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

Geochemistry, Geophysics, Geosystems     Full-text available via subscription   (Followers: 21, SJR: 2.156, h-index: 61)
Geophysical Research Letters     Full-text available via subscription   (Followers: 46, SJR: 2.668, h-index: 142)
Global Biogeochemical Cycles     Full-text available via subscription   (Followers: 3, SJR: 2.4, h-index: 109)
J. of Advances in Modeling Earth Systems     Open Access   (Followers: 2, SJR: 0.126, h-index: 2)
J. of Geophysical Research : Atmospheres     Partially Free   (Followers: 20)
J. of Geophysical Research : Biogeosciences     Full-text available via subscription   (Followers: 5)
J. of Geophysical Research : Earth Surface     Partially Free   (Followers: 22)
J. of Geophysical Research : Oceans     Partially Free   (Followers: 15)
J. of Geophysical Research : Planets     Full-text available via subscription   (Followers: 13)
J. of Geophysical Research : Solid Earth     Full-text available via subscription   (Followers: 22)
J. of Geophysical Research : Space Physics     Full-text available via subscription   (Followers: 14)
Paleoceanography     Full-text available via subscription   (Followers: 4, SJR: 2.16, h-index: 82)
Radio Science     Full-text available via subscription   (Followers: 2, SJR: 0.527, h-index: 47)
Reviews of Geophysics     Full-text available via subscription   (Followers: 19, SJR: 8.837, h-index: 87)
Space Weather     Full-text available via subscription   (Followers: 3, SJR: 0.496, h-index: 16)
Tectonics     Full-text available via subscription   (Followers: 7, SJR: 2.16, h-index: 79)
Water Resources Research     Full-text available via subscription   (Followers: 136, SJR: 1.769, h-index: 110)
Journal Cover Geochemistry, Geophysics, Geosystems
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     ISSN (Online) 1525-2027
     Published by American Geophysical Union (AGU) Homepage  [17 journals]   [SJR: 2.156]   [H-I: 61]
  • Reconciling mantle attenuation‐temperature relationships from
           seismology, petrology, and laboratory measurements
    • Authors: GA Abers; KM Fischer, G Hirth, DA Wiens, T Plank, BK Holtzman, C McCarthy, E. Gazel
      Pages: n/a - n/a
      Abstract: Seismic attenuation measurements provide a powerful tool for sampling mantle properties. Laboratory experiments provide calibrations at seismic frequencies and mantle temperatures for dry melt‐free rocks, but require ~102 ‐ 103 extrapolations in grain size to mantle conditions; also, the effects of water and melt are not well understood. At the same time, body‐wave attenuation measured from dense broadband arrays provides reliable estimates of shear‐wave attenuation (QS‐1), affording an opportunity for calibration. We reanalyze seismic datasets that sample arc and back‐arc mantle, in Central America, the Marianas and the Lau basin, confirming very high attenuation (QS ~ 25‐80) at 1 Hz and depths of 50‐100 km. At each of these sites, independent petrological studies constrain the temperature and water content where basaltic magmas last equilibrated with the mantle, 1300‐1450°C. The QS measurements correlate inversely with the petrologically‐inferred temperatures, as expected. However, dry attenuation models predict QS too high by a factor of 1.5‐5. Modifying models to include effects of H2O and rheology‐dependent grain size shows that the effects of water‐enhanced dissipation and water‐enhanced grain growth nearly cancel, so H2O effects are modest. Therefore high H2O in the arc source region cannot explain the low QS, nor in the backarc where lavas show modest water content. Most likely the high attenuation reflects the presence of melt, and some models of melt effects come close to reproducing observations. Overall, body‐wave QS can be reconciled with petrologic and laboratory inferences of mantle conditions if melt has a strong influence beneath arcs and backarcs.
      PubDate: 2014-08-16T01:24:55.445493-05:
      DOI: 10.1002/2014GC005444
       
  • Identification and environmental interpretation of diagenetic and biogenic
           greigite in sediments: A lesson from the Messinian Black Sea
    • Authors: Liao Chang; Iuliana Vasiliev, Christiaan van Baak, Wout Krijgsman, Mark J. Dekkers, Andrew P. Roberts, John D. Fitz Gerald, Annelies van Hoesel, Michael Winklhofer
      Pages: n/a - n/a
      Abstract: Greigite (Fe3S4) is a widespread authigenic magnetic mineral in anoxic sediments, and is also commonly biosynthesized by magnetotactic bacteria in aqueous environments. While the presence of fossilized bacterial magnetite (Fe3O4) has now been widely demonstrated, the preservation of greigite magnetofossils in the geological record is only poorly constrained. Here, we investigate Mio‐Pliocene sediments of the former Black Sea to test whether we can detect greigite magnetofossils, and to unravel potential environmental controls on greigite formation. Our magnetic analyses and transmission electron microscope (TEM) observations indicate the presence of both diagenetic and bacterial greigite, and suggest a potentially widespread preservation of greigite magnetofossils in ancient sediments, which has important implications for assessing the reliability of paleomagnetic records carried by greigite. TEM‐based chemical and structural analyses also indicate the common presence of nickel‐substituted diagenetic iron sulfide crystals with a ferrimagnetic greigite structure. In addition, our cyclostratigraphic framework allows correlation of magnetic properties of Messinian former Black Sea sediments (Taman Peninsula, Russia) to global climate records. Diagenetic greigite enhancements appear to be climatically controlled, with greigite mainly occurring in warm/wet periods. Diagenetic greigite formation can be explained by variations in terrigenous inputs and dissolved pore water sulfate concentrations in different sedimentary environments. Our analysis demonstrates the usefulness of greigite for studying long‐term climate variability in anoxic environments.
      PubDate: 2014-08-16T01:23:37.2059-05:00
      DOI: 10.1002/2014GC005411
       
  • Issue Information
    • Pages: i - i
      PubDate: 2014-08-14T13:10:52.95441-05:0
      DOI: 10.1002/ggge.20323
       
  • Deciphering bottom current velocity and paleoclimate signals from
           contourite deposits in the Gulf of Cádiz during the last 140 kyr: An
           inorganic geochemical approach
    • Authors: André Bahr; Francisco J. Jiménez‐Espejo, Nada Kolasinac, Patrick Grunert, F. Javier Hernández‐Molina, Ursula Röhl, Antje H. L. Voelker, Carlota Escutia, Dorrik A. V. Stow, David Hodell, Carlos A. Alvarez‐Zarikian
      Pages: n/a - n/a
      Abstract: Contourites in the Gulf of Cádiz (GC) preserve a unique archive of Mediterranean Outflow Water (MOW) variability over the past 5.3 Ma. In our study, we investigate the potential of geochemical data obtained by XRF scanning to decipher bottom current processes and paleoclimatic evolution at two different sites drilled during IODP Expedition 339 through contourites in the northern GC: Site U1387, which is bathed by the upper MOW core, and Site U1389, located more proximal to the Strait of Gibraltar. The lack of major downslope transport during the Pleistocene makes both locations ideally suited for our study. The results indicate that the Zr/Al ratio, representing the relative enrichment of heavy minerals (zircon) over less dense alumnosilicates under fast bottom current flow, is the most useful indicator for a semiquantitative assessment of current velocity. Although most elements are biased by current‐related processes, the bromine (Br) record, representing organic content, preserves the most pristine climate signal rather independent of grain‐size changes. Hence, Br can be used for chronostratigraphy and site‐to‐site correlation in addition to stable isotope stratigraphy. Based on these findings, we reconstructed MOW variability for Marine Isotope Stages (MIS) 1–5 using the Zr/Al ratio from Site U1387. The results reveal abrupt, millennial‐scale variations of MOW strength during Greenland Stadials (GS) and Interstadials (GI) with strong MOW during GS and glacial Terminations and a complex behavior during Heinrich Stadials. Millennial‐scale variability persisting during periods of poorly expressed GS/GI cyclicities implies a strong internal oscillation of the Mediterranean/North Atlantic climate system.
      PubDate: 2014-08-14T13:03:56.17845-05:0
      DOI: 10.1002/2014GC005356
       
  • Stable isotopes of surface water across the Longmenshan margin of the
           eastern Tibetan Plateau
    • Authors: Qiang Xu; Gregory D. Hoke, Jing Liu‐Zeng, Lin Ding, Wei Wang, Yang Yang
      Pages: n/a - n/a
      Abstract: Characterization of the stable isotope compositions (δ18O and δD) of modern‐day surface waters traversing mountain ranges and bordering continental plateaus is important for refining climate models and establishing modern isotope‐elevation gradients along mountain ranges. The Longmenshan margin of the Tibetan Plateau is a steep, 4 km topographic front situated near the boundaries between westerlies and Asian monsoon moisture sources, and is previously unexplored with respect to the variation in water isotopic composition with elevation. This study reports stable isotope data from 101 water samples collected from streams, springs, and ponds along the Min River (Minjiang) watershed, which traverses the Longmenshan margin. Local meteoric water lines, d‐excess values, and surrounding precipitation and river water datasets suggest that precipitation across the Longmenshan margin is dominated by the East Asian summer monsoon. The increase in d‐excess values with increasing catchment elevation breaks down as local moisture recycling becomes important at elevations > 3 km a.s.l.. Along the Min River, however, the δ18Ow and δDw values decrease with increasing catchment elevation, which fit second order polynomial curves and are well approximated by a simple Rayleigh fractionation processes. The temperature‐corrected oxygen values in authigenic carbonates from the Pleistocene Zoige Basin, north of the Min River watershed, yield elevations equivalent to present by the oxygen isotope‐elevation relationship of the Longmenshan margin.
      PubDate: 2014-08-05T02:46:03.461866-05:
      DOI: 10.1002/2014GC005252
       
  • Nd and Sr isotope compositions of different phases of surface sediments in
           the South Pacific: Extraction of seawater signatures, boundary exchange,
           and detrital/dust provenance
    • Authors: Mario Molina‐Kescher; Martin Frank, Ed C. Hathorne
      Pages: n/a - n/a
      Abstract: The radiogenic isotope composition of neodymium (Nd) and strontium (Sr) are useful tools to investigate present and past oceanic circulation or input of terrigenous material. We present Nd and Sr isotope compositions extracted from different sedimentary phases, including early diagenetic Fe‐Mn coatings, ‘unclean' foraminiferal shells, fossil fish teeth, and detritus of marine surface sediments (core‐tops) covering the entire mid‐latitude South Pacific. Comparison of detrital Nd isotope compositions to deep‐water values from the same locations suggest that ‘boundary exchange' has little influence on the Nd isotope composition of western South Pacific seawater. Concentrations of Rare Earth Elements (REE) and Al/Ca ratios of ‘unclean' planktonic foraminifera suggest that this phase is a reliable recorder of seawater Nd isotope composition. The signatures obtained from fish teeth and ‘non‐decarbonated' leachates of bulk sediment Fe‐Mn oxyhydroxide coatings also agree with ‘unclean' foraminifera. Direct comparison of Nd isotope compositions extracted using these methods with seawater Nd isotope compositions is complicated by the low accumulation rates yielding radiocarbon ages of up to 24 kyrs, thus mixing the signal of different ocean circulation modes. This suggests that different past seawater Nd isotope compositions have been integrated in authigenic sediments from regions with low sedimentation rates. Combined detrital Nd and Sr isotope signatures indicate a dominant role of the Westerly winds transporting lithogenic material from South New Zealand and Southeastern Australia to the open South Pacific. The proportion of this material decreases towards the east, where supply from the Andes increases and contributions from Antarctica cannot be ruled out.
      PubDate: 2014-08-05T02:43:27.981471-05:
      DOI: 10.1002/2014GC005443
       
  • Eruptive and tectonic history of the Endeavour segment, Juan de Fuca
           Ridge, based on AUV mapping data and lava flow ages
    • Authors: David A. Clague; Brian M. Dreyer, Jennifer B. Paduan, Julie F. Martin, David W. Caress, James B. Gill, Deborah S. Kelley, Hans Thomas, Ryan A. Portner, John R. Delaney, Thomas P. Guilderson, Mary L. McGann
      Pages: n/a - n/a
      Abstract: High‐resolution bathymetric surveys from autonomous underwater vehicles ABE and D. Allan B. were merged to create a co‐registered map of 71.7 km2 of the Endeavour Segment of the Juan de Fuca Ridge. Radiocarbon dating of foraminifera in cores from three dives of remotely operated vehicle Doc Ricketts provide minimum eruption ages for 40 lava flows that are combined with the bathymetric data to outline the eruptive and tectonic history. The ages range from Modern to 10,700 marine‐calibrated years before present (yr BP). During a robust magmatic phase from >10,700 yr BP to ˜4300 yr BP, flows erupted from an axial high and many flowed >5 km down the flanks; some partly buried adjacent valleys. Axial magma chambers (AMCs) may have been wider than today to supply dike intrusions over a 2‐km‐wide axial zone. Summit Seamount formed by ˜4770 yr BP and was subsequently dismembered during a period of extension with little volcanism starting ˜4300 yr BP. This tectonic phase with only rare volcanic eruptions lasted until ˜2300 yr BP and may have resulted in near‐solidification of the AMCs. The axial graben formed by crustal extension during this period of low magmatic activity. Infrequent eruptions occurred on the flanks between 2620‐1760 yr BP and within the axial graben since ˜1750 yr BP. This most recent phase of limited volcanic and intense hydrothermal activity that began ˜2300 yr BP defines a hydrothermal phase of ridge development that coincides with the present‐day 1‐km wide AMCs and overlying hydrothermal vent fields.
      PubDate: 2014-08-02T05:36:58.085282-05:
      DOI: 10.1002/2014GC005415
       
  • Metasomatized ancient lithospheric mantle beneath the young Zealandia
           microcontinent and its role in HIMU‐like intraplate magmatism
    • Authors: JM Scott; TE Waight, QHA van der Meer, JM Palin, AF Cooper, C Münker
      Pages: n/a - n/a
      Abstract: There has been long debate on the asthenospheric versus lithospheric source for numerous intraplate basalts with ocean island basalt (OIB) and high time‐integrated U/Pb (HIMU)‐like source signatures that have erupted through the Zealandia continental crust. Analysis of 157 spinel facies peridotitic mantle xenoliths from 25 localities across Zealandia permits the first comprehensive regional description of the sub‐continental lithospheric mantle (SCLM) and insights into whether it could be a source to the intraplate basalts. Contrary to previous assumptions, the Oligocene‐Miocene Zealandia SCLM is highly heterogeneous. It is composed of a refractory craton‐like domain (West Otago) adjacent to several moderately fertile domains (East Otago, North Otago, Auckland Islands). Each domain has an early history decoupled from the overlying Carboniferous and younger continental crust, and each domain has undergone varying degrees of depletion followed by re‐enrichment. Clinopyroxene grains reveal trace element characteristics (low Ti/Eu, high Th/U) consistent with enrichment through reaction with carbonatite. This metasomatic overprint has a composition that closely matches HIMU in Sr, Pb + Nd isotopes. However, clinopyroxene Hf isotopes are in part highly radiogenic and decoupled from the other isotope systems, and also mostly more radiogenic than the intraplate basalts. If the studied spinel facies xenoliths are representative of the thin Zealandia SCLM, the melting of garnet facies lithosphere could only be the intraplate basalt source if it had a less radiogenic Hf‐Nd isotope composition than the investigated spinel facies, or was mixed asthenospheric‐derived melts containing less radiogenic Hf.
      PubDate: 2014-08-02T03:49:45.763415-05:
      DOI: 10.1002/2014GC005300
       
  • Correlation of cycles in lava lake motion and degassing at Erebus volcano,
           Antarctica
    • Authors: Nial Peters; Clive Oppenheimer, Drea Rae Killingsworth, Jed Frechette, Philip Kyle
      Pages: n/a - n/a
      Abstract: Several studies at Erebus volcano have recorded pulsatory behaviour in many of the observable properties of its active lava lake. A strong correlation between the variations in surface speed of the lake and the composition of gas emitted has previously been noted. While previous studies have shown that the SO2 flux and the surface elevation exhibit pulsatory behaviour with a similar period to that of the surface speed and gas composition, suggesting they are linked, a lack of overlap between the different measurements has prevented direct comparisons from being made. Using high time‐resolution measurements of surface elevation, surface speed, gas composition and SO2 flux we demonstrate for the first time an unambiguous link between the cyclic behaviour in each of these properties. We also show that the variation in gas composition may be explained by a subtle change in oxygen fugacity. The cycles are found to be in‐phase with each other, with a small but consistent lag of 1–3 min between the peaks in surface elevation and surface speed. Explosive events are found to have no observable effect on the pulsatory behaviour beyond the ~5 min period required for lake refill. The close correspondences between the varying lake surface motion, gas flux and composition, and modelled oxygen fugacity suggest strong links between magma degassing, redox change and the fluid dynamics of the shallow magmatic system.
      PubDate: 2014-07-31T03:55:49.446131-05:
      DOI: 10.1002/2014GC005399
       
  • Alteration of volcaniclastic deposits at Minna Bluff: Geochemical insights
           on mineralizing environment and climate during the Late Miocene in
           Antarctica
    • Authors: Joanne V. Antibus; Kurt S. Panter, Thomas I. Wilch, Nelia Dunbar, William McIntosh, Aradhna Tripati, Ilya Bindeman, Jerzy Blusztajn
      Pages: n/a - n/a
      Abstract: Secondary minerals in volcaniclastic deposits at Minna Bluff, a 45‐km‐long peninsula in the Ross Sea, are used to infer processes of alteration and environmental conditions in the Late Miocene. Glassy volcaniclastic deposits are altered and contain phillipsite and chabazite, low‐ to high‐Mg carbonates, chalcedony and clay. The δ18O of carbonates and chalcedony is variable, ranging from ‐0.50 to 21.53‰ and 0.68 to 10.37‰, respectively, and δD for chalcedony is light (‐187.8 to ‐220.6‰), corresponding to Antarctic meteoric water. A mean carbonate 87Sr/86Sr ratio of 0.70327 ±0.0009 (1σ, n = 12) is comparable to lava and suggests fresh water, as opposed to seawater, caused the alteration. Minerals were precipitated at elevated temperatures based on quartz‐calcite (91º and 104ºC) equilibrium, carbonate 13C‐18O (Δ47 = 5º to 43ºC) thermometry, and stability of zeolites in geothermal systems (>10º to ~100ºC). The alteration was a result of isolated, ephemeral events involving the exchange between heated meteoric water and glass during or soon after the formation of each deposit. Near surface evaporative distillation can explain 18O‐enriched compositions for some Mg‐rich carbonates and chalcedony. The δ18Owater calculated for carbonates (‐15.8 to ‐22.9‰) reveals a broad change, becoming heavier between ~12 and ~7 Ma, consistent with a warming climate. These findings are independently corroborated by the interpretation of Late Miocene sedimentary sequences recovered from nearby sediment cores. However, in contrast to a cold‐based thermal regime proposed for ice flow at core sites, wet‐based conditions prevailed at Minna Bluff; a likely consequence of high heat flow associated with an active magma system.
      PubDate: 2014-07-31T03:51:50.370357-05:
      DOI: 10.1002/2014GC005422
       
  • Persistent uplift of the Lazufre volcanic complex (Central Andes): New
           insights from PCAIM inversion of InSAR time series and GPS data
    • Authors: D. Remy; J.L Froger, H Perfettini, S. Bonvalot, G. Gabalda, F. Albino, V. Cayol, D. Legrand, M. De Saint Blanquat
      Pages: n/a - n/a
      Abstract: We reanalyzed the surface Displacements observed at the Lazufre Volcanic Complex in the Southern Andean Central Volcanic Zone using GPS measurements made between 2006 and 2008 and a large InSAR dataset. We performed a detailed spatiotemporal analysis of the displacements using a Principal Component Analysis Inversion Method (PCAIM). The PCAIM reveals a source with no significant changes in shape and dimension and with a remarkably linear strength increase over the whole period of observation (i.e. 2003 to 2010). Then we used a three‐dimensional Mixed Boundary Element Method (MBEM) to invert the first component of surface displacement as obtained from PCAIM. We explored a continuum of geometries from a shallow elliptic crack to a deep massive truncated elliptical cone that could represent a sill or a large magma chamber, respectively. The best models indicate a large flat‐topped source with a roof area between 40 and 670 km2 and a depth of between 2 and 14 km below ground surface. Lastly, on the basis of the limited data available for the thermo‐mechanical structure of the crust in the Southern Andean Central Volcanic Zone we consider some possible scenarios to explain the spatial and temporal pattern of displacements at Lazufre.
      PubDate: 2014-07-31T03:17:28.850604-05:
      DOI: 10.1002/2014GC005370
       
  • Influences of surface processes on fold growth during 3‐D detachment
           folding
    • Authors: M. Collignon; B.J.P. Kaus, D.A. May, N. Fernandez
      Pages: n/a - n/a
      Abstract: In order to understand the interactions between surface processes and multilayer folding systems, we here present fully coupled three‐dimensional numerical simulations. The mechanical model represents a sedimentary cover with internal weak layers, detached over a much weaker basal layer representing salt or evaporites. Applying compression in one direction results in a series of three‐dimensional buckle folds, of which the topographic expression consists of anticlines and synclines. This topography is modified through time by mass redistribution, which is achieved by a combination of fluvial and hillslope erosion, as well as deposition, and which can in return influence the subsequent deformation. Model results show that surface processes do not have a significant influence on folding patterns and aspect ratio of the folds. Nevertheless, erosion reduces the amount of shortening required to initiate folding and increases the exhumation rates. Increased sedimentation in the synclines contributes to this effect by amplifying the fold growth rate by gravity. The main contribution of surface processes is rather due to their ability to strongly modify the initial topography and hence the initial random noise, prior to deformation. If larger initial random noise is present, folds amplify faster, which is consistent with previous detachment folding theory. Variations in thickness of the sedimentary cover (in one or two directions) also have a significant influence on the folding pattern, resulting in linear, large aspect ratio folds.
      PubDate: 2014-07-31T03:17:23.993107-05:
      DOI: 10.1002/2014GC005450
       
  • Antarctic icebergs: A significant natural ocean sound source in the
           Southern Hemisphere
    • Authors: Haru Matsumoto; DelWayne R. Bohnenstiehl, Jean Tournadre, Robert P. Dziak, Joseph H. Haxel, T‐K A. Lau, Matt Fowler, Sigrid A. Salo
      Pages: n/a - n/a
      Abstract: In late 2007, two massive icebergs, C19a and B15a, drifted into open water and slowly disintegrated in the southernmost Pacific Ocean. Archived acoustic records show that the high‐intensity underwater sounds accompanying this breakup increased ocean noise levels at mid‐to‐equatorial latitudes over a period of ˜1.5 years. More typically, seasonal variations in ocean noise, which are characterized by austral summer‐highs and winter‐lows, appear to be modulated by the annual cycle of Antarctic iceberg drift and subsequent disintegration. This seasonal pattern is observed in all three Oceans of the Southern Hemisphere. The life cycle of Antarctic icebergs affects not only marine ecosystem but also the sound environment in far‐reaching areas and must be accounted for in any effort to isolate anthropogenic or climate‐induced noise contributions to the ocean soundscape.
      PubDate: 2014-07-30T04:30:46.434119-05:
      DOI: 10.1002/2014GC005454
       
  • Late Pleistocene stratigraphy of IODP Site U1396 and compiled chronology
           offshore of south and south west Montserrat, Lesser Antilles
    • Authors: Deborah Wall‐Palmer; Maya Coussens, Peter J. Talling, Martin Jutzeler, Michael Cassidy, Isabelle Marchant, Martin R. Palmer, Sebastian F. L. Watt, Christopher W. Smart, Jodie K. Fisher, Malcolm B. Hart, Andrew Fraass, Jessica Trofimovs, Anne Le Friant, Osamu Ishizuka, Tatsuya Adachi, Mohammed Aljahdali, Georges Boudon, Christoph Breitkreuz, Daisuke Endo, Akihiko Fujinawa, Robert Hatfield, Matthew J. Hornbach, Kyoko Kataoka, Sara Lafuerza, Fukashi Maeno, Michael Manga, Michael Martinez‐Colon, Molly McCanta, Sally Morgan, Takeshi Saito, Angela L. Slagle, Adam J. Stinton, K. S. V. Subramanyam, Yoshihiko Tamura, Benoit Villemant, Fei Wang
      Pages: n/a - n/a
      Abstract: Marine sediments around volcanic islands contain an archive of volcaniclastic deposits, which can be used to reconstruct the volcanic history of an area. Such records hold many advantages over often incomplete terrestrial data sets. This includes the potential for precise and continuous dating of intervening sediment packages, which allow a correlatable and temporally constrained stratigraphic framework to be constructed across multiple marine sediment cores. Here we discuss a marine record of eruptive and mass‐wasting events spanning ∼250 ka offshore of Montserrat, using new data from IODP Expedition 340, as well as previously collected cores. By using a combination of high‐resolution oxygen isotope stratigraphy, AMS radiocarbon dating, biostratigraphy of foraminifera and calcareous nannofossils, and clast componentry, we identify five major events at Soufriere Hills volcano since 250 ka. Lateral correlations of these events across sediment cores collected offshore of the south and south west of Montserrat have improved our understanding of the timing, extent and associations between events in this area. Correlations reveal that powerful and potentially erosive density‐currents traveled at least 33 km offshore and demonstrate that marine deposits, produced by eruption‐fed and mass‐wasting events on volcanic islands, are heterogeneous in their spatial distribution. Thus, multiple drilling/coring sites are needed to reconstruct the full chronostratigraphy of volcanic islands. This multidisciplinary study will be vital to interpreting the chaotic records of submarine landslides at other sites drilled during Expedition 340 and provides a framework that can be applied to the stratigraphic analysis of sediments surrounding other volcanic islands.
      PubDate: 2014-07-25T11:12:51.967581-05:
      DOI: 10.1002/2014GC005402
       
  • Low‐frequency earthquakes reveal punctuated slow slip on the deep
           extent of the Alpine Fault, New Zealand
    • Authors: Calum J. Chamberlain; David R. Shelly, John Townend, Tim A. Stern
      Pages: n/a - n/a
      Abstract: We present the first evidence of low‐frequency earthquakes (LFEs) associated with the deep extension of the transpressional Alpine Fault beneath the central Southern Alps of New Zealand. Our database comprises a temporally continuous 36 month‐long catalog of 8760 LFEs within 14 families. To generate this catalog, we first identify 14 primary template LFEs within known periods of seismic tremor and use these templates to detect similar events in an iterative stacking and cross‐correlation routine. The hypocentres of 12 of the 14 LFE families lie within 10 km of the inferred location of the Alpine Fault at depths of approximately 20–30 km, in a zone of high P‐wave attenuation, low P‐wave speeds, and high seismic reflectivity. The LFE catalog consists of persistent, discrete events punctuated by swarm‐like bursts of activity associated with previously and newly identified tremor periods. The magnitudes of the LFEs range between ML – 0.8 and ML 1.8, with an average of ML 0.5. We find that the frequency‐magnitude distribution of the LFE catalog both as a whole and within individual families is not consistent with a power law, but that individual families' frequency‐amplitude distributions approximate an exponential relationship, suggestive of a characteristic length‐scale of failure. We interpret this LFE activity to represent quasi‐continuous slip on the deep extent of the Alpine Fault, with LFEs highlighting asperities within an otherwise steadily creeping region of the fault.
      PubDate: 2014-07-25T11:12:20.430978-05:
      DOI: 10.1002/2014GC005436
       
  • Evidence of an upper mantle seismic anomaly opposing the Cocos slab
           beneath the Isthmus of Tehuantepec, Mexico
    • Authors: YoungHee Kim; Hobin Lim, Meghan S. Miller, Fred Pearce, Robert W. Clayton
      Pages: n/a - n/a
      Abstract: Subduction of the Cocos plate beneath southern Mexico is characterized by several unusual features, such as a discontinuous volcanic arc, unusual arc chemistry, and anomalously low topography of Tehuantepec Isthmus. Recent seismic images from both receiver functions and seismic tomography suggest that there may be an additional, opposing structure dipping to the southwest from the Gulf of Mexico, and these images have been previously explained by a southwest‐dipping slab. However, standard models of the Caribbean tectonic history do not support this interpretation. To better define the Cocos slab's structure and the possible existence of a structure dipping in the opposite direction, dense seismic data across southern Mexico are used to form high‐resolution seismic images, based on the 2‐D generalized radon transform method, and to relocate regional earthquakes. Our images show the Cocos plate dipping at 30° to the northeast encounters the anomaly that is dipping in the opposite sense at ∼150 km depth. Relocated seismicity clearly delineates a Wadati‐Benioff zone that marks the subducting Cocos plate. A cluster of seismicity also appears at ∼150 km depth which may be related to the subduction of the Tehuantepec ridge and/or to the imaged seismic structure with opposite polarity.
      PubDate: 2014-07-25T11:04:43.127986-05:
      DOI: 10.1002/2014GC005320
       
  • Composition and flux of explosive gas release at LUSI mud volcano (East
           Java, Indonesia)
    • Authors: Loÿc Vanderkluysen; Michael R. Burton, Amanda B. Clarke, Hilairy E. Hartnett, Jean‐François Smekens
      Pages: n/a - n/a
      Abstract: The LUSI mud volcano has been erupting since May 2006 in the densely populated Sidoarjo regency (East Java, Indonesia), forcing the evacuation of 40,000 people and destroying industry, farmland, and over 10,000 homes. Mud extrusion rates of 180,000 m3 d−1 were measured in the first few months of the eruption, decreasing to a loosely documented 4000 m for methane and approximately 600 m for carbon dioxide; however, the mass fractions of these gases are insufficient to explain the observed dynamics. Rather, the primary driver of the cyclic bubble‐bursting activity is decompressional boiling of water, which initiates a few tens of meters below the surface, setting up slug flow in the upper conduit. Our measured gas flux and conceptual model lead to a corresponding upper‐bound estimate for the mud‐water mass flux of 105 m3 d−1.
      PubDate: 2014-07-24T14:56:14.927819-05:
      DOI: 10.1002/2014GC005275
       
  • Normal faulting sequence in the Pumqu‐Xainza Rift constrained by
           InSAR and teleseismic body‐wave seismology
    • Authors: Hua Wang; John R. Elliott, Timothy J. Craig, Tim J. Wright, Jing Liu‐Zeng, Andrew Hooper
      Pages: n/a - n/a
      Abstract: Normal faulting earthquakes play an important role in the deformation of continents, and pose significant seismic hazard, yet important questions remain about their mechanics. We use InSAR and body‐wave seismology to compute dislocation models and centroid moment solutions for four normal‐faulting earthquakes (Mw 5.7–6.2) that occurred in the Pumqu‐Xainza Rift (PXR), southern Tibet, a region where low‐angle normal faulting has previously been inferred. We also use the fault locations and slip to investigate the correlation between earthquakes and surface topography, and to calculate stress interactions between the earthquakes. The InSAR and body‐wave models give consistent focal mechanisms except for the magnitude of the 1996 event, which may be overestimated due to postseismic deformation in the long‐interval interferograms. We calculate the static stress changes due to coseismic slip and find that the 1993 event was too distant to cause triggering of the later events, but that the 1998 event pair occurred in regions of increased Coulomb stress resulting from the 1996 event. All the fault planes found here dip at 40–60°, reinforcing the absence in observations for low‐angle normal faulting earthquakes (dip 
      PubDate: 2014-07-24T14:55:17.643005-05:
      DOI: 10.1002/2014GC005369
       
  • Sea level and climate forcing of the Sr isotope composition of late
           Miocene Mediterranean marine basins
    • Authors: T. F. Schildgen; D. Cosentino, G. Frijia, F. Castorina, F. Ö. Dudas, A. Iadanza, G. Sampalmieri, P. Cipollari, A. Caruso, S. A. Bowring, M. R. Strecker
      Pages: n/a - n/a
      Abstract: Sr isotope records from marginal marine basins track the mixing between seawater and local continental runoff, potentially recording the effects of sea level, tectonic, and climate forcing in marine fossils and sediments. Our 110 new 87Sr/86Sr analyses on oyster and foraminifera samples from six late Miocene stratigraphic sections in southern Turkey, Crete, and Sicily show that 87Sr/86Sr fell below global seawater values in the basins several million years before the Messinian Salinity Crisis, coinciding with tectonic uplift and basin shallowing. 87Sr/86Sr from more centrally located basins (away from the Mediterranean coast) drop below global seawater values only during the Messinian Salinity Crisis. In addition to this general trend, 55 new 87Sr/86Sr analyses from the astronomically tuned Lower Evaporites in the central Apennines (Italy) allow us to explore the effect of glacio‐eustatic sea level and precipitation changes on 87Sr/86Sr. Most variation in our data can be explained by changes in sea level, with greatest negative excursions from global seawater values occurring during relative sea level lowstands, which generally coincided with arid conditions in the Mediterranean realm. We suggest that this greater sensitivity to lowered sea level compared with higher runoff could relate to the inverse relationship between Sr concentration and river discharge. Variations in the residence time of groundwater within the karst terrain of the circum‐Mediterranean region during arid and wet phases may help to explain the single (robust) occurrence of a negative excursion during a sea level highstand, but this explanation remains speculative without more detailed paleoclimatic data for the region.
      PubDate: 2014-07-24T14:52:43.557557-05:
      DOI: 10.1002/2014GC005332
       
  • Coring disturbances in IODP piston cores with implications for offshore
           record of volcanic events and the Missoula megafloods
    • Authors: Martin Jutzeler; James D. L. White, Peter J. Talling, Molly McCanta, Sally Morgan, Anne Le Friant, Osamu Ishizuka
      Pages: n/a - n/a
      Abstract: Piston cores collected from IODP drilling platforms (and its predecessors) provide the best long‐term geological and climatic record of marine sediments worldwide. Coring disturbances affecting the original sediment texture have been recognized since the early days of coring, and include deformation resulting from shear of sediment against the core barrel, basal flow‐in due to partial stroke, loss of stratigraphy, fall‐in, sediment loss through core catchers, and structures formed during core recovery and on‐deck transport. The most severe disturbances occur in non‐cohesive (sandy) facies, which are particularly common in volcanogenic environments and submarine fans. Although all of these types of coring disturbances have been recognized previously, our contribution is novel because it provides an easily accessible summary of methods for their identification. This contribution gives two specific examples on the importance of these coring disturbances. We show how suck‐in of sediments during coring artificially created very thick volcaniclastic sand layers in cores offshore Montserrat and Martinique (Lesser Antilles). We then analyze very thick, structureless sand layers from the Escanaba Trough inferred to be a record of the Missoula mega‐floods. These sand layers tend to coincide with the base of core sections, and their facies suggest coring disturbance by basal flow‐in, destroying the original structure and texture of the beds. We conclude by outlining and supporting IODP‐led initiatives to further reduce and identify coring disturbances, and acknowledge their recent successes in drilling challenging sand‐rich settings, such as during IODP Expedition 340.
      PubDate: 2014-07-24T04:22:23.637509-05:
      DOI: 10.1002/2014GC005447
       
  • Reactive halogens (BrO and OClO) detected in the plume of Soufrière
           Hills Volcano during an eruption hiatus
    • Authors: Amy Donovan; Vitchko Tsanev, Clive Oppenheimer, Marie Edmonds
      Pages: n/a - n/a
      Abstract: Volcanic plumes are sites of dynamic chemistry involving halogen gases. Here we present new data on the relative abundances of SO2, BrO and OClO gases emitted from Soufrière Hills Volcano (SHV). They were collected during an eruptive hiatus but during sustained degassing at this halogen‐rich volcano. By comparison with data from a previous study during an eruptive phase (Bobrowski et al, 2003) and application of the data and modelling of Villemant et al. (2008), we suggest that, after consideration of errors, either the rate of HBr conversion to BrO is variable, ranging from ~30% to ~15%, and/or the relative partitioning of Cl and Br into the gas phase from the melt changes according to eruptive activity. We examine the potential implications of this for fluid‐melt partitioning, and compare our results with data from the experimental literature. Our work contributes towards understanding the controls on the BrO/SO2 ratio for volcano monitoring purposes; the changes in plume chemistry with regard to bromine at the onset of lava extrusion may be large and rapid. OClO was detected in the plume at SHV for the first time. This species has only previously been detected in emissions from Mount Etna (Bobrowski et al., 2007, using ground‐based methods) and from Puyehue Cordon Caulle (Theys et al., 2014, using satellite‐based methods). No HCHO or NOy species were detected in the spectra.
      PubDate: 2014-07-23T02:05:44.622788-05:
      DOI: 10.1002/2014GC005419
       
  • Extent and distribution of aseismic slip on the Ismetpaşa segment of
           the North Anatolian Fault (Turkey) from Persistent Scatterer InSAR
    • Authors: Esra Cetin; Ziyadin Cakir, Mustapha Meghraoui, Semih Ergintav, Ahmet M. Akoglu
      Pages: n/a - n/a
      Abstract: We use the Persistent Scatterer InSAR (PSI) technique with elastic dislocation models and geology along the creeping section of the North Anatolian Fault (NAF) at Ismetpaşa, to map and deduce the velocity field and the aseismic slip distribution. Revealing the spatiotemporal nature of the creep helped us associate the creep with potential lithological controls, hence providing a new perspective to better understand the underlying causes and mechanisms. The PSI analysis of Envisat ASAR images between 2003 and 2010 reveals a clear picture of surface creep along the fault and a new interseismic velocity field transitioning gradually between the creeping and the locked fault sections. The creep rate is found to fluctuate along a 100 km long section of the fault in a manner similar to that along the Hayward fault, reaching a maximum of ∼20 ± 2 mm/yr, close to the far field plate velocity (∼25 ± 1.5 mm/yr). At Ismetpaşa, it is in the range of 8 ± 2 mm/yr, consistent with the previous geodetic observations. The creeping section appears to extend 30 km further east than those previously reported. Modeling of the PSI data reveals a heterogeneous creep distribution at depth with two main patches confined mostly to the uppermost 5 km portion of the seismogenic crust, releasing annually 6.2 × 1016 Nm (Mw = 5.1) geodetic moment. Our analysis combined with previous studies suggests that creep might have commenced as postseismic deformation following the 1944 earthquake and has evolved to stable fault creep with time. There is a correlation between aseismic surface creep and the geology along the fault as it is in major part associated to rocks with low frictional strength such as the andesitic‐basaltic, limestone, and serpentine bodies within the fault zone.
      PubDate: 2014-07-22T10:24:55.958357-05:
      DOI: 10.1002/2014GC005307
       
  • Along‐axis hydrothermal flow at the axis of slow spreading
           Mid‐Ocean Ridges: Insights from numerical models of the Lucky Strike
           vent field (MAR)
    • Authors: Fabrice J. Fontaine; Mathilde Cannat, Javier Escartin, Wayne C. Crawford
      Pages: n/a - n/a
      Abstract: The processes and efficiency of hydrothermal heat extraction along the axis of mid‐ocean ridges are controlled by lithospheric thermal and permeability structures. Hydrothermal circulation models based on the structure of fast and intermediate spreading ridges predict that hydrothermal cell organization and vent site distribution are primarily controlled by the thermodynamics of high‐temperature mid‐ocean ridge hydrothermal fluids. Using recent constraints on shallow structure at the slow spreading Lucky Strike segment along the Mid‐Atlantic Ridge, we present a physical model of hydrothermal cooling that incorporates the specificities of a magma‐rich slow spreading environment. Using three‐dimensional numerical models, we show that, in contrast to the aforementioned models, the subsurface flow at Lucky Strike is primarily controlled by across‐axis permeability variations. Models with across‐axis permeability gradients produce along‐axis oriented hydrothermal cells and an alternating pattern of heat extraction highs and lows that match the distribution of microseismic clusters recorded at the Lucky Strike axial volcano. The flow is also influenced by temperature gradients at the base of the permeable hydrothermal domain. Although our models are based on the structure and seismicity of the Lucky Strike segment, across‐axis permeability gradients are also likely to occur at faster spreading ridges and these results may also have important implications for the cooling of young crust at fast and intermediate spreading centers.
      PubDate: 2014-07-22T10:24:45.538763-05:
      DOI: 10.1002/2014GC005372
       
  • Three‐dimensional inversion of seafloor magnetotelluric data
           collected in the Philippine Sea and the western margin of the northwest
           Pacific Ocean
    • Authors: Noriko Tada; Kiyoshi Baba, Hisashi Utada
      Pages: n/a - n/a
      Abstract: We report a result of three‐dimensional (3‐D) upper mantle electrical conductivity inversion of seafloor magnetotelluric data. We used existing data at 25 sites in the Philippine Sea and the western margin of the Pacific Ocean. In order to obtain a reliable model by 3‐D inversion, we evaluated the large and small‐scale topographic effects. We also conducted a comprehensive search of the one‐dimensional (1‐D) profiles of the study area in order to determine the best initial and prior models. A two‐phase inversion method was applied so that the error floors for the diagonal and off‐diagonal elements of the impedance tensor could be separately controlled. Through this first attempt at inverting real data, we obtained basic knowledge about tuning the inversion parameters and conditions. We also proposed a procedure to evaluate the reliability of the 3‐D conductivity anomalies imaged by the inversion by conducting checkerboard and sensitivity tests. After the iterations converged, 13 distinct anomalies were found in the inverted 3‐D conductivity model; four conductive and two resistive anomalies were confirmed to be resolved enough by the data through the checkerboard test. Then the sensitivity tests were conducted to quantify how each anomaly was required by the observed data, and we confirmed that the intensities of three conductive anomalies and one resistive anomaly were statistically significant. This paper presented an example of possible approach in 3‐D seafloor electromagnetic inversion procedure for imaging reliable electrical conductivity structure of the oceanic mantle, which will be useful in understanding dynamics and evolution of solid Earth.
      PubDate: 2014-07-22T10:24:35.311913-05:
      DOI: 10.1002/2014GC005421
       
  • Atmospheric transport of mineral dust from the Indo‐Gangetic Plain:
           Temporal variability, acid processing, and iron solubility
    • Authors: Bikkina Srinivas; M. M. Sarin, R. Rengarajan
      Pages: n/a - n/a
      Abstract: Atmospheric transport of chemical constituents from the Indo‐Gangetic Plain (IGP) to the Bay of Bengal is a conspicuous seasonal feature that occurs during the late NE‐monsoon (December‐March). With this perspective, aerosol composition and abundance of mineral dust have been studied during November 2009 ‐ March 2010 from a sampling site (Kharagpur: 22.3 N, 87.3E) in the IGP, representing the atmospheric outflow to the Bay of Bengal. The chemical composition of PM2.5 suggests the dominance of nss‐SO42‐ (6.9 – 24.3 µg m‐3); whereas the abundance of mineral dust varied from 3 to 18 µg m‐3. The concentration of aerosol iron (FeTot) and its fractional solubility (Fews % = Fews/FeTot *100, where Fews is the water‐soluble fraction of FeTot) varied from 60 to 1144 ng m‐3 and from 6.7 to 26.5 %, respectively. A striking similarity in the temporal variability of total inorganic acidity (TIA = NO3‐ + nss‐SO42‐) and Fews (%) provides evidence for acid processing of mineral dust (alluvium) during atmospheric transport from the IGP. The contribution of TIA to water‐soluble inorganic species [(nss‐SO42‐ + NO3‐)/ΣWSIS], mass ratios of Ca/Al and Fe/Al, abundance of dust (%) and Fews (%) in the IGP‐outflow are similar to the aerosol composition over the Bay of Bengal. With the rapid increase in anthropogenic activities over south and south‐east Asia, the enhanced fractional solubility of aerosol iron (attributed to acid processing of mineral dust) has implications to further increase the air‐sea deposition of Fe to the surface ocean.
      PubDate: 2014-07-22T02:09:59.622471-05:
      DOI: 10.1002/2014GC005395
       
  • Evolution of stress and fault patterns in oblique rift systems: 3‐D
           numerical lithospheric‐scale experiments from rift to breakup
    • Authors: Sascha Brune
      Pages: n/a - n/a
      Abstract: Rifting involves complex normal fault systems that are controlled by extension direction, reactivation of pre‐rift structures, sedimentation, and dyke dynamics. The relative impact of these factors on the observed fault pattern, however, is difficult to deduce from field‐based studies alone. This study provides insight in crustal stress patterns and fault orientations by employing a laterally homogeneous, 3D rift setup with constant extension velocity. The presented numerical forward experiments cover the whole spectrum of oblique extension. They are conducted using an elasto‐visco‐plastic finite element model and involve crustal and mantle layers accounting for self‐consistent necking of the lithosphere. Despite recent advances, 3D numerical experiments still require relatively coarse resolution so that individual faults are poorly resolved. This issue is addressed by applying a post‐processing method that identifies the stress regime and preferred fault azimuth at each surface element. The simple model setup results in a surprising variety of fault orientations that are solely caused by the three‐dimensionality of oblique rift systems. Depending on rift obliquity, these orientations can be grouped in terms of rift‐parallel, extension‐orthogonal, and intermediate normal fault directions as well as strike‐slip faults. While results compare well with analog rift models of low to moderate obliquity, new insight is gained in advanced rift stages and highly oblique settings. Individual fault populations are activated in a characteristic multi‐phase evolution driven by lateral density variations of the evolving rift system. In natural rift systems this pattern might be modified by additional heterogeneities, surface processes and dyke dynamics.
      PubDate: 2014-07-22T01:10:50.108671-05:
      DOI: 10.1002/2014GC005446
       
  • Robust coupled fluid‐particle simulation scheme in Stokes‐flow
           regime: Toward the geodynamic simulation including granular media
    • Authors: Mikito Furuichi; Daisuke Nishiura
      Pages: n/a - n/a
      Abstract: We present a simulation scheme for solving high‐viscosity fluid and particle dynamics in a coupled computational fluid dynamics and discrete element method (CFD‐DEM) framework. This simulation scheme is intended to be used for geodynamical magmatic studies such as crystal settling at the melting roof of a magma chamber. The high‐viscosity fluid is treated by the Stokes‐flow approximation, where the fluid interacts with particles via the drag force in a cell‐averaged manner. The particles are tracked with contact forces by DEM. To efficiently solve such Stokes‐DEM coupled equations, we propose two key techniques. One is formulation of particle motion without the inertial term, allowing a larger time step at higher viscosities. The other is a semi‐implicit treatment of the cell‐averaged particle velocity in the fluid equation to stabilize the calculation. We simulate the settling particles in strongly viscous fluids in three dimensions and compare the results with the experimental and theoretical results. Our solution strategy is found to be robust and successfully captures the collective behavior of the particles. The simulation method presented here will be useful in various fields interested in long‐term dynamics of high‐viscosity granular media.
      PubDate: 2014-07-17T11:07:30.809469-05:
      DOI: 10.1002/2014GC005281
       
  • Crustal and upper mantle structure beneath south‐western margin of
           the Arabian Peninsula from teleseismic tomography
    • Authors: Félicie Korostelev; Clémence Basuyau, Sylvie Leroy, Christel Tiberi, Abdulhakim Ahmed, Graham W. Stuart, Derek Keir, Frédérique Rolandone, Ismail Ganad, Khaled Khanbari, Lapo Boschi
      Pages: n/a - n/a
      Abstract: We image the lithospheric and upper asthenospheric structure of western continental Yemen with 24 broadband stations to evaluate the role of the Afar plume on the evolution of the continental margin and its extent eastward along the Gulf of Aden. We use teleseismic tomography to compute relative P wave velocity variations in south‐western Yemen down to 300 km depth. Published receiver function analysis suggest a dramatic and localized thinning of the crust in the vicinity of the Red Sea and the Gulf of Aden, consistent with the velocity structure that we retrieve in our model. The mantle part of the model is dominated by the presence of a low‐velocity anomaly in which we infer partial melting just below thick Oligocene flood basalts and recent off‐axis volcanic events (from 15 Ma to present). This low‐velocity anomaly could correspond to an abnormally hot mantle and could be responsible for dynamic topography and recent magmatism in western Yemen. Our new P wave velocity model beneath western Yemen suggests the young rift flank volcanoes beneath margins and on the flanks of the Red Sea rift are caused by focused small‐scale diapiric upwelling from a broad region of hot mantle beneath the area. Our work shows that relatively hot mantle, along with partial melting of the mantle, can persist beneath rifted margins after breakup has occurred.
      PubDate: 2014-07-17T10:54:05.416847-05:
      DOI: 10.1002/2014GC005316
       
  • Multi‐scale convection in a geodynamo simulation with uniform heat
           flux along the outer boundary
    • Authors: Hiroaki Matsui; Eric King, Bruce Buffett
      Pages: n/a - n/a
      Abstract: It is generally expected that Earth's magnetic field, which is generated by convecting liquid metal within its core, will substantially alter that convection through the action of Lorentz forces. In most dynamo simulations, however, Lorentz forces do very little to change convective flow, which is predominantly fine‐scaled. An important exception to this observation is in dynamo models that employ uniform heat flux boundary conditions, rather than the usual uniform temperature conditions, in which multi‐scale convection is observed. We investigate the combined influence of thermal boundary conditions and magnetic fields using four simulations: two dynamos and two non‐magnetic models, with either uniform temperature or heat flux fixed at the outer boundary. Of the four, only the fixed‐heat‐flux dynamo simulation produces multi‐scale convective flow patterns. Comparison between the models suggests that the fixed‐flux dynamo generates large patches of strong azimuthal magnetic field that suppress small‐scale convective motions. By allowing temperature to vary along the outer boundary, the fixed‐flux dynamo generates stronger azimuthal flow and, in turn, stronger magnetic field, and the resulting Lorentz forces alter the nature of convective flow. Extrapolation of the analyses presented here suggests that magnetic fields may also suppress small‐scale convection in the Earth's core.
      PubDate: 2014-07-17T04:08:41.726933-05:
      DOI: 10.1002/2014GC005432
       
  • Correlated patterns in hydrothermal plume distribution and apparent
           magmatic budget along 2500 km of the Southeast Indian Ridge
    • Authors: Edward T. Baker; Christophe Hémond, Anne Briais, Marcia Maia, Daniel S. Scheirer, Sharon L. Walker, Tingting Wang, Yongshun John Chen
      Pages: n/a - n/a
      Abstract: Multiple geological processes affect the distribution of hydrothermal venting along a mid‐ocean ridge. Deciphering the role of a specific process is often frustrated by simultaneous changes in other influences. Here we take advantage of the almost constant spreading rate (65‐71 mm/yr) along 2500 km of the Southeast Indian Ridge (SEIR) between 77°‐99°E to examine the spatial density of hydrothermal venting relative to regional and segment‐scale changes in the apparent magmatic budget. We use 227 vertical profiles of light backscatter and (on 41 profiles) oxidation‐reduction potential along 27 1st‐ and 2nd‐order ridge segments on and adjacent to the Amsterdam‐St. Paul (ASP) Plateau to map ph, the fraction of casts detecting a plume. At the regional scale, venting on the five segments crossing the magma‐thickened hotspot plateau is almost entirely suppressed (ph = 0.02). Conversely, the combined ph (0.34) from all other segments follows the global trend of ph versus spreading rate. Off the ASP Plateau, multi‐segment trends in ph track trends in the regional axial depth, high where regional depth increases and low where it decreases. At the individual segment scale, a robust correlation between ph and cross‐axis inflation for 1st‐order segments shows that different magmatic budgets among 1st‐order segments are expressed as different levels of hydrothermal spatial density. This correlation is absent among 2nd‐order segments. Eighty‐five percent of the plumes occur in eight clusters totaling ~350 km. We hypothesize that these clusters are a minimum estimate of the length of axial melt lenses underlying this section of the SEIR.
      PubDate: 2014-07-17T04:04:55.240659-05:
      DOI: 10.1002/2014GC005344
       
  • Effect of latent heat of freezing on crustal generation at low spreading
           rates
    • Authors: Norman H. Sleep; Jessica M. Warren
      Pages: n/a - n/a
      Abstract: Lithospheric structure changes at low spreading rates (
      PubDate: 2014-07-16T04:17:54.238951-05:
      DOI: 10.1002/2014GC005423
       
  • Insights into magmatic processes and hydrothermal alteration of in situ
           superfast spreading ocean crust at ODP/IODP Site 1256 from a cluster
           analysis of rock magnetic properties
    • Authors: Mark J. Dekkers; David Heslop, Emilio Herrero‐Bervera, Gary Acton, David Krasa
      Pages: n/a - n/a
      Abstract: We analyze magnetic properties from Ocean Drilling Program (ODP)/Integrated ODP (IODP) Hole 1256D (6°44.1' N, 91°56.1' W) on the Cocos Plate in ~15.2 Ma oceanic crust generated by superfast seafloor spreading, the only drill hole that has sampled all three oceanic crust layers in a tectonically undisturbed setting. Fuzzy c‐means cluster analysis and non‐linear mapping are utilized to study down‐hole trends in the ratio of the saturation remanent magnetization and the saturation magnetization, the coercive force, the ratio of the remanent coercive force and coercive force, the low‐field magnetic susceptibility, and the Curie temperature, to evaluate the effects of magmatic and hydrothermal processes on magnetic properties. A statistically robust five‐cluster solution separates the data predominantly into three clusters that express increasing hydrothermal alteration of the lavas, which differ from two distinct clusters mainly representing the dikes and gabbros. Extensive alteration can obliterate magnetic property differences between lavas, dikes, and gabbros. The imprint of thermochemical alteration on the iron‐titanium oxides is only partially related to the porosity of the rocks. Thus, the analysis complements interpretation based on electrofacies analysis. All clusters display rock magnetic characteristics compatible with an ability to retain a stable natural remanent magnetization suggesting that the entire sampled sequence of ocean crust can contribute to marine magnetic anomalies. Paleointensity determination is difficult because of the propensity of oxy‐exsolution during laboratory heating and/or the presence of intergrowths. The upper part of the extrusive sequence, the granoblastic dikes, and moderately altered gabbros may contain a comparatively uncontaminated thermoremanent magnetization.
      PubDate: 2014-07-16T04:16:57.890206-05:
      DOI: 10.1002/2014GC005343
       
  • Dynamics of lithospheric thinning and mantle melting by edge‐driven
           convection: Application to Moroccan Atlas mountains
    • Authors: Lars Kaislaniemi; Jeroen van Hunen
      Pages: n/a - n/a
      Abstract: Edge‐driven convection (EDC) forms in the upper mantle at locations of lithosphere thickness gradients, e.g. craton edges. In this study we show how the traditional style of EDC, a convection cell governed by the cold downwelling below an edge alternates with another style of EDC, in which the convection cell forms as a secondary feature with a hot asthenospheric shear flow from underneath the thicker lithosphere. These alternating EDC styles produce episodic lithosphere erosion and decompression melting. Three‐dimensional models of EDC show that convection rolls form perpendicular to the thickness gradient at the lithosphere‐asthenosphere boundary. Stagnant‐lid convection scaling laws are used to gain further insight in the underlying physical processes. Application of our models to the Moroccan Atlas mountains region shows that the combination of these two styles of EDC can reproduce many of the observations from the Atlas mountains, including two distinct periods of Cenozoic volcanism, a semi‐continuous corridor of thinned lithosphere under the Atlas mountains, and piecewise delamination of the lithosphere. A very good match between observations and numerical models is found for the lithosphere thicknesses across the study area, amounts of melts produced, and the length of the quiet gap in between volcanic episodes show quantitative match to observations.
      PubDate: 2014-07-16T04:16:56.447272-05:
      DOI: 10.1002/2014GC005414
       
  • Seafloor basalt alteration and chemical change in the ultra thinly
           sedimented South Pacific
    • Authors: Guo‐Liang Zhang; Christopher Smith‐Duque
      Pages: n/a - n/a
      Abstract: Determining the relationship between ocean floor basalt alteration and sedimentation is fundamental to understanding how oceanic crust evolves with time. Ocean floor basalts recovered at IODP Sites U1365 (~100 Ma) and U1368 (~13.5 Ma) in the South Pacific have been subjected to remarkably low sedimentation rates (0.71 to 1.1 m·Myr‐1, respectively). We report detailed petrographic and geochemical analysis of basalt cores from these sites in order to investigate what impact sediment insulation has on seafloor alteration beyond 10‐15 Myr of ocean crust formation. Both sites exhibit low temperature (
      PubDate: 2014-07-15T03:46:27.20656-05:0
      DOI: 10.1002/2013GC005141
       
  • Skeletal oxygen and carbon isotope compositions of Acropora coral primary
           polyps experimentally cultured at different temperatures
    • Authors: Kozue Nishida; Kei Ishikawa, Akira Iguchi, Yasuaki Tanaka, Mizuho Sato, Toyoho Ishimura, Mayuri Inoue, Takashi Nakamura, Kazuhiko Sakai, Atsushi Suzuki
      Pages: n/a - n/a
      Abstract: We investigated temperature and growth‐rate dependency of skeletal oxygen and carbon isotopes in primary polyps of Acropora digitifera (Scleractinia: Acroporidae) by culturing them at 20, 23, 27, or 31°C. Calcification was most rapid at 27 and 31°C. We obtained a δ18O‐temperature relationship (−0.18‰ °C−1) consistent with reported ranges for Porites, indicating that juvenile Acropora polyps can be used for temperature reconstruction. A growth‐rate dependency of skeletal isotopes was detected in the experimental polyps cultured at lower water temperatures, when the skeletal growth rate of these polyps was also low. The estimated upper calcification flux limit for a kinetic isotope effect to be observed in the δ18O‐growth rate relationship (∼0.4–0.7 g CaCO3 cm−2 yr−1) was similar to the calcification flux in Porites corresponding to a linear extension rate of 5 mm yr−1, the maximum rate at which the kinetic isotope effect is evident. This result suggests that the calcification flux can be used as a measure of growth rate‐related isotope fractionation, that is, the kinetic isotope effect, in corals of different genera and at different growth stages.
      PubDate: 2014-07-14T11:57:09.778277-05:
      DOI: 10.1002/2014GC005322
       
  • Active alkaline traps to determine acidic‐gas ratios in volcanic
           plumes: Sampling techniques and analytical methods
    • Authors: J. Wittmer; N. Bobrowski, M. Liotta, G. Giuffrida, S. Calabrese, U. Platt
      Pages: n/a - n/a
      Abstract: In situ measurements have been the basis for monitoring volcanic gas emissions for many years and—being complemented by remote sensing techniques—still play an important role to date. Concerning in situ techniques for sampling a dilute plume, an increase in accuracy and a reduction of detection limits are still necessary for most gases (e.g., CO2, SO2, HCl, HF, HBr, HI). In this work, the Raschig‐Tube technique (RT) is modified and utilized for application on volcanic plumes. The theoretical and experimental absorption properties of the RT and the Drechsel bottle (DB) setups are characterized and both are applied simultaneously to the well‐established Filter packs technique (FP) in the field (on Stromboli Island and Mount Etna). The comparison points out that FPs are the most practical to apply but the results are error‐prone compared to RT and DB, whereas the RT results in up to 13 times higher analyte concentrations than the DB in the same sampling time. An optimization of the analytical procedure, including sample pretreatment and analysis by titration, Ion Chromatography, and Inductively Coupled Plasma Mass Spectrometry, led to a comprehensive data set covering a wide range of compounds. In particular, less abundant species were quantified more accurately and iodine was detected for the first time in Stromboli's plume. Simultaneously applying Multiaxis Differential Optical Absorption Spectroscopy (MAX‐DOAS) the chemical transformation of emitted bromide into bromine monoxide (BrO) from Stromboli and Etna was determined to 3–6% and 7%, respectively, within less than 5 min after the gas release from the active vents.
      PubDate: 2014-07-14T11:57:07.061102-05:
      DOI: 10.1002/2013GC005133
       
  • Anisotropy of magnetic susceptibility in natural olivine single crystals
    • Authors: Andrea R. Biedermann; Thomas Pettke, Eric Reusser, Ann M. Hirt
      Pages: n/a - n/a
      Abstract: Mantle flow dynamics can cause preferential alignment of olivine crystals that results in anisotropy of physical properties. To interpret anisotropy in mantle rocks, it is necessary to understand the anisotropy of olivine single crystals. We determined anisotropy of magnetic susceptibility (AMS) for natural olivine crystals. High‐field AMS allows for the isolation of the anisotropy due to olivine alone. The orientations of the principal susceptibility axes are related to the olivine's crystallographic structure as soon as it contains > 3 wt.% FeO. The maximum susceptibility is parallel to the c‐axis both at room temperature (RT) and at 77 K. The orientation of the minimum axis at RT depends on iron content; it is generally parallel to the a‐axis in crystals with 3 – 5 wt.% FeO, and along b in samples with 6 – 10 wt.% FeO. The AMS ellipsoid is prolate and the standard deviatoric susceptibility, k', is on the order of 8*10‐10 m3/kg for the samples with < 1wt.% FeO, and ranges from 3.1*10‐9 m3/kg to 5.7*10‐9 m3/kg for samples with 3‐10 wt.% FeO. At 77 K, the minimum susceptibility is along b, independent of iron content. The shape of the AMS ellipsoid is prolate for samples with < 5 wt.% FeO, but can be prolate or oblate for higher iron content. The degree of anisotropy increases at 77 K with p77' = 7.1 ± 0.5. The results from this study will allow AMS fabrics to be used as a proxy for olivine texture in ultramafic rocks with high olivine content.
      PubDate: 2014-07-12T06:30:47.442401-05:
      DOI: 10.1002/2014GC005386
       
  • Approximately 1.78 Ga mafic dykes in the Lüliang Complex, North China
           Craton: Zircon ages and Lu‐Hf isotopes, geochemistry, and
           implications
    • Authors: Xi Wang; Wenbin Zhu, Meng Luo, Xingmin Ren, Xiang Cui
      Pages: n/a - n/a
      Abstract: Mafic dyke swarms are excellent time markers and paleo‐stress indicators. Numerous late Paleoproterozoic mafic dykes are exposed throughout the Trans‐North China Orogen (TNCO). Most of these dykes trend NW‐SE or NNW‐SSE, nearly parallel to the orogen, while a series of E‐W‐trending mafic dykes are restricted in the Lüliang and southern Taihang areas in the central segment of the TNCO. These dykes were mostly considered to be linked with break‐up of the supercontinent Columbia previously. In this study, sixteen mafic dykes were investigated in the Lüliang Complex. Zircon LA‐ICP‐MS dating of four samples yields magmatic crystallization ages of 1.78‐1.79 Ga. These dykes belong to the tholeiite series and consist of basalt, basaltic andesite and andesite. They are enriched in LREE and LILE and depleted in HFSE, and have negative zircon εHf(t) values of ‐1.7 to ‐12.2. The E‐W‐trending mafic dykes show similar geochemical and isotopic features compare to the NW‐SE‐trending dykes in other complexes. They were most likely originated from a lithospheric mantle metasomatised by subduction‐related fluids and later emplaced along extensional fractures in a post‐collisional setting. NW‐SE‐trending fractures were formed due to gravitational collapse and thinning of the lithosphere. E‐W‐trending fractures in the central segment of the orogen constitute a transverse accommodation belt to equilibrate the different amounts of extension between the northern and southern TNCO. The impact of the post‐orogenic extension might have continued to ca. 1680 Ma as evidenced by the presence of abundant ca. 1750‐1680 Ma anorthosite‐ gabbro‐ mangerite‐ rapakivi granite suites (AMCG‐like) occurring in the northern NCC.
      PubDate: 2014-07-12T06:29:57.781042-05:
      DOI: 10.1002/2014GC005378
       
  • Interplate seismicity at the CRISP drilling site: The 2002 Mw 6.4 Osa
           Earthquake at the southeastern end of the Middle America Trench
    • Authors: Ivonne G. Arroyo; Ingo Grevemeyer, Cesar R. Ranero, Roland von Huene
      Pages: n/a - n/a
      Abstract: We investigate potential relations between variations in seafloor relief and age of the incoming plate and interplate seismicity. Westward from Osa Peninsula in Costa Rica a major change in the character of the incoming Cocos Plate is displayed by abrupt lateral variations in seafloor depth and thermal structure. Here, a Mw 6.4 thrust earthquake was followed by three aftershock clusters in June 2002. Initial relocations indicate that the main shock occurred fairly trenchward of most large earthquakes along the Middle America Trench off central Costa Rica. The earthquake sequence occurred while a temporary network of OBH and land stations ~80 km to the northwest were deployed. By adding readings from permanent local stations, we obtain uncommon P wave coverage of a large subduction zone earthquake. We relocate this catalog using a non‐linear probabilistic approach within both, a 1‐D and a 3‐D P wave velocity models. The main shock occurred ~25 km from the trench and probably along the plate interface at 5 to 10 km depth. We analyze teleseismic data to further constrain the rupture process of the main shock. The best depth estimates indicate that most of the seismic energy was radiated at shallow depth below the continental slope, supporting the nucleation of the Osa earthquake at ~6 km depth. The location and depth coincide with the plate boundary imaged in pre‐stack depth‐migrated reflection lines shot near the nucleation area. Aftershocks propagated downdip to the area of a 1999 Mw 6.9 sequence and partially overlapped it. The results indicate that underthrusting of the young and buoyant Cocos Ridge has created conditions for interplate seismogenesis shallower and closer to the trench axis than elsewhere along the central Costa Rica margin.
      PubDate: 2014-07-10T10:56:32.451961-05:
      DOI: 10.1002/2014GC005359
       
  • Controlled‐atmosphere thermal demagnetization and paleointensity
           analyses of extraterrestrial rocks
    • Authors: Clément Suavet; Benjamin P. Weiss, Timothy L. Grove
      Pages: n/a - n/a
      Abstract: We describe an apparatus to conduct thermal demagnetization of extraterrestrial rocks in a controlled atmosphere appropriate for a wide range of oxygen fugacities within the stability domain of iron. Thermal demagnetization and Thellier‐Thellier paleointensity experiments on lunar basalt synthetic analogs show that the controlled atmosphere prevents oxidation of magnetic carriers. When combined with multidomain paleointensity techniques, this opens the possibility of highly accurate thermal demagnetization and paleointensity measurements on rocks from the Moon and asteroids.
      PubDate: 2014-07-01T04:21:33.167587-05:
      DOI: 10.1002/2013GC005215
       
  • The Cobb hotspot: HIMU‐DMM mixing and melting controlled by a
           progressively thinning lithospheric lid
    • Authors: John Chadwick; Randall Keller, George Kamenov, Gene Yogodzinski, John Lupton
      Pages: n/a - n/a
      Abstract: The Cobb Seamount Chain in the northeast Pacific basin records the composition of the Cobb hotspot for the past 33 Myr, as the migrating Juan de Fuca Ridge approached and ultimately overran it ca. 0.5 Myr ago. In this first comprehensive geochemical study of the Cobb chain, major and trace element compositions and Sr, Nd, Pb, and Hf isotopic ratios were measured for whole‐rock samples from throughout the chain, and He isotopes were acquired for olivine phenocrysts from one seamount. Trace element modeling indicates increased melting along the chain over time, with progressively more depleted lavas as the ridge approached the hotspot. The isotopic data reveal the first evidence of the high µ (µ = 238U/204Pb) (HIMU) mantle component in the north Pacific basin, and are consistent with a progressively decreasing mixing proportion of HIMU melts relative to those from depleted mid‐ocean ridge basalt mantle (DMM) in the chain over time. Decreasing lithospheric thickness over the Cobb hotspot due to the approach of the migrating Juan de Fuca ridge allowed adiabatic melting to continue to shallower depths, leading to increased melt fractions of the refractory DMM component in the hotspot and more depleted and MORB‐like lavas in the younger Cobb seamounts.
      PubDate: 2014-06-26T12:02:57.987204-05:
      DOI: 10.1002/2014GC005334
       
  • Reply to comment by Hu et al. on “Holocene evolution in weathering
           and erosion patterns in the Pearl River delta"
    • Authors: Peter D. Clift
      Pages: n/a - n/a
      PubDate: 2014-05-16T03:56:53.074639-05:
      DOI: 10.1002/2014GC005371
       
  • Correction to “Extreme crustal thinning in the Bay of Biscay and the
           Western Pyrenees: From observations to modeling”
    • Authors: Suzon Jammes; Patricia Persaud, Luc Lavier, Gianreto Manatschal
      Pages: n/a - n/a
      PubDate: 2014-03-01T17:32:52.11764-05:0
      DOI: 10.1002/2013GC00505
       
  • In situ geochemistry of Lower Paleozoic dolomites in the northwestern
           Tarim basin: Implications for the nature, origin, and evolution of
           diagenetic fluids
    • Authors: Wei Zhang; Ping Guan, Xing Jian, Fan Feng, Caineng Zou
      Pages: 2744 - 2764
      Abstract: Lower Paleozoic sedimentary rocks in the northwestern Tarim basin were strongly altered by complicated geofluids, which resulted in the occurrence of various diagenetic minerals (e.g., dolomite). Here, in situ major, trace, and rare earth element geochemistry of Lower Ordovician diagenetic dolomite grains as well as petrography were performed to unravel the geochemical features, the nature, and origin of the diagenetic fluids. The results indicate that different geochemical information can be detected within a single sample, even within a single dolomite grain. Five generations of diagenetic dolomite have been identified based on geochemical signatures, resulting from four distinct types of diagenetic fluids: (1) HREE enrichment (PAAS‐normalized), low ΣREE, no Eu anomaly, low Mn, Ba, moderate Fe, and high Sr contents are probably due to early burial dolomitizing fluids; (2) MREE enrichment, high ΣREE, high Mn, Fe, and low Sr content are likely to be associated with Devonian deep‐circulating crustal hydrothermal fluids; (3) flat or LREE enrichment pattern with obviously positive Eu anomaly is inferred to be linked to Permian magmatic hydrothermal fluids; and (4) flat REE pattern, moderate ΣREE, no Eu anomaly, low Mn, Ba, moderate Fe, and high Sr contents are probably due to late burial dolomitizing fluids. The significances of in situ method demonstrated in this study, compared with the whole rock analysis, include not only contamination‐free analysis but also unraveling the internal geochemical variation within a single sample or a mineral grain. Thus, for the geochemical study of complicated diagenetic process, in situ method should be preferentially considered.
      PubDate: 2014-07-10T15:39:15.080718-05:
      DOI: 10.1002/2013GC005194
       
  • Pb isotope compositions of detrital K‐feldspar grains in the
           upper‐middle Yangtze River system: Implications for sediment
           provenance and drainage evolution
    • Authors: Zengjie Zhang; Shane Tyrrell, Chang'an Li, J. Stephen Daly, Xilin Sun, Qiwen Li
      Pages: 2765 - 2779
      Abstract: The upper‐middle Yangtze River drains the Qiangtang Block, the Songpan‐Ganzi, the Yangtze Craton, and the Qinling‐Dabie orogenic belt. These tectonic units have been shown to have heterogeneous Pb isotopic compositions, which allow this isotope system to be used as a sediment provenance tool. In this study we have employed laser ablation multiple collector inductively coupled plasma mass spectrometry (LA‐MC‐ICPMS) to measure Pb isotope compositions of sand‐sized K‐feldspar grains from the upper‐middle Yangtze River. Data are presented from four major tributaries: Yalongjiang, Minjiang, Jialingjiang, and Hanjiang, as well as from the main Yangtze River near Yichang. A portion of K‐feldspar grains in the Yalongjiang shows an ultraradiogenic character (206Pb/204Pb > 20), which is unique in the upper‐middle Yangtze. Moreover, these ultraradiogenic grains were transported as far as Yichang, just downstream from the Three Gorges, suggesting that the Pb‐in‐K‐feldspar method could be applied to the sediments within the Jianghan Basin to date the formation of the Three Gorges. Pb isotopic data from Yichang indicate that erosion in the Longmen Shan and neighboring regions is more important than the Jinshajiang in supplying sediment. The grains in Hanoi Basin have little overlap with the Songpan‐Ganzi, but show a good match with the Yangtze Craton in its range of lower 206Pb/204Pb ratios. These observations support the idea that the “Middle Yangtze” used to be a tributary of the paleo‐Red River and that there has been no drainage linking the Songpan‐Ganzi and the Red River since the Eocene.
      PubDate: 2014-07-12T06:23:36.759815-05:
      DOI: 10.1002/2014GC005391
       
  • Variable remanence acquisition efficiency in sediments containing biogenic
           and detrital magnetites: Implications for relative paleointensity signal
           recording
    • Authors: Tingping Ouyang; David Heslop, Andrew P. Roberts, Chengjing Tian, Zhaoyu Zhu, Yan Qiu, Xuechao Peng
      Pages: 2780 - 2796
      Abstract: Widespread geological preservation of biogenic magnetite makes it important to assess how such particles contribute to sedimentary paleomagnetic signals. We studied a sediment core from the South China Sea that passes the strict empirical criteria for magnetic “uniformity” used in relative paleointensity studies. Such assessments are based routinely on bulk magnetic parameters that often fail to enable identification of mixed magnetic mineral assemblages. Using techniques that enable component‐specific magnetic mineral identification, we find that biogenic and detrital magnetites occur in approximately equal concentrations within the studied sediments. We analyzed normalized remanence signals associated with the two magnetite components to assess whether co‐occurring biogenic and detrital magnetites record geomagnetic information in the same way and with the same efficiency. Paleomagnetic directions for the two components have no phase lag, which suggests that the biogenic and detrital magnetites acquired their magnetizations at equivalent times. However, we find that the biogenic magnetite is generally 2–4 times more efficient as the detrital magnetite in contributing to the natural remanent magnetization (NRM) despite their approximately equal magnetic contributions. Variations in the concentration and efficiency of remanence acquisition of the two components suggest that a significant part of the NRM is controlled by nongeomagnetic factors that will affect relative paleointensity recording. We recommend that methods suited to the detection of variable recording efficiency associated with biogenic and detrital magnetites should be used on a routine basis in relative paleointensity studies.
      PubDate: 2014-07-12T06:23:39.394157-05:
      DOI: 10.1002/2014GC005301
       
  • Consolidation state of incoming sediments to the Nankai Trough subduction
           zone: Implications for sediment deformation and properties
    • Authors: Hiroko Kitajima; Demian M. Saffer
      Pages: 2821 - 2839
      Abstract: The hydromechanical properties of accreted and underthrust sediments are key parameters controlling the mechanics of earthquakes and the development of fluid pressure in subduction zones. We conducted consolidation tests on sediments from the Philippine Sea Plate (PSP) in the Nankai Trough to understand the consolidation state and hydraulic properties of the incoming sediment section before its incorporation into the subduction zone. We used mudstone and sandstone cores sampled from the Integrated Ocean Drilling Program Nankai Trough Seismogenic Zone Experiment at two reference sites (Site C0011 located on a basement low; and Site C0012 located on a basement high). Our experimental results indicate that most of the mudstone samples are normally consolidated or overconsolidated, with overconsolidation ratios (OCR) ranging from 0.89 to 2.52 at Site C0011 and 0.86 to 3.85 at Site C0012. Higher OCR values at Site C0012, at least at shallow depths, are likely caused by erosional unloading. This implies that Site C0011 may serve as a better geotechnical reference site. We also find that mudstones accreted along the frontal thrust are severely overconsolidated relative to coeval mudstones at Site C0011, which likely reflects enhanced consolidation due to increased horizontal tectonic stress. Sandstones in the incoming section on the PSP exhibit 2–3 orders of magnitude higher in situ permeability than the mudstones, and the siliciclastic sandstone we tested maintains a high permeability at stresses up to at least 70 MPa, suggesting that the sandstones may act as important pathways for drainage or pore pressure translation from depths of several kilometers.
      PubDate: 2014-07-14T11:24:07.52904-05:0
      DOI: 10.1002/2014GC005360
       
  • Why are the δ13Corg values in Phanerozoic black shales more negative
           than in modern marine organic matter?
    • Authors: Philip A. Meyers
      Pages: 3085 - 3106
      Abstract: The δ13Corg values of Phanerozoic black shales average −27‰, whereas those of modern marine organic matter average −20‰. The black shale isotopic values mimic those of continental organic matter, yet their organic geochemical properties mandate that they contain predominantly marine organic matter. Hypotheses that proposed to explain the low δ13C values of black shales include diagenetic losses of isotopically heavier organic matter components, releases of isotopically light carbon from methane clathrates or extensive magmatic events, greater photosynthetic discrimination against 13C during times of higher atmospheric pCO2, and greenhouse climate stratification of the surface ocean that magnified photic zone recycling of isotopically light organic matter. Although the last possibility seems contrary to the vertical mixing that leads to the high productivity of modern oceanic upwelling systems, it is consistent with the strongly stratified conditions that accompanied deposition of the organic carbon‐rich Pliocene‐Pleistocene sapropels of the Mediterranean Sea. Because most Phanerozoic black shales contain evidence of photic zone anoxia similar to the sapropels, well‐developed surface stratification of the oceans was likely involved in their formation. Existence of isotopically light land plant organic matter during several episodes of extensive magmatism that accompanied black shale deposition implies massive release of mantle CO2 that added to the greenhouse conditions that favored oceanic stratification. The 13C depletion common to most Phanerozoic black shales apparently resulted from a greenhouse climate associated with elevated atmospheric pCO2 that led to a strongly stratified ocean and photic zone recycling of organic matter in, augmented by magmatic CO2 releases.
      PubDate: 2014-07-12T06:23:34.09141-05:0
      DOI: 10.1002/2014GC005305
       
 
 
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