for Journals by Title or ISSN
for Articles by Keywords

Publisher: American Geophysical Union (AGU)   (Total: 17 journals)

Geochemistry, Geophysics, Geosystems     Full-text available via subscription   (Followers: 26, SJR: 2.56, h-index: 69)
Geophysical Research Letters     Full-text available via subscription   (Followers: 55, SJR: 3.493, h-index: 157)
Global Biogeochemical Cycles     Full-text available via subscription   (Followers: 5, SJR: 3.239, h-index: 119)
J. of Advances in Modeling Earth Systems     Open Access   (Followers: 2, SJR: 1.944, h-index: 7)
J. of Geophysical Research : Atmospheres     Partially Free   (Followers: 22)
J. of Geophysical Research : Biogeosciences     Full-text available via subscription   (Followers: 7)
J. of Geophysical Research : Earth Surface     Partially Free   (Followers: 25)
J. of Geophysical Research : Oceans     Partially Free   (Followers: 14)
J. of Geophysical Research : Planets     Full-text available via subscription   (Followers: 15)
J. of Geophysical Research : Solid Earth     Full-text available via subscription   (Followers: 26)
J. of Geophysical Research : Space Physics     Full-text available via subscription   (Followers: 15)
Paleoceanography     Full-text available via subscription   (Followers: 3, SJR: 3.22, h-index: 88)
Radio Science     Full-text available via subscription   (Followers: 3, SJR: 0.959, h-index: 51)
Reviews of Geophysics     Full-text available via subscription   (Followers: 20, SJR: 9.68, h-index: 94)
Space Weather     Full-text available via subscription   (Followers: 3, SJR: 1.319, h-index: 19)
Tectonics     Full-text available via subscription   (Followers: 9, SJR: 2.748, h-index: 85)
Water Resources Research     Full-text available via subscription   (Followers: 85, SJR: 2.189, h-index: 121)
Journal Cover   Geochemistry, Geophysics, Geosystems
  [SJR: 2.56]   [H-I: 69]   [26 followers]  Follow
   Full-text available via subscription Subscription journal
   ISSN (Online) 1525-2027
   Published by American Geophysical Union (AGU) Homepage  [17 journals]
  • Analysis of bubble plume distributions to evaluate methane hydrate
           decomposition on the continental slope
    • Authors: H. Paul Johnson; Una K. Miller, Marie S. Salmi, Evan A. Solomon
      Abstract: Cascadia margin sediments contain a rich reservoir of carbon derived both from terrestrial input and sea surface productivity. A portion of this carbon exists as solid gas hydrate within sediment pore spaces which previous studies have shown to be a methane reservoir of substantial size on both the Vancouver Island and Oregon portions of the Cascadia margin. Multi‐channel seismic reflection profiles on the Cascadia margin show the widespread presence of Bottom Simulating Reflectors (BSRs) within the sediment column, indicating the gas hydrate reservoir extends from the deformation front at 3000 meters depth to the upper limit of gas hydrate stability near 500 meters water depth. In this study, we compile an inventory of methane bubble plume sites on the Cascadia margin identified in investigations carried out for a range of interdisciplinary goals that also include sites volunteered by commercial fishermen. High plume density anomalies are associated with both the continental shelf (
      PubDate: 2015-10-01T11:31:04.138528-05:
      DOI: 10.1002/2015GC005955
  • A high‐order numerical study of reactive dissolution in an upwelling
           heterogeneous mantle: 2. Effect of shear deformation
    • Authors: Conroy Baltzell; E.M. Parmentier, Yan Liang, Seshu Tirupathi
      Abstract: High‐porosity dunite channels produced by orthopyroxene dissolution may provide pathways for orthopyroxene‐undersaturated melt generated in the deep mantle to reach shallower depth without extensive chemical reequilibration with surrounding mantle. Previous studies have considered these high porosity channels and melt localization in the presence of a uniform upwelling mantle flow through the process of melt‐rock reaction as well as shear deformation; but not both simultaneously. In this Part II of a numerical study of high‐porosity melt and dunite channel formation during reactive dissolution, we considered the effect of shear deformation on channel distribution and channel geometry in an upwelling and viscously compacting mantle column. We formulated a high‐order numerical experiment using conditions similar to those in Part I, but with an additional prescribed horizontal shearing component in the solid matrix, as could be present in flowing mantle beneath spreading centers. Our focus was to examine orthopyroxene dissolution to determine the behavior of dunite formation and its interaction with melt flow field, by varying the upwelling and shear rate, orthopyroxene solubility gradient, and domain height. Introduction of shearing tilts the developing dunite, causing asymmetry in the orthopyroxene gradient between the dunite channels and the surrounding harzburgite. The downwind gradient is sharp, nearly discontinuous; whereas the upwind gradient is more gradual. For higher shear rates, a wave‐like pattern of alternating high and low porosity bands form on the downwind side of the channel. The band spacing increases with increasing shear rate, relative melt flow, and orthopyroxene solubility gradient, whereas the band angle is independent of solubility gradient and increases with increasing shear rate and decreasing relative melt flow rate. Such features could be observable in the field and provide evidence for mantle shearing. Standing wave‐like patterns of melt fraction also develop on the downwind side with possible implications for the interpretation of seismic velocities in upwelling mantle. This article is protected by copyright. All rights reserved.
      PubDate: 2015-10-01T11:30:42.237561-05:
      DOI: 10.1002/2015GC006038
  • Anisotropic upper crust above the aftershock zone of the 2013 Ms 7.0
           Lushan earthquake from the shear wave splitting analysis
    • Authors: Ying Liu; Haijiang Zhang, Xin Zhang, Shunping Pei, Meijian An, Shuwen Dong
      Abstract: We have conducted a systematic shear wave splitting analysis using 1000 selected aftershocks with M >2 from the 2013 Ms 7.0 Lushan earthquake along the Longmenshan fault system in southwest China. Polarization directions of fast shear waves show a bimodal distribution with one dominant direction approximately parallel to the fault strike and the other close to the regional maximum horizontal compressive stress direction. This indicates that in this area mechanisms causing crustal seismic anisotropy are both stress induced and fault zone structure controlled. Delay times between fast and slow shear waves do not show a clear trend of increase for deeper events, suggesting the anisotropic zone is mostly above the aftershocks, which are generally located below 8 km. We further applied a shear wave splitting tomography method to measured delay times to characterize the spatial distribution of seismic anisotropy. The three‐dimensional anisotropic percentage model shows strong anisotropy above 8 km but low anisotropy below it. The mainshock slip zone and its aftershocks are associated with very low or negligible anisotropy and high velocity, indicating that the zones with high anisotropy and low velocity are mechanically weak and it is difficult for stress to accumulate there. The main and back reverse fault zones are associated with high anisotropic anomalies above ∼8 km, likely caused by shear fabric or microfractures aligned parallel to the fault zone. This article is protected by copyright. All rights reserved.
      PubDate: 2015-10-01T10:58:01.462905-05:
      DOI: 10.1002/2015GC005972
  • Geomagnetic paleointensity in historical pyroclastic density currents:
           Testing the effects of emplacement temperature and postemplacement
    • Authors: Julie A. Bowles; Jeffrey S. Gee, Mike J. Jackson, Margaret S. Avery
      Abstract: Thellier‐type paleointensity experiments were conducted on welded ash matrix or pumice from the 1912 Novarupta (NV) and 1980 Mt. St. Helens (MSH) pyroclastic density currents (PDCs) with the intention of evaluating their suitability for geomagnetic paleointensity studies. PDCs are common worldwide, but can have complicated thermal and alteration histories. We attempt to address the role that emplacement temperature and post‐emplacement hydrothermal alteration may play in non‐ideal paleointensity behavior of PDCs. Results demonstrate two types of non‐ideal behavior: unstable remanence in multi‐domain (MD) titanomagnetite, and non‐ideal behavior linked to fumarolic and vapor phase alteration. Emplacement temperature indirectly influences MSH results by controlling the fraction of homogenous MD vs. oxyexsolved pseudo‐single domain titanomagnetite. NV samples are more directly influenced by vapor phase alteration. The majority of NV samples show distinct two‐slope behavior in the natural remanent magnetization – partial thermal remanent magnetization plots. We interpret this to arise from a (thermo)chemical remanent magnetization associated with vapor phase alteration, and samples with high water content (>0.75% loss on ignition) generate paleointensities that deviate most strongly from the true value. We find that PDCs can be productively used for paleointensity, but that – as with all paleointensity studies – care should be taken in identifying potential post‐emplacement alteration below the Curie temperature, and that large, welded flows may be more alteration‐prone. One advantage in using PDCs is that they typically have greater areal (spatial) exposure than a basalt flow, allowing for more extensive sampling and better assessment of errors and uncertainty. This article is protected by copyright. All rights reserved.
      PubDate: 2015-10-01T10:57:44.952264-05:
      DOI: 10.1002/2015GC005910
  • Neodymium isotopic composition in foraminifera and authigenic phases of
           South China Sea sediments: Implications for the hydrology of the North
           Pacific Ocean over the past 25 kyr
    • Abstract: εNd and normalized Rare Earth Elements (REE) patterns of benthic and planktonic foraminifera and Fe‐Mn coatings precipitated on sediments have been investigated for the South China Sea (SCS) to (1) assess the reliability of the extraction of past seawater εNd in the SCS and to (2) reconstruct past hydrological changes during the last 25 kyr. Reductively cleaned mono‐specific planktonic foraminifera (G. ruber) and mixed benthic foraminifera in core‐top sediments from 1500 to 2400 m display similar εNd values to those of the modern Pacific Deep Water (εNd of ‐3.9 ∼ ‐4.4). Furthermore, the εNd of the reductive cleaning solutions shows similar εNd values to ones obtained on cleaned foraminifera. Combined with PAAS‐normalized REE patterns, these results confirm that the oxidative and reductive cleaning procedure applied to foraminifera does not totally remove all of the Fe‐Mn coating and that εNd values yielded by cleaned planktonic foraminifera retain the εNd imprint of the bottom and/or pore water. εNd values obtained from a leaching procedure carried out on the bulk non‐decarbonated sediments are comparable to the εNd values of the modern PDW, whereas a similar leaching procedure applied to decarbonated sediments reveals a bias due to contamination with Nd deriving from lithogenic particles. In core MD05‐2904, seawater εNd, reconstructed from planktonic foraminifera, indicates that the last glacial period is characterized by lower εNd (‐5.2±0.2 to ‐6.4±0.3) than the late Holocene (‐4.1±0.2). Assuming that Nd input from river does not change strongly the εNd of the PDW of the northern SCS, these εNd variations suggest a higher relative proportions of southern source water in the deep‐water of the western subtropical Pacific Ocean during the last glacial period. This article is protected by copyright. All rights reserved.
      PubDate: 2015-10-01T10:57:29.589346-05:
      DOI: 10.1002/2015GC005871
  • Tectonic slicing of subducting oceanic crust along plate interfaces:
           Numerical modeling
    • Authors: J.B. Ruh; L. Le Pourhiet, Ph. Agard, E. Burov, T. Gerya
      Abstract: Multikilometer‐sized slivers of high‐pressure low‐temperature metamorphic oceanic crust and mantle are observed in many mountain belts. These blueschist and eclogite units were detached from the descending plate during subduction. Large‐scale thermo‐mechanical numerical models based on finite difference marker‐in‐cell staggered grid technique are implemented to investigate slicing processes that lead to the detachment of oceanic slivers and their exhumation before the onset of the continental collision phase. In particular, we investigate the role of the serpentinized sub‐crustal slab mantle in the mechanisms of shallow and deep crustal slicing. Results show that spatially homogeneous serpentinization of the sub‐Moho slab mantle leads to complete accretion of oceanic crust within the accretionary wedge. Spatially discontinuous serpentinization of the slab mantle in form of unconnected patches can lead to shallow slicing of the oceanic crust below the accretionary wedge and to its deep slicing at mantle depths depending on the patch length, slab angle, convergence velocity and continental geothermal gradient. P‐T paths obtained in this study are compared to natural examples of shallow slicing of the Crescent Terrane below Vancouver Island and deeply sliced crust of the Lago Superiore and Saas‐Zermatt units in the Western Alps. This article is protected by copyright. All rights reserved.
      PubDate: 2015-09-29T11:20:04.550608-05:
      DOI: 10.1002/2015GC005998
  • An efficient and general approach for implementing thermodynamic
           phase‐equilibria information in geophysical and geodynamic studies
    • Authors: Juan Carlos Afonso; Sergio Zlotnik, Pedro Diez
      Abstract: We present a flexible, general and efficient approach for implementing thermodynamic phase equilibria information (in the form of sets of physical parameters) into geophysical and geodynamic studies. The approach is based on tensor rank decomposition methods, which transform the original multi‐dimensional discrete information into a separated representation that contains significantly fewer terms, thus drastically reducing the amount of information to be stored in memory during a numerical simulation or geophysical inversion. Accordingly, the amount and resolution of the thermodynamic information that can be used in a simulation or inversion increases substantially. In addition, the method is independent of the actual software used to obtain the primary thermodynamic information, and therefore it can be used in conjunction with any thermodynamic modeling program and/or database. Also, the errors associated with the decomposition procedure are readily controlled by the user, depending on her/his actual needs (e.g. preliminary runs vs full resolution runs). We illustrate the benefits, generality and applicability of our approach with several examples of practical interest for both geodynamic modeling and geophysical inversion/modeling. Our results demonstrate that the proposed method is a competitive and attractive candidate for implementing thermodynamic constraints into a broad range of geophysical and geodynamic studies. MATLAB implementations of the method and examples are provided as supplementary material and can be downloaded from the journal's website. This article is protected by copyright. All rights reserved.
      PubDate: 2015-09-23T09:48:56.706727-05:
      DOI: 10.1002/2015GC006031
  • Architecture of North Atlantic contourite drifts modified by transient
           circulation of the Icelandic mantle plume
    • Abstract: Overflow of Northern Component Water, the precursor of North Atlantic Deep Water, appears to have varied during Neogene times. It has been suggested that this variation is moderated by transient behavior of the Icelandic mantle plume, which has influenced North Atlantic bathymetry through time. Thus pathways and intensities of bottom currents that control deposition of contourite drifts could be affected by mantle processes. Here, we present regional seismic reflection profiles that cross sedimentary accumulations (Björn, Gardar, Eirik and Hatton Drifts). Prominent reflections were mapped and calibrated using a combination of boreholes and legacy seismic profiles. Interpreted seismic profiles were used to reconstruct solid sedimentation rates. Björn Drift began to accumulate in late Miocene times. Its average sedimentation rate decreased at ∼2.5 Ma and increased again at ∼0.75 Ma. In contrast, Eirik Drift started to accumulate in early Miocene times. Its average sedimentation rate increased at ∼5.5 Ma and decreased at ∼2.2 Ma. In both cases, there is a good correlation between sedimentation rates, inferred Northern Component Water overflow, and the variation of Icelandic plume temperature independently obtained from the geometry of diachronous V‐shaped ridges. Between 5.5 and 2.5 Ma, the plume cooled, which probably caused subsidence of the Greenland‐Iceland‐Scotland Ridge, allowing drift accumulation to increase. When the plume became hotter at 2.5 Ma, drift accumulation rate fell. We infer that deep‐water current strength is modulated by fluctuating dynamic support of the Greenland‐Scotland Ridge. Our results highlight the potential link between mantle convective processes and ocean circulation. This article is protected by copyright. All rights reserved.
      PubDate: 2015-09-23T09:46:44.751972-05:
      DOI: 10.1002/2015GC005947
  • Petrofabric development during experimental partial melting and
           recrystallization of a mica‐schist analogue
    • Authors: Bjarne S. G. Almqvist; Andrea R. Biedermann, Iwona Klonowska, Santanu Misra
      Abstract: Magnetic properties and the anisotropy of magnetic susceptibility (AMS) present promising methods to track mineral orientation and petrofabric in rocks that have undergone partial melting. In order to better understand the source of the magnetic signal in these types of rocks, the interpretation of field observations may be integrated with laboratory experiments, designed to re‐create conditions of partial melting. A set of experiments is presented in this study, where synthetic foliated quartz‐muscovite aggregates undergo partial melting at 300 MPa hydrostatic confining pressure and 750°C. Magnetic properties and AMS are measured before and after partial melting. Prior to partial melting, the synthetic aggregate shows a compaction‐related oblate magnetic fabric, dominated by paramagnetic muscovite that contains small amounts of iron. Post‐experiment samples show neoblasts that crystallize from incongruent melt reactions. Most notably for the magnetic fabric, the breakdown of muscovite results in growth of secondary phases of Fe‐bearing spinel and biotite. Isothermal remanence acquisition and temperature‐dependence of susceptibility indicate that the spinel is magnetite. The degree of magnetic anisotropy reduces significantly after partial melting, but notably the orientation of the principal axes of susceptibility mimic the AMS of the original quartz‐muscovite aggregate. Additionally, the post‐experiment samples show a relationship between the amount of sample shortening (compaction) and the degree of magnetic anisotropy and susceptibility ellipsoid shape factor. These results suggest that petrofabrics in rocks that undergo partial melting at near hydrostatic pressure conditions may in part be inherited, or mimic, the original petrofabric of a sedimentary or metasedimentary rock. This article is protected by copyright. All rights reserved.
      PubDate: 2015-09-23T09:43:29.13753-05:0
      DOI: 10.1002/2015GC005962
  • Tremor‐genic slow slip regions may be deeper and warmer and may slip
           slower than nontremor‐genic regions
    • Abstract: Slow slip events (SSEs) are observed worldwide and often coincide with tectonic tremor. Notable examples of SSEs lacking observed tectonic tremor, however, occur beneath Kīlauea Volcano, Hawaii, the Boso Peninsula, Japan, {near San Juan Bautista on the San Andreas Fault, California, and recently in Central Ecuador. These SSEs are similar to other worldwide SSEs in many ways (e.g., size or duration), but lack the concurrent tectonic tremor observed elsewhere; instead they trigger swarms of regular earthquakes. We investigate the physical conditions that may distinguish these non‐tremor‐genic SSEs from those associated with tectonic tremor including: slip velocity, pressure, temperature, fluids and fault asperities, although we cannot eliminate the possibility that tectonic tremor may be obscured in highly attenuating regions. Slip velocities of SSEs at Kīlauea Volcano (∼10−6 m/s) and Boso Peninsula (∼10−7 m/s) are among the fastest SSEs worldwide. Kīlauea Volcano, the Boso Peninsula and Central Ecuador are also among the shallowest SSEs worldwide, and thus have lower confining pressures and cooler temperatures in their respective slow slip zones. {Fluids also likely contribute to tremor generation, and no corresponding zone of high vp/vs has been noted at Kīlauea or Boso. We suggest that the relatively faster slip velocities at Kīlauea Volcano and the Boso Peninsula result from specific physical conditions that may also be responsible for triggering swarms of regular earthquakes adjacent to the slow slip, while different conditions produce slower SSE velocities elsewhere and trigger tectonic tremor. This article is protected by copyright. All rights reserved.
      PubDate: 2015-09-21T11:59:57.677185-05:
      DOI: 10.1002/2015GC005895
  • Which is the better proxy for paleo‐current strength:
           Sortable‐silt mean size (SS‐) or sortable‐silt mean
           grain diameter (d‐SS)? A case study from the Nordic Seas
    • Authors: Andrea D. Tegzes; Eystein Jansen, Richard J. Telford
      Abstract: The coarseness of the 10‐63 µm terrigenous silt (i.e. sortable silt) fraction tends to vary independently of sediment supply in current‐sorted muds in the world's oceans, with coarser sediments representing relatively greater near‐bottom flow speeds. Traditionally the coarseness of this size fraction is described using an index called sortable‐silt mean size (SS‐), which is an arithmetic average calculated from the differential volume or mass distribution of grains within the 10‐63 µm terrigenous silt fraction, where the relative weights of the individual size bins become increasingly disproportionate, with respect to the actual number of grains within those size bins, towards the coarse end of the size range. This not only increases the absolute value of the apparent “mean size” within the 10‐63 μm terrigenous silt fraction, but it may also affect the apparent pattern of relative changes in the coarseness of the sortable‐silt fraction along the core. In addition, it makes SS‐ more prone to biases due to, for example, analytical errors. Here we present a detailed analysis of grain‐size distributions over three selected Holocene time intervals from two complementary sediment cores (JM97‐948/2A, MD95‐2011), extracted from the center of a high‐accumulation area along the flow path of the main branch of the Atlantic Inflow into the Nordic Seas and show that differential‐number‐based statistics, which likely better describes variations in the actual coarseness of the sortable‐silt fraction, may provide a more robust alternative to SS‐. This article is protected by copyright. All rights reserved.
      PubDate: 2015-09-16T11:06:52.292795-05:
      DOI: 10.1002/2014GC005655
  • Issue Information
    • PubDate: 2015-09-16T03:10:04.990145-05:
      DOI: 10.1002/ggge.20564
  • Teleseismic P wave spectra from USArray and implications for upper mantle
           attenuation and scattering
    • Authors: Samantha Cafferky; Brandon Schmandt
      Abstract: Teleseismic P wave amplitude spectra from deep earthquakes recorded by USArray are inverted for maps of upper mantle Δt* for multiple frequency bands within 0.08–2 Hz. All frequency bands show high Δt* regions in the southwestern U.S., southern Rocky Mountains and Appalachian margin. Low Δt* is more common across the cratonic interior. Inversions with narrower frequency bands yield similar patterns, but greater Δt* magnitudes. Even the two standard deviation Δt* magnitude for the widest band is ∼2–7 times greater than predicted by global QS tomography or an anelastic olivine thermal model, suggesting that much of the Δt* signal is non‐thermal in origin. Non‐thermal contributions are further indicated by only a moderate correlation between Δt* and P travel times. Some geographic variations, such as high Δt* in parts of the cratonic interior with high mantle velocities and low heat flow, demonstrate that the influence of temperature is regionally overwhelmed. Transverse spectra are used to investigate the importance of scattering because they would receive no P energy in the absence of 3‐D heterogeneity or anisotropy. Transverse to vertical (T/Z) spectral ratios for stations with high Δt* are higher and exhibit steeper increases with frequency compared to T/Z spectra for low Δt* stations. The large magnitude of Δt* estimates and the T/Z spectra are consistent with major contributions to Δt* from scattering. A weak positive correlation between intrinsic attenuation and apparent attenuation due to scattering may contribute to Δt* magnitude and the moderate correlation of Δt* with travel times. This article is protected by copyright. All rights reserved.
      PubDate: 2015-09-15T03:08:21.060734-05:
      DOI: 10.1002/2015GC005993
  • Missing western half of the Pacific Plate: Geochemical nature of the
           Izanagi–Pacific Ridge interaction with a stationary boundary between
           the Indian and Pacific mantles
    • Abstract: The source mantle of the basaltic ocean crust on the western half of the Pacific Plate was examined using Pb–Nd–Hf isotopes. The results showed that the subducted Izanagi–Pacific Ridge (IPR) formed from both Pacific (180–∼80 Ma) and Indian (∼80–70 Ma) mantles. The western Pacific Plate becomes younger westward and is thought to have formed from the IPR. The ridge was subducted along the Kurile–Japan–Nankai–Ryukyu (KJNR) Trench at 60–55 Ma and leading edge of the Pacific Plate is currently stagnated in the mantle transition zone. Conversely, the entire eastern half of the Pacific Plate, formed from isotopically distinct Pacific mantle along the East Pacific Rise and the Juan de Fuca Ridge, largely remains on the seafloor. The subducted IPR is inaccessible; therefore, questions regarding which mantle might be responsible for the formation of the western half of the Pacific Plate remain controversial. Knowing the source of the IPR basalts provides insight into the Indian–Pacific mantle boundary before the Cenozoic. Isotopic compositions of the basalts from borehole cores (165–130 Ma) in the western Pacific show that the surface oceanic crust is of Pacific mantle origin. However, the accreted ocean floor basalts (∼80–70 Ma) in the accretionary prism along the KJNR Trench have Indian mantle signatures. This indicates the younger western Pacific Plate of IPR origin formed partly from Indian mantle and that the Indian–Pacific mantle boundary has been stationary in the western Pacific at least since the Cretaceous. This article is protected by copyright. All rights reserved.
      PubDate: 2015-09-14T03:54:03.683285-05:
      DOI: 10.1002/2015GC005911
  • Glacioeustasy, Meteoric Diagenesis, and the Carbon Cycle During the
    • Authors: Blake Dyer; Adam C. Maloof, John A. Higgins
      Abstract: Mid‐Carboniferous carbonates in the western United States have undergone Pleistocene Bahamas‐style meteoric diagenesis that may be associated with expanding late Paleozoic ice sheets. Fourteen stratigraphic sections from carbonate platforms illustrate the regional distribution and variable intensity of physical and chemical diagenesis just below the mid‐Carboniferous unconformity. Each section contains top‐negative carbon isotope excursions that terminate in regional exposure surfaces that are associated with some combination of karst towers, desiccation cracks, fabric destructive recrystallization, or extensive root systems. The timing of the diagenesis is synchronous with similarly‐scaled top‐negative carbon isotope excursions observed by others in England, Kazakhstan, and China. The mass flux of negative carbon required to generate similar isotopic profiles across the areal extent of middle Carboniferous platform carbonates is a significant component of the global carbon cycle. We present a simple carbon box model to illustrate that the δ13C of dissolved inorganic carbon in the ocean could be elevated by ∼1.4 textperthousand∼as isotopically lower carbon from the terrestrial organic weathering reacts with exposed platforms before reaching the ocean and atmosphere. These results represent an improvement on global biogeochemical models that have struggled to provide a congruent solution to the high δ13C of the late Paleozoic icehouse. This article is protected by copyright. All rights reserved.
      PubDate: 2015-09-14T03:53:16.26003-05:0
      DOI: 10.1002/2015GC006002
  • Deep subsurface carbon cycling in the Nankai Trough (Japan) –
           evidence of tectonically induced stimulation of a deep microbial
    • Authors: N. Riedinger; M. Strasser, R.N. Harris, G. Klockgether, T.W. Lyons, E. Screaton
      Abstract: The abundance of microbial life and the sources of energy necessary for deep subsurface microbial communities remain enigmatic. Here we investigate deep microbial processes and their potential relationships to tectonic events in sediments from the Nankai Trough offshore Japan, drilled and sampled during IODP (Integrated Ocean Drilling Program) Expedition 316. Observed methane isotope profiles indicate that microbially mediated methane production occurs at Sites C0006 and C0007 in sediments below ∼450 meters below seafloor (mbsf) and ∼425 mbsf, respectively. The active carbon cycling in these deep subsurface sediments is likely related to the highly dynamic tectonic regime at Nankai Trough. We propose that transient increases in temperature have re‐stimulated organic matter degradation at these distinct depths and explore several candidate processes for transient heating. Our favored hypothesis is frictional heating associated with earthquakes. In concert with transient heating leading to the reactivation of recalcitrant organic matter, the heterogeneous sedimentary system provides niches for microbial life. The newly available/accessible organic carbon compounds fuel the microbial community – resulting in an onset of methanogenesis several hundred meters below the seafloor. This process is captured in the methane C‐isotope signal, showing the efficacy of methane C‐isotopes for delineating locations of active microbial processes in deeply buried sediments. Additionally, simple model approaches applied to observed chemical pore water profiles can potentially constrain timing relationships, which can then be linked to causative tectonic events. Our results suggest the occurrence of slip‐to‐the‐trench earthquake(s) 200‐400 year ago, which could relate to historical earthquakes (1707 Hoei and/or 1605 Keicho earthquakes). This article is protected by copyright. All rights reserved.
      PubDate: 2015-09-10T02:29:13.531219-05:
      DOI: 10.1002/2015GC006050
  • Erosion patterns and mantle sources of topographic change across the
           southern African Plateau derived from the shallow and deep records of
    • Authors: Jessica R. Stanley; Rebecca M. Flowers, David R. Bell
      Abstract: Flow in the sub‐lithospheric mantle is increasingly invoked as a mechanism to explain both modern and past surface topography, but the importance of this phenomenon and its influence at different localities are debated. Southern Africa is an elevated continental shield proposed to represent dynamically supported topography. However, this region is also characterized by a complex lithospheric architecture variably affected by Cretaceous heating, thinning, and metasomatic alteration. We used apatite (U‐Th)/He thermochronometry on fifteen Cretaceous kimberlites from an ∼600 km long transect across the Kaapvaal Craton, combined with information from xenoliths in these pipes, to determine the plateau interior erosion history. The goal was to determine the relationships with lithospheric modification patterns and thereby better isolate the sub‐lithospheric contribution to elevation. The results document a wave of erosion from west to east across the craton from ∼120 Ma to
      PubDate: 2015-09-07T07:58:25.560615-05:
      DOI: 10.1002/2015GC005969
  • Inter‐laboratory comparison of magnesium isotopic compositions of 12
           felsic to ultramafic igneous rock standards analyzed by MC‐ICPMS
    • Abstract: To evaluate the inter‐laboratory mass bias for high‐precision stable Mg isotopic analysis of natural materials, a suite of silicate standards ranging in composition from felsic to ultramafic were analyzed in five laboratories by using three types of multi‐collector inductively coupled plasma mass spectrometer (MC‐ICPMS). Magnesium isotopic compositions from all labs were in agreement for most rocks within quoted uncertainties but are significantly (up to 0.3‰ in 26Mg/24Mg, > 4 times of uncertainties) different for some mafic samples. The inter‐laboratory mass bias does not correlate with matrix element/Mg ratios and the mechanism for producing it is uncertain but very likely arises from column chemistry. Our results suggest that standards with different matrices are needed to calibrate the efficiency of column chemistry and caution should be taken when dealing with samples with complicated matrices. Well‐calibrated standards with matrix elements matching samples should be used to reduce the inter‐laboratory mass bias. This article is protected by copyright. All rights reserved.
      PubDate: 2015-09-07T07:25:29.016917-05:
      DOI: 10.1002/2015GC005939
  • Late Miocene to recent plate tectonic history of the southern central
           America convergent margin
    • Authors: K. D. Morell
      Abstract: New plate reconstructions constrain the tectonic evolution of the subducting Cocos and Nazca plates across the southern Central American subduction zone from late Miocene to Recent. Because of the strong relationships between lower and upper (Caribbean) plate dynamics along this margin, these constraints have wide‐ranging implications for the timing and growth of upper plate deformation and volcanism in southern Central America. The reconstructions outline three important events in the Neogene history of this margin: 1) the coeval development of the Panama Triple Junction with the initiation of oblique subduction of the Nazca plate at ∼8.5 Ma; 2) the initiation of seamount and rough crust subduction beginning at ∼3‐4 Ma; and 3) Cocos Ridge subduction from ∼2‐3 Ma. A comparison of these events with independent geologic, geomorphic, volcanic, and stratigraphic datasets reveals that the timing, rates and origin of subducting crust directly impacted the Neogene growth of upper plate deformation and volcanism in southern Central America. These analyses constrain the timing, geometry and causes of a number of significant tectonic and volcanic processes, including rapid Plio‐Quaternary arc‐forearc contraction due to Cocos Ridge subduction, the detachment of the Panama microplate at ∼1‐3 Ma, and the late Miocene cessation of mantle‐wedge‐derived volcanism across ∼300 km of the subduction zone. This article is protected by copyright. All rights reserved.
      PubDate: 2015-09-07T03:49:35.114864-05:
      DOI: 10.1002/2015GC005971
  • Quantitative estimates of Asian dust input to the western Philippine Sea
           in the mid‐late quaternary and its potential significance for
    • Authors: Zhaokai Xu; Tiegang Li, Peter D. Clift, Dhongil Lim, Shiming Wan, Hongjin Chen, Zheng Tang, Fuqing Jiang, Zhifang Xiong
      Abstract: We present a new high‐resolution multiproxy data set of Sr‐Nd isotopes, rare earth element, soluble iron, and total organic carbon data from International Marine Global Change Study Core MD06‐3047 located in the western Philippine Sea. We integrate our new data with published clay mineralogy, rare earth element chemistry, thermocline depth, and δ13C differences between benthic and planktonic foraminifera, in order to quantitatively constrain Asian dust input to the basin. We explore the relationship between Philippine Sea and high‐latitude Pacific eolian fluxes, as well as its significance for marine productivity and atmospheric CO2 during the mid–late Quaternary. Three different indices indicate that Asian dust contributes between ∼15% and ∼50% to the detrital fraction of the sediments. Eolian dust flux in Core MD06‐3047 is similar to that in the polar Southern Pacific sediment. Coherent changes for most dust flux maximum/minimum indicate that dust generation in interhemispheric source areas might have a common response to climatic variation over the mid–late Quaternary. Furthermore, we note relatively good coherence between Asian dust input, soluble iron concentration, local marine productivity, and even global atmospheric CO2 concentration over the entire study interval. This suggests that dust‐borne iron fertilization of marine phytoplankton might have been a periodic process operating at glacial/interglacial time scales over the past 700 ka. We suggest that strengthening of the biological pump in the Philippine Sea, and elsewhere in the tropical Western Pacific during the mid–late Quaternary glacial periods may contribute to the lowering of atmospheric CO2 concentrations during ice ages. This article is protected by copyright. All rights reserved.
      PubDate: 2015-09-07T03:48:54.877371-05:
      DOI: 10.1002/2015GC005929
  • Toward the reconciliation of seismological and petrological perspectives
           on oceanic lithosphere heterogeneity
    • Authors: B.L.N. Kennett; T. Furumura
      Abstract: The character of the high‐frequency seismic phases Po and So, observed after propagation for long distances in the oceanic lithosphere, requires the presence of scattering from complex structure in 3‐D. Current models use stochastic representations of seismic structure in the oceanic lithosphere. The observations are compatible with quasi‐laminate features with horizontal correlation length around 10 km and vertical correlation length 0.5 km, with a uniform level of about 2% variation through the full thickness of the lithosphere. Such structures are difficult to explain with petrological models, which would favour stronger heterogeneity at the base of the lithosphere associated with underplating from frozen melts. Petrological evidence mostly points to smaller‐scale features than suggested by seismology. The models from the different fields have been derived independently, with various levels of simplification. Fortunately it is possible to gently modify the seismological model towards stronger basal heterogeneity, but there remains a need for some quasi‐laminate structure throughout the mantle component of the oceanic lithosphere. The new models help to bridge the gulf between the different viewpoints, but ambiguities remain. This article is protected by copyright. All rights reserved.
      PubDate: 2015-09-07T02:54:57.567973-05:
      DOI: 10.1002/2015GC006017
  • Active mud volcanoes on the continental slope of the Canadian Beaufort Sea
    • Abstract: Morphologic features, 600 to 1,100 m across and elevated up to 30 m above the surrounding seafloor, interpreted to be mud volcanoes were investigated on the continental slope in the Beaufort Sea in the Canadian Arctic. Sediment cores, detailed mapping with an autonomous underwater vehicle and exploration with a remotely operated vehicle show that these are young and actively forming features experiencing ongoing eruptions. Biogenic methane and low‐chloride, sodium‐bicarbonate‐rich waters are extruded with warm sediment that accumulates to form cones and low relief circular plateaus. The chemical and isotopic compositions of the ascending water indicate that a mixture of meteoric water, seawater, and water from clay dehydration has played a significant role in the evolution of these fluids. The venting methane supports extensive siboglinid tubeworms communities and forms some gas hydrates within the near seafloor. We believe that these are the first documented living chemosynthetic biological communities in the continental slope of the western Arctic Ocean. This article is protected by copyright. All rights reserved.
      PubDate: 2015-09-04T18:15:54.741342-05:
      DOI: 10.1002/2015GC005928
  • Influence of plate tectonic mode on the coupled thermochemical evolution
           of Earth's mantle and core
    • Authors: Takashi Nakagawa; Paul J. Tackley
      Abstract: We investigate the influence of tectonic mode on the thermo‐chemical evolution of simulated mantle convection coupled to a parameterized core cooling model. The tectonic mode is controlled by varying the friction coefficient for brittle behavior, producing the three tectonic modes mobile lid (plate tectonics), stagnant lid and episodic lid. The resulting compositional structure of the deep mantle is strongly dependent on tectonic mode, with episodic lid resulting in a thick layer of subducted basalt in the deep mantle, whereas mobile lid produces only isolated piles and stagnant lid no basaltic layering. The tectonic mode is established early on, with subduction initiating at around 60 Myr from the initial state in mobile and episodic cases, triggered by the arrival of plumes at the base of the lithosphere. Crustal production assists subduction initiation, increasing the critical friction coefficient. The tectonic mode has a strong effect on core evolution via its influence on deep mantle structure; episodic cases in which a thick layer of basalt builds up experience less core heat flow and cooling and a failed geodynamo. Thus, a continuous mobile lid mode existing from early times matches Earth's mantle structure and core evolution better than an episodic mode characterized by large‐scale flushing (overturn) events. This article is protected by copyright. All rights reserved.
      PubDate: 2015-09-04T18:15:42.170868-05:
      DOI: 10.1002/2015GC005996
  • The three‐dimensional construction of the Rae craton, central Canada
    • Authors: David B. Snyder; James A. Craven, Mark Pilkington, Michael J. Hillier
      Abstract: Reconstruction of the 3‐dimensional tectonic assembly of early continents, first as Archean cratons and then Proterozoic shields, remains poorly understood. In this paper, all readily available geophysical and geochemical data are assembled in a 3‐D model with the most accurate bedrock geology in order to understand better the geometry of major structures within the Rae craton of central Canada. Analysis of geophysical observations of gravity and seismic wave speed variations revealed several lithospheric‐scale discontinuities in physical properties. Where these discontinuities project upward to correlate with mapped upper crustal geological structures, the discontinuities can be interpreted as shear zones. Radiometric dating of xenoliths provides estimates of rock types and ages at depth beneath sparse kimberlite occurrences. These ages can also be correlated to surface rocks. The 3.6–2.6 Ga Rae craton comprises at least three smaller continental terranes, that ‘cratonized' during a granitic bloom. Cratonization probably represents final differentiation of early crust into a relatively homogeneous, uniformly thin (35–42 km), tonalite‐trondhjemite‐granodiorite crust with pyroxenite layers near the Moho. The peak thermo‐tectonic event at 1.86–1.7 Ga was associated with the Hudsonian orogeny that assembled several cratons and lesser continental blocks into the Canadian Shield using a number of southeast‐dipping megathrusts. This orogeny metasomatized, mineralized and recrystallized mantle and lower crustal rocks, apparently making them more conductive by introducing or concentrating sulfides or graphite. Little evidence exists of thin slabs similar to modern oceanic lithosphere in this Precambrian construction history whereas underthrusting and wedging of continental lithosphere is inferred from multiple dipping discontinuities. This article is protected by copyright. All rights reserved.
      PubDate: 2015-09-03T11:34:39.854031-05:
      DOI: 10.1002/2015GC005957
  • Supercontinental inheritance and its influence on supercontinental
           breakup: The central Atlantic magmatic province and the break up of Pangea
    • Authors: Lisa Whalen; Esteban Gazel, Christopher Vidito, John Puffer, Michael Bizimis, William Henika, Mark J. Caddick
      Abstract: The Central Atlantic Magmatic Province (CAMP) is the large igneous province (LIP) that coincides with the breakup of the supercontinent Pangea. Major and trace element data, Sr‐Nd‐Pb radiogenic isotopes, and high‐precision olivine chemistry were collected on primitive CAMP dikes from Virginia (VA). These new samples were used in conjunction with a global CAMP dataset to elucidate different mechanisms for supercontinent breakup and LIP formation. In the Eastern North America Margin, CAMP flows are found primarily in rift basins that can be divided into northern or southern groups based on different tectonic evolution, rifting histories, and supercontinental inheritance. Geochemical signatures of CAMP suggest an upper mantle source modified by subduction processes. We propose that the greater number of accretionary events or metasomatism by sediment melts as opposed to fluids on the northern versus the southern Laurentian margin during the formation of Pangea led to different subduction‐related signatures in the mantle source of the northern versus southern CAMP lavas. CAMP samples have elevated Ni and low Ca in olivine phenocrysts indicating a significant pyroxenite component in the source, interpreted here as a result from subduction metasomatism. Different collisional styles during the Alleghanian orogeny in the North and South may have led to the diachroneity of the rifting of Pangea. Furthermore, due to a low angle of subduction, the Rheic plate may have underplated the lithosphere, then delaminated thus triggering both the breakup of Pangea and the formation of CAMP. This article is protected by copyright. All rights reserved.
      PubDate: 2015-09-03T11:34:23.730804-05:
      DOI: 10.1002/2015GC005885
  • Modeling olivine CPO evolution with complex deformation
           histories—Implications for the interpretation of seismic anisotropy
           in the mantle
    • Authors: Yuval Boneh; Luiz F.G. Morales, Edouard Kaminski, Philip Skemer
      Abstract: Relating seismic anisotropy to mantle flow requires detailed understanding of the development and evolution of olivine crystallographic preferred orientation (CPO). Recent experimental and field studies have shown that olivine CPO evolution depends strongly on the integrated deformation history, which may lead to differences in how the corresponding seismic anisotropy should be interpreted. In this study, two widely‐used numerical models for CPO evolution – D‐Rex and VPSC – are evaluated to further examine the effect of deformation history on olivine texture and seismic anisotropy. Building on previous experimental work, models are initiated with several different CPOs to simulate unique deformation histories. Significantly, models initiated with a pre‐existing CPO evolve differently than the CPOs generated without pre‐existing texture. Moreover, the CPO in each model evolves differently as a function of strain. Numerical simulations are compared to laboratory experiments by Boneh and Skemer (2014). In general, the D‐Rex and VPSC models are able to reproduce the experimentally observed CPOs, although the models significantly over‐estimate the strength of the CPO and in some instances produce different CPO from what is observed experimentally. Based on comparison with experiments, recommended parameters for D‐Rex are: M* = 10, λ* = 5, and χ = 0.3, and for VPSC: α = 10 ‐ 100. Numerical modeling confirms that CPO evolution in olivine is highly sensitive to the details of the initial CPO, even at strains greater than 2. These observations imply that there is a long transient interval of CPO realignment which must be considered carefully in the modeling or interpretation of seismic anisotropy in complex tectonic settings. This article is protected by copyright. All rights reserved.
      PubDate: 2015-09-01T11:15:15.675919-05:
      DOI: 10.1002/2015GC005964
  • A new method for calibrating a boron isotope paleo‐pH proxy using
           massive Porites corals
    • Authors: Kaoru Kubota; Yusuke Yokoyama, Tsuyoshi Ishikawa, Atsushi Suzuki
      Abstract: The boron isotope ratio (δ11B) of marine biogenic carbonates can reconstruct pH and pCO2 of seawater, and potentially CO2 concentration in the atmosphere. To date, δ11B‐pHSW calibration has been proposed via culturing experiments, where calcifying organisms are cultured under artificially acidified seawater. However, in scleractinian corals, reconstructed pH values using culture‐based calibrations do not agree well with actual observations of seawater CO2 chemistry. Thus, another approach is needed to establish a more reliable calibration method. In this study, we established field‐based calibrations for Chichijima and Tahiti, both located in subtropical gyres where surface seawater is close to CO2 equilibrium. We suggest a new approach to calibration of δ11B‐pH in which the long‐term δ11B variation of massive Porites corals is compared with the decreasing pH trend (i.e., ocean acidification) that has occurred since the Industrial Revolution. This calibration will offer a new avenue for studying seawater CO2 chemistry using coral δ11B in diverse settings, such as upwelling regions, coral reefs, and coastal areas. This article is protected by copyright. All rights reserved.
      PubDate: 2015-09-01T10:49:25.705661-05:
      DOI: 10.1002/2015GC005975
  • Characterization and petrological constraints of the
           mid‐lithospheric discontinuity
    • Abstract: Within continental lithosphere, widespread seismic evidence suggests a sharp discontinuous downward decrease in seismic velocity at 60‐160 km depth. This mid‐lithospheric discontinuity (MLD) may be due to anisotropy, melt, hydration, and/or mantle metasomatism. We survey global seismologic observations of the MLD, including observed depths, velocity contrasts, gradients, and locales across multiple seismic techniques. The MLD is primarily found in regions of thick continental lithosphere and is a decrease in seismic shear velocity (2‐7% over 10‐20 km) at 60‐160 km depth, the majority of observations clustering at 80‐100 km. Of xenoliths in online databases, 25% of amphibole‐bearing xenoliths, 90% of phlogopite‐bearing xenoliths, and none of carbonate‐bearing xenoliths were formed at pressures associated with these depth (2‐5 GPa). We used Perple_X modeling to evaluate the elastic moduli and densities of multiple petrologies to test if the MLD is a layer of crystallized melt. The fractional addition of 5‐10% phlogopite, 10‐15% carbonate, or 45‐100% pyroxenite produce a 2‐7% velocity decrease. We postulate this layer of crystallized melt would originate at active margins of continents and crystallize in place as the lithosphere cools. The concentration of mildly‐incompatible elements (Y, Ho, Er, Yb, and Lu) in xenoliths near the MLD is consistent with higher degrees of melting. Thus, we postulate that the MLD is the seismological signature of a chemical interface related to the paleo‐intersection of a volatile‐rich solidus and progressively cooling lithosphere. Furthermore, the MLD may represent a remnant chemical tracer of the lithosphere‐asthenosphere boundary (LAB) from when the lithosphere was active and young. This article is protected by copyright. All rights reserved.
      PubDate: 2015-09-01T10:47:21.517926-05:
      DOI: 10.1002/2015GC005943
  • Improving shallow‐water carbonate chemostratigraphy by means of
           rudist bivalve sclerochemistry
    • Authors: S. Huck; U. Heimhofer
      Abstract: Deep‐time shallow marine carbonate platforms record distinct biotic responses to climatic and environmental stressors. Unfortunately, precise temporal assignment of these biotic responses is often problematical due to poor biostratigraphic control and/or a significant diagenetic overprint of the neritic bulk carbonate chemostratigraphic inventory. An accurate stratigraphic framework is essential to better understand the causal relation between biotic events recorded by carbonate platforms and environmental changes that, for instance, culminated in mass extinction events or prolonged episodes of oceanic anoxia. Here, we provide an integrated carbon and strontium‐isotope stratigraphy of the Early Cretaceous subtropical Provence carbonate platform in SE France that is based solely on pristine low‐Mg calcite from rudist bivalves. Carbon‐isotope data of geochemically screened rudist fragments enabled reconstruction of a characteristic Barremian pattern including the Mid‐Barremian Event (MBE) that allowed for a precise correlation with stratigraphically well‐constrained Tethyan shallow‐water and hemipelagic reference sections. In order to evaluate ontogenetic carbon‐isotope changes and the overall variability of the shell‐derived carbon‐isotope data, numerous sclerochronological carbon‐isotope profiles of individual large rudist shells are presented. Strontium‐isotope stratigraphy supports the carbon‐isotope based age of the studied sections, but also provides unequivocal evidence for a major hiatus in the depositional record covering large parts of the Late Barremian. In contrast to biostratigraphic and bulk carbonate chemostratigraphic archives, the here established chronostratigraphy of carbonate platform evolution in the southern Provence region demonstrates a two‐fold resurgence of rudist‐rich carbonate platform production during the Early Aptian and arguably the latest Early Aptian. This article is protected by copyright. All rights reserved.
      PubDate: 2015-08-27T03:59:39.319544-05:
      DOI: 10.1002/2015GC005988
  • Ventilation and dissolved oxygen cycle in Lake Superior: Insights from a
           numerical model
    • Authors: Katsumi Matsumoto; Kathy S. Tokos, Chad Gregory
      Abstract: Ventilation and dissolved oxygen in Lake Superior are key factors that determine the fate of various natural and anthropogenic inputs to the lake. We employ an idealized age tracer and biogeochemical tracers in a realistically configured numerical model of Lake Superior to characterize its ventilation and dissolved O2 cycle. Our results indicate that Lake Superior is preferentially ventilated over rough bathymetry and that spring overturning following a very cold winter does not completely ventilate the lake interior. While this is unexpected for a dimictic lake, no part of the lake remains isolated from the atmosphere for more than 300 days. Our results also show that Lake Superior's oxygen cycle is dominated by solubility changes; as a result, the expected relationship between biological consumption of dissolved O2 and ventilation age does not manifest. This article is protected by copyright. All rights reserved.
      PubDate: 2015-08-27T03:50:06.364321-05:
      DOI: 10.1002/2015GC005916
  • Continent scale strike‐slip on a low‐angle fault beneath New
           Zealand's Southern Alps: Implications for crustal thickening in oblique
           collision zones
    • Abstract: New Zealand's Southern Alps lie adjacent to the continent‐scale dextral strike‐slip Alpine Fault, on the boundary between the Pacific and Australian plates. We show with a simple 2‐D model of crustal balancing that the observed crustal root and erosion (expressed as equivalent crustal shortening) is up to twice that predicted by the orthogonal plate convergence since ∼11 Ma, and even since ∼23 Ma when the Alpine Fault formed. We consider two explanations for this, involving a strong component of motion along the length of the plate‐boundary zone. Geophysical data indicate that the Alpine Fault has a listric geometry, flattening at mid‐crustal levels, and has accommodated sideways underthrusting of Australian plate crust beneath Pacific plate crust. The geometry of the crustal root, together with plate reconstructions, require the underthrust crust to be the hyper‐extended part of an asymmetric rift system which formed over 500 km farther south during the Eocene – the narrow remnant part today forms the western margin of the Campbell Plateau. At ∼10 Ma, the hyper‐extended margin underwent shallow subduction in the Puysegur subduction zone, and then was dragged over 300 km along the length of the Southern Alps beneath a low angle (
      PubDate: 2015-08-22T01:08:10.434771-05:
      DOI: 10.1002/2015GC005990
  • Deeply dredged submarine HIMU glasses from the Tuvalu Islands, Polynesia:
           Implications for volatile budgets of recycled oceanic crust
    • Abstract: Ocean island basalts (OIB) with extremely radiogenic Pb‐isotopic signatures are melts of a mantle component called HIMU (high µ, high 238U/204Pb). Until now, deeply‐dredged submarine HIMU glasses have not been available, which has inhibited complete geochemical (in particular, volatile element) characterization of the HIMU mantle. We report major, trace and volatile element abundances in a suite of deeply‐dredged glasses from the Tuvalu Islands. Three Tuvalu glasses with the most extreme HIMU signatures have F/Nd ratios (35.6±3.6) that are higher than the ratio (∼21) for global OIB and MORB, consistent with elevated F/Nd ratios in endmember HIMU Mangaia melt inclusions. The Tuvalu glasses with the most extreme HIMU composition have Cl/K (0.11 to 0.12), Br/Cl (0.0024) and I/Cl (5‐6 × 10−5) ratios that preclude significant assimilation of seawater‐derived Cl. The new HIMU glasses that are least degassed for H2O have low H2O/Ce ratios (75‐84), similar to ratios identified in endmember OIB glasses with EM1 and EM2 signatures, but significantly lower than H2O/Ce ratios (119 to 245) previously measured in melt inclusions from Mangaia. CO2‐H2O equilibrium solubility models suggest that these HIMU glasses (recovered in two different dredges at 2,500 to 3,600 meters water depth) have eruption pressures of 295 to 400 bars. We argue that degassing is unlikely to significantly reduce the primary melt H2O. Thus, the lower H2O/Ce in the HIMU Tuvalu glasses is a mantle signature. We explore oceanic crust recycling as the origin of the low H2O/Ce (∼50 to 80) in the EM1, EM2 and HIMU mantle domains. This article is protected by copyright. All rights reserved.
      PubDate: 2015-08-21T03:16:24.19131-05:0
      DOI: 10.1002/2015GC005966
  • Evolution of permeability across the transition from brittle failure to
           cataclastic flow in porous siltstone
    • Authors: Marco M. Scuderi; Hiroko Kitajima, Brett M. Carpenter, Demian M. Saffer, Chris Marone
      Abstract: Porous sedimentary rocks fail in a variety of modes ranging from localized, brittle deformation to pervasive, cataclastic flow. To improve our understanding of this transition and its affect on fluid flow and permeability, we investigated the mechanical behavior of a siltstone unit within the Marcellus Formation, PA USA, characterized by an initial porosity ranging from 41 to 45%. We explored both hydrostatic loading paths (σ1=σ2=σ3) and triaxial loading paths (σ1>σ2=σ3) while maintaining constant effective pressure (Pe=Pc‐Pp). Samples were deformed with an axial displacement rate of 0.1 μm/s (strain rate of 2x10−6 s−1). Changes in pore water volume were monitored (drained conditions) to measure the evolution of porosity. Permeability was measured at several stages of each experiment. Under hydrostatic loading, we find the onset of macroscropic grain crushing (P*) at 39 MPa. Triaxial loading experiments show a transition from brittle behavior with shear localization and compaction to cataclastic‐flow as confining pressure increases. When samples fail by shear localization, permeability decreases abruptly without significant changes in porosity. Conversely, for cataclastic deformation, permeability reduction is associated with significant porosity reduction. Post‐experiment observation of brittle samples show localized shear zones characterized by grain comminution. Our data show how zones of shear localization can act as barriers to fluid flow and thus modify the hydrological and mechanical properties of the surrounding rocks. Our results have important implications for deformation behavior and permeability evolution in sedimentary systems, and in particular where the stress field is influenced by injection or pumping. This article is protected by copyright. All rights reserved.
      PubDate: 2015-08-20T11:12:37.356814-05:
      DOI: 10.1002/2015GC005932
  • A 1400 year environmental magnetic record from varved sediments of Lake
           Xiaolongwan (northeast China) reflecting natural and anthropogenic soil
    • Authors: Youliang Su; Guoqiang Chu, Qingsong Liu, Zhaoxia Jiang, Xing Gao, Torsten Haberzettl
      Abstract: Lake sediments can provide high‐quality information about human activities. In this study, we investigate a sediment core from Lake Xiaolongwan using magnetic and geochemical methods. The dominant magnetic minerals of this sediment core are stable single domain (SSD) and superparamagnetic (SP) magnetite particles. The increasing amount of SP particles reflected by the rise of magnetic susceptibility and frequency dependent magnetic susceptibility since AD 1500 can be attributed to an increasing influx in pedogenic soil, which is related to a regional‐scale increase in the intensity of human activity in Northeastern China. This extends the timing of human activities, which is independent from climate changes and its effects on local ecosystems in Northeastern China significantly. This article is protected by copyright. All rights reserved.
      PubDate: 2015-08-20T01:38:45.352313-05:
      DOI: 10.1002/2015GC005880
  • First hydrothermal discoveries on the Australian‐Antarctic Ridge:
           Discharge sites, plume chemistry, and vent organisms
    • Abstract: The Australian‐Antarctic Ridge (AAR) is one of the largest unexplored regions of the global mid‐ocean ridge system. Here, we report a multi‐year effort to locate and characterize hydrothermal activity on two 1st‐order segments of the AAR: KR1 and KR2. To locate vent sites on each segment, we used profiles collected by Miniature Autonomous Plume Recorders on rock corers during R/V Araon cruises in March and December of 2011. Optical and oxidation‐reduction‐potential anomalies indicate multiple active sites on both segments. Seven profiles on KR2 found 3 sites, each separated by ∼25 km. Forty profiles on KR1 identified 13 sites, some within a few km of each other. The spatial density of hydrothermal activity along KR1 and KR2 (plume incidence of 0.34) is consistent with the global trend for a spreading rate of ∼70 mm/yr. The densest area of hydrothermal activity, named “Mujin”, occurred along the 20‐km‐long inflated section near the segment center of KR1. Continuous plume surveys conducted in January‐February of 2013 on R/V Araon found CH4/3He (1‐15 × 106) and CH4/Mn (0.01‐0.5) ratios in the plume samples, consistent with a basaltic‐hosted system and typical of ridges with intermediate spreading rates. Additionally, some of the plume samples exhibited slightly higher ratios of H2/3He and Fe/Mn than others, suggesting that those plumes are supported by a younger hydrothermal system that may have experienced a recent eruption. The Mujin‐field was populated by Kiwa crabs and seven‐armed Paulasterias starfish previously recorded on the East Scotia Ridge, raising the possibility of circum‐Antarctic biogeographic connections of vent fauna. This article is protected by copyright. All rights reserved.
      PubDate: 2015-08-18T18:33:52.560033-05:
      DOI: 10.1002/2015GC005926
  • Magnetic unmixing of first‐order reversal curve diagrams using
           principal component analysis
    • Authors: Ioan Lascu; Richard J. Harrison, Yuting Li, Joy R. Muraszko, James E. T. Channell, Alexander M. Piotrowski, David A. Hodell
      Abstract: We describe a quantitative magnetic unmixing method based on principal component analysis (PCA) of first‐order reversal curve (FORC) diagrams. For PCA we resample FORC distributions on grids that capture diagnostic signatures of single‐domain (SD), pseudo‐single‐domain (PSD), and multi‐domain (MD) magnetite, as well as of minerals such as hematite. Individual FORC diagrams are recast as linear combinations of end‐member (EM) FORC diagrams, located at user‐defined positions in PCA space. The EM selection is guided by constraints derived from physical modeling and imposed by data scatter. We investigate temporal variations of two EMs in bulk North Atlantic sediment cores collected from the Rockall Trough and the Iberian Continental Margin. Sediments from each site contain a mixture of magnetosomes and granulometrically distinct detrital magnetite. We also quantify the spatial variation of three EM components (a coarse silt‐sized MD component, a fine silt‐sized PSD component, and a mixed clay‐sized component containing both SD magnetite and hematite) in surficial sediments along the flow path of the North Atlantic Deep Water (NADW). These samples were separated into granulometric fractions, which helped constrain EM definition. PCA‐based unmixing reveals systematic variations in EM relative abundance as a function of distance along NADW flow. Finally, we apply PCA to the combined dataset of Rockall Trough and NADW sediments, which can be recast as a four‐EM mixture, providing enhanced discrimination between components. Our method forms the foundation of a general solution to the problem of unmixing multi‐component magnetic mixtures, a fundamental task of rock magnetic studies. This article is protected by copyright. All rights reserved.
      PubDate: 2015-08-14T03:36:41.971052-05:
      DOI: 10.1002/2015GC005909
  • Intraplate volcanism of the western Pacific: New insights from geological
           and geophysical observations in the Pigafetta Basin
    • Authors: Timothy J. Stadler; Masako Tominaga
      Abstract: Understanding intraplate volcanism is a key to deciphering the Earth's magmatic history. One of the largest intraplate volcanic events occurred during the mid Cretaceous, roughly 75 to 125 Ma in the western Pacific. To investigate the origin and effects of this volcanism on various Earth systems, we present the first comprehensive study of volcanism in the Pigafetta Basin using seismic surveys, magnetic and gravity modeling, and Ocean Drilling Program drill core and well log data from Site 801. Our results show that intraplate volcanism in the Pigafetta Basin coincides with the rest of the western Pacific seamount provinces, supporting the previously suggested plumelets scenario for the origin of intraplate volcanism during the mid Cretaceous volcanic events. We also discover that the late stage volcanism does not overprint the remanant magnetization acquired by the Jurassic ocean crust in the Pigafetta Basin, and hence, marine magnetic anomalies recorded in the Jurassic basement are preserved. Also, the formerly identified Rough Smooth Boundary (RSB) is indistinguishable from any other rough‐smooth topographic boundaries throughout the survey area suggesting that the RSB is unlikely to be a Cretaceous sill‐Jurassic basement boundary. Lastly, the apparent ages and spatial distribution of volcanic features suggests a dynamic history of hydrothermal circulation in the Pigafetta Basin, indicating that hydrothermal circulation was ongoing well past 100 Ma. This article is protected by copyright. All rights reserved.
      PubDate: 2015-08-12T03:42:07.870133-05:
      DOI: 10.1002/2015GC005873
  • Exploring the mineralogical heterogeneities of the Louisville Seamount
    • Authors: Michael J. Dorais
      Abstract: Diopside phenocrysts of the Louisville Seamount Trail show an increase in Ti, Al, and Na with decreasing Mg/(Mg+Fe) as is typical for clinopyroxene in alkalic basalts. Chondrite‐normalized REE patterns of calculated liquids from LA‐ICPMS analyses are comparable to whole‐rock and glass values. Exceptions are clinopyroxene crystals from the Rigil Seamount, the second oldest seamount drilled at the northern end of the chain. Some crystals from this site are strongly zoned with distinct compositional boundaries between cores and mantles. The cores have high Mg/(Mg+Fe) and low Al and Ti concentrations compared to the mantles and phenocrysts. Major element, clinopyroxene discrimination diagrams indicate that the clinopyroxene mantles and phenocrysts crystallized from alkalic basalts. In contrast, the Mg‐rich cores have tholeiitic affinities. The REE abundances of the cores are similar to that of clinopyroxene from transitional tholeiites of the Kerguelen Archipelago. Calculated liquid La/Yb values for the cores have ratios that are similar to transitional tholeiites in Hawaii, whereas the mantles have higher La/Yb values similar to Hawaiian alkalic basalts. The major and trace element compositions of clinopyroxene cores from the Rigil seamount suggest that a transitional tholeiitic magma was present, but no evidence for Hawaiian shield‐type tholeiites was found. Plagioclase crystals from the Rigil seamount have 86Sr/86Sr from 0.70306 to 0.70363, within the range of FOZO. The transitional tholeiitic signature of the Mg‐rich clinopyroxene cores probably did not have a distinct source compared to other Louisville magmas, but more likely indicates a higher degree of partial melting (2‐5%) of that FOZO source. This article is protected by copyright. All rights reserved.
      PubDate: 2015-08-12T03:41:44.858747-05:
      DOI: 10.1002/2015GC005917
  • Tracing mantle‐reacted fluids in magma‐poor rifted margins:
           The example of Alpine Tethyan rifted margins
    • Authors: Victor Hugo; G. Pinto, Gianreto Manatschal, Anne Marie Karpoff, Adriano Viana
      Abstract: The thinning of the crust and the exhumation of subcontinental mantle in magma‐poor rifted margins is accompanied by a series of extensional detachment faults. We show that exhumation along these detachments is intimately related to migration of fluids leading to changes in mineralogy and chemistry of the mantle, crustal and sedimentary rocks. Using field observation and analytical methods, we investigate the role of fluids in the fossil distal margins of the Alpine Tethys. Using Cr‐Ni‐V, Fe and Mn as tracers, we show that fluids used detachment faults as pathways and interacted with the overlying crust and sediments. These observations allow us to discuss when, where and how this interaction happened during the formation of the rifted margin. The results show that: (i) serpentinization of mantle rocks during their exhumation results in the depletion of elements and migration of mantle‐reacted fluids that are channeled along active detachment system; (ii) in earlier‐stages, these fluids affected the overlying syn‐tectonic sediments by direct migration from the underlying detachments; (iii) in later‐stages, these fluids arrived at the seafloor, were introduced into, or “polluted” the seawater and were absorbed by post‐tectonic sediments. We conclude that a significant amount of serpentinization occurred underneath the hyperextended continental crust, and that the mantle‐reacted fluids might have modified the chemical composition of the sediments and seawater. We propose that the chemical signature of serpentinization related to mantle exhumation is recorded in the sediments and may serve as a proxy to date serpentinization and mantle exhumation at present‐day magma‐poor rifted margins. This article is protected by copyright. All rights reserved.
      PubDate: 2015-08-12T03:40:26.956968-05:
      DOI: 10.1002/2015GC005830
  • Geochemistry of river‐borne clays entering the East China Sea
           indicates two contrasting types of weathering and sediment transport
    • Authors: Lei Bi; Shouye Yang, Chao Li, Yulong Guo, Quan Wang, James T. Liu, Ping Yin
      Abstract: The East China Sea is characterized by wide continental shelf receiving a huge input of terrigenous matter from both large rivers and mountainous rivers, which makes it an ideal natural laboratory for studying sediment source‐to‐sink transport processes. This paper presents mineralogical and geochemical data of the clays and bulk sediments from the rivers entering the East China Sea, aiming to investigate the general driving mechanism of silicate weathering and sediment transport processes in East Asian continental margin. Two types of river systems, tectonically‐stable continental rivers and tectonically‐active mountainous rivers, co‐exist in East Asia. As the direct weathering products, clays can better reflect the silicate weathering regimes within the two river systems. Provenance rock types are not the dominant factor causing silicate weathering intensity difference existed in the East Asian rivers. The silicate weathering intensity of tectonically‐stable river basins is primarily driven by monsoon climate, and the sediment transfer is relatively slow because of natural trapping process and increasing damming effect. The geochemistry of these river‐borne sediments can thus indicate paleo‐weathering intensities in East Asian continent. In contrast, silicate weathering intensity in tectonically‐active mountainous rivers is greatly limited by strong physical erosion despite the high temperature and highest monsoon rainfall. The factors controlling silicate weathering in tectonically‐active catchments are complex and thus, it should be prudent to use river sediment records to decipher paleoclimate change. These two different silicate weathering regimes and sediment transport processes are manifestations of the landscape evolution and overall dominate the sedimentation in Asian continental margin. This article is protected by copyright. All rights reserved.
      PubDate: 2015-08-12T03:39:40.424822-05:
      DOI: 10.1002/2015GC005867
  • Heterogeneous and asymmetric crustal accretion: New constraints from
           multibeam bathymetry and potential field data from the Rainbow area of the
           Mid‐Atlantic Ridge (36°15′N)
    • Authors: M. Paulatto; J. P. Canales, R. A. Dunn, R. A. Sohn
      Abstract: At slow‐spreading mid‐ocean ridges, crustal accretion style can vary significantly along and across ridge segments. In magma‐poor regions, seafloor spreading can be accommodated largely by tectonic processes, however, the internal structure and formation mechanism of such highly tectonized crust are not fully understood. We analyze multi‐beam bathymetry and potential field data from the Rainbow area of the Mid‐Atlantic Ridge (35º40'N‐37º40'N), a section of the ridge that shows diverse accretion styles. We identify volcanic, tectonized and sedimented terrain and measure exposed fault area to estimate the tectonic strain, T, and the fraction of magmatic accretion, M. Estimated T values range from 0.2‐0.4 on ridge segments to 0.6‐0.8 at the Rainbow non‐transform discontinuity (NTD). At segment ends T is asymmetric, reflecting asymmetries in accretion rate, topography and faulting between inside and outside offset corners. Detachment faults have formed preferentially at inside corners, where tectonic strain is higher. We identify at least two oceanic core complexes on the fossil trace of the NTD, in addition to the Rainbow massif, which occupies the offset today. A gravity high and low magnetization suggest that the Rainbow massif, which hosts a high‐temperature hydrothermal system, was uplifted by a west dipping detachment fault. Asymmetric plate ages indicate localization of tectonic strain at the inside corners and migration of the detachment towards and across the ridge axis, which may have caused emplacement of magma into the footwall. Hydrothermal circulation and heat extraction is possibly favored by increased permeability generated by fracturing of the footwall and deep‐penetrating second‐generation faults. This article is protected by copyright. All rights reserved.
      PubDate: 2015-08-10T03:51:23.495754-05:
      DOI: 10.1002/2015GC005743
  • Influence of cratonic lithosphere on the formation and evolution of flat
           slabs: Insights from 3‐D time‐dependent modeling
    • Abstract: Several mechanisms have been suggested for the formation of flat slabs including buoyant features on the subducting plate, trenchward motion and thermal or cratonic structure of the overriding plate. Analysis of episodes of flat subduction indicate that not all flat slabs can be attributed to only one of these mechanisms and it is likely that multiple mechanisms work together to create the necessary conditions for flat slab subduction. In this study we examine the role of localized regions of cratonic lithosphere in the overriding plate in the formation and evolution of flat slabs. We explicitly build on previous models, by using time‐dependent simulations with three‐dimensional variation in overriding plate structure. We find that there are two modes of flat subduction: permanent underplating occurs when the slab is more buoyant (shorter or younger), while transient flattening occurs when there is more negative buoyancy (longer or older slabs). Our models show how regions of the slab adjacent to the sub‐cratonic flat portion continue to pull the slab into the mantle leading to highly contorted slab shapes with apparent slab gaps beneath the craton. These results show how the interpretation of seismic images of subduction zones can be complicated by the occurrence of either permanent or transient flattening of the slab, and how the signature of a recent flat slab episode may persist as the slab resumes normal subduction. Our models suggest that permanent underplating of slabs may preferentially occur below thick and cold lithosphere providing a built‐in mechanism for regeneration of cratons. This article is protected by copyright. All rights reserved.
      PubDate: 2015-08-07T04:33:07.19802-05:0
      DOI: 10.1002/2015GC005940
  • Coupled C‐S‐Fe geochemistry in a rapidly accumulating marine
           sedimentary system: Diagenetic and depositional implications
    • Authors: A. Peketi; A. Mazumdar, H.M. Joao, D. J. Patil, A. Usapkar, P. Dewangan
      Abstract: In the present study, we have investigated the C‐S‐Fe systematics in a sediment core (MD161‐13) from the Krishna‐Godavari (K‐G) basin, Bay of Bengal. The core covers the late Holocene period with high overall sedimentation rate of ∼573 cm ky−1. Pore fluid chemical analyses indicate that the depth of the present sulfate methane transition zone (SMTZ) is at ∼6 mbsf. The (ΔTA+ΔCa+ΔMg)/ΔSO42‐ ratios suggest that both organoclastic degradation and anaerobic oxidation of methane (AOM) drive sulfate reduction at the study site. The positive correlation between total organic carbon content (TOC) and chromium reducible sulfur (CRS) content indicates marked influence of organoclastic sulfate reduction on sulfidization. Coupled occurrence of 34S enriched iron sulfide (pyrite) with 12C enriched authigenic carbonate zones are the possible records of paleo‐sulfate methane transition zones where AOM driven focused sulfate reduction was likely fueled by sustained high methane flux from underlying gas rich zone. Aluminium normalized poorly reactive iron (FePR/Al) and La/Yb ratios suggest increasing contribution from Deccan basalts relative to that of Archean‐ Proterozoic granitic complex in sediment flux of Krishna‐Godavari basin during the last 4 ky. This article is protected by copyright. All rights reserved.
      PubDate: 2015-08-07T04:31:25.214918-05:
      DOI: 10.1002/2015GC005754
  • Effect of maghemization on the magnetic properties of nonstoichiometric
           pseudosingle‐domain magnetite particles
    • Abstract: The effect of maghemization on the magnetic properties of magnetite (Fe3O4) grains in the pseudo‐single‐domain (PSD) size range is investigated as a function of annealing temperature. X‐ray diffraction and transmission electron microscopy confirms the precursor grains as Fe3O4 ranging from ∼ 150 nm to ∼ 250 nm in diameter, whilst Mössbauer spectrometry suggests the grains are initially near‐stoichiometric. The Fe3O4 grains are heated to increasing reaction temperatures of 120 – 220 ºC to investigate their oxidation to maghemite (γ‐Fe2O3). High‐angle annular dark field imaging and localized electron energy‐loss spectroscopy reveals slightly oxidized Fe3O4 grains, heated to 140 ºC, exhibit higher oxygen content at the surface. Off‐axis electron holography allows for construction of magnetic induction maps of individual Fe3O4 and γ‐Fe2O3 grains, revealing their PSD (vortex) nature, which is supported by magnetic hysteresis measurements, including first‐order reversal curve analysis. The coercivity of the grains is shown to increase with reaction temperature up to 180 ºC, but subsequently decreases after heating above 200 ºC; this magnetic behavior is attributed to the growth of a γ‐Fe2O3 shell with magnetic properties distinct from the Fe3O4 core. It is suggested there is exchange coupling between these separate components that results in a vortex state with reduced vorticity. Once fully oxidized to γ‐Fe2O3, the domain states revert back to vortices with slightly reduced coercivity. It is argued that due to a core/shell coupling mechanism during maghemization, the directional magnetic information will still be correct, however, the intensity information will not be retained. This article is protected by copyright. All rights reserved.
      PubDate: 2015-08-06T11:12:26.282034-05:
      DOI: 10.1002/2015GC005858
  • S wave splitting in the offshore South Island, New Zealand: Insights into
           plate‐boundary deformation
    • Authors: S. C. Karalliyadda; M. K. Savage, A. Sheehan, J. Collins, D. Zietlow, A. Shelley
      Abstract: Local and regional S‐wave splitting in the offshore South Island of the New Zealand plate‐boundary zone provides constraints on the spatial and depth extent of the anisotropic structure with an enhanced resolution relative to land‐based and SKS studies. The combined analysis of offshore and land measurements using splitting tomography suggests plate‐boundary shear dominates in the central and northern South Island. The width of this shear zone in the central South Island is about 200 km, but is complicated by stress‐controlled anisotropy at shallow levels. In northern South Island, a broader (>200 km) zone of plate‐boundary parallel anisotropy is associated with the transitional faulting between the Alpine fault and Hikurangi subduction and the Hikurangi subduction zone itself. These results suggest S‐phases of deep events (∼ 90 km) in the central South Island are sensitive to plate‐boundary derived NE‐SW aligned anisotropic media in the upper‐lithosphere, supporting a “thin viscous sheet” deformation model. This article is protected by copyright. All rights reserved.
      PubDate: 2015-08-06T11:12:08.057655-05:
      DOI: 10.1002/2015GC005882
  • Wettability measurement under high P‐T conditions using X‐ray
           imaging with application to the brine‐supercritical CO2 system
    • Authors: Kuldeep Chaudhary; Eric J. Guiltinan, M. Bayani Cardenas, Jessica A. Maisano, Richard A. Ketcham, Philip C. Bennett
      Abstract: We present a new method for measuring wettability or contact angle of minerals at reservoir pressure‐temperature conditions using high‐resolution X‐ray computed tomography (HRXCT) and radiography. In this method, a capillary or a narrow slot is constructed from a mineral or a rock sample of interest wherein two fluids are allowed to form an interface that is imaged using X‐rays. After some validation measurements at room pressure‐temperature conditions, we illustrate this method by measuring the contact angle of CO2–brine on quartz, muscovite, shale, borosilicate glass, polytetrafluoroethylene (PTFE or Teflon), and polyether ether ketone (PEEK) surfaces at 60‐71°C and 13.8 – 22.8 MPa. At reservoir conditions, PTFE and PEEK surfaces were found to be CO2–wet with contact angles of 140° and 127°, respectively. Quartz and muscovite were found to be water–wet with contact angles of 26° and 58°, respectively under similar conditions. Borosilicate glass–air–brine at room conditions showed strong water‐wet characteristics with a contact angle of 9°, whereas borosilicate glass‐CO2–brine at 13.8 MPa and 60°C showed a decrease in its water‐wetness with contact angle of 54°. This method provides a new application for X‐ray imaging and an alternative to other methods. This article is protected by copyright. All rights reserved.
      PubDate: 2015-08-06T11:09:33.875019-05:
      DOI: 10.1002/2015GC005936
  • Late Pleistocene ages for the most recent volcanism and
           glacial‐pluvial deposits at Big Pine volcanic field, California,
           USA, from cosmogenic 36Cl dating
    • Authors: J.A. Vazquez; J. M. Woolford
      Abstract: The Big Pine volcanic field is one of several Quaternary volcanic fields that poses a potential volcanic hazard along the tectonically active Owens Valley of east‐central California, and whose lavas are interbedded with deposits from Pleistocene glaciations in the Sierra Nevada range. Previous geochronology indicates an ∼1.2 Ma history of volcanism, but the eruption ages and distribution of volcanic products associated with the most‐recent eruptions are poorly resolved. To delimit the timing and products of the youngest volcanism, we combine field mapping and cosmogenic 36Cl dating of basaltic lava flows in the area where lavas with youthful morphology and well‐preserved flow structures are concentrated. Field mapping and petrology reveal approximately fifteen vents and six principal flow units with variable geochemical composition and mineralogy. Cosmogenic 36Cl exposure ages for lava flow units from the top, middle, and bottom of the volcanic stratigraphy indicate eruptions at ca. 17 ka, 27 ka, and 40 ka, revealing several different and previously unrecognized episodes of late Pleistocene volcanism. Olivine to plagioclase‐pyroxene phyric basalt erupted from several vents during the most recent episode of volcanism at ca. 17 ka, and produced a lava flow field covering ∼ 35 km2. The late Pleistocene 36Cl exposure ages indicate that moraine and pluvial shoreline deposits that overly or modify the youngest Big Pine lavas reflect Tioga stage glaciation in the Sierra Nevada and the shore of Owens Lake during the last glacial cycle. This article is protected by copyright. All rights reserved.
      PubDate: 2015-08-01T10:44:20.049941-05:
      DOI: 10.1002/2015GC005889
  • Propagation of back‐arc extension into the arc lithosphere in the
           southern New Hebrides volcanic arc
    • Authors: M. Patriat; J. Collot, M. Fabre, L. Danyushevsky, S. Meffre, T. Falloon, P. Rouillard, B. Pelletier, M. Roach, M. Fournier
      Abstract: New geophysical data acquired during three expeditions of the R/V Southern Surveyor in the southern part of the North Fiji Basin allow us to characterize the deformation of the upper plate at the southern termination of the New Hebrides subduction zone, where it bends eastward along the Hunter Ridge. Unlike the northern end of the Tonga subduction zone, on the other side of the North Fiji Basin, the 90° bend does not correspond to the transition from a subduction zone to a transform fault, but it is due to the progressive retreat of the New Hebrides trench. The subduction trench retreat is accommodated in the upper plate by the migration toward the southwest of the New Hebrides arc and toward the south of the Hunter Ridge, so that the direction of convergence remains everywhere orthogonal to the trench. In the back‐arc domain, the active deformation is characterised by propagation of the back‐arc spreading ridge into the Hunter volcanic arc. The N‐S spreading axis propagates southward and penetrates in the arc, where it connects to a sinistral strike‐slip zone via an oblique rift. The collision of the Loyalty Ridge with the New Hebrides arc, less than two million years ago, likely initiated this deformation pattern and the fragmentation of the upper plate. In this particular geodynamic setting, with an oceanic lithosphere subducting beneath a highly sheared volcanic arc, a wide range of primitive subduction‐related magmas has been produced including adakites, island arc tholeiites, back‐arc basin basalts, and medium‐K subduction‐related lavas. This article is protected by copyright. All rights reserved.
      PubDate: 2015-08-01T10:43:49.688434-05:
      DOI: 10.1002/2015GC005717
  • Multiple mantle upwellings in the transition zone beneath the northern
           East African Rift System from relative P wave travel time tomography
    • Authors: Chiara Civiero; James O. S. Hammond, Saskia Goes, Stewart Fishwick, Abdulhakim Ahmed, Atalay Ayele, Cecile Doubre, Berhe Goitom, Derek Keir, Michael Kendall, Sylvie Leroy, Ghebrebrhan Ogubazghi, Georg Rumpker, Graham W. Stuart
      Abstract: Mantle plumes and consequent plate extension have been invoked as the likely cause of East African Rift volcanism. However, the nature of mantle upwelling is debated, with proposed configurations ranging from a single broad plume connected to the large low‐shear‐velocity province beneath Southern Africa, the so‐called African Superplume, to multiple lower‐mantle sources along the rift. We present a new P‐wave travel‐time tomography model below the northern East‐African, Red Sea and Gulf of Aden rifts and surrounding areas. Data are from stations that span an area from Madagascar to Saudi Arabia. The aperture of the integrated dataset allows us to image structures of ∼100 km length scale down to depths of 700‐800 km beneath the study region. Our images provide evidence of two clusters of low‐velocity structures consisting of features with diameter of 100‐200 km that extend through the transition zone, the first beneath Afar and a second just west of the Main Ethiopian Rift, a region with off‐rift volcanism. Considering seismic sensitivity to temperature, we interpret these features as upwellings with excess temperatures of 100±50 K. The scale of the upwellings is smaller than expected for lower mantle plume sources. This, together with the change in pattern of the low‐velocity anomalies across the base of the transition zone, suggests that ponding or flow of deep‐plume material below the transition zone may be spawning these upper‐mantle upwellings. This article is protected by copyright. All rights reserved.
      PubDate: 2015-08-01T10:42:37.541849-05:
      DOI: 10.1002/2015GC005948
  • Factors affecting the rare earth element compositions in massive sulfides
           from deep‐sea hydrothermal systems
    • Authors: Zhigang Zeng; Yao Ma, Xuebo Yin, David Selby, Fancui Kong, Shuai Chen
      Abstract: To reconstruct the evolution of ore‐forming fluids and determine the physicochemical conditions of deposition associated with seafloor massive sulfides, we must better understand the sources of rare earth elements (REEs), the factors that affect the REE abundance in the sulfides, and the REE flux from hydrothermal fluids to the sulfides. Here, we examine the REE profiles of 46 massive sulfide samples collected from seven seafloor hydrothermal systems. These profiles feature variable total REE concentrations (37.2–4,092 ppb) and REE distribution patterns (LaCN/LuCN ratios = 2.00–73.8; (Eu/Eu*)CN ratios = 0.34–7.60). The majority of the REE distribution patterns in the sulfides are similar to those of vent fluids, with the sulfides also exhibiting light REE enrichment. We demonstrate that the variable REE concentrations, Eu anomalies, and fractionation between light REEs and heavy REEs in the sulfides exhibit a relationship with the REE properties of the sulfide‐forming fluids and the massive sulfide chemistry. Based on the sulfide REE data, we estimate that modern seafloor sulfide deposits contain approximately 280 tons of REEs. According to the flux of hydrothermal fluids at mid‐ocean ridges (MORs) and an average REE concentration of 3 ng/g in these fluids, hydrothermal vents at MORs alone transport more REEs (>360 tons) to the oceans over the course of just two years than the total quantity of REEs in seafloor sulfides. The excess REEs (i.e., the quantity not captured by massive sulfides) may be transported away from the systems and become bound in sulfate deposits and metalliferous sediments. This article is protected by copyright. All rights reserved.
      PubDate: 2015-08-01T10:42:11.191306-05:
      DOI: 10.1002/2015GC005812
  • Hydrogeological responses to incoming materials at the erosional
           subduction margin, offshore Osa Peninsula, Costa Rica
    • Authors: Jun Kameda; Robert N. Harris, Mayuko Shimizu, Kohtaro Ujiie, Akito Tsutsumi, Minoru Ikehara, Masaoki Uno, Asuka Yamaguchi, Yohei Hamada, Yuka Namiki, Gaku Kimura
      Abstract: Bulk mineral assemblages of sediments and igneous basement rocks on the incoming Cocos Plate at the Costa Rica subduction zone are examined by X‐ray diffraction analyses on core samples. These samples are from Integrated Ocean Drilling Program Expedition 334 reference Site U1381, ∼ 5 km seaward of the trench. Drilling recovered approximately 100 m of sediment and 70 m of igneous oceanic basement. The sediment includes two lithologic units: hemipelagic clayey mud and siliceous to calcareous pelagic ooze. The hemipelagic unit is composed of clay minerals (∼50 wt.%), quartz (∼5 wt.%), plagioclase (∼5 wt.%), calcite (∼15 wt.%) and ∼30 wt.% of amorphous materials, while the pelagic unit is mostly made up of biogenic amorphous silica (∼50 wt.%) and calcite (∼50 wt.%). The igneous basement rock consists of plagioclase (∼50‐60 wt.%), clinopyroxene (∼>25 wt.%), and saponite (∼15‐40 wt.%). Saponite is more abundant in pillow basalt than in the massive section, reflecting the variable intensity of alteration. We estimate the total water influx of the sedimentary package is 6.9 m3/yr per m of trench length. Fluid expulsion models indicate that sediment compaction during shallow subduction causes the release of pore water while peak mineral dehydration occurs at temperatures of approximately ∼100 °C, 40‐30 km landward of the trench. This region is landward of the observed updip extent of seismicity. We posit that in this region the presence of subducting bathymetric relief capped by velocity weakening nannofossil chalk is more important in influencing the updip extent of seismicity than the thermal regime. This article is protected by copyright. All rights reserved.
      PubDate: 2015-08-01T10:41:59.942064-05:
      DOI: 10.1002/2015GC005837
  • Chemical and Pb‐isotope composition of phenocrysts from bentonites
           constrain the chronostratigraphy around the Cretaceous‐Paleogene
           boundary in the Hell Creek region, Montana
    • Authors: Ryan B. Ickert; Sean R. Mulcahy, Courtney J. Sprain, Jessica F. Banaszak, Paul R. Renne
      Abstract: An excellent record of environmental and paleobiological change around the Cretaceous‐Paleogene boundary is preserved in the Hell Creek and Fort Union Formations in the western Williston Basin of northeastern Montana. These records are present in fluvial deposits whose lateral discontinuity hampers longdistance correlation. Geochronology has been focused on bentonite beds that are often present in lignites. To better identify unique bentonites for correlation across the region, the chemical and Pb isotopic composition of feldspar and titanite have been measured on 46 samples. Many of these samples have been dated by 40Ar/39Ar. The combination of chemical and isotopic compositions of phenocrysts has enabled the identification of several unique bentonite beds. In particular, three horizons located at and above the Cretaceous‐Paleogene boundary can now be traced – based on their unique compositions – across the region, clarifying previously ambiguous stratigraphic relationships. Other bentonites show unusual features, such as Pb isotope variations consistent with magma‐mixing or assimilation, that will make them easy to recognize in future studies. This technique is limited in some cases by more than one bentonite having compositions that cannot be distinguished, or bentonites with abundant xenocrysts. The Pb isotopes are consistent with a derivation from the Bitterroot Batholith, whose age range overlaps that of the tephra. These data provide an improved stratigraphic framework for the Hell Creek region and provide a basis for more focused tephrostratigraphic work, and more generally demonstrate that the combination of mineral chemistry and Pb isotope compositions is an effective technique for tephra correlation. This article is protected by copyright. All rights reserved.
      PubDate: 2015-08-01T10:41:45.112356-05:
      DOI: 10.1002/2015GC005898
  • Multiple‐scale hydrothermal circulation in 135 Ma oceanic crust of
           the Japan Trench outer rise: Numerical models constrained with heat flow
    • Authors: Labani Ray; Yoshifumi Kawada, Hideki Hamamoto, Makoto Yamano
      Abstract: Anomalous high heat flow is observed within 150 km seaward of the trench axis at the Japan Trench offshore of Sanriku, where the old Pacific plate (∼135 Ma) is subducting. Individual heat flow values range between 42 and 114 mW m−2, with an average of ∼70 mW m−2. These values are higher than those expected from the seafloor age based on thermal models of the oceanic plate, i.e., ∼50 mW m−2. The heat flow exhibits spatial variations at multiple scales: regional high average heat flow (∼100 km) and smaller‐scale heat flow peaks (∼1 km). We found that hydrothermal mining of heat from depth due to gradual thickening of an aquifer in the oceanic crust toward the trench axis can yield elevated heat flow of the spatial scale of ∼100 km. Topographic effects combined with hydrothermal circulation may account for the observed smaller‐scale heat flow variations. Hydrothermal circulation in high‐permeability faults may result in heat flow peaks of a sub‐kilometer spatial scale. Volcanic intrusions are unlikely to be a major source of heat flow variations at any scale because of limited occurrence of young volcanoes in the study area. Hydrothermal heat transport may work at various scales on outer rises of other subduction zones as well, since fractures and faults have been well developed due to bending of the incoming plate. This article is protected by copyright. All rights reserved.
      PubDate: 2015-07-30T03:59:32.900082-05:
      DOI: 10.1002/2015GC005771
  • Cobalt‐based age models of pelagic clay in the South Pacific Gyre
    • Authors: Ann G. Dunlea; Richard W. Murray, Justine Sauvage, Robert A. Pockalny, Arthur J. Spivack, Steven D'Hondt, Robert N. Harris
      Abstract: Dating pelagic clay can be a challenge due to its slow sedimentation rate, post‐depositional alteration, and lack of biogenic deposition. Co‐based dating techniques have the potential to create age models in pelagic clay under the assumption that the flux of non‐detrital Co to the seafloor is spatially and temporally constant, resulting in the non‐detrital Co concentrations being inversely proportional to sedimentation rate. We apply a Co‐based method to the pelagic clay sequences from Sites U1365, U1366, U1369, and U1370 drilled during Integrated Ocean Drilling Program (IODP) Expedition 329 in the South Pacific Gyre. We distinguished non‐detrital Co from detrital Co using multivariate statistical partitioning techniques. We found that the non‐detrital flux of Co at Site U1370 is approximately twice as high than at the other sites, implying that the non‐detrital Co flux is not regionally constant. This regional variation reflects the heterogeneous distribution of Co in the water column, as is observed in the present day. We present an improved approach to Co‐based age modeling throughout the South Pacific Gyre and determine that the Co‐based method can effectively date oxygenated pelagic clay deposited in the distal open‐ocean, but is less reliable for deposition closer to continents. When extending the method to geologically old sediment, it is important to consider the paleolocation of a given site to ensure these conditions are met. This article is protected by copyright. All rights reserved.
      PubDate: 2015-07-29T19:48:36.917079-05:
      DOI: 10.1002/2015GC005892
  • A Bayesian, multivariate calibration for Globigerinoides ruber Mg/Ca
    • Abstract: The use of Mg/Ca in marine carbonates as a paleothermometer has been challenged by observations that implicate salinity as a contributing influence on Mg incorporation into biotic calcite and that dissolution at the sea‐floor alters the original Mg/Ca. Yet, these factors have not yet been incorporated into a single calibration model. We introduce a new Bayesian calibration for Globigerinoides ruber Mg/Ca based on 186 globally‐distributed core top samples, which explicitly takes into account the effect of temperature, salinity, and dissolution on this proxy. Our reported temperature, salinity, and dissolution (here expressed as deep‐water ΔCO32‐) sensitivities are (±2σ) 8.7±0.9%/°C, 3.9±1.2%/psu, and 3.3±1.3%/μ−1 below a critical threshold of 21 μmol/kg, in good agreement with previous culturing and core‐top studies. We then perform a sensitivity experiment on a published record from the western tropical Pacific to investigate the bias introduced by these secondary influences on the interpretation of past temperature variability. This experiment highlights the potential for misinterpretations of past oceanographic changes when the secondary influences of salinity and dissolution are not accounted for. Multi‐proxy approaches could potentially help deconvolve the contributing influences but this awaits better characterization of the spatio‐temporal relationship between salinity and δ18Osw over millennial and orbital timescales. This article is protected by copyright. All rights reserved.
      PubDate: 2015-07-22T03:58:44.749955-05:
      DOI: 10.1002/2015GC005844
  • Geology, sulphide geochemistry, and supercritical venting at the Beebe
           Hydrothermal Vent Field, Cayman Trough
    • Authors: Alexander P. Webber; Stephen Roberts, Bramley J. Murton, Matthew R.S. Hodgkinson
      Abstract: The Beebe Vent Field (BVF) is the world's deepest known hydrothermal system, at 4960m below sea level. Located on the Mid‐Cayman Spreading Centre, Caribbean, the BVF hosts high temperature (∼401°C) ‘black smoker' vents that build Cu, Zn and Au‐rich sulphide mounds and chimneys. The BVF is highly gold‐rich, with Au values up to 93 ppm and an average Au:Ag ratio of 0.15. Gold precipitation is directly associated with diffuse flow through ‘beehive' chimneys. Significant mass‐wasting of sulphide material at the BVF, accompanied by changes in metal content, results in metaliferous talus and sediment deposits. Situated on very thin (2‐3km thick) oceanic crust, at an ultraslow spreading centre, the hydrothermal system circulates fluids to a depth of ∼1.8km in a basement that is likely to include a mixture of both mafic and ultramafic lithologies. We suggest hydrothermal interaction with chalcophile‐bearing sulphides in the mantle rocks, together with precipitation of Au in beehive chimney structures, has resulted in the formation of a Au‐rich volcanogenic massive sulphide (VMS) deposit. With its spatial distribution of deposit materials and metal contents, the BVF represents a modern day analogue for basalt hosted, Au‐rich VMS systems. This article is protected by copyright. All rights reserved.
      PubDate: 2015-07-22T03:40:30.387821-05:
      DOI: 10.1002/2015GC005879
  • Water‐rich bending faults at the Middle America Trench
    • Authors: Samer Naif; Kerry Key, Steven Constable, Rob L. Evans
      Abstract: The portion of the Central American margin that encompasses Nicaragua is considered to represent an end‐member system where multiple lines of evidence point to a substantial flux of subducted fluids. The seafloor spreading fabric of the incoming Cocos plate is oriented parallel to the trench such that flexural bending at the outer rise optimally reactivates a dense network of normal faults that extend several kilometers into the upper mantle. Bending faults are thought to provide fluid pathways that lead to serpentinization of the upper mantle. While geophysical anomalies detected beneath the outer rise have been interpreted as broad crustal and upper mantle hydration, no observational evidence exists to confirm that bending faults behave as fluid pathways. Here, we use seafloor electromagnetic data collected across the Middle America Trench (MAT) offshore of Nicaragua to create a comprehensive electrical resistivity image that illuminates the infiltration of seawater along bending faults. We quantify porosity from the resistivity with Archie's law and find that our estimates for the abyssal plain oceanic crust are in good agreement with independent observations. As the Cocos crust traverses the outer rise, the porosity of the dikes and gabbros progressively increase from 2.7% and 0.7% to 4.8% and 1.7%, peaking within 20 km of the trench axis. We conclude that the intrusive crust subducts twice as much pore water as previously thought, significantly raising the flux of fluid to the seismogenic zone and the mantle wedge. This article is protected by copyright. All rights reserved.
      PubDate: 2015-07-21T08:23:13.310045-05:
      DOI: 10.1002/2015GC005927
  • Eduction, extension, and exhumation of ultrahigh‐pressure rocks in
           metamorphic core complexes due to subduction initiation
    • Authors: Kenni Dinesen Petersen; W. Roger Buck
      Abstract: The controversy over the exhumation of ultra‐high pressure (UHP) rocks centers on whether it involves rising of pieces of crust detached from subducted continental lithosphere or an entire subducted plate that undergoes “eduction”, i.e. reverse subduction. We present a new thermomechanical model of continental subduction showing that these apparently contrasting mechanisms can occur together: Crust subducted deep enough is heated and weakened, causing limited diapiric rise, while crust subducted to shallower depths retains strength and is exhumed only by eduction. The model also shows for the first time how eduction followed by seafloor spreading can occur in a zone of regional convergence. This occurs spontaneously when subduction of buoyant crust causes a subduction zone to “lock up” in one place causing a new subduction zone to form in another. The model is consistent with many features of the youngest region of UHP rock exhumation on earth: the D'Entrecasteaux Islands. UHP exhumation and the amount of regional extension, as well as the seismic structure around the islands, can be explained by eduction. Ductile flow fabrics, seen on the islands, would result from exhumation of the most deeply subducted crust heated enough to undergo partial melting. Reversal of motion on the north‐dipping continental subduction zone, required by this model, was likely triggered by initiation of the New Britain Trench, as suggested previously. Our model implies that the crust of Goodenough Basin, south of the islands, was exhumed by eduction in the last 5 Ma and this hypothesis can be tested by drilling. This article is protected by copyright. All rights reserved.
      PubDate: 2015-07-17T03:28:24.269522-05:
      DOI: 10.1002/2015GC005847
  • Reply to comment by Ivan Pineda‐Velasco, Tai T. Nguyen, Hiroshi
           Kitagawa, and Eizo Nakamura on “Diverse magmatic effects of
           subducting a hot slab in SW Japan: Results from forward modeling”
    • Abstract: The Comment by Pineda‐Velasco et al. [2015] examined Pb isotope data presented by Kimura et al. [2014]. The authors' points are that (1) there is uncertainty in the analytical results of Kimura et al. [2014] due to the effect of mass fractionation, and therefore, (2) the interpretations of Kimura et al. [2014] based on the extent of crustal assimilation and the estimated Pb isotopic composition of the crustal component are erroneous. In response to the Comment, we report a flaw in the original paper that the samples from the Aono, Daisen, and Kannabe regions were analyzed using conventional TIMS methods and all other samples were analyzed using thallium‐spiked multi‐collector inductively‐coupled‐plasma mass spectrometry (TS‐MC‐ICP‐MS). We have re‐analyzed the sample powders from Karasugasen, Daisen, and Aono using TS‐MC‐ICP‐MS. Our new results showed considerable overlap with the data in Pineda‐Velasco et al. [2015]. Therefore, the isotopic trends shown by the conventional TIMS in Kimura et al. [2014] were analytical artifacts from mass bias. We conclude that the crustal assimilation proposed by Kimura et al. [2014] was erroneous in terms of Pb isotopes, nevertheless some crustal assimilation in the Karasugasen lava is evident from the chemical zoning of hornblende phenocrysts. Although the original Pb isotope argument for crustal contamination was wrong, the ABS4 modeling is unaffected because of no to subtle changes in estimated mantle source compositions in their ABS4 model. This article is protected by copyright. All rights reserved.
      PubDate: 2015-07-17T02:22:07.781949-05:
      DOI: 10.1002/2015GC005984
  • Comment on “Diverse magmatic effects of subducting a hot slab in SW
           Japan: Results from forward modeling'' by J.‐I. Kimura et al.
    • PubDate: 2015-07-16T04:33:10.722884-05:
      DOI: 10.1002/2015GC005914
  • Varying styles of magmatic strain accommodation across the East African
    • Authors: James D. Muirhead; Simon A. Kattenhorn, Nicolas Le Corvec
      Abstract: Observations of active dike intrusions provide present day snapshots of the magmatic contribution to continental rifting. However, unravelling the contributions of upper crustal dikes over the timescale of continental rift evolution is a significant challenge. To address this issue, we analyzed the morphologies and alignments of >1,500 volcanic cones to infer the distribution and trends of upper crustal dikes in various rift basins across the East African Rift (EAR). Cone lineament data reveal along‐axis variations in the distribution and geometries of dike intrusions as a result of changing tectono‐magmatic conditions. In younger (
      PubDate: 2015-07-16T04:28:13.092083-05:
      DOI: 10.1002/2015GC005918
  • River geochemistry, chemical weathering, and atmospheric CO2 consumption
           rates in the Virunga Volcanic Province (East Africa)
    • Abstract: We report a water chemistry data set from 13 rivers of the Virunga Volcanic Province (VVP) (Democratic Republic of Congo), sampled between December 2010 and February 2013. Most parameters showed no pronounced seasonal variation, whereas their spatial variation suggests a strong control by lithology, soil type, slope and vegetation. High total suspended matter (289‐1467 mg L−1) was recorded in rivers in the Lake Kivu catchment, indicating high soil erodibility, partly as a consequence of deforestation and farming activities. Dissolved and particulate organic carbon (DOC and POC) were lower in rivers from lava fields, and higher in non‐volcanic sub‐catchments. Stable carbon isotope signatures (δ13C) of POC and DOC mean δ13C of ‐22.5 and ‐23.5 ‰, respectively), are the first data to be reported for the highland of the Congo River basin and showed a much higher C4 contribution than in lowland areas. Rivers of the VVP were net sources of CH4 to the atmosphere (4‐ 5052 nmol L−1). Most rivers show N2O concentrations close to equilibrium, but some rivers showed high N2O concentrations related to denitrification in groundwaters. δ13C signatures of dissolved inorganic carbon suggested magmatic CO2 inputs to aquifers/soil, which could have contributed to increase basalt weathering rates. This magmatic CO2‐mediated basalt weathering strongly contributed to the high major cation concentrations and total alkalinity. Thus, chemical weathering (39.0 ‐2779.9 t km−2 yr−1) and atmospheric CO2 consumption (0.4‐ 37.0 x 106 mol km−2 yr−1) rates were higher than previously reported in literature for basaltic terrains. This article is protected by copyright. All rights reserved.
      PubDate: 2015-07-16T04:27:14.648344-05:
      DOI: 10.1002/2015GC005999
  • LA‐ICPMS Ba/Ca analyses of planktic foraminifera from the Bay of
           Bengal: Implications for late Pleistocene orbital control on monsoon
           freshwater flux
    • Abstract: Indian Summer Monsoon (ISM) indices are characterised by large secular variation during both glacials and interglacials. Although much information about palaeo‐monsoon intensity is derived from such indicies, current datasets do not relate simply to precipitation. In order to directly constrain the variability of ISM freshwater flux to the Bay of Bengal, we report Ba/Ca LA‐ICPMS data of the surface‐dwelling foraminifera G. ruber from core RC12‐343 (central Bay of Bengal) between 68‐47 ka. Planktic foraminifera Ba/Ca directly relates to seawater Ba/Ca, in turn principally controlled by freshwater flux. Our foraminifera‐derived Ba/Casw record for the central Bay of Bengal is highly coherent with that derived from δ18O measurements of the same material, implying that these reconstructions are not significantly biased by potential shifts in δ18Ofreshwater. Validating this method allows us to produce a freshwater stack for the last 80 ka for the Bay of Bengal, enabling the orbital controls on ISM precipitation to be examined for the first time. The highest freshwater flux in the last 80 ka was ∼3× larger than present and occurred during the early‐mid Holocene. We show that the orbital timing of this record is best explained by a combination of factors with a weighting of ∼45% given to the 30° N‐equator mean summer insolation gradient and ∼55% given to the variability of May‐July insolation at the equator and June‐July insolation at 30° N. These processes are consistent with current mechanistic understanding of ISM forcings and demonstrate a dominant orbital control on monsoon precipitation amount on millennial‐timescales. This article is protected by copyright. All rights reserved.
      PubDate: 2015-07-16T04:26:23.921548-05:
      DOI: 10.1002/2015GC005822
  • Crystallographic preferred orientations may develop in nanocrystalline
           materials on fault planes due to surface energy interactions
    • Authors: Virginia G. Toy; Thomas M. Mitchell, Anthony Druiventak, Richard Wirth
      Abstract: A layer of substantially non‐crystalline material, composed of partially annealed nanopowder with local melt, was experimentally generated by comminution during ∼1.5 mm total slip at ∼2.5x10−6 ms−1, Pconf ∼ 0.5 GPa, and 450°C or 600°C, on sawcut surfaces in novaculite. The partially annealed nanopowder comprises angular grains mostly 5‐200 nm diameter in a variably dense packing arrangement. A sharp transition from wall rock to partially annealed nanopowder illustrates the nanopowder effectively localizes shear, consistent with generation of nanoparticles during initial fragmentation, not by progressive grain size reduction. Dislocation densities in nanopowder grains or immediate wall rock are not significantly high, but there are planar plastic defects spaced at 5‐200 nm parallel to the host quartz grain's basal plane. We propose these plastic defects developed into through‐going fractures to generate nanocrystals. The partially annealed nanopowder has a crystallographic preferred orientation (CPO) that we hypothesise developed due to surface energy interactions to maximize coincident site lattices (CSL) during annealing. This mechanism may also have generated CPO recently described in micro/nanocrystalline calcite fault gouges. This article is protected by copyright. All rights reserved.
      PubDate: 2015-07-14T10:01:35.047042-05:
      DOI: 10.1002/2015GC005857
  • The Manihiki Plateau—A multistage volcanic emplacement history
    • Abstract: The formation history of the Manihiki Plateau, a Large Igneous Province, is poorly understood. New high resolution seismic reflection data across the High Plateau, the largest edifice of the Manihiki Plateau, provides evidence for multistage magmatic emplacement. Improved data quality allows for an identification of an earlier volcanic phase, the initial formation phase (>125 Ma), in addition to the previously known volcanic formation phases: the expansion phase (125‐116) formerly called main‐phase and the secondary volcanic phase (100‐65 Ma). This enhances the understanding of the emplacement scenario. An intrabasement reflection band IB1 reveals the end of initial volcanic formation and forms the nucleus of the High Plateau. This feature provides indications that it continued beyond the Manihiki Scarp and thus supports the hypothesis of an extension of the Manihiki Plateau to the East during the initial formation and expansion phases. The expansion phase is characterized by massive volcanic outpourings leveling and extending the basement throughout the High Plateau and the neighboring Western Plateaus, which in contrast shows massive tectonic alteration. Extrusion centers formed within the secondary volcanic phase (ending ∼65 Ma) are mainly concentrated along the margins of the High Plateau, suggesting the magmatic sources shifted from those being related to the initial emplacement and expansion phases of the High Plateau to induced volcanism at the tectonically altered margins. This article is protected by copyright. All rights reserved.
      PubDate: 2015-07-14T07:25:43.009957-05:
      DOI: 10.1002/2015GC005852
  • Particle dynamics in the rising plume at Piccard Hydrothermal Field,
           Mid‐Cayman Rise
    • Authors: M. L. Estapa; J. A. Breier, C. R. German
      Abstract: Processes active in rising hydrothermal plumes, such as precipitation, particle aggregation, and biological growth, affect particle size distributions and can exert important influences on the biogeochemical impact of submarine venting of iron to the oceans and their sediments. However, observations of particle size distribution within these systems, to date, are both limited and conflicting. In a novel buoyant hydrothermal plume study at the recently discovered high‐temperature (398°C) Piccard Hydrothermal Field, Mid‐Cayman Rise, we report optical measurements of particle size distributions (PSDs). We describe the plume PSD in terms of a simple, power‐law model commonly used in studies of upper‐ and coastal ocean particle dynamics. Observed PSD slopes, derived from spectral beam attenuation and laser diffraction measurements, are among the highest found to date anywhere in the ocean and ranged from 2.9 to 8.5. Beam attenuation at 650 nm ranged from near zero to a rarely‐observed maximum of 192 m−1 at 3.5 m above the vent. We did not find large (>100 μm) particles that would settle rapidly to the sediments. Instead, beam attenuation was well‐correlated to total iron, suggesting the first‐order importance of particle dilution, rather than precipitation or dissolution, in the rising plume at Piccard. Our observations at Piccard caution against the assumption of rapid deposition of hydrothermal, particulate metal fluxes, and illustrate the need for more particle size and composition measurements across a broader range of sites, globally. This article is protected by copyright. All rights reserved.
      PubDate: 2015-07-14T07:25:40.4561-05:00
      DOI: 10.1002/2015GC005831
  • Imaging continental breakup using teleseismic body waves: The Woodlark
           Rift, Papua New Guinea
    • Authors: Zachary Eilon; Geoffrey A Abers, James B Gaherty, Ge Jin
      Abstract: This study images the upper mantle beneath the D'Entrecasteax Islands, Papua New Guinea, providing insight into mantle deformation beneath a highly rifted continent adjacent to propagating spreading centers. Differential travel times from P‐ and S‐wave teleseisms recorded during the 2010‐2011 CDPapua passive seismic experiment are used to invert for separate VP and VS velocity models of the continental rift. A low‐velocity structure marks the E‐W axis of the rift, correlating with the thinnest crust, high heat flow, and a linear trend of volcanoes. This slow region extends 250 km along strike from the oceanic spreading centers, demonstrating significant mantle extension ahead of seafloor breakup. The rift remains narrow to depth indicating localization of extension, perhaps as a result of mantle hydration. A high‐VP structure at depths of 90‐120 km beneath the north of the array is more than 6.5% faster than the rift axis and contains well‐located intermediate depth earthquakes. These independent observations place firm constraints on the lateral thermal contrast at depth between the rift axis and cold lithosphere to the north that may be related to recent subduction, although the polarity of subduction cannot be resolved. This geometry is gravitationally unstable; downwelling or small‐scale convection could have facilitated rifting and rapid lithospheric removal, although this may require a wet mantle to be realistic on the required timescales. The high‐V structure agrees with the maximum P,T conditions recorded by young ultra‐high pressure rocks exposed on the rift axis and may be implicated in their genesis. This article is protected by copyright. All rights reserved.
      PubDate: 2015-07-03T09:50:57.704778-05:
      DOI: 10.1002/2015GC005835
  • Effects of inherited cores and magmatic overgrowths on zircon
           (U‐Th)/He ages and age‐eU trends from Greater Himalayan
           sequence rocks, Mt. Everest region, Tibet
    • Authors: D.A. Orme; P.W. Reiners, J.K. Hourigan, B. Carrapa
      Abstract: Previous constraints on the timing and rate of exhumation of the footwall of the South Tibetan detachment system (STDS) north of Mt. Everest suggest rapid Miocene cooling from ∼ 700°C to 120°C between ca. 14‐17 Ma. However, twenty‐five new single grain zircon He ages from leucogranites intruding Greater Himalayan Sequence rocks in the footwall of the STDS are between 9.9 to 15 Ma, with weighted mean ages between 10 and 12 Ma. Zircon grains exhibit a positive correlation between age and effective uranium (eU). Laser ablation zircon U‐Pb geochronology, detailed SEM observations and laser ablation depth‐profiling of these zircons reveal low‐eU 0.5‐2.5 Ga inherited cores overgrown by high‐eU 17‐22 Ma rims. This intragranular zonation produces ages as much as 32% too young when a standard alpha‐ejection correction assuming uniform eU distribution is applied. Modeling of the effects of varying rim thickness and rim eU concentration on the bulk grain eU and alpha‐ejection correction suggests that zonation also exerts the primary control on the form of the age‐eU correlation observed. Application of grain‐specific zonation‐dependent age corrections to our data yields zircon He ages between 14‐17 Ma, in agreement with AFT and 40Ar/39Ar ages. Growth of magmatic rims followed by cooling to 
      PubDate: 2015-07-02T07:54:12.871204-05:
      DOI: 10.1002/2015GC005818
  • Age of the Lava Creek supereruption and magma chamber assembly from
           combined 40Ar/39Ar and U‐Pb dating of sanidine and zircon crystals
    • Authors: Naomi E. Matthews; Jorge A. Vazquez, Andrew T. Calvert
      Abstract: The last supereruption from the Yellowstone Plateau formed Yellowstone caldera and ejected the >1000 km3 of rhyolite that composes the Lava Creek Tuff. Tephra from the Lava Creek eruption is a key Quaternary chronostratigraphic marker, in particular for dating the deposition of mid‐Pleistocene glacial and pluvial deposits in western North America. To resolve the timing of eruption and crystallization history for the Lava Creek magma, we performed (1) 40Ar‐39Ar dating of single sanidine crystals to delimit eruption age and (2) ion microprobe U‐Pb and trace‐element analyses of the crystal faces and interiors of single zircons to date the interval of zircon crystallization and characterize magmatic evolution. Sanidines from the two informal members composing Lava Creek Tuff yield a preferred 40Ar/39Ar isochron date of 631.3 ± 4.3 ka. Crystal faces on zircons from both members yield a weighted mean 206Pb/238U date of 626.5 ± 5.8 ka, and have trace element concentrations that vary with the eruptive stratigraphy. Zircon interiors yield a mean 206Pb/238U date of 659.8 ± 5.5 ka, and reveal reverse and/or oscillatory zoning of trace element concentrations, with many crystals containing high U concentration cores that likely grew from highly evolved melt. The occurrence of distal Lava Creek tephra in stratigraphic sequences marking the marine isotope stage 16—15 transition supports the apparent eruption age of ca. 631 ka. The combined results reveal that Lava Creek zircons record episodic heating, renewed crystallization, and an overall up‐temperature evolution for Yellowstone's subvolcanic reservoir in the 103—104 interval before eruption. This article is protected by copyright. All rights reserved.
      PubDate: 2015-07-02T06:36:59.453925-05:
      DOI: 10.1002/2015GC005881
  • Appreciation of peer reviewers for 2014
    • Abstract: The editorial and scientific publishing process relies on the sustained work of volunteer reviewers, and evaluating the inter‐disciplinary and broad interest papers published in G‐Cubed can be a particular challenge. As editors and associated editors, we are therefore hugely appreciative of the efforts of our reviewers, and would like to thank and acknowledge them in this editorial. G‐Cubed published 257 manuscripts out of 431 submissions in 2014, and for this we were able to rely on the efforts of 710 dedicated reviewers. Their names are listed below, in italics those 27 who provided three or more reviews. A big thank you from the G‐Cubed team!. This article is protected by copyright. All rights reserved.
      PubDate: 2015-07-01T10:36:24.123782-05:
      DOI: 10.1002/2015GC005982
  • Lithospheric shear wave velocity and radial anisotropy beneath the
           northern part of North China from surface wave dispersion analysis
    • Authors: Yuanyuan V. Fu; Yuan Gao, Aibing Li, Yutao Shi
      Abstract: Rayleigh and Love wave phase velocities in the northern part of the North China are obtained from ambient noise tomography in the period range of 8 to 35 s and two plane wave earthquake tomography at periods of 20 to 91 s using data recorded at 222 broadband seismic stations from the temporary North China Seismic Array and permanent China Digital Seismic Array. The dispersion curves of Rayleigh and Love wave from 8 to 91 s are jointly inverted for the 3‐D shear wave structure and radial anisotropy in the lithosphere to 140 km depth. Distinct seismic structure are observed from the Fenhe Graben and Taihang Mountain to North China Basin. The North China Basin from the lower crust to the depth of 140 km is characterized by high velocity anomaly, reflecting mafic intrusion and residual materials after the extraction of melt, and by strong radial anisotropy with Vsh > Vsv implying horizontal layering of intrusion and alignment of minerals due to vigorous extensional deformation and subsequent thermal annealing. However, low velocity anomaly and positive radial anisotropy are observed in the Fenhe Graben and Taihang Mountain, suggesting the presence of partial melt in the lithosphere due to the mantle upwelling and horizontal flow pull. This article is protected by copyright. All rights reserved.
      PubDate: 2015-06-19T17:51:39.82515-05:0
      DOI: 10.1002/2015GC005825
  • Crustal‐scale degassing due to magma system destabilization and
           magma‐gas decoupling at Soufrière Hills Volcano, Montserrat
    • Authors: T.E. Christopher; J. Blundy, K. Cashman, P. Cole, M. Edmonds, P.J. Smith, R.S.J. Sparks, A. Stinton
      Abstract: Activity since 1995 at Soufrière Hills Volcano (SHV), Montserrat has alternated between andesite lava extrusion and quiescence, which are well‐correlated with seismicity and ground deformation cycles. Large variations in SO2 flux do not correlate with these alternations, but high and low HCl/SO2 characterising lava dome extrusion and quiescent periods respectively. Since lava extrusion ceased (February 2010) steady SO2 emissions have continued at an average rate of 374 tonnes/day (± 140 t/d), and incandescent fumaroles (temperatures up to 610oC) on the dome have not changed position or cooled. Occasional short bursts (over several hours) of higher (∼ 10x) SO2 flux have been accompanied by swarms of volcano‐tectonic earthquakes. Strain data from these bursts indicate activation of the magma system to depths up to 10 km. SO2 emissions since 1995 greatly exceed the amounts that could be derived from 1.1 km3 of erupted andesite, and indicating extensive partitioning of sulfur into a vapour phase, as well as efficient decoupling and outgassing of sulfur‐rich gases from the magma. These observations are consistent with a vertically‐extensive, crustal magmatic mush beneath SHV. Three states of the magmatic system are postulated to control degassing. During dormant periods (103 to 104 years) magmatic vapour and melts separate as layers from the mush and decouple from each other. In periods of unrest (years) without eruption, melt and fluid layers become unstable, ascend and can amalgamate. Major destabilisation of the mush system leads to eruption, characterised by magma mixing and release of volatiles with different ages, compositions and sources. This article is protected by copyright. All rights reserved.
      PubDate: 2015-06-11T11:17:24.359999-05:
      DOI: 10.1002/2015GC005791
School of Mathematical and Computer Sciences
Heriot-Watt University
Edinburgh, EH14 4AS, UK
Tel: +00 44 (0)131 4513762
Fax: +00 44 (0)131 4513327
About JournalTOCs
News (blog, publications)
JournalTOCs on Twitter   JournalTOCs on Facebook

JournalTOCs © 2009-2015