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Geochemistry, Geophysics, Geosystems     Full-text available via subscription   (Followers: 25, SJR: 2.439, h-index: 91)
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Journal Cover Geochemistry, Geophysics, Geosystems
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   ISSN (Online) 1525-2027
   Published by AGU Homepage  [17 journals]
  • Olivine fabrics and tectonic evolution of forearc mantles: A natural
           perspective from the Songshugou dunite and harzburgite in the Qinling
           orogenic belt, central China
    • Authors: Yi Cao; Haemyeong Jung, Shuguang Song
      Abstract: To advance our understanding of deformation characteristics, rheological behaviors and tectonic evolution of the forearc lithospheric mantle, we analyzed mineral fabrics for a large spinel-bearing ultramafic massif in the Songshugou area in the Qinling orogenic belt, central China. In the spinel-poor coarse-grained dunite, stronger A-/D-type and weaker C-type-like fabrics were found, whereas the spinel-rich coarse-grained dunite displayed a comparatively stronger B-type-like fabric. These olivine fabrics are high-T fabrics influenced by the presence of melt, in which B- and C-type-like fabrics are inferred to be produced by melt-assisted grain boundary sliding during synkinematic high-T melt‒rock reactions. In contrast, the spinel-poor porphyroclastic and fine-grained dunites present weak AG- and B-type-like fabrics, respectively. Their olivine fabrics (low-T fabrics) are inferred to transform from A-/D-type fabric in their coarse-grained counterparts possibly through mylonitization process assisted by low-T fluid‒rock reactions, during which strain was accommodated by the fluid-enhanced dislocation slip and/or fluid-assisted grain boundary sliding processes. Combined with the tectonic results of our previous work [Cao et al., 2016], the high-T olivine fabrics are probably related to a young and warm forearc mantle where intense partial melting and high-T boninitic melt‒rock reactions prevalently occurred, whereas the low-T olivine fabrics likely reflect the evolving tectonic settings through the cooling forearc mantle to a continental lower crust in a collisional orogeny where low-T fluid‒rock reactions were pervasively activated. These low-T olivine fabrics imply that, though cold, the forearc lithospheric mantle may be locally weak (∼20‒30 MPa), allowing ductile deformation to occur at a geologically significant strain rate. This article is protected by copyright. All rights reserved.
      PubDate: 2017-02-17T03:32:17.450918-05:
      DOI: 10.1002/2016GC006614
       
  • Iron isotopic evolution during fractional crystallization of the uppermost
           Bushveld Complex layered mafic intrusion
    • Authors: Laura D. Bilenker; Jill A. VanTongeren, Craig C. Lundstrom, Adam C. Simon
      Abstract: We present δ56Fe (56Fe/54Fe relative to standard IRMM-014) data from whole rock and magnetite of the Upper and Upper Main Zones (UUMZ) of the Bushveld Complex. With it, we assess the role of fractional crystallization in controlling the Fe isotopic evolution of a mafic magma. The UUMZ evolved by fractional crystallization of a dry tholeiitic magma to produce gabbros and diorites with cumulus magnetite and fayalitic olivine. Despite previous experimental work indicating a potential for magnetite crystallization to drastically change magma δ56Fe, we observe no change in whole rock δ56Fe above and below magnetite saturation. We also observe no systematic change in whole rock δ56Fe with increasing stratigraphic height, and only a small variation in δ56Fe in magnetite separates above magnetite saturation. Whole rock δ56Fe (errors twice standard deviation, ±2σ) throughout the UUMZ ranges from -0.01 ±0.03‰ to 0.21 ±0.09‰ (δ56FeaverageWR = 0.10 ±0.09‰; n=21, isotopically light outlier: δ56FeWR = -0.15‰), and magnetites range from 0.28 ±0.04‰ to 0.86 ±0.07‰ (δ56FeaverageMgt = 0.50 ±0.15‰; n=20), similar to values previously reported for other layered intrusions. We compare our measured δ56FeWR to a model that incorporates the changing normative mineralogy, calculated temperatures, and published fractionation factors of Fe-bearing phases throughout the UUMZ and produces δ56FeWR values that evolve only in response to fractional crystallization. Our results show that the Fe isotopic composition of a multiply-saturated (multiple phases on the liquidus) magma is unlikely to change significantly during fractional crystallization of magnetite due to the competing fractionation of other Fe-bearing cumulus phases. This article is protected by copyright. All rights reserved.
      PubDate: 2017-02-16T01:05:56.84891-05:0
      DOI: 10.1002/2016GC006660
       
  • Composition law of oblique anhysteretic remanent magnetization and its
           relation to the magnetostatic interaction
    • Authors: Masahiko Sato; Nobutatsu Mochizuki, Minako Watanabe, Hideo Tsunakawa
      Abstract: The basic properties of oblique anhysteretic remanent magnetization (OARM) acquired in a weak and steady magnetic field with an arbitrary angle to the alternating field direction were studied. OARM and rock-magnetic experiments were conducted on samples of basalt, granite, and sediment containing non-interacting single-domain (SD), interacting SD, pseudo-single-domain, and multidomain low-Ti titanomagnetites. The intensity of OARM (MOARM) systematically increased or decreased with increasing angle between alternating and steady field directions (θSF), while the angle between alternating field and OARM directions (θOARM) increased with increasing θSF for all samples. During stepwise alternating field demagnetization, the OARM vector shows a single component parallel to the steady field direction for θSF = 0° (ARMǁ) and 90° (ARM⊥). The median destructive field of ARM⊥ is larger than that of ARMǁ. For intermediate angles (θSF = 30°, 45°, and 60°), the OARM vector was not parallel to the applied steady field; instead, it gradually increased with coercivity. These experiments indicate that the OARM vector is approximately given by the sum of two orthogonal magnetizations coinciding with ARMǁ and ARM⊥, respectively. Thus, the OARM vector can be determined by acquisition efficiencies of ARMǁ and ARM⊥ in an individual sample. Based on these experiments and associated rock-magnetic measurements, non-interacting SD samples show lower ARM⊥/ARMǁ ratios, compared to other samples. This result suggests that OARM can be used as a useful tool to detect non-interacting SD particles in the paleomagnetic samples. This article is protected by copyright. All rights reserved.
      PubDate: 2017-02-14T18:05:31.630523-05:
      DOI: 10.1002/2016GC006671
       
  • Geodynamic implications for zonal and meridional isotopic patterns across
           the northern Lau and North Fiji Basins
    • Authors: Allison A. Price; Matthew G. Jackson, Janne Blichert-Toft, Mark D. Kurz, Jim Gill, Jerzy Blusztajn, Frances Jenner, Raul Brens, Richard Arculus
      Abstract: We present new Sr-Nd-Pb-Hf-He isotopic data for sixty-five volcanic samples from the northern Lau and North Fiji Basin. This includes forty-seven lavas obtained from forty dredge sites spanning an east-west transect across the Lau and North Fiji basins, ten ocean island basalt (OIB)-type lavas collected from seven Fijian islands, and eight OIB lavas sampled on Rotuma. For the first time we are able to map clear north-south and east-west geochemical gradients in 87Sr/86Sr across the northern Lau and North Fiji Basins: lavas with the most geochemically enriched radiogenic isotopic signatures are located in the northeast Lau Basin, while signatures of geochemical enrichment are diminished to the south and west away from the Samoan hotspot. Based on these geochemical patterns and plate reconstructions of the region, these observations are best explained by the addition of Samoa, Rurutu, and Rarotonga hotspot material over the past 4 Ma. We suggest that underplated Samoan material has been advected into the Lau Basin over the past ∼4 Ma. As the slab migrated west (and toward the Samoan plume) via rollback over time, younger and hotter (and therefore less viscous) underplated Samoan plume material was entrained. Thus, entrainment efficiency of underplated plume material was enhanced, and Samoan plume signatures in the Lau Basin became stronger as the trench approached the Samoan hotspot. The addition of subducted volcanoes to the Cook-Austral Volcanic Lineament material, first from the Rarotonga hotspot, then followed by the Rurutu hotspot, contributes to the extreme geochemical signatures observed in the northeast Lau Basin. This article is protected by copyright. All rights reserved.
      PubDate: 2017-02-11T03:45:32.295224-05:
      DOI: 10.1002/2016GC006651
       
  • Neogene paleoceanography of the eastern equatorial Pacific based on the
           radiolarian record of IODP drill sites off Costa Rica
    • Authors: María I. Sandoval; Demetrio Boltovskoy, Alan T. Baxter, Peter O. Baumgartner
      Abstract: The Integrated Ocean Drilling Program (IODP) Expedition 344 drilled cores following a transect across the convergent margin off Costa Rica. Two of the five sites (U1381 and U1414) are the subject of the present study. Major radiolarian faunal breaks and characteristic species groups were defined with the aid of cluster analysis, nodal analysis and discriminant analysis of principal components. A middle-late Miocene to Pleistocene age (radiolarian zones RN5 to RN16) was determined for the sites, which agrees with the nannofossil zonations and 40Ar/39Ar and tephra layers. Considering the northward movement of the Cocos plate (∼7.3 cm/year), and a paleolatitude calculator, it is assumed that during the Miocene the two sites were located ∼1000 km to the southwest of their current position, slightly south of the equator. The radiolarian faunas retrieved were thus seemingly formed under the influence of different oceanic currents and sources of nutrients. Changes in the radiolarian assemblages at Site U1414 point at dissimilar environmental settings associated with the colder South Equatorial Current and the warmer Equatorial Countercurrent, as well as to coastal upwelling. These differences are best reflected by changes in the abundance of the morphotype Spongurus spp., with noticeably higher values during the Miocene, than in the Pliocene and the Pleistocene. Because Spongurus spp. is generally associated with cooler waters, these abundance variations (as well as those of several other species) suggest that during the Miocene the area had a stronger influence of colder waters than during younger periods. During the Pliocene and the lowermost Pleistocene, biogenic remains are scarce, presumably due to the terrigenous input, which could have diluted and affected the preservation of pelagic fossils, as well as to the displacement of the site to warmer waters. A typically tropical fauna characterized the Pleistocene, yet with widespread presence of colder water species, most probably indicative of the influence of coastal upwelling processes. This article is protected by copyright. All rights reserved.
      PubDate: 2017-02-11T03:40:58.811024-05:
      DOI: 10.1002/2016GC006623
       
  • Boron isotope variations in Tonga-Kermadec-New Zealand arc lavas:
           Implications for origin of subduction components and mantle influences
    • Authors: William P. Leeman; Sonia Tonarini, Simon Turner
      Abstract: The Tonga-Kermadec-New Zealand volcanic arc is an end-member of arc systems with fast subduction suggesting that the Tonga sector should have the coolest modern slab thermal structure on Earth. New data for boron concentration and isotopic composition are used to evaluate the contrasting roles of postulated subduction components (sediments and oceanic slab lithologies) in magma genesis. Major observations include: (a) Tonga-Kermadec volcanic front lavas are enriched in B (as recorded by B/Nb and similar ratios) and most have relatively high δ11B (> +4‰), whereas basaltic lavas from New Zealand have relatively low B/Nb and δ11B (< -3.5‰); (b) both δ11B and B/Nb generally increase northward from New Zealand along with convergence rate and overall slab flux; (c) δ11B and B/Nb decrease toward the back-arc, as observed elsewhere; and (d) low δ11B is observed in volcanic front samples from Ata, an anomalous sector where the back-arc Valu Fa Spreading Center impinges on the arc and the Louisville Seamount Chain is presently subducting. Otherwise, volcanic front lavas exhibit positive correlations for both B/Nb and δ11B with other plausible indicators of slab-derived fluid contributions (e.g., Ba/Nb, U/Th, (230Th/232Th) and 10Be/9Be), and with estimated degree of melting to produce the mafic lavas. Inferred B-enrichments in the arc magma sources are likely dominated by serpentinite domains deeper within the subducting slab (± altered oceanic crust), and B systematics are consistent with dominant transport by slab-derived aqueous fluids. Effects of this process are amplified by mantle wedge source depletion due to prior melt extraction. . This article is protected by copyright. All rights reserved.
      PubDate: 2017-02-11T03:40:50.41082-05:0
      DOI: 10.1002/2016GC006523
       
  • Variations in pockmark composition at the Vestnesa Ridge: Insights from
           marine controlled source electromagnetic and seismic data
    • Authors: Bedanta K. Goswami; Karen A. Weitemeyer, Stefan Bünz, Timothy A. Minshull, Graham K. Westbrook, Stephan Ker, Martin C. Sinha
      Abstract: The Vestnesa Ridge marks the northern boundary of a known submarine gas hydrate province in the west Svalbard margin. Several seafloor pockmarks at the eastern segment of the ridge are sites of active methane venting. Until recently, seismic reflection data was the main tool for imaging beneath the ridge. Coincident controlled source electromagnetic (CSEM), high-resolution two-dimensional (2D) airgun, sweep frequency SYSIF and three-dimensional (3D) p-cable seismic reflection data were acquired at the south-eastern part of the ridge between 2011 and 2013. The CSEM and seismic data contains profiles across and along the ridge, passing several active and inactive pockmarks. Joint interpretation of resistivity models obtained from CSEM and seismic reflection data provides new information regarding the fluid composition beneath the pockmarks. There is considerable variation in transverse resistance and seismic reflection characteristics of the gas hydrate stability zone (GHSZ) between the ridge flanks and chimneys beneath pockmarks. Layered seismic reflectors on the flanks are associated with around 300Ωm2 transverse resistance, whereas the seismic reflectors within the chimneys exhibit amplitude blanking and chaotic patterns. The transverse resistance of the GHSZ within the chimneys vary between 400 and 1200 Ωm2. Variance attributes obtained from the 3D p-cable data also highlight faults and chimneys, which coincide with the resistivity anomalies. Based on the joint data interpretation, widespread gas hydrate presence is likely at the ridge, with both hydrates and free gas contained within the faults and chimneys. However, at the active chimneys the effect of gas likely dominate the resistive anomalies. This article is protected by copyright. All rights reserved.
      PubDate: 2017-02-10T17:35:40.546073-05:
      DOI: 10.1002/2016GC006700
       
  • Three-dimensional resistivity structure in Ishikari Lowland, Hokkaido,
           northeastern Japan - Implications to strain concentration mechanism
    • Authors: Yusuke Yamaya; Toru Mogi, Ryo Honda, Hideaki Hase, Takeshi Hashimoto, Makoto Uyeshima
      Abstract: The Ishikari Lowland on the island of Hokkaido in northeastern Japan is situated at the end of a westward-moving foreland fold-and-thrust belt from the Hidaka collision zone, where the northeastern Japan and Kurile arcs meet. This activity forms a tectonic zone under an east–west compression field in this region. A magnetotelluric resistivity survey was performed to investigate the mechanism for the strain concentration in this region. A three-dimensional (3D) resistivity inversion showed a conductive thick sedimentary layer and an underlying resistive basement. Remarkable conductors were found in the resistive basement beneath the Ishikari-teichi-toen fault zone (ITFZ) and the Shikotsu caldera. The conductors beneath the ITFZ were interpreted as aqueous fluids that accumulated in the damaged zone in connection with the formation of pull-apart faults and horst. In contrast, the conductor beneath the Shikotsu caldera corresponds to a magmatic fluid path from the upper mantle. These features suggest that the ductile deformation in the upper crust contribute to the strain concentration in this region. The soft thick sediments allow ductile deformations to occur. Furthermore, local fluid-rich zones in the basement cause the crustal strength to be heterogeneous. These thick sediments and local fluids in the basement both contribute to the strain concentration in this region. This article is protected by copyright. All rights reserved.
      PubDate: 2017-02-10T17:35:29.727286-05:
      DOI: 10.1002/2016GC006771
       
  • Origin of geochemical mantle components: Role of spreading ridges and
           thermal evolution of mantle
    • Authors: Jun-Ichi Kimura; James B. Gill, Peter E. van Keken, Hiroshi Kawabata, Susanne Skora
      Abstract: We explore the element redistribution at mid-ocean ridges (MOR) using a numerical model to evaluate the role of decompression melting of the mantle in Earth's geochemical cycle, with focus on the formation of the depleted mantle component. Our model uses a trace element mass balance based on an internally consistent thermodynamic-petrologic computation to explain the composition of MOR basalt (MORB) and residual peridotite. Model results for MORB-like basalts from 3.5 to 0 Ga indicate a high mantle potential temperature (Tp) of 1650–1500°C during 3.5–1.5 Ga before decreasing gradually to ∼1300°C today. The source mantle composition changed from primitive (PM) to depleted as Tp decreased, but this source mantle is variable with an early depleted reservoir (EDR) mantle periodically present. We examine a two-stage Sr-Nd-Hf-Pb isotopic evolution of mantle residues from melting of PM or EDR at MORs. At high-Tp (3.5–1.5 Ga), the MOR process formed extremely depleted DMM. This coincided with formation of the majority of the continental crust, the sub-continental lithospheric mantle, and the enriched mantle components formed at subduction zones and now found in OIB. During cooler mantle conditions (1.5–0 Ga), the MOR process formed most of the modern ocean basin DMM. Changes in the mode of mantle convection from vigorous deep mantle recharge before ∼1.5 Ga to less vigorous afterwards is suggested to explain the thermochemical mantle evolution. This article is protected by copyright. All rights reserved.
      PubDate: 2017-02-09T10:45:39.896012-05:
      DOI: 10.1002/2016GC006696
       
  • Primary magmas and mantle temperatures through time
    • Authors: Jérôme Ganne; Xiaojun Feng
      Abstract: Chemical composition of mafic magmas is a critical indicator of physico-chemical conditions, such as pressure, temperature and fluid availability, accompanying melt production in the mantle and its evolution in the continental or oceanic lithosphere. Recovering this information has fundamental implications in constraining the thermal state of the mantle and the physics of mantle convection throughout the Earth's history. Here, a statistical approach is applied to a geochemical database of about 22,000 samples from the mafic magma record. Potential temperatures (Tps) of the mantle derived from this database, assuming melting by adiabatic decompression and a Ti-dependent (Fe2O3/TiO2=0.5) or constant redox condition (Fe2+/∑Fe = 0.9 or 0.8) in the magmatic source, are thought to be representative of different thermal “horizons” (or thermal heterogeneities) in the ambient mantle, ranging in depth from a shallow sublithospheric mantle (Tp minima) to a lower thermal boundary layer (Tp maxima). The difference of temperature (Δ Tp) observed between Tp maxima and minima did not change significantly with time (∼170°C). Conversely, a progressive but limited cooling of ∼150°C is proposed since ∼2.5 Gyr for the Earth's ambient mantle, which falls in the lower limit proposed by Herzberg et al [2010] (∼ 150 to 250°C hotter than today). Cooling of the ambient mantle after 2.5 Ga is preceded by a high-temperature plateau evolution and a transition from dominant plumes to a plate tectonics geodynamic regime, suggesting that subductions stabilized temperatures in the Archaean mantle that was in warming mode at that time. This article is protected by copyright. All rights reserved.
      PubDate: 2017-02-09T10:45:30.478804-05:
      DOI: 10.1002/2016GC006787
       
  • Linking basin-scale and pore-scale gas hydrate distribution patterns in
           diffusion-dominated marine hydrate systems
    • Authors: Michael Nole; Hugh Daigle, Ann E. Cook, Jess I.T. Hillman, Alberto Malinverno
      Abstract: The goal of this study is to computationally determine the potential distribution patterns of diffusion-driven methane hydrate accumulations in coarse-grained marine sediments. Diffusion of dissolved methane in marine gas hydrate systems has been proposed as a potential transport mechanism through which large concentrations of hydrate can preferentially accumulate in coarse-grained sediments over geologic time. Using one-dimensional compositional reservoir simulations, we examine hydrate distribution patterns at the scale of individual sand layers (1 to 20 m thick) that are deposited between microbially active fine-grained material buried through the gas hydrate stability zone (GHSZ). We then extrapolate to two- dimensional and basin-scale three-dimensional simulations, where we model dipping sands and multilayered systems. We find that properties of a sand layer including pore size distribution, layer thickness, dip, and proximity to other layers in multilayered systems all exert control on diffusive methane fluxes toward and within a sand, which in turn impact the distribution of hydrate throughout a sand unit. In all of these simulations, we incorporate data on physical properties and sand layer geometries from the Terrebonne Basin gas hydrate system in the Gulf of Mexico. We demonstrate that diffusion can generate high hydrate saturations (upward of 90%) at the edges of thin sands at shallow depths within the GHSZ, but that it is ineffective at producing high hydrate saturations throughout thick (greater than 10 m) sands buried deep within the GHSZ. Furthermore, we find that hydrate in fine-grained material can preserve high hydrate saturations in nearby thin sands with burial. This article is protected by copyright. All rights reserved.
      PubDate: 2017-02-07T11:31:21.528571-05:
      DOI: 10.1002/2016GC006662
       
  • The undatables: Quantifying uncertainty in a highly expanded Late
           Glacial-Holocene sediment sequence recovered from the deepest Baltic Sea
           basin: IODP Site M0063
    • Authors: S.P. Obrochta; T. Andrén, S.Z. Fazekas, B.C. Lougheed, I. Snowball, Y. Yokoyama, Y. Miyairi, R. Kondo, A.T. Kotilainen, O. Hyttinen, A Fehr
      Abstract: Laminated, organic-rich silts and clays with high dissolved gas content characterize sediments at IODP Site M0063 in the Landsort Deep, which at 459 m is the deepest basin in the Baltic Sea. Cores recovered from Hole M0063A experienced significant expansion as gas was released during the recovery process, resulting in high sediment loss. Therefore during operations at subsequent holes, penetration was reduced to 2 m per 3.3 m core, permitting expansion into 1.3 m of initially empty liner. Fully filled liners were recovered from Holes B through E, indicating that the length of recovered intervals exceeded the penetrated distance by a factor of >1.5. A typical down-core logarithmic trend in gamma density profiles, with anomalously low density values within the upper ∼1 m of each core, suggests that expansion primarily occurred in this upper interval. Thus, we suggest that a simple linear correction is inappropriate. This interpretation is supported by anisotropy of magnetic susceptibility data that indicate vertical stretching in the upper ∼1.5 m of expanded cores. Based on the mean gamma density profiles of cores from Holes M0063C and D, we obtain an expansion function that is used to adjust the depth of each core to conform to its known penetration. The variance in these profiles allows for quantification of uncertainty in the adjusted depth scale. Using a number of bulk 14C dates, we explore how the presence of multiple carbon source pathways leads to poorly constrained radiocarbon reservoir age variability that significantly affects age and sedimentation rate calculations. This article is protected by copyright. All rights reserved.
      PubDate: 2017-02-07T03:20:46.276395-05:
      DOI: 10.1002/2016GC006697
       
  • Geochemistry of Zr, Hf, and REE in a wide spectrum of Eh and water
           composition: The case of Dead Sea fault system (Israel)
    • Authors: P. Censi; M. Raso, Y. Yechieli, H. Ginat, F. Saiano, P. Zuddas, L. Brusca, W. D'Alessandro, C. Inguaggiato
      Abstract: Along the Jordan Valley-Dead Sea fault area several natural waters in springs, wells and catchments occur. The chemical-physical characters of the studied waters allowed for the first time the investigation of the Zr and Hf geochemical behaviour, apart from REE, extended to a wide range of Eh, temperature, salinity and pH conditions. The results of this study indicate that the dissolved Zr and Hf distribution in natural waters is strongly influenced by redox conditions since these in turn drive the deposition of Fe-oxyhydroxides or pyrite. In oxidizing waters saturated or oversaturated in Fe-oxyhydroxides [Group-1], superchondritic Zr/Hf values are measured. On the contrary, in waters where Eh
      PubDate: 2017-02-07T03:20:40.473874-05:
      DOI: 10.1002/2016GC006704
       
  • A geochemical and geophysical reappraisal to the significance of the
           recent unrest at Campi Flegrei caldera (Southern Italy)
    • Authors: Roberto Moretti; Giuseppe De Natale, Claudia Troise
      Abstract: Volcanic unrest at calderas involve complex interaction between magma, hydrothermal fluids and crustal stress and strain. Campi Flegrei caldera (CFc), located in the Naples (Italy) area and characterised by the highest volcanic risk on Earth for the extreme urbanisation, undergoes unrest phenomena involving several meters of uplift and intense shallow micro-seismicity since several decades. Despite unrest episodes display in the last decade only moderate ground deformation and seismicity, current interpretations of geochemical data point to a highly pressurized hydrothermal system. We show that at CFc, the usual assumption of vapour-liquid coexistence in the fumarole plumes leads to largely overestimated hydrothermal pressures and, accordingly, interpretations of elevated unrest. By relaxing unconstrained geochemical assumptions, we infer an alternative model yielding better agreement between geophysical and geochemical observations. The model reconciles discrepancies between what observed 1) for two decades since the 1982-84 large unrest, when shallow magma was supplying heat and fluids to the hydrothermal system, and 2) in the last decade. Compared to the 1980's unrest, the post-2005 phenomena are characterized by much lower aquifers overpressure and magmatic involvement, as indicated by geophysical data and despite large changes in geochemical indicators. Our interpretation points out a model in which shallow sills, intruded during 1969-1984, have completely cooled, so that fumarole emissions are affected now by deeper, CO2-richer, magmatic gases producing the modest heating and overpressure of the hydrothermal system. Our results have important implications on the short-term eruption hazard assessment and on the best strategies for monitoring and interpreting geochemical data. This article is protected by copyright. All rights reserved.
      PubDate: 2017-02-03T20:00:41.742925-05:
      DOI: 10.1002/2016GC006569
       
  • Mantle exhumation and metamorphism in the Basque-Cantabrian Basin (N
           Spain): Stable and clumped isotopic analysis in carbonates and comparison
           with ophicalcites in the North-Pyrenean Zone (Urdach and Lherz)
    • Authors: I. DeFelipe; D. Pedreira, J. A. Pulgar, E. Iriarte, M. Mendia
      Abstract: The presence of ophicalcites in serpentinized peridotites together with fragments of these rocks in Cretaceous breccias along several North-Pyrenean basins, led to the interpretation of complete mantle exhumation to the seafloor. The westernmost outcrop of peridotites in the Pyrenean-Cantabrian belt is located in Ziga (eastern Basque-Cantabrian Basin), associated to a band of HT metamorphism related to the Leiza fault (Marble Unit). We present a petrological and geochemical study of the marbles and Ziga peridotite-hosted calcite, including standard stable isotope composition and clumped isotope geothermometry. These isotopic techniques allow the determination of different types of formational fluids and crystallization temperatures, and are a useful tool for studying carbonation processes in hyperextended basins. Fieldwork and analytical studies lead us to conclude that during the opening of the Bay of Biscay, mantle rocks were unroofed at the base of the sedimentary pile of the eastern Basque-Cantabrian Basin. However, the ophicalcite veins were recrystallized from meteoric fluids at low temperatures (≤ 49 ºC). The primary carbonate phase may have been formed either during the mid-Cretaceous unroofing of the mantle or in a post-exhumation stage. The process of mantle exhumation was accompanied with HT-LP metamorphism and fluid circulation along major faults that reset the marine isotopic signature in the nearest marbles. For comparison, ophicalcites from Urdach and Lherz (North-Pyrenean Zone) were included in the clumped isotope study. Results show that they were recrystallized from hot (∼200-230 ºC), saline fluids, and from meteoric fluids at near ambient temperatures (∼32-42 ºC), respectively. This article is protected by copyright. All rights reserved.
      PubDate: 2017-01-31T06:52:42.549131-05:
      DOI: 10.1002/2016GC006690
       
  • Conductivity structure of the lithosphere-asthenosphere boundary beneath
           the eastern North American margin
    • Authors: Eric Attias; Rob. L. Evans, Samer Naif, Jimmy Elsenbeck, Kerry Key
      Abstract: Tectonic plate motion and mantle dynamics processes are heavily influenced by the characteristics of the lithosphere-asthenosphere boundary (LAB), yet this boundary remains enigmatic regarding its properties and geometry. The processes involved in rifting at passive margins result in substantial alteration of the lithosphere through the transition from continental to oceanic lithologies. Here we employ marine magnetotelluric (MT) data acquired along a ∼135 km long profile, offshore Martha's Vineyard, New England, USA, to image the electrical conductivity structure beneath the New England continental margin for the first time. We invert the data using two different MT 2-D inversion algorithms and present a series of models that are obtained using three different parameterizations: fully unconstrained, unconstrained with an imposed LAB discontinuity and a-priori constrained lithosphere resistivity. This suite of models infers variability in the depth of the LAB, with an average depth of 115 km at the eastern North America passive margin. Models robustly detect a ∼350 Ωm lithospheric anomalous conductivity zone (LACZ) that extends vertically through the entire lithosphere. Our preferred conductivity model is consistent with regional P-to-S receiver function data, shear-wave velocity, gravity anomalies and prominent geological features. We propose that the LACZ is indicative of paleo lithospheric thinning, either resulting from Kimberlite intrusions associated with rifting and the New England Great Meteor hotspot track, or from shear-driven localized deformation related to rifting. This article is protected by copyright. All rights reserved.
      PubDate: 2017-01-31T06:52:26.125212-05:
      DOI: 10.1002/2016GC006667
       
  • Classification of geochemical data based on multivariate statistical
           analyses: Complementary roles of cluster, principal component, and
           independent component analyses
    • Authors: Hikaru Iwamori; Kenta Yoshida, Hitomi Nakamura, Tatsu Kuwatani, Morihisa Hamada, Satoru Haraguchi, Kenta Ueki
      Abstract: Identifying the data structure including trends and groups/clusters in geochemical problems is essential to discuss the origin of sources and processes from the observed variability of data. An increasing number and high dimensionality of recent geochemical data require efficient and accurate multivariate statistical analysis methods. In this paper, we show the relationship and complementary roles of k-means cluster analysis (KCA), principal component analysis (PCA), and independent component analysis (ICA) to capture the true data structure. When the data are preprocessed by primary standardization (i.e., with the zero mean and normalized by the standard deviation), KCA and PCA provide essentially the same results, although the former returns the solution in a discretized space. When the data are preprocessed by whitening (i.e., normalized by eigenvalues along the principal components), KCA and ICA may identify a set of independent trends and groups, irrespective of the amplitude (power) of variance. As an example, basalt isotopic compositions have been analyzed with KCA on the whitened data, demonstrating clear rock‒tectonic occurrence‒mantle end-member discrimination. Therefore, the combination of these methods, particularly KCA on whitened data, is useful to capture and discuss the data structure of various geochemical systems, for which an Excel program is provided. This article is protected by copyright. All rights reserved.
      PubDate: 2017-01-28T03:21:49.561039-05:
      DOI: 10.1002/2016GC006663
       
  • An evaluation of benthic foraminiferal U/Ca and U/Mn proxies for deep
           ocean carbonate chemistry and redox conditions
    • Authors: Pujiao Chen; Jimin Yu, Zhangdong Jin
      Abstract: The deep ocean is thought to have played a crucial role in modulating atmospheric CO2 changes, and thus reconstructions of deep ocean conditions can place important constraints on the past global carbon cycle. Some previous studies suggested that foraminiferal U/Ca could be used to infer seawater carbonate chemistry changes, but others showed complications from diagenesis and temperature. A recent down-core study suggested that foraminiferal U/Mn may be used for sedimentary redox-conditions, but no core-top work has been done to investigate factors affecting U/Mn. We investigate controlling factors on U/Ca and U/Mn in two benthic foraminiferal species from 120 global core-tops and three Atlantic sediment cores. Our core-top data reveal no significant correlation between core-top benthic U/Ca and carbonate system parameters. The lack of an influence of deep-water [CO32-] on U/Ca is further supported by our down-core results. Together, our data highlight complications to use benthic U/Ca for deep-water carbonate chemistry reconstructions. Although no correlation is found between core-top U/Mn and hydrographic data, high-resolution U/Mn and U/Ca in core TNO57-21 show similar patterns to authigenic U (aU) and vary in tandem with atmospheric CO2 on millennial timescales. Changes in U/Mn, U/Ca and aU in TNO57-21 may reflect post-depositional diagenesis linked to sedimentary oxygen, which is controlled by subantarctic surface productivity and ventilation of deep South Atlantic in the past. We suggest that benthic U/Mn and U/Ca may be used as auxiliary indicators for past sedimentary redox-conditions and along with other proxies could reflect deep-water oxygenation. This article is protected by copyright. All rights reserved.
      PubDate: 2017-01-28T03:21:44.224751-05:
      DOI: 10.1002/2016GC006730
       
  • How can satellite imagery be used for mineral exploration in thick
           vegetation areas?
    • Authors: A. N. H. Hede; K. Koike, K. Kashiwaya, S. Sakurai, R. Yamada, D. A. Singer
      Abstract: The Hokuroku district, northern Japan, is globally recognized for rich ore deposits (kuroko and vein types), which have been thoroughly explored under thick vegetation cover. This situation is ideal to evaluate the effects of ore deposits on vegetation anomalies through geobotanical remote sensing. Here we present novel methods to detect vegetation anomalies caused by ore deposits and verify their usefulness by comparing the anomalies with a deposit potential map produced from multiple geological data. We use the reflectance spectra of Landsat ETM+ images acquired in summer and autumn to calculate a vegetation index for plant physiological activity. A key variable to detect the anomalies is a variation of vegetation index with time at each pixel. Difference in variation is enlarged by a sequence of image enhancement methods for the detection. We find that the vegetation anomalies, defined by the large ratios, correspond well to the high potential zones of ore deposits and known major deposits. Consequently, our methods can extend the applicability of remote sensing-based mineral exploration to the areas covered by thick vegetation, in addition to traditional arid and semi-arid areas. This article is protected by copyright. All rights reserved.
      PubDate: 2017-01-28T03:21:39.857568-05:
      DOI: 10.1002/2016GC006501
       
  • The GMT/MATLAB toolbox
    • Authors: Paul Wessel; Joaquim F. Luis
      Abstract: The GMT/MATLAB toolbox is a basic interface between MATLAB® (or Octave) and GMT, the Generic Mapping Tools, which allows MATLAB users full access to all GMT modules. Data may be passed between the two programs using intermediate MATLAB structures that organize the metadata needed; these are produced when GMT modules are run. In addition, standard MATLAB matrix data can be used directly as input to GMT modules. The toolbox improves interoperability between two widely used tools in the geosciences and extends the capability of both tools: GMT gains access to the powerful computational capabilities of MATLAB while the latter gains the ability to access specialized gridding algorithms and can produce publication-quality PostScript-based illustrations. The toolbox is available on all platforms and may be downloaded from the GMT website. This article is protected by copyright. All rights reserved.
      PubDate: 2017-01-28T03:21:30.437174-05:
      DOI: 10.1002/2016GC006723
       
  • Upper mantle seismic anisotropy beneath the northern transantarctic
           mountains, Antarctica from PKS, SKS, and SKKS splitting analysis
    • Authors: Jordan H. Graw; Samantha E. Hansen
      Abstract: Using data from the new Transantarctic Mountains Northern Network, this study aims to constrain azimuthal anisotropy beneath a previously unexplored portion of the Transantarctic Mountains (TAMs) to assess both past and present deformational processes occurring in this region. Shear-wave splitting parameters have been measured for PKS, SKS, and SKKS phases using the eigenvalue method within the SplitLab software package. Results show two distinct geographic regions of anisotropy within our study area: one behind the TAMs front, with an average fast axis direction of 42 ± 3° and an average delay time of 0.9 ± 0.04 s, and the other within the TAMs near the Ross Sea coastline, with an average fast axis oriented at 51 ± 5° and an average delay time of 1.5 ± 0.08 s. Behind the TAMs front, our results are best explained by a single anisotropic layer that is estimated to be 81-135 km thick, thereby constraining the anisotropic signature within the East Antarctic lithosphere. We interpret the anisotropy behind the TAMs front as relict fabric associated with tectonic episodes occurring early in Antarctica's geologic history. For the coastal stations, our results are best explained by a single anisotropic layer estimated to be 135-225 km thick. This places the anisotropic source within the viscous asthenosphere, which correlates with low seismic velocities along the edge of the West Antarctic Rift System. We interpret the coastal anisotropic signature as resulting from active mantle flow associated with rift-related decompression melting and Cenozoic extension. This article is protected by copyright. All rights reserved.
      PubDate: 2017-01-28T03:21:11.861349-05:
      DOI: 10.1002/2016GC006729
       
  • Bubble formation and decrepitation control the CO2 content of
           olivine-hosted melt inclusions
    • Authors: J. Maclennan
      Abstract: The CO2 contents of olivine-hosted melt inclusions have previously been used to constrain the depth of magma chambers in basaltic systems. However, the vast majority of inclusions have CO2 contents which imply entrapment pressures that are significantly lower than those obtained from independent petrological barometers. Furthermore, a global database of melt inclusions compositions from low H2O settings, indicates that the distribution of saturation pressures varies surprisingly little between mid-ocean ridges, ocean islands and continental rift zones. 95% of the inclusions in the database have saturation pressures of 200 MPa or less, indicating that melt inclusion CO2, does not generally provide an accurate estimate of magma chamber depths. A model of the P-V-T-X evolution of olivine-hosted melt inclusions was developed so that the properties of the inclusion system could be tracked as the hosts follow a model P-T path. The models indicate that the principal control on the saturation of CO2 in the inclusion and the formation of vapour bubbles is the effect of post-entrapment crystallisation on the major element composition of the inclusions and how this translates into variation in CO2 solubility. The pressure difference between external melt and the inclusion is likely to be sufficiently high to cause decrepitation of inclusions in most settings. Decrepitation can account for the apparent mismatch between CO2-based barometry and other petrological barometers, and can also account for the observed global distribution of saturation pressures. Only when substantial post-entrapment crystallisation occurs can reconstructed inclusion compositions provide an accurate estimate of magma chamber depth. This article is protected by copyright. All rights reserved.
      PubDate: 2017-01-25T20:20:40.772611-05:
      DOI: 10.1002/2016GC006633
       
  • M2Di: Concise and efficient MATLAB 2D Stokes solvers using the Finite
           Difference method
    • Authors: Ludovic Räss; Thibault Duretz, Yury Y. Podladchikov, Stefan M. Schmalholz
      Abstract: Recent development of many multiphysics modeling tools reflects the currently growing interest for studying coupled processes in Earth Sciences. The core of such tools should rely on fast and robust mechanical solvers. Here we provide M2Di, a set of routines for 2D linear and power law incompressible viscous flow based on Finite Difference discretizations. The 2D codes are written in a concise vectorized MATLAB fashion and can achieve a time to solution of 22 seconds for linear viscous flow on 10002 grid points using a standard personal computer. We provide application examples spanning from finely resolved crystal-melt dynamics, deformation of heterogeneous power law viscous fluids to instantaneous models of mantle flow in cylindrical coordinates. The routines are validated against analytical solution for linear viscous flow with highly variable viscosity and compared against analytical and numerical solutions of power law viscous folding and necking. In the power law case, both Picard and Newton iterations schemes are implemented. For linear Stokes flow and Picard linearization, the discretization results in symmetric positive-definite matrix operators on Cartesian grids with either regular or variable grid spacing allowing for an optimized solving procedure. For Newton linearization, the matrix operator is no longer symmetric and an adequate solving procedure is provided. The reported performance of linear and power law Stokes flow are finally analyzed in terms of wall time. All MATLAB codes are provided and can readily be used for educational as well as research purposes. This article is protected by copyright. All rights reserved.
      PubDate: 2017-01-20T05:00:47.983454-05:
      DOI: 10.1002/2016GC006727
       
  • Biomarkers in Lake Van sediments reveal dry conditions in Eastern Anatolia
           during 110.000-10.000 years B.P.
    • Authors: Marie-Eve Randlett; Achim Bechtel, Marcel T.J. van der Meer, Francien Peterse, Thomas Litt, Nadine Pickarski, Ola Kwiecien, Mona Stockhecke, Bernhard Wehrli, Carsten J. Schubert
      Abstract: Lipid biomarkers were analyzed in Lake Van sediments covering the last 600 ka, with a focus on the period between 110 and 10 ka, when a broad maximum in pore water salinity as a relict from the past suggests dry conditions. The occurrence and distribution of biomarkers indicative for terrestrial plants (long-chain n-alkane C29), haptophyte algae (methyl alkenones C37) and halophilic archaea (archaeol) all point towards a dry climate in Lake Van region during this time interval. The hydrogen isotopic composition of C29 n-alkanes (δDC29) and C37 alkenones (δDC37) is enriched between MIS 4 and MIS 2, which is interpreted as a decrease in the regional ratio of precipitation to evaporation. Similarly, the low abundance of the acyclic glycerol dialkyl glycerol tetraether GDGT-0 relative to archaeol, quantified by the Archaeol and Caldarchaeol Ecometric (ACE) is assumed to reflect the presence of halophilic euryarchaeota adapted to high salinity water. The climate around Lake Van appears in phase with the Yammouneh basin 800 km southwest and Lake Urmia 250 km southeast of Lake Van over the last two glacial periods. The results highlight the potential of combining ACE, δDC29, and δDC37 for reconstructing salinity changes and regional precipitation to evaporation ratio from lake sediments. This article is protected by copyright. All rights reserved.
      PubDate: 2017-01-16T04:15:54.883525-05:
      DOI: 10.1002/2016GC006621
       
  • Mid-Brunhes magnetic excursions in marine isotope stages 9, 13, 14, and 15
           (286, 495, 540, and 590 ka) at North Atlantic IODP Sites U1302/3, U1305
           and U1306
    • Authors: J.E.T. Channell
      Abstract: Integrated Ocean Drilling Program (IODP) Site U1302/3 (Orphan Knoll, off Newfoundland) recorded magnetic excursions in marine isotope stages (MIS) 9a (at 286 ka) and 13a (at 495 ka). Sites U1306 and U1305 (Eirik Drift, off SE Greenland) record excursions in MIS 14a/b (at 540 ka) and 15b/c (at 590 ka). In the excursion intervals, magnetic measurements of continuous “u-channel” samples from multiple holes within site are augmented by measurements of cubic (8-cm3) discrete samples. The excursions lie in relative paleointensity (RPI) minima at each site and in RPI reference stacks, and correspond to dated intervals of 10Be overproduction in other deep-sea sediment records. Although observed at multiple holes at each site, and from u-channel and discrete samples, the excursions are not observed at all three sites, and often at only one of the three sites. Sporadic recording of these magnetic excursions, and excursions in general, is attributed to a combination of filtering by the process of acquisition of detrital remanent magnetization (DRM), post-depositional overprint of weak excursion magnetizations, the millennial or even centennial duration of directional excursions, and non-uniform sedimentation rates at these timescales in North Atlantic sediment drifts. This article is protected by copyright. All rights reserved.
      PubDate: 2017-01-13T05:06:23.809131-05:
      DOI: 10.1002/2016GC006626
       
  • Anisotropy in the lowermost mantle beneath the Indian Ocean Geoid Low from
           ScS splitting measurements
    • Authors: B. Padma Rao; M. Ravi Kumar, Arun Singh
      Abstract: The Indian Ocean Geoid Low (IOGL) to the south of Indian sub-continent is the world's largest geoid anomaly. In this study, we investigate the seismic anisotropy of the lowermost mantle beneath the IOGL by analyzing splitting of high quality ScS phases corrected for source and receiver side upper mantle anisotropy. Results reveal significant anisotropy (1.01 in the $D^{\prime\prime}$layer. The observed fast axis polarization azimuths in the ray coordinate system indicate a TTI (transverse isotropy with a tilted axis of symmetry) style of anisotropy. Lattice Preferred Orientation (LPO) deformation of the palaeo-subducted slabs experiencing high shear strain is a plausible explanation for the observed anisotropy beneath the IOGL. This article is protected by copyright. All rights reserved.
      PubDate: 2017-01-13T03:40:25.071435-05:
      DOI: 10.1002/2016GC006604
       
  • Shear localization in a mature mylonitic rock analogue during fast slip
    • Authors: M. Takahashi; M. van den Ende, A. R. Niemeijer, C. J. Spiers
      Abstract: Highly localized slip zones developed within ductile shear zones, such as pseudotachylite bands occurring within mylonitic fabric rocks, are frequently interpreted as evidence for earthquake nucleation and/or propagation within the ductile regime. To understand brittle/frictional shear localization processes in ductile shear zones and to relate these to earthquake nucleation and propagation, we performed tests with large change in velocity on a brine-saturated, 80:20 (wt.%) mixture of halite and muscovite gouge after forming a mature mylonitic structure through frictional-viscous flow. The direct effect a on shear strength that occurs in response to an instantaneous upward velocity-step is an important parameter in determining the nature of seismic rupture nucleation and propagation. We obtained reproducible results regarding low velocity mechanical behavior compared with previous work, but also obtained new insights into effects of sudden increases in slip velocity on localization and strength evolution, at velocities above a critical velocity Vc (∼20 μm/s). We found that once a ductile, mylonitic structure has developed in a shear zone, subsequent cataclastic deformation is consistently localized in a narrow zone. This switch to localized deformation is controlled by the imposed velocity, and becomes most apparent at velocities above Vc. In addition, the direct effect drops rapidly when the velocity exceeds Vc. This implies that slip can accelerate towards seismic velocities almost instantly and without much loss of fracture energy, once Vc is exceeded. Obtaining a measure for Vc in natural faults is therefore of key importance for understanding earthquake nucleation and propagation in the brittle-ductile transitional regime.
      PubDate: 2017-01-10T18:00:33.237004-05:
      DOI: 10.1002/2016GC006687
       
  • A new high-resolution seafloor age grid for the South Atlantic
    • Authors: L. Pérez-Díaz; G. Eagles
      Abstract: Digital grids of basement age of the world's oceans are essential for modern geodynamic and paleoceanographic studies. Any such grid is built using a plate kinematic model, whose accuracy and reliability directly influence the accuracy and reliability of the grid. We present a seafloor age grid for the South Atlantic based on a recent high-resolution plate kinematic model. The grid is built from a dataset of points whose ages are defined in or for the plate kinematic model, incorporating breaks at tectonic boundaries like fracture zones where the age function is discontinuous. We compare predictions of the new grid and of a previously published one, which is based on an older plate kinematic model, to magnetic isochron pick datasets. The comparison shows the new grid to provide a more reliable depiction of seafloor age in the South Atlantic. Numerical estimates of the new grid's uncertainty are determined by interpolation between (1) misfits at grid cells coinciding with magnetic isochron ages, (2) misfits implied by locational uncertainties in predicted isochrons propagated from uncertainties in the plate kinematic model and (3) by the proximities of cells to fracture zone traces or ridge-jump scars. Estimated total uncertainty is
      PubDate: 2017-01-10T17:20:28.416999-05:
      DOI: 10.1002/2016GC006750
       
  • Reaction kinetics of alkenone and n-alkane thermal alteration at seismic
           timescales
    • Authors: H. S. Rabinowitz; P. J. Polissar, H. M. Savage
      Abstract: Recent experiments and field observations have indicated that biomarker molecules can react over short timescales relevant to seismic slip, thereby making these compounds a useful tool in studying temperature rise in fault zones. However, short-timescale biomarker reaction kinetics studies have previously focused on compounds that have already experienced burial heating. Here, we present a set of hydrous pyrolysis experiments on Pleistocene-aged shallow marine sediment to develop the reaction kinetics of long-chain alkenone destruction, change in the alkenone unsaturation ratio (Uk'37), and change in the n-alkane chain length distribution. Our results show that biomarker thermal maturity provides a useful method for detecting temperature rise in the shallow reaches of faults, such as subduction zone trench environments. Through the course of our work, we also noted the alteration of total alkenone concentrations and Uk'37 values in crushed sediments stored dry at room temperature for durations of months to years but not in the solvent extracts of these materials. This result, though parenthetical for our work in fault zones, has important implications for proper storage of sedimentary samples to be used for alkenone paleotemperature and productivity analysis. This article is protected by copyright. All rights reserved.
      PubDate: 2017-01-07T03:50:24.15317-05:0
      DOI: 10.1002/2016GC006553
       
  • MeltMigrator: A MATLAB-based software for modeling three-dimensional melt
           migration and crustal thickness variations at mid-ocean ridges following a
           rules-based approach
    • Authors: Hailong Bai; Laurent G. J. Montési, Mark D. Behn
      Abstract: MeltMigrator is a MATLAB®-based melt migration software developed to process three-dimensional mantle temperature and velocity data from user-supplied numerical models of mid-ocean ridges, calculate melt production and melt migration trajectories in the mantle, estimate melt flux along plate boundaries and predict crustal thickness distribution on the seafloor. MeltMigrator is also capable of calculating compositional evolution depending on the choice of petrologic melting model. Programmed in modules, MeltMigrator is highly customizable and can be expanded to a wide range of applications. We have applied it to complex mid-ocean ridge model settings, including transform faults, oblique segments, ridge migration, asymmetrical spreading, background mantle flow and ridge-plume interaction. In this technical report, we include an example application to a segmented mid-ocean ridge. MeltMigrator is available as a supplement to this paper, and it's also available from GitHub and the University of Maryland Geodynamics Group website. This article is protected by copyright. All rights reserved.
      PubDate: 2017-01-03T03:25:25.96655-05:0
      DOI: 10.1002/2016GC006686
       
  • Influence of mantle flow on the drainage of eastern Australia since the
           Jurassic Period
    • Authors: T. Salles; N. Flament, D. Müller
      Abstract: Recent studies of the past eastern Australian landscape from present-day longitudinal river profiles and from mantle flow models suggest that the interaction of plate motion with mantle convection accounts for the two phases of large scale uplift of the region since 120 Ma. We coupled the dynamic topography predicted from one of these mantle flow models to a surface process model to study the evolution of the eastern Australian landscape since the Jurassic Period. We varied the rainfall regime, erodibility, sea level variations, dynamic topography magnitude and elastic thickness accross a series of experiments. The approach accounts for erosion and sedimentation and simulates catchment dynamics. Despite the relative simplicity of our model, the results provide insights on the fundamental links between dynamic topography and continental-scale drainage evolution. Based on temporal and spatial changes in longitudinal river profiles as well as erosion and deposition maps, we show that the motion of the Australian plate over the convecting mantle has resulted in significant reorganization of the eastern Australian drainage. The model predicts that the Murray river drained eastward between 150 and ∼120 Ma, and switched to westward draining due to the tilting of the Australian plate from ∼120 Ma. First order comparisons of eight modelled river profiles and of the catchment shape of modelled Murray-Darling Basin are in agreement with present-day observations. The predicted denudation of the eastern highlands is compatible with thermochronology data and sedimentation rates along the southern Australian margin are consistent with cumulative sediment thickness. This article is protected by copyright. All rights reserved.
      PubDate: 2017-01-03T03:05:59.551976-05:
      DOI: 10.1002/2016GC006617
       
  • The impact of rapid sediment accumulation on pore pressure development and
           dehydration reactions during shallow subduction in the Gulf of Alaska
    • Authors: L.N. Meridth; E. J. Screaton, J.M. Jaeger, S. R. James, T. Villaseñor
      Abstract: In the Gulf of Alaska region, sediment has rapidly accumulated (>1 km/my) in the trench sourced from intensified glaciation in the past ∼1.2 million years. This rapid sediment accumulation increases overburden and should accelerate dehydration of hydrous minerals by insulating the underlying sediment column. These processes have the potential to generate fluid overpressures in the low permeability sediments entering the subduction zone. A 1-D model was developed to simulate dehydration reaction progress and investigate excess pore pressures as sediments approach the trench and are subducted. At the deformation front, simulated temperatures increase by ∼30°C due to the insulating effect of trench sediments. As a result, opal-A begins to react to form quartz while smectite remains mostly unreacted. Loading due to the trench sediments elevates excess pore pressures to ∼30% of lithostatic pressure at the deformation front; however, deformation front excess pore pressures are sensitive to assumptions about the permeability of outer wedge sediments. If the outer wedge sediments are coarse-grained and high-permeability rather than mud-dominated, excess pore pressures are lower but still have an insulating effect. During early subduction, simulated pore pressures continue to rise and reach ∼70% of lithostatic by 60 km landward. The 1-D modeling results suggest that the elevated pore pressures are primarily due to loading and that dehydration reactions are not a significant component of excess pore pressure generation at this margin. This article is protected by copyright. All rights reserved.
      PubDate: 2017-01-03T03:01:08.70781-05:0
      DOI: 10.1002/2016GC006693
       
  • Mid-crustal shearing and doming in a Cenozoic compressive setting along
           the Ailao Shan-Red River shear zone
    • Authors: B. Zhang; C. Y. Yin, J. J. Zhang, J. M. Wang, D. L. Zhong, Y. Wang, Q. Z. Lai, Y. H. Yue, Q. Y. Zhou
      Abstract: The Cenozoic Xuelong Shan antiformal dome is located along the northern segment of the Ailao Shan-Red River shear zone in Yunnan, China. Subhorizontal foliation in the gneiss core is recognized, representing a broad top-to-NE shear initiated under amphibolite facies conditions and propagating into greenschist facies in the mantling schist and strike-slip shear zone. Microfabrics of crystallographic preferred orientations (CPOs) in quartz suggest that the deformation temperatures increased with increasing structural depth from the upper crust (300-500°C) in the mantling schist to the mid-crust (15 km or more, ≥650°C) in the gneissic core. This trend is mirrored by variations in the metamorphic grade of the syn-kinematic mineral assemblages and microstructures, which range from garnet + amphibole + biotite + sillimanite + rutile + feldspar in the core to garnet + staurolite + biotite + epidote + muscovite within the limb units. The dome experienced the following deformation history: (1) a broad top-to-NE shear in the subhorizontal foliation of the gneiss core during the first stage of deformation (D1); (2) opposing reverse-sense shear along the two schist limbs of the dome during contraction-related doming (D2-D3); (3) sinistral strike-slip shearing within the eastern limb (D4); and (4) extensional deformation (D5). The structural-thermal patterns suggest the antiformal dome formation was roughly coeval with top-to-NE ductile shearing in the mid-crust of Tibet at 32 Ma or earlier. A major implication is that there was a phase of contractional ductile deformation in the region prior to the initiation of strike-slip deformation. This article is protected by copyright. All rights reserved.
      PubDate: 2017-01-03T03:01:07.464885-05:
      DOI: 10.1002/2016GC006520
       
  • Mineralogical, geochemical and magnetic signatures of surface sediments
           from the Canadian Beaufort Shelf and Amundsen Gulf (Canadian Arctic)
    • Authors: Adriana Gamboa; Jean-Carlos Montero-Serrano, Guillaume St-Onge, André Rochon, Pierre-Arnaud Desiage
      Abstract: Mineralogical, geochemical, magnetic, and siliciclastic grain-size signatures of 34 surface sediment samples from the Mackenzie-Beaufort Sea Slope and Amundsen Gulf were studied in order to better constrain the redox status, detrital particle provenance, and sediment dynamics in the western Canadian Arctic. Redox-sensitive elements (Mn, Fe, V, Cr, Zn) indicate that modern sedimentary deposition within the Mackenzie-Beaufort Sea Slope and Amundsen Gulf took place under oxic bottom-water conditions, with more turbulent mixing conditions and thus a well-oxygenated water column prevailing within the Amundsen Gulf. The analytical data obtained, combined with multivariate statistical (notably, principal component and fuzzy c-means clustering analyses) and spatial analyses, allowed the division of the study area into four provinces with distinct sedimentary compositions: (1) the Mackenzie Trough-Canadian Beaufort Shelf with high phyllosilicate-Fe oxide-magnetite and Al-K-Ti-Fe-Cr-V-Zn-P contents; (2) Southwestern Banks Island, characterized by high dolomite-K-feldspar and Ca-Mg-LOI contents; (3) the Central Amundsen Gulf, a transitional zone typified by intermediate phyllosilicate-magnetite-K-feldspar-dolomite and Al-K-Ti-Fe-Mn-V-Zn-Sr-Ca-Mg-LOI contents; and (4) mud volcanoes on the Canadian Beaufort Shelf distinguished by poorly sorted coarse-silt with high quartz-plagioclase-authigenic carbonate and Si-Zr contents, as well as high magnetic susceptibility. Our results also confirm that the present-day sedimentary dynamics on the Canadian Beaufort Shelf is mainly controlled by sediment supply from the Mackenzie River. Overall, these insights provide a basis for future studies using mineralogical, geochemical, and magnetic signatures of Canadian Arctic sediments in order to reconstruct past variations in sediment inputs and transport pathways related to late Quaternary climate and oceanographic changes. This article is protected by copyright. All rights reserved.
      PubDate: 2017-01-03T03:01:02.726976-05:
      DOI: 10.1002/2016GC006477
       
  • TerraFERMA: The Transparent Finite Element Rapid Model Assembler for
           multiphysics problems in Earth sciences
    • Authors: Cian R. Wilson; Marc Spiegelman, Peter E. van Keken
      Abstract: We introduce and describe a new software infrastructure TerraFERMA, the\emph{Transparent Finite Element Rapid Model Assembler}, for the rapid and reproducible description and solution of coupled multiphysics problems. The design of TerraFERMA is driven by two computational needs in Earth sciences. The first is the need for increased flexibility in both problem description and solution strategies for coupled problems where small changes in model assumptions can lead to dramatic changes in physical behavior. The second is the need for software and models that are more transparent so that results can be verified, reproduced, and modified in a manner such that the best ideas in computation and Earth science can be more easily shared and reused. TerraFERMA leverages three advanced open-source libraries for scientific computation that provide high-level problem description (FEniCS), composable solvers for coupled multiphysics problems (PETSc), and an options handling system (SPuD) that allows the hierarchical management of all model options. TerraFERMA integrates these libraries into an interface that organizes the scientific and computational choices required in a model into a single options file from which a custom compiled application is generated and run. Because all models share the same infrastructure, models become more reusable and reproducible, while still permitting the individual researcher considerable latitude in model construction. TerraFERMA solves partial differential equations using the finite element method. It is particularly well suited for nonlinear problems with complex coupling between components. TerraFERMA is open-source and available at http://terraferma.github.io, which includes links to documentation and example input files. This article is protected by copyright. All rights reserved.
      PubDate: 2017-01-03T03:00:41.43675-05:0
      DOI: 10.1002/2016GC006702
       
  • Fidelity of the Sr/Ca proxy in recording ocean temperature in the western
           Atlantic coral Siderastrea siderea
    • Authors: Ilsa B. Kuffner; Kelsey E. Roberts, Jennifer A. Flannery, Jennifer M. Morrison, Julie N. Richey
      Abstract: Massive corals provide a useful archive of environmental variability, but careful testing of geochemical proxies in corals is necessary to validate the relationship between each proxy and environmental parameter throughout the full range of conditions experienced by the recording organisms. Here we use samples from a coral-growth study to test the hypothesis that Sr/Ca in the coral Siderastrea siderea accurately records sea-surface temperature (SST) in the subtropics (Florida, USA) along 350 km of reef tract. We test calcification rate, measured via buoyant weight, and linear extension (LE) rate, estimated with Alizarin Red-S staining, as predictors of variance in the Sr/Ca records of 39 individual S. siderea corals grown at four outer-reef locations next to in-situ temperature loggers during two, year-long periods. We found that corals with calcification rates 
      PubDate: 2017-01-03T03:00:27.388429-05:
      DOI: 10.1002/2016GC006640
       
  • Metamorphic records for subduction erosion and subsequent underplating
           processes revealed by garnet-staurolite-muscovite schists in central
           Qiangtang, Tibet
    • Authors: Xiu-Zheng Zhang; Yong-Sheng Dong, Qiang Wang, Wei Dan, Chunfu Zhang, Wang Xu, Ming-Liang Huang
      Abstract: Subduction erosion is confirmed as a crucial geodynamic process of crustal recycling based on geological, geochemical, and geophysical observations at modern convergent plate margins. So far, not a single metamorphic record has been used for constraining a general tectonic evolution for subduction erosion. Here we first revealed metamorphic records for a subduction erosion process based on our study of the Late Paleozoic garnet-staurolite-muscovite schists in the central Qiangtang block, Tibet. Provenance analyses suggest that the protoliths of garnet-staurolite-muscovite schists have the Northern Qiangtang-affinity and were deposited in an active continental margin setting. Mineral inclusion data show that the early metamorphic stage (M1) recorded blueschist facies pressure–temperature (P–T) conditions of 0.8–1.1 GPa and 402–441°C, indicating that a part of the material from the overriding plate had been abraded into the subduction channel and undergone high-pressure/low-temperature metamorphism. The peak metamorphic stage (M2) recorded amphibolite facies P–T conditions of 0.3–0.5 GPa and 470–520°C. The 40Ar/39Ar cooling ages (263–259 Ma) yielded from muscovite suggest the amphibolite facies metamorphism (> 263 Ma) occurred at oceanic subduction stage. The distinctly-staged metamorphism defines a clockwise and warming decompression P–T–t path which reveals an underplating process following the early subduction erosion. During the tectonic process, the eroded low-density material escaped from the cold subduction channel and rise upwards into the warm middle-lower crust of the upper plate, undergoing amphibolite facies metamorphism. Our new results revealed a complete evolutional process from the early subduction erosion to the subsequent underplating during the northward subduction of the Paleo-Tethys Ocean. This article is protected by copyright. All rights reserved.
      PubDate: 2017-01-03T02:55:52.333201-05:
      DOI: 10.1002/2016GC006576
       
  • Seismic structure of the lithosphere beneath NW Namibia: Impact of the
           Tristan da Cunha mantle plume
    • Authors: Xiaohui Yuan; Benjamin Heit, Sascha Brune, Bernhard Steinberger, Wolfram H. Geissler, Wilfried Jokat, Michael Weber
      Abstract: Northwestern Namibia, at the landfall of the Walvis Ridge, was affected by the Tristan da Cunha mantle plume during continental rupture between Africa and South America, as evidenced by the presence of the Etendeka continental flood basalts. Here we use data from a passive-source seismological network to investigate the upper mantle structure and to elucidate the Cretaceous mantle plume-lithosphere interaction. Receiver functions reveal an interface associated with a negative velocity contrast within the lithosphere at an average depth of 80 km. We interpret this interface as the relic of the lithosphere-asthenosphere boundary (LAB) formed during the Mesozoic by interaction of the Tristan da Cunha plume head with the pre-existing lithosphere. The velocity contrast might be explained by stagnated and “frozen” melts beneath an intensively depleted and dehydrated peridotitic mantle. The present-day LAB is poorly visible with converted waves, indicating a gradual impedance contrast. Beneath much of the study area, converted phases of the 410 and 660 km mantle transition zone discontinuities arrive 1.5 s earlier than in the landward plume-unaffected continental interior, suggesting high velocities in the upper mantle caused by a thick lithosphere. This indicates that after lithospheric thinning during continental breakup, the lithosphere has increased in thickness during the last 132 Myr. Thermal cooling of the continental lithosphere alone cannot produce the lithospheric thickness required here. We propose that the remnant plume material, which has a higher seismic velocity than the ambient mantle due to melt depletion and dehydration, significantly contributed to the thickening of the mantle lithosphere. This article is protected by copyright. All rights reserved.
      PubDate: 2017-01-03T02:55:38.265192-05:
      DOI: 10.1002/2016GC006645
       
  • Issue Information
    • Pages: 4815 - 4816
      PubDate: 2017-01-22T13:15:51.1098-05:00
      DOI: 10.1002/ggge.20839
       
  • Formation of hydrothermal pits and the role of seamounts in the Guatemala
           Basin (Equatorial East Pacific) from heat flow, seismic and core studies
    • Authors: H. W. Villinger; T. Pichler, N. Kaul, S. Stephan, H. Pälike, F. Stephan
      Abstract: We acquired seismic and heat flow data and collected sediment cores in three areas in the Guatemala Basin (Cocos Plate, Eastern Pacific) to investigate the process by which depressions (pits) in the sedimentary cover on young oceanic crust were formed. Median heat flow of 55 mW/m2 for the three areas is about half of the expected conductive cooling value. The heat deficit is caused by massive recharge of cold seawater into the upper crust through seamounts which is inferred from depressed heat flow in the vicinity of seamounts. Heat flow inside of pits is always elevated, in some cases up to three times (max. 300 mW/m2) relative to background. None of the geochemical pore water profiles from cores inside and outside of the pits show any evidence of active fluid flow inside the pits. All three areas originated within the high productivity equatorial zone and moved northwest over the past 15 to 18 Ma. Pits found in the working areas are likely dissolution structures formed by diffuse hydrothermal venting in a zone of high biogenic carbonate production which were sealed when they moved north. It is likely that these pits were discharge sites of 'hydrothermal siphons' where recharging seamounts could feed cold seawater via the upper crust to several discharging pits. Probably pit density on the whole Cocos Plate is similar to the three working areas and which may explain the huge heat deficit of the Cocos Plate. This article is protected by copyright. All rights reserved.
      PubDate: 2016-12-29T18:26:17.034835-05:
      DOI: 10.1002/2016GC006665
       
  • Trade winds drive pronounced seasonality in carbonate chemistry in a
           tropical Western Pacific island cave - implications for speleothem
           paleoclimatology
    • Authors: Alexandra L. Noronha; Benjamin F. Hardt, Jay L. Banner, John W. Jenson, Judson W. Partin, Eric W. James, Mark A. Lander, Kaylyn K. Bautista
      Abstract: Carbon dioxide concentrations in caves are a primary driver of rates of carbonate dissolution and precipitation, exerting strong control on speleothem growth rate and geochemistry. Long-term cave monitoring studies in mid-latitude caves have observed seasonal variability in cave pCO2, whereby airflow is driven by temperature contrasts between the surface and subsurface. In tropical settings, where diurnal temperature cycles are larger than seasonal temperature cycles, it is has been proposed caves will ventilate on daily timescales, preventing cave pCO2 from increasing substantially above atmospheric pCO2. By contrast, the relatively small temperature difference between the surface and subsurface may be insufficient to drive complete ventilation of tropical caves. Here we present results of an 8-year cave monitoring study, including observations of cave pCO2 and carbonate chemistry, at Jinapsan Cave, Guam (13.4°N, 144.5°E). We find that cave pCO2 in Jinapsan Cave is both relatively high and strongly seasonal, with cave pCO2 ranging from 500 - 5000 ppm. The seasonality of cave pCO2 cannot be explained by temperature contrasts, instead we find evidence that seasonal trade winds drive cave ventilation and modulate cave pCO2. Calcite deposition rates at seven drip sites in Jinapsan Cave are shown to be seasonally variable, demonstrating that speleothem growth rates in Jinapsan Cave are strongly affected by seasonal variations in cave pCO2. These results highlight the importance that advection can have on cave ventilation processes and carbonate chemistry. Seasonality in carbonate chemistry and calcite deposition in this cave effect the interpretation of speleothem-based paleoclimate records. This article is protected by copyright. All rights reserved.
      PubDate: 2016-12-29T18:10:26.996805-05:
      DOI: 10.1002/2016GC006644
       
  • Geochemical and microstructural evidence for interseismic changes in fault
           zone permeability and strength, Alpine Fault, New Zealand
    • Authors: Carolyn Boulton; Catriona D. Menzies, Virginia G. Toy, John Townend, Rupert Sutherland
      Abstract: Oblique dextral motion on the central Alpine Fault in the last c. 5 Myr has exhumed garnet-oligoclase facies mylonitic fault rocks from c. 35 km depth. During exhumation, deformation, accompanied by fluid infiltration, has generated complex lithological variations in fault-related rocks retrieved during Deep Fault Drilling Project (DFDP-1) drilling at Gaunt Creek, South Island, New Zealand. Lithological, geochemical, and mineralogical results reveal that the fault comprises a core of highly comminuted cataclasites and fault gouges bounded by a damage zone containing cataclasites, protocataclasites, and fractured mylonites. The fault core-alteration zone extends c. 20-30 m from the principal slip zone (PSZ) and is characterized by alteration of primary phases to phyllosilicate minerals. Alteration associated with distinct mineral phases occurred proximal the brittle-to-plastic transition (T≤300-400°C, 6-10 km depth) and at shallow depths (T=20-150°C, 0-3 km depth). Within the fault core-alteration zone, fractures have been sealed by authigenic precipitation of calcite and phyllosilicates. This sealing has decreased fault normal permeability and increased rock mass competency, potentially promoting interseismic strain buildup. This article is protected by copyright. All rights reserved.
      PubDate: 2016-12-29T05:36:59.690503-05:
      DOI: 10.1002/2016GC006588
       
  • Influence of diatom microfossils on sediment shear strength and slope
           stability
    • Authors: G. Wiemer; A. Kopf
      Abstract: Diatom microfossils have been detected in many natural marine sediment deposits around the globe and are held responsible for the disobedience to well-established geotechnical relationships between index-properties and shear strength. We revisit the static shear strength and present the first cyclic undrained shear strength experiments on diatom microfossil – clayey-silt mixtures to study the role of diatoms on submarine slope stability. It is attested that the angle of internal friction (Φ) increases with diatom content, however, we provide evidence for a significant overestimation of Φ in previous studies. Based on direct shear tests at varying normal stresses ≤ 600 kPa we find Φ = 32° in contrast to 43° in pure diatom. Similarly, to static shear strength, cyclic shear strength increases with diatom content, however, in contrast to static shear strength the most drastic increase does not occur from 0% to 25% diatoms but from 75% to 100%. Interestingly, diatomaceous sediments tend to fail by liquefaction although well-established relations between index properties and liquefaction susceptibility predict the opposite. Liquefaction failure is observed solely in samples containing ≥ 50% diatoms whereas samples with lower diatom content fail by cyclic softening. We conclude diatom microfossils in marine sediments significantly contribute to an increased slope stability under static and cyclic loading conditions since diatoms lead to higher resistance independently of the loading mode. The strength increase is interpreted as a result of particle interlocking and surface roughness, which is very efficient given the highly variable habitus of diatom species. This article is protected by copyright. All rights reserved.
      PubDate: 2016-12-29T05:30:36.67717-05:0
      DOI: 10.1002/2016GC006568
       
  • Focused hydrocarbon-migration in shallow sediments of a pockmark cluster
           in the Niger Delta (Off Nigeria)
    • Authors: Alexis de Prunelé; Livio Ruffine, Vincent Riboulot, Carl A. Peters, Claire Croguennec, Vivien Guyader, Thomas Pape, Claire Bollinger, Germain Bayon, Jean-Claude Caprais, Yoan Germain, Jean-Pierre Donval, Tania Marsset, Gerhard Bohrmann, Louis Géli, Abdulkarim Rabiu, Marc Lescanne, Eric Cauquil, Nabil Sultan
      Abstract: The Niger Delta is one of the largest hydrocarbon basin offshore Africa and it is well known for the presence of active pockmarks on the seabed. During the Guineco-MeBo cruise in 2011, long cores were taken from a pockmark cluster in order to investigate the state of its current activity. Gas hydrates, oil and pore-water were sampled for geochemical studies.The resulting dataset combined with seismic data reveal that shallow hydrocarbon migration in the upper sedimentary section was focused exclusively within the pockmarks. There is a clear tendency for gas migration within the hydrate-bearing pockmarks, and oil migration within the carbonate-rich one. This trend is interpreted as a consequence of hydrate dissolution followed by carbonate precipitation in the course of the evolution of these pockmarks. We also demonstrate that Anaerobic Oxidation of Methane (AOM) is the main process responsible for the depletion of pore-water sulfate, with depths of the Sulfate-Methane Transition Zone (SMTZ) ranging between 1.8 and 33.4 m. In addition, a numerical transport-reaction model was used to estimate the age of hydrate-layer formation from the present-day sulfate profiles. The results show that the sampled hydrate-layers were formed between 21 and 3750 years before present.Overall, this work shows the importance of fluid flow on the dynamics of pockmarks, and the investigated cluster offers new opportunities for future cross-site comparison studies. Our results imply that sudden discharges of gas can create hydrate layers within the upper sedimentary column which can affect the seafloor morphology over few decades. This article is protected by copyright. All rights reserved.
      PubDate: 2016-12-29T05:30:33.492413-05:
      DOI: 10.1002/2016GC006554
       
  • Dawsonite occurrences related to the age and origin of CO2 influx in
           sandstone reservoirs: A case study in the Songliao Basin, NE China
    • Authors: Fulai Li; Wenshuai Li, Na Liu, Zhichao Yu, Huidong Yang, Li Liu
      Abstract: Dawsonite is often associated with CO2-rich gas reservoirs, and it is regarded as a “trace mineral” for recording migration and accumulation of CO2. Following accepted petrological, mineralogical and geochemical principles, we used several methods (described herein) to study the Cretaceous dawsonite-bearing sandstone reservoirs in the Songliao Basin, China. We used the ideas of “sequencing” and “timing” to verify the influx stages of CO2 and hydrocarbons, dividing their influx sequence pattern and building a CO2-influx timeframe. First, we determined the stable isotopic ratios of dawsonite and CO2 in gas and oil reservoirs, and found that the CO2 in? dawsonite is of a mantle-derived magma origin. Second, we differentiated an early/late-stage oil and gas influx and a mid-mantle source influx through the study of diagenetic paragenetic sequences, formation water, and fluid inclusions in the dawsonite-bearing sandstones. Combining burial/thermal-history curves and illite K-Ar dates from the study area, we determined that the early-stage oil and gas influx, late oil and gas influx and medium CO2 influx occurred at 85 - 58.8 Ma, 41 - 20 Ma and 58.8 - 41 Ma (Paleocene and Eocene), respectively. Finally, we observed a coupling relationship between CO2 influx and Shuangliao volcanic activities and material compositions, as constrained by volcanic activity history in the basin since the Late Cretaceous. This article is protected by copyright. All rights reserved.
      PubDate: 2016-12-29T05:30:30.256954-05:
      DOI: 10.1002/2016GC006555
       
  • Additive effects of acidification and mineralogy on calcium isotopes in
           Triassic/Jurassic boundary limestones
    • Authors: Adam B. Jost; Aviv Bachan, Bas van de Schootbrugge, Shaun T. Brown, Donald J. DePaolo, Jonathan L. Payne
      Abstract: The end-Triassic mass extinction coincided with a negative δ13C excursion, consistent with release of 13C-depleted CO2 from the Central Atlantic Magmatic Province. However, the amount of carbon released and its effects on ocean chemistry are poorly constrained. The coupled nature of the carbon and calcium cycles allows calcium isotopes to be used for constraining carbon cycle dynamics and vice versa. We present a high-resolution calcium isotope (δ44/40Ca) record from 100 m of marine limestone spanning the Triassic/Jurassic boundary in two stratigraphic sections from northern Italy. Immediately above the extinction horizon and the associated negative excursion in δ13C, δ44/40Ca decreases by ca. 0.8‰ in 20 m of section and then recovers to pre-excursion values. Coupled numerical models of the geological carbon and calcium cycles demonstrate that this δ44/40Ca excursion is too large to be explained by changes to seawater δ44/40Ca alone, regardless of CO2 injection volume and duration. Less than 20% of the δ44/40Ca excursion can be attributed to acidification. The remaining 80% likely reflects a higher proportion of aragonite in the original sediment, based largely on high concentrations of Sr in the samples. Our study demonstrates that coupled models of the carbon and calcium cycles have the potential to help distinguish contributions of primary seawater isotopic changes from local or diagenetic effects on the δ44/40Ca of carbonate sediments. Differentiating between these effects is critical for constraining the impact of ocean acidification during the end-Triassic mass extinction, as well as for interpreting other environmental events in the geologic past. This article is protected by copyright. All rights reserved.
      PubDate: 2016-12-29T05:30:26.875021-05:
      DOI: 10.1002/2016GC006724
       
  • High-sensitivity multifunctional spinner magnetometer using a
           magnetoimpedance sensor
    • Authors: Kazuto Kodama
      Abstract: A novel spinner magnetometer was developed with a wide dynamic range from 10−10−10−4 Am2 and a resolution of 10−11 Am2. High sensitivity was achieved with the use of a magneto-impedance (MI) sensor, which is a compact, sensitive magnetic sensor used industrially. Its slow spinning rate (5 Hz) and the incorporation of a unique mechanism for adjusting the spacing between the sensing unit and the spinning axis allows the measurement of fragile samples sized 10–50 mm. The sensor configuration, in which a pair of MI sensors is connected in opposite serial, along with an amplification circuit with a programmable low-pass filter, reduces the problems of external noise and sensor drift. The signal, with reference to the spinning frequency, is detected with a lock-in amplifier. The MI spinner has two selectable measurement modes: the fundamental mode (F mode) and the harmonic mode (H mode). Measurements in the F mode detect signals of the fundamental frequency (5 Hz), in the same way as conventional spinner magnetometers. In the H mode, the second (10 Hz) and the third (15 Hz) harmonic components are measured, in addition to the fundamental component. Tests in the H mode were performed using a small coil and a natural sample to simulate dipoles with various degrees of offset. The results revealed that the magnitude of the fundamental component of the offset dipole was systematically larger (by several percent) than that of the non-offset dipole. These findings suggest that this novel MI spinner will be useful in estimating the inhomogeneity of the magnetization of a sample that can equivalently be described by an offset dipole. This article is protected by copyright. All rights reserved.
      PubDate: 2016-12-29T05:30:23.050893-05:
      DOI: 10.1002/2016GC006615
       
  • Rapid variations in fluid chemistry constrain hydrothermal phase
           separation at the Main Endeavour Field
    • Authors: Brooke Love; Marvin Lilley, David Butterfield, Eric Olson, Benjamin Larson
      Abstract: Previous work at the Main Endeavour Field (MEF) has shown that chloride concentration in high-temperature vent fluids has not exceeded 510 mmol/kg (94% of seawater), which is consistent with brine condensation and loss at depth, followed by upward flow of a vapor phase toward the seafloor. Magmatic and seismic events have been shown to affect fluid temperature and composition and these effects help narrow the possibilities for sub-surface processes. However, chloride-temperature data alone are insufficient to determine details of phase separation in the upflow zone. Here we use variation in chloride and gas content in a set of fluid samples collected over several days from one sulfide chimney structure in the MEF to constrain processes of mixing and phase separation. The combination of gas (primarily magmatic CO2 and seawater-derived Ar) and chloride data, indicate that neither variation in the amount of brine lost, nor mixing of the vapor phase produced at depth with variable quantities of (i) brine or (ii) altered gas rich seawater that has not undergone phase separation, can explain the co-variation of gas and chloride content. The gas-chloride data require additional phase separation of the ascending vapor-like fluid. Mixing and gas partitioning calculations show that near-critical temperature and pressure conditions can produce the fluid compositions observed at Sully vent as a vapor-liquid conjugate pair or as vapor-liquid pair with some remixing, and that the gas partition coefficients implied agree with theoretically predicted values. This article is protected by copyright. All rights reserved.
      PubDate: 2016-12-29T05:28:27.915162-05:
      DOI: 10.1002/2016GC006550
       
  • A preliminary 1-D model investigation of tidal variations of temperature
           and chlorinity at the Grotto mound, Endeavour segment, Juan de Fuca ridge7
           
    • Authors: G. Xu; B. I. Larson, K. G. Bemis, Marvin D. Lilley
      Abstract: Tidal oscillations of venting temperature and chlorinity have been observed in the long-term times-series data recorded by the Benthic and Resistivity Sensors (BARS) at the Grotto mound on the Juan de Fuca Ridge. In this study, we use a one-dimensional two-layer poroelastic model to conduct a preliminary investigation of three hypothetical scenarios in which seafloor tidal loading can modulate the venting temperature and chlorinity at Grotto through the mechanisms of subsurface tidal mixing and/or subsurface tidal pumping. For the first scenario, our results demonstrate that it is unlikely for subsurface tidal mixing to cause coupled tidal oscillations in venting temperature and chlorinity of the observed amplitudes. For the second scenario, the model results suggest it is plausible that the tidal oscillations in venting temperature and chlorinity are decoupled with the former caused by subsurface tidal pumping and the latter caused by subsurface tidal mixing, although the mixing depth is not well constrained. For the third scenario, out results suggest it is plausible for subsurface tidal pumping to cause coupled tidal oscillations in venting temperature and chlorinity. In this case, the observed tidal phase lag between venting temperature and chlorinity is close to the poroelastic model prediction if brine storage occurs throughout the upflow zone under the premise that layer 2A and 2B have similar crustal permeabilities. However, the predicted phase lag is poorly constrained if brine storage is limited to layer 2B as would be expected when its crustal permeability is much smaller than that of layer 2A. This article is protected by copyright. All rights reserved.
      PubDate: 2016-12-29T05:25:34.269619-05:
      DOI: 10.1002/2016GC006537
       
  • The biokarst system and its carbon sinks in response to pH changes: A
           simulation experiment with microalgae
    • Authors: Tengxiang Xie; Yanyou Wu
      Abstract: This study aims to explore the changes in a microalgal biokarst system as a potential carbon sink system in response to pH changes. The bidirectional isotope labeling method and mass balance calculation were adopted in a simulated biokarst environment with a series of set pH conditions and three microalgal species. Three key processes of the microalgal biokarst system, including calcite dissolution, CaCO3 reprecipitation, and inorganic carbon assimilation by microalgae, were completely quantitatively described. The combined effects of chemical dissolution and species-specific bio-dissolution caused a decrease in overall dissolution rate when the pH increased from 7 to 9. CaCO3 reprecipitation and the utilization of dissolved inorganic carbon originating from calcite dissolution decreased when the pH increased from 7 to 9. The three processes exhibited different effects in changing the CO2 atmosphere. The amount of photosynthetic carbon sink was larger at high pH values than at low pH values. However, the CO2 sequestration related to the biokarst process (biokarst carbon sink) increased with decreasing pH. Overall, the total amount of sequestered CO2 produced by the biokarst system (CaCO3-CO2-microalgae) shows a minimum at a specific pH then increases with decreasing pH. Therefore, various processes and carbon sinks in the biokarst system are sensitive to pH changes, and biokarst processes play an important negative feedback role in the release of CO2 by acidification. The results also suggest that the carbon sink associated with carbonate weathering cannot be neglected when considering the global carbon cycle on the scale of thousands of years (
      PubDate: 2016-12-20T13:21:08.2333-05:00
      DOI: 10.1002/2016GC006628
       
  • Fitful and protracted magma assembly leading to a Giant eruption, Youngest
           Toba Tuff, Indonesia
    • Authors: Mary R. Reid; Jorge A. Vazquez
      Abstract: The paroxysmal eruption of the 74 ka Youngest Toba Tuff (YTT) of northern Sumatra produced an extraordinary 2800 km3 of non-welded to densely welded ignimbrite and co-ignimbrite ash-fall. We report insights into the duration of YTT magma assembly obtained from ion microprobe U-Th and U-Pb dates, including continuous age spectra over >50% of final zircon growth, for pumices and a welded tuff spanning the compositional range of the YTT. A relatively large subpopulation of zircon crystals nucleated before the penultimate caldera-related eruption at 501 ka, but most zircons yielded interior dates 100-300 ka thereafter. Zircon nucleation and growth was likely episodic and from diverse conditions over protracted time intervals of >100 to >500 ka. Final zircon growth is evident as thin rim plateaus that are in Th/U chemical equilibrium with hosts, and that give crystallization ages within tens of ka of eruption. The longevity and chemical characteristics of the YTT zircons, as well as evidence for intermittent zircon isolation and remobilization associated with magma recharge, is especially favored at the cool and wet eutectoid conditions that characterize at least half of the YTT, wherein heat fluxes could dissolve major phases but have only a minor effect on larger zircon crystals. Repeated magma recharge may have contributed to the development of compositional zoning in the YTT but, considered together with limited allanite, quartz, and other mineral dating and geospeedometry, regular perturbations to the magma reservoir over >400 ka did not lead to eruption until 74 ka ago. This article is protected by copyright. All rights reserved.
      PubDate: 2016-12-20T13:21:04.763827-05:
      DOI: 10.1002/2016GC006641
       
  • Controls on evolution of gas-hydrate system in the Krishna-Godavari Basin,
           offshore India
    • Authors: F. Badesab; P. Dewangan, A. Usapkar, M. Kocherla, A. Peketi, K. Mohite, S.J. Sangode, K. Deenadayalan
      Abstract: In this study, we integrate environmental magnetic, sedimentological and geochemical records of sediment core of Hole NGHP-01-10D overlying methane hydrate deposits to decipher the controls on the evolution of fracture-filled gas hydrate system in the Krishna-Godavari (K-G) basin. Four distinct sedimentary units have been identified, based on the sediment magnetic signatures. An anomalous zone of enhanced magnetic susceptibility (Unit – III: 51.9 - 160.4 mbsf) coinciding with the gas hydrate bearing intervals is due to the presence of magnetite-rich detrital minerals brought-in by the river systems as a result of higher sedimentation events in K-G basin and has no influence over hydrate formation. A strong to moderate correlation between magnetite concentration and chromium reducible sulphur (CRS) content indicates significant influence of sulfidization on the magnetic record and could be further exploited as a proxy to decipher paleo-H2S seepage events. Analysis of high-resolution seismic, bathymetry and sub-bottom profiler data reveals the existence of a regional fault system in K-G basin. The opening and closing dynamics of the faults facilitated the migration and trapping of required gas concentrations resulting in accumulation of gas hydrates at the studied site. The seismic data provides support to the rock-magnetic interpretations. The observed variations in magnetic and geochemical properties have resulted from the episodic flow of methane and sulphide-enriched fluids through the fracture-filled network formed as a result of shale-tectonism. Our study demonstrated the potential of using an enviro-magnetic approach in combination with other proxies to constrain the evolution of gas hydrate system in marine environments. This article is protected by copyright. All rights reserved.
      PubDate: 2016-12-20T13:20:55.91911-05:0
      DOI: 10.1002/2016GC006606
       
  • Short organic carbon turnover time and narrow 14C Age spectra in early
           Holocene wetland paleosols
    • Authors: Lael Vetter; Brad E. Rosenheim, Alvaro Fernandez, Torbjörn E. Törnqvist
      Abstract: Paleosols contain information about the rates of soil organic carbon turnover when the soil was actively forming. However, this temporal information is often difficult to interpret without tight stratigraphic control on the age of the paleosol. Here we apply ramped pyrolysis/oxidation (Ramped PyrOx) 14C analyses to evaluate age spectra of transgressive early Holocene paleosols from the Mississippi Delta in southeastern Louisiana, U.S.A. We find 14C age spectra from soil organic matter (SOM) in both paleosols and overlying basal peats that represent variability in age that is close to, or only slightly greater than, analytical uncertainty of 14C measurements. Such age spectra have not previously been observed in the sedimentary record. Here, they indicate vigorous soil carbon turnover prior to burial, which homogenized 14C ages within SOM across the entire thermochemical spectrum. The weighted bulk 14C ages from Ramped PyrOx of paleosols and overlying peats are identical within analytical and process-associated uncertainty, and corroborate 14C ages from charcoal fragments and plant macrofossils from the overlying peat. The youngest ages from Ramped PyrOx age spectra may also potentially be applied as chronometers for stratigraphic burial ages. Our results suggest rapid turnover (≪300 yr) of carbon in these soils relative to input of allochthonous carbon, indicating that the 14C age of different soil components is decoupled from thermochemical stability and instead reflects vigorous turnover processes. The concurrence of paleosol and peat 14C ages also suggests that pedogenic processes were linked with the development of coastal marshes, and that the priming effect potentially masked the signal of allochthonous carbon inputs during sea-level rise. This article is protected by copyright. All rights reserved.
      PubDate: 2016-12-20T13:20:52.913745-05:
      DOI: 10.1002/2016GC006526
       
  • Variation in magnetic properties of serpentinized peridotites exposed on
           the Yokoniwa Rise, Central Indian Ridge: Insights into the role of
           magnetite in serpentinization
    • Authors: Masakazu Fujii; Kyoko Okino, Hiroshi Sato, Kentaro Nakamura, Taichi Sato, Toshitsugu Yamazaki
      Abstract: Magnetic properties in serpentinized peridotites are of increasing interest in seafloor mapping and petrologic studies because such data can promote the understanding of serpentinization reactions and hydrogen creation in ultramafic rocks. In order to reveal the magnetic properties and magnetite growth in serpentinized peridotites, we analyzed 30 serpentinized peridotite samples from a non-transform offset massif called the Yokoniwa Rise in the Central Indian Ridge. The results from multiple rock magnetic analyses and petrological observations illustrate the details of the creation and growth of magnetite in serpentinized peridotites that have undergone 17 − 100% serpentinzation. The magnetic carrier of these samples is pure magnetite, which did not suffer from maghemitization (low-temperature oxidation). The magnetic susceptibility ranged from 0.002 − 0.087 SI and increased nonlinearly with the progression of the serpentinization reaction. The natural remanent magnetization intensities of 0.2 − 8.4 A/m are comparable to those of basalts, which suggests that the remanence as well as induced magnetization of highly serpentinized peridotite can contribute to magnetization of the oceanic lithosphere. The amount of magnetite estimated from saturation magnetization increased nonlinearly from 0.1 wt% to 5.5 wt% with the progression of the serpentinization. Highly serpentinized peridotites have a well-developed serpentine mesh texture. Pseudo-single-domain (PSD) and multi-domain (MD) grains were formed during igneous processes in the mantle and/or during the initial stages of serpentinization. Super-paramagnetic (SP) particles were formed during the initial stages of serpentinization. Single-domain (SD) magnetite was formed during the later stage of serpentinization, and it is assembled inside of mesh structures with strong magnetostatic interactions. This article is protected by copyright. All rights reserved.
      PubDate: 2016-12-20T13:20:51.120621-05:
      DOI: 10.1002/2016GC006511
       
  • Subduction, high–P metamorphism, and collision fingerprints in SW Iran:
           Constraints from zircon U–Pb and mica Rb–Sr geochronology
    • Authors: H. Shafaii Moghadam; M. Bröcker, W. L. Griffin, X. H– Li, R. X. Chen, S. Y. O'Reilly
      Abstract: The Esfandagheh region of the Zagros Orogenic Belt is an ideal area to address many aspects of continental convergence between Arabia and Eurasia, including incorporation of Late Neoproterozoic (Ediacaran) basement, subduction–related magmatism and the formation of HP/LT rocks. The rock units exposed here represent a presumably Jurassic magmatic arc within the Sanandaj–Sirjan Zone (SSNZ), remnants of oceanic lithosphere, blueschist– and greenschist–facies rocks, and a distinct group of poorly characterized rocks.U–Pb ages define four populations, related to Paleoproterozoic, Ediacaran, Carboniferous and Jurassic magmatic events. U–Pb ages of ca. 1.8–1.7 Ga for a pegmatite represent the first report of Paleoproterozoic rocks in Iran. Zircon U–Pb ages from the SSNZ provide evidence for Ediacaran (547 Ma), Carboniferous (326–312 Ma) and Jurassic (194–186 Ma) magmatic activity. Zircons from the Haji–Abad ophiolites yielded Jurassic ages.The new Rb–Sr results from white micas provide indications of a poorly–constrained >85 Ma high–pressure metamorphic history. Rb–Sr ages of two chlorite–epidote–actinolite schists indicate that greenschist–facies P–T conditions had already been attained around 130–125 Ma.The new results are consistent with a model in which the closure of the Esfandagheh Ocean and the subsequent collision between Arabia and Iran led to incorporation of Paleoproterozoic and Cadomian rock units and tectonic juxtaposition of all lithotectonic elements, from oceanic lithosphere to continental crust, along the Main Zagros Suture Zone. This article is protected by copyright. All rights reserved.
      PubDate: 2016-12-20T13:20:48.757196-05:
      DOI: 10.1002/2016GC006585
       
  • Madagascar's escape from Africa: A high-resolution plate reconstruction
           for the Western Somali Basin and implications for supercontinent dispersal
           
    • Authors: Jordan J.J. Phethean; Lara M. Kalnins, Jeroen van Hunen, Paolo G. Biffi, Richard J. Davies, Ken J.W. McCaffrey
      Abstract: Accurate reconstructions of the dispersal of supercontinent blocks are essential for testing continental breakup models. Here, we provide a new plate tectonic reconstruction of the opening of the Western Somali Basin during the breakup of East and West Gondwana. The model is constrained by a new comprehensive set of spreading lineaments, detected in this heavily sedimented basin using a novel technique based on directional derivatives of free-air gravity anomalies. Vertical gravity gradient and free-air gravity anomaly maps also enable the detection of extinct mid-ocean ridge segments which can be directly compared to several previous ocean magnetic anomaly interpretations of the Western Somali Basin. The best-matching interpretations have basin symmetry around the M0 anomaly; these are then used to temporally constrain our plate tectonic reconstruction. The reconstruction supports a tight fit for Gondwana fragments prior to breakup, and predicts that the continent-ocean transform margin lies along the Rovuma Basin, not along the Davie Fracture Zone (DFZ) as commonly thought. According to our reconstruction, the DFZ represents a major ocean-ocean fracture zone formed by the coalescence of several smaller fracture zones during evolving plate motions as Madagascar drifted southwards, and offshore Tanzania is an obliquely rifted, rather than transform, margin. New seismic reflection evidence for oceanic crust inboard of the DFZ strongly supports these conclusions. Our results provide important new constraints on the still enigmatic driving mechanism of continental rifting, the nature of the lithosphere in the Western Somali Basin, and its resource potential. This article is protected by copyright. All rights reserved.
      PubDate: 2016-12-20T13:20:30.107079-05:
      DOI: 10.1002/2016GC006624
       
  • Paleoweathering and paleoenvironmental change recorded in lacustrine
           sediments of the early to middle Eocene in Fushun Basin, Northeast China
    • Authors: Zuoling Chen; Zhongli Ding, Zihua Tang, Shiling Yang, Xu Wang, Linlin Cui
      Abstract: Deciphering the long-term interaction between continental silicate weathering, global climate, and atmospheric CO2 concentrations is helpful in understanding the mechanisms of the Cenozoic climate change and accessing the future climatic and environmental response to anthropogenic carbon emissions. The Eocene, which is characterized by the Early Eocene Climatic Optimum (EECO) and the following global cooling, represents an ideal test case. Here we generate geochemical data of the Eocene lacustrine sediments from the Fushun Basin, northeast China, to explore the regional climatic response to the global climate change. The chemical index of alteration (CIA) and plagioclase index of alteration (PIA) consistently show a gradual, long-term decrease, indicating a climatic transition from warm and humid to relatively cold and arid during the Eocene in the Fushun Basin. This climatic trend is broadly coincident with the global cooling and decreasing CO2 concentration, implying that the regional climate is closely correlated with the global climate change over geological time scales. Additionally, the extreme silicate weathering and high lake productivity as reflected by relatively positive δ13C values of lacustrine organic matter are associated with the EECO. This consistency may demonstrate that enhanced continental weathering and lake productivity had served as effective sinks to lower atmospheric CO2 across the EECO. Collectively, our new geochemical data add supporting evidence for a long-term, close coupling between continental silicate weathering, climate and global carbon cycle during the Eocene. This article is protected by copyright. All rights reserved.
      PubDate: 2016-12-20T12:47:06.240895-05:
      DOI: 10.1002/2016GC006573
       
  • Effect of graphite on the electrical conductivity of the lithospheric
           mantle
    • Authors: Baohua Zhang; Takashi Yoshino
      Abstract: Graphite is considered as one of candidate to explain the high-conductivity anomalies revealed through magnetotelluric (MT) observations. To investigate the effect of interfacial energy on the interconnection of graphite in olivine matrix, we measured the electrical conductivity of polycrystalline San Carlos olivine mixed with 0.8 vol.% graphite on the grain boundaries via impedance spectroscopy at 1 GPa and 300–1700 K in a cubic multi-anvil apparatus. The olivine–graphite dihedral angle of the recovered sample was also measured to determine interfacial energy between graphite and olivine. The bulk electrical conductivities and large activation enthalpy (∼1.32 eV) of the carbon-bearing sample were consistent with those of dry polycrystalline olivine. This behavior implies that graphite cannot be interconnected on olivine grain boundaries, which is also supported by the large dihedral angle (98°) of the olivine/graphite system. Impedance spectroscopy measurements were performed at 3 GPa and a temperature of up to 1700 K for carbon-coated olivine bicrystal samples to investigate the stability of graphite films on the grain boundaries of silicate minerals under upper-mantle conditions. The conductivities rapidly or slowly dropped as a function of time and graphite film thickness during annealing at the target temperature. This phenomenon exhibits that graphite film on the olivine grain boundary is readily destroyed under upper-mantle conditions as supported by microstructural observations on the recovered carbon-coated olivine bicrystal samples. Higher interfacial energy and larger dihedral angle (∼ 98°) between graphite and olivine would not allow the maintenance of graphite film on olivine grain boundaries. The activation enthalpy for the apparent disconnection rate of a graphite film on olivine grain boundaries is close to that of carbon diffusion in olivine grain boundaries, which implies suggests that the disconnection of the graphite film is likely to be controlled by carbon grain boundary diffusion. Therefore, graphite is an unlikely candidate to explain the high-conductivity anomalies revealed by MT surveys in the upper mantle. This article is protected by copyright. All rights reserved.
      PubDate: 2016-12-20T12:40:40.05325-05:0
      DOI: 10.1002/2016GC006530
       
  • Splay fault branching from the Hikurangi subduction shear zone:
           Implications for slow slip and fluid flow
    • Authors: A. Plaza-Faverola; S. Henrys, I. Pecher, L. Wallace, D. Klaeschen
      Abstract: Pre-stack depth migration data across the Hikurangi margin, East Coast of the North Island, New Zealand, are used to derive subducting slab geometry, upper crustal structure and seismic velocities resolved to ∼14 km depth. We investigate the potential relationship between the crustal architecture, fluid migration and short-term geodetically determined slow-slip events. The subduction interface is a shallow dipping thrust at 
      PubDate: 2016-12-01T08:47:43.122429-05:
      DOI: 10.1002/2016GC006563
       
  • Effects of titanomagnetite reordering processes on thermal demagnetization
           and paleointensity experiments
    • Authors: Julie A. Bowles; Mike J. Jackson
      Abstract: Titanomagnetite (Fe3-xTixO4, 0 ≤ x ≤ 1) is a common, naturally-occurring magnetic mineral critical to many paleomagnetic studies. Underlying most interpretations is the assumption that, lacking chemical alteration, Curie temperature (Tc) remains constant. However, recent work has demonstrated that Tc of many natural titanomagnetites varies strongly as a function of thermal history, independent of chemical alteration. This is inferred to arise from reordering of cations and/or vacancies in the crystal structure, and changes occur at temperatures and times relevant to standard paleomagnetic thermal treatments. Because changes take place at T  Tclose will have their original Tb spectrum truncated at T ≈ Tclose. Predicted behavior during Thellier-type paleointensity experiments results in only modest deviations in NRM-lost or pTRM*-gained from the non-reordering case. Much larger deviations are predicted for pTRM checks. Compared to paleointensity results from titanomagnetite-bearing pyroclastic deposits, modeled non-ideal behavior occurs in the same temperature intervals, but is much more systematic. Reordering is likely one contributing factor to failure of paleointensity experiments. This article is protected by copyright. All rights reserved.
      PubDate: 2016-12-01T02:25:22.951611-05:
      DOI: 10.1002/2016GC006607
       
  • Postrift magmatic evolution of the eastern North American
           “passive-aggressive” margin
    • Authors: Sarah E. Mazza; Esteban Gazel, Elizabeth A. Johnson, Michael Bizimis, Ryan McAleer, C. Berk Biryol
      Abstract: Understanding the evolution of passive margins requires knowledge of temporal and chemical constraints on magmatism following the transition from super-continent to rifting, to post-rifting evolution. The Eastern North American Margin (ENAM) is an ideal study location as several magmatic pulses occurred in the 200 My following rifting. In particular, the Virginia-West Virginia region of the ENAM has experienced two post-rift magmatic pulses at ∼152 Ma and 47 Ma, and thus provides a unique opportunity to study the long-term magmatic evolution of passive margins. Here we present a comprehensive set of geochemical data that includes new 40Ar/39Ar ages, major and trace-element compositions, and analysis of radiogenic isotopes to further constrain their magmatic history. The Late Jurassic volcanics are bi-modal, from basanites to phonolites, while the Eocene volcanics range from picrobasalt to rhyolite. Modeling suggests that the felsic volcanics from both the Late Jurassic and Eocene events are consistent with fractional crystallization. Sr-Nd-Pb systematics for the Late Jurassic event suggests HIMU and EMII components in the magma source that we interpret as upper mantle components rather than crustal interaction. Lithospheric delamination is the best hypothesis for magmatism in Virginia/West Virginia, due to tectonic instabilities that are remnant from the long-term evolution of this margin, resulting in a “passive-aggressive” margin that records multiple magmatic events long after rifting ended. This article is protected by copyright. All rights reserved.
      PubDate: 2016-11-26T04:08:07.788298-05:
      DOI: 10.1002/2016GC006646
       
  • Boron desorption and fractionation in Subduction Zone Forearcs:
           Implications for the sources and transport of deep fluids
    • Authors: Demian M. Saffer; Achim J. Kopf
      Abstract: At many subduction zones, pore water geochemical anomalies at seafloor seeps and in shallow boreholes indicate fluid flow and chemical transport from depths of several km. Identifying the source regions for these fluids is essential toward quantifying flow pathways and volatile fluxes through forearcs, and in understanding their connection to the loci of excess pore pressure at depth. Here, we develop a model to track the coupled effects of boron desorption, smectite dehydration, and progressive consolidation within sediment at the top of the subducting slab, where such deep fluid signals likely originate. Our analysis demonstrates that the relative timing of heating and consolidation is a dominant control on pore water composition. For cold slabs, pore water freshening is maximized because dehydration releases bound water into low porosity sediment, whereas boron concentrations and isotopic signatures are modest because desorption is strongly sensitive to temperature and is only partially complete. For warmer slabs, freshening is smaller, because dehydration occurs earlier and into larger porosities, but the boron signatures are larger. The former scenario is typical of non-accretionary margins where insulating sediment on the subducting plate is commonly thin. This result provides a quantitative explanation for the global observation that signatures of deeply-sourced fluids are generally strongest at non-accretionary margins. Application of our multi-tracer approach to the Costa Rica, N. Japan, N. Barbados, and Mediterranean Ridge subduction zones illustrates that desorption and dehydration are viable explanations for observed geochemical signals, and suggest up-dip fluid migration from these source regions over tens of km. This article is protected by copyright. All rights reserved.
      PubDate: 2016-11-23T03:40:39.676815-05:
      DOI: 10.1002/2016GC006635
       
  • Physical Properties and Seismic Structure of Izu-Bonin-Mariana Fore Arc
           
    • Authors: G.L. Christeson; S. Morgan, S. Kodaira, M. Yamashita, R.R. Almeev, K. Michibayashi, T. Sakuyama, E.C. Ferré, W. Kurz
      Abstract: Most of the well-preserved ophiolite complexes are believed to form in supra-subduction zone (SSZ) settings. We compare physical properties and seismic structure of SSZ crust at the Izu-Bonin-Mariana (IBM) fore arc with oceanic crust drilled at Holes 504B and 1256D to evaluate the similarities of SSZ and oceanic crust. Expedition 352 basement consists of fore arc basalt (FAB) and boninite lavas and dikes. P-wave sonic log velocities are substantially lower for the IBM fore arc (mean values 3.1-3.4 km/s) compared to Holes 504B and 1256D (mean values 5.0-5.2 km/s) at depths of 0-300 m below the sediment-basement interface. For similar porosities, lower P-wave sonic log velocities are observed at the IBM fore arc than at Holes 504B and 1256D. We use a theoretical asperity compression model to calculate the fractional area of asperity contact Af across cracks. Af values are 0.021-0.025 at the IBM fore arc and 0.074-0.080 at Holes 504B and 1256D for similar depth intervals (0-300 m within basement). The Af values indicate more open (but not necessarily wider) cracks in the IBM fore arc than for the oceanic crust at Holes 504B and 1256D, which is consistent with observations of fracturing and alteration at the Expedition 352 sites. Seismic refraction data constrains a crustal thickness of 10-15 km along the IBM fore arc. Implications and inferences are that crust composing ophiolites formed at SSZ settings could be thick and modified after accretion, and these processes should be considered when using ophiolites as an analog for oceanic crust. This article is protected by copyright. All rights reserved.
      PubDate: 2016-11-22T03:35:40.415991-05:
      DOI: 10.1002/2016GC006638
       
  • Dynamics of primary productivity in the northern South China Sea over the
           past 24,000 years
    • Authors: Hongrui Zhang; Chuanlian Liu, Xiaobo Jin, Jiangnan Shi, Shaohua Zhao, Zhimin Jian
      Abstract: In this study, paleoproductivity on millennial scales was precisely reconstructed from core MD12-3428cq in the northern South China Sea (SCS) over the past 24 kyr, based on a transfer function derived from the strong exponential negative correlation between relative abundance of Florisphaera profunda (%FP) in core top sediments and basin-wide satellite-based primary productivity (PP) in the SCS. To detect the potential driving mechanisms of PP, correlation analyses were carried out among our PP records and other paleoenvironment parameters. PP peaked during 18–15 ka in parallel with the strong East Asian Winter Monsoon (EAWM). From 15 ka to the early Holocene, a decrease in PP coincided with sea level progradation and weakening of EAWM, which ultimately reduced fluvial nutrient levels and wind-driven upper water column mixing. Since the middle Holocene, gradually more frequent El Niño-Southern Oscillation (ENSO) events have taken place, further decreasing PP by injecting oligotrophic Kuroshio water masses into the northern SCS. Associated findings conclusively indicated that the main controlling factors of PP in the northern SCS have shifted from EAWM (glacial) to ENSO (interglacial) over the past 24 kyr. This article is protected by copyright. All rights reserved.
      PubDate: 2016-11-18T09:40:21.994586-05:
      DOI: 10.1002/2016GC006602
       
  • Log-ratio transformed major element based multidimensional classification
           for altered High-Mg igneous rocks
    • Authors: Surendra P. Verma; M. Abdelaly Rivera-Gómez, Lorena Díaz-González, Alfredo Quiroz-Ruiz
      Abstract: A new multidimensional classification scheme consistent with the chemical classification of the International Union of Geological Sciences (IUGS) is proposed for the nomenclature of High-Mg altered rocks. Our procedure is based on an extensive database of major element (SiO2, TiO2, Al2O3, Fe2O3t, MnO, MgO, CaO, Na2O, K2O, and P2O5) compositions of a total of 33868 (920 High-Mg and 32948 “Common”) relatively fresh igneous rock samples. The database consisting of these multinormally distributed samples in terms of their isometric log-ratios was used to propose a set of 11 discriminant functions and 6 diagrams to facilitate High-Mg rock classification. The multinormality required by linear discriminant and canonical analysis was ascertained by a new computer program DOMuDaF. One multidimensional function can distinguish the High-Mg and Common igneous rocks with high percent success values of about 86.4% and 98.9%, respectively. Similarly, from 10 discriminant functions the High-Mg rocks can also be classified as one of the four rock types (komatiite, meimechite, picrite, and boninite), with high success values of about 88% to 100%. Satisfactory functioning of this new classification scheme was confirmed by seven independent tests. Five further case studies involving application to highly altered rocks illustrate the usefulness of our proposal. A computer program HMgClaMSys was written to efficiently apply the proposed classification scheme, which will be available for online processing of High-Mg igneous rock compositional data. Monte Carlo simulation modelling and mass-balance computations confirmed the robustness of our classification with respect to analytical errors and post-emplacement compositional changes. This article is protected by copyright. All rights reserved.
      PubDate: 2016-11-17T18:25:33.009641-05:
      DOI: 10.1002/2016GC006652
       
  • Trace element and Sr isotope records of multi-episode carbonatite
           metasomatism on the eastern margin of the North China Craton
    • Authors: Lixu Deng; Yongsheng Liu, Keqing Zong, Lüyun Zhu, Rong Xu, Zhaochu Hu, Shan Gao
      Abstract: Lherzolite xenoliths entrained in the Changle Cenozoic basalts were analyzed to infer mantle process beneath the eastern block of the North China Craton. These xenoliths were classified into two types. Clinopyroxenes (Cpx) in the type-1 xenoliths are strongly enriched in large ion lithophile elements and light rare earth elements (LREE) but depleted in high field-strength elements and heavy rare earth elements, and show high Ca/Al, Zr/Hf and (La/Yb)N ratios but low Ti/Eu ratios. These features indicate that they were crystallized from a carbonatitic melt. Cpx in the type-2 xenoliths are mostly characterized by a chemical zonation, i.e., LREE and Sr contents and (La/Yb)N and Eu/Ti ratios gradually increase from the cores to the rims. Some fresh cores preserve the original signatures of the depleted mantle. These observations indicate partial modification of pre-existing Cpx by carbonatite metasomatism.Two episodes of metasomatism were identified based on Sr isotopic compositions of Cpx and carbonate inclusions within olivines. Both the carbonate inclusions and Cpx cores in the type-2 xenoliths have relatively high 87Sr/86Sr ratios (>0.7033), suggesting metasomatism due to CO2-rich silicate melt derived from the recycled oceanic crust. However, low 87Sr/86Sr ratios of Cpx rims in the type-2 xenoliths suggest a late stage of metasomatism by a low-87Sr/86Sr carbonatitic melt (
      PubDate: 2016-11-17T18:25:29.985081-05:
      DOI: 10.1002/2016GC006618
       
  • A practical tool for examining paleoerosion rates from sedimentary
           deposits using cosmogenic radionuclides: Examples from hypothetical
           scenarios and data
    • Authors: Pedro Val; Greg Hoke
      Abstract: We provide a MatlabTM-based algorithm that calculates paleo-erosion rates based on measured Cosmogenic Radionuclide (CRN) concentrations from sedimentary deposits and additional geological constraints provided by the user. Based on the input data, the algorithm models CRN concentrations accumulated due to sediment burial and the subsequent exhumation of these deposits. Description of the methods and applications of the algorithm is provided, which includes single or coupled CRNs (i.e. 10Be or 10Be + 26Al). Results from the literature are reproduced. Further applicability of this algorithm is demonstrated with hypothetical scenarios of paleo-erosion rates. The tool can be used towards the exploration of necessary assumptions, feasibility of case-studies, and method limits. This contribution accompanies an overview of the paleo-erosion rate method, its applications, and necessary assumptions. This article is protected by copyright. All rights reserved.
      PubDate: 2016-11-17T18:25:24.107505-05:
      DOI: 10.1002/2016GC006608
       
  • Cenozoic epeirogeny of the Indian Peninsula
    • Authors: F. D. Richards; M. J. Hoggard, N. J. White
      Abstract: Peninsular India is a cratonic region with asymmetric relief manifest by eastward tilting from the 1.5 km high Western Ghats escarpment toward the floodplains of eastward-draining rivers. Oceanic residual depth measurements on either side of India show that this west-east asymmetry is broader scale, occurring over distances of >2,000 km. Admittance analysis of free-air gravity and topography shows that the elastic thickness is 10 ±3 km, suggesting that regional uplift is not solely caused by flexural loading. To investigate how Indian physiography is generated, we have jointly inverted 530 river profiles to determine rock uplift rate as a function of space and time. Key erosional parameters are calibrated using independent geologic constraints (e.g. emergent marine deposits, elevated paleosurfaces, uplifted lignite deposits). Our results suggest that regional tilt grew at rates of up to 0.1 mm a– 1 between 25 Ma and the present day. Neogene uplift initiated in the south and propagated northward along the western margin. This calculated history is corroborated by low-temperature thermochronologic observations, by sedimentary flux of clastic deposits into the Krishna-Godavari delta, and by sequence stratigraphic architecture along adjacent rifted margins. Onset of regional uplift predates intensification of the Indian monsoon at 8 Ma, suggesting that rock uplift rather than climatic change is responsible for modern-day relief. A positive correlation between residual depth measurements and shear wave velocities beneath the lithosphere suggests that regional uplift is generated and maintained by temperature anomalies of ±100°C within a 200 ±25 km thick asthenospheric channel. This article is protected by copyright. All rights reserved.
      PubDate: 2016-11-17T11:00:42.57124-05:0
      DOI: 10.1002/2016GC006545
       
  • Estimating the duration of geologic intervals from a small number of age
           determinations: A challenge common to petrology and paleobiology
    • Authors: Allen F. Glazner; Peter M. Sadler
      Abstract: The duration of a geologic interval, such as the time over which a given volume of magma accumulated to form a pluton, or the lifespan of a large igneous province, is commonly determined from a relatively small number of geochronologic determinations (e.g., 4-10) within that interval. Such sample sets can underestimate the true length of the interval by a significant amount. For example, the average interval determined from a sample of size n = 5, drawn from a uniform random distribution, will underestimate the true interval by 50%. Even for n = 10 the average sample only captures ∼80% of the interval. If the underlying distribution is known then a correction factor can be determined from theory or Monte Carlo analysis; for a uniform random distribution this factor is . Systematic undersampling of interval lengths can have a large effect on calculated magma fluxes in plutonic systems. The problem is analogous to determining the duration of an extinct species from its fossil occurrences. Confidence interval statistics developed for species origination and extinction times are applicable to the onset and cessation of magmatic events. This article is protected by copyright. All rights reserved.
      PubDate: 2016-11-17T10:34:35.380231-05:
      DOI: 10.1002/2016GC006542
       
  • Sediment melting during subduction initiation: Geochronological and
           geochemical evidence from the Darutso high-Mg andesites within ophiolite
           melange, central Tibet
    • Authors: Zeng Yunchuan; Chen Jianlin, Xu Jifeng, Wang Baodi, Huang Feng
      Abstract: In addition to fluids, the concept of sediment-derived melts infiltrating the fore-arc mantle during subduction initiation has been proposed based on studies of modern subduction zones and ophiolite mélange. However, outcrops that contain the products of such melts are rare, especially in conjunction with boninite. New U–Pb zircon dating reveals that the Darutso volcanic rocks (DVRs) within ophiolitic mélange in the Beila area, central Tibet, crystallized at ∼164–162 Ma. This is the first recognition of Jurassic volcanic rocks in the middle section of the Bangong–Nujiang Suture Zone. Geochemically, the DVRs are high-Mg andesites with moderate SiO2 (59.03–63.62 wt%) and high MgO (3.74–6.53 wt%), Cr (up to 395 ppm), and Mg# (50.3–67.9). They also have high Th contents, (La/Sm)N ratios, and (87Sr/86Sr)i values (0.7085–0.7147); low Ba/Th, U/Th, and Sr/Y ratios; and negative values of εNd(t) (−8.7 to −9.8) and zircon εHf(t) (−7.4 to −9.9). The εNd(t) values of the DVRs overlap those of regional sediments. Detailed analyses of these geochemical characteristics indicate that the DVRs were derived from partial melting of subducted sediments and subsequent interaction with overlying mantle peridotite in a shallow and hot setting. In combination with the regional geology, in particular adjacent ophiolites that contain MORB-like and boninite mafic lavas, these rocks collectively recorded the evolution of a fore-arc setting during the initiation of the northward subduction of the south branch of the Bangong–Nujiang Ocean. Therefore, the results provide direct evidence for sediment melting during subduction initiation and constrain the Jurassic tectonic evolution of the Lhasa terrane. This article is protected by copyright. All rights reserved.
      PubDate: 2016-11-17T10:34:32.471811-05:
      DOI: 10.1002/2016GC006456
       
  • Variations in slow slip moment rate associated with rapid tremor reversals
           in Cascadia
    • Authors: Jessica C. Hawthorne; Michael G. Bostock, Alexandra A. Royer, Amanda M. Thomas
      Abstract: During large slow slip events, tremor sometimes propagates in the reverse along-strike direction for a few hours, at speeds 10 to 40 times faster than the forward propagation. We examine the aseismic slip that underlies this rapidly propagating tremor. We use PBO (Plate Boundary Observatory) borehole strainmeter data to search for variations in the slow slip moment rate during 35 rapid tremor reversals (RTRs) that occurred beneath Vancouver Island. and were identified via low frequency earthquake (LFE) analysis of tremor. The strain records reveal that, on average, the strain rate increases by about 100% (±30%) during RTRs. Given the Green's functions expected for slip in the RTR locations, these strain rate increases imply 50 to 130% increases in the aseismic moment rate. The median moment released per RTR is between 8 and 21% of the daily slow slip moment, equivalent to that of a MW 5.0 to 5.1 earthquake. By combining the RTR moments with the spatial extents suggested by tremor, we estimate that a typical RTR has peak slip of roughly one-sixth of the peak slip in the main slow slip event, near-front slip rate of a few to ten times the main front slip rate, stress drop around half the main event stress drop, and strain energy release rate around one-tenth that of the main front. Our observations support a picture of RTRs as aseismic subevents with high slip rates but modest strain energy release. RTRs appear to contribute to but not dominate the overall slow slip moment, though they may accommodate most of the slip in certain locations. This article is protected by copyright. All rights reserved.
      PubDate: 2016-11-14T03:50:56.590098-05:
      DOI: 10.1002/2016GC006489
       
  • Compositional layering within the large low shear-wave velocity provinces
           in the lower mantle
    • Authors: Maxim D. Ballmer; Lina Schumacher, Vedran Lekic, Christine Thomas, Garrett Ito
      Abstract: The large low shear-wave velocity provinces (LLSVP) are thermochemical anomalies in the deep Earth's mantle, thousands of km wide and ∼1,800 km high. This study explores the hypothesis that the LLSVPs are compositionally subdivided into two domains: a primordial bottom domain near the core-mantle boundary and a basaltic shallow domain extending from 1,100∼2,300 km depth. This hypothesis reconciles published observations in that it predicts that the two domains have different physical properties (bulk-sound vs. shear-wave speed vs. density anomalies), the transition in seismic velocities separating them is abrupt, and both domains remain seismically distinct from the ambient mantle. We here report underside reflections from the top of the LLSVP shallow domain, supporting a compositional origin. By exploring a suite of two-dimensional geodynamic models, we constrain the conditions under which well-separated “double-layered” piles with realistic geometry can persist for billions of years. Results show that long-term separation requires density differences of ∼100 kg/m3 between LLSVP materials, providing a constraint for origin and composition. The models further predict short-lived “secondary” plumelets to rise from LLSVP roofs and to entrain basaltic material that has evolved in the lower mantle. Long-lived, vigorous “primary” plumes instead rise from LLSVP margins and entrain a mix of materials, including small fractions of primordial material. These predictions are consistent with the locations of hotspots relative to LLSVPs, and address the geochemical and geochronological record of (oceanic) hotspot volcanism. The study of large-scale heterogeneity within LLSVPs has important implications for our understanding of the evolution and composition of the mantle. This article is protected by copyright. All rights reserved.
      PubDate: 2016-11-12T03:50:41.169931-05:
      DOI: 10.1002/2016GC006605
       
  • A quantitative assessment of methane cycling in Hikurangi Margin sediments
           (New Zealand) using geophysical imaging and biogeochemical modeling
    • Authors: Min Luo; Andrew W. Dale, Laura Haffert, Matthias Haeckel, Stephanie Koch, Gareth Crutchley, Henko De Stigter, Duofu Chen, Jens Greinert
      Abstract: Takahe seep, located on the Opouawe Bank, Hikurangi Margin, is characterized by a well-defined subsurface seismic chimney structure ca. 80,500 m2 in area. Sub-seafloor geophysical data based on acoustic anomaly layers indicated the presence of gas hydrate and free gas layers within the chimney structure. Reaction-transport modeling was applied to porewater data from 11 gravity cores to constrain methane turnover rates and benthic methane fluxes in the upper 10 m. Model results show that methane dynamics were highly variable due to transport and dissolution of ascending gas. The dissolution of gas (up to 3761 mmol m−2 yr−1) dwarfed the rate of methanogenesis within the simulated sediment column (2.6 mmol m−2 yr−1). Dissolved methane is mainly consumed by anaerobic oxidation of methane (AOM) at the base of the sulfate reduction zone and trapped by methane hydrate formation below it, with maximum rates in the central part of the chimney (946 and 2420 mmol m−2 yr−1, respectively). A seep-wide methane budget was constrained by combining the biogeochemical model results with geophysical data and led to estimates of AOM rates, gas hydrate formation and benthic dissolved methane fluxes of 3.68 × 104 mol yr−1, 73.85 × 104 mol yr−1and 1.19 × 104 mol yr−1, respectively. A much larger flux of methane probably escapes in gaseous form through focused bubble vents. The approach of linking geochemical model results with spatial geophysical data put forward here can be applied elsewhere to improve benthic methane turnover rates from limited single spot measurements to larger spatial scales. This article is protected by copyright. All rights reserved.
      PubDate: 2016-11-07T09:21:27.390467-05:
      DOI: 10.1002/2016GC006643
       
  • The Campi Flegrei Deep Drilling Project (CFDDP): New insight on caldera
           structure, evolution and hazard implications for the Naples area (Southern
           Italy)
    • Authors: Giuseppe De Natale; Claudia Troise, Darren Mark, Angela Mormone, Monica Piochi, Mauro Antonio Di Vito, Roberto Isaia, Stefano Carlino, Diana Barra, Renato Somma
      Abstract: The 501m-deep hole of the Campi Flegrei Deep Drilling Project, located west of the Naples metropolitan area and inside the Campi Flegrei caldera, gives new insight to reconstruct the volcano-tectonic evolution of this highly populated volcano. It is one of the highest risk volcanic areas in the world, but its tectonic structure, eruptive history and size of the largest eruptions are intensely debated in literature. New stratigraphic and 40Ar/39Ar geochronological dating allow us to determine, for the first time, the age of intra-caldera deposits belonging to the two highest magnitude caldera-forming eruptions (i.e. Campanian Ignimbrite, CI, 39 ka, and Neapolitan Yellow Tuff, NYT, 14.9 ka) and to estimate the amount of collapse. Tuffs from 439 m of depth yield the first 40Ar/39Ar age of c. 39 ka within the caldera, consistent with the CI. Volcanic rocks from the NYT were, moreover, detected between 250 m and 160 m. Our findings highlight: i) a reduction of the area affected by caldera collapse, which appears to not include the city of Naples; ii) a small volume of the infilling caldera deposits, particularly for the CI and iii) the need for reassessment of the collapse amounts and mechanisms related to larger eruptions. Our results also imply a revaluation of volcanic risk for the eastern caldera area, including the city of Naples. The results of this study point out that large calderas are characterized by complex collapse mechanisms and dynamics, whose understanding needs more robust constraints, which can be obtained from scientific drilling. This article is protected by copyright. All rights reserved.
      PubDate: 2016-10-27T03:30:29.546041-05:
      DOI: 10.1002/2015GC006183
       
 
 
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