for Journals by Title or ISSN
for Articles by Keywords

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

Showing 1 - 17 of 17 Journals sorted alphabetically
Geochemistry, Geophysics, Geosystems     Full-text available via subscription   (Followers: 32, SJR: 2.439, h-index: 91)
Geophysical Research Letters     Full-text available via subscription   (Followers: 146, SJR: 3.323, h-index: 185)
Global Biogeochemical Cycles     Full-text available via subscription   (Followers: 17, SJR: 3.22, h-index: 136)
J. of Advances in Modeling Earth Systems     Open Access   (Followers: 5, SJR: 4.444, h-index: 18)
J. of Geophysical Research : Atmospheres     Partially Free   (Followers: 142)
J. of Geophysical Research : Biogeosciences     Full-text available via subscription   (Followers: 32)
J. of Geophysical Research : Earth Surface     Partially Free   (Followers: 56)
J. of Geophysical Research : Oceans     Partially Free   (Followers: 55)
J. of Geophysical Research : Planets     Full-text available via subscription   (Followers: 133)
J. of Geophysical Research : Solid Earth     Full-text available via subscription   (Followers: 52)
J. of Geophysical Research : Space Physics     Full-text available via subscription   (Followers: 140)
Paleoceanography     Full-text available via subscription   (Followers: 5, SJR: 3.067, h-index: 100)
Radio Science     Full-text available via subscription   (Followers: 43, SJR: 1.072, h-index: 59)
Reviews of Geophysics     Full-text available via subscription   (Followers: 37, SJR: 8.833, h-index: 107)
Space Weather     Full-text available via subscription   (Followers: 20, SJR: 1.341, h-index: 26)
Tectonics     Full-text available via subscription   (Followers: 16, SJR: 2.628, h-index: 96)
Water Resources Research     Full-text available via subscription   (Followers: 82, SJR: 2.661, h-index: 144)
Journal Cover Geochemistry, Geophysics, Geosystems
  [SJR: 2.439]   [H-I: 91]   [32 followers]  Follow
   Full-text available via subscription Subscription journal
   ISSN (Online) 1525-2027
   Published by AGU Homepage  [17 journals]
  • Uplift, Emergence, and Subsidence of the Gorda Escarpment Basement Ridge
           Offshore Cape Mendocino, CA
    • Authors: Susan M. Hoover; Anne M. Tréhu
      Abstract: The Gorda Escarpment is a topographic step that characterizes the south side of the Mendocino Transform Fault east of 126oW and forms the northern edge of the Vizcaino Block. Seismic reflection data suggest that the basement beneath the northern edge of the Vizcaino is composed of east-west trending slivers of oceanic crust that form a 15-km wide band of buried ridges we call the Gorda Escarpment Basement Ridge (GEBR) to distinguish it from the northwest-trending basement structure that characterizes most of the Vizcaino Block. The history of uplift and subsidence of the GEBR is reconstructed by combining the seismic reflection profiles with age and lithological constraints from ODP Site 1022, gravity cores, and grab samples from the northern face of the Escarpment recovered using a remotely operated vehicle. Uplift of the GEBR began prior to 6 Ma, and it was above sea level 3.7-2.5 Ma. GEBR uplift and emergence coincided with sediment deposition on the southern flank of the GEBR that we interpret as indicative of strong upwelling and turbulence in the lee of a shallow ridge and island chain. A bright reflection, interpreted to be a sill, is observed south of the shallowest part of the GEBR. We speculate that this sill may reflect a larger, hidden intrusion at depth and that thermal expansion of the crust combined with tectonic forces to drive enhanced uplift of this segment of the plate boundary. The GEBR has been subsiding since 2.7 Ma, and its shallowest point is now 1400 meters below sea level.
      PubDate: 2017-11-24T11:01:34.223068-05:
      DOI: 10.1002/2017GC007128
  • A Comparison of Geodetic and Geologic Rates Prior to Large Strike-Slip
           Earthquakes: A Diversity of Earthquake Cycle Behaviors'
    • Authors: James F. Dolan; Brendan J. Meade
      Abstract: Comparison of pre-event geodetic and geologic rates in three large-magnitude (Mw=7.6-7.9) strike-slip earthquakes reveals a wide range of behaviors. Specifically, geodetic rates of 26-28 mm/yr for the North Anatolian fault along the 1999 MW=7.6 Izmit rupture are ∼40% faster than Holocene geologic rates. In contrast, geodetic rates of ∼6-8 mm/yr along the Denali fault prior to the 2002 MW=7.9 Denali earthquake are only ∼half as fast as the latest Pleistocene-Holocene geologic rate of ∼12 mm/yr. In the third example where a sufficiently long pre-earthquake geodetic time series exists, the geodetic and geologic rates along the 2001 MW=7.8 Kokoxili rupture on the Kunlun fault are approximately equal at ∼11 mm/yr. These results are not readily explicable with extant earthquake cycle modeling, suggesting that they may instead be due to some combination of regional kinematic fault interactions, temporal variations in the strength of lithospheric-scale shear zones, and/or variations in local relative plate motion rate. Whatever the exact causes of these variable behaviors, these observations indicate that either the ratio of geodetic to geologic rates before an earthquake may not be diagnostic of the time to the next earthquake, as predicted by many rheologically based geodynamic models of earthquake cycle behavior, or that different behaviors characterize different fault systems in a manner that is not yet understood or predictable.
      PubDate: 2017-11-22T10:20:20.60846-05:0
      DOI: 10.1002/2017GC007014
  • Mantle Flow as a Trigger for Subduction Initiation: A Missing Element of
           the Wilson Cycle Concept
    • Authors: M. Baes; S.V. Sobolev
      Abstract: The classical Wilson Cycle concept, describing repeated opening and closing of ocean basins, hypothesizes spontaneous conversion of passive continental margins into subduction zones. This process, however, is impeded by the high strength of passive margins, and it has never occurred in Cenozoic times. Here, using thermomechanical models, we show that additional forcing, provided by mantle flow, which is induced by neighboring subduction zones and mid-mantle slab remnants, can convert a passive margin into a subduction zone. Models suggest that this is a long-term process, thus explaining the lack of Cenozoic examples. We speculate that new subduction zones may form in the next few tens of millions of years along the Argentine passive margin and the US East Coast. Mantle suction force can similarly trigger subduction initiation along large oceanic fracture zones. We propose that new subduction zones will preferentially originate where subduction zones were active in the past, thus explaining the remarkable co-location of subduction zones during at least the last 400 Myr.
      PubDate: 2017-11-20T17:47:28.377868-05:
      DOI: 10.1002/2017GC006962
  • Modeling Seismic Cycles of Great Megathrust Earthquakes Across the Scales
           With Focus at Postseismic Phase
    • Authors: Stephan V. Sobolev; Iskander A. Muldashev
      Abstract: Subduction is substantially multi-scale process where the stresses are built by long-term tectonic motions, modified by sudden jerky deformations during earthquakes, and then restored by following multiple relaxation processes. Here, we develop a cross-scale thermomechanical model aimed to simulate the subduction process from 1 minute to million years' time scale. The model employs elasticity, nonlinear transient viscous rheology, and rate-and-state friction. It generates spontaneous earthquake sequences and by using an adaptive time-step algorithm, recreates the deformation process as observed naturally during the seismic cycle and multiple seismic cycles. The model predicts that viscosity in the mantle wedge drops by more than three orders of magnitude during the great earthquake with a magnitude above 9. As a result, the surface velocities just an hour or day after the earthquake are controlled by viscoelastic relaxation in the several hundred km of mantle landward of the trench and not by the afterslip localized at the fault as is currently believed. Our model replicates centuries-long seismic cycles exhibited by the greatest earthquakes and is consistent with the postseismic surface displacements recorded after the Great Tohoku Earthquake. We demonstrate that there is no contradiction between extremely low mechanical coupling at the subduction megathrust in South Chile inferred from long-term geodynamic models and appearance of the largest earthquakes, like the Great Chile 1960 Earthquake.
      PubDate: 2017-11-15T10:40:36.116295-05:
      DOI: 10.1002/2017GC007230
  • REY-Th-U Solute Dynamics in the Critical Zone: Combined Influence of
           Chemical Weathering, Atmospheric Deposit Leaching and Vegetation Cycling
           (Mule Hole Watershed, South India)
    • Authors: Jean-Jacques Braun; Jean Riotte, Shrema Battacharya, Aurélie Violette, Jonathan Prunier, Vincent Bouvier, Frédéric Candaudap, Jean-Christophe Maréchal, Laurent Ruiz, Smruthi Rekha Panda, S. Subramanian
      Abstract: The source and proportion of REY, Th and U exported by groundwater and by the ephemeral stream along with the elemental proportions passing through vegetation have been assessed in the sub-humid tropical forested CZO of Mule Hole, Southern India. The study relies on a pluri-annual hydro-geochemical monitoring combined with a hydrological model. The significant difference between the soil input (SI) and output (SO) solute fluxes (mmol/km2/yr) of LREE (SI-SO = 13250-1500), HREE (1930-235), Th (64-12) and U (63-25) indicates a strong uptake by roots carried by canopy and forest floor processes. The contribution of atmospheric dust leaching can reach about 60% of LREE and 80% of HREE. At the watershed scale, the U solute flux exported by groundwater (180 mmol/km2/yr) mainly originates from the breakdown of primary U-bearing accessory minerals and dominates by a factor of 25 the stream flux. The precipitation of authigenic U bearing phases and adsorption onto Fe-oxides and oxyhydroxides plays a significant role for limiting the U mobility. In the groundwater, the plagioclase chemical weathering is efficiently traced by the positive Eu-anomaly. The very low (REY) to nil (Th) contents are explained by the precipitation of authigenic phases. In the stream flow, dominated by the overland flow (87% of the yearly stream flow), the solute exports (in mmol/km2/yr) of REY (1080 for LREE and 160 for HREE) and of Th (14) dominate those by groundwater. Their mobility is enhanced by chelation with organic ligands produced by forest floor and canopy processes.
      PubDate: 2017-11-15T10:40:26.659252-05:
      DOI: 10.1002/2017GC007158
  • Earthquakes as Precursors of Ductile Shear Zones in the Dry and Strong
           Lower Crust
    • Authors: L. Menegon; G. Pennacchioni, N. Malaspina, K. Harris, E. Wood
      Abstract: The rheology and the conditions for viscous flow of the dry granulite facies lower crust are still poorly understood. Viscous shearing in the dry and strong lower crust commonly localizes in pseudotachylyte veins, but the deformation mechanisms responsible for the weakening and viscous shear localization in pseudotachylytes are yet to be explored. We investigated examples of pristine and mylonitized pseudotachylytes in anorthosites from Nusfjord (Lofoten, Norway). Mutual overprinting relationships indicate that pristine- and mylonitized pseudotachylytes are coeval and resulted from the cyclical interplay between brittle and viscous deformation. The stable mineral assemblage in the mylonitized pseudotachylytes consists of plagioclase, amphibole, clinopyroxene, quartz, biotite, ± garnet ± K-feldspar. Amphibole-plagioclase geothermobarometry and thermodynamic modelling indicate that pristine- and mylonitized pseudotachylytes formed at 650-750°C and 0.7-0.8 GPa. Thermodynamic modelling indicates that a limited amount of H2O infiltration (0.20-0.40 wt%) was necessary to stabilize the mineral assemblage in the mylonite. Diffusion creep is identified as the main deformation mechanisms in the mylonitized pseudotachylytes based on the lack of crystallographic preferred orientation in plagioclase, the high degree of phase mixing, and the synkinematic nucleation of amphiboles in dilatant sites. Extrapolation of flow laws to natural conditions indicates that mylonitized pseudotachylytes are up to 3 orders of magnitude weaker than anorthosites deforming by dislocation creep, thus highlighting the fundamental role of lower crustal earthquakes as agents of weakening in strong granulites.
      PubDate: 2017-11-15T10:40:18.118666-05:
      DOI: 10.1002/2017GC007189
  • Magmatic Focusing to Mid-Ocean Ridges: The Role of Grain Size Variability
           and Non-Newtonian Viscosity
    • Authors: Andrew J. Turner; Richard F. Katz, Mark D. Behn, Tobias Keller
      Abstract: Melting beneath mid-ocean ridges occurs over a region that is much broader than the zone of magmatic emplacement that forms the oceanic crust. Magma is focused into this zone by lateral transport. This focusing has typically been explained by dynamic pressure gradients associated with corner flow, or by a sub-lithospheric channel sloping upward toward the ridge axis. Here we discuss a novel mechanism for magmatic focusing: lateral transport driven by gradients in compaction pressure within the asthenosphere. These gradients arise from the co-variation of melting rate and compaction viscosity. The compaction viscosity, in previous models, was given as a function of melt fraction and temperature. In contrast, we show that the viscosity variations relevant to melt focusing arise from grain-size variability and non-Newtonian creep. The asthenospheric distribution of melt fraction predicted by our models provides an improved explanation of the electrical resistivity structure beneath one location on the East Pacific Rise. More generally, we find that although grain size and non-Newtonian viscosity are properties of the solid phase, their effect on melt transport beneath mid-ocean ridges is more profound than their effect on the mantle corner-flow.
      PubDate: 2017-11-15T10:39:50.781725-05:
      DOI: 10.1002/2017GC007048
  • Perceiving the Crust in 3D: A Model Integrating Geological, Geochemical,
           and Geophysical Data
    • Authors: Virginia Strati; Scott A. Wipperfurth, Marica Baldoncini, William F. McDonough, Fabio Mantovani
      Abstract: Regional characterization of the continental crust has classically been performed through either geologic mapping, geochemical sampling, or geophysical surveys. Rarely are these techniques fully integrated, due to limits of data coverage, quality, and/or incompatible datasets. We combine geologic observations, geochemical sampling, and geophysical surveys to create a coherent 3D geologic model of a 50 × 50 km upper crustal region surrounding the Sudbury Neutrino Observatory (SNO) in Canada, which includes, the Southern Province, the Superior Province, the Sudbury Structure and the Grenville Front Tectonic Zone. Nine representative, aggregate units, of exposed lithologies, are geologically characterized, geophysically constrained, and probed with 109 rock samples supported by compiled geochemical databases. A detailed study of the lognormal distributions of U and Th abundances and of their correlation permits a bivariate analysis for a robust treatment of the uncertainties. A downloadable 3D numerical model of U and Th distribution, defines an average heat production of 1.5+1.4-0.7 µW/m3, and predicts a contribution of 7.8+8.4-3.2 TNU (a Terrestrial Neutrino Unit is one geoneutrino event per 1032 target protons per year) out of a crustal geoneutrino signal of 31.2+8.6-4.7 TNU. The relatively high local crust geoneutrino signal together with its large variability strongly restrict the SNO+ capability of experimentally discriminating among BSE compositional models of the mantle. Future work to constrain the crustal heat production and the geoneutrino signal at SNO+ will be inefficient without more detailed geophysical characterization of the 3D structure of the heterogeneous Huronian Supergroup, which contributes the largest uncertainty to the calculation.
      PubDate: 2017-11-15T10:39:45.248802-05:
      DOI: 10.1002/2017GC007067
  • Bay of Bengal Exhibits Warming Trend During the Younger Dryas:
           Implications of AMOC
    • Authors: Champoungam Panmei; Pothuri Divakar Naidu, Mahyar Mohtadi
      Abstract: A sharp decline in temperature during the Younger Dryas (YD) preceding the current warmer Holocene is well documented in climate archives from the Northern Hemisphere high latitudes. Although the magnitude of YD cooling varied spatially, the response of YD cooling was well documented in the Atlantic and Pacific Oceans but not in the Indian Ocean. Here we investigate whether the modern remote forcing of tropical Indian Ocean sea surface temperature (SST) by Northern Hemisphere climate changes holds true for events such as the YD. Our SST reconstruction from the western Bay of Bengal exhibits an overall warming of ∼1.8°C during the YD. We further compared our data with other existing Mg/Ca-based SST records from the Northern Indian Ocean and found no significant negative SST anomalies in both the Arabian Sea and the Bay of Bengal compared to pre and post YD, suggesting that no apparent cooling occurred during the YD in the Northern Indian Ocean. In contrast, most part of the YD exhibits positive SST anomalies in the Northern Indian Ocean that coincide with the slowdown of the Atlantic Meridional Overturning Circulation during this period.
      PubDate: 2017-11-15T10:39:26.262237-05:
      DOI: 10.1002/2017GC007075
  • A Reassessment of the Precision of Carbonate Clumped Isotope Measurements:
           Implications for Calibrations and Paleoclimate Reconstructions
    • Authors: Alvaro Fernandez; Inigo A. Müller, Laura Rodríguez-Sanz, Joep van Dijk, Nathan Looser, Stefano M. Bernasconi
      Abstract: Carbonate clumped isotopes offer a potentially transformational tool to interpret Earth's history, but the proxy is still limited by poor inter-laboratory reproducibility. Here, we focus on the uncertainties that result from the analysis of only a few replicate measurements to understand the extent to which unconstrained errors affect calibration relationships and paleoclimate reconstructions. We find that highly precise data can be routinely obtained with multiple replicate analyses, but this is not always done in many laboratories. For instance, using published estimates of external reproducibilities we find that typical clumped isotope measurements (3 replicate analyses) have margins of error at the 95% confidence level (CL) that are too large for many applications. These errors, however, can be systematically reduced with more replicate measurements. Second, using a Monte Carlo-type simulation we demonstrate that the degree of disagreement on published calibration slopes is about what we should expect considering the precision of Δ47 data, the number of samples and replicate analyses, and the temperature range covered in published calibration. Finally, we show that the way errors are typically reported in clumped isotope data can be problematic and lead to the impression that data is more precise than warranted. We recommend that uncertainties in Δ47 data should no longer be reported as the standard error of a few replicate measurements. Instead, uncertainties should be reported as margins of error at a specified confidence level (e.g., 68% or 95% CL). These error bars are a more realistic indication of the reliability of a measurement.
      PubDate: 2017-11-15T10:39:20.640056-05:
      DOI: 10.1002/2017GC007106
  • Gas Hydrate Formation Amid Submarine Canyon Incision: Investigations From
           New Zealand's Hikurangi Subduction Margin
    • Authors: G.J. Crutchley; K.F. Kroeger, I.A. Pecher, J.J. Mountjoy, A.R. Gorman
      Abstract: We investigate gas hydrate system dynamics beneath a submarine canyon on New Zealand's Hikurangi subduction margin using seismic reflection data and petroleum systems modelling. High seismic velocities just above the base of gas hydrate stability (BGHS) indicate that concentrated gas hydrates exist beneath the canyon. Two-dimensional gas hydrate formation modelling shows how the process of canyon incision at this location alters the distribution and concentration of gas hydrate. The key modelling result is that free gas is trapped beneath the gas hydrate layer and then ‘captured' into a concentrated gas hydrate deposit as a result of a downward-shift in the BGHS driven by canyon incision. Our study thus provides new insight into the functioning of this process. From our data, we also conceptualise two other models to describe how canyons could significantly change gas hydrate distribution and concentration. One scenario is related to deflection of fluid flow pathways from over-pressured regions at the BGHS toward the canyon, and the other is based on relationships between simultaneous seafloor uplift and canyon incision. The relationships and processes described are of global relevance because of considerations of gas hydrate as an energy resource and the influence of both submarine canyons and gas hydrate systems on seafloor biodiversity.
      PubDate: 2017-11-15T10:39:04.60434-05:0
      DOI: 10.1002/2017GC007021
  • Goethite Concentration Variations in the Red Clay Sequence on the Chinese
           Loess Plateau
    • Authors: R. Zhang; J. S. Nie
      Abstract: The Goethite is a common magnetic mineral in soils and is important for understanding past climatic and environmental change. Although goethite concentration records are available for the Quaternary loess-paleosol sequences from the Chinese Loess Plateau (CLP), no goethite concentration record is available for the underlying Red Clay sequence. Here, we generate a record of goethite concentration from the Chaona section on the CLP and investigate the relationship between goethite and ferrimagnetic mineral concentration and climate parameters on the Red Clay sequence deposited from 6-2.7 Ma. We found that increasing goethite concentration generally corresponds to decreasing magnetic susceptibility for the Red Clay sequence, with the exception of an anomalous section from 4.3-3.7 Ma where both records show increasing trends. Precipitation seasonality could explain this opposite variation pattern between magnetic susceptibility and goethite concentration in the Red Clay sequence. From 3.6-2.7 Ma, goethite concentration has a decreasing trend while magnetic susceptibility increased, which we interpret as more seasonally distributed precipitation associated with intensification of the East Asian summer monsoon (EASM). Similar pattern was observed from 0.65-0 Ma from the Lingtai section. Taken together, the goethite record reveals that the EASM experienced two stages of rapid intensification with apparent seasonally-distributed precipitation since 6 Ma.
      PubDate: 2017-11-08T01:15:48.770411-05:
      DOI: 10.1002/2017GC007148
  • A New Sulfur and Carbon Degassing Inventory for the Southern Central
           American Volcanic Arc: The Importance of Accurate Time-Series Datasets and
           Possible Tectonic Processes Responsible for Temporal Variations in
           Arc-Scale Volatile Emissions
    • Authors: J.M. de Moor; C. Kern, G. Avard, C. Muller, A. Aiuppa, A. Saballos, M. Ibarra, P. LaFemina, M. Protti, T.P. Fischer
      Abstract: This work presents a new database of SO2 and CO2 fluxes from the Southern Central American Volcanic Arc (SCAVA) for the period 2015-2016. We present ∼300 SO2 flux measurements from 10 volcanoes and gas ratios from 11 volcanoes in Costa Rica and Nicaragua representing the most extensive available assessment of this ∼500 km arc segment. The SO2 flux from SCAVA is estimated at 6,240±1,150 T/d, about a factor of three higher than previous estimations (1972-2013). We attribute this increase in part to our more complete assessment of the arc. Another consideration in interpreting the difference is the context of increased volcanic activity, as there were more eruptions in 2015-2016 than in any period since ∼1980. A potential explanation for increased degassing and volcanic activity is a change in crustal stress regime (from compression to extension, opening volcanic conduits) following two large (Mw>7) earthquakes in the region in 2012. The CO2 flux from the arc is estimated at 22,500±4,900 T/d, which is equal to or greater than estimates of C input into the SCAVA subduction zone. Time-series datasets for arc degassing need to be improved in temporal and spatial coverage to robustly constrain volatile budgets and tectonic controls. Arc volatile budgets are strongly influenced by short-lived degassing events and arc systems likely display significant short-term variations in volatile output, calling for expansion of nascent geochemical monitoring networks to achieve spatial and temporal coverage similar to traditional geophysical networks.
      PubDate: 2017-11-08T01:15:34.892038-05:
      DOI: 10.1002/2017GC007141
  • P- and S-Wave Receiver Function Imaging of Subduction With Scattering
    • Authors: S. M. Hansen; B. Schmandt
      Abstract: A method for scattered wave imaging in 3D with both teleseismic P- and S-wave receiver function data is introduced. The approach relies on body-wave scattering kernels that are derived from the adjoint data sensitivity kernels which are typically used for full waveform tomography. The forward problem is approximated using ray theory, yielding a computationally efficient imaging algorithm that can resolve dipping and discontinuous interfaces using both P- and S-wave receiver functions. Travel-time fields for the incident teleseismic arrivals and the receiver point-sources are obtained by solving the Eikonal equation using a Fast Marching code which can handle a 3D reference velocity model. An energy stable finite-difference method is used to simulate elastic wave propagation in a 2D hypothetical subduction zone model. The resulting synthetic P- and S-wave receiver function datasets are used to validate the imaging method. The kernel images are compared with those generated by the Generalized Radon Transform and Common Conversion Point Stacking methods. These results demonstrate the potential of the kernel imaging approach for constraining lithospheric structure in complex geologic environments with sufficiently dense recordings of teleseismic data. Potential imaging targets include short-wavelength compositional variations in the mantle wedge and the slab's lithosphere-asthenosphere boundary.
      PubDate: 2017-11-03T01:10:46.544655-05:
      DOI: 10.1002/2017GC007120
  • Gravity-Driven Deposits in an Active Margin (Ionian Sea) Over the Last 330
           000 Years
    • Authors: Eléonore Köng; Sébastien Zaragosi, Jean-Luc Schneider, Thierry Garlan, Patrick Bachèlery, Marjolaine Sabine, Laurine San Pedro
      Abstract: In the Ionian Sea, the subduction of the Nubia plate underneath the Eurasia plate leads to an important sediment remobilisation on the Calabrian Arc and the Mediterranean Ridge. These events are often associated with earthquakes and tsunamis. In this study, we analyse gravity-driven deposits in order to establish their recurrence time on the Calabrian Arc and the western Mediterranean Ridge. Four gravity cores collected on ridges and slope basins of accretionary prisms record turbidites, megaturbidites, slumping and micro-faults over the last 330,000 years. These turbidites were dated by correlation with a hemipelagic core with a multi-proxy approach: radiometric dating, δ18O, b* colour curve, sapropels and tephrochronology. The origin of the gravity-driven deposits was studied with a sedimentary approach: grain-size, lithology, thin section, geochemistry of volcanic glass. The results suggest three periods of presence/absence of gravity-driven deposits: a first on the western lobe of the Calabrian Arc between 330 000 and 250 000 years, a second between 120 000 years and present day on the eastern lobe of the Calabrian Arc and over the last 60 000 years on the western lobe, and a third on the Mediterranean Ridge over the last 37 000 years. Return times for gravity-driven deposits are around 1000 years during the most important record periods. The turbidite activity also highlights the presence of volcaniclastic turbidites that seems to be link to the Etna changing morphology over the last 320 000 years.
      PubDate: 2017-11-02T11:02:39.383141-05:
      DOI: 10.1002/2017GC006950
  • The Effect of Arc Proximity on Hydrothermal Activity Along Spreading
           Centers: New Evidence From the Mariana Back-Arc (12.7°–18.3°N)
    • Authors: Edward T. Baker; Sharon L. Walker, Joseph A. Resing, William W. Chadwick, Susan G. Merle, Melissa O. Anderson, David A. Butterfield, Nathan J. Buck, Susanna Michael
      Abstract: Back-arc spreading centers (BASCs) form a distinct class of ocean spreading ridges distinguished by steep along-axis gradients in spreading rate and by additional magma supplied through subduction. These characteristics can affect the population and distribution of hydrothermal activity on BASCs compared to mid-ocean ridges (MORs). To investigate this hypothesis we comprehensively explored 600 km of the southern half of the Mariana BASC. We used water column mapping and seafloor imaging to identify 19 active vent sites, an increase of 13 over the current listing in the InterRidge Database (IRDB), on the bathymetric highs of 7 of the 11 segments. We identified both high- and low- (i.e., characterized by a weak or negligible particle plume) temperature discharge occurring on segment types spanning dominantly magmatic to dominantly tectonic. Active sites are concentrated on the two southernmost segments, where distance to the adjacent arc is shortest (48 mm/yr), and tectonic extension is pervasive. Re-examination of hydrothermal data from other BASCs supports the generalization that hydrothermal site density increases on segments
      PubDate: 2017-11-02T10:56:06.794623-05:
      DOI: 10.1002/2017GC007234
  • Lateral Variations in SKS Splitting Across the MAGIC Array, Central
    • Authors: John C. Aragon; Maureen D. Long, Margaret H. Benoit
      Abstract: The eastern margin of North America has been shaped by several cycles of supercontinent assembly. These past episodes of orogenesis and continental rifting have likely deformed the lithosphere, but the extent, style, and geometry of this deformation remain poorly known. Measurements of seismic anisotropy in the upper mantle can shed light on past lithospheric deformation, but may also reveal contributions from present-day mantle flow in the asthenosphere. Here we examine SKS waveforms and measure splitting of SKS phases recorded by the MAGIC experiment, a dense transect of seismic stations across the central Appalachians. Our measurements constrain small-scale lateral variations in azimuthal anisotropy and reveal distinct regions of upper mantle anisotropy. Stations within the present-day Appalachian Mountains exhibit fast splitting directions roughly parallel to the strike of the mountains and delay times of about 1.0 sec. To the west, transverse component waveforms for individual events reveal lateral variability in anisotropic structure. Stations immediately to the east of the mountains exhibit complicated splitting patterns, more null SKS arrivals, and a distinct clockwise rotation of fast directions. The observed variability in splitting behavior argue for contributions from both the lithosphere and the asthenospheric mantle. We infer that the sharp lateral transition in splitting behavior at the eastern edge of the Appalachians is controlled by a change in anisotropy in the lithospheric mantle. We hypothesize that beneath the Appalachians, SKS splitting reflects lithospheric deformation associated with Appalachian orogenesis, while just to the east this anisotropic signature was modified by Mesozoic rifting.
      PubDate: 2017-11-02T10:50:34.898767-05:
      DOI: 10.1002/2017GC007169
  • Influence of Exsolved Volatiles on Reheating Silicic Magmas by Recharge
           and Consequences for Eruptive Style at Volcan Quizapu (Chile)
    • Authors: W. Degruyter; C. Huber, O. Bachmann, K. M. Cooper, A. J. R. Kent
      Abstract: The two most recent eruptions of Volcàn Quizapu (southern Andes, Chile), only 86 years apart, were both triggered by magma recharge and extruded the same volume (about 5 km3) of the same volatile-rich dacitic magma, but showed a remarkable shift from effusive (1846-47) to explosive (1932) behavior. We demonstrate, using a newly developed model, that the presence or absence of an exsolved volatile phase in the reservoir strongly influences its mechanical and thermal response to new inputs of magma. We propose that, prior to the 1846-47 effusive eruption, gas bubbles damped the build-up of excess pressure and allowed recharge of a significant volume of magma before triggering the 1846-1847 eruption. The strong temperature increase that resulted enhanced syn-eruptive outgassing leading to an effusive eruption. In contrast, during the repose period between the 1847 and 1932 eruptions new recharges found a much less compressible host reservoir as the exsolved gas phase was largely removed in response to the prior eruption, yielding rapid pressurization, minor reheating, and comparatively less syneruptive outgassing. The combination of these effects culminated in an explosive eruption.
      PubDate: 2017-11-01T10:55:46.335172-05:
      DOI: 10.1002/2017GC007219
  • Significance of Northeast-Trending Features in Canada Basin, Arctic Ocean
    • Authors: D.R. Hutchinson; H.R. Jackson, D.W. Houseknecht, Q. Li, J.W. Shimeld, D. Mosher, D. Chian, R.W. Saltus, G.N. Oakey
      Abstract: Synthesis of seismic velocity, potential field, and geological data from Canada Basin and its surrounding continental margins suggest that a northeast-trending structural fabric has influenced the origin, evolution, and current tectonics of the basin. This structural fabric has a crustal origin, based on the persistence of these trends in upward continuation of total magnetic intensity data and vertical derivative analysis of free air gravity data. Three sub-parallel northeast trending features are described. Northwind Escarpment, bounding the east side of the Chukchi Borderland, extends ∼600 km and separates continental crust of Northwind Ridge from high-velocity transitional crust in Canada Basin. A second, shorter northeast-trending zone extends ∼300 km in northern Canada Basin and separates inferred continental crust of Sever Spur from magmatically intruded crust of the High Arctic Large Igneous Province. A third northeast-trending feature, here called the Alaska-Prince Patrick magnetic lineament (APPL) is inferred from magnetic data and its larger regional geologic setting. Analysis of these three features suggests strike-slip or transtensional deformation played a role in the opening of Canada Basin. These features can be explained by initial Jurassic-Early Cretaceous strike slip deformation (phase 1) followed in the Early Cretaceous (∼134 to ∼124 Ma) by rotation of Arctic Alaska with sea-floor spreading orthogonal to the fossil spreading axis preserved in the central Canada Basin (phase 2). In this model, the Chukchi Borderland is part of Arctic Alaska.
      PubDate: 2017-11-01T10:50:49.749805-05:
      DOI: 10.1002/2017GC007099
  • Environmental Controls on Mg/Ca in Neogloboquadrina incompta: A Core-Top
           Study From the Subpolar North Atlantic
    • Authors: Audrey Morley; Tali Babila, James Wright, Ulysses Ninnemann, Kikki Kleiven, Nil Irvali, Yair Rosenthal
      Abstract: Magnesium/Calcium paleothermometry is an established tool for reconstructing past surface and deep-sea temperatures. However, our understanding of non-thermal environmental controls on the uptake of Mg into the calcitic lattice of foraminiferal tests remains limited. Here we present a combined analysis of multiple trace element/calcium ratios and stable isotope (δ18O and δ13C) geochemistry on the subpolar planktonic foraminifera Neogloboquadrina incompta to assess the validity of Mg/Ca as a proxy for surface ocean temperature. We identify small size-specific offsets in Mg/Ca and δ18Oc values for N. incompta that are consistent with depth habitat migration patterns throughout the life cycle of this species. Additionally, an assessment of non-thermal controls on Mg/Ca values reveals that (1) the presence of volcanic ash, (2) the addition of high-Mg abiotic overgrowths, and (3) ambient seawater carbonate chemistry can have a significant impact on the Mg/Ca-to-temperature relationship. For carbonate ion concentrations of values > 200 μmol*kg−1, we find that temperature exerts the dominant control on Mg/Ca values, while at values 
      PubDate: 2017-11-01T10:50:36.642676-05:
      DOI: 10.1002/2017GC007111
  • Sedimentary Mercury Enrichments as a Marker for Submarine Large Igneous
           Province volcanism' Evidence From the Mid-Cenomanian Event and Oceanic
           Anoxic Event 2 (Late Cretaceous)
    • Authors: J. D. Scaife; M. Ruhl, A. J. Dickson, T. A. Mather, H. C. Jenkyns, L. M. E. Percival, S. P. Hesselbo, J. Cartwright, J. S. Eldrett, S. C. Bergman, D. Minisini
      Abstract: Oceanic Anoxic Event 2 (OAE 2), during the Cenomanian–Turonian transition (∼94 Ma), was the largest perturbation of the global carbon cycle in the mid-Cretaceous and can be recognized by a positive carbon-isotope excursion in sedimentary strata. Although OAE 2 has been linked to large-scale volcanism, several large igneous provinces (LIPs) were active at this time (e.g. Caribbean, High Arctic, Madagascan, Ontong-Java) and little clear evidence links OAE 2 to a specific LIP. The Mid-Cenomanian Event (MCE, ∼96 Ma), identified by a small, 1 ‰ positive carbon-isotope excursion, is often referred to as a prelude to OAE 2. However, no underlying cause has yet been demonstrated and its relationship to OAE 2 is poorly constrained. Here, we report sedimentary mercury (Hg) concentration data from four sites, three from the southern margin of the Western Interior Seaway and one from Demerara Rise, in the equatorial proto-North Atlantic Ocean. We find that, in both areas, increases in mercury concentrations and Hg/TOC ratios coincide with the MCE and the OAE 2. However, the increases found in these sites are of a lower magnitude than those found in records of many other Mesozoic events, possibly characteristic of a marine rather than atmospheric dispersal of mercury for both events. Combined, the new mercury data presented here are consistent with an initial magmatic pulse at the time of the MCE, with a second, greater pulse at the onset of OAE 2, possibly related to the emplacement of LIPs in the Pacific Ocean and/or the High Arctic.
      PubDate: 2017-11-01T10:50:28.769806-05:
      DOI: 10.1002/2017GC007153
  • Slab Breakoff of the Neo-Tethys Ocean in the Lhasa Terrane Inferred From
           Contemporaneous Melting of the Mantle and Crust
    • Authors: Feng Huang; Jifeng Xu, Yunchuan Zeng, Jianlin Chen, Baodi Wang, Hongxia Yu, Ling Chen, Wenlong Huang, Rongyu Tan
      Abstract: Oceanic slab breakoff significantly affects the thermal regime of the lithosphere during continental collision. This often triggers extension-related mafic magmatism and crustal melting. It is generally accepted that the Neo-Tethyan lithosphere subducted beneath the southern Lhasa Subterrane, resulting in the formation of the Gangdese magmatic arc. However, the timing of slab breakoff is still disputed, due to a lack of evidence for extension-related mafic magmatism. In this study, we provide comprehensive age, element and Sr–Nd–Hf isotopic data of mafic dikes, felsic intrusions, and enclaves from the Daju area, southern Lhasa Subterrane. The timing of mafic dikes and granitoids are contemporaneous at ca. 57 Ma. The mafic dikes are characterized by high Th/U, and Zr/Y ratios, their geochemistry indicates an intraplate affinity rather than arc magmas. Furthermore, the mafic dikes show strongly variable igneous zircon εHf(t), and lower whole-rock εNd(t) than granitoids. This evidence suggests that the mafic dikes represent asthenosphere-derived melts contaminated by various degrees of ancient lithosphere. However, the granitoids were directly derived from the juvenile lower crust. Given the abrupt decrease in the convergence rate between India and Asia, and the surface uplift and sedimentation cessation in the southern Lhasa Subterrane in the early Cenozoic, the occurrence of synchronous mafic dikes and granitoids is best explained by a slab breakoff model. The occurrence of intraplate-type magmas likely corresponds to the magmatic expression of the initial stage of Neo-Tethyan slab breakoff. The slab breakoff concept also explains the onset of the magmatic ‘flare-up' and crustal growth after 57 Ma.
      PubDate: 2017-10-25T10:55:23.386206-05:
      DOI: 10.1002/2017GC007039
  • Evolving Mantle Sources in Postcollisional Early Permian-Triassic Magmatic
           Rocks in the Heart of of Tianshan Orogen (Western China)
    • Authors: Gong-Jian Tang; Peter A. Cawood, Derek A. Wyman, Qiang Wang, Zhen-Hua Zhao
      Abstract: Magmatism post-dating the initiation of continental collision provides insight into the late stage evolution of orogenic belts including the composition of the contemporaneous underlying subcontinental mantle. The Awulale Mountains, in the heart of the Tianshan Orogen, display three types of post-collisional mafic magmatic rocks. (1) a medium to high K calc-alkaline mafic volcanic suite (∼ 280 Ma), which display low La/Yb ratios (2.2 – 11.8) and a wide range of εNd(t) values from +1.9 to +7.4. This suite of rocks was derived from melting of depleted metasomatized asthenospheric mantle followed by upper crustal contamination. (2) Mafic shoshonitic basalts (∼ 272 Ma), characterized by high La/Yb ratios (14.4 – 20.5) and more enriched isotope compositions (εNd(t) = +0.2 – +0.8). These rocks are considered to have been generated by melting of lithospheric mantle enriched by melts from the Tarim continental crust that was subducted beneath the Tianshan during final collisional suturing. (3) Mafic dikes (∼ 240 Ma), with geochemical and isotope compositions similiar to the ∼280 Ma basaltic rocks. This succession of post-collision mafic rock types suggests there were two stages of magma generation involving the sampling of different mantle sources. The first stage, which occurred in the early Permian, involved a shift from depleted asthenospheric sources to enriched lithospheric mantle. It was most likely triggered by the subduction of Tarim continental crust and thickening of the Tianshan lithospheric mantle. During the second stage, in the middle Triassic, there was a reversion to more asthenospheric sources, related to post-collision lithospheric thinning.
      PubDate: 2017-10-25T10:50:35.398259-05:
      DOI: 10.1002/2017GC006977
  • The Influence of Crystal Size Distributions on the Rheology of Magmas: New
           Insights From Analogue Experiments
    • Authors: Johannes Klein; Sebastian P. Mueller, Jonathan M. Castro
      Abstract: This study examines the influence of particle size distributions on the rheology of particle suspensions by using analogue experiments with spherical glass beads in silicone oil as magma equivalent. The analyses of 274 individual particle-bearing suspensions of varying modality (uni-, bi- tri- and tetramodality), as well as of polymodal suspensions with specific defined skewness and variance, are the first data set of its kind and provide important insights in the relationship between the solid particles of a suspension and its rheological behaviour. Since the relationship between the rheology of particle bearing suspensions and its maximum packing fraction ϕm is well established by several theoretical models, the results of the analogue experiments of this study reveal that the polydispersity γ exerts the largest influence on ϕm. Consequently, the estimation of the polydispersity γ of a particle size distribution is essential for estimating the viscosity of that given suspension.
      PubDate: 2017-10-24T11:03:40.177857-05:
      DOI: 10.1002/2017GC007114
  • High Pressure Experiments on Metal-Silicate Partitioning of Chlorine in A
           Magma Ocean: Implications for Terrestrial Chlorine Depletion
    • Authors: Hideharu Kuwahara; Hirotada Gotou, Toru Shinmei, Nobuhiro Ogawa, Asuka Yamaguchi, Naoto Takahata, Yuji Sano, Takehiko Yagi, Seiji Sugita
      Abstract: In the bulk silicate Earth, chlorine is more depleted than other elements with similar volatilities; however the cause of terrestrial chlorine depletion is not well understood. Two major hypotheses have been proposed to explain this depletion: Incorporation into the Earth's metallic core and escape to space. The former hypothesis can be tested by investigating the partitioning of chlorine between iron-rich metallic liquids and silicate melts. In this study, we investigated the experimental partitioning of chlorine between iron-rich metallic liquids and silicate melts at pressures from 4 to 23 GPa and temperatures from 1650 to 2400°C using multi-anvil presses. The results demonstrate that chlorine is moderately to highly lithophile under the experimental conditions. In sulfur-free experiments, chlorine becomes slightly more siderophile as temperature increases and less siderophile as pressure increases. For sulfur-bearing experiments, no significant effects of pressure or temperature were observed. Based on these data and thermodynamic considerations, we obtained empirical laws to estimate chlorine partition coefficients between iron-rich metallic liquids and silicate melts. Under the P-T conditions that would have controlled metal-silicate equilibration during core segregation in the Earth, the calculated metal-silicate partition coefficients for chlorine are much lower than unity. This result suggests that terrestrial chlorine that may have been present in the accreting Earth was not partitioned into its core, supporting that escape to space is the more likely hypothesis. If terrestrial chlorine was lost to space, chlorine depletion may have resulted from the loss of the primordial hydrosphere during the formation of the Earth.
      PubDate: 2017-10-23T17:30:44.36253-05:0
      DOI: 10.1002/2017GC007159
  • Physical and Transport Property Variations Within Carbonate-Bearing Fault
           Zones: Insights From the Monte Maggio Fault (Central Italy)
    • Authors: F. Trippetta; B.M. Carpenter, S. Mollo, M.M. Scuderi, P. Scarlato, C. Collettini
      Abstract: The physical characterization of carbonate–bearing normal faults is fundamental for resource development and seismic hazard. Here we report laboratory measurements of density, porosity, Vp, Vs, elastic moduli and permeability for a range of effective confining pressures (0.1-100 MPa), conducted on samples representing different structural domains of a carbonate-bearing fault. We find a reduction in porosity from the fault breccia (11.7% total and 6.2% connected) to the main fault plane (9% total and 3.5% connected), with both domains showing higher porosity compared to the protolith (6.8% total and 1.1% connected). With increasing confining pressure, P-wave velocity evolves from 4.5 km/s to 5.9 km/s in the fault breccia, is constant at 5.9 km/s approaching the fault plane and is low (4.9 km/s) in clay-rich fault domains. We find that while the fault breccia shows pressure sensitive behaviour (a reduction in permeability from 2*10−16 m2 to 2*10−17 m2), the cemented cataclasite close to the fault plane is characterized by pressure independent behaviour (permeability 4*10−17 m2). Our results indicate that the deformation processes occurring within the different fault structural domains influence the physical and transport properties of the fault zone. In-situ Vp profiles match well the laboratory measurements demonstrating that laboratory data are valuable for implications at larger scale. Combining the experimental values of elastic moduli and frictional properties it results that at shallow crustal levels M ≤ 1 earthquakes are less favoured, in agreement with earthquake-depth distribution during the L'Aquila 2009 seismic sequence that occurred on carbonates.
      PubDate: 2017-10-20T10:56:23.717002-05:
      DOI: 10.1002/2017GC007097
  • High Throughput Petrochronology and Sedimentary Provenance Analysis by
           Automated Phase Mapping and LAICPMS
    • Authors: Pieter Vermeesch; Martin Rittner, Ethan Petrou, Jenny Omma, Chris Mattinson, Eduardo Garzanti
      Abstract: The first step in most geochronological studies is to extract dateable minerals from the host rock, which is time consuming, removes textural context, and increases the chance for sample cross-contamination. We here present a new method to rapidly perform in-situ analyses by coupling a fast Scanning Electron Microscope (SEM) with Energy Dispersive X-ray Spectrometer (EDS) to a Laser Ablation Inductively Coupled Plasma Mass Spectrometer (LAICPMS) instrument. Given a polished hand specimen, a petrographic thin section, or a grain mount, Automated Phase Mapping (APM) by SEM/EDS produces chemical and mineralogical maps from which the X-Y coordinates of the datable minerals are extracted. These coordinates are subsequently passed on to the laser ablation system for isotopic analysis. We apply the APM+LAICPMS method to three igneous, metamorphic and sedimentary case studies. In the first case study, a polished slab of granite from Guernsey was scanned for zircon, producing a 609 ± 8 Ma weighted mean age. The second case study investigates a paragneiss from an ultra-high pressure terrane in the North Qaidam terrane (Qinghai, China). 107 small (25 μm) metamorphic zircons were analysed by LAICPMS to confirm a 419 ± 4 Ma age of peak metamorphism. The third and final case study uses APM+LAICPMS to generate a large provenance dataset and trace the provenance of 25 modern sediments from Angola, documenting longshore drift of Orange River sediments over a distance of 1500 km. These examples demonstrate that APM+LAICPMS is an efficient and cost effective way to improve the quantity and quality of geochronological data.
      PubDate: 2017-10-20T10:55:44.545789-05:
      DOI: 10.1002/2017GC007109
  • GrowYourIC: A Step Toward a Coherent Model of the Earth's Inner Core
           Seismic Structure
    • Authors: Marine Lasbleis; Lauren Waszek, Elizabeth A. Day
      Abstract: A complex inner core structure has been well-established from seismic studies, showing radial and lateral heterogeneities at various length scales. Yet, no geodynamic model is able to explain all the features observed. One of the main limits for this is the lack of tools to compare seismic observations and numerical models successfully. We use here a new Python tool called GrowYourIC to compare models of inner core structure. We calculate properties of geodynamic models of the inner core along seismic ray paths, for random or user-specified datasets. We test kinematic models which simulate fast lateral translation, super-rotation, and differential growth. We explore first the influence on a real inner core data set, which has a sparse coverage of the inner core boundary. Such a data set is however able to successfully constrain the hemispherical boundaries due to a good sampling of latitudes. Combining translation and rotation could explain some of the features of the boundaries separating the inner core hemispheres. The depth shift of the boundaries, observed by some authors, seems unlikely to be modelled by a fast translation, but could be produced by slow translation associated to super-rotation.
      PubDate: 2017-10-20T10:52:06.862924-05:
      DOI: 10.1002/2017GC007149
  • The Iceland Plate Boundary Zone: Propagating Rifts, Migrating Transforms,
           and Rift-Parallel Strike-Slip Faults
    • Authors: J.A. Karson
      Abstract: Unlike most of the Mid-Atlantic Ridge, the North America/Eurasia plate boundary in Iceland lies above sea level where magmatic and tectonic processes can be directly investigated in subaerial exposures. Accordingly, geologic processes in Iceland have long been recognized as possible analogs for seafloor spreading in the submerged parts of the mid-ocean ridge system. Combining existing and new data from across Iceland provides an integrated view of this active, mostly subaerial plate boundary. The broad Iceland plate boundary zone includes segmented rift zones linked by transform fault zones. Rift propagation and transform fault migration away from the Iceland hotspot rearrange the plate boundary configuration resulting in widespread deformation of older crust and reactivation of spreading-related structures. Rift propagation results in block rotations that are accommodated by widespread, rift-parallel, strike-slip faulting. The geometry and kinematics of faulting in Iceland may have implications for spreading processes elsewhere on the mid-ocean ridge system where rift propagation and transform migration occur.
      PubDate: 2017-10-20T10:50:44.548404-05:
      DOI: 10.1002/2017GC007045
  • MeBo70 Seabed Drilling on a Polar Continental Shelf: Operational Report
           and Lessons From Drilling in the Amundsen Sea Embayment of West Antarctica
    • Authors: K. Gohl; T. Freudenthal, C.-D. Hillenbrand, J. Klages, R. Larter, T. Bickert, S. Bohaty, W. Ehrmann, O. Esper, T. Frederichs, C. Gebhardt, K. Küssner, G. Kuhn, H. Pälike, T. Ronge, P. Simões Pereira, J. Smith, G. Uenzelmann-Neben, C. van de Flierdt,
      Abstract: A multi-barrel seabed drill rig was used for the first time to drill unconsolidated sediments and consolidated sedimentary rocks from an Antarctic shelf with core recoveries between 7 and 76%. We deployed the MARUM-MeBo70 drill device at nine drill sites in the Amundsen Sea Embayment. Three sites were located on the inner shelf of Pine Island Bay from which soft sediments, presumably deposited at high sedimentation rates in isolated small basins, were recovered from drill depths of up to 36 m below seafloor. Six sites were located on the middle shelf of the eastern and western embayment. Drilling at five of these sites recovered consolidated sediments and sedimentary rocks from dipping strata spanning ages from Late Cretaceous to Miocene. This report describes the initial coring results, the challenges posed by drifting icebergs and sea ice, and technical issues related to deployment of the MeBo70. We also present recommendations for similar future drilling campaigns on polar continental shelves.
      PubDate: 2017-10-16T01:17:07.314434-05:
      DOI: 10.1002/2017GC007081
  • Understanding Copper Isotope Behavior in the High Temperature
           Magmatic-Hydrothermal Porphyry Environment
    • Authors: Melissa J. Gregory; Ryan Mathur
      Abstract: Copper stable isotope geochemistry has the potential to constrain aspects of ore deposit formation once variations in the isotopic data can be related to the physiochemical conditions during metal deposition. This study presents Cu isotope ratios for copper sulfides from samples from the Pebble porphyry Cu-Au-Mo deposit in Alaska. The δ65Cu values range from -2.09 to 1.11 ‰ and show a series of linear correlations with the δ18O isotope ratios calculated for the fluid in equilibrium with the hydrothermal alteration in each sample. Samples with sodic-potassic, potassic and illite alteration display a negative linear correlation between the Cu and O isotope results. This suggests that fractionation of Cu isotopes between the fluid and precipitating chalcopyrite is positive as the hydrothermal fluid is evolving from magmatic to mixed magmatic-meteoric compositions. Samples with advanced argillic alteration display a weak positive linear correlation between Cu and O isotope results consistent with small negative fluid-chalcopyrite Cu isotope fractionation during fluid evolution. The hydrothermal fluids that formed sodic-potassic, potassic and illite alteration likely transported Cu as CuHS0. Hydrothermal fluids that resulted in advanced argillic alteration likely transport Cu as CuCl2-. The pH conditions also control Cu isotope fractionation, consistent with previous experimental work. Larger fractionation factors were found between fluids and chalcopyrite precipitating under neutral conditions contrasting with small fractionation factors calculated between fluids and chalcopyrite precipitating under acidic conditions. Therefore, this study proposes that hydrothermal fluid compositions and pH conditions are related to Cu isotope variations in high temperature magmatic-hydrothermal deposits.
      PubDate: 2017-10-16T01:16:33.667356-05:
      DOI: 10.1002/2017GC007026
  • Possible Icelandic Tephra Found in European Colle Gnifetti Glacier
    • Authors: M. T. Luongo; A. V. Kurbatov, T. Erhardt, P. A. Mayewski, M. McCormick, A. F. More, N. E. Spaulding, S. D. Wheatley, M. G. Yates, P. D. Bohleber
      Abstract: Volcanic ash (tephra) provides unique time markers (isochrons) that are often used as an independent age-control tool for stratigraphic correlations of paleoclimate archives from ice cores. However, little credence has been given to the notion of finding tephra in ice cores collected in the European Alps because of the relatively large distance from volcanic sources and the presumed nature of regional atmospheric circulation patterns. We filtered particles from melted ice core drilling chips gathered roughly every meter during a 2013 drilling operation at Colle Gnifetti glacier in the Swiss-Italian Alps (45°55.74′N, 7°52.58′E, 4450 m asl). One filter, preliminarily dated to the nineteenth century by annual layer counting, contained a group of six visually similar tephra particles. Analyzing their chemistry using a scanning electron microscope equipped with an energy-dispersive x-ray spectrometer established that the six particles were volcanic in origin and are very similar in composition (a distinctive geochemical signature), pointing to a single volcanic eruption source. We proposed that one of several massive nineteenth century Eastern Icelandic eruptions is a potential source given eruption timing, size, tephra dispersion area, and similarities in chemical composition. This first finding of tephra in an Alpine ice core contributes to a regional tephrochronological framework that can be adapted for future correlation among different paleoclimate sequences.
      PubDate: 2017-10-16T01:11:50.482623-05:
      DOI: 10.1002/2017GC007022
  • Registration of Precession Signal in the Last Interglacial Paleosol (S1)
           on the Chinese Loess Plateau
    • Authors: Long Ma; Ying Li, Xingxing Liu, Youbin Sun
      Abstract: Solar insolation plays an essential role in driving orbital-scale East Asian Monsoon (EAM) variability. Unlike Chinese speleothem δ18O records characterized by dominant precessional cycles, many loess proxies from the Chinese Loess Plateau (CLP) usually display damped precessional-scale variability. To reconcile the precessional monsoon variability recorded in two classic archives, it's critical to investigate which factors can significantly affect the sensitivity of loess proxies to precessional forcing. In this study, we synthesize grain size (GS) and magnetic susceptibility (χ) results of eighteen paleosol (S1) profiles over the CLP, with effort to evaluating the relative impacts of sedimentation rate (SR) and pedogenesis on the precessional signal intensity (PSI) during the last interglaciation. Based on unified grain-size age models, amplitude contrast between normalized GS and χ variations of five substages of the last interglaciation is employed to infer the PSI. The results indicate that precessional signals are evident in the GS/χ records of the high-SR loess profiles on the northern and western CLP. Comparison of the PSI variations with the SR and mean annual precipitation (MAP) changes suggests that the GS-PSI is positively correlated to the SR changes, whilst the χ-PSI is negatively/positively related to the SR/MAP changes. Our results confirm that distinct precessional cycles in high-resolution loess proxies permit a plausible reconciliation of monsoon variability recorded in Chinese loess and speleothem.
      PubDate: 2017-10-16T01:11:35.805163-05:
      DOI: 10.1002/2017GC006964
  • Reconstructing the Evolution of the Submarine Monterey Canyon System From
           Os, Nd, and Pb Isotopes in Hydrogenetic Fe-Mn Crusts
    • Authors: T. A. Conrad; S. G. Nielsen, B. Peucker-Ehrenbrink, J. Blusztajn, D. Winslow, J. R. Hein, A. Paytan
      Abstract: The sources of terrestrial material delivered to the California margin over the past 7 Myr were assessed using 187Os/188Os, Nd, and Pb isotopes in hydrogenetic ferromanganese crusts from three seamounts along the central and southern California margin. From 6.8 to 4.5 (± 0.5) Ma, all three isotope systems show more radiogenic values at Davidson Seamount, located near the base of the Monterey Canyon System, than in Fe-Mn crusts from the more remote Taney and Hoss seamounts. At the Taney seamounts, approximately 225 km farther offshore from Davidson Seamount, 187Os/188Os values, but not Pb and Nd isotope ratios, also deviate from the Cenozoic seawater curve towards more radiogenic values from 6.8 to 4.5 (± 0.5) Ma. However, none of the isotope systems in Fe-Mn crusts deviate from seawater at Hoss Seamount located approximately 450 km to the south. The regional gradients in isotope ratios indicate that substantial input of dissolved and particulate terrestrial material into the Monterey Canyon System is responsible for the local deviations in the seawater Nd, Pb, and Os isotope compositions from 6.8 to 4.5 (± 0.5) Ma. The isotope ratios recorded in Fe-Mn crusts are consistent with a southern Sierra Nevada or western Basin and Range provenance of the terrestrial material which was delivered by rivers to the canyon. The exhumation of the modern Monterey Canyon must have begun between 10 and 6.8 ± 0.5 Ma, as indicated by our data, the age of incised strata, and paleo-location of the Monterey Canyon relative to the paleo-coastline.
      PubDate: 2017-10-16T01:11:13.978882-05:
      DOI: 10.1002/2017GC007071
  • Using Detrital Zircon Geochronology to Constrain Paleogene Provenance and
           its Relationship to Rifting in the Zhu 1 Depression, Pearl River Mouth
           Basin, South China Sea
    • Authors: Wei Wang; Jiaren Ye, Tandis Bidgoli, Xianghua Yang, Hesheng Shi, Yu Shu
      Abstract: Paleogene syn-rift successions in the South China Sea are poorly understood and systematic provenance analysis, which could provide clues to their history, is lacking. Here we report 409 new concordant U-Pb ages from detrital zircons separated from the Paleogene Wenchang, Enping, and Zhuhai formations in the Zhu 1 depression, Pearl River Mouth Basin. The new data, combined with the published age data from the region, document changes in the provenance of syn-rift successions. Detrital zircons from the Eocene Wenchang Formation are unimodal, with Jurassic-Cretaceous (180-80 Ma) ages making up>80% of grains. The ages are consistent with the geochronology of intrabasinal highs, dominated by igneous rocks emplaced during the Yanshanian orogeny, and suggest local provenance. By contrast, detrital zircons from the upper Eocene to lower Oligocene Enping Formation form three well-recognized age-clusters, with peaks at 150 Ma, 254 Ma, and 438 Ma that match documented tectonomagmatism in South China Block (SCB). Combined with increasing numbers of Precambrian zircons, the data suggest increasing influence of regional provenance of the SCB. Similar age peaks are also recognized from the limited number of zircons analyzed from the upper Oligocene Zhuhai Formation and comparability with modern shelf and river sediment indicates the unit was mainly sourced from the SCB and likely transported by a paleo-Pearl River. We infer that the change in provenance, from local uplifts within the Zhu 1 to the SCB, is related to distinct phases of PRMB rift development; however, later changes are best explained by SCB drainage evolution.
      PubDate: 2017-10-15T19:40:26.968814-05:
      DOI: 10.1002/2017GC007110
  • 3-D Seismic Imaging of Ancient Submarine Lava Flows: An Example From the
           Southern Australian Margin
    • Authors: P. Reynolds; S. Holford, N. Schofield, A. Ross
      Abstract: Submarine lava flows are the most common surficial igneous rock on the Earth. However, they are inherently more difficult to study than their subaerial counterparts due to their inaccessibility. In this study we use newly-acquired 3D (three-dimensional) seismic reflection data to document the distribution and morphology of 26 ancient, buried lava flows within the Middle Eocene-aged Bight Basin Igneous Complex, offshore southern Australia. Many of these lava flows are associated with volcanoes that vary from 60 − 625 m in height and 0.3 − 10 km in diameter. Well data and seismic-stratigraphic relationships suggest that the lava flows and volcanoes were emplaced offshore in water depths of
      PubDate: 2017-10-03T10:30:57.403555-05:
      DOI: 10.1002/2017GC007178
  • A Hybrid Approach to Data Assimilation for Reconstructing the Evolution of
           Mantle Dynamics
    • Authors: Quan Zhou; Lijun Liu
      Abstract: Quantifying past mantle dynamic processes represents a major challenge in understanding the temporal evolution of the solid earth. Mantle convection modeling with data assimilation is one of the most powerful tools to investigate the dynamics of plate subduction and mantle convection. Although various data assimilation methods, both forward and inverse, have been created, these methods all have limitations in their capabilities to represent the real earth. Pure forward models tend to miss important mantle structures due to the incorrect initial condition and thus may lead to incorrect mantle evolution. In contrast, pure tomography-based models cannot effectively resolve the fine slab structure and would fail to predict important subduction-zone dynamic processes. Here we propose a hybrid data assimilation approach that combines the unique power of the sequential and adjoint algorithms, which can properly capture the detailed evolution of the downgoing slab and the tomographically constrained mantle structures, respectively. We apply this new method to reconstructing mantle dynamics below the western U.S. while considering large lateral viscosity variations. By comparing this result with those from several existing data assimilation methods, we demonstrate that the hybrid modeling approach recovers the realistic 4-D mantle dynamics the best.
      PubDate: 2017-10-03T10:30:42.067151-05:
      DOI: 10.1002/2017GC007116
  • Paleoproterozoic Geomagnetic Field Strength From the Avanavero Mafic
           Sills, Amazonian Craton, Brazil
    • Authors: A. Di Chiara; A. R. Muxworthy, R.I.F. Trindade, F. Bispo-Santos
      Abstract: A recent hypothesis has suggested that Earth's inner core nucleated during the Mesoproterozoic, as evidenced by a rapid increase in the paleointensity (ancient geomagnetic field intensity) record; however, paleointensity data during the Paleoproterozoic and Mesoproterozoic period are limited. To address this problem, we have determined paleointensity from samples from three Paleoproterozoic Avanavero mafic sills (Amazonian Craton, Brazil): Cotingo, 1782 Ma, Puiuà 1788, and Pedra Preta, 1795 Ma. We adopted a multi-protocol approach for paleointensity estimates combining Thellier-type IZZI and LTD-IZZI methods, and the non-heating Preisach protocol. We obtained an average VDM value of 1.3 ± 0.7 × 1022Am2 (Cotingo) of 2.0 ± 0.4 × 1022Am2 (Puiuà) and 6 ± 4 × 1022Am2 (Pedra Preta); it is argued that the Cotingo estimate is the most robust. Our results are the first data from the upper Paleoproterozoic for South America and are comparable to data available from other regions and similar periods. The new data do not invalidate the hypothesis of that Earth's inner core nucleated during the Mesoproterozoic.
      PubDate: 2017-09-29T11:46:51.426935-05:
      DOI: 10.1002/2017GC007175
  • Holocene Paleointensity of the Island of Hawai'i From Glassy Volcanics
    • Authors: G. Cromwell; F. Trusdell, L. Tauxe, H. Staudigel, H. Ron
      Abstract: This study presents new high-quality paleointensity records and 14C radiocarbon age determinations from the Island of Hawai'i during the Holocene. Previous studies on Hawai'i use experimental methods and statistical selection criteria that may produce inaccurate geomagnetic field strength estimates. Additional high-quality paleointensity results can be used to evaluate the existing Hawaiian dataset and investigate Holocene geomagnetic field behavior. New paleointensity sites from 22 lava flows were calculated using the IZZI-Thellier laboratory technique and a strict set of selection criteria. Rapidly cooled, glassy volcanic material was collected for all sites. Isotopic age determinations range from 270- > 10,000 years before present (nine new 14C ages are also presented as part of this study). The median intensity for the 22 flows is 47.5 μT, with a median absolute deviation uncertainty of 5.6 μT; substantially greater than the present day field strength at Hawai'i (∼36 μT). These new results are comparable to previously published data from this location and are consistent with global paleointensity models. There is no evidence of an intensity “spike” at 3,000 years before present, as seen in the Levant and elsewhere. Previously published data vary in intensity by experimental technique relative to data using glassy material and strict selection criteria. Non-Thellier-type data are biased low, a result of these techniques estimating intensity from possibly non-single domain magnetic carriers. Thellier-Thellier data are biased high, the reasons for which remain unclear as no cooling rate effect was demonstrated and we were unable to reproduce the high bias with different selection criteria.
      PubDate: 2017-09-29T11:46:16.303262-05:
      DOI: 10.1002/2017GC006927
  • Extracting a Detailed Magnetostratigraphy From Weakly Magnetized,
           Oligocene to Early Miocene Sediment Drifts Recovered at IODP Site U1406
           (Newfoundland Margin, Northwest Atlantic Ocean)
    • Authors: Tim E. van Peer; Chuang Xuan, Peter C. Lippert, Diederik Liebrand, Claudia Agnini, Paul A. Wilson
      Abstract: Fine-grained magnetic particles in deep-sea sediments often statistically align with the ambient magnetic field during (and shortly after) deposition and can therefore record geomagnetic reversals. Correlation of these reversals to a geomagnetic polarity time scale is an important geochronological tool that facilitates precise stratigraphic correlation and dating of geological records globally. Sediments often carry a remanence strong enough for confident identification of polarity reversals, but in some cases a low signal-to-noise ratio prevents the construction of a reliable and robust magnetostratigraphy. Here we implement a data-filtering protocol, which can be integrated with the UPmag software package, to automatically reduce the maximum angular deviation and statistically mask noisy data and outliers deemed unsuitable for magnetostratigraphic interpretation. This protocol thus extracts a clearer signal from weakly magnetized sediments recovered at Integrated Ocean Drilling Program (IODP) Expedition 342 Site U1406 (Newfoundland margin, northwest Atlantic Ocean). The resulting magnetostratigraphy, in combination with shipboard and shore-based biostratigraphy, provides an age model for the study interval from IODP Site U1406 between Chrons C6Ar and C9n (∼21-27 Ma). We identify rarely observed geomagnetic directional changes within Chrons C6Br, C7r, and C7Ar, and perhaps within Subchron C8n.1n. Our magnetostratigraphy dates three intervals of unusual stratigraphic behavior within the sediment drifts at IODP Site U1406 on the Newfoundland margin. These lithostratigraphic changes are broadly concurrent with the coldest climatic phases of the middle Oligocene to early Miocene and we hypothesize that they reflect changes in bottom-water circulation.
      PubDate: 2017-09-29T11:45:34.767422-05:
      DOI: 10.1002/2017GC007185
  • Tibetan Magmatism Database
    • Authors: James B. Chapman; Paul Kapp
      Abstract: A database containing previously published geochronologic, geochemical, and isotopic data on Mesozoic to Quaternary igneous rocks in the Himalayan-Tibetan orogenic system is presented. The database is intended to serve as a repository for new and existing igneous rock data and is publicly accessible through a web-based platform that includes an interactive map and data table interface with search, filtering, and download options. To illustrate the utility of the database, the age, location, and εHft composition of magmatism from the central Gangdese batholith in the southern Lhasa terrane are compared. The data identify three high-flux events, which peak at 93 Ma, 50 Ma, and 15 Ma. They are characterized by inboard arc migration and a temporal and spatial shift to more evolved isotopic compositions.
      PubDate: 2017-09-29T11:44:57.880911-05:
      DOI: 10.1002/2017GC007217
  • Redistribution of Iron and Titanium in High-Pressure Ultramafic Rocks
    • Authors: Rosalind J. Crossley; Katy A. Evans, Steven M. Reddy, Gregory W. Lester
      Abstract: The redox state of iron in high-pressure serpentinites, which host a significant proportion of Fe3+ in subduction zones, can be used to provide an insight into iron cycling and constrain the composition of subduction zone fluids. In this study, we use oxide and silicate mineral textures, interpretation of mineral parageneses, mineral composition data, and whole rock geochemistry of high-pressure retrogressed ultramafic rocks from the Zermatt-Saas Zone to constrain the distribution of iron and titanium, and iron oxidation state. These data provide an insight on the oxidation state and composition of fluids at depth in subduction zones. Oxide minerals host the bulk of iron, particularly Fe3+. The increase in mode of magnetite and observation of magnetite within antigorite veins in the investigated ultramafic samples during initial retrogression is most consistent with oxidation of existing iron within the samples during the infiltration of an oxidizing fluid since it is difficult to reconcile addition of Fe3+ with the known limited solubility of this species. However, high Ti contents are not typical of serpentinites and also cannot be accounted for by simple mixing of a depleted mantle protolith with the nearby Allalin gabbro. Titanium-rich phases coincide with prograde metamorphism and initial exhumation, implying the early seafloor and/or prograde addition and late mobilization of Ti. If Ti addition has occurred, then the introduction of Fe3+, also generally considered to be immobile, cannot be disregarded. We explore possible transport vectors for Ti and Fe through mineral texture analysis.
      PubDate: 2017-09-28T10:36:55.195733-05:
      DOI: 10.1002/2017GC007145
  • Spatially Variable CO2 Degassing in The Main Ethiopian Rift: Implications
           For Magma Storage, Volatile Transport And Rift-Related Emissions
    • Authors: Jonathan A. Hunt; Amdemichael Zafu, Tamsin A. Mather, David M. Pyle, Peter H. Barry
      Abstract: Deep carbon emissions from historically inactive volcanoes, hydrothermal and tectonic structures are among the greatest unknowns in the long-term (∼Myr) carbon cycle. Recent estimates of diffuse CO2 flux from the Eastern Rift of the East African Rift System (EARS) suggest this could equal emissions from the entire mid-ocean ridge system. We report new CO2 surveys from the Main Ethiopian Rift (MER, northernmost EARS), and reassess the rift-related CO2 flux. Since degassing in the MER is concentrated in discrete areas of volcanic and off-edifice activity, characterisation of such areas is important for extrapolation to a rift-scale budget. Locations of hot springs and fumaroles along the rift show numerous geothermal areas away from volcanic edifices. With these new data we estimate total CO2 emissions from the central and northern MER as 0.52—4.36 Mt yr−1. Our extrapolated flux from the Eastern Rift is 3.9—32.7 Mt yr−1 CO2, overlapping with lower end of the range presented in recent estimates. By scaling, we suggest that 6—18 Mt yr−1 CO2 flux can be accounted for by magmatic extension, which implies an important role for volatile-enriched lithosphere, crustal assimilation and/or additional magmatic intrusion to account for the upper range of flux estimates. Our results also have implications for the nature of volcanism in the MER. Many geothermal areas are found>10 km from the nearest volcanic centre, suggesting ongoing hazards associated with regional volcanism.
      PubDate: 2017-09-28T10:32:09.261285-05:
      DOI: 10.1002/2017GC006975
  • Paleomagnetic Constraints on the Middle Miocene-Early Pliocene
           Stratigraphy in the Xining Basin, NE Tibetan Plateau, and the Geologic
    • Authors: Rongsheng Yang; Xiaomin Fang, Qingquan Meng, Jinbo Zan, Weilin Zhang, Tao Deng, Yibo Yang, Xiaobai Ruan, Liye Yang, Bingshuai Li
      Abstract: The Xining Basin lies in the transitional zone between the arid Asian interior and the East Asian monsoon region. The continuous Cenozoic sediments in the basin provide a unique archive recording the uplift of the Tibetan Plateau and its environmental effects on central Asian aridification and Asian monsoon evolution. However, sediments deposited since the middle Middle Miocene have not been precisely dated, hindering our ability to address these issues. Here, we dated a 336-m-thick section containing many Late Miocene fossil mammals from the eastern basin. High-resolution paleomagnetism revealed 16 normal and 16 reversed zones that correlate well with chrons 3n to 5Ar.1r of the Geomagnetic Polarity Time Scale, constraining the section to ∼12.7-4.8 Ma. The changes in lithofacies from floodplain to braided river at ∼8.6 Ma and to thick alluvial fan at ∼6.3 Ma with predominantly southerly paleocurrent directions occur simultaneously with an increase in the sedimentation rates, representing two periods of rapid uplift in the eastern Qilian Shan to the north. Our results provide a robust oldest age constraint (
      PubDate: 2017-09-28T10:30:28.086514-05:
      DOI: 10.1002/2017GC006945
  • Morphological Expressions of Crater Infill Collapse: Model Simulations of
           Chaotic Terrains on Mars
    • Authors: Manuel Roda; George Marketos, Jan Westerweel, Rob Govers
      Abstract: Martian chaotic terrains are characterized by deeply depressed intensively fractured areas that contain a large number of low-strain tilted blocks. Stronger deformation (e.g. higher number of fractures) is generally observed in the rims when compared to the middle regions of the terrains. The distribution and number of fractures and tilted blocks are correlated with the size of the chaotic terrains. Smaller chaotic terrains are characterized by few fractures between undeformed blocks. Larger terrains show an elevated number of fractures uniformly distributed with single blocks. We investigate whether this surface morphology may be a consequence of the collapse of the infill of a crater. We perform numerical simulations with the Discrete Element Method and we evaluate the distribution of fractures within the crater and the influence of the crater size, infill thickness and collapsing depth on the final morphology.The comparison between model predictions and the morphology of the Martian chaotic terrains shows strong statistical similarities in terms of both number of fractures and correlation between fractures and crater diameters. No or very weak correlation is observed between fractures and the infill thickness or collapsing depth. The strong correspondence between model results and observations suggests that the collapse of an infill layer within a crater is a viable mechanism for the peculiar morphology of the martian chaotic terrains.
      PubDate: 2017-09-25T11:24:27.674575-05:
      DOI: 10.1002/2017GC006933
  • Great Salt Lake (Utah) Microbialite δ13C, δ18O, and δ15N Record
           Fluctuations in Lake Biogeochemistry Since the Late Pleistocene
    • Authors: D. L. Newell; J. L. Jensen, C. M. Frantz, M. D. Vanden Berg
      Abstract: Extensive lacustrine microbialite deposits exposed along the shores of Great Salt Lake (GSL), Utah preserve a rich continental paleoenvironmental record. Newly-reported microbialite carbon and oxygen stable isotope ratios in carbonate, nitrogen isotope ratios in organic matter, and organic matter radiocarbon ages archive paleolake hydrological and biogeochemical changes from the late Pleistocene through the Holocene. Positive correlations between δ18O and δ13C in ∼15 – 7.6 cal ka microbialite carbonate are consistent with a hydrologically closed-basin lake with fluctuations in volume, chemistry, and associated changes in lake primary production. The δ15N of microbialite bulk organic matter (5 – 18 ‰ vs. AIR) shows that the balance between nitrogen fixation and assimilation of dissolved inorganic nitrogen has varied significantly. Inverse δ18O and δ13C correlations in combination with high δ15N in some carbonate deposits may imply periods of higher salinity and stable lake stratification similar to modern GSL conditions. We compare our C and O datasets with Pleistocene Lake Bonneville carbonate stable isotope records and demonstrate progressive development of spatially-isolated hydrological basins during the shift to warmer and drier conditions in the Holocene.
      PubDate: 2017-09-21T11:37:36.823971-05:
      DOI: 10.1002/2017GC007078
  • Hydrothermal Venting at Hinepuia Submarine Volcano, Kermadec Arc:
           Understanding Magmatic-Hydrothermal Fluid Chemistry
    • Authors: Valerie K. Stucker; Sharon L. Walker, Cornel E. J. de Ronde, Fabio Caratori Tontini, Shinji Tsuchida
      Abstract: The Hinepuia volcanic center is made up of two distinct edifices aligned northwest to southeast, with an active cone complex in the SE. Hinepuia is one of several active volcanoes in the northern segment of the Kermadec arc. Regional magnetic data shows no evidence for large-scale hydrothermal alteration at Hinepuia, yet plume data confirm present-day hydrothermal discharge, suggesting that the hydrothermal system may be too young to have altered the host rocks with respect to measurable changes in magnetic signal. Gravity data are consistent with crustal thinning and shallow mantle under the volcanic center. Following the discovery of hydrothermal plumes over Hinepuia, the submersible Shinkai 6500 was used to explore the SE cone and sample hydrothermal fluids.The chemistry of hydrothermal fluids from submarine arc and backarc volcanoes are typically dominated by water-rock interactions and/or magmatic degassing. Chemical analyses of vent fluids show that Hinepuia does not quite fit either traditional model. Moreover, the Hinepuia samples fall between those typically ascribed to both end-member fluid types when plotted on a K-Mg-SO4 ternary diagram. Due to evidence of strong degassing, abundant native sulfur deposition, and H2S presence, the vent sampled at Hinepuia is ultimately classified as a magmatic-hydrothermal system with a water-rock influence. This vent is releasing water vapor and magmatic volatiles with a notable lack of salinity due to subcritical boiling and phase separation. Magmatic-hydrothermal fluid chemistry appears to be controlled by a combination of gas flux, phase separation processes, and volcano evolution and/or distance from the magma source.
      PubDate: 2017-09-21T11:30:45.627151-05:
      DOI: 10.1002/2016GC006713
  • Arctic Deep-Water Ferromanganese-Oxide Deposits Reflect the Unique
           Characteristics of the Arctic Ocean
    • Authors: James R. Hein; Natalia Konstantinova, Mariah Mikesell, Kira Mizell, Jessica N. Fitzsimmons, Phoebe Lam, Laramie T. Jensen, Yang Xiang, Amy Gartman, Georgy Cherkashov, Deborah R. Hutchinson, Claire P. Till
      Abstract: Little is known about marine mineral deposits in the Arctic Ocean, an ocean dominated by continental shelf and basins semi-closed to deep-water circulation. Here, we present data for ferromanganese crusts and nodules collected from the Amerasia Arctic Ocean in 2008, 2009, and 2012 (HLY0805, HLY0905, HLY1202). We determined mineral and chemical compositions of the crusts and nodules and the onset of their formation. Water column samples from the GEOTRACES program were analyzed for dissolved and particulate scandium concentrations, an element uniquely enriched in these deposits.The Arctic crusts and nodules are characterized by unique mineral and chemical compositions with atypically high growth rates, detrital contents, Fe/Mn ratios, and low Si/Al ratios, compared to deposits found elsewhere. High detritus reflects erosion of submarine outcrops and North America and Siberia cratons, transport by rivers and glaciers to the sea, and distribution by sea ice, brines, and currents. Uniquely high Fe/Mn ratios are attributed to expansive continental shelves, where diagenetic cycling releases Fe to bottom waters, and density flows transport shelf bottom water to the open Arctic Ocean. Low Mn contents reflect the lack of a mid-water oxygen minimum zone that would act as a reservoir for dissolved Mn. The potential host phases and sources for elements with uniquely high contents are discussed with an emphasis on scandium. Scandium sorption onto Fe oxyhydroxides and Sc-rich detritus account for atypically high scandium contents. The opening of Fram Strait in the Miocene and ventilation of the deep basins initiated Fe-Mn crust growth ∼15 Myr ago.
      PubDate: 2017-09-21T11:26:34.751794-05:
      DOI: 10.1002/2017GC007186
  • Light Stable Isotopic Compositions of Enriched Mantle Sources: Resolving
           the Dehydration Paradox
    • Authors: J. E. Dixon; I. N. Bindeman, R. H. Kingsley, K. K. Simons, P. J. le Roux, T. R. Hajewski, P. Swart, C. H. Langmuir, J. G. Ryan, K. J. Walowski, I. Wada, P. J. Wallace
      Abstract: Volatile and stable isotope data provide tests of mantle processes that give rise to mantle heterogeneity. New data on enriched mid-oceanic ridge basalts (MORB) show a diversity of enriched components. Pacific PREMA-type basalts (H2O/Ce = 215 ± 30, δDSMOW = -45 ± 5 ‰) are similar to those in the northern Atlantic (H2O/Ce = 220 ± 30; δDSMOW = -30 to -40 ‰). Basalts with EM-type signatures have regionally variable volatile compositions. Northern Atlantic EM-type basalts are wetter (H2O/Ce = 330 ± 30) and have isotopically heavier hydrogen (δDSMOW = -57 ± 5 ‰) than northern Atlantic MORB. Southern Atlantic EM-type basalts are damp (H2O/Ce = 120 ± 10) with intermediate δDSMOW (-68 ± 2 ‰), similar to δDSMOW for Pacific MORB. Northern Pacific EM-type basalts are dry (H2O/Ce = 110 ± 20) and isotopically light (δDSMOW = -94 ± 3 ‰).A multi-stage metasomatic and melting model accounts for the origin of the enriched components by extending the subduction factory concept down through the mantle transition zone, with slab temperature a key variable. Volatiles and their stable isotopes are decoupled from lithophile elements, reflecting primary dehydration of the slab followed by secondary rehydration, infiltration and re-equilibration by fluids derived from dehydrating subcrustal hydrous phases (e.g., antigorite) in cooler, deeper parts of the slab. Enriched mantle sources form by addition of
      PubDate: 2017-09-21T11:25:40.005031-05:
      DOI: 10.1002/2016GC006743
  • In situ Raman Raman Detection of Gas Hydrates Exposed on the Seafloor of
           the South China Sea
    • Authors: Xin Zhang; Zengfeng Du, Zhendong Luan, Xiujuan Wang, Shichuan Xi, Bing Wang, Lianfu Li, Chao Lian, Jun Yan
      Abstract: Gas hydrates are usually buried in sediments. Here, we report the first discovery of gas hydrates exposed on the seafloor of the South China Sea. The in situ chemical compositions and cage structures of these hydrates were measured at the depth of 1130 m below sea level using a Raman insertion probe (RiP-Gh) that was carried and controlled by a remotely operated vehicle (ROV) Faxian. This in situ analytical technique can avoid the physical and chemical changes associated with the transport of samples from the deep sea to the surface. Natural gas hydrate samples were analyzed at two sites. The in situ spectra suggest that the newly formed hydrate was Structure I but contains a small amount of C3H8 and H2S. Pure gas spectra of CH4, C3H8 and H2S were also observed at the SCS-SGH02 site. These data represent the first in situ proof that free gas can be trapped within the hydrate fabric during rapid hydrate formation. We provide the first in situ confirmation of the hydrate growth model for the early stages of formation of crystalline hydrates in a methane-rich seafloor environment. Our work demonstrates that natural hydrate deposits, particularly those in the early stages of formation, are not monolithic single structures but instead exhibit significant small-scale heterogeneities due to inclusions of free gas and the surrounding seawater, there inclusions also serve as indicators of the likely hydrate formation mechanism. These data also reinforce the importance of correlating visual and in situ measurements when characterizing a sampling site.
      PubDate: 2017-09-21T11:20:48.813706-05:
      DOI: 10.1002/2017GC006987
  • The Impacts of Seawater Mg/Ca and Temperature on Element Incorporation in
           Benthic Foraminiferal Calcite
    • Authors: L.J. de Nooijer; I. van Dijk, T. Toyofuku, G.J. Reichart
      Abstract: On geological timescales, oceanic [Mg2+] and [Ca2+] vary with changing rates of weathering, seafloor spreading and dolomite formation. Accurate reconstruction of the ratio between [Mg2+] and [Ca2+] in seawater (Mg/Casw), may potentially be reconstructed using foraminiferal Mg/Ca ratios. Since both temperature and seawater Mg/Ca impact foraminiferal Mg/Ca, successful reconstruction of Mg/Casw requires quantification of both these parameters independently on foraminiferal Mg/Ca, as well as their combined effect on Mg-incorporation. Here we present the combined and isolated impacts of temperature and Mg/Casw on Mg incorporation in two model species, the benthic hyaline (i.e. perforate) foraminifer Elphidium crispum and porcelaneous (i.e. miliolid) foraminifer Quinqueloculina sp. using controlled growth experiments. Specimens of these two species were kept at four different temperatures (ranging from 10 to 27°C) and three Mg/Casw's (3.4, 6.4 and 8.5 mol/mol), resulting in 12 experimental conditions. Newly grown calcite was analyzed for a number of elements (Na, Mg and Sr) by laser ablation-ICP-MS. Results show that although the Mg/Ca varied by more than an order of magnitude between species, the sensitivity of Mg incorporation with respect to temperature appeared not to be influenced by Mg/Casw. By extension, these results may also help improving accuracy in the reconstruction of past Mg/Casw based on foraminifera with contrasting Mg/Ca.
      PubDate: 2017-09-21T11:20:40.503805-05:
      DOI: 10.1002/2017GC007183
  • Stress State in the Kumano Basin and in Slope Sediment Determined From
           Anelastic Strain Recovery: Results From IODP Expedition 338 to the Nankai
    • Authors: Kiyokazu Oohashi; Weiren Lin, Hung-Yu Wu, Asuka Yamaguchi, Yuhji Yamamoto
      Abstract: Three-dimensional, in situ stresses in the Kumano Basin and slope sediment (IODP Sites C0002 and C0022) in the Nankai Trough, southwest Japan, have been determined using the anelastic strain recovery (ASR) of core samples. Two samples taken from Hole C0002J, located in the bottom of the Kumano Basin, indicate that the maximum principal stress, σ1, is vertical. The intermediate principal stress, σ2, is oriented ENE–WSW, parallel to the trench axis. These stress orientations are similar to those obtained using ASR and borehole breakout methods in previous expeditions. In contrast, a sample from the lower section of the slope sediment (Hole C0022B), located beneath the megasplay fault, is characterized by σ1 plunging moderately to the ESE and σ3 oriented near-horizontally, trending NNE–SSW. The direction of maximum horizontal stress obtained from ASR (WNW–ESE) is similar to that inferred from borehole breakouts in an adjacent hole (NW–SE). Trench-normal compression and a near-vertical σ2 are also inferred from focal mechanisms of very-low-frequency earthquakes within the Nankai accretionary prism, and from borehole breakouts in the hanging wall of the megasplay fault. These observations suggest that the horizontal compressional regime extends to a shallower level than previously thought, likely due to the shallow portion of the megasplay fault accumulating tectonic stress in response to plate convergence.
      PubDate: 2017-09-12T11:55:55.558981-05:
      DOI: 10.1002/2017GC007137
  • Investigating Segmentation in Cascadia: Anisotropic Crustal Structure and
           Mantle Wedge Serpentinization from Receiver Functions
    • Authors: Hannah E. Krueger; Erin A. Wirth
      Abstract: The Cascadia subduction zone exhibits along-strike segmentation in structure, processes, and seismogenic behavior. While characterization of seismic anisotropy can constrain deformation processes at depth, the character of seismic anisotropy in Cascadia remains poorly understood. This is primarily due to a lack of seismicity in the subducting Juan de Fuca slab, which limits shear wave splitting and other seismological analyses that interrogate the fine-scale anisotropic structure of the crust and mantle wedge. We investigate lower crustal anisotropy and mantle wedge structure by computing P-to-S receiver functions at 12 broadband seismic stations along the Cascadia subduction zone. We observe P-to-SV converted energy consistent with previously estimated Moho depths. Several stations exhibit evidence of an “inverted Moho” (i.e., a downward velocity decrease across the crust-mantle boundary), indicative of a serpentinized mantle wedge. Stations with an underlying hydrated mantle wedge appear prevalent from northern Washington to central Oregon, but sparse in southern Oregon and northern California. Transverse component receiver functions are complex, suggesting anisotropic and/or dipping crustal structure. To constrain the orientation of crustal anisotropy we compute synthetic receiver functions using manual forward modeling. We determine that the lower crust shows variable orientations of anisotropy along-strike, with highly complex anisotropy in northern Cascadia, and generally NW-SE and NE-SW orientations of slow-axis anisotropy in central and southern Cascadia, respectively. The orientations of anisotropy from this work generally agree with those inferred from shear wave splitting of tremor studies at similar locations, lending confidence to this relatively new method of inferring seismic anisotropy from slow earthquakes.
      PubDate: 2017-09-11T11:32:08.056537-05:
      DOI: 10.1002/2017GC007064
  • Extreme monsoon rainfall signatures preserved in the invasive terrestrial
           gastropod Lissachatina fulica
    • Authors: Prosenjit Ghosh; Ravi Rangarajan, Kaustubh Thirumalai, Fred Naggs
      Abstract: Indian summer monsoon (ISM) rainfall lasts for a period of four months with large variations recorded in terms of rainfall intensity during its period between June to September. Proxy reconstructions of past ISM rainfall variability are required due to the paucity of long instrumental records. However, reconstructing sub-seasonal rainfall is extremely difficult using conventional hydroclimate proxies due to inadequate sample resolution. Here, we demonstrate the utility of the stable oxygen isotope composition of gastropod shells in reconstructing past rainfall on sub-seasonal timescales. We present a comparative isotopic study on present day rainwater and stable isotope ratios of precipitate found in the incremental growth bands of giant African land snail Lissachatina fulica (Bowdich) from modern day (2009) and in the historical past (1918). Isotopic signatures present in the growth bands allowed for the identification of ISM rainfall variability in terms of its active and dry spells in the modern as well as past gastropod record. Our results demonstrate the utility of gastropod growth band stable isotope ratios in semi-quantitative reconstructions of seasonal rainfall patterns. High resolution climate records extracted from gastropod growth band stable isotopes (museum and archived specimens) can expand the scope for understanding past sub-seasonal-to-seasonal climate variability.
      PubDate: 2017-09-07T10:40:33.147307-05:
      DOI: 10.1002/2017GC007041
  • Generation of silicic melts in the early Izu-Bonin arc recorded by
           detrital zircons in proximal arc volcaniclastic rocks from the Philippine
    • Authors: A.P. Barth; K. Tani, S. Meffre, J.L. Wooden, M.A. Coble, R.J. Arculus, O. Ishizuka, J.T. Shukle
      Abstract: A 1.2 kilometer thick Paleogene volcaniclastic section at International Ocean Discovery Program Site 351-U1438 preserves the deep-marine, proximal record of Izu-Bonin oceanic arc initiation and volcano evolution along the Kyushu-Palau Ridge (KPR). Pb/U ages and trace element compositions of zircons recovered from volcaniclastic sandstones preserve a remarkable temporal record of juvenile island arc evolution. Pb/U ages ranging from 43 to 27 Ma are compatible with provenance in one or more active arc edifices of the northern KPR. The abundances of selected trace elements with high concentrations provide insight into the genesis of U1438 detrital zircon host melts, and represent useful indicators of both short and long-term variations in melt compositions in arc settings. The Site U1438 zircons span the compositional range between zircons from mid-ocean ridge gabbros and zircons from relatively enriched continental arcs, as predicted for melts in a primitive oceanic arc setting derived from a highly depleted mantle source. Melt zircon saturation temperatures and Ti-in-zircon thermometry suggest a provenance in relatively cool and silicic melts that evolved toward more Th and U-rich compositions with time. Th, U and light rare earth element enrichments beginning about 35 Ma are consistent with detrital zircons recording development of regional arc asymmetry and selective trace element-enriched rear arc silicic melts as the juvenile Izu-Bonin arc evolved.
      PubDate: 2017-09-06T16:30:33.753004-05:
      DOI: 10.1002/2017GC006948
  • Fault-magma interactions during early continental rifting: Seismicity of
           the Magadi-Natron-Manyara basins, Africa
    • Authors: A. Weinstein; S. J. Oliva, C. J. Ebinger, S. Roecker, C. Tiberi, M. Aman, C. Lambert, E. Witkin, J. Albaric, S. Gautier, S. Peyrat, J. D. Muirhead, A. N. N. Muzuka, G. Mulibo, G. Kianji, R. Ferdinand-Wambura, M. Msabi, A. Rodzianko, R. Hadfield, F. Illsley-Kemp, T.P. Fischer
      Abstract: Although magmatism may occur during the earliest stages of continental rifting, its role in strain accommodation remains weakly constrained by largely 2D studies. We analyze seismicity data from a 13-month, 39-station broadband seismic array to determine the role of magma intrusion on state-of-stress and strain localization, and their along-strike variations. Precise earthquake locations using cluster analyses and a new 3D velocity model reveal lower crustal earthquakes beneath the central basins and along projections of steep border faults that degas CO2. Seismicity forms several disks interpreted as sills at 6-10 km below a monogenetic cone field. The sills overlie a lower crustal magma chamber that may feed eruptions at Oldoinyo Lengai volcano. After determining a new ML scaling relation, we determine a b-value of 0.87 ± 0.03. Focal mechanisms for 65 earthquakes, and 13 from a catalogue prior to our array reveal an along-axis stress rotation of ∼60° in the magmatically active zone. New and prior mechanisms show predominantly normal slip along steep nodal planes, with extension directions ∼ N90°E north and south of an active volcanic chain consistent with geodetic data, and ∼ N150°E in the volcanic chain. The stress rotation facilitates strain transfer from border fault systems, the locus of early stage deformation, to the zone of magma intrusion in the central rift. Our seismic, structural, and geochemistry results indicate that frequent lower crustal earthquakes are promoted by elevated pore pressures from volatile degassing along border faults, and hydraulic fracture around the margins of magma bodies. Results indicate that earthquakes are largely driven by stress state around inflating magma bodies.
      PubDate: 2017-08-21T10:32:57.501624-05:
      DOI: 10.1002/2017GC007027
  • Geochemical and geophysical constrains on the dynamic topography of the
           Southern African Plateau
    • Authors: Alan G. Jones; Juan Carlos Afonso, Javier Fullea
      Abstract: The deep mantle African Superswell is considered to contribute to the topographic uplift of the Southern African Plateau, but dynamic support estimates vary wildly depending on the approach and data used. One reason for these large disparities is that the role of lithospheric structure, key in modulating deep dynamic contributions to elevation, is commonly ignored or oversimplified in convection studies. We use multiple high-quality geophysical data coupled with xenolith-based geochemical constraints to compute the isostatic lithospheric contribution to the elevation of the Plateau, facilitating isolation of the current dynamic component from the total observed elevation. We employ a multi-observable stochastic algorithm to invert geoid anomaly, surface-wave dispersion data, magnetotelluric data and surface heat flow to predict elevation in a fully thermodynamically and internally-consistent manner. We find that a compositionally-layered 230 ±7 km thick lithosphere is required to simultaneously fit all four data types, in agreement with abundant independent xenolith evidence. Our stochastic modelling indicates a lithospheric contribution to elevation of the order of 670 m, which implies dynamic support arising from the convecting sub-lithospheric mantle of ∼650 m. Our results have important implications for the understanding of lithospheric-deep mantle feedback mechanisms and for calibrating dynamic topography estimates from global convection studies.
      PubDate: 2017-08-14T11:10:28.18306-05:0
      DOI: 10.1002/2017GC006908
  • Issue Information
    • Pages: 3555 - 3555
      PubDate: 2017-11-16T01:05:31.176006-05:
      DOI: 10.1002/ggge.21126
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
Home (Search)
Subjects A-Z
Publishers A-Z
Your IP address:
About JournalTOCs
News (blog, publications)
JournalTOCs on Twitter   JournalTOCs on Facebook

JournalTOCs © 2009-