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

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Geochemistry, Geophysics, Geosystems     Full-text available via subscription   (Followers: 28, SJR: 2.439, h-index: 91)
Geophysical Research Letters     Full-text available via subscription   (Followers: 121, SJR: 3.323, h-index: 185)
Global Biogeochemical Cycles     Full-text available via subscription   (Followers: 15, SJR: 3.22, h-index: 136)
J. of Advances in Modeling Earth Systems     Open Access   (Followers: 4, SJR: 4.444, h-index: 18)
J. of Geophysical Research : Atmospheres     Partially Free   (Followers: 131)
J. of Geophysical Research : Biogeosciences     Full-text available via subscription   (Followers: 29)
J. of Geophysical Research : Earth Surface     Partially Free   (Followers: 54)
J. of Geophysical Research : Oceans     Partially Free   (Followers: 50)
J. of Geophysical Research : Planets     Full-text available via subscription   (Followers: 111)
J. of Geophysical Research : Solid Earth     Full-text available via subscription   (Followers: 47)
J. of Geophysical Research : Space Physics     Full-text available via subscription   (Followers: 125)
Paleoceanography     Full-text available via subscription   (Followers: 5, SJR: 3.067, h-index: 100)
Radio Science     Full-text available via subscription   (Followers: 39, SJR: 1.072, h-index: 59)
Reviews of Geophysics     Full-text available via subscription   (Followers: 36, SJR: 8.833, h-index: 107)
Space Weather     Full-text available via subscription   (Followers: 17, SJR: 1.341, h-index: 26)
Tectonics     Full-text available via subscription   (Followers: 15, SJR: 2.628, h-index: 96)
Water Resources Research     Full-text available via subscription   (Followers: 81, SJR: 2.661, h-index: 144)
Journal Cover Geochemistry, Geophysics, Geosystems
  [SJR: 2.439]   [H-I: 91]   [28 followers]  Follow
    
   Full-text available via subscription Subscription journal
   ISSN (Online) 1525-2027
   Published by AGU Homepage  [17 journals]
  • Evolution of submarine eruptive activity during the 2011-2012 El Hierro
           event as documented by hydroacoustic images and remotely operated vehicle
           observations
    • Authors: L. Somoza; F.J. González, S.J. Barker, P. Madureira, T. Medialdea, C. de Ignacio, N. Lourenço, R. León, J.T. Vázquez, D. Palomino
      Abstract: Submarine volcanic eruptions are frequent and important events, yet they are rarely observed. Here we relate bathymetric and hydroacoustic images from the 2011-2012 El Hierro eruption with surface observations and deposits imaged and sampled by ROV. As result of the shallow submarine eruption, a new volcano named Tagoro grew from 375 to 89 m depth. The eruption consisted of two main phases of edifice construction intercalated with collapse events. Hydroacoustic images show that the eruptions ranged from explosive to effusive with variable plume types and resulting deposits, even over short time intervals. At the base of the edifice, ROV observations show large accumulations of lava balloons changing in size and type downslope, coinciding with the area where floating lava balloon fallout was observed. Peaks in eruption intensity during explosive phases generated vigorous bubbling at the surface, extensive ash,vesicular lapilli and formed high-density currents, which together with periods of edifice gravitational collapse, produced extensive deep volcaniclastic aprons. Secondary cones developed in the last stages and show evidence for effusive activity with lava ponds and lava flows that cover deposits of stacked lava balloons. Chaotic masses of heterometric boulders around the summit of the principal cone are related to progressive sealing of the vent with decreasing or variable magma supply. Hornitos represent the final eruptive activity with hydrothermal alteration and bacterial mats at the summit. Our study documents the distinct evolution of a submarine volcano and highlights the range of deposit types that may form and be rapidly destroyed in such eruptions.
      PubDate: 2017-07-26T11:42:00.757534-05:
      DOI: 10.1002/2016GC006733
       
  • Paleothermal structure of the Nankai inner accretionary wedge estimated
           from vitrinite reflectance of cuttings
    • Authors: Rina Fukuchi; Asuka Yamaguchi, Yuzuru Yamamoto, Juichiro Ashi
      Abstract: The paleothermal structure and tectonic evolution of an accretionary prism is basic information for understanding subduction zone seismogenesis. To evaluate the entire paleotemperature profile of the Integrated Ocean Drilling Program (IODP) Site C0002 located in the off-Kumano region of the Nankai Trough and penetrates the inner accretionary wedge down to 3058.5 m below the seafloor (mbsf), we performed a vitrinite reflectance analysis for cuttings and core samples during IODP Expeditions 338 and 348: Nankai Trough Seismogenic Zone Experiment. Although vitrinite reflectance values (Ro) tend to increase with depth, two reversals of these values suggested the existence of thrust fault zones with sufficient displacements to offset the paleothermal structure. The estimated maximum paleotemperatures are 42–70°C at 1200–1300 mbsf, 44–100°C at 1600–2400 mbsf, and 56–115°C at 2600–3000 mbsf, respectively. These temperatures roughly coincide with estimated modern temperatures; however, at a smaller scale, the reconstructed partial paleogeothermal gradient (∼60–150°C/km) recorded at the hanging- and footwall of the presumed thrust fault zone is higher than the modern geothermal gradient (∼30–40°C/km). This high paleogeothermal gradient was possibly obtained prior to subduction, reflecting the large heat flow of the young Philippine Sea Plate.
      PubDate: 2017-07-26T10:16:20.237744-05:
      DOI: 10.1002/2017GC006928
       
  • Driving magma to the surface: The 2011-2012 El Hierro volcanic eruption
    • Authors: Carmen López; Maria A. Benito-Saz, Joan Martí, Carmen del-Fresno, Laura García-Cañada, Helena Albert, Héctor Lamolda
      Abstract: We reanalyzed the seismic and deformation data corresponding to the pre-eruptive unrest on El Hierro (Canary Islands) in 2011. We considered new information about the internal structure of the island. We updated the seismic catalogue to estimate the full evolution of the released seismic energy and demonstrate the importance of non-located earthquakes. Using seismic data and GPS displacements, we characterized the shear-tensile type of the predominant fracturing and modelled the strain and stress fields for different time periods. This enabled us to identify a prolonged first phase characterized by hydraulic tensile fracturing, which we interpret as being related to the emplacement of new magma below the volcanic edifice on El Hierro. This was followed by post-injection unidirectional migration, probably controlled by the stress field and the distribution of the structural discontinuities. We identified the effects of energetic magmatic pulses occurring a few days before the eruption that induced shear seismicity on pre-existing faults within the volcano and raised the Coulomb stress over the whole crust. We suggest that these magmatic pulses reflect the crossing of the Moho discontinuity, as well as changes in the path geometry of the dyke migration towards the surface. The final phase involved magma ascent through a pre-fractured crust.
      PubDate: 2017-07-26T10:15:53.731871-05:
      DOI: 10.1002/2017GC007023
       
  • The origin of volatiles in the earth's mantle
    • Authors: Saswata Hier-Majumder; Marc M. Hirschmann
      Abstract: The Earth's deep interior contains significant reservoirs of volatiles such as H, C, and N. Due to the incompatible nature of these volatile species, it has been difficult to reconcile their storage in the residual mantle immediately following crystallization of the terrestrial magma ocean (MO). As the magma ocean freezes, it is commonly assumed, very small amounts of melt is retained in the residual mantle, limiting the trapped volatile concentration in the primordial mantle. In this article, we show that inefficient melt drainage out of the freezing front can retain large amounts of volatiles hosted in the trapped melt in the residual mantle while creating a thick early atmosphere. Using a two-phase flow model, we demonstrate that compaction within the moving freezing front is inefficient over time scales characteristic of magma ocean solidification. We employ a scaling relation between the trapped melt fraction, the rate of compaction, and the rate of freezing in our magma ocean evolution model. For cosmochemically plausible fractions of volatiles delivered during the later stages of accretion, our calculations suggest that up to 77% of total H2O and 12% of CO2 could have been trapped in the mantle during magma ocean crystallization. The assumption of a constant trapped melt fraction underestimates the mass of volatiles in the residual mantle by more than an order of magnitude.
      PubDate: 2017-07-26T10:15:24.870311-05:
      DOI: 10.1002/2017GC006937
       
  • Micrometer-scale magnetic imaging of geological samples using a quantum
           diamond microscope
    • Authors: D. R. Glenn; R. R. Fu, P. Kehayias, D. Le Sage, E. A. Lima, B. P. Weiss, R. L. Walsworth
      Abstract: Remanent magnetization in geological samples may record the past intensity and direction of planetary magnetic fields. Traditionally, this magnetization is analyzed through measurements of the net magnetic moment of bulk millimeter to centimeter sized samples. However, geological samples are often mineralogically and texturally heterogeneous at submillimeter scales, with only a fraction of the ferromagnetic grains carrying the remanent magnetization of interest. Therefore, characterizing this magnetization in such cases requires a technique capable of imaging magnetic fields at fine spatial scales and with high sensitivity. To address this challenge, we developed a new instrument, based on nitrogen-vacancy centers in diamond, which enables direct imaging of magnetic fields due to both remanent and induced magnetization, as well as optical imaging, of room-temperature geological samples with spatial resolution approaching the optical diffraction limit. We describe the operating principles of this device, which we call the quantum diamond microscope (QDM), and report its optimized image-area-normalized magnetic field sensitivity (20 µT⋅µm/Hz½), spatial resolution (5 µm), and field of view (4 mm), as well as trade-offs between these parameters. We also perform an absolute magnetic field calibration for the device in different modes of operation, including three-axis (vector) and single-axis (projective) magnetic field imaging. Finally, we use the QDM to obtain magnetic images of several terrestrial and meteoritic rock samples, demonstrating its ability to resolve spatially distinct populations of ferromagnetic carriers.
      PubDate: 2017-07-26T10:11:12.790832-05:
      DOI: 10.1002/2017GC006946
       
  • Uranium isotope evidence for an expansion of marine anoxia during the
           end-Triassic extinction
    • Authors: Adam B. Jost; Aviv Bachan, Bas van de Schootbrugge, Kimberly V. Lau, Karrie L. Weaver, Kate Maher, Jonathan L. Payne
      Abstract: The end-Triassic extinction coincided with an increase in marine black shale deposition and biomarkers for photic zone euxinia, suggesting that anoxia played a role in suppressing marine biodiversity. However, global changes in ocean anoxia are difficult to quantify using proxies for local anoxia. Uranium isotopes (δ238U) in CaCO3 sediments deposited under locally well-oxygenated bottom waters can passively track seawater δ238U, which is sensitive to the global areal extent of seafloor anoxia due to preferential reduction of 238U(VI) relative to 235U(VI) in anoxic marine sediments. We measured δ238U in shallow-marine limestones from two stratigraphic sections in the Lombardy Basin, northern Italy, spanning over 400 m. We observe a ∼0.7‰ negative excursion in δ238U beginning in the lowermost Jurassic, coeval with the onset of the initial negative δ13C excursion and persisting for the duration of subsequent high δ13C values in the lower-middle Hettangian Stage. The δ238U excursion cannot be realistically explained by local mixing of uranium in primary marine carbonate and reduced authigenic uranium. Based on output from a forward model of the uranium cycle, the excursion is consistent with a 40–100-fold increase in the extent of anoxic deposition occurring worldwide. Additionally, relatively constant uranium concentrations point towards increased uranium delivery to the oceans from continental weathering, which is consistent with weathering-induced eutrophication following the rapid increase in pCO2 during emplacement of the Central Atlantic Magmatic Province. The relative timing and duration of the excursion in δ238U implies that anoxia could have delayed biotic recovery well into the Hettangian stage.
      PubDate: 2017-07-26T10:10:23.385348-05:
      DOI: 10.1002/2017GC006941
       
  • Evidence of a modern deep-water magmatic hydrothermal system in the Canary
           Basin (Eastern Central Atlantic Ocean)
    • Authors: T. Medialdea; L. Somoza, F.J. González, J.T. Vázquez, C. de Ignacio, H. Sumino, O. Sánchez-Guillamón, Y. Orihashi, R. León, D. Palomino
      Abstract: New seismic profiles, bathymetric data and sediment-rock sampling document for the first time the discovery of hydrothermal vent complexes and volcanic cones at 4800-5200 m depth related to recent volcanic and intrusive activity in an unexplored area of the Canary Basin (Eastern Atlantic Ocean, 500 km west of the Canary Islands). A complex of sill intrusions is imaged on seismic profiles showing saucer-shaped, parallel or inclined geometries. Three main types of structures are related to these intrusions. Type I consists of cone-shaped depressions developed above inclined sills interpreted as hydrothermal vents. Type II is the most abundant and is represented by isolated or clustered hydrothermal domes bounded by faults rooted at the tips of saucer-shaped sills. Domes are interpreted as seabed expressions of reservoirs of CH4- and CO2-rich fluids formed by degassing and contact metamorphism of organic-rich sediments around sill intrusions. Type III are hydrothermal-volcanic complexes originated above stratified or branched inclined sills connected by a chimney to the seabed volcanic edifice. Parallel sills sourced from the magmatic chimney formed also domes surrounding the volcanic cones. Core and dredges revealed that these volcanoes, which must be among the deepest in the world, are constituted by OIB-type, basanites with an outer ring of blue-green hydrothermal Al-rich smectite muds. Magmatic activity is dated, based on lava samples, at 0.78±0.05 and 1.61±0.09 Ma (K/Ar methods) and on tephra layers within cores at 25-237 ky. The Subvent hydrothermal-volcanic complex constitutes the first modern system reported in deep-water oceanic basins related to intraplate hotspot activity.
      PubDate: 2017-07-21T11:42:22.156145-05:
      DOI: 10.1002/2017GC006889
       
  • Temporal variations in air permeability and soil CO2 flux in volcanic ash
           soils (island of Vulcano, Italy)
    • Authors: Marco Camarda; Vincenzo Prano, Santo Cappuzzo, Sergio Gurrieri, Mariano Valenza
      Abstract: Air permeability is a major physical factor affecting the advective transport of a gas through the soil, and variations in this parameter can strongly influence the emission of endogenous gases from the soil to the atmosphere. In this paper we illustrated a new and simple method for measuring in situ air permeability based on the measurement of air pressure inside a special probe inserted into the soil. The method was designed and developed primarily to study the relationship between air permeability and the soil CO2 flux in an active volcanic area. The method was used for continuous monitoring of the air permeability at two different locations on the island of Vulcano. At the same time, the values of the atmospheric pressure, temperature, rain and volumetric water content of the soil were also acquired to investigate their effect on soil air permeability and soil CO2 flux. The results showed that during the monitoring period, soil air permeability exhibited minor variations at each site, while larger variations in the soil CO2 flux were recorded. The effect of soil air permeability on soil CO2 flux was negligible at both sites, whereas a strong dependence of soil CO2 flux on volumetric water content and on atmospheric pressure was found. Furthermore, the variation in air permeability recorded at both sites was much lower than that predicted using some well-known predictive models, showing that the relationship among different soil transport parameters is more complex in real field conditions than would be expected by semi-empirical models.
      PubDate: 2017-07-21T11:21:03.533967-05:
      DOI: 10.1002/2017GC006857
       
  • A comparison of cooling- and volume-limited flow systems: Examples from
           channels in the Piton de la Fournaise April 2007 lava flow field
    • Authors: Maéva Rhéty; Andrew Harris, Nicolas Villeneuve, Lucia Gurioli, Etienne Médard, Oryaëlle Chevrel, Patrick Bachélery
      Abstract: Channel-fed lava flow systems lack detailed thermal and textural studies through the boundary between channelized and dispersed flow, and out to the flow front. Here, chemical, textural and morphological analyses were made to define cooling and crystallisation rates down the entire system, especially through the zone of dispersed flow. We compare two channel systems active during the 2007 eruption of Piton de la Fournaise, one of which was cooling-limited and one of which as volume-limited. In the cooling-limited case, rapid changes in rheology occurred across the zone of dispersed flow, where viscosity increased from 1000 to 1600 Pa s over the last 100 m of the channel system. This was due to an increase in cooling rate from 7°C km−1 over the first 500 m of the system, to 42°C km−1 over the last 100 m, and an increase in microcryst content from 13% to 25%. In the volume-limited case, the exponentially increasing segment of the down flow cooling and viscosity trend is absent. Instead, lava arriving at the flow front is still relatively hot (1161°C compared with a near-vent temperature of 1167°C), and is thus of relatively low viscosity (1125 Pa s). In the volume-limited case, because the channel was still in extension when supply to the system was cut, the zone of dispersed flow was extremely short. However, because lava behind the stalled flow front was still hot and fluid, break-outs from the static front resulted in a complex flow front morphology.
      PubDate: 2017-07-21T11:20:48.713077-05:
      DOI: 10.1002/2017GC006839
       
  • Link between Indian monsoon rainfall and physical erosion in the Himalayan
           system during the Holocene
    • Authors: Ronan Joussain; Zhifei Liu, Christophe Colin, Stéphanie Duchamp-Alphonse, Zhaojie Yu, Eva Moréno, Léa Fournier, Sébastien Zaragosi, Arnaud Dapoigny, Laure Meynadier, Franck Bassinot
      Abstract: Mineralogical and geochemical analyses conducted on cores located on the active channel-levee system of the northern Bengal Fan are used to establish changes in the weathering pattern and the sediment transport of the Himalayan system, and evaluate the effect of Indian summer monsoon rainfall during the Holocene. Our data indicate that during the Holocene, sediments from the northern Bengal Fan originate mainly from the G-B river system without any significant changes in the relative contribution of these rivers. From 9.8 to around 6 ka, relatively low smectite/(illite+chlorite) ratios and relatively high K/Si* ratios indicate high physical denudation rates of the Himalayan highlands together with a rapid transfer of the detrital material to the Bengal Fan. The period between 9.2 and 7 ka is associated to lower values of K/Si* and corresponds to the maximum of Indian monsoon rainfall which indicates a more important chemical weathering material that rapidly transits by the G-B river system without a long storage in the Indo-Gangetic plain. From 6.0 ka to present day, higher smectite/(illite+chlorite) ratio and lower K/Si* ratio document a gradual increase of sediments originated from the Indo-Gangetic plain, characterized by higher degree of chemical weathering. During the last 2.5 ka, the drastic increase in the smectite/(illite+chlorite) ratio could be associated to enhanced alteration of the plain soils due to anthropogenic activity. The comparison of mineralogical and geochemical data with previous reconstructions of the Indian monsoon dynamic indicates a rapid response of erosion and sediment transfer of the G-B river system to changes of monsoon rainfall intensity.
      PubDate: 2017-07-21T11:20:33.915601-05:
      DOI: 10.1002/2016GC006762
       
  • Constraints on the anisotropic contributions to velocity discontinuities
           at ∼60 km depth beneath the Pacific
    • Authors: Catherine A. Rychert; Nicholas Harmon
      Abstract: Strong, sharp, negative seismic discontinuities, velocity decreases with depth, are observed beneath the Pacific seafloor at ∼60 km depth. It has been suggested that these are caused by an increase in radial anisotropy with depth, which occurs in global surface wave models. Here we test this hypothesis in two ways. We evaluate whether an increase in surface wave radial anisotropy with depth is robust with synthetic resolution tests. We do this by fitting an example surface wave dataset near the East Pacific Rise. We also estimate the apparent isotropic seismic velocity discontinuities that could be caused by changes in radial anisotropy in S-to-P and P-to-S receiver functions and SS precursors using synthetic seismograms. We test one model where radial anisotropy is caused by olivine alignment and one model where it is caused by compositional layering. The result of our surface wave inversion suggests strong shallow azimuthal anisotropy beneath 0-10 Ma seafloor, which would also have a radial anisotropy signature. An increase in radial anisotropy with depth at 60 km depth is not well-resolved in surface wave models, and could be artificially observed. Shallow isotropy underlain by strong radial anisotropy could explain moderate apparent velocity drops (< 6%) in SS precursor imaging, but not receiver functions. The effect is diminished if strong anisotropy also exists at 0 – 60 km depth as suggested by surface waves. Overall, an increase in radial anisotropy with depth may not exist at 60 km beneath the oceans and does not explain the scattered wave observations.
      PubDate: 2017-07-21T04:49:49.228612-05:
      DOI: 10.1002/2017GC006850
       
  • Production, consumption, and migration of methane in accretionary prism of
           southwestern Taiwan
    • Authors: Nai-Chen Chen; Tsanyao Frank Yang, Wei-Li Hong, Hsuan-Wen Chen, Hsiao-Chi Chen, Chin-Yi Hu, Yu-Chun Huang, Saulwood Lin, Li-Hung Lin, Chih-Chieh Su, Wei-Zhi Liao, Chih-Hsien Sun, Pei-Ling Wang, Tao Yang, Shao-yong Jiang, Char-Shine Liu, Yunshuen Wang, San-Hsiung Chung
      Abstract: To systematically quantify the production, consumption and migration of methane along a continental margin, 210 sediment cores were collected from offshore southwestern Taiwan and analyzed for their gas and aqueous geochemistry. These data, combined with published results, were used to calculate the diffusive methane fluxes across different geochemical transitions and to develop scenarios of mass balance to constrain deep microbial and thermogenic methane production rates within the accretionary prism. The results showed that methane diffusive fluxes ranged from 2.71 × 10−3 to 2.78 × 10−1 and from ‒1.88 × 10−1 to 3.97 mmol m−2 d−1 at the sulfate-methane-transition-zone (SMTZ) and sediment-seawater interfaces, respectively. High methane fluxes tend to be associated with structural features, suggesting a strong structural control on the methane transport. A significant portion of ascending methane (>50%) is consumed by anaerobic oxidation of methane at the SMTZ at most sites, indicating effective biological filtration. Gas compositions and isotopes revealed a transition from the predominance of microbial methane in the passive margin to thermogenic methane at the upper slope of the active margin and onshore mud volcanoes. Methane production and consumption at shallow depths were nearly offset with a small fraction of residual methane discharged into seawater. The flux imbalance arose primarily due to the larger production of methane through deep microbial and thermogenic processes at a magnitude of 1,512 – 43,096 Tg Myr−1 methane and could be likely accounted for by the sequestration of methane into hydrate forms, and clay absorption.
      PubDate: 2017-07-15T03:24:03.733573-05:
      DOI: 10.1002/2017GC006798
       
  • Modeling phase separation and phase change for magma ocean solidification
           dynamics
    • Authors: C.-E. Boukaré; Y. Ricard
      Abstract: Just after accretion, the Earth's mantle was significantly molten by the heat dissipation due to large impacts and to the segregation of the core. The mineralogical observations and thermodynamics models of solid-liquid equilibrium of silicates show that several types of crystallization may have happened at different depths in the mantle. Solids were probably formed first at the bottom of the lower mantle or at mid mantle leaving two possible magma oceans, a shallow one and an abyssal one. Near the bottom of the mantle, the liquid phase might become denser than solids due to iron enrichment. In the shallow magma ocean, the crystallizing solid phase was denser and sank through the magma to settle and compact at depth. To understand these complex dynamics, we develop a two phase numerical code that can handle simultaneously convection in each phase and in the slurry, and the compaction or decompaction of the two phases. Although our code can only run in a parameter range (Rayleigh number, viscosity contrast between phases, Prandlt number) far from what would be realistic, we think it already provides a rich dynamics that illustrates what could have happened. We show situations in which the crystallization front is gravitationally stable and situations were the newly formed solids are gravitationally unstable and can snow across the magma. Our study suggests that the location of a density contrast between solid and magma must be considered of equal importance with that of the intersection between liquidus and isentrope for what concerns mantle solidification.
      PubDate: 2017-07-15T03:23:53.619128-05:
      DOI: 10.1002/2017GC006902
       
  • How plume-ridge interaction shapes the crustal thickness pattern of the
           Réunion hotspot track
    • Authors: Eva Bredow; Bernhard Steinberger, Rene Gassmöller, Juliane Dannberg
      Abstract: The Réunion mantle plume has shaped a large area of the Earth's surface over the past 65 million years: from the Deccan Traps in India along the hotspot track comprising the island chains of the Laccadives, Maldives and Chagos Bank on the Indian plate and the Mascarene Plateau on the African plate up to the currently active volcanism at La Réunion Island. This study addresses the question how the Réunion plume, especially in interaction with the Central Indian Ridge, created the complex crustal thickness pattern of the hotspot track. For this purpose, the mantle convection code ASPECT was used to design three-dimensional numerical models, which consider the specific location of the plume underneath moving plates and surrounded by large-scale mantle flow. The results show the crustal thickness pattern produced by the plume, which altogether agrees well with topographic maps. Especially two features are consistently reproduced by the models: the distinctive gap in the hotspot track between the Maldives and Chagos is created by the combination of the ridge geometry and plume-ridge interaction; and the Rodrigues Ridge, a narrow crustal structure which connects the hotspot track and the Central Indian Ridge, appears as the surface expression of a long-distance sub-lithospheric flow channel. This study therefore provides further insight how small-scale surface features are generated by the complex interplay between mantle and lithospheric processes.
      PubDate: 2017-07-15T03:23:22.358627-05:
      DOI: 10.1002/2017GC006875
       
  • Forearc seafloor unconformities and geology: Insight from 3D seismic
           geomorphology analysis, Peru
    • Authors: Gérôme Calvès; Constance Auguy, Léopold de Lavaissière, Stéphane Brusset, Ysabel Calderon, Patrice Baby
      Abstract: New 3D seismic data collected over 4870 km2 in the 3°45'–12°30'S Peruvian segment of the East Pacific subduction system image seafloor erosional surfaces that can be mapped across the forearc basins. Forearc basins experience various stresses, from their base where basal tectonic erosion acts to the seafloor which is influenced by aerial, shallow and deep water currents driven by waves or thermohaline oceanic currents. Previously there has been little interest in stresses on the upper layer and there is a lack of documentation of unconformities and the erosive processes in certain bathymetric domains in forearc basins. We address this with the study of examples sourced from 3D seismic reflection surveys of the seafloor offshore Peru. Unconformities occur in two distinctive bathymetric domains associated with the continental shelf and the upper slope of the margin. Identification and characterization of unconformity surfaces yield estimates of the amount of erosion at the modern seafloor that range from 18 to 100%. Regional physical oceanography allows us to calibrate potential candidates for these two distinctive domains. The first control on erosion is the dynamics of deep to intermediate oceanic currents related to the Humboldt-Peru Chile water masses, while the second is wave action in the shallower erosional surfaces. This study illustrates the unseen landscape of the forearc basins of South America and helps to highlight the importance of erosive surficial processes in subduction landscapes.
      PubDate: 2017-07-15T03:22:13.043249-05:
      DOI: 10.1002/2017GC007036
       
  • Spreading rate-dependent variations in crystallization along the global
           mid-ocean ridge system
    • Authors: V.D. Wanless; Mark D. Behn
      Abstract: We investigate crustal accretion at mid-ocean ridges by combining crystallization pressures calculated from major element contents in mid-ocean ridge basalt (MORB) glasses and vapor-saturation pressures from melt inclusions and MORB glasses. Specifically, we use established major element barometers and pressures estimated from 192 fractional crystallization trends to calculate crystallization pressures from>9,000 MORB glasses across the global range of mid-ocean ridge spreading rates. Additionally, we estimate vapor-saturation pressures from>400 MORB glasses from PETDB and>400 olivine-hosted melt inclusions compiled from five ridges with variable spreading rates. Both major element and vapor-saturation pressures increase and become more variable with decreasing spreading rate. Vapor saturation pressures indicate that crystallization occurs in the lower crust and upper mantle at all ridges, even when a melt lens is present. We suggest that the broad peaks in major element crystallization pressures at all spreading rates reflects significant crystallization of on- and off-axis magmas along the base of a sloping lithosphere. Combining our observations with ridge thermal models we show that crystallization occurs over a range of pressures at all ridges, but it is enhanced at thermal/rheologic boundaries, such as the melt lens and the base of the lithosphere. Finally, we suggest that the remarkable similarity in the maximum vapor-saturation pressures (∼3 kbars) recorded in melt inclusions from a wide range of spreading rates reflects a relatively uniform CO2 content of 50–85 ppm for the depleted upper mantle feeding the global mid-ocean ridge system.
      PubDate: 2017-07-15T03:15:44.307403-05:
      DOI: 10.1002/2017GC006924
       
  • Distinct control mechanism of fine-grained sediments from Yellow River and
           Kyushu supply in the northern Okinawa Trough since the last glacial
    • Authors: Debo Zhao; Shiming Wan, Samuel Toucanne, Peter D. Clift, Ryuji Tada, Sidonie Révillon, Yoshimi Kubota, Xufeng Zheng, Zhaojie Yu, Jie Huang, Hanchao Jiang, Zhaokai Xu, Xuefa Shi, Anchun Li
      Abstract: High-resolution multi-proxy records, including clay minerals and Sr-Nd-Pb isotopes of the clay-sized silicate fraction of sediments from IODP Site U1429 in the northern Okinawa Trough, provide reliable evidence for distinct control mechanism on fine-grained sediments input from the Yellow River and the southern Japanese Islands to the northern Okinawa Trough since 34 ka BP. Provenance analysis indicates that the sediments were mainly derived from the Yellow River and the island of Kyushu. Since the last glacial, clay-sized sediments transported from the Yellow River to the study site were strongly influenced by sea-level fluctuation. During low sea-level stage (∼34‒14 ka BP), the paleo-Yellow River mouth was positioned closer to the northern Okinawa Trough, favoring large fluvial discharge or even direct input of detrital sediments, which resulted about four times more flux of clay-sized sediments supply to the study area as during the relatively high sea-level stage (∼14‒0 ka BP). The input of Kyushu-derived clay-sized sediments to the study site was mainly controlled by the Kuroshio Current and Tsushima Warm Current intensity, with increased input in phase with weakened Kuroshio Current/Tsushima Warm Current. Our study suggests that the Kuroshio Current was very likely flowed into the Okinawa Trough and thus influenced the fine-grained sediment transport in the area throughout the last glacial and deglacial. During ∼34‒11 ka BP, the Kyushu clay-sized sediment input was mainly controlled by the Kuroshio Current. Since ∼11 ka BP, the occurrence of Tsushima Warm Current became important in influencing the Kyushu fine-grained sediment input to the northern Okinawa Trough.
      PubDate: 2017-07-15T03:15:37.975514-05:
      DOI: 10.1002/2016GC006764
       
  • High-resolution chronology of sediment below CCD based on Holocene
           paleomagnetic secular variations in the Tohoku-oki earthquake rupture zone
           
    • Authors: Toshiya Kanamatsu; Kazuko Usami, Cecilia M. G. McHugh, Ken Ikehara
      Abstract: Using high-resolution paleomagnetic data, we examined the potential for obtaining precise ages from sediment core samples recovered from deep-sea basins close to rupture zones of the 2011 and earlier earthquakes off Tohoku, Japan. Obtaining detailed stratigraphic ages from deep-sea sediments below the calcium compensation depth (CCD) is difficult, but we found that the samples contain excellent paleomagnetic secular variation records to constrain age models. Variations in paleomagnetic directions obtained from the sediments reveal systematic changes in the cores. A stacked paleomagnetic profile closely matches the Lake Biwa data sets in southwest Japan for the past 7,000 years, one can establish age models based on secular variations of the geomagnetic field on sediments recovered uniquely below the CCD. Comparison of paleomagnetic directions near a tephra and a paleomagnetic direction of contemporaneous pyroclastic flow deposits acquired by different magnetization processes shows precise depositional ages reflecting the magnetization delay of the marine sediment record.
      PubDate: 2017-07-06T01:00:57.599035-05:
      DOI: 10.1002/2017GC006878
       
  • Unmixing detrital geochronology age distributions
    • Authors: Kurt Sundell; Joel E. Saylor
      Abstract: Despite recent advances in quantitative methods of detrital provenance analysis, there is currently no widely accepted method of unmixing detrital geochronology data. We developed a mixing model that determines mixing proportions for source samples through inverse Monte Carlo modeling, wherein mixed samples are compared to randomly generated combinations of source distributions, and a range of best mixing proportions are retained. Results may then be used to constrain a forward optimization routine to find a single best-fit mixture. Quantitative comparison is based on the Kolmogorov-Smirnov (KS) test D statistic and Kuiper test V statistic for cumulative distribution functions, and the Cross-correlation coefficient for finite mixture distributions (probability density plots or kernel density estimates). We demonstrate the capacity of this model through a series of tests on synthetic data sets and a published empirical data set from North America mixed in known proportions; this proof-of-concept testing shows the model is capable of accurately unmixing highly complex distributions. We apply the model to two published empirical data sets mixed in unknown proportions from Colombia and central China. Neither data set yields perfect model fits, which provides a cautionary note of potentially inadequate characterization of source and/or mixed samples, and highlights the importance of such characterization for accurate interpretation of sediment provenance. Data set size appears to be a major control on mixture model results; small (n 
      PubDate: 2017-07-06T00:56:06.871737-05:
      DOI: 10.1002/2016GC006774
       
  • Origin of primitive ocean island basalts by crustal gabbro assimilation
           and multiple recharge of plume-derived melts
    • Authors: Anastassia Y. Borisova; Wendy A. Bohrson, Michel Grégoire
      Abstract: Chemical Geodynamics relies on a paradigm that the isotopic composition of ocean island basalt (OIB) represents equilibrium with its primary mantle sources. However, the discovery of huge isotopic heterogeneity within olivine-hosted melt inclusions in primitive basalts from Kerguelen, Iceland, Hawaii and South Pacific Polynesia islands implies open-system behavior of OIBs, where during magma residence and transport, basaltic melts are contaminated by surrounding lithosphere. To constrain the processes of crustal assimilation by OIBs, we employed the Magma Chamber Simulator (MCS), an energy-constrained thermodynamic model of recharge, assimilation and fractional crystallization. For a case study of the 21 – 19 Ma basaltic series, the most primitive series ever found among the Kerguelen OIBs, we performed sixty-seven simulations in the pressure range from 0.2 to 1.0 GPa using compositions of olivine-hosted melt inclusions as parental magmas, and metagabbro xenoliths from the Kerguelen Archipelago as wallrock. MCS modeling requires that the assimilant is anatectic crustal melts (P2O5 ≤ 0.4 wt.% contents) derived from the Kerguelen oceanic metagabbro wallrock. To best fit the phenocryst assemblage observed in the investigated basaltic series, recharge of relatively large masses of hydrous primitive basaltic melts (H2O = 2 – 3 wt%; MgO = 7 – 10 wt.%) into a middle crustal chamber at 0.2 to 0.3 GPa is required. Our results thus highlight the important impact that crustal gabbro assimilation and mantle recharge can have on the geochemistry of mantle-derived olivine-phyric OIBs. The importance of crustal assimilation affecting primitive plume-derived basaltic melts underscores that isotopic and chemical equilibrium between ocean island basalts and associated deep plume mantle source(s) may be the exception rather than the rule.
      PubDate: 2017-07-06T00:55:24.463886-05:
      DOI: 10.1002/2017GC006986
       
  • Toward a new paradigm for Boulder dislodgement during storms
    • Authors: Robert Weiss; Alex Sheremet
      Abstract: Boulders are an important coastal hazard event deposit because they can only be moved by tsunamis and energetic storms effects of storms. Storms and tsunami are competing processes for coastal change along many shorelines. Therefore, distinguishing the boulders that were moved during a storm from those moved by a tsunami is important. In this contribution, we present the results of a parameter study based on the TRIADS model for wave shoaling on mildly sloping beaches, coupled with a boulder dislodgement model that is based on Newton's Second Law of Motion. The results show how smaller slopes expose the waves longer to the nonlinear processes, thus increasing the energy in the infragravity wave band. More energy in the infragravity wave band means that there is more energy wave lengths that can dislodge larger boulders. At the same time, a steeper slope lowers the threshold for boulder dislodgement (critical angle of dislodgement), making it more likely for larger boulders to be dislodged on a steeper slope. The competition between these two processes govern boulder dislodgement during storms and is investigated inhere.
      PubDate: 2017-07-05T12:46:51.939691-05:
      DOI: 10.1002/2017GC006926
       
  • Geothermal heat flux in the Amundsen Sea sector of West Antarctica: New
           insights from temperature measurements, depth to the bottom of the
           magnetic source estimation, and thermal modeling
    • Authors: R. Dziadek; K. Gohl, A. Diehl, N. Kaul
      Abstract: Focused research on the Pine Island and Thwaites glaciers, which drain the West Antarctic Ice Shelf (WAIS) into the Amundsen Sea Embayment (ASE), revealed strong signs of instability in recent decades that result from variety of reasons, such as inflow of warmer ocean currents and reverse bedrock topography and has been established as the Marine Ice Sheet Instability hypothesis. Geothermal heat flux (GHF) is a poorly constrained parameter in Antarctica and suspected to affect basal conditions of ice sheets, i.e. basal melting and subglacial hydrology. Thermomechanical models demonstrate the influential boundary condition of geothermal heat flux for (paleo) ice sheet stability. Due to a complex tectonic and magmatic history of West Antarctica, the region is suspected to exhibit strong heterogeneous geothermal heat flux variations. We present an approach to investigate ranges of realistic heat fluxes in the ASE by different methods, discuss direct observations, and 3D numerical models that incorporate boundary conditions derived from various geophysical studies, including our new Depth to the Bottom of the Magnetic Source (DBMS) estimates. Our in-situ temperature measurements at 26 sites in the ASE more than triples the number of direct GHF observations in West Antarctica. We demonstrate by our numerical 3D models that GHF spatially varies from 68 mWm−2 up to 110 mWm−2.
      PubDate: 2017-07-05T12:46:47.870722-05:
      DOI: 10.1002/2016GC006755
       
  • The mechanics of shallow magma reservoir outgassing
    • Authors: A. Parmigiani; W Degruyter, S. Leclaire, C. Huber, O. Bachmann
      Abstract: Magma degassing fundamentally controls the Earth's volatile cycles. The large amount of gas expelled into the atmosphere during volcanic eruptions (i.e. volcanic outgassing) is the most obvious display of magmatic volatile release. However, owing to the large intrusive:extrusive ratio, and considering the paucity of volatiles left in intrusive rocks after final solidification, volcanic outgassing likely constitutes only a small fraction of the overall mass of magmatic volatiles released to the Earth's surface. Therefore, as most magmas stall on their way to the surface, outgassing of uneruptible, crystal-rich magma storage regions will play a dominant role in closing the balance of volatile element cycling between the mantle and the surface. We use a numerical approach to study the migration of a magmatic volatile phase (MVP) in crystal-rich magma bodies (“mush zones”) at the pore-scale. Our results suggest that buoyancy driven outgassing is efficient over crystal volume fractions between 0.4 and 0.7 (for mm-sized crystals). We parameterize our pore-scale results for MVP migration in a thermo-mechanical magma reservoir model to study outgassing under dynamical conditions where cooling controls the evolution of the proportion of crystal, gas and melt phases and to investigate the role of the reservoir size and the temperature-dependent visco-elastic response of the crust on outgassing efficiency. We find that buoyancy-driven outgassing allows for a maximum of 40-50% volatiles to leave the reservoir over the 0.4-0.7 crystal volume fractions, implying that a significant amount of outgassing must occur at high crystal content (>0.7) through veining and/or capillary fracturing.
      PubDate: 2017-07-05T12:45:54.9002-05:00
      DOI: 10.1002/2017GC006912
       
  • Downstream and seasonal changes of lithium isotope ratios in the
           Ganges-Brahmaputra river system
    • Authors: Takuya Manaka; Daisuke Araoka, Toshihiro Yoshimura, H. M. Zakir Hossain, Yoshiro Nishio, Atsushi Suzuki, Hodaka Kawahata
      Abstract: The Li isotope ratio (δ7Li) is expected to be a useful tracer of silicate weathering in river and groundwater systems, which is an important contributor to the seawater compositional changes that accompany the evolution of the Earth's surface environment. To obtain accurate estimates of continental Li fluxes to the ocean, we determined δ7Li values of dissolved Li in the lower Ganges–Brahmaputra river system in both the dry and rainy seasons, and in deep groundwater in the Bengal basin. Dissolved Li and δ7Li values in the lower reaches of the rivers (0.04–0.66 µmol kg−1 and +19.1‰ to +34.2‰, respectively) were predominantly derived from silicate weathering, as is the case in the upper parts of these rivers. We observed large changes in δ7Li over a distance of more than 1000 km downstream that were due mainly to Rayleigh-type removal of Li from river water. Extremely high Li concentrations (1.15–1.67 µmol kg−1) and low δ7Li values (+5.1‰ to +11.6‰) in groundwater samples indicate congruent isotope leaching and dissolution of silicate minerals in the deep aquifer, where the water residence time is long. In the rainy season, Li concentrations and δ7Li values were lower than in the dry season, owing to the shorter residence time of river water and the substantial input of local subsurface flow through lowland alluvium. These results suggest that accurate estimation of continental Li fluxes to the ocean should take account of downstream and seasonal changes, as well as aquifer depth variations, in δ7Li values.
      PubDate: 2017-07-05T12:45:25.256062-05:
      DOI: 10.1002/2016GC006738
       
  • SedCT: MATLABTM tools for standardized and quantitative processing of
           sediment core computed tomography (CT) data collected using a medical CT
           scanner
    • Authors: B. T. Reilly; J. S. Stoner, J. Wiest
      Abstract: Computed Tomography (CT) of sediment cores allows for high resolution images, three dimensional volumes, and down core profiles. These quantitative data are generated through the attenuation of X-rays, which are sensitive to sediment density and atomic number, and are stored in pixels as relative grayscale values or Hounsfield units (HU). We present a suite of MATLABTM tools specifically designed for routine sediment core analysis as a means to standardize and better quantify the products of CT data collected on medical CT scanners. SedCT uses a graphical interface to process Digital Imaging and Communications in Medicine (DICOM) files, stitch overlapping scanned intervals, and create down core HU profiles in a manner robust to normal coring imperfections. Utilizing a random sampling technique, SedCT reduces data size and allows for quick processing on typical laptop computers. SedCTimage uses a graphical interface to create quality tiff files of CT slices that are scaled to a user defined HU range, preserving the quantitative nature of CT images and easily allowing for comparison between sediment cores with different HU means and variance. These tools are presented along with examples from lacustrine and marine sediment cores to highlight the robustness and quantitative nature of this method.
      PubDate: 2017-07-05T12:42:31.646633-05:
      DOI: 10.1002/2017GC006884
       
  • Searching for patterns in caldera unrest
    • Authors: Laura Sandri; Valerio Acocella, Chris Newhall
      Abstract: The ultimate goal of volcanology is forecasting eruptions. This task is particularly challenging at calderas, where unrest is frequent, affects wider areas and its evidence is often masked by the activity of hydrothermal systems. A recent study has compiled a database on caldera unrest, derived from seismicity, geodetic, gravity and geochemical monitoring data at calderas worldwide, from 1988 to 2014. Here we exploit this database, searching for the most recurring features of unrest and, in turn, its possible dynamics. In particular, we focus on: a) the duration of unrest at calderas; b) recurring patterns in unrest; c) unrest episodes culminating in eruptions, including time- or size-predictability and a multivariate regression analysis. Our analysis indicates that pre-eruptive unrest is shorter than non-eruptive unrest, particularly with open or semi-open calderas, calderas with mafic or mixed composition of past eruptive products, or unrest driven by mafic magma; conversely, lack of data on pre-eruptive unrest driven by felsic magma and/or at felsic or plugged calderas prevents an analysis of these specific subsets. In addition, 72% of pre-eruptive unrest lasts
      PubDate: 2017-07-05T12:42:13.844228-05:
      DOI: 10.1002/2017GC006870
       
  • Carbon isotope systematics of Turrialba volcano, Costa Rica, using a
           portable cavity ring-down spectrometer
    • Authors: K. S. Malowany; J. Stix, J. M. de Moor, K. Chu, G. Lacrampe-Couloume, B. Sherwood Lollar
      Abstract: Over the past two decades, activity at Turrialba volcano, Costa Rica, has shifted from hydrothermal to increasingly magmatic in character, with enhanced degassing and eruption potential. We have conducted a survey of the δ13C signatures of gases at Turrialba using a portable field-based CRDS with comparison to standard IRMS techniques. Our δ13C results of the volcanic plume, high temperature vents and soil gases reveal isotopic heterogeneity in the CO2 gas composition at Turrialba prior to its recent phase of eruptive activity. The isotopic value of the regional fault system, Falla Ariete (-3.4±0.1‰), is in distinct contrast with the Central crater gases (-3.9±0.1‰) and the 2012 high temperature vent (-4.4±0.2‰), an indication that spatial variability in δ13C may be linked to hydrothermal transport of volcanic gases, heterogeneities in the source composition, or magmatic degassing. Isotopic values of CO2 samples collected in the plume vary from δ13C of -5.2 to -10.0 ‰, indicative of mixing between atmospheric CO2 (-9.2 ± 0.1‰), and a volcanic source. We compare the Keeling method to a traditional mixing model (hyperbolic mixing curve) to estimate the volcanic source composition at Turrialba from the plume measurements. The predicted source compositions from the Keeling and hyperbolic methods (-3.0±0.5‰ and -3.9±0.4‰, respectively) illustrate two potential interpretations of the volcanic source at Turrialba. As of the 29 October 2014, Turrialba has entered a new eruptive period, and continued monitoring of the summit gases for δ13C should be conducted to better understand the dominant processes controlling δ13C fractionation at Turrialba.
      PubDate: 2017-07-05T12:41:53.934472-05:
      DOI: 10.1002/2017GC006856
       
  • Reconciling magma-ocean crystallization models with the present-day
           structure of the earth's mantle
    • Authors: Maxim D. Ballmer; Diogo L. Lourenço, Kei Hirose, Razvan Caracas, Ryuichi Nomura
      Abstract: Terrestrial planets are thought to experience episode(s) of large-scale melting early in their history. Fractionation during magma-ocean freezing leads to unstable stratification within the related cumulate layers due to progressive iron enrichment upwards, but the effects of incremental cumulate overturns during MO crystallization remain to be explored. Here, we use geodynamic models with a moving-boundary approach to study convection and mixing within the growing cumulate layer, and thereafter within the fully-crystallized mantle. For fractional crystallization, cumulates are efficiently stirred due to subsequent incremental overturns, except for strongly iron-enriched late-stage cumulates, which persist as a stably stratified layer at the base of the mantle for billions of years. Less extreme crystallization scenarios can lead to somewhat more subtle stratification. In any case, the long-term preservation of at least a thin layer of extremely enriched cumulates with Fe#>0.4, as predicted by all our models, is inconsistent with seismic constraints. Based on scaling relationships, however, we infer that final-stage Fe-rich magma-ocean cumulates originally formed near the surface should have overturned as small diapirs, and hence undergone melting and reaction with the host rock during sinking. The resulting moderately iron-enriched metasomatized/hybrid rock assemblages should have accumulated at the base of the mantle, potentially fed an intermittent basal magma ocean, and be preserved through the present-day. Such moderately iron-enriched rock assemblages can reconcile the physical properties of the large low shear-wave velocity provinces in the present-day lower mantle. Thus, we reveal Hadean melting and rock-reaction processes by integrating magma-ocean crystallization models with the seismic-tomography snapshot.
      PubDate: 2017-07-05T12:41:13.987325-05:
      DOI: 10.1002/2017GC006917
       
  • Early diagenetic greigite as an indicator of paleosalinity changes in the
           middle Miocene Paratethys Sea of central Europe
    • Authors: Suzhen Liu; Wout Krijgsman, Mark J. Dekkers, Dan Palcu
      Abstract: The Miocene epicontinental Paratethys Sea of central Eurasia has experienced multiple restriction and reconnection events to the open ocean. Magnetostratigraphy is an important dating tool to better understand the temporal and spatial paleoenvironmental variations associated with these changes. Magnetostratigraphy in the Paratethys domain, however, is complicated by the presence of greigite (Fe3S4). Here, we report rock magnetic and X-ray fluorescence data of the Tisa section (Romania) which was previously magnetostratigraphically dated at the middle Miocene (base at 12.8 Ma and top at 12.2 Ma). This section comprises the Badenian Sarmatian Extinction Event (BSEE), which is marked by a major salinity change from marine to brackish environments, related to the opening of the connection between the Central and the Eastern Paratethys basins. In the marine Badenian sediments below the BSEE, the pyritization process is shown to be complete because of abundant sulfate supply. In the brackish Sarmatian deposits, four intervals with early diagenetic greigite are observed, and linked to insufficient sulfate in the water column. These four greigite intervals appear to correspond to maxima in the ∼100 kyr eccentricity cycle. We propose that increased fresh water from the Eastern Paratethys basin during eccentricity maxima restricted the sulfate availability in the Tisa area, leading to a reduced HS- production and enhanced greigite preservation. The early diagenetic formation of greigite enables a quasi syn-depositional recording of the paleomagnetic field, which allows reliable paleomagnetic dating in this section. Our results further suggest greigite as a potential indicator for salinity changes during marine/brackish transitions.
      PubDate: 2017-07-01T07:33:31.661071-05:
      DOI: 10.1002/2017GC006988
       
  • Dynamic topography and lithospheric stresses since 400 Ma
    • Authors: Marianne Greff-Lefftz; Boris Robert, Jean Besse, Dominique Frizon de Lamotte, Sophie Vicente de Gouveia
      Abstract: We present a global model of dynamic topography and lithospheric stresses for the last 400 Ma. Our starting point is a simple geodynamic model combining both contributions of subducted lithosphere and long wavelength upwellings in a reference frame linked to the fixed african plate. A dominant feature of plate tectonics is the quasi permanence of a girdle of subductions around the Pacific ocean (or its ancestor), which creates large-wavelength positive topography anomaly within the ring they form. The superimposition of the resultant extension with the one induced by the dome leads to a permanent extensional regime over Africa and the future Indian ocean which creates faults with azimuth directions depending on the direction of the most active part of the ring of subductions. We thus obtain fractures with NW-SE azimuth during the period 275-165 Ma parallel to the strike of the subduction zone of the West South American active margin, which appears to be very active during this period. Between 155-95 Ma, subduction became more active along the Eastern Australian coast involving a change in the direction of the faults toward an E-W direction, in agreement with the observed fault systems between Africa and India, Antartica and Australia. During the Mesozoic and the Cenozoic, we correlate the permanent extensional regime over Africa and Indian ocean with the observed rift systems. Finally we emphasize the role of three primary hotspots as local additional contributors to the stress field imposed by our proposed subduction-doming system, which help in the opening of Indian and South Atlantic oceans.
      PubDate: 2017-07-01T02:51:49.960771-05:
      DOI: 10.1002/2017GC006936
       
  • Mantle dynamics beneath the discrete and diffuse plate boundaries of the
           
    • Authors: Joseph S. Byrnes; Douglas R. Toomey, Emilie E. E. Hooft, John Nábělek, Jochen Braunmiller
      Abstract: We use the delay times of teleseismic S phases recorded by ocean bottom seismometers during the plate-scale Cascadia Initiative community experiment to constrain the heterogeneity of seismic velocity structure beneath young oceanic lithosphere. Our study area covers the entire Juan de Fuca (JdF) and Gorda plates, from their creation at the JdF and Gorda Ridges to their subduction beneath the North American continent, and the entire length of the Blanco transform fault. The range of the observed Vs anomalies requires variations in the melt fraction of the asthenosphere. The data require that low Vs anomalies extend to depths of at least 200 km, which is within the carbonatite melting regime. In the upper 200 km of the mantle, Vs increases rapidly to the east of the JdF Ridge, while there is no clear relationship with the age of the lithosphere in the Gorda region. The distribution of melt is asymmetric about both the JdF and Gorda Ridges. Dynamic upwelling – due to the buoyancy of the mantle – and accompanying downwelling can explain the rapid decrease in melt fraction to the east of the JdF Ridge, the asymmetry about the JdF Ridge, and the sinuous pattern of upwelling near the Blanco transform fault. Finally, mantle flow beneath the diffuse Gorda and Explorer plate boundaries is distinct from that beneath the discrete plate boundary of the JdF Ridge. In particular, shear between the Pacific and JdF plates appears to dominate mantle deformation over seafloor spreading beneath the Gorda Ridge.
      PubDate: 2017-07-01T02:46:50.604905-05:
      DOI: 10.1002/2017GC006980
       
  • The North America mid-Cretaceous kimberlite corridor: Wet, edge-driven
           decompression melting of an OIB-type deep mantle source
    • Authors: B.A. Kjarsgaard; L.M. Heaman, C. Sarkar, D.G. Pearson
      Abstract: Thirty new high precision U-Pb perovskite and zircon ages from kimberlites in central North America delineate a corridor of mid-Cretaceous (115 to 92 Ma) magmatism that extends ∼4000 km from Somerset Island in Arctic Canada through central Saskatchewan to Kansas, U.S.A. The least contaminated whole rock Sr, Nd and Hf isotopic data, coupled with Sr isotopic data from groundmass perovskite indicates an exceptionally limited range in Sr-Nd-Hf isotopic compositions, clustering at the low εNd end of the OIB array. These isotopic compositions are distinct from other studied North American kimberlites and point to a sub-lithospheric source region. This mid-Cretaceous kimberlite magmatism cannot be related to mantle plumes associated with the African or Pacific large low-shearwave velocity province (LLSVP). All three kimberlite fields are adjacent to strongly attenuated lithosphere at the edge of the North American craton. This facilitated edge-driven convection, a top-down driven processes that caused decompression melting of the transition zone or overlying asthenosphere. The inversion of ringwoodite and/or wadsleyite and release of H2O, with subsequent metasomatism and synchronous wet partial melting generates a hot CO2- and H2O-rich proto-kimberlite melt. Emplacement in the crust is controlled by local lithospheric factors; all three kimberlite fields have mid-Cretaceous age, re-activated major deep-seated structures that facilitated kimberlite melt transit through the lithosphere.
      PubDate: 2017-07-01T02:46:24.874966-05:
      DOI: 10.1002/2016GC006761
       
  • Epsilon iron oxide: Origin of the high coercivity stable low Curie
           temperature magnetic phase found in heated archeological materials
    • Authors: J. López-Sánchez; G. McIntosh, M. L. Osete, A. del Campo, J. J. Villalaín, L. Perez, M. Kovacheva, O. Rodriguez de la Fuente
      Abstract: The identification of epsilon iron oxide (ε-Fe2O3) as the low Curie temperature high coercivity stable phase (HCLST) carrying the remanence in heated archaeological samples has been achieved in samples from two archaeological sites that exhibited the clearest evidence of the presence of the HCSLT. This uncommon iron oxide has been detected by Confocal Raman Spectroscopy (CRS) and characterized by rock magnetic measurements. Large numbers of ε-Fe2O3 micro-aggregates (in CO) or isolated clusters (in HEL) could be recognized, distributed over the whole sample and embedded within the ceramic matrix, along with hematite and pseudobrookite and with minor amounts of anatase, rutile and maghemite. Curie temperature estimates of around 170°C for CO and 190°C for HEL are lower than for pure, synthetic ε-Fe2O3 (227°C). This, together with structural differences between the Raman spectra of the archeologically-derived and synthetic samples, is likely due to Ti substitution in the ε-Fe2O3 crystal lattice. The γ-Fe2O3 – ε-Fe2O3 – α-Fe2O3 transformation series has been recognised in heated archeological samples, which may have implications in terms of their thermal history and in the factors that govern the formation of ε-Fe2O3.
      PubDate: 2017-07-01T02:40:28.749557-05:
      DOI: 10.1002/2017GC006929
       
  • Normal faulting and mass movement during ridge subduction inferred from
           porosity transition and zeolitization in the Costa Rica subduction zone
    • Authors: Mari Hamahashi; Elizabeth Screaton, Wataru Tanikawa, Yoshitaka Hashimoto, Kylara Martin, Saneatsu Saito, Gaku Kimura
      Abstract: Subduction of the buoyant Cocos Ridge offshore the Osa Peninsula, Costa Rica substantially affects the upper plate structure through a variety of processes, including outer forearc uplift, erosion, and focused fluid flow. To investigate the nature of a major seismic reflector (MSR) developed between slope sediments (late Pliocene∼late Pleistocene silty clay) and underlying higher velocity upper plate materials (late Pliocene∼early Pleistocene clayey siltstone), we infer possible mechanisms of sediment removal by examining the consolidation state, microstructure, and zeolite assemblages of sediments recovered from Integrated Ocean Drilling Program Expedition 344 Site U1380. Formation of Ca-type zeolites laumontite and heulandite, inferred to form in the presence of Ca-rich fluids, has caused porosity reduction. We adjust measured porosity values for these pore-filling zeolites and evaluated the new porosity profile to estimate how much material was removed at the MSR. Based on the composite porosity-depth curve, we infer the past burial depth of the sediments directly below the MSR. The corrected and uncorrected porosity-depth curves yield values of 800±70 m and 900±70 m, respectively. We argue that deposition and removal of this entire estimated thickness in 0.49 m.y. would require unrealistically large sedimentation rates and suggest that normal faulting at the MSR must contribute. The porosity offset could be explained with maximum 250±70 m of normal fault throw, or 350±70 m if the porosity were not corrected. The porosity correction significantly reduces the amount of sediment removal needed for the combination of mass movement and normal faulting that characterize the slope in this margin.
      PubDate: 2017-06-16T11:35:35.549751-05:
      DOI: 10.1002/2016GC006577
       
  • Assessing marine gas emission activity and contribution to the atmospheric
           methane inventory: A multidisciplinary approach from the Dutch Dogger Bank
           seep area (North Sea)
    • Authors: M. Römer; S. Wenau, S. Mau, M. Veloso, J. Greinert, M. Schlüter, G. Bohrmann
      Abstract: We present a comprehensive study showing new results from a shallow gas seep area in ∼40 m water depth located in the North Sea, Netherlands sector B13 that we call ‘Dutch Dogger Bank seep area'. It has been postulated that methane presumably originating from a gas reservoir in ∼600 m depth below the seafloor is naturally leaking to the seafloor. Our ship-based subbottom echosounder data indicate that the migrating gas is trapped in numerous gas pockets in the shallow sediments. The gas pockets are located at the boundary between the top of the Late Pliocene section and overlying fine grained sediments, which were deposited during the early Holocene marine transgression after the last glaciation. We mapped gas emissions during three R/V Heincke cruises in 2014, 2015, and 2016 and repeatedly observed up to 850 flares in the study area. Most of them (∼80%) were concentrated at five flare clusters. Our repeated analysis revealed spatial similarities of seep clusters, but also heterogeneities in emission intensities. A first calculation of the methane released from these clusters into the water column revealed a flow rate of 277 L/min (SD=140), with two clusters emitting 132 and 142 L/min representing the most significant seepage sites. Above these two flare clusters, elevated methane concentrations were recorded in atmospheric measurements. Our results illustrate the effective transport of methane via gas bubbles through a ∼40 m water column, and furthermore provide an estimate of the emission rate needed to allow for a contribution to the atmospheric methane concentration.
      PubDate: 2017-06-16T05:45:20.95936-05:0
      DOI: 10.1002/2017GC006995
       
  • Regional-scale development of opening-mode calcite veins due to silica
           diagenesis
    • Authors: John N. Hooker; Jennifer M. Huggett, Joe Cartwright, Mohammad Ali Hussein
      Abstract: The formation and distribution of natural fractures in Cretaceous–Paleogene strata in Jordan are strongly tied to diagenetic processes, which in turn reflect the lithology of the host material. Observations collected from subsurface cores show that widespread fracturing began before compaction of the host sediment was complete, based on ptygmatic folding of one set of mineral-filled fractures (veins). Non-folded veins are preferentially developed within heavily cemented layers. Calcium carbonate is the greatest volumetric component of the host sediment, and most fractures are at least partially filled by calcite. Dolomite- and silica-bearing fractures are present in dolomitized and silicified host beds, respectively. Horizontal veins are filled by cone-in-cone calcite or, rarely, silica or dolomite. The stratigraphic arrangement and degree of compaction around ptygmatically folded calcite veins and chert nodules suggest that silica diagenesis was an important driver of early fractures. Nevertheless, those fractures were filled with carbonate cements as they opened, based on crack-seal texture of the vein fill. The volume loss associated with silica diagenesis created fracture porosity, which was filled coevally by carbonate cements. The distribution of later veins reflects embrittlement of host layers by cementation and is consistent with crustal deformation as the primary fracture driver.
      PubDate: 2017-06-15T18:05:45.773205-05:
      DOI: 10.1002/2017GC006888
       
  • The global systematics of primitive arc melts
    • Authors: M.W. Schmidt; O. Jagoutz
      Abstract: We extracted all volcanic arc rock analyses calculated to be in equilibrium with mantle olivine from the global georoc database. This results in 938 primitive melt compositions from 30 arcs. Based on geochemical criteria six principal types of primitive arc melts can be distinguished: calc-alkaline basalts and andesites, tholeiitic basalts, highly depleted tholeiitic andesites, shoshonites and low-Si basalts. Their major element systematics indicates that last mantle equilibration occurred mostly at 1.0-2.5 GPa, 1220-1350°C for tholeiitic and calc-alkaline basalts, at 0.5-1.2 GPa and ∼1200°C for depleted tholeiitic andesites, and at 0.7-1.2 GPa, 1050-1150°C for calc-alkaline andesites. Quantitative treatment of major and trace elements suggests that the different melt types can be explained by a combination of variable mantle wedge preconditioning (degree of depletion prior to slab component addition, metasomatism in the lithosphere), variation in the amount and nature of the slab component added, and - for primitive calc-alkaline andesites - reactive fractionation in the lithospheric top of the mantle wedge.The different slab components are best characterized by high Na2O, TiO2, Zr and Th for slab melts; high K2O/Na2O and more pronounced Nb, Sr, and Pb anomalies for fluids; and high K2O at high K2O/Na2O for supercritical liquids. A slab component that is dominantly a slab melt is common in continental but rare in intra-oceanic arcs, consistent with comparatively cooler slabs in intra-oceanic subduction zones. A majority of the arcs has more than one melt type, testifying for heterogeneity in the mantle wedge and added slab component.
      PubDate: 2017-06-15T18:00:38.593099-05:
      DOI: 10.1002/2016GC006699
       
  • Rates of mantle cooling and exhumation during rifting constrained by
           REE-in-pyroxene speedometry
    • Authors: A. Smye; S. Seman, M. Hudak, K. Crispin
      Abstract: Ocean basins are formed when continents are broken apart. Adiabatic melt generation that is driven by rifting of continental lithosphere is strongly dependent on the rate of extension. Slow extension results in conductive heat loss from the upwelling mantle, whereas cooling is limited during fast extension and can result in the geotherm intersecting the peridotite solidus. However, there are few direct constraints on the rates of mantle upwelling during extension of continental lithosphere. Here, we use diffusion modelling of subsolidus REE re-equilibration between orthopyroxene and clinopyroxene to show that the Lanzo peridotite massif —lithospheric mantle exhumed during opening of the Ligurian Tethys—cooled at rates between 5 and 25°C/Myr across the spinel-to-plagioclase peridotite facies transition. We show that these rates are sufficiently slow to suppress significant adiabatic melt generation, providing an explanation for the magma-poor nature of the Alpine Tethys margin.
      PubDate: 2017-06-15T17:20:44.011185-05:
      DOI: 10.1002/2017GC006957
       
  • Flexural isostasy of the carbonate platform in North central Florida
    • Authors: H. B. Woo; M. Panning, Peter N. Adams, A. Dutton
      Abstract: Deformed marine terraces can be used to explore a region's uplift history. Trail Ridge is a marine terrace in north Florida that is nearly 80 meters above modern sea level and contains Quaternary marine fossils, a fact that is inconsistent with estimates of paleo-sea level history since the early Pleistocene. This implies that the terrace has experienced uplift since its formation, as well as non-uniform deformation recorded by the warping of its previously horizontal state. The Florida carbonate platform, located on the passive margin of eastern North America, is a setting where non-tectonic influences (e.g. isostatic adjustment, dynamic topography) can be examined. We present a single-transect, numerical model of vertical displacement, derived from elastic flexure, to assess the influence of karst-driven isostatic uplift on present day topography of Trail Ridge in north Florida. Flexural modeling predicts elevations in central Florida not observed today, most likely because surface erosion and karst cavity collapse have obliterated this high topography. Older subsurface stratigraphic units, however, display the arched profile predicted from flexural modeling. Mass loss, calculated by differencing modeled topography and observed topography, was found to be 6.75 × 1012 kg, since emplacement of Trail Ridge. Uplift rates, assuming karst-driven flexural isostasy alone, using previously estimated ages of Trail Ridge of 0.125, 1.4, 3, or 3.5 Ma were found to be 0.535, 0.048, 0.022, and 0.019 mm/yr, respectively. A more likely explanation of uplift includes contributions from dynamic topography and glacial isostatic adjustment which should be further explored with more advanced geophysical modeling.
      PubDate: 2017-06-15T17:20:39.694771-05:
      DOI: 10.1002/2017GC006934
       
  • Human-induced seismicity and large-scale hydrocarbon production in the USA
           and Canada
    • Authors: Mirko van der Baan; Frank J. Calixto
      Abstract: We compare current and historic seismicity rates in six States in the USA and three Provinces in Canada to past and present hydrocarbon production. All States/Provinces are major hydrocarbon producers. Our analyses span three to five decades depending on data availability. Total hydrocarbon production has significantly increased in the past few years in these regions. Increased production in most areas is due to large-scale hydraulic fracturing and thus underground fluid injection. Furthermore, increased hydrocarbon production generally leads to increased water production, which must be treated, recycled or disposed of underground. Increased fluid injection enhances the likelihood of fault reactivation, which may affect current seismicity rates.We find that increased seismicity in Oklahoma, likely due to salt-water disposal, has an 85% correlation with oil production. Yet, the other areas do not display State/Province-wide correlations between increased seismicity and production, despite 8-16 fold increases in production in some States. However in various cases seismicity has locally increased.Multiple factors play an important role in determining the likelihood of anthropogenic activities influencing earthquake rates, including (i) the near-surface tectonic background rate, (ii) the existence of critically stressed and favorably oriented faults, which must be hydraulically connected to injection wells, (iii) the orientation and magnitudes of the in situ stress field, combined with (iv) the injection volumes and implemented depletion strategies. A comparison with the seismic hazard maps for the USA and Canada shows that induced seismicity is less likely in areas with a lower hazard. The opposite however is not necessarily true.
      PubDate: 2017-06-09T11:00:46.374601-05:
      DOI: 10.1002/2017GC006915
       
  • Observed correlation between the depth to base and top of gas hydrate
           occurrence from review of global drilling data
    • Authors: M. Riedel; T.S. Collett
      Abstract: A global inventory of data from gas hydrate drilling expeditions is used to develop relationships between the base of structure I gas hydrate stability, top of gas hydrate occurrence, sulfate-methane transition depth, pressure (water depth), and geothermal gradients. The motivation of this study is to provide first-order estimates of the top of gas hydrate occurrence and associated thickness of the gas hydrate occurrence zone for climate-change scenarios, global carbon budget analyses, or gas hydrate resource assessments. Results from publically available drilling campaigns (21 expeditions, 52 drill sites) off Cascadia, Blake Ridge, India, Korea, South China Sea, Japan, Chile, Peru, Costa Rica, Gulf of Mexico, and Borneo reveal a first-order linear relationship between the depth to the top and base of gas hydrate occurrence. The reason for these nearly linear relationships is believed to be the strong pressure- and temperature dependence of methane solubility in the absence of large difference in thermal gradients between the various sites assessed. In addition, a statistically robust relationship was defined between the thickness of the gas hydrate occurrence zone and the base of gas hydrate stability (in meters below seafloor). The relationship developed is able to predict the depth of the top of gas hydrate occurrence zone using observed depths of the base of gas hydrate stability within less than 50 meters at most locations examined in this study. No clear correlation of the depth to the top and base of gas hydrate occurrences with geothermal gradient and sulfate-methane transition depth were identified.
      PubDate: 2017-06-09T11:00:44.952881-05:
      DOI: 10.1002/2017GC006805
       
  • Block motion changes in Japan triggered by the 2011 Great Tohoku
           Earthquake
    • Authors: Brendan J. Meade; John P. Loveless
      Abstract: Plate motions are governed by equilibrium between basal and edge forces. Great earthquakes may induce differential static stress changes across tectonic plates, enabling a new equilibrium state. Here we consider the torque balance for idealized circular plates and find a simple scalar relationship for changes in relative plate speed as a function of its size, upper mantle viscosity, and coseismic stress changes. Applied to Japan, the 2011 MW = 9.0 Tohoku earthquake generated coseismic stresses of 102 – 105 Pa that could have induced changes in motion of small (radius ∼ 100 km) crustal blocks within Honshu. Analysis of time-dependent GPS velocities, with corrections for earthquake cycle effects, reveals that plate speeds may have changed by up to ∼ 3 mm/yr between ∼ 3.75-year epochs bracketing this earthquake, consistent with an upper mantle viscosity of ∼ 5 × 1018 Pa·s, suggesting that great earthquakes may modulate motions of proximal crustal blocks at frequencies as high as 10−8 Hz.
      PubDate: 2017-06-09T11:00:43.463414-05:
      DOI: 10.1002/2017GC006983
       
  • The behaviour of iron and zinc stable isotopes accompanying the subduction
           of mafic oceanic crust: A case study from Western Alpine Ophiolites
    • Authors: Edward C. Inglis; Baptiste Debret, Kevin W. Burton, Marc-Alban Millet, Marie-Laure Pons, Christopher W. Dale, Pierre Bouilhol, Matthew Cooper, Geoffrey M. Nowell, Alex McCoy-West, Helen M. Williams
      Abstract: Arc lavas display elevated Fe3+/ΣFe ratios relative to MORB. One mechanism to explain this is the mobilization and transfer of oxidised or oxidising components from the subducting slab to the mantle wedge. Here we use iron and zinc isotopes, which are fractionated upon complexation by sulfide, chloride and carbonate ligands, to remark on the chemistry and oxidation state of fluids released during prograde metamorphism of subducted oceanic crust. We present data for metagabbros and metabasalts from the Chenaillet massif, Queyras complex and the Zermatt-Saas ophiolite (Western European Alps), which have been metamorphosed at typical subduction zone P-T conditions and preserve their prograde metamorphic history. There is no systematic, detectable fractionation of either Fe or Zn isotopes across metamorphic facies, rather the isotope composition of the eclogites overlaps with published data for MORB. The lack of resolvable Fe isotope fractionation with increasing prograde metamorphism likely reflects the mass balance of the system, and in this scenario Fe mobility is not traceable with Fe isotopes. Given that Zn isotopes are fractionated by S- and C-bearing fluids, this suggests that relatively small amounts of Zn are mobilised from the mafic lithologies in within these types of dehydration fluids. Conversely, metagabbros from the Queyras that are in close proximity to metasediments display a significant Fe isotope fractionation. The covariation of δ56Fe of these samples with selected fluid mobile elements suggests the infiltration of sediment derived fluids with an isotopically light signature during subduction.
      PubDate: 2017-06-09T11:00:35.955397-05:
      DOI: 10.1002/2016GC006735
       
  • The role of mud volcanism and deep-seated dewatering processes in the
           Nankai Trough accretionary prism and Kumano Basin, Japan
    • Authors: Walter Menapace; David Völker, Norbert Kaul, Michael D. Tryon, Achim J. Kopf
      Abstract: Circulation of water at moderate depths in subduction zones is dominantly driven by clay mineral dehydration over distinct pressure and temperature gradients. The signature of these dehydration reactions is found in mud volcano pore waters, however, it is largely unknown, how much of the deep-seated fluids are emitted at mud volcanoes. To unravel this relation for the region off the Kii Peninsula, Japan, we calculated the water volume that is subducted in the Nankai Trough using input data from IODP holes C0011 and C0012 and the correspondent water volume released from the subducted plate under the Kumano Basin, in an area where 13 mud volcanoes are located. According to our model, water released at depth in the mud volcano area is derived almost entirely from basaltic saponite and sedimentary smectite transformation (up to 96%). Nonetheless, the mud volcanoes themselves expel ≪1% of the total volume. To test the contribution of the accreted strata and the Kumano Basin fill to the water budget, we run a second model. Water loss due to compaction of sediments and smectite-illite transition below the basin floor have been calculated. The results were compared with salinity measurements on background cores scattered in the study area to extrapolate the volume of water loss at depth. The comparison of the two methods yielded similar results and led us to conclude that the bulk part of the deep-seated fluid re-enters the hydrosphere via the basin floor, a mechanism rarely taken into account in fluid budgets in the literature.
      PubDate: 2017-06-09T11:00:32.752309-05:
      DOI: 10.1002/2016GC006763
       
  • Stratigraphic Signatures of forearc basin formation mechanisms
    • Authors: Utsav Mannu; Kosuke Ueda, Sean D. Willett, Taras V. Gerya, Michael Strasser
      Abstract: Tectonic deformation of accretionary wedges is often interpreted using stratigraphic patterns observed in forearc basins. However, similar stratigraphic patterns could evolve from different deformational processes making it difficult to uniquely reconcile stratigraphic patterns and the geodynamic history of natural accretionary wedges. Therefore, it is important to test the dynamic consistency of interpreted deformation histories. One approach is to compare synthetic stratigraphy of forearc basins generated in numerical accretionary wedge models, to the stratal patterns observed in natural forearcs. We present a simple method to simulate synthetic stratigraphy in numerical accretionary wedge models, comparable to the stratigraphic patterns observed in seismic reflection data. As calibration, we use reflection seismic and borehole data for the Kumano forearc basin in Nankai. We observe that the stratigraphy in retro-forearc basins remains predominantly undisturbed and unaffected by wedge deformation. Sediment-stabilized wedge-top basins in wedges with trenchward surface slope typically form landward of an active out-of-sequence thrust, leading to a landward tilting of the basin. These stratigraphic characteristics are diagnostic of the forearc basin forming mechanism. We also infer that sedimentation in the trench can potentially influence the generation/reactivation of Megasplay Fault activity in the Nankai accretionary wedge. Activity on the Megasplay Fault helps create accommodation space landward to it that leads to the formation of Kumano forearc basin.
      PubDate: 2017-06-05T11:01:19.054413-05:
      DOI: 10.1002/2017GC006810
       
  • An experimental study of CO2-oil-brine-rock interaction under in situ
           reservoir conditions
    • Authors: Zhichao Yu; Keyu Liu, Li Liu, Siyu Yang, Yongzhi Yang
      Abstract: To understand the mineralogical and chemical changes in oil–bearing reservoirs (e.g. depleted oil reservoirs) during massive CO2 injection, we have carried out a core-flooding experimental study of CO2–oil–brine–rock interactions under a simulated reservoir condition of 100°C and 24 MPa. The experimental condition is based on field data from a CO2–EOR project in the southern Songliao Basin. This oil–bearing CO2–flooding experiment used the same experimental setup, reservoir conditions and workflow as the oil–free experiment reported by Yu et al. (2012). The sandstone core samples used in the experiment have similar mineralogical compositions as that used in the previous experiment. Compared with the oil–free experiment, the presence of oil appears to substantially reduce the reaction degree between the CO2 fluid and some sensitive minerals. The dissolution rates of the K–feldspar and carbonate minerals for the oil–bearing experiment are 1/5 and 1/4 of that for the oil–free experiments, respectively. For the silicate minerals represented by the K–feldspar, the presence of oil mainly delays the dissolution during the experiment, and reduces the equilibrium dissolution rate. For the carbonate minerals, the presence of oil appears to primarily affect the dissolution at the beginning of the experiments, and reduce the maximum dissolution rate attained. The core permeabilities for the oil–free and oil–bearing cases are both reduced after experiments. The reduction in permeability is probably due to the precipitation of fine siliceous mineral and clay particles released by the dissolution of the carbonate cement, which may clog some pore throats. The results provide some new insights on the fluid-rock interaction during CO2 injection in depleted oil reservoirs or during CO2–EOR.
      PubDate: 2017-06-05T10:56:32.980102-05:
      DOI: 10.1002/2017GC006858
       
  • Spatial and temporal uplift history of South America from calibrated
           drainage analysis
    • Authors: V. Rodríguez Tribaldos; N. J. White, G.G. Roberts, M. J. Hoggard
      Abstract: A multi-disciplinary approach is used to analyze the Cenozoic uplift history of South America. Residual depth anomalies of oceanic crust abutting this continent help to determine the pattern of present-day dynamic topography. Admittance analysis and crustal thickness measurements indicate that the elastic thickness of the Borborema and Altiplano regions is ≤ 10 km with evidence for sub-plate support at longer wavelengths. A drainage inventory of 1827 river profiles is assembled and used to investigate landscape development. Linear inverse modeling enables river profiles to be fitted as a function of the spatial and temporal history of regional uplift. Erosional parameters are calibrated using observations from the Borborema Plateau and tested against continent-wide stratigraphic and thermochronologic constraints. Our results predict that two phases of regional uplift of the Altiplano plateau occurred in Neogene times. Regional uplift of the southern Patagonian Andes also appears to have occurred in Early Miocene times. The consistency between observed and predicted histories for the Borborema, Altiplano and Patagonian plateaux implies that drainage networks record coherent signals that are amenable to simple modeling strategies. Finally, the predicted pattern of incision across the Amazon catchment constrains solid sedimentary flux at the Foz do Amazonas. Observed and calculated flux estimates match, suggesting that erosion and deposition were triggered by regional Andean uplift during Miocene times.
      PubDate: 2017-06-02T07:05:46.559832-05:
      DOI: 10.1002/2017GC006909
       
  • Marine redox stratification during the early Cambrian (ca. 529-509 Ma) and
           
    • Authors: Yuying Zhang; Zhiliang He, Shu Jiang, Bo Gao, Zhongbao Liu, Bo Han, Hu Wang
      Abstract: High resolution geochemical data from nine sections representing shelf to basinal environments in the Yangtze Platform were analyzed to reconstruct the marine redox environment during early Cambrian. Based on Fe species and Mo/TOC ratios, we have supplemented marine redox stratification during Stage 4 (late Canglangpuian-Longwangmiaoan, ∼514-509 Ma) on basis of the previously studied Stage 2-Stage 3 (Meishucunian-Qiongzhusian, ∼529-514 Ma). A new proposed marine stratified redox model indicates that the mid-depth “euxinic wedge” developed at the base of slope during ∼514-509 Ma in contrast to that the “euxinic wedge” prevailed at the shelf margin during ∼529-514 Ma, even though these mid-depth euxinic waters both occurred between the oxic surface waters and ferruginous deep waters. This marine redox stratification resulted in high production and good preservation of organic matter during early Cambrian. TOC values in euxinic waters in the middle are generally higher than in ferruginous waters due to upwelling in slope. Therefore, the lower Cambrian organic-rich shales in the Yangtze Platform are inferred to be deposited under the anoxic-ferruginous and euxinic bottom waters with moderate-strong restriction.
      PubDate: 2017-06-02T07:05:40.24199-05:0
      DOI: 10.1002/2017GC006864
       
  • Magnetic signature of the June 22nd, 1932 tsunami deposits (Jalisco,
           Mexican Pacific coast)
    • Authors: M.F. Bógalo; M-T. Ramírez-Herrera, A. Goguitchaichvili, D. Rey, K.J. Mohamed, M. Calvo-Rathert, N. Corona
      Abstract: Recent studies have demonstrated that rock-magnetic analysis may provide additional information to distinguish and characterize extreme marine inundation events such as tsunamis. Rock-magnetic proxies reinforce and improve the environmental evidences supplied by other methods, adding some decisive clues for the interpretation of the origin and genesis of the sedimentary deposits. Here we report rock-magnetic, XRD and SEM microscopy results obtained in the Palo Verde estuary (Colima Pacific coast, Mexico) in order to enhance the tools for identification and reconstruction of two tsunami-induced deposits. The sedimentary sequence includes two sand units, a tsunami deposit (PV1) associated with the 22 June 1932 tsunami and a deeper sandy layer (PV2) related to a possible palaeotsunami that occurred around 1300 CE. Both sandy units are topped by finer grained units.Magnetic properties exhibit a significant correlation with the stratigraphy. High susceptibility (χ) and high saturation isothermal remanence (SIRM) values typical of high concentrations of (titano)magnetite are a distinctive feature of the most recent sandy tsunamigenic unit PV1 and the overlaying soil. The lower sandy tsunamigenic unit PV2 shows significantly lower χ and SIRM values, indicating lower concentration of (titano)magnetite in this unit and the overlaying clayey-silt unit. The latter also shows a higher coercivity component associated to (titano)hematite. Magnetic grain-size differences are also observed between PV1 and PV2 suggesting differences in hydraulic conditions at the time of deposition. The bulk mineralogical composition and sediment texture of these units also supports the hypothesis of different provenances for each tsunamigenic unit as inferred from magnetic properties.
      PubDate: 2017-06-02T06:56:04.228683-05:
      DOI: 10.1002/2016GC006752
       
  • Faulting and off-axis submarine massive sulfide accumulation at
           slow-spreading mid-ocean ridges: A numerical modeling perspective
    • Authors: C. Andersen; S. Theissen-Krah, M. Hannington, L. Rüpke, S. Petersen
      Abstract: The potential of mining Seafloor Massive Sulfide deposits for metals such as Cu, Zn, and Au is currently debated. One key challenge is to predict where the largest deposits worth mining might form, which in turn requires understanding the pattern of sub-seafloor hydrothermal mass and energy transport. Numerical models of heat and fluid flow are applied to illustrate the important role of fault zone properties (permeability and width) in controlling mass accumulation at hydrothermal vents at slow-spreading ridges. We combine modeled mass-flow rates, vent temperatures and vent field dimensions with the known fluid chemistry at the fault-controlled Logatchev 1 hydrothermal field of the Mid-Atlantic Ridge. We predict that the 135 kilotons of SMS at this site (estimated by other studies) can have accumulated with a minimum depositional efficiency of 5% in the known duration of hydrothermal venting (58,200 year age of the deposit). In general, the most productive faults must provide an efficient fluid pathway while at the same time limit cooling due to mixing with entrained cold seawater. This balance is best met by faults that are just wide and permeable enough to control a hydrothermal plume rising through the oceanic crust. Model runs with increased basal heat input, mimicking a heat flow contribution from along-axis, lead to higher mass fluxes and vent temperatures, capable of significantly higher SMS accumulation rates. Non-steady state conditions, such as the influence of a cooling magmatic intrusion beneath the fault zone, also can temporarily increase the mass flux while sustaining high vent temperatures.
      PubDate: 2017-06-02T06:55:49.191764-05:
      DOI: 10.1002/2017GC006880
       
  • The Gondou hydrothermal field in the Ryukyu Arc: A huge hydrothermal
           system on the flank of a caldera volcano
    • Authors: H. Minami; Y. Ohara
      Abstract: High-resolution geophysical mapping was conducted from an autonomous underwater vehicle on the flank of Daisan-Kume Knoll in the Ryukyu Arc, southwest of Japan. 1-m resolution bathymetry identified 264 spires, 173 large mounds and 268 small mounds within a depression that is up to 1600 m wide and up to 60 m deep, at water depths between 1330 and 1470 m. Hydrothermal venting is strongly inferred from the observation of plumes in sidescan sonar imagery and positive temperature anomalies over the spires and mounds. This field, named the Gondou Field, has a giant mound G1 with a diameter of 280 m and a height of 80 m. Mound G1 has distinctive summit ridges comprised of multiple spires where acoustic plumes with temperature anomalies up to 1.12°C are observed, indicative of high-temperature venting. Other than mound G1, a number of active large mounds more than 30 m wide and spires over 10-22 m tall are common and they concentrate in the central and southern areas of the field, suggesting that these areas are the center of present hydrothermal activity. Acoustic plumes imaged by side-scan sonar at the Gondou Field are different in character from bubble plumes imaged in other hydrothermal fields in the Ryukyu Arc. The plumes are diffused and deflected as they rise through the water column and have a shape consistent with black smokers.
      PubDate: 2017-05-30T11:36:43.696855-05:
      DOI: 10.1002/2017GC006868
       
  • New insights into the magmatism in the northern margin of the South China
           Sea: Spatial features and volume of intraplate seamounts
    • Authors: Chaoyan Fan; Shaohong Xia, Fang Zhao, Jinlong Sun, Jinghe Cao, Huilong Xu, Kuiyuan Wan
      Abstract: The extensive intraplate seamounts are obvious features in the northern South China Sea (SCS). However, the distribution, volume, and origin of these seamounts are not well understood, which greatly hinders our understanding of magmatism in the SCS. Based on high-resolution bathymetric data and 147 seismic profiles, and combining gravity and magnetic data, we first identify 45 seamounts in the northern margin of the SCS and simulate their shape with elliptical cones. Results show that the total volume of these 45 seamounts above seafloor is estimated at about 1885–3078 km3 and the total volume of intrusive magma above Moho is about 0.15 Mkm3, which is close to the estimates for classic large igneous provinces across the world. These seamounts are mostly located on the continental slope with thin crust (approximately 12–18 km), which reduces the overlying pressure and shortens the magmatic conduits. The dominant azimuth of elliptical major axis in seamounts is consistent with the synrift and synspreading fault strikes (NE-NEE), indicating that these pre-existing faults provide magmatic conduits for the subsequent postrift intraplate seamounts. Based on three existing clues, i.e., (1) the intraplate seamounts, high velocity layer and Hainan mantle plume are contiguous in 3-D space, (2) the high velocity layer is thicker beneath the continental shelf but thinner beneath the slope and (3) the basalts dredged from certain seamounts show OIB-type geochemical features, we propose a magmatic upwelling pattern which contains Hainan mantle plume to explain the spatial and morphological characteristics of these intraplate seamounts.
      PubDate: 2017-05-30T11:15:56.473593-05:
      DOI: 10.1002/2016GC006792
       
  • Submarine landslides triggered by destabilization of high-saturation
           hydrate anomalies
    • Authors: Alexander L. Handwerger; Alan W. Rempel, Rob M. Skarbek
      Abstract: Submarine landslides occur along continental margins at depths that often intersect the gas hydrate stability zone, prompting suggestions that slope stability may be affected by perturbations that arise from changes in hydrate stability. Here, we develop a numerical model to identify the conditions under which the destabilization of hydrates results in slope failure. Specifically, we focus on high-saturation hydrate anomalies at fine- to coarse-grained stratigraphic boundaries that can transmit bridging stresses that decrease the effective stress at sediment contacts and disrupt normal sediment consolidation. We evaluate slope stability before and after hydrate destabilization. Hydrate anomalies act to significantly increase the overall slope stability due to large increases in effective cohesion. However, when hydrate anomalies destabilize there is a loss of cohesion and increase in effective stress that causes the sediment grains to rapidly consolidate and generate pore pressures that can either trigger immediate slope failure or weaken the surrounding sediment until the pore pressure diffuses away. In cases where failure does not occur, the sediment can remain weakened for months. In cases where failure does occur, we quantify landslide dynamics using a rate and state frictional model and find that landslides can display either slow or dynamic (i.e. catastrophic) motion depending on the rate-dependent properties, size of the stress perturbation, and the size of the slip patch relative to a critical nucleation length scale. Our results illustrate the fundamental mechanisms through which the destabilization of gas hydrates can pose a significant geohazard.
      PubDate: 2017-05-30T11:10:47.055013-05:
      DOI: 10.1002/2016GC006706
       
  • Contrasted hydrothermal activity along the South-East Indian Ridge
           (130°E–140°E): From crustal to ultramafic circulation
    • Authors: Cédric Boulart; Anne Briais, Valérie Chavagnac, Sidonie Révillon, Georges Ceuleneer, Jean-Pierre Donval, Vivien Guyader,
      Abstract: Using a combined approach of seafloor mapping, MAPR and CTD survey, we report evidence for active hydrothermal venting along the 130°-140°E section of the poorly-known South-East Indian Ridge (SEIR) from the Australia-Antarctic Discordance (AAD) to the George V Fracture Zone (FZ). Along the latter, we report Eh and CH4 anomalies in the water column above a serpentinite massif, which unambiguously testify for ultramafic-related fluid flow. This is the first time that such circulation is observed on an intermediate-spreading ridge. The ridge axis itself is characterized by numerous off-axis volcanoes, suggesting a high magma supply. The water column survey indicates the presence of at least ten distinct hydrothermal plumes along the axis. The CH4:Mn ratios of the plumes vary from 0.37 to 0.65 denoting different underlying processes, from typical basalt-hosted to ultramafic-hosted high-temperature hydrothermal circulation. Our data suggest that the change of mantle temperature along the SEIR not only regulates the magma supply, but also the hydrothermal activity. The distribution of hydrothermal plumes from a ridge segment to another implies secondary controls such as the presence of fractures and faults along the axis or in the axial discontinuities. We conclude from these results that hydrothermal activity along the SEIR is controlled by magmatic processes at the regional scale and by the tectonics at the segment scale, which influences the type of hydrothermal circulation and leads to various chemical compositions. Such variety may impact global biogeochemical cycles, especially in the Southern Ocean where hydrothermal venting might be the only source of nutrients.
      PubDate: 2017-05-30T11:10:43.007143-05:
      DOI: 10.1002/2016GC006683
       
  • A calcite reference material for LA-ICP-MS U-Pb geochronology
    • Authors: Nick M. W. Roberts; E. Troy Rasbury, Randall R Parrish, Christopher J Smith, Matthew S. A. Horstwood, Daniel J Condon
      Abstract: U-Pb dating of calcite is an emerging but rapidly growing field of application in geochronology with great potential to inform problems in landscape, basin and mountain belt evolution, through age determination of diagenetic cements, vein mineralisation and geological formations difficult to date otherwise. In this brief, we present isotope dilution U-Pb isotope measurements on a sample of calcite (WC-1) that has been and will continue to be used as a reference material for in-situ U-Pb Laser Ablation Inductively Coupled Plasma Mass Spectrometry (LA-ICP-MS) dating, and which is suitable to be distributed to the geochronological community. We present in-situ measurements using LA-ICP-MS to demonstrate the suitability of WC-1 for use as a U-Pb dating reference material, in spite of it not being isotopically homogeneous. The WC-1 calcite sample is 254.4 ± 6.4 Ma old, and comprised of 85 to 98% radiogenic lead. It presents a suitable reference material that can facilitate dating of calcite ranging in age from Precambrian to late Neogene age.
      PubDate: 2017-05-30T11:10:39.071915-05:
      DOI: 10.1002/2016GC006784
       
  • What happens to in-soil Radon activity during a long-lasting eruption?
           Insights from Etna by multidisciplinary data analysis
    • Authors: S. Falsaperla; M. Neri, G. Di Grazia, H. Langer, S. Spampinato
      Abstract: We analyze short- to long-term changes (from days to months) in Radon (Rn) activity measured nearby (
      PubDate: 2017-05-30T10:50:47.284909-05:
      DOI: 10.1002/2017GC006825
       
  • Multianalytical provenance analysis of eastern Ross Sea LGM till sediments
           (Antarctica): Petrography, geochronology, and thermochronology detrital
           data
    • Authors: Matteo Perotti; Benedetta Andreucci, Franco Talarico, Massimiliano Zattin, Antonio Langone
      Abstract: In order to reveal provenance of detrital sediments supplied by West Antarctic Ice Sheet (WAIS), 19 glaciomarine cores of Last Glacial Maximum age were analyzed from Eastern Ross Sea and Sulzberger Bay. Analytical techniques included petrographic analysis of gravel-sized clasts, geochronology (zircon U-Pb: Zrn-UPb) and thermochronology (apatite fission track: AFT) of sand-sized fractions. Petrographic analysis revealed a similarity with the lithologies presently exposed in western Marie Byrd Land (MBL), with major roles played by low grade metamorphic rocks and granitoids. Furthermore Zrn-UPb and AFT data allowed to identify the ages of formation and cooling of sedimentary source area, consisting of Cambrian-Precambrian basement (i.e. Swanson Formation in western MBL) which underwent at least two episodes of magma intrusion, migmatization and cooling during Devonian-Carboniferous and Cretaceous-Paleocene times. Scarcity of volcanic clasts in the region of Ross Sea along the front of West Antarctica Ice Streams in association with the occurrence of AFT Oligocene-Pliocene dates suggests a localized tectonic exhumation of portions of MBL, as already documented for the opposite side of West Antarctic Rift System in the Transantarctic Mountains. Furthermore, a Zrn-UPb and AFT population of Late Triassic-Jurassic age indicates the presence of unexposed rocks that formed or metamorphosed at that time in the sedimentary source area, which could be identified in McAyeal Ice Stream and Bindschadler Ice Stream catchment areas.
      PubDate: 2017-05-30T10:50:44.215514-05:
      DOI: 10.1002/2016GC006728
       
  • The onset of modern-like Atlantic meridional overturning circulation at
           the Eocene-Oligocene transition: Evidence, causes, and possible
           implications for global cooling
    • Authors: Meir Abelson; Jonathan Erez
      Abstract: A compilation of benthic δ18O from the whole Atlantic and the Southern Ocean (Atlantic sector), shows two major jumps in the interbasinal gradient of δ18O (Δδ18O) during the Eocene and the Oligocene: One at ∼40 Ma and the second concomitant with the isotopic event of the Eocene-Oligocene transition (EOT), ∼33.7 Ma ago. From previously published circulation models and proxies, we show that the first Δδ18O jump reflects the thermal isolation of Antarctica associated with the proto-Antarctic circumpolar current (ACC). The second marks the onset of interhemispheric northern-sourced circulation cell, similar to the modern Atlantic meridional overturning circulation (AMOC). The onset of AMOC-like circulation slightly preceded (100-300 ky) the EOT, as we show by the high resolution profiles of δ18O and δ13C previously published from DSDP/ODP sites in the Southern Ocean and South Atlantic. These events coincide with the onset of anti-estuarine circulation between the Nordic seas and the North Atlantic which started around the EOT and may be connected to the deepening of the Greenland-Scotland Ridge. We suggest that while the shallow proto-ACC supplied the energy for deep ocean convection in the Southern Hemisphere, the onset of the interhemispheric northern circulation cell was due to the significant EOT intensification of deepwater formation in the North Atlantic driven by the Nordic anti-estuarine circulation. This onset of the interhemispheric northern-sourced circulation cell could have prompted the EOT global cooling.
      PubDate: 2017-05-30T10:50:37.306625-05:
      DOI: 10.1002/2017GC006826
       
  • Seismic structure and segmentation of the axial valley of the Mid-Cayman
           Spreading Center
    • Authors: Harm J.A. Van Avendonk; Nicholas W. Hayman, Jennifer L. Harding, Ingo Grevemeyer, Christine Peirce, Anke Dannowski
      Abstract: We report the results of a two-dimensional tomographic inversion of marine seismic refraction data from an array of ocean-bottom seismographs (OBSs), which produced an image of the crustal structure along the axial valley of the ultraslow-spreading Mid-Cayman Spreading Center (MCSC). The seismic velocity model shows variations in the thickness and properties of the young oceanic crust that are consistent with the existence of two magmatic-tectonic segments along the 110 km-long spreading center. Seismic wave speeds are consistent with exhumed mantle at the boundary between these two segments, but changes in the vertical gradient of seismic velocity suggest that volcanic crust occupies most of the axial valley seafloor along the seismic transect. The two spreading segments both have a low-velocity zone (LVZ) several kilometers beneath the seafloor, which may indicate the presence of shallow melt. However, the northern segment also has low seismic velocities (3 km/s) in a thick upper crustal layer (1.5-2.0 km), which we interpret as an extrusive volcanic section with high porosity and permeability. This segment hosts the Beebe vent field, the deepest known high-temperature black smoker hydrothermal vent system. In contrast, the southern spreading segment has seismic velocities as high as 4.0 km/s near the seafloor. We suggest that the porosity and permeability of the volcanic crust in the southern segment are much lower, thus limiting deep seawater penetration and hydrothermal recharge. This may explain why no hydrothermal vent system has been found in the southern half of the MCSC.
      PubDate: 2017-05-30T10:47:14.854635-05:
      DOI: 10.1002/2017GC006873
       
  • A kinetic model to explain the grainsize and organic matter content
           dependence of magnetic susceptibility in transitional marine environments:
           A case study in Ría de Muros (NW Iberia)
    • Authors: Kais J. Mohamed; Alba Andrade, Daniel Rey, Belén Rubio, Ana María Bernabeu
      Abstract: Magnetic minerals in marine sediments are sensitive indicators of processes such as provenance changes, climatic controls, pollution and postdepositional geochemical changes. Magnetic susceptibility is the bulk property of the sediments most commonly used to understand the magnetic characteristics of sediments. Before conclusions can be drawn from changes in this parameter, it is important to understand what factors and to what extent control changes in magnetic susceptibility. The magnetic susceptibility of surficial sediments in the Galician Rias Baixas, in NW Spain, has been shown to covary with sediment texture and organic matter content. Downcore, the magnetic properties of these sediments experience drastic changes as a result of strong dissolution caused by early diagenesis. In this paper, we further explore the relationship between these factors and formalise the observed covariations as the result of a simple second order kinetic model dependent on the content of organic matter in surficial sediments in the Ria de Muros. The reanalysis of previously reported data from the Rias de Vigo and Pontevedra confirmed the validity of this model, and suggested that further controls such as wave climate and water depth in the rates at which magnetic susceptibility changes are controlled by organic matter content.
      PubDate: 2017-05-30T10:46:57.98981-05:0
      DOI: 10.1002/2017GC006823
       
  • Geological interpretation of volcanism and segmentation of the Mariana
           back-arc spreading center between 12.7°N and 18.3°N
    • Authors: Melissa O. Anderson; William W. Chadwick, Mark D. Hannington, Susan G. Merle, Joseph A. Resing, Edward T. Baker, David A. Butterfield, Sharon L. Walker, Nico Augustin
      Abstract: The relationships between tectonic processes, magmatism, and hydrothermal venting along ∼600 km of the slow-spreading Mariana back-arc between 12.7°N and 18.3°N reveal a number of similarities and differences compared to slow-spreading mid-ocean ridges. Analysis of the volcanic geomorphology and structure highlights the complexity of the back-arc spreading center. Here, ridge segmentation is controlled by large-scale basement structures that appear to pre-date back-arc rifting. These structures also control the orientation of the chains of cross-arc volcanoes that characterize this region. Segment-scale faulting is oriented perpendicular to the spreading direction, allowing precise spreading directions to be determined. Four morphologically-distinct segment types are identified: dominantly magmatic segments (Type I); magmatic segments currently undergoing tectonic extension (Type II); dominantly tectonic segments (Type III); and tectonic segments currently undergoing magmatic extension (Type IV). Variations in axial morphology (including eruption styles, neovolcanic eruption volumes, and faulting) reflect magma supply, which is locally enhanced by cross-arc volcanism associated with N-S compression along the 16.5°N and 17.0°N segments. In contrast, cross-arc seismicity is associated with N-S extension and increased faulting along the 14.5°N segment, with structures that are interpreted to be oceanic core complexes—the first with high-resolution bathymetry described in an active back-arc basin. Hydrothermal venting associated with recent magmatism has been discovered along all segment types.
      PubDate: 2017-05-23T05:20:31.927232-05:
      DOI: 10.1002/2017GC006813
       
  • Halogen variations through the quenched margin of a MORB lava: Evidence
           for direct assimilation of seawater during eruption
    • Authors: Michael W. Broadley; Ray. Burgess, Hidenori. Kumagai, Natalie M. Curran, Chris J. Ballentine
      Abstract: Halogens and noble gases within submarine basaltic glasses are critical tracers of interactions between the surface volatile reservoirs and the mantle. However, as the halogens and noble gases are concentrated within seawater, sediments and the oceanic crust this makes the original volatile signature of submarine basaltic lavas susceptible to geochemical overprinting. This study combines halogen (Cl, Br and I), noble gas, and K concentrations within a single submarine basaltic quenched margin to quantify the amount of seawater assimilation during eruption, and to further elucidate the mechanisms of overprinting. The outer sections of the glass rim are enriched in Cl compared to the interior of the margin, which maintains mantle-like Br/Cl, I/Cl and K/Cl ratios. Low Br/Cl and K/Cl in the outer sections of the basaltic glass margin indicate that the Cl enrichment in the outer glass is derived from the assimilation of a saline brine component with up to 70% of the Cl within the glass being derived from brine assimilation. Atmospheric noble gas contamination is decoupled from halogen contamination with contaminated outer sections maintaining MORB-like 40Ar/36Ar, suggesting seawater derived brine assimilation during eruption is not the dominant source of atmospheric noble gases in submarine basalts. Volatile heterogeneities in submarine basalts introduced during and after eruption, as we have shown in this study, have the potential to expand the range of mantle halogen compositions and only by better understanding these heterogeneities can the Br/Cl and I/Cl variance in mantle derived samples be determined accurately.
      PubDate: 2017-05-22T05:35:41.410744-05:
      DOI: 10.1002/2016GC006711
       
  • Development of branching brittle and ductile shear zones: A numerical
           study
    • Authors: Sven Erik Meyer; Boris Kaus, Cees Passchier
      Abstract: Continental collision zones are usually associated with large-scale strike-slip shear zones. In most cases these shear zones are complex and consist of multiple strands, varying in width, length, and total displacement. Here we present 2-D numerical models to simulate the formation of such shear zones at different depth levels within the crust, under either brittle (frictional/plastic) or ductile conditions. Localization of shear zones is initiated by a material contrast (heterogeneity) of the material parameters. We systematically test the rate of strain-weakening in brittle and in ductile regimes to understand its influence on the development of shear zone networks. Our simulations suggest that the development of antithetic faults in a brittle shear zone system is closely linked to a decrease in the angle of friction during deformation. In general, variation of the strain-weakening also has a significant influence on ductile shear zones. Numerical results show that the geometry and thickness of the localized high strain zone are especially affected by weakening mechanisms during deformation. Furthermore, the interconnection and interaction of the shear strands lead to a more complex kinematic pattern, which lead to a local change in the maximum principal stress axis. These interaction of shear strands may explain the occurrence of shear related structures (e.g. folds) or differing characteristics of shear zones, such as the thickness of shear zones or the orientation of the faults to the stress field, which are consistent with field observations.
      PubDate: 2017-05-22T05:25:54.060713-05:
      DOI: 10.1002/2016GC006793
       
  • Sand supply to the Lake Albert Basin (Uganda) during the Miocene-Pliocene:
           A multiproxy provenance approach
    • Authors: D. Gagnevin; S. Tyrrell, A.C. Morton, J. Leather, N. Lee, N. Bordas-Le Floch, D. Frei, J. Lukaye
      Abstract: A multi-proxy provenance approach (heavy mineral analysis, U-Pb zircon geochronology and Pb isotopic analysis of K-feldspar) has constrained sediment supply within the Upper Nile drainage system in the Miocene - Pliocene. Provenance data from sandstones were obtained from three exploration wells, two situated on the north-eastern margin and one on the eastern flank of the Lake Albert Basin, NW Uganda. Data suggest that high- to low-grade metamorphic rocks and granitoids have variably supplied the heavy mineral assemblages around the Lake Albert Basin during the Miocene-Pliocene, with contributions from the isotopically-heterogeneous Archean Cratons (including the local Ugandan Craton, Tanzanian and Congo Cratons) and the Pan-African rocks (the Mozambique Belt) with possible contributions from the Neoproterozoic and Paleoproterozoic rocks. These data also highlight clear differences between supply to the eastern basin margin, compared with the northeast, which is reconcilable with current models for Miocene-Pliocene drainage in the region. Supply to northeastern Lake Albert during the Miocene-Pliocene appears to have been through a proto-Albert Nile (draining from NE to SW) and from a proto-Victoria Nile or similarly oriented palaeo-river systems draining from the east. In contrast, the eastern flanks of the basin were likely supplied via the palaeo-Nkusi river, tapping local hinterland sources and more distal basement to the far-east (Mozambique Belt). This study highlights the importance of utilizing a multi-proxy approach in provenance analysis as no one signal is capable of distinguishing the different source lands and constraining the evolving drainage patterns.
      PubDate: 2017-05-22T05:20:46.257961-05:
      DOI: 10.1002/2016GC006650
       
  • Rock magnetic and geochemical evidence for authigenic magnetite formation
           via iron reduction in coal-bearing sediments offshore Shimokita Peninsula,
           Japan (IODP Site C0020)
    • Authors: Stephen C. Phillips; Joel E. Johnson, William C. Clyde, Jacob B. Setera, Daniel P. Maxbauer, Silke Severmann, Natascha Riedinger
      Abstract: Sediments recovered at Integrated Ocean Drilling Program (IODP) Site C0020, in a forearc basin offshore Shimokita Peninsula, Japan, include numerous coal beds (0.3 – 7 m thick) that are associated with a transition from a terrestrial to marine depositional environment. Within the primary coal-bearing unit (∼2 km depth below seafloor) there are sharp increases in magnetic susceptibility in close proximity to the coal beds, superimposed on a background of consistently low magnetic susceptibility throughout the remainder of the recovered stratigraphic sequence. We investigate the source of the magnetic susceptibility variability and characterize the dominant magnetic assemblage throughout the entire cored record, using isothermal remanent magnetization (IRM), thermal demagnetization, anhysteretic remanent magnetization (ARM), iron speciation, and iron isotopes. Magnetic mineral assemblages in all samples are dominated by very low-coercivity minerals with unblocking temperatures between 350-580°C that are interpreted to be magnetite. Samples with lower unblocking temperatures (300-400°C), higher ARM, higher frequency dependence, and isotopically heavy δ56Fe across a range of lithologies in the coal-bearing unit (between 1925-1995 mbsf), indicate the presence of fine-grained authigenic magnetite. We suggest that iron-reducing bacteria facilitated the production of fine-grained magnetite within the coal-bearing unit during burial and interaction with pore waters. The coal/peat acted as a source of electron donors during burial, mediated by humic acids, to supply iron reducing bacteria in the surrounding siliciclastic sediments. These results indicate that coal-bearing sediments may play an important role in iron cycling in subsiding peat environments and if buried deeply through time, within the subsequent deep biosphere.
      PubDate: 2017-05-22T05:20:42.482476-05:
      DOI: 10.1002/2017GC006943
       
  • A CO2-gas precursor to the March 2015 Villarrica volcano eruption
    • Authors: Alessandro Aiuppa; Marcello Bitetto, Vincenzo Francofonte, Gabriela Velasquez, Claudia Bucarey Parra, Gaetano Giudice, Marco Liuzzo, Roberto Moretti, Yves Moussallam, Nial Peters, Giancarlo Tamburello, Oscar. A. Valderrama, Aaron Curtis
      Abstract: We present here the first volcanic gas compositional time-series taken prior to a paroxysmal eruption of Villarrica volcano (Chile). Our gas plume observations were obtained using a fully autonomous Multi-component Gas Analyser System (Multi-GAS) in the 3 month-long phase of escalating volcanic activity that culminated into the March 3 2015 paroxysm, the largest since 1985. Our results demonstrate a temporal evolution of volcanic plume composition, from low CO2/SO2 ratios (0.65-2.7) during November 2014-January 2015 to CO2/SO2 ratios up to ≈ 9 then after. The H2O/CO2 ratio simultaneously declined to
      PubDate: 2017-05-22T05:20:40.074385-05:
      DOI: 10.1002/2017GC006892
       
  • Pervasive silicification and hanging wall overplating along the 13°20'N
           oceanic detachment fault (Mid-Atlantic Ridge)
    • Authors: D. Bonnemains; J. Escartín, C. Mével, M. Andreani, A. Verlaguet
      Abstract: The corrugated detachment fault zone of the active 13°20'N oceanic core complex (Mid Atlantic Ridge) was investigated with a deep-sea vehicle to assess the links between deformation, alteration, and magmatism at detachment fault zones. We present a study of eighteen in-situ fault rock samples from striated fault outcrops on the flanks of microbathymetric corrugations. All the samples are mafic breccias that are mostly derived from a diabase protolith, with two of them also showing mixing with ultramafic clasts. Breccias are cataclastic and display variable deformation textures, recording numerous slip events, and showing pervasive silicification throughout the fault zone. Deformation-silicification relationships are also complex, showing both static and syntectonic quartz precipitation; undeformed quartz overprints the fault breccia textures, and reflective and striated fault surfaces cross-cut silicified rocks. In-situ detachment fault rocks are mainly fault breccias with almost exclusively basaltic clasts, with rare ultramafic ones, a lithology and texture never observed previously at other oceanic detachment fault zones. We propose the lower dyke complex in the hanging wall crust at the volcanic rift valley floor is the most plausible diabase source. Mechanical mixing of predominantly mafic and rare ultramafic clasts suggests an underlying ultramafic footwall, and that mafic accretion operates in the shallowest crust (1-2 km), at the base of the dyke complex at temperatures >400°C. Silicification is produced by silica-rich fluids syntectonically channeled along the fault zone, and likely derived from hydrothermal alteration of basaltic rocks, likely mixed with serpentinization-derived fluids.
      PubDate: 2017-05-22T05:20:34.069562-05:
      DOI: 10.1002/2017GC006846
       
  • Short-term variations of Icelandic ice cap mass inferred from cGPS
           coordinate time series
    • Authors: Kathleen Compton; Richard A. Bennett, Sigrún Hreinsdóttir, Tonie van Dam, Andrea Bordoni, Valentina Barletta, Giorgio Spada
      Abstract: As the global climate changes, understanding short-term variations in water storage is increasingly important. Continuously operating Global Positioning System (cGPS) stations in Iceland record annual periodic motion – the elastic response to winter accumulation and spring melt seasons – with peak-to-peak vertical amplitudes over 20 mm for those sites in the Central Highlands. Here, for the first time for Iceland, we demonstrate the utility of these cGPS-measured displacements for estimating seasonal and shorter-term ice cap mass changes. We calculate unit responses to each of the five largest ice caps in central Iceland at each of the 62 cGPS locations using an elastic half-space model and estimate ice mass variations from the cGPS time series using a simple least squares inversion scheme. We utilize all three components of motion, taking advantage of the seasonal motion recorded in the horizontal. We remove secular velocities and accelerations and explore the impact that seasonal motions due to atmospheric, hydrologic, and non-tidal ocean loading have on our inversion results. Our results match available summer and winter mass balance measurements well, and we reproduce the seasonal stake-based observations of loading and melting within the 1σ confidence bounds of the inversion. We identify non-periodic ice mass changes associated with interannual variability in precipitation and other processes such as increased melting due to reduced ice surface albedo or decreased melting due to ice cap insulation in response to tephra deposition following volcanic eruptions, processes that are not resolved with once- or twice- yearly stake measurements.
      PubDate: 2017-05-22T05:20:29.304937-05:
      DOI: 10.1002/2017GC006831
       
  • The formation of gold-rich seafloor sulfide deposits: Evidence from the
           Beebe Hydrothermal Vent Field, Cayman Trough
    • Authors: Alexander P. Webber; Stephen Roberts, Bramley J. Murton, Rachel A. Mills, Matthew R. S. Hodgkinson
      Abstract: The Beebe vent field (BVF) in the Cayman Trough has built an auriferous massive sulfide deposit on the ultra-slow spreading mid-Cayman spreading centre. The genesis of auriferous sulfide deposits at mid-ocean ridges is not fully understood, although there is a growing recognition that slow and ultra-slow spreading centres are conducive to gold mineralization. Analysis of hydrothermal precipitates from the BVF indicates that the highest gold contents are present within “beehive diffusers”, which have developed a highly porous pyrrhotite framework. The beehive structure allows vent fluids to effuse slowly, while allowing ingress of seawater to cool the fluid. The prevalence of pyrrhotite in the beehive samples, lack of sulfates, association between pyrrhotite and gold grains, and results of thermodynamic modelling, suggests gold precipitation occurred under highly reduced conditions even during mixing with seawater. In contrast, high temperature chimneys, with a single orifice, maintain high temperatures to the primary vent orifice and much of the gold is lost to seawater. Despite this, both chimney types are relatively gold enriched, which points to a further underlying cause for high gold at the BVF such as interaction of hydrothermal fluids with ultramafic lithologies in the basement. The final gold composition of the deposit is partially controlled by loss of gold during mass-wasting of the material, with gold depletion most prevalent in blocks formed at beehive-type chimneys. The BVF demonstrates that the overall gold content of a massive sulfide deposit is the sum of basement, precipitation, and surface processes.
      PubDate: 2017-05-08T04:50:44.982809-05:
      DOI: 10.1002/2017GC006922
       
  • Issue Information
    • Pages: 2009 - 2010
      PubDate: 2017-07-16T08:55:06.401873-05:
      DOI: 10.1002/ggge.21122
       
 
 
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