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  Subjects -> EARTH SCIENCES (Total: 650 journals)
    - EARTH SCIENCES (469 journals)
    - GEOLOGY (71 journals)
    - GEOPHYSICS (27 journals)
    - HYDROLOGY (21 journals)
    - OCEANOGRAPHY (62 journals)

EARTH SCIENCES (469 journals)                  1 2 3 | Last

Showing 1 - 200 of 371 Journals sorted alphabetically
Acta Geochimica     Hybrid Journal   (Followers: 3)
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: 6)
Acta Geotechnica     Hybrid Journal   (Followers: 6)
Acta Meteorologica Sinica     Hybrid Journal   (Followers: 3)
Advances in High Energy Physics     Open Access   (Followers: 16)
Advances In Physics     Hybrid Journal   (Followers: 10)
Aeolian Research     Hybrid Journal   (Followers: 5)
African Journal of Aquatic Science     Hybrid Journal   (Followers: 12)
Algological Studies     Full-text available via subscription   (Followers: 2)
Alpine Botany     Hybrid Journal   (Followers: 3)
AMBIO     Hybrid Journal   (Followers: 13)
Anadolu University Journal of Science and Technology     Open Access  
Anales del Instituto de la Patagonia     Open Access   (Followers: 1)
Andean geology     Open Access   (Followers: 3)
Annales Henri Poincaré     Hybrid Journal   (Followers: 3)
Annales UMCS, Geographia, Geologia, Mineralogia et Petrographia     Open Access  
Annals of Geophysics     Full-text available via subscription   (Followers: 12)
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: 9)
Anthropocene     Hybrid Journal   (Followers: 2)
Anthropocene Review     Hybrid Journal   (Followers: 4)
Applied Clay Science     Hybrid Journal   (Followers: 4)
Applied Geochemistry     Hybrid Journal   (Followers: 11)
Applied Geomatics     Hybrid Journal   (Followers: 3)
Applied Geophysics     Hybrid Journal   (Followers: 7)
Applied Ocean Research     Hybrid Journal   (Followers: 4)
Applied Petrochemical Research     Open Access   (Followers: 2)
Applied Remote Sensing Journal     Open Access   (Followers: 29)
Aquatic Conservation Marine and Freshwater Ecosystems     Hybrid Journal   (Followers: 34)
Arctic Science     Open Access   (Followers: 6)
Arctic, Antarctic, and Alpine Research     Full-text available via subscription   (Followers: 10)
Artificial Satellites : The Journal of Space Research Centre of Polish Academy of Sciences     Open Access   (Followers: 16)
Asia-Pacific Journal of Atmospheric Sciences     Hybrid Journal   (Followers: 8)
Asian Journal of Earth Sciences     Open Access   (Followers: 20)
Asian Review of Environmental and Earth Sciences     Open Access   (Followers: 1)
Atlantic Geology : Journal of the Atlantic Geoscience Society / Atlantic Geology : revue de la Société Géoscientifique de l'Atlantique     Full-text available via subscription   (Followers: 2)
Atmosphere-Ocean     Full-text available via subscription   (Followers: 12)
Atmospheric and Climate Sciences     Open Access   (Followers: 24)
Australian Journal of Earth Sciences: An International Geoscience Journal of the Geological Society of Australia     Hybrid Journal   (Followers: 11)
Boletim de Ciências Geodésicas     Open Access  
Boreas: An International Journal of Quaternary Research     Hybrid Journal   (Followers: 10)
Bragantia     Open Access   (Followers: 2)
Bulletin of Earthquake Engineering     Hybrid Journal   (Followers: 10)
Bulletin of Geosciences     Open Access   (Followers: 8)
Bulletin of the Lebedev Physics Institute     Hybrid Journal   (Followers: 1)
Bulletin of the Seismological Society of America     Full-text available via subscription   (Followers: 20)
Bulletin of Volcanology     Hybrid Journal   (Followers: 17)
Cadernos de Geociências     Open Access  
Canadian Journal of Plant Science     Full-text available via subscription   (Followers: 14)
Canadian Mineralogist     Full-text available via subscription   (Followers: 3)
Canadian Water Resources Journal     Hybrid Journal   (Followers: 20)
Carbonates and Evaporites     Hybrid Journal   (Followers: 3)
CATENA     Hybrid Journal   (Followers: 4)
Chemical Geology     Hybrid Journal   (Followers: 14)
Chemie der Erde - Geochemistry     Hybrid Journal   (Followers: 4)
Chinese Geographical Science     Hybrid Journal   (Followers: 5)
Chinese Journal of Oceanology and Limnology     Hybrid Journal   (Followers: 3)
Ciencias Espaciales     Open Access  
Climate and Development     Hybrid Journal   (Followers: 12)
Coastal Management     Hybrid Journal   (Followers: 23)
Cogent Geoscience     Open Access  
Comptes Rendus Geoscience     Full-text available via subscription   (Followers: 7)
Computational Geosciences     Hybrid Journal   (Followers: 13)
Computational Mathematics and Mathematical Physics     Hybrid Journal   (Followers: 1)
Computers and Geotechnics     Hybrid Journal   (Followers: 8)
Contemporary Trends in Geoscience     Open Access   (Followers: 2)
Continental Shelf Research     Hybrid Journal   (Followers: 8)
Contributions to Mineralogy and Petrology     Hybrid Journal   (Followers: 10)
Contributions to Plasma Physics     Hybrid Journal   (Followers: 3)
Coral Reefs     Hybrid Journal   (Followers: 16)
Cretaceous Research     Hybrid Journal   (Followers: 6)
Cybergeo : European Journal of Geography     Open Access   (Followers: 4)
Depositional Record     Open Access  
Developments in Geotectonics     Full-text available via subscription   (Followers: 3)
Developments in Quaternary Science     Full-text available via subscription   (Followers: 3)
Développement durable et territoires     Open Access   (Followers: 2)
Diatom Research     Hybrid Journal  
Doklady Physics     Hybrid Journal   (Followers: 1)
Dynamics of Atmospheres and Oceans     Hybrid Journal   (Followers: 10)
E&S Engineering and Science     Open Access  
E3S Web of Conferences     Open Access  
Earth and Planetary Science Letters     Hybrid Journal   (Followers: 68)
Earth and Space Science     Open Access   (Followers: 9)
Earth Interactions     Full-text available via subscription   (Followers: 11)
Earth Science Research     Open Access   (Followers: 6)
Earth Surface Dynamics (ESurf)     Open Access   (Followers: 3)
Earth Surface Processes and Landforms     Hybrid Journal   (Followers: 19)
Earth System Dynamics     Open Access   (Followers: 6)
Earth System Dynamics Discussions     Open Access   (Followers: 4)
Earth's Future     Open Access   (Followers: 1)
Earth, Planets and Space     Open Access   (Followers: 10)
Earthquake Engineering and Engineering Vibration     Hybrid Journal   (Followers: 6)
Earthquake Science     Hybrid Journal   (Followers: 9)
Earthquake Spectra     Full-text available via subscription   (Followers: 17)
Ecohydrology     Hybrid Journal   (Followers: 9)
Ecological Questions     Open Access   (Followers: 5)
Electromagnetics     Hybrid Journal   (Followers: 4)
Energy Efficiency     Hybrid Journal   (Followers: 11)
Energy Exploration & Exploitation     Full-text available via subscription   (Followers: 4)
Environmental Earth Sciences     Hybrid Journal   (Followers: 21)
Environmental Geology     Hybrid Journal   (Followers: 9)
Environmental Geosciences     Full-text available via subscription   (Followers: 4)
Environmental Geotechnics     Hybrid Journal   (Followers: 3)
Erwerbs-Obstbau     Hybrid Journal  
Estuaries and Coasts     Hybrid Journal   (Followers: 17)
Estuarine, Coastal and Shelf Science     Hybrid Journal   (Followers: 33)
Estudios Geográficos     Open Access  
European Journal of Mineralogy     Full-text available via subscription   (Followers: 11)
Exploration Geophysics     Hybrid Journal   (Followers: 3)
Facies     Hybrid Journal   (Followers: 9)
Fieldiana Life and Earth Sciences     Full-text available via subscription   (Followers: 1)
Física de la Tierra     Open Access  
Folia Musei rerum naturalium Bohemiae occidentalis. Geologica et Paleobiologica     Open Access  
Folia Quaternaria     Open Access  
Forestry Chronicle     Full-text available via subscription   (Followers: 10)
Frontiers in Earth Science     Open Access   (Followers: 5)
Frontiers in Geotechnical Engineering     Open Access   (Followers: 2)
Frontiers of Earth Science     Hybrid Journal   (Followers: 8)
Fundamental and Applied Limnology / Archiv für Hydrobiologie     Full-text available via subscription   (Followers: 4)
GEM - International Journal on Geomathematics     Hybrid Journal   (Followers: 1)
Geo-Marine Letters     Hybrid Journal   (Followers: 7)
Geoacta     Open Access   (Followers: 4)
Geobiology     Hybrid Journal   (Followers: 7)
Geocarto International     Hybrid Journal   (Followers: 17)
Geochemical Perspectives     Hybrid Journal  
Geochemistry : Exploration, Environment, Analysis     Hybrid Journal   (Followers: 7)
Geochemistry, Geophysics, Geosystems     Full-text available via subscription   (Followers: 25)
Geochimica et Cosmochimica Acta     Hybrid Journal   (Followers: 25)
Geochronometria     Hybrid Journal   (Followers: 3)
Geoderma Regional : The International Journal for Regional Soil Research     Full-text available via subscription   (Followers: 3)
Geodinamica Acta     Hybrid Journal   (Followers: 3)
Geodynamics & Tectonophysics     Open Access  
Geoenvironmental Disasters     Open Access   (Followers: 3)
Geofluids     Hybrid Journal   (Followers: 4)
Geoforum     Hybrid Journal   (Followers: 20)
Géographie physique et Quaternaire     Full-text available via subscription  
Geography and Natural Resources     Hybrid Journal   (Followers: 5)
Geoheritage     Hybrid Journal   (Followers: 1)
Geoinformatica Polonica : The Journal of Polish Academy of Arts and Sciences     Open Access  
Geoinformatics & Geostatistics     Hybrid Journal   (Followers: 8)
Geological Journal     Hybrid Journal   (Followers: 14)
Geology Today     Hybrid Journal   (Followers: 18)
Geomagnetism and Aeronomy     Hybrid Journal   (Followers: 3)
Geomatics, Natural Hazards and Risk     Hybrid Journal   (Followers: 8)
Geomechanics for Energy and the Environment     Full-text available via subscription  
GEOmedia     Open Access   (Followers: 1)
Geomorphology     Hybrid Journal   (Followers: 24)
Geophysical & Astrophysical Fluid Dynamics     Hybrid Journal   (Followers: 2)
Geophysical Journal International     Hybrid Journal   (Followers: 31)
Geophysical Prospecting     Hybrid Journal   (Followers: 7)
Geophysics     Full-text available via subscription   (Followers: 18)
GeoResJ     Hybrid Journal  
Georisk: Assessment and Management of Risk for Engineered Systems and Geohazards     Hybrid Journal   (Followers: 8)
Geoscience Canada : Journal of the Geological Association of Canada / Geoscience Canada : journal de l'Association Géologique du Canada     Full-text available via subscription   (Followers: 3)
Geoscience Data Journal     Open Access   (Followers: 2)
Geoscience Frontiers     Open Access   (Followers: 8)
Geoscience Letters     Open Access  
Geoscience Records     Open Access  
Geosciences     Open Access   (Followers: 2)
Geosciences Journal     Hybrid Journal   (Followers: 9)
Geoscientific Instrumentation, Methods and Data Systems     Open Access   (Followers: 2)
Geoscientific Model Development     Open Access   (Followers: 2)
Geostandards and Geoanalytical Research     Hybrid Journal   (Followers: 2)
Geosystem Engineering     Hybrid Journal   (Followers: 1)
Geotectonic Research     Full-text available via subscription   (Followers: 4)
Geotectonics     Hybrid Journal   (Followers: 7)
GISAP : Earth and Space Sciences     Open Access   (Followers: 1)
Glass Physics and Chemistry     Hybrid Journal   (Followers: 3)
Global and Planetary Change     Hybrid Journal   (Followers: 12)
Global Biogeochemical Cycles     Full-text available via subscription   (Followers: 11)
Gondwana Research     Hybrid Journal   (Followers: 6)
Grassland Science     Hybrid Journal   (Followers: 1)
Ground Water     Hybrid Journal   (Followers: 23)
Ground Water Monitoring & Remediation     Hybrid Journal   (Followers: 15)
Groundwater for Sustainable Development     Full-text available via subscription  
GSA Today     Partially Free  
Helgoland Marine Research     Open Access   (Followers: 3)
History of Geo- and Space Sciences     Open Access   (Followers: 3)
Hydrobiologia     Hybrid Journal   (Followers: 18)
Hydrogeology Journal     Hybrid Journal   (Followers: 16)
Hydrological Processes     Hybrid Journal   (Followers: 23)
Hydrology and Earth System Sciences     Open Access   (Followers: 26)
ICES Journal of Marine Science: Journal du Conseil     Hybrid Journal   (Followers: 53)
IEEE Journal of Oceanic Engineering     Hybrid Journal   (Followers: 11)
Indian Geotechnical Journal     Hybrid Journal   (Followers: 2)
Indonesian Journal on Geoscience     Open Access   (Followers: 2)
Interdisciplinary Environmental Review     Hybrid Journal   (Followers: 3)
International Geology Review     Hybrid Journal   (Followers: 5)
International Journal of Advanced Geosciences     Open Access  
International Journal of Advanced Remote Sensing and GIS     Open Access   (Followers: 29)
International Journal of Advancement in Earth and Enviromental Sciences     Open Access   (Followers: 2)
International Journal of Advancement in Remote Sensing, GIS, and Geography     Open Access   (Followers: 23)
International Journal of Applied Earth Observation and Geoinformation     Hybrid Journal   (Followers: 28)
International Journal of Coal Geology     Hybrid Journal   (Followers: 2)
International Journal of Disaster Risk Reduction     Hybrid Journal   (Followers: 10)
International Journal of Earth Sciences     Hybrid Journal   (Followers: 30)
International Journal of Forest, Soil and Erosion     Open Access   (Followers: 4)

        1 2 3 | Last

Journal Cover Chemical Geology
  [SJR: 2.346]   [H-I: 145]   [14 followers]  Follow
    
   Hybrid Journal Hybrid journal (It can contain Open Access articles)
   ISSN (Print) 0009-2541
   Published by Elsevier Homepage  [3039 journals]
  • Ice core record of dust sources in the western United States over the last
           300years
    • Abstract: Publication date: 28 November 2016
      Source:Chemical Geology, Volume 442
      Author(s): S.M. Aarons, S.M. Aciego, P. Gabrielli, B. Delmonte, J.M. Koornneef, C. Uglietti, A. Wegner, M.A. Blakowski, C. Bouman
      Over the past ~5000years, amplified dust generation and deposition in the American West has been linked to human activity. In recent decades, intensified rates of agriculture and livestock grazing have been correlated with greater dust production detected on seasonal to annual timescales. The combination of land use intensification and climate change (i.e. increased drought frequency) in North America highlights the importance of characterizing the sources of dust both before and after the influence of anthropogenic activity. We apply high-precision geochemical and isotopic (Sr and Nd isotopes) techniques to an ice core from the Upper Fremont Glacier (Wyoming, USA) to produce the first glacial dataset from the American West. Our Sr-Nd isotopic composition data indicates the evolving dust provenance to the Upper Fremont Glacier (UFG) from a long-range transport of mineral dust to a local source. This increasing input of dust from a local source is supported by a rise in average dust particle diameter combined with greater average dust concentration throughout the record. The greater presence of dust particles smaller than 2.5μm in the most recent samples from UFG ice core record support existing satellite and sediment core data regarding the effects of anthropogenic activity upon dust sources and pathways in the American West. Although the Sr-Nd isotope database in North America needs be expanded, our results provide a survey of windborne dust through the past 270years.


      PubDate: 2016-09-22T23:29:42Z
       
  • Consumption of atmospheric methane in a limestone cave in Indiana, USA
    • Abstract: Publication date: 2 December 2016
      Source:Chemical Geology, Volume 443
      Author(s): Kevin D. Webster, Anmar Mirza, Jessica M. Deli, Peter E. Sauer, Arndt Schimmelmann
      Recent observations suggest that karst landscapes may be an unaccounted sink for atmospheric CH4, but questions remain about the processes contributing to sub-atmospheric CH4 mole fractions in caves. The CH4 dynamics associated with karst environments were studied over 18months at 6 locations in Buckner Cave, Southern Indiana by measuring the mole fractions and stable isotopic composition of CH4 and carbon dioxide (CO2). CO2 mole fractions were used to infer seasonal changes in airflow. Samples were obtained on a monthly basis. CH4 mole fractions ranged from 1.9±0.1ppm near the cave entrance to 0.1±0.1ppm in the more remote parts of the cave. The carbon and hydrogen stable isotopic compositions of CH4 in the cave ranged from −58.7 to +7 ‰ (VPDB) and −170 to +10 ‰ (VSMOW), respectively. The isotopic data suggest that the CH4 dynamics of Buckner Cave can be described by a seasonally variable mixing system in which atmospheric CH4 enters primarily through the main entrance and is subsequently consumed by methanotrophs. Additionally, at least two smaller CH4 sources are evident when air has been stagnant. The results suggest that subterranean karst cavities are an important sink for atmospheric CH4.


      PubDate: 2016-09-22T23:29:42Z
       
  • The effect of melt composition on mineral-melt partition coefficients: The
           case of beryllium
    • Abstract: Publication date: 28 November 2016
      Source:Chemical Geology, Volume 442
      Author(s): A.D. Burnham, H. St C. O'Neill
      The divalent cation Be2+ is considerably smaller than other divalent cations (Mg2+, Fe2+, Ca2+, et cetera), leading to a strong preference for tetrahedral coordination in minerals. Its thermodynamic properties in silicate melts may accordingly differ from these other divalent cations, potentially distinguishing its mineral/melt partition coefficients. In order to investigate this possibility, the partitioning of Be between silicate melt and forsterite was examined for 16 melt compositions in the systems CaO–MgO–Al2O3–SiO2 at 1400°C with additional experiments to investigate the effect of added Na2O and TiO2, and temperature at 1300°C. The relative activity coefficient of BeO in the melts decreases with increasing CaO and NaO1.5. The results are compared to Mg and Ca partitioning in the same experiments, and to the partitioning of other divalent cations (Ni, Co, Mn) from the literature. While the partition coefficient of the latter correlate positively with the Mg partition coefficient, Be shows only a weak negative correlation. Compared to Ca, Be partitions less strongly into forsterite when the melt has high Na and/or Ca. Partition coefficients for Na, Al and Ti are also reported.


      PubDate: 2016-09-22T23:29:42Z
       
  • pH-dependent control of feldspar dissolution rate by altered surface
           layers
    • Abstract: Publication date: 28 November 2016
      Source:Chemical Geology, Volume 442
      Author(s): Bastien Wild, Damien Daval, François Guyot, Kevin G. Knauss, Marion Pollet-Villard, Gwenaël Imfeld
      Relevant modeling of mass and energy fluxes involved in pedogenesis, sequestration of atmospheric CO2 or geochemical cycling of elements partly relies on kinetic rate laws of mineral dissolution obtained in the laboratory. Deriving an accurate and unified description of mineral dissolution has therefore become a prerequisite of crucial importance. However, the impact of amorphous silica-rich surface layers on the dissolution kinetics of silicate minerals remains poorly understood, and ignored in most reactive transport codes. In the present study, the dissolution of oriented cleavage surfaces and powders of labradorite feldspar was investigated as a function of pH and time at 80°C in batch reactors. Electron microscopy observations confirmed the formation of silica-rich surface layers on all samples. At pH=1.5, the dissolution rate of labradorite remained constant over time. In contrast, at pH=3, both the dissolution rates at the external layer/solution interface and the internal layer/mineral interface dramatically decreased over time. The dissolution rate at the external interface was hardly measurable after 4weeks of reaction, and decreased by an order of magnitude at the internal interface. In another set of experiments conducted in aqueous silica-rich solutions, the stabilization of silica-rich surface layers controlled the dissolution rate of labradorite at pH=3. The reduction of labradorite dissolution rate may result from a gradual modification of the textural properties of the amorphous surface layer at the fluid/mineral interface. The passivation of the main cleavage of labradorite feldspar was consistent with that observed on powders. Overall, our results demonstrate that the nature of the fluid/mineral interface to be considered in the rate limiting step of the process, as well as the properties of the interfacial layer (i.e. its chemical composition, structure and texture) to be taken into account for an accurate determination of the dissolution kinetics may depend on several parameters, such as pH or time. The dramatic impact of the stabilization of surface layers with increasing pH implies that the formation and the role of surface layers on dissolving feldspar minerals should be accounted for in the future.


      PubDate: 2016-09-22T23:29:42Z
       
  • High-strain deformation and fluid infiltration diachronism of the middle
           crust: New Devonian–Permian Alice Springs ages (365–290Ma) of shear
           zones in the Strangways Metamorphic Complex, Central Australia
    • Abstract: Publication date: 2 December 2016
      Source:Chemical Geology, Volume 443
      Author(s): H.W. Fournier, A. Camacho, J.K.W. Lee
      The Proterozoic granulite-facies Strangways Metamorphic Complex (SMC) in the Arunta Region of Central Australia was partially hydrated during the Devonian–Carboniferous (380–320Ma) Alice Springs Orogeny by amphibolite-greenschist-facies high-strain deformation. Stable and radiogenic isotope analyses on coeval muscovite and biotite from shear-zone samples of the SMC were performed to analyse the composition and date the timing of fluid circulation and cooling. Stable isotope data (δ18O and δD) indicate that in the south of the SMC, the source of fluids appear to be the thick successions of Neoproterozoic Heavitree Quartzite, while another source in the north and central part of the SMC may be related to meteoric fluids. The heterogeneous δ18OFluid and homogenous δDFluid values for muscovite reflect changes in water/rock ratios with initial high values in the south and lower in the north, possibly suggesting a migration of fluids from south to north (as indicated by the evolving δ18OFluid and homogenous δDFluid values). Rb/Sr muscovite ages of 365–340Ma approximate the time of crystallization and fluid infiltration. They also indicate that the shear zones were active at different times during the Alice Springs Orogeny and that some were active prior to the peak Carboniferous amphibolite-facies metamorphism. 40Ar/39Ar muscovite ages of 350–325Ma are mostly interpreted as cooling ages. Rb/Sr biotite ages of 335–290Ma are interpreted as cooling ages at ~300°C (Sr closure temperature for biotite) and extend the activity of the Alice Springs Orogeny until the Permian. Cooling histories of the Wallaby Knob and Harry Creek shear zones, which serve as the tectonic boundaries of the SMC to the north and south respectively, indicate slow cooling (<3.5°CMa−1) for c. 30Ma (360–330Ma) at both boundaries continuing until 290Ma in the western portion of the Harry Creek shear zone. This slow cooling is associated with passive exhumation (i.e. erosion). Rapid cooling (20–10°CMa−1) then occurred over c. 10Ma (330–320Ma) in the Wallaby Knob shear zone and in the east portion of the Harry Creek shear zone. This change in cooling rate is associated with the combined initiation in the Carboniferous of dextral and thrusting deformation by the Pinnacles Bore and Erontonga shear zones.


      PubDate: 2016-09-22T23:29:42Z
       
  • Calibration of dolomite clumped isotope thermometry
    • Abstract: Publication date: 2 December 2016
      Source:Chemical Geology, Volume 443
      Author(s): Ian Z. Winkelstern, Stephen E. Kaczmarek, Kyger C Lohmann, John D. Humphrey
      Use of the clumped isotope thermometer to decipher environments of dolomite formation has been inhibited by a lack of empirical data from dolomites formed at known temperatures. Calibrations for aragonite, calcite, siderite, and apatite indicate that a universal Δ47 – temperature relationship may exist across all carbonate-bearing mineral phases. However, theoretical modeling and proposed acid fractionation differences have suggested that dolomites may have different Δ47 values relative to calcites precipitated at the same temperature. To resolve this question, we analyzed five synthetic and four natural dolomites formed at known temperatures. These included synthetic dolomites grown in Mg-Ca-Cl solutions at temperatures of 200–250°C, and natural samples constrained by fluid inclusion analyses (~70°C), climate (~28 and ~27°C), and deep ocean borehole temperature (~21°C). When using calcite Δ47 acid fractionation values, these data result in a calibration line that is statistically indistinguishable from other clumped isotope calibrations. At least with current precision, we find no evidence for a consistent dolomite Δ47 offset. These results further support a universal calibration for carbonate clumped isotope thermometry and enable new investigations into conditions of dolomite formation.


      PubDate: 2016-09-22T23:29:42Z
       
  • Biosignatures and microbial fossils in endolithic microbial communities
           colonizing Ca-sulfate crusts in the Atacama Desert
    • Abstract: Publication date: 2 December 2016
      Source:Chemical Geology, Volume 443
      Author(s): Beatríz Cámara, Virginia Souza-Egipsy, Carmen Ascaso, Octavio Artieda, Asunción De Los Ríos, Jacek Wierzchos
      Since the description of microbial communities colonizing Ca-sulfate crusts in the Atacama Desert, there has been much interest in the mechanisms that could lead to the formation and preservation of biosignatures or microbial fossils of these communities. A key to understanding physico-chemical processes of taphonomy and early diagenesis is to examine microfossils in their natural environment. In this study, we characterize organomineral traces and microbial fossils found around microbial communities present in these Ca-sulfate crusts. Through scanning electron microscopy, microanalytical (EDS) and Raman spectroscopy techniques, calcium carbonate precipitates were detected around remnants of cryptoendolithic algae beneath the crust surface. As what seems to be the final step in the organomineralization of these cryptoendolithic communities, we also observed alga cell remains permineralized by Mg-Si-rich minerals inside gypsum crystals. Additionally, Mg-Si bearing minerals formed a web-like structure within the hypoendolithic cyanobacterial habitat via permineralization of extracellular polymeric substances. Our observations indicate that despite the extremely hyperarid environment, microenvironmental conditions may be appropriate for the formation of biosignatures and microbial fossils of extinct endolithic microbial communities. A model of the possible organomineralization processes involved is presented.
      Graphical abstract image

      PubDate: 2016-09-22T23:29:42Z
       
  • Dissolved inert gases (He, Ne and N2) as markers of groundwater flow and
           degassing areas at Mt Etna volcano (Italy)
    • Abstract: Publication date: 2 December 2016
      Source:Chemical Geology, Volume 443
      Author(s): A. Paonita, M. Longo, S. Bellomo, W. D'Alessandro, L. Brusca
      Fractions of the volatiles ascending from magma chambers meet groundwaters flowing away from the volcano summit and are carried to great distance as dissolved gases. The complex interactions between ascending magmatic volatiles, tectonic structures, heterogeneities in rock permeability and flow lines of aquifers deeply affect the dispersion of the dissolved species. Studying the spatial distribution of such species can therefore provide valuable information on the circulation of fluids inside volcanic edifices. Our study focussed on the composition of dissolved inert gases (He, Ne and N2) and He isotope ratio (3He/4He) in groundwaters circulating at Mt Etna volcano (Italy), because the concentrations of these species differ markedly between magmatic and shallow (crustal and atmospheric) sources, and they do not interact chemically with rocks. We identified groundwaters that flow through anomalously degassing areas associated with clearly evident or known tectonic structures. These waters show a typically magmatic He isotope composition (high 3He/4He ratios) and high proportions of dissolved magmatic gases (He and CO2) compared to the atmospheric ones (Ne and N2). Downstream of the degassing structures, along the hydrological outflows, we found groundwaters that are progressively enriched in atmospheric-derived gases (Ne and N2) and exhibited lower 3He/4He ratios. On this basis, we set up a model of unidimensional dispersion-advection of inert volatile solutes, coupled with a two-layer model for the dynamic exchange of volatiles through the aquifer–atmosphere interface. The model is able to quantitatively explain the progressive dilution of the magmatic signal over distances of several kilometres from the source location of the anomaly towards the final part of the flow lines at the coast. Typical hydrogeological parameters such as the flow velocity, rock permeability and rate of air–groundwater interaction can be constrained, and underground pathways of waters can be identified. Waters that are anomalously rich in magmatic tracers with respect to their peripheral position along the flow lines reflect arrival of deep gases from below, and they therefore offer a powerful tool for revealing hidden tectonic structures.


      PubDate: 2016-09-22T23:29:42Z
       
  • A synthetic fluid inclusion study of the solubility of monazite-(La) and
           xenotime-(Y) in H2O-Na-K-Cl-F-CO2 fluids at 800°C and 0.5GPa
    • Abstract: Publication date: 28 November 2016
      Source:Chemical Geology, Volume 442
      Author(s): Li Zhou, John Mavrogenes, Carl Spandler, Heping Li
      Alkali-rich F-Cl-CO2-bearing aqueous fluids are thought to be common ore-forming fluids of carbonatite-associated rare earth element (REE) deposits. Solubilities of synthetic monazite-(La) and xenotime-(Y) end-members in such fluids were determined at 800°C and 0.5GPa in a series of piston-cylinder experiments. At run conditions, solutions of NaCl/KCl (1.9mol/kg)±CO2 (5% mole), NaCl/KCl (1.7mol/kg)±NaF/KF (0.2mol/kg) were trapped as synthetic fluid inclusions in quartz, and subsequently analyzed by laser ablation inductively coupled plasma mass spectrometry (LA-ICPMS). The resultant concentrations in these fluids are extremely low, ranging from 2.92×10−6 to 3.05×10−5 molLa/kg H2O and 3.97×10−6 to 2.74×10−5 molY/kg H2O. Such low solubilities indicate that fluids expelled from carbonatite melts contain insufficient phosphate and REE simultaneously to form REE deposits. Mixing REE-bearing fluids with phosphate-bearing aqueous fluids or interacting with apatite in the wall rock may be a key control on the deposition of REE phosphate minerals. In addition, our results show that, at nearly equal total ligand molality, solubilities of La and Y are both higher in F or CO2-bearing solutions than in Cl-only solutions, and the highest solubilities obtained overall were in NaCl+NaF solutions, which significantly increase by a factor of 9.4 for La and 6.1 for Y relative to NaCl-only solutions. Moreover, the solubility differences between xenotime and monazite in F/CO2-bearing solutions, shown as Y/La (molality ratio), are higher in K-bearing solutions than in Na-bearing solutions. Y/La in the NaCl+F/CO2 solutions are close to 1, whereas in the KCl+F/CO2 solutions Y/La increase to 2.9 and 2.0, respectively. These results indicate that during the evolution of carbonatitic fluids, enrichment of Na and F can significantly increase solubilities of all REE, whilst enrichment of K may promote the fractionation of HREE from LREE in the presence of F or CO2.


      PubDate: 2016-09-18T19:55:54Z
       
  • Surface reactivity of the anaerobic phototrophic Fe(II)-oxidizing
           bacterium Rhodovulum iodosum: Implications for trace metal budgets in
           ancient oceans and banded iron formations
    • Abstract: Publication date: 28 November 2016
      Source:Chemical Geology, Volume 442
      Author(s): Raul E. Martinez, Kurt O. Konhauser, Nataliya Paunova, Wenfang Wu, Daniel S. Alessi, Andreas Kappler
      It is widely accepted that anoxygenic photosynthetic bacteria (photoferrotrophs) oxidized dissolved Fe(II) in anoxic Archean seawater, leading to the sedimentation of cellular biomass and the Fe(III) minerals that comprised banded iron formations (BIF). However, it remains unknown to which extent the sedimented bacteria may have transported trace metals to BIF through their ability to sorb ions from seawater. We therefore chose the marine anoxygenic photoferrotroph Rhodovulum iodosum to quantify Cd2+, Co2+, Cu2+, Mn2+, Ni2+ and Zn2+ adsorption to bacteria/Fe(III)-oxyhydroxide composites at neutral pH and an ionic strength of 0.1M. Acid base titration data were modeled using a linear programming optimization method to yield pKa values of 4.83±0.13, 6.21±0.18, 7.74±0.24 and 9.28±0.27 and corresponding site densities of 5.7, 12.0, 3.3 and 6.5×10−4 mol/g, respectively. FTIR spectroscopy confirmed the presence of carboxyl groups as the most acidic sites corresponding to the lowest pKa values. The analysis of Cd2+ sorption data showed two metal complexing sites with pKS values of 3.44±0.14 and 4.80±0.21 and corresponding binding site concentrations of 1.1±0.3×10−4 and 0.9±0.2×10−4 mol/g on the ferrihydrite/R. iodosum composite. The pKS values were used to calculate metal-ligand binding constants, Km. This confirmed Cd2+ binding to the most acidic carboxyl groups on the ferrihydrite/R. iodosum composite surface, as the calculated Km was consistent with reported Cd2+ binding constants for simple organic acids (e.g., lactic, acetic, pyruvic and citric acid). This was used to further calculate the concentration of Co2+, Cu2+, Mn2+, Ni2+ and Zn2+ sorbed to ferrihydrite/R. iodosum composites. The resulting concentrations were ultimately compared to the concentrations of trace elements in BIF to demonstrate that is plausible that the trace metal content in BIF was derived from plankton growing in the ancient ocean's photic zone.


      PubDate: 2016-09-18T19:55:54Z
       
  • Experimental study on the kinetics of silica polymerization during cooling
           of the Bouillante geothermal fluid (Guadeloupe, French West Indies)
    • Abstract: Publication date: 28 November 2016
      Source:Chemical Geology, Volume 442
      Author(s): Christelle Dixit, Marie-Lise Bernard, Bernard Sanjuan, Laurent André, Sarra Gaspard
      Despite many studies, our understanding of silica precipitation from natural waters remains limited, in particular for geothermal waters. Here we present a detailed study on the kinetics of silica polymerization as a function of fluid temperature and pH using high-temperature (250-260°C) seawater-derived geothermal fluids as those discharged from the Bouillante geothermal site. We monitored the on-site decrease in monomeric silica concentration (initial SiO2 concentration of about 600mgl−1) with time using the molybdenum blue spectrophotometric method on samples of separated water collected from the high-pressure separator at 167°C and cooled to 25, 50, 75, and 90°C and for pH values ranging from 5 to 8. During all these experiments, only silica polymerization was observed, with the formation of colloidal particles in suspension in the solutions. No scaling of amorphous silica was formed. The collected data were after modeled in order to determine the useful kinetic parameters for predicting and preventing amorphous silica precipitation in the production wells during fluid exploitation in the specific context of Bouillante. Results under the investigated experimental conditions show that the kinetics of silica precipitation is affected more strongly by pH than by temperature change. The reaction in the acidic Bouillante solution begins with a transition period that significantly decreases the kinetics of silica polymerization. Modeling the experimental data indicates that the silica polymerization up to the state of equilibrium is characterized by a 2nd-order kinetic law relative to dissolved silica; this could indicate that the polymerization is controlled mainly by the formation of dimers. However, the first hour of the experiments is better characterized by a 4th-order kinetic law, suggesting more complex polymerization reactions in the initial stage with the formation of nanocolloidal particles containing 3 to 4 monomers. In both cases, the corresponding kinetics rate constant k is linearly dependent on pH for pH between 5 and 7. The activation energy E a for the overall reaction, calculated using the Arrhenius equation under the considered pH and temperature conditions, ranges between 41±9.8 and 54±9.6kJmol−1. To complete this work, the colloid particles formed at the end of the kinetics experiments were extracted and analyzed by SEM and TEM microscopy, X-ray Diffraction and the BET method. Results show that the size of the colloids increase and their specific surface decrease with increasing pH and are thus dependent on pH. Globally, our work provides a reliable database for understanding silica polymerization kinetics in natural geothermal brines or geologic waters characterized by a near neutral pH and moderate dissolved silica concentrations.


      PubDate: 2016-09-18T19:55:54Z
       
  • Absolute isotopic abundance ratios and the accuracy of Δ47
           measurements
    • Abstract: Publication date: 28 November 2016
      Source:Chemical Geology, Volume 442
      Author(s): M. Daëron, D. Blamart, M. Peral, H.P. Affek
      Clumped isotope measurements aim to quantify some statistical properties of the isotopologue population in a given sample, which requires prior knowledge of the absolute isotopic abundance ratios in reference materials such as VSMOW or VPDB. In the case of CO2, matters are further complicated by the need to define a mass-dependent fractionation law linking the three stable isotopes of oxygen. Conversion from raw mass spectrometric data to mass-47 anomalies (Δ47) thus relies on four external parameters: the (13C/12C) ratio of VPDB, the (17O/16O) and (18O/16O) ratios of VSMOW (or VPDB-CO2), and the slope of the triple oxygen isotope line (λ). Here we investigate the influence that these isotopic parameters exert on measured Δ47 values, using (a) real-world data corresponding to seven months of measurements; (b) simple simulations based on randomly generated data; (c) precise comparisons between water-equilibrated CO2 samples and between carbonate standards believed to share quasi-identical Δ47 values; and (d) reprocessing of two carbonate calibration data sets with different slopes of Δ47 versus T. We demonstrate that the use of different sets of isotopic parameters generally produces systematic offsets as large as 0.04‰ in measured Δ47 values, even after following the established standardization procedures. What's more, even using a single set of isotopic parameters can produce intra- and inter-laboratory discrepancies in measured Δ47 values, if some of these parameters are inaccurate, and depending on the isotopic compositions of the standards used for conversion to the absolute scale of Dennis et al. (2011), these errors should correlate strongly with either δ13C or δ18O, or more weakly with both. Based on measurements of samples expected to display quasi-identical Δ47 values, such as 25°C water-equilibrated CO2 with different carbon and oxygen isotope compositions, or high-temperature standards ETH-1 and ETH-2, we conclude that the traditional set of isotopic parameters used in all early clumped isotope studies produces large, systematic errors controlled by the relative bulk isotopic compositions of samples and standards. These errors are likely to be one of the key factors responsible for current inter-laboratory discrepancies, but cannot easily explain the conflicting carbonate calibration slopes obtained by different groups. By contrast, the isotopic parameters of Brand et al. (2010) appear to yield accurate Δ47 values regardless of bulk isotopic composition. Based on these findings, we offer recommendations aiming to minimize errors related to the choice of isotopic parameters.


      PubDate: 2016-09-18T19:55:54Z
       
  • Viscosity of pantelleritic and alkali-silicate melts: Effect of Fe redox
           state and Na/(Na+K) ratio
    • Abstract: Publication date: 28 November 2016
      Source:Chemical Geology, Volume 442
      Author(s): Paola Stabile, Sharon Webb, Jaayke L. Knipping, Harald Behrens, Eleonora Paris, Gabriele Giuli
      The viscosity of two series of synthetic alkali silicates, corresponding to Al-bearing pantelleritic and Al-free tri-silicate compositions, has been investigated as a function of temperature, iron redox and Na/(Na+K) ratio. Low temperature (708–1000K) viscosities were determined by the micropenetration technique in the 109.6 to 1013.6 Pas range. The effect of Fe2+/Fetot, from 0.15 to 0.86, was explored for [Na/(Na+K)] ratios from 0 to 1. The results demonstrate a strong decrease of viscosity with the replacement of Na for K in both the Al-bearing and Al-free compositions. In the Al-bearing pantelleritic glasses (Ebu) the viscosity as well as the activation energy for viscous flow decrease with an increase of Fe2+/Fetot from 0.15 to 0.76. In contrast, no measurable changes in viscosity occur in the Al-free tri-silicate glasses (NFS and KFS) as Fe2+/Fetot ratio varies from 0.15 to 0.86. The comparison of the pantelleritic glasses with peralkaline compositions from the literature stresses the strong influence of iron redox on the overall viscosity of such melts. This suggests that the contribution of iron species should be accounted for in the calculation of agpaitic index (AI) if magma Fe3+ contents are known. One peculiarity of peralkaline magmas is the excess of alkali to alumina, which generally results in lower viscosities of pantelleritic liquids compared with more common metaluminous rhyolites and exerts a primary control of the rheological behaviour of these melts. Our new viscosity data, combined with existing literature, allow more accurate constraints on the nature and eruption of the pantelleritic magmas. Moreover, the present experimental study can be used to improve the viscosity models that mostly do not take in consideration parameters as iron redox in the viscosity prediction.


      PubDate: 2016-09-18T19:55:54Z
       
  • Editorial Board
    • Abstract: Publication date: 21 November 2016
      Source:Chemical Geology, Volume 441




      PubDate: 2016-09-18T19:55:54Z
       
  • Isotopic behavior during the aragonite-calcite transition: Implications
           for sample preparation and proxy interpretation
    • Abstract: Publication date: 28 November 2016
      Source:Chemical Geology, Volume 442
      Author(s): Philip T. Staudigel, Peter K. Swart
      At the Earth's surface, aragonite, a polymorph of calcium carbonate, is thermodynamically unstable, and readily converts to calcite, a process accelerated by heating and mechanical stress. This study has investigated the behavior of the 13C/12C, 18O/16O ratios, and the clumped isotope temperature proxy (∆47) during this mineral transition. Aragonite samples were partially or completely converted to calcite in an oven (normal atmospheric composition and pressure) or in a sealed tube in the presence of a vacuum or pure CO2 of varying isotopic compositions. Other samples were converted from aragonite to calcite using a drill. These experiments demonstrate a relationship between the δ13C and δ18O values and percent transition from aragonite to calcite. The ∆47 value of the aragonite changed independently from the mineral transition at temperatures as low as 125°C and was accompanied by a 0.5‰ decrease in the δ18O value. This change occurred irrespective of the presence of CO2 suggesting that water trapped in the mineral may facilitate some of this alteration. Results show that sampling techniques, such as drilling, produce sufficient heat to convert aragonite to calcite and can considerably alter ∆47. Even small degrees of conversion (~10%) can increase clumped isotope derived paleotemperature estimates by nearly 10°C.


      PubDate: 2016-09-18T19:55:54Z
       
  • Early Jurassic carbon-isotope excursion in the Qiangtang Basin (Tibet),
           the eastern Tethys: Implications for the Toarcian Oceanic anoxic event
    • Abstract: Publication date: 28 November 2016
      Source:Chemical Geology, Volume 442
      Author(s): Xiugen Fu, Jian Wang, Xinglei Feng, Dong Wang, Wenbin Chen, Chunyan Song, Shengqiang Zeng
      The Early Jurassic was characterized by a global disturbance of the carbon cycle known as the Toarcian oceanic anoxic event (T-OAE). This event is recorded by a large negative carbon-isotope excursion (CIE) in marine carbonates and marine organic matter, as well as fossil wood. Here, we present new high-resolution carbonate (δ13Ccarb) and organic (δ13Corg) carbon-isotope records and total organic content from the Bilong Co section in the Qiangtang Basin, the eastern Tethys. This section is high in organic carbon, and black shale facies are also identified. The Bilong Co section shows heavy δ13Corg values compared to other reported marine organic carbon-isotope values for the T-OAE. This is explained by variable mixing of terrestrial and marine organic matter. Compared to European sections, the magnitude of the negative CIE in the Bilong Co section is slightly different, which could be explained by different local paleoenvironmental conditions and strata missing. Although these, the carbon-isotope profile from the Bilong Co area displays a similar trend to those in contemporaneous strata in the western Tethyan and Boreal realms of NW Europe, which is characterized by a distinct negative excursion in the Early Toarcian. Our new data from the Bilong Co section in N Tibet provide the first high-resolution record of the Early Toarcian Oceanic Anoxic Event in the eastern Tethys.


      PubDate: 2016-09-18T19:55:54Z
       
  • Three whole-wood isotopic reference materials, USGS54, USGS55, and USGS56,
           for δ2H, δ18O, δ13C, and δ15N measurements
    • Abstract: Publication date: 28 November 2016
      Source:Chemical Geology, Volume 442
      Author(s): Haiping Qi, Tyler B. Coplen, James A. Jordan
      Comparative measurements of stable hydrogen and oxygen isotopes in wood are hampered by the lack of proper reference materials (RMs). The U.S. Geological Survey (USGS) has prepared three powdered, whole-wood RMs, USGS54 (Pinus contorta, Canadian lodgepole pine), USGS55 (Cordia cf. dodecandra, Mexican ziricote), and USGS56 (Berchemia cf. zeyheri, South African red ivorywood). The stable isotopes of hydrogen, oxygen, carbon, and nitrogen in these RMs span ranges as δ 2HVSMOW from –150.4 to –28.2mUr or ‰, as δ 18OVSMOW from +17.79 to +27.23mUr, as δ 13CVPDB from –27.13 to –24.34mUr, and as δ 15N AIR-N2 from –2.42 to +1.8mUr. These RMs will enable users to normalize measurements of wood samples to isotope–delta scales, and they are intended primarily for the normalization of δ 2H and δ 18O measurements of unknown wood samples. However, they also are suitable for normalization of stable isotope measurements of carbon and nitrogen in wood samples. In addition, these RMs are suitable for inter-laboratory calibration for the dual-water suilibration procedure for the measurements of δ 2HVSMOW values of non-exchangeable hydrogen. The isotopic compositions with 1-σ uncertainties, mass fractions of each element, and fractions of exchangeable hydrogen of these materials are: USGS54 (Pinus contorta, Canadian Lodgepole pine) δ 2HVSMOW =–150.4±1.1mUr (n=29), hydrogen mass fraction=6.00±0.04 % (n=10) Fraction of exchangeable hydrogen=5.4±0.6 % (n=29) δ 18OVSMOW =+17.79±0.15mUr (n=18), oxygen mass fraction=40.4±0.2 % (n=6) δ 13CVPDB =–24.43±0.02mUr (n=18), carbon mass fraction=48.3±0.4 % (n=12) δ 15NAIR- N2 =–2.42±0.32mUr (n=17), nitrogen mass fraction=0.05 % (n=4) USGS55 (Cordia cf. dodecandra, Mexican ziricote) δ 2HVSMOW =–28.2±1.7mUr (n=30), hydrogen mass fraction=5.65±0.06 % (n=10) Fraction of exchangeable hydrogen=4.1±0.5 % (n=30) δ 18OVSMOW =+19.12±0.07mUr (n=18), oxygen mass fraction=35.3±0.2 % (n=6) δ 13CVPDB =–27.13± 0.02mUr (n=18), carbon mass fraction=53.3±0.6 % (n=12) δ 15NAIR-N2
      PubDate: 2016-09-11T05:59:54Z
       
  • Rare earth elements concentrations and speciation in rainwater from
           Guiyang, an acid rain impacted zone of Southwest China
    • Abstract: Publication date: 28 November 2016
      Source:Chemical Geology, Volume 442
      Author(s): Zhaozhou Zhu, Cong-Qiang Liu, Zhong-Liang Wang, Xiaolong Liu, Jun Li
      Rare earth elements (REEs) and major ions were determined in rainwater samples collected from Guiyang in the acid rain impacted zone of Southwest China. The concentrations of total dissolved and acid-soluble REEs in rainwater are higher than those of most of the world's cities as well as the local surface water. The dissolved REE concentrations are negatively correlated to pH in rainwater. This is consistent with acidification of rainwater resulting in increasing REE concentrations. Speciation calculations using the PHREEQC-Model predict that the free REE metal ion (i.e., Ln3+), sulfate (LnSO4 +), oxalate (LnOx+ and LnOx2 −) and fluoride (LnF2+) complexes were relatively important forms of dissolved REE. Although the contribution to rainwater acidity of oxalic acid is much lower than that of sulfuric acid, the proportion of REE-oxalate complexes is not lower than that of REE-sulfate complexes in rain waters. The rainwater is enriched in the middle REEs (MREE) compared to both the light REEs (LREE) and heavy REEs (HREE). REE-phosphate complexes, phosphatic minerals and a Fe-Mn-oxyhydroxide coating are more likely controls on the development of MREE enrichment in the rainwater. Complexation of sulfate and REE played little role in the development of MREE enrichment, even though sulfate is the most important anion in acidic rainwater. In the shale-normalized REE pattern plot, La showed a clear positive anomaly. La might have two main anthropogenic sources in rainwater. One of the important sources could be automobile emission; another main source of excess La in rainwater could be the REE fertilizer that has been widely used in the agriculture of China for approximately 30years.


      PubDate: 2016-09-11T05:59:54Z
       
  • Celestine in a sulfidic spring barite deposit — A potential
           biomarker?
    • Abstract: Publication date: 28 November 2016
      Source:Chemical Geology, Volume 442
      Author(s): David M. Singer, Elizabeth M. Griffith, John M. Senko, Kaci Fitzgibbon, Inoka H. Widanagamage
      We have documented the presence of celestine (SrSO4) within sediment accumulating at an artesian sulfidic spring (Zodletone Spring, Oklahoma) dominated by barite (BaSO4) precipitation associated with microbiological activity. The distribution and speciation of Sr in solid phases was determined by synchrotron-based micro-X-ray fluorescence spectroscopy and micro-X-ray diffraction, and particle morphology and texture was determined using electron microscopy. In all the natural sulfidic spring samples and lithified tufa sample, celestine was detected in fine-grained micron-scale Sr-rich phases but not in euhedral, Sr-poor grains. In parallel laboratory-based precipitation experiments, celestine was not observed even when solutions contained high Sr/Ba. Thermodynamic predictions alone do not account for the presence of micron-scale celestine in the sulfidic spring, and they do not account for the differences in Sr presence and distribution in naturally-occurring versus synthetic grains. While the mechanism is unclear, based on this evidence we hypothesize that the combination of bacterial surfaces and microenvironments within the crusts and microbial mats creates a synergistic effect where Sr is preferentially exchanged over Ba between the overlying stream water and the pore water within the mats allowing celestine to precipitate. Ultimately, our results point to an important role of biological activity for preferential Sr uptake. The presence of micron-scale celestine in ancient barite deposits can therefore potentially be used as a biomarker for conditions similar to modern sulfidic springs.
      Graphical abstract image

      PubDate: 2016-09-11T05:59:54Z
       
  • Geochemistry of Campanian–Maastrichtian brachiopods from the Rørdal-1
           core (Denmark): Differential responses to environmental change and
           diagenesis
    • Abstract: Publication date: 28 November 2016
      Source:Chemical Geology, Volume 442
      Author(s): R. Harlou, C.V. Ullmann, C. Korte, B.W. Lauridsen, N.H. Schovsbo, F. Surlyk, N. Thibault, L. Stemmerik
      Past marine environmental conditions can be reconstructed from geochemical signatures of both benthic and planktic organisms and particularly brachiopods have been extensively used for this purpose. However, it is seldom possible to investigate the links between environmental conditions and shell geochemistry in monospecific records over extended periods of time. Here, we present geochemical data from the ~100m thick, brachiopod-rich upper Campanian – upper Maastrichtian chalk (~73.8–68.6Ma) of the Rørdal-1 drill core (northern Denmark). Two species that lived attached to small substrate (Argyrotheca bronnii Roemer and Gisilina jasmundi Steinich) and one secondarily free-living species (Magas chitoniformis Schlotheim) have been studied for element concentrations as well as C and O isotope signatures. Each species has a unique chemical signature, with M. chitoniformis showing lowest Mg and Mn concentrations, A. bronnii the highest Mn concentrations and G. jasmundi the highest Sr concentrations. All three species display decreasing Mg/Ca ratios throughout the studied interval and distinct Mn enrichments in the Campanian–Maastrichtian boundary (CMB) interval. In the Campanian part of the succession, the three species are slightly enriched in 13C (median Δ13C values of +0.0 to +0.5‰) and 18O (median Δ18O values of +0.3 to +0.6‰) with respect to coeval chalk. In the course of the Maastrichtian, the two attached species acquire progressively lighter isotopic compositions with A. bronnii reaching δ13C and δ18O values >1.0‰ lower than coeval chalk. The secondarily free-living species M. chitoniformis, on the other hand, becomes isotopically heavier and reaches δ13C and δ18O enrichments of ~1.0‰ at the top of the core. The observed differences between the species illustrate species-specific signatures of biomineralisation, diagenesis and response to environmental change. This geochemical complexity illustrates that a comprehensive geochemistry-based picture of the Late Cretaceous Chalk Sea can only be painted using detailed multi-species and multi-proxy datasets.


      PubDate: 2016-09-11T05:59:54Z
       
  • 186Os/188Os variations in upper mantle peridotites: Constraints on the
           Pt/Os ratio of primitive upper mantle, and implications for late veneer
           accretion and mantle mixing timescales
    • Abstract: Publication date: 28 November 2016
      Source:Chemical Geology, Volume 442
      Author(s): Rudra Chatterjee, John C. Lassiter
      186Os/188Os variations in mantle peridotites provide constraints on the long-term Pt/Os evolution of the depleted mantle and the Pt/Os ratio of the primitive upper mantle (PUM). We report new 186Os/188Os data for mantle peridotites from continental (Rio Grande Rift and Colorado Plateau) and oceanic (Lena Trough and Hawaiian Islands) settings that span a wide range in fertility (Al2O3 ≈0.67-4.42 wt.%) and 187Os/188Os ratios (0.1138-0.1305). Although peridotite 186Os/188Os values span only a narrow range (from 0.1198345 to 0.1198384), 186Os/188Os broadly correlates with indices of melt depletion including bulk rock Al2O3, spinel Cr#, and clinopyroxene Cr#, consistent with Pt depletion in residual peridotites. PUM 186Os/188Os is estimated to be 0.1198378±23 (2 SD) based on extrapolation of 186Os/188Os-fertility trends, which is very slightly lower than H-chondrites [≈0.1198398±16 (2 SD); Brandon et al., 2006]. This value is consistent with a PUM Pt/Os of 1.7±0.2, similar to average Pt/Os ratios of fertile continental peridotites. The inferred PUM Pt/Os is slightly lower than but within error of Pt/Os values measured in several classes of chondrites [Carbonaceous ≈1.8±0.2, Ordinary ≈1.9±0.1, and Enstatite ≈1.9±0.1 (Brandon et al., 2006)] indicating that PUM Pt/Os is broadly chondritic. In contrast, estimates for PUM Ru/Ir and Pd/Ir (cf. Becker et al., 2006) are suprachondritic. The addition of a chondritic late veneer alone cannot create a combination of chondritic and suprachondritic HSE ratios for the PUM. Instead, minor core segregation occurring concurrently with the addition of a late veneer may explain the observed mantle HSE abundances and ratios. Combined 186Os/188Os-187Os/188Os isotopic and Pt/Os and Re/Os variability in peridotites suggest an average mantle homogenization timescale of ~1.2 Ga. In contrast, combined Hf-Nd isotopic and Lu/Hf and Sm/Nd variability in peridotites indicate much shorter homogenization timescales (<0.4 Ga), potentially reflecting enhanced homogenization by melt-rock interaction to which the Pt-Os and Re-Os systems are relatively immune. The mechanical mixing timescale inferred from Os isotopes is consistent with timescales predicted for whole mantle convection.


      PubDate: 2016-09-11T05:59:54Z
       
  • Elemental and isotopic composition of surface soils from key Saharan dust
           sources
    • Abstract: Publication date: 28 November 2016
      Source:Chemical Geology, Volume 442
      Author(s): A. Gross, D. Palchan, M.D. Krom, A. Angert
      Saharan dust contains significant amount of P, an important macronutrient to all living organisms, which has been shown to exert large effects on nearby and remote ecosystems located across the dust transport pathways. The biological effect of Saharan dust depends on the amount and nature of the P speciation of the dust. However, thus far relatively small numbers of samples from potential source areas (PSA) has been analyzed. Here we report the P speciation (resin-P, HCl-P, Fe-bound-P and organic-P), the δ18OP values, the elemental composition, and the 87Sr/86Sr and 143Nd/144Nd of the fine fraction and bulk soil from 5 important PSAs across Northern Africa. We found the HCl-P concentrations between different source areas were relatively constrained but that these concentrations were higher in the fine fraction, which here is used a surrogate for dust. The δ18OP values for soils from sand dunes varied from 15.0 to 21.4‰, which is in the range of phosphate minerals from sedimentary origin. The δ18OP values of soils from dry lakes were significantly higher (24.0–28.5‰), probably since their P is derived from fossilized plankton that lived in the lake as it dried up. The 87Sr/86Sr and εNd values ranged from 0.7219 to 0.7276 and −12.7 to −14.0 in eastern samples and from 0.7146 to 0.7185 and −11.9 to −13.4 in western samples, suggesting a different source for the siliciclastic material of eastern and western samples. Our analysis indicates that the δ18OP values are decoupled from the Sr and Nd isotopic systems. Together, the new chemical and isotope data are specific for different PSAs and thus are used for source apportionment purposes. Such data can be used to provide more accurate estimates of the flux of potentially bioavailable P to marine and terrestrial ecosystems. These estimates can be used in global climate models to determine the magnitude and distribution of P control on carbon uptake.


      PubDate: 2016-09-11T05:59:54Z
       
  • Mantle depletion and metasomatism recorded in orthopyroxene in highly
           depleted peridotites
    • Abstract: Publication date: 21 November 2016
      Source:Chemical Geology, Volume 441
      Author(s): James M. Scott, Jingao Liu, D. Graham Pearson, Tod E. Waight
      Although trace element concentrations in clinopyroxene serve as a useful tool for assessing the depletion and enrichment history of mantle peridotites, this is not applicable for peridotites in which the clinopyroxene component has been consumed (~25% partial melting). Orthopyroxene persists in mantle residues until ~40% melting and it is therefore this mineral that offers petrological insights into the evolution of refractory peridotites. Major and trace element concentrations in orthopyroxene±clinopyroxene from two spinel facies harzburgitic xenolith suites from New Zealand are examined. Samples from Cape L'Evique (CLEV) on Chatham Island contain traces of clinopyroxene (<2 modal %) but a suite from Lake Moana (MOA) in the South Island is devoid of this mineral. When compared with modelled orthopyroxene trace element budgets, which are constructed from a review of published source modes, melting modes and element/melt partition co-efficients, the measured orthopyroxene rare earth element data in both suites generally indicate minimums of 25–30% partial melting. These results are consistent with co-existing elevated Mg# in olivine (mostly 91.4 to 93.0) and orthopyroxene (mostly 91.3 to 93.6), high spinel Cr# (commonly >45) and low orthopyroxene Al2O3 (generally <3.1wt%). However, comparison of modelled and measured orthopyroxene compositions shows that all samples, even the most refractory, have undergone metasomatism by small volume light rare earth element-bearing agents. Measured orthopyroxene Ti concentrations show that the metasomatic agent that affected the CLEV suite carried Ti, but that the MOA suite metasomatiser was Ti-poor. Orthopyroxene trace elements in the inspected rocks are therefore partly decoupled from the major element abundances, with the results demonstrating that even highly refractory peridotites can record evidence for mantle metasomatism.


      PubDate: 2016-09-11T05:59:54Z
       
  • Editorial Board
    • Abstract: Publication date: 15 November 2016
      Source:Chemical Geology, Volume 440




      PubDate: 2016-09-11T05:59:54Z
       
  • Definition of new trace-metal proxies for the controls on organic matter
           enrichment in marine sediments based on Mn, Co, Mo and Cd concentrations
    • Abstract: Publication date: 21 November 2016
      Source:Chemical Geology, Volume 441
      Author(s): Tim Sweere, Sander van den Boorn, Alexander J. Dickson, Gert-Jan Reichart
      Trace metal enrichments in sedimentary deposits are of prime interest because they are governed by processes that also control the production and preservation of organic matter. Consequently, trace metals have been used in reconstructions of the (palaeo)depositional environment of organic-rich deposits, but most of these studies have primarily focused on hydrographically restricted basins and the response of trace metals to changing redox conditions whereas the role of trace metals in the nutrient cycle and primary productivity in upwelling settings remains relatively unexplored. In this study we present a comprehensive database of published trace metal concentration data in modern organic-rich deposits from a variety of marine settings. Scrutiny of the compiled dataset has resulted in the development of novel trace metal based proxies that allow the distinction between two marine end-member depositional settings that are associated with enhanced organic carbon burial: open marine settings on the continental margin associated with upwelling (e.g. Namibian Margin) and hydrographically restricted marginal marine basins (e.g. Black Sea). It is shown that high Cd/Mo values are typical for sediments from continental margin upwelling settings whereas Co and Mn concentrations (expressed as Co∗Mn values) are high in samples from hydrographically restricted marine basins. The Cd/Mo ratios are thought to track the relative importance of productivity versus preservation with high values in productivity driven systems attributed to the transfer of Cd to the sediments from re-mineralised plankton biomass. Co∗Mn values, on the other hand, are believed to reflect the supply and reactive behaviour of Co and Mn and can be used to assess circulation patterns in the water column and the relative contribution of deep versus surface/river water influx to the basin. It is demonstrated that the combined use of the Cd/Mo and Co∗Mn proxies provides a highly effective way to distinguish modern/recent marine sedimentary environments, which holds promise for its use in palaeo-environmental reconstructions.
      Graphical abstract image

      PubDate: 2016-09-01T17:18:50Z
       
  • Contrasted effect of aluminum on the serpentinization rate of olivine and
           orthopyroxene under hydrothermal conditions
    • Abstract: Publication date: 21 November 2016
      Source:Chemical Geology, Volume 441
      Author(s): Maria Pens, Muriel Andreani, Isabelle Daniel, Jean-Phillipe Perrillat, Herve Cardon
      Olivine and pyroxene are the major minerals of ultramafic rocks. The hydrothermal alteration of these rocks leads to the serpentinization reaction that mainly forms serpentine and variable amounts of talc, brucite and magnetite, as well as hydrogen. The serpentinization kinetics of pyroxene under hydrothermal conditions has been very little studied in comparison with olivine, and both have been evaluated experimentally only in simple aqueous fluids. Here, we evaluate the effect of aluminum on the serpentinization rate of olivine and orthopyroxene at 200°C, 340°C and 200MPa to simulate natural hydrothermal conditions. We used low-pressure diamond-anvil cells (lp-DAC) and time-resolved X-ray diffraction to monitor in situ the progress of the serpentinization reaction in four experiments. We also performed two long-lasting additional experiments with orthopyroxene for six days at 340°C and 200MPa, for which in situ monitoring was not possible. At 340°C in presence of Al, olivine conversion into lizardite is extremely fast (half-time reaction t1/2 =7h) while orthopyroxene did not react much even after 6days (11%). In contrast to olivine, orthopyroxene conversion to serpentine was faster without Al (48% in 6days). Magnetite was also observed in the run with olivine only at 340°C. In experiments run with orthopyroxene only, we observed the exclusive formation of proto-serpentine instead of lizardite. We propose that the contrasted effect of Al on the serpentinization rate of olivine and orthopyroxene results from the complexation of Al in the solution that reacts differently with the different mineral surfaces during their dissolution. The positively charged olivine surface allows the adsorption of the dominant negatively charged Al(OH)4 − complex, while the neutral surface of orthopyroxene does not. This adsorption process could facilitate both the dissolution of olivine and the nucleation-growth of an Al-enriched lizardite.


      PubDate: 2016-09-01T17:18:50Z
       
  • Using elemental and boron isotopic compositions of tourmaline to trace
           fluid evolutions of IOCG systems: The worldclass Dahongshan FeCu deposit
           in SW China
    • Abstract: Publication date: 21 November 2016
      Source:Chemical Geology, Volume 441
      Author(s): Zhi-Kun Su, Xin-Fu Zhao, Xiao-Chun Li, Mei-Fu Zhou
      The worldclass Dahongshan iron-oxide copper gold (IOCG) deposit is hosted within the late Paleoproterozoic Dahongshan Group in the Kangdian region, SW China. The orebodies are spatially associated with sub-volcanic intrusions and are structurally controlled. Extensive metamorphic overprinting during the Neoproterozoic makes it difficult to determine the initial compositions of the ore fluids. Tourmaline, however, is a useful phase for this purpose because of its refractory nature and chemical diversity. It occurs widely in the four stages of mineralization and alteration of the Dahongshan deposit, including pre-mineralization sodic alteration (stage I), iron-oxide mineralization (stage II), sulfide mineralization (stage III), and late quartz-calcite veining (stage IV). The temporal association between tourmaline and hydrothermal alteration provides an excellent opportunity to investigate the fluid evolution. Tourmaline from all four stages belongs to the alkali group and ranges from dravite to schorl in composition. In-situ B-isotope analyses of tourmaline conducted by LA-MC-ICPMS yielded a total range of δ11B values from −14.7‰ to +5.9‰ and the values show significant variations among the different stages. Tourmaline of stage I albitization has δ11B values from −14.7‰ to −5.7‰. Stage II tourmaline from Fe-oxide ores shows δ11B values of −11.6‰ to −6.1‰, similar to those of stage I, whereas stage III tourmaline from Cu-sulfide ores displays significantly heavier isotopes with δ11B values from −4.4‰ to −0.6‰. Tourmaline of stage IV, represented by barren quartz-calcite-tourmaline veins in intensely-altered wall rocks, has positive δ11B values from +2.9‰ to +5.9‰. The observed variations of B isotopes cannot be produced by fractionation of the ore fluids alone. Mixing of B from at least two distinct sources, a magmatic source and external sources with highly positive δ11B values, are hence proposed on the basis of geological constraints and modal calculations. The calculated oxygen isotope compositions of fluids in equilibrium with hydrothermal minerals that associated with tourmaline from stage II, stage III, and stage IV are 7.4‰ to 10.2‰, 8.4‰ to 8.6‰, and 3.8‰ to 8.0‰. The decreasing trend of δ18O of fluids from stage II to stage IV is consistent with the incursion of external basinal brines by fluid mixing. Our dataset thus demonstrates that ore fluids of Dahongshan IOCG system were initially of magmatic origin (stages I and II) and were later progressively modified by mixing with external brines (stages III and IV).


      PubDate: 2016-09-01T17:18:50Z
       
  • Discrimination and quantification of Fe and Ni abundances in Genesis solar
           wind implanted collectors using X-ray standing wave fluorescence yield
           depth profiling with internal referencing
    • Abstract: Publication date: 21 November 2016
      Source:Chemical Geology, Volume 441
      Author(s): Y. Choi, P. Eng, J. Stubbs, S.R. Sutton, M. Schmeling, I.V. Veryovkin, D. Burnett
      X-ray standing wave fluorescence yield depth profiling was used to determine the solar wind implanted Fe and Ni fluences in a silicon-on-sapphire (SoS) Genesis collector (60326). An internal reference standardization method was developed based on fluorescence from Si and Al in the collector materials. Measured Fe fluence agreed well with that measured previously by us on a sapphire collector (50722) as well as SIMS results by Jurewicz et al. Measured Ni fluence was higher than expected by a factor of two; neither instrumental errors nor solar wind fractionation effects are considered significant perturbations to this value. Impurity Ni within the epitaxial Si layer, if present, could explain the high Ni fluences and therefore needs further investigation. As they stand, these results are consistent with minor temporally-variable Fe and Ni fractionation on the timescale of a year.


      PubDate: 2016-09-01T17:18:50Z
       
  • The geochemical filter of large river confluences
    • Abstract: Publication date: 21 November 2016
      Source:Chemical Geology, Volume 441
      Author(s): Damien Guinoiseau, Julien Bouchez, Alexandre Gélabert, Pascale Louvat, Naziano Filizola, Marc F. Benedetti
      This study reports the behaviour and fate of major and trace elements in solid and solution in the Encontro das Aguas mixing zone in the Amazonian basin. This area results from the confluence of acidic, organic-rich Rio Negro and Andean sediment-rich Rio Solimões. The differences existing between these two water masses are likely to modify the partitioning of elements between solid and solution or to induce losses by physical or chemical removal. To account for lateral and vertical heterogeneities in the river sections, the fluxes of elements in dissolved and solid loads were estimated as accurately as possible using an interpolation technique. A general loss of suspended particulate matter (SPM) of 23% is measured 80km after the confluence but the various elements and their associated hosted minerals are not affected in a similar way. SiSPM, CaSPM, NaSPM or SrSPM, mostly associated with coarse phases (quartz and Ca-Na feldspars) are preferentially lost. Refractory elements (CoSPM, CrSPM, TiSPM, AlSPM, NiSPM), other alkali (A) and alkali earth (AE) elements (KSPM, MgSPM, BaSPM), FeSPM or organic carbon (OC) are lost to a lesser extent due to their occurrence in finer fractions such as clays (smectite, illite or kaolinite), oxides and particulate organic matter (POM). In solution A-AE and Cudiss, mostly originating from the Rio Solimões behave conservatively. By contrast, refractory elements, coming mostly from the Rio Negro, exhibit important losses (from −10% for DOC to −60% for Codiss). Changes in pH and ionic strength during the mixing may induce a sorption of these elements to particles supplied by the Rio Solimões, a clustering and/or a settling of nanokaolinites phases or a sorption of Aldiss, Fediss, Codiss, Mndiss or Zndiss by dissolved organic matter (DOM). The differential elemental settling at a river confluence could thus induce a phase shift between element supply and export out of a watershed. The physical and chemical processes reported here for a river confluence could be applied in other mixing zones such as estuaries or mining exhausts.


      PubDate: 2016-08-27T20:43:40Z
       
  • Geochemistry and iron isotope systematics of hydrothermal plume fall-out
           at East Pacific Rise 9°50′N
    • Abstract: Publication date: 21 November 2016
      Source:Chemical Geology, Volume 441
      Author(s): Olivier Rouxel, Brandy M. Toner, Steven J. Manganini, Christopher R. German
      While gross hydrothermal fluxes entering the ocean are known to be significant, much remains unknown about the fate of this material as it disperses through the oceans, and its impact upon ocean biogeochemistry. Mineral precipitation within hydrothermal plumes removes hydrothermally-sourced metals from solution and also acts to scavenge trace elements from the surrounding water column. Here, we investigate the fate of particulate Fe released from high-temperature hydrothermal venting at EPR 9°50′N and its potential impact on local deep-ocean Fe-isotopic and geochemical budgets. We measured the geochemical composition, mineralogy and Fe isotope systematics of hydrothermal plume products in order to determine whether mineral precipitation imposes characteristic Fe-isotope “fingerprints” for hydrothermally sourced Fe in the deep ocean. Our sampling includes sediment trap deployments after the eruptive event of Jan. 2006, allowing the examination of temporal changes of hydrothermal fluxes over a 160day period. Results show that Fe isotope composition in the high-temperature vent fluids is rather constant over the sampling period 2004–2008, and that secular variations of δ56Fe values of plume particles from −0.03 to −0.91‰ (relative to IRMM-14 standard) could be explained by local processes leading to variable mixing extents of hydrothermal, biogenic and lithogenic particles. Through geochemical modeling, we have calculated the relative abundances of hydrothermal plume components such as sulfides, Fe oxyhydroxides, organic matter, biogenic and lithogenic phases. We demonstrate that Fe isotope fractionation in the hydrothermal plume occurs during the formation and rapid settling of Fe-sulfides that are characterized by δ56Fe values ranging from −0.73±0.13‰ to −0.86±0.13‰, which is systematically lower than the end-member hydrothermal fluids (δ56Fe=−0.4‰). This study suggests that both the initial Fe isotope composition of the high-temperature vent fluids and its initial Fe/H2S ratio (i.e. Fe-sulfide precipitation versus Fe-oxyhydroxide precipitation) should impose characteristic Fe isotope “fingerprints” for hydrothermally derived Fe in the deep ocean.


      PubDate: 2016-08-27T20:43:40Z
       
  • First-principles calculations of sulphur isotope fractionation in MX2
           minerals, with M=Fe, Co, Ni and X2=AsS, SbS
    • Abstract: Publication date: 21 November 2016
      Source:Chemical Geology, Volume 441
      Author(s): Shanqi Liu, Yongbing Li, Hujun Gong, Caiyun Chen, Jianming Liu, Yaolin Shi
      Iron, Co and Ni are first row transition metals and are all able to combine with S, As, and Sb to form disulphides, sulpharsenides and sulphantimonides, respectively, all of which exhibit substitutions of metal and metalloid elements in their structures. As an important tracer in the geochemistry of ore deposits, S isotope fractionation in sulphides can be used to analyse the ore-forming process and the source of ore-forming elements. However, there have been few studies of S isotope fractionation in sulpharsenides and sulphantimonides. Studying the isotope fractionation of the various members of this structural group can provide systematic information regarding S isotope fractionation in transition metal disulphides, sulpharsenides and sulphantimonides. In this paper, the S isotope fractionation parameters for Fe, Co and Ni sulpharsenides and sulphantimonides were calculated using first-principles methods based on density functional theory in the temperature range of 0–1000°C. Our calculations show that the order of heavy S isotope enrichment is arsenopyrite>cobaltite>gudmundite>costibite>gersdorffite>ullmannite. For sulphides with the same anion pair, the main factor affecting S isotope fractionation is the metal-sulphur bond strength. For most of the sulphides studied in this paper, S isotope fractionation arising from substitution of metalloid elements is much smaller than that of metal elements.


      PubDate: 2016-08-27T20:43:40Z
       
  • Cassiterite dissolution and Sn diffusion in silicate melts of variable
           water content
    • Abstract: Publication date: 21 November 2016
      Source:Chemical Geology, Volume 441
      Author(s): Yuping Yang, Youxue Zhang, Adam Simon, Peng Ni
      Experiments to constrain cassiterite dissolution kinetics in rhyolitic melts with 0.1–5.9wt% H2O were conducted at 1023–1373K and 0.5GPa in a piston-cylinder apparatus. Care was taken to minimize convection in the experimental charge. Tin diffusivity was found to be concentration dependent. The diffusion profiles were fit well by assuming that tin diffusivity depends exponentially on tin concentration, which is roughly linearly related to SiO2 or SiO2 +Al2O3 concentration. Tin diffusivity was found to increase with temperature following the Arrhenius relation, with activation energy decreasing from 161kJ/mol in dry rhyolite to 93kJ/mol in hydrous rhyolite containing 5.9wt% H2O. Tin diffusivity increases exponentially with H2O concentration, by about 3.2 orders of magnitude at 1123K from 0 to 6wt% H2O, and decreases exponentially with SiO2 concentration, by about 0.7 orders of magnitude when SiO2 concentration increases by 10wt%. The equation to describe Sn2+ diffusivity in rhyolitic to dacitic melt (64–76wt% SiO2) at about 0.5GPa is: ln D Sn = − 18.194 + 0.17 76 – C SiO 2 − 19418 − 1389 w / T , where D Sn is in m2/s, C SiO2 and w are SiO2 and H2O concentrations in wt%, and T is in Kelvin. The solubility of cassiterite (or tin concentration at cassiterite saturation) in rhyolitic melts increases strongly with increasing temperature.


      PubDate: 2016-08-27T20:43:40Z
       
  • Ionic molal conductivities, activity coefficients, and dissociation
           constants of HAsO42− and H2AsO4− from 5 to 90°C and ionic strengths
           from 0.001 up to 3molkg−1 and applications in natural systems
    • Abstract: Publication date: 21 November 2016
      Source:Chemical Geology, Volume 441
      Author(s): Xiangyu Zhu, D. Kirk Nordstrom, R. Blaine McCleskey, Rucheng Wang
      Arsenic is known to be one of the most toxic inorganic elements, causing worldwide environmental contamination. However, many fundamental properties related to aqueous arsenic species are not well known which will inhibit our ability to understand the geochemical behavior of arsenic (e.g. speciation, transport, and solubility). Here, the electrical conductivity of Na2HAsO4 solutions has been measured over the concentration range of 0.001–1molkg−1 and the temperature range of 5–90°C. Ionic strength and temperature-dependent equations were derived for the molal conductivity of HAsO4 2− and H2AsO4 − aqueous ions. Combined with speciation calculations and the approach used by McCleskey et al. (2012b), these equations can be used to calculate the electrical conductivities of arsenic-rich waters having a large range of effective ionic strengths (0.001–3molkg−1) and temperatures (5–90°C). Individual ion activity coefficients for HAsO4 2− and H2AsO4 − in the form of the Hückel equation were also derived using the mean salt method and the mean activity coefficients of K2HAsO4 (0.001–1molkg−1) and KH2AsO4 (0.001–1.3molkg−1). A check on these activity coefficients was made by calculating mean activity coefficients for Na2HAsO4 and NaH2AsO4 solutions and comparing them to measured values. At the same time Na-arsenate complexes were evaluated. The NaH2AsO4 0 ion pair is negligible in NaH2AsO4 solutions up to 1.3molkg−1. The NaHAsO4 − ion pair is important in NaHAsO4 solutions >0.1molkg−1 and the formation constant of 100.69 was confirmed. The enthalpy, entropy, free energy and heat capacity for the second and third arsenic acid dissociation reactions were calculated from pH measurements. These properties have been incorporated into a widely used geochemical calculation code WATEQ4F and applied to natural arsenic waters. For arsenic spiked water samples from Yellowstone National Park, the mean difference between the calculated and measured conductivities have been improved from −18% to −1.0% with a standard deviation of 2.4% and the mean charge balances have been improved from 28% to 0.6% with a standard deviation of 1.5%.
      Graphical abstract image

      PubDate: 2016-08-27T20:43:40Z
       
  • Compound-specific carbon and nitrogen isotopic compositions of chlorophyll
           a and its derivatives reveal the eutrophication history of Lake Zurich
           (Switzerland)
    • Abstract: Publication date: 21 November 2016
      Source:Chemical Geology, Volume 441
      Author(s): Sebastian Naeher, Hisami Suga, Nanako O. Ogawa, Carsten J. Schubert, Kliti Grice, Naohiko Ohkouchi
      To reconstruct the impact of eutrophication on phototrophic communities and the biogeochemical cycling of carbon and nitrogen in the surface water, we investigated the distributions and carbon and nitrogen isotopic compositions (δ13C and δ15N values) of chlorins in the sediments of Lake Zurich. The chlorin distributions were dominated by chlorophyll a (Chl a) and its derivatives, which reflect rapid degradation to the pheopigments in the water column and sediments of the lake. The δ13C values of these sedimentary chlorins followed the historical trends of eutrophication and reoligotrophication, except in the surface sediments, which were characterised by higher relative contributions of aged, redeposited organic matter (OM). The δ13C values of the sedimentary chlorins together with bulk sediment δ13C values and C/N ratios indicate that the phototrophic communities in the lake used a 13C-depleted carbon source, which is mainly of aquatic origin. The δ15N values of chlorins reflect the predominance of nitrate assimilating phototrophs, especially the non-N2-fixing cyanobacterium Planktothrix rubescens prevalent during sediment deposition. Shifts in δ15N values of Chl a followed mostly the trends in eutrophication and reoligotrophication, but were also affected by community assemblage shifts to diatoms and/or other cyanobacteria at the end of the 19th century and during the eutrophication maximum in the 1970s. The lower δ15NChl-a values in the surface sediments coincide with increasing nitrogen to phosphorus ratios and reduced water column mixing that characterise the recent reoligotrophication period and may explain the predominance of P. rubescens in Lake Zurich. In contrast, the higher contributions of laterally transported OM explains the large offset of δ15N values of the pheopigments relative to Chl a, which is supported by the high radiocarbon age of the surface sediments.


      PubDate: 2016-08-24T02:49:41Z
       
  • Oxygen isotope and trace element geochemistry of zircons from porphyry
           copper system: Implications for Late Triassic metallogenesis within the
           Yidun Terrane, southeastern Tibetan Plateau
    • Abstract: Publication date: 21 November 2016
      Source:Chemical Geology, Volume 441
      Author(s): De-Xin Kong, Ji-Feng Xu, Jian-Lin Chen
      It is intriguing to investigate the generation of the magmatic system and the geochemical signatures that distinguish the ore-bearing ones from barren ones, especially by examining the oxygen isotopic and trace element compositions of zircon. The oxygen isotopic composition and trace element abundances of zircons from the mineralized Pulang porphyries and the coeval Disuga and Lannitang andesites of the Yidun Terrane, eastern Tibetan Plateau, have been determined by secondary ion mass spectrometry (SIMS) and laser ablation–inductively coupled plasma–mass spectrometry (LA–ICP–MS) in this study, respectively. Analyzed zircons have relatively high but invariant δ18O values (~5.8‰–6.9‰) that are indicative of derivation from homogeneous mantle or mantle-derived magmas together with contribution of mature crustal materials. We propose a model for the Late Triassic magmatism in the Yidun Terrane involving magmas derived from partial melting of subduction-metasomatized mantle peridotites that subsequently experienced melting–assimilation–storage–homogenization (MASH) processes. The low calculated Ti-in-zircon temperatures (644°C–863°C, generally between 640°C and 690°C) for these zircons may be caused by the lower solidus temperature for zircon in hydrous magmas, and partly controlled by the crystallization of amphibole. Zircon grains from porphyry copper system display small negative europium anomalies (mostly Eu/Eu*>0.5), contrasting with pronounced negative Eu anomalies in zircons (mostly Eu/Eu*<0.5) associated with magmato-hydrothermal quartz-vein Cu-Mo-W/Mo-W deposits. Calculated REE compositions of the melt that was in equilibrium with zircons from porphyry copper system display depletion in the middle rare earth elements (MREE) without negative Eu anomaly, while that of zircons from magmato-hydrothermal quartz-vein Cu-Mo-W/Mo-W system show depletion in the MREE with negative Eu anomaly. It is speculated that the parental melts of zircon have decreasing magmatic oxidation state in the following sequence: porphyry copper system>magmato-hydrothermal quartz-vein Cu-Mo-W deposit>magmato-hydrothermal quartz-vein Mo-W deposit. This study suggests that zircon trace element geochemistry probably bears significant implications for elucidating the origin of the arc magmatism and guiding exploration strategies for porphyry copper deposits.


      PubDate: 2016-08-24T02:49:41Z
       
  • Geochemistry of dissolved aluminum at low pH: Hydrobasaluminite formation
           and interaction with trace metals, silica and microbial cells under anoxic
           conditions
    • Abstract: Publication date: 21 November 2016
      Source:Chemical Geology, Volume 441
      Author(s): Javier Sánchez-España, Iñaki Yusta, William D. Burgos
      The precipitation of dissolved aluminum is among the most important geochemical processes affecting the water chemistry and trace metal dynamics of acid mine drainage (AMD). In this study, we focus on the in situ formation of Al precipitates and their subsequent interaction with trace metals, dissolved silica and microbial cells under anoxic conditions (e.g., deep strata of pit lakes, flooded underground mines). In the presence of sulfate (0.1–0.2M), dissolved aluminum precipitates at pH ~4.0 and forms an amorphous, globular hydroxysulfate precipitate with composition corresponding to hydrobasaluminite. This proto-hydrobasaluminite shows an important capacity to incorporate SiO2 (aq), divalent metal cations (FeII, PbII, BeII), and many anion-forming trace elements (As, Cr, Sb, V, Se, U) by coprecipitation. Removal efficiencies obtained experimentally with freshly formed hydrobasaluminite under anoxic and oxic conditions were consistent with metal profiles obtained in the pit lakes and suggest that the behaviour of many toxic metals is closely linked to the Al particles. Detailed transmission electron microscopy (TEM) shows that hydrobasaluminite may coexist with more crystalline minerals like gibbsite, bayerite and/or allophane, which are present as nanocrystals embedded in the amorphous matrix and appear to form by transformation of the former. Hydrobasaluminite precipitation is favoured by the activity of iron- and sulfate-reducing microbes in the water column and at the sediment/water interface. Our study shows that, at solubility equilibrium, microbial cells often serve as nuclei for Al precipitation. Further, chemical mapping of Al-coated cells by scanning/TEM shows an important incorporation of SiO2, Fe2+ (valence confirmed by electron energy loss spectroscopy) and other metal cations (e.g., Mg2+) in the Al cell coatings as compared to Al particles presumed to form via abiotic precipitation. These Al cell coatings are amorphous to X-rays and their compositions likely result from coprecipitation of SiO2 and Fe2+ resulting in Si-rich and Fe(II)-rich hydrobasaluminite. The disparity between field and laboratory data, the serendipitous detection of nontronite-like smectite in precipitates formed in the acidic waters under laboratory conditions, and the apparent saturation with respect to different clay minerals (e.g., kaolinite, halloysite) are all evidences suggesting thermodynamic feasibility of aluminosilicate precursor formation in these systems.
      Graphical abstract image

      PubDate: 2016-08-24T02:49:41Z
       
  • Behavior of U, Th and Ra isotopes in soils during a land cover change
    • Abstract: Publication date: 21 November 2016
      Source:Chemical Geology, Volume 441
      Author(s): Sophie Rihs, Adrien Gontier, Eric Pelt, David Fries, Marie-Pierre Turpault, François Chabaux
      Over the last decades, the U- and Th-series nuclides were successfully used to determine weathering rates in various environments. The objective of this study is to assess the potential impact of the vegetation change on the U- and Th-series signal recorded in forested soils. This study was carried out from the experimental forest site of Breuil-Chenue (Morvan, France) developed by the INRA-BEF team. The native forest of the site was partially clear-felled in 1976 and replaced by monospecific plantations stands (Oak and Douglas fir). U- and Th-series disequilibria were measured in 2011 in the podzolic soils developed under the native forest, and in the two replanted stands. Separation of primary minerals (biotite, muscovite and perthitic feldspar) and selective extractions of the Fe and Al oxides were performed to investigate the distribution of U and Th among these soil fractions. The selective extractions suggest that a significant part of U and Th is primarily held by Fe-bearing silicates. Our results suggest that the tree substitution seems to produce a large dissolution of these minerals under the Oaks, resulting to a release of U and Th. However, below 25cm no impact of this release was observed on U-series disequilibria. A scenario allowing to reconcile the significant mobilization of U and the constancy of U-series disequilibria is proposed. Above 25cm, additional pedogenic redistribution of U and Th isotopes occurs in all the profiles, inducing some discrepancies between U-series disequilibria. A clear correlation between the (230Th/234U) ratios and the proportions of amorphous and interlayer Al hydroxides has been highlighted. This correlation suggests a mobility of U and Th isotopes strongly associated to Al dynamics in these soils rather than Fe, despite the primary location of U in the Fe-bearing silicates and the overwhelming reported control of UVI by Fe-oxides in oxidized environments. These pedogenic processes make the shallowest horizons of podzolic soils unsuitable for U-series dating. In contrast, a soil production rate can be deduced from the deepest soil layers which do not show such effects on the U-series nuclides. The reproducible U-series disequilibria measured in four whole-profile replicates emphasize the robustness and the significance of the “long-lived” U-series disequilibria in deep soil layers relative to long-term weathering rates, independent of transient perturbations such as land cover changes. Finally, because Ra can strongly accumulates in plants, the (226Ra/230Th) ratios in the different soils were affected by the flux of 226Ra released by litter degradation. The use of this ratio as a long-term chronometer should therefore be performed with caution in such contexts. No direct impact of the vegetation type on the (228Ra/232Th) ratios was identified, due to the short 228Ra half-life.
      Graphical abstract image

      PubDate: 2016-08-24T02:49:41Z
       
  • Elevated uranium concentrations in Lake Baikal sediments: Burial and early
           diagenesis
    • Abstract: Publication date: 21 November 2016
      Source:Chemical Geology, Volume 441
      Author(s): Lawrence M. Och, Beat Müller, Christian März, Adrian Wichser, Elena G. Vologina, Michael Sturm
      The water column of Lake Baikal (Siberia) is pervasively oxic and O2 penetrates several cm into the sediment, followed by distinct layers of Fe/Mn oxide that undergo reductive-dissolution/oxidative-precipitation cycles. Uranium (U) contents of the oxic surface sediment layers were ~15μgg−1, which is unparalleled in oxygenated lakes. To understand the processes leading to this enrichment we investigated the geochemical composition of the particulate matter and pore water of four sediment cores from different locations in the lake and performed mass balance calculations based on sediment mass accumulation rates and published loads from major tributaries. The comparison of loads and export of U in Lake Baikal suggested that current estimates of loads are too low by a factor of about 3 compared to sediment mass accumulation rates. Peak loads during spring ice melt in tributaries that are difficult to monitor and quantify might be the main cause for the deviation. The high U concentrations in the lake sediments originated from the scavenging of U in the water column through association with settling organic particles and particulate Fe(III)- and, to a lesser extent, Mn(IV)-oxides. We outline the hypothesis that two distinct U phases, lithogenic and non-lithogenic U reach the lake sediment and that authigenic U is subsequently formed under reducing conditions within the sediment. In some cores we found that most U was remobilized during the degradation of organic matter, in particular within the top oxygenated layer of the sediment. Significant enrichments prevailed due to U adsorption to and/or co-precipitation with Fe-oxides. When Fe-oxides and, to a lesser extent, Mn-oxides were reductively dissolved, they released U to the pore water, leading to peak dissolved U concentrations in the anoxic sediment, which in turn, precipitated as authigenic U under predominantly sulphate-reducing conditions. The onset of the accumulation of authigenic U coincided with the formation of distinct Fe/Mn oxide layers above. We argue that the resilience of Fe-oxides (especially crystalline goethite and hematite), in association with phosphate, even within reducing (but non-sulfidic) sediments support the burial of substantial amounts of U.


      PubDate: 2016-08-24T02:49:41Z
       
  • Simultaneous quantitative analysis of Ni, VO, Cu, Zn and Mn geoporphyrins
           by liquid chromatography-high resolution multistage mass spectrometry:
           Method development and validation
    • Abstract: Publication date: 21 November 2016
      Source:Chemical Geology, Volume 441
      Author(s): Martijn Woltering, Svenja Tulipani, Chris J. Boreham, John Walshe, Lorenz Schwark, Kliti Grice
      A method is described for the identification and quantification of Cu, Ni, VO, Zn and Mn metalloporphyrins in geological samples using a high performance liquid chromatography (HPLC) system coupled to a high resolution Thermo Orbitrap XL mass spectrometer (MS). The linear correlation of the detector response to the compound concentrations in Cu, Ni, VO, Zn and Mn porphyrin standards (R2 values between 0.9975 and 0.9994), verified the suitability of the methodology for the quantification of these compounds. Furthermore, the method was validated by the analysis of complex porphyrin distributions in geological sample isolates from the Australian Toolebuc Formation and Bight Basin. By using the high resolution of the Orbitrap MS detector it was not only possible to reproduce the porphyrin distributions reported from previous analyses of the same isolates, but also to identify and resolve a range of additional compounds such as an iso-butyl C34 VO porphyrin indicative of palaeoenvironmental photic zone euxinia and several Cu and Zn porphyrins. The methodology described here provides a new high resolution tool for routine analysis of complex metalloporphyrin distributions in geological sample extracts, enabling the simultaneous quantitative analysis of Cu, Ni, VO, Zn and Mn porphyrins without the need of prior de-metalation or further fractionation of the porphyrin extract. The high resolution of the Orbitrap MS combined with the ability to perform multistage mass spectrometry leads to a significant improvement in compound detection and identification, which shows a high potential in the analysis of low abundance porphyrins, such as high-molecular-weight porphyrins with extended alkyl side-chains.


      PubDate: 2016-08-24T02:49:41Z
       
  • Aluminous gneiss derived by weathering of basaltic source rocks in the
           Neoarchean Storø Supracrustal Belt, southern West Greenland
    • Abstract: Publication date: 21 November 2016
      Source:Chemical Geology, Volume 441
      Author(s): Kristoffer Szilas, Kate Maher, Dennis K. Bird
      The origin of amphibolite-facies aluminous gneiss from the gold-hosting Neoarchean Storø Supracrustal Belt in the Nuuk region of southern West Greenland is investigated in this study. An improved understanding of the formation of such aluminous gneiss has implications for genetic models (epithermal vs. orogenic style) for a local gold occurrence, which is hosted by sheeted quartz-veins within amphibolite in the hanging wall adjacent to the aluminous gneiss on the island of Storø. The aluminous gneiss mainly consists of garnet, plagioclase, sillimanite, quartz and biotite, which suggest a pelitic protolith. However, it has previously been postulated that the aluminous gneiss represents a hydrothermal alteration product, formed by leaching of a mafic precursor that was subsequently transformed to the current mineral assemblage during later regional metamorphism. In support of this hypothesis are decimeter-scale relicts of amphibolite, found within the decameter-wide aluminous gneiss, that share similar ratios of commonly fluid immobile elements, such as Al, Ti, Zr, Hf, Nb and Lu. Metasedimentary rocks sensu stricto are also present within the Storø Supracrustal Belt. These mostly comprise biotite schist with a similar mineralogy as the aluminous gneiss, and are located adjacent to the latter in the footwall. The aluminous gneiss has high bulk-rock Al2O3 contents and low SiO2 relative to the biotite schist, which is less aluminous, has higher SiO2, and is more potassic than the aluminous gneiss. The immobile element ratios of the biotite schist are significantly different from those of the amphibolite and the aluminous gneiss. Additionally, the biotite schist yields distinct detrital zircon age populations, whereas only metamorphic zircon has been found in the aluminous gneiss and amphibolite. In the present study, the isocon method was applied to provide mass balance constraints on the alteration of basalt to the protolith of the aluminous gneiss prior to regional metamorphism of the entire Storø supracrustal sequence. Accepting minor fractionation among otherwise immobile elements, an error of at least ±20% is estimated for the isocon mass-balance model. The results indicate that all major elements, except for K2O were leached from the basaltic precursor, resulting in a net mass loss ranging from −20 to −40wt.%. However, despite the similar trace element patterns and ratios of the aluminous gneiss and the adjacent amphibolite, in situ alteration or weathering of the basaltic precursor rock is not possible given the small, but persistent, fractionation among fluid immobile elements, due to the unusual accumulation of Cr, U, Ni and Th, as well as the observed modal layering within the aluminous gneiss. Instead, this points to physical transport during sedimentary reworking of a mafic protolith and potentially the addition of redox-sensitive elements such as U and Cr from the water column, and therefore suggest that this aluminous gneiss simply represents a metasediment with a mafic provenance. The implication of this study for gold exploration within the Archean supracrustal belts of the SW Greenland is that aluminous gneiss is unlikely to represent an indicator of acidic hydrothermal alteration as previously postulated. Instead, aluminous gneisses within these supracrustal belts are likely of sedimentary origin and may provide a venue to further understand the exogenous environments of the Archean Earth, and thus further geochemical studies of such rocks are recommended in order to place constraints on the composition of the hydrosphere at that time.


      PubDate: 2016-08-19T18:19:08Z
       
  • Multiple sulfur isotopes (δ34S, Δ33S) and trace elements (Mo, U, V)
           reveal changing palaeoenvironments in the mid-Carboniferous Chokier
           Formation, Belgium
    • Abstract: Publication date: 21 November 2016
      Source:Chemical Geology, Volume 441
      Author(s): Katharina Siedenberg, Harald Strauss, Ralf Littke
      In this study, multiple sulfur isotopes are combined with widely-used redox-sensitive trace element concentrations (Mo, V, and U) in order to further validate the use of δ 34S and Δ 33S as a paleoredox proxy. The approach is applied to the mid-Carboniferous Chokier Formation, for which a detailed microfacies study identified a marked change in the environmental conditions during sediment deposition. Sediments from the upper part of the Chokier Formation exhibit negative δ 34S (mean −24.6‰) and positive Δ 33S (mean +0.040‰) values that are attributed to microbial sulfate reduction (MSR) and/or microbial sulfur disproportionation (MSD). Samples from the lower part of the Chokier Formation display less negative δ 34S (mean −9.7‰) and negative Δ 33S (mean −0.029‰) values. Paired δ 34S-Δ 33S values are located on a mixing curve between two endmembers: (1) sulfur generated by MSR and/or MSD, and (2) seawater sulfate. This latter observation is attributed to a restricted seawater exchange that limited the sulfate supply, thus, promoting closed-system conditions. Consequently, sulfur isotope values approached the isotopic composition of contemporaneous seawater. Trace element abundances exhibit a stronger enrichment in the upper Chokier Formation (EFMo =9–77, EFV =1–4; EF=Enrichment Factor) than in the lower Chokier Formation (EFMo =16–21, EFV ≈1). However, a hydrographic rather than a redox control is inferred for both sections based on Mo/TOC≤15 and Mo/U ratios well below seawater values. For the upper Chokier Formation, Mo/TOC≈15 indicates a moderately restricted basin, whereas the lower part is more strongly restricted (Mo/TOC≪15). Multiple sulfur isotopes and trace element abundances/ratios reveal a consistent picture of restricted basinal conditions during deposition of the lower Chokier Formation and more open conditions for the upper Chokier Formation. Hence, it can be concluded that results support the application of multiple sulfur isotope analyses for revealing paleoenvironmental conditions.


      PubDate: 2016-08-19T18:19:08Z
       
  • Trace elements (Li, B, Mn and Ba) as sensitive indicators for salinization
           and freshening events in coastal aquifers
    • Abstract: Publication date: 21 November 2016
      Source:Chemical Geology, Volume 441
      Author(s): A. Russak, O. Sivan, Y. Yechieli
      The current global intrusion of seawater into coastal aquifers causes salinization of groundwater and thus significant degradation of its quality. This study quantified the effect of seawater intrusion and freshening events in coastal aquifers on trace elements (Li, B, Mn and Ba) across the fresh-saline water interface (FSI) and their possible use as indicators for these events. This was done by combining field data and column experiments simulating these events. The experiments enabled quantification of the processes affecting the trace element composition and examination of whether salinization and freshening events are geochemically reversible, which has been seldom investigated. The dominant process affecting trace element composition during salinization and freshening is ion exchange. The results of the experiments show that the concentrations of major cations and Li+ were reversible during salinization and freshening, whereas B, Mn2+ and Ba2+ were not. During salinization, Li+ and B were depleted due to sorption by 10 and 100μmol·L−1, respectively, to about half of their expected conservative concentrations. The relative depletion of Li+ increased with distance from the shore, representing the propagation of salinization. Ba2+ and Mn2+ were desorbed from the sediment during salinization and enriched by tenfold in the aqueous phase compared to their concentration in seawater (~0.1 μeq·L−1). During freshening both were depleted by almost tenfold compared to their concentration in fresh groundwater (~0.7 μeq·L−1). The depletion of Mn2+ is a sensitive marker for freshening because Mn2+ has a strong affinity to the solid phase. Moreover, this study shows that both Mn2+ and Ba2+ can be used as sensitive hydrogeochemical tools to distinguish between salinization and freshening events in the FSI zone in coastal aquifers.


      PubDate: 2016-08-19T18:19:08Z
       
  • SIMS and NanoSIMS analyses of well-preserved microfossils imply
           oxygen-producing photosynthesis in the Mesoproterozoic anoxic ocean
    • Abstract: Publication date: 21 November 2016
      Source:Chemical Geology, Volume 441
      Author(s): Xiaotong Peng, Zixiao Guo, Christopher H. House, Shun Chen, Kaiwen Ta
      Well-preserved microfossils in the stromatolitic cherts from the Gaoyuzhuang Formation (~1500Ma), which have one of the best state of preservation in the Proterozoic Era, may play key roles in systematizing information about the evolution of early life in the Proterozoic Ocean. Here, a combination of light microscopy (LM), scanning electron microscopy (SEM), nano-scale secondary ion mass spectrometry (NanoSIMS), secondary ion mass spectrometry (SIMS) and geochemical techniques was employed to characterize the morphology, elemental distributions and carbon isotope values of individual Gaoyuzhuang microfossils, and their growing environment. Light microscopy analyses show that abundant filamentous and coccoid microfossils, classified as oscillatoriacean and chroococcacean cyanobacteria, respectively, are exceptionally well preserved in chert. NanoSIMS analyses show that metabolically important elements, such as 12C−, 13C−, 12C14N−, 32S−, and 34S−, are concentrated in these microfossils and that the variations in the concentrations of these elements are similar to each other, establishing the elemental patterns in undoubtedly biogenic microstructures. The carbon isotope (δ13C) values of individual microfossils range from −31.7‰±0.9‰ to −23.1‰±1.0‰ (weighted mean=−28.5‰±0.1‰), consistent not only with a Calvin Cycle-based photosynthesis, but typically for chroococcacean cyanobacteria as well. Our results, for the first time, provide the element distributions and cell specific carbon isotope values on convincing Mesoproterozoic cyanobacterial fossils, supporting sustained oxygen-producing photosynthesis in the Proterozoic Ocean. The geochemical data indicate these fossil microorganisms may grow in an anoxic seawater, potentially supporting the delayed oxygenation on the Earth. In this regard, this integrated approach may be a step towards a more comprehensive picture of the evolution of early life on the Earth.


      PubDate: 2016-08-15T02:56:05Z
       
  • Stable hydrogen and oxygen isotopes in mineral-bound water and the
           indication for chemical weathering intensity
    • Abstract: Publication date: 21 November 2016
      Source:Chemical Geology, Volume 441
      Author(s): Chengfan Yang, Shouye Yang, Ni Su
      Chemical weathering plays a key role in the long-term climate change and global biogeochemical cycle. In this study, a 5.1m-long basalt-developed weathering profile from Hunan Province, China, was selected for the analyses of major elements, grain size, pH, total organic carbon, thermogravimetric (TG) and differential thermogravimetric properties (DTG), as well as stable isotopes (δD and δ18O) in mineral-bound waters. The major purpose is to distinguish different types of mineral-bound water and to verify whether hydrogen and oxygen isotopes of mineral-bound water can indicate chemical weathering intensity. Based on the TG and DTG results, different types of mineral-bound water were extracted by a sequential heating extraction technique (40–120°C, 120–300°C, 300–600°C and 600–900°C). The mineral-bound water lost below 120°C is absorbed water, a mixture of interlayer and crystal waters occurs at 120–300°C, and above 300°C is mostly constitution water. Hydrogen and oxygen isotopic ratios at top ~2m are subject to the influences of eolian and pedogenic processes. The absorbed water has been exchanged and reached isotopic equilibrium with ambient soil water, while the crystal water and interlayer water cannot be separated by heating method in this study. Hydrogen and oxygen isotopes of constitution water can indicate chemical weathering, showing positive relationships with weathering intensity. Although the mechanism for their indication to chemical weathering remains further investigation, we suggest that the isotopic fractionation between the formation of clay minerals and surface water should be taken into account. This research sheds new light on mineral-bound water phase and a potential proxy for tracing the process of chemical weathering.


      PubDate: 2016-08-15T02:56:05Z
       
  • Spatial and temporal variations of base cation release from chemical
           weathering on a hillslope scale
    • Abstract: Publication date: 21 November 2016
      Source:Chemical Geology, Volume 441
      Author(s): M. Erlandsson, E.H. Oelkers, K. Bishop, H. Sverdrup, S. Belyazid, J.L.J. Ledesma, S.J. Köhler
      Cation release rates to catchment runoff from chemical weathering were assessed using an integrated catchment model that included the soil's unsaturated, saturated and riparian zones. In-situ mineral dissolution rates were calculated in these zones as a function of pH, aluminum and dissolved organic carbon (DOC) concentrations along a hillslope in Northern Sweden where soil water was monitored over nine years. Three independent sets of mineral dissolution equations of varying complexity were used: PROFILE, Transition-State Theory (TST), and the Palandri & Kharaka database. Normalization of the rate-coefficients was necessary to compare the equations, as published rate-coefficients gave base cation release rates differing by several orders of magnitude. After normalizing the TST- and Palandri & Kharaka-rate coefficients to match the base cation release rates calculated from the PROFILE-equations, calculated Ca2+ and Mg2+ release rates are consistent with mass balance calculations, whereas those of Na+ and K+ are overestimated. Our calculations further indicate that a significant proportion of base cations are released from the organic soils in the near-stream zone, in part due to its finer texture. Of the three sets of rate equations, the base cation release rates calculated from the normalized TST-equations were more variable than those calculated using the other two sets of equations, both spatially and temporally, due to its higher sensitivity to pH. In contrast, the normalized Palandri & Kharaka-equations were more sensitive to variations in soil temperature.


      PubDate: 2016-08-15T02:56:05Z
       
  • Simulation of geochemical banding I: Acidization-precipitation experiments
           in a ferruginous limestone rock
    • Abstract: Publication date: 15 November 2016
      Source:Chemical Geology, Volume 440
      Author(s): Maysam Msharrafieh, Mazen Al-Ghoul, Farah Zaknoun, Houssam El-Rassy, Samia El-Joubeily, Rabih Sultan
      This paper is the first among two articles which aim at exploring the possible similarities between the well-known Liesegang banding phenomenon in precipitate systems, and the stripe formation observed in a large number of rocks. In the present (first) article, we review a comprehensive and long study wherein patterning experiments were performed in-situ (real rock systems), to simulate the band formation through the acidization of a ferruginous limestone rock, causing dissolution and precipitation reactions. The results are analyzed by microscopy, AA and XRD techniques. The correlation between the Liesegang gel experiment and the processes taking place inside the rock medium is established. In the second part (following paper), a theoretical model is set forth to support our experiments. The fractal nature of the contours of the various regions will be explored.
      Graphical abstract image

      PubDate: 2016-07-24T06:13:25Z
       
  • High-precision measurement and standard calibration of triple oxygen
           isotopic compositions (δ18O, Δ′17O) of sulfate by F2 laser
           fluorination
    • Abstract: Publication date: 15 November 2016
      Source:Chemical Geology, Volume 440
      Author(s): Benjamin R. Cowie, David T. Johnston
      A new approach for measuring the triple oxygen isotope composition of sulfate minerals was developed using fluorine gas and an infrared laser to generate O2. A correction for the mass-dependent isotope effect observed during barite fluorination was rigorously calibrated. Analyte gas purification was performed via numerous cryofocus steps and the introduction of an in-line gas chromatograph. As with previous methods, our fluorination technique still requires pairing with independent δ 18O measurements made by TC/EA conversion to carbon monoxide, but leads to higher yields than BrF5-based methods. We first calibrated our method against known silicate standards (UWG-2, SCO, NBS-28). Following from this, we report high-precision triple oxygen isotope ratios for international sulfate standards (IAEA-SO-5, IAEA-S O-6, NBS-127), and an internal laboratory standard, JMG. Replicate analyses of JMG yielded a Δ ′17O value of −0.057 ± 0.004‰ (standard error), demonstrating per-meg level precision that approaches instrumental limits. We provide δ 17O and δ 18O values that can be translated into any preferred reference frame for comparison with gases, water, or rocks/minerals. Quantification of differences in triple oxygen isotope composition of the standard reference materials further enables application of this measure and approach on environmental and geological materials.


      PubDate: 2016-07-24T06:13:25Z
       
  • How sulfate-driven anaerobic oxidation of methane affects the sulfur
           isotopic composition of pyrite: A SIMS study from the South China Sea
    • Abstract: Publication date: 15 November 2016
      Source:Chemical Geology, Volume 440
      Author(s): Zhiyong Lin, Xiaoming Sun, Jörn Peckmann, Yang Lu, Li Xu, Harald Strauss, Haoyang Zhou, Junli Gong, Hongfeng Lu, Barbara M.A. Teichert
      Sulfate-driven anaerobic oxidation of methane (SO4-AOM) in marine sediments commonly leads to the precipitation of pyrite. It is, however, frequently challenging to unequivocally unravel the entire history of pyritization, because of the common coexistence of SO4-AOM derived pyrite with pyrite resulting from organiclastic sulfate reduction (OSR). To better understand how SO4-AOM affects pyritization in methane-bearing sediments and how this can be identified, we applied secondary ion mass spectroscopy (SIMS) to analyze the sulfur isotope composition (δ34S) of authigenic pyrite in addition to sulfur isotope measurements of bulk sulfide and hand-picked pyrite aggregates from the two seafloor sites, HS148 and HS217, in the Shenhu seepage area, South China Sea. Authigenic, mostly tubular pyrite aggregates from these sites consist of three types of pyrite: framboids, zoned aggregates with radial overgrowths surrounding a framboidal core, and euhedral pyrite crystals. Framboids with low SIMS δ34S values (as low as −41.6‰ at HS148, and −38.8‰ at HS217) are dispersed throughout the cores, but are especially abundant in the shallow part of the sedimentary column (i.e. above 483cmbsf in HS148; above 670cmbsf in HS217). These patterns are interpreted to reflect the dominance of OSR during early diagenetic processes in the shallow sediments. With increasing depth, both δ34S values of bulk sulfide minerals and hand-picked pyrite aggregates increase sharply at 483cmbsf in core HS148, and at 700cmbsf in core HS217, respectively. Radial pyrite overgrowths and euhedral crystals become abundant at depth typified by high δ34S values for hand-picked pyrite. Moreover, SIMS analysis reveals an extreme variability of δ34S values for the three pyrite types on a small scale in these zones. Besides some moderately 34S enriched framboids, most of the overgrowths and euhedral crystals display extremely high SIMS δ34S values (as high as +114.8‰ at HS148, and +74.3‰ at HS217), representing the heaviest stable sulfur isotope composition of pyrite ever reported to the best of our knowledge. Such an abrupt and extreme increase in δ34Spyrite values with depth is best explained by an enrichment of 34S in the pool of dissolved sulfide caused by SO4-AOM in the sulfate methane transition zone (SMTZ). The increase in δ34S values from framboidal cores to overgrowth layers and euhedral crystals indicates continuous, and finally near to complete exhaustion of dissolved sulfate at the SMTZ following a Rayleigh distillation process. SO4-AOM allowed for subsequent growth of later stage pyrite over the initial framboids, part of which formed earlier and at shallower depth by OSR. The combination of a detailed petrographic study of authigenic pyrite with SIMS analysis of stable sulfur isotopes in organic-rich strata proves to be a powerful tool for reconstructing the dynamics of sulfur cycling in modern and, potentially, ancient sedimentary sequences.
      Graphical abstract image

      PubDate: 2016-07-24T06:13:25Z
       
  • Celestine in a sulfidic spring barite deposit - A potential biomarker?
    • Abstract: Publication date: 15 November 2016
      Source:Chemical Geology, Volume 440
      Author(s): David M. Singer, Elizabeth M. Griffith, John M. Senko, Kaci Fitzgibbon, Inoka H. Widanagamage
      We have documented the presence of celestine (SrSO4) within sediment accumulating at an artesian sulfidic spring (Zodletone Spring, Oklahoma) dominated by barite (BaSO4) precipitation associated with microbiological activity. The distribution and speciation of Sr in solid phases was determined by synchrotron-based micro-X-ray fluorescence spectroscopy and micro-X-ray diffraction, and particle morphology and texture was determined using electron microscopy. In all the natural sulfidic spring samples and lithified tufa sample, celestine was detected in fine-grained micron-scale Sr-rich phases but not in euhedral, Sr-poor grains. In parallel laboratory-based precipitation experiments, celestine was not observed even when solutions contained high Sr/Ba. Thermodynamic predictions alone do not account for the presence of micron-scale celestine in the sulfidic spring, and they do not account for the differences in Sr presence and distribution in naturally-occurring versus synthetic grains. While the mechanism is unclear, based on this evidence we hypothesize that the combination of bacterial surfaces and microenvironments within the crusts and microbial mats creates a synergistic effect where Sr is preferentially exchanged over Ba between the overlying stream water and the pore water within the mats allowing celestine to precipitate. Ultimately, our results point to an important role of biological activity for preferential Sr uptake. The presence of micron-scale celestine in ancient barite deposits can therefore potentially be used as a biomarker for conditions similar to modern sulfidic springs.
      Graphical abstract image

      PubDate: 2016-07-24T06:13:25Z
       
  • Redox conditions across the G–L boundary in South China: Evidence from
           pyrite morphology and sulfur isotopic compositions
    • Abstract: Publication date: 15 November 2016
      Source:Chemical Geology, Volume 440
      Author(s): Hengye Wei, Xuemei Wei, Zhen Qiu, Huyue Song, Guo Shi
      The end-Guadalupian (middle Permian) mass extinction recorded the disappearance of shelf faunas such as fusulinids, small foraminifers, brachiopods, rugose corals which flourished in the warm shallow marine habitats after the late Pennsylvanian–early Permian glaciation. The causes for this mass extinction are still unclear. Marine anoxia, often as a cause for other mass extinctions, has been paid less attention in this extinction. Here we present a comprehensive study including pyrite morphology and sulfur isotopic compositions of pyrite across the Guadalupian–Lopingian (middle–upper Permian; G–L) boundary at Tianfengping in northern Yangtze Platform and at Penglaitan in southern Yangtze Platform in South China. Our results show that the sulfur isotope ratios of pyrite mainly track the framboid size distributions in these two sections. Anoxia and intermittent euxinia occur at the G–L boundary evidenced by the coincidence between reduction of framboid size and the extremely negative sulfur isotope values of pyrite involving disproportionation, and corresponds to the main mass extinction of the end-Guadalupian, suggesting a causal link between them. Long-term dysoxic conditions during the middle Capitanian are evidenced by the relative small framboid size and lower sulfur isotope ratios of pyrite, and coincide with the first pulse of the end-Guadalupian mass extinction, also suggesting a causal link between them. The oxygen-depletion conditions in the Capitanian during the global sea level falling may be the main cause for the end-Guadalupian mass extinction.


      PubDate: 2016-07-24T06:13:25Z
       
 
 
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