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EARTH SCIENCES (466 journals)                  1 2 3 | Last

Showing 1 - 200 of 371 Journals sorted alphabetically
Acta Geochimica     Hybrid Journal   (Followers: 4)
Acta Geodaetica et Geophysica     Hybrid Journal   (Followers: 1)
Acta Geodaetica et Geophysica Hungarica     Full-text available via subscription   (Followers: 2)
Acta Geophysica     Open Access   (Followers: 7)
Acta Geotechnica     Hybrid Journal   (Followers: 7)
Acta Meteorologica Sinica     Hybrid Journal   (Followers: 3)
Advances in High Energy Physics     Open Access   (Followers: 19)
Advances In Physics     Hybrid Journal   (Followers: 19)
Aeolian Research     Hybrid Journal   (Followers: 5)
African Journal of Aquatic Science     Hybrid Journal   (Followers: 13)
Algological Studies     Full-text available via subscription   (Followers: 2)
Alpine Botany     Hybrid Journal   (Followers: 4)
AMBIO     Hybrid Journal   (Followers: 15)
Anales del Instituto de la Patagonia     Open Access   (Followers: 1)
Andean geology     Open Access   (Followers: 13)
Annales Henri Poincaré     Hybrid Journal   (Followers: 3)
Annales UMCS, Geographia, Geologia, Mineralogia et Petrographia     Open Access  
Annals of Geophysics     Open Access   (Followers: 13)
Annals of GIS     Hybrid Journal   (Followers: 21)
Annual Review of Marine Science     Full-text available via subscription   (Followers: 11)
Anthropocene     Hybrid Journal   (Followers: 3)
Anthropocene Review     Hybrid Journal   (Followers: 7)
Applied Clay Science     Hybrid Journal   (Followers: 5)
Applied Geochemistry     Hybrid Journal   (Followers: 13)
Applied Geomatics     Hybrid Journal   (Followers: 3)
Applied Geophysics     Hybrid Journal   (Followers: 8)
Applied Ocean Research     Hybrid Journal   (Followers: 5)
Applied Petrochemical Research     Open Access   (Followers: 2)
Aquatic Conservation Marine and Freshwater Ecosystems     Hybrid Journal   (Followers: 35)
Arctic Science     Open Access   (Followers: 6)
Arctic, Antarctic, and Alpine Research     Full-text available via subscription   (Followers: 9)
Artificial Satellites : The Journal of Space Research Centre of Polish Academy of Sciences     Open Access   (Followers: 19)
Asia-Pacific Journal of Atmospheric Sciences     Hybrid Journal   (Followers: 20)
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: 10)
Atmosphere-Ocean     Full-text available via subscription   (Followers: 12)
Atmospheric and Climate Sciences     Open Access   (Followers: 28)
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: 11)
Bulletin of Geosciences     Open Access   (Followers: 9)
Bulletin of the Lebedev Physics Institute     Hybrid Journal   (Followers: 1)
Bulletin of the Seismological Society of America     Full-text available via subscription   (Followers: 21)
Bulletin of Volcanology     Hybrid Journal   (Followers: 18)
Cadernos de Geociências     Open Access  
Canadian Mineralogist     Full-text available via subscription   (Followers: 3)
Canadian Water Resources Journal     Hybrid Journal   (Followers: 21)
Carbonates and Evaporites     Hybrid Journal   (Followers: 3)
CATENA     Hybrid Journal   (Followers: 5)
Chemical Geology     Hybrid Journal   (Followers: 19)
Chemie der Erde - Geochemistry     Hybrid Journal   (Followers: 4)
Chinese Geographical Science     Hybrid Journal   (Followers: 5)
Chinese Journal of Oceanology and Limnology     Hybrid Journal   (Followers: 4)
Ciencia del suelo     Open Access   (Followers: 2)
Ciencias Espaciales     Open Access  
Climate and Development     Hybrid Journal   (Followers: 12)
Coastal Management     Hybrid Journal   (Followers: 25)
Cogent Geoscience     Open Access  
Comptes Rendus Geoscience     Full-text available via subscription   (Followers: 8)
Computational Geosciences     Hybrid Journal   (Followers: 14)
Computational Mathematics and Mathematical Physics     Hybrid Journal   (Followers: 2)
Computers and Geotechnics     Hybrid Journal   (Followers: 10)
Contemporary Trends in Geoscience     Open Access   (Followers: 3)
Continental Shelf Research     Hybrid Journal   (Followers: 9)
Contributions to Mineralogy and Petrology     Hybrid Journal   (Followers: 11)
Contributions to Plasma Physics     Hybrid Journal   (Followers: 3)
Coral Reefs     Hybrid Journal   (Followers: 16)
Cretaceous Research     Hybrid Journal   (Followers: 8)
Cybergeo : European Journal of Geography     Open Access   (Followers: 5)
Depositional Record     Open Access  
Developments in Geotectonics     Full-text available via subscription   (Followers: 3)
Developments in Quaternary Science     Full-text available via subscription   (Followers: 3)
Développement durable et territoires     Open Access   (Followers: 3)
Diatom Research     Hybrid Journal   (Followers: 2)
Doklady Physics     Hybrid Journal   (Followers: 1)
Dynamics of Atmospheres and Oceans     Hybrid Journal   (Followers: 11)
E&S Engineering and Science     Open Access  
E3S Web of Conferences     Open Access  
Earth and Planetary Science Letters     Hybrid Journal   (Followers: 122)
Earth and Space Science     Open Access   (Followers: 13)
Earth Interactions     Full-text available via subscription   (Followers: 12)
Earth Science Research     Open Access   (Followers: 7)
Earth Surface Dynamics (ESurf)     Open Access   (Followers: 5)
Earth Surface Processes and Landforms     Hybrid Journal   (Followers: 19)
Earth System Dynamics     Open Access   (Followers: 7)
Earth System Dynamics Discussions     Open Access   (Followers: 5)
Earth's Future     Open Access   (Followers: 1)
Earth, Planets and Space     Open Access   (Followers: 71)
Earthquake Engineering and Engineering Vibration     Hybrid Journal   (Followers: 8)
Earthquake Science     Hybrid Journal   (Followers: 11)
Earthquake Spectra     Full-text available via subscription   (Followers: 18)
Ecohydrology     Hybrid Journal   (Followers: 11)
Ecological Questions     Open Access   (Followers: 7)
Electromagnetics     Hybrid Journal   (Followers: 5)
Energy Efficiency     Hybrid Journal   (Followers: 11)
Energy Exploration & Exploitation     Hybrid Journal   (Followers: 4)
Environmental Earth Sciences     Hybrid Journal   (Followers: 27)
Environmental Geology     Hybrid Journal   (Followers: 20)
Environmental Geosciences     Full-text available via subscription   (Followers: 4)
Environmental Geotechnics     Hybrid Journal   (Followers: 5)
Erwerbs-Obstbau     Hybrid Journal  
Estuaries and Coasts     Hybrid Journal   (Followers: 18)
Estuarine, Coastal and Shelf Science     Hybrid Journal   (Followers: 33)
Estudios Geográficos     Open Access  
European Journal of Mineralogy     Full-text available via subscription   (Followers: 13)
European Journal of Remote Sensing     Open Access  
Exploration Geophysics     Hybrid Journal   (Followers: 4)
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: 3)
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: 8)
Geocarto International     Hybrid Journal   (Followers: 22)
Geochemical Perspectives     Hybrid Journal  
Geochemistry : Exploration, Environment, Analysis     Hybrid Journal   (Followers: 7)
Geochemistry, Geophysics, Geosystems     Full-text available via subscription   (Followers: 28)
Geochimica et Cosmochimica Acta     Hybrid Journal   (Followers: 47)
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   (Followers: 2)
Geoenvironmental Disasters     Open Access   (Followers: 3)
Geofluids     Open Access   (Followers: 4)
Geoforum     Hybrid Journal   (Followers: 23)
Géographie physique et Quaternaire     Full-text available via subscription   (Followers: 1)
Geography and Natural Resources     Hybrid Journal   (Followers: 7)
Geoheritage     Hybrid Journal   (Followers: 1)
Geoinformatica Polonica : The Journal of Polish Academy of Arts and Sciences     Open Access  
Geoinformatics & Geostatistics     Hybrid Journal   (Followers: 9)
Geological Journal     Hybrid Journal   (Followers: 14)
Geology Today     Hybrid Journal   (Followers: 27)
Geology, Geophysics and Environment     Open Access   (Followers: 1)
Geomagnetism and Aeronomy     Hybrid Journal   (Followers: 3)
Geomatics, Natural Hazards and Risk     Hybrid Journal   (Followers: 9)
Geomechanics for Energy and the Environment     Full-text available via subscription   (Followers: 1)
GEOmedia     Open Access   (Followers: 1)
Geomorphology     Hybrid Journal   (Followers: 27)
Geophysical & Astrophysical Fluid Dynamics     Hybrid Journal   (Followers: 2)
Geophysical Journal International     Hybrid Journal   (Followers: 34)
Geophysical Prospecting     Hybrid Journal   (Followers: 7)
Geophysics     Full-text available via subscription   (Followers: 19)
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: 4)
Geoscience Data Journal     Open Access   (Followers: 2)
Geoscience Frontiers     Open Access   (Followers: 9)
Geoscience Letters     Open Access   (Followers: 1)
Geoscience Records     Open Access  
Geosciences     Open Access   (Followers: 2)
Geosciences Journal     Hybrid Journal   (Followers: 10)
Geoscientific Instrumentation, Methods and Data Systems     Open Access   (Followers: 4)
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: 5)
Geotectonics     Hybrid Journal   (Followers: 7)
GISAP : Earth and Space Sciences     Open Access   (Followers: 2)
Glass Physics and Chemistry     Hybrid Journal   (Followers: 4)
Global and Planetary Change     Hybrid Journal   (Followers: 19)
Global Biogeochemical Cycles     Full-text available via subscription   (Followers: 15)
Gondwana Research     Hybrid Journal   (Followers: 8)
GPS Solutions     Hybrid Journal   (Followers: 17)
Grassland Science     Hybrid Journal  
Ground Water     Hybrid Journal   (Followers: 34)
Ground Water Monitoring & Remediation     Hybrid Journal   (Followers: 19)
Groundwater for Sustainable Development     Full-text available via subscription   (Followers: 3)
Helgoland Marine Research     Open Access   (Followers: 3)
History of Geo- and Space Sciences     Open Access   (Followers: 4)
Hydrobiologia     Hybrid Journal   (Followers: 18)
Hydrogeology Journal     Hybrid Journal   (Followers: 21)
Hydrological Processes     Hybrid Journal   (Followers: 28)
Hydrology and Earth System Sciences     Open Access   (Followers: 28)
ICES Journal of Marine Science: Journal du Conseil     Hybrid Journal   (Followers: 53)
IEEE Journal of Oceanic Engineering     Hybrid Journal   (Followers: 12)
Indian Geotechnical Journal     Hybrid Journal   (Followers: 4)
Indonesian Journal on Geoscience     Open Access   (Followers: 2)
Interdisciplinary Environmental Review     Hybrid Journal   (Followers: 3)
International Geology Review     Hybrid Journal   (Followers: 16)
International Journal of Advanced Geosciences     Open Access  
International Journal of Advanced Remote Sensing and GIS     Open Access   (Followers: 36)
International Journal of Advancement in Earth and Enviromental Sciences     Open Access   (Followers: 3)
International Journal of Advancement in Remote Sensing, GIS, and Geography     Open Access   (Followers: 27)
International Journal of Applied Earth Observation and Geoinformation     Hybrid Journal   (Followers: 36)
International Journal of Coal Geology     Hybrid Journal   (Followers: 2)
International Journal of Disaster Risk Reduction     Hybrid Journal   (Followers: 15)
International Journal of Earth Sciences     Hybrid Journal   (Followers: 38)
International Journal of Earthquake and Impact Engineering     Hybrid Journal   (Followers: 2)
International Journal of Geo-Engineering     Open Access   (Followers: 2)

        1 2 3 | Last

Journal Cover Chemical Geology
  [SJR: 2.346]   [H-I: 145]   [19 followers]  Follow
   Hybrid Journal Hybrid journal (It can contain Open Access articles)
   ISSN (Print) 0009-2541
   Published by Elsevier Homepage  [3043 journals]
  • Rapid reactions between CO2, brine and silicate minerals during geological
           carbon storage: Modelling based on a field CO2 injection experiment
    • Abstract: Publication date: Available online 2 August 2017
      Source:Chemical Geology
      Author(s): Mike Bickle, Niko Kampman, Hazel Chapman, Chris Ballentine, Benoit Dubacq, Albert Galy, Tule Sirikitputtisak, Oliver Warr, Max Wigley, Zheng Zhou
      The dissolution of CO2 into formation brines and the subsequent reactions of the CO2-charged brines with reservoir minerals are two key processes likely to increase the security of geological carbon-dioxide storage. These processes will be dependent on the permeability structure and mineral compositions of the reservoirs, but there is limited observational data on their rates. In this paper we report the cation and anion concentrations and Sr, oxygen and carbon isotopic compositions of formation waters from four extraction wells sampled at surface, over ~6months after commencement of CO2 injection in a five spot pattern for enhanced oil recovery at the Salt Creek field, Wyoming. Sampled fluids, separated from the minor oil component, exhibit near-monotonic increases in alkalinity and concentrations of cations but little change in Cl and Br concentrations and oxygen and deuterium isotope ratios. The increases in alkalinity are modelled in terms of reaction with reservoir calcite and silicate minerals as the changes in fluid chemistry and Sr-isotopic compositions are inconsistent with simple addition of injected fluids sampled over the course of the experiment. The reservoir mineral chemical and isotopic compositions are characterised by sampling core as well as surface exposures of the Frontier Formation elsewhere in Wyoming. The evolution of the fluid chemistries reflects extensive dissolution of both carbonate and silicate minerals over the course of the six months sampling implying rapid dissolution of CO2 in the formation waters and reaction of CO2-bearing brines with formation minerals. Rates of CO2 diffusion into the brines and advection of CO2 charged brines in the reservoir are sufficiently slow that, if present, calcite should react to be close to equilibrium with the fluids. This allows estimation of the CO2 partial pressures in the sampled fluids and comparison with the thermodynamic driving force for the relatively rapid average plagioclase dissolution rates of ~10−12 mol·m−2·s−1.

      PubDate: 2017-08-03T08:05:29Z
  • Climatic zonation and weathering control on sediment composition (Angola)
    • Abstract: Publication date: Available online 1 August 2017
      Source:Chemical Geology
      Author(s): Pedro Dinis, Eduardo Garzanti, Pieter Vermeesch, João Huvi
      Complementary mineralogical and geochemical datasets on fluvial, beach and dune samples collected along the Atlantic margin of subequatorial southwestern Africa are used to investigate the relationships between provenance and climatic controls on sediment composition and to test the reliability of different geochemical and mineralogical weathering proxies as climatic indicators. The studied N/S-trending coastal region is characterized by strong latitudinal and inland climatic gradients, and thus represents an excellent natural laboratory in which to study the effects of climatic-induced weathering on sediment composition. Although the mineralogy and geochemistry of suspended-load muds closely reflects the different weathering intensities over both latitudinal and inland climatic gradients, the composition of mud and sand samples are strongly affected by sediment provenance. Consequently, weathering parameters such as the αAlE values (estimating the degree of depletion in element E relative to the UCC standard), display complex patterns of variation especially for sand samples. By assuming a typical order of bulk-sediment mobility Na>Ca>Sr>Mg>K>Ba≈Rb, anomalously high or low αAl values placing a specific element off the expected mobility order are considered as an indicator of source-rock control on sediment composition. The composition of detritus recycled from Meso-Cenozoic strata reflects the cumulative effect of successive sediment cycles, with recycling processes affecting to a different extent the diverse weathering proxies. In particular, αAlNa appears to be more strongly affected by recycling in muds than in sands. Among all mineralogical and chemical parameters, those that correlate best with rainfall in the drainage areas are αAlNa for sands, αAlMg for muds and smectite content (only in areas of low rainfall). In the geological and geomorphological setting of SW Africa these proxies turn out to be better climate estimators than the classical weathering indices CIA or WIP. This case study reminds us to carefully consider source-rock control and mixing with recycled detritus when drawing inferences on climatic conditions based on weathering indices.

      PubDate: 2017-08-03T08:05:29Z
  • Seasonal progression of uranium series isotopes in subglacial meltwater:
           Implications for subglacial storage time
    • Abstract: Publication date: Available online 31 July 2017
      Source:Chemical Geology
      Author(s): Carli A. Arendt, Sarah M. Aciego, Kenneth W.W. Sims, Sarah M. Aarons
      The residence time of subglacial meltwater impacts aquifer recharge, nutrient production, and chemical signals that reflect underlying bedrock/substrate, but is inaccessible to direct observation. Here we report the seasonal evolution of subglacial meltwater chemistry from the 2011 melt season at the terminus of the Athabasca Glacier, Canada. We measured major and trace analytes and U-series isotopes for twenty-nine bulk meltwater samples collected over the duration of the melt season. This dataset, which is the longest time-series record of (234U/238U) isotopes in a glacial meltwater system, provides insight into the hydrologic evolution of the subglacial system during active melting. Meltwater samples, measured from the outflow, were analyzed for (238U), (222Rn) and (234U/238U)activity, conductivity, alkalinity, pH and major cations. Subglacial meltwater varied in [238U] and (222Rn) from 23 to 832ppt and 9 to 171 pCi/L, respectively. Activity ratios of (234U/238U) ranged from 1.003 to 1.040, with the highest (238U), (222Rn) and (234U/238U)activity values occurring in early May when delayed-flow basal meltwater composed a significant portion of the bulk melt. From the chemical evolution of the meltwater, we posit that the relative subglacial water residence times decrease over the course of the melt season. This decrease in qualitative residence time during active melt is consistent with prior field studies and model-predicted channel switching from a delayed, distributed network to a fast, channelized network flow. As such, our study provides support for linking U-series isotopes to storage lengths of meltwater beneath glacial systems as subglacial hydrologic networks evolve with increased melting and channel network efficiency.

      PubDate: 2017-08-03T08:05:29Z
  • Dissolution and surface roughening of Columbia River flood basalt at
           geologic carbon sequestration conditions
    • Abstract: Publication date: Available online 31 July 2017
      Source:Chemical Geology
      Author(s): Rachel K. Wells, Wei Xiong, Daniel Giammar, Philip Skemer
      Carbon storage in basalt reservoirs can mitigate CO2 emissions to the atmosphere that contribute to climate change. For basalt reservoirs, CO2 injection leads to Ca2+, Fe2+, and Mg2+-rich solutions that may result in carbonate precipitation for long-term stable carbon sequestration. Examination of basalt surfaces as dissolution progresses can offer important information about which minerals are dissolving, the timing and sequence of dissolution, and the effects these processes have on surface roughness and morphology. We carried out two series of experiments using two polished Columbia River flood basalt samples in CO2-rich water at 150°C and 100bar to observe the physical and geochemical changes during dissolution. One series reacted a sample over short time increments, while the other series examined a separate sample over longer time increments. Scanning electron microscopy, 2D profilometer analysis, and 3D laser confocal microscopy were combined with ICP-MS to characterize dissolution. Dissolution resulted in pitting, dissolution along fractures and grain boundaries, and an increase in species concentrations in the bulk solution over incremented reaction times. Based on these observations, early dissolution of olivine grains contribute Mg2+ and Fe2+ to aqueous solution in initial stages (<1week at a pH<4), while slower continuous dissolution of Ca-rich pyroxene contributes Mg2+, Fe2+, and Ca2+ to the bulk solution over a longer period of time. The complete dissolution of olivine grains resulted in pits up to 200μm deep. Dissolution of plagioclase and matrix was slower and resulted in the formation of micro-sized textures (<10μm). Following 1–2months of reaction, the surface roughness parameters (mean and root mean squared) increased by factors of 42 and 28, respectively, while surface area of the flood basalt increased 20% relative to the starting polished surface. The results of this study indicate 1) pyroxene is the sustaining contributor of divalent metal cations during dissolution of basalt and 2) the limited connectivity limits of olivine and pyroxene grains the exposure of new reactive surface areas.

      PubDate: 2017-08-03T08:05:29Z
  • New biotite and muscovite isotopic reference materials, USGS57 and USGS58,
           for δ2H measurements–A replacement for NBS 30
    • Abstract: Publication date: Available online 29 July 2017
      Source:Chemical Geology
      Author(s): Haiping Qi, Tyler B. Coplen, Matthias Gehre, Torsten W. Vennemann, Willi A. Brand, Heike Geilmann, Gerard Olack, Ilya N. Bindeman, Jim Palandri, Li Huang, Fred J. Longstaffe
      The advent of continuous-flow isotope-ratio mass spectrometry (CF-IRMS) coupled with a high temperature conversion (HTC) system enabled faster, more cost effective, and more precise δ 2H analysis of hydrogen-bearing solids. Accurate hydrogen isotopic analysis by on-line or off-line techniques requires appropriate isotopic reference materials (RMs). A strategy of two-point calibrations spanning δ 2H range of the unknowns using two RMs is recommended. Unfortunately, the supply of the previously widely used isotopic RM, NBS 30 biotite, is exhausted. In addition, recent measurements have shown that the determination of δ 2H values of NBS 30 biotite on the VSMOW-SLAP isotope-delta scale by on-line HTC systems with CF-IRMS may be unreliable because hydrogen in this biotite may not be converted quantitatively to molecular hydrogen. The δ 2HVSMOW-SLAP values of NBS 30 biotite analyzed by on-line HTC systems can be as much as 21mUr (or ‰) too positive compared to the accepted value of −65.7mUr, determined by only a few conventional off-line measurements. To ensure accurate and traceable on-line hydrogen isotope-ratio determinations in mineral samples, we here propose two isotopically homogeneous, hydrous mineral RMs with well-characterized isotope-ratio values, which are urgently needed. The U.S. Geological Survey (USGS) has prepared two such RMs, USGS57 biotite and USGS58 muscovite. The δ 2H values were determined by both glassy carbon-based on-line conversion and chromium-based on-line conversion, and results were confirmed by off-line conversion. The quantitative conversion of hydrogen from the two RMs using the on-line HTC method was carefully evaluated in this study. The isotopic compositions of these new RMs with 1-σ uncertainties and mass fractions of hydrogen are: USGS57 (biotite) δ 2HVSMOW-SLAP =−91.5±2.4mUr (n =24) Mass fraction hydrogen=0.416±0.002% (n =4) Mass fraction water=3.74±0.02% (n =4) USGS58 (muscovite) δ 2HVSMOW-SLAP =−28.4±1.6mUr (n =24) Mass fraction hydrogen=0.448±0.002% (n =4) Mass fraction water=4.03±0.02% (n =4). These δ 2HVSMOW-SLAP values encompass typical ranges for solid unknowns of crustal and mantle origin and are available to users for recommended two-point calibration.

      PubDate: 2017-08-03T08:05:29Z
  • Carbon and nutrient fluxes from floodplains and reservoirs in the Zambezi
    • Abstract: Publication date: Available online 29 July 2017
      Source:Chemical Geology
      Author(s): Alissa Zuijdgeest, Bernhard Wehrli
      Inland waters are under increasing anthropogenic stresses. In the last century, roughly two thirds of the world's wetland area disappeared, and so many dams have been constructed that currently 50% of river water passes through reservoirs before reaching the ocean. Large river systems in tropical and subtropical areas often develop extensive floodplain areas that will suffer from modifications in the flow regime as another boom in dam construction is under way. In the Zambezi catchment, we developed a comparative analysis of the biogeochemical effects of floodplains (Barotse Plains) and reservoirs (Lake Kariba) on tropical river biogeochemistry, to provide a basis to assess the net effect of eliminating wetland areas and transforming rivers into artificial lakes. To support such analyses, we propose a combination of specific sampling campaigns with sensor deployments to capture the seasonality of fluxes over large regional scales in remote areas. Dams and reservoirs alter the riverine biogeochemistry in distinct, and often different ways. While the Barotse Plains floodplain releases particles during a flood cycle, this suspended material is effectively trapped in Kariba reservoir. Seasonal production of biomass on the floodplain binds nutrients in the form of organic matter that sustains biological productivity in downstream ecosystems. Degradation of the biomass can lead to significant greenhouse gas emissions from the floodplain. The reservoir traps particles and nutrients, but carbon burial (120·103 tCperyear) is offset by annual emissions of methane to the atmosphere with about 3000·103 t C-CO2-equivalents. Therefore, building new dams will add permanent sinks of particles and nutrients to the land-ocean aquatic continuum, while draining riparian wetlands will disrupt their functions as temporal storage systems and source of terrestrial biomass for aquatic food chains.
      Graphical abstract image

      PubDate: 2017-08-03T08:05:29Z
  • Multiple oxygen (16O, 17O and 18O) and sulfur (32S, 33S, 34S and 36S)
           isotope signatures of the dissolved sulfate from Deception Island,
           Antarctic Peninsula: Implications on sulfate formation, transportation and
           deposition in the Antarctic region
    • Abstract: Publication date: Available online 29 July 2017
      Source:Chemical Geology
      Author(s): Yeongmin Kim, Insung Lee, Jung Hun Seo, Jong Ik Lee, James Farquhar
      Oxygen (16O, 17O and 18O) and sulfur (32S, 33S, 34S and 36S) isotope ratios of and major ion (Na+, Ca2+, Cl−, NO3 − and SO4 2−) concentrations in lakes, ponds and creeks from Deception Island, Antarctic Peninsula were analyzed to study the sources of sulfate, its oxidation, and the surficial processes of the dissolved sulfate. The positive relationship between the δ34Ssulfate (8.1‰ to 17.3‰) and the Cl−/SO4 2− molar ratio suggests mixing of sulfate from atmospheric deposition and from oxidation of sulfide minerals. The average sea salt fraction (28%) and δ34Snss values (from 5.6‰ to 15.9‰) indicate that a combination of sea salt and marine biogenic sulfide provide the high δ34S end-member of the dissolved sulfates. The relatively low δ18Osulfate (from −4.6‰ to 0.7‰) of Deception Island water suggests a role of local water in the formation of sulfate. Slightly negative but mass-dependent Δ17Osulfate values imply that atmospheric oxidation by O3 and H2O2 are negligible, while these values might suggest a significant role of oxidation by molecular oxygen and OH. The distinctly low δ34Ssulfate value of two samples (DCW-2 and DCW-3) suggests the input of sulfate from sulfide oxidation. Slight elevation of δ34Ssulfate values up to 17.3‰ compared to a typical atmospheric value indicates a minimal role for dissimilatory microbial sulfate reduction of Deception Island water and sediments. Both Δ33Ssulfate and Δ36Ssulfate values are homogeneous and near zero, implying that the dominant atmospheric oxidation process is tropospheric and that there are minimal to no contributions of stratospheric sulfate to Deception Island water.

      PubDate: 2017-08-03T08:05:29Z
  • Ca and Mg isotope fractionation during the stoichiometric dissolution of
           dolomite at temperatures from 51 to 126°C and 5bars CO2 pressure
    • Abstract: Publication date: Available online 29 July 2017
      Source:Chemical Geology
      Author(s): A. Perez-Fernandez, U.-N. Berninger, V. Mavromatis, P.A.E. Pogge von Strandmann, E.H. Oelkers
      Natural polycrystalline hydrothermal Sainte Colombe dolomite was dissolved in stirred titanium closed system reactors in aqueous 0.1mol/kg NaCl solutions at 51, 75, 121, and 126°C and a pressure of 5bars CO2. In total, 52, 27, 16, and 12%, respectively, of the dolomite placed in the reactors dissolved into the fluid phase during these experiments. Each experiment lasted from 12 to 47days and the fluid phase in each evolved towards, but did not exceed, ordered dolomite equilibrium at a pH of 5.9±0.3. All aqueous reactive fluids were undersaturated with respect to all potential secondary phases including calcite and magnesite. The reactive fluid compositions at the end of the experiments had a molar Ca/Mg ratio equal to that of the dissolving dolomite, and the dolomite recovered after the experiments contained only pure dolomite as verified by scanning electron microscopy. The Ca and Mg isotopic ratios of the reactive fluids remained within uncertainty equal to that of the dissolving dolomite in the experiments performed at 51 and 75°C. In contrast, the Ca isotopic composition of the reactive fluid in the experiment performed at 121 and 126°C was significantly greater such that Δ44/42Casolid-fluid =−0.6±0.1‰, whereas that of Mg was within uncertainty equal to that of the dissolving mineral. The equilibrium fractionation factors for both divalent cations favor the incorporation of isotopically light metals into the dolomite structure. Our results at 121 and 126°C, therefore, are consistent with the one-way transfer of Mg from dolomite to the fluid but the two-way transfer of Ca from and to dolomite as equilibrium is approached during its stoichiometric dissolution. The lack of Mg returning to the dolomite structure at these conditions is attributed to the slow dehydration kinetics of aqueous Mg. As more than 12% of the dolomite dissolved during the 121 and 126°C closed system experiments, our observations indicate a significant change in the Ca isotopic signature of the dolomite during its stoichiometric dissolution. Moreover, as there is no visual evidence for dolomite recrystallization during this experiment, it seems likely that the resetting of Ca isotopic signatures of carbonate minerals can be readily overlooked in the interpretation of natural systems.
      Graphical abstract image

      PubDate: 2017-08-03T08:05:29Z
  • Partial melting control of lithium concentrations and isotopes in the
           Cenozoic lithospheric mantle beneath Jiande area, the Cathaysia block of
           SE China
    • Abstract: Publication date: Available online 27 July 2017
      Source:Chemical Geology
      Author(s): Yan Xiao, Hong-Fu Zhang, Ben-Xun Su, Bin Zhu, Bei-Bei Chen, Chen Chen, Patrick Asamoah Sakyi
      To investigate whether Li isotopic fractionation occurred during high temperature partial melting in the mantle, major and trace element, and Li isotopes were measured in minerals (olivine, Opx, and Cpx) from lherzolite xenoliths entrained in the Jiande Cenozoic basalts in the Cathaysia block. These lherzolites show correlations between Al2O3 and Mg# in Opx and between Fo and Cr# in spinel, indicating they experienced variable degrees of partial melting. Most of them show LREE-depleted patterns in Cpx, indicating little imprint from subsequent incipient metasomatism. The minerals in these lherzolites have narrow Li concentrations (olivine: 1.81–2.82ppm; Cpx: 0.85–1.85ppm; Opx: 0.45–1.44ppm) but variable Li isotopic compositions (olivine: 2.06–10.00‰; Cpx: −5.05–3.58‰; Opx: −4.11–9.18‰). The relative concentrations among the different mineral phases in the Jiande lherzolites (LiOl >LiCpx ≥LiOpx) are similar to those found in “equilibrated” mantle peridotites. The Li concentrations of olivine are a bit higher than the range of normal mantle (1.0–2.2ppm). The negative correlations between indices of partial melting (Fo content and Cr# in spinel) and Li concentrations of olivine also implies that partial melting was the main process controlling the variations of olivine Li concentration and it generated depletion in Li concentration of <1ppm. In addition, the negative correlations between indices of partial melting and δ7Li values in olivine also occur. Thus, the observed variable δ7Li values in olivine could not have been produced by recent diffusive-driven isotopic fractionation, but potentially be explained through variable degree of partial melting of their source. Therefore, partial melting process could account for small but systematic Li isotopic fractionation in the mantle. In contrast to olivine, δ7Li values of Cpx and Opx in the Jiande lherzolites are a bit variable. They display isotopically heavy cores and light rims, suggesting a recent diffusive ingress of Li into a crystal from the entraining magma or during related intrusive magmatism. The restricted Li concentration in pyroxenes relative to isotope zonation may suggest faster isotopic self-diffusion relative to elemental diffusion.

      PubDate: 2017-08-03T08:05:29Z
  • Raman spectroscopic quantification of sulfur species in aqueous fluids:
           Ratios of relative molal scattering factors of Raman bands of H2S, HS−,
           SO2, HSO4−, SO42−, S2O32−, S3− and H2O at ambient conditions and
           information on changes with pressure and temperature
    • Abstract: Publication date: Available online 26 July 2017
      Source:Chemical Geology
      Author(s): Christian Schmidt, Terry M. Seward
      We determined ratios of relative molar scattering factors J (or j) of Raman bands of sulfur species in aqueous solutions at 22°C, 0.1MPa, specifically those of H2S(aq) at ~2590cm−1 (J 2590), HS−(aq) at ~2570cm−1 (J 2570), SO2(aq) at ~1150cm−1 (J 1150), HSO4 −(aq) at ~1050cm−1 (J 1050), SO4 2−(aq) at ~980cm−1 (J 980), and S2O3 2−(aq) at ~445cm−1 (J 445) and of water (i.e. the bending mode at ~1640cm−1 (J 1640) and the OH stretching band at ~3400cm−1 (J 3400)). From experiments at elevated temperatures using a hydrothermal diamond-anvil cell, we further estimated ratios of the relative molar scattering factor of the Raman band of S3 −(aq) at ~535cm−1 (J 535) for excitation at wavelengths of 473 and 532nm, and discussed the effect of the blue coloration caused by S3 − on Raman spectroscopic analysis of sulfur speciation. Another experiment suggested a significant decrease in the Raman scattering cross section of the band of HSO4 −(aq) at ~1050cm−1 with temperature. A systematic study on integrated ν1(SO4) and νS(OH) intensities and their ratio as function of temperature to 600°C and pressure to 2.02GPa for a 2.33 m Na2SO4 solution revealed a relationship between hydrogen bonding and νS(OH) Raman scattering efficiency. Moreover, it demonstrated that the integrated νS(OH) intensity can be used with reasonable accuracy as internal standard but only if corrected for the response function of the spectrometer, the frequency and scattering factor and the theoretical temperature dependence of the Raman scattering efficiency from the Bose-Einstein factor. The results of our study can be used to obtain the sulfur speciation in Raman spectroscopic studies on aqueous fluids in natural samples and experiments.

      PubDate: 2017-08-03T08:05:29Z
  • Retention of 226Ra by barite: The role of internal porosity
    • Abstract: Publication date: Available online 26 July 2017
      Source:Chemical Geology
      Author(s): Juliane Weber, Juri Barthel, Martina Klinkenberg, Dirk Bosbach, Maximilian Kruth, Felix Brandt
      The role of internal macropores and nano-scale pores for the uptake of 226Ra into barite was studied via scanning and transmission electron microscopy as well as focused ion beam methods. A temporal evolution of the internal microstructure and the Ra distribution was observed on samples taken from long-term Ra uptake experiments. The results of this study clearly show a significant impact of the presence of Ra leading to a complete reconstruction of the internal barite microstructure, whereas the microstructure of Ra-free reference samples remained unchanged. The initial internal barite microstructure contains a connected network of macropores and a layered structure of nano-scale pores which, in the presence of Ra, coalesced in favor of larger pores during the experiment. A clear relationship between the Ra uptake and the internal porosity was observed by high-resolution STEM-EDX mappings. Starting from strongly enhanced Ra concentrations in the solid in the vicinity of the pores, Ra is temporarily inhomogeneously distributed within the barite particles. At later stages of the long-term experiment the Ra distribution becomes homogenous while nano-scale and macro-scale pores disappear. In conclusion, the uptake of Ra into barite takes place by a special case of dissolution/reprecipitation from the inside of the particle to the outside.
      Graphical abstract image

      PubDate: 2017-08-03T08:05:29Z
  • Application of flow-through time-resolved analysis (FT-TRA) to isolate the
           elemental composition in ostracod calcite
    • Abstract: Publication date: Available online 25 July 2017
      Source:Chemical Geology
      Author(s): Nicole Börner, Bart De Baere, Roger Francois, Antje Schwalb
      Trace element concentrations in ostracod shells are a vital tool for paleoenvironmental reconstructions. The most common approach used to measure trace element concentrations in ostracod shells is batch dissolution followed by analysis of the resulting solution. However, oxide coatings, secondary carbonate crusts, or adhering clay minerals can contaminate their surface with significant quantities of the trace elements of interest, which requires thorough cleaning to prevent biasing the results. Cleaning ostracod shells is time consuming and, more importantly, it is difficult to verify whether the cleaning procedure used has effectively removed all contamination. There is also the possibility that chemical cleaning procedures affect the elemental composition of the biogenic calcite, so that manual removal of contaminant phases is often the method of choice. To address this cleaning conundrum, it would be beneficial to develop a method which could identify and correct for contamination, instead of attempting to remove contaminants as has been done so far. In this study, we explore the applicability of flow-through time-resolved analysis (FT-TRA) to determine the contamination-free trace element composition of biogenic carbonate in ostracod shells. In this system, for shells with a relatively simple surface geometry, such as most ostracods, dissolution proceeds gradually from their outer to inner layers. Time-resolved eluate composition thus documents the initial dissolution of the surface layers, often contaminated with Mn-carbonates and Fe-Mn oxides coatings, followed by the dissolution of the inner layers of uncontaminated biogenic carbonate, whose elemental composition can thus be retrieved from the time series data. Partial dissolution of adhering clays can also be corrected using elemental ratios measured after total dissolution of the carbonates. As a result, unbiased elemental data can be obtained even when the shells have not been sufficiently cleaned, and the cleaning step could even be omitted. The method is sensitive enough to allow single shell measurements for many elements of interest (Mg, Sr, Ba, Mn), although elements with lower concentration (U) or higher background (Fe) require multiple shell measurements. Comparing FT-TRA and laser ablation measurements of manually cleaned ostracod shells indicate that Mg, Ba, Mn, Fe and U surface contamination can still remain despite prior manual cleaning and bias the laser ablation data. FT-TRA demonstrates that, while thorough manual cleaning can remove adhering clay minerals, it cannot effectively remove Fe-Mn oxides or Mn-carbonates incrustations. FT-TRA is thus preferable to LA-ICP-MS because it does not rely on questionable cleaning procedures.

      PubDate: 2017-08-03T08:05:29Z
  • Influence of exchangeable oxygen on biogenic silica oxygen isotope data
    • Abstract: Publication date: Available online 25 July 2017
      Source:Chemical Geology
      Author(s): Anthony J. Menicucci, Howard J. Spero, Joy Matthews, Sanjai J. Parikh
      Application of biogenic opal oxygen isotope ratios from fossil and subfossil diatoms to paleoceanographic problems has been hampered by analytical and calibration issues in concert with a lack of experimental data to test fundamental assumptions about sample processing techniques. Here we present experiments where we react purified sediment trap and sediment core diatom samples to waters of different oxygen isotopic composition to quantify oxygen isotope exchange between laboratory processing solutions and δ18Odiatom values. We generate δ18Odiatom data using a microfluorination technique, and present FTIR data for samples both before and after vacuum dehydroxylation in order to investigate the mineralogic behavior of biogenic silica after dehydroxylation occurs. Our data demonstrate that exposure of diatoms to different δ18Oequil. water solutions during sample preparation alters final δ18Odiatom values and this alteration occurs during sample dehydroxylation prior to IRMS analysis. In addition, we present data that show structural diatom hydroxyl is persistent in sediment core samples, but the degree of equilibration with surrounding water decreases with sample age and/or core depth increase. Based on FTIR data, we propose that OH– loss from biogenic silica occurs post depositionally and may occur after heating in the laboratory, producing molecular scale reorganization of the silica tetrahedra within the biogenic opaline structure. Finally, we provide preliminary estimates of the time necessary for complete dehydroxylation of diatom silica, which would result in fully mature biogenic opal. These data suggest that high latitude marine sediments are slow to mature, and that the rate of exchangeable oxygen reduction varies exponentially at the locations we examined. Maturation time estimates require millions of years, and likely vary depending on core location.

      PubDate: 2017-08-03T08:05:29Z
  • The transition zone as a host for recycled volatiles: Evidence from
           nitrogen and carbon isotopes in ultra-deep diamonds from Monastery and
           Jagersfontein (South Africa)
    • Abstract: Publication date: Available online 25 July 2017
      Source:Chemical Geology
      Author(s): M. Palot, D.G. Pearson, T. Stachel, R.A. Stern, A. Le Pioufle, J.J. Gurney, J.W. Harris
      Sublithospheric (ultra-deep) diamonds provide a unique window into the deepest parts of Earth's mantle, which otherwise remain inaccessible. Here, we report the first combined C- and N-isotopic data for diamonds from the Monastery and Jagersfontein kimberlites that sample the deep asthenosphere and transition zone beneath the Kaapvaal Craton, in the mid Cretaceous, to investigate the nature of mantle fluids at these depths and the constraints they provide on the deep volatile cycle. Both diamond suites exhibit very light δ13C values (down to −26‰) and heavy δ15N (up to +10.3‰), with nitrogen abundances generally below 70at.ppm but varying up to very high concentrations (2520at.ppm) in rare cases. Combined, these signatures are consistent with derivation from subducted crustal materials. Both suites exhibit variable nitrogen aggregation states from 25 to 100% B defects. Internal growth structures, revealed in cathodoluminescence (CL) images, vary from faintly layered, through distinct cores to concentric growth patterns with intermittent evidence for dissolution and regular octahedral growth layers in places. Modelling the internal co-variations in δ13C-δ15N-N revealed that diamonds grew from diverse C-H-O-N fluids involving both oxidised and reduced carbon species. The diversity of the modelled diamond-forming fluids highlights the complexity of the volatile sources and the likely heterogeneity of the deep asthenosphere and transition zone. We propose that the Monastery and Jagersfontein diamonds form in subducted slabs, where carbon is converted into either oxidised or reduced species during fluid-aided dissolution of subducted carbon before being re-precipitated as diamond. The common occurrence of recycled C and N isotopic signatures in super-deep diamonds world-wide indicates that a significant amount of carbon and nitrogen is recycled back to the deep asthenosphere and transition zone via subducting slabs, and that the transition zone may be dominated by recycled C and N.

      PubDate: 2017-08-03T08:05:29Z
  • Fluid-mobile elements in serpentinites: Constraints on serpentinisation
           environments and element cycling in subduction zones
    • Abstract: Publication date: Available online 20 July 2017
      Source:Chemical Geology
      Author(s): Daniel Peters, Annette Bretscher, Timm John, Marco Scambelluri, Thomas Pettke
      Fluid-mobile element (FME) systematics in serpentinites are key to unravel the environments of mantle rock hydration, dehydration, and element recycling in subduction zones. Here we compile serpentinite geochemical data and, for the first time, report discriminative FME enrichment trends for mid ocean ridge vs. forearc serpentinisation by applying alkali element-U ratios. Characteristic element fractionations are thereby governed by redox-dependent differential U mobility at mid ocean ridges and in forearcs, and by high Cs input in forearcs due to fluids equilibrated with sediments. Simple modelling reproduces the observed enrichment trends in serpentinites that range over several orders of magnitude. From these systematics, first constraints on potentially discriminative fractionation trends for unconventional fluid tracers such as B, As, and Sb can be deduced. Prominent W enrichments that correlate with FMEs suggest significant W mobility in low-temperature serpentinising environments. Application of the alkali element-U systematics to the subducted metaperidotites of Erro Tobbio (recording initial brucite+antigorite breakdown during subduction) and Almirez (recording final antigorite breakdown) reveal that pre-subduction FME enrichment signatures are retained in progressively subducted hydrous mantle rocks to beyond subarc levels. Associated dehydration veins and fluid inclusions reveal subordinate alkali element-U fractionation trends during dehydration. Subducted hydrous mantle rocks therefore may introduce characteristic element signatures and thus contribute towards mantle geochemical heterogeneities.
      Graphical abstract image

      PubDate: 2017-07-23T14:27:32Z
  • The analytical limits of modeling short diffusion timescales
    • Abstract: Publication date: Available online 20 July 2017
      Source:Chemical Geology
      Author(s): Richard W. Bradshaw, Adam J.R. Kent
      Diffusion modeling of chemical zonation in crystals and glasses is a widely used method of quantifying the timescales of a variety of geological processes. Obtaining timescale information from diffusion modeling relies on fitting modeled diffusion profiles to measured compositional gradients. Thus analytical factors, such as spatial resolution, analysis location and analytical uncertainty have the potential to limit the accuracy and precision of calculated diffusion timescales, especially when the resolution of the individual analyses approaches the width of the observed diffusion gradient. Herein we use a probabilistic modeling approach to assess the accuracy of timescales based on diffusion modeling at various spatial resolutions and diffusivities. We have calculated synthetic 1D diffusion profiles produced from simple step function geometries at various diffusivities and timescales and then resampled these at common analytical spatial resolutions. Standard diffusion modeling techniques were used to estimate apparent diffusion timescales from the resampled profiles to compare with the “true” synthetic times. Results confirm that for a given diffusivity, higher analytical spatial resolution gives access to shorter timescales, and that for each analytical resolution there is a minimum threshold timescale, below which diffusion modeling significantly overestimates the true timescale. The accuracy of short diffusion timescales depends on the width of the diffusion gradient (the portion of a profile that has been modified by diffusion) relative to the analytical spatial resolution, with models becoming accurate (within 20%) at a gradient width/spot size ratio>2. The precision of diffusion timescales depends on the gradient width to spot size ratio, the magnitude of analytical uncertainty and the magnitude of the compositional difference across the step function boundary. Precision is improved with a large gradient width/spot size ratio, and where the magnitude of the compositional difference across the step function boundary is large relative to the analytical uncertainty. We present a generalized method to quantify these threshold timescales for any given spatial resolution and diffusivity, and present simple guidelines to aid in selecting compositional profiles that will produce more accurate diffusion timescale estimates.

      PubDate: 2017-07-23T14:27:32Z
  • Carbonated sediment recycling and its contribution to lithospheric
           refertilization under the northern North China Craton
    • Abstract: Publication date: Available online 20 July 2017
      Source:Chemical Geology
      Author(s): Chunfei Chen, Yongsheng Liu, Stephen F. Foley, Mihai N. Ducea, Xianlei Geng, Wen Zhang, Rong Xu, Zhaochu Hu, Lian Zhou, Zaicong Wang
      Subduction of carbonated sediments is the principal mechanism for the replenishment of mantle carbon reservoirs, but the processes of transport of carbonated sediments into the mantle and their incorporation into mantle chemical and isotopic heterogeneities are poorly constrained. Here we use in-situ major and trace element and Sr isotopic compositions and mineral Sr-Nd isotopes of the peridotite xenoliths (lherzolites with minor harzburgites) in the Oligocene Fanshi basalts from the northern North China Craton (NCC) to characterize and decode two metasomatic events caused by subduction of carbonated sediments. Clinopyroxenes (Cpx) in the harzburgites are characterized by high (La/Yb)N (8.4–66), Ca/Al (4.7–6.4) and Zr/Hf (30–66) ratios but low Ti/Eu ratios (478–1268) coupled with strongly enriched in large ion lithophile elements (LILE) and light rare earth elements (LREE) but depleted high field-strength elements (HFSE), suggesting that they have been metasomatized by carbonatitic melt. Highly enriched Sr-Nd isotopic compositions of these Cpx (87Sr/86Sr=0.70640–0.70716, 143Nd/144Nd=0.512304) imply that metasomatic carbonatitic melt was derived from carbonated sediments. Compositional zonation of Cpx in a harzburgite shows decreasing 87Sr/86Sr ratio, increasing Sr content and decreasing Zr/Hf ratio from cores to the rims, documenting a later stage of metasomatism. In situ Sr isotopes in Cpx from both harzburgites and lherzolites converge in diagrams of 87Sr/86Sr vs Sr, Zr/Hf, Ti/Eu and (La/Yb)N, indicating that these peridotites could have been extensively affected by the second metasomatic event. Cpx in the lherzolites display lower (La/Yb)N (0.16–10.6) and Zr/Hf ratios (25–39), suggesting that the later metasomatic agent is probably a carbonate-rich silicate melt and not carbonatite. Sr and Nd isotopes of these Cpx (87Sr/86Sr=0.702075–0.706148, 143Nd/144Nd=0.512410–0.513286) exhibit negative correlation along a simple mixing line between depleted mantle and carbonated sediments. The low Ce/Pb ratio (<12) of the metasomatic agent suggests that the carbonate-rich silicate melt also originated from carbonated sediments. The timing of the two metasomatic events and the regional tectonic setting are consistent with derivation of the carbonated sediments from the Paleo-Asian Oceanic slab: low degree melting formed early carbonatite melts, which caused decreasing carbonate content of carbonated sediments, whereas subsequent higher degree melting of residual carbonated sediments produced the later carbonate-rich silicate melt. Both contributed significantly to lithospheric refertilization under the northern NCC.

      PubDate: 2017-07-23T14:27:32Z
  • Geochemistry, geothermometry and influence of the concentration of mobile
           elements in the chemical characteristics of carbonate-evaporitic thermal
           systems. The case of the Tiermas geothermal system (Spain)
    • Abstract: Publication date: Available online 19 July 2017
      Source:Chemical Geology
      Author(s): M. Blasco, L.F. Auqué, M.J. Gimeno, P. Acero, M.P. Asta
      The Tiermas low temperature geothermal system, hosted in the Paleocene-Eocene carbonates of the Jaca-Pamplona basin, has been studied to evaluate the geochemistry and the temperature of the waters in the deep reservoir. These waters are of chloride-sodium type and emerge with a temperature of about 37°C. Two hydrogeochemical groups of waters have been distinguished: one with lower sulphate concentration and lower TDS (about 7500ppm) and the other with higher sulphate content and TDS values (close to 11,000ppm). There are also slight differences in the reservoir temperature estimated for each group. These temperatures have been determined by combining several geothermometrical techniques: (1) classical chemical geothermometers (SiO2-quartz, Na-K, K-Mg and Na-K-Ca), (2) specific geothermometers for carbonate systems (Ca-Mg), (3) isotopic geothermometers and, (4) geothermometrical modelling. The good agreement in the temperature obtained by these techniques, including the cationic geothermometers which are not usually considered suitable for this type of systems, allows establishing a reliable range of temperature of 90±20°C for the low-sulphate waters and 82±15°C for the high-sulphate waters. The mineral assemblage in equilibrium in the reservoir is assumed to be the same for both groups of waters (calcite, dolomite, quartz, anhydrite, albite, K-feldspar and other aluminosilicate phases); therefore, the differences found in the reservoir temperature and, mostly, in the geochemical characteristics of each group of waters must be due to the existence of two flow patterns, with slightly different temperatures and intensity of water-rock interaction. Anhydrite is at equilibrium in the reservoir suggesting that, although this system is hosted in carbonates, evaporites may also be present. The dissolution of halite (the increase in the chloride concentration) conditions the chemical characteristics of the waters and the equilibrium situations in the reservoir and waters acquire their chloride-sodium affinity at depth and not during their ascent to the surface. Finally, a favourable tectonic structure for CO2 storage has been recognised in the Paleocene-Eocene carbonates of this area. Therefore, considering the characteristics of these waters (in equilibrium with calcite, dolomite and anhydrite in the reservoir), the results of this work are useful to understand some of the geochemical processes that might take place during the CO2 injection: 1) precipitation of carbonates and sulphates in the vicinity of the injection well due to desiccation of the waters and, 2) carbonate dissolution and sulphate precipitation in the long term.

      PubDate: 2017-07-23T14:27:32Z
  • Molybdenum dynamics in sediments of a seasonally-hypoxic coastal marine
    • Abstract: Publication date: Available online 19 July 2017
      Source:Chemical Geology
      Author(s): Fatimah Sulu-Gambari, Anne Roepert, Tom Jilbert, Mathilde Hagens, Filip J.R. Meysman, Caroline P. Slomp
      Molybdenum (Mo) enrichments in marine sediments are a common indicator of the presence of sulphide near the sediment-water interface and can thereby record historic bottom-water oxygen depletion. Here, we assess the impact of temporal changes in manganese (Mn) cycling and bottom-water oxygen on sedimentary Mo dynamics in a seasonally-hypoxic coastal marine basin (Lake Grevelingen, the Netherlands). High resolution line scans obtained with LA-ICP-MS and discrete sample analyses reveal distinct oscillations in Mo with depth in the sediment. These oscillations and high sediment Mo concentrations (up to ~130ppm) are attributed to deposition of Mo-bearing Mn-oxide-rich particles from the overlying water, the release of molybdate (MoO4 2−) to the pore water upon reduction of these Mn-oxides, and subsequent sequestration of Mo. The latter process only occurs in summer when sulphide concentrations near the sediment-water interface are elevated. Gravitational focussing of Mn oxides explains the observed increased input of Mo with increasing water depth. Diffusion of MoO4 2− from the overlying water contributes only a small amount to the sediment Mo enrichments. Cable bacteria may indirectly impact sediment Mo dynamics by dissolving Mn-carbonates and thereby enhancing the pool of Mn-oxides in the system, and by contributing to remobilisation of sediment Mo during oxic periods. A sediment record that spans the past ~45years indicates that sediment Mo concentrations have increased over the past decades, despite less frequent occurrences of anoxia in the bottom waters based on oxygen measurements from water column monitoring. We suggest that the elevated Mo in recent sediments reflects both enhanced rates of sulphate reduction and sulphide production in the surface sediment as a result of increased input of organic matter into the basin from the adjacent North Sea since 1999, and an associated enhanced “Mn refluxing” in the marine lake in summer.

      PubDate: 2017-07-23T14:27:32Z
  • Pb isotopes in the impact melt breccia 66095: Association with the Imbrium
           basin and the isotopic composition of lithologies at the Apollo 16 landing
    • Abstract: Publication date: Available online 19 July 2017
      Source:Chemical Geology
      Author(s): Joshua F. Snape, Alexander A. Nemchin, Jeremy J. Bellucci, Martin J. Whitehouse
      Recent in situ Secondary Ion Mass Spectrometry (SIMS) Pb isotope analyses of lunar basalts have provided precise crystallisation ages and initial Pb isotopic compositions for these samples. In this study, the same approach has been tested in the Apollo 16 impact melt breccia 66095, referred to as the “Rusty Rock” due to its enrichments in volatile elements, including Pb. Based on these analyses of the breccia, a Pb-Pb isochron age of 3909±17Ma (at the 95% confidence level) and an initial Pb composition for 66095 have been determined. This age is interpreted as representing the time of breccia formation that, when combined with recent studies of lunar breccias, can be linked to the Imbrium basin forming impact. The directly measured initial Pb composition of the breccia from this work is similar a modelled compositions presented previously, and likely reflects an average value for the lithologies present at the Apollo 16 landing site at the time that the Imbrium ejecta was emplaced. The 66095 initial Pb isotopic composition is compared with the compositions in other lunar samples and the nature of the endmember lithologies in this mixture has been discussed within the framework of a multiple stage model of Pb isotope evolution on the Moon. This study demonstrates the effectiveness of this technique beyond its application in crystalline basalts, opening up the possibility of obtaining precise geochronological and Pb isotopic compositions from a broader sample set than was previously recognised.

      PubDate: 2017-07-23T14:27:32Z
  • Chemical and boron isotopic composition of hydrothermal tourmaline from
           the Panasqueira W-Sn-Cu deposit, Portugal
    • Abstract: Publication date: Available online 14 July 2017
      Source:Chemical Geology
      Author(s): Marta S. Codeço, Philipp Weis, Robert B. Trumbull, Filipe Pinto, Pilar Lecumberri-Sanchez, Franziska D.H. Wilke
      Tourmaline is a locally abundant hydrothermal mineral in the wallrocks surrounding the W-Sn-Cu mineralized veins in the Panasqueira deposit (Portugal) and a minor phase within the veins themselves. Tourmaline chemical and boron-isotope compositions have been determined from three settings: (1) pervasive fine-grained tourmalinization zones in wallrocks within 10cm of the veins, (2) coarser tourmaline in wallrock-hosted fault zones, and (3) needle-shaped tourmaline from late-stage vugs in the quartz veins. All tourmalines from Panasqueira have ferromagnesian compositions with significant octahedral aluminum contents and variable X-site vacancies. Tourmaline compositions show significant chemical variations at the grain and sample scale but are homogeneous on the deposit scale. Tourmaline is typically optically and chemically zoned, showing significant increases in Fe and F and decrease in Mg, Ca and Al from core to rim. Moreover, as a general trend, Al increases and Mg decreases with proximity to the vein contact. The observed chemical variation seems to have been controlled mainly by the vectors: FeMg−1; NaR2+(X□Al)−1; NaR2+ F(X□AlOH)−1; (R2+)(OH,F)(AlO)−1 and Na(R2+) 2(OH,F)X□−1Al−2O−1. The total range in tourmaline δ11B values from the Panasqueira W-Sn-Cu deposit is from −3.7 to −13.3‰, including tourmaline from the three settings. The calculated B-isotope composition of the hydrothermal fluid based on the average tourmaline composition of −9‰ and an estimated alteration temperature in the wallrocks, based on Ti-in-quartz thermometry, of 500°C is −7.1‰. This isotopic composition is permissive of a boron source from local metasediments or an S-type granite, but the high boron concentration in the wallrocks, with >50vol% tourmaline, along with the typical element association of Panasqueira (W, Sn, F, Nb, Ta) makes a granitic source more likely. Late-stage tourmaline needles in vugs within the mineralized veins show the same patterns and ranges in both chemical and boron isotope composition, suggesting that exsolved magmatic fluids were injected in at least two pulses. The variations in chemical and boron isotopic compositions of tourmaline combined with estimates from Ti-in-quartz thermometry suggest that hydrothermal ore formation is associated with fluid cooling and geochemical fluid-rock interactions.

      PubDate: 2017-07-23T14:27:32Z
  • Synthesis and characterization of redox-active ferric nontronite
    • Abstract: Publication date: Available online 12 July 2017
      Source:Chemical Geology
      Author(s): A.G. Ilgen, R.K. Kukkadapu, D.R. Dunphy, K. Artyushkova, J.M. Cerrato, J.N. Kruichak, M.T. Janish, C.J. Sun, J.M. Argo, R.E. Washington
      Heterogeneous redox reactions on clay mineral surfaces control mobility and bioavailability of redox-sensitive nutrients and contaminants. Iron (Fe) residing in clay mineral structures can either catalyze or directly participate in redox reactions; however, chemical controls over its reactivity are not fully understood. In our previous work we demonstrated that converting a minor portion of Fe(III) to Fe(II) (partial reduction) in the octahedral sheet of natural Fe-rich clay mineral nontronite (NAu-1) activates its surface, making it redox-active. In this study we produced and characterized synthetic ferric nontronite (SIP), highlighting structural and chemical similarities and differences between this synthetic nontronite and its natural counterpart NAu-1, and probed whether mineral surface is redox-active by reacting it with arsenic As(III) under oxic and anoxic conditions. We demonstrate that synthetic nontronite SIP undergoes the same activation as natural nontronite NAu-1 following the partial reduction treatment. Similar to NAu-1, SIP oxidized As(III) to As(V) under both oxic (catalytic pathway) and anoxic (direct oxidation) conditions. The similar reactivity trends observed for synthetic nontronite and its natural counterpart make SIP an appropriate analog for laboratory studies. The development of chemically pure analogs for ubiquitous soil minerals will allow for systematic research of the fundamental properties of these minerals.
      Graphical abstract image

      PubDate: 2017-07-23T14:27:32Z
  • Coupled reactive transport model study of pore size effects on solubility
           during cement-bicarbonate water interaction
    • Abstract: Publication date: Available online 12 July 2017
      Source:Chemical Geology
      Author(s): Sanheng Liu, Diederik Jacques
      We investigate the interaction between Ca rich cementitious materials and bicarbonate rich clay pore water with the primary focus on pore size effect on the dissolution of the main hydration products, portlandite and calcium silicate hydrates (C-S-H), and the precipitation of the secondary mineral, calcite, in a coupled reactive transport modelling framework. The concept of pore-size dependent solubility is extended to reactive transport modelling within cementitious materials. For the two pore sizes 1e-6 and 1e-8m considered here the effect of pore size on the solubility of portlandite, C-S-H and calcite is different, with portlandite and calcite less affected and C-S-H more affected due to a higher water/C-S-H interfacial tension. This difference leads to a different evolution of porosity at the interface of a cementitious material and bicarbonate-enriched pore water between systems with large or small pores. In systems with mixed large and small pores, if diffusion through large pores is much faster than through small pores, precipitation will only occur in large pores because the pore solution diffused into small pores has already become under-saturated with respect to calcite due to precipitation in large pores first. These results might also explain why cementation of porous rocks occurs preferentially in large pores relative to small pores observed in nature.

      PubDate: 2017-07-23T14:27:32Z
  • Geochemistry of thermal waters in the Southern Volcanic Zone, Chile –
           Implications for structural controls on geothermal fluid composition
    • Abstract: Publication date: Available online 10 July 2017
      Source:Chemical Geology
      Author(s): Jackie Wrage, Daniele Tardani, Martin Reich, Linda Daniele, Gloria Arancibia, José Cembrano, Pablo Sánchez-Alfaro, Diego Morata, Rodrigo Pérez-Moreno
      Thermal waters in the Southern Volcanic Zone (SVZ) of Chile between 36°–41°S were studied using major and selected trace element relationships to characterize their geochemistry, formation mechanisms, and to explore the influence of regional structural controls on fluid composition. Three sets of waters were identified based on physicochemical characteristics: (i) NaCl waters, (ii) acid-sulfate waters, and (iii) bicarbonate (HCO3) waters. NaCl waters are the most abundant type in the studied region and their chemistry is controlled by significant water-rock interaction. They are characterized by an alkaline pH (7.2–9.3), generally lower temperatures (avg: 55°C), and relatively high concentrations of Cl, Na, B, As, Li, and Cs. Acid-sulfate waters are typically associated with volcanoes and have a strong magmatic/volcanic component due to the absorption of magmatic vapors. They are acidic (pH<4), generally higher in temperature (avg: 85°C), and have elevated concentrations of SO4, Mg, and Ba. Bicarbonate waters are characterized by the highest concentrations of HCO3 (>892ppm) in the region and are similar in temperature (<47°C) and pH (>6.2) to NaCl waters. They have elevated concentrations of most cations (Ca, K, Na, Mg, Ba, Sr) as a result of intense shallow cation leaching due to the absorption of CO2-rich volcanic vapors on the peripheries of geothermal systems. The thermal waters were also characterized according to their spatial relation with the dominant fault systems of the region: the NNE-striking intra-arc Liquiñe-Ofqui Fault System (LOFS) and the WNW-striking Andean Transverse Faults (ATF). The inherent differences in fault nature between these fault systems constitutes the primary structural control influencing geothermal fluid development in the SVZ. The chemistry of waters spatially associated with the LOFS as a whole is defined by high vertical permeability networks and lack magmatic reservoir development. Therefore, these waters tend to have higher Cl/B ratios and strong correlations between trace alkali metals and Cl due to rapid, efficient upflow pathways. In contrast, waters spatially associated with the ATF have lower Cl/B ratios and show no correlation between trace alkali metals and Cl due to degassing magma chambers and decreased vertical permeability. The relationship between water type and structural domain in Cl/B ratios and trace metal behavior provides evidence that fault geometry and kinematics exert a fundamental control on geothermal fluid development in the SVZ of Chile.

      PubDate: 2017-07-23T14:27:32Z
  • Inside front cover (Editorial Board)
    • Abstract: Publication date: 20 August 2017
      Source:Chemical Geology, Volume 465

      PubDate: 2017-06-15T14:25:12Z
  • Skeletal growth controls on Mg/Ca and P/Ca ratios in tropical Eastern
           Pacific rhodoliths (coralline red algae)
    • Abstract: Publication date: 20 August 2017
      Source:Chemical Geology, Volume 465
      Author(s): Hillary R. Sletten, David P. Gillikin, Jochen Halfar, C. Fred T. Andrus, Héctor M. Guzmán
      The Mg/Ca ratios of non-geniculate coralline red algae (CA) are often assumed to be solely dependent on the sea surface temperature (SST) in which they grow, suggesting that they are ideal proxies for SST reconstruction. Mg/Ca ratio cyclicity is also commonly used as a marker for annual growth band identification in many CA species. However, other controls on elemental uptake such as light, calcification mechanisms, and vital effects may control elemental ratios (e.g. Mg/Ca and P/Ca) in certain genera or species of CA, thus complicating the use of Mg/Ca and P/Ca ratios as a proxy source for SST and paleo-environmental reconstructions. Here we present evidence showing that the Mg/Ca and P/Ca ratio cyclicity in unattached living tropical Lithothamnion sp. (n=8) rhodoliths correlate >90% to the number of sub-annual growth bands (n=1 to 20) formed during a 172-day controlled growth experiment and not to temperature or other environmental conditions (e.g. salinity, pH, etc.). This pattern is also observed in the wild, pre-experimental growth further supporting that the bands are not annual. The Mg/Caalga (R2 =0.67; p-value=0.013; n=8) and P/Caalga (R2 =0.64; p-value=0.018; n=8) significantly correlate to skeletal extension during the experiment. This work has implications for trace elemental proxy development using CA as well as the influence of skeletal extension on elemental incorporation in biogenic carbonates, particularly rhodoliths of Lithothamnion sp. from the Gulf of Panama, Panama.

      PubDate: 2017-06-15T14:25:12Z
  • The chemical and isotopic compositions of volatiles in magmatic
           hydrothermal fluids beneath the Songliao Basin, northeastern China
    • Abstract: Publication date: 20 August 2017
      Source:Chemical Geology, Volume 465
      Author(s): Jinliang Gao, Jiaqi Liu, David R. Hilton, Fanchao Meng, Zhongping Li, Lina Zhai, Chunqing Sun, Lei Zhang
      It is controversial whether deep fluids can make contributions to natural gas accumulations in the Songliao Basin, NE China. The occurrence of magmatic hydrothermal quartz veins in crystalline basement of the basin provides a way to understand the characteristics of deep fluids and their significance for natural gas accumulation. Three types of fluid inclusions were identified in hydrothermal quartz: aqueous inclusions, H2O-CO2-rich inclusions and H2O-CO2-CH4-rich inclusions. The primary inclusions show high homogenization temperatures (280–433°C) which are typical of magmatic hydrothermal fluids. The oxygen and hydrogen isotopic compositions (δ18O and δD) of water are 2.0‰ to 4.6‰ and −91.6‰ to −75.7‰, respectively. The OH isotopic compositions suggest that hydrothermal fluids associated with the quartz veins were derived from the primary magmatic system and mixed by small amounts of meteoric water. Two types of magmatic hydrothermal volatiles were identified in the study. Both types of volatiles are dominated by CO2. CO2 in type I volatiles shows more negative δ13C values (−13.8‰ to −9.7‰) than typical magmatic CO2, which likely resulted from carbon isotopic fractionation during magma degassing. Type II volatiles have higher δ13C values (−5.5‰ to −3.3‰) and are similar to typical magmatic CO2. Small amounts of hydrocarbons were observed in both types of magmatic hydrothermal fluids. Hydrocarbons in type I volatiles have relatively high δ13CCH4 values (−30.6‰ to −24.1‰) and reversed carbon isotopic trends while hydrocarbons in type II volatiles have more negative δ13CCH4 values (−49.7‰ to −37.6‰) and orderly carbon isotopic trends. CO2 in hydrothermal fluids is a potential source for CO2 gas accumulations. Hydrocarbons in hydrothermal fluids, however, can hardly make significant contributions to the commercial gas reservoirs due to their low concentration.

      PubDate: 2017-06-15T14:25:12Z
  • Building biogenic beachrock: Visualizing microbially-mediated carbonate
           cement precipitation using XFM and a strontium tracer
    • Abstract: Publication date: 20 August 2017
      Source:Chemical Geology, Volume 465
      Author(s): Jenine McCutcheon, Luke D. Nothdurft, Gregory E. Webb, Jeremiah Shuster, Linda Nothdurft, David Paterson, Gordon Southam
      The fate of reef islands is a topic of ongoing debate in the face of climate change-induced sea-level rise and increased cyclone intensity. Increased erosion and changes to the supply of reef-derived sediment may put sand reef cays at risk of dramatic morphological changes. These changes may have negative implications for the existence of reef cay environments, which host vital sea turtle and bird rookery habitats. Beachrock, consolidated carbonate beach sediment in the intertidal zone, forms naturally on many tropical beaches and reduces the erosion rates of these beaches when compared to unconsolidated sand. In spite of the critical role beachrock plays in stabilizing some reef cay shores, the method of beachrock formation is still incompletely understood. In this investigation, beachrock was synthesized using beach sand and beachrock samples from Heron Island (Great Barrier Reef, Australia) in aquarium experiments in which natural beachrock formation conditions were simulated. Beachrock was produced in two aquaria wherein the water chemistry was influenced by microorganisms derived from the natural beachrock ‘inoculum’, whereas no cementation occurred in an aquarium that lacked a microbial inoculum and was controlled only by physicochemical evapoconcentration. The new cements in the synthesized beachrock were analyzed using synchrotron-based X-ray fluorescence microscopy and were identified using a Sr-tracer added to the experimental seawater. The resulting precipitates cement sand grains together and contain abundant microfossils of the microorganisms on whose exopolymer they nucleated, demonstrating the fundamental role microbes play in beachrock formation. These results are of interest because beachrock could be utilized as a natural coastline stabilization strategy on sand reef cays, and in turn, protect the unique habitats reef islands support.
      Graphical abstract image

      PubDate: 2017-06-15T14:25:12Z
  • The impact of partition coefficient data on the interpretation of chemical
           tracer behaviour in carbon geosequestration projects
    • Abstract: Publication date: 20 August 2017
      Source:Chemical Geology, Volume 465
      Author(s): Matthew Myers, Cameron White, Tara La Force, Charles Heath, Se Gong, Linda Stalker, Bobby Pejcic
      Partition (or distribution) coefficients determine the relative equilibrium concentrations of chemical constituents (or chemical tracers) in each of the phases of a multi-phase system under dilute conditions. The various fluid phases in a reservoir have differing transport properties (e.g. varying relative permeability) and to correctly interpret the behaviour of injected chemical tracers it is essential that accurate partition coefficients are known. In the context of carbon geosequestration or long-term storage of CO2, chemical tracers will be predominantly exposed to an environment consisting of supercritical CO2 and formation water as the main fluid phases. To estimate/simulate the reservoir properties relevant to injected CO2 tagged with chemical tracers, it is therefore necessary to incorporate high pressure/temperature CO2/water partition coefficients into any model/simulation. In this paper, we present a method to determine these partition coefficients for gaseous chemical tracers using a variation of the widely used EPICS (or equilibrium partitioning in a closed system) method. With this method, only the concentration in one phase (in this case, CO2) needs to be measured. We then report these values for a series of representative chemical tracers (i.e. krypton, xenon, sulfur hexafluoride, perdeuterated methane and R134a) at pressure/temperature conditions that have been previously used at the CO2CRC's Otway CCS demonstration project in Victoria, Australia. These values were generally lower than the corresponding Henry's Law coefficients at comparable temperatures. Experiments also examined the impact of adding CH4 to the system to mimic feedstock gas at the CO2CRC Otway project and provide data pertinent to scenarios where CO2 is injected into depleted CH4 gas fields. These values are compared with Henry's Law coefficients and another recently published set of high pressure/temperature partition coefficients. With computational simulations, we have shown that these differences are potentially significant and demonstrate their impact in three typical CO2 geo-sequestration scenarios (i.e. injection into two types of aquifers and injection into a depleted reservoir with a gas cap).

      PubDate: 2017-06-15T14:25:12Z
  • Dolomite dissolution in aqueous solutions in the presence of nucleotides
           and their structural components at 25°C and pCO2 ~1atm
    • Abstract: Publication date: 20 August 2017
      Source:Chemical Geology, Volume 465
      Author(s): Sanoopkumar Puthiya Veetil, Alfonso Mucci, Takeshi Arakaki
      The kinetics and stoichiometry of dolomite dissolution were investigated in deionized water at 25°C and pCO2 ~1atm in the absence (control) and presence of various nucleotides and their structural components at low concentrations (1mM). Dissolutions were conducted in “free-drift” mode and rates estimated based on the accumulation rate of calcium and magnesium ions in solution. The additives considered in this study are likely present at low concentrations (<mM) in deep, carbonate aquifers targeted for CO2 geological sequestration. Results of our control experiment are consistent with rates and stoichiometries reported in earlier studies. In the presence of nucleotides, dolomite dissolution was inhibited and proceeded incongruently, yielding a Mg:Ca solution ratio ≥2. An investigation of the nucleotide structural components (nucleosides, nitrogenous bases and phosphates) revealed that, whereas they nearly all inhibited dolomite dissolution, only phosphate salts (mono-, di-, tri-, and hexametaphosphate) lead to incongruent dissolution. Examination of the surface morphology and phosphate analysis of the solids reacted in the presence of nucleotides and phosphate salts revealed the formation of a solid phosphate phase on the dolomite surface, likely a Ca rich-phosphate phase that accounts for the observed dissolution incongruency.

      PubDate: 2017-06-15T14:25:12Z
  • Inside front cover (Editorial Board)
    • Abstract: Publication date: 5 August 2017
      Source:Chemical Geology, Volume 464

      PubDate: 2017-06-01T15:20:29Z
  • Dirac's dream: Understanding metal sorption by geomedia using density
           functional theory
    • Abstract: Publication date: 5 August 2017
      Source:Chemical Geology, Volume 464
      Author(s): Kideok D. Kwon, Garrison Sposito
      Density functional theory (DFT) can provide accurate, computationally-manageable, first-principles characterizations of mineral surface speciation at molecular scales. This is accomplished by solving the Kohn-Sham equation, a tractable method of applying quantum mechanics to multi-electron systems that was inspired by some visionary remarks of P. A. M. Dirac, one of the founders of quantum theory. Fifty years of advances in DFT and the development of massively-parallel computing platforms have paved the way for molecular-scale modeling of the mineral surface that is directly relevant to the study of natural nanoparticles. In fact, DFT has matured sufficiently to be applied to metal sorption by geomedia as a “computer experiment,” yielding quantitative molecular-scale information that fully complements insights gained from surface spectroscopy. This powerful approach is illustrated first through a series of structural simulations of chalcophanite group minerals and hetaerolite–hausmannite solid solutions, then through several recent studies by the authors and their collaborators focusing on transition and heavy metal sorption by layer-type Mn(IV) oxide and Fe(II) sulfide nanoparticles. Our results show how DFT simulations reveal the mechanistic basis of metal partitioning trends observed experimentally while creating useful models for resolving experimental surface speciation conundrums.

      PubDate: 2017-06-01T15:20:29Z
  • A density functional theory investigation of oxalate and Fe(II) adsorption
           onto the (010) goethite surface with implications for ligand- and
           reduction-promoted dissolution
    • Abstract: Publication date: 5 August 2017
      Source:Chemical Geology, Volume 464
      Author(s): James D. Kubicki, Daniel Tunega, Stephan Kraemer
      Oxalic acid is an important, biologically-produced species in the natural environment. The deprotonated form, oxalate, is dominant in aqueous solutions under circumneutral pH conditions and is a strong ligand for Fe(III). The high affinity of oxalate for Fe(III) means that Fe(III)-oxalate surface and aqueous complexes are common and can lead to ligand-enhanced dissolution. Fe(II) adsorption onto goethite (α-FeOOH) has been shown to enhance dissolution-recrystallization reactions. The goethite (010) face is one of the more common and reactive surfaces on this environmentally critical Fe-hydroxide phase. Hence, this study models both separate and coordinated adsorption of oxalate and Fe(II) onto the (010) face of goethite in order to test for synergistic effects of ligand-promoted and reductive dissolution. Periodic and cluster density functional theory (DFT) energy minimizations were performed to determine the structure, vibrational frequencies and energies of various configurations. The adsorption mechanism of oxalate is verified via comparison to observed IR spectra. The potential roles of oxalate and Fe(II) in ligand-enhanced reductive dissolution of goethite are discussed.
      Graphical abstract image

      PubDate: 2017-06-01T15:20:29Z
  • An integrated flow microcalorimetry, infrared spectroscopy and density
           functional theory approach to the study of chromate complexation on
           hematite and ferrihdyrite
    • Abstract: Publication date: 5 August 2017
      Source:Chemical Geology, Volume 464
      Author(s): Nadine J. Kabengi, Maria Chrysochoou, Nefeli Bompoti, James D. Kubicki
      An approach to constructing comprehensive predictive models for contaminant interactions with mineral surfaces is to obtain multiples lines of evidence for surface properties and the types of complexes formed under different geochemical conditions. In this study, we used flow adsorption microcalorimetry (FAMC), attenuated total reflection infrared (ATR FTIR) spectroscopy, and density functional theory (DFT) calculations to study chromate complexation on ferrihydrite (FH) and hematite (HT). Under the experimental conditions used, chromate binds via an exothermic inner-sphere complexation on both surfaces, with similar enthalpy values that do not reveal dramatic differences in the sorption mechanism. Due to their underlying surface structure, FH favors more monodentate and HT more bidentate complexation. Chromate complexes were found by ATR and FAMC to be completely reversible, with substantially slower desorption compared to sorption. Both the FAMC and DFT indicated the presence of surface sites with different energetics, whereby lower surface coverages corresponded to higher enthalpies on both FH and HT. Both flow-based ATR and FAMC yielded different surface coverages than batch isotherms under the same conditions, highlighting the need to assess contaminant sorption under realistic conditions. Overall, this integrated approach proved to be an improved paradigm to study ion sorption on mineral surfaces.

      PubDate: 2017-06-01T15:20:29Z
  • Formation of a mixed Fe(II)-Zn-Al layered hydroxide: Effects of Zn
           co-sorption on Fe(II) layered hydroxide formation and kinetics
    • Abstract: Publication date: 5 August 2017
      Source:Chemical Geology, Volume 464
      Author(s): Autumn N. Starcher, Evert J. Elzinga, Donald L. Sparks
      Previous research demonstrated the formation of single divalent metal (Co, Ni, and ZnAl) and mixed divalent metal (NiZnAl) layered double hydroxide (LDH) phases from reactions of the divalent metal with Al-bearing substrates and soils in both laboratory experiments and in the natural environment. Recently Fe(II)-Al-LDH phases have been found in laboratory batch reaction studies, and although they have yet to be found in the natural environment. Potential locations of Fe(II)-Al-LDH phases in nature include areas with suboxic and anoxic conditions. Because these areas can be environments of significant contaminant accumulation, it is important to understand the possible interactions and impacts of contaminant elements on LDH phase formation. One such contaminant, Zn, can also form as an LDH and has been found to form as a mixed divalent layered hydroxide phase. To understand how Zn impacts the formation of Fe(II)-Al-LDH phase formation and kinetics, 3mM or 0.8mM Fe(II) and 0.8mM Zn were batch reacted with either 10g/L pyrophyllite or 7.5g/L γ-Al2O3 for up to three months under anoxic conditions. Aqueous samples were analyzed by inductively coupled plasma optical emission spectrometry (ICP-OES) and solid samples were analyzed with X-ray absorption spectroscopy (XAS). Shell-by-shell fits of Fe(II) and co-sorption samples with pyrophyllite show the formation of a mixed divalent metal (Fe(II)-Zn-Al) layered hydroxide phase, while Fe(II) and Zn co-sorption samples with γ-Al2O3 produce Fe(II)-Al-LDH phases and Zn in inner-sphere complexation with the γ-Al2O3. This study demonstrates the formation of a mixed divalent metal layered hydroxide and further iterates the importance of sorbent reactivity on LDH phase formation.

      PubDate: 2017-06-01T15:20:29Z
  • Comparison of tungstate and tetrathiotungstate adsorption onto pyrite
    • Abstract: Publication date: 5 August 2017
      Source:Chemical Geology, Volume 464
      Author(s): Minming Cui, Karen H. Johannesson
      Adsorption of tungstate (WO4 2−) and tetrathiotungstate (WS4 2−) onto pyrite was investigated as a function of W species concentrations, pH, and ionic strength. Additional experiments examined the adsorption of these W species onto goethite. The adsorption experiments indicate that pyrite is a strong scavenger of W in aquatic environments. Specifically, results showed that WO4 2− and WS4 2− adsorption onto pyrite increased with decreasing pH. The greatest amounts of WO4 2− and WS4 2− adsorbed onto pyrite occurred at pH4.95 and 5.2, respectively. Kinetic experiments indicate that WO4 2− adsorption onto pyrite occurs more rapidly than WS4 2−. The kinetic behavior of the adsorption of both W species onto pyrite is well described with a pseudo-second-order model. More specifically, rapid external adsorption followed by intraparticle diffusion were the rate-controlling steps during both WO4 2− and WS4 2− adsorption onto pyrite, and intraparticle diffusion of both W species onto pyrite took up >99% of time, suggesting that it was the major rate-limiting step. The mono-surface Langmuir model provided the best fit to the adsorption data indicating that WO4 2− and WS4 2− were mainly adsorbed onto one specific site on the pyrite surface. Our model calculations suggest that the specific adsorption of WO4 2− onto pyrite was greater than the corresponding adsorption of WS4 2− onto pyrite in all cases. The difference of specific adsorption between WO4 2− and WS4 2− may be attributed to their different inner-sphere complexation on the pyrite surface. The adsorption experiments also showed that W species were less adsorbed onto pyrite than goethite. The results of the adsorption experiments demonstrate that pyrite plays an important role in determining the fate and transport of WO4 2− and WS4 2− in natural waters. Furthermore, comparison of our results with previous studies of Mo indicates that WS4 2− is less particle reactive with respect to pyrite than MoS4 2−, which we propose can explain, in part, the apparent stability of W in sulfidic waters compared to Mo as well as the elevated Mo/W ratios in Black Sea sapropels.

      PubDate: 2017-06-01T15:20:29Z
  • Molecular-level understanding of malic acid retention mechanisms in
           ternary kaolinite-Fe(III)-malic acid systems: The importance of Fe
    • Abstract: Publication date: 5 August 2017
      Source:Chemical Geology, Volume 464
      Author(s): Jianjun Yang, Jin Liu, Yongfeng Hu, Cornelia Rumpel, Nanthi Bolan, Donald Sparks
      Multivalent cation interaction and mineral association play a vital role in organic carbon (OC) stability in tropical soils. However, the retention mechanisms of OC in mineral-cation-OC systems remain largely unclear at the molecular level. Of all OC forms, low molecular weight organic acids (LMWOAs) are readily biodegradable. Given the general acidic conditions and dominance of kaolinite in tropical soils, we investigated the retention mechanisms of malic acid (MA) in kaolinite-Fe(III)-MA systems with various Fe/MA molar ratios at pH~3.5 using Fe K-edge extended X-ray absorption fine structure (EXAFS) spectroscopy. Our results indicated the formed ferrihydrite mainly contributed to the enhanced MA retention through adsorption and/or coprecipitation at high Fe/MA molar ratios in kaolinite-Fe(III)-MA systems relative to kaolinite-MA systems. With the decrease of Fe/MA molar ratios, ternary complexation of MA with kaolinite via a tetrahedral Fe bridge occurred in kaolinite-Fe(III)-MA systems. This study highlighted the significant impact of Fe speciation on MA retention mechanisms in kaolinite-Fe(III)-MA systems under acidic conditions, and clearly showed the impact of ferrihydrite-induced adsorption/coprecipitation and Fe-bridged ternary complexation on MA retention in ternary kaolinite-Fe(III)-MA systems, which will enhance our understanding of the dynamics of MA and other LMWOAs in tropical soils.
      Graphical abstract image

      PubDate: 2017-06-01T15:20:29Z
  • Sorption of Se(IV) and Se(VI) to coal fly ash/cement composite: Effect of
           Ca2+ and high ionic strength
    • Abstract: Publication date: 5 August 2017
      Source:Chemical Geology, Volume 464
      Author(s): Weiling Sun, Jay E. Renew, Wenlong Zhang, Yuanzhi Tang, Ching-Hua Huang
      The sorption behavior and mechanisms of Se from wastewater with high salinity are unclear, a scenario that can be encountered in the flue-gas-desulfurization (FGD) blowdown at coal-fired power plants. In this study, we investigated the uptake behavior and mechanisms of Se(IV) and Se(VI) by a coal fly ash/cement composite at high ionic strengths (IS) of up to 3.0M NaCl or CaCl2 using batch sorption experiments, Fourier-transform infrared spectroscopy, X-ray photoelectron spectroscopy, X-ray diffraction, and X-ray absorption spectroscopy. NaCl, regardless of IS, only caused slight changes in Se(IV) and Se(VI) uptake. Presence of CaCl2 significantly enhanced Se(IV) uptake, but had minor effect at low IS (≤0.3M) and no effect at high IS (>0.3M) on Se(VI) uptake. Both Se(IV) and Se(VI) mainly formed outer-sphere surface complexes in the presence of NaCl. The enhanced Se(IV) uptake by CaCl2 is likely due to the formation of Ca-SeO3 precipitate(s). Outer-sphere complexation is the main mechanism for Se(VI) sorption in the presence of CaCl2, with additional formation of minor amounts of inner-sphere complex at IS >0.3M. The results added new mechanistic insights for Se removal from industrial wastewaters with high salinity and will be useful for immobilization of Se in the co-management of FGD blowdown and coal fly ash wastes.
      Graphical abstract image

      PubDate: 2017-06-01T15:20:29Z
  • Comparison of europium and neptunium adsorption to aluminum (hydr)oxide
    • Abstract: Publication date: 5 August 2017
      Source:Chemical Geology, Volume 464
      Author(s): Teresa Baumer, Patricia Kay, Amy E. Hixon
      Batch experiments and detailed solid-phase characterization (i.e., powder X-ray diffraction, scanning electron microscopy, and infrared spectroscopy) were used to determine the effect of secondary phase formation on the adsorption of europium and neptunium to a suite of aluminum (hydr)oxide minerals. Europium experiments were conducted as a function of gibbsite (γ-Al(OH)3), bayerite (α-Al(OH)3), corundum (α-Al2O3), and γ-alumina (γ-Al2O3) concentration (2.5–30m2 L−1), europium concentration (10−8–10−5 M), pH (3−12), and ionic strength (0.01–0.1M NaCl). Neptunium experiments were conducted at constant neptunium and mineral concentrations. The composition of the mineral phase had no apparent effect on europium adsorption whereas preferential adsorption of neptunium followed the trend bayerite>corundum>γ-alumina. The data presented here suggest that there are at least two different mechanisms controlling the adsorption of europium and neptunium and highlight the need to study both aqueous-phase chemistry and the properties of mineral surfaces in order to understand the behavior of lanthanides and actinides at the mineral-water interface.

      PubDate: 2017-06-01T15:20:29Z
  • Bonding properties of humic acid with attapulgite and its influence on
           U(VI) sorption
    • Abstract: Publication date: 5 August 2017
      Source:Chemical Geology, Volume 464
      Author(s): Liqiang Tan, Xiangxue Wang, Xiaoli Tan, Huiyang Mei, Changlun Chen, Tasawar Hayat, Ahmed Alsaedi, Tao Wen, Songsheng Lu, Xiangke Wang
      Humic acid (HA) can markedly change the surface charge of clay minerals and influences the sorption of uranium(U) on clay minerals in aquatic environments. Furthermore, HA can potentially affect the mobility of U(VI) in the natural environment. The effect of HA on the surface charge, aggregation, sorption and bonding properties of attapulgite [(Mg,Al)4(Si)8(O,OH,H2O)26·nH2O] was investigated using fluorescence spectroscopy technique in combination with other analytical tools. The UV–vis absorbance spectra and fluorescence peaks obtained in the experiments are attributed to a large number of electron donating substitutes, such as carboxylic acid-like and phenolic acid-like fluorophores, in HA. Electrostatic attraction and ligand exchanges are the primary interaction mechanisms between attapulgite and HA. A significant modification of excitation-emission matrix (EEM) spectra and emission intensity of HA are observed after the interaction, indicating a different binding complex of HA on attapulgite at the molecular level. The sorption kinetics indicate that strong chemical interactions and/or surface complexation primarily contributed to U(VI) sorption. An analysis of U(VI) sorption isotherms and model fitting reveals that the sorption of U(VI) onto attapulgite is a spontaneous and endothermic process. The presence of HA is an important factor that greatly influences the immobilization and removal of U(VI) ions from aqueous solutions.
      Graphical abstract image

      PubDate: 2017-06-01T15:20:29Z
  • The reactivity of Fe(II) associated with goethite formed during short
           redox cycles toward Cr(VI) reduction under oxic conditions
    • Abstract: Publication date: 5 August 2017
      Source:Chemical Geology, Volume 464
      Author(s): Elizabeth J. Tomaszewski, Seungyeol Lee, Jared Rudolph, Huifang Xu, Matthew Ginder-Vogel
      Chromium (Cr) is a toxic metal that causes a myriad of health problems and enters the environment as a result of anthropogenic activities and/or natural processes. The toxicity and solubility of chromium is linked to its oxidation state; Cr(III) is poorly soluble and relatively nontoxic, while Cr(VI) is soluble and a known carcinogen. Solid Fe(II) in iron-bearing minerals, such as pyrite, magnetite, and green rusts, reduce the oxidation state of chromium, reducing its toxicity and mobility. However, these minerals are not the only potential sources of solid-associated Fe(II) available for Cr(VI) reduction. For example, ferric (Fe(III)) (hydr)oxides, such as goethite or hematite, can have Fe(II) in the solid without phase transformation; however, the reactivity of Fe(II) within Fe(III) (hydr)oxides with contaminants, has not been previously investigated. Here, we cyclically react goethite with dissolved Fe(II) followed by dissolved O2, leading to the formation of reactive Fe(II) associated with goethite. In separate reactors, the reactivity of this Fe(II) is probed under oxic conditions, by exposure to chromate (CrO4 2−) after either one, two, three or four redox cycles. Cr is not present during redox cycling; rather, it is introduced to a subset of the solid after each oxidation half-cycle. Analysis of X-ray absorption near edge structure (XANES) spectra reveals that the extent of Cr(VI) reduction to Cr(III) depends not only on solid Fe(II) content but also surface area and mean size of ordered crystalline domains, determined by BET surface area analysis and X-ray diffraction (XRD), respectively. Shell-by-shell fitting of the extended X-ray absorption fine structure (EXAFS) spectra demonstrates chromium forms both single and double corner sharing complexes on the surface of goethite, in addition to sorbed Cr(III) species. Finally, transmission electron microscope (TEM) imaging and X-ray energy-dispersive spectroscopy (EDS) illustrate that Cr preferentially localizes on the (100) face of goethite, independent of the number of redox cycles goethite undergoes. This work demonstrates that under oxic conditions, solid Fe(II) associated with goethite resulting from rapid redox cycling is reactive and available for electron transfer to Cr(VI), suggesting Fe(III) (hydr)oxides may act as reservoirs of reactive electron density, even in oxygen saturated environments.
      Graphical abstract image

      PubDate: 2017-06-01T15:20:29Z
  • Surface area effects on the reduction of UVI in the presence of synthetic
    • Abstract: Publication date: 5 August 2017
      Source:Chemical Geology, Volume 464
      Author(s): Maxim I. Boyanov, Drew E. Latta, Michelle M. Scherer, Edward J. O'Loughlin, Kenneth M. Kemner
      The redox transformations that affect the environmental mobility of metal or radionuclide contaminants typically take place in the presence of mineral or biological surfaces. Adsorption can alter the speciation and free energy of a dissolved ion and can thus change the occurrence, rate, or products of redox reactions relative to those expected in homogenous solution. Here, we investigated the effect of SYn-1, a redox-inactive synthetic montomorillonite clay mineral, on the reduction of UVI by dihydroanthraquinone-2,6 disulfonate (AH2QDS, the reduced form of the soluble electron shuttle AQDS). We varied the surface:U ratio in a circumneutral bicarbonate solution and measured the valence and atomic coordination of U in the solids using X-ray absorption spectroscopy (XANES and EXAFS). The spectra show that UIV was the predominant product both in the presence or absence of the clay mineral, indicating that adsorption of UVI to clay mineral surface sites does not affect its ability to be reduced by AH2QDS. In the absence of the clay mineral and at low-to-intermediate surface:U ratios the predominant UIV product was nanoparticulate uraninite, UO2. At the highest surface:U ratio tested (100g/L clay mineral and 50μMU) we observe a decrease in the proportion of uraninite down to 50%, with the remaining UIV present as adsorbed, non-uraninite species. These results are similar to previous findings with magnetite and rutile, where adsorbed UIV were the predominant species below a specific surface coverage and U in excess of this coverage was precipitated as uraninite. The threshold coverage determined here for the SYn-1 clay mineral is 10-100× lower than that determined for magnetite and rutile, suggesting that clay mineral surfaces may be less important than metal oxides for stabilizing non-uraninite UIV species in natural sediments.
      Graphical abstract image

      PubDate: 2017-06-01T15:20:29Z
  • Coupled dynamics of iron and iron-bound organic carbon in forest soils
           during anaerobic reduction
    • Abstract: Publication date: 5 August 2017
      Source:Chemical Geology, Volume 464
      Author(s): Qian Zhao, Dinesh Adhikari, Rixiang Huang, Aman Patel, Xilong Wang, Yuanzhi Tang, Daniel Obrist, Eric E. Roden, Yu Yang
      The behavior of iron (Fe)-bound organic carbon (OC) under anoxic conditions in natural soils and sediments represents a critical knowledge gap for understanding the biogeochemical cycles of OC and Fe. In this study, we investigated the dynamics of Fe and OC in four forest soils in the presence of the dissimilatory Fe-reducing bacterium, Shewanella oneidensis MR-1. Over an 8-day reduction period, 3.8–9.9% of total OC was released to solution in conjunction with the reduction of 12.5–37.7% of reactive Fe. The fraction of OC released was correlated with the fraction of Fe reduced, indicating that the reductive release was the controlling factor for the mobilization of OC upon the anaerobic microbial reaction. During the reduction, the fractions of poorly crystalline Fe oxides decreased, coupling with an increase in the relative abundance of crystalline Fe oxides. Lability of OC (as reflected by water-extractable OC content) increased after microbial reduction, indicating the decreased stability of OC because of changes in mineral-OC interactions and the conformation of mineral-OC complexes. The reduction of Fe was closely related to bulk soil electron accepting capacity (0.15–0.34mmole−/mol C). Our findings demonstrate that the redox reactions of Fe, modified by the redox reactivity of OC, play an important role in regulating the stability and transformation of OC.

      PubDate: 2017-06-01T15:20:29Z
  • Advanced biotic ligand models: Using surface complexation modeling to
           quantify metal bioavailability to bacteria in geologic systems
    • Abstract: Publication date: 5 August 2017
      Source:Chemical Geology, Volume 464
      Author(s): Jeremy B. Fein
      There is considerable evidence that metal adsorption onto microbial cell envelopes represents the first step in bioavailability of the metal to the cell. Traditional biotic ligand models were the first attempts to relate metal bioavailability, as manifested by metal toxicity, to the concentration of adsorbed metal on organisms, using a single generic ‘biotic’ ligand as the lone metal binding site on the organism. Because biotic ligand models represent the bound metal using a non-mechanistic reaction, these models cannot be extrapolated accurately to predict metal adsorption, and hence metal bioavailability, under conditions that were not directly measured in the laboratory. Over the past 20years, surface complexation modeling has been applied to model metal adsorption onto bacteria and other microorganisms, yielding a mechanistic understanding of the adsorption reactions that have been studied. The development of mechanistic surface complexation models for bacteria provides an opportunity to develop advanced biotic ligand models which are similar to traditional biotic ligand models but which incorporate the mechanistic approach of surface complexation modeling to quantify metal adsorption. This review summarizes the studies that have been conducted to date which explore and calibrate this approach, and each of these studies demonstrates a strong correlation between the concentration and/or speciation of adsorbed metal and the bioavailability of that metal to bacteria. Using a surface complexation approach for modeling metal bioavailability to bacteria creates powerful quantitative models of metal bioavailability which can be extrapolated to complex realistic geologic or engineered systems.

      PubDate: 2017-06-01T15:20:29Z
  • Stoichiometry of mercury-thiol complexes on bacterial cell envelopes
    • Abstract: Publication date: 5 August 2017
      Source:Chemical Geology, Volume 464
      Author(s): Bhoopesh Mishra, Elizabeth Shoenfelt, Qiang Yu, Nathan Yee, Jeremy B. Fein, Satish C.B. Myneni
      We have examined the speciation of Hg(II) complexed with intact cell suspensions (1013 cellsL−1) of Bacillus subtilis, a common gram-positive soil bacterium, Shewanella oneidensis MR-1, a facultative gram-negative aquatic organism, and Geobacter sulfurreducens, a gram-negative anaerobic bacterium capable of Hg-methylation at Hg(II) loadings spanning four orders of magnitude (120nM to 350μM) at pH5.5 (±0.2). The coordination environments of Hg on bacterial cells were analyzed using synchrotron based X-ray Absorption Near Edge Structure (XANES) and Extended X-ray Absorption Fine Structure (EXAFS) spectroscopy at the Hg LIII edge. The abundance of thiols on intact cells was determined by a fluorescence-spectroscopy based method using a soluble bromobimane, monobromo(trimethylammonio)bimane (qBBr) to block thiol sites, and potentiometric titrations of biomass with and without qBBr treatment. The chemical forms of S on intact bacterial cells were determined using S k-edge XANES spectroscopy. Hg(II) was found to complex entirely with cell bound thiols at low Hg:biomass ratios. For Bacillus subtilis and Shewanella oneidensis MR-1 cells, the HgS stoichiometry changed from HgS3 to HgS2 and HgS (where ‘S’ represents a thiol site such as is present on cysteine) progressively as the Hg(II) loading increased on the cells. However, Geobacter sulfurreducens did not form HgS3 complexes. Because the abundance of thiol was highest for Geobacter sulfurreducens (75μM/g wet weight) followed by Shewanella oneidensis MR-1 (50μM/g wet weight) and Bacillus subtilis (25μM/g wet weight), the inability of Hg(II) to form HgS3 complexes on Geobacter sulfurreducens suggests that the density and reactivity of S-amino acid containing cell membrane proteins on Geobacter sulfurreducens are different from those of Bacillus subtilis and Shewanella oneidensis MR-1. Upon saturation of the high affinity thiol sites at higher Hg:biomass ratios, Hg(II) was found to form a chelate with α-hydroxy carboxylate anion. The stoichiometry of cell envelope bound Hg-thiol complexes and the associated abundance of thiols on the cell envelopes provide important insights for understanding the differences in the rate and extent of uptake and redox transformations of Hg in the environment.
      Graphical abstract image

      PubDate: 2017-06-01T15:20:29Z
  • Thermodynamic modeling of Mn(II) adsorption onto manganese oxidizing
    • Abstract: Publication date: 5 August 2017
      Source:Chemical Geology, Volume 464
      Author(s): Angélica Vázquez-Ortega, Jeremy B. Fein
      Bacterial cell membranes display a strong affinity for a wide variety of aqueous metal cations and have the potential to affect the mass transport of these cations through adsorption reactions in water–rock systems. Prior studies have focused on determining the thermodynamic stability constants of heavy metals and radionuclides; however, the constants for Mn-bacterial surface complexes formed on manganese oxidizing bacteria remain unmeasured. We measured the rate, extent, and reversibility of Mn(II) adsorption onto a bacterial species capable of Mn-oxidization (Pseudomonas putida), and onto a bacterial species that does not promote Mn-oxidization (Bacillus subtilis). The extent of adsorption was measured as a function of both pH and metal loading in order to determine the thermodynamic stability constants of the Mn-bacterial surface complexes that form on the bacteria and to test whether Mn oxidizers exhibit unusual Mn adsorption behavior relative to species that do not oxidize Mn. Furthermore, we determined the effect of bacterial extracellular polymeric substances (EPS) on Mn(II) adsorption by conducting experiments with and without EPS removal from the biomass. The experimental results indicated that Mn(II) adsorption onto B. subtilis and P. putida was rapid and reversible. The extent of Mn(II) adsorption onto both bacterial species increased with increasing pH, but P. putida adsorbed significantly more Mn(II) than did B. subtilis across most of the pH range studied. Both the adsorption measurements and the calculated stability constants indicate that P. putida, a Mn oxidizing bacterial species, exhibited significantly enhanced Mn adsorption relative to that observed for B. subtilis. The enhanced Mn(II) adsorption exhibited by P. putida suggests that bacteria may adapt the metal binding environments within their cell envelopes in order to optimize bioavailability of metals that are beneficial to their metabolism. Experiments conducted at low metal-loading conditions yielded stability constants for the Mn(II)-bacterial surface complexes that were less than or similar to those calculated for the high metal-loading conditions, suggesting that Mn(II)-sulfhydryl binding does not dominate under low metal loading conditions as it does for other metals. Removal of EPS molecules from P. putida led to significantly reduced extents of Mn(II) adsorption, suggesting that Mn(II)-EPS binding plays at least some role in the overall adsorption of Mn(II) onto P. putida biomass.

      PubDate: 2017-06-01T15:20:29Z
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