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

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Journal Cover Geochemistry, Geophysics, Geosystems
  [SJR: 2.56]   [H-I: 69]   [27 followers]  Follow
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   ISSN (Online) 1525-2027
   Published by American Geophysical Union (AGU) Homepage  [17 journals]
  • Heat flow, strong near‐fault seismic waves, and near‐fault
           tectonics on the central San Andreas fault
    • Authors: Norman H. Sleep
      Abstract: The main San Andreas Fault strikes subparallel to compressional folds and thrust faults. Its fault normal traction is on average a factor of γ=1+2μthr(1+μthr2+μthr), where µthris the coefficient of friction for thrust faults, times the effective lithostatic pressure. A useful upper limit for µthr of 0.6 (where γ is 3.12) is obtained from the lack of heat flow anomalies by considering off‐fault convergence at a rate of 1 mm/yr for 10 km across strike. If the fault‐normal traction is in fact this high, the well‐known heat‐flow constraint of average stresses of 10‐20 MPa during strike‐slip on the main fault becomes more severe. Only a few percent of the total slip during earthquakes can occur at the peak stress before dynamic mechanisms weaken the fault. The spatial dimension of the high‐stress rupture‐tip zone is ∼10 m for γ = 3.12 and, for comparison, ∼100 m for γ = 1. High dynamic stresses during shaking occur within these distances of the fault plane. In terms of scalars, fine‐scale tectonic stresses cannot exceed the difference between failure stress and dynamic stress. Plate scale slip causes stresses to build up near geometrical irregularities of the fault plane. Strong dynamic stresses near the rupture tip facilitate anelastic deformation with the net effects of relaxing the local deviatoric tectonic stress and accommodating deformation around the irregularities. There also is a mild tendency for near‐fault material to extrude upward. Slip on minor thrust faults causes the normal traction on the main fault to be spatially variable. This article is protected by copyright. All rights reserved.
      PubDate: 2016-04-27T19:00:32.937246-05:
      DOI: 10.1002/2016GC006280
  • PmagPy: Software package for paleomagnetic data analysis and a bridge to
           the Magnetics Information Consortium (MagIC) Database
    • Abstract: The Magnetics Information Consortium (MagIC) database provides an archive with a flexible data model for paleomagnetic and rock magnetic data. The PmagPy software package is a cross‐platform and open‐source set of tools written in Python for the analysis of paleomagnetic data that serves as one interface to MagIC, accommodating various levels of user expertise. PmagPy facilitates thorough documentation of sampling, measurements, datasets, visualization, and interpretation of paleomagnetic and rock magnetic experimental data. Although not the only route into the MagIC database, PmagPy makes preparation of newly published datasets for contribution to MagIC as a byproduct of normal data analysis and allows manipulation as well as reanalysis of datasets downloaded from MagIC with a single software package. The graphical user interface (GUI), Pmag GUI enables use of much of PmagPy's functionality, but the full capabilities of PmagPy extend well beyond that. Over 400 programs and functions can be called from the command line interface mode, or from within the interactive Jupyter notebooks. Use of PmagPy within a notebook allows for documentation of the workflow from the laboratory to the production of each published figure or data table, making research results fully reproducible. The PmagPy design and its development using GitHub accommodates extensions to its capabilities through development of new tools by the user community. Here we describe the PmagPy software package and illustrate the power of data discovery and re‐use through a re‐analysis of published paleointensity data which illustrates how the effectiveness of selection criteria can be tested. This article is protected by copyright. All rights reserved.
      PubDate: 2016-04-27T18:55:34.525654-05:
      DOI: 10.1002/2016GC006307
  • Mantle control of the geodynamo: Consequences of top‐down regulation
    • Authors: Peter Olson
      Abstract: The mantle global circulation, including deep subduction and lower mantle superplumes, exerts first‐order controls on the evolution of the core, the history of the geodynamo, and the structure of the geomagnetic field. Mantle global circulation models that include realistic plate motions, deep subduction, and compositional heterogeneity similar to the observed large low seismic velocity provinces in the lower mantle predict that the present‐day global average heat flux at the core‐mantle boundary (CMB) exceeds 85 mW m−2. This is sufficient to drive the present‐day geodynamo by thermochemical convection and implies a very young inner core, with inner core nucleation between 400 and 1000 Ma. The mantle global circulation also generates spatially heterogeneous heat flux at the CMB, with peak‐to‐peak lateral variations exceeding 100 mW m−2. Such extreme lateral variability in CMB heat flux, in conjunction with the high thermal conductivity of the core, implies that the liquid outer core is thermally unstable beneath the high seismic velocity regions in the lower mantle but thermally stable beneath the large low seismic velocity provinces. Numerical dynamo simulations show how this pattern of heterogeneous boundary heat flux affects flow in the outer core, producing localized circulation patterns beneath the CMB tied to the mantle heterogeneity and long‐lived deviations from axial symmetry in the geomagnetic field. This article is protected by copyright. All rights reserved.
      PubDate: 2016-04-25T09:52:07.798165-05:
      DOI: 10.1002/2016GC006334
  • Fractionation of Cu and Mo isotopes caused by vapor‐liquid
           partitioning, evidence from the Dahutang W‐Cu‐Mo deposit
    • Authors: Junming Yao; Ryan Mathur, Weidong Sun, Weile Song, Huayong Chen, Laurence Mutti, Xinkui Xiang, Xiaohong Luo
      Abstract: The study presents δ65Cu and δ97Mo isotope values from cogenetic chalcopyrite and molybdenite found in veins and breccias of the Dahutang W‐Cu‐Mo ore field in China. The samples span a 3 to 4 Km range. Both isotopes show a significant degree of fractionation. Cu isotope values in the chalcopyrite range from ‐0.31‰ to +1.48‰, and Mo isotope values in the molybdenite range from ‐0.03‰ to +1.06‰. For the cogenetic sulfide veined samples, a negative slope relationship exists between δ65Cu and δ97Mo values, which suggest a similar fluid history. Rayleigh distillation models the vein samples' change in isotope values. The breccia samples do not fall on the trend, thus indicating a different source mineralization event. Measured fluid inclusion and δD and δ18O data from cogenetic quartz indicate changes in temperature, and mixing of fluids do not appear to cause the isotopic shifts measure. Related equilibrium processes associated with the partitioning of metal between the vapor‐fluid in the hydrothermal system could be the probable cause for the relationship seen between the two isotope systems. This article is protected by copyright. All rights reserved.
      PubDate: 2016-04-22T08:46:03.292767-05:
      DOI: 10.1002/2016GC006328
  • Inference of the viscosity structure and mantle conditions beneath the
           Central Nevada Seismic Belt from combined postseismic and lake unloading
    • Authors: Haylee Dickinson; Andrew M. Freed, Christopher Andronicos
      Abstract: We test whether a single depth‐dependent Newtonian viscosity structure can be found to explain measured surface deformation in Western Nevada from two separate loading events: tectonic loading from a series of 7 historic earthquakes in the Central Nevada Seismic Belt and non‐tectonic loading from the formation and evaporation of co‐located Pleistocene‐aged Lake Lahontan. Rheologic studies are generally plagued with non‐uniqueness issues due to the limitations of observational constraints. Here, we reduce non‐uniqueness by solving for a single rheologic structure that can simultaneously satisfy all observational constraints associated with all events. Model results suggest that Western Nevada is underlain by a strong lower crust (order 1020 Pa s), a relatively weak mantle (order 5 x 1018 Pa s) from 40 to 80 km, and a much weaker mantle (order 1018 Pa s) below 80 km. We would thus place the mechanical lithosphere/asthenosphere boundary (LAB) at 40 km depth. Thermal modeling of conductive geothermal gradients, combined with melting curves calculated for enriched and depleted mantle compositions suggest that the viscosity decrease at 40 km depth (the LAB) is associated with the onset of wet melting of mantle lithosphere hydrated by past subduction and is about 10 km shallower than the inferred transition from conduction to convection. This article is protected by copyright. All rights reserved.
      PubDate: 2016-04-22T05:02:27.928779-05:
      DOI: 10.1002/2015GC006207
  • Exhumation of the Panama basement complex and basins: Implications for the
           closure of the cCentral American seaway
    • Abstract: The emergence of the Central American isthmus occurred episodically from Eocene to Pliocene time and was caused by a series of tectonic and volcanic processes. Results from zircon U‐Pb geochronology, zircon (U‐Th)/He (ZHe) and apatite (U‐Th)/He (AHe) thermochronology, and zircon Lu‐Hf isotopic data from sedimentary (sandstones and recent river sands) and plutonic rocks from the Azuero Peninsula and Central Panama document the exhumation and uplift history of the Panamanian basement complex. Our data supports previous paleobotanical and thermochronological studies that suggest that by middle Eocene time some areas of Central Panama and Azuero Peninsula were exposed above sea level as a series of islands surrounded by shallow open marine waters. The Gatuncillo, Cobachón and Tonosí formations were deposited during this partial emergence. Transtension in the Oligocene‐early Miocene produced various pull‐apart basins (e.g., the Canal Basin) and local uplift that exhumed the Eocene strata (Gatuncillo and Cobachón formations). This event probably reduced circulation between the Pacific Ocean and the Caribbean Sea. The Tonosí Formation records late Miocene to Pleistocene cooling and exhumation, which may be related to uplift above the subducting Coiba Ridge. These results suggest that the emergence of the Isthmus of Panama followed a series of diachronous events that led to the final closure of the Central American seaway. This article is protected by copyright. All rights reserved.
      PubDate: 2016-04-20T18:05:47.60936-05:0
      DOI: 10.1002/2016GC006289
  • Correction of interstitial water changes in calibration methods applied to
           XRF core‐scanning major elements in long sediment cores: Case study
           from the South China Sea
    • Authors: Quan Chen; Catherine Kissel, Aline Govin, Zhifei Liu, Xin Xie
      Abstract: Fast and non‐destructive X‐ray fluorescence (XRF) core scanning provides high‐resolution element data that are widely used in paleoclimate studies. However, various matrix and specimen effects prevent the use of semi‐quantitative raw XRF core‐scanning intensities for robust paleoenvironmental interpretations. We present here a case study of a 50.8‐m‐long piston Core MD12‐3432 retrieved from the northern South China Sea. The absorption effect of interstitial water is identified as the major source of deviations between XRF core‐scanning intensities and measured element concentrations. The existing two calibration methods, i.e. normalized median‐scaled calibration (NMS) and multivariate log‐ratio calibration (MLC), are tested with this sequence after the application of water absorption correction. The results indicate that an improvement is still required to appropriately correct the influence of downcore changes in interstitial water content in the long sediment core. Consequently, we implement a new polynomial water content correction in NMS and MLC methods, referred as NPS and P_MLC calibrations. Results calibrated by these two improved methods indicate that the influence of downcore water content changes is now appropriately corrected. We therefore recommend either of the two methods to be applied for robust paleoenvironmental interpretations of major elements measured by XRF‐scanning in long sediment sequences with significant downcore interstitial water content changes. This article is protected by copyright. All rights reserved.
      PubDate: 2016-04-15T19:00:51.312838-05:
      DOI: 10.1002/2016GC006320
  • A microphysical interpretation of rate‐dependent and
           state‐dependent friction for fault gouge
    • Authors: Matt J. Ikari; Brett M. Carpenter, Chris Marone
      Abstract: The evolution of fault strength during the seismic cycle plays a key role in the mode of fault slip, nature of earthquake stress drop, and earthquake nucleation. Laboratory‐based rate‐ and state‐dependent friction (RSF) laws can describe changes in fault strength during slip, but the connections between fault strength and the mechanisms that dictate the mode of failure, from aseismic creep to earthquake rupture, remain poorly understood. The empirical nature of RSF laws remains a drawback to their application in nature. Here, we analyze an extensive friction data set of constitutive parameters with the goal of illuminating the microphysical processes controlling RSF. We document robust relationships between: (1) the initial value of sliding (or kinetic) friction, (2) RSF parameters, and (3) the time rates of frictional strengthening (aging). We derive a microphysical model based on asperity contact mechanics and show that these relationships are dictated by: (1) an activation energy that controls the rate of asperity growth by plastic creep, and (2) an inverse relationship between material hardness and the activation volume of plastic deformation. Collectively, our results illuminate the physics expressed by the RSF parameters, and which describe the absolute value of frictional strength and its dependence on time and slip rate. Moreover, we demonstrate that seismogenic fault behavior may be dictated by the interplay between grain properties and ambient conditions controlling the local shear strength of grain‐scale asperity contacts. This article is protected by copyright. All rights reserved.
      PubDate: 2016-04-12T20:31:28.922989-05:
      DOI: 10.1002/2016GC006286
  • Magnetic minerals in three Asian rivers draining into the South China Sea:
           Pearl, Red, and Mekong Rivers
    • Authors: Catherine Kissel; Zhifei Liu, Jinhua Li, Camille Wandres
      Abstract: The use of the marine sedimentary magnetic properties as tracers for changes in precipitation rate and in oceanic water masses transport and exchanges, implies to identify and to characterize the different sources of the detrital fraction. This is of particular importance in closed and/or marginal seas such as the South China Sea. We report on the magnetic properties of sedimentary samples collected in three main Asian rivers draining into the South China Sea: the Pearl, Red and Mekong Rivers. The geological formations as well as the present climatic conditions are different from one catchment to another. The entire set of performed magnetic analyses (low‐field magnetic susceptibility, ARM acquisition and decay, IRM acquisition and decay, back‐field acquisition, thermal demagnetization of 3‐axes IRM, hysteresis parameters, FORC diagrams and low‐temperature magnetic measurements) allow us to identify the magnetic mineralogy and the grain size distribution when magnetite is dominant. Some degree of variability is observed in each basin, illustrating different parent rocks and degree of weathering. On average it appears that the Pearl River is rich in magnetite along the main stream while the Mekong River is rich in hematite. The Red River is a mixture of the two. Compared to clay mineral assemblages and major element contents previously determined on the same samples, these new findings indicate that the magnetic fraction brings complementary information of great interest for environmental reconstructions based on marine sediments from the South China Sea. This article is protected by copyright. All rights reserved.
      PubDate: 2016-04-08T13:04:24.552368-05:
      DOI: 10.1002/2016GC006283
  • Isotope dilution analysis of Ca and Zr in apatite and zircon
           (U‐Th)/He chronometry
    • Authors: William R. Guenthner; Peter W. Reiners, Uttam Chowdhury
      Abstract: Because radiation damage influences He diffusivity, correlations between (U‐Th)/He ages and effective uranium (eU, eU=U+0.235×Th) concentrations of single apatite and zircon grains are important for understanding thermal histories. Here we describe a method for quantifying eU concentrations in apatite and zircon grains using isotope dilution ICP‐MS measurements of Zr and Ca and stoichiometry of zircon (ZrSiO4) and apatite (Ca5(PO4)3F) to obtain grain masses. Combined with independent U and Th measurements, these yield eU concentrations not based on the traditional morphologic measurements and assumptions. Additional benefits of this method include correct identification of an apatite or zircon and volume estimates for crystal shards. In some cases, this method gives eU concentrations consistent with those calculated with the morphologic approach, but often significant differences are observed between concentrations calculated from the two methods. Differences in eU concentrations for our apatite grains are greater and less than those morphology estimates, and the majority are between 0.7 and 31%. With the exception of two grains, all of our zircon grains have differences between 3 and 34% less than morphology estimates. These differences could result from incorrect grain width measurements, mischaracterized grain shape, or incorrect volume calculations of the pure mineral phase due to inclusions. These morphologic errors—combined with evidence for the accuracy of our isotope dilution method from analyses of reference materials—suggest that eU concentrations calculated from morphology may often be significantly inaccurate. Finally, we demonstrate that differences between the two measurements of eU cause age‐eU correlation variations for representative thermal histories. This article is protected by copyright. All rights reserved.
      PubDate: 2016-04-07T17:46:17.861797-05:
      DOI: 10.1002/2016GC006311
  • New analytic solutions for modeling vertical gravity gradient anomalies
    • Abstract: Modern processing of satellite altimetry for use in marine gravimetry involves computing the along‐track slopes of observed sea‐surface heights, projecting them into east‐west and north‐south deflection of the vertical grids, and using Laplace's equation to algebraically obtain a grid of the vertical gravity gradient (VGG). The VGG grid is then integrated via overlapping, flat Earth Fourier transforms to yield a free‐air anomaly grid. Because of this integration and associated edge effects, the VGG grid retains more short‐wavelength information (e.g., fracture zone and seamount signatures) that is of particular importance for plate tectonic investigations. While modeling of gravity anomalies over arbitrary bodies has long been a standard undertaking, similar modeling of VGG anomalies over oceanic features is not commonplace yet. Here, we derive analytic solutions for VGG anomalies over simple bodies and arbitrary 2‐D and 3‐D sources. We demonstrate their usability in determining mass excess and deficiency across the Mendocino fracture zone (a 2‐D feature) and find the best bulk density estimate for Jasper seamount (a 3‐D feature). The methodologies used herein are implemented in the Generic Mapping Tools, available from This article is protected by copyright. All rights reserved.
      PubDate: 2016-04-06T14:55:49.112418-05:
      DOI: 10.1002/2016GC006263
  • Constraints on Lu‐Hf and Nb‐Ta systematics in globally
           subducted oceanic crust from a survey of orogenic eclogites and
    • Authors: N. Alex Zirakparvar
      Abstract: To further understand Lu‐Hf and Nb‐Ta systematics in globally subducted oceanic crust, this paper evaluates all available Lu‐Hf garnet isochron ages and initial εHf values in conjunction with present‐day bulk‐rock Lu‐Hf isotope and trace element (K, Nb, Ta, Zr, and Ti in addition to Lu‐Hf) data from the world's orogenic eclogites and amphibolites (OEAs). Approximately half of OEAs exhibit Lu‐Hf and Nb‐Ta systematics mimicking those of unsubducted oceanic crust whereas the rest exhibit variability in one or both systems. For the Lu‐Hf system, mixing calculations demonstrate that subduction‐related phase transformations, in conjunction with open system behavior, can shift subducted oceanic crust towards higher Lu/Hf, or towards lower Lu/Hf that can also be associated with unradiogenic εHf values. However, evaluation of potential mechanisms for fractionating Nb from Ta is more complicated because many of the OEA's have Nb‐Ta systematics that are decoupled from Lu‐Hf and the behavior of K, Zr, and Ti. Nonetheless, the global dataset demonstrates that the association between unradiogenic εHf and elevated Nb/Ta observed in some kimberlitic eclogite xenoliths can be inherited from processes that occurred during subduction of their oceanic crustal protoliths. This allows for a geologically based estimate of the Nb concentration in a reservoir composed of deeply subducted oceanic crust. However, mass balance calculations confirm that such a reservoir, when considered as a whole, likely has a Nb concentration similar to un‐subducted oceanic crust and is therefore not the solution to the problem of the Earth's ‘missing' Nb. This article is protected by copyright. All rights reserved.
      PubDate: 2016-04-06T10:23:34.533804-05:
      DOI: 10.1002/2016GC006377
  • Early Miocene subduction in the western Mediterranean: Constraints from
           Rb‐Sr multimineral isochron geochronology
    • Authors: Kory L. Kirchner; Whitney M. Behr, Staci Loewy, Daniel F. Stockli
      Abstract: The Betic Cordillera of southern Spain is a complex orogen formed in the context of convergence between Africa and Iberia from the Mesozoic to the present. The internal zone of the orogen includes three tectonic complexes, two of which have been subducted to high pressure conditions, then exhumed back to the surface during subsequent extension. Subduction in the structurally lower complex, known as the Nevado‐Filabride Complex (NFC), has been a topic of debate for several years due to conflicting geochronological data. Here we use multi‐mineral isochron 87Rb/86Sr dating on carefully selected mineral samples from high pressure metamorphic rocks in the NFC to better constrain the timing of high pressure metamorphism and subduction in the region. Out of five samples analyzed, statistically valid multi‐mineral isochrons were obtained for one eclogite and two schists, yielding ages of 20.1+/‐1.1 Ma, 16.0+/‐0.3 Ma, and 13.3+/‐1.3 Ma, respectively. Despite that the other two eclogite samples appeared to preserve prograde mineral assemblages, low 87Rb/86Sr ratios in white mica precluded precise age calculations. These new ages are in close agreement with previously published Lu‐Hf ages on garnet and U‐Pb ages on metamorphic zircon overgrowths for the same rocks, but are substantially younger than published data from the 40Ar/39Ar technique. Combined with recently published tomographic images of slab structure beneath the Alboran Sea, the new ages support a tectonic model in which subduction occurred both prior to the Miocene and during the early to mid‐Miocene, but that it was punctuated in time by a pulse of extensional exhumation in the early Miocene associated with lithospheric delamination and/or slab tearing. This article is protected by copyright. All rights reserved.
      PubDate: 2016-04-01T01:56:58.593725-05:
      DOI: 10.1002/2015GC006208
  • Orbitally paced phosphogenesis in Mediterranean shallow marine carbonates
           during the middle Miocene Monterey event
    • Authors: Gerald Auer; Christoph A. Hauzenberger, Markus Reuter, Werner E. Piller
      Abstract: During the Oligo‐Miocene major phases of phosphogenesis occurred in the Earth's oceans. However, most phosphate deposits represent condensed or allochthonous hemipelagic deposits, formed by complex physical and chemical enrichment processes, limiting their applicability for the study regarding the temporal pacing of Miocene phosphogenesis. The Oligo‐Miocene Decontra section located on the Maiella Platform (central Apennines, Italy) is a widely continuous carbonate succession deposited in a mostly middle to outer neritic setting. Of particular interest are the well‐winnowed grain‐ to packstones of the middle Miocene Bryozoan Limestone, where occurrences of authigenic phosphate grains coincide with the prominent carbon isotope excursion of the Monterey event. This unique setting allows the analysis of orbital forcing on phosphogenesis, within a bio‐, chemo‐ and cyclostratigraphically constrained age‐model. LA‐ICP‐MS analyses revealed a significant enrichment of uranium in the studied authigenic phosphates compared to the surrounding carbonates, allowing natural gamma‐radiation (GR) to be used as a qualitative proxy for autochthonous phosphate content. Time series analyses indicate a strong 405‐kyr‐eccentricity‐forcing of GR in the Bryozoan Limestone. These results link maxima in the GR record and thus phosphate content to orbitally paced increases in the burial of organic carbon, particularly during the carbon isotope maxima of the Monterey event. Thus, phosphogenesis during the middle Miocene in the Mediterranean was controlled by the 405‐kyr‐eccentricity and its influence on large‐scale paleoproductivity patterns. Rare earth element data were used as a tool to reconstruct the formation conditions of the investigated phosphates, indicating generally oxic formation conditions, which are consistent with microbially mediated phosphogenesis. This article is protected by copyright. All rights reserved.
      PubDate: 2016-03-31T03:32:27.283307-05:
      DOI: 10.1002/2016GC006299
  • Crustal structure and fluid distribution beneath the southern part of the
           Hidaka collision zone revealed by 3‐D electrical resistivity
    • Authors: Hiroshi Ichihara; Toru Mogi, Kengo Tanimoto, Yusuke Yamaya, Takeshi Hashimoto, Makoto Uyeshima, Yasuo Ogawa
      Abstract: The Hidaka collision zone, where the Kurile and northeastern (NE) Japan arcs collide, provides a useful study area for elucidating the processes of arc‐continent evolution and inland earthquakes. To produce an image of the collision structure and elucidate the mechanisms of anomalously deep inland earthquakes such as the 1970 Hidaka earthquake (M6.7), we conducted magnetotelluric observations and generated a three‐dimensional resistivity distribution in the southern part of the Hidaka collision zone. The modeled resistivity was characterized by a high resistivity area in the upper crust of the Kurile arc corresponding to metamorphic rocks. The model also showed conductive zones beneath the center of the collision zone. The boundary between the resistive and conductive areas corresponds geometrically to the Hidaka main thrust, which is regarded as the arc‐arc boundary. The correspondence supports the collision model that the upper‐middle part of crust in the Kurile arc is obducting over the NE Japan arc. The conductive areas were interpreted as fluid‐filled zones associated with collision processes and upwelling of dehydrated fluid from the subducting Pacific slab. The fluid flow possibly contributes to over‐pressurized conduction that produces deep inland earthquakes. We also observed a significant conductive anomaly beneath the area of Horoman peridotite, which may be related to the uplift of mantle materials to the surface. This article is protected by copyright. All rights reserved.
      PubDate: 2016-03-31T03:26:19.814241-05:
      DOI: 10.1002/2015GC006222
  • Diffuse venting at the ASHES hydrothermal field: Heat flux and tidally
           modulated flow variability derived from in situ time‐series
    • Authors: Eric Mittelstaedt; Daniel J. Fornari, Timothy J. Crone, James Kinsey, Deborah Kelley, Mitch Elend
      Abstract: Time‐series measurements of diffuse exit‐fluid temperature and velocity collected with a new, deep‐sea camera and temperature measurement system, the Diffuse Effluent Measurement System (DEMS), were examined from a fracture network within the ASHES hydrothermal field located in the caldera of Axial Seamount, Juan de Fuca Ridge. The DEMS was installed using the HOV Alvin above a fracture near the Phoenix vent. The system collected 20 seconds of 20 Hz video imagery and 24 seconds of 1 Hz temperature measurements each hour between July 22 and August 2, 2014. Fluid velocities were calculated using the Diffuse Fluid Velocimetry (DFV) technique. Over the ∼12 day deployment, median upwelling rates and mean fluid temperature anomalies ranged from 0.5 to 6 cm/s and 0°C to ∼6.5°C above ambient, yielding a heat flux of 0.29 ± 0.22 MW m−2 and heat output of 3.1± 2.5 kW. Using a photo mosaic to measure fracture dimensions, the total diffuse heat output from cracks across ASHES field is estimated to be 2.05 ± 1.95 MW. Variability in temperatures and velocities are strongest at semi‐diurnal periods and show significant coherence with tidal height variations. These data indicate that periodic variability near Phoenix vent is modulated both by tidally controlled bottom currents and seafloor pressure, with seafloor pressures being the dominant influence. These results emphasize the importance of local permeability on diffuse hydrothermal venting at mid‐ocean ridges and the need to better quantify heat flux associated with young oceanic crust. This article is protected by copyright. All rights reserved.
      PubDate: 2016-03-24T03:35:36.278573-05:
      DOI: 10.1002/2015GC006144
  • Whole planet coupling between climate, mantle, and core: Implications for
           rocky planet evolution
    • Authors: Bradford J. Foley; Peter E. Driscoll
      Abstract: Earth's climate, mantle, and core interact over geologic timescales. Climate influences whether plate tectonics can take place on a planet, with cool climates being favorable for plate tectonics because they enhance stresses in the lithosphere, suppress plate boundary annealing, and promote hydration and weakening of the lithosphere. Plate tectonics plays a vital role in the long‐term carbon cycle, which helps to maintain a temperate climate. Plate tectonics provides long‐term cooling of the core, which is vital for generating a magnetic field, and the magnetic field is capable of shielding atmospheric volatiles from the solar wind. Coupling between climate, mantle, and core can potentially explain the divergent evolution of Earth and Venus. As Venus lies too close to the sun for liquid water to exist, there is no long‐term carbon cycle and thus an extremely hot climate. Therefore plate tectonics cannot operate and a long‐lived core dynamo cannot be sustained due to insufficient core cooling. On planets within the habitable zone where liquid water is possible, a wide range of evolutionary scenarios can take place depending on initial atmospheric composition, bulk volatile content, or the timing of when plate tectonics initiates, among other factors. Many of these evolutionary trajectories would render the planet uninhabitable. However, there is still significant uncertainty over the nature of the coupling between climate, mantle, and core. Future work is needed to constrain potential evolutionary scenarios and the likelihood of an Earth‐like evolution. This article is protected by copyright. All rights reserved.
      PubDate: 2016-03-24T03:31:19.396868-05:
      DOI: 10.1002/2015GC006210
  • Fluid inclusion petrology and microthermometry of the Cocos Ridge
           hydrothermal system, IODP Expedition 344 (CRISP 2), Site U1414
    • Abstract: In this study we present new data from microthermometry of fluid inclusions entrapped in hydrothermal veins along the Cocos Ridge from the IODP Expedition 344 Site U1414. The results of our study concern a primary task of IODP Expedition 344 to evaluate fluid/rock interaction linked with the tectonic evolution of the incoming Cocos Plate from the Early Miocene up to recent times. Aqueous, low saline fluids are concentrated within veins from both the Cocos Ridge basalt and the overlying lithified sediments of Unit III. Mineralization and crosscutting relationships give constraints for different vein generations. Isochores from primary, re‐equilibrated and secondary fluid inclusions crossed with litho‐/hydrostatic pressures indicate an anti‐clockwise PT evolution during vein precipitation and modification by isobaric heating and subsequent cooling at pressures between ca. 210 and 350 bar. Internal over‐ and underpressures in the inclusions enabled decrepitation and re‐equilibration of early inclusions but also modification of vein generations in the Cocos Ridge basalt and in the lithified sediments. We propose that lithification of the sediments was accompanied with a first stage of vein development (VU1 and VC1) that resulted from Galapagos hotspot activity in the Middle Miocene. Heat advection, either related to the Cocos‐Nazca spreading center or to hotspot activity closer to the Middle America Trench, led to subsequent vein modification (VC2, VU2/3) related to isobaric heating. The latest mineralization (VC3, VU3) within aragonite and calcite veins and some vesicles of the Cocos Ridge basalt occurred during crustal cooling up to recent times. Fluid inclusion analyses and published isotope data show evidence for communication with deeper sourced, high‐temperature hydrothermal fluids within the Cocos Plate. The fluid source of the hydrothermal veins reflects aqueous low saline pore water mixed with invaded seawater. This article is protected by copyright. All rights reserved.
      PubDate: 2016-03-24T03:31:06.876868-05:
      DOI: 10.1002/2015GC006212
  • Thermokinematic evolution of the Annapurna‐Dhaulagiri Himalaya,
           central Nepal: The composite orogenic system
    • Authors: A.J. Parsons; R.D. Law, G.E. Lloyd, R.J. Phillips, M.P. Searle
      Abstract: The Himalayan orogen represents a 'Composite Orogenic System' in which channel flow, wedge extrusion and thrust stacking operate in separate 'Orogenic Domains' with distinct rheologies and crustal positions. We analyze 104 samples from the metamorphic core (Greater Himalayan Sequence, GHS) and bounding units of the Annapurna‐Dhaulagiri Himalaya, central Nepal. Optical microscopy and electron backscatter diffraction (EBSD) analyses provide a record of deformation microstructures and an indication of active crystal slip systems, strain geometries and deformation temperatures. These data, combined with existing thermobarometry and geochronology data are used to construct detailed deformation temperature profiles for the GHS. The profiles define a three‐stage thermo‐kinematic evolution from mid‐crustal channel flow (Stage 1, >700 oC to 550‐650 oC), to rigid wedge extrusion (Stage 2, 400‐600 oC) and duplexing (Stage 3,
      PubDate: 2016-03-24T03:30:51.299074-05:
      DOI: 10.1002/2015GC006184
  • Surface heat flow measurements from the East Siberian continental slope
           and southern Lomonosov Ridge, Arctic Ocean
    • Authors: Matt O'Regan; Pedro Preto, Christian Stranne, Martin Jakobsson, Andrey Koshurnikov
      Abstract: Surface heat flow data in the Arctic Ocean is needed to assess hydrocarbon and methane hydrate distributions, and provide constraints into the tectonic origins and nature of underlying crust. However, across broad areas of the Arctic, few published measurements exist. This is true for the outer continental shelf and slope of the East Siberian Sea, and the adjoining deep water ridges and basins. Here we present 21 new surface heat flow measurements from this region of the Arctic Ocean. On the Southern Lomonosov Ridge, the average measured heat flow, uncorrected for effects of sedimentation and topography, is 57 ± 4 mW/m2 (n=4). On the outer continental shelf and slope of the East Siberian Sea (ESS), The average is 57 ± 10 mW/m2 (n=16). An anomalously high heat flow of 203 ± 28 mW/m2 was measured at a single station in the Herald Canyon. With the exception of this high heat flow, the new data from the ESS is consistent with predictions for thermally equilibrated lithosphere of continental origin that was last affected by thermo‐tectonic processes in the Cretaceous to early Cenozoic. Variability within the data likely arises from differences in radiogenic heat production within the continental crust and overlying sediments. This can be further explored by comparing the data with geophysical constraints on sediment and crustal thicknesses. This article is protected by copyright. All rights reserved.
      PubDate: 2016-03-22T03:30:55.999517-05:
      DOI: 10.1002/2016GC006284
  • Geochemical evidence in the northeast Lau Basin for subduction of the
           Cook‐Austral volcanic chain in the Tonga Trench
    • Abstract: Lau Basin basalts host an array of geochemical signatures that suggest incorporation of enriched mantle source material often associated with intraplate hotspots, but the origin of these signatures remain uncertain. Geochemical signatures associated with mantle material entrained from the nearby Samoan hotspot are present in northwest Lau Basin lavas, and subducted seamounts from the Louisville hotspot track may contribute geochemical signatures to the Tonga Arc. However, lavas in the northeast Lau Basin (NELB) have unique enriched geochemical signatures that cannot be related to these hotspots, but can be attributed to the subduction of seamounts associated with the Cook‐Austral volcanic lineament. Here we present geochemical data on a new suite of NELB lavas—ranging in 40Ar/39Ar age from 1.3 Ma to 0.365 ka—that have extreme signatures of geochemical enrichment, including lavas with the highest 206Pb/204Pb (19.580) and among the lowest 143Nd/144Nd (0.512697) encountered in the Lau Basin to date. These signatures are linked to the canonical EM1 (enriched mantle 1) and HIMU (high‐μ= 238U/204Pb) mantle end‐members, respectively. Using a plate reconstruction model, we show that older portions of the traces of two of the Cook‐Austral hotspots that contributed volcanism to the Cook‐Austral volcanic lineament—the Rarotonga and Rurutu hotspots—were potentially subducted in the Tonga Trench beneath the NELB. The geochemical signatures of the Rarotonga, Rurutu, and Samoan hotspots provide a compelling match to the extreme geochemical components observed in the new NELB lavas. This article is protected by copyright. All rights reserved.
      PubDate: 2016-03-22T03:30:54.116557-05:
      DOI: 10.1002/2015GC006237
  • Major influence of plume‐ridge interaction, lithosphere thickness
           variations, and global mantle flow on hotspot volcanism—The example
           of Tristan
    • Abstract: Hotspot tracks are thought to originate when mantle plumes impinge moving plates. However, many observed cases close to mid‐ocean ridges do not form a single age‐progressive line, but vary in width, are separated into several volcanic chains, or are distributed over different plates. Here we study plume‐ridge interaction at the example of the Tristan plume, which features all of these complexities. Additionally, the South Atlantic formed close to where plume volcanism began, opening from the south and progressing northwards with a notable decrease in magmatism across the Florianopolis Fracture Zone. We study the full evolution of the Tristan plume in a series of three‐dimensional regional models created with the convection code ASPECT. We then compute crustal thickness maps and compare them to seismic profiles and the topography of the South Atlantic. We find that the separation of volcanism into the Tristan and Gough chain can be explained by the position of the plume relative to the ridge and the influence of the global flow field. Plume material below the off‐ridge track can flow towards the ridge and regions of thinner lithosphere, where decompression melting leads to the development of a second volcanic chain resembling the Tristan and Gough hotspot tracks. Agreement with the observations is best for a small plume buoyancy flux of 500kg/s or a low excess temperature of 150K. The model explains the distribution of syn‐rift magmatism by hot plume material that flows into the rift and increases melt generation. This article is protected by copyright. All rights reserved.
      PubDate: 2016-03-21T10:59:52.077882-05:
      DOI: 10.1002/2015GC006177
  • An X‐ray spectroscopic perspective on Messinian evaporite from
           Sicily: Sedimentary fabrics, element distributions, and chemical
           environments of S and Mg
    • Abstract: The Messinian salinity crisis is a dramatic hydrological and biological crisis that occurred in the Mediterranean basin at 5.97‐5.33 Ma. The interpretation of the facies and stratigraphic associations of the Messinian salt deposits is still the object of active research because of the absence of modern depositional analogues of comparable scale. In this study, the spatial distributions of Na, Mg, S, O, Si, and Al in a potassic–magnesian salt and a halite layers of Messinian evaporites from the Realmonte mine on Sicily were determined using synchrotron based micro‐X‐ray fluorescence. The dominant molecular host site of Mg and S obtained by X‐ray absorption near edge structure (XANES) is applied to specify the hydrochemistry of hypersaline brines and the presence of diagenetic minerals, thus shedding light on evaporative concentration processes in the Caltanissetta Basin of Sicily. Mg and S K‐edge XANES spectra revealed the presence of highly soluble Mg‐bearing sulfates. The massive halite layer “unit C”, contains less soluble minerals, thus did not exceed the stage of halite crystallization. We infer that as evaporative concentration increased, the density of the brine at the shallow margin of the basin increased as salinity increased to concentrations over 70 times the starting values, creating brines that were oversaturated with Mg‐sulfate. Density stratification of the deep basin caused heavy brines to sink to the bottom and become overlain by more dilute brines. We propose lateral advection of dense Mg‐sulfate brines that certainly affected marine biota. This article is protected by copyright. All rights reserved.
      PubDate: 2016-03-21T10:51:47.936167-05:
      DOI: 10.1002/2015GC006233
  • Carbon cycling fed by methane seepage at the shallow Cumberland Bay, South
           Georgia, sub‐Antarctic
    • Abstract: Recent studies have suggested that the marine contribution of methane from shallow regions and melting marine terminating glaciers may have been underestimated. Here we report on methane sources and potential sinks associated with methane seeps in Cumberland Bay, South Georgia's largest fjord system. The average organic carbon content in the upper 8 meters of the sediment is around 0.65 wt.%; this observation combined with Parasound data suggest that the methane gas accumulations probably originate from peat‐bearing sediments currently located several tens of meters below the seafloor. Only one of our cores indicates upward advection; instead most of the methane is transported via diffusion. Sulfate and methane flux estimates indicate that a large fraction of methane is consumed by anaerobic oxidation of methane (AOM). Carbon cycling at the sulfate‐methane transition (SMT) results in a marked fractionation of the δ13C‐CH4 from an estimated source value of ‐65‰ to a value as low as ‐96‰ just below the SMT. Methane concentrations in sediments are high, especially close to the seepage sites (∼40 mM); however, concentrations in the water column are relatively low (max. 58 nM) and can be observed only close to the seafloor. Methane is trapped in the lowermost water mass, however, measured microbial oxidation rates reveal very low activity with an average turnover of 3.1 years. We therefore infer that methane must be transported out of the bay in the bottom water layer. A mean sea‐air flux of only 0.005 nM/m2s confirms that almost no methane reaches the atmosphere. This article is protected by copyright. All rights reserved.
      PubDate: 2016-03-21T10:36:19.620352-05:
      DOI: 10.1002/2016GC006276
  • Choosing optimal exposure times for XRF core‐scanning: Suggestions
           based on the analysis of geological reference materials
    • Abstract: X‐ray fluorescence (XRF) core‐scanning is a fast and non‐destructive technique to assess elemental variations of unprocessed sediments. However, although the exposure time of XRF‐scanning directly affects the scanning counts and total measurement time, only a few studies have considered the influence of exposure time during the scan. How to select an optimal exposure time to achieve reliable results and reduce the total measurement time is an important issue. To address this question, six geological reference materials from the Geological Survey of Japan (JLK‐1, JMS‐1, JMS‐2, JSD‐1, JSD‐2, and JSD‐3) were scanned by the Itrax‐XRF core scanner using the Mo‐ and the Cr‐tube with different exposure times to allow a comparison of scanning counts with absolute concentrations. The regression lines and correlation coefficients of elements that are generally used in paleoenvironmental studies were examined for the different exposure times and X‐ray tubes. The results show that for those elements with relatively high concentrations or high detectability, the correlation coefficients are higher than 0.90 for all exposure times. In contrast, for the low detectability or low concentration elements, the correlation coefficients are relatively low, and improve little with increased exposure time. Therefore, we suggest that the influence of different exposure times is insignificant for the accuracy of the measurements. Thus, caution must be taken when interpreting the results of elements with low detectability, even when the exposure times are long and scanning counts are reasonably high. This article is protected by copyright. All rights reserved.
      PubDate: 2016-03-21T10:26:20.64494-05:0
      DOI: 10.1002/2016GC006256
  • Alkenone paleothermometry in the North Atlantic: A review and synthesis of
           surface sediment data and calibrations
    • Authors: A. Filippova; M. Kienast, M. Frank, R. R. Schneider
      Abstract: Despite a clear correlation of alkenone unsaturation and sea surface temperatures (SST) throughout most parts of the ocean, scatter of the regression for various calibration equations has been shown to increase significantly at low SSTs. In this study we combine previously published (n = 101) and new (n = 51) surface sediment data from the northern North Atlantic to constrain uncertainties of alkenone paleothermometry at low SSTs and to discuss possible sources of the increased scatter in the regression. The correlation between alkenone unsaturation and SSTs is strongest, in particular at the cold end (SSTs
      PubDate: 2016-03-18T10:56:57.739586-05:
      DOI: 10.1002/2015GC006106
  • Variation of ocean sediment thickness with crustal age
    • Authors: Peter Olson; Evan Reynolds, Linda Hinnov, Arghya Goswami
      Abstract: Global ocean sediment thickness and present‐day ocean sediment accumulation rates are analyzed with respect to the age of the underlying ocean crust. Trends in average sediment thickness and present‐day accumulation rate are well‐fit by cubic polynomials in crustal age for the global ocean and for individual ocean basins. Sediment thickness and accumulation rates are larger in the North and South Atlantic and Indian Oceans compared to the Pacific Ocean, primarily because the anomalous sediment accumulations that followed continental rifting and collision in the Atlantic and Indian Ocean basins are missing in the Pacific Ocean. Modern ocean sediment accumulation rates, extrapolated into the past assuming steady state conditions, account within uncertainties for the global average sediment thickness on 0‐65 Ma (Cenozoic age) ocean crust, while the profile of anomalously thick sediments on older (Mesozoic age) ocean crust is well‐fit by adding localized, diffusive sediment transport from a steady state source referenced to the adjacent continental margin. Apart from a distinct 0‐5 Ma (Quaternary age) sediment pulse, deviations in average sediment thickness from this simple model are generally small and are uncorrelated across ocean basins. This article is protected by copyright. All rights reserved.
      PubDate: 2016-03-18T10:50:53.553109-05:
      DOI: 10.1002/2015GC006143
  • Distinct responses of planktonic foraminiferal B/Ca to dissolution on
    • Authors: Yuhao Dai; Jimin Yu, Heather J. H. Johnstone
      Abstract: We have measured B/Ca in four core‐top planktonic foraminiferal species (Globigerinoides ruber (white), Globigerinoides sacculifer (without final sac‐like chamber), Neogloboquadrina dutertrei, and Pulleniatina obliquiloculata) from three depth transects (the Caribbean Sea, the Southwestern Indian Ocean, and the Ontong Java Plateau) to evaluate dissolution effects on planktonic B/Ca. At each transect, G. ruber (w) and G. sacculifer (w/o sac) show decreasing B/Ca with increasing water depth. B/Ca decreases are accompanied with decreases in shell weights, Mg/Ca, and bottom water calcite saturation state. This indicates a post‐depositional dissolution effect on B/Ca in these two species. The strong correlation observed between changes in B/Ca and bottom water calcite saturation state offers an approach to correct for the dissolution bias. By contrast, B/Ca in N. dutertrei and P. obliquiloculata remains unchanged along depth transects, although shell weights and Mg/Ca display significant declines. Overall, our core‐top results suggest species‐specific dissolution effects on B/Ca in different planktonic foraminiferal species. This article is protected by copyright. All rights reserved.
      PubDate: 2016-03-14T10:42:09.509364-05:
      DOI: 10.1002/2015GC006199
  • Thermal structure and melting conditions in the mantle beneath the basin
           and range province from seismology and petrology
    • Authors: T. Plank; D. W. Forsyth
      Abstract: To better constrain the temperature structure in the upper mantle, we jointly invert seismic surface wave velocities and basalt thermobarometry. New measurements of the water concentration (1.0‐3.5 wt%) and oxygen fugacity (FMQ+0.5 to +1.5) of basalts from seven volcanic fields in the Basin and Range (Cima, Pisgah, Amboy, Big Pine, Black Rock, Snow Canyon, W. Grand Canyon) enable more accurate equilibration pressure (P) and temperature (T) estimates of the mantle melts. We developed a revised thermobarometer that more precisely predicts the results of laboratory experiments on melts equilibrated with olivine and orthopyroxene and accounts for the effects of water and CO2. Applying these methods to basalts from recently active volcanic fields in the Basin and Range province, we find that most equilibrated near the dry solidus in P‐T space and at depths in the vicinity of the lithosphere‐asthenosphere boundary (LAB) inferred from receiver function analysis and Rayleigh surface wave tomography. The wet basalts should have begun melting well below the dry solidus, so the depths of equilibration probably reflect ponding of rising melts beneath the nominally dry lithosphere. A two‐parameter thermal model is sufficient to simultaneously satisfy both the seismological and petrological constraints. In the model, the depth to the dry solidus defines the bottom boundary of the conductive lid, while the potential temperature (Tp) controls the asthenosphere and LAB thermal structure. The optimum estimates of Tp range from  1500°C, and depths to the LAB range from ∼ 55 to 75 km, with uncertainties on the order of +/‐ 50°C and +/‐ 10 km. In contrast to standard tomographic images or basalt thermobarometry, the output of the joint inversion is a geotherm that can be tested quantitatively against other observations. This article is protected by copyright. All rights reserved.
      PubDate: 2016-03-14T10:38:31.618639-05:
      DOI: 10.1002/2015GC006205
  • Environmental rock‐magnetism of Cenozoic red clay in the South
           Pacific Gyre
    • Authors: Takaya Shimono; Toshitsugu Yamazaki
      Abstract: Nonfossiliferous red clay can be used for elucidating long‐range environmental changes, although such studies were limited so far because of the difficulty in precise age estimation and extremely low sedimentation rates. We conducted an environmental rock‐magnetic study of Cenozoic red clay at the Integrated Ocean Drilling Program Site U1365 in the South Pacific Gyre. Magnetostratigraphy could be established only above ∼6 meters below the seafloor (mbsf) (∼5 Ma). Below ∼6 mbsf, the ages of the cores were transferred from the published ages of nearby Deep Sea Drilling Project Site 596, which is based mainly on a constant Cobalt flux model, by inter‐core correlation using magnetic susceptibility and rare earth element content variation patterns. Rock‐magnetic analyses including first‐order reversal curve diagrams, the ratio of anhysteretic remanent magnetization susceptibility to saturation isothermal remanent magnetization (SIRM), and IRM component analyses revealed that magnetic minerals consist mainly of biogenic magnetite and terrigenous maghemite, and that the proportion of the terrigenous component increased since ∼23 Ma. We consider that the increase reflects a growth of eolian dust flux associated with a northward shift of Australia and the site to an arid region of the middle latitudes. The increase of the terrigenous component accelerated after ∼5 Ma, which may be associated with a further growth of the Antarctic glaciation at that time. This is coeval with the onset of the preservation of magnetostratigraphy, suggesting that the primary remanent magnetization is carried by the terrigenous component. This article is protected by copyright. All rights reserved.
      PubDate: 2016-03-08T17:47:27.741527-05:
      DOI: 10.1002/2015GC006062
  • Nature of the seismic lithosphere‐asthenosphere boundary within
           normal oceanic mantle from high‐resolution receiver functions
    • Abstract: Receiver‐function observations in the oceanic upper mantle can test causal mechanisms for the depth, sharpness and age‐dependence of the seismic wavespeed decrease thought to mark the lithosphere‐asthenosphere boundary (LAB). We use a combination of frequency‐dependent harmonic decomposition of receiver functions and synthetic forward‐modeling to provide new seismological constraints on this “seismic LAB” from 17 ocean‐bottom stations and 2 borehole stations in the Philippine Sea and northwest Pacific Ocean. Underneath young oceanic crust, the seismic LAB depth follows the ∼1300 K isotherm but a lower isotherm (∼1000 K) is suggested in the Daito ridge, the Izu‐Bonin‐Mariana trench and the northern Shikoku basin. Underneath old oceanic crust, the seismic LAB lies at a constant depth ∼70 km. The age‐dependence of the seismic LAB depth is consistent with either a transition to partial‐melt conditions or a sub‐solidus rheological change as the causative factor. The age‐dependence of interface sharpness provides critical information to distinguish these two models. Underneath young oceanic crust, the velocity gradient is gradational, while for old oceanic crust a sharper velocity gradient is suggested by the receiver functions. This behavior is consistent with the prediction of the sub‐solidus model invoking anelastic relaxation mediated by temperature and water‐content, but is not readily explained by a partial‐melt model. The Ps conversions display negligible two‐lobed or four‐lobed back‐azimuth dependence in harmonic stacks, suggesting that a sharp change in azimuthal anisotropy with depth is not responsible for them. We conclude that these ocean‐bottom observations indicate a sub‐solidus elastically‐accommodated grain‐boundary sliding (EAGBS) model for the seismic LAB. Because EAGBS does not facilitate long‐term ductile deformation, the Seismic LAB may not coincide with the conventional transition from lithosphere to asthenosphere. This article is protected by copyright. All rights reserved.
      PubDate: 2016-03-07T04:42:10.483336-05:
      DOI: 10.1002/2015GC006214
  • Algorithms and software for U‐Pb geochronology by LA‐ICPMS
    • Authors: Noah McLean; James Bowring, George Gehrels
      Abstract: The past fifteen years have produced numerous innovations in geochronology, including experimental methods, instrumentation, and software that are revolutionizing the acquisition and application of geochronological data. For example, exciting advances are being driven by Laser‐Ablation ICP Mass Spectrometry (LA‐ICPMS), which allows for rapid determination of U‐Th‐Pb ages with 10s of μm‐scale spatial resolution. This method has become the most commonly applied tool for dating zircons, constraining a host of geological problems. The LA‐ICPMS community is now faced with archiving these data with associated analytical results and, more importantly, ensuring that data meet the highest standards for precision and accuracy and that inter‐laboratory biases are minimized. However, there is little consensus with regard to analytical strategies and data reduction protocols for LA‐ICPMS geochronology. The result is systematic inter‐laboratory bias and both under‐ and over‐estimation of uncertainties on calculated dates that, in turn, decrease the value of data in repositories such as EarthChem, which archives data and analytical results from participating laboratories. We present free open source software software that implements new algorithms for evaluating and resolving many of these discrepancies. This solution is the result of a collaborative effort to extend the U‐Pb_Redux software for the ID‐TIMS community to the LA‐ICPMS community. Now named ET_Redux, our new software automates the analytical and scientific workflows of data acquisition, statistical filtering, data analysis and interpretation, publication, community‐based archiving, and the compilation and comparison of data from different laboratories to support collaborative science. This article is protected by copyright. All rights reserved.
      PubDate: 2016-03-03T20:07:37.488273-05:
      DOI: 10.1002/2015GC006097
  • Development of a 0.5 T magnetic‐core alternating‐field
    • Authors: W. E. Schillinger; E. R. Morris, R. S. Coe, D. R. Finn
      Abstract: We have constructed an alternating‐field (AF) demagnetizer with a magnetic core in a passively air‐cooled coil that can routinely operate at fields up to 0.5 tesla, almost three times higher than we could attain before in our commercial instrument. The field is powered by a commercial 1 kW power amplifier and is transverse to the bore, uniform to ±2% over a 25 mm paleomagnetic sample, and compatible with our existing sample handler for automated demagnetization and measurement. Even harmonics are ≤1 ppm of the fundamental and so generate negligible anhysteretic remanence. The much higher peak alternating field, 2 and 5 times that commonly available in air‐core solenoidal and Helmhotz coil configurations, respectively, enables successful AF demagnetization of many samples that could not be completely demagnetized with commercially available equipment. This capability is especially useful for high‐coercivity sedimentary and igneous rocks and extra‐terrestrial materials that contain magnetic minerals that alter during thermal demagnetization. In addition to the benefits this instrument brings to our own research, a much broader potential impact is that it could replace the transverse coils of most automated AF demagnetization systems in use today, whether for discrete or continuous U‐channel measurements, which are commonly limited to peak fields of ∼100 mT. Manual and tumbling demagnetizers would benefit as well by the ∼2‐times increase in maximum field over those that can be attained by commercial solenoidal coils. Furthermore, we expect that it and similarly designed magnetic‐core instruments will be capable of attaining even higher fields, of order 1 tesla. This article is protected by copyright. All rights reserved.
      PubDate: 2016-03-03T20:07:23.001253-05:
      DOI: 10.1002/2015GC006204
  • Mg/Ca thermometry in planktic foraminifera: Improving paleotemperature
           estimations for G. bulloides and N. pachyderma left
    • Abstract: Planktic foraminiferal Mg/Ca ratios have become a fundamental seawater temperature proxy in past climate reconstructions, due to the temperature dependence of Mg uptake into foraminiferal calcite. However, empirical calibrations for single species from methodologically consistent data are still lacking. Here we present species‐specific calibrations of Mg/Ca vs. calcification temperature for two commonly used species of planktic foraminifera: Globigerina bulloides and Neogloboquadrina pachyderma left, based on a series of Southern Ocean and North Atlantic core tops. Combining these new data with previously published data, we derive an integrated G. bulloides Mg/Ca‐temperature calibration for mid and high latitudes of both hemispheres between 2 ‐ 18°C, where Mg/Ca = 1.006 ± 0.032 * e0.065 ± 0.003*Tiso (R2 = 0.82). G. bulloides is found to calcify deeper in the Southern Ocean (∼ 200 m) than in the North Atlantic (top 50 m). We also propose a Mg/Ca temperature calibration to describe the temperature response in N. pachyderma left that calcified away from the influence of sea ice in the Southern Ocean, valid between ∼ ‐1 and 9°C, of the form Mg/Ca = 0.580 ± 0.016 * e0.084 ± 0.006*Tiso (R2 = 0.70). These calibrations account for uncertainties on Mg/Ca measurements and calcification temperature that were carefully estimated and propagated using Monte Carlo iterations. The 1σ propagated error in Mg/Ca‐derived temperatures is 1.1°C for G. bulloides and 0.9°C for N. pachyderma left for the presented datasets. Geographical extension of genotypes must be assessed when choosing to develop regional or global calibrations. This article is protected by copyright. All rights reserved.
      PubDate: 2016-03-03T20:02:24.232058-05:
      DOI: 10.1002/2015GC006234
  • Rock magnetic expression of fluid infiltration in the
           Yingxiu‐Beichuan fault (Longmen Shan thrust belt, China)
    • Authors: Tao Yang; Xiaosong Yang, Qingbao Duan, Jianye Chen, Mark J. Dekkers
      Abstract: Fluid infiltration within fault zones is an important process in earthquake rupture. Magnetic properties of fault rocks convey essential clues pertaining to physicochemical processes in fault zones. In 2011, two shallow holes (134 and 54 m depth, respectively) were drilled into the Yingxiu‐Beichuan fault (Longmen Shan thrust belt, China), which accommodated most of the displacement of the 2008 Mw 7.9 Wenchuan earthquake. Fifty‐eight drill core samples, including granitic host rock and various fault rocks, were analyzed rock‐magnetically, mineralogically and geochemically. The magnetic behavior of fault rocks appears to be dominated by paramagnetic clay minerals. Magnetite in trace amounts is identified as the predominant ferrimagnetic fraction in all samples, decreasing from the host rock, via fault breccia to (proto‐)cataclasite. Significant mass‐losses (10.7‐45.6%) are determined for the latter two with the ‘isocon' method. Volatile contents and alteration products (i.e., chlorite) are enriched toward the fault core relative to the host rocks. These observations suggest that magnetite depletion occurred in these fault rocks – exhumed from the shallow crust – plumbed by fluid‐assisted processes. Chlorite, interpreted to result from hydrothermal activity, occurs throughout almost the entire fault core and shows high coefficients of determination (R2 > 0.6) with both low‐ and high‐field magnetic susceptibility. Close relationships, with R2 > 0.70, are also observed between both low‐ and high‐field magnetic susceptibility and the immobile elements (e.g., TiO2, P2O5, MnO), H2O+, and the calculated mass‐losses of fault rocks. Hence, magnetic properties of fault rocks can serve as proxy indicators of fluid infiltration within shallow fault zones. This article is protected by copyright. All rights reserved.
      PubDate: 2016-03-01T19:32:43.697777-05:
      DOI: 10.1002/2015GC006095
  • Time series measurements of bubble plume variability and water column
           methane distribution above Southern Hydrate Ridge, Oregon
    • Authors: Brendan T. Philip; Alden R. Denny, Evan A. Solomon, Deborah S. Kelley
      Abstract: An estimated 500‐2500 gigatons of methane carbon is sequestered in gas hydrate at continental margins and some of these deposits are associated with overlying methane seeps. To constrain the impact that seeps have on methane concentrations in overlying ocean waters and to characterize the bubble plumes that transport methane vertically into the ocean, water samples and time‐series acoustic images were collected above Southern Hydrate Ridge (SHR), a well‐studied hydrate‐bearing seep site ∼90 km west of Newport, Oregon. These data were co‐registered with robotic vehicle observations to determine the origin of the seeps, the plume rise heights above the seafloor, and the temporal variability in bubble emissions. Results show that the locations of seep activity and bubble release remained unchanged over the 3 year time‐series investigation, however, the magnitude of gas release was highly variable on hourly time scales. Bubble plumes were detected to depths of 320‐620 m below sea level (mbsl), in several cases exceeding the upper limit of hydrate stability by ∼190 m. For the first time, sustained gas release was imaged at the Pinnacle site and in‐between the Pinnacle and the Summit area of venting, indicating that the sub‐seafloor transport of fluid and gas is not restricted to the Summit at SHR, requiring a revision of fluid‐flow models. Dissolved methane concentrations above background levels from 100‐300 mbsl are consistent with long‐term seep gas transport into the upper water column, which may lead to the build‐up of seep‐derived carbon in regional subsurface waters and to increases in associated biological activity. This article is protected by copyright. All rights reserved.
      PubDate: 2016-03-01T19:19:03.200575-05:
      DOI: 10.1002/2016GC006250
  • Stabilization of large drainage basins over geological time scales:
           Cenozoic West Africa, hot spot swell growth, and the Niger River
    • Abstract: Reconstructing the evolving geometry of large river catchments over geological timescales is crucial to constraining yields to sedimentary basins. In the case of Africa, it should further help deciphering the response of large cratonic sediment routing systems to Cenozoic growth of the basin‐and‐swell topography of the continent. Mapping of dated and regionally correlated lateritic paleolandscape remnants complemented by onshore sedimentological archives allows the reconstruction of two physiographic configurations of West Africa in the Paleogene. Those reconstructions show that the geometry of the drainage stabilized by the Late Early Oligocene (29 Ma) and probably by the end of the Eocene (34 Ma), allowing to effectively link the inland morphoclimatic record to offshore sedimentation since that time, particularly in the case of the Niger catchment – delta system. Mid‐Eocene paleogeography reveals the antiquity of the Senegambia catchment back to at least 45 Ma and suggests that a marginal upwarp forming a continental divide preexisted Early Oligocene connection of the Niger and Volta catchments to the Equatorial Atlantic Ocean. Such a drainage rearrangement was primarily enhanced by the topographic growth of the Hoggar hot spot swell and caused a major stratigraphic turnover along the Equatorial margin of West Africa. This article is protected by copyright. All rights reserved.
      PubDate: 2016-03-01T19:18:52.928126-05:
      DOI: 10.1002/2015GC006169
  • Focusing of relative plate motion at a continental transform fault:
           Cenozoic dextral displacement >700 km on New Zealand's Alpine Fault,
           reversing >225 km of Late Cretaceous sinistral motion
    • Authors: Simon Lamb; Nick Mortimer, Euan Smith, Gillian Turner
      Abstract: The widely accepted ∼450 km Cenozoic dextral strike‐slip displacement on New Zealand's Alpine Fault is large for continental strike‐slip faults, but it is still less than 60% of the Cenozoic relative plate motion between the Australian and Pacific plates through Zealandia, with the remaining motion assumed to be taken up by rotation and displacement on other faults in a zone up to 300 km wide. We show here that the 450 km total displacement across the Alpine Fault is an artifact of assumptions about the geometry of New Zealand's basement terranes in the Eocene, and the actual Cenozoic dextral displacement across the active trace is greater than 665 km, with more than 700 km (and 94%) of the relative plate motion in the last 25 Ma at an average rate in excess of 28 mm/yr. It reverses more than 225 km (and
      PubDate: 2016-03-01T19:17:39.53719-05:0
      DOI: 10.1002/2015GC006225
  • Variations in timing of lithospheric failure on terrestrial planets due to
           chaotic nature of mantle convection
    • Authors: Teresa Wong; Viatcheslav S. Solomatov
      Abstract: We perform numerical simulations of lithospheric failure in the stagnant lid regime of temperature‐dependent viscosity convection, using the yield stress approach. We find that the time of failure can vary significantly for the same values of the controlling parameters due to the chaotic nature of the convective system. The general trend of the dependence of the time of lithospheric failure on the yield stress can be explained by treating lithospheric failure as a type of Rayleigh‐Taylor instability. This study suggests that it is important to address not only the question of whether plate tectonics can occur on a planet but also when it would occur if conditions are favorable. This article is protected by copyright. All rights reserved.
      PubDate: 2016-03-01T19:17:03.895603-05:
      DOI: 10.1002/2015GC006158
  • A failure to reject: Testing the correlation between large igneous
           provinces and deep mantle structures with EDF statistics
    • Authors: Pavel V. Doubrovine; Bernhard Steinberger, Trond H. Torsvik
      Abstract: Absolute reconstructions of large igneous provinces (LIPs) for the past 300 Ma reveal a remarkable spatial pattern suggesting that almost all LIPs have erupted over the margins of the two large‐scale structures in the Earth's lower mantle commonly referred to as the Large Low Shear‐wave Velocity Provinces (LLSVPs). This correlation suggests that mantle plumes that have triggered LIP eruptions rose from the margins of LLSVPs, implying long‐term stability of these structures and suggesting that they may be chemically distinct from the bulk of the mantle. Yet, some researchers consider the LLSVPs to be purely thermal upwellings, arguing that the observed distribution of LIPs can be explained by plumes randomly forming over the entire areas of LLSVPs. Here we examine the correlation between the LIPs and LLSVPs using nonparametric statistical tests, updated plate reconstructions, and a large number of alternative definitions of LLSVPs based on seismic tomography. We show that probability models assuming plume sources originating at the margins of LLSVPs adequately explain the observed distribution of reconstructed LIPs. In contrast, we find strong evidence against the models seeking to link LIPs with plumes randomly forming over the entire LLSVP areas. However, the hypothesis proposing that the correlation can be explained by plumes randomly forming over a larger area of slower‐than‐average shear wave velocities in the lowermost mantle cannot be ruled out formally. Our analysis suggests that there is no statistically sound reason for questioning the hypothesis that the LIPs correlate with the margins of LLSVP globally. This article is protected by copyright. All rights reserved.
      PubDate: 2016-03-01T19:16:11.191148-05:
      DOI: 10.1002/2015GC006044
  • Potential and limits of InSAR to characterize interseismic deformation
           independently of GPS data: Application to the southern San Andreas Fault
    • Abstract: The evaluation of long‐wavelength deformation associated with interseismic strain accumulation traditionally relies on spatially sparse GPS measurements, or on high spatial‐resolution InSAR velocity fields aligned to a GPS‐based model. In this approach the InSAR contributes only short‐wavelength deformation and the two datasets are dependent, thereby challenging the evaluation of the InSAR uncertainties and the justification of atmospheric corrections. Here, we present an analysis using seven years of Envisat InSAR data to characterize interseismic deformation along the southern San Andreas Fault (SAF) and the San Jacinto Fault (SJF) in southern California, where the SAF bifurcates onto the Mission Creek (MCF) and the Banning (BF) fault strands. We outline the processing steps for using InSAR alone to characterize both the short‐ and long‐wavelength deformation, and evaluate the velocity field uncertainties with independent continuous GPS data. InSAR line‐of‐sight (LOS) and continuous GPS velocities agree within ∼1‐2 mm/yr in the study area, suggesting that multi‐year InSAR time series can be used to characterize interseismic deformation with a higher spatial resolution than GPS. We investigate with dislocation models the ability of this mean LOS velocity field to constrain fault slip rates and show that a single viewing geometry can help distinguish between different slip‐rate scenarios on the SAF and SJF (∼35 km apart) but multiple viewing geometries are needed to differentiate slip on the MCF and BF (
      PubDate: 2016-03-01T19:15:48.85301-05:0
      DOI: 10.1002/2015GC006246
  • Are quartz LPOs predictably oriented with respect to the shear zone
           boundary?— a test from the Alpine Fault mylonites, New Zealand
    • Authors: Timothy A. Little; David J. Prior, Virginia G. Toy
      Abstract: The Alpine fault self‐exhumes its own ductile shear zone roots and has a known slip kinematics. Within ∼1 km of the fault, the mylonitic foliation is subparallel to the boundary of the amphibolite‐facies ductile shear zone in which it formed. Using EBSD, we analysed quartz Lattice Preferred Orientations (LPOs) of mylonites along a central part of the Alpine Fault. All LPOs feature a strongest girdle of [c]‐axes that is forward‐inclined ∼28 ±4° away from the pole to the fault. A maximum of axes is inclined at the same angle relative the fault. The [c]‐axis girdle is perpendicular to extensional (C') shear bands and the maximum is parallel to their slip direction. [c]‐axis girdles do not form perpendicular to the SZB. Schmid factor analysis suggests that σ1 was arranged at 60 to 80° to the Alpine Fault. These observations indicate ductile transpression in the shear zone. The inclined arrangement of [c]‐axis girdles, axes, and C' planes relative to the fault can be explained by their alignment relative to planes of maximum shear‐strain‐rate in a general shear zone, a significant new insight regarding shear zones and how LPO fabrics may generally develop within them. For the Alpine mylonite zone, our data imply a kinematic vorticity number (Wk) of ∼0.7 to ∼0.85. Inversions of seismic focal mechanisms in the brittle crust of the Southern Alps indicate that σ1 is oriented ∼60° to the Alpine Fault; that shear bands form at ∼30° to this direction, and that σ2 and σ3 flip positions between the brittle and ductile parts of the crust. This article is protected by copyright. All rights reserved.
      PubDate: 2016-02-23T18:09:07.978548-05:
      DOI: 10.1002/2015GC006145
  • Constraining dolomitization by Mg isotopes: A case study from partially
           dolomitized limestones of the middle Cambrian Xuzhuang Formation, North
    • Abstract: The ‘dolomite problem' refers to the rare dolomite formation in modern oceans that is in sharp contrast to the widespread ancient dolostone in rock record, as well as failure of laboratory inorganic dolomite precipitation at near Earth‐surface temperature. Novel Mg isotope systematics provides a promising tool in resolving the ‘dolomite problem'. Here, we develop a protocol to place constraints on the dolomitization process by using Mg isotopes. In this study, we measured Mg isotopic compositions (δ26Mg) of two batches of partially dolomitized limestone samples from the middle Cambrian Xuzhuang Formation in North China. δ26Mg varies between ‐0.55‰ and ‐3.18‰, and shows a negative linear correlation with 1[Mg], suggesting that δ26Mg can be described by a binary mixing between the calcite and dolomite components. Mg isotopic composition of the dolomite component (δ26Mgdol) for the lower sample set that is collected from a 4‐meter stratigraphic interval containing 3 high frequency ribbon rock‐packstone cycles is ‐1.6‰, while δ26Mgdol for the upper sample set (from a thick sequence of ribbon rock) is significantly higher (‐0.3‰). However, neither mineralogical and elemental compositions, carbon and oxygen isotopes, nor crystal morphologies of dolomite provides diagnostic criteria to differentiate these two batches of samples. δ26Mgdol of the Xuzhuang limestone is simulated by the Advective Flow (AF) and Diffusion‐Advection‐Reaction (DAR) models. The AF model assumes that Mg is transported by advective fluid flows, while the DAR model simulates a contemporaneous seawater dolomitization process, in which Mg is delivered by diffusion. The AF modeling result indicates that δ26Mg of the dolomitization fluid is +0.4‰ and +1.7‰ for the lower and upper sample sets, respectively. These values are significantly higher than modern and Cenozoic seawater Mg isotopic composition, suggesting that the dolomitization fluid is not contemporaneous seawater. The AF model also predicts spatially heterogeneous δ26Mgdol with progressive enrichment in 26Mg along the fluid flow pathway. In the DAR model, both dolomite content and δ26Mgdol of the lower sample set can be simulated by using seawater Mg isotopic composition of ‐0.75‰, thus contemporaneous seawater dolomitization may explain δ26Mgdol of the Xuzhuang limestone. Furthermore, the DAR model demonstrates spatially homogeneous δ26Mgdol. To differentiate the AF and DAR models, samples from multiple sections are required. Nevertheless, this study implies that Mg isotope might be a useful tool in the study of dolomitization. This article is protected by copyright. All rights reserved.
      PubDate: 2016-02-23T18:08:42.970519-05:
      DOI: 10.1002/2015GC006057
  • Shear velocity structure beneath the central United States: Implications
           for the origin of the Illinois Basin and intraplate seismicity
    • Authors: Chen Chen; Hersh Gilbert, Christopher Andronicos, Michael Hamburger, Timothy Larson, Stephen Marshak, Gary Pavlis, Xiaotao Yang
      Abstract: We present new estimates of lithospheric shear velocities for the intraplate seismic zones and the Illinois Basin in the US midcontinent by analyzing teleseismic Rayleigh waves. We find that relatively high crustal shear velocities (VS) characterize the southern Illinois Basin, while relatively low crustal velocities characterize the middle and lower crust of the central and northern Illinois Basin. The observed high crustal velocities may correspond to high‐density mafic intrusions emplaced into the crust during the development of the Reelfoot Rift, which may have contributed to the subsidence of the Illinois Basin. The low crustal VS beneath the central and northern basin follow the La Salle deformation belt. We also observe relatively low velocities in the mantle beneath the New Madrid seismic zone where VS decreases by about 7% compared to those outside of the rift. The low VS in the upper mantle also extends beneath the Wabash Valley and Ste. Genevieve seismic zones. Testing expected VS reductions based on plausible thermal heterogeneities for the midcontinent indicates that the 7% velocity reduction would not result from elevated temperatures alone. Instead this scale of anomaly requires a contribution from some combination of increased iron and water content. Both rifting and interaction with a mantle plume could introduce these compositional heterogeneities. Similar orientations for the NE‐SW low‐velocity zone and the Reelfoot Rift suggest a rift origin to the reduced velocities. The low VS upper mantle represents a weak region and the intraplate seismic zones would correspond to concentrated crustal deformation above weak mantle. This article is protected by copyright. All rights reserved.
      PubDate: 2016-02-22T18:19:01.683233-05:
      DOI: 10.1002/2015GC006206
  • Vulcan: A deep‐towed CSEM receiver
    • Authors: Steven Constable; Peter K. Kannberg, Karen Weitemeyer
      Abstract: We have developed a three‐axis electric field receiver designed to be towed behind a marine electromagnetic transmitter for the purpose of mapping the electrical resistivity in the upper 1,000 m of seafloor geology. By careful adjustment of buoyancy and the use of real‐time monitoring of depth and altitude, we are able to deep‐tow multiple receivers on arrays up to 1200 m long within 50 m of the seafloor, thereby obtaining good coupling to geology. The rigid body of the receiver is designed to reduce noise associated with lateral motion of flexible antennas during towing, and allows the measurement of the vertical electric field component, which modeling shows to be particularly sensitive to near‐seafloor resistivity variations. The positions and orientations of the receivers are continuously measured, and realistic estimates of positioning errors can be used to build an error model for the data. During a test in the San Diego Trough, offshore California, inversions of the data were able to fit amplitude and phase of horizontal electric fields at three frequencies on three receivers to about 1% in amplitude and 1° in phase and vertical fields to about 5% in amplitude and 5° in phase. The geological target of the tests was a known cold seep and methane vent in 1,000 m water depth, which inversions show to be associated with a 1 km wide resistor at a depth between 50 m and 150 m below seafloor. Given the high resistivity (30 Ωm) and position within the gas hydrate stability field, we interpret this to be massive methane hydrate. This article is protected by copyright. All rights reserved.
      PubDate: 2016-02-22T18:18:39.500696-05:
      DOI: 10.1002/2015GC006174
  • Iron and magnesium isotopic constraints on the origin of chemical
           heterogeneity in podiform chromitite from the Luobusa ophiolite, Tibet
    • Abstract: We present high‐precision measurements of iron (Fe) and magnesium (Mg) isotopic compositions of olivine, orthopyroxene and chromite separates from harzburgites, dunites and chromitites in the mantle section of the Luobusa ophiolite, southern Tibet, to investigate the origins of podiform chromitite. Two harzburgites in the Zedong ophiolite, southern Tibet, are also reported for comparison. The olivine and orthopyroxene in the Luobusa and Zedong harzburgites have similar Fe and Mg isotopic compositions, with δ56Fe values ranging from 0‰ to +0.083‰ in olivine, from ‐0.034‰ to +0.081‰ in orthopyroxene and δ26Mg values ranging from ‐0.25‰ to ‐0.20‰ in olivine, from ‐0.29‰ to ‐0.26‰ in orthopyroxene, respectively. The olivines of two dunites from the Luobusa display small Fe and Mg isotopic variations, with δ56Fe values of +0.014‰ and +0.116‰ and δ26Mg values of ‐0.21‰ and ‐0.29‰. All chromites in the Luobusa chromitites have lighter Fe isotopic compositions than the coexisting olivines, with δ56Fe values ranging from ‐0.247‰ to +0.043‰ in chromite and from ‐0.146‰ to +0.215‰ in olivine (Δ56FeChr‐Ol=‐0.294 to ‐0.101‰). The chromite δ26Mg values span a significant range from ‐0.41‰ to +0.14‰. Large disequilibrium Fe and Mg isotope fractionation between chromite and olivine, as well as positive correlation of chromite δ56Fe values with their MgO contents, could be attributed to Fe‐Mg exchange between chromite and olivine. In the disseminated chromitites, the higher modal abundances of olivine than chromite would result in a more extensive Fe‐Mg exchange, whereas chromite in the massive chromitite where olivine is rare could not be affected by this process. This article is protected by copyright. All rights reserved.
      PubDate: 2016-02-17T18:43:35.666274-05:
      DOI: 10.1002/2015GC006223
  • A permeability and compliance contrast measured hydrogeologically on the
           San Andreas Fault
    • Authors: Lian Xue; Emily Brodsky, Jon Erskine, Patrick Fulton, Reed Carter
      Abstract: Hydrogeologic properties of fault zones are critical to faulting processes; however, they are not well understood and difficult to measure in situ, particularly in low permeability fractured bedrock formations. Analysis of continuous water level response to Earth tides in monitoring wells provides a method to measure the in‐situ hydrogeologic properties. We utilize four monitoring wells within the San Andreas Fault zone near Logan Quarry to study the fault zone hydrogeologic architecture by measuring the water level tidal response. The specific storage and permeability inferred from the tidal response suggest that there is a difference in properties at different distances from the fault. The sites closer to the fault have higher specific storage and higher permeability than farther from the fault. This difference of properties might be related to the fault zone fracture distribution decreasing away from the fault. Although permeability channels near faults have been documented before, the difference in specific storage near the fault is a new observation. The inferred compliance contrast is consistent with prior estimates of elastic moduli in the near‐fault environment, but the direct measurements are new. The combination of measured permeability and storage yields a diffusivity of about 10−2 m2/s at all the sites both near and far from the fault as a result of the competing effects of permeability and specific storage. This uniform diffusivity structure suggests that the permeability contrast might not efficiently trap fluids during the interseismic period. This article is protected by copyright. All rights reserved.
      PubDate: 2016-02-17T12:25:34.669902-05:
      DOI: 10.1002/2015GC006167
  • Dynamic simulations of potential methane release from East Siberian
           continental slope sediments
    • Authors: C. Stranne; M. O'Regan, G. R. Dickens, P. Crill, C. Miller, P. Preto, M. Jakobsson
      Abstract: Sediments deposited along continental margins of the Arctic Ocean presumably host large amounts of methane (CH4) in gas hydrates. Here we apply numerical simulations to assess the potential of gas hydrate dissociation and methane release from the East Siberian slope over the next 100 years. Simulations are based on a hypothesized bottom water warming of 3°C, and an assumed starting distribution of gas hydrate. The simulation results show that gas hydrate dissociation in these sediments is relatively slow, and that CH4 fluxes toward the seafloor are limited by low sediment permeability. The latter is true even when sediment fractures are permitted to form in response to overpressure in pore space. With an initial gas hydrate distribution dictated by present‐day pressure and temperature conditions, nominally 0.35 gigaton (Gt) of CH4 are released from the East Siberian slope during the first 100 years of the simulation. However, this CH4 discharge becomes significantly smaller (to ∼0.05 Gt) if glacial sea‐level changes in the Arctic Ocean are considered. This is because a lower sea level during the last glacial maximum (LGM) must result in depleted gas hydrate abundance within the most sensitive region of the modern gas hydrate stability zone. Even if all released CH4 reached the atmosphere, the amount coming from East Siberian slopes would be trivial compared to present‐day atmospheric CH4 inputs from other sources. This article is protected by copyright. All rights reserved.
      PubDate: 2016-02-17T12:23:55.631999-05:
      DOI: 10.1002/2015GC006119
  • Persistent multitiered magma plumbing beneath Katla volcano, Iceland
    • Abstract: Recent seismic unrest and a persistent Holocene eruption record at Katla volcano, Iceland indicate that a near‐future eruption is possible. Previous petrological investigations suggest that Katla is supplied by a simple plumbing system that delivers magma directly from depth, while seismic and geodetic data also point towards the existence of upper‐crustal magma storage. To characterise Katla's recent plumbing system, we established mineral‐melt equilibrium crystallisation pressures from four age‐constrained Katla tephras spanning from 8 ky BP to 1918. The results point to persistent shallow‐ (≤8 km depth) as well as deep‐crustal (ca. 10 – 25 km depth) magma storage beneath Katla throughout the last 8 ky. The presence of multiple magma storage regions implies that mafic magma from the deeper reservoir system may become gas‐rich during ascent and storage in the shallow crust and erupt explosively. Alternatively, it might intersect evolved magma pockets in the shallow‐level storage region, and so increase the potential for explosive mixed‐magma ash eruptions. This article is protected by copyright. All rights reserved.
      PubDate: 2016-02-17T12:23:44.072458-05:
      DOI: 10.1002/2015GC006118
  • Fault‐controlled lithospheric detachment of the volcanic southern
           South Atlantic rift
    • Authors: Katharina Becker; David C. Tanner, Dieter Franke, Charlotte M. Krawczyk
      Abstract: We present structural models of two exemplary conjugate seismic lines of the southernmost South Atlantic margins to examine their initial evolution, especially the seaward‐dipping reflectors (SDRs). Modelling illustrates the different structure and inclination angles of the SDRs, which therefore require different subsidence histories. Since typical symmetrical subsidence models are not applicable, we suggest a model with a westward‐dipping detachment fault that offsets the SDRs on the South American margin and we speculate on passively‐subsided SDRs on the South African margin. We propose a simple‐shear rifting mechanism to explain the initial break‐up of the South Atlantic. This article is protected by copyright. All rights reserved.
      PubDate: 2016-02-17T12:23:11.802548-05:
      DOI: 10.1002/2015GC006081
  • Hydrothermal venting and mineralization in the crater of Kick'em Jenny
           submarine volcano, Grenada (Lesser Antilles)
    • Authors: Steven Carey; Rene Olsen, Katherine L.C. Bell, Robert Ballard, Frederic Dondin, Chris Roman, Clara Smart, Marvin Lilley, John Lupton, Brad Seibel, Winton Cornell, Craig Moyer
      Abstract: Kick'em Jenny is a frequently‐erupting, shallow submarine volcano located 7.5 km off the northwest coast of Grenada in the Lesser Antilles subduction zone. Focused and diffuse hydrothermal venting is taking place mainly within a small (∼70 x 110 m) depression within the 300 m diameter crater of the volcano at depths of about 265 meters. Much of the crater is blanketed with a layer of fine‐grained tephra that has undergone hydrothermal alteration. Clear fluids and gas are being discharged near the center of the depression from mound‐like vents at a maximum temperature of 180° C. The gas consists of 93‐96% CO2 with trace amounts of methane and hydrogen. Gas flux measurements of individual bubble streams range from 10 to 100 kg of CO2 per day. Diffuse venting with temperatures 5 to 35° C above ambient occurs throughout the depression and over large areas of the main crater. These zones are colonized by reddish‐yellow bacteria with the production of Fe‐oxyhydroxides as surface coatings, fragile spires up to several meters in height, and elongated mounds up to tens of centimeters thick. A high‐resolution photomosaic of the inner crater depression shows fluid flow patterns descending the sides of the depression towards the crater floor. We suggest that the negatively buoyant fluid flow is the result of phase separation of hydrothermal fluids at Kick'em Jenny generating a dense saline component that does not rise despite its elevated temperature. This article is protected by copyright. All rights reserved.
      PubDate: 2016-02-15T22:54:59.354191-05:
      DOI: 10.1002/2015GC006060
  • Impact of warming events on reef‐scale temperature variability as
           captured in two Little Cayman coral Sr/Ca records
    • Abstract: The rising temperature of the world's oceans is affecting coral reef ecosystems by increasing the frequency and severity of bleaching and mortality events. The susceptibility of corals to temperature stress varies on local and regional scales. Insights into potential controlling parameters are hampered by a lack of long term in situ data in most coral reef environments and sea surface temperature (SST) products often do not resolve reef‐scale variations. Here we use 42 years (1970‐2012) of coral Sr/Ca data to reconstruct seasonal‐ to decadal‐scale SST variations in two adjacent but distinct reef environments at Little Cayman, Cayman Islands. Our results indicate that two massive Diploria strigosa corals growing in the lagoon and in the fore reef responded differently to past warming events. Coral Sr/Ca data from the shallow lagoon successfully record high summer temperatures confirmed by in situ observations (>33°C). Surprisingly, coral Sr/Ca from the deeper fore reef is strongly affected by thermal stress events, although seasonal temperature extremes and mean SSTs at this site are reduced compared to the lagoon. The shallow lagoon coral showed decadal variations in Sr/Ca, supposedly related to the modulation of lagoonal temperature through varying tidal water exchange, influenced by the 18.6‐year lunar nodal cycle. Our results show that reef‐scale SST variability can be much larger than suggested by satellite SST measurements. Thus, using coral SST proxy records from different reef zones combined with in situ observations will improve conservation programs that are developed to monitor and predict potential thermal stress on coral reefs. This article is protected by copyright. All rights reserved.
      PubDate: 2016-02-15T11:17:09.778071-05:
      DOI: 10.1002/2015GC006194
  • Performance benchmarks for a next generation numerical dynamo model
    • Abstract: Numerical simulations of the geodynamo have successfully represented many observable characteristics of the geomagnetic field, yielding insight into the fundamental processes that generate magnetic fields in the Earth's core. Because of limited spatial resolution, however, the diffusivities in numerical dynamo models are much larger than those in the Earth's core, and consequently, questions remain about how realistic these models are. The typical strategy used to address this issue has been to continue to increase the resolution of these quasi‐laminar models with increasing computational resources, thus pushing them toward more realistic parameter regimes. We assess which methods are most promising for the next generation of supercomputers, which will offer access to O(106) processor cores for large problems. Here we report performance and accuracy benchmarks from 15 dynamo codes that employ a range of numerical and parallelization methods. Computational performance is assessed on the basis of weak and strong scaling behavior up to 16,384 processor cores. Extrapolations of our weak scaling results indicate that dynamo codes that employ two‐ or three‐dimensional domain decompositions can perform efficiently on up to ∼106 processor cores, paving the way for more realistic simulations in the next model generation. This article is protected by copyright. All rights reserved.
      PubDate: 2016-02-12T08:20:31.344886-05:
      DOI: 10.1002/2015GC006159
  • Robustness of fossil fish teeth for seawater neodymium isotope
           reconstructions under variable redox conditions in an ancient shallow
           marine setting
    • Abstract: Fossil fish teeth from pelagic open ocean settings are considered a robust archive for preserving the neodymium (Nd) isotopic composition of ancient seawater. However, using fossil fish teeth as an archive to reconstruct seawater Nd isotopic compositions in different sedimentary redox environments and in terrigenous‐dominated, shallow marine settings is less proven. To address these uncertainties, fish tooth and sediment samples from a middle Eocene section deposited proximal to the East Antarctic margin at Integrated Ocean Drilling Program Site U1356 were analyzed for major and trace element geochemistry, and Nd isotopes. Major and trace element analyses of the sediments reveal changing redox conditions throughout deposition in a shallow marine environment. However, variations in the Nd isotopic composition and rare earth element (REE) patterns of the associated fish teeth do not correspond to redox changes in the sediments. REE patterns in fish teeth at Site U1356 carry a typical mid‐REE‐enriched signature. However, a consistently positive Ce anomaly marks a deviation from a pure authigenic origin of REEs to the fish tooth. Neodymium isotopic compositions of cleaned and uncleaned fish teeth fall between modern seawater and local sediments and hence could be authigenic in nature, but could also be influenced by sedimentary fluxes. We conclude that the fossil fish tooth Nd isotope proxy is not sensitive to moderate changes in pore water oxygenation. However, combined studies on sediments, pore waters, fish teeth and seawater are needed to fully understand processes driving the reconstructed signature from shallow marine sections in proximity to continental sources. This article is protected by copyright. All rights reserved.
      PubDate: 2016-02-12T03:12:52.265677-05:
      DOI: 10.1002/2015GC006218
  • Age and paleoenvironmental reconstruction of partially remagnetized
           lacustrine sedimentary rocks (Oligocene Aktoprak basin, central Anatolia,
    • Abstract: The age and paleoenvironmental record of lacustrine deposits in the Aktoprak basin of south‐central Turkey provides information about the evolution of topography, including the timing of development of an orographic rain shadow caused by uplift of the mountain ranges fringing the Central Anatolian Plateau. New magnetostratigraphy‐based age estimates, in combination with existing biostratigraphic ages, suggest that the partially remagnetized Kurtulmuş Tepe section of the basin is Chattian (Upper Oligocene). The mean carbon and oxygen stable isotope ratios (δ18O= 24.6 ± 2.0 ‰, δ13C= ‐4.9 ± 1.1‰) are largely constant through the section and indicative of a subtropical, open freshwater lake. These isotopic values are also similar to those of the Chattian Mut basin to the south, on the Mediterranean side of the modern orographic barrier (Tauride Mountains), and indicate absence of an orographic barrier during Late Oligocene basin deposition. Post‐depositional partial remagnetization occurred after tilting of the basin sequence and was mineralogically controlled, affecting grey, carbonate‐rich rocks (average %CaCO3= 82), whereas interlayered pink carbonate‐poor rocks (average %CaCO3= 38) carry a primary, pre‐tilt magnetization. The pink rocks are rich in clay minerals that may have reduced the permeability of these rocks that carry a primary magnetization, concentrating basinal fluid flow in the carbonate‐rich grey layers and leading to the removal and reprecipitation of magnetic minerals. The normal and reverse polarities recorded by the remagnetized rocks suggest that remagnetization occurred over a protracted period of time. This article is protected by copyright. All rights reserved.
      PubDate: 2016-02-12T03:11:56.425476-05:
      DOI: 10.1002/2015GC006209
  • A comparison of U/Th and rapid‐screen 14C dates from Line Island
           fossil corals
    • Authors: Pamela R. Grothe; Kim M. Cobb, Shari L. Bush, Hai Cheng, Guaciara M. Santos, John R. Southon, R. Lawrence Edwards, Daniel M. Deocampo, Hussein R. Sayani
      Abstract: Time‐consuming and expensive radiometric dating techniques limit the number of dates available to construct absolute chronologies for high‐resolution paleoclimate reconstructions. A recently developed rapid‐screen 14C dating technique reduces sample preparation time and per sample costs by 90%, but its accuracy has not yet been tested on shallow‐water corals. In this study, we test the rapid‐screen 14C dating technique on shallow‐water corals by comparing 44 rapid‐screen 14C dates to both high‐precision 14C dates and U/Th dates from mid‐ to late‐Holocene fossil corals collected from the central tropical Pacific (2‐4°N, 157‐160°W). Our results show that 42 rapid‐screen 14C and U/Th dates agree within uncertainties, confirming closed‐system behavior and ensuring chronological accuracy. However, two samples that grew ∼6500yrs ago have calibrated 14C ages ∼1000yrs younger than the corresponding U/Th ages, consistent with diagenetic alteration as indicated by the presence of 15‐23% calcite. Mass balance calculations confirm that the observed dating discrepancies are consistent with 14C addition and U removal, both of which occur during diagenetic calcite recrystallization. Under the assumption that aragonite‐to‐calcite replacement is linear through time, we estimate the samples' true ages using the measured 14C and U/Th dates and percent calcite values. Results illustrate that the rapid‐screen 14C dates of Holocene‐aged fossil corals are accurate for samples with less than 2% calcite. Application of this rapid‐screen 14C method to the fossil coral rubble fields from Kiritimati Island reveal significant chronological clustering of fossil coral across the landscape, with older ages farther from the water's edge. This article is protected by copyright. All rights reserved.
      PubDate: 2016-02-12T03:11:37.531305-05:
      DOI: 10.1002/2015GC005893
  • Multiscale seismic heterogeneity in the continental lithosphere
    • Authors: B.L.N. Kennett; T. Furumura
      Abstract: We examine the nature of seismic heterogeneity in the continental lithosphere, with particular reference to Australia. With the inclusion of deterministic large‐scale structure and realistic medium‐scale features there is not a need for strong fine‐scale variations. The resulting multi‐scale heterogeneity model gives a good representation of the character of observed seismograms and their geographic variation, and is also in good agreement with recent direct results on P‐wave reflectivity in the lithosphere. Fine‐scale heterogeneity is pervasive, but strongest in the crust. There is a weak quasi‐laminar component above the lithosphere‐asthenosphere transition with horizontal correlation length of 10 km and vertical correlation length of 0.5 km. Within the transition the aspect ratio of heterogeneity changes and can be well represented with a horizontal correlation length of 5 km and vertical correlation length of 1 km. For the Australian cratons this transition zone needs low intrinsic attenuation (high Q) to sustain the long high‐frequency coda of both P and S waves. The interaction of the different aspects of the heterogeneity is complex and produces a diversity of behaviour depending on the relative thickness of the different lithospheric zones. The multi‐scale model reconciles many of the divergent concepts of the character of heterogeneity based on interpretations of particular aspects of the seismic wavefield. The varying nature of the heterogeneity also ties well with the variations in tectonic character across the Australian continent. This article is protected by copyright. All rights reserved.
      PubDate: 2016-02-06T12:31:42.857977-05:
      DOI: 10.1002/2015GC006200
  • Importance of titanohematite in detrital remanent magnetizations of strata
           spanning the Cretaceous‐Paleogene boundary, Hell Creek region,
    • Authors: Courtney J. Sprain; Joshua M. Feinberg, Paul R. Renne, Mike Jackson
      Abstract: Intermediate composition titanohematite, Fe2‐yTiyO3 with 0.5 ≤ y ≤ 0.7, is seldom the focus of paleomagnetic study and is commonly believed to be rare in nature. While largely overlooked in magnetostratigraphic studies, intermediate titanohematite has been identified as the dominant ferrimagnetic mineral in an array of Late Mesozoic and Early Cenozoic Laramide clastic deposits throughout the central United States. Intermediate titanohematite is ferrimagnetic and has similar magnetic properties to titanomagnetite, except its unique self‐reversing property. Due to these similarities, and with detrital remanent magnetizations masking its self‐reversing nature, intermediate titanohematite is often misidentified in sedimentary deposits. Past studies relied upon non‐magnetic techniques including X‐Ray diffraction and electron microprobe analysis. While these techniques can identify the presence of intermediate titanohematite, they fail to test whether the mineral is the primary recorder. To facilitate the identification of intermediate titanohematite in sedimentary deposits, we characterize this mineral using low‐temperature magnetometry and high‐temperature susceptibility experiments, and present a new identification technique based on titanohematite's self‐reversing property, for sediments that span the Cretaceous‐Paleogene boundary (Hell Creek region, Montana). Results from the self‐reversal test indicate that the majority of remanence is held by minerals that become magnetized parallel to an applied field, but that intermediate, self‐reversing titanohematite (y = 0.53–0.63) is an important ancillary carrier of remanence. While earlier literature suggests that intermediate titanohematite is rare in nature, reanalysis using specialized rock magnetic techniques may reveal that it's more abundant in the rock record, particularly within depositional basins adjacent to calc‐alkaline volcanics, than previously thought. This article is protected by copyright. All rights reserved.
      PubDate: 2016-02-05T10:45:50.824336-05:
      DOI: 10.1002/2015GC006191
  • An evolving view of transition zone and mid‐mantle viscosity
    • Authors: Scott D. King
      PubDate: 2016-02-02T17:44:24.564192-05:
      DOI: 10.1002/2016GC006279
  • P wave velocity structure below India and Tibet incorporating anisotropic
           delay time effects
    • Authors: Debasis D. Mohanty; Arun Singh, Leland J. O'Driscoll, M. Ravi Kumar, D. Srinagesh, Eugene D. Humphreys
      Abstract: We incorporate the effects of anisotropy to refine the continental scale 3D isotropic velocity model previously produced for India and Tibet by inverting 52,050 teleseismic P wave residuals. We have exploited a total of 1648 individual SKS splitting parameters to calculate the P wave travel time corrections due to azimuthal anisotropy. Our results suggest that anisotropy affects the P wave delays significantly (‐0.3 to +0.5 s). Integration of these corrections into the 3D modelling is achieved in two ways a) a priori adjustment to the delay time vector b) inverting only for anisotropic delays by introducing strong damping above 80 km and below 360 km depths and then subtracting the obtained anisotropic artifact image from the isotropic image, to get the corrected image. Under the assumption of azimuthal anisotropy resulting from lattice preferred orientation (LPO) alignment due to horizontal flow, the bias in isotropic P‐wave tomographic images is clear. The anisotropy corrected velocity perturbations are in the range of ±1.2% at depths of around 150 km and reduced further at deeper levels. Although the bias due to anisotropy does not affect the gross features, it does introduce certain artifacts at deeper levels. This article is protected by copyright. All rights reserved.
      PubDate: 2016-02-02T17:41:18.05163-05:0
      DOI: 10.1002/2015GC006064
  • Comprehensive Pb‐Sr‐Nd‐Hf isotopic, trace element, and
           mineralogical characterization of mafic to ultramafic rock reference
    • Abstract: Controlling the accuracy and precision of geochemical analyses requires the use of characterized reference materials with matrices similar to those of the unknown samples being analyzed. We report a comprehensive Pb‐Sr‐Nd‐Hf isotopic and trace element concentration dataset, combined with quantitative phase analysis by XRD Rietveld refinement, for a wide range of mafic to ultramafic rock reference materials analyzed at the Pacific Centre for Isotopic and Geochemical Research, University of British Columbia. The samples include a pyroxenite (NIM‐P), five basalts (BHVO‐2, BIR‐1a, JB‐3, BE‐N, GSR‐3), a diabase (W‐2), a dolerite (DNC‐1), a norite (NIM‐N), and an anorthosite (AN‐G); results from a leucogabbro (Stillwater) are also reported. Individual isotopic ratios determined by MC‐ICP‐MS and TIMS, and multi‐element analyses by HR‐ICP‐MS are reported with 4‐12 complete analytical duplicates for each sample. The basaltic reference materials have coherent Sr and Nd isotopic ratios with external precision below 50 ppm (2SD) and below 100 ppm for Hf isotopes (except BIR‐1a). For Pb isotopic reproducibility, several of the basalts (JB‐3, BHVO‐2) require acid leaching prior to dissolution. The plutonic reference materials also have coherent Sr and Nd isotopic ratios (
      PubDate: 2016-02-02T17:40:40.326903-05:
      DOI: 10.1002/2015GC006181
  • High‐resolution paleomagnetic and sedimentological investigations on
           the Tibetan Plateau for the past 16 ka cal B.P.—The Tangra Yumco
    • Abstract: The spatial distribution of paleomagnetic secular variation (PSV) records on the Tibetan Plateau and adjacent areas covering the Holocene and Late Glacial is sparse. In order to reconstruct PSV in this area, a piston core covering the past 17.5 ka cal BP retrieved from Lake Tangra Yumco, southern‐central Tibetan Plateau, was analyzed. In the laminated sediments several event layers are intercalated. Those were identified by sedimentological analysis and excluded for age‐depth modelling and interpretation. Paleomagnetic measurements on u‐channels reveal two contrasting core sections. The lower section (dated to 17.5‐15.9 ka cal BP) is very coarse grained (up to 220 µm) and characterized by low intensities (0.8 mA m−1) and high maximum angular deviation values (mean 25°), making it unsuitable for PSV reconstruction. In contrast, the upper unit (dated to
      PubDate: 2016-02-02T17:40:25.943231-05:
      DOI: 10.1002/2015GC006023
  • Constraining mantle viscosity structure for a thermochemical mantle using
           the geoid observation
    • Authors: Xi Liu; Shijie Zhong
      Abstract: Long‐wavelength geoid anomalies provide important constraints on mantle dynamics and viscosity structure. Previous studies have successfully reproduced the observed geoid using seismically inferred buoyancy in whole‐mantle convection models. However, it has been suggested that large low shear velocity provinces (LLSVPs) underneath Pacific and Africa in the lower mantle are chemically distinct and are likely denser than the ambient mantle. We formulate instantaneous flow models based on seismic tomographic models to compute the geoid and constrain mantle viscosity by assuming both thermochemical and whole‐mantle convection. Geoid modelling for the thermochemical model is performed by considering the compensation effect (Liu & Zhong, 2015) of dense thermochemical piles and removing buoyancy structure of the compensation layer in the lower mantle. Thermochemical models well reproduce the observed geoid, thus reconciling the geoid with the interpretation of LLSVPs as dense thermochemical piles. The viscosity structure inverted for thermochemical models is nearly identical to that of whole‐mantle models. In the preferred model, the lower mantle viscosity is ∼10 times higher than the upper mantle viscosity that is ∼10 times higher than the transition zone viscosity. The weak transition zone is consistent with the proposed high water content there. The geoid in thermochemical mantle models is sensitive to seismic structure at mid‐mantle depths, suggesting a need to improve seismic imaging resolution there. The geoid modelling constrains the vertical extent of dense and stable chemical piles to be within ∼500 km above CMB. Our results have implications for mineral physics, seismic tomographic studies, and mantle convection modelling. This article is protected by copyright. All rights reserved.
      PubDate: 2016-02-02T17:40:11.947433-05:
      DOI: 10.1002/2015GC006161
  • Comment on “Determination of low B/Ca ratios in carbonates using
           ICP‐QQQ)” by S. D. Fernandez et al.
    • Abstract: The use of 46Ca to determine the amount of Ca will have large uncertainties in B/Ca ratios, due to large uncertainty in isotope abundance of 46Ca. Also the large discrepancy of results between ICP‐QQQ MS data and those obtained by TIMS and SIMS cannot be explained by the tailing effect and isobaric interference from 11C at 11B in TIMS. This article is protected by copyright. All rights reserved.
      PubDate: 2016-01-25T03:15:54.939174-05:
      DOI: 10.1002/2015GC006139
  • Reply to comments by S. K. Aggarwal and C.‐F. You on
           “Determination of low B/Ca ratios in carbonates using
    • PubDate: 2016-01-25T02:57:22.324903-05:
      DOI: 10.1002/2015GC006187
  • Crustal anisotropy beneath Pacific Ocean islands from harmonic
           decomposition of receiver functions
    • Authors: Tolulope Morayo Olugboji; Jeffrey Park
      Abstract: Crustal anisotropy beneath ocean islands can be attributed to preferentially aligned minerals, cracks, or dike structures. Stacked with harmonic weighting, receiver functions from permanent ocean‐island stations display evidence of strong and distinct anisotropy parameters in the underlying crust and underplated layer. We analyze data for eleven IRIS‐GSN stations in the Pacific Ocean. We observe the prevalence of two‐lobed receiver function (RF) amplitude variations with back‐azimuth, consistent with “slow” tilted‐axis anisotropy. In most cases the anisotropy is accommodated in the underplated crust. Synthetic modeling of a representative station indicates that the strength of anisotropy of Vp=10% and Vs=5% is possible. The strike direction of the inferred symmetry axis tends to align with plate motion, with some scatter. At stations in the northwest Pacific i.e. KWAJ, TARA, and WAKE, the strike direction of the symmetry axis aligns with plate motion at the time of volcano emplacement. Beneath station POHA and the closest stations to the present‐day Hawaiian hotspot, alignment of the symmetry axis is almost orthogonal to the plate motion. We attribute the crustal anisotropy to the preferred alignment of dike structures that transported asthenospheric magma toward the seafloor volcanic edifice. Our results suggest that the thermal‐plume origin for ocean islands must be supplemented by tectonic‐stress heterogeneities that allow magma to penetrate the lithosphere via fractures. Magma‐transport fractures should align normal to the least‐compressive direction, which are predicted by theoretical models to align approximately with plate motion at the time of emplacement. This article is protected by copyright. All rights reserved.
      PubDate: 2016-01-20T18:06:29.002305-05:
      DOI: 10.1002/2015GC006166
  • Eocene to Pleistocene magmatic evolution of the Delarof Islands, Aleutian
    • Authors: Allen J. Schaen; Brian R. Jicha, Suzanne M. Kay, Brad S. Singer, Ashley Tibbetts
      Abstract: The Delarof Islands in the Aleutian Arc near 179º W record ∼37 million years of discontinuous arc magmatism along a SW‐NE cross‐arc transect from near the trench to the active volcanic front. Geochemical and geochronologic data from the pre‐Pleistocene volcanic record in this region are limited and the 40Ar/39Ar, isotopic, and trace element data presented here are the first from units older than the Pleistocene‐Holocene volcanoes (Tanaga, Gareloi). Twenty‐two new 40Ar/39Ar ages establish a temporal framework for geochemical data and reveal that magmatism in the Delarof region was coincident with two arc‐wide magmatic flare ups in the late Eocene/early Oligocene and latest Miocene/Pliocene. Mafic lavas and plutons in the southern Delarofs give 40Ar/39Ar plateau ages ranging from 36.8 ± 0.2 to 26.9 ± 0.6 Ma on Amatignak Island and 37.0 ± 0.2 to 29.3 ± 1.0 Ma on Ulak Island. To the north 25 km, 40Ar/39Ar ages from the central Delarof Islands, Kavalga, Ogliuga, and Skagul are late Miocene (6.28 ± 0.04 Ma) to Pliocene (4.77 ± 0.18 Ma) with younger ages to the northeast. A significant transition in arc chemistry occurs in the Pleistocene where lavas from active volcanoes Gareloi and Tanaga exhibit higher sediment and hydrous fluid signatures (Th/La, Cs/Ta, La/Sm, LILE abundances) and lower 143Nd/144Nd than older Delarof Island units closer to the trench. Similar findings from Eocene‐Miocene lavas from Amchitka to Adak suggest that a previously minor sediment melt component become more pronounced in the Quaternary. This article is protected by copyright. All rights reserved.
      PubDate: 2016-01-20T17:50:35.731893-05:
      DOI: 10.1002/2015GC006067
  • Small‐scale spatial variation in near‐surface turbidites
           around the JFAST site near the Japan Trench
    • Authors: Shuro Yoshikawa; Toshiya Kanamatsu, Takafumi Kasaya
      Abstract: This paper aims to improve our understanding of the depositional processes associated with turbidites related to recent earthquake events. A series of short sediment cores (ca. 20–30 cm long) were recovered from the landward slope of the Japan Trench around JFAST (Japan Trench Fast Drilling Project) site C0019 by a remotely operated vehicle, KAIKO 7000 II, and the sample sites were accurately located using an LBL (long base line) acoustic navigation system. The properties of the cores were analyzed using visual observations, soft X‐ray radiographs, smear slides, measurement of anisotropy of magnetic susceptibility, and analysis of radioactive elements (134Cs, 137Cs, and excess 210Pb). For the first time, small‐scale (ca. 200–1000 m) spatial variations in recent earthquake‐triggered deep‐sea turbidites, the formation of which was probably linked to the 2011 Tohoku‐oki earthquake, are described. We also examine the submarine landslide that probably generated the sediment unit below the turbidites, which is thought to be an important process in the study area. The spatial distribution and characteristics of the near‐surface seismoturbidite obtained immediately after the earthquake, presented here, will enable precise calibration of offshore evidence of recent earthquakes, and thus facilitate the use of the sedimentary archive for paleoseismic interpretations. Furthermore, although sampling for turbidite seismology on steep slopes has not been widely performed previously, our results suggest that the recent event deposits may be continuously tracked from the slope to the basin using a combination of the present sampling method and conventional large‐scale investigation techniques. This article is protected by copyright. All rights reserved.
      PubDate: 2016-01-11T17:51:04.299981-05:
      DOI: 10.1002/2015GC006114
  • Surface Slip during Large Owens Valley Earthquakes
    • Abstract: The 1872 Owens Valley earthquake is the third largest known historical earthquake in California. Relatively sparse field data and a complex rupture trace, however, inhibited attempts to fully resolve the slip distribution and reconcile the total moment release. We present a new, comprehensive record of surface slip based on lidar and field investigation, documenting 162 new measurements of laterally and vertically displaced landforms for 1872 and prehistoric Owens Valley earthquakes. Our lidar analysis uses a newly developed analytical tool to measure fault slip based on cross‐correlation of sub‐linear topographic features and produce a uniquely shaped probability density function (PDF) for each measurement. Stacking PDFs along strike to form cumulative offset probability distribution plots (COPDs) highlights common values corresponding to single‐ and multiple‐event displacements. Lateral offsets for 1872 vary systematically from ∼1.0–6.0 m and average 3.3 ± 1.1 m. Vertical offsets are predominantly east‐side down between ∼0.1–2.4 m, with a mean of 0.8 ± 0.5 m. The average lateral‐to‐vertical ratio compiled at specific sites is ∼6:1. Summing displacements across sub‐parallel, overlapping ruptures implies a maximum of 7–11 m and net average of 4.4 ± 1.5 m, corresponding to a geologic Mw ∼7.5 for the 1872 event. We attribute progressively higher‐offset lateral COPD peaks at 7.1 ± 2.0 m, 12.8 ± 1.5 m, and 16.6 ± 1.4 m to three earlier large surface ruptures. Evaluating cumulative displacements in context with previously dated landforms in Owens Valley suggests relatively modest rates of fault slip, averaging between ∼0.6–1.6 mm/yr (1σ) over the late Quaternary. This article is protected by copyright. All rights reserved.
      PubDate: 2016-01-08T19:13:33.397692-05:
      DOI: 10.1002/2015GC006033
  • Issue Information
    • Pages: 643 - 644
      PubDate: 2016-04-26T02:05:16.300906-05:
      DOI: 10.1002/ggge.20830
  • A possible transoceanic tsunami directed toward the U.S. west coast from
           the Semidi segment, Alaska convergent margin
    • Authors: Roland von Huene; John J. Miller, Peter Dartnell
      Abstract: The Semidi segment of the Alaska convergent margin appears capable of generating a giant tsunami like the one produced along the nearby Unimak segment in 1946. Reprocessed legacy seismic reflection data and a compilation of multibeam bathymetric surveys reveal structures that could generate such a tsunami. A 200 km long ridge or escarpment with crests >1 km high is the surface expression of an active out‐of‐sequence fault zone, recently referred to as a splay fault. Such faults are potentially tsunamigenic. This type of fault zone separates the relatively rigid rock of the margin framework from the anelastic accreted sediment prism. Seafloor relief of the ridge exceeds that of similar age accretionary prism ridges indicating preferential slip along the splay fault zone. The greater slip may derive from Quaternary subduction of the Patton Murray hot spot ridge that extends 200 km toward the east across the north Pacific. Estimates of tsunami repeat times from paleotsunami studies indicate that the Semidi segment could be near the end of its current inter‐seismic cycle. GPS records from Chirikof Island at the shelf edge indicate 90% locking of plate interface faults. An earthquake in the shallow Semidi subduction zone could generate a tsunami that will inundate the US west coast more than the 1946 and 1964 earthquakes because the Semidi continental slope azimuth directs a tsunami southeastward. This article is protected by copyright. All rights reserved.
      PubDate: 2015-12-15T10:17:00.430602-05:
      DOI: 10.1002/2015GC006147
  • Seismicity distribution and locking depth along the Main Marmara Fault,
    • Abstract: The seismicity along the Main Marmara Fault (MMF) below the Marmara Sea is analyzed during the 2007‐2012 period to provide insights on the recent evolution of this important regional seismic gap. High precision locations show that seismicity is strongly varying along strike and depth providing fine details of the fault behavior that are unaccessible from geodetic inversions. The activity strongly clusters at the regions of transition between basins. The Central basin shows significant seismicity located below the shallow locking depth inferred from GPS measurements. Its b‐value is low and the average seismic slip is high. All observations are consistent with a deep creep of this segment. On the contrary, the Kumburgaz basin at the center of the fault shows sparse seismicity with the hallmarks of a locked segment. In the eastern Marmara Sea, the seismicity distribution along the Princes Island segment in the Cinarcik basin, is consistent with the geodetic locking depth of 10km and a low contribution to the regional seismic energy release. The assessment of the locked segment areas provide an estimate of the magnitude of the main forthcoming event to be about 7.3 assuming that the rupture will not enter significantly within creeping domains. This article is protected by copyright. All rights reserved.
      PubDate: 2015-12-09T06:39:04.370361-05:
      DOI: 10.1002/2015GC006120
  • Composition, geometry, and emplacement dynamics of a large volcanic island
           landslide offshore Martinique: From volcano flank‐collapse to
           seafloor sediment failure
    • Abstract: Landslides are common features in the vicinity of volcanic islands. In this contribution, we investigate landslides emplacement and dynamics around the volcanic island of Martinique based on the first scientific drilling of such deposits. The evolution of the active Montagne Pelée volcano on this island has been marked by three major flank‐collapses that removed much of the western flank of the volcano. Subaerial collapse volumes vary from 2 to 25 km3 and debris avalanches flowed into the Grenada Basin. High‐resolution seismic data (AGUADOMAR – 1999, CARAVAL – 2002 and GWADASEIS – 2009) is combined with new drill cores that penetrate up to 430 m through the three submarine landslide deposits previously associated to the aerial flank‐collapses (Site U1399, Site U1400, Site U1401, IODP Expedition 340, Joides Resolution, March‐April 2012). This combined geophysical and core data provide an improved understanding of landslide processes offshore a volcanic island. The integrated analysis shows a large submarine landslide deposit, without debris avalanche deposits coming from the volcano, comprising up to 300 km3 of remobilized seafloor sediment that extends for 70 km away from the coast and covers an area of 2100 km2. Our new data suggest that the aerial debris avalanche deposit enter the sea but stop at the base of submarine flank. We propose a new model dealing with seafloor sediment failures and landslide propagation mechanisms, triggered by volcanic flank‐collapse events affecting Montagne Pelée volcano. Newly recognized landslide deposits occur deeper in the stratigraphy, suggesting the recurrence of large‐scale mass‐wasting processes offshore the island and thus, the necessity to better assess the associated tsunami hazards in the region. This article is protected by copyright. All rights reserved.
      PubDate: 2015-11-25T17:44:35.763497-05:
      DOI: 10.1002/2015GC006034
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