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  Subjects -> EARTH SCIENCES (Total: 634 journals)
    - EARTH SCIENCES (462 journals)
    - GEOLOGY (67 journals)
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EARTH SCIENCES (462 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     Open Access   (Followers: 7)
Acta Geotechnica     Hybrid Journal   (Followers: 9)
Acta Meteorologica Sinica     Hybrid Journal   (Followers: 2)
Advances in High Energy Physics     Open Access   (Followers: 13)
Advances In Physics     Hybrid Journal   (Followers: 9)
Aeolian Research     Hybrid Journal   (Followers: 3)
African Journal of Aquatic Science     Hybrid Journal   (Followers: 12)
Algological Studies     Full-text available via subscription   (Followers: 2)
Alpine Botany     Hybrid Journal   (Followers: 5)
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: 1)
Annales UMCS, Geographia, Geologia, Mineralogia et Petrographia     Open Access   (Followers: 2)
Annals of Geophysics     Full-text available via subscription   (Followers: 10)
Annals of GIS     Hybrid Journal   (Followers: 18)
Annals of Glaciology     Full-text available via subscription   (Followers: 2)
Annual Review of Marine Science     Full-text available via subscription   (Followers: 11)
Anthropocene     Hybrid Journal  
Anthropocene Review     Hybrid Journal   (Followers: 3)
Applied Clay Science     Hybrid Journal   (Followers: 2)
Applied Geochemistry     Hybrid Journal   (Followers: 7)
Applied Geomatics     Hybrid Journal   (Followers: 7)
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: 10)
Aquatic Conservation Marine and Freshwater Ecosystems     Hybrid Journal   (Followers: 20)
Arctic Science     Open Access   (Followers: 4)
Arctic, Antarctic, and Alpine Research     Full-text available via subscription   (Followers: 9)
Artificial Satellites     Open Access   (Followers: 15)
Asia-Pacific Journal of Atmospheric Sciences     Hybrid Journal   (Followers: 2)
Asian Journal of Earth Sciences     Open Access   (Followers: 20)
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: 7)
Atmospheric and Climate Sciences     Open Access   (Followers: 16)
Australian Journal of Earth Sciences: An International Geoscience Journal of the Geological Society of Australia     Hybrid Journal   (Followers: 12)
Boletim de Ciências Geodésicas     Open Access  
Boreas: An International Journal of Quaternary Research     Hybrid Journal   (Followers: 12)
Bragantia     Open Access   (Followers: 2)
Bulletin of Earthquake Engineering     Hybrid Journal   (Followers: 10)
Bulletin of Geosciences     Open Access   (Followers: 9)
Bulletin of the Lebedev Physics Institute     Hybrid Journal   (Followers: 1)
Bulletin of the Seismological Society of America     Full-text available via subscription   (Followers: 18)
Bulletin of Volcanology     Hybrid Journal   (Followers: 16)
Canadian Journal of Plant Science     Full-text available via subscription   (Followers: 13)
Canadian Mineralogist     Full-text available via subscription   (Followers: 1)
Canadian Water Resources Journal     Hybrid Journal   (Followers: 20)
Carbonates and Evaporites     Hybrid Journal   (Followers: 4)
CATENA     Hybrid Journal   (Followers: 4)
Central European Journal of Geosciences     Hybrid Journal   (Followers: 6)
Central European Journal of Physics     Hybrid Journal   (Followers: 2)
Chemical Geology     Hybrid Journal   (Followers: 11)
Chemie der Erde - Geochemistry     Hybrid Journal   (Followers: 3)
Chinese Geographical Science     Hybrid Journal   (Followers: 5)
Chinese Journal of Geochemistry     Hybrid Journal   (Followers: 3)
Chinese Journal of Oceanology and Limnology     Hybrid Journal   (Followers: 2)
Ciencia del suelo     Open Access  
Ciencias Espaciales     Open Access  
Climate and Development     Hybrid Journal   (Followers: 12)
Coastal Management     Hybrid Journal   (Followers: 18)
Cogent Geoscience     Open Access  
Comptes Rendus Geoscience     Full-text available via subscription   (Followers: 6)
Computational Geosciences     Hybrid Journal   (Followers: 12)
Computational Mathematics and Mathematical Physics     Hybrid Journal   (Followers: 1)
Computers and Geotechnics     Hybrid Journal   (Followers: 7)
Contemporary Trends in Geoscience     Open Access   (Followers: 2)
Continental Shelf Research     Hybrid Journal   (Followers: 8)
Contributions to Mineralogy and Petrology     Hybrid Journal   (Followers: 8)
Contributions to Plasma Physics     Hybrid Journal   (Followers: 2)
Coral Reefs     Hybrid Journal   (Followers: 18)
Cretaceous Research     Hybrid Journal   (Followers: 6)
Cybergeo : European Journal of Geography     Open Access   (Followers: 5)
Depositional Record     Open Access  
Developments in Geotectonics     Full-text available via subscription   (Followers: 3)
Developments in Quaternary Science     Full-text available via subscription   (Followers: 4)
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: 74)
Earth and Space Science     Open Access  
Earth Interactions     Full-text available via subscription   (Followers: 11)
Earth Science Research     Open Access   (Followers: 8)
Earth Surface Dynamics (ESurf)     Open Access   (Followers: 3)
Earth Surface Processes and Landforms     Hybrid Journal   (Followers: 14)
Earth System Dynamics     Open Access   (Followers: 7)
Earth System Dynamics Discussions     Open Access   (Followers: 4)
Earth's Future     Open Access   (Followers: 1)
Earth, Planets and Space     Open Access   (Followers: 3)
Earthquake Engineering and Engineering Vibration     Hybrid Journal   (Followers: 7)
Earthquake Science     Hybrid Journal   (Followers: 8)
Earthquake Spectra     Full-text available via subscription   (Followers: 13)
Ecohydrology     Hybrid Journal   (Followers: 10)
Electromagnetics     Hybrid Journal   (Followers: 1)
Energy Efficiency     Hybrid Journal   (Followers: 12)
Energy Exploration & Exploitation     Full-text available via subscription   (Followers: 4)

        1 2 3 4 5 | Last

Journal Cover   Chemical Geology
  [SJR: 1.927]   [H-I: 123]   [11 followers]  Follow
    
   Hybrid Journal Hybrid journal (It can contain Open Access articles)
   ISSN (Print) 0009-2541
   Published by Elsevier Homepage  [2812 journals]
  • Editorial Board
    • Abstract: Publication date: 22 June 2015
      Source:Chemical Geology, Volumes 407–408




      PubDate: 2015-07-03T00:23:31Z
       
  • Optimizing LA-ICP-MS analytical procedures for elemental depth profiling
           of foraminifera shells
    • Abstract: Publication date: 22 June 2015
      Source:Chemical Geology, Volumes 407–408
      Author(s): Jennifer S. Fehrenbacher , Howard J. Spero , Ann D. Russell , Lael Vetter , Stephen Eggins
      Laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) is becoming a widespread technique for analyzing elemental ratios in foraminiferal calcite. Here we focus on optimizing LA-ICP-MS for high-resolution depth profiling of elemental ratios through shell walls. This application reveals intrashell variability and provides a unique opportunity to quantify trace element incorporation over short time scales of calcification by an individual foraminifer. High-resolution depth profiling requires careful consideration of both ablation and analytical conditions required to resolve differences in shell chemistry across sub-micron shell thickness. We present laser ablation profiles of NIST SRM 610 standard glass data (in cps) and elemental/Ca ratios (in mmol/mol) from foraminiferal calcite obtained over a range of operating conditions using a Photon Machines 193nm UV excimer laser-ablation system, equipped with a dual-volume ANU HelEx chamber, coupled to an Agilent 7700x quadrupole ICP-MS. Different combinations of energy density, repetition rate, and mass spectrometer cycle time can yield varying elemental profiles. This variability can mimic and/or mask real intrashell trace element heterogeneity in foraminifer shells. At low (<3Hz) laser repetition rates, real intrashell element variation can be obscured depending on the laser energy, whereas using moderate (≥3Hz) laser repetition rates and/or a signal-smoothing device improves the accuracy and precision of intrashell trace element profiles. Shell material is ablated rapidly when using a 5Hz or greater repetition rate and an energy density of 3J/cm2 or greater, resulting in reduced spatial resolution.


      PubDate: 2015-07-03T00:23:31Z
       
  • Application of radon and radium isotopes to groundwater flow dynamics: an
           example from the Dead Sea
    • Abstract: Publication date: Available online 23 June 2015
      Source:Chemical Geology
      Author(s): Yael Kiro , Yishai Weinstein , Abraham Starinsky , Yoseph Yechieli
      This study presents the behavior of radon and radium isotopes and their application to groundwater age and flow dynamics. The research was conducted in the complex Dead Sea groundwater system, which includes a large variety of sediments, groundwater salinities, flow mechanisms and groundwater ages. Groundwater around the Dead Sea contains high activities of radon (up to tens of thousands dpm/L) and radium (up to hundreds dpm/L). Adsorption of radium, which is partially salinity controlled, is an important source of unsupported 222Rn, which is used for estimating the adsorption partition coefficient of radium. In addition to salinity, the concentration of Mn and Fe oxides and aquifer heterogeneity are important factors controlling the adsorption partition coefficient. The different nature of the rocks on both sides of the Dead Sea transform, with lower Th/U ratios in the carbonate rocks on the western catchment of the Dead Sea compared to higher ratios in the sandstone aquifer east of the Dead Sea, is reflected in a higher 228Ra/226Ra activity ratio in the eastern compared with the western groundwaters (averages of 0.76 and 0.15, respectively). The different groundwater groups around the Dead Sea contain secular or non-secular equilibrium ratios, which depend on the age of the groundwater (the time since the groundwater entered the aquifer) or whether the groundwater system is in a steady state (the age of the groundwater system).Young groundwater, such as the Dead Sea water that circulates in the aquifer or freshwater springs, is depleted in the long-lived radium isotopes compared to the short-lived isotopes, whereas old groundwater contains relatively high activity of 226Ra (∼500 dpm/L) and the radium activity ratios are close to secular equilibrium. The common secular equilibrium ratios between all four radium isotopes in the Dead Sea groundwaters suggest that many of the groundwater flow paths did not change significantly during the past 8,000 years.


      PubDate: 2015-07-03T00:23:31Z
       
  • A nitrogen isotope fractionation factor between diamond and its parental
           fluid derived from detailed SIMS analysis of a gem diamond and theoretical
           calculations
    • Abstract: Publication date: Available online 25 June 2015
      Source:Chemical Geology
      Author(s): D.C. Petts , T. Chacko , T. Stachel , R.A. Stern , L.M. Heaman
      To determine the magnitude of N-isotope fractionation between diamond and its parental fluid, detailed C- and N-isotope analyses of a complexly-zoned, eclogitic diamond (JDE-25) were undertaken using secondary ion mass spectrometry. Combined C- and N-isotope and N-abundance measurements were made across four distinct growth zones and show the following range of values: δ 13C=−5.7 to −2.1‰; δ 15N=−7.0 to +5.5‰; [N]=104 to 5420at.ppm. The core zone displays a continuous, rimward increase in δ 13C and δ 15N values and decreases in N-abundance, and is interpreted to have formed by fractional crystallization of diamond from a single pulse of fluid (i.e., closed system). Modelling of the isotopic and abundance data from the core zone yields a diamond–fluid nitrogen partition coefficient (K N) of 4.4 and a N-isotope fractionation factor (∆15Ndiam–fluid) of −4.0±1.2‰ (2σ) at ~1100°C, for precipitation from a pure carbonate fluid. Calculated K N and ∆15Ndiam–fluid values would have larger magnitudes if JDE-25 formed from a more complex fluid, in which the carbonate species formed only a minor component. Theoretical calculations of N-isotope fractionation between the principal N-species associated with upper mantle fluids (N2, NH3 or NH4 +) and the CN− molecule, as an analogue for the carbon–nitrogen bond in diamond, yield the following ∆15Ndiam (CN)–fluid estimates at 1100°C: −3.6‰ for NH4 +, −2.1‰ for N2 and −1.4‰ for NH3. The theoretical calculations provide only minimum estimates of the true diamond–fluid N-isotope fractionation factor, given that the C–N single bond in diamond would have a lower affinity for 15 N than the stronger C–N triple bond in the CN− molecule. Accordingly, the theoretical N-isotope fractionation factors are consistent with the empirical fractionation factor derived from diamond JDE-25. As a consequence of the large magnitude of ∆15Ndiam–fluid, intracrystalline N-isotope variations in diamond should provide a sensitive test for fluid-related, fractional crystallization processes. Furthermore, the large magnitude of ∆15Ndiam–fluid could be reflected in the wide range of δ 15N values for natural diamonds and the absence of clearly defined modes for the N-isotope compositions of peridotitic and eclogitic diamonds.


      PubDate: 2015-07-03T00:23:31Z
       
  • Refining the extraction methodology of carbonate associated sulfate:
           Evidence from synthetic and natural carbonate samples
    • Abstract: Publication date: Available online 25 June 2015
      Source:Chemical Geology
      Author(s): Bethany P. Theiling , Max Coleman
      Sulfur and oxygen isotope analyses of trace and whole mineral sulfate are valuable in investigating diagenetic processes and the microbial communities that produced them, seawater anoxia, and paleoclimate. Oxygen isotopes are particularly useful in that they may also record alterations to the original isotope ratio, be it from post-depositional processes or oxidation of sulfide minerals during the chemical extraction procedure. Here we rigorously test several common methodological procedures of extracting carbonate associated sulfate (CAS) for sulfur and oxygen isotope analysis in order to generate a method that will extract only the CAS, while preserving associated organic reduced sulfur and sulfides for analysis. The results of these experiments on synthetically generated carbonates demonstrate that our proposed protocol sufficiently removes all non-CAS sulfate and does not result in oxidation of included sulfides. Analytical reproducibility (standard deviation) of synthetic carbonates is 0.1‰ (1σ) for δ34S and 0.3‰ (1σ) for δ18O. Extractions of low pyrite, high organic matter geologic samples from the Monterey Formation across a range of facies types demonstrate a reproducibility (1σ) of 0.4‰-0.7‰ for δ34S and 0.8‰-1.3‰ for δ18O, resulting from sample heterogeneity. δ34S and δ18O from Monterey Formation samples do not demonstrate oxidation of organic matter, suggesting our proposed protocol will preserve organic sulfur. A high pyrite-concentration Jet Rock concretion demonstrates that additional sulfate can be produced during the non-CAS leaching processes from oxidation of pyrite. We show that pyrite from the Jet Rock concretion ceases to oxidize when the sample is leached under an anoxic environment.


      PubDate: 2015-07-03T00:23:31Z
       
  • Post-earthquake anomalies in He-CO2 isotope and relative abundance
           systematics of thermal waters: the case of the 2011 Van earthquake,
           eastern Anatolia, Turkey
    • Abstract: Publication date: Available online 25 June 2015
      Source:Chemical Geology
      Author(s): Harun Aydın , David R. Hilton , Nilgün Gülecxç , Halim Mutlu
      We report the helium and carbon isotope (3He/4He, δ13C) and relative abundance (CO2/3He) characteristics of hydrothermal gases from eastern Anatolia sampled ~1 month after the October 23, 2011 Van earthquake (Mw: 7.2, focal depth: 19 km). Seven sites were sampled which comprise three localities along the Çaldıran fault zone, at a distance of 58-66 km to the epicenter (Group I), two localities north of the Çaldıran fault, about 90-113 km from the epicenter (Group II), and two localities in vicinity of the historically-active Nemrut Caldera at a distance of 110-126 km from the epicenter (Group III). All sites were previously sampled for their He-CO2 systematics in 2009 (Mutlu et al., 2012) facilitating direct comparison with the post-earthquake dataset. The post-earthquake values cover a wide range of 3He/4He, δ13C and CO2/3He ratios, from 0.84 to 6.37 RA (where RA =air 3He/4He), -5.30 to +0.49‰ (vs. VPDB), and 4.9x1010 to 6.85x1013, respectively. Group I samples show a consistent post-earthquake increase in 3He/4He whereas both Group II localities decreased in 3He/4He. No change was recorded for the two Group III localities. He isotope variations are consistent with simple changes in the proportions of mantle and crustal volatiles, with all Group I sites showing an increase in the mantle He contribution. We hypothesize that the enhanced mantle He signal is derived from asthenospheric melts intruded into the crust, with seismic perturbations responsible for bubble formation and growth leading to overpressure and gas loss. The strike-slip Çaldıran fault zone provides the permeable pathway for the liberated volatiles to reach hydrothermal systems at shallow levels of the crust and the surface. Release of crustal He dominates the He mass balance of Group II samples as locations are further from the earthquake epicenter. Group III samples are even further away from the earthquake and show no perturbations in He isotopes. Whereas binary mixing dominates the He isotope systematics, CO2 shows additional effects involving the hydrothermal system. Consequently, changes in the balance between mantle and crustal CO2 are masked and more difficult to discern. The results emphasize the sensitivity of He isotopes to seismic perturbations in the crust and illustrate how location of sampling sites – on permeable segments of faults – and distance from seismic events influence resulting changes involving gas chemistry.


      PubDate: 2015-07-03T00:23:31Z
       
  • Carbon isotope composition in modern brachiopod calcite: a case of
           equilibrium with seawater?
    • Abstract: Publication date: Available online 26 June 2015
      Source:Chemical Geology
      Author(s): Uwe Brand , K. Azmy , E. Griesshaber , M.A. Bitner , A. Logan , M. Zuschin , E. Ruggiero , P.L. Colin
      We examined a large number of modern, shallow-water articulated brachiopods representing the orders Terebratulida, Rhynchonellida, Thecideida and one inarticulated brachiopod of the order Craniida from polar to tropical regions for their carbon isotope compositions. Based on our detailed investigation, we recommend avoiding fast growth areas such as the youngest shell increments; in addition, the primary layer and transition zone calcites of brachiopods must be avoided because they are in carbon and oxygen isotope disequilibrium with ambient seawater. After adjusting isotope compositions for the Mg effect, we observed no significant difference (p > 0.05) in δ13C values between dorsal and ventral valves of our articulated brachiopods. Using the calcite-bicarbonate enrichment factor (ε) in conjunction with δ13C values of dissolved inorganic carbon of habitat seawater, we conclude that modern shallow-water articulated and some inarticulated brachiopods incorporate oxygen (Brand et al., 2013) and carbon isotopes into shell calcite of secondary and/or tertiary layers in apparent equilibrium with ambient seawater. Within the general concept of equilibrium incorporation, with seawater, shell δ13C values are an excellent recorder of local/global seawater environments and water mass circulation. Thus, application of the Mg-effect permits brachiopods to be an extremely powerful archive, and δ13C values more precise proxy and tracer of past changes in marine productivity, evolution of seawater carbon chemistry and variation in the global carbon cycle.


      PubDate: 2015-07-03T00:23:31Z
       
  • Chemical weathering controls on variations in the molybdenum isotopic
           composition of river water: evidence from large rivers in China
    • Abstract: Publication date: Available online 26 June 2015
      Source:Chemical Geology
      Author(s): Zhibing Wang , Jinlong Ma , Jie Li , Gangjian Wei , Xuefei Chen , Wenfeng Deng , Luhua Xie , Weijian Lu , Liang Zou
      Mo isotopic composition in large rivers is very important for understanding global Mo cycle. At present, temporal variation signatures in the δ98/95Mo in large rivers have not been investigated, which hinder a comprehensive understanding on the mechanism for the variations of Mo isotopic compositions in river water. In this study, we report a one-year-long time series (March 2010 to March 2011) of the δ98/95Mo of both the water and suspended particles collected at Guiping, from the middle reaches of the Xijiang River (XJR), and of the water δ98/95Mo from the lower reaches of the Huanghe River (HHR, or Yellow River) collected at Lijin, China. The results indicate that the temporal variations in the concentration and δ98/95Mo of dissolved Mo in the XJR range from 4.32 to 10.5nmol/L (mean 7.31nmol/L) and 1.04‰ to 1.31‰ (relative to NIST 3134) (mean 1.20‰), respectively, but that the particulates have a lower δ98/95Mo (-0.18‰ to 0.58‰). Analysis of the suspended particulates and other chemical parameters of the river water suggest that the weathering of silicates and sulfides is the main contributor to the dissolved Mo content in the XJR. Subsequently, the highly efficient preferential trapping of lighter δ98/95Mo by weathering products such as clay minerals, Fe–Mn oxides, and organic materials in soils and saprolites, which are abundant in the tropical catchment, is the key to the heavy Mo isotope signatures in the XJR. Furthermore, it seems that neither Mo-scavenging by suspended particulates during riverine transportation, nor the weathering of sulfates, significantly influence the δ98/95Mo of the XJR water. Cross comparison among the δ98/95Mo in the waters of the three largest rivers in China (the Changjiang (CJR), HHR, and XJR) indicates that the HHR has the highest Mo concentrations, but the lightest δ98/95Mo, the XJR has the lowest Mo concentrations but the heaviest δ98/95Mo, and the values for the CJR are intermediate between those for the other two rivers. This supports the conclusion that the efficiency of the selective trapping of Mo by soils and saprolites is the main factor controlling δ98/95Mo in these large rivers. The tropical/subtropical XJR catchment experiences intense chemical weathering, the semi-arid temperate HHR catchment experiences very little, and the CJR catchment falls somewhere between the two. Such a relationship between water Mo concentrations and δ98/95Mo agrees with previous observations from many large rivers worldwide. It is therefore suggested that chemical weathering on continents is the key to variations in the isotopic composition of Mo in the waters of large rivers.


      PubDate: 2015-07-03T00:23:31Z
       
  • In-situ production of natural 236U in groundwaters and ores in high-grade
           uranium deposits
    • Abstract: Publication date: Available online 26 June 2015
      Source:Chemical Geology
      Author(s): Melissa J. Murphy , Michaela B. Froehlich , L. Keith Fifield , Simon P. Turner , Bruce F. Schaefer
      In nature, primordial 236U has long since decayed to concentrations below detection. However, measurement of 236U produced in-situ by neutron capture on 235U in high-grade uranium deposits is made possible by recent advances in accelerator mass spectrometry (AMS). The detection of appreciable quantities of 236U in groundwaters may reflect local uranium mineralisation, and thus prove useful in uranium exploration and potential age and ore grade estimations. Nine mineralised sediments from the South Australian Beverley North sandstone-hosted uranium deposits have 236U/238U ratios ranging from (1.57±0.43)×10−12 to (9.09±0.55)×10−12, and U concentrations that vary by almost three orders of magnitude, ranging from 78.9 to 24,200μg/g. Overall, the samples with the highest [U] have higher 236U/238U ratios, consistent with the generation of higher neutron fluxes with one notable exception with anomalously high [U] and a relatively low 236U/238U ratio. The observed variability in the 236U/238U ratio both within the deposits themselves, and between deposits may reflect heterogeneous mineralogy, elemental composition and water contents, which can affect the neutron flux generated within the samples. A single groundwater sampled within mineralisation from the Pepegoona West deposit yielded a 236U/238U ratio of (6.57±2.97)×10−12. This is the first published data detecting natural, non-anthropogenic 236U in groundwater in contact with a uranium deposit. The 236U/238U isotopic composition of the single groundwater sample is indistinguishable from that of the mineralised sediments from the same deposit. This is interpreted to reflect isotopic equilibration between the mineralisation and groundwater, rather than the in-situ production of 236U by neutron capture on dissolved 235U in the waters due to the low [U] typical of these highly reducing groundwaters. 236U appears to have limited mobility in the Pepegoona West groundwater system, as evidenced by the lack of signature in groundwaters sampled from nearby wells in low-grade and un-mineralised portions of the deposit. This suggests that the detection of 236U in the highly reducing groundwaters prevalent in this area may not be applicable as a proxy for uranium mineralisation. However, use of this technique as a potential exploration tool may have greater success in other areas with different hydrogeological conditions, specifically where the groundwaters are oxidising and uranium has a greater solubility as U(VI) complexes.


      PubDate: 2015-07-03T00:23:31Z
       
  • [Cobalt(III)-EDTA]− reduction by thermophilic methanogen
           Methanothermobacter thermautotrophicus
    • Abstract: Publication date: Available online 30 June 2015
      Source:Chemical Geology
      Author(s): Rajesh Singh , Hailiang Dong , Deng Liu , Amy R. Marts , David L. Tierney , Catherine Almquist
      Cobalt is a metal contaminant at high temperature radioactive waste disposal sites. Past studies have largely focused on mesophilic microorganisms to remediate cobalt, despite the presence of thermophilic microorganisms at such sites. In this study, Methanothermobacter thermautotrophicus, a thermophilic methanogen, was used to reduce Co(III) in the form of [Co(III)-EDTA]−. Bioreduction experiments were conducted in a growth medium with H2/CO2 as a growth substrate at initial Co(III) concentrations of 1, 2, 4, 7, and 10 mM. At low Co(III) concentrations (<4 mM), a complete reduction was observed within a week. Wet chemistry, X-ray absorption near-edge structure (XANES) and electron paramagnetic resonance (EPR) analyses were all consistent in revealing the reduction kinetics. However, at higher concentrations (7 and 10 mM) the reduction extents only reached 69.8% and 48.5%, respectively, likely due to the toxic effect of Co(III) to the methanogen cells as evidenced by a decrease in total cellular protein at these Co(III) concentrations. Methanogenesis was inhibited by Co(III) bioreduction, possibly due to impaired cell growth and electron diversion from CO2 to Co(III). Overall, our results demonstrated the ability of M. thermautotrophicus to reduce Co(III) to Co(II) and its potential application for remediating 60Co contaminant at high temperature subsurface radioactive waste disposal sites.


      PubDate: 2015-07-03T00:23:31Z
       
  • Microanalytical methods for in-situ high-resolution analysis of rock
           varnish at the micrometer to nanometer scale
    • Abstract: Publication date: Available online 30 June 2015
      Source:Chemical Geology
      Author(s): D.S. Macholdt , K.P. Jochum , C. Pöhlker , B. Stoll , U. Weis , B. Weber , M. Müller , M. Kappl , S. Buhre , A.L.D. Kilcoyne , M. Weigand , D. Scholz , A.M. Al-Amri , M.O. Andreae
      A wide range of analytical techniques were used to investigate rock varnish from different locations (Negev, Israel; Knersvlakte, South Africa; Death Valley and Mojave Desert, California): a 200nm-femtosecond laser ablation-inductively coupled plasma-mass spectrometer (LA-ICP-MS), an electron probe microanalyzer (EPMA), focused ion beam (FIB) slicing, and scanning transmission X-ray microscopy–near edge X-ray absorption fine structure spectroscopy (STXM–NEXAFS). This combination enables comprehensive high-spatial-resolution analysis of rock varnish. Femtosecond LA-ICP-MS and EPMA were used for quantitative determination of element concentrations. In-situ measurements were conducted on thick and thin sections with a resolution of 10–40μm and 2μm, respectively. The results demonstrate that some elements, such as Mn, Co, Pb, Ni, and Cu, are highly enriched in varnish relative to the upper continental crust (up to a factor of 100). The varnish composition is not influenced by the composition of the underlying rock, which is witnessed by plots of MnO2 vs. SiO2 contents. Furthermore, the Mn-free end members fall in the range of average dust compositions. The varnishes from the various locations show distinct differences in some elemental ratios, in particular Mn/Fe (0.3–25.1), Mn/Ba (4–170), Ni/Co (0.03–1.8) and Pb/Ni (0.4–23). The rare earth element (REE) patterns vary with LaN/YbN =3.5–12 and different degrees of Ce anomalies (Ce/Ce*=1.5–5.3). To study the internal structure of the varnish, 100–200nm thick FIB slices were prepared and mappings of Fe, Mn, N, CO3 2−, Ca, C, and Si at the nm scale performed. Banded internal structures of Mn, Fe and organic C were observed in the Israeli and Californian samples, however, no Fe-rich layers are present in the South African rock varnish samples. Furthermore, cavities were found that are partly filled by C, Fe, and Mn rich material. Internal structures are different for varnish from different locations, which might reflect different types of genesis. The results of the combined microanalytical techniques give important detailed insights towards unraveling the genesis of rock varnish.


      PubDate: 2015-07-03T00:23:31Z
       
  • In situ spectroscopic study of water speciation in the depolymerized
           Na2Si2O5 melt
    • Abstract: Publication date: 20 August 2015
      Source:Chemical Geology, Volume 409
      Author(s): Nadezda Chertkova , Shigeru Yamashita
      Water speciation in the sodium disilicate melts containing 1.3–8.1wt.% of total water was studied by near-infrared spectroscopy at 900°C and pressures from 0.2 to 1.7GPa using externally heated diamond anvil cell. Hydrous sodium disilicate melt served both as a sample and as a pressure medium in the experiments, which allowed to keep the total water content of the melt constant and thereby to perform an internally consistent calibration of molar absorption coefficients for the near-infrared bands of water species. No pressure dependence of water speciation was observed in the studied pressure range, indicating that formation of structurally bound OH groups at the expense of H2O molecules in the melt is accompanied by a negligible volume change. At 900°C, compositional dependence of water speciation was found to be much weaker than that expected from the previously reported data for low-temperature range near the glass transition. This can be explained by significant weakening of hydrogen bonding in the sodium silicate melts at high temperatures.


      PubDate: 2015-07-03T00:23:31Z
       
  • High-resolution LA-ICP-MS trace element mapping of igneous minerals: In
           search of magma histories
    • Abstract: Publication date: 20 August 2015
      Source:Chemical Geology, Volume 409
      Author(s): Teresa Ubide , Cora A. McKenna , David M. Chew , Balz S. Kamber
      We report experiments on optimisation of LA-ICP-MS mapping as a tool for visualising and quantifying internal structure of trace element concentration in igneous minerals. The experimental design was refined with maps on clinopyroxene and amphibole macrocrysts (mainly antecrysts) from a porphyritic lamprophyre in NE Spain, as well as on a high precision metal wire grid. In terms of spatial resolution, we demonstrate with scanning electron microscope and white light interferometry that a full ablation removes between 0.4 and 0.7μm of material, depending on ablation parameters. Maps were produced with square laser beam spots of 12 and 24μm. It was found that complexities can be resolved in the sample even though they are smaller than the beam diameter (e.g., 7–10μm discontinuities using 12μm laser beam). Resolution in x and y was found to be identical, probably reflecting the fast washout of the two-volume ablation cell and the short total dwell time of the analyte menu selected. Due to the excellent stage reproducibility and the limited ablation depth, it is feasible to re-ablate the identical map area many times employing different instrument parameters or analyte menus. On the magmatic crystals, LA-ICP-MS maps define very sharp compositional zoning in trace elements, highlighting complex crystallisation histories where ‘normal’ magmatic fractionation is not the only process. Events of mafic recharge are easily recognised as zones enriched in compatible metals such as Cr, Ni or Sc. Further, trace element maps reveal complexities in mineral zoning previously undetectable with petrography or major element data. These include resorbed primitive cores and oscillatory zoning within apparently homogeneous mineral zones. Therefore, LA-ICP-MS mapping opens a new window of opportunity for analysis of magmatic histories. The wide combination of instrumental parameters, such as laser beam size, scan speed and repetition rate, make it possible to carry out experiments at different levels of detail. We recommend a two-step approach to mapping. The initial step involves rapid maps to gain an overview of potential complexities in the sample; this enhances representativeness of the analysed materials, as a large number of crystals and trace elements can be tested in little time. Subsequently, detailed maps can be carried out on areas of interest. An additional functionality is to create 1D-profiles from 2D-maps. The potential of the technique to unveil compositional complexities efficiently and at greater detail than traditional microanalysis will help to improve our understanding of processes in the magmatic environment and beyond.
      Graphical abstract image

      PubDate: 2015-07-03T00:23:31Z
       
  • Cu(II) removal by E. coli–iron oxide composites during the addition
           and oxidation of Fe(II)
    • Abstract: Publication date: 20 August 2015
      Source:Chemical Geology, Volume 409
      Author(s): Rachel E. Franzblau , Christopher J. Daughney , Magali Moreau , Christopher G. Weisener
      Copper removal by Escherichia coli–iron oxide composites was investigated over time during the addition of Fe(II)aq. Cu removal by the composites occurred mainly via adsorption onto cell surface functional groups under the conditions used in these experiments. Cu removal by the composites was less than under similar conditions in the single-sorbent end-member systems. This non-additive sorption behavior was attributed to physical blockage of bacterial surface sites by the iron oxide particles. Cu removal by the composites was well described by a surface complexation model that accounted for the following processes: Cu adsorption onto the cells; formation of aqueous Cu complexes with ligands released over time by the bacteria; ternary cell and mineral surface complexes involving Cu and these bacterial exudate ligands; and Cu sorption onto and co-precipitation within the iron oxide. Overall this study has shown that Cu adsorption in bacteria–mineral systems cannot be evaluated without considering redox processes and sorbent–sorbent interactions.


      PubDate: 2015-07-03T00:23:31Z
       
  • The impact of fluoride on Al abundance and speciation in boreal streams
    • Abstract: Publication date: 20 August 2015
      Source:Chemical Geology, Volume 409
      Author(s): Tobias Berger , Frédéric A. Mathurin , Jon Petter Gustafsson , Pasi Peltola , Mats E. Åström
      The impact of fluoride on the abundance and speciation of aluminium (Al) was investigated in three boreal streams characterised by overall high concentrations of fluoride and dissolved organic matter. Stream-water sampling was carried out several times a year for at least 4years, and a chemical equilibrium model (Visual MINTEQ) was applied in order to model the proportion of colloidal and organically/inorganically complexed Al in the waters. The Al concentrations in filtered (0.45μm) water samples were inversely correlated with pH, and reached values up to approximately 1mg/L during low pH conditions (pH<6.0). In a stream with high fluoride concentrations, as compared to a similar stream with only moderately elevated fluoride concentrations, the Al concentrations were consistently elevated. For the stream with high concentrations of fluoride and Al, the model predicted both high concentrations and proportions of Al-fluoride complexation. This prediction indicates that high fluoride levels contribute to raise both the Al abundance and the ratio of inorganic to organic Al complexation in stream water. In contrast, for another stream with high fluoride concentrations and consistently high (near neutral) pH, there was no evidence of fluoride affecting Al concentration or complexation. These results show that it is important to focus future studies on the role of high levels of dissolved fluoride on both the speciation and the toxicity of Al in stream water.


      PubDate: 2015-07-03T00:23:31Z
       
  • A method for measurement of 36Ar without H35Cl interference
    • Abstract: Publication date: 20 August 2015
      Source:Chemical Geology, Volume 409
      Author(s): J.M. Saxton
      Most noble gas analyses are made in static mode when instrument volume is minimized to maintain adequate sensitivity. This makes the building of large instruments to obtain high resolving power impracticable. A method is presented which makes improved use of the available resolving power to remove isobaric interferences, which may be used on multicollector instruments. By arranging that the target mass position on a minor isotope (e.g. 36Ar), from which the interference must be removed, coincides with the approximately 50% point on the side of a major isotope (e.g. 40Ar), it is possible both to set the mass accurately and to verify the mass position and stability during measurements. The peak top of the major isotope is measured in a separate mass step. Calibration measurements are necessary, using different relative amounts of target/interference, to assess residual tailing to the measurement position and also the relative efficiency at the extreme edge of the target peak. The method is demonstrated by using it to obtain 36Ar measurements free of H35Cl. With samples containing 4×10−15 to 3×10−14 mol of 40Ar, 36Ar/40Ar was measured, without HCl interference, to a 1σ precision of 0.5%, only slightly worse than counting statistics. This is potentially useful for 40Ar/39Ar dating, where 36Ar is used to correct for trapped air, and may be particularly significant for smaller or younger samples.


      PubDate: 2015-07-03T00:23:31Z
       
  • Monazite trace-element and isotopic signatures of (ultra)high-pressure
           metamorphism: Examples from the Western Gneiss Region, Norway
    • Abstract: Publication date: 20 August 2015
      Source:Chemical Geology, Volume 409
      Author(s): Robert M. Holder , Bradley R. Hacker , Andrew R.C. Kylander-Clark , John M. Cottle
      Monazite U–Pb and trace-element data were gathered from six high- to ultrahigh-pressure (UHP) samples from the Western Gneiss Region, Norway, using LASS (laser-ablation split-stream ICP-MS) to investigate variations in monazite composition during high-pressure metamorphism. The UHP monazites were found to contain up to 7600ppm Sr, 110ppm non-radiogenic Pb, relatively weak negative Eu anomalies, and Y concentrations as low as 500ppm. Amphibolite-facies monazite that rims the UHP monazite in one sample contains Y concentrations up to 1.6wt.%, Sr as low as 13ppm, and no detectable non-radiogenic Pb. The UHP monazite composition (high Sr–Pb) is interpreted to result from growth in the absence of feldspar, possibly aided by increased compatibility of Sr–Pb–Eu2+ in the monazite crystal structure at high pressure. Sr in monazite as a proxy for feldspar stability may be a useful tool not only in studying high-pressure metamorphism, but also in determining timescales of melting and crystallization, when the amount of feldspar changes over time.


      PubDate: 2015-07-03T00:23:31Z
       
  • Oxygen isotope signatures from land snail (Helix melanostoma) shells and
           body fluid: Proxies for reconstructing Mediterranean and North African
           rainfall
    • Abstract: Publication date: 20 August 2015
      Source:Chemical Geology, Volume 409
      Author(s): Amy L. Prendergast , Rhiannon E. Stevens , Graeme Barker , Tamsin C. O'Connell
      In this paper, we show that oxygen isotope ratios from the shell carbonate of the land snail Helix melanostoma reflect local rainfall parameters. Stable oxygen isotopes were measured in the body fluid (δ18Obody) and shells (δ18Oshell) of live-collected H. melanostoma along a north–south transect across the Gebel Akhdar in northeast Libya. δ18Obody ranged between −8.1‰ and 13.0‰ whilst δ18Oshell ranged between −1.0‰ and +2.8‰. To investigate the relationship between snail oxygen isotope composition and climate, these measurements were correlated with ambient air temperature, relative humidity, rainfall amount, and the δ18O composition of rainwater (δ18Orain). Strong and significant correlations were found between δ18Oshell and δ18Orain, and between δ18Oshell and rainfall amount (R2 =0.79 and 0.85 respectively) suggesting that H. melanostoma shells can be used as a proxy for past rainfall conditions. When combined in multiple linear regressions, δ18Orain and rainfall amount explained 90% of the variance in δ18Oshell (R2 =0.90). Simple and multiple linear regressions were used to develop predictive models for these climate parameters. As H. melanostoma shells are commonly found in Mediterranean and North African archaeological and geological deposits, the predictive equations developed in this study hold great potential for reconstructing Pleistocene to Holocene rainfall and aridity patterns in the Gebel Akhdar and for assessing the relationship between environmental changes and human behavioural changes.


      PubDate: 2015-07-03T00:23:31Z
       
  • Differentiating marine vs hydrothermal processes in Devonian
           carbonatemounds using rare earth elements (Kess Kess mounds, Anti-Atlas,
           Morocco)
    • Abstract: Publication date: 20 August 2015
      Source:Chemical Geology, Volume 409
      Author(s): Fulvio Franchi , Axel Hofmann , Barbara Cavalazzi , Allan Wilson , Roberto Barbieri
      Petrological and geochemical analyses were carried out on Early Devonian Kess Kess mound limestones of the Seheb el Rhassel Group exposed in the Hamar Laghdad Ridge (Tafilalt Platform, Eastern Anti-Atlas, Morocco) in order to evaluate marine vs hydrothermal processes for the origin of the mounds. Hydrothermal fluid circulation affected limestone deposition resulting in the formation of a plumbing system preserved in the mound facies and inter-mound facies as cavities, veins, and dykes. Shale-normalized rare earth element (REE) patterns for limestones display pronounced light REE depletion compared to middle REE (average PrSN /DySN =0.46) and heavy REE (average PrSN /YbSN =0.55), a super-chondritic Y/Ho ratio (up to 87) and positive La anomaly consistent with precipitation from normal marine seawater. Within our data set the Ce anomaly, as well as U and other trace element concentrations vary with facies providing evidence for variable redox conditions. The presence of positive Ce anomaly in carbonates of the plumbing system is consistent with precipitation under anoxic condition. Samples from mound facies and quartz veinlets in the upper part of the group show REE patterns consistent with precipitation from hydrothermal fluids. Early Devonian hypothetical seawater REE patterns were calculated from samples showing normal marine REE patterns (i.e. inter-mound facies) and from samples with weaker marine signature (i.e. fossiliferous mound limestones and plumbing system deposits). Hypothetical patterns are slightly enriched in ΣREE compared to modern open ocean seawater. This study reveals that REE analysis is a powerful tool for understanding polygenetic carbonate systems. It sheds light into the genesis of the Kess Kess mounds by reconstructing fluid pathways and palaeo-redox conditions.


      PubDate: 2015-07-03T00:23:31Z
       
  • Molecular proxies as indicators of freshwater incursion-driven salinity
           stratification
    • Abstract: Publication date: 20 August 2015
      Source:Chemical Geology, Volume 409
      Author(s): Svenja Tulipani , Kliti Grice , Paul F. Greenwood , Lorenz Schwark , Michael E. Böttcher , Roger E. Summons , Clinton B. Foster
      Salinity-related stratification in marine environments is one of the main drivers for the development of persistent anoxia and euxinia. It therefore plays a vital role in the sedimentary preservation of organic matter and has also been associated with Oceanic Anoxic Events during several mass extinctions in Earth's history. Here we present a novel molecular proxy using relative abundances of methyltrimethyltridecylchromans (MTTCs) as indicators of riverine freshwater incursions into Middle to Late Devonian paleoreefs exposed to conditions of prevailing anoxia, photic zone euxinia and water column stratification. This paper aims at re-instigating a discussion about the origin of these compounds to potentially broaden their utilization as paleoproxies, adding freshwater stratification to the general salinity reconstructions. The co-variation of MTTC abundance profiles and δ13C values with other biomarker parameters in the sediments studied here indicate a link of MTTCs to terrigenous input, pointing towards a previously suggested early diagenetic formation of these compounds from phytol with higher plant-derived alkylphenols.
      Graphical abstract image

      PubDate: 2015-07-03T00:23:31Z
       
  • Characterising the stable (δ88/86Sr) and radiogenic (87Sr/86Sr)
           isotopic composition of strontium in rainwater
    • Abstract: Publication date: 20 August 2015
      Source:Chemical Geology, Volume 409
      Author(s): Christopher R. Pearce , Ian J. Parkinson , Jérôme Gaillardet , Benjamin Chetelat , Kevin W. Burton
      Rainwater plays a major role in determining the transfer of elements and pollutants to soils and hydrological catchments, with compositional variations reflecting the extent of mixing between marine (sea-salt), terrestrial (dust and vegetation) and anthropogenic aerosols. While the radiogenic strontium isotope system (87Sr/86Sr) is often used to help constrain the different sources of atmospheric Sr, the degree of variability in the stable strontium isotope system (δ88/86Sr) remains unresolved. In order to determine how atmospheric Sr affects the δ88/86Sr composition of water entering the hydrological cycle this study analysed a suite of rainwater samples collected over the course of a year in central Paris. Rainwater δ88/86Sr compositions were found to range from 0.13‰ to 0.32‰, with 87Sr/86Sr ratios varying between 0.70796 and 0.71093. No seasonality was observed in δ88/86Sr, whereas 87Sr/86Sr ratios were observed to be generally more radiogenic in the summer and less radiogenic in the winter. Strontium enrichment factors significantly above seawater implied a predominantly terrestrial (non-sea-salt) origin of Sr in rainwater, with the δ88/86Sr and 87Sr/86Sr composition of this terrestrial fraction consistent with a mixture of Sr derived from carbonate dust and agricultural fertiliser. Evidence for a ~0.1‰ decrease in the δ88/86Sr composition of the River Seine as it passes through Paris suggests that anthropogenic pollutants may contribute to significant regional variations in the composition of Sr in the hydrological cycle. Together these results demonstrate that there is significant potential for both temporal and spatial variations in the δ88/86Sr and 87Sr/86Sr composition of rainwater that need to be taken into consideration when studying δ88/86Sr variations in localised catchments.


      PubDate: 2015-07-03T00:23:31Z
       
  • Vivianite formation and its role in phosphorus retention in Lake Ørn,
           Denmark
    • Abstract: Publication date: 20 August 2015
      Source:Chemical Geology, Volume 409
      Author(s): David W. O'Connell , Marlene Mark Jensen , Rasmus Jakobsen , Bo Thamdrup , Thorbjørn Joest Andersen , Andras Kovacs , Hans Christian Bruun Hansen
      Vivianite [(Fe3(PO4)2·8H2O)] may precipitate in anoxic lake sediments affecting the porewater orthophosphate concentration, and thereby the trophic status of lakes. We have investigated changes in lake diagenesis of Fe and P (1969–2009), with particular attention focused on vivianite formation with sediment depth (0–20cm) in an iron–silica–carbon rich lake sediment (Ørnsø, Denmark). Porewaters were supersaturated for vivianite by two to five orders of magnitude (upper 10cm) with porewater phosphate concentrations ranging between 0.69 and 10μmoll−1, in winter, and summer concentrations ranging between 9.8 and 40μmoll−1. Significant formation of vivianite was confirmed by X-ray diffraction while scanning electron microscopy and electron dispersive X-ray spectroscopy indicated an increase in vivianite crystal size with depth (~20 to ~70μm across). Variations in elemental composition of vivianite crystals in relation to at.% P and Fe were especially seen going from 9.5cm to 24.5cm. The total sediment Fe pool was very large ~3000μmolg−1 and total P increased from 200μmolg−1 to 400μmolg−1 descending down the sediment profile. Differential extraction experiments of P release at pH3 estimated that vivianite amounts to between 3 and 5% of the total Fe pool. The total P burial fluxes estimate that ~38μmolcm−2 yr−1 or ~26% of sedimentary P in the lower sediments is sequestered as vivianite. There are seasonal variations in the porewater composition with lower Fe, orthophosphate and higher sulfate concentrations during winter (5°C), than during summer (15°C). This suggests that temperature modulates the rate of organic matter degradation which in turns affects the rate of Fe(III) phase reduction, release of phosphate, and thereby the porewater Fe2+ and orthophosphate concentrations and hence vivianite formation. This work highlights the role vivianite can play for P retention in a Si–Fe–C rich lake sediment.


      PubDate: 2015-07-03T00:23:31Z
       
  • Monazite response to ultrahigh-pressure subduction from U–Pb dating
           by laser ablation split stream
    • Abstract: Publication date: 20 August 2015
      Source:Chemical Geology, Volume 409
      Author(s): Bradley R. Hacker , Andrew R.C. Kylander-Clark , Robert Holder , Torgeir B. Andersen , Emily M. Peterman , Emily O. Walsh , Jonathan K. Munnikhuis
      To assess the response of monazite during subduction of continental crust to mantle depths, U–Pb isotopic ratios and elemental abundances were measured simultaneously by laser-ablation split-stream inductively-coupled plasma mass spectrometry (LASS) in rocks from the ultrahigh-pressure Western Gneiss Region of the Scandinavian Caledonides. Nearly seventy different samples of quartzofeldspathic basement and overlying metasedimentary rocks were studied. Pre-subduction monazite (chiefly 1.6Ga and 1.0Ga) is preserved locally in the structurally lowest, basement rocks because earlier, Precambrian tectonism produced coarse-grained, high-grade rocks that were resistant to further recrystallization in spite of syn-subduction temperatures and pressures of 650–800°C and 2–3.5GPa. A few of the monazite in the metasedimentary rocks atop the basement preserve syn-subduction U–Pb dates, but the majority continued to recrystallize during post-subduction exhumation and record a general westward decrease in age related to westward-progressing exhumation. The absence of Precambrian monazite in the metasedimentary rock atop the basement suggests that sedimentation postdated the 1.0–0.9Ga high-grade metamorphism and was late Proterozoic to early Paleozoic.


      PubDate: 2015-07-03T00:23:31Z
       
  • Making geological sense of ‘Big Data’ in sedimentary
           provenance analysis
    • Abstract: Publication date: 20 August 2015
      Source:Chemical Geology, Volume 409
      Author(s): Pieter Vermeesch , Eduardo Garzanti
      Sedimentary provenance studies increasingly apply multiple chemical, mineralogical and isotopic proxies to many samples. The resulting datasets are often so large (containing thousands of numerical values) and complex (comprising multiple dimensions) that it is warranted to use the Internet-era term ‘Big Data’ to describe them. This paper introduces Multidimensional Scaling (MDS), Generalised Procrustes Analysis (GPA) and Individual Differences Scaling (INDSCAL, a type of ‘3-way MDS’ algorithm) as simple yet powerful tools to extract geological insights from ‘Big Data’ in a provenance context. Using a dataset from the Namib Sand Sea as a test case, we show how MDS can be used to visualise the similarities and differences between 16 fluvial and aeolian sand samples for five different provenance proxies, resulting in five different ‘configurations’. These configurations can be fed into a GPA algorithm, which translates, rotates, scales and reflects them to extract a ‘consensus view’ for all the data considered together. Alternatively, the five proxies can be jointly analysed by INDSCAL, which fits the data with not one but two sets of coordinates: the ‘group configuration’, which strongly resembles the graphical output produced by GPA, and the ‘source weights’, which can be used to attach geological meaning to the group configuration. For the Namib study, the three methods paint a detailed and self-consistent picture of a sediment routing system in which sand composition is determined by the combination of provenance and hydraulic sorting effects.


      PubDate: 2015-07-03T00:23:31Z
       
  • Am(III) coprecipitation with and adsorption on the smectite hectorite
    • Abstract: Publication date: 20 August 2015
      Source:Chemical Geology, Volume 409
      Author(s): N. Finck , K. Dardenne , H. Geckeis
      Clay minerals, such as the smectite hectorite, have been detected in the alteration layer of nuclear waste glass corroded in laboratory experiments. The neoformation of such secondary phases upon dissolution and re-precipitation represents a significant retention potential for radionuclide released during the waste matrix alteration. Hectorite was synthesized from a brucite precursor phase in the presence of trivalent americium, and separately, the Am(III) aqua ions were adsorbed on the smectite in order to decipher the actual retention mechanism(s). X-ray diffractograms indicated that both brucite and hectorite formed and that the actinide had no significant influence on the syntheses. No separate phase was detected by XRD and no Am-containing precipitate was detected by SEM-EDX. The Am local chemical environment was probed by EXAFS spectroscopy. In the precursor, the polarized EXAFS data are consistent with Am(III) very likely substituting for Mg at octahedral site. The weak angular dependence on the data and the split nearest octahedral neighboring shell suggest that the substitution significantly distorts the local brucite structure. In hectorite, americium is 6-fold coordinated by oxygen atoms and next nearest Mg and Si shells are detected at distances strongly pointing to an octahedral clay-like environment. EXAFS data of the actinide adsorbed onto hectorite are consistent with the formation of monomeric inner-sphere surface complexes at the platelet edges. The formation of Am(III) surface complexes during the coprecipitation experiment is marginal. The results show that clay minerals synthesized under controlled conditions can accommodate trivalent actinides, and thus lanthanides, at octahedral position within the structure. The identified retention mechanism can in principle be applied to all sheet silicates, including those occurring in nature and for which no such structural investigation has been reported.


      PubDate: 2015-07-03T00:23:31Z
       
  • Noble gases in sulfide deposits of modern deep-sea hydrothermal systems:
           Implications for heat fluxes and hydrothermal fluid processes
    • Abstract: Publication date: 20 August 2015
      Source:Chemical Geology, Volume 409
      Author(s): Zhigang Zeng , Samuel Niedermann , Shuai Chen , Xiaoyuan Wang , Zhaoxue Li
      Studies of noble gases in seafloor sulfide deposits are an important tool for understanding the origin of ore-forming fluids, both enabling the determination of noble gas sources and revealing the degree of fluid–rock interaction and mantle degassing. The noble gas concentrations and isotopic compositions of 27 sulfide, 3 sulfate, and 2 opal mineral aggregate samples have been studied in different hydrothermal fields from the East Pacific Rise, Mid-Atlantic Ridge, Central Indian Ridge, Southwest Indian Ridge, and North Fiji Back-Arc Basin. The helium concentrations and isotopic ratios in the sulfide aggregate samples are variable (4He 0.12 to 22×10−8 cm3 STP/g; 3He/4He ~0.6 to 10.4 Ra), and most of the sulfide helium concentrations are higher than those in the opal mineral samples (4He 0.017 and 0.028×10−8 cm3 STP/g), suggesting that the low-temperature fluids have lost their mantle helium during cooling. The helium of high (>7 Ra), medium (1–7 Ra), and low (≈1 Ra) 3He/4He ratio samples is derived mainly from the MORB or OIB mantle by magma degassing, from mixing between hydrothermal fluid and seawater during ore-forming processes, and from ambient seawater, respectively. The high 3He/4He ratios (>7 Ra) of sulfides imply that high-temperature sulfides retain the helium isotopic compositions of the primary hydrothermal fluid, whereas low-temperature sulfides, sulfates, and opal minerals do not. The neon, argon, krypton, and xenon concentrations in the sulfide aggregate samples are also variable; in most of the sulfide aggregates, they are significantly lower than in the sulfate and opal mineral samples. It is known that barite and opal minerals are characteristic of low-temperature (<200°C) paragenetic associations, indicating that heavier noble gases (Ne, Ar, Kr and Xe) are enriched under low-temperature conditions. Most of the sulfide, sulfate and opal mineral aggregate samples possess heavy noble gas elemental abundances similar to those of air-saturated seawater and Ne, Ar, and Xe isotopic compositions that span narrow ranges around atmospheric values, an observation which is most easily explained by the dominance of a seawater-derived component. In addition, based on the calculated helium/heat ratios, global helium and heat fluxes to high-temperature hydrothermal vents are approximately 0.05–6×104 kg per year and 0.1–12×1012 W, meaning that roughly 0.3% of ocean heat is supplied by seafloor high-temperature hydrothermal activity.


      PubDate: 2015-07-03T00:23:31Z
       
  • Impact of heterotrophic bacterium Pseudomonas aureofaciens on the release
           of major and trace elements from podzol soil into aqueous solution
    • Abstract: Publication date: 2 September 2015
      Source:Chemical Geology, Volume 410
      Author(s): Olga Yu. Drozdova , Liudmila S. Shirokova , Audrey Сarrein , Sergey A. Lapitskiy , Oleg S. Pokrovsky
      The release of major and trace elements (TEs) from mineral and organic-rich podzol soil horizons has been studied in batch reactors with live and dead soil heterotrophic Pseudomonas aureofaciens bacteria and in bacteria-free systems. Dissolved organic carbon (DOC) concentrations decreased and dissolved inorganic carbon concentrations increased over the course of experiments with live bacteria due to on-going DOC mineralization processes. Several families of major and trace elements could be distinguished, depending on their release patterns in bacteria-free systems and live bacteria-bearing systems. Live bacteria enhanced the release of Mg, Rb, Cd, Pb, Al, Fe and V from the mineral soil horizon and the release of Rb, Ni, Pb, As, Fe, V and La from the organic soil horizon relative to bacteria-free soil or dead bacteria experiments. Unexpectedly, K, Ca, Sr, Cu, Ti, Mn, Zn and As release from the mineral horizon and Mg, K, Ca, Sr, Ba, Cr, Ti, Mn and Zn release from the organic horizon decreased in the presence of live P. aureofaciens compared to bacteria-free and dead bacteria systems. Finally, live bacteria exhibited no effect on the release of Si, Al, Cu and Mo from the humic horizon and Ni, Mo, Cr, Ba, Si and La from the mineral horizon relative to the bacteria-free system. These results can be interpreted via a combination of several simultaneous processes, which occur in the soil–bacteria suspension and lead to changes in TE speciation and affinities to mineral surfaces and bacteria and include the following: 1) a slight decrease in the pH due to exometabolite production; 2) degradation of DOC and TE organic complexes by heterotrophic bacteria; 3) element adsorption at cell surfaces and biological uptake; and 4) element release from the soil mineral and organic particles. Compared to abiotic systems, the observed decreases in the concentrations of major elements and many heavy metals that leach from the soil in the presence of bacteria have important consequences regarding our understanding of the role of bacteria in element mobilizations from soil to rivers. It follows that chemical weathering in both organic and mineral horizons of podzol soil may not be strongly affected by heterotrophic bacterial activity; rather, the solution pH and DOC levels may control the intensity of element mobilization. The aqueous concentrations of many TEs (Fe, Al, La, Cr, Ni, Cd, Pb, Cu, and Rb) at the end of live-bacteria experiments were comparable with reported compositions of interstitial podzol soil solutions.
      Graphical abstract image

      PubDate: 2015-07-03T00:23:31Z
       
  • Approaching the geochemical complexity of As(V)-contaminated systems
           through thermodynamic modeling
    • Abstract: Publication date: 2 September 2015
      Source:Chemical Geology, Volume 410
      Author(s): Katherine Vaca-Escobar , Mario Villalobos , Teresa Pi-Puig , Rodolfo Zanella
      Arsenate mobility in oxic environments is largely controlled by its adsorption to iron (hydr)oxides, but precipitation as heavy metal arsenates represents a potentially significant competing mechanism. Predicting As geochemical behavior in heterogeneous contaminated systems where various simultaneous equilibria are taking place may be achieved in a thermodynamically sound manner by coupling adsorption and solid-aqueous equilibria, provided that accurate equilibrium constants are employed; especially challenging is the surface complexation model segment. The influence of adsorption and precipitation processes on As(V) mobility was modeled in the presence of Pb(II) and goethite by varying the As/Fe and As/Pb ratios, the goethite particle size, pH, and the inclusion of chloride and sulfate ions. A bottom-up approach is adopted here to gradually approximate the geochemical complexity of real contaminated scenarios. A unified surface complexation model for goethites of different particle sizes was used, which accounts for differences in their reactivity, and was coupled with a thermodynamic speciation model for aqueous and solid-phase equilibria. The geochemical conditions found that favor As(V) precipitation as Pb(II) arsenates were high As/Fe, low As/Pb, goethites of small particle size, and especially the presence of chloride, and low sulfate concentrations (at low pH). The opposite conditions favor As(V) adsorption. Surprisingly, precipitation processes are more prevalent than expected and are favored from relatively low As/Fe in the presence of chloride, or with small particle-sized goethites.
      Graphical abstract image

      PubDate: 2015-07-03T00:23:31Z
       
  • Sulfide metasomatism and the mobility of gold in the lithospheric mantle
    • Abstract: Publication date: 2 September 2015
      Source:Chemical Geology, Volume 410
      Author(s): J. Edward Saunders , N.J. Pearson , Suzanne Y. O'Reilly , W.L. Griffin
      Gold is a strongly chalcophile element and its concentration and behaviour in the mantle are intimately connected to the nature, abundance and mobility of sulfide phases. In this study Au has been analysed together with a suite of chalcophile and siderophile elements in sulfides hosted in 31 mantle-derived xenoliths from Spitsbergen in the Svalbard Archipelago. These data have been combined with analysis of silicate phases to assess the mobility of Au, and sulfides in general, during mantle processes. The 31 samples comprise lherzolites, harzburgites, dunites and a wehrlite. Several lherzolites contain amphibole and apatite, and two are cross-cut by amphibole veins. The REE patterns in clinopyroxene reveal three geochemically distinct groups of peridotites. Group I samples have flat or LREE depleted patterns ((La/Yb)n <1); Group II samples show sinuous REE patterns; and Group III samples show LREE enrichment with moderately steep slopes ((La/Yb)n >1). The sulfides hosted in the Group I samples have very heterogeneous Au compositions (1.9±1.4ppm), whereas those hosted in Groups II and III samples have lower and more homogenous Au concentrations (Group II Au: 0.66±0.60ppm; Group III Au: 0.53±0.57ppm). A chromatographic metasomatic model is proposed to explain the variations between the three groups of samples. The LREE enrichment, and common inclusion of amphibole in the Group III samples is consistent with modification by a carbonate-rich melt. It is interpreted that these samples were located close to the metasomatic conduit. The sulfide homogeneity in these samples results from extensive equilibration with the metasomatic agent; Au has partitioned into the fluid phase, reducing the Au content of these sulfides. The Group I samples show little to no metasomatic alteration in the silicate phases, indicating they were located far from the conduit. The agents that have affected these samples have been modified by extensive fractionation and wall–rock interaction during percolation through the mantle, becoming enriched in Au. Group II samples are inferred to have been located at an intermediate distance from the conduit with silicate and sulfide characteristics that fall in between those from the other two groups.


      PubDate: 2015-07-03T00:23:31Z
       
  • Carbonate composition and its impact on fluvial geochemistry in the NE
           Tibetan Plateau region
    • Abstract: Publication date: 2 September 2015
      Source:Chemical Geology, Volume 410
      Author(s): Yibo Yang , Xiaomin Fang , Albert Galy , Gengxin Zhang , Shaochen Liu , Jinbo Zan , Fuli Wu , Qingquan Meng , Chengcheng Ye , Rongsheng Yang , Xiaoming Liu
      Using co-variations of Sr/Ca and Mg/Ca, we examined the carbonate compositions of various bedrocks (silicate and carbonate rocks) and sediments (eolian and fluvial sediments, sand, and topsoil) found in the NE Tibetan Plateau (TP) region. A combined carbonate composition dataset based on our results and other reported data shows that bedrock carbonate composition on the NE TP displays a much broader range of Sr/Ca and Mg/Ca ratios than restricted source carbonate endmembers reported upon in previous studies. This has clear implications for modern weathering studies in addition to paleo-reconstructions in this tectonically active and climatically variable area during the Late Cenozoic. Bedrock carbonate compositions are characterized by disseminated carbonates with higher Sr/Ca and Mg/Ca ratios, and sedimentary carbonates (mostly marine) with lower Sr/Ca, but variable Mg/Ca, ratios. The mostly authigenic carbonates found in sediments show similar trends, with a gradient ~0.97–1.00 in a plot of log (Sr/Ca) versus log (Mg/Ca), suggesting that ‘calcite precipitation’ processes – i.e. the sources of the dissolved cations in the water – control their chemistry. Based on observations and modeling, we conclude that the mixing of authigenic and bedrock carbonate endmembers, plus the incongruent dissolution of bedrock carbonates, accounts for the bulk carbonate composition of sediments (e.g. loess, sand and topsoil). A comparison of bedrock and sedimentary carbonate composition with reported fluvial water data in the NE TP suggests that weathering of carbonates in terrigenous sediments, rather than in bedrock, is mostly responsible for the changes in fluvial Sr, Mg and Ca compositions. Our study suggests that interactions between carbonates and water occur widely during the exposure, transport and deposition of sediments, significantly modifying regional carbonate compositions and fluvial geochemistry.


      PubDate: 2015-07-03T00:23:31Z
       
  • Abiotic and candidate biotic micro-alteration textures in subseafloor
           basaltic glass: A high-resolution in-situ textural and geochemical
           investigation
    • Abstract: Publication date: 2 September 2015
      Source:Chemical Geology, Volume 410
      Author(s): Leif-Erik Rydland Pedersen , Nicola McLoughlin , Per Erik Vullum , Ingunn H. Thorseth
      The oceanic crust provides one of the largest habitats for subsurface microbial life on earth, where lithoautotrophs utilize redox gradients between reduced elements in volcanic rocks and oxygenated seawater to form the basis of a deep microbial biosphere. Progressive alteration of the oceanic crust is argued to be “in part” microbially mediated, but identifying robust textural and geochemical biosignatures with good fossilization potential is challenging. This study investigates pillow basalts from the Antarctic Australian Discordance (AAD) at the South East Indian Ridge (SEIR) containing candidate textural biosignatures in alteration products of the glassy margins (Thorseth et al., 2003). Samples include 2.5Ma dredged seafloor basalts, and 18–28Ma drill core samples from the Ocean Drilling Program (ODP) Leg 187. The focused ion beam (FIB) technique was used to prepare electron transparent foils across spherical microtextures in zeolite filled fractures and altered glass (palagonite), and across microtunnels at the interface of fresh and altered glass. Transmission electron microscopy (TEM) was used to map chemical and ultrastructural variations and to evaluate both biotic and abiotic origins of the candidate textural biosignatures in the FIB prepared foils. Three foils were cut from zeolite hosted, hollow microspheres, which comprise purely Fe-oxyhydroxides, or mixed Mn–Mg, and Fe–Mn oxyhydroxides. The microspheres are 1 to 4μm across, with a radiating ultrastructure, and have a denser inner surface and a more porous outer surface, suggesting outwards growth from a spherical initial surface. Amorphous organic carbon is associated with some of the microtextures both on the inner and outer walls. These microtextures are interpreted as mineral encrusted microbial cells. A FIB-foil was also cut from palagonite-hosted microspheres, which are more irregular in shape and partially infilled by palagonite. Amorphous organic carbon is abundant in the vicinity of the microtextures but is spatially unrelated, and may be derived from several sources. The results indicate that maturation of the palagonite, involving dehydration and recrystallization, overprints and destroys potential biosignatures in this alteration phase. In contrast, the zeolite-hosted microtextures appear to have a higher preservation potential. Tubular microtextures in the glass at the alteration front comparable to argued “bioalteration” textures are also abundant in the AAD basalts. However, their angular cross-sectional shape and lack of “bio-elements” in the palagonite infill, mean that an abiotic origin cannot be excluded. In summary, FIB-TEM provides multiple high-resolution lines of information to characterize alteration textures in ocean floor basalts. Comparing the evidence obtained from glass hosted microtunnels, zeolite and palagonite hosted microspheres we conclude that the zeolite hosted microtextures are the strongest candidate biosignature. The combination of the size, rim ultrastructure, and elemental composition is consistent with an origin as cell encrustations, resulting from the biologically induced mineralization of microbial cells that inhabited fractures in pillow lavas both at the seafloor and the subseafloor stage.


      PubDate: 2015-07-03T00:23:31Z
       
  • Preservation of successive diagenetic stages in Middle Triassic bonebeds:
           Evidence from in situ trace element and strontium isotope analysis of
           vertebrate fossils
    • Abstract: Publication date: 2 September 2015
      Source:Chemical Geology, Volume 410
      Author(s): Jeremy Michael McCormack , André Bahr , Axel Gerdes , Thomas Tütken , Peter Prinz-Grimm
      Bonebeds comprise reworked and time averaged constituents of different phosphatic vertebrate remains and, hence, provide an ideal substrate for the study of long and short term diagenetic processes. To test whether trace elements (U, Sr, REE) and 87Sr/86Sr ratios can be used for distinguishing between successive diagenetic signals, we performed geochemical analyses on vertebrate remains (bones, teeth, scales and coprolites) from two Triassic bonebeds located near Palzem (Germany). Trace element analysis was done on 170 vertebrate remains by LA-ICP-MS while 87Sr/86Sr ratios were determined for 39 bioclasts using LA-MC-ICP-MS. Although a low inter- and intra-bioclast REE variability indicates long-term open system behaviour, the observed REE patterns suggest the preservation of different diagenetic stages. A distinct difference in the geochemical composition exists between dark luminescent SEM-CL domains and greyish ones. Dark domains represent dense, diagenetically more resistant material of low porosities such as enamel and ganoine. These domains display distinctly lower ƩREE contents and a higher inter-clast variability than those of various grey tones. Together with lower, hence more seawater-like 87Sr/86Sr ratios this argues for a preservation of an earlier diagenetic signal. Notably, most dark domains are characterised by negative Eu anomalies while all grey ones display positive Eu anomalies. With the exception of Eu both domains display almost identical REE patterns with different concentrations indicating that late diagenetic REE uptake is not necessarily accompanied by fractionation processes in vertebrate bioclasts. Negative Eu anomalies are most likely inherited from Eu-depleted surface waters, possibly as a result of Eu-depleted aeolian input. Conversely, positive Eu anomalies were presumably affected by Eu-enriched late diagenetic fluids with a possible high temperature origin causing a fractionation of europium. Our study therefore demonstrates the potential of high-resolution geochemical analysis in bonebed strata to disentangle different stages of diagenesis and to detect least altered areas in vertebrate fossils for recovering pristine palaeoenvironmental signals.


      PubDate: 2015-07-03T00:23:31Z
       
  • Interpretations and implications of LA ICP-MS analysis of chert for the
           origin of geochemical signatures in banded iron formations (BIFs) from the
           Meadowbank gold deposit, Western Churchill Province, Nunavut
    • Abstract: Publication date: 2 September 2015
      Source:Chemical Geology, Volume 410
      Author(s): B. Gourcerol , P.C. Thurston , D.J. Kontak , O. Côté-Mantha
      Among the many types of mineral deposits within Archean cratons, gold mineralization is an important economic commodity with over 20,000metric tons of gold produced from greenstone belts in 2001. Of the Archean–early Paleoproterozoic gold deposits, several different types of mineralization are known, which includes the important Algoma-type banded iron formation (BIF) where gold is locally associated with sulfide-facies zones within regionally extensive oxide-facies. It is commonly accepted that the shale-normalized chemical signature of REE+Y of chert bands in Algoma-type BIFs may reflect one of the three processes, each of which may be relevant to the nature and origin of the gold mineralization: (1) direct seawater precipitation; (2) involvement of and contribution from hydrothermal fluids; and (3) replacement of precursor volcanic units due to silicification. An essential question in regard to the mineralization is, therefore, whether the gold mineralizing fluids have a preference for one geochemical type of iron formation versus another. In order to assess the relevance of these competing models, we report herein the results of a LA ICP-MS study of chert samples within different Algoma-type BIFs from the Meadowbank deposit (24.5Mt proven/probable ore reserves grading 2.8g/t (2011)) hosted in the Neoarchean Woodburn Lake Group of the Rae Domain of the western Churchill Province, Canada. This study used 39 carefully selected and characterized (i.e., petrography and SEM-EDS imaging) chert samples from the main deposit, the Central BIF, and four additional BIFs, the Far West, West, East and Grizzly zones, with data collected using line traverses along the chert bands. The geochemical data indicate that an ambient seawater signature (i.e., enrichment in HREE relative to LREE associated with positive La and Y anomalies) dominates the samples with a lesser hydrothermal component (characterized by a positive Eu anomaly) and the influence of detrital contamination can also be detected. These initial results indicate that the methodology and protocol employed provides a reliable means to assess and interpret the chemical signature of BIFs hosting gold mineralization. In the present case, the results for the Meadowbank deposit suggest that chert from mineralized BIF units does not record a typical chemical signature that may be used as a vector for potential gold mineralization.


      PubDate: 2015-07-03T00:23:31Z
       
  • Contrasting effects of extracellular polymeric substances on the surface
           characteristics of bacterial pathogens and cell attachment to soil
           particles
    • Abstract: Publication date: 2 September 2015
      Source:Chemical Geology, Volume 410
      Author(s): Wenqiang Zhao , Sharon L. Walker , Qiaoyun Huang , Peng Cai
      Extracellular polymeric substances (EPSs) have been confirmed to affect bacterial surface properties and cell attachment to minerals. However, no systematic work has been done to clarify the contrasting roles of EPS in cell attachment to natural soil between different pathogenic strains. This study compared the different surface properties and attachment behaviors of two bacterial pathogens (with full or partial EPS) using attenuated total reflection Fourier transform infrared (ATR-FTIR) spectroscopy, potentiometric titration, zeta potential, hydrophobicity analysis, DLVO theory, and attachment tests. Cation exchange resin (CER) was employed to remove the EPS on Streptococcus suis and Escherichia coli such that the contribution of EPS to cell attachment to soil could be determined. ATR-FTIR confirmed the binding sites differed between S. suis and E. coli EPS. Notably, after partial EPS removal the absorption bands of S. suis between 1800cm−1 and 800cm−1 shifted or disappeared, whereas the lack of EPS did not affect the infrared absorption peaks for E. coli. This result suggests the overall surface site types within the E. coli EPS were similar to the residual EPS fractions or cell wall. The partial removal of EPS also changed the proton-active site concentrations of both cell types, and reduced the bacterial surface charge densities by 7%–17%. The negative charges on bacterial surfaces followed the order of full EPS-S. suis <partial EPS-S. suis <partial EPS-E. coli <full EPS-E. coli (ionic strength 1–100mM; pH5.6–5.8). With the removal of EPS, the average hydrophobicities of S. suis increased by 5% while those of E. coli decreased by 11%. EPS removal inhibited the attachment of S. suis to soil particles (<2mm) but enhanced E. coli attachment across the IS range of 1–100mM, which was attributed to the alteration in electrostatic repulsion. At IS 60–100mM, a sudden reduction in the attachment was observed only for full EPS-S. suis, which could be ascribed to the steric hindrance derived from EPS. However, full EPS-E. coli and partial EPS-E. coli showed similar increasing attachment trends at IS 1–100mM. This study clearly showed the distinct contribution of EPS to pathogen attachment to soil as a function of cell type and EPS present.
      Graphical abstract image

      PubDate: 2015-07-03T00:23:31Z
       
  • Vanadium geochemistry along groundwater flow paths in contrasting aquifers
           of the United States: Carrizo Sand (Texas) and Oasis Valley (Nevada)
           aquifers
    • Abstract: Publication date: 2 September 2015
      Source:Chemical Geology, Volume 410
      Author(s): Katherine Telfeyan , Karen H. Johannesson , T. Jade Mohajerin , C. Dianne Palmore
      Vanadium (V) concentrations were measured along the flow paths in two contrasting aquifer systems to study how changing solution composition and redox conditions affect V cycling and transport in groundwater flow systems. The Oasis Valley groundwater system in Nevada is an unconfined system where groundwaters generally remain oxic along the flow path, whereas the Carrizo Sand aquifer is a deep, confined coastal plain aquifer in Texas that contains well-defined Fe(III) and SO4 2− reduction zones along the flow path. Speciation modeling predicts that in groundwaters from the Oasis Valley, V chiefly occurs as the vanadate oxyanion (e.g., H2VO4 −, HVO4 2−), which is mobile along the entire flow path because of vanadate's solubility in oxic and alkaline waters. Furthermore, the data indicate that V concentrations increase in groundwaters along the flow path suggesting that mineral dissolution and/or desorption reactions progressively contribute V to groundwaters of the Oasis Valley system. Geochemical modeling is consistent with dissolution of mafic phenocrysts that are common in the local tuffaceous aquifer rocks as a probable source of V to Oasis Valley groundwaters. Within the Carrizo Sand aquifer, groundwater V concentrations decrease with flow beyond the recharge zone and into the confined portion of the aquifer. Thereafter, total V concentrations remain low and relatively constant along the remainder of the studied flow path, exhibiting no systematic variations across the Fe(III) and SO4 2− reduction zones. Groundwaters from the Carrizo Sand aquifer are generally reducing, and as such, geochemical modeling suggests that both V(V) and V(IV) species occur in Carrizo Sand groundwaters, and furthermore, that the proportion of V(IV) species increases concurrent with the decrease in total V concentrations along the flow path. Results from sequential extraction experiments on Carrizo Sand aquifer sediment samples suggest that V adsorption onto Fe(III)/Mn(IV) oxides/oxyhydroxides acts to decrease the aqueous V concentrations in Carrizo Sand groundwater. Furthermore, geochemical modeling is consistent with complexation of V(IV) with dissolved organic matter (DOM) within the Carrizo Sand aquifer groundwaters, which, owing to the low DOM concentrations, appears to buffer V at low, but relatively constant concentrations within the confined and reducing Carrizo Sand groundwaters. These results support previous reports of V(IV)–DOM binding and imply that aquifer material maintains an essential role in not only determining the availability of V to enter aqueous solution but also in influencing partitioning between solid and aqueous phases.


      PubDate: 2015-07-03T00:23:31Z
       
  • Arsenate and phosphate adsorption on ferrihydrite nanoparticles.
           Synergetic interaction with calcium ions
    • Abstract: Publication date: 2 September 2015
      Source:Chemical Geology, Volume 410
      Author(s): Juan Antelo , Florencio Arce , Sarah Fiol
      The geochemical behaviour of phosphate and arsenate ions in soil and aquatic systems is determined by the presence of mineral surfaces and major ions. Information about the distribution of oxyanions over the solid and solution phases is essential for understanding the transport, bioavailability and toxicity of these compounds in the environment. Here, we studied the adsorption of both arsenate and phosphate on ferrihydrite nanoparticles in the presence of calcium ions. The presence of calcium ions enhanced the retention of these oxyanions on ferrihydrite and vice versa. The arsenate–calcium and phosphate–calcium multi-component systems were described using a mechanistic surface complexation model. Use of this type of model enables prediction of the solution and surface speciation, along with analysis of oxyanion mobility in relation to environmental conditions. We were able to calibrate the charge distribution model with the macroscopic data obtained for the single-component systems, thus obtaining surface complexation constants for later use to simulate multi-component systems. The mutual interactions between arsenate and calcium were successfully described with these parameters, indicating that changes in the electrostatic forces at the solid/solution interface caused the observed enhanced adsorption. However, adsorption in the phosphate–calcium system was underestimated with the parameters obtained for the single-component systems, indicating that additional mechanisms or processes should be considered. Formation of insoluble mineral phases was ruled out, but the inclusion of a phosphate–calcium ternary surface complex improved the modelling predictions.


      PubDate: 2015-07-03T00:23:31Z
       
  • Ge/Si ratios indicating hydrothermal and sulfide weathering input to
           rivers of the Eastern Tibetan Plateau and Mt. Baekdu
    • Abstract: Publication date: 2 September 2015
      Source:Chemical Geology, Volume 410
      Author(s): Yeongcheol Han , Youngsook Huh , Louis Derry
      Concentrations of dissolved silicon in river waters reflect a complex interplay among chemical weathering of primary silicate minerals, formation and weathering of secondary clay minerals, hydrothermal input and biological cycling (formation and dissolution of opal phytoliths and growth of diatoms). We applied the Ge/Si ratio to assess the different sources of dissolved Si in rivers hailing from the eastern Tibetan Plateau — the Salween, Mekong, Chang Jiang (Yangtze), Hong (Red) and Huang He (Yellow) and from Mt. Baekdu — the Duman. Elevated riverine Ge/Si ratios were observed in arid regions with high geothermal activity in the Salween, Chang Jiang and Mt. Baekdu streams. In the Huang He and Hong River basins geothermal influence was not as pronounced, but weathering of sulfide- and coal-bearing minerals may be responsible for the high Ge/Si ratios. In rivers where inputs from hydrothermal and sulfide weathering are minimal, our data mostly fall in the weathering-limited regime of high riverine Si concentrations and low Ge/Si ratios.


      PubDate: 2015-07-03T00:23:31Z
       
  • Selenium isotope ratios, redox changes and biological productivity across
           the end-Permian mass extinction
    • Abstract: Publication date: 2 September 2015
      Source:Chemical Geology, Volume 410
      Author(s): Eva E. Stüeken , Julien Foriel , Roger Buick , Shane D. Schoepfer
      The causes of the greatest mass extinction in Earth's history, in the latest Permian, remain actively debated. Here we use Se isotopes and abundances in marine sediments from an outer-shelf environment to test one of the most common hypotheses for the collapse of the biosphere, i.e. widespread euxinia in the open ocean. Our data show a small positive excursion in δ82/78Se prior to the extinction, consistent with local euxinia. However, this is followed by a significant negative excursion with a minimum of −1.8‰ (relative to NIST SRM 3149), immediately preceding the principal extinction horizon. A net fractionation of this magnitude likely resulted from partial reduction of Se oxyanions dissolved in the water column. Due to their low abundance, Se oxyanions are rapidly scavenged in anoxic basins or regions of high biological productivity with little net isotopic fractionation. We therefore interpret the uniquely negative fractionations in this section as an indicator for relatively oxygenated conditions in this marine basin at the time when biological productivity declined. The offset between the peak excursion and the major extinction horizon possibly reflects a slow-down in ocean circulation leading to nutrient limitation, which may thus have prohibited a rapid recovery of the local biosphere in the early Triassic. Although we are unable to extrapolate to the global ocean due to the short residence time of Se in seawater, our data are consistent with the newly emerging view that euxinia developed along ocean margins and in oxygen minimum zones before the extinction, but was probably replaced by (sub-)oxic conditions during the ~1kyr peak productivity decline and was thus not solely responsible for the extinction event.


      PubDate: 2015-07-03T00:23:31Z
       
  • Fate of sulfide in the Frasassi cave system and implications for sulfuric
           acid speleogenesis
    • Abstract: Publication date: 2 September 2015
      Source:Chemical Geology, Volume 410
      Author(s): Daniel S. Jones , Lubos Polerecky , Sandro Galdenzi , Brian A. Dempsey , Jennifer L. Macalady
      The oxidation of hydrogen sulfide (H2S) has led to the formation of some of the world's largest caves through a process known as sulfuric acid speleogenesis (SAS). Here we present a multi-year study of the large, sulfidic, and actively-forming Frasassi cave system, Italy. We show that despite the presence of abundant sulfide-oxidizing biofilms in Frasassi streams, H2S(g) degassing to the cave atmosphere was the major sink for dissolved sulfide. Degassing rates ranged from 0.9 to 80μmolm−2 s−1, whereas microbial oxidation rates were between 0.15 and 2.0μmolm−2 s−1. Furthermore, microsensor measurements showed that sulfuric acid is not a major end product of microbial sulfide oxidation in the streams. Our results suggest that subaerial SAS will be important for karstification, and more important than subaqueous SAS, wherever ground waters with high sulfide concentrations emerge as flowing streams in contact with cave air.


      PubDate: 2015-07-03T00:23:31Z
       
  • High Co-doping promotes the transition of birnessite layer symmetry from
           orthogonal to hexagonal
    • Abstract: Publication date: 2 September 2015
      Source:Chemical Geology, Volume 410
      Author(s): Hui Yin , Yu Liu , Luuk K. Koopal , Xionghan Feng , Shengqi Chu , Mengqiang Zhu , Fan Liu
      Despite its presence in limited amounts, birnessite has a wide spread distribution and is often highly enriched in trace metals such as Co in diverse geological environments. This study investigated the effects of Co doping on the layer structure and properties of birnessites synthesized through the oxidation of Mn2+ by O2 under alkaline conditions, by using powder X-ray diffraction (XRD) and X-ray absorption spectroscopy (both XANES and EXAFS). The Co doped, high-pH birnessites are composed of platy crystals, and have lower crystallinity, larger specific surface areas (SSAs) and higher Mn average oxidation states (AOSs) than pure birnessite. Cobalt K-edge EXAFS analysis reveals that no CoOOH is formed, and ~76% of the total Co is located in the layers of the Co-doped birnessites. Careful examination of these Co-doped samples by XRD and EXAFS analyses demonstrates that the presence of Co during the synthesis of high-pH birnessite promotes the structural transition of the birnessite layer symmetry from orthogonal to hexagonal. This is due to the decrease in Mn(III) in the layers of the doped solids, leading to the attenuation of Jahn–Teller effect. The decrease in Mn(III) in the layers might be attributed to the substitution of Mn(III) by Co(III) in the layers. The competitive adsorption of Co2+/3+ with Mn2+ might also decrease the oxidation of Mn2+ to Mn(III) and the subsequent migration of Mn(III) into the birnessite layers. The results provide new insights into the interaction mechanisms between transition metals and birnessite-like minerals, and improve our understanding of the abiotic oxidation of Mn2+ as well as the prevalence of birnessites with hexagonal symmetry in natural environments.


      PubDate: 2015-07-03T00:23:31Z
       
  • The effect of lithium on the viscosity of pegmatite forming liquids
    • Abstract: Publication date: 2 September 2015
      Source:Chemical Geology, Volume 410
      Author(s): Alexander Bartels , Harald Behrens , Francois Holtz , Burkhard C. Schmidt
      Viscosity experiments were conducted to investigate the effect of Li2O on the viscosity of pegmatite forming melts and the possible difference to K2O. The individual effects of Li2O and K2O on melt viscosity were investigated by adding the same molar proportion of both components to a starting composition representing natural pegmatite forming melts. The viscosity experiments on hydrous melts (~1 to ~6wt.% H2O) were carried out in the low temperature range at ambient pressure using the micropenetration technique and in the high temperature range at 200–300MPa pressure using the falling sphere method. The addition of Li2O and K2O results in a decrease of melt viscosity and, on a molar basis, the effect of Li2O is comparable to that of K2O. Viscosity-related properties such as activation energy of viscous flow, melt fragility and glass transition temperature are affected in a similar way by both alkali oxides. The observed effects of alkalis on the viscosity of melt are smaller when compared to an equal molar amount of OH− groups at low water content but become equal with increasing water content. Finally, it is shown that the low viscosity of pegmatite forming melts can only be explained by the combined effects of H2O, F and of non-charge balancing alkalis.


      PubDate: 2015-07-03T00:23:31Z
       
  • Effect of manganese oxide minerals and complexes on gold mobilization and
           speciation
    • Abstract: Publication date: 22 June 2015
      Source:Chemical Geology, Volumes 407–408
      Author(s): Christine Ta , Joël Brugger , Allan Pring , Rosalie K. Hocking , Claire E. Lenehan , Frank Reith
      (Bio)geochemical processes mediate the dispersion and re-concentration of gold (Au) in Earth surface environments. The fundamental mechanisms underlying the mobilization of Au in surface environments and the chemical speciation of mobile Au remain largely unexplored. For more than half a century authors have argued that highly reactive manganese (Mn) oxides are involved in oxidizing, and hence mobilizing, Au in waters, soils and sediments. However, no systematic study has assessed the mechanisms of Au oxidation under conditions occurring in natural environments. Therefore, this paper explores the links between Mn-oxides and the oxidation/mobilization of Au under acidic to circum-neutral conditions. Using a newly developed high performance liquid chromatography–inductively coupled plasma-mass spectrometry (HPLC–ICP-MS) method, a range of Au(I/III)-complexes occurring in aqueous solution were directly quantified. We show that the oxidation of Au(I) to Au(III) with birnessite occurs in acidic environments, and that the rapid release of Au(III) sorbed (on Mn- or Fe-minerals) via influx of neutral waters transfers Au(III) into solution, making it mobile and transportable. This provides a viable pathway for explaining the occurrence of mobile Au(III)-complexes under neutral to alkaline conditions, as was reported from a Western Australian salt lake environment. In addition, we show that the [(bpy)2MnIII(μ-O)2MnIV(bpy)2]3− complex (bpy=2,2′-bipyridyl), a useful model compound for the interaction of Au with bioorganic Mn in natural systems, mediates the formation of soluble Au(III)-complexes from metallic Au and from Au(I)-complexes under oxic and anoxic conditions. In conclusion, this study shows that reactive Mn-oxide minerals are important drivers of Au mobilization, and further suggests that natural organometallic compounds play an important role for the mobilization of Au in Earth surface environments.


      PubDate: 2015-07-03T00:23:31Z
       
  • Subsurface hydrothermal processes and the bioenergetics of
           chemolithoautotrophy at the shallow-sea vents off Panarea Island (Italy)
    • Abstract: Publication date: 22 June 2015
      Source:Chemical Geology, Volumes 407–408
      Author(s): Roy E. Price , Douglas E. LaRowe , Francesco Italiano , Ivan Savov , Thomas Pichler , Jan P. Amend
      The subsurface evolution of shallow-sea hydrothermal fluids is a function of many factors including fluid–mineral equilibria, phase separation, magmatic inputs, and mineral precipitation, all of which influence discharging fluid chemistry and consequently associated seafloor microbial communities. Shallow-sea vent systems, however, are understudied in this regard. In order to investigate subsurface processes in a shallow-sea hydrothermal vent, and determine how these physical and chemical parameters influence the metabolic potential of the microbial communities, three shallow-sea hydrothermal vents associated with Panarea Island (Italy) were characterized. Vent fluids, pore fluids and gases at the three sites were sampled and analyzed for major and minor elements, redox-sensitive compounds, free gas compositions, and strontium isotopes. The corresponding data were used to 1) describe the subsurface geochemical evolution of the fluids and 2) to evaluate the catabolic potential of 61 inorganic redox reactions for in situ microbial communities. Generally, the vent fluids can be hot (up to 135°C), acidic (pH1.9–5.7), and sulfidic (up to 2.5mM H2S). Three distinct types of hydrothermal fluids were identified, each with higher temperatures and lower pH, Mg and SO4, relative to seawater. Type 1 was consistently more saline than Type 2, and both were more saline than seawater. Type 3 fluids were similar to or slightly depleted in most major ions relative to seawater. End-member calculations of conservative elements indicate that Type 1 and Type 2 fluids are derived from two different sources, most likely 1) a deeper, higher salinity reservoir and 2) a shallower, lower salinity reservoir, respectively, in a layered hydrothermal system. The deeper reservoir records some of the highest end-member Cl concentrations to date, and developed as a result of recirculation of brine fluids with long term loss of steam and volatiles due to past phase separation. No strong evidence for ongoing phase separation is observed. Type 3 fluids are suggested to be mostly influenced by degassing of volatiles and subsequently dissolution of CO2, H2S, and other gases into the aqueous phase. Gibbs energies (ΔGr ) of redox reactions that couple potential terminal electron acceptors (O2, NO3 −, MnIV, FeIII, SO4 2−, S0, CO2) with potential electron donors (H2, NH4 +, Fe2+, Mn2+, H2S, CH4) were evaluated at in situ temperatures and compositions for each site and by fluid type. When Gibbs energies of reaction are normalized per kilogram of hydrothermal fluid, sulfur oxidation reactions are the most exergonic, while the oxidation of Fe2+, NH4 +, CH4, and Mn2+ is moderately energy yielding. The energetic calculations indicate that the most robust microbial communities in the Panarea hot springs combine H2S from deep water–rock–gas interactions with O2 that is entrained via seawater mixing to fuel their activities, regardless of site location or fluid type.


      PubDate: 2015-07-03T00:23:31Z
       
  • Adsorption of monothioarsenate on amorphous aluminum hydroxide under
           anaerobic conditions
    • Abstract: Publication date: 22 June 2015
      Source:Chemical Geology, Volumes 407–408
      Author(s): Fan Xiao , Shaofeng Wang , Liying Xu , Ying Wang , Zidan Yuan , Yongfeng Jia
      Thioarsenates have been found to be major arsenic species in some sulfidic waters and their interactions with minerals play an important role in the geochemical cycling of arsenic. This work investigated the sorption behavior of monothioarsenate (mtAsV) on amorphous aluminum hydroxide (am-Al(OH)3) at various pHs and characterized the adsorbed arsenic by using X-ray absorption spectroscopy (XAS) and Fourier transform infrared spectroscopy (FTIR). The uptake of arsenic by am-Al(OH)3 from mtAsV solution (86–16mg·g−1) was found to be much less than that from arsenate (AsV) solution (112–50.5mg·g−1) at all pHs applied (4–10). MtAsV was partly reduced to arsenite with concurrent production of elemental sulfur, and the interaction with am-Al(OH)3 at acidic pH enhanced the reduction, which is attributed to acid-catalysis at the am-Al(OH)3 surface. Transformation of mtAsV to arsenate also occurred during the adsorption process. The linear combination fitting (LCF) of As K-edge X-ray absorption near edge structure (XANES) spectra indicated that the arsenic adsorbed on am-Al(OH)3 was present dominantly as mtAsV, with minor amounts as arsenite and arsenate. The As K-edge extended X-ray absorption fine structure (EXAFS) and FTIR results suggested that bidentate binuclear complexation was involved in the adsorption of mtAsV on am-Al(OH)3. This study is the first documentation of mtAsV sorption on aluminum hydroxides and is necessary to assess geochemical cycling of arsenic in sulfidic natural or mining-impacted environments.


      PubDate: 2015-07-03T00:23:31Z
       
  • Gas blowout from shallow boreholes near Fiumicino International Airport
           (Rome): Gas origin and hazard assessment
    • Abstract: Publication date: 22 June 2015
      Source:Chemical Geology, Volumes 407–408
      Author(s): Maria Luisa Carapezza , Luca Tarchini , Domenico Granieri , Mauro Martelli , Alessandro Gattuso , Nicola Pagliuca , Massimo Ranaldi , Tullio Ricci , Fausto Grassa , Andrea Rizzo , Luca Pizzino , Alessandra Sciarra
      In summer 2013 a toxic and polluting gas blowout (19tonnesday−1 CO2, 95kgday−1 CH4) occurred from two shallow boreholes drilled at only 50m from the International Airport of Rome (Italy), in the town of Fiumicino. Another gas blowout occurred in the same period from a borehole located offshore, 2km away, also generating sea-water acidification; it lasted only a couple of days. Onshore, CO2 was also diffusing from holes within the soil, particularly toward the airport, generating a soil flux up to 1.8tonnesday−1. In 3.5months ~1500tonnes of CO2 and 5.4tonnes of CH4 were emitted in the atmosphere. Temporal monitoring of gas geochemistry indicates that in this area a mixing occurs between shallow and pressurized gas pockets, CO2-dominated, but with different chemical (i.e., He/CH4 ratio) and isotopic (3He/4He, δ13C-δDCH4) characteristics. Numerical simulation of CO2 dispersion in the atmosphere showed that dangerous air CO2 concentrations, up to lethal values, were only found near the vents at a height of 0.2m. Fiumicino is a high blowout risk area, as CO2 rising through deep reaching faults pressurizes the shallow aquifer contained in gravels confined underneath shales of the Tiber delta deposits. The Fiumicino blowout is a typical example of dangerous phenomenon that may occur in urban context lying nearby active or recent volcanoes and requires quick response on hazard assessment by scientists to be addressed to civil protection and administrators.


      PubDate: 2015-07-03T00:23:31Z
       
  • Corrigendum to “Origin of calcareous dust in Argentinean Pleistocene
           periglacial deposits traced by Sr, C and O isotopic compositions, and REE
           distribution” [Chem. Geol. 380 (2014) 119–132]
    • Abstract: Publication date: 22 June 2015
      Source:Chemical Geology, Volumes 407–408
      Author(s): Isabelle Techer , Norbert Clauer , Thea Vogt



      PubDate: 2015-07-03T00:23:31Z
       
  • Separation of monomethylmercury from estuarine sediments for mercury
           isotope analysis
    • Abstract: Publication date: Available online 20 June 2015
      Source:Chemical Geology
      Author(s): Sarah E. Janssen , Marcus W. Johnson , Joel D. Blum , Tamar Barkay , John R. Reinfelder
      Estuarine sediments support the production of monomethylmercury (MeHg) which accumulates in aquatic organisms. While natural variation in mercury stable isotope ratios can potentially be used to track sources and transformations of mercury in the environment, the isotopic signature of MeHg in sediments has not been measured directly. The isotopic composition of MeHg has been studied in laboratory experiments and fish using tandem gas chromatography-multicollector inductively coupled plasma mass spectrometry (MC-ICP-MS) systems; however, the precision and sensitivity of this method may be too low for the analysis of many environmental samples including sediments in which MeHg constitutes 1% or less of the total mercury. In this study, we developed an offline separation method for the precise measurement of the Hg isotopic composition of MeHg in estuarine sediments. Separation of MeHg from inorganic species was accomplished by distillation and chemical ethylation-GC, and was followed by gold amalgam trapping to collect and preconcentrate pyrolyzed MeHg, which was then released into an oxidizing solution. MeHg standards processed in this way were collected with an average yield of 97.5%. External precision for all replicate isotope analyses of MeHg process standards was ±0.14‰ (2SD, n=8) for δ202Hg and no detectable fractionation of Hg stable isotopes occurred during the separation. δ202Hg values for MeHg separated from estuarine sediments using our approach varied from -0.41 to +0.41‰ and were generally higher, and spatially and temporally more variable, than those for total Hg (-0.21 to -0.48‰).


      PubDate: 2015-07-03T00:23:31Z
       
  • Trace Element Characterization of USGS Reference Materials by HR-ICP-MS
           and Q-ICP-MS
    • Abstract: Publication date: Available online 20 June 2015
      Source:Chemical Geology
      Author(s): Gary Schudel , Vivian Lai , Kathy Gordon , Dominique Weis
      A trace-element characterization of United States Geological Survey (USGS) reference materials has been carried out using three different inductively coupled plasma mass spectrometer instruments: Nu Instruments AttoM, Thermo Scientific Element 2, and Agilent 7700x. The six USGS reference materials (BHVO-2, BCR-2, AGV-2, STM-2, RGM-2, and G-3) include both extrusive and intrusive rocks with compositions ranging from mafic to felsic that encompass a wide range of trace element concentrations and mineral phases. The prevalence of these 2nd (and 3rd) generation reference materials among geochemical laboratories and the added care that was taken during sample preparation makes this generation of reference materials more suitable for characterization and use by the geochemical community. The mafic-intermediate reference materials (BHVO-2, BCR-2, and AGV-2) had the most abundant published characterization prior to this study, which allowed for thorough and direct comparison with previous studies (and USGS certificates). Published data for these reference materials are relatively well constrained for most elements (i.e., not highly variable) and the results of this study generally correspond well with the literature. This can, at least partially, be attributed to the relatively simple matrices of these reference materials together with their overall smaller range of trace element concentrations. Unlike the mafic-intermediate reference materials, STM-2 lacked any published trace element concentrations prior to this study and data for RGM-2 and G-3 were relatively sparse. Data for STM-2 and RGM-2 from this study are compared to preliminary USGS certificates, while data for G-3 are compared to the G-2 certificate values; for comparison with published data, RGM-1 and G-2 are included. Overall, The published data for RGM and G are variable across the entire suite of trace elements and display poorer agreement with the data from this study. This can be explained by the fact that, in addition to the comparison between generations that may add heterogeneity, the felsic reference materials have relatively complex matrices and larger ranges in trace element concentration. Given the new data that this study provides, the felsic reference materials can now be used by the geochemical community to produce more accurate trace element concentration analyses of felsic materials. By using three instruments with different configurations, this study also serves as a comparison of the performance of each instrument in response to an array of geologic matrices. The data reveal that the tested instruments are all capable of producing accurate and precise trace element concentration data over a range of geologic matrices, provided that each instrument is carefully operated within its dynamic range. In many cases, this could be accomplished simply by ensuring proper dilution of samples; however, when matrices are more complex, the use of medium and high resolutions can be useful in reducing the amount of preparative chemistry that is required to manage highly variable trace element concentrations within a single sample as well as any potential isobaric interferences.


      PubDate: 2015-07-03T00:23:31Z
       
  • Size distribution of alkali elements in riverbed sediment and its
           relevance to fractionation of alkali elements during chemical weathering
    • Abstract: Publication date: Available online 19 June 2015
      Source:Chemical Geology
      Author(s): Kazuya Tanaka , Naoko Watanabe
      In this study, we analyzed major elements, Rb and Cs in riverbed sediments after size separation to investigate the release and retention of alkali elements (K, Rb, and Cs) during chemical weathering. The degree of chemical weathering (measured using the chemical index of alteration) increased with decreasing particle size of the sediment, which is consistent with the mineralogy of the size-fractioned sediments. The analytical results from the size-fractioned sediments show that K and Rb are released from primary minerals during chemical weathering, whereas Cs is retained and accumulates in the weathering products (i.e., fine particle fractions), possibly because of a strong fixation by clay minerals. The analytical results show that the durability toward chemical weathering increases in the order of K, Rb, and Cs. As a result, the Rb/K and Cs/K ratios in the river water samples were low relative to the sediments, reflecting ongoing chemical weathering in the catchment basin. The Rb/K and Cs/K ratios of the sediments and soils would increase with progressive chemical weathering. Thus, the degree of fractionation between K, Rb, and Cs can be used to evaluate the degree of chemical weathering for granitic rocks. The behavior of stable Cs and radiocesium isotopes, both of which were distributed more in the fine particles of sediments, was coherent. However, isotope equilibrium was not reached, possibly because of the presence of stable Cs at the inner (nonreactive) sites of the minerals, depending on the particle size. Thus, the Cs isotope ratio of 137Cs/133Cs decreased with increasing particle size. In conclusion, the strong affinity of Cs for the weathering products of clay minerals causes a large fractionation of alkali elements during chemical weathering, and the transportation of particulates and dissolved fractions results in a further fractionation between the alkali elements.


      PubDate: 2015-07-03T00:23:31Z
       
  • The stability of microbially reduced U(IV); impact of residual electron
           donor and sediment ageing
    • Abstract: Publication date: Available online 1 June 2015
      Source:Chemical Geology
      Author(s): Laura Newsome , Katherine Morris , Samuel Shaw , Divyesh Trivedi , Jonathan R. Lloyd
      The stimulation of microbial U(VI) reduction to precipitate insoluble U(IV) has been proposed as a means of remediating mobile uranium groundwater contamination. Crucial to the success of such a remediation strategy is determining the longevity of U(IV) biominerals in the subsurface, particularly if the groundwater becomes oxidising. Here we describe experiments to assess the susceptibility of microbially-reduced U(IV) to oxidative remobilisation both via aeration and by the addition of nitrate at environmentally-relevant conditions. Additional factors examined include the possibility of biogenic U(IV) becoming more crystalline (and potentially more recalcitrant) during a period of ageing, and the role played by residual electron donor in controlling the long-term fate of the uranium. Biogenic U(IV) was precipitated as a non-crystalline U(IV) or “monomeric” phase, with a small but increasing contribution to the EXAFS spectra from nanocrystalline uraninite occurring during 15 months of ageing. Despite this, no evidence was observed for an increase in recalcitrance to oxidative remobilisation. However, the presence of residual electron donor post-biostimulation was shown to exert a strong control on U(IV) reoxidation kinetics, highlighting the importance of maintaining the presence of electron donor in the subsurface, in order to protect biogenic U(IV) from oxidative remobilisation.


      PubDate: 2015-06-06T16:39:04Z
       
 
 
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