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  Subjects -> EARTH SCIENCES (Total: 586 journals)
    - EARTH SCIENCES (433 journals)
    - GEOLOGY (65 journals)
    - GEOPHYSICS (27 journals)
    - HYDROLOGY (17 journals)
    - OCEANOGRAPHY (44 journals)

EARTH SCIENCES (433 journals)                  1 2 3 4 5 | Last

Acta Geodaetica et Geophysica     Hybrid Journal   (Followers: 1)
Acta Geodaetica et Geophysica Hungarica     Full-text available via subscription   (Followers: 2)
Acta Geophysica     Hybrid Journal   (Followers: 7)
Acta Geotechnica     Hybrid Journal   (Followers: 8)
Acta Meteorologica Sinica     Hybrid Journal   (Followers: 2)
Advances in High Energy Physics     Open Access   (Followers: 12)
Advances In Physics     Hybrid Journal   (Followers: 6)
Aeolian Research     Hybrid Journal   (Followers: 1)
African Journal of Aquatic Science     Hybrid Journal   (Followers: 13)
Algological Studies     Full-text available via subscription   (Followers: 2)
Alpine Botany     Hybrid Journal   (Followers: 3)
AMBIO     Hybrid Journal   (Followers: 13)
Anales del Instituto de la Patagonia     Open Access   (Followers: 2)
Andean geology     Open Access   (Followers: 5)
Annales Henri Poincaré     Hybrid Journal   (Followers: 2)
Annales UMCS, Geographia, Geologia, Mineralogia et Petrographia     Open Access   (Followers: 1)
Annals of Geophysics     Full-text available via subscription   (Followers: 9)
Annals of GIS     Hybrid Journal   (Followers: 14)
Annals of Glaciology     Full-text available via subscription  
Annual Review of Marine Science     Full-text available via subscription   (Followers: 9)
Anthropocene Review     Hybrid Journal  
Applied Clay Science     Hybrid Journal   (Followers: 2)
Applied Geochemistry     Hybrid Journal   (Followers: 6)
Applied Geomatics     Hybrid Journal   (Followers: 6)
Applied Geophysics     Hybrid Journal   (Followers: 6)
Applied Ocean Research     Hybrid Journal   (Followers: 6)
Applied Petrochemical Research     Open Access   (Followers: 3)
Applied Remote Sensing Journal     Open Access   (Followers: 8)
Aquatic Conservation Marine and Freshwater Ecosystems     Hybrid Journal   (Followers: 20)
Arctic, Antarctic, and Alpine Research     Full-text available via subscription   (Followers: 7)
Artificial Satellites     Open Access   (Followers: 13)
Asia-Pacific Journal of Atmospheric Sciences     Hybrid Journal   (Followers: 2)
Asian Journal of Earth Sciences     Open Access   (Followers: 17)
Atlantic Geology : Journal of the Atlantic Geoscience Society / Atlantic Geology : revue de la Société Géoscientifique de l'Atlantique     Full-text available via subscription   (Followers: 3)
Atmosphere-Ocean     Full-text available via subscription   (Followers: 3)
Atmospheric and Climate Sciences     Open Access   (Followers: 14)
Australian Journal of Earth Sciences: An International Geoscience Journal of the Geological Society of Australia     Hybrid Journal   (Followers: 11)
Boletim de Ciências Geodésicas     Open Access  
Boreas: An International Journal of Quaternary Research     Hybrid Journal   (Followers: 8)
Bragantia     Open Access   (Followers: 2)
Bulletin of Earthquake Engineering     Hybrid Journal   (Followers: 9)
Bulletin of Geosciences     Open Access   (Followers: 8)
Bulletin of Marine Science     Full-text available via subscription   (Followers: 13)
Bulletin of the Lebedev Physics Institute     Hybrid Journal   (Followers: 1)
Bulletin of the Seismological Society of America     Full-text available via subscription   (Followers: 16)
Bulletin of Volcanology     Hybrid Journal   (Followers: 11)
Canadian Journal of Plant Science     Full-text available via subscription   (Followers: 11)
Canadian Mineralogist     Full-text available via subscription   (Followers: 1)
Canadian Water Resources Journal     Hybrid Journal   (Followers: 17)
Carbonates and Evaporites     Hybrid Journal   (Followers: 3)
CATENA     Hybrid Journal   (Followers: 3)
Central European Journal of Geosciences     Hybrid Journal   (Followers: 5)
Central European Journal of Physics     Hybrid Journal   (Followers: 1)
Chemical Geology     Hybrid Journal   (Followers: 9)
Chemie der Erde - Geochemistry     Hybrid Journal   (Followers: 3)
Chinese Geographical Science     Hybrid Journal   (Followers: 3)
Chinese Journal of Geochemistry     Hybrid Journal   (Followers: 2)
Chinese Journal of Oceanology and Limnology     Hybrid Journal   (Followers: 3)
Ciencia del suelo     Open Access  
Climate and Development     Hybrid Journal   (Followers: 9)
Coastal Management     Hybrid Journal   (Followers: 18)
Comptes Rendus Geoscience     Full-text available via subscription   (Followers: 5)
Computational Geosciences     Hybrid Journal   (Followers: 10)
Computational Mathematics and Mathematical Physics     Hybrid Journal   (Followers: 1)
Computers and Geotechnics     Hybrid Journal   (Followers: 5)
Contemporary Trends in Geoscience     Open Access  
Continental Journal of Earth Sciences     Open Access   (Followers: 2)
Continental Shelf Research     Hybrid Journal   (Followers: 7)
Contributions to Mineralogy and Petrology     Hybrid Journal   (Followers: 7)
Contributions to Plasma Physics     Hybrid Journal   (Followers: 2)
Coral Reefs     Hybrid Journal   (Followers: 18)
Cretaceous Research     Hybrid Journal   (Followers: 4)
Cybergeo : European Journal of Geography     Open Access   (Followers: 4)
Developments in Geotectonics     Full-text available via subscription   (Followers: 2)
Developments in Quaternary Science     Full-text available via subscription   (Followers: 3)
Développement durable et territoires     Open Access   (Followers: 2)
Diatom Research     Hybrid Journal  
Doklady Physics     Hybrid Journal   (Followers: 1)
Dynamics of Atmospheres and Oceans     Hybrid Journal   (Followers: 3)
E3S Web of Conferences     Open Access  
Earth and Planetary Science Letters     Hybrid Journal   (Followers: 140)
Earth Interactions     Full-text available via subscription   (Followers: 8)
Earth Science Research     Open Access   (Followers: 6)
Earth Surface Dynamics (ESurf)     Open Access   (Followers: 1)
Earth Surface Processes and Landforms     Hybrid Journal   (Followers: 11)
Earth System Dynamics     Open Access   (Followers: 4)
Earth System Dynamics Discussions     Open Access   (Followers: 3)
Earth's Future     Open Access   (Followers: 1)
Earth, Planets and Space     Open Access  
Earthquake Engineering and Engineering Vibration     Hybrid Journal   (Followers: 7)
Earthquake Spectra     Full-text available via subscription   (Followers: 11)
Ecohydrology     Hybrid Journal   (Followers: 11)
Ecological Questions     Open Access   (Followers: 6)
Electromagnetics     Hybrid Journal   (Followers: 1)
Energy Efficiency     Hybrid Journal   (Followers: 9)
Energy Exploration & Exploitation     Full-text available via subscription   (Followers: 4)
Environmental Earth Sciences     Hybrid Journal   (Followers: 10)
Environmental Geology     Hybrid Journal   (Followers: 11)
Environmental Geosciences     Full-text available via subscription   (Followers: 4)
Environmental Geotechnics     Open Access  

        1 2 3 4 5 | Last

Journal Cover Chemical Geology
   [11 followers]  Follow    
   Hybrid Journal Hybrid journal (It can contain Open Access articles)
     ISSN (Print) 0009-2541
     Published by Elsevier Homepage  [2563 journals]   [SJR: 1.714]   [H-I: 111]
  • Redox-driven dissolution of clay minerals by uranium under high pressure
           CO2 conditions
    • Abstract: Publication date: 15 September 2014
      Source:Chemical Geology, Volume 383
      Author(s): Yan Liu , Fubo Luan , William D. Burgos
      Geologic sequestration of supercritical CO2 is one technology proposed to mitigate global warming. Increased acidity of brine due to CO2 injection could lead to mineral dissolution of cap rock and well seals and mobilization of contaminants (e.g., U, Pb, As). In this study we examined the dissolution of nontronite NAu-2, an Fe(III)-rich clay mineral, and partially-reduced nontronite (R-NAu-2) in a synthetic brine (0.33M Na2SO4) under high pressure CO2 conditions (PT =9.66bar, PCO2 ≥8.66bar CO2, T=20°C) and in 1.40M H3PO4–0.50M H2SO4. Uranyl(VI) or biogenic uraninite(IV) was added as a redox-active contaminant and reaction kinetics were measured over a 15d period. Unaltered nontronite [3.4% Fe(II)] dissolved very little under high pressure CO2 conditions. However, chemically-reduced nontronite [48% Fe(II)] dissolved more rapidly (half-life of 78.4d under high pressure CO2 conditions, 17.8h in H3PO4–H2SO4). Structural Fe(II) in reduced nontronite [R-NAu-2 Fe(II)] was preferentially dissolved compared to structural Fe(III) in unaltered nontronite [NAu-2 Fe(III)]. No redox reactions were observed between R-NAu-2 Fe(II) and U(VI). In contrast, uraninite was oxidized by NAu-2 Fe(III) faster and to a greater extent under high pressure CO2 conditions as compared to ambient pressure conditions (PT =1.0bar, 95:5% N2:H2). Redox reactions between uraninite and NAu-2 Fe(III) enhanced the dissolution of both clay and U, indicative of potential risks associated with geologic carbon sequestration.


      PubDate: 2014-07-27T19:19:23Z
       
  • Editorial Board
    • Abstract: Publication date: 14 August 2014
      Source:Chemical Geology, Volume 381




      PubDate: 2014-07-27T19:19:23Z
       
  • Development of in situ sulfur four-isotope analysis with multiple Faraday
           cup detectors by SIMS and application to pyrite grains in a
           Paleoproterozoic glaciogenic sandstone
    • Abstract: Publication date: 15 September 2014
      Source:Chemical Geology, Volume 383
      Author(s): Takayuki Ushikubo , Kenneth H. Williford , James Farquhar , David T. Johnston , Martin J. Van Kranendonk , John W. Valley
      An in situ sulfur four-isotope analysis technique with multiple Faraday cup detectors by ion microprobe was developed and applied to detrital pyrite grains in ~2.4Ga glaciogenic sandstone from the Meteorite Bore Member of the Turee Creek Group, Western Australia. Data are standardized with the UWPy-1 pyrite standard (δ34S=16.04±0.18‰, Δ33S=−0.003±0.009‰, and Δ36S=−0.21±0.24‰, 2 SD) whose sulfur four isotopes were newly determined by gas-source mass spectrometry. Typical reproducibility at two standard deviations (2 SD) of spot-to-spot analyses of standard UWPy-1 pyrite with a primary beam size of ~20μm were ±0.23, ±0.05, and ±0.86‰ for δ34S, Δ33S, and Δ36S, respectively. The measured 36S/32S ratio [1/(6641±27)] is approximately 19‰ lower than the published ratio for VCDT, and we propose a revision of the 36S abundance in VCDT. Pyrite grains in ~2.4Ga glaciogenic sandstone have wide ranging sulfur isotope ratios (−32.7 to 13.5 for δ34S, −3.03 to 11.66 for Δ33S, and −9.7 to 4.6 for Δ36S, respectively). Some pyrite grains are zoned in δ34S values within a grain. Sulfur isotope ratios of most pyrite grains are distributed along a line with slope=−0.9 for Δ33S vs. Δ36S, suggesting that pyrite grains mostly derived from a limited range of source rocks and near-surface sulfur reservoirs. One pyrite aggregate has a distinct texture from other pyrite grains in the same sandstone, and yields a significant mass-independent deficit in 36S with a small excess in 33S (Δ36S/Δ33S~−4‰). This is used to suggest that this grain authigenically formed by biological activity during or after sedimentation. This work demonstrates that the use of multiple Faraday cup detectors provides improved accuracy and precision for in situ sulfur four-isotope analysis with secondary ion mass spectrometry.


      PubDate: 2014-07-27T19:19:23Z
       
  • Chemical abrasion of zircon and ilmenite megacrysts in the Monastery
           kimberlite: Implications for the composition of kimberlite melts
    • Abstract: Publication date: 15 September 2014
      Source:Chemical Geology, Volume 383
      Author(s): Vadim S. Kamenetsky , Elena A. Belousova , Andrea Giuliani , Maya B. Kamenetsky , Karsten Goemann , William L. Griffin
      Ilmenite and zircon megacrysts, among other minerals representing the subcontinental lithospheric mantle, are exclusively delivered to the surface by kimberlite magmas. The intimate association of ilmenite and zircon with their transporting kimberlite melts still remains perplexing, as these minerals do not belong to the kimberlite liquidus assemblage at crustal pressures. The ilmenite and zircon megacrysts from the Monastery kimberlite (South Africa) represent a textbook example of the megacryst suite. The megacrysts show substantial chemical modification along contacts with the host kimberlite. Fine-grained “reaction” assemblages, comprising minerals rich in Zr (baddeleyite and sodium–zirconium silicates) and Ti (Ti–Fe oxides, perovskite, sphene, kassite), are present around zircon and ilmenite, respectively. At the zircon–ilmenite contact, chemical contributions from both minerals are recorded in Zr–Ti-rich phases such as calzirtite and zirkelite. The megacrysts contain crystallised melt pools and secondary melt inclusions in healed fractures; their mineral assemblage is dominated by alkali-bearing phases, including silicates (nepheline, kalsilite, sodalite, phlogopite–tetraferriphlogopite), titanates (priderite, freudenbergite), zirconium silicates (khibinskite, parakeldyshite), carbonates (zemkorite, eitelite), phosphates (apatite, bradleyite, nahpoite), sulfates (aphthitalite) and chlorides (halite, sylvite). These inclusions and melt pools are interpreted to be produced by reaction between the megacrysts and the transporting kimberlite melt, which infiltrated fractures in the megacrysts. Most secondary minerals at contacts with kimberlite require a supply of Ca, which is readily available in the carbonatite component of the kimberlite magma. The enrichment of the encapsulated mineral assemblages in alkali and volatile elements (Na, K, S, Cl) also appears to originate from the kimberlite melt. The similar U–Pb ages and identical Hf-isotope compositions of the megacryst assemblage (89.2±2.8Ma; εHf −0.4 to +1.3), the reaction assemblage (98±7Ma) and the host kimberlite (90±4Ma; εHf −0.6 to +1.7), imply their close genetic affinity. Although the megacrysts and kimberlite magma originated from the same source at the same time, the chemical disequilibrium recorded in the alteration of megacrysts precludes a simple “parental melt–cognate crystal” relationships. This apparent paradox can be resolved by considering the unmixing of a protokimberlite melt into silicate-oxide and carbonate liquids at mantle conditions.


      PubDate: 2014-07-27T19:19:23Z
       
  • New insights into the Hadean mantle revealed by 182W and highly
           siderophile element abundances of supracrustal rocks from the Nuvvuagittuq
           Greenstone Belt, Quebec, Canada
    • Abstract: Publication date: 15 September 2014
      Source:Chemical Geology, Volume 383
      Author(s): Mathieu Touboul , Jingao Liu , Jonathan O'Neil , Igor S. Puchtel , Richard J. Walker
      Tungsten concentration and isotopic data, coupled with highly siderophile element (HSE) concentration and Os isotopic data for ≥3.66billionyear-old ultramafic, mafic, and felsic supracrustal rocks from the Nuvvuagittuq Greenstone Belt, were investigated to place additional constraints on the nature and origin of 182W heterogeneities in the early Earth. The absolute and relative abundances of HSE in the mafic and ultramafic rocks are generally similar to those in modern rocks with comparable MgO contents. Further, most samples plot close to 3.8 to 4.4Ga reference lines on a 187Re–187Os isochron diagram, indicating that HSE abundances in most Nuvvuagittuq samples remained undisturbed by post-Eoarchean metamorphic events. All Nuvvuagittuq samples analyzed show well-resolved 182W excesses, ranging from +6 to +17ppm, compared with the modern isotopic composition of W. The observed level of HSE abundances, coupled with the 182W enrichments of these rocks is seemingly inconsistent with their derivation from mantle that was isolated from a HSE-rich and 182W-depleted late accretionary component. However, the absence of correlation between W and MgO contents, as well as variable W enrichment relative to elements with similar incompatibilities suggest that the W in the Nuvvuagittuq samples involved fluid transport of the W in either the crust or the mantle, and that it has little genetic relationship with the HSE. Given the lack of evidence for extensive redistribution of W in the crust, the HSE and W elemental and isotopic systematics of the Nuvvuagittuq rocks may be explained by a model whereby peridotitic mantle, with modern-like HSE abundances, was metasomatized by fluids derived from a 182W-rich crustal component that had been recycled into the mantle via subduction or delamination. The source of the 182W excess carried by this crustal component remains enigmatic. It was most likely inherited from either pre-late accretionary, or early-depleted parental mantle reservoirs.


      PubDate: 2014-07-27T19:19:23Z
       
  • Surface complexation of the phototrophic anoxygenic non-sulfur bacterium
           Rhodopseudomonas palustris
    • Abstract: Publication date: 15 September 2014
      Source:Chemical Geology, Volume 383
      Author(s): O.S. Pokrovsky , R.E. Martinez , E.I. Kompantzeva , L.S. Shirokova
      This work extends our knowledge of anoxygenic phototrophic bacteria (APB) interactions and contributes to the database of aquatic microorganism trace element adsorption parameters, such as heavy metal pollutants and limiting micronutrients. The surface properties of a phototrophic anaerobic non-sulfur bacterium (Rhodopseudomonas palustris KR-95p), which is often considered a model microorganism for Precambrian anoxygenic primary producers, were studied using potentiometric acid-base titration at various ionic strengths. The electrophoretic mobility of viable cells was measured as a function of pH and ionic strength, as well as cation (Zn, Cd, and Pb) and anion (Mo, W, and Cr) concentration. Zn, Cd, Sr, Pb, Cu, Cs, Al, Ga, Sb, As, Ge, Tl, W, and Mo adsorption was studied as a function of pH at a constant initial element concentration, yielding the pH-dependent adsorption edge. Zn, Cd, Sr, Cu and Pb adsorption was also studied as a function of metal concentration in solution at a constant pH, producing a “Langmuirian” adsorption isotherm. The non-electrostatic linear programming method (LPM) was used to fit the experimental data and assess the number of surface sites and adsorption reaction constants involved in heavy metal and anion binding to the cyanobacteria surface. The total H/OH binding site number (80–480μmol/gwet) for Rps. palustris encompasses the values reported for other APB (60–120μmol/gwet) and cyanobacteria (50–200μmol/gwet) examined using the same technique. For the cations, the adsorption edge follows the order Pb2+ >Cu2+ ≫Cd2+ ≈Zn2+ >Sr2+ >Tl+ >Cs+, which is similar to the element hydrolysis order. Trivalent hydrolysates (Al3+ and Ga3+) demonstrated increased adsorption with a pH increase at 2≤pH≤4 followed by desorption from the bacterial surface at pH>5–6 as negatively charged Al(OH)4 − and Ga(OH)4 −, similar to anions, such as MoO4 2− and WO4 2−. This behavior directly reflects the level of trace element complexation with two primary binding sites at the surface, carboxylate and phosphorylate, and certain electrostatic-like interactions between negatively charged anions and positive amine groups in acidic solutions. The main results of the present study are that the Rps. palustris exhibit trace element binding properties through their pH-dependence and overall adsorption capacity, which are similar to properties described earlier for other phototrophic bacteria, both anoxygenic bacteria and cyanobacteria. The absolute adsorption capacities for metal cations follow the order Rps. palustris < APB Rdb. blasticus <cyanobacteria Gloeocapsa sp, which presumably reflects a greater concentration of free metal cations in the external milieu for this species. Correspondingly, the adsorbed anion concentration is greatest for Rps. palustris, which therefore yields a higher proportion of positively charged amine groups or fewer negative charges overall due to a lower fraction of carboxylates and phosphorylates.
      Graphical abstract image

      PubDate: 2014-07-27T19:19:23Z
       
  • Biokinetic modeling of ureolysis in Sporosarcina pasteurii and its
           integration into a numerical chemodynamic biocalcification model
    • Abstract: Publication date: 15 September 2014
      Source:Chemical Geology, Volume 383
      Author(s): Biswanath Mahanty , Subin Kim , Chang Gyun Kim
      A dynamic model for Sporosarcina pasteurii-mediated ureolytic biocalcification process has been proposed, which incorporates microbial growth-dependent ureolysis, equilibrium speciation of dissolved inorganic carbon (DIC), dissolved inorganic nitrogen (DIN), different sodium and calcium containing species, kinetics of gas/solution exchange (for CO2, NH3), and a dynamic charge balance. The kinetics of microbial growth and ureolysis (including parameters) were selected from a set of relevant representative models using designed batch ureolysis experiments and were incorporated into the dynamic model. The gas/solution exchange rate constants for CO2 and NH3 were assessed from independent sets of abiotic experiments while regressing their measured DIC-pH profiles or DIN-pH profiles, respectively. Subsequently, residual parameters associated with CaCO3 precipitation were estimated by calibrating one or more measured profiles (DIN, urea, Ca2+, and pH) from the ureolytic-biocalcification experiment with the dynamic model. Bootstrap confidence intervals and sensitivity analysis of these model parameters are presented.


      PubDate: 2014-07-27T19:19:23Z
       
  • Enrichment and heterogeneity of trace elements at the redox-interface of
           Fe-rich intertidal sediments
    • Abstract: Publication date: 15 September 2014
      Source:Chemical Geology, Volume 383
      Author(s): Annabelle F. Keene , Scott G. Johnston , Richard T. Bush , Edward D. Burton , Leigh A. Sullivan , Matthew Dundon , Angus E. McElnea , C. Doug Smith , Col R. Ahern , Bernard Powell
      Redox-interfacial sediments can undergo radical geochemical changes with oscillating tides. In this study, we examine trace element enrichment and availability, at both landscape and pedon-scales, in the surface sediments of a remediating acidic tidal wetland. Fe-rich sediments at the surface-water interface (0–10mm in depth) were collected across an elevation gradient spanning the supratidal to subtidal range. These sediments were analysed for solid phase Fe fractions and trace elements (As, Pb, Cr, Cu, Mn, Ni, Zn, V, B, Co, Mo, Ba and U) via dilute HCl-extractions and total digests. Their concentrations were compared with those of underlying (0.05–0.65m in depth) former sulfuric horizon sediments of a coastal acid sulfate soil (CASS). Reactive Fe was enriched at the redox interface by up to 16 times (197gFe/kg) that of the former sulfuric horizon. The proportion of total trace elements associated with reactive phases was high in interfacial sediments, representing over 90% of B and U and 50% of Pb, Cu, Zn, V and Ba extractable by dilute HCl. The interfacial sediments were particularly enriched in reactive Cr, Cu, Ni, Zn, B, Mo and U, with reactive B, Mo and U concentrations between 5 and 10 times greater than in the former sulfuric horizon. Surface enrichment of trace elements is strongly co-associated with Fe(III) mineralisation, likely via sorption and co-precipitation processes. Enrichment is highly spatially heterogeneous and is strongly influenced by elevation and tidal zonation at a landscape-scale and by sediment micro-topography and preferential advective transport via surface connected macropores at the pedon-scale. The results from this study provide new insights to the processes influencing trace element enrichment in Fe-rich redox-interfacial sediments across a remediating acidic tidal wetland.
      Graphical abstract image

      PubDate: 2014-07-27T19:19:23Z
       
  • Coupled dissolution and precipitation at mineral–fluid interfaces
    • Abstract: Publication date: 15 September 2014
      Source:Chemical Geology, Volume 383
      Author(s): E. Ruiz-Agudo , C.V. Putnis , A. Putnis
      Reactions occurring at mineral–fluid interfaces are important in all geochemical processes and essential for the cycling of elements within the Earth. Understanding the mechanism of the transformation of one solid phase to another and the role of fluids is fundamental to many natural and industrial processes. Problems such as the interaction of minerals with CO2-saturated water, the durability of nuclear waste materials, the remediation of polluted water, and mineral reactions that can destroy our stone-based cultural heritage, are related by the common feature that a mineral assemblage in contact with a fluid may be replaced by a more stable assemblage.


      PubDate: 2014-07-27T19:19:23Z
       
  • Geochemical effects of an oxycombustion stream containing SO2 and O2 on
           carbonate rocks in the context of CO2 storage
    • Abstract: Publication date: 29 August 2014
      Source:Chemical Geology, Volume 382
      Author(s): Stéphane Renard , Jérôme Sterpenich , Jacques Pironon , Pierre Chiquet , Aurélien Randi
      This paper describes the effects of the injection of a CO2-dominated gas mixture into a geologic reservoir rock through experimental work in the context of limiting greenhouse gas emissions into the atmosphere. The injected gas mixture consists of the exhaust fumes from an oxyboiler without desulphurisation with the following mole fraction composition: CO2 =0.82, SO2 =0.04, O2 =0.04, N2 =0.04 and Ar=0.06. Corresponding experiments using pure CO2 and N2 were performed as a benchmark. The rock sample was obtained by drilling to a depth of 4600m into a low-porosity dolostone reservoir containing micrometric to centimetric fractures in the south-west of France (northern Pyrenees). The fracture network represents the primary volume available for CO2 storage and is partly filled with dolomite separated from the rock matrix by a thin layer of calcite covering the wall rocks. Experimental reactivity of the rock was tested in 2-cm3 batch reactors in the presence of saline water (25g/l NaCl) and a gas phase (pure N2, pure CO2 and gas mixture). Chemical analyses of the reacting solutions indicated that the mineralogic assemblage during exposure to pure CO2 was in equilibrium with the aqueous solution. Raman analyses of the gas phase revealed only the presence of CO2. Optical and electronic microscopy of the resultant solid phases indicated partial dissolution of carbonates and oxidation of the pyrite surfaces. In the presence of the gas mixture, important mineralogic alteration occurred together with the consumption of half of the O2 and total consumption of the SO2. This high reactivity with the gas mixture leads to the complete dissolution of calcite and partial dissolution of dolomite and the precipitation of anhydrite and barite, particularly in the zones where the calcite was initially present. Similarly, pyrite was completely oxidised to hematite. Analyses of the rock samples indicated partial alteration of the clay minerals in the matrix to potassic beidellite in the experiment involving pure CO2 and solely to vermiculites in the gas mixture experiment. In conclusion, the presence of SO2 in the injection stream associated with the presence of O2 results in an early strong acidification of the water, which was buffered by the significant reactivity of the carbonates (dissolution of all of the calcite and 6% of the dolomite) and partial alteration of the clay minerals (87% of illite and 100% of smectite) to vermiculites. Pyrite and aqueous Fe from clays were completely oxidised by O2, resulting in hematite and Fe3+. The mineralogic alteration and consequent volume changes under experimental conditions led to a slight increase in the porosity of the dolomite matrix and an average pore volume loss of 11% in the fractures caused by the replacement of calcite with anhydrite. Due to its high reactivity with carbonate, SO2 can react early during the injection phase. The spatial distribution of calcite in the fractures of the reservoir has to be considered as one of the primary parameters controlling the evolution of the reservoir in terms of injectivity and petrophysical properties, particularly in the zone near the injection wellbores. The integrity of the calcite-rich caprock is, however, ensured due to its thickness (more than 1000m) and its calcite content, which leads to pH buffering and anhydrite precipitation, which, in turn, induces a porosity reduction and a possible coating of the rock formation.


      PubDate: 2014-07-27T19:19:23Z
       
  • A method for determining the extent of bulk 210Po and 210Pb adsorption and
           retardation in aquifers
    • Abstract: Publication date: 29 August 2014
      Source:Chemical Geology, Volume 382
      Author(s): Don Porcelli
      The naturally-occurring radionuclides 210Po and 210Pb are normally found in very low concentrations in the environment. However, under some conditions 210Po contributes significantly to the radioactivity in ground waters and can constitute a health risk. Naturally-occurring 210Pb can be used in predicting the behavior of contaminant Pb. While studies have identified the processes controlling 210Po and 210Pb behavior, the extent of adsorption and rates of removal in an aquifer cannot be determined without quantifying the rates of supply of these isotopes from decay of parent nuclides on and within aquifer solids. This study demonstrates how the isotope systematics of the 238U decay series, which includes 210Po and 210Pb, can be used to quantify the adsorption and retardation factors of 210Po and 210Pb in aquifers. The rates of 210Po and 210Pb supply to ground waters are determined from the concentrations of other radionuclides in the series, especially 222Rn, and so the distribution coefficients between aquifer solids and groundwater can be calculated. The effects of different supply rates due to different distributions of parent radionuclides in the aquifer can then be separated from differences in adsorption, which is controlled by aquifer water chemistry. A review of groundwaters where 210Po, 210Pb, and 222Rn data are available indicates that systematic variations can be found in bulk effective adsorption coefficients and retardation factors. Overall, a method is provided for future studies of 210Po and 210Pb behavior, and greater understanding of the occurrence and migration of these radionuclides.


      PubDate: 2014-07-27T19:19:23Z
       
  • Editorial Board
    • Abstract: Publication date: 15 September 2014
      Source:Chemical Geology, Volume 383




      PubDate: 2014-07-27T19:19:23Z
       
  • Genesis of nanometric illite crystals elucidated by light-element
           (hydrogen, lithium, boron and oxygen) isotope tracing, and K–Ar and
           Rb–Sr dating
    • Abstract: Publication date: 15 September 2014
      Source:Chemical Geology, Volume 383
      Author(s): Norbert Clauer , Lynda B. Williams , Anthony E. Fallick
      Illitization is a widely used tracer for evaluation of the thermal evolution in volcano-sedimentary sequences during burial, metamorphic and tectonic events. However, no agreement exists about how the process proceeds at the crystal scale, which initiated perspectives based on the challenging separation of nanometric “fundamental” illite-rich particles in the mid 1980s. In 1997, the first isotopic study on such nanometric crystals was published, followed by others that raised new potential to improve understanding of the illitization process. The present review focuses on the promising and still unsolved aspects of light-element isotopic (δD, δ7Li, δ11B, δ18O) tracing, and K–Ar and Rb–Sr dating of nanometric illite-rich crystals extracted not only from bentonites, but also from sandstones and shales in diagenetic to low-grade metamorphic conditions. If the study of nanocrystals from bentonite and sandstone beds now appears successful, problems remain in separating mechanically authigenic illite-type crystals from detrital minerals of shales, even at the nanometric size. An indirect way to distinguish the data of these components is alkylammonium leaching, which has the specificity of stochiometrically replacing K in dioctahedral mica-type particles, and therefore to modify significantly their K–Ar balance. The overall separation technique for illite nanocrystals is specific with chemical treatments to remove the soluble phases, including organics, that are mixed with illite, “infinite” dispersion, high-speed fluid-flow centrifugation and removal of excess reagent by dialysis. Importantly, the initial K–Ar studies showed that no age information is lost during crystal nucleation and growth, with the whole illitization history apparently retained in the illite particles of different sizes. Based on combined K–Ar, Rb–Sr, δ18O and δD studies, reaction rates and durations of illitization can be reconstructed, together with changing crystallization temperature and fluid chemistry depending on the water/rock ratio. On the basis of K–Ar dating, nucleation and growth can be continuous during a given period of time, or episodic. The Rb–Sr method complements the information about the origin of the interacting fluids by recalculating 87Sr/86Sr ratios that depend on the amount of the fluids and their interaction with their pore environment. Combined with oxygen isotope data, such Sr isotope information points to differences in the origin of the solvents (by δ18O) and the solutes (by 87Sr/86Sr). The δ18O values of the illite crystals provide information about nucleation or growth temperature, either increasing during burial, or set abnormally high and constant during thermal episodes, or about changing fluid composition. Occurrence of organic matter could potentially bias the δ18O of nucleating and growing illite crystals, when involved in initial organic maturation. The largest B reservoir in sedimentary deposits is probably within the organic matter. When released with oil and gas after maturation, most of this organic B is isotopically light in the fluids relative to other natural waters, whereas 11B fractionates preferentially into the bitumen. Put in context with oxygen and hydrogen isotope variations, the B data also confirm that the illite tetrahedral BO bonds are as strong as the SiO bonds, thus preserving the large isotopic variations that occur during thermal maturation of kerogen. The changing clay–organic interactions in sedimentary environments can, therefore, be traced by the B content and isotope composition. The presently limited results on Li contents and isotope composition of nanometric illite crystals that potentially interact with organics outline trends that are similar to those of B.


      PubDate: 2014-07-27T19:19:23Z
       
  • Geochemical variation of amphiboles in A-type granites as an indicator of
           complex magmatic systems: Wentworth pluton, Nova Scotia, Canada
    • Abstract: Publication date: 25 September 2014
      Source:Chemical Geology, Volume 384
      Author(s): Angeliki Papoutsa , Georgia Pe-Piper
      The Wentworth pluton is the most complex intrusion among a series of A-type granitic plutons, emplaced along an active shear zone in the Cobequid Highlands of Nova Scotia during the latest Devonian–earliest Carboniferous. This pluton consists of A-type granites of different generations and a large gabbroic body. Among all late Paleozoic plutons in the Cobequid Highlands, only the Wentworth granites contain both primary calcic (edenite, hornblende) and sodic amphiboles (arfvedsonite). Whole-rock, and mineral chemical data were examined along with estimated magmatic parameters, such as temperature, emplacement pressure, oxygen fugacity and volatile contents, in order to understand the geological factors responsible for this mineralogical variation. The granites with calcic amphiboles show systematically lower εNd values, magmatic temperatures, and F-in-melt contents and appear geochemically less evolved, compared to the granites with the sodic amphiboles. All granites, however, appear geochemically homogeneous with limited evidence of fractionation. Specific chemical differences between these two types of granites indicate the presence of two coeval but distinct granitic systems in the Wentworth pluton. The granites with the calcic amphiboles were formed from a geochemically less evolved, hydrous, relatively calcic melt, whereas the granites with sodic amphiboles were emplaced at structurally deeper levels and were derived from a relatively drier sodic melt, which had a larger mantle component and was enriched in fluorine. Coexistence of these two magmas is indicated by the presence of interstitial sodic–calcic amphiboles in a late granitic dyke. This study provides evidence that in A-type granites, where fractional crystallization is limited, extreme variation in the type of amphibole can be the result of a very complex magmatic history.


      PubDate: 2014-07-27T19:19:23Z
       
  • Incipient weathering by Stereocaulon vulcani at Réunion volcanic
           island
    • Abstract: Publication date: 29 August 2014
      Source:Chemical Geology, Volume 382
      Author(s): J.D. Meunier , S. Kirman , D. Strasberg , O. Grauby , P. Dussouillez
      The impact of early land plants and fungi in increasing global weathering is still debated, particularly before the advent of vascular plants during the Devonian. Here we present a study of the incipient weathering of basalt by Stereocaulon vulcani, a native colonizing lichen on Réunion Island (Indian Ocean). We analyzed the chemistry and mineralogy of a 24-year-old flow located at low altitude that was mostly covered by S. vulcani with aboveground biomass of 6249kgha−1. The chemical composition of S. vulcani showed that besides C and N, Si and Fe were the dominant elements. The Si stored in the aboveground pioneer vegetation gives 27kgha−1, comparable to the Si stored in the old-growth at the Marelongue Reserve (Meunier et al., 2010). On thin sections, the inner part of S. vulcani was mostly composed of Si while Fe coatings observed at the base of the thallus may be the result of wind blow dust interception as suggested by Cochran and Berner (1992). Using BSE images on SEM, we showed evidence of dissolution of the glass matrix at the basalt–thallus contact. The quantification of porosity by digital imagery showed a variation between 7% in the unweathered zone to near to 40% at the surface. A maximum denudation rate of 6.7μmyear−1 is estimated to fall within the range of the values reported in the literature. Using our data for analog to the past, we suggest that early land plants would have been capable to mobilize Si in a similar proportion as vascular plants and should have significantly affected the weathering of land before the advent of vascular plants.


      PubDate: 2014-07-27T19:19:23Z
       
  • Editorial Board
    • Abstract: Publication date: 29 August 2014
      Source:Chemical Geology, Volume 382




      PubDate: 2014-07-27T19:19:23Z
       
  • Assessing the utility of Fe/Al and Fe-speciation to record water column
           redox conditions in carbonate-rich sediments
    • Abstract: Publication date: 29 August 2014
      Source:Chemical Geology, Volume 382
      Author(s): M.O. Clarkson , S.W. Poulton , R. Guilbaud , R.A. Wood
      Geochemical proxies based on Fe abundance (Fe/Al) and Fe-speciation have been widely applied to marine sediments in order to unravel paleo-depositional redox conditions though geological time. To date, however, these Fe proxies have only been calibrated in relation to modern and ancient fine-grained siliciclastic marine sediments. This clearly limits their use, particularly in relation to carbonate-rich sediments and rocks. To address this, we here explore the applicability of Fe-based redox proxies in carbonates through three approaches. First, we have compiled Fe/Al data for modern marine sediments to investigate variability in Fe-enrichments as a function of carbonate content and depositional setting. Second, we have expanded this approach with a compilation of new and existing Fe-speciation data for modern and ancient marine sediments deposited under oxic and euxinic (anoxic sulfidic) water column conditions. Finally, we show new data from paired limestone and dolomite sample sets to demonstrate the potential significance of deep burial dolomitization on the Fe-speciation redox proxy. Modern marine sediments deposited under oxic conditions show no relationship between Fe/Al and carbonate content. These sediments have an average Fe/Al ratio of 0.55±0.11, with some higher values potentially being attributable to steady-state early diagenetic remobilization of Fe towards the sediment–water interface. In contrast, significant Fe/Al enrichments occur as a consequence of water column Fe mineral formation and deposition, either under anoxic conditions, or due to input of anoxic hydrothermal fluids into oxic seawater. Iron speciation data also show no direct correlation with carbonate content, and instead three groups can be distinguished based on total Fe (FeT) and organic C contents. Sediments deposited under oxic water column conditions, with FeT >0.5wt.%, generally plot below the lower FeHR/FeT siliciclastic reference threshold (0.38) for distinguishing anoxic environments, regardless of carbonate content. Also consistent with siliciclastic calibrations, carbonate-rich sediments deposited under anoxic water column conditions tend to have FeHR/FeT ratios >0.38. In contrast, oxic carbonate-rich sediments with low FeT (<0.5wt.%) and low organic C (<0.5wt.%) routinely give spuriously high FeHR/FeT ratios, suggesting that the use of Fe-speciation for such samples is not appropriate for evaluating water column redox conditions. Taken together, our data suggest that an FeT content of >0.5wt.%, rather than carbonate content, represents the most suitable basis for identifying whether carbonate-rich sediments are appropriate for Fe-based redox proxy analyses. Analysis of burial dolostones suggests that the Fe-speciation proxy may also be compromised by deep burial dolomitization, where there has been a source of mobile Fe to enrich rocks during recrystallization. However, pre-screening can identify such samples. This new assessment expands the utility of Fe-based redox proxies to also incorporate appropriate carbonate-rich rocks, provided that care is taken to assess the possible impact of deep burial dolomitization.


      PubDate: 2014-07-27T19:19:23Z
       
  • Selenate adsorption to composites of Escherichia coli and iron oxide
           during the addition, oxidation, and hydrolysis of Fe(II)
    • Abstract: Publication date: 15 September 2014
      Source:Chemical Geology, Volume 383
      Author(s): Rachel E. Franzblau , Christopher J. Daughney , Magali Moreau , Christopher G. Weisener
      Adsorption onto iron oxide and bacterial surfaces can affect the fate of many dissolved ions, but currently there is a poor understanding of ion adsorption onto mixtures of these two sorbents. This study used chemical analyses of the aqueous and solid phases and electron microscopy to observe the adsorption of dissolved selenate (SeO4 2−) onto composites of iron oxide and Escherichia coli during the addition, oxidation, hydrolysis of Fe(II)aq and precipitation of Fe(III)-oxide. No SeO4 2− adsorption onto E. coli was observed under the conditions of this study ([SeO4]total =3ppm, biomass concentration 0.11–0.44 dry g/l, ionic strength=0.01M, pH3–8, reaction time 0–24h). SeO4 2− adsorption onto abiotic and biotic iron oxides decreased with time which was attributed to blockage of iron oxide surface sites by other mineral particles. Adsorption onto bacteria–iron oxide composites was significantly reduced compared to the end-member systems. Surface complexation models (SCMs) were developed to fit the experimental data and suggested that the reduction in SeO4 2− sorption by the composites was due to masking of iron oxide surface sites by both bacteria and other iron oxides and also via interactions with the bacterial supernatant. This study has shown that SeO4 2− adsorption in bacteria-bearing systems cannot be evaluated without considering redox processes like Fe(II)aq oxidation, hydrolysis and precipitation.


      PubDate: 2014-07-27T19:19:23Z
       
  • Origin of gaseous hydrocarbons, noble gases, carbon dioxide and nitrogen
           in Carboniferous and Permian strata of the distal part of the Polish
           Basin: Geological and isotopic approach
    • Abstract: Publication date: 15 September 2014
      Source:Chemical Geology, Volume 383
      Author(s): Maciej J. Kotarba , Keisuke Nagao , Paweł H. Karnkowski
      Sixteen natural gas samples collected from Pennsylvanian and Permian (Rotliegend and Zechstein Limestone) strata of the distal part of the Polish Basin in Wielkopolska and Lower Silesia were analysed for molecular composition and stable carbon isotope composition of hydrocarbons (CH4, C2H6, C3H8, n-C4H10 and i-C4H10) and CO2, stable hydrogen isotope composition of methane, stable nitrogen isotope composition of N2, and stable isotope composition of noble gases (He, Ne, Ar, Kr, and Xe). Thirteen of analysed hydrocarbon gases reveal complete [δ 13C(CH4)> δ 13C(C2H6)> δ 13C(C3H8)] and partial inversed isotopic trends from methane to propane thus they have a very complicated generation, migration and accumulation history and range of their source rock horizons. Two source rock horizons occur NE from Wolsztyn Ridge. They contain mixed types III/II and II/III kerogens of Pennsylvanian age and Mississippian and/or Devonian age, respectively. One source rock horizon occurs SW from Wolsztyn Ridge. It contains type III and mixed type III/II kerogens of Pennsylvanian age. The kerogens in all source rock horizons generated gaseous hydrocarbons at two separate maturity stages: about 0.5 to 0.8% in vitrinite reflectance scale at the first maturity stage, and over 1.3% in vitrinite reflectance scale at the second maturity stage. High concentrations of He in analysed gases are mostly a product of α-decay of U and Th enriched in crustal materials. A small contribution of He and Ne of mantle origin to the gas reservoirs was inferred. Radiogenic 4He/40Ar ratios are higher than the average production rate ratio of about 5 for crustal materials, which might have been caused by a selective supply of lighter He through crustal rocks surrounding the gas reservoirs, or higher (U+Th)/K ratio than the crustal average. CO2 from analysed gases was mainly generated during thermogenic processes of transformation of organic matter, although some gases can contain components from endogenic processes and from thermal destruction of Zechstein Limestone and probably Precambrian carbonates. N2 was mainly generated during thermal transformation of organic matter and had a large component released from NH4-rich illites. Very high N2/40Ar for the gases might be caused by accelerated thermogenic production of N2 under the condition of high heat flux from volcanic activity at late Pennsylvanian and early Permian age. Deep-seated faults, mainly framing the Wolsztyn Ridge, perform important role in migration and mixed in different proportions of noble gases originated from upper mantle and from mineralisation by radioactive minerals their fault surfaces and occurrence of Pennsylvanian–Lower Permian volcanites in crust as well as hydrocarbon gases, CO2 and N2, also from thermal destruction of NH4-rich illites, from “gas generation kitchens” of source rock horizons to the traps.


      PubDate: 2014-07-27T19:19:23Z
       
  • Zn(II) sequestration by fungal biogenic manganese oxide through enzymatic
           and abiotic processes
    • Abstract: Publication date: 15 September 2014
      Source:Chemical Geology, Volume 383
      Author(s): Jianing Chang , Yukinori Tani , Hirotaka Naitou , Naoyuki Miyata , Fuyumi Tojo , Haruhiko Seyama
      Here, we examined the ability of biogenic manganese oxide (BMO), formed in the cultures of a Mn(II) oxidizing fungus Acremonium strictum strain KR21-2, to sequester Zn(II) in the presence and absence of extra Mn(II) and found that BMOs sequester Zn(II) through various paths involving enzymatic and abiotic processes. Newly formed BMOs were observed to effectively sequester Zn(II), with virtually no release of Mn(II), when treated with Zn(II) in 4-(2-hydroxyethyl)-1-piperazineethanesulphonic acid (HEPES) buffer (pH7.0) under aerobic conditions. Under anaerobic conditions, smaller amounts of Zn(II) were sequestered, whereas a significant amount of Mn(II) was released. Similar trends were observed when the BMOs were heated at 85°C for 1h to inactivate the Mn(II) oxidase associated with the BMOs. Combined with a two-step extraction of the resulting solid phase, these results demonstrated that the Mn(II) oxidase associated with BMOs can oxidize Mn(II) which is released through an ion exchange at the BMO surface and can subsequently reduce the competition for sorption with released Mn(II). Assays using the concentrated Mn(II) oxidase crude solution indicated that coexisting Zn(II) had a decreased inhibitory effect on enzymatic Mn(II) oxidation if the preformed Mn oxide phase was present. XRD revealed that no alteration of the bulk structure occurred after Zn(II) treatment under either aerobic or anaerobic conditions. When the newly formed BMOs were repeatedly treated with mixed solutions of Zn(II) and Mn(II), the Mn(II) was effectively oxidized to an oxide phase, thereby sequestering Zn(II). The XRD of the resulting solid phases indicated the formation of woodruffite (ZnMn3 IVO7 ·2H2O) in addition to the original vernadite (birnessite) structure, which suggested that ongoing Mn(II) oxidation by the associated Mn(II) oxidase in the presence of Zn(II) may participate in the formation of mixed Zn–Mn oxide phases. Following repeated treatment of the heated BMOs in the mixed Zn(II)/Mn(II) solution at a Mn(II)/Zn(II) ratio of ~4.0, the resulting solid phase possessed well-defined XRD peaks consistent with hetaerolite (ZnMn2 IIIO4) formation through an abiotic comproportionation reaction between added Mn(II) and structural Mn(IV). With a decrease in the Mn(II)/Zn(II) ratio in the bathing solution, the XRD peaks weakened and shifted toward a pattern that was identified as hydrohetaerolite rather than hetaerolite. Thus, the higher concentration of Zn(II) relative to Mn(II) in the bathing solution may have inhibited the proper formation of crystalline hetaerolite. No comproportionation reaction occurred in the absence of Zn(II) under the evaluated experimental conditions (pH7.0, ~1mM added Mn(II)), indicating that Zn(II) triggered the comproportionation reaction at the BMO surface. The results presented here increase our understanding of the role of BMO in Zn(II) sequestration through microbial (enzymatic) and abiotic processes in natural environments and provide new insights into the application of enzymatically active BMOs for Zn(II) removal processes.
      Graphical abstract image

      PubDate: 2014-07-27T19:19:23Z
       
  • Sulphur isotope composition of dissolved sulphate in the
           Cambrian–Vendian aquifer system in the northern part of the Baltic
           Artesian Basin
    • Abstract: Publication date: 15 September 2014
      Source:Chemical Geology, Volume 383
      Author(s): Valle Raidla , Kalle Kirsimäe , Jüri Ivask , Enn Kaup , Kay Knöller , Andres Marandi , Tõnu Martma , Rein Vaikmäe
      The groundwater in the Cambrian–Vendian aquifer system with its δ18Owater values of about −22‰ (VSMOW) and a low radiocarbon concentration is of glacial origin from the Last Ice Age. Earlier surveys have highlighted a negative co-variance of sulphate and bicarbonate content in the groundwater of the Cambrian–Vendian aquifer system, whereas the most depleted dissolved inorganic carbon δ13C values have been measured mainly in groundwater samples with the lowest sulphate concentrations. In this paper we studied the origin of sulphate and the factors controlling the sulphur and carbon isotope geochemistry in the aquifer system. Direct sources of sulphate were not found, but relying upon δ18OSO4 measurements we suggest that the sulphate originates from oxidation of sulphide minerals whereas the δ34S of the dissolved SO4 2− in the groundwater is more enriched than the δ34S of the surrounding rocks. We show that bacterial activity may have caused the enrichment of δ34S of sulphate.


      PubDate: 2014-07-27T19:19:23Z
       
  • A new technique to determine element amounts down to femtograms in dust
           using femtosecond laser ablation-inductively coupled plasma-mass
           spectrometry
    • Abstract: Publication date: 15 September 2014
      Source:Chemical Geology, Volume 383
      Author(s): Dorothea S. Macholdt , Klaus Peter Jochum , Brigitte Stoll , Ulrike Weis , Meinrat O. Andreae
      Atmospheric dust has a substantial impact on climate, and climate in turn affects the dust flux and origin. Knowing its composition helps to determine dust sources and climate processes. We have therefore determined 46 major and trace elements in dust samples, using in-situ 200nm-femtosecond laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS). Very small test portions of 4–7μg were enough to enable analyses down to detection limits of femtograms and ngg−1, respectively. The data agree within 40 to 15% with the reference values for test portions of 1 to 100μg, respectively. The femtosecond laser results are compared to those obtained using a 213nm nanosecond laser, which show the advantages of the femtosecond laser, such as the ability to use non-matrix-matched calibration, reliable measurements of volatile elements, and smaller quantities of material ablation with similar results for the repeatability. The technique is especially useful for measurements of small amounts of dust on filters, and for 2D-distribution maps of selected elements to identify minerals or contamination. To demonstrate the applicability, five dust samples from Niger and Chad, in the Sahara zone, were analyzed. The samples are characterized by enrichment in light REEs and a distinct Eu anomaly, with surface soils as their source.


      PubDate: 2014-07-27T19:19:23Z
       
  • Quantifying biomineralization of zinc in the Rio Naracauli (Sardinia,
           Italy), using a tracer injection and synoptic sampling
    • Abstract: Publication date: 25 September 2014
      Source:Chemical Geology, Volume 384
      Author(s): G. De Giudici , R.B. Wanty , F. Podda , B.A. Kimball , P.L. Verplanck , P. Lattanzi , R. Cidu , D. Medas
      Streams draining mined areas throughout the world commonly have high concentrations of Zn. Because Zn is not easily removed from stream water and because it can be toxic to aquatic organisms, its presence is a persistent problem. The discovery of biomineralization of Zn-bearing solids in the mine drainage of Rio Naracauli, in Sardinia, Italy, provides insights into strategies for removing Zn and improving water quality in streams affected by mine drainage. Until now, the transport and attenuation of Zn has not been quantified in this stream setting. A continuous tracer injection experiment was conducted to quantify the biomineralization process and to identify the loading of constituents that causes a change from precipitation of hydrozincite [Zn5(CO3)2(OH)6] in the upstream reach to precipitation of a Zn-silicate phase downstream. Based on the mass-load calculations derived from the tracer experiment, about 1.2kg/day of Zn is sequestered in hydrozincite. This biomineralization represents nearly 90% removal of Zn. Other elements such as Pb and Cd also are sequestered, either in the hydrozincite, or in a separate phase that forms simultaneously. In the lower 600m of the stream, where the Zn-silicate forms, as much as 0.7kg/day Zn are sequestered in this solid, but additions of Zn to the stream from groundwater discharge lead to an overall increase in load in that portion of the Rio Naracauli.


      PubDate: 2014-07-27T19:19:23Z
       
  • Interaction between CO2-rich sulfate solutions and carbonate reservoir
           rocks from atmospheric to supercritical CO2 conditions: Experiments and
           modeling
    • Abstract: Publication date: 15 September 2014
      Source:Chemical Geology, Volume 383
      Author(s): Maria Garcia-Rios , Jordi Cama , Linda Luquot , Josep M. Soler
      A test site for CO2 geological storage is situated in Hontomín (Burgos, northern Spain) with a reservoir rock that is mainly composed of limestone. During and after CO2 injection, the resulting CO2-rich acid brine gives rise to the dissolution of carbonate minerals (calcite and dolomite) and gypsum (or anhydrite at depth) may precipitate since the reservoir brine contains sulfate. Experiments using columns filled with crushed limestone or dolostone were conducted under different P–pCO2 conditions (atmospheric: 1–10−3.5 bar; subcritical: 10–10bar; and supercritical: 150–34bar), T (25, 40 and 60°C) and input solution compositions (gypsum-undersaturated and gypsum-equilibrated solutions). We evaluated the effect of these parameters on the coupled reactions of calcite/dolomite dissolution and gypsum/anhydrite precipitation. The CrunchFlow and PhreeqC (v.3) numerical codes were used to perform reactive transport simulations of the experiments. Within the range of P–pCO2 and T of this study only gypsum precipitation took place (no anhydrite was detected) and this only occurred when the injected solution was equilibrated with gypsum. Under the P–pCO2–T conditions, the volume of precipitated gypsum was smaller than the volume of dissolved carbonate minerals, yielding an increase in porosity (Δϕ up to ≈4%). A decrease in T favored limestone dissolution regardless of pCO2 owing to increasing undersaturation with decreasing temperature. However, gypsum precipitation was favored at high T and under atmospheric pCO2 conditions but not at high T and under 10bar of pCO2 conditions. The increase in limestone dissolution with pCO2 was directly attributed to pH, which was more acidic at higher pCO2. Limestone dissolution induced late gypsum precipitation (long induction time) in contrast to dolostone dissolution, which promoted rapid gypsum precipitation. Moreover, owing to the slow kinetics of dolomite dissolution with respect to that of calcite, both the volume of dissolved mineral and the increase in porosity were larger in the limestone experiments than in the dolostone ones under all pCO2 conditions (10−3.5 and 10bar). By increasing pCO2, carbonate dissolution occurred along the column whereas it was localized in the very inlet under atmospheric conditions. This was due to the buffer capacity of the carbonic acid, which maintains pH at around 5 and keeps the solution undersaturated with respect to calcite and dolomite along the column. 1D reactive transport simulations reproduced the experimental data (carbonate dissolution and gypsum precipitation for different P–pCO2–T conditions). Drawing on reaction rate laws in the literature, we used the reactive surface area to fit the models to the experimental data. The values of the reactive surface area were much smaller than those calculated from the geometric areas.


      PubDate: 2014-07-27T19:19:23Z
       
  • Partitioning behavior of chlorine and fluorine in felsic
           melt–fluid(s)–apatite systems at 50MPa and
           850–950°C
    • Abstract: Publication date: 25 September 2014
      Source:Chemical Geology, Volume 384
      Author(s): Angela L. Doherty , James D. Webster , Beth A. Goldoff , Philip M. Piccoli
      The partitioning of Cl between apatite, felsic melts, and aqueous fluids (vapor±brine), and the partitioning of F between apatite and felsic melt were investigated experimentally at 50–58MPa and 850–950°C. Chlorine partitioning between apatite, melt, and fluid(s) is a function of the Cl content of the system, melt composition, Cl solubility in the melt, and pressure. For most of the 23 equilibrium experiments, the Cl content of apatite increases as a simple and linear function of the Cl concentration of the melt. The partitioning behavior of Cl between apatite and melt (DCl apat/mt by wt. fractions of [Cl in apatite/Cl in melt]) ranges from 3 to 32. Furthermore, weight fractions of DCl apat/fluid(s) range from 0.07 to 1.3 and most are less than unity. Both DCl apat/mt and DCl apat/fluid(s) vary with the molar (Al2O3/(Na2O+K2O+CaO)) ratio of the melt at 50MPa. Chlorine partitions more strongly in favor of melt as the molar (Al2O3/(Na2O+K2O+CaO)) ratio decreases below 0.9, and Cl distribution between apatite and fluid(s) varies non-linearly with the molar (Na2O/(Na2O+K2O)) ratio of the coexisting silicate melts. Weight-based values of DCl fluid(s)/mt range from 3 to 31. Chlorine partitioning exhibits non-Nernstian behavior; the distribution of Cl between fluid(s) and melt at 50MPa varies with the Cl concentration of the system and the Cl solubility in the silicate melt. Similar behavior was observed previously with this rhyodacitic melt composition at 200MPa (Webster et al., 2009b). Comparison with prior research at 200MPa shows that Cl is distributed increasingly in favor of apatite, relative to felsic melt or vapor±saline liquid, as pressure decreases from 200 to 50MPa. The partitioning of F between apatite and silicate melt is masked by analytical imprecision. No distinct relationships with system composition or pressure are apparent for molar DXF apat/mt ranging from 13 to 219; F partitions in favor of apatite relative to melt at all conditions. We estimate pressures of apatite crystallization and Cl contents of melts and fluid(s) coexisting with apatite by comparing these 50-MPa results with published data involving apatite, rhyodacitic melt, and fluid(s) at 200MPa. These experimental constraints are applied to prehistoric and recent Cl-enriched apatites and silicate melt inclusions of magmas of Augustine volcano to investigate pre-eruptive Cl concentrations of magma and pressures of apatite crystallization.


      PubDate: 2014-07-27T19:19:23Z
       
  • Steam and gas emission rate from La Soufriere volcano, Guadeloupe (Lesser
           Antilles): Implications for the magmatic supply during degassing unrest
    • Abstract: Publication date: 25 September 2014
      Source:Chemical Geology, Volume 384
      Author(s): Patrick Allard , Alessandro Aiuppa , François Beatuducel , Damien Gaudin , Rossella Di Napoli , Sergio Calabrese , Francesco Parello , Olivier Crispi , Gilbert Hammouya , Giancarlo Tamburello
      Since its last magmatic eruption in 1530AD, La Soufrière andesitic volcano in Guadeloupe has displayed intense hydrothermal activity and six phreatic eruptive crises. Here we report on the first direct quantification of gas plume emissions from its summit vents, which gradually intensified during the past 20years. Gas fluxes were determined in March 2006 and March 2012 by measuring the horizontal and vertical distributions of volcanic gas concentrations in the air-diluted plume and scaling to the speed of plume transport. Fluxes in 2006 combine real-time measurements of volcanic H2S concentrations and plume parameters with the composition of the hot (108.5°C) fumarolic fluid at exit. Fluxes in 2012 result from MultiGAS analysis of H2S, H2O, CO2, SO2 and H2 concentrations, combined with thermal imaging of the plume geometry and dynamics. Measurements were not only focused on the most active South crater (SC) vent, but also targeted Tarissan crater and other reactivating vents. We first demonstrate that all vents are fed by a common H2O-rich (97–98mol%) fluid end-member, emitted almost unmodified at SC but affected by secondary shallow alterations at other vents. Daily fluxes in 2012 averaged 200tons of H2O, 15tons of CO2, ~4tons of H2S and 1ton of HCl, increased by a factor ~3 compared to 2006. Even though modest, such fluxes make La Soufrière the second most important volcanic gas emitter in the Lesser Antilles arc, after Soufriere Hills of Montserrat. Taking account of other hydrothermal manifestations (hot springs and diffuse soil degassing), the summit fumarolic activity is shown to contribute most of the bulk volatile and heat budget of the volcano. The hydrothermal heat output (8MW) exceeds by orders of magnitude the contemporaneous seismic energy release. Isotopic evidences support that La Soufrière hydrothermal emissions are sustained by a variable but continuous heat and gas supply from a magma reservoir confined at 6–7km depth. By using petro-geochemical data for La Soufrière magma(s) and their dissolved volatile content, and assuming a magmatic derivation of sulfur, we estimate that the volcanic gas fluxes measured in 2012 could result from the underground release of magmatic gas exsolved from ~1400m3 d−1 of basaltic melt feeding the system at depth. We recommend that fumarolic gas flux at La Soufrière becomes regularly measured in the future in order to carefully monitor the temporal evolution of that magmatic supply.


      PubDate: 2014-07-27T19:19:23Z
       
  • Hydrochemical and isotopic (δ18O, δ2H, 87Sr/86Sr, δ37Cl and
           δ81Br) evidence for the origin of saline formation water in a gas
           reservoir
    • Abstract: Publication date: 25 September 2014
      Source:Chemical Geology, Volume 384
      Author(s): R. Bagheri , A. Nadri , E. Raeisi , H.G.M. Eggenkamp , G.A. Kazemi , A. Montaseri
      The Permo-Triassic Kangan gasfield in southern Iran is composed of an aquifer, the Kangan Aquifer (KA), and an overlying gas reservoir. It is located in the Kangan and Dalan Formations and consists predominantly of limestone and dolomite. The Kangan gasfield is exploited from 36 wells at depths from 2300 to 2860m. The quality of the produced formation waters (PFW) varies from fresh to saline, with salinities of up to 60,000mg/L. The present study aims to identify the origin of the salinity of the PFWs. The concentration of major ions, Li, Sr and Br, as well as δ18O, δD, 87Sr/86Sr, δ37Cl and δ81Br isotope compositions of the KA water and PFW samples, a fresh and a brine spring, and the Persian Gulf water were all measured. The suite of isotopic (δ37Cl and δ81Br) and hydrochemical data (Li, Cl and Br concentrations and CF index) in this study indicates that the salinity origin of the PFWs is evaporated seawater, which could be either the KA water or the intergranular saturated brine in the Kangan Gas Reservoir (KGR), both derived from Permo-Triassic evaporated seawater. δ18O and 87Sr/86Sr data exclude the KA water as the source of the saline PFW, leaving the intergranular brine of the KGR as the most likely source although further isotopic and geochemical tests are needed to verify this. This work also proposes, for the first time, the use of a Li/Cl versus Br/Cl diagram to distinguish between evaporate formation water, water modified by halite dissolution and fresh water. It is recommended to apply these comprehensive methods to deep brine aquifers, especially related to gasfields.


      PubDate: 2014-07-27T19:19:23Z
       
  • Reaction rind formation in the Catalina Schist: Deciphering a history of
           mechanical mixing and metasomatic alteration
    • Abstract: Publication date: 25 September 2014
      Source:Chemical Geology, Volume 384
      Author(s): Sarah C. Penniston-Dorland , Julia K. Gorman , Gray E. Bebout , Philip M. Piccoli , Richard J. Walker
      In the subduction-related Catalina Schist, reaction zones or ‘rinds’ developed at contacts of metamorphosed mafic blocks with chemically distinct mélange matrix. These rinds exhibit complex chemical compositions that defy simple, single-process explanations. A comparison of high-grade and low-grade reaction rinds, including field observations, bulk-rock geochemistry, thin section textural observations, mineral compositional analyses, and thermodynamic modeling provides constraints on the nature and timing of the rind-forming processes. Bulk-rock variations in the highly siderophile elements (HSE, including Os, Ir, Ru, Pt, Pd and Re) and MgO, Cr, and Ni provide insight into the physical processes of mixing of ultramafic material of the mélange matrix, rich in these elements, with mafic material derived from the blocks. Enrichments in the concentrations of these elements are found in both low-grade and high-grade reaction rinds suggesting that the processes that enrich these elements and create the reaction rinds occur over a wide range of P–T conditions in subduction zones. Differences in the chemical composition of garnets in an amphibolite-grade metamafic block, compared with garnets in the associated reaction rind, suggest that at least some of the mixing occurred before the growth of garnet in the blocks, perhaps at P–T conditions different from those producing the garnet. Bulk-rock variations in elements commonly considered fluid-mobile, such as K2O, Ba and Rb, are observed in all reaction rinds. At high-grade, enrichments of K2O, Ba and Rb are associated with replacement of garnet and amphibole by phengite and chlorite. Because phengite is the major host for K, Ba and Rb, the addition of these elements by fluids appears to have been a post-garnet growth phenomenon. This study highlights the significance of processes of metasomatism and mechanical mixing in mélange zones for influencing the geochemical evolution of the slab–mantle interface. Any “fluids” (hydrous or silicate melts) emanating from subducting slabs and entering the mantle wedge and arc lava source regions would necessarily reflect the hybrid compositions created by these processes.


      PubDate: 2014-07-27T19:19:23Z
       
  • Re-evaluating digestion methods for highly siderophile element and 187Os
           isotope analysis: Evidence from geological reference materials
    • Abstract: Publication date: 25 September 2014
      Source:Chemical Geology, Volume 384
      Author(s): Akira Ishikawa , Ryoko Senda , Katsuhiko Suzuki , Christopher W. Dale , Thomas Meisel
      Here we evaluate measurement procedures currently used for the combined determination of highly siderophile element concentrations (HSEs: Re, Ir, Os, Ru, Pt and Pd) and Os isotope ratios in geological samples by isotope dilution mass spectrometry (ID-MS) applying high-temperature sample digestion using inverse aqua regia in closed glass vessels, such as Carius tubes or a high-pressure asher system (HPA). In particular, we address the question of whether an additional hydrofluoric acid dissolution step is required to release HSEs hosted in the silicate portions of rock samples. To do so, we conducted systematic experiments for basaltic (TDB-1 and BIR-1), ultramafic (UB-N and JP-1) and sedimentary (SCo-1, SDO-1 and JCh-1) reference materials with and without HF desilicification. Through the extensive tests on TDB-1—varying apparatus (microwave, Carius tube, high-pressure asher), conditions (temperature, duration, sample size) and protocol (HF desilicification before or after aqua regia attack)—we find the optimum digestion method is to use inverse aqua regia for digesting 1–2g of powder over long durations, such as Carius tubes heated to 240˚C for 72h, followed by an HF desilicification step after CCl4 solvent extraction of Os. For basaltic reference materials, the method provides significantly improved HSE recoveries—particularly Ru—largely due to the use of HF. Strong linear correlations between Os and Ir–Ru–Pt concentrations and isochronous behavior on a 187Re/188Os vs. 187Os/188Os diagram (1240±59Ma, MSWD=1.4) are observed for repeat dissolutions of TDB-1, reflecting sample heterogeneity due to minor minerals enriched in Os, Ir, Ru and Pt—most likely sulphides. In comparison, excellent reproducibilities were obtained for BIR-1: relative standard deviations (RSDs) for 1–2g aliquots were 6.9% Os, 5.1% Ir, 2.0% Ru, 5.1% Pt, 1.5% Pd and 0.7% Re (n=9). Thus, BIR-1 might be a useful candidate reference material for obtaining certified values with small measurement uncertainties. Unlike the basaltic reference materials, extraction of HSEs from ultramafic and sedimentary reference materials is largely independent of the use of HF. The data obtained by Carius tube for UB-N and JP-1 agree well with the literature data obtained using apparently more aggressive digestion techniques. The precision of our data for 0.5–2g aliquots of the sedimentary reference materials also compares favourably with limited data available in the literature.


      PubDate: 2014-07-27T19:19:23Z
       
  • Solid-phase phosphorus speciation in Saharan Bodélé Depression
           dusts and source sediments
    • Abstract: Publication date: 25 September 2014
      Source:Chemical Geology, Volume 384
      Author(s): Karen A. Hudson-Edwards , Charlie S. Bristow , Giannantonio Cibin , Gary Mason , Caroline L. Peacock
      Phosphorus (P) is one of the most important limiting nutrients for the growth of oceanic phytoplankton and terrestrial ecosystems, which in turn contributes to CO2 sequestration. The solid-phase speciation of P will influence its solubility and hence its availability to such ecosystems. This study reports on the results of X-ray diffraction, electron microprobe chemical analysis and X-ray mapping, chemical extractions and X-ray absorption near-edge spectroscopy analysis carried out to determine the solid-phase speciation of P in dusts and their source sediments from the Saharan Bodélé Depression, the world's greatest single source of dust. Chemical extraction data suggest that the Bodélé dusts contain 28 to 60% (mean 49%) P sorbed to, or co-precipitated with, Fe (hydr)oxides, <10% organic P, 21–50% (mean 32%) detrital apatite P, and 10–22% (mean 15%) authigenic–biogenic apatite P. This is confirmed by the other analyses, which also suggest that the authigenic–biogenic apatite P is likely fish bone and scale, and that this might form a larger proportion of the apatite pool (33+/−22%) than given by the extraction data. This is the first-ever report of fish material in aeolian dust, and it is significant because P derived from fish bone and scale is relatively soluble and is often used as a soil fertilizer. Therefore, the fish-P will likely be the most readily consumed form of Bodélé P during soil weathering and atmospheric processing, but given time and acid dissolution, the detrital apatite, Fe-P and organic-P will also be made available. The Bodélé dust input of P to global ecosystems will only have a limited life, however, because its major source materials, diatomite in the Bodélé Depression, undergo persistent deflation and have a finite thickness.
      Graphical abstract image

      PubDate: 2014-07-27T19:19:23Z
       
  • The entropy paradox and overstepping in metamorphic reactions
    • Abstract: Publication date: 25 September 2014
      Source:Chemical Geology, Volume 384
      Author(s): G.M. Anderson
      It is commonly asserted that entropy tends towards a maximum at equilibrium in isolated systems. But entropy is only defined in equilibrium states, so in an isolated system the entropy is constant, if it is defined at all. The solution to this paradox is the existence of metastable equilibrium states, so common in mineralogy and petrology. Consideration of such states in a thermodynamic context also involves the concept of constraints and of the affinity, and because the affinity is a measure of the change in Gibbs energy in spontaneous reactions involving metastable phases, the entropy paradox and the overstepping of reactions in metamorphic rocks are closely related subjects.


      PubDate: 2014-07-27T19:19:23Z
       
  • Thermal diffusivity of Fe-rich pyroxene glasses and their melts
    • Abstract: Publication date: 25 September 2014
      Source:Chemical Geology, Volume 384
      Author(s): Anne M. Hofmeister , Alexander Sehlke , Alan G. Whittington
      We provide new measurements of thermal diffusivity (D) of 8 synthetic glasses and melts along the enstatite–ferrosilite compositional binary, 4 remelted crystals which are close to the binary, plus one synthetic ternary composition (Fs15Wo40En45). Thermal diffusivity of these glasses is between 0.50 and 0.62mm2 s−1 at room temperature, decreases to ~0.45 to 0.50mm2 s−1 near 1000K, then drops upon melting at 1100K to ~0.34mm2 s−1. A roughly linear trend in D exists across the En–Fs binary at 298K, with Fe-rich compositions having lower D, whereas at high temperatures all melts have similar D. The trend is not smooth, due to variable amounts of crystallites and residual strain in glasses, both of which affect D-values. Above about 700K, ∂D/∂T is visibly positive for moderate Fe contents (i.e., Fe/(Fe+Mg)=0.1 to 0.3), which we attribute to diffusive radiative transfer being enhanced by electronic–vibronic coupling. All data are well represented by equations of the form FT −G +HT, where F, G and H are fitting parameters. The fitting reveals a weak increase in D at high T for all of our glasses, consistent with weak radiative transfer in the infrared. Although data could only be collected on melts over a narrow temperature range, they also appear to have a positive ∂D/∂T, which is consistent with previous results on less mafic iron-bearing glasses and melts. Our results suggest that Fe-rich melts in the lower mantle could have thermal diffusivity higher than their crystalline counterparts due to the response of D to temperature.


      PubDate: 2014-07-27T19:19:23Z
       
  • Melting versus contamination effects on 238U–230Th–226Ra and
           235U–231Pa disequilibria in lavas from São Miguel, Azores
    • Abstract: Publication date: 14 August 2014
      Source:Chemical Geology, Volume 381
      Author(s): Julie Prytulak , Riccardo Avanzinelli , Govert Koetsier , Katharina Kreissig , Christoph Beier , Tim Elliott
      The island of São Miguel, Azores piques geochemical interest due to a strikingly large range in the long-lived radiogenic isotope ratios of its lavas. The ‘enriched’ signatures (e.g., radiogenic 87Sr/86Sr and unradiogenic 143Nd/144Nd) observed in lavas found in the east of the island have been proposed to originate from recycled crustal components in their mantle sources. Such fertile lithologies should have higher melt productivities (amount of melt generated per decrement in pressure) than peridotitic mantle and this heritage ought to be evident in the U-series signatures of melts derived from such materials. Specifically, combined 238U–230Th and 235U–231Pa disequilibria in erupted lavas should identify significant differences in the melting rate and/or the mineralogical makeup of their sources (e.g., the amount of garnet). To this end, new 238U–230Th–226Ra and 235U–231Pa disequilibria are presented for sixteen mafic lavas (<50wt.% SiO2) that encompass the full range of São Miguel's long-lived radiogenic isotope variability. However, primary U-series signatures of mafic lavas can be influenced by assimilation of more differentiated products. Notably, syenite nodules and cumulate alkali feldspar xenocrysts have been identified in São Miguel basalts. We argue that secondary contamination of basalts is most evident in the hugely variable 230Th–226Ra disequilibria (~18% deficits to >300% excesses of 226Ra) that strongly correlate with Ba/Th ratios. The longer-lived U-series nuclides are only slightly perturbed in a number of the more enriched lavas, resulting in increased noise in the dataset. In apparently uncontaminated samples, isotopically depleted lavas from the west of São Miguel display slightly higher 230Th and 231Pa excesses than the enriched samples from the east. The U-series signature of the enriched lavas can be modeled with a combination of higher absolute values of D ¯ U U and D ¯ Th , coupled with higher melting rates compared to the depleted western source. These parameters are consistent with a peridotite source enriched with contributions from more fusible lithologies such as eclogite and/or pyroxenite in the eastern source, but not with direct derivation from such fusible lithologies. The only subtle variations in U-series isotopes coupled with large, systematic variations in radiogenic isotope compositions suggest that enriched components and/or their melts are well-mixed with mantle peridotite, thus masking their distinctive melting behavior. Finally, basalts from Sete Cidades volcano in the west of São Miguel have very similar long-lived isotope compositions and U-series disequilibria to basalts from the island of Pico, Azores. The geochemical similarity of lavas from islands at the center and periphery of the archipelago argue against systematic differences in melting behavior related to sampling different portions of an underlying plume structure.


      PubDate: 2014-06-14T15:32:48Z
       
  • A new aqueous activity model for geothermal brines in the system
           Na-K-Ca-Mg-H-Cl-SO4-H2O from 25 to 300°C
    • Abstract: Publication date: 14 August 2014
      Source:Chemical Geology, Volume 381
      Author(s): Ferdinand F. Hingerl , Thomas Wagner , Dmitrii A. Kulik , Kaj Thomsen , Thomas Driesner
      A revised formulation (named REUNIQUAC) of the Extended Universal QUAsiChemical (EUNIQUAC) activity model has been developed, which fits excess thermodynamic properties of binary and selected aqueous ternary electrolyte solutions in the system Na-K-Ca-Mg-H-Cl-SO4-H2O over temperatures from 298 to 573K and concentrations to 5 molal (or up to saturation if solubility is below 5 molal) for saturated water vapor conditions. Compared to the original EUNIQUAC model, REUNIQUAC employs an extended version of the Debye–Hückel model using effective ionic radii of solute species, a concentration dependence of a UNIQUAC parameter, an additional empirical term for strongly complexing salts, as well as a simple quadratic temperature dependence of the fitting parameters. REUNIQUAC considers only pairwise interactions between solute species, as opposed to the Pitzer activity model, which additionally needs to account for ternary interactions. Since REUNIQUAC uses also species-specific parameters, extension of the existing parameter set to solutions that are composed of different combinations of the fitted species, is straightforward, and involves only the parameterization of the pairwise interaction terms. All systems could be fitted with accuracy comparable to the Pitzer model or better, although with much fewer parameters.


      PubDate: 2014-06-14T15:32:48Z
       
  • Distinguishing silicate and carbonatite mantle metasomatism by using
           lithium and its isotopes
    • Abstract: Publication date: 14 August 2014
      Source:Chemical Geology, Volume 381
      Author(s): Ben-Xun Su , Hong-Fu Zhang , Etienne Deloule , Nathalie Vigier , Yan Hu , Yan-Jie Tang , Yan Xiao , Patrick Asamoah Sakyi
      To investigate the effects of silicate and carbonatite metasomatism on mantle heterogeneity, we report lithium (Li) concentrations and isotopic compositions for olivine (Ol), orthopyroxene (Opx) and clinopyroxene (Cpx) from two suites of mantle xenoliths (Hannuoba, the North China Craton, and Haoti, the Western Qinling Orogen). The Hannuoba xenoliths range from lherzolite to pyroxenite and were affected by silicate metasomatism, whereas the Haoti xenoliths vary from harzburgite to wehrlite and were affected by carbonatite metasomatism. Lithium concentrations and isotopic compositions display a dichotomy between Hannuoba and Haoti xenoliths, and the overall variation exceeds what was previously reported. The minerals from Haoti xenoliths are more enriched in Li (Ol: 1.23–13.2ppm; Opx: 3.00–82.8ppm; Cpx: 1.39–112ppm) than those from Hannuoba samples (Ol: 1.34–5.52ppm; Opx: 0.23–16.1ppm; Cpx: 1.18–79.8ppm). Lithium isotopic compositions of these samples are highly variable in both suites of samples. δ7Li ranges from +3.0‰ to +41.9‰ in Ol, from −21.0‰ to +20.2‰ in Opx and from −17.4‰ to +18.9‰ in Cpx for Hannuoba samples. Haoti minerals display a similar degree of variation with δ7Li ranging from −29.1‰ to +19.9‰ in Ol, −16.9‰ to +18.0‰ in Opx and −45.1‰ to +19.6‰ in Cpx. On average, Li isotopic compositions of minerals from Hannuoba xenoliths follow the sequence of δ7LiOl >δ7LiOpx >δ7LiCpx, whereas those from Haoti xenoliths are characterized by the opposite sequence of δ7LiCpx >δ7LiOpx >δ7LiOl; in particular there is considerable difference in δ7Li values of Ol. The Li elemental and isotopic data suggest that mantle metasomatism by distinct agents is an important process for generating the large heterogeneity of Li abundances and isotopic distribution in the lithospheric mantle. The distinct geochemical characteristics of Li isotopes in silicate and carbonatite metasomatism are closely related to the preferential incorporation of Li into minerals from distinct melts. These findings further demonstrate that the Li isotopic systematics may in turn help to discriminate between silicate and carbonatite metasomatism.


      PubDate: 2014-06-14T15:32:48Z
       
  • New insights into cerium anomalies and mechanisms of trace metal
           enrichment in authigenic carbonate from hydrocarbon seeps
    • Abstract: Publication date: 14 August 2014
      Source:Chemical Geology, Volume 381
      Author(s): Yu Hu , Dong Feng , Jörn Peckmann , Harry H. Roberts , Duofu Chen
      Authigenic carbonates that form at marine hydrocarbon seeps provide a unique geological archive of past local environmental conditions and pore fluid geochemistry. Recent work on such carbonates revealed variable cerium (Ce) anomalies and anomalous enrichments of certain trace metals. However, the mechanisms accounting for such anomalies remain poorly constrained. Here, we characterize the rare earth element (REE) patterns of carbonate phases and the trace metal patterns of bulk carbonate rocks sampled at three hydrocarbon seeps located at Congo Fan pockmarks (CF) and the Gulf of Mexico sites AC645 and GB425. The analyzed CF, GB425, and AC645 carbonates yielded different REE patterns, displaying positive, no, as well as negative Ce anomalies. The covariation of molybdenum (Mo) with uranium (U), including authigenic Mo (Moauth) and U (Uauth) enrichments as well as (Mo/U)auth ratios proved useful to obtain new insight into the applicability of Ce anomalies to constrain past redox conditions. Trace element patterns suggest that (1) CF carbonates formed in a restricted sulfidic environment, while (2) the AC645 site experienced intermittent oxygenation causing negative Ce anomalies, and (3) environmental conditions were variable at the GB425 mud volcano site. Interestingly, GB425 carbonates show significant Mo, arsenic (As), and antimony (Sb) enrichments with the enrichment factor of As (AsEF) correlating well with the authigenic Fe fraction. These results suggest that iron oxyhydroxides played an important role in the adsorption of Mo, As, and Sb in the water column and their transfer to the sediment. The combination of trace metal and REE geochemistry of authigenic carbonates used here is a promising tool to better assess past variability of redox conditions and biogeochemical processes at marine hydrocarbon seeps.


      PubDate: 2014-06-14T15:32:48Z
       
  • An eclogitic diamond from Mir pipe (Yakutia), recording two growth events
           from different isotopic sources
    • Abstract: Publication date: 14 August 2014
      Source:Chemical Geology, Volume 381
      Author(s): G.P. Bulanova , D.F. Wiggers de Vries , D.G. Pearson , A. Beard , S. Mikhail , A.P. Smelov , G.R. Davies
      An eclogitic octahedral macrodiamond from the Mir kimberlite (Yakutia) has a complex growth structure with distinctive core, intermediate and rim zones. Carbon isotope ratios change abruptly from depleted δ13C values of −16.6‰ in the core to a mean mantle-like signature of −6.8‰ in the intermediate and −7.5‰ in the rim areas, establishing that two main stages of diamond formation took place from fluids/melts derived from different carbon isotope reservoirs. The core and intermediate growth areas are separated by a zone with oscillatory growth that records an episode of diamond dissolution and regrowth. The Mir kimberlite pipe is known to contain two populations of eclogitic diamonds, with light and heavy carbon isotopes, but this is the first case where both populations have been found in a single diamond monocrystal. Multiple sulphides and two silicate inclusions occur at the boundary of the oscillatory zone with the intermediate area of the diamond. Silicate inclusions are composite and there is textural and compositional disequilibrium between the mineral phases. A primary omphacite phase has Mg# 66, a high jadeite content, 0.3wt.% K2O and contains no Cr. Texturally later omphacites in the composite inclusions have Mg# 70, lower jadeite, no potassium, 0.3wt.% Cr2O3 and occur together with interstitial albite and single grains of phlogopite. Late omphacites are 1.6–3 times higher in Ba, Ti, and Yb. This later mineral assemblage was formed as the result of decompression of primary omphacite in a partly open system in the presence of a fluid/melt enriched in Ba, K, Ti, Yb, Cr, and possibly water. Estimated formation pressure for the primary omphacite based on experimental data is 6GPa and significantly less than 6GPa for the later omphacites. A mantle residence time of 1.7Ga is implied for the core formation of the studied diamond, using published results that couple a 2.1Ga Re–Os sulphide inclusion age for Mir eclogitic diamonds with δ13C values of <−10‰, relative to the 0.36Ga emplacement age of the kimberlite. The re–growth and formation of the intermediate and rim areas took place at 0.9Ga based on a Re–Os isochron age determined from the sulphide inclusions of these zones, from a carbon source with mean mantle δ13C values. It is proposed that an early eclogitic diamond population characterised by low N content, negative δ15N and low δ13C values formed at a mantle depth of ~180km during the final assembly of the Siberian Craton associated with active subduction at ~2.1Ga. Formation of the second, 0.9Ga aged population of eclogitic diamonds characterised by moderate N content, less negative δ15N and typical mean-mantle δ13C values (−5‰) took place at a shallower depth of ~120km and is associated with rift-related magmatism at the end of the Proterozoic. Evidence for both episodes of Mir eclogitic diamond growth is found recording its exhumation from lower to shallower upper mantle depths and formation in two distinct geochemical environments.


      PubDate: 2014-06-14T15:32:48Z
       
  • Linking soil element-mass-transfer to microscale mineral weathering across
           a semiarid environmental gradient
    • Abstract: Publication date: 14 August 2014
      Source:Chemical Geology, Volume 381
      Author(s): Rebecca A. Lybrand , Craig Rasmussen
      Understanding controls on silicate weathering is critical to characterizing critical zone evolution. The objective of this study was to investigate how climate, vegetation, and landscape position control feldspar transformations across a semiarid environmental gradient. Granitic surface soil and saprock samples were collected from desert scrub and mixed conifer sites within the Santa Catalina Mountain Critical Zone Observatory where mean annual temperature ranges from 24°C to 10°C and mean annual precipitation from 25 to 85cm. Quantitative X-ray diffraction, X-ray fluorescence, and electron microprobe analyses were employed to quantify elemental changes in bulk soils and across plagioclase grains. The chemical depletion of Na in bulk soils ranged from 5.4 – 15% in the desert scrub sites relative to 16–33% in the mixed conifer sites. Plagioclase grain alteration was classified into unaltered, edge, and altered sections to compare microscale weathering and elemental variation. The Na/Al and Si/Al ratios decreased from unaltered, to edge, to altered grain sections in the mixed conifer sites, whereas the element ratios of the desert scrub system were similar between unaltered and edge grain sections, and only exhibited significant decreases in Na/Al and Si/Al ratios between edge and altered materials. The microscale depletion of Na and Si suggested increased silicate weathering in the cooler, wetter, and more biologically productive mixed conifer system compared to the hot, dry desert scrub system. The results also demonstrated a topographic control on mineral transformation where increased plagioclase weathering occurred in convergent footslope landscapes with little change in elemental depletion of soils in divergent summit sites.


      PubDate: 2014-06-14T15:32:48Z
       
  • Separation and analysis of chlorine isotopes in higher plants
    • Abstract: Publication date: 14 August 2014
      Source:Chemical Geology, Volume 381
      Author(s): Aide Sun , Qingcai Xu , Shujian Xu , Hongyuan Shen , Jing Sun , Yanling Zhang
      A novel chemical mass spectrographic method was used in the determination of chlorine (Cl) isotopes in plant tissues. The procedure includes dry ashing, three-step ion chromatographic separation of Cl isotopes, and isotope ratio determination based on Cs2Cl+ ion in positive thermal ionization mass spectrometry. The recovery of the method and the fractionation of Cl isotopes were validated using certified reference standard materials. The pretreatment strongly eliminated the effects of organic impurities and other anionic interferences, especially soluble nitrates and sulfates. The results show that there was severe fractionation of Cl isotopic composition in the tissues of plant samples, which might be caused by different molecular mechanisms of uptake and translocation of Cl within plants. The observed Cl isotopic variation is considered to be a useful isotope signature of living systems, which may be used to understand better the Cl cycling process in the environment.
      Graphical abstract image

      PubDate: 2014-06-14T15:32:48Z
       
  • Effects of Co and Ni co-doping on the structure and reactivity of
           hexagonal birnessite
    • Abstract: Publication date: 14 August 2014
      Source:Chemical Geology, Volume 381
      Author(s): Hui Yin , Hui Li , Yan Wang , Matthew Ginder-Vogel , Guohong Qiu , Xionghan Feng , Lirong Zheng , Fan Liu
      Natural hexagonal birnessites are enriched in various transition metals (TMs). Many studies have examined the effects of single metal doping on the structures and properties of birnessites, but none focused on the simultaneous interaction mechanism of coprecipitation of two different TMs with birnessite. In this work Co and Ni co-doped hexagonal birnessites were synthesized and characterized by powder X-ray diffraction (XRD), elemental analysis, field emission scanning electron microscopy (FE-SEM), X-ray photoelectron spectroscopy (XPS) and X-ray absorption fine structure (XAFS) spectroscopy to investigate the effects of co-doping on the structure and reactivity of birnessite and the crystal chemistry of Co and Ni. These co-doped birnessites have lower crystallinity, i.e., fewer manganese layers stacking in the c* direction, larger specific surface areas (SSAs) and increased Mn average oxidation states (AOSs) than the undoped birnessite, and Co exists in a valence of +3. Co, Ni and Mn K-edge extended X-ray absorption fine structure spectroscopy (EXAFS) spectra demonstrate an increase in edge-sharing Ni–Me (Me=Ni, Co and Mn) distances in birnessite layers with the increase of the contents of dopants while Mn–Me distances first decrease and then increase while those of Co–Me pairs are nearly constant, coupled with first a decrease and then increase of the in-plane unit-cell parameter b. The effect of co-doping on the amounts of structural Mn and K+, numbers of [MnO6] layers stacked in c* axis, and SSAs, is larger than the effects of doping with Co alone, but less than singly Ni doping. In birnessites doped with both Co and Ni, ~74–79% of the total Co and ~23–39% of the total Ni are present within the manganese layers. Compared with the spatial distribution of TM in singly doped birnessites, the coexistence of Ni hinders the incorporation of Co into the layers during birnessite crystallization; however, coprecipitation with Co has little effects, neither hindrance nor promotion, on the insertion of Ni into the layers. These results provide insight into the interaction mechanism between coexisting Co, Ni within layered Mn oxides. It further helps us to interpret the geochemical characteristics of multi-metal incorporation into natural Mn oxides and their effects on the structures and physicochemical properties of these minerals.


      PubDate: 2014-06-14T15:32:48Z
       
  • Discriminating between different genetic types of marine ferro-manganese
           crusts and nodules based on rare earth elements and yttrium
    • Abstract: Publication date: 14 August 2014
      Source:Chemical Geology, Volume 381
      Author(s): M. Bau , K. Schmidt , A. Koschinsky , J. Hein , T. Kuhn , A. Usui
      Marine ferro-manganese (Fe–Mn) crusts and nodules are metal-rich chemical sediments that are archives of paleoceanographic proxies and potential metal resources and targets of deep-sea mining. Traditionally, crusts and nodules are subdivided into hydrogenetic, diagenetic, and hydrothermal types. Because these are characterized by different compositions and hence economic resource potential, a useable genetic classification is not only a tool for geochemical studies but may also help to characterize potential exploration targets. We propose two easy-to-use, yet robust discrimination diagrams based on geochemical relationships controlling the rare earths and yttrium (REY) inventory of marine Fe–Mn (oxyhydr)oxide deposits. The REY are a set of trace elements that show coherent behavior, are routinely determined in geochemical studies, and for which the analytical quality can easily be evaluated. Graphs of Ce anomaly vs Nd concentration and Ce anomaly vs Y anomaly effectively discriminate between the different types of precipitates, regardless of their diverse mineralogical composition. Both hydrogenetic crusts and nodules show positive Ce anomalies, negative Y anomalies and high Nd concentrations of >100mgkg−1, although nodules tend to have slightly lower Nd concentrations than crusts. In marked contrast, hydrothermal deposits generally yield negative Ce anomalies, positive Y anomalies, and low Nd concentrations of <10mgkg−1, in spite of their large geochemical, mineralogical and sedimentological diversity. Diagenetic nodules show negative Ce anomalies, negative Y anomalies, and intermediate Nd concentrations, between 10 and 100mgkg−1. We discuss the geochemical background of these diagrams, the respective sources of REY in the different precipitates, and address the processes that control their REY inventory. Besides the three end-members, we also use REY systematics to define mixed-type diagenetic–hydrogenetic nodules and classify well-studied international reference standards.


      PubDate: 2014-06-14T15:32:48Z
       
  • Dating iron skarn mineralization using hydrothermal allanite-(La)
           U–Th–Pb isotopes by laser ablation ICP-MS
    • Abstract: Publication date: 29 August 2014
      Source:Chemical Geology, Volume 382
      Author(s): Xiao-Dong Deng , Jian-Wei Li , Guang Wen
      Trace elements and U–Th–Pb isotopes of hydrothermal allanite from the Beiminghe iron skarn deposit (eastern North China Craton) were simultaneously analyzed using laser ablation inductively coupled plasma mass spectrometry (LA-ICPMS) to demonstrate the suitability and robustness of the U–Th–Pb system in this phase as a geochronometer of ore-forming processes. Allanite grains from the Beiminghe deposit, typically exhibit optical and chemical zoning and are intergrown or texturally equilibrated with prograde and/or retrograde skarn assemblages mainly consisting of tremolite, diopside, magnetite, and garnet. In addition, they contain abundant aqueous two-phase or daughter mineral-bearing three-phase aqueous fluid inclusions, confirming their hydrothermal origin. Most grains have high La/Ce atomic ratios (1.19–1.74) and thus are classified as allanite-(La). A notable feature of the hydrothermal allanite-(La) is the lack of common Pb, with values comparable with those of the zircon standards 91500 and GJ-1 used in this study. LA-ICPMS spot analyses reveal remarkable variation in Ca, REEs (rare earth elements), Fe, Al, and Mg within a single grain, consistent with the substitution of REE3+ +Mg+Fe2+ for Ca+Al+Fe3+ in the mineral structure. Traverses of zoned grains show that the rims have Th/U ratios ranging from 0.73 to 11.55, significantly higher than the core areas (0.07–1.81). Fifty-nine spot analyses (beam size of 32μm or 60μm) on 5 allanite-(La) grains yield reproducible 206Pb/238U ages of 134±7 to 139±6Ma, with a weighted mean 206Pb*/238U age (206Pbexcess-corrected 206Pb/238U age) of 136±1Ma (2σ). Of these, 34 analyses on allanite rims have a weighted mean 208Pb/232Th age of 139±2Ma (2σ) that is within error of the 206Pb*/238U age. The dating results showed that an increase of beam size from 32 to 60μm significantly reduces the analytical uncertainties (2σ). Petrographic and textural data have demonstrated that the allanite-(La) grains formed during skarn alteration and notably have close paragenetic relationships with retrograde skarn assemblages and iron oxide minerals; their U–Th–Pb ages therefore are interpreted as the timing of iron skarn mineralization at Beiminghe. Thirteen zircon grains from the ore-related diorite intrusion have a weighted mean 206Pb/238U age of 136±2Ma (2σ), consistent with and thus confirming the reliability of the allanite-(La) U–Th–Pb ages when interpreted as the formation age of the Beiminghe iron deposit. Our results suggest that hydrothermal allanite with low Th and U is a promising U–Th–Pb geochronometer and can provide reliable constraints on the timing of hydrothermal mineralization without requiring a matrix-matched external standard. The present dating results also indicate that the U–Pb, rather than Th–Pb, isotope system is preferable for U–Th–Pb dating of low Th/U allanite.


      PubDate: 2014-06-14T15:32:48Z
       
  • Geochemical response to pulsed sedimentation: Implications for the use of
           Mo as a paleo-proxy
    • Abstract: Publication date: 29 August 2014
      Source:Chemical Geology, Volume 382
      Author(s): J.L. McKay , T.F. Pedersen
      Deglacial sediments in piston core JT96-02 from the continental slope off western Canada contain numerous centimeter-thick, iron sulfide-rich layers. Major and minor element chemistry, as well as C and N isotopic data suggest that these layers represent background hemipelagic sedimentation. Intervening gray silty clay layers, each of which exhibits a fining-upward trend, are distal turbidites. The iron sulfides precipitated in response to the emplacement of each turbidite, which slowed oxygen diffusion into the sediment allowing anoxic conditions to develop in the underlying relatively organic-rich deposits. Resulting sulfate reduction and precipitation of Fe-sulfide fostered Mo enrichment (>2μg/g). It may also explain Ni, Cr, V, and Co enrichments, although differences in detrital mineralogy cannot be ruled out, particularly for Ni and Cr. Rhenium is also enriched, albeit slightly, within the sulfide layers, but low Re/Mo ratios (~0.78ng/g/μg/g) suggest that anoxic conditions developed very rapidly. Metals with high lithogenic concentrations (Cd and U) are not obviously enriched in the sulfide layers because there was insufficient time for noticeable authigenic accumulation before deposition of the next turbidite which severed diffusive communication with overlying seawater, the primary source of metals. Silver is also slightly higher in the sulfide layers, possibly linked to higher marine organic matter content. In summary, the pulsed character of sedimentation during the deglaciation led to episodic enrichment of Mo and occurred independently of changes in bottom water oxygen or organic carbon content. This observation potentially complicates the use of Mo as a paleo-proxy in some continental margin environments.


      PubDate: 2014-06-14T15:32:48Z
       
  • Fractionation of Sr and Hf isotopes by mineral sorting in Cascadia Basin
           terrigenous sediments
    • Abstract: Publication date: 29 August 2014
      Source:Chemical Geology, Volume 382
      Author(s): Marion Carpentier , Dominique Weis , Catherine Chauvel
      Oceanic sediments deposited on continental margins consist mainly of erosion products of the nearby exposed continental areas. Detrital input usually dominates their geochemical budget, and the composition of these sediments should record potential changes in their continental sources. However, along margins, mineral sorting associated with transport and sedimentary processes induces significant chemical and isotopic fractionation over a few tens of kilometers. The study of margin sediments should help to quantify the extent of modification of the continental terrigenous supply when it reaches deep oceans. Reported Sr, Nd, Hf and Pb isotopic compositions of fifty-seven sediments from the northernmost part of the Cascade forearc (Ocean Drilling Program, ODP, Sites 888 and 1027) suggest the involvement of two dominant end-members coming from the nearby Canadian Cordillera. Erosion products of the depleted, western part of the Cordillera dominate the detrital input, while the eastern enriched terranes of the Cordillera contribute only 10 to 28% of the input. There is no marked change of provenance of sediments during the last 3.5Myr and they all appear unaffected by glacial–interglacial climate cycles. The average isotopic compositions of the two sites are slightly different, but are both dominated by continental signature; these values can be used in future studies to identify any subducted sediment contribution to the Cascades Arc. On a finer scale, there are differences in the isotopic signature between samples dominated by clay minerals and those with coarser lithologies. For a given Nd isotopic composition, fine sediments have more radiogenic Sr and Hf isotope ratios than sands, and we interpret the difference as resulting from mineral sorting during transport of the particles. Fine sediments concentrate minerals with radiogenic Sr and Hf such as clays and micas, while coarse-grained detritus carry the unradiogenic mineral component of a given source rock through plagioclase–epidote and zircon. ODP Site 1027 is located 100km further away from continent than ODP Site 888 and contains more clay. As a consequence, it has significantly more radiogenic Sr and Hf bulk composition than ODP Site 888. Similar differences in isotopic signatures related to the distance to continent certainly occur in other areas in the word, and will account for a large part of differences known between continental sources and deep-sea sediments.


      PubDate: 2014-06-14T15:32:48Z
       
  • Carbon dioxide triggered metal(loid) mobilisation in a mofette
    • Abstract: Publication date: 29 August 2014
      Source:Chemical Geology, Volume 382
      Author(s): Judith Mehlhorn , Felix Beulig , Kirsten Küsel , Britta Planer-Friedrich
      Carbon capture and geologic storage is a frequently discussed option to reduce atmospheric CO2 concentrations with the long-term risk of leakage from storage sites to overlying aquifers and soils. We chose natural CO2 exhalations, so-called mofettes, in a wetland area in the Czech Republic as analogues to follow the fate of metal(loid)s under CO2-saturated conditions. Compared to the reference fluvisol at the study site, mofette soils exhibited lower pH (4.9±0.05) and redox potential (300±40mV), as well as higher organic carbon contents. Poorly crystalline and crystalline Fe (hydr)oxides, the most important metal(loid) sorbents in the CO2-unaffected soils (7.9±5.9gkg−1), showed significantly lower concentrations under the acidic and reducing conditions in the mofettes (1.2±0.4gkg−1). In turn, this increased the mobility of As and resulting concentrations were up to 2.5 times higher than in the CO2-unaffected pore water (58±18μgL−1). Methylation (up to 11% of total As) and thiolation (up to 9%) contributed to net As mobilisation. Dissolved Mn (131±53μgL−1), Ni (9.1±3.1μgL−1) and especially Cu (2.2±1.0μgL−1) concentrations remained low, likely due to complexation and/or adsorption to organic carbon and the small amount of Fe (hydr)oxides. A one-month-in-situ mobilisation experiment showed mobilisation of all investigated elements to the aqueous phase suggesting that desorption is the faster and initially dominating process while resorption is a secondary, slower process. We conclude that the CO2-induced mobilisation of toxic As and net-immobilisation of essential micro-nutrients (Mn, Ni, Cu) constitute serious risks and must be tested for transferability and relevance at geologic carbon storage sites.


      PubDate: 2014-06-14T15:32:48Z
       
  • Understanding biogeochemical gradients of sulfur, iron and carbon in an
           oil sands tailings pond
    • Abstract: Publication date: 29 August 2014
      Source:Chemical Geology, Volume 382
      Author(s): Sebastian Stasik , Nadine Loick , Kay Knöller , Christopher Weisener , Katrin Wendt-Potthoff
      Oil sands tailings ponds in Alberta (Canada) are strongly stratified ecosystems structured in an upper water layer and underlying mud layers that harbour a diversity of microorganisms, contributing to hydrocarbon degradation and elemental cycling. Until now not much is known about the biogeochemistry of the ponds and their spatial structure is not well explored yet. An understanding of microbial activity and community composition is important, in particular, in order to determine potential effects on pond properties and long term development. Therefore, the purpose of the present study was to identify reactive zones of iron, carbon and sulfur cycling in an active tailings pond, by comparing biogeochemical data along two depth profiles. For both profiles a zone of intense sulfur cycling was substantiated by maxima of: (a) dissolved and solid sulfides; (b) sulfate reduction rates and thiosulfate oxidation potentials; and (c) viable counts of sulfate reducers and relative abundances of functional genes. In addition, methanogenesis and microbial iron reduction were shown to be important electron accepting processes in the ponds. All processes coexisted in a zone of intense elemental cycling at a depth of 1–4m below the water–mud interface, where fresh tailings are likely to accumulate. Microbial activity and biomass decreased with depth, where tailings had higher age and density. While the upper mud layers were influenced by the presence of different archaea, the microbial communities showed an increased presence of bacterial species at depth. Insights from qPCR, 35S radiotracer technique and stable isotope analysis mirrored some differences between the profiles, regarding sulfur and carbon cycling. Despite this, both profiles showed remarkably similar patterns of microbial community composition and activity, revealing a good reproducibility of biogeochemical cycling within a few metres.


      PubDate: 2014-06-14T15:32:48Z
       
  • Records of trace metals in sediments from the Oregon shelf and slope:
           Investigating the occurrence of hypoxia over the past several thousand
           years
    • Abstract: Publication date: 29 August 2014
      Source:Chemical Geology, Volume 382
      Author(s): Andrea M. Erhardt , Clare E. Reimers , David Kadko , Adina Paytan
      Hypoxic (<62μmol/kg or 1.43mL/L O2) to anoxic conditions have been repeatedly observed over the last 10years on the Oregon shelf, while similar conditions are absent in historical records from 1950 to 1999. This study seeks to identify whether similar instances of decadal length hypoxia/anoxia have occurred in the Oregon coastal zone prior to recorded history and to shed light on potential causes for these events. We have measured redox-sensitive metals, uranium, vanadium, and molybdenum concentration profiles in 7 cores across the coastal affected region and in 3 cores from deeper water sites. Results indicate regional variability in redox conditions through time. The northern sites show no metal enrichment throughout the cores, while the southern sites show strong metal enrichment at the base of the cores, indicative of previous hypoxic/anoxic conditions. The southern sites indicate progression in time toward less hypoxic/anoxic burial, in conflict with recent hydrographic trends. Analysis of offshore sediments representing sites beneath the California Undercurrent shows an opposite trend to that observed in the coastal sites. Excess Mo concentrations generally increase toward the present in cores collected within the upwelling source waters at ~300m water depth, suggesting a trend toward oxygen depletion. The Mo enrichment corresponds to increases in δ13C, total organic carbon (TOC), and declines in carbon:nitrogen (C:N) ratios which may indicate that a localized rise in marine productivity has contributed to oxygen drawdown. However, these metal and associated geochemical enrichments are not clearly seen in other cores collected in deeper water in the same region, suggesting that widespread changes in productivity or hypoxia may not have occurred. When the Mo enrichment records for two dated mid-depth sites are compared to climatic indicators such as the Pacific Decadal Oscillation no clear relationships are found on decadal time scales. These results are consistent with the hypothesis that modern hypoxic conditions observed on the continental shelf throughout the region are driven by modern climate changes, not observed in this region for the past hundreds to thousands of years.


      PubDate: 2014-06-14T15:32:48Z
       
  • Methane sources and sinks in the subtropical South Pacific along 17°S
           as traced by stable isotope ratios
    • Abstract: Publication date: 29 August 2014
      Source:Chemical Geology, Volume 382
      Author(s): Chisato Yoshikawa , Elena Hayashi , Keita Yamada , Osamu Yoshida , Sakae Toyoda , Naohiro Yoshida
      We analyzed the concentration and stable carbon isotopic ratio (δ13C-CH4) of methane in the atmosphere and in dissolved methane in water column along 17°S in the subtropical South Pacific. Additionally, the hydrogen isotopic ratios (δD-CH4) of some water samples were analyzed. The sea–air CH4 flux is high in the eastern region and off the west coast of Australia, which is related to the high concentrations of dissolved CH4 and high wind speeds. Moreover, there is a positive correlation between the CH4 and chlorophyll a concentrations at the surface. This consistency suggested that active CH4 productions related to the primary production cause surface CH4 accumulation. CH4 shows a decrease in concentration and an increase in δ13C-CH4 and δD-CH4 values from the surface to the depth of about 1000m. The relationship between δ13C-CH4 values and CH4 concentration indicates that the isotopic enrichment of CH4 reflects microbial oxidation of CH4 with isotopic fractionation during vertical transport via vertical sinking and/or zooplankton migration. East of 120°W, δ13C-CH4 values at around 1000m exceed −30.0‰. The relationships among the δ13C-CH4 values, CH4 concentrations, and oxygen concentrations indicate that the 13C-enriched CH4 originates not only from in situ CH4 production and oxidation but also from CH4 transported from the eastern margin off Peru. Furthermore, at a site near the Central Lau Spreading Centers in the Lau Basin, high δ13C-CH4 values (up to −21.4‰) are observed in the benthic water, suggesting a hydrothermal field source.


      PubDate: 2014-06-14T15:32:48Z
       
  • Chalcophile elemental compositions of MORBs from the ultraslow-spreading
           Southwest Indian Ridge and controls of lithospheric structure on
           S-saturated differentiation
    • Abstract: Publication date: 29 August 2014
      Source:Chemical Geology, Volume 382
      Author(s): Alexandra Yang Yang , Mei-Fu Zhou , Tai-Ping Zhao , Xi-Guang Deng , Liang Qi , Ji-Feng Xu
      Sulfide globules in mid-ocean ridge basalts (MORBs) from the ultra-slow spreading Southwest Indian Ridge (SWIR) are present in olivine crystals and in the matrix glass, indicating S-saturated fractionation during magmatic evolution. Most of the sulfides were in equilibrium with the magmas based on the Ni contents of the sulfides. The major element trends of MORBs from Zone C (48–51°E) of the SWIR define two distinct liquid lines of descent (LLD), including one which is composed of MORBs depleted in HREE. The MORBs have variable PGE concentrations with Pd contents ranging from 0.056 to 1.572ppb. They are PGE-depleted relative to Ni and Cu with high Cu/Pd ratios (55,000–1,785,000). Quantitative modeling using Pd vs. Ni correlations yields relatively low amounts of segregated sulfides for both groups assuming complete equilibration between the sulfides and the melt. However, the correlation of calculated sulfur contents with MgO suggests much higher amounts of segregated sulfides, indicating that the segregated sulfides are incompletely equilibrated with the magma. There are no obvious correlations between PGE concentrations and spreading rates for MORBs globally. MORBs from the SWIR show slowly decreasing PGE contents during magmatic evolution but exhibit higher sulfide segregation rates compared to MORBs from faster-spreading ridges, largely due to the poor equilibration between the segregated sulfides and the magma. Such incomplete equilibration could result from thicker lithosphere and an absence of steady magma chambers and conduits beneath ultra-slow spreading ridges.


      PubDate: 2014-06-14T15:32:48Z
       
  • Evaluation of foraminiferal trace element cleaning protocols on the Mg/Ca
           of marine ostracod genus Krithe
    • Abstract: Publication date: 29 August 2014
      Source:Chemical Geology, Volume 382
      Author(s): William Gray , Jonathan Holmes , Amelia Shevenell
      The Mg/Ca of calcite from the marine ostracod genus Krithe may be an important tool for reconstructing past changes in oceanic bottom water temperature (150–4000m water depth). Rigorous cleaning procedures, routinely used to remove clays, organic matter and Fe–Mn oxyhydroxide coatings in trace element studies of foraminifera, are not regularly applied to marine ostracods despite the potential for Mg contamination. Here we apply standard oxidative and reductive foraminiferal cleaning procedures to core top Krithe pernoides valves from boxcore OCE205-50BC (26.23°N, 77.7°W, 817m water depth) to evaluate the effects of contamination on Mg/Ca ratios and assess the impact of cleaning techniques on contaminant removal and ostracod valve chemistry. Our results show that clays and Fe–Mn oxyhydroxides influence the Mg/Ca of Krithe. Following sonication in methanol/ultrapure water, there is a 1.6mmol/mol (11%) decrease in Mg/Ca (equivalent to a reduction in reconstructed temperature of 1.5°C), indicating that this is a critical step in the preparation of Krithe valves for Mg/Ca analyses. Oxidation with buffered hydrogen peroxide has little effect on the Mg/Ca of valves from our site. Reductive cleaning reduces inter-valve variability from 12% to 5%, resulting in an equivalent temperature precision of ±0.6°C. However, reductive cleaning also decreases Mg/Ca ratios due to the partial dissolution of the valve surface. Reductive cleaning offers the potential to improve Krithe Mg/Ca paleotemperature reconstructions and should be utilised in future Krithe Mg/Ca studies. Future work should also aim to constrain the effects of partial dissolution of the valve surface.


      PubDate: 2014-06-14T15:32:48Z
       
 
 
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