisation of such areas is important for extrapolation to a rift-scale budget. Locations of hot springs and fumaroles along the rift show numerous geothermal areas away from volcanic edifices. With these new data we estimate total CO2 emissions from the central and northern MER as 0.52—4.36 Mt yr−1. Our extrapolated flux from the Eastern Rift is 3.9—32.7 Mt yr−1 CO2, overlapping with lower end of the range presented in recent estimates. By scaling, we suggest that 6—18 Mt yr−1 CO2 flux can be accounted for by magmatic extension, which implies an important role for volatile-enriched lithosphere, crustal assimilation and/or additional magmatic intrusion to account for the upper range of flux estimates. Our results also have implications for the nature of volcanism in the MER. Many geothermal areas are found>10 km from the nearest volcanic centre, suggesting ongoing hazards associated with regional volcanism.
      PubDate: 2017-09-28T10:32:09.261285-05:
      DOI: 10.1002/2017GC006975
  • Paleomagnetic Constraints on the Middle Miocene-Early Pliocene
           Stratigraphy in the Xining Basin, NE Tibetan Plateau, and the Geologic
    • Authors: Rongsheng Yang; Xiaomin Fang, Qingquan Meng, Jinbo Zan, Weilin Zhang, Tao Deng, Yibo Yang, Xiaobai Ruan, Liye Yang, Bingshuai Li
      Abstract: The Xining Basin lies in the transitional zone between the arid Asian interior and the East Asian monsoon region. The continuous Cenozoic sediments in the basin provide a unique archive recording the uplift of the Tibetan Plateau and its environmental effects on central Asian aridification and Asian monsoon evolution. However, sediments deposited since the middle Middle Miocene have not been precisely dated, hindering our ability to address these issues. Here, we dated a 336-m-thick section containing many Late Miocene fossil mammals from the eastern basin. High-resolution paleomagnetism revealed 16 normal and 16 reversed zones that correlate well with chrons 3n to 5Ar.1r of the Geomagnetic Polarity Time Scale, constraining the section to ∼12.7-4.8 Ma. The changes in lithofacies from floodplain to braided river at ∼8.6 Ma and to thick alluvial fan at ∼6.3 Ma with predominantly southerly paleocurrent directions occur simultaneously with an increase in the sedimentation rates, representing two periods of rapid uplift in the eastern Qilian Shan to the north. Our results provide a robust oldest age constraint (
      PubDate: 2017-09-28T10:30:28.086514-05:
      DOI: 10.1002/2017GC006945
  • Morphological Expressions of Crater Infill Collapse: Model Simulations of
           Chaotic Terrains on Mars
    • Authors: Manuel Roda; George Marketos, Jan Westerweel, Rob Govers
      Abstract: Martian chaotic terrains are characterized by deeply depressed intensively fractured areas that contain a large number of low-strain tilted blocks. Stronger deformation (e.g. higher number of fractures) is generally observed in the rims when compared to the middle regions of the terrains. The distribution and number of fractures and tilted blocks are correlated with the size of the chaotic terrains. Smaller chaotic terrains are characterized by few fractures between undeformed blocks. Larger terrains show an elevated number of fractures uniformly distributed with single blocks. We investigate whether this surface morphology may be a consequence of the collapse of the infill of a crater. We perform numerical simulations with the Discrete Element Method and we evaluate the distribution of fractures within the crater and the influence of the crater size, infill thickness and collapsing depth on the final morphology.The comparison between model predictions and the morphology of the Martian chaotic terrains shows strong statistical similarities in terms of both number of fractures and correlation between fractures and crater diameters. No or very weak correlation is observed between fractures and the infill thickness or collapsing depth. The strong correspondence between model results and observations suggests that the collapse of an infill layer within a crater is a viable mechanism for the peculiar morphology of the martian chaotic terrains.
      PubDate: 2017-09-25T11:24:27.674575-05:
      DOI: 10.1002/2017GC006933
  • Great Salt Lake (Utah) Microbialite δ13C, δ18O, and δ15N Record
           Fluctuations in Lake Biogeochemistry Since the Late Pleistocene
    • Authors: D. L. Newell; J. L. Jensen, C. M. Frantz, M. D. Vanden Berg
      Abstract: Extensive lacustrine microbialite deposits exposed along the shores of Great Salt Lake (GSL), Utah preserve a rich continental paleoenvironmental record. Newly-reported microbialite carbon and oxygen stable isotope ratios in carbonate, nitrogen isotope ratios in organic matter, and organic matter radiocarbon ages archive paleolake hydrological and biogeochemical changes from the late Pleistocene through the Holocene. Positive correlations between δ18O and δ13C in ∼15 – 7.6 cal ka microbialite carbonate are consistent with a hydrologically closed-basin lake with fluctuations in volume, chemistry, and associated changes in lake primary production. The δ15N of microbialite bulk organic matter (5 – 18 ‰ vs. AIR) shows that the balance between nitrogen fixation and assimilation of dissolved inorganic nitrogen has varied significantly. Inverse δ18O and δ13C correlations in combination with high δ15N in some carbonate deposits may imply periods of higher salinity and stable lake stratification similar to modern GSL conditions. We compare our C and O datasets with Pleistocene Lake Bonneville carbonate stable isotope records and demonstrate progressive development of spatially-isolated hydrological basins during the shift to warmer and drier conditions in the Holocene.
      PubDate: 2017-09-21T11:37:36.823971-05:
      DOI: 10.1002/2017GC007078
  • Hydrothermal Venting at Hinepuia Submarine Volcano, Kermadec Arc:
           Understanding Magmatic-Hydrothermal Fluid Chemistry
    • Authors: Valerie K. Stucker; Sharon L. Walker, Cornel E. J. de Ronde, Fabio Caratori Tontini, Shinji Tsuchida
      Abstract: The Hinepuia volcanic center is made up of two distinct edifices aligned northwest to southeast, with an active cone complex in the SE. Hinepuia is one of several active volcanoes in the northern segment of the Kermadec arc. Regional magnetic data shows no evidence for large-scale hydrothermal alteration at Hinepuia, yet plume data confirm present-day hydrothermal discharge, suggesting that the hydrothermal system may be too young to have altered the host rocks with respect to measurable changes in magnetic signal. Gravity data are consistent with crustal thinning and shallow mantle under the volcanic center. Following the discovery of hydrothermal plumes over Hinepuia, the submersible Shinkai 6500 was used to explore the SE cone and sample hydrothermal fluids.The chemistry of hydrothermal fluids from submarine arc and backarc volcanoes are typically dominated by water-rock interactions and/or magmatic degassing. Chemical analyses of vent fluids show that Hinepuia does not quite fit either traditional model. Moreover, the Hinepuia samples fall between those typically ascribed to both end-member fluid types when plotted on a K-Mg-SO4 ternary diagram. Due to evidence of strong degassing, abundant native sulfur deposition, and H2S presence, the vent sampled at Hinepuia is ultimately classified as a magmatic-hydrothermal system with a water-rock influence. This vent is releasing water vapor and magmatic volatiles with a notable lack of salinity due to subcritical boiling and phase separation. Magmatic-hydrothermal fluid chemistry appears to be controlled by a combination of gas flux, phase separation processes, and volcano evolution and/or distance from the magma source.
      PubDate: 2017-09-21T11:30:45.627151-05:
      DOI: 10.1002/2016GC006713
  • Arctic Deep-Water Ferromanganese-Oxide Deposits Reflect the Unique
           Characteristics of the Arctic Ocean
    • Authors: James R. Hein; Natalia Konstantinova, Mariah Mikesell, Kira Mizell, Jessica N. Fitzsimmons, Phoebe Lam, Laramie T. Jensen, Yang Xiang, Amy Gartman, Georgy Cherkashov, Deborah R. Hutchinson, Claire P. Till
      Abstract: Little is known about marine mineral deposits in the Arctic Ocean, an ocean dominated by continental shelf and basins semi-closed to deep-water circulation. Here, we present data for ferromanganese crusts and nodules collected from the Amerasia Arctic Ocean in 2008, 2009, and 2012 (HLY0805, HLY0905, HLY1202). We determined mineral and chemical compositions of the crusts and nodules and the onset of their formation. Water column samples from the GEOTRACES program were analyzed for dissolved and particulate scandium concentrations, an element uniquely enriched in these deposits.The Arctic crusts and nodules are characterized by unique mineral and chemical compositions with atypically high growth rates, detrital contents, Fe/Mn ratios, and low Si/Al ratios, compared to deposits found elsewhere. High detritus reflects erosion of submarine outcrops and North America and Siberia cratons, transport by rivers and glaciers to the sea, and distribution by sea ice, brines, and currents. Uniquely high Fe/Mn ratios are attributed to expansive continental shelves, where diagenetic cycling releases Fe to bottom waters, and density flows transport shelf bottom water to the open Arctic Ocean. Low Mn contents reflect the lack of a mid-water oxygen minimum zone that would act as a reservoir for dissolved Mn. The potential host phases and sources for elements with uniquely high contents are discussed with an emphasis on scandium. Scandium sorption onto Fe oxyhydroxides and Sc-rich detritus account for atypically high scandium contents. The opening of Fram Strait in the Miocene and ventilation of the deep basins initiated Fe-Mn crust growth ∼15 Myr ago.
      PubDate: 2017-09-21T11:26:34.751794-05:
      DOI: 10.1002/2017GC007186
  • Light Stable Isotopic Compositions of Enriched Mantle Sources: Resolving
           the Dehydration Paradox
    • Authors: J. E. Dixon; I. N. Bindeman, R. H. Kingsley, K. K. Simons, P. J. le Roux, T. R. Hajewski, P. Swart, C. H. Langmuir, J. G. Ryan, K. J. Walowski, I. Wada, P. J. Wallace
      Abstract: Volatile and stable isotope data provide tests of mantle processes that give rise to mantle heterogeneity. New data on enriched mid-oceanic ridge basalts (MORB) show a diversity of enriched components. Pacific PREMA-type basalts (H2O/Ce = 215 ± 30, δDSMOW = -45 ± 5 ‰) are similar to those in the northern Atlantic (H2O/Ce = 220 ± 30; δDSMOW = -30 to -40 ‰). Basalts with EM-type signatures have regionally variable volatile compositions. Northern Atlantic EM-type basalts are wetter (H2O/Ce = 330 ± 30) and have isotopically heavier hydrogen (δDSMOW = -57 ± 5 ‰) than northern Atlantic MORB. Southern Atlantic EM-type basalts are damp (H2O/Ce = 120 ± 10) with intermediate δDSMOW (-68 ± 2 ‰), similar to δDSMOW for Pacific MORB. Northern Pacific EM-type basalts are dry (H2O/Ce = 110 ± 20) and isotopically light (δDSMOW = -94 ± 3 ‰).A multi-stage metasomatic and melting model accounts for the origin of the enriched components by extending the subduction factory concept down through the mantle transition zone, with slab temperature a key variable. Volatiles and their stable isotopes are decoupled from lithophile elements, reflecting primary dehydration of the slab followed by secondary rehydration, infiltration and re-equilibration by fluids derived from dehydrating subcrustal hydrous phases (e.g., antigorite) in cooler, deeper parts of the slab. Enriched mantle sources form by addition of
      PubDate: 2017-09-21T11:25:40.005031-05:
      DOI: 10.1002/2016GC006743
  • In situ Raman Raman Detection of Gas Hydrates Exposed on the Seafloor of
           the South China Sea
    • Authors: Xin Zhang; Zengfeng Du, Zhendong Luan, Xiujuan Wang, Shichuan Xi, Bing Wang, Lianfu Li, Chao Lian, Jun Yan
      Abstract: Gas hydrates are usually buried in sediments. Here, we report the first discovery of gas hydrates exposed on the seafloor of the South China Sea. The in situ chemical compositions and cage structures of these hydrates were measured at the depth of 1130 m below sea level using a Raman insertion probe (RiP-Gh) that was carried and controlled by a remotely operated vehicle (ROV) Faxian. This in situ analytical technique can avoid the physical and chemical changes associated with the transport of samples from the deep sea to the surface. Natural gas hydrate samples were analyzed at two sites. The in situ spectra suggest that the newly formed hydrate was Structure I but contains a small amount of C3H8 and H2S. Pure gas spectra of CH4, C3H8 and H2S were also observed at the SCS-SGH02 site. These data represent the first in situ proof that free gas can be trapped within the hydrate fabric during rapid hydrate formation. We provide the first in situ confirmation of the hydrate growth model for the early stages of formation of crystalline hydrates in a methane-rich seafloor environment. Our work demonstrates that natural hydrate deposits, particularly those in the early stages of formation, are not monolithic single structures but instead exhibit significant small-scale heterogeneities due to inclusions of free gas and the surrounding seawater, there inclusions also serve as indicators of the likely hydrate formation mechanism. These data also reinforce the importance of correlating visual and in situ measurements when characterizing a sampling site.
      PubDate: 2017-09-21T11:20:48.813706-05:
      DOI: 10.1002/2017GC006987
  • The Impacts of Seawater Mg/Ca and Temperature on Element Incorporation in
           Benthic Foraminiferal Calcite
    • Authors: L.J. de Nooijer; I. van Dijk, T. Toyofuku, G.J. Reichart
      Abstract: On geological timescales, oceanic [Mg2+] and [Ca2+] vary with changing rates of weathering, seafloor spreading and dolomite formation. Accurate reconstruction of the ratio between [Mg2+] and [Ca2+] in seawater (Mg/Casw), may potentially be reconstructed using foraminiferal Mg/Ca ratios. Since both temperature and seawater Mg/Ca impact foraminiferal Mg/Ca, successful reconstruction of Mg/Casw requires quantification of both these parameters independently on foraminiferal Mg/Ca, as well as their combined effect on Mg-incorporation. Here we present the combined and isolated impacts of temperature and Mg/Casw on Mg incorporation in two model species, the benthic hyaline (i.e. perforate) foraminifer Elphidium crispum and porcelaneous (i.e. miliolid) foraminifer Quinqueloculina sp. using controlled growth experiments. Specimens of these two species were kept at four different temperatures (ranging from 10 to 27°C) and three Mg/Casw's (3.4, 6.4 and 8.5 mol/mol), resulting in 12 experimental conditions. Newly grown calcite was analyzed for a number of elements (Na, Mg and Sr) by laser ablation-ICP-MS. Results show that although the Mg/Ca varied by more than an order of magnitude between species, the sensitivity of Mg incorporation with respect to temperature appeared not to be influenced by Mg/Casw. By extension, these results may also help improving accuracy in the reconstruction of past Mg/Casw based on foraminifera with contrasting Mg/Ca.
      PubDate: 2017-09-21T11:20:40.503805-05:
      DOI: 10.1002/2017GC007183
  • Stress State in the Kumano Basin and in Slope Sediment Determined From
           Anelastic Strain Recovery: Results From IODP Expedition 338 to the Nankai
    • Authors: Kiyokazu Oohashi; Weiren Lin, Hung-Yu Wu, Asuka Yamaguchi, Yuhji Yamamoto
      Abstract: Three-dimensional, in situ stresses in the Kumano Basin and slope sediment (IODP Sites C0002 and C0022) in the Nankai Trough, southwest Japan, have been determined using the anelastic strain recovery (ASR) of core samples. Two samples taken from Hole C0002J, located in the bottom of the Kumano Basin, indicate that the maximum principal stress, σ1, is vertical. The intermediate principal stress, σ2, is oriented ENE–WSW, parallel to the trench axis. These stress orientations are similar to those obtained using ASR and borehole breakout methods in previous expeditions. In contrast, a sample from the lower section of the slope sediment (Hole C0022B), located beneath the megasplay fault, is characterized by σ1 plunging moderately to the ESE and σ3 oriented near-horizontally, trending NNE–SSW. The direction of maximum horizontal stress obtained from ASR (WNW–ESE) is similar to that inferred from borehole breakouts in an adjacent hole (NW–SE). Trench-normal compression and a near-vertical σ2 are also inferred from focal mechanisms of very-low-frequency earthquakes within the Nankai accretionary prism, and from borehole breakouts in the hanging wall of the megasplay fault. These observations suggest that the horizontal compressional regime extends to a shallower level than previously thought, likely due to the shallow portion of the megasplay fault accumulating tectonic stress in response to plate convergence.
      PubDate: 2017-09-12T11:55:55.558981-05:
      DOI: 10.1002/2017GC007137
  • Investigating Segmentation in Cascadia: Anisotropic Crustal Structure and
           Mantle Wedge Serpentinization from Receiver Functions
    • Authors: Hannah E. Krueger; Erin A. Wirth
      Abstract: The Cascadia subduction zone exhibits along-strike segmentation in structure, processes, and seismogenic behavior. While characterization of seismic anisotropy can constrain deformation processes at depth, the character of seismic anisotropy in Cascadia remains poorly understood. This is primarily due to a lack of seismicity in the subducting Juan de Fuca slab, which limits shear wave splitting and other seismological analyses that interrogate the fine-scale anisotropic structure of the crust and mantle wedge. We investigate lower crustal anisotropy and mantle wedge structure by computing P-to-S receiver functions at 12 broadband seismic stations along the Cascadia subduction zone. We observe P-to-SV converted energy consistent with previously estimated Moho depths. Several stations exhibit evidence of an “inverted Moho” (i.e., a downward velocity decrease across the crust-mantle boundary), indicative of a serpentinized mantle wedge. Stations with an underlying hydrated mantle wedge appear prevalent from northern Washington to central Oregon, but sparse in southern Oregon and northern California. Transverse component receiver functions are complex, suggesting anisotropic and/or dipping crustal structure. To constrain the orientation of crustal anisotropy we compute synthetic receiver functions using manual forward modeling. We determine that the lower crust shows variable orientations of anisotropy along-strike, with highly complex anisotropy in northern Cascadia, and generally NW-SE and NE-SW orientations of slow-axis anisotropy in central and southern Cascadia, respectively. The orientations of anisotropy from this work generally agree with those inferred from shear wave splitting of tremor studies at similar locations, lending confidence to this relatively new method of inferring seismic anisotropy from slow earthquakes.
      PubDate: 2017-09-11T11:32:08.056537-05:
      DOI: 10.1002/2017GC007064
  • Extreme monsoon rainfall signatures preserved in the invasive terrestrial
           gastropod Lissachatina fulica
    • Authors: Prosenjit Ghosh; Ravi Rangarajan, Kaustubh Thirumalai, Fred Naggs
      Abstract: Indian summer monsoon (ISM) rainfall lasts for a period of four months with large variations recorded in terms of rainfall intensity during its period between June to September. Proxy reconstructions of past ISM rainfall variability are required due to the paucity of long instrumental records. However, reconstructing sub-seasonal rainfall is extremely difficult using conventional hydroclimate proxies due to inadequate sample resolution. Here, we demonstrate the utility of the stable oxygen isotope composition of gastropod shells in reconstructing past rainfall on sub-seasonal timescales. We present a comparative isotopic study on present day rainwater and stable isotope ratios of precipitate found in the incremental growth bands of giant African land snail Lissachatina fulica (Bowdich) from modern day (2009) and in the historical past (1918). Isotopic signatures present in the growth bands allowed for the identification of ISM rainfall variability in terms of its active and dry spells in the modern as well as past gastropod record. Our results demonstrate the utility of gastropod growth band stable isotope ratios in semi-quantitative reconstructions of seasonal rainfall patterns. High resolution climate records extracted from gastropod growth band stable isotopes (museum and archived specimens) can expand the scope for understanding past sub-seasonal-to-seasonal climate variability.
      PubDate: 2017-09-07T10:40:33.147307-05:
      DOI: 10.1002/2017GC007041
  • Generation of silicic melts in the early Izu-Bonin arc recorded by
           detrital zircons in proximal arc volcaniclastic rocks from the Philippine
    • Authors: A.P. Barth; K. Tani, S. Meffre, J.L. Wooden, M.A. Coble, R.J. Arculus, O. Ishizuka, J.T. Shukle
      Abstract: A 1.2 kilometer thick Paleogene volcaniclastic section at International Ocean Discovery Program Site 351-U1438 preserves the deep-marine, proximal record of Izu-Bonin oceanic arc initiation and volcano evolution along the Kyushu-Palau Ridge (KPR). Pb/U ages and trace element compositions of zircons recovered from volcaniclastic sandstones preserve a remarkable temporal record of juvenile island arc evolution. Pb/U ages ranging from 43 to 27 Ma are compatible with provenance in one or more active arc edifices of the northern KPR. The abundances of selected trace elements with high concentrations provide insight into the genesis of U1438 detrital zircon host melts, and represent useful indicators of both short and long-term variations in melt compositions in arc settings. The Site U1438 zircons span the compositional range between zircons from mid-ocean ridge gabbros and zircons from relatively enriched continental arcs, as predicted for melts in a primitive oceanic arc setting derived from a highly depleted mantle source. Melt zircon saturation temperatures and Ti-in-zircon thermometry suggest a provenance in relatively cool and silicic melts that evolved toward more Th and U-rich compositions with time. Th, U and light rare earth element enrichments beginning about 35 Ma are consistent with detrital zircons recording development of regional arc asymmetry and selective trace element-enriched rear arc silicic melts as the juvenile Izu-Bonin arc evolved.
      PubDate: 2017-09-06T16:30:33.753004-05:
      DOI: 10.1002/2017GC006948
  • Magma buoyancy and volatile ascent driving autocyclic eruptivity at Hekla
           Volcano (Iceland)
    • Authors: Stefanie Hautmann; I. Selwyn Sacks, Alan T. Linde, Matthew J. Roberts
      Abstract: Volcanic eruptions are typically accompanied by ground deflation due to the withdrawal of magma from depth and its effusion at the surface. Here, based on continuous high-resolution borehole strain data, we show that ground deformation was absent during the major effusion phases of the 1991 and 2000 eruptions of Hekla Volcano, Iceland. This lack of surface deformation challenges the classic model of magma intrusion/withdrawal as source for volcanic ground uplift/subsidence. We incorporate geodetic and geochemical observables into theoretical models of magma chamber dynamics in order to constrain quantitatively alternative co- and inter-eruptive physical mechanisms that govern magma propagation and system pressurization. We find the lack of surface deformation during lava effusion to be linked to chamber replenishment from below whilst magma migrates as a buoyancy-driven flow from the magma chamber towards the surface. We further demonstrate that inter-eruptive pressure build-up is likely to be generated by volatile ascent within the chamber rather than magma injection. Our model explains the persistent periodic eruptivity at Hekla throughout historic times with self-initiating cycles and is conceptually relevant to other volcanic systems.
      PubDate: 2017-08-31T11:40:23.341566-05:
      DOI: 10.1002/2017GC007061
  • High-resolution hyperspectral imaging of diagenesis and clays in fossil
           coral reef material: a nondestructive tool for improving environmental and
           climate reconstructions
    • Authors: R. J. Murphy; J. M. Webster, L. Nothdurft, B. Dechnik, H. V. McGregor, M. A. Patterson, K. L. Sanborn, G. E. Webb, L. I. Kearney, L. Rintoul, D. V. Erler
      Abstract: Hyperspectral imagery (1000–2500 nm) was used to quantitatively map carbonate and clay minerals in fossil reef cores that are relevant to accurately reconstructing past environmental and climatic conditions. Techniques were developed using hyperspectral imagery of fossil reef corals and cores acquired from three different geological settings, and were validated against independent measures of calcite to aragonite ratios. Aragonite, calcite, and dolomite were distinguished using a combination of the wavelength position and asymmetry of the primary carbonate absorption between 2300 and 2350 nm. Areas of core containing small amounts of calcite (>2–5%) were distinguished from aragonite in imagery of two cores, enabling quantitative maps of these minerals to be constructed. Dolomite was found to be the dominant mineral in another core. Trace amounts of the aluminium-rich clay mineral kaolinite were detected, quantified, and mapped in one core using its diagnostic absorption feature near 2200 nm. The amounts of clay detected from hyperspectral imagery were below the limits of detection by standard X-ray diffraction techniques but its presence was confirmed by Fourier Transform Infrared Spectroscopy. Hyperspectral imagery acquired at high spatial resolution simplifies vetting procedures for secondary carbonate minerals in coral reef cores, significantly reduces sampling time and costs, and is a powerful nondestructive tool to identify well-preserved coral aragonite in cores for uses in paleoclimate, paleoenvironment and paleoecosystem reconstruction.
      PubDate: 2017-08-30T06:51:17.423162-05:
      DOI: 10.1002/2017GC006949
  • ADOPT: A tool for automatic detection of tectonic plates at the surface of
           convection models
    • Authors: C. Mallard; B. Jacquet, N. Coltice
      Abstract: Mantle convection models with plate-like behavior produce surface structures comparable to Earth's plate boundaries. However, analyzing those structures is a difficult task, since convection models produce, as on Earth, diffuse deformation and elusive plate boundaries. Therefore we present here and share a quantitative tool to identify plate boundaries and produce plate polygon layouts from results of numerical models of convection: Automatic Detection Of Plate Tectonics (ADOPT). This digital tool operates within the free open-source visualization software Paraview. It is based on image segmentation techniques to detect objects. The fundamental algorithm used in ADOPT is the watershed transform. We transform the output of convection models into a topographic map, the crest lines being the regions of deformation (plate boundaries) and the catchment basins being the plate interiors. We propose two generic protocols (the field and the distance methods) that we test against an independent visual detection of plate polygons. We show that ADOPT is effective to identify the smaller plates and to close plate polygons in areas where boundaries are diffuse or elusive. ADOPT allows the export of plate polygons in the standard OGR-GMT format for visualization, modification, and analysis under generic softwares like GMT or GPlates.
      PubDate: 2017-08-30T06:46:56.385131-05:
      DOI: 10.1002/2017GC007030
  • Micro-computed tomography: Applications for high-resolution skeletal
           density determinations. An example using annually banded crustose
           coralline algae
    • Authors: P. Chan; J. Halfar, C.J.D. Norley, S.I. Pollmann, W. Adey, D.W. Holdsworth
      Abstract: Warming and acidification of the world's oceans are expected to have widespread consequences for marine biodiversity and ecosystem functioning. However, due to the relatively short record of instrumental observations, one has to rely upon geochemical and physical proxy information stored in biomineralized shells and skeletons of calcareous marine organisms as in-situ recorders of past environments. Of particular interest is the response of marine calcifiers to ocean acidification through the examination of structural growth characteristics. Here we demonstrate the application of micro-computed tomography (micro-CT) for three-dimensional visualization and analysis of growth, skeletal density, and calcification in a slow-growing, annually-banded crustose coralline alga Clathromorphum nereostratum (increment width ∼380 µm). X-ray images and time series of skeletal density were generated at 20 µm resolution and rebinned to 40, 60, 80, and 100 µm for comparison in a sensitivity analysis. Calcification rates were subsequently calculated as the product of density and growth (linear extension). While both skeletal density and calcification rates do not significantly differ at varying spatial resolutions (the latter being strongly influenced by growth rates), clear visualization of micron-scale growth features and the quantification of structural changes on subannual timescales requires higher scanning resolutions. In the present study, imaging at 20 µm resolution reveals seasonal cycles in density that correspond to summer/winter variations in skeletal structure observed using scanning electron microscopy (SEM). Micro-CT is a fast, non-destructive, and high-resolution technique for structural and morphometric analyses of temporally-banded paleoclimate archives, particularly those that exhibit slow or compressed growth or micron-scale structures.
      PubDate: 2017-08-24T11:25:37.595044-05:
      DOI: 10.1002/2017GC006966
  • Radioactive heat production of six geologically important nuclides
    • Authors: Thomas Ruedas
      Abstract: Heat production rates for the geologically important nuclides 26Al, 40K, 60Fe, 232Th, 235U, and 238U are calculated on the basis of recent data on atomic and nuclear properties. The revised data differ by several per cent from some older values, but indicate that more recent analyses converge toward values with an accuracy sufficient for all common geoscience applications, although some possibilities for improvement still remain, especially in the case of 40K and with regard to the determination of half-lives. A Python script is provided for calculating heat production (
      PubDate: 2017-08-24T11:25:25.986533-05:
      DOI: 10.1002/2017GC006997
  • Fault-magma interactions during early continental rifting: Seismicity of
           the Magadi-Natron-Manyara basins, Africa
    • Authors: A. Weinstein; S. J. Oliva, C. J. Ebinger, S. Roecker, C. Tiberi, M. Aman, C. Lambert, E. Witkin, J. Albaric, S. Gautier, S. Peyrat, J. D. Muirhead, A. N. N. Muzuka, G. Mulibo, G. Kianji, R. Ferdinand-Wambura, M. Msabi, A. Rodzianko, R. Hadfield, F. Illsley-Kemp, T.P. Fischer
      Abstract: Although magmatism may occur during the earliest stages of continental rifting, its role in strain accommodation remains weakly constrained by largely 2D studies. We analyze seismicity data from a 13-month, 39-station broadband seismic array to determine the role of magma intrusion on state-of-stress and strain localization, and their along-strike variations. Precise earthquake locations using cluster analyses and a new 3D velocity model reveal lower crustal earthquakes beneath the central basins and along projections of steep border faults that degas CO2. Seismicity forms several disks interpreted as sills at 6-10 km below a monogenetic cone field. The sills overlie a lower crustal magma chamber that may feed eruptions at Oldoinyo Lengai volcano. After determining a new ML scaling relation, we determine a b-value of 0.87 ± 0.03. Focal mechanisms for 65 earthquakes, and 13 from a catalogue prior to our array reveal an along-axis stress rotation of ∼60° in the magmatically active zone. New and prior mechanisms show predominantly normal slip along steep nodal planes, with extension directions ∼ N90°E north and south of an active volcanic chain consistent with geodetic data, and ∼ N150°E in the volcanic chain. The stress rotation facilitates strain transfer from border fault systems, the locus of early stage deformation, to the zone of magma intrusion in the central rift. Our seismic, structural, and geochemistry results indicate that frequent lower crustal earthquakes are promoted by elevated pore pressures from volatile degassing along border faults, and hydraulic fracture around the margins of magma bodies. Results indicate that earthquakes are largely driven by stress state around inflating magma bodies.
      PubDate: 2017-08-21T10:32:57.501624-05:
      DOI: 10.1002/2017GC007027
  • Sr-Nd-Hf-O isotope geochemistry of the Ertaibei pluton, East Junggar, NW
           China: Implications for development of crustal-scale granitoid pluton and
           crustal growth
    • Authors: Gong-Jian Tang; Qiang Wang, Chunfu Zhang, Derek A. Wyman, Wei Dan, Xiao-Ping Xia, Hong-Yi Chen, Zhen-Hua Zhao
      Abstract: To better understand the compositional diversity of plutonic complexes and crustal growth of the Central Asian Orogenic Belt (CAOB), we conducted an integrated study of the Ertaibei pluton, which obtained geochronological, petrological, geochemical, and isotopic (including whole rock Sr-Nd, in-situ zircon Hf-O) data. The pluton (ca. 300 Ma) is composed of granodiorites that contain mafic microgranular enclaves (MMEs), dolerite dikes, and granite dikes containing quartz–tourmaline orbicules. The dolerite dikes were possibly generated by melting of an asthenospheric mantle source, with discrete assimilation of lower crustal components in the MASH (melting, assimilation, storage, and homogenization) zone. The MMEs originated from hybridization between mantle- and crust-derived magmas, which spanned a range of melting depths (∼25 – 30 km) in the MASH zone and were episodically tapped. Melting of the basaltic lower crust in the core of the MASH zone generated magmas to form the granodiorites. The granite dikes originated from melting of an arc-derived volcanogenic sedimentary source with a minor underplated basaltic source in the roof of the MASH zone (∼25 km). The compositional diversity reflects both the magma sources and the degree of maturation of the MASH zone. Although having mantle-like radiogenic isotope compositions, the Ertaibei and other post-collisional granitoids show high zircon δ18O values (mostly between +6 and +9‰), indicating a negligible contribution to the CAOB crustal growth during the post-collisional period.
      PubDate: 2017-08-21T10:25:49.696052-05:
      DOI: 10.1002/2017GC006998
  • Testing a thermo-chemo-hydro-geomechanical model for gas hydrate bearing
           sediments using triaxial compression lab experiments
    • Authors: S. Gupta; C. Deusner, M. Haeckel, R. Helmig, B. Wohlmuth
      Abstract: Natural gas hydrates are considered a potential resource for gas production on industrial scales. Gas hydrates contribute to the strength and stiffness of the hydrate-bearing sediments. During gas production, the geomechanical stability of the sediment is compromised. Due to the potential geotechnical risks and process management issues, the mechanical behavior of the gas hydrate-bearing sediments needs to be carefully considered. In this study, we describe a coupling concept that simplifies the mathematical description of the complex interactions occuring during gas production by isolating the effects of sediment deformation and hydrate phase changes. Central to this coupling concept is the assumption that the soil grains form the load-bearing solid skeleton, while the gas hydrate enhances the mechanical properties of this skeleton. We focus on testing this coupling concept in capturing the overall impact of geomechanics on gas production behavior though numerical simulation of a high-pressure isotropic compression experiment combined with methane hydrate formation and dissociation. We consider a linear-elastic stress-strain relationship because it is uniquely defined and easy to calibrate. Since, in reality, the geomechanical response of the hydrate bearing sediment is typically inelastic and is characterized by a significant shear-volumetric coupling, we control the experiment very carefully in order to keep the sample deformations small and well within the assumptions of poro-elasticity. The closely co-ordinated experimental and numerical procedures enable us to validate the proposed simplified geomechanics-to-flow coupling, and set an important precursor towards enhancing our coupled hydro-geomechanical hydrate reservoir simulator with more suitable elasto-plastic constitutive models.
      PubDate: 2017-08-14T11:10:42.88686-05:0
      DOI: 10.1002/2017GC006901
  • Organic carbon preservation in Southeastern Arabian Sea sediments since
           mid-Holocene: Implications to South Asian Summer Monsoon variability
    • Authors: Siddhesh S. Nagoji; Manish Tiwari
      Abstract: The earlier studies show a contrasting long-term trend of the South Asian Summer Monsoon (SASM) after attaining the precessional forcing induced mid-Holocene maximum. The increasing total organic carbon (TOC) concentration of marine sediments in the Southeastern Arabian Sea (SEAS) has been interpreted to imply strengthening SASM since mid-Holocene by a few studies. However, TOC concentration is also influenced by redox conditions, sedimentation rate, and an influx of terrigenous matter depending on the regional settings. So, it needs to be ascertained whether the TOC concentration of the sediments in the SEAS is a signal of productivity related to the SASM strength or preservation. Therefore, we studied multiple proxies (TOC, total nitrogen, atomic C/N, δ13Corg, CaCO3, and major and trace elements concentration) for determining the productivity, redox conditions, detrital supply, and provenance in a sediment core from the upper continental slope of the SEAS spanning the past ∼4700 years at centennial scale resolution. The present study shows that the observed TOC increase is not a result of enhanced productivity but is because of better preservation due to the increased sedimentation rate along with increasingly reducing conditions since mid-Holocene. We further show that the SASM has been declining since mid-Holocene after attaining a precession-forced maximum, which corroborates the earlier model ensemble studies.
      PubDate: 2017-08-14T11:10:32.141698-05:
      DOI: 10.1002/2017GC006804
  • Geochemical and geophysical constrains on the dynamic topography of the
           Southern African Plateau
    • Authors: Alan G. Jones; Juan Carlos Afonso, Javier Fullea
      Abstract: The deep mantle African Superswell is considered to contribute to the topographic uplift of the Southern African Plateau, but dynamic support estimates vary wildly depending on the approach and data used. One reason for these large disparities is that the role of lithospheric structure, key in modulating deep dynamic contributions to elevation, is commonly ignored or oversimplified in convection studies. We use multiple high-quality geophysical data coupled with xenolith-based geochemical constraints to compute the isostatic lithospheric contribution to the elevation of the Plateau, facilitating isolation of the current dynamic component from the total observed elevation. We employ a multi-observable stochastic algorithm to invert geoid anomaly, surface-wave dispersion data, magnetotelluric data and surface heat flow to predict elevation in a fully thermodynamically and internally-consistent manner. We find that a compositionally-layered 230 ±7 km thick lithosphere is required to simultaneously fit all four data types, in agreement with abundant independent xenolith evidence. Our stochastic modelling indicates a lithospheric contribution to elevation of the order of 670 m, which implies dynamic support arising from the convecting sub-lithospheric mantle of ∼650 m. Our results have important implications for the understanding of lithospheric-deep mantle feedback mechanisms and for calibrating dynamic topography estimates from global convection studies.
      PubDate: 2017-08-14T11:10:28.18306-05:0
      DOI: 10.1002/2017GC006908
  • The importance of grain size to mantle dynamics and seismological
    • Authors: J. Dannberg; Z. Eilon, Ulrich Faul, Rene Gassmöller, Pritwiraj Moulik, Robert Myhill
      Abstract: Grain size plays a key role in controlling the mechanical properties of the Earth's mantle, affecting both long-time-scale flow patterns and anelasticity on the time scales of seismic wave propagation. However, dynamic models of Earth's convecting mantle usually implement flow laws with constant grain size, stress-independent viscosity, and a limited treatment of changes in mineral assemblage. We study grain size evolution, its interplay with stress and strain rate in the convecting mantle, and its influence on seismic velocities and attenuation. Our geodynamic models include the simultaneous and competing effects of dynamic recrystallization resulting from dislocation creep, grain growth in multiphase assemblages, and recrystallization at phase transitions. They show that grain size evolution drastically affects the dynamics of mantle convection and the rheology of the mantle, leading to lateral viscosity variations of 6 orders of magnitude due to grain size alone, and controlling the shape of upwellings and downwellings. Using laboratory-derived scaling relationships, we convert model output to seismologically observable parameters (velocity and attenuation) facilitating comparison to Earth structure. Reproducing the fundamental features of the Earth's attenuation profile requires reduced activation volume and relaxed shear moduli in the lower mantle compared to the upper mantle, in agreement with geodynamic constraints. Faster lower mantle grain growth yields best fit to seismic observations, consistent with our reexamination of high-pressure grain growth parameters. We also show that ignoring grain size in interpretations of seismic anomalies may underestimate the Earth's true temperature variations.
      PubDate: 2017-08-13T23:33:00.151406-05:
      DOI: 10.1002/2017GC006944
  • Trace element partitioning between plagioclase and Melt: An investigation
           of the impact of experimental and analytical procedures
    • Authors: Roger L. Nielsen; Gokce Ustunisik, Allison B. Weinsteiger, Frank J. Tepley, A. Dana Johnston, Adam J. R. Kent
      Abstract: Quantitative models of petrologic processes require accurate partition coefficients. Our ability to obtain accurate partition coefficients is constrained by their dependence on pressure temperature and composition, and on the experimental and analytical techniques we apply. The source and magnitude of error in experimental studies of trace element partitioning may go unrecognized if one examines only the processed published data. The most important sources of error are relict crystals, and analyses of more than one phase in the analytical volume. Because we have typically published averaged data, identification of compromised data is difficult if not impossible. We addressed this problem by examining unprocessed data from plagioclase/melt partitioning experiments, by comparing models based on that data with existing partitioning models, and evaluated the degree to which the partitioning models are dependent on the calibration data. We found that partitioning models are dependent on the calibration data in ways that result in erroneous model values, and that the error will be systematic and dependent on the value of the partition coefficient. In effect, use of different calibration datasets will result in partitioning models whose results are systematically biased, and that one can arrive at different and conflicting conclusions depending on how a model is calibrated, defeating the purpose of applying the models. Ultimately this is an experimental data problem, which can be solved if we publish individual analyses (not averages) or use a projection method wherein we use an independent compositional constraint to identify and estimate the uncontaminated composition of each phase.
      PubDate: 2017-08-12T10:37:01.423971-05:
      DOI: 10.1002/2017GC007080
  • Directional change during a Miocene R-N geomagnetic polarity reversal
           recorded by mafic lava flows, Sheep Creek Range, north central Nevada, USA
    • Authors: S. W. Bogue; J. M. G. Glen, N. A. Jarboe
      Abstract: Recurring transitional field directions during three Miocene geomagnetic reversals provide evidence that lateral inhomogeneity of the lower mantle affects flow in the outer core. We compare new paleomagnetic results from a composite sequence of 15.2 Ma lava flows in north central Nevada (Sheep Creek Range; 40.7N, 243.2E), erupted during a polarity reversal, to published data from Steens Mountain (250 km to the northwest in Oregon) and the Newberry Mountains (650 km to the south in California) that document reversals occurring millions of years and many polarity switches earlier. Alternating field demagnetization, followed by thermal demagnetization in half the samples, clearly isolated the primary thermoremanent magnetization of Sheep Creek Range flows. We correlated results from our three sampled sections to produce a composite record that begins with a single virtual geomagnetic pole (VGP) at low latitude in the Atlantic, followed by two VGPs situated near latitude 30N in NE Africa. After jumping to 83N (one VGP), the pole moves to equatorial South America (one VGP), back to NE Africa (three VGPs), to high southern latitudes (two VGPs), back to equatorial South America (three VGPs), and finally to high northern latitudes (nine VGPs). The repeated visits of the transitional VGP to positions in South America and near NE Africa, as well as the similar behavior recorded at Steens Mountain and the Newberry Mountains, suggest that lower mantle or core-mantle boundary features localize core flow structures, thereby imparting a discernible regional structure on the transitional geomagnetic field that persists for millions of years.
      PubDate: 2017-08-12T10:36:44.741208-05:
      DOI: 10.1002/2017GC007049
  • Influence of static alternating field demagnetization on anisotropy of
           magnetic susceptibility: Experiments and implications
    • Authors: Andrea R. Biedermann; Mike Jackson, Dario Bilardello, Joshua M. Feinberg, Maxwell C. Brown, Suzanne A. McEnroe
      Abstract: Anisotropy of magnetic susceptibility (AMS) indicates the preferred orientation of a rock's constituent minerals. However, other factors can influence the AMS, e.g. domain wall pinning or domain alignment in ferromagnetic minerals. Therefore, it is controversial whether samples should be alternating field (AF) demagnetized prior to AMS characterization. This may remove the influence of natural remanent magnetization (NRM) or domain wall pinning on AMS; however, it may also result in field-induced anisotropy. This study investigates the influence of stepwise AF and low-temperature demagnetization on mean susceptibility, principal susceptibility directions, AMS degree and shape for sedimentary, metamorphic and igneous rocks. Alternating fields up to 200 mT were applied along the sample x, y and z axes, rotating the order for each step, to characterize the relationship between AMS principal directions and the last AF orientation. The changes in anisotropy, defined by the mean deviatoric susceptibility of the difference tensors, are between
      PubDate: 2017-08-12T10:36:40.957782-05:
      DOI: 10.1002/2017GC007073
  • Thermal segmentation of mid-ocean ridge transform faults
    • Authors: Monica Wolfson-Schwehr; Margaret S. Boettcher, Mark D. Behn
      Abstract: 3D finite element simulations are used to calculate thermal structures and mantle flow fields underlying mid-ocean ridge transform faults (RTFs) composed of two fault segments separated by an orthogonal step-over. Using fault lengths and slip rates, we derive an empirical scaling relation for the critical step-over length (LS∼), which marks the transition from predominantly horizontal to predominantly vertical mantle flow at the base of the lithosphere under a step-over. Using the ratio of step-over length (LS) to LS∼, we define 3 degrees of segmentation: first-degree, corresponding to type I step-overs (LS/LS∼ ≥ 3); second-degree, corresponding to type II step-overs (1 ≤ LS/LS∼ 
      PubDate: 2017-08-12T10:36:09.238745-05:
      DOI: 10.1002/2017GC006967
  • Heat flow bounds over the Cascadia margin derived from bottom simulating
           reflectors and implications for thermal Models of subduction
    • Authors: Benjamin J. Phrampus; Robert N. Harris, Anne M. Tréhu
      Abstract: Understanding the thermal structure of the Cascadia subduction zone is important for understanding megathrust earthquake processes and seismogenic potential. Currently our understanding of the thermal structure of Cascadia is limited by a lack of high spatial resolution heat flow data and by poor understanding of thermal processes such as hydrothermal fluid circulation in the subducting basement, sediment thickening and dewatering, and frictional heat generation on the plate boundary. Here, using a dataset of publically available seismic lines combined with new interpretations of bottom simulating reflector (BSR) distributions, we derive heat flow estimates across the Cascadia margin. Thermal models that account for hydrothermal circulation predict BSR-derived heat flow bounds better than purely conductive models, but still over-predict surface heat flows. We show that when the thermal effects of in-situ sedimentation and of sediment thickening and dewatering due to accretion are included, models with hydrothermal circulation become consistent with our BSR-derived heat flow bounds.
      PubDate: 2017-08-12T10:35:47.815267-05:
      DOI: 10.1002/2017GC007077
  • Evolution of submarine eruptive activity during the 2011-2012 El Hierro
           event as documented by hydroacoustic images and remotely operated vehicle
    • Authors: L. Somoza; F.J. González, S.J. Barker, P. Madureira, T. Medialdea, C. de Ignacio, N. Lourenço, R. León, J.T. Vázquez, D. Palomino
      Abstract: Submarine volcanic eruptions are frequent and important events, yet they are rarely observed. Here we relate bathymetric and hydroacoustic images from the 2011-2012 El Hierro eruption with surface observations and deposits imaged and sampled by ROV. As result of the shallow submarine eruption, a new volcano named Tagoro grew from 375 to 89 m depth. The eruption consisted of two main phases of edifice construction intercalated with collapse events. Hydroacoustic images show that the eruptions ranged from explosive to effusive with variable plume types and resulting deposits, even over short time intervals. At the base of the edifice, ROV observations show large accumulations of lava balloons changing in size and type downslope, coinciding with the area where floating lava balloon fallout was observed. Peaks in eruption intensity during explosive phases generated vigorous bubbling at the surface, extensive ash,vesicular lapilli and formed high-density currents, which together with periods of edifice gravitational collapse, produced extensive deep volcaniclastic aprons. Secondary cones developed in the last stages and show evidence for effusive activity with lava ponds and lava flows that cover deposits of stacked lava balloons. Chaotic masses of heterometric boulders around the summit of the principal cone are related to progressive sealing of the vent with decreasing or variable magma supply. Hornitos represent the final eruptive activity with hydrothermal alteration and bacterial mats at the summit. Our study documents the distinct evolution of a submarine volcano and highlights the range of deposit types that may form and be rapidly destroyed in such eruptions.
      PubDate: 2017-07-26T11:42:00.757534-05:
      DOI: 10.1002/2016GC006733
  • Paleothermal structure of the Nankai inner accretionary wedge estimated
           from vitrinite reflectance of cuttings
    • Authors: Rina Fukuchi; Asuka Yamaguchi, Yuzuru Yamamoto, Juichiro Ashi
      Abstract: The paleothermal structure and tectonic evolution of an accretionary prism is basic information for understanding subduction zone seismogenesis. To evaluate the entire paleotemperature profile of the Integrated Ocean Drilling Program (IODP) Site C0002 located in the off-Kumano region of the Nankai Trough and penetrates the inner accretionary wedge down to 3058.5 m below the seafloor (mbsf), we performed a vitrinite reflectance analysis for cuttings and core samples during IODP Expeditions 338 and 348: Nankai Trough Seismogenic Zone Experiment. Although vitrinite reflectance values (Ro) tend to increase with depth, two reversals of these values suggested the existence of thrust fault zones with sufficient displacements to offset the paleothermal structure. The estimated maximum paleotemperatures are 42–70°C at 1200–1300 mbsf, 44–100°C at 1600–2400 mbsf, and 56–115°C at 2600–3000 mbsf, respectively. These temperatures roughly coincide with estimated modern temperatures; however, at a smaller scale, the reconstructed partial paleogeothermal gradient (∼60–150°C/km) recorded at the hanging- and footwall of the presumed thrust fault zone is higher than the modern geothermal gradient (∼30–40°C/km). This high paleogeothermal gradient was possibly obtained prior to subduction, reflecting the large heat flow of the young Philippine Sea Plate.
      PubDate: 2017-07-26T10:16:20.237744-05:
      DOI: 10.1002/2017GC006928
  • Driving magma to the surface: The 2011-2012 El Hierro volcanic eruption
    • Authors: Carmen López; Maria A. Benito-Saz, Joan Martí, Carmen del-Fresno, Laura García-Cañada, Helena Albert, Héctor Lamolda
      Abstract: We reanalyzed the seismic and deformation data corresponding to the pre-eruptive unrest on El Hierro (Canary Islands) in 2011. We considered new information about the internal structure of the island. We updated the seismic catalogue to estimate the full evolution of the released seismic energy and demonstrate the importance of non-located earthquakes. Using seismic data and GPS displacements, we characterized the shear-tensile type of the predominant fracturing and modelled the strain and stress fields for different time periods. This enabled us to identify a prolonged first phase characterized by hydraulic tensile fracturing, which we interpret as being related to the emplacement of new magma below the volcanic edifice on El Hierro. This was followed by post-injection unidirectional migration, probably controlled by the stress field and the distribution of the structural discontinuities. We identified the effects of energetic magmatic pulses occurring a few days before the eruption that induced shear seismicity on pre-existing faults within the volcano and raised the Coulomb stress over the whole crust. We suggest that these magmatic pulses reflect the crossing of the Moho discontinuity, as well as changes in the path geometry of the dyke migration towards the surface. The final phase involved magma ascent through a pre-fractured crust.
      PubDate: 2017-07-26T10:15:53.731871-05:
      DOI: 10.1002/2017GC007023
  • The origin of volatiles in the earth's mantle
    • Authors: Saswata Hier-Majumder; Marc M. Hirschmann
      Abstract: The Earth's deep interior contains significant reservoirs of volatiles such as H, C, and N. Due to the incompatible nature of these volatile species, it has been difficult to reconcile their storage in the residual mantle immediately following crystallization of the terrestrial magma ocean (MO). As the magma ocean freezes, it is commonly assumed, very small amounts of melt is retained in the residual mantle, limiting the trapped volatile concentration in the primordial mantle. In this article, we show that inefficient melt drainage out of the freezing front can retain large amounts of volatiles hosted in the trapped melt in the residual mantle while creating a thick early atmosphere. Using a two-phase flow model, we demonstrate that compaction within the moving freezing front is inefficient over time scales characteristic of magma ocean solidification. We employ a scaling relation between the trapped melt fraction, the rate of compaction, and the rate of freezing in our magma ocean evolution model. For cosmochemically plausible fractions of volatiles delivered during the later stages of accretion, our calculations suggest that up to 77% of total H2O and 12% of CO2 could have been trapped in the mantle during magma ocean crystallization. The assumption of a constant trapped melt fraction underestimates the mass of volatiles in the residual mantle by more than an order of magnitude.
      PubDate: 2017-07-26T10:15:24.870311-05:
      DOI: 10.1002/2017GC006937
  • Micrometer-scale magnetic imaging of geological samples using a quantum
           diamond microscope
    • Authors: D. R. Glenn; R. R. Fu, P. Kehayias, D. Le Sage, E. A. Lima, B. P. Weiss, R. L. Walsworth
      Abstract: Remanent magnetization in geological samples may record the past intensity and direction of planetary magnetic fields. Traditionally, this magnetization is analyzed through measurements of the net magnetic moment of bulk millimeter to centimeter sized samples. However, geological samples are often mineralogically and texturally heterogeneous at submillimeter scales, with only a fraction of the ferromagnetic grains carrying the remanent magnetization of interest. Therefore, characterizing this magnetization in such cases requires a technique capable of imaging magnetic fields at fine spatial scales and with high sensitivity. To address this challenge, we developed a new instrument, based on nitrogen-vacancy centers in diamond, which enables direct imaging of magnetic fields due to both remanent and induced magnetization, as well as optical imaging, of room-temperature geological samples with spatial resolution approaching the optical diffraction limit. We describe the operating principles of this device, which we call the quantum diamond microscope (QDM), and report its optimized image-area-normalized magnetic field sensitivity (20 µT⋅µm/Hz½), spatial resolution (5 µm), and field of view (4 mm), as well as trade-offs between these parameters. We also perform an absolute magnetic field calibration for the device in different modes of operation, including three-axis (vector) and single-axis (projective) magnetic field imaging. Finally, we use the QDM to obtain magnetic images of several terrestrial and meteoritic rock samples, demonstrating its ability to resolve spatially distinct populations of ferromagnetic carriers.
      PubDate: 2017-07-26T10:11:12.790832-05:
      DOI: 10.1002/2017GC006946
  • Uranium isotope evidence for an expansion of marine anoxia during the
           end-Triassic extinction
    • Authors: Adam B. Jost; Aviv Bachan, Bas van de Schootbrugge, Kimberly V. Lau, Karrie L. Weaver, Kate Maher, Jonathan L. Payne
      Abstract: The end-Triassic extinction coincided with an increase in marine black shale deposition and biomarkers for photic zone euxinia, suggesting that anoxia played a role in suppressing marine biodiversity. However, global changes in ocean anoxia are difficult to quantify using proxies for local anoxia. Uranium isotopes (δ238U) in CaCO3 sediments deposited under locally well-oxygenated bottom waters can passively track seawater δ238U, which is sensitive to the global areal extent of seafloor anoxia due to preferential reduction of 238U(VI) relative to 235U(VI) in anoxic marine sediments. We measured δ238U in shallow-marine limestones from two stratigraphic sections in the Lombardy Basin, northern Italy, spanning over 400 m. We observe a ∼0.7‰ negative excursion in δ238U beginning in the lowermost Jurassic, coeval with the onset of the initial negative δ13C excursion and persisting for the duration of subsequent high δ13C values in the lower-middle Hettangian Stage. The δ238U excursion cannot be realistically explained by local mixing of uranium in primary marine carbonate and reduced authigenic uranium. Based on output from a forward model of the uranium cycle, the excursion is consistent with a 40–100-fold increase in the extent of anoxic deposition occurring worldwide. Additionally, relatively constant uranium concentrations point towards increased uranium delivery to the oceans from continental weathering, which is consistent with weathering-induced eutrophication following the rapid increase in pCO2 during emplacement of the Central Atlantic Magmatic Province. The relative timing and duration of the excursion in δ238U implies that anoxia could have delayed biotic recovery well into the Hettangian stage.
      PubDate: 2017-07-26T10:10:23.385348-05:
      DOI: 10.1002/2017GC006941
  • Evidence of a modern deep-water magmatic hydrothermal system in the Canary
           Basin (Eastern Central Atlantic Ocean)
    • Authors: T. Medialdea; L. Somoza, F.J. González, J.T. Vázquez, C. de Ignacio, H. Sumino, O. Sánchez-Guillamón, Y. Orihashi, R. León, D. Palomino
      Abstract: New seismic profiles, bathymetric data and sediment-rock sampling document for the first time the discovery of hydrothermal vent complexes and volcanic cones at 4800-5200 m depth related to recent volcanic and intrusive activity in an unexplored area of the Canary Basin (Eastern Atlantic Ocean, 500 km west of the Canary Islands). A complex of sill intrusions is imaged on seismic profiles showing saucer-shaped, parallel or inclined geometries. Three main types of structures are related to these intrusions. Type I consists of cone-shaped depressions developed above inclined sills interpreted as hydrothermal vents. Type II is the most abundant and is represented by isolated or clustered hydrothermal domes bounded by faults rooted at the tips of saucer-shaped sills. Domes are interpreted as seabed expressions of reservoirs of CH4- and CO2-rich fluids formed by degassing and contact metamorphism of organic-rich sediments around sill intrusions. Type III are hydrothermal-volcanic complexes originated above stratified or branched inclined sills connected by a chimney to the seabed volcanic edifice. Parallel sills sourced from the magmatic chimney formed also domes surrounding the volcanic cones. Core and dredges revealed that these volcanoes, which must be among the deepest in the world, are constituted by OIB-type, basanites with an outer ring of blue-green hydrothermal Al-rich smectite muds. Magmatic activity is dated, based on lava samples, at 0.78±0.05 and 1.61±0.09 Ma (K/Ar methods) and on tephra layers within cores at 25-237 ky. The Subvent hydrothermal-volcanic complex constitutes the first modern system reported in deep-water oceanic basins related to intraplate hotspot activity.
      PubDate: 2017-07-21T11:42:22.156145-05:
      DOI: 10.1002/2017GC006889
  • Temporal variations in air permeability and soil CO2 flux in volcanic ash
           soils (island of Vulcano, Italy)
    • Authors: Marco Camarda; Vincenzo Prano, Santo Cappuzzo, Sergio Gurrieri, Mariano Valenza
      Abstract: Air permeability is a major physical factor affecting the advective transport of a gas through the soil, and variations in this parameter can strongly influence the emission of endogenous gases from the soil to the atmosphere. In this paper we illustrated a new and simple method for measuring in situ air permeability based on the measurement of air pressure inside a special probe inserted into the soil. The method was designed and developed primarily to study the relationship between air permeability and the soil CO2 flux in an active volcanic area. The method was used for continuous monitoring of the air permeability at two different locations on the island of Vulcano. At the same time, the values of the atmospheric pressure, temperature, rain and volumetric water content of the soil were also acquired to investigate their effect on soil air permeability and soil CO2 flux. The results showed that during the monitoring period, soil air permeability exhibited minor variations at each site, while larger variations in the soil CO2 flux were recorded. The effect of soil air permeability on soil CO2 flux was negligible at both sites, whereas a strong dependence of soil CO2 flux on volumetric water content and on atmospheric pressure was found. Furthermore, the variation in air permeability recorded at both sites was much lower than that predicted using some well-known predictive models, showing that the relationship among different soil transport parameters is more complex in real field conditions than would be expected by semi-empirical models.
      PubDate: 2017-07-21T11:21:03.533967-05:
      DOI: 10.1002/2017GC006857
  • A comparison of cooling- and volume-limited flow systems: Examples from
           channels in the Piton de la Fournaise April 2007 lava flow field
    • Authors: Maéva Rhéty; Andrew Harris, Nicolas Villeneuve, Lucia Gurioli, Etienne Médard, Oryaëlle Chevrel, Patrick Bachélery
      Abstract: Channel-fed lava flow systems lack detailed thermal and textural studies through the boundary between channelized and dispersed flow, and out to the flow front. Here, chemical, textural and morphological analyses were made to define cooling and crystallisation rates down the entire system, especially through the zone of dispersed flow. We compare two channel systems active during the 2007 eruption of Piton de la Fournaise, one of which was cooling-limited and one of which as volume-limited. In the cooling-limited case, rapid changes in rheology occurred across the zone of dispersed flow, where viscosity increased from 1000 to 1600 Pa s over the last 100 m of the channel system. This was due to an increase in cooling rate from 7°C km−1 over the first 500 m of the system, to 42°C km−1 over the last 100 m, and an increase in microcryst content from 13% to 25%. In the volume-limited case, the exponentially increasing segment of the down flow cooling and viscosity trend is absent. Instead, lava arriving at the flow front is still relatively hot (1161°C compared with a near-vent temperature of 1167°C), and is thus of relatively low viscosity (1125 Pa s). In the volume-limited case, because the channel was still in extension when supply to the system was cut, the zone of dispersed flow was extremely short. However, because lava behind the stalled flow front was still hot and fluid, break-outs from the static front resulted in a complex flow front morphology.
      PubDate: 2017-07-21T11:20:48.713077-05:
      DOI: 10.1002/2017GC006839
  • Link between Indian monsoon rainfall and physical erosion in the Himalayan
           system during the Holocene
    • Authors: Ronan Joussain; Zhifei Liu, Christophe Colin, Stéphanie Duchamp-Alphonse, Zhaojie Yu, Eva Moréno, Léa Fournier, Sébastien Zaragosi, Arnaud Dapoigny, Laure Meynadier, Franck Bassinot
      Abstract: Mineralogical and geochemical analyses conducted on cores located on the active channel-levee system of the northern Bengal Fan are used to establish changes in the weathering pattern and the sediment transport of the Himalayan system, and evaluate the effect of Indian summer monsoon rainfall during the Holocene. Our data indicate that during the Holocene, sediments from the northern Bengal Fan originate mainly from the G-B river system without any significant changes in the relative contribution of these rivers. From 9.8 to around 6 ka, relatively low smectite/(illite+chlorite) ratios and relatively high K/Si* ratios indicate high physical denudation rates of the Himalayan highlands together with a rapid transfer of the detrital material to the Bengal Fan. The period between 9.2 and 7 ka is associated to lower values of K/Si* and corresponds to the maximum of Indian monsoon rainfall which indicates a more important chemical weathering material that rapidly transits by the G-B river system without a long storage in the Indo-Gangetic plain. From 6.0 ka to present day, higher smectite/(illite+chlorite) ratio and lower K/Si* ratio document a gradual increase of sediments originated from the Indo-Gangetic plain, characterized by higher degree of chemical weathering. During the last 2.5 ka, the drastic increase in the smectite/(illite+chlorite) ratio could be associated to enhanced alteration of the plain soils due to anthropogenic activity. The comparison of mineralogical and geochemical data with previous reconstructions of the Indian monsoon dynamic indicates a rapid response of erosion and sediment transfer of the G-B river system to changes of monsoon rainfall intensity.
      PubDate: 2017-07-21T11:20:33.915601-05:
      DOI: 10.1002/2016GC006762
  • Constraints on the anisotropic contributions to velocity discontinuities
           at ∼60 km depth beneath the Pacific
    • Authors: Catherine A. Rychert; Nicholas Harmon
      Abstract: Strong, sharp, negative seismic discontinuities, velocity decreases with depth, are observed beneath the Pacific seafloor at ∼60 km depth. It has been suggested that these are caused by an increase in radial anisotropy with depth, which occurs in global surface wave models. Here we test this hypothesis in two ways. We evaluate whether an increase in surface wave radial anisotropy with depth is robust with synthetic resolution tests. We do this by fitting an example surface wave dataset near the East Pacific Rise. We also estimate the apparent isotropic seismic velocity discontinuities that could be caused by changes in radial anisotropy in S-to-P and P-to-S receiver functions and SS precursors using synthetic seismograms. We test one model where radial anisotropy is caused by olivine alignment and one model where it is caused by compositional layering. The result of our surface wave inversion suggests strong shallow azimuthal anisotropy beneath 0-10 Ma seafloor, which would also have a radial anisotropy signature. An increase in radial anisotropy with depth at 60 km depth is not well-resolved in surface wave models, and could be artificially observed. Shallow isotropy underlain by strong radial anisotropy could explain moderate apparent velocity drops (< 6%) in SS precursor imaging, but not receiver functions. The effect is diminished if strong anisotropy also exists at 0 – 60 km depth as suggested by surface waves. Overall, an increase in radial anisotropy with depth may not exist at 60 km beneath the oceans and does not explain the scattered wave observations.
      PubDate: 2017-07-21T04:49:49.228612-05:
      DOI: 10.1002/2017GC006850
  • Production, consumption, and migration of methane in accretionary prism of
           southwestern Taiwan
    • Authors: Nai-Chen Chen; Tsanyao Frank Yang, Wei-Li Hong, Hsuan-Wen Chen, Hsiao-Chi Chen, Chin-Yi Hu, Yu-Chun Huang, Saulwood Lin, Li-Hung Lin, Chih-Chieh Su, Wei-Zhi Liao, Chih-Hsien Sun, Pei-Ling Wang, Tao Yang, Shao-yong Jiang, Char-Shine Liu, Yunshuen Wang, San-Hsiung Chung
      Abstract: To systematically quantify the production, consumption and migration of methane along a continental margin, 210 sediment cores were collected from offshore southwestern Taiwan and analyzed for their gas and aqueous geochemistry. These data, combined with published results, were used to calculate the diffusive methane fluxes across different geochemical transitions and to develop scenarios of mass balance to constrain deep microbial and thermogenic methane production rates within the accretionary prism. The results showed that methane diffusive fluxes ranged from 2.71 × 10−3 to 2.78 × 10−1 and from ‒1.88 × 10−1 to 3.97 mmol m−2 d−1 at the sulfate-methane-transition-zone (SMTZ) and sediment-seawater interfaces, respectively. High methane fluxes tend to be associated with structural features, suggesting a strong structural control on the methane transport. A significant portion of ascending methane (>50%) is consumed by anaerobic oxidation of methane at the SMTZ at most sites, indicating effective biological filtration. Gas compositions and isotopes revealed a transition from the predominance of microbial methane in the passive margin to thermogenic methane at the upper slope of the active margin and onshore mud volcanoes. Methane production and consumption at shallow depths were nearly offset with a small fraction of residual methane discharged into seawater. The flux imbalance arose primarily due to the larger production of methane through deep microbial and thermogenic processes at a magnitude of 1,512 – 43,096 Tg Myr−1 methane and could be likely accounted for by the sequestration of methane into hydrate forms, and clay absorption.
      PubDate: 2017-07-15T03:24:03.733573-05:
      DOI: 10.1002/2017GC006798
  • Modeling phase separation and phase change for magma ocean solidification
    • Authors: C.-E. Boukaré; Y. Ricard
      Abstract: Just after accretion, the Earth's mantle was significantly molten by the heat dissipation due to large impacts and to the segregation of the core. The mineralogical observations and thermodynamics models of solid-liquid equilibrium of silicates show that several types of crystallization may have happened at different depths in the mantle. Solids were probably formed first at the bottom of the lower mantle or at mid mantle leaving two possible magma oceans, a shallow one and an abyssal one. Near the bottom of the mantle, the liquid phase might become denser than solids due to iron enrichment. In the shallow magma ocean, the crystallizing solid phase was denser and sank through the magma to settle and compact at depth. To understand these complex dynamics, we develop a two phase numerical code that can handle simultaneously convection in each phase and in the slurry, and the compaction or decompaction of the two phases. Although our code can only run in a parameter range (Rayleigh number, viscosity contrast between phases, Prandlt number) far from what would be realistic, we think it already provides a rich dynamics that illustrates what could have happened. We show situations in which the crystallization front is gravitationally stable and situations were the newly formed solids are gravitationally unstable and can snow across the magma. Our study suggests that the location of a density contrast between solid and magma must be considered of equal importance with that of the intersection between liquidus and isentrope for what concerns mantle solidification.
      PubDate: 2017-07-15T03:23:53.619128-05:
      DOI: 10.1002/2017GC006902
  • How plume-ridge interaction shapes the crustal thickness pattern of the
           Réunion hotspot track
    • Authors: Eva Bredow; Bernhard Steinberger, Rene Gassmöller, Juliane Dannberg
      Abstract: The Réunion mantle plume has shaped a large area of the Earth's surface over the past 65 million years: from the Deccan Traps in India along the hotspot track comprising the island chains of the Laccadives, Maldives and Chagos Bank on the Indian plate and the Mascarene Plateau on the African plate up to the currently active volcanism at La Réunion Island. This study addresses the question how the Réunion plume, especially in interaction with the Central Indian Ridge, created the complex crustal thickness pattern of the hotspot track. For this purpose, the mantle convection code ASPECT was used to design three-dimensional numerical models, which consider the specific location of the plume underneath moving plates and surrounded by large-scale mantle flow. The results show the crustal thickness pattern produced by the plume, which altogether agrees well with topographic maps. Especially two features are consistently reproduced by the models: the distinctive gap in the hotspot track between the Maldives and Chagos is created by the combination of the ridge geometry and plume-ridge interaction; and the Rodrigues Ridge, a narrow crustal structure which connects the hotspot track and the Central Indian Ridge, appears as the surface expression of a long-distance sub-lithospheric flow channel. This study therefore provides further insight how small-scale surface features are generated by the complex interplay between mantle and lithospheric processes.
      PubDate: 2017-07-15T03:23:22.358627-05:
      DOI: 10.1002/2017GC006875
  • Forearc seafloor unconformities and geology: Insight from 3D seismic
           geomorphology analysis, Peru
    • Authors: Gérôme Calvès; Constance Auguy, Léopold de Lavaissière, Stéphane Brusset, Ysabel Calderon, Patrice Baby
      Abstract: New 3D seismic data collected over 4870 km2 in the 3°45'–12°30'S Peruvian segment of the East Pacific subduction system image seafloor erosional surfaces that can be mapped across the forearc basins. Forearc basins experience various stresses, from their base where basal tectonic erosion acts to the seafloor which is influenced by aerial, shallow and deep water currents driven by waves or thermohaline oceanic currents. Previously there has been little interest in stresses on the upper layer and there is a lack of documentation of unconformities and the erosive processes in certain bathymetric domains in forearc basins. We address this with the study of examples sourced from 3D seismic reflection surveys of the seafloor offshore Peru. Unconformities occur in two distinctive bathymetric domains associated with the continental shelf and the upper slope of the margin. Identification and characterization of unconformity surfaces yield estimates of the amount of erosion at the modern seafloor that range from 18 to 100%. Regional physical oceanography allows us to calibrate potential candidates for these two distinctive domains. The first control on erosion is the dynamics of deep to intermediate oceanic currents related to the Humboldt-Peru Chile water masses, while the second is wave action in the shallower erosional surfaces. This study illustrates the unseen landscape of the forearc basins of South America and helps to highlight the importance of erosive surficial processes in subduction landscapes.
      PubDate: 2017-07-15T03:22:13.043249-05:
      DOI: 10.1002/2017GC007036
  • Spreading rate-dependent variations in crystallization along the global
           mid-ocean ridge system
    • Authors: V.D. Wanless; Mark D. Behn
      Abstract: We investigate crustal accretion at mid-ocean ridges by combining crystallization pressures calculated from major element contents in mid-ocean ridge basalt (MORB) glasses and vapor-saturation pressures from melt inclusions and MORB glasses. Specifically, we use established major element barometers and pressures estimated from 192 fractional crystallization trends to calculate crystallization pressures from>9,000 MORB glasses across the global range of mid-ocean ridge spreading rates. Additionally, we estimate vapor-saturation pressures from>400 MORB glasses from PETDB and>400 olivine-hosted melt inclusions compiled from five ridges with variable spreading rates. Both major element and vapor-saturation pressures increase and become more variable with decreasing spreading rate. Vapor saturation pressures indicate that crystallization occurs in the lower crust and upper mantle at all ridges, even when a melt lens is present. We suggest that the broad peaks in major element crystallization pressures at all spreading rates reflects significant crystallization of on- and off-axis magmas along the base of a sloping lithosphere. Combining our observations with ridge thermal models we show that crystallization occurs over a range of pressures at all ridges, but it is enhanced at thermal/rheologic boundaries, such as the melt lens and the base of the lithosphere. Finally, we suggest that the remarkable similarity in the maximum vapor-saturation pressures (∼3 kbars) recorded in melt inclusions from a wide range of spreading rates reflects a relatively uniform CO2 content of 50–85 ppm for the depleted upper mantle feeding the global mid-ocean ridge system.
      PubDate: 2017-07-15T03:15:44.307403-05:
      DOI: 10.1002/2017GC006924
  • Distinct control mechanism of fine-grained sediments from Yellow River and
           Kyushu supply in the northern Okinawa Trough since the last glacial
    • Authors: Debo Zhao; Shiming Wan, Samuel Toucanne, Peter D. Clift, Ryuji Tada, Sidonie Révillon, Yoshimi Kubota, Xufeng Zheng, Zhaojie Yu, Jie Huang, Hanchao Jiang, Zhaokai Xu, Xuefa Shi, Anchun Li
      Abstract: High-resolution multi-proxy records, including clay minerals and Sr-Nd-Pb isotopes of the clay-sized silicate fraction of sediments from IODP Site U1429 in the northern Okinawa Trough, provide reliable evidence for distinct control mechanism on fine-grained sediments input from the Yellow River and the southern Japanese Islands to the northern Okinawa Trough since 34 ka BP. Provenance analysis indicates that the sediments were mainly derived from the Yellow River and the island of Kyushu. Since the last glacial, clay-sized sediments transported from the Yellow River to the study site were strongly influenced by sea-level fluctuation. During low sea-level stage (∼34‒14 ka BP), the paleo-Yellow River mouth was positioned closer to the northern Okinawa Trough, favoring large fluvial discharge or even direct input of detrital sediments, which resulted about four times more flux of clay-sized sediments supply to the study area as during the relatively high sea-level stage (∼14‒0 ka BP). The input of Kyushu-derived clay-sized sediments to the study site was mainly controlled by the Kuroshio Current and Tsushima Warm Current intensity, with increased input in phase with weakened Kuroshio Current/Tsushima Warm Current. Our study suggests that the Kuroshio Current was very likely flowed into the Okinawa Trough and thus influenced the fine-grained sediment transport in the area throughout the last glacial and deglacial. During ∼34‒11 ka BP, the Kyushu clay-sized sediment input was mainly controlled by the Kuroshio Current. Since ∼11 ka BP, the occurrence of Tsushima Warm Current became important in influencing the Kyushu fine-grained sediment input to the northern Okinawa Trough.
      PubDate: 2017-07-15T03:15:37.975514-05:
      DOI: 10.1002/2016GC006764
  • High-resolution chronology of sediment below CCD based on Holocene
           paleomagnetic secular variations in the Tohoku-oki earthquake rupture zone
    • Authors: Toshiya Kanamatsu; Kazuko Usami, Cecilia M. G. McHugh, Ken Ikehara
      Abstract: Using high-resolution paleomagnetic data, we examined the potential for obtaining precise ages from sediment core samples recovered from deep-sea basins close to rupture zones of the 2011 and earlier earthquakes off Tohoku, Japan. Obtaining detailed stratigraphic ages from deep-sea sediments below the calcium compensation depth (CCD) is difficult, but we found that the samples contain excellent paleomagnetic secular variation records to constrain age models. Variations in paleomagnetic directions obtained from the sediments reveal systematic changes in the cores. A stacked paleomagnetic profile closely matches the Lake Biwa data sets in southwest Japan for the past 7,000 years, one can establish age models based on secular variations of the geomagnetic field on sediments recovered uniquely below the CCD. Comparison of paleomagnetic directions near a tephra and a paleomagnetic direction of contemporaneous pyroclastic flow deposits acquired by different magnetization processes shows precise depositional ages reflecting the magnetization delay of the marine sediment record.
      PubDate: 2017-07-06T01:00:57.599035-05:
      DOI: 10.1002/2017GC006878
  • Unmixing detrital geochronology age distributions
    • Authors: Kurt Sundell; Joel E. Saylor
      Abstract: Despite recent advances in quantitative methods of detrital provenance analysis, there is currently no widely accepted method of unmixing detrital geochronology data. We developed a mixing model that determines mixing proportions for source samples through inverse Monte Carlo modeling, wherein mixed samples are compared to randomly generated combinations of source distributions, and a range of best mixing proportions are retained. Results may then be used to constrain a forward optimization routine to find a single best-fit mixture. Quantitative comparison is based on the Kolmogorov-Smirnov (KS) test D statistic and Kuiper test V statistic for cumulative distribution functions, and the Cross-correlation coefficient for finite mixture distributions (probability density plots or kernel density estimates). We demonstrate the capacity of this model through a series of tests on synthetic data sets and a published empirical data set from North America mixed in known proportions; this proof-of-concept testing shows the model is capable of accurately unmixing highly complex distributions. We apply the model to two published empirical data sets mixed in unknown proportions from Colombia and central China. Neither data set yields perfect model fits, which provides a cautionary note of potentially inadequate characterization of source and/or mixed samples, and highlights the importance of such characterization for accurate interpretation of sediment provenance. Data set size appears to be a major control on mixture model results; small (n 
      PubDate: 2017-07-06T00:56:06.871737-05:
      DOI: 10.1002/2016GC006774
  • The mechanics of shallow magma reservoir outgassing
    • Authors: A. Parmigiani; W Degruyter, S. Leclaire, C. Huber, O. Bachmann
      Abstract: Magma degassing fundamentally controls the Earth's volatile cycles. The large amount of gas expelled into the atmosphere during volcanic eruptions (i.e. volcanic outgassing) is the most obvious display of magmatic volatile release. However, owing to the large intrusive:extrusive ratio, and considering the paucity of volatiles left in intrusive rocks after final solidification, volcanic outgassing likely constitutes only a small fraction of the overall mass of magmatic volatiles released to the Earth's surface. Therefore, as most magmas stall on their way to the surface, outgassing of uneruptible, crystal-rich magma storage regions will play a dominant role in closing the balance of volatile element cycling between the mantle and the surface. We use a numerical approach to study the migration of a magmatic volatile phase (MVP) in crystal-rich magma bodies (“mush zones”) at the pore-scale. Our results suggest that buoyancy driven outgassing is efficient over crystal volume fractions between 0.4 and 0.7 (for mm-sized crystals). We parameterize our pore-scale results for MVP migration in a thermo-mechanical magma reservoir model to study outgassing under dynamical conditions where cooling controls the evolution of the proportion of crystal, gas and melt phases and to investigate the role of the reservoir size and the temperature-dependent visco-elastic response of the crust on outgassing efficiency. We find that buoyancy-driven outgassing allows for a maximum of 40-50% volatiles to leave the reservoir over the 0.4-0.7 crystal volume fractions, implying that a significant amount of outgassing must occur at high crystal content (>0.7) through veining and/or capillary fracturing.
      PubDate: 2017-07-05T12:45:54.9002-05:00
      DOI: 10.1002/2017GC006912
  • Downstream and seasonal changes of lithium isotope ratios in the
           Ganges-Brahmaputra river system
    • Authors: Takuya Manaka; Daisuke Araoka, Toshihiro Yoshimura, H. M. Zakir Hossain, Yoshiro Nishio, Atsushi Suzuki, Hodaka Kawahata
      Abstract: The Li isotope ratio (δ7Li) is expected to be a useful tracer of silicate weathering in river and groundwater systems, which is an important contributor to the seawater compositional changes that accompany the evolution of the Earth's surface environment. To obtain accurate estimates of continental Li fluxes to the ocean, we determined δ7Li values of dissolved Li in the lower Ganges–Brahmaputra river system in both the dry and rainy seasons, and in deep groundwater in the Bengal basin. Dissolved Li and δ7Li values in the lower reaches of the rivers (0.04–0.66 µmol kg−1 and +19.1‰ to +34.2‰, respectively) were predominantly derived from silicate weathering, as is the case in the upper parts of these rivers. We observed large changes in δ7Li over a distance of more than 1000 km downstream that were due mainly to Rayleigh-type removal of Li from river water. Extremely high Li concentrations (1.15–1.67 µmol kg−1) and low δ7Li values (+5.1‰ to +11.6‰) in groundwater samples indicate congruent isotope leaching and dissolution of silicate minerals in the deep aquifer, where the water residence time is long. In the rainy season, Li concentrations and δ7Li values were lower than in the dry season, owing to the shorter residence time of river water and the substantial input of local subsurface flow through lowland alluvium. These results suggest that accurate estimation of continental Li fluxes to the ocean should take account of downstream and seasonal changes, as well as aquifer depth variations, in δ7Li values.
      PubDate: 2017-07-05T12:45:25.256062-05:
      DOI: 10.1002/2016GC006738
  • SedCT: MATLABTM tools for standardized and quantitative processing of
           sediment core computed tomography (CT) data collected using a medical CT
    • Authors: B. T. Reilly; J. S. Stoner, J. Wiest
      Abstract: Computed Tomography (CT) of sediment cores allows for high resolution images, three dimensional volumes, and down core profiles. These quantitative data are generated through the attenuation of X-rays, which are sensitive to sediment density and atomic number, and are stored in pixels as relative grayscale values or Hounsfield units (HU). We present a suite of MATLABTM tools specifically designed for routine sediment core analysis as a means to standardize and better quantify the products of CT data collected on medical CT scanners. SedCT uses a graphical interface to process Digital Imaging and Communications in Medicine (DICOM) files, stitch overlapping scanned intervals, and create down core HU profiles in a manner robust to normal coring imperfections. Utilizing a random sampling technique, SedCT reduces data size and allows for quick processing on typical laptop computers. SedCTimage uses a graphical interface to create quality tiff files of CT slices that are scaled to a user defined HU range, preserving the quantitative nature of CT images and easily allowing for comparison between sediment cores with different HU means and variance. These tools are presented along with examples from lacustrine and marine sediment cores to highlight the robustness and quantitative nature of this method.
      PubDate: 2017-07-05T12:42:31.646633-05:
      DOI: 10.1002/2017GC006884
  • Mantle dynamics beneath the discrete and diffuse plate boundaries of the
    • Authors: Joseph S. Byrnes; Douglas R. Toomey, Emilie E. E. Hooft, John Nábělek, Jochen Braunmiller
      Abstract: We use the delay times of teleseismic S phases recorded by ocean bottom seismometers during the plate-scale Cascadia Initiative community experiment to constrain the heterogeneity of seismic velocity structure beneath young oceanic lithosphere. Our study area covers the entire Juan de Fuca (JdF) and Gorda plates, from their creation at the JdF and Gorda Ridges to their subduction beneath the North American continent, and the entire length of the Blanco transform fault. The range of the observed Vs anomalies requires variations in the melt fraction of the asthenosphere. The data require that low Vs anomalies extend to depths of at least 200 km, which is within the carbonatite melting regime. In the upper 200 km of the mantle, Vs increases rapidly to the east of the JdF Ridge, while there is no clear relationship with the age of the lithosphere in the Gorda region. The distribution of melt is asymmetric about both the JdF and Gorda Ridges. Dynamic upwelling – due to the buoyancy of the mantle – and accompanying downwelling can explain the rapid decrease in melt fraction to the east of the JdF Ridge, the asymmetry about the JdF Ridge, and the sinuous pattern of upwelling near the Blanco transform fault. Finally, mantle flow beneath the diffuse Gorda and Explorer plate boundaries is distinct from that beneath the discrete plate boundary of the JdF Ridge. In particular, shear between the Pacific and JdF plates appears to dominate mantle deformation over seafloor spreading beneath the Gorda Ridge.
      PubDate: 2017-07-01T02:46:50.604905-05:
      DOI: 10.1002/2017GC006980
  • The global systematics of primitive arc melts
    • Authors: M.W. Schmidt; O. Jagoutz
      Abstract: We extracted all volcanic arc rock analyses calculated to be in equilibrium with mantle olivine from the global georoc database. This results in 938 primitive melt compositions from 30 arcs. Based on geochemical criteria six principal types of primitive arc melts can be distinguished: calc-alkaline basalts and andesites, tholeiitic basalts, highly depleted tholeiitic andesites, shoshonites and low-Si basalts. Their major element systematics indicates that last mantle equilibration occurred mostly at 1.0-2.5 GPa, 1220-1350°C for tholeiitic and calc-alkaline basalts, at 0.5-1.2 GPa and ∼1200°C for depleted tholeiitic andesites, and at 0.7-1.2 GPa, 1050-1150°C for calc-alkaline andesites. Quantitative treatment of major and trace elements suggests that the different melt types can be explained by a combination of variable mantle wedge preconditioning (degree of depletion prior to slab component addition, metasomatism in the lithosphere), variation in the amount and nature of the slab component added, and - for primitive calc-alkaline andesites - reactive fractionation in the lithospheric top of the mantle wedge.The different slab components are best characterized by high Na2O, TiO2, Zr and Th for slab melts; high K2O/Na2O and more pronounced Nb, Sr, and Pb anomalies for fluids; and high K2O at high K2O/Na2O for supercritical liquids. A slab component that is dominantly a slab melt is common in continental but rare in intra-oceanic arcs, consistent with comparatively cooler slabs in intra-oceanic subduction zones. A majority of the arcs has more than one melt type, testifying for heterogeneity in the mantle wedge and added slab component.
      PubDate: 2017-06-15T18:00:38.593099-05:
      DOI: 10.1002/2016GC006699
  • Flexural isostasy of the carbonate platform in North central Florida
    • Authors: H. B. Woo; M. Panning, Peter N. Adams, A. Dutton
      Abstract: Deformed marine terraces can be used to explore a region's uplift history. Trail Ridge is a marine terrace in north Florida that is nearly 80 meters above modern sea level and contains Quaternary marine fossils, a fact that is inconsistent with estimates of paleo-sea level history since the early Pleistocene. This implies that the terrace has experienced uplift since its formation, as well as non-uniform deformation recorded by the warping of its previously horizontal state. The Florida carbonate platform, located on the passive margin of eastern North America, is a setting where non-tectonic influences (e.g. isostatic adjustment, dynamic topography) can be examined. We present a single-transect, numerical model of vertical displacement, derived from elastic flexure, to assess the influence of karst-driven isostatic uplift on present day topography of Trail Ridge in north Florida. Flexural modeling predicts elevations in central Florida not observed today, most likely because surface erosion and karst cavity collapse have obliterated this high topography. Older subsurface stratigraphic units, however, display the arched profile predicted from flexural modeling. Mass loss, calculated by differencing modeled topography and observed topography, was found to be 6.75 × 1012 kg, since emplacement of Trail Ridge. Uplift rates, assuming karst-driven flexural isostasy alone, using previously estimated ages of Trail Ridge of 0.125, 1.4, 3, or 3.5 Ma were found to be 0.535, 0.048, 0.022, and 0.019 mm/yr, respectively. A more likely explanation of uplift includes contributions from dynamic topography and glacial isostatic adjustment which should be further explored with more advanced geophysical modeling.
      PubDate: 2017-06-15T17:20:39.694771-05:
      DOI: 10.1002/2017GC006934
  • The Gondou hydrothermal field in the Ryukyu Arc: A huge hydrothermal
           system on the flank of a caldera volcano
    • Authors: H. Minami; Y. Ohara
      Abstract: High-resolution geophysical mapping was conducted from an autonomous underwater vehicle on the flank of Daisan-Kume Knoll in the Ryukyu Arc, southwest of Japan. 1-m resolution bathymetry identified 264 spires, 173 large mounds and 268 small mounds within a depression that is up to 1600 m wide and up to 60 m deep, at water depths between 1330 and 1470 m. Hydrothermal venting is strongly inferred from the observation of plumes in sidescan sonar imagery and positive temperature anomalies over the spires and mounds. This field, named the Gondou Field, has a giant mound G1 with a diameter of 280 m and a height of 80 m. Mound G1 has distinctive summit ridges comprised of multiple spires where acoustic plumes with temperature anomalies up to 1.12°C are observed, indicative of high-temperature venting. Other than mound G1, a number of active large mounds more than 30 m wide and spires over 10-22 m tall are common and they concentrate in the central and southern areas of the field, suggesting that these areas are the center of present hydrothermal activity. Acoustic plumes imaged by side-scan sonar at the Gondou Field are different in character from bubble plumes imaged in other hydrothermal fields in the Ryukyu Arc. The plumes are diffused and deflected as they rise through the water column and have a shape consistent with black smokers.
      PubDate: 2017-05-30T11:36:43.696855-05:
      DOI: 10.1002/2017GC006868
  • Issue Information
    • Pages: 2815 - 2816
      PubDate: 2017-09-20T04:30:44.459787-05:
      DOI: 10.1002/ggge.21124
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