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    - EARTH SCIENCES (470 journals)
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EARTH SCIENCES (470 journals)                  1 2 3 | Last

Showing 1 - 200 of 371 Journals sorted alphabetically
Acta Geochimica     Hybrid Journal   (Followers: 3)
Acta Geodaetica et Geophysica     Hybrid Journal   (Followers: 1)
Acta Geodaetica et Geophysica Hungarica     Full-text available via subscription   (Followers: 2)
Acta Geophysica     Open Access   (Followers: 6)
Acta Geotechnica     Hybrid Journal   (Followers: 6)
Acta Meteorologica Sinica     Hybrid Journal   (Followers: 3)
Advances in High Energy Physics     Open Access   (Followers: 21)
Advances In Physics     Hybrid Journal   (Followers: 18)
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: 3)
AMBIO     Hybrid Journal   (Followers: 14)
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: 20)
Annals of Glaciology     Full-text available via subscription   (Followers: 3)
Annual Review of Marine Science     Full-text available via subscription   (Followers: 11)
Anthropocene     Hybrid Journal   (Followers: 3)
Anthropocene Review     Hybrid Journal   (Followers: 4)
Applied Clay Science     Hybrid Journal   (Followers: 4)
Applied Geochemistry     Hybrid Journal   (Followers: 11)
Applied Geomatics     Hybrid Journal   (Followers: 3)
Applied Geophysics     Hybrid Journal   (Followers: 7)
Applied Ocean Research     Hybrid Journal   (Followers: 5)
Applied Petrochemical Research     Open Access   (Followers: 2)
Applied Remote Sensing Journal     Open Access   (Followers: 34)
Aquatic Conservation Marine and Freshwater Ecosystems     Hybrid Journal   (Followers: 34)
Arctic Science     Open Access   (Followers: 6)
Arctic, Antarctic, and Alpine Research     Full-text available via subscription   (Followers: 10)
Artificial Satellites : The Journal of Space Research Centre of Polish Academy of Sciences     Open Access   (Followers: 17)
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: 8)
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: 17)
Cadernos de Geociências     Open Access  
Canadian Journal of Plant Science     Full-text available via subscription   (Followers: 15)
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: 4)
Chemical Geology     Hybrid Journal   (Followers: 18)
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: 12)
Computational Mathematics and Mathematical Physics     Hybrid Journal   (Followers: 1)
Computers and Geotechnics     Hybrid Journal   (Followers: 8)
Contemporary Trends in Geoscience     Open Access   (Followers: 3)
Continental Shelf Research     Hybrid Journal   (Followers: 9)
Contributions to Mineralogy and Petrology     Hybrid Journal   (Followers: 10)
Contributions to Plasma Physics     Hybrid Journal   (Followers: 3)
Coral Reefs     Hybrid Journal   (Followers: 16)
Cretaceous Research     Hybrid Journal   (Followers: 7)
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: 109)
Earth and Space Science     Open Access   (Followers: 11)
Earth Interactions     Full-text available via subscription   (Followers: 11)
Earth Science Research     Open Access   (Followers: 6)
Earth Surface Dynamics (ESurf)     Open Access   (Followers: 4)
Earth Surface Processes and Landforms     Hybrid Journal   (Followers: 19)
Earth System Dynamics     Open Access   (Followers: 6)
Earth System Dynamics Discussions     Open Access   (Followers: 4)
Earth's Future     Open Access   (Followers: 1)
Earth, Planets and Space     Open Access   (Followers: 65)
Earthquake Engineering and Engineering Vibration     Hybrid Journal   (Followers: 7)
Earthquake Science     Hybrid Journal   (Followers: 10)
Earthquake Spectra     Full-text available via subscription   (Followers: 18)
Ecohydrology     Hybrid Journal   (Followers: 10)
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: 22)
Environmental Geology     Hybrid Journal   (Followers: 20)
Environmental Geosciences     Full-text available via subscription   (Followers: 3)
Environmental Geotechnics     Hybrid Journal   (Followers: 4)
Erwerbs-Obstbau     Hybrid Journal  
Estuaries and Coasts     Hybrid Journal   (Followers: 18)
Estuarine, Coastal and Shelf Science     Hybrid Journal   (Followers: 34)
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: 11)
Frontiers in Earth Science     Open Access   (Followers: 5)
Frontiers in Geotechnical Engineering     Open Access   (Followers: 2)
Frontiers of Earth Science     Hybrid Journal   (Followers: 8)
Fundamental and Applied Limnology / Archiv für Hydrobiologie     Full-text available via subscription   (Followers: 4)
GEM - International Journal on Geomathematics     Hybrid Journal   (Followers: 1)
Geo-Marine Letters     Hybrid Journal   (Followers: 7)
Geoacta     Open Access   (Followers: 4)
Geobiology     Hybrid Journal   (Followers: 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: 25)
Geochimica et Cosmochimica Acta     Hybrid Journal   (Followers: 48)
Geochronometria     Hybrid Journal   (Followers: 3)
Geoderma Regional : The International Journal for Regional Soil Research     Full-text available via subscription   (Followers: 4)
Geodinamica Acta     Hybrid Journal   (Followers: 3)
Geodynamics & Tectonophysics     Open Access   (Followers: 1)
Geoenvironmental Disasters     Open Access   (Followers: 3)
Geofluids     Hybrid Journal   (Followers: 4)
Geoforum     Hybrid Journal   (Followers: 21)
Géographie physique et Quaternaire     Full-text available via subscription   (Followers: 1)
Geography and Natural Resources     Hybrid Journal   (Followers: 6)
Geoheritage     Hybrid Journal   (Followers: 1)
Geoinformatica Polonica : The Journal of Polish Academy of Arts and Sciences     Open Access  
Geoinformatics & Geostatistics     Hybrid Journal   (Followers: 8)
Geological Journal     Hybrid Journal   (Followers: 14)
Geology Today     Hybrid Journal   (Followers: 25)
Geology, Geophysics and Environment     Open Access  
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: 26)
Geophysical & Astrophysical Fluid Dynamics     Hybrid Journal   (Followers: 2)
Geophysical Journal International     Hybrid Journal   (Followers: 31)
Geophysical Prospecting     Hybrid Journal   (Followers: 7)
Geophysics     Full-text available via subscription   (Followers: 18)
GeoResJ     Hybrid Journal  
Georisk: Assessment and Management of Risk for Engineered Systems and Geohazards     Hybrid Journal   (Followers: 8)
Geoscience Canada : Journal of the Geological Association of Canada / Geoscience Canada : journal de l'Association Géologique du Canada     Full-text available via subscription   (Followers: 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: 4)
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: 17)
Global Biogeochemical Cycles     Full-text available via subscription   (Followers: 12)
Gondwana Research     Hybrid Journal   (Followers: 7)
Grassland Science     Hybrid Journal   (Followers: 1)
Ground Water     Hybrid Journal   (Followers: 30)
Ground Water Monitoring & Remediation     Hybrid Journal   (Followers: 17)
Groundwater for Sustainable Development     Full-text available via subscription  
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: 16)
Hydrological Processes     Hybrid Journal   (Followers: 24)
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: 11)
Indian Geotechnical Journal     Hybrid Journal   (Followers: 3)
Indonesian Journal on Geoscience     Open Access   (Followers: 2)
Interdisciplinary Environmental Review     Hybrid Journal   (Followers: 3)
International Geology Review     Hybrid Journal   (Followers: 14)
International Journal of Advanced Geosciences     Open Access  
International Journal of Advanced Remote Sensing and GIS     Open Access   (Followers: 33)
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: 35)
International Journal of Coal Geology     Hybrid Journal   (Followers: 2)
International Journal of Disaster Risk Reduction     Hybrid Journal   (Followers: 12)
International Journal of Earth Sciences     Hybrid Journal   (Followers: 31)

        1 2 3 | Last

Journal Cover Chemical Geology
  [SJR: 2.346]   [H-I: 145]   [18 followers]  Follow
   Hybrid Journal Hybrid journal (It can contain Open Access articles)
   ISSN (Print) 0009-2541
   Published by Elsevier Homepage  [3043 journals]
  • Aspartate transformation at 200°C with brucite [Mg(OH)2], NH3, and H2:
           Implications for prebiotic molecules in hydrothermal systems
    • Abstract: Publication date: Available online 25 March 2017
      Source:Chemical Geology
      Author(s): Charlene F. Estrada, Irena Mamajanov, Jihua Hao, Dimitri A. Sverjensky, George D. Cody, Robert M. Hazen
      Hydrothermal systems may have been favorable environments for the evolution of prebiotic chemistry on early Earth due to the presence of chemical, temperature, and redox gradients that could promote the formation of biomolecules. However, the relevance of these environments in origins of life scenarios has been debated due to rapid decomposition of biologically essential species, such as amino acids, at high temperatures. Little is known about the reactivity of amino acids in the presence of mineral surfaces and reducing conditions, which reflect the geochemical complexities of environments such as serpentinite-hosted hydrothermal vents. We investigated the decomposition of 25mM aspartate at 200°C and 15.5bars (PSAT) in gold capsules both with and without brucite [Mg(OH)2], a mineral product of serpentinization, and reducing conditions (NH4Cl and H2(aq)). We observed that the reaction products of aspartate vary significantly with the initial reaction conditions. Fluids containing aspartate only decomposed to fumarate, maleate, malate, acetate, and trace amounts of succinate and glycine. However, under reducing conditions, the main product was succinate (8mM) and also approximately 1mM total of the amino acids glycine, α-alanine, and β-alanine. The amount of α-alanine increased three-fold with brucite. Furthermore, we detected a two-fold decrease in the fumarate concentration, whereas total maleate concentration dramatically decreased over ten-fold and resulted in an overall increase in the trans/cis ratio of these deamination products of aspartate from 0.9 to 4.5 as a function of brucite loading. This net decrease in fumarate and maleate concentration and the five-fold increase in the trans/cis ratio might have been caused by a combination of a pH increase and the formation of magnesite due to increased Mg2+ ion concentration. The results of this study provide evidence that the fundamental properties of a hydrothermal system, including mineral assemblages, reducing conditions, and dissolved species concentrations, will influence the fate of amino acids at high temperature.

      PubDate: 2017-03-27T07:54:43Z
  • Recycled noble gases preserved in podiform chromitites from Luobusa, Tibet
    • Abstract: Publication date: Available online 25 March 2017
      Source:Chemical Geology
      Author(s): Wei Guo, Huaiyu He, David R. Hilton, Yongfei Zheng, Fei Su, Yan Liu, Rixiang Zhu
      We report noble gas (He, Ne, Ar) signatures of chromite and olivine separates from the Luobusa chromitites in Tibet to better understand the volatile compositions trapped in the minerals, and further to trace the origin of melts responsible for formation of the chromite deposits. The studied samples can be divided into two groups based on petrography and distinct noble gas signatures. Group I samples are free of carbonates and have 3He/4He ratios from 0.81 to 2.36 Ra (where Ra is the 3He/4He ratio of air=1.4×10−6) and air-like Ne and Ar isotopic compositions irrespective of chromitite structure types. Most 3He/4He ratios of Group I samples are higher than air, suggesting apparent presence of mantle volatiles. Given the 4He/20Ne and 3He/36Ar several orders of magnitude higher than air, negligible contributions are from the atmospheric helium. The observed He isotope compositions thus can be regarded as a two-component mixture of mantle-derived and radiogenic He. A broadly positive correlation between 3He and 36Ar in nodular chromitite samples indicates a source mixing between mantle and recycled noble gases but not due to shallow air contamination. In addition, the wide distribution range of 20Ne/36Ar also supports a subduction-related origin of neon and argon. Combined with major element data, the most appropriate tectonic setting to generate such noble gas signatures in Group I samples is subduction zone (probably forearc) where favorable conditions are present for the formation of the chromitites. In contrast, Group II samples containing carbonates have much more radiogenic 3He/4He ratios of 0.03 to 0.3 Ra but much less radiogenic 40Ar/36Ar ratios of 344 to 420. In combination with the occurrence of carbonate veins it is suggested that Group II samples are predominated by supracrustal components that may be imparted during or after the emplacement stage. A comparison of these two group samples indicates that the primary noble gas signatures reflecting the characteristics of ore-forming melts can be preserved in chromite and olivine grains (Group I samples) and thus used to trace the origin of podiform chromitites.

      PubDate: 2017-03-27T07:54:43Z
  • The influence of O2 availability and Fe(III) mineralogy on Fe metabolism
           by an acidophilic bacterial culture
    • Abstract: Publication date: Available online 23 March 2017
      Source:Chemical Geology
      Author(s): John E. Burwick, Shagun Sharma, Christopher M. Menge, Alper Buldum, John M. Senko
      Microbiological Fe(II) oxidation in emergent acid mine drainage (AMD) at the terrestrial surface, and continued precipitation of biogenic Fe(III) can lead to massive deposits of Fe(III) phases. Fe(II) oxidizing bacteria that mediate Fe(III) deposition are challenged to continue to metabolize despite burial in these Fe(III) phases, which increases the distance from the sediment-water interface where O2 is most readily available to support Fe(II) oxidation. Microbial communities associated with such deposits vary little with depth, indicating that the associated organisms can adapt to the physicochemical changes associated with Fe(III) deposition. A stable Alicyclobacillacea bacterium-containing culture (A06) was obtained under Fe(III) reducing conditions, and was also capable of aerobic growth with Fe(II) or glucose as electron donors. To evaluate the influence of Fe(III) mineralogy and O2 availability on microbial activities in an iron mound setting, we incubated the A06 culture in agarose-solidified suspensions of Fe(III) phases that mimicked the Fe(III) deposits. During the incubations, we measured O2 and Fe(II) concentrations at various depth intervals within these “artificial cores.” In incubations that included goethite, an Fe(II) oxidation front preceded maximal O2 diffusion into the incubations, indicating that Fe(II) oxidation could occur at extremely low O2 concentrations. This finding is consistent with previous field measurements. Computational modeling of activities in the incubations indicated that Fe(II) oxidation is partially controlled by diffusive O2 availability as well as Fe(II) diffusion, which may be influenced by the mineralogy of Fe(III) phases where Fe(II) oxidation is occurring. In incubations amended with glucose, schwertmannite-Fe(III) reduction occurred despite available O2, which is also consistent with previous field measurements. Our results indicate that Fe metabolism may be more heavily influenced by the mineralogical context in which activity occurs than the availability of O2.

      PubDate: 2017-03-27T07:54:43Z
  • Redox properties of clay-rich sediments as assessed by mediated
           electrochemical analysis: Separating pyrite, siderite and structural Fe in
           clay minerals
    • Abstract: Publication date: Available online 23 March 2017
      Source:Chemical Geology
      Author(s): Alwina L. Hoving, Michael Sander, Christophe Bruggeman, Thilo Behrends
      Redox reactions with Fe-containing minerals in clay-rich sediments largely affect the speciation, mobility, and (bio-) availability of redox-sensitive contaminants. Here, we use mediated electrochemical oxidation (MEO) and reduction (MER), to quantify the electron accepting and donating capacities (EAC and EDC) of Boom Clay, a potential host formation for radioactive waste disposal. The relevant redox-active minerals pyrite, siderite, smectite and illite were first studied separately. MEO and MER of smectites and illites resulted in sharp current peak responses, reflecting fast electron transfer kinetics. Conversely, broad current peaks were obtained from MEO of pyrite. The current response to MEO of siderite was very small. Under the applied electrochemical conditions in MEO, pyrite was not completely oxidized and only a marginal fraction of siderite was oxidized. All structural Fe (Festruct) in smectites SWa-1 and SWy-1 was redox-active in MER and MEO, whereas in Fithian Illite and IMt-1 only 12–22% of the total Festruct was available. An empirical equation was used to describe the current curves of the tested minerals. This equation allowed to delineate the relative contributions of these minerals to MEO of their mixtures. The EDC of Boom Clay determined by MEO was 0.2±0.05mmol e−/g and predominantly consisted of contributions of pyrite, Festruct in clays and natural organic matter (NOM). Applying the empirical equation allowed to separate the oxidative current response into the contribution of pyrite with slower oxidation kinetics and the combined contribution of faster reacting Festruct and natural organic matter (NOM). Due to the absence of NOM isolates from Boom Clay, the EDC of NOM was estimated based on MEO measurements of dissolved organic matter in Boom Clay pore water and the organic carbon content of Boom Clay. The EDC of Festruct in clays was then obtained by subtracting the contributions of NOM and pyrite from the measured EDC. About 14% of the measured EDC can be attributed to Festruct which implies that about 50% of the structural FeII in Boom Clay is redox-active. In contrast, EAC measurements indicate that FeIII struct in Boom Clay is electrochemically inactive.
      Graphical abstract image

      PubDate: 2017-03-27T07:54:43Z
  • Modeling controls on the chemical weathering of marine mudrocks from the
           Middle Jurassic in Southern Germany
    • Abstract: Publication date: Available online 22 March 2017
      Source:Chemical Geology
      Author(s): Zhongwen Bao, Christina M. Haberer, Uli Maier, Richard T. Amos, David W. Blowes, Peter Grathwohl
      Chemical weathering of sedimentary rocks is of great importance in determining seepage water chemistry, carbon, iron, calcium and sulfur turnover, as well as mineral transformation. In this study, we used the numerical code MIN3P to investigate controls on seepage water chemistry during chemical weathering of marine mudrocks. In particular, we focused on the pyrite- and kerogen-bearing formation, Opalinus Clay (with outcrops in the area of the Swabian and Franconian Alb in Southern Germany), a typical fine-grained sedimentary mudrock that had been deposited during the Middle Jurassic in a shallow marine environment. In the geochemical model we considered four reactive minerals, i.e., pyrite, kerogen, calcite and siderite (assuming silicate minerals to be stable), and ran model scenarios over a time period of 10kyrs (since the last ice age). Our numerical results show that chemical weathering of Opalinus Clay is driven by oxygen ingress (which depends on effective gas diffusion, and thus on water saturation). Due to oxidation of pyrite and kerogen seepage water acidifies, which leads to dissolution of carbonate minerals, i.e., calcite and siderite. As a consequence, porosity and groundwater alkalinity increase, and CO2 is released into the atmosphere at early decades. Following the consumption of primary reactive minerals, iron oxides precipitate in the oxic zone. We compared our model results with field data of water saturation, porosity, and water chemistry. The overall reasonable fit between model results and field data demonstrates the applicability of the numerical code MIN3P to quantify chemical weathering of pyrite-bearing sedimentary mudrocks and to predict seepage water chemistry that is impacted by geochemical water-rock interactions.
      Graphical abstract image

      PubDate: 2017-03-27T07:54:43Z
  • The large-scale evolution of neodymium isotopic composition in the global
           modern and Holocene ocean revealed from seawater and archive data
    • Abstract: Publication date: Available online 21 March 2017
      Source:Chemical Geology
      Author(s): Kazuyo Tachikawa, Thomas Arsouze, Germain Bayon, Aloys Bory, Christophe Colin, Jean-Claude Dutay, Norbert Frank, Xavier Giraud, Alexandra T. Gourlan, Catherine Jeandel, François Lacan, Laure Meynadier, Paolo Montagna, Alexander M. Piotrowski, Yves Plancherel, Emmanuelle Pucéat, Matthieu Roy-Barman, Claire Waelbroeck
      Neodymium isotopic compositions (143Nd/144Nd or εNd) have been used as a tracer of water masses and lithogenic inputs to the ocean. To further evaluate the faithfulness of this tracer, we have updated a global seawater εNd database and combined it with hydrography parameters (temperature, salinity, nutrients and oxygen concentrations), carbon isotopic ratio and radiocarbon of dissolved inorganic carbon. Archive εNd data are also compiled for leachates, foraminiferal tests, deep-sea corals and fish teeth/debris from the Holocene period (<10,000years). At water depths ≥1500m, property-property plots show clear correlations between seawater εNd and the other variables, suggesting that large-scale water mass mixing is a primary control of deepwater εNd distribution. At ≥200m, basin-scale seawater T-S-εNd diagrams demonstrate the isotopic evolution of different water masses. Seawater and archive εNd values are compared using property-property plots and T-S-εNd diagrams. Archive values generally agree with corresponding seawater values although they tend to be at the upper limit in the Pacific. Both positive and negative offsets exist in the northern North Atlantic. Applying multiple regression analysis to deep (≥1500m) seawater data, we established empirical equations that predict the main, large-scale, deepwater εNd trends from hydrography parameters. Large offsets from the predicted values are interpreted as a sign of significant local/regional influence. Dominant continental influence on seawater and archive εNd is observed mainly within 1000km from the continents. Generally, seawater and archive εNd values form gradual latitudinal trend in the Atlantic and Pacific at depths ≥600m, consistent with the idea that Nd isotopes help distinguish between northern/southern sourced water contributions at intermediate and deep water depths.

      PubDate: 2017-03-27T07:54:43Z
  • Tin stable isotope analysis of geological materials by double-spike
    • Abstract: Publication date: Available online 20 March 2017
      Source:Chemical Geology
      Author(s): J.B. Creech, F. Moynier, N. Badullovich
      Tin is a volatile as well as chalcophile and siderophile element, and this geochemical behaviour gives rise to a broad range of potential applications for Sn as a stable isotope tracer in geological processes. We present the first high-precision method to analyse the stable isotopic composition of Sn in geological materials using ion-exchange chromatography and double-spike multi-collector inductively-coupled-plasma mass-spectrometry (MC-ICPMS). We apply these methods to analyse the Sn stable isotopic composition of four geological reference materials, including a number of replicate digestions of BHVO-2 and BCR-2 to assess the reproducibility of the technique. Tin was purified prior to analysis using TRU resin to remove matrix and interfering elements. Isotopic ratios were measured using a Thermo-Fischer Neptune Plus MC-ICPMS combined with a 117Sn–122Sn double-spike to correct for any mass dependent fractionation resulting from instrumental mass bias or incomplete yields from chemical purification. Results are expressed in delta notation as δ 122/118Sn, representing the per mil (‰) difference in the 122/118Sn ratio of the sample relative to our in-house standard Sn_IPGP. Based on replicate analyses of the USGS reference materials BHVO-2 and BCR-2, we estimate the external reproducibility to be ca. ± 0.065‰ (2 sd) on the δ 122/118Sn ratio, or ca. ± 0.016‰ per amu. Analyses of these plus two additional USGS reference materials, AGV-2 and GSP-2, show a large range (>0.2‰ on δ 122/118Sn) of Sn stable isotopic compositions that are correlated with igneous differentiation indicators (e.g. SiO2 content), indicating that Sn isotopes are fractionated during igneous processes. These results indicate significant potential for Sn stable isotopes as a tracer of magmatic differentiation and the redox state of the mantle. In addition, Sn stable isotopes may prove useful in tracing diverse geological processes such as volatilisation/condensation and metal–silicate/metal–sulphide differentiation.
      Graphical abstract image

      PubDate: 2017-03-27T07:54:43Z
  • The distribution of fluid mobile and other incompatible trace elements in
           orthopyroxene from mantle wedge peridotites
    • Abstract: Publication date: Available online 18 March 2017
      Source:Chemical Geology
      Author(s): Marlon M. Jean, John W. Shervais
      Orthopyroxene is especially suited to decode and testify to the behavior of highly immobile elements during hydrous mantle melting. Laser ablation ICP-MS analyses from orthopyroxene hosted within peridotite from the Coast Range ophiolite (CRO) demonstrates that Group A peridotites (lherzolites) have similar compositions to mid-ocean-ridge abyssal peridotite, whereas other peridotites (Groups B and C; harzburgites) retain depleted signatures, but display ‘spoon-shaped’ enrichments for the light-REE. These patterns are consistent with variable degrees of partial melting of MORB-source asthenosphere initiated within the garnet stability field (<10%) and continuing into the spinel stability field (<15%). A few samples may have been subjected to subsequent melt/rock interaction. The supra-subduction zone (SSZ) environment represented by the CRO is illustrated by enriched fluid mobile elements (Li, Be, B, Pb) in all samples - up to 200× depleted-MORB mantle (DMM). New applications of trace-element addition calculations [Shervais J. and Jean M.M. (2012) inside the subduction factory: Modeling fluid mobile element enrichment in the mantle wedge above a subduction zone. GCA 95, 270–285] modified for orthopyroxene reveals that tens to hundreds of ppm were added to the DMM-source region. Our purpose is to demonstrate that orthopyroxene, in the absence of clinopyroxene, can be a constructive (and perhaps better) indicator of tectonic environment and magmatic processes that occurred within the North American Cordillera mantle wedge. Through this investigation we have captured all three stages of Coast Range ophiolite petrogenesis: starting with initial SSZ-coupled forearc spreading dominated by decompression melting, to a mature subduction zone with fluid-assisted partial melting, and the transition between the two.

      PubDate: 2017-03-20T11:24:09Z
  • Temperature versus hydrologic controls of chemical weathering fluxes from
           United States forests
    • Abstract: Publication date: Available online 18 March 2017
      Source:Chemical Geology
      Author(s): Peter A. Raymond
      Chemical weathering is a dominant control on modern inland water chemistry and global element budgets over geologic time scales. Due to its central role in the earth's biogeochemistry it remains an intense area of interest. Understanding the controls on chemical weathering is difficult because it has many drivers with overlapping temporal and spatial signals. Of particular interest is the relationship between chemical weathering fluxes and global temperatures due to a negative feedback loop where warmer temperatures leads to more chemical weathering and its associated atmospheric CO2 consumption (Berner et al., 1983). Recently it has been proposed that this negative feedback loop is indirect where the acceleration of the hydrologic cycle by increased global temperatures leads to higher chemical weathering and atmospheric CO2 consumption (Maher and Chamberlain, 2014). Here, fluxes of all major cations and anions are calculated for 150 forested watersheds smaller than 500km2 in order to explore controls on chemical weathering from United States forests. Relationships between watershed hydrology, ion ratios and pH are reported that explain a large amount of across site variation in bicarbonate fluxes. Furthermore, across all watersheds ~32% of the cation flux is not balanced by bicarbonate but by sulfate. Paired alkalinity, temperature and discharge data are used to determine a temperature sensitivity of chemical weathering across 51 forested watersheds with a minimum of 70 measurements. The temperature sensitivity of bicarbonate fluxes is absent at low flow conditions because long residence times leads to chemical equilibrium and transport limitation. As discharge increases and residence time decreases, the temperature sensitivity of chemical weathering is released from transport limitation. The temperature sensitivity then increases with discharge until it levels off at high discharges as the system becomes reaction limited. Records of daily discharge, watershed discharge to flux relationships, and the temperature sensitivity are then used to estimate how chemical fluxes would change with a 20% increase in discharge, and 10° increase in temperature global change scenario. Not surprisingly it is found that increased discharge leads to higher weathering fluxes. Interestingly, wetter years have a higher temperature sensitivity due to a release of the temperature sensitivity from transport limitation. This coupled with a strong direct temperature effect leads to an approximately equal response from temperature and increased discharge using this scenario of global change. Thus periods of time and regions that are subject to both warm and wet conditions may have a particularly strong control on weathering fluxes.
      Graphical abstract image

      PubDate: 2017-03-20T11:24:09Z
  • Organically bound iodine as a bottom-water redox proxy: Preliminary
           validation and application
    • Abstract: Publication date: Available online 18 March 2017
      Source:Chemical Geology
      Author(s): Xiaoli Zhou, Hugh C. Jenkyns, Wanyi Lu, Dalton S. Hardisty, Jeremy D. Owens, Timothy W. Lyons, Zunli Lu
      Carbonate-associated iodine (I/Ca) has been used as a proxy of local, upper-ocean redox conditions, and has successfully demonstrated highly dynamic spatial and temporal patterns across different time scales of Earth history. To further explore the utility of iodine as a paleo-environmental proxy, we present here a new method of extracting organically bound iodine (Iorg) from shale using volumes of samples on the order of tens of milligrams, thus offering the potential for high-resolution work across thin shale beds. The ratio of Iorg to total organic carbon (I/TOC) in modern surface and subsurface sediments decreases with decreasing bottom-water oxygen, suggesting that iodine burial may have been influenced by redox changes. As a proof of concept, we evaluate the I/TOC proxy in Holocene sediments from the Baltic Sea, Landsort Deep (IODP 347) and discuss those data within a framework of additional independent redox proxies, e.g., iron speciation and [Mo]. The results imply that I/TOC may be sensitive to hypoxic–suboxic conditions, complementary to proxies sensitive to more reducing, anoxic–euxinic conditions. Then, we test the usage of I/TOC in sediments deposited during Late Cretaceous, Cenomanian–Turonian Oceanic Anoxic Event (OAE) 2 from ~94millionyears ago (Ma). We generated I/TOC and Iorg records from six OAE 2 sections: Tarfaya (Morocco), Furlo (central Italy), Demerara Rise (western equatorial Atlantic), Cape Verde Basin (eastern equatorial Atlantic), South Ferriby (UK), and Kerguelen Plateau (southern Indian Ocean), which provides a broad spatial coverage. Generally, I/TOC decreases over the interval recorded by the positive carbon-isotope excursion, the global signature of OAE 2, suggesting an expansion of more reducing bottom-water conditions and consistent with independent constraints from iron speciation and redox-sensitive trace-metals (e.g., Mo). Relatively higher I/TOC values (thus more oxic conditions) are recorded at two high latitude sites for OAE 2, supporting previous model simulations (cGENIE) that indicated higher bottom water oxygen concentrations in these regions. Our results also indicate that organic-rich and oxygenated seafloors are likely a major sink of iodine and correspondingly influence its global seawater inventory.

      PubDate: 2017-03-20T11:24:09Z
  • Marine redox conditions during deposition of Late Ordovician and Early
           Silurian organic-rich mudrocks in the Siljan ring district, central Sweden
    • Abstract: Publication date: Available online 18 March 2017
      Source:Chemical Geology
      Author(s): Xinze Lu, Brian Kendall, Holly J. Stein, Chao Li, Judith L. Hannah, Gwyneth W. Gordon, Jan Ove R. Ebbestad
      The Late Ordovician Period witnessed the second largest mass extinction in the Phanerozoic Eon and the Hirnantian glaciation. To infer ocean redox conditions across the Ordovician-Silurian transition, we measured the U (as δ238U relative to standard CRM145=0‰) and Mo (as δ98Mo relative to standard NIST SRM 3134=+0.25‰) isotope compositions of 26 organic-rich mudrock samples from the Late Ordovician (Katian) Fjäcka Shale and the Early Silurian (Aeronian-Telychian) Kallholn Formation (Siljan ring district, Sweden). The magnitude of Re, Mo, and U enrichments, ReEF/MoEF and UEF/MoEF ratios, and sedimentary Fe speciation point to locally euxinic bottom water conditions during deposition of the Fjäcka Shale. The same proxies suggest that black shales of the Kallholn Formation were deposited under transiently euxinic conditions with the chemocline situated near the sediment-water interface, whereas gray shales stratigraphically equivalent to the upper Kallholn Formation were deposited from oxygenated bottom waters. These observations are consistent with higher δ98Mo and δ238U in the Fjäcka Shale compared with the Kallholn Formation. Because the Fjäcka Shale was deposited from persistently euxinic bottom waters, the Mo and U isotope compositions from these rocks can be used to estimate the extent of global ocean euxinia and ocean anoxia (euxinic plus ferruginous conditions), respectively. Elevated MoEF and Mo/TOC ratios in the euxinic Fjäcka Shale suggest no more than moderate basin restriction from the open ocean as well as non-quantitative removal of Mo from the euxinic bottom waters, thus pointing to Mo isotope fractionation between seawater and the euxinic sediments. Hence, we infer that even the highest δ98Mo (+1.28‰) preserved in the Fjäcka Shale is only a minimum estimate for the Mo isotope composition of coeval global seawater. Correcting for seawater-sediment Mo isotope fractionation, the δ98Mo of late Katian seawater may have been +1.4–2.1‰, which corresponds to ~10–70% Mo removal into the euxinic sink. The average authigenic δ238U of the Fjäcka Shale is −0.05‰ to +0.02‰ after correcting for a range of possible detrital δ238U values, thus yielding an overall average of ~0‰. Taking into account isotope fractionation during U removal to euxinic sediments, we infer that late Katian seawater δ238U was about −0.85‰ to −0.60‰. A steady-state U isotope mass balance model reveals that 46–63% of riverine U input was removed in anoxic settings. Based on the Mo and U isotope data, we infer that euxinic and anoxic waters may have covered <1% and at least 5% (potentially tens of percent) of the total seafloor area during the late Katian, respectively, based on previously published models that relate the magnitude of Mo and U burial fluxes to the areal extent of euxinic and anoxic seafloor. By comparison, only 0.21–0.35% and <1% of the total seafloor area was covered by anoxic waters today and during the Cenozoic, respectively. The difference between the estimated extent of ocean anoxia (euxinic plus ferruginous) and ocean euxinia points to an appreciable extent of ferruginous water masses during the late Katian. Integration of our data with previous studies thus supports the hypothesis that ocean oxygenation intensified during the subsequent Hirnantian glaciation (when seawater δ98Mo temporarily reached values similar to today). Hence, environmental stresses related to glaciation, not an expansion of ocean anoxia, may have triggered the first phase of the Hirnantian mass extinction.

      PubDate: 2017-03-20T11:24:09Z
  • A new occurrence of titanian (hydro)andradite from the Nagaland ophiolite,
           India: Implications for element mobility in hydrothermal environments
    • Abstract: Publication date: Available online 18 March 2017
      Source:Chemical Geology
      Author(s): Biswajit Ghosh, Tomoaki Morishita, Jyotisankar Ray, Akihiro Tamura, Tomoyuki Mizukami, Yusuke Soda, Thungyani N. Ovung
      Titanium-bearing (hydro)andradites from hydrothermally altered mantle section of the Nagaland ophiolite, India have been investigated for mineral chemical study. In serpentinized peridotite they occur in association with accessory chromites or dusty magnetites and the TiO2 content in them is relatively low (<0.35wt%). (Hydro)andradites in magnetite pods, hosted within serpentinized peridotite have a range of TiO2 content (up to 13.77wt%) and occur typically within veins and veinlets intricately traversing the pods. The TiO2-rich variants of the (hydro)andradites within the veins occur either as large, polyhedral, margin-parallel zoned, mostly isolated grains within an andradite-rich porous matrix, or in clusters of equant, complexly zoned spherulites, resembling a fissure-fill. Besides (hydro)andradites as the most abundant component, the remaining of the veins consists mostly of serpentine and chlorite with relict titanites and perovskites. Magnetite pods are almost monomineralic, composed of large chromian magnetite crystals with accessory ilmenites occurring at the grain boundaries. Textural and mineralogical evidences suggest the transformation of these pods prior to the formation of (hydro)andradites from chromitite protoliths that have undergone extensive Ca-metasomatism together with host peridotites. We envisage at least two hydrothermal episodes for the mineralogical modifications where the formation of (hydro)andradites is related to the last event. The textural and mineral chemical characteristics of the entire assemblage as a whole demonstrate a gradual, temporal evolution in the composition of the metasomatic fluid from the onset of serpentinization to the end. This new occurrence of titanian andradites from chromitite protolith, hitherto not reported has major implications on the mobility of elements in hydrothermal environments.

      PubDate: 2017-03-20T11:24:09Z
  • The use of chromium reduction in the analysis of organic carbon and
           inorganic sulfur isotope compositions in Archean rocks
    • Abstract: Publication date: Available online 15 March 2017
      Source:Chemical Geology
      Author(s): Élodie Muller, Magali Ader, Carine Chaduteau, Pierre Cartigny, Franck Baton, Pascal Philippot
      One of the most serious issues with deciphering the evolution of organisms and their biogeochemical environments from the ancient rock record is the difficulty in obtaining well-preserved samples. Although not much can be done to avoid diagenetic and metamorphic alteration when they have occurred, alteration due to weathering can be avoided by working on drill core samples. This implies however that the amount of sample is limited, which may in turn restrain the number of possible chemical and isotopic analyses that can be performed. In order to save sample we show here that the chemical protocol used for the sulfur sulfide extraction (for later sulfur isotope analyses) is also suitable to decarbonate samples (for later organic carbon isotope analyses). In the case of carbonated rocks, both sulfur sulfide extraction and decarbonation require high amounts of sample so that coupling them may save a significant amount of sample and time. In addition it allows both organic carbon (TOC and δ13C) and sulfur isotope composition measurements to be performed on the exact same powder, which is essential when trying to understand couplings between S and C cycles in heterogeneous samples. We thus tested the efficiency of the acidic chromium solution, commonly used to extract sulfur from sulfide, for sample decarbonation on various Archean rocks. Our results show that no significant carbon isotope fractionation is caused by this new decarbonation protocol, even for the samples with low organic carbon content. The chromium solution seems to be perfectly adapted for the analysis of organic matter in the ancient rock record, at least when the rock samples have experienced low greenschist facies metamorphism. Further tests will be needed to verify if this protocol can also be used for less mature organic matter.

      PubDate: 2017-03-20T11:24:09Z
  • Impact of low denudation rates on soil chemical weathering intensity: A
           multiproxy approach
    • Abstract: Publication date: Available online 8 March 2017
      Source:Chemical Geology
      Author(s): Yolanda Ameijeiras-Mariño, Sophie Opfergelt, Jérôme Schoonejans, Veerle Vanacker, Philippe Sonnet, Jeroen de Jong, Pierre Delmelle
      Quantifying the influence of denudation, i.e., physical erosion and chemical weathering, on soil weathering intensity is an important component for a comprehensive understanding of element biogeochemical cycles. The relation between the weathering intensity and the denudation rate is not clear and requires further investigation in a variety of climatic and erosional settings. Here, in the Betic Cordillera (southern Spain), we assess the soil chemical weathering intensity with a multiproxy approach combining different indicators of chemical weathering of the soil: the Total Reserve in Bases (TRB), the content in Fe-oxides, the quartz and clay content, the soil cation exchange capacity (CEC), and the silicon (Si) isotope composition of the clay-sized fraction. Our multiproxy approach demonstrates that in this semi-arid environment at low denudation rates, an increasing denudation rate decreases the soil weathering intensity, whereas Si mobility remains limited. Our results converge with previous conclusions based on chemical mass balance methods in the same geological setting. Mass balance methods, and particularly Chemical Depletion Fractions (CDF), are based on the immobility of a refractory element (commonly zirconium, Zr) relative to major cations in soils. Interestingly, our study suggests that a weathering index such as the TRB may provide a useful complement to assess soil chemical weathering intensity in eroding landscapes where the application of chemical mass balances may be hampered by potential Zr mobility in the soil or by heterogeneity of Zr concentrations in the bedrock.

      PubDate: 2017-03-20T11:24:09Z
  • Chromium isotope systematics in the Connecticut River
    • Abstract: Publication date: Available online 7 March 2017
      Source:Chemical Geology
      Author(s): Weihua Wu, Xiangli Wang, Christopher T. Reinhard, Noah J. Planavsky
      Limited constraints on Cr isotope fractionation during weathering and river transport is currently a gap in our understanding of the chromium (Cr) isotope system, which is an emerging proxy in environmental and paleoenvironmental studies. Here, we investigate Cr mobility and isotope fractionation from the temperate Connecticut River, USA, including Cr concentrations and isotopic compositions of river water, suspended particles, riverbed sediments, and weathering profiles. The δ53Cr values of the Connecticut River water range from −0.17‰ to +0.92‰, which are similar to or higher than the weathered rocks in the catchment (−0.08‰ to −0.29‰). We find seasonal variations in dissolved δ53Cr values in some but not all tributaries, suggesting that dissolved δ53Cr is not a simple function of seasonality but may also be influenced by sub-catchment heterogeneity in lithology. In contrast to dissolved Cr, we found consistent seasonal difference in suspended Cr concentration and δ53Cr. Suspended δ53Cr is 0.1‰ higher than the unfractionated BSE in the fall (0.01–0.13‰), but indistinguishable from the BSE in the spring (−0.11‰ to 0.00‰). The suspended Cr concentration is also lower in the spring, and with higher Al-Mn-Fe concentrations. The lower suspended Cr concentration and δ53Cr in spring may be linked to increased silicate and oxide load with depleted Cr due to stronger hydrological flux. Building from our dataset, there is not a consistent correlation with climate zones in a compilation of δ53Cr and Cr concentration data from river water and weathering profiles, suggesting that climate is not a dominating factor controlling Cr isotopic behavior during weathering, suggesting that other factors (e.g., local catchment conditions and dissolved organic matter) may also be responsible for the observed river water δ53Cr variability.

      PubDate: 2017-03-20T11:24:09Z
  • The implications of overstepping for metamorphic assemblage diagrams
    • Abstract: Publication date: Available online 7 March 2017
      Source:Chemical Geology
      Author(s): Frank S. Spear, David R.M. Pattison
      Metamorphic assemblage diagrams (MADs, also known as pseudosections) are based on the assumption of chemical equilibrium throughout. Nucleation of a porphyroblast, however, requires a driving force that is achieved through overstepping of the isograd reaction. By making an assumption about the degree of overstepping required to nucleate garnet, aluminosilicate, staurolite, and cordierite, a modified MAD can be constructed that offers insights into metamorphic parageneses beyond those provided by an equilibrium phase diagram. Assuming a representative value of 300J/mol oxygen of affinity for the nucleation of a porphyroblast, modified MADs have been constructed for a typical pelitic bulk rock composition. The resulting diagrams show a displacement of the garnet isograd to higher temperature (T), a shrinking of the stability field for garnet+chlorite, an expansion of the field for aluminosilicate, and a shrinking of the stability field for cordierite. Furthermore, the size of the stability field for staurolite+garnet depends on which reaction produces aluminosilicate: if aluminosilicate nucleates from an assemblage of garnet+staurolite the field of staurolite+garnet is greatly expanded; if aluminosilicate nucleates from an assemblage of chlorite+muscovite, the field shrinks dramatically and may be eliminated entirely. If garnet is a reactive phase, then kyanite is predicted to nucleate prior to or nearly simultaneously with staurolite in typical Barrovian metamorphic trajectories. Finally, certain equilibrium assemblages present in only small regions of P–T space may not be realized unless sufficient driving force for nucleation or progressive reaction can occur over limited changes in P and T. The key consideration is the amount of affinity required to drive the reaction at the P–T conditions of interest.

      PubDate: 2017-03-20T11:24:09Z
  • Trace elements in anatectic products at the roof of mid-ocean ridge magma
           chambers: An experimental study
    • Abstract: Publication date: Available online 7 March 2017
      Source:Chemical Geology
      Author(s): Martin Erdmann, Lydéric France, Lennart A. Fischer, Etienne Deloule, Jürgen Koepke
      At fast-spreading mid-ocean ridges (MORs), the horizon between the axial melt lens (AML) and the overlying sheeted dikes is characterized by extensive anatectic processes. The heat flux of the AML in combination with hydrothermal fluids from above causes high-grade contact metamorphism, which may result in anatexis of the roof rocks above the AML. The products of this process are silica-rich anatectic melts that have the potential to contaminate MOR basalts and residual hornfels. Here, we simulate the complex igneous and metamorphic processes occurring at the AML roof by hydrous partial melting experiments and provide corresponding trace element partition coefficients between melt and residues, which are useful to quantify those processes. We present trace element patterns from experimental anatectic felsic melts and the related residue produced by hydrous partial melting of various types of AML roof rocks. The starting materials used are sheeted dikes and hornfelses from Hole 1256D drilled by the Integrated Ocean Drilling Program. Results are compared with directly-related natural lithologies (i.e., felsic veins and granoblastic hornfels) from the same site. The trace element contents generally overlap with natural examples and experimental melts produced at low water activity (aH2O<0.5) can be highly enriched in trace elements despite relatively low SiO2 contents (58.9 to 65.7wt%). A low aH2O is required to reproduce the low Al2O3 contents observed in natural silica-rich rocks. However, low aH2O implies that the presence of residual amphibole is not required for anatectic processes Even though residual amphibole is often used as an important phase for explaining trace element characteristics in relevant felsic rocks formed at MORs when modeling anatexis. Because amphibole is lacking in any experimental residue, which is in agreement with natural hornfelses from the dike/gabbro transition at Site 1256, we assume that partial melting within the AML roof rocks proceeds without the participation of amphibole as residual phase. We present a comprehensive set of trace element compositions as well as bulk and mineral/melt trace element partition coefficients obtained from our amphibole-free experimental results for different potential protoliths over a large range of temperature and at different aH2Os.

      PubDate: 2017-03-20T11:24:09Z
  • Tracking the onset of Phanerozoic-style redox-sensitive trace metal
           enrichments: New results from basal Ediacaran post-glacial strata in NW
    • Abstract: Publication date: Available online 6 March 2017
      Source:Chemical Geology
      Author(s): Austin J. Miller, Justin V. Strauss, Galen P. Halverson, Francis A. Macdonald, David T. Johnston, Erik A. Sperling
      A global rise in oxygen levels has been proposed to coincide with the Ediacaran to Cambrian radiation of animals, yet the precise timing and nature of this change remains unresolved. One hypothesis is that the ocean/atmosphere system became temporarily well-oxygenated in the earliest Ediacaran, directly following the Marinoan Snowball Earth glaciation (~635Ma). The evidence for oxygenation is based on large enrichments of redox-sensitive trace elements in black shale from South China presumably deposited under a euxinic water column. These enrichments meet or exceed the bulk concentrations of redox-sensitive trace elements found in Phanerozoic shale deposited under euxinic water columns. Here we test the early Ediacaran post-Snowball oxygenation hypothesis with new data from a high-resolution, multi-proxy geochemical and sedimentological study of three stratigraphic sections in the earliest Ediacaran Sheepbed Formation of the Mackenzie and Wernecke Mountains, NW Canada. Iron speciation data from all sections suggest that the local water column was dominantly ferruginous, with a notable exception of probable euxinic conditions recorded in part of one section. Redox-sensitive elements show no appreciable enrichments in the basal Sheepbed Formation, with maximum concentrations of molybdenum (Mo), vanadium (V), uranium (U) and chromium (Cr) only slightly above world average shale values. The lack of substantial elemental enrichments within the Sheepbed Formation is consistent with results from coeval ferruginous strata in Svalbard. The dominance of local ferruginous conditions in NW Canada cannot alone account for the muted redox-sensitive element enrichments as V, U, and Cr are thought to be enriched under these conditions, and samples deposited under euxinic conditions also lack Phanerozoic-style enrichments. These contrasting results from different localities highlight a need to further investigate the veracity of trace metal enrichments in all localities as representing global redox conditions as the results have important implications for the timing of oxygenation with respect to early animal evolution.

      PubDate: 2017-03-08T07:30:26Z
  • Coupled control of land uses and aquatic biological processes on the
           diurnal hydrochemical variations in the five ponds at the Shawan Karst
           Test Site, China: Implications for the carbonate weathering-related carbon
    • Abstract: Publication date: Available online 6 March 2017
      Source:Chemical Geology
      Author(s): Bo Chen, Rui Yang, Zaihua Liu, Hailong Sun, Hao Yan, Qingrui Zeng, Sibo Zeng, Cheng Zeng, Min Zhao
      High-resolution hydrochemical data from five spring-fed ponds are presented to demonstrate the effect of different land uses and aquatic biological processes on the carbon cycle at a karst-analog test site. The results show that hydrochemical parameters including pH and the concentrations of HCO3 −, Ca2+, NO3 −, partial pressures of CO2 (pCO2) and dissolved O2 (DO) as well as carbon isotopic compositions (δ13C) of HCO3 – in the pond water all displayed distinct diurnal variations, while those of the spring water itself were rather stable. The coupled dynamic behaviors of pCO2, DO and NO3 − indicate a significant influence from the metabolism of submerged plants in the ponds. In the afternoon, when photosynthesis is the strongest, the pCO2 of the five pond waters was lower even than that of the ambient atmosphere, demonstrating the existence of a “biological carbon pumping (BCP) effect”, similar to that in the oceans. It was determined that, in October (autumn), the BCP fluxes in the five spring-fed ponds were 156±51tCkm−2 a−1 in P1 (Pond 1 – adjoining a bare rock shore), 239±83tCkm−2 a−1 in P2 (adjoining uncultivated soil), 414±139tCkm−2 a−1 in P3 (adjoining land cultivated with corn), 493±165tCkm−2 a−1 in P4 (adjoining grassland) and 399±124tCkm−2 a−1 of P5 (adjoining brushland), indicating the potentially significant role of aquatic photosynthesis in stabilizing the carbonate weathering-related carbon sink. In addition, by comparing the DIC concentrations and fluxes of DIC transformed into autochthonous organic matter (AOC) in the five ponds, the so-called “DIC fertilization effect” was found in which more AOC is produced in pond waters with higher concentrations of DIC. This implies that the carbon cycle driven by aquatic biological processes can be regulated by changing land use and cover, the latter determining the DIC concentrations. Further, the rock weathering-related carbon sink is underestimated if one only considers the DIC component in surface waters instead of both DIC and AOC.

      PubDate: 2017-03-08T07:30:26Z
  • Effect of calcite precipitation on stable strontium isotopic compositions:
           Insights from riverine and pool waters in a karst cave
    • Abstract: Publication date: Available online 6 March 2017
      Source:Chemical Geology
      Author(s): Hou-Chun Liu, Chen-Feng You, Houyun Zhou, Kuo-Fang Huang, Chuan-Hsiung Chung, Wei-Jen Huang, Jing Tang
      Radiogenic and stable Sr isotope ratios (87Sr/86Sr, δ88/86Sr) serve as potential tracers for evaluation of weathering sources and associated physicochemical processes in terrestrial environments. Despite carbonate weathering sourcing two-thirds of the total continental Sr flux to the ocean, however, few attempts have been aimed at studying stable Sr isotope fractionation in carbonate-dominated aquifers. Here we present a monthly-resolved monitoring dataset of major ion concentrations and Sr isotope ratios for karst riverine and pool waters at the Luofang Cave in the Central China to understand the potential controls responsible for water δ88/86Sr compositions under various climatic conditions. With the exception of some pool waters in the dry period, significantly heavier δ88/86Sr were detected in the riverine and pool waters (0.20‰ to 0.42‰) compared to bedrock δ88/86Sr (0.24‰ to 0.28‰), which can't be interpreted solely by lithology. A sequential leaching experiment of the local soils suggests limited stable Sr isotope fractionation during incongruent silicate mineral dissolution. Local plant utilization and biological activity in the soil overlying the cave are possible processes for causing the observed heavier δ88/86Sr in cave water. However, air partial pressure of CO2 in cave, monitoring water pH, and Sr isotopic mass balance calculations suggest that plant utilization is unlikely to be the primary control on the heavier water δ88/86Sr. On the other hand, significant stable Sr isotope fractionation was found between the cave dripping waters and calcite precipitates, and the Δ88/86Srcar-aq was estimated to be −0.15‰±0.07‰ (2SD). Precipitation of secondary calcites might be a potential process responsible for the heavier δ88/86Sr in the waters. The negative correlation between δ88/86Sr and Sr/Na ratios in the cave waters is consistent with a scenario of preferential incorporation of the light Sr isotope by solid phase. Furthermore, by combining the calcite saturation states and water pH, our results suggest that calcite precipitation is the most likely process controlling δ88/86Sr compositions in cave water. We demonstrate that secondary calcite precipitation in carbonate-dominated catchments could be important in controlling the riverine water δ88/86Sr compositions.

      PubDate: 2017-03-08T07:30:26Z
  • Statistics for LA-ICP-MS based fission track dating
    • Abstract: Publication date: Available online 6 March 2017
      Source:Chemical Geology
      Author(s): Pieter Vermeesch
      Despite the conceptual elegance and simplicity of the External Detector Method (EDM) for fission track dating, an increasing number of laboratories are switching to LA-ICP-MS as a means of measuring the uranium content of apatite, zircon and sphene. LA-ICP-MS based fission track (LAFT) dating offers shorter turnaround times, removes the need to handle radioactive materials and hydrofluoric acid, and facilitates double-dating with the U-Pb method. This paper aims to bring the statistical treatment of LAFT data on an equal footing with the EDM by formulating four different analytical protocols, depending on the accuracy and reproducibility of the uranium measurements. Under the ‘absolute’ dating approach, the spontaneous track densities and uranium concentrations are directly plugged into the fundamental fission track age equation, assuming that both of these measured quantities have been determined accurately, and that the fission decay constant and equivalent isotopic track length are known. The ‘zeta calibration’ approach avoids making these assumptions by normalisation to a reference material of known age. Uranium zoning has a detrimental effect on the accuracy of LAFT ages. This effect can be removed by counting only those fission tracks located within the laser ablation pit. Alternatively, the uranium heterogeneity may be quantified by fitting multiple ablation spots in some or all the analysed grains, using a (lognormal) distributional assumption for the uranium concentration. LAFT dating is arguably less well suited than the EDM to young and U-poor samples that lack sufficient spontaneous fission tracks to reveal visual evidence for uranium zoning. Such samples occasionally contain no fission tracks at all, resulting in infinite analytical uncertainties. This paper introduces a pragmatic solution to this problem, in which the uranium measurement are converted to ‘virtual’ induced fission track counts to produce strictly positive ages with finite standard errors. With the age equation and zero-track strategy in place, LAFT ages can be subjected to more sophisticated statistical analysis. Using a logarithmic transformation, these ages can be visualised on radial plots and deconvolved into finite and continuous mixtures. The methods proposed in this paper have been implemented in a software package called IsoplotR that is available free of charge at

      PubDate: 2017-03-08T07:30:26Z
  • Subduction channel fluid–rock interaction and mass transfer: Constraints
           from a retrograde vein in blueschist (SW Tianshan, China)
    • Abstract: Publication date: Available online 6 March 2017
      Source:Chemical Geology
      Author(s): Ji-Lei Li, Timm John, Jun Gao, Reiner Klemd, Xin-Shui Wang
      One widely neglected part of the subduction zone fluid cycle is the retrograde fluid flow along the plate interface. This fluid flow may facilitate the exhumation of slab-derived rocks within the subduction channel. However, our understanding on the nature and behavior of these retrograde fluids is still insufficient. Retrograde veins preserved in HP–UHP rocks can provide valuable information about fluid activities and mass transfer at the plate interface and exhumation processes. In this study, we present a petrological and geochemical investigation of a typical retrograde vein and its blueschist host rock, which occurs throughout the whole Akeyazi (U)HP terrane in the Chinese southwestern Tianshan. The magnetite-bearing albite–calcite vein, representing an external fluid pathway, crosscuts the host blueschist that mainly consists of glaucophane, garnet, epidote and titanite. Along the fluid conduit the immediate wall-rock experienced greenschist-facies overprinting as displayed by an albite-rich selvage that grades into a chlorite–biotite selvage towards the host rock. The replacements of garnet by chlorite and glaucophane by albite are thought to have occurred due to an interface-coupled dissolution-precipitation mechanism. Mass balance calculations show that during fluid–rock interaction large amounts of LILE (K–Rb–Ba–Cs and Pb), Cu and CO2 were added to the selvages (mass gain >200%), whereas the altered wall-rock released significant amounts of Ca, Li, Sr, REE and transition metal elements (V–Mn–Co–Ni–Zn) (mass loss mainly between 40 and 100%) due to the dissolution of glaucophane, garnet and epidote. The chlorite–biotite selvage experienced additional SiO2 and Na2O mass losses as well as FeO, MgO and H2O mass gains, whereas the albite-rich selvage underwent exactly the opposite mass transfer with respect to these components. The Al, Ti, Nb, Ta and Cr contents were constant during the alteration. Therefore, the retrograde fluid causing greenschist-facies overprinting of the blueschist host in the subduction channel (at ca. 30km depth) is proposed to be oxidized and enriched in LILE, Cu and CO2 components, whereas Ca, Li, Sr, REE, Y and transition metal elements (V–Mn–Co–Ni–Zn) are thought to have been mobilized during fluid-rock interaction. In addition, the main fluid sources (sediments, oceanic crust or serpentinites) diversely contribute to retrograde fluids at different depths in the subduction channel.

      PubDate: 2017-03-08T07:30:26Z
  • Experimental determination of solubilities of di-calcium
           ethylenediaminetetraacetic acid hydrate [Ca2C10H12N2O8·7H2O(s)] in NaCl
           and MgCl2 solutions to high ionic strengths and its Pitzer model:
           Applications to geological disposal of nuclear waste and other low
           temperature environments
    • Abstract: Publication date: 5 April 2017
      Source:Chemical Geology, Volume 454
      Author(s): Yongliang Xiong, Leslie Kirkes, Terry Westfall
      In this study, solubility measurements on di-calcium ethylenediaminetetraacetic acid hydrate [Ca2C10H12N2O8·7H2O(s), abbreviated as Ca2EDTA·7H2O(s)] as a function of ionic strength are conducted in NaCl solutions up to I =4.4mol·kg−1 and in MgCl2 solutions up to I =7.5mol·kg−1, at room temperature (22.5±0.5°C). The solubility constant (logK sp 0) for Ca2EDTA·7H2O(s) and formation constant (logβ 1 0) for CaEDTA2−, Ca2EDTA·7H2O(s)=2Ca2+ +EDTA4− +7H2O(1) Ca2+ +EDTA4− =CaEDTA2− (2) are determined as −15.57±0.10 and 11.50±0.05, respectively, based on the Pitzer model with a set of Pitzer parameters describing the specific interactions in NaCl and MgCl2 media. The solubility measurements and thermodynamic modeling indicate that Ca2EDTA·7H2O(s) could become a solubility-controlling phase for EDTA in geological repositories for nuclear waste when the inventories of EDTA reach the saturation concentrations for Ca2EDTA·7H2O(s). The model developed in this work would also enable researchers to calculate the optimal EDTA concentrations to be used for remediation of soils contaminated with heavy metals, and to calculate the maximum EDTA concentrations that could be present in soils after an ETDA washing technology has been applied.

      PubDate: 2017-03-02T07:54:39Z
  • Whole rock basalt alteration from CO2-rich brine during flow-through
           experiments at 150°C and 150bar
    • Abstract: Publication date: 20 March 2017
      Source:Chemical Geology, Volume 453
      Author(s): Andrew J. Luhmann, Benjamin M. Tutolo, Chunyang Tan, Bruce M. Moskowitz, Martin O. Saar, William E. Seyfried
      Four flow-through experiments at 150°C were conducted on intact cores of basalt to assess alteration and mass transfer during reaction with CO2-rich fluid. Two experiments used a flow rate of 0.1ml/min, and two used a flow rate of 0.01ml/min. Permeability increased for both experiments at the higher flow rate, but decreased for the lower flow rate experiments. The experimental fluid (initial pH of 3.3) became enriched in Si, Mg, and Fe upon passing through the cores, primarily from olivine and titanomagnetite dissolution and possibly pyroxene dissolution. Secondary minerals enriched in Al and Si were present on post-experimental cores, and an Fe2O3-rich phase was identified on the downstream ends of the cores from the experiments at the lower flow rate. While we could not specifically identify if siderite (FeCO3) was present in the post-experimental basalt cores, siderite was generally saturated or supersaturated in outlet fluid samples, suggesting a thermodynamic drive for Fe carbonation from basalt-H2O-CO2 reaction. Reaction path models that employ dissolution kinetics of olivine, labradorite, and enstatite also suggest siderite formation at low pH. Furthermore, fluid-rock interaction caused a relatively high mobility of the alkali metals; up to 29% and 99% of the K and Cs present in the core, respectively, were preferentially dissolved from the cores, likely due to fractional crystallization effects that made alkali metals highly accessible. Together, these datasets illustrate changes in chemical parameters that arise due to fluid-basalt interaction in relatively low pH environments with elevated CO2.

      PubDate: 2017-03-02T07:54:39Z
  • In-situ trace element and Fe-isotope studies on magnetite of the
           volcanic-hosted Zhibo and Chagangnuoer iron ore deposits in the Western
           Tianshan, NW China
    • Abstract: Publication date: 20 March 2017
      Source:Chemical Geology, Volume 453
      Author(s): T. Günther, R. Klemd, X. Zhang, I. Horn, S. Weyer
      The Carboniferous Zhibo and Chagangnuoer iron deposits are situated within a caldera centre and along the flank of the same volcanic edifice, respectively, in the Awulale Iron Metallogenic Belt of the Western Tianshan orogen. Several stratiform 10 to 100m large, tabular to lenticular shaped magnetite-dominated ore-bodies occur in (trachy-) andesitic to rhyolitic host rocks. The magnetite mineralization mainly occurs as massive iron ores, partly with columnar-network or flow textures, and as disseminated magnetite ores. Trace element and isotope investigations of the different ore types reveal two major groups of magnetite: Group I, represented by the massive, partly brecciated ores from both deposits, is enriched in Ti, V, Ni, and HFSE such as Y, with concentrations similar to Iron Oxide-Copper-Gold (IOCG) ores. The δ56Fe values (up to 0.4‰) support an ortho-magmatic origin corresponding with an isotopic source calculation at ~800°C. Positive correlations between Fetotal and δ56Fe (from +0.4‰ to −0.1‰) and incompatible trace element contents (e.g. Si, Al, Nb, Ti and Y) in Group I magnetite are interpreted to be the consequence of a Raleigh-type fractionation process. Decreasing V, Ni and Mn values indicate changing fO2 conditions at the time of ore genesis. Group II, which is represented by the disseminated ores from Chagangnuoer, is - compared to Group I - relatively depleted in elements like Ti, V, Ni and Y and further spans a dominant δ56Fe range from about 0‰ to −0.5‰. These textural and chemical characteristics and the garnet-actinolite-diopside-epidote-carbonate-K-feldspar paragenesis are in accordance with hydrothermal Fe-skarn ores. The similar multi-element patterns of magnetite from all investigated samples, the overlapping δ56Fe ratios of the same massive ore-type from Zhibo and Chagangnuoer and the close proximity of both deposits indicate a common source of Fe-enrichment for the different iron ore types. In contrast to the ortho-magmatic Group I magnetite, reverse trace element trends with decreasing δ56Fe ratios (from 0‰ to −0.5‰) among the disseminated ores cannot simply be explained by a straightforward Raleigh fractionation or alteration processes. Therefore, a bimodal formation model is suggested for the Group II magnetite formation, including a partial remobilization of iron from the proximal, ortho-magmatic ore bodies and a subsequent distal re-precipitation. These processes were driven by late-stage hydrothermal fluids, which originated from deeper- seated granitic/granodioritic intrusions in the immediate vicinity.

      PubDate: 2017-03-02T07:54:39Z
  • Microbial methane from in situ biodegradation of coal and shale: A review
           and reevaluation of hydrogen and carbon isotope signatures
    • Abstract: Publication date: 20 March 2017
      Source:Chemical Geology, Volume 453
      Author(s): David S. Vinson, Neal E. Blair, Anna M. Martini, Steve Larter, William H. Orem, Jennifer C. McIntosh
      Stable carbon and hydrogen isotope signatures of methane, water, and inorganic carbon are widely utilized in natural gas systems for distinguishing microbial and thermogenic methane and for delineating methanogenic pathways (acetoclastic, hydrogenotrophic, and/or methylotrophic methanogenesis). Recent studies of coal and shale gas systems have characterized in situ microbial communities and provided stable isotope data (δD-CH4, δD-H2O, δ13C-CH4, and δ13C-CO2) from a wider range of environments than available previously. Here we review the principal biogenic methane-yielding pathways in coal beds and shales and the isotope effects imparted on methane, document the uncertainties and inconsistencies in established isotopic fingerprinting techniques, and identify the knowledge gaps in understanding the subsurface processes that govern H and C isotope signatures of biogenic methane. We also compare established isotopic interpretations with recent microbial community characterization techniques, which reveal additional inconsistencies in the interpretation of microbial metabolic pathways in coal beds and shales. Collectively, the re-assessed data show that widely-utilized isotopic fingerprinting techniques neglect important complications in coal beds and shales. Isotopic fingerprinting techniques that combine δ13C-CH4 with δD-CH4 and/or δ13C-CO2 have significant limitations: (1) The consistent ~160‰ offset between δD-H2O and δD-CH4 could imply that hydrogenotrophic methanogenesis is the dominant metabolic pathway in microbial gas systems. However, hydrogen isotopes can equilibrate between methane precursors and coexisting water, yielding a similar apparent H isotope signal as hydrogenotrophic methanogenesis, regardless of the actual methane formation pathway. (2) Non-methanogenic processes such as sulfate reduction, Fe oxide reduction, inputs of thermogenic methane, anaerobic methane oxidation, and/or formation water interaction can cause the apparent carbon isotope fractionation between δ13C-CH4 and δ13C-CO2 (α13CCO2-CH4) to differ from the true methanogenic fractionation, complicating interpretation of methanogenic pathways. (3) Where little-fractionating non-methanogenic bacterial processes compete with highly-fractionating methanogenesis, the mass balance between CH4 and CO2 is affected. This has implications for δ13C values and provides an alternative interpretation for net C isotope signatures than solely the pathways used by active methanogens. (4) While most of the reviewed values of δD-H2O - δD-CH4 and α13CCO2-CH4 are apparently consistent with hydrogenotrophic methanogenesis as the dominant pathway in coal beds and shales, recent microbial community characterization techniques suggest a possible role for acetoclastic or methylotrophic methanogenesis in some basins.

      PubDate: 2017-03-02T07:54:39Z
  • Neodymium isotopes and concentrations in aragonitic scleractinian
           cold-water coral skeletons - Modern calibration and evaluation of
    • Abstract: Publication date: 20 March 2017
      Source:Chemical Geology, Volume 453
      Author(s): Torben Struve, Tina van de Flierdt, Andrea Burke, Laura F. Robinson, Samantha J. Hammond, Kirsty C. Crocket, Louisa I. Bradtmiller, Maureen E. Auro, Kais J. Mohamed, Nicholas J. White
      Cold-water corals (CWCs) are unique archives of mid-depth ocean chemistry and have been used successfully to reconstruct the neodymium (Nd) isotopic composition of seawater from a number of species. High and variable Nd concentrations in fossil corals however pose the question as to how Nd is incorporated into their skeletons. We here present new results on modern specimens of Desmophyllum dianthus, Balanophyllia malouinensis, and Flabellum curvatum, collected from the Drake Passage, and Madrepora oculata, collected from the North Atlantic. All modern individuals were either collected alive or uranium-series dated to be <500years old for comparison with local surface sediments and seawater profiles. Modern coral Nd isotopic compositions generally agree with ambient seawater values, which in turn are consistent with previously published seawater analyses, supporting small vertical and lateral Nd isotope gradients in modern Drake Passage waters. Two Balanophyllia malouinensis specimens collected live however deviate by up to 0.6 epsilon units from ambient seawater. We therefore recommend that this species should be treated with caution for the reconstruction of past seawater Nd isotopic compositions. Seventy fossil Drake Passage CWCs were furthermore analysed for their Nd concentrations, revealing a large range from 7.3 to 964.5ng/g. Samples of the species D. dianthus and Caryophyllia spp. show minor covariation of Nd with 232Th content, utilised to monitor contaminant phases in cleaned coral aragonite. Strong covariations between Nd and Th concentrations are however observed in the species B. malouinensis and G. antarctica. In order to better constrain the source and nature of Nd in the cleaned aragonitic skeletons, a subset of sixteen corals was investigated for its rare earth element (REE) content, as well as major and trace element geochemistry. Our new data provide supporting evidence that the applied cleaning protocol efficiently removes contaminant lithogenic and ferromanganese oxyhydroxide phases. Mass balance calculations and seawater-like REE patterns rule out lithogenic and ferromanganese oxyhydroxide phases as a major contributor to elevated Nd concentrations in coral aragonite. Based on mass balance considerations, geochemical evidence, and previously published independent work by solid-state nuclear magnetic resonance (NMR) spectroscopy, we suggest authigenic phosphate phases as a significant carrier of skeletal Nd. Such a carrier phase could explain sporadic appearance of high Nd concentrations in corals and would be coupled with seawater-derived Nd isotopic compositions, lending further confidence to the application of Nd isotopes as a water mass proxy in CWCs.

      PubDate: 2017-03-02T07:54:39Z
  • Iron K-edge X-ray absorption near-edge structure spectroscopy of
           aerodynamically levitated silicate melts and glasses
    • Abstract: Publication date: 20 March 2017
      Source:Chemical Geology, Volume 453
      Author(s): O.L.G. Alderman, M.C. Wilding, A. Tamalonis, S. Sendelbach, S.M. Heald, C.J. Benmore, C.E. Johnson, J.A. Johnson, H.-Y. Hah, J.K.R. Weber
      The local structure about Fe(II) and Fe(III) in silicate melts was investigated in-situ using iron K-edge X-ray absorption near-edge structure (XANES) spectroscopy. An aerodynamic levitation and laser heating system was used to allow access to high temperatures without contamination, and was combined with a chamber and gas mixing system to allow the iron oxidation state, Fe3+/ΣFe, to be varied by systematic control of the atmospheric oxygen fugacity. Eleven alkali-free, mostly iron-rich and depolymerized base compositions were chosen for the experiments, including pure oxide FeO, olivines (Fe,Mg)2SiO4, pyroxenes (Fe,Mg)SiO3, calcic FeO-CaSiO3, and a calcium aluminosilicate composition, where total iron content is denoted by FeO for convenience. Melt temperatures varied between 1410 and 2160K and oxygen fugacities between FMQ – 2.3(3) to FMQ+9.1(3) log units (uncertainties in parentheses) relative to the fayalite-magnetite-β-quartz (FMQ) buffer. Remarkably, XANES pre-edge peak areas imply mean Fe-O coordination numbers (n FeO) close to 5 in all cases, with only a slight tendency toward higher values in the most iron rich melts, suggesting an intermediate role for both Fe(II) and Fe(III) in terms of network formation. End member coordination numbers for Fe(II)-O and Fe(III)-O are estimated to be similar, having means (and standard deviations) of 5.0(2) and 4.9(1), respectively. As such, the preference for ferric iron to occupy lower coordination sites than ferrous is weak, in contrast to published behavior in some alkali-rich systems, which may explain the larger published viscosity variations with Fe3+/ΣFe in alkali-, compared to alkaline earth-iron silicates. Temperature effects on n FeO are inferred to be small based on the melt data, as well as by comparison to glasses formed on quenching. Positive shifts of the pre-edge peak centroids observed in many cases on quenching are attributed to rapid oxidation enabled by the stirring of the melt droplets by the levitation gas jet. Fe3+/ΣFe values were estimated from XANES pre-edge peaks using published calibrations and compared to semi-empirical thermodynamic model calculations and Mössbauer measurements on quench products. Whilst showing positive correlation, the comparisons highlight the limitations involved in applying XANES calibrations and models for Fe3+/ΣFe derived from measurements on glasses, to high temperature basic melts. Fe3+/ΣFe varies from approximately zero up to about 65% in the high temperature melts and 75% in the glasses.
      Graphical abstract image

      PubDate: 2017-03-02T07:54:39Z
  • LGC-1: A zircon reference material for in-situ (U-Th)/He dating
    • Abstract: Publication date: Available online 1 March 2017
      Source:Chemical Geology
      Author(s): Yuntao Tian, Pieter Vermeesch, Martin Danišík, Daniel J. Condon, Wen Chen, Barry Kohn, James Schwanethal, Martin Rittner
      A pairwise in-situ (U-Th)/He dating method has been proposed for mitigating matrix-related bias in U and Th measurements using synthetic reference materials. This method requires a natural zircon reference material whose (U-Th)/He age should be homogeneous on the scale (~10–100μm) to be used in such dating experiments. A newly characterized zircon LGC-1 megacryst fulfils this requirement. This pale-yellowish, flawless Sri Lanka gem specimen is about 1.2∗0.8∗0.8cm in size. Optical microscopy, cathodoluminescence-imaging, X-ray elemental mapping, and Raman spectroscopy on a large number of random shards did not reveal any detectable textural and compositional heterogeneity. Laser ablation inductively-coupled plasma mass spectrometry (LA-ICP-MS) analyses on a large number of randomly selected fragments yield 266 measurements of U, Th and Pb concentrations, which are within the corresponding experimental uncertainties. The weighted mean U, Th and Pb concentrations are 357.7±1.8ppm, 740.9±5.0ppm, and 39.06±0.18ppm, respectively, with a weighted mean Th/U ratio of 2.07±0.01, indistinguishable from Isotope Dilution ICP-MS (ID-ICP-MS) and Thermal Ionization Mass Spectrometry (ID-TIMS) results. ID-TIMS U/Pb ages are concordant within uncertainties of decay constants, with a concordia age of 541.70±0.70Ma. Conventional (U-Th)/He dating on 28 random shards from the crystal in different laboratories gives a central age of 476.4±5.7Ma. Six in-situ (U-Th)/He analyses yield consistent 4He concentrations and ages with weighted mean values of 1248±46nmol/g and 462±21Ma, respectively. Fractions of this zircon have been shared with several laboratories in the Australia, China, UK and US, and are expected to serve as a reference for both in-situ and conventional (U-Th)/He analyses. The combination of analytical methods used to characterize LGC-1 zircon may be used as a template for future age reference calibration.

      PubDate: 2017-03-02T07:54:39Z
  • Gas source attribution techniques for assessing leakage at geologic CO2
           storage sites: Evaluating a CO2 and CH4 soil gas anomaly at the Cranfield
           CO2-EOR site
    • Abstract: Publication date: Available online 28 February 2017
      Source:Chemical Geology
      Author(s): Jacob S. Anderson, Katherine D. Romanak, Changbing Yang, Jiemin Lu, Susan D. Hovorka, Michael H. Young
      At the Cranfield CO2 enhanced oil recovery (CO2-EOR) site, a localized area of high concentrations of CO2 (up to 44%) and CH4 (up to 47%) in soil gas was detected near a plugged and abandoned well. The complexity of attributing this anomaly, especially in a CO2-EOR setting, underscores the need for careful attribution techniques and provides rare and valuable experiential knowledge on attributing blind anomalies. An extensive geochemical monitoring program utilizing process-based soil gas ratios, stable and radioactive isotopes of CO2 and CH4, light hydrocarbon concentrations, noble gases, and perfluorocarbon and sulfur hexafluoride tracers was undertaken from 2009 through 2014. The goals were to attribute source, assess the usefulness of various attribution techniques, and begin to develop a framework for attribution in complex CO2-EOR settings. Initial process-based assessment indicated an “exogenous” source meaning that it was not the result of natural in-situ processes (Romanak et al., 2012). We report on the additional analyses used to determine the degree to which the anomaly was related to CO2 injection. This work included characterization of potential non-reservoir gas sources within the overburden using mud-gas samples collected during a new drill and downhole fluids collected from wells within the field. Two hydrocarbon gas sources, one within the reservoir (Tuscaloosa) and one in the above-zone (Wilcox) were geochemically distinct. Stable carbon isotopes (δ13C) of CH4 in the anomaly were similar to those of the reservoir, but stable hydrogen isotopes (δD) indicated that anomalous gases originate from an undetermined microbial source rather than either of the subsurface gas reservoirs. Hydrocarbon geochemical parameters were therefore not only useful for attribution, but were also found to have a high potential for leading to inaccurate conclusions because of alteration via CH4 oxidation. Noble gases and introduced tracers proved least effective for attribution in this case. The most useful indicator was radioactive isotopes of CO2 and CH4, which contained >100% modern carbon indicating a negligible input, if any, from the reservoir.
      Graphical abstract image

      PubDate: 2017-03-02T07:54:39Z
  • Geochemical characteristics, speciation and size-fractionation of iron
           (Fe) in two marine shallow-water hydrothermal systems, Dominica, Lesser
    • Abstract: Publication date: Available online 24 February 2017
      Source:Chemical Geology
      Author(s): Charlotte Kleint, Thomas Pichler, Andrea Koschinsky
      Marine shallow-water hydrothermal systems have so far largely been neglected with respect to trace metal fluxes and possible stabilizing complexation processes, even though they emit their fluids directly into the photic zone. The impact of stabilized dissolved metal input by shallow vents into surface waters as well as the effect on the transport and bioavailability of bioactive trace metals within the area of highest primary production rates in the world oceans is therefore, at present, mostly unknown. In this study, we investigated the concentration, size fraction distribution (colloidal and soluble) as well as redox speciation and labile concentrations of the limiting micronutrient iron (Fe) at two marine shallow-water hydrothermal systems (Champagne Hot Springs and Soufriere) off the coast of Dominica, Lesser Antilles Island Arc, Caribbean. Geochemical characterization of the two different vent sites showed that both are affected by meteoric and seawater influence, with a stronger meteoric influence at Soufriere than at Champagne Hot Springs. Measurements of soluble and labile Fe were performed using a modified competitive ligand exchange – adsorptive cathodic stripping voltammetry (CLE-AdCSV) with salicylaldoxime (SA) as the artificial ligand. Our results show that focused fluids discharging at the seafloor, as well as hydrothermal pore fluids are, despite a calculated theoretical oxidation half-life of only 6.4min, highly enriched in Fe(II), indicating a strong complexation of Fe(II), strong enough to prevent Fe(II) from oxidation and precipitation. Since these fluids show enriched dissolved organic carbon (DOC) concentrations, and very low fractions of chemically labile Fe, complexation may occur by organic carbon, which was recently suggested to also be a factor in stabilizing particulate Fe(II) in deep-sea hydrothermal non-buoyant plumes. Our results indicate that shallow-water hydrothermalism off the coast of Dominica releases high concentrations of stabilized, bioavailable Fe(II) into the photic zone, which influences the biogeochemical cycle of Fe in surface waters. Considering the abundance of marine shallow-water hydrothermal systems in many regions, such processes may even play a role in the global oceanic dFe cycle.

      PubDate: 2017-03-02T07:54:39Z
  • Partial melting of the orogenic lower crust: Geochemical insights from
           post-collisional alkaline volcanics in the Dabie orogen
    • Abstract: Publication date: Available online 24 February 2017
      Source:Chemical Geology
      Author(s): Fu-Qiang Dai, Zi-Fu Zhao, Yong-Fei Zheng
      Alkaline igneous rocks are a minor component in common associations of felsic to mafic rocks. Although they are usually regarded as the product of mantle-derived magmatism, they are possibly produced by partial melting of the overthickened orogenic crust. This is illustrated by post-collisional alkaline volcanics from the Dabie orogen in China. A combined study of whole-rock major and trace elements and Sr-Nd-Hf isotopes, and zircon U-Pb ages and Hf-O isotopes for these volcanics provides geochemical constraints on the nature of their magma sources and thus insights into reworking of the lower continental crust in a collisional orogen. SIMS zircon U-Pb dating yields consistent Early Cretaceous ages of 136±2Ma and 133±2Ma for magma emplacement, contemporaneous with early-stage adakitic rocks in the Dabie orogen. Relict zircons record Neoproterozoic and Triassic U-Pb ages, respectively, in agreement with ages for protolith formation and ultrahigh-pressure metamorphism in the orogen. The alkaline volcanics are composed of phonolite and trachyte, with variable contents of SiO2 (55.94–64.10wt%), MgO (0.16–0.95wt%), Al2O3 (16.80–21.32wt%), K2O (4.65–14.55wt%) and Na2O+K2O (11.51–15.01wt%). They show arc-like trace element distribution patterns, with enrichment in LILE and LREE but depletion in HFSE. They exhibit enriched whole-rock Sr-Nd-Hf isotope compositions, with generally high (87Sr/86Sr)i ratios of 0.7074 to 0.7104, but negative εNd(t) values of −16.3 to −15.1 and negative εHf(t) values of −18.4 to −16.3. Zircon Hf-O isotope analyses yield less variable εHf(t) values of −27.8 to −23.0 and δ18O values of 4.9 to 5.9‰ for magmatic zircons, but more variable εHf(t) values of −32.7 to −6.5 and δ18O values of 3.0 to 10.2‰ for Neoproterozoic and Triassic relict zircons. Such geochemical features indicate that these alkaline volcanics were derived from partial melting of the subducted continental crust of the South China Block in the Dabie orogen. The magma sources would be dominated by the mafic igneous rocks of middle Neoproterozoic age. Additional contributions were from supracrustal rocks with high δ18O and variable εHf(t) values, which were incorporated into the overthickened orogenic crust during the Triassic continental collision. The orogenic crust underwent low-degree partial melting during its thinning at the post-collisional stage, giving rise to the alkaline volcanics in the Early Cretaceous.

      PubDate: 2017-03-02T07:54:39Z
  • PXRF calibrations for volcanic rocks and the application of in-field
           analysis to the geosciences
    • Abstract: Publication date: 20 March 2017
      Source:Chemical Geology, Volume 453
      Author(s): A.E. Steiner, R.M. Conrey, J.A. Wolff
      Advances in portable X-ray fluorescence (pXRF) technologies have made such instruments an attractive choice for geologists who wish to collect chemical data in the field. The acceptance of pXRF data for research purposes such as, for example, petrogenetic studies by the geologic community has been hampered by a perception of poor quality. Here, we show that current-generation pXRF instruments can yield high-quality data when deployed either in the laboratory, or as a base-camp field instrument. We have developed four calibrations for major, minor and trace elements in geological materials using minimal sample preparation: one for the broad range of geologic compositions (0–100% SiO2) using rock powders, and three with an emphasis on volcanic rock samples: rock billets ranging from primitive basalt to rhyolite, basalt rock powder, and rhyolitic pumice powder. The calibrations use influence coefficients to correct for absorbance and secondary enhancement of one element by X-rays of other elements present in the sample. Nugget effects can have a significant influence over the determination of the concentration of elements such as Zr if proper care in sample homogenization is not taken. We evaluate data quality for each element in each calibration, and find that for several elements (Ti, Mn, Fe, Ca, K, P, Cr, Ni, Zn, Rb, Sr, Y, Zr, Nb), precision and accuracy approach those obtainable from full-size wavelength-dispersive XRF analysis. Maximization of data quality is achieved by matching of calibration material types and unknowns.

      PubDate: 2017-02-23T08:27:00Z
  • Impact of electrogenic sulfur oxidation on trace metal cycling in a
           coastal sediment
    • Abstract: Publication date: 5 March 2017
      Source:Chemical Geology, Volume 452
      Author(s): Sebastiaan van de Velde, Ine Callebaut, Yue Gao, Filip J.R. Meysman
      Electrogenic sulfur oxidation (e-SOx) is a newly discovered pathway of microbial sulfide oxidation, mediated by so-called cable bacteria. The reduction of oxygen near the sediment-water interface is coupled by long-distance electron transport to the oxidation of sulfide in deeper sediment, and in this way, electrical currents are induced within the seafloor that range over centimeter scale distances. Previously, electrogenic sulfur oxidation has been shown to generate extreme pH excursions in the pore water, and as a result, the process strongly amplifies the cycling of various pH-sensitive minerals, such sulfide minerals and carbonates. Here we show that e-SOx also strongly influence the early diagenesis of trace metals in coastal sediments. For this, field observations at a shallow subtidal site in the North Sea were combined with dedicated laboratory incubations of repacked sediments. High resolution microsensor profiling (pH, H2S and O2) confirmed the typical geochemical signature of e-SOx both in situ as in the laboratory experiments. Pore water analysis revealed a strong mobilization of both arsenic and cobalt within the electro-active sediment zone. The dissolution of iron sulfides, resulting from the acidification of the pore water by e-SOx, appears to be the main driver for the observed release of cobalt and arsenic. The current generated by e-SOx affects charged species. In this manuscript, we have proposed a new estimation method for the associated current density, based on the rate of sulfide oxidation. Overall, electrogenic sulfur oxidation has a major impact on the cycling of arsenic and cobalt in coastal sediments, and may substantially increase the effluxes of these trace metals to the coastal ocean.

      PubDate: 2017-02-23T08:27:00Z
  • The hyper-enrichment of V and Zn in black shales of the Late
           Devonian-Early Mississippian Bakken Formation (USA)
    • Abstract: Publication date: 5 March 2017
      Source:Chemical Geology, Volume 452
      Author(s): Clint Scott, John F. Slack, Karen D. Kelley
      Black shales of the Late Devonian to Early Mississippian Bakken Formation are characterized by high concentrations of organic carbon and the hyper-enrichment (>500 to 1000s of mg/kg) of V and Zn. Deposition of black shales resulted from shallow seafloor depths that promoted rapid development of euxinic conditions. Vanadium hyper-enrichments, which are unknown in modern environments, are likely the result of very high levels of dissolved H2S (~10mM) in bottom waters or sediments. Because modern hyper-enrichments of Zn are documented only in Framvaren Fjord (Norway), it is likely that the biogeochemical trigger responsible for Zn hyper-enrichment in Framvaren Fjord was also present in the Bakken basin. With Framvaren Fjord as an analogue, we propose a causal link between the activity of phototrophic sulfide oxidizing bacteria, related to the development of photic-zone euxinia, and the hyper-enrichment of Zn in black shales of the Bakken Formation.

      PubDate: 2017-02-23T08:27:00Z
  • Using stable isotopes (δ17O, δ18O and δD) of gypsum hydration water to
           ascertain the role of water condensation in the formation of subaerial
           gypsum speleothems
    • Abstract: Publication date: 5 March 2017
      Source:Chemical Geology, Volume 452
      Author(s): Fernando Gázquez, José María Calaforra, Nicholas P. Evans, David A. Hodell
      We analyzed the stable isotopes (δ17O, δ18O and δD) of gypsum hydration water (GHW) in a variety of speleothems, as well as condensation and infiltration waters in five caves of the semiarid gypsum karst of Sorbas basin (Almeria, SE Spain). Microclimate parameters (air temperature, relative humidity and effective condensation rate) were also monitored over an annual cycle. We found that the mother solution from which the majority of gypsum speleothems grow is composed of a mixture of condensation (~60%) and infiltration water (~40%) that undergoes evaporation. Although evaporation of infiltration water alone was thought to be responsible for secondary gypsum precipitation in vadose caves, our results suggest that condensation can be a major source of water for the formation of gypsum speleothems. The modelled d-excess and Δ17O trajectories of water during the evaporative process confirm that the majority of speleothems precipitate from a mixture of condensation and infiltration water under relative humidity of 75–85%, similar to that measured in the cave atmosphere during winter. These findings have important implications for future studies of gypsum speleothems as paleoenvironmental archives.

      PubDate: 2017-02-23T08:27:00Z
  • Sulfur diagenesis under rapid accumulation of organic-rich sediments in a
           marine mangrove from Guadeloupe (French West Indies)
    • Abstract: Publication date: Available online 21 February 2017
      Source:Chemical Geology
      Author(s): Antoine Crémière, Harald Strauss, Mathieu Sebilo, Wei-Li Hong, Olivier Gros, Sabine Schmidt, Jennifer Tocny, Francoise Henry, Swanne Gontharet, Anniet M. Laverman
      The biogeochemistry of sulfur and carbon during early-diagenetic processes within organic-rich marine mangrove sediments was studied in the “Manche à Eau” lagoon, Guadeloupe, West Indies. These sediments are characterized by a total organic carbon content (TOC) mostly above 11wt%, δ13CTOC below −25‰ VPDB and C/Nmolar ratios exceeding 15. Rates of mangrove-derived organic carbon accumulation vary between ~200 and 400gOC·m−2·yr−1, with highest rates at the shore of the lagoon. On the lagoon border, where colorless filamentous sulfur-oxidizing bacteria colonize the sediments, active sulfate reduction within the upper 20cm, with a net removal rate of ~0.5μmol·cm−3·d−1, is assumed to be essentially driven by organic carbon oxidation. This is expressed by relatively low apparent sulfur isotope fractionation (34εnet =−33‰) and a gentle δ18O/δ34Ssulfate apparent slope of 0.36±0.03 (2s). Further inside the lagoon, in the absence of sulfur-oxidizing bacteria, higher apparent sulfur isotope fractionation and a steeper δ18O/δ34Ssulfate slope (0.67±0.20) suggest an overall lower sulfate removal rates that may be coupled to minor sulfur disproportionation. Spatial and vertical variation in sulfur cycling, reflected by oxygen and sulfur isotopic characteristics, seem to be mainly controlled by unsteady to relatively steady organic matter deposition and its reactivity. In all sediments, δ34S values of pyrite are positively correlated with the TOC/TS ratio and negatively correlated with δ13CTOC; suggesting a primary control of the quantity and quality of organic matter on the pyrite isotope records, despite potential iron-limiting conditions for the most active sites. Our results provide insights into the role of organic carbon input on sulfur cycling; stimulating sulfate reduction and in turn the presence of sulfur-oxidizing microbial mats, resulting in an intense cycling of both carbon and sulfur in these marine mangrove sediments.

      PubDate: 2017-02-23T08:27:00Z
  • Sulfur isotope signatures in the lower crust: A SIMS study on S-rich
           scapolite of granulites
    • Abstract: Publication date: Available online 20 February 2017
      Source:Chemical Geology
      Author(s): Johannes Hammerli, Anthony I.S. Kemp, Natasha Barrett, Boswell A. Wing, Malcolm Roberts, Richard J. Arculus, Pierre Boivin, Prosper M. Nude, Kai Rankenburg
      Scapolite is an important reservoir for volatiles in the deep crust and provides unique insights into the S isotope signatures at the mantle/crust interface. Here, we document the first scapolite reference material (herein referred to as CB1) for in situ S isotope analysis. The chemical and isotope composition of this euhedral, S-rich scapolite megacryst was characterized via LA-ICP-MS, EPMA, SIMS, and bulk fluorination gas source isotope ratio mass spectrometry. The CB1 scapolite is isotopically homogeneous and our results show that crystal orientation does not affect in situ S isotope SIMS analysis. This makes CB1 an ideal primary calibration standard for in situ analysis of S isotope ratios (36S/32S, 34S/32S and 33S/32S) in scapolite. With this reference material in hand, we then applied in situ SIMS analysis of S isotopes for the first time on scapolite in granulite samples from the lower crust/upper mantle. The analysed sample suite comprises rocks from classic granulite xenolith locations in southeastern Australia, as well as a sample from the high-grade suture zone of the Dahomeyides in south-eastern Ghana. The results show that scapolites in the lower crust have δ34S values between ~−0.5 and +4 (‰ VCDT). These values fall within the range of S isotope signatures present in mantle rocks and provide no evidence for the recycling of seawater-derived S into the lower crust. We propose that scapolite formed during granulite facies metamorphism of igneous cumulates, where S was sourced from precursor igneous sulfides. Sulfur isotope heterogeneities between individual scapolite grains in some of the studied samples may reflect non-uniform S-isotope compositions of igneous S-phases, which precipitated from mantle-derived melt.

      PubDate: 2017-02-23T08:27:00Z
  • Linear correlations in bamboo coral δ13C and δ18O sampled by SIMS and
           micromill: Evaluating paleoceanographic potential and biomineralization
           mechanisms using δ11B and ∆47 composition
    • Abstract: Publication date: Available online 16 February 2017
      Source:Chemical Geology
      Author(s): Casey Saenger, Rinat I. Gabitov, Jesse Farmer, James M. Watkins, Robert Stone
      Bamboo corals represent an intriguing paleoceanographic archive with the potential to reconstruct variations throughout the water column over multiple centuries. Realizing this potential partially depends on if, and at what resolution, timeseries of variability can be generated. Recent work demonstrates that bamboo coral growth temperature, averaged over its entire lifespan, can be derived from linear correlations in its carbon and oxygen isotope composition (δ13C, δ18O) when the apparent equilibrium fractionations for a coral's growth rate and calcifying pH are used. Building on this method, this study applies it to coeval coral skeleton to assess the possibility of extracting paleoceanographic timeseries from bamboo coral skeletons. Using boron isotope (δ11B) based pH estimates, micromilled samples yield accurate paleotemperatures with uncertainties of <2°C, whose precision could be improved to <1°C if additional sampling yielded more robust regressions. This provides strong evidence that decadal-scale temperature reconstructions may be extracted from bamboo corals. Complementary SIMS data generated at annual to inter-annual resolution often yield accurate temperatures, but with greater uncertainty that is always >2°C. This is attributed to the inability to measure δ13C and δ18O in exactly the same skeleton, as is the case for micromilled samples. A micromilled sampling strategy is therefore likely the most practical means of applying the method. Carbonate clumped isotopes (∆47) estimate temperatures slightly warmer than observed, suggesting they may not accurately record the subtle variations of the latest Holocene. When interpreted in conjunction, δ18O, δ11B and ∆47 data suggest that pH up-regulation plays a role in generating linear δ13C-δ18O trends. However, oxygen isotope fractionation and ∆47 are lower than would be predicted by pH alone. Potential explanations for this discrepancy include biological processes that favor the incorporation of carbonate ion into coral skeleton, uncertainties in the δ11B-pH proxy, the influence of magnesium on calcite-fluid isotope fractionation, or uncertainties in the fractionation factors used to calculate apparent equilibrium. Despite some remaining uncertainties, the method can resolve many observed decadal-scale thermocline temperature anomalies, suggesting that appropriately located bamboo corals could help constrain the behavior of similar temporal variability prior to instrumental monitoring.

      PubDate: 2017-02-23T08:27:00Z
  • Carbon isotope analysis of whole-coal and vitrinite from intruded coals
           from the Illinois Basin: No isotopic evidence for thermogenic methane
    • Abstract: Publication date: 20 March 2017
      Source:Chemical Geology, Volume 453
      Author(s): Mohammad W. Rahman, Susan M. Rimmer, Harold D. Rowe, William W. Huggett
      Igneous intrusions into coals and organic-rich rocks may have contributed to global warming in the geologic past through the release of greenhouse gases. Evidence for a large release of thermogenic CH4 from the organics would include significant δ13Corg enrichment in the residual organic matter (OM). However, δ13Corg values of thermally altered OM in coals and shales adjacent to intrusions often show negative trends or, in some cases, ambiguous or positive trends. Previous studies have evaluated the δ13Corg of whole-coal samples rather than that of individual organic components, or macerals. As different macerals have different isotopic compositions, maceral-specific trends may be masked by variations in maceral composition of the bulk samples. This study evaluates the hypothesis that, if a large-scale release of 13C-depleted thermogenic CH4 resulted from intrusion of the coal, then it should have produced 13C-enriched coal residuum and, specifically, vitrinite (the most abundant component of the coal) adjacent to the intrusion. This study reports geochemical and petrographic data for whole-coal samples and vitrinite macerals (separated via density-gradient centrifugation, DGC) from a transect of thermally altered Springfield (No. 5) Coal (Pennsylvanian) in the Illinois Basin. Approaching the dike contact, mean vitrinite reflectance (Rr) increases from background levels of 0.55% up to ~4.8% and liptinites become indistinguishable from vitrinite. Isotropic and fine-grained circular mosaic coke is observed in samples within ~1.3m of the coal/intrusion contact. Approaching the intrusion, volatile matter (VM) decreases and fixed carbon (FC) and %C increase, whereas %H decreases. Total organic carbon (TOC) for whole coal decreases from 77% to 34%, whereas TOC for separated vitrinite fractions increases from ~66% to 93% toward the coal/intrusion contact. Density of DGC-separated vitrinites ranges from 1.27g/mL in the unaltered coal to 1.52g/mL in the sample adjacent to the coal/intrusion contact. Vitrinite density increases with increased Rr and %C, and decreases with increased %H and H/C. Despite these marked geochemical and petrographic changes, no significant changes in δ13Corg values of the whole coal (−25.3‰ to −24.9‰) or the separated vitrinites (−25.3‰ to −25.0‰) occur proximal to the intrusion. Changes in the isotopic signatures are not of a magnitude that would be expected if significant quantities of isotopically depleted thermogenic CH4 had been generated by the intrusive event. Moreover, no petrographic evidence supports the occurrence of condensed or immobilized thermal products due to rapid pyrolysis (such as 12C-rich pyrolytic carbon) close to the intrusion that could have moderated any changes in δ13Corg. Geochemical and petrographic results suggest that only minimal loss of CH4 was associated with the rapid heating of the Springfield (No. 5) Coal by the intrusion. These findings have significant implications for models linking past global warming to the intrusion of coals and carbonaceous shales.

      PubDate: 2017-02-16T07:08:54Z
  • Arsenic and thioarsenic species in the hot springs of the Rehai magmatic
           geothermal system, Tengchong volcanic region, China
    • Abstract: Publication date: 20 March 2017
      Source:Chemical Geology, Volume 453
      Author(s): Qinghai Guo, Britta Planer-Friedrich, Mingliang Liu, Jiexiang Li, Chao Zhou, Yanxin Wang
      Rehai is the sole geothermal area in mainland China that discharges both acidic and alkaline hot springs with pHs ranging from 1.4 to 10.0. The hot springs in Rehai are also characterized by a wide range of arsenic concentrations, with the highest being 1.35mg/L. Analysis of the arsenic geochemistry indicates that the alkaline and neutral hot springs in Rehai have high arsenic concentrations. The alkaline hot springs represent arsenic-rich boiled reservoir fluids, and the neutral hot springs represent mixtures of these reservoir fluids with non-thermal waters and/or condensed steam. The enrichment of arsenic in the deep reservoir fluids, also called “primary fluids”, is primarily attributable to the leaching of reservoir rocks at high temperatures, but a possible arsenic contribution from magmatic fluids cannot be ruled out. The acidic springs, in contrast, have much lower arsenic concentrations because they are a mixture of non-thermal oxygenated waters and condensed steam, both of which are depleted in arsenic. In addition to arsenite and arsenate, mono-, di-, and tri-thioarsenates have been detected in the Rehai geothermal waters. The arsenic speciation in the Rehai hot springs is primarily linked to the formation processes of the various secondary fluids at the surface and their sulfide concentrations. In addition, the speciation is also affected by the pH values and the redox potentials of the waters. Compared to the springs along secondary E-W-trending faults, the chloride-rich springs along the main N-S-trending faults are generally much less affected by mixing with non-thermal waters and/or condensed steam. They are also more enriched in sulfide, and thioarsenates consequently represent a greater proportion of the total arsenic. This is the first study to report the thioarsenic geochemistry of hot springs in China.

      PubDate: 2017-02-16T07:08:54Z
  • The role of Fe on the formation and diagenesis of interstratified
           glauconite-smectite and illite-smectite: A case study of Lower Cretaceous
           shallow-water carbonates
    • Abstract: Publication date: 20 March 2017
      Source:Chemical Geology, Volume 453
      Author(s): Andre Baldermann, Martin Dietzel, Vasileios Mavromatis, Florian Mittermayr, Laurence N. Warr, Klaus Wemmer
      The widespread formation of interstratified glauconite-smectite (Gl-S) and illite-smectite (I-S) in modern and ancient diagenetic settings records the physicochemical conditions prevailing during clay mineral authigenesis. To date, however, significant gaps in our knowledge persist in respect to the influence of interstitial solution chemistry, temperature and reaction kinetics on the evolution of composition, mineralogy and microstructure of Gl-S and I-S. Herein, we present a study on the reaction mechanisms and the physicochemical conditions that led to the precipitation of early diagenetic Gl-S and late diagenetic I-S on a stable carbonate platform during the Cenomanian at Langenstein in the Northern German Basin. The texture and the K-Ar age (95.0±1.8Ma) of the green glauconitized grains revealed that green-clay authigenesis progressed in initially organic-rich, semi-confined micromilieus, i.e., in fecal pellets and in foraminifera, close to the sediment-seawater interface. The composition of Gl-S varied in the range (K+ 0.20–0.74Na+ 0–0.10Ca2+ 0–0.05)0.28–0.75 (Fe3+ 0.63–1.20Fe2+ 0.08–0.24Al3+ 0.19–0.97 Mg2+ 0.29–0.52)2.01–2.12 [Al3+ 0.09–0.35Si4+ 3.65–3.91O10](OH2), and depended on the rate of aqueous Fe2+ and K+ ion diffusion, the micromilieu of glauconitization and on the bulk sedimentation rate. The mineralogical, microstructural and chemical changes of the ongoing Gl-S products revealed the following reaction for green-clay authigenesis at Langenstein: Fe(III)-smectite reacted with monosilicic acid, goethite and aqueous K+, Mg2+ and Fe2+ to form glauconite and aqueous Na+, Ca2+ and H+ ions. This process considers complex mineral transformations commonly associated with glauconitization, such as early diagenetic oxidation of organic matter and microbial-catalyzed dissolution of Fe-(oxy)hydroxides, carbonates and detrital silicates. In contrast, the K-Ar age of I-S (68.0±1.6Ma) and its compositional variability, (K+ 0.29–0.45Na+ 0–0.10Ca2+ 0–0.06)0.30–0.55 (Fe3+ 0.16–0.29Fe2+ 0–0.10Al3+ 1.37–1.68Mg2+ 0.18–0.43)2.00–2.12 [Al3+ 0.17–0.39Si4+ 3.61–3.83O10](OH2), indicate a burial diagenetic origin for this mineral phase, rather than transformation of illitic clays into I-S during weathering under warm and humid climatic conditions. The results from kinetic modelling support a diagenetic origin of I-S (50–60%I layers and 50–40%S layers) and imply its formation by the replacement of pre-existing K-feldspar at high pore-fluid activity K/Na ratios and at low Fe2+ concentrations. We propose that the substitution of Al3+ for Fe3+, Fe2+ and Mg2+ in the octahedral sheet shifts the stability field of the kaolinite–Fe-Al-Mg-smectite–Fe-Al-Mg-illite (or glauconite) triple point to much lower monosilicic acid activities, and stabilizes the I-S (or Gl-S) structure. This reaction supports the idea that the (bio)availability of Fe is the rate-limiting factor for glauconitization, which is not the case for the diagenetic growth of I-S, whereby the pore water Fe2+ concentration may be limited by the competing formation of Fe-(oxy)hydroxides and/or Fe-sulfides.

      PubDate: 2017-02-16T07:08:54Z
  • Conodont apatite δ88/86Sr and δ44/40Ca compositions and implications for
           the evolution of Palaeozoic to early Mesozoic seawater
    • Abstract: Publication date: Available online 14 February 2017
      Source:Chemical Geology
      Author(s): Sandrine Le Houedec, Malcolm McCulloch, Julie Trotter, Kai Rankenburg
      We present high precision (TIMS double spike) stable isotope measurements of both δ44/40Ca and δ88/86Sr together with radiogenic 87Sr/86Sr ratios determined from conodont apatite. These data represent five intervals ranging from the early Ordovician to late Triassic. The conodont δ44/40Ca values (relative to NIST 915a) range from −0.47‰ to +0.15‰, with an apparent shift to more positive values between the early Silurian and late Devonian/early Carboniferous, similar to the brachiopod-based marine δ44/40Ca record (Farkaš et al., 2007a). We calculated a δ44/40Ca seawater-bio-apatite fractionation factor of about −1.9‰, which allowed us to reconstruct a palaeo-seawater δ44/40Ca record from bio-apatites. Despite a slightly positive offset of about +0.2 to +0.5‰, the δ44/40Ca record obtained from bio-apatites is consistent with the previously reported δ44/40Ca seawater record inferred from carbonates. We find that unlike the carbonate δ44/40Ca records, the δ88/86Sr measurements from conodont apatite show unexpectedly large variations (up to ~1‰), with ratios ranging from −0.6‰ to 0.3‰. These reconnaissance data reveal a reasonable correlation between δ88/86Sr and radiogenic 87Sr/86Sr* (r2 =0.60, n =13, p =0.002), suggesting that the controls from differential weathering regimes and/or continental crustal compositions buffered ancient seawater compositions.

      PubDate: 2017-02-16T07:08:54Z
  • Setting a limit on anthropogenic sources of atmospheric 81Kr through Atom
           Trap Trace Analysis
    • Abstract: Publication date: Available online 9 February 2017
      Source:Chemical Geology
      Author(s): J.C. Zappala, K. Bailey, W. Jiang, B. Micklich, P. Mueller, T.P. O’Connor, R. Purtschert
      We place a 2.5% limit on the anthropogenic contribution to the modern abundance of 81Kr/Kr in the atmosphere at the 90% confidence level. Due to its simple production and transport in the terrestrial environment, 81Kr (half-life=230,000years) is an ideal tracer for old water and ice with mean residence times in the range of 105–106 years. In recent years, 81Kr-dating has been made available to the earth science community thanks to the development of Atom Trap Trace Analysis (ATTA), a laser-based atom counting technique. Further upgrades and improvements to the ATTA technique now allow us to demonstrate 81Kr/Kr measurements with relative uncertainties of 1% and place this new limit on anthropogenic 81Kr. As a result of this limit, we have removed a potential systematic constraint for 81Kr-dating.

      PubDate: 2017-02-16T07:08:54Z
  • The behaviour of zirconium, hafnium and rare earth elements during the
           crystallisation of halite and other salt minerals
    • Abstract: Publication date: Available online 9 February 2017
      Source:Chemical Geology
      Author(s): P. Censi, C. Inguaggiato, S. Chiavetta, C. Schembri, F. Sposito, V. Censi, P. Zuddas
      Halite crystals from Messinian and Tortonian evaporites from Sicily and Spain and current precipitated halite crystals and the relative parent brines (active evaporation systems) were investigated in order to evaluate the Zr, Hf and Rare Earth Element (REE) behaviour. Halite crystallisation from evaporating brines fractionates Zr, Hf and REE through a two-step process. During the first step, dissolved complexes of studied elements are scavenged onto the surfaces of crystallising halite. During the second step, elements are co-precipitated into the crystal lattice as it grows. The first step mechanism is determined by the dissolved REE speciation. In saltworks where carbonate-REE complexes occur, surface complexation of REE onto halite crystals does not occur. On the contrary, surface REE-complexes onto halite crystals are forming in the Dead Sea water where aqueous REE speciation is dominated by chloride-complexes. Under the latter conditions, halite crystallises with cubic and cubic-octahedral composite habitus. Octahedral planes involve the formation of strong coulombic interactions, mainly with [Hf(H2O)3(OH)5]− rather than with [Zr(H2O)4(OH)4]0 complexes. As a consequence, newly formed halite in the Dead Sea shows strong subchondritic Zr/Hf ratios. Based on these indications, analyses carried out on salt minerals from Messinian and Tortonian evaporites in Sicily and Spain show that their overall REE content can be considered a discriminating parameter between authigenic minerals and diagenetic modified materials. However, features of shale-normalised REE patterns are driven by the mineralogical composition of evaporites rather than their authigenic or secondary nature. On the contrary, the Zr/Hf signature of salt minerals is influenced by their origin. Indeed, subchondritic Zr/Hf values are found in primary salt minerals, whereas larger Zr/Hf values are recognised in those diagenetically modified. Calculated distribution coefficients of Zr, Hf and REE are employed for modelling the REE distribution in halite equilibrated with the deep-sea brines from Typo, Medee and Thetis basins (Eastern Mediterranean). The obtained indications allow us to discriminate brines formed by dissolution of evaporites relative to those representing relics of fossil evaporated seawater.

      PubDate: 2017-02-16T07:08:54Z
  • Immobilization of nanoparticles by occlusion into microbial calcite
    • Abstract: Publication date: Available online 6 February 2017
      Source:Chemical Geology
      Author(s): Rebecca L. Skuce, Dominique J. Tobler, Ian MacLaren, Martin R. Lee, Vernon R. Phoenix
      Binding of nanoparticles (NPs) to mineral surfaces influences their transport through the environment. The potential, however, for growing minerals to immobilize NPs via occlusion (the process of trapping particles inside the growing mineral) has yet to be explored in environmentally relevant systems. In this study, the ureolytic bacteria Sporosarcina pasteurii was used to induce calcium carbonate precipitation in the presence of organo-metallic manufactured nanoparticles. As calcite crystals grew the nanoparticles in the solution became trapped inside these crystals. Capture of NPs within the calcite via occlusion was verified by transmission electron microscopy of thin foils. Nanoparticles with a negative surface charge were captured with greater efficiency than those with a positive surface charge, resulting from stronger attachment of negative nanoparticles to the positively charged calcite surfaces, which in turn facilitated occlusion. Thermodynamic and kinetic analysis, however, did not reveal a significant difference in k p (calcite precipitation rate constant) or the critical saturation at which precipitation initiates (S crit ), indicating the presence of different charged nanoparticles did not influence calcite precipitation at the concentrations used here. Overall, these findings demonstrate that microbially driven mineral precipitation has potential to immobilize nanoparticles in the environment via occlusion.

      PubDate: 2017-02-16T07:08:54Z
  • Probing the aluminum complexation by Siberian riverine organic matter
           using solid-state DNP-NMR
    • Abstract: Publication date: Available online 4 February 2017
      Source:Chemical Geology
      Author(s): Frédérique Pourpoint, Joëlle Templier, Christelle Anquetil, Hervé Vezin, Julien Trébosc, Xavier Trivelli, François Chabaux, Oleg S. Pokrovsky, Anatoly S. Prokushkin, Jean-Paul Amoureux, Olivier Lafon, Sylvie Derenne
      In a Siberian river, the concentrations of chemical species vary with the hydrological regime. Dissolved Organic Matter (DOM) and aluminum ions show a parallel trend in the course of the hydrological year. However, the speciation of aluminum in this natural environment remains an open question. We propose here a combination of spectroscopic techniques to investigate the proximity between the aluminum atoms and DOM. First, one-dimensional (1D) solid-state Nuclear Magnetic Resonance (NMR), Electron Paramagnetic Resonance (EPR) and 1D and 2D solution NMR spectra were acquired, providing a clear overview of the DOM composition. Second, the sensitivity enhancement yielded by Dynamic Nuclear Polarization enabled the NMR detection of proximities between the 27Al and 13C nuclei. Hence, we show that 8.3±1.3% of the carboxylate groups observed by NMR are connected to the Al3+ ions in the DOM sample. We here demonstrate for the first time how advanced solid-state NMR methods can provide key information about the localization of aluminum in such complex natural materials.
      Graphical abstract image

      PubDate: 2017-02-16T07:08:54Z
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