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  Subjects -> EARTH SCIENCES (Total: 588 journals)
    - EARTH SCIENCES (435 journals)
    - GEOLOGY (66 journals)
    - GEOPHYSICS (26 journals)
    - HYDROLOGY (17 journals)
    - OCEANOGRAPHY (44 journals)

EARTH SCIENCES (435 journals)                  1 2 3 4 5 | Last

Acta Geodaetica et Geophysica     Hybrid Journal   (Followers: 1)
Acta Geodaetica et Geophysica Hungarica     Full-text available via subscription   (Followers: 2)
Acta Geophysica     Hybrid Journal   (Followers: 7)
Acta Geotechnica     Hybrid Journal   (Followers: 8)
Acta Meteorologica Sinica     Hybrid Journal   (Followers: 2)
Advances in High Energy Physics     Open Access   (Followers: 12)
Advances In Physics     Hybrid Journal   (Followers: 7)
Aeolian Research     Hybrid Journal   (Followers: 1)
African Journal of Aquatic Science     Hybrid Journal   (Followers: 13)
Algological Studies     Full-text available via subscription   (Followers: 2)
Alpine Botany     Hybrid Journal   (Followers: 3)
AMBIO     Hybrid Journal   (Followers: 13)
Anales del Instituto de la Patagonia     Open Access   (Followers: 2)
Andean geology     Open Access   (Followers: 5)
Annales Henri Poincaré     Hybrid Journal   (Followers: 2)
Annales UMCS, Geographia, Geologia, Mineralogia et Petrographia     Open Access   (Followers: 1)
Annals of Geophysics     Full-text available via subscription   (Followers: 9)
Annals of GIS     Hybrid Journal   (Followers: 15)
Annals of Glaciology     Full-text available via subscription  
Annual Review of Marine Science     Full-text available via subscription   (Followers: 9)
Anthropocene Review     Hybrid Journal   (Followers: 1)
Applied Clay Science     Hybrid Journal   (Followers: 2)
Applied Geochemistry     Hybrid Journal   (Followers: 6)
Applied Geomatics     Hybrid Journal   (Followers: 6)
Applied Geophysics     Hybrid Journal   (Followers: 6)
Applied Ocean Research     Hybrid Journal   (Followers: 6)
Applied Petrochemical Research     Open Access   (Followers: 3)
Applied Remote Sensing Journal     Open Access   (Followers: 8)
Aquatic Conservation Marine and Freshwater Ecosystems     Hybrid Journal   (Followers: 20)
Arctic, Antarctic, and Alpine Research     Full-text available via subscription   (Followers: 7)
Artificial Satellites     Open Access   (Followers: 14)
Asia-Pacific Journal of Atmospheric Sciences     Hybrid Journal   (Followers: 2)
Asian Journal of Earth Sciences     Open Access   (Followers: 17)
Atlantic Geology : Journal of the Atlantic Geoscience Society / Atlantic Geology : revue de la Société Géoscientifique de l'Atlantique     Full-text available via subscription   (Followers: 3)
Atmosphere-Ocean     Full-text available via subscription   (Followers: 4)
Atmospheric and Climate Sciences     Open Access   (Followers: 14)
Australian Journal of Earth Sciences: An International Geoscience Journal of the Geological Society of Australia     Hybrid Journal   (Followers: 11)
Boletim de Ciências Geodésicas     Open Access  
Boreas: An International Journal of Quaternary Research     Hybrid Journal   (Followers: 8)
Bragantia     Open Access   (Followers: 2)
Bulletin of Earthquake Engineering     Hybrid Journal   (Followers: 9)
Bulletin of Geosciences     Open Access   (Followers: 8)
Bulletin of Marine Science     Full-text available via subscription   (Followers: 13)
Bulletin of the Lebedev Physics Institute     Hybrid Journal   (Followers: 1)
Bulletin of the Seismological Society of America     Full-text available via subscription   (Followers: 16)
Bulletin of Volcanology     Hybrid Journal   (Followers: 11)
Canadian Journal of Plant Science     Full-text available via subscription   (Followers: 12)
Canadian Mineralogist     Full-text available via subscription   (Followers: 1)
Canadian Water Resources Journal     Hybrid Journal   (Followers: 19)
Carbonates and Evaporites     Hybrid Journal   (Followers: 3)
CATENA     Hybrid Journal   (Followers: 3)
Central European Journal of Geosciences     Hybrid Journal   (Followers: 5)
Central European Journal of Physics     Hybrid Journal   (Followers: 1)
Chemical Geology     Hybrid Journal   (Followers: 9)
Chemie der Erde - Geochemistry     Hybrid Journal   (Followers: 3)
Chinese Geographical Science     Hybrid Journal   (Followers: 3)
Chinese Journal of Geochemistry     Hybrid Journal   (Followers: 2)
Chinese Journal of Oceanology and Limnology     Hybrid Journal   (Followers: 3)
Ciencia del suelo     Open Access  
Climate and Development     Hybrid Journal   (Followers: 9)
Coastal Management     Hybrid Journal   (Followers: 18)
Comptes Rendus Geoscience     Full-text available via subscription   (Followers: 5)
Computational Geosciences     Hybrid Journal   (Followers: 10)
Computational Mathematics and Mathematical Physics     Hybrid Journal   (Followers: 1)
Computers and Geotechnics     Hybrid Journal   (Followers: 5)
Contemporary Trends in Geoscience     Open Access  
Continental Journal of Earth Sciences     Open Access   (Followers: 2)
Continental Shelf Research     Hybrid Journal   (Followers: 8)
Contributions to Mineralogy and Petrology     Hybrid Journal   (Followers: 7)
Contributions to Plasma Physics     Hybrid Journal   (Followers: 2)
Coral Reefs     Hybrid Journal   (Followers: 18)
Cretaceous Research     Hybrid Journal   (Followers: 4)
Cybergeo : European Journal of Geography     Open Access   (Followers: 4)
Developments in Geotectonics     Full-text available via subscription   (Followers: 2)
Developments in Quaternary Science     Full-text available via subscription   (Followers: 3)
Développement durable et territoires     Open Access   (Followers: 2)
Diatom Research     Hybrid Journal  
Doklady Physics     Hybrid Journal   (Followers: 1)
Dynamics of Atmospheres and Oceans     Hybrid Journal   (Followers: 3)
E3S Web of Conferences     Open Access  
Earth and Planetary Science Letters     Hybrid Journal   (Followers: 152)
Earth Interactions     Full-text available via subscription   (Followers: 8)
Earth Science Research     Open Access   (Followers: 6)
Earth Surface Dynamics (ESurf)     Open Access   (Followers: 1)
Earth Surface Processes and Landforms     Hybrid Journal   (Followers: 11)
Earth System Dynamics     Open Access   (Followers: 4)
Earth System Dynamics Discussions     Open Access   (Followers: 3)
Earth's Future     Open Access   (Followers: 1)
Earth, Planets and Space     Open Access   (Followers: 1)
Earthquake Engineering and Engineering Vibration     Hybrid Journal   (Followers: 7)
Earthquake Spectra     Full-text available via subscription   (Followers: 11)
Ecohydrology     Hybrid Journal   (Followers: 11)
Ecological Questions     Open Access   (Followers: 6)
Electromagnetics     Hybrid Journal   (Followers: 1)
Energy Efficiency     Hybrid Journal   (Followers: 10)
Energy Exploration & Exploitation     Full-text available via subscription   (Followers: 4)
Environmental Earth Sciences     Hybrid Journal   (Followers: 10)
Environmental Geology     Hybrid Journal   (Followers: 11)
Environmental Geosciences     Full-text available via subscription   (Followers: 4)
Environmental Geotechnics     Open Access  

        1 2 3 4 5 | Last

Journal Cover Chemical Geology
   Journal TOC RSS feeds Export to Zotero [11 followers]  Follow    
   Hybrid Journal Hybrid journal (It can contain Open Access articles)
     ISSN (Print) 0009-2541
     Published by Elsevier Homepage  [2563 journals]   [SJR: 1.714]   [H-I: 111]
  • Evolution of quartz cementation and burial history of the Eau Claire
           Formation based on in situ oxygen isotope analysis of quartz overgrowths
    • Abstract: Publication date: 25 September 2014
      Source:Chemical Geology, Volume 384
      Author(s): Ayumi Hyodo , Reinhard Kozdon , Anthony D. Pollington , John W. Valley
      Individual quartz overgrowths in siltstone of the late Cambrian Eau Claire Formation (Fm.) are systematically zoned in oxygen isotope ratio (δ18O). In situ analysis of δ18O was performed with 3 and 15μm beam spots by secondary ion mass spectrometer (SIMS) on detrital quartz grains and quartz overgrowths. These results from thin lenses within impermeable mudstones reflect samples that were sealed from basin-wide fluid flow and compliment previous studies of more permeable sandstones. Individual grains of detrital quartz (DQ) are homogeneous in δ18O. The average δ18O values in fine-grained detrital quartz in mudstones and siltstones and in coarser-grained quartz in the Eau Claire Fm., Mt. Simon and St. Peter Sandstones (Ss.) are essentially identical at δ18O=10‰ VSMOW, suggesting that detrital quartz is dominantly igneous in origin. The δ18O values of overgrowth quartz (OQ) of buried samples from the Illinois Basin are higher and quartz overgrowths are systematically zoned outward from the detrital cores. These gradients are similar to those from the underlying Mt. Simon Ss., and are best explained by increasing temperatures during burial. Pressure solution is evident in thin section and may have supplied significant silica for overgrowths. In contrast to the deeply buried samples from the Illinois Basin, quartz overgrowths in samples from the Wisconsin Arch are homogeneous and higher in δ18O. Those overgrowths are interpreted as quartz cements formed in a near-surface environment (<40°C), which is consistent with geological evidence that these rocks were only shallowly buried (<500m). Based on these δ18O(OQ) results and the modeled thermal history during burial of the basin, the earliest-formed quartz overgrowths were produced at low temperature from low δ18O(water) around 450Ma. The δ18O values in traverses of single overgrowths decrease by up to 9.1‰, showing continued cementation with increased burial, pressure solution, and heating until ~250Ma. In traverses of the outermost zone of some overgrowths, oxygen isotope values become constant or increase slightly, possibly due to clay mineral dehydration reactions or later fluid infiltration. We present a new cementation and basin evolution model, in which the δ18O of cement correlates to the age of formation and the late overgrowths formed between 270 and 250Ma, during and/or after the migration of brines that formed the Pb–Zn deposits of the Upper Mississippi Valley District (270Ma). Cementation around 270Ma would have reduced permeability, possibly ending the flow of ore forming brines.


      PubDate: 2014-08-17T21:04:42Z
       
  • Time integrated variation of sources of fluids and seepage dynamics
           archived in authigenic carbonates from Gulf of Mexico Gas Hydrate Seafloor
           Observatory
    • Abstract: Publication date: 14 October 2014
      Source:Chemical Geology, Volume 385
      Author(s): Dong Feng , Daniel Birgel , Jörn Peckmann , Harry H. Roberts , Samantha B. Joye , Roger Sassen , Xiao-Lei Liu , Kai-Uwe Hinrichs , Duofu Chen
      Authigenic carbonate rocks recovered from the Gulf of Mexico Gas Hydrate Seafloor Observatory in Mississippi Canyon block 118 (MC118) at approximately 900m water depth were studied using mineralogical, bulk geochemical, and lipid biomarker analyses. Carbonate rocks occurred as fractured blocks and nodular masses incorporated in carbonate breccias. The carbonates were comprised mainly of high-Mg-calcite and aragonite. The stable carbon isotope composition (δ13C) of authigenic carbonate varied from −29.8‰ to −18.1‰ vs. V-PDB, suggesting a complex mixture of various carbon sources, including dissolved marine inorganic carbon (DIC), oil, as well as methane. Oxygen isotopes (δ18O) varied from +3.4‰ to +5.8‰. The observed 18O-enrichment in relation to calculated equilibrium values in the carbonates probably reflects decomposition of gas hydrates. The most abundant lipid biomarkers in the carbonates were isoprenoidal glycerol dibiphytanyl glycerol tetraethers (GDGTs), predominated by GDGT-2 and GDGT-3, which are typically indicators of anaerobic methane oxidizing archaea (ANMEs). Mono- and bicyclic biphytanes (derived after ether cleavage of GDGT-2 and GDGT-3) showed strong 13C-depletion, which is characteristic for ANMEs. Interestingly, large differences between the δ13C values of the archaeal diether archaeol and acyclic biphytane on the one hand and monocyclic biphytane on the other hand suggest the presence of archaea other than ANMEs. Archaeol and GDGT-0 (containing two acyclic biphytane moieties) are commonly assigned to various methanogenic archaea. Where methane seepage activity is intermediate or low within acoustic wipeout zones at the MC118 gas hydrate site nowadays, microbial communities must have coped with changing conditions as well as longer-term fluctuations in oil and gas seepage or the temporary cessation of hydrocarbon flux in the past. The change from methane seepage to oil seepage or vice versa in addition to flux variability apparently favors the establishment of complex prokaryotic communities dominated by archaea. In addition to anaerobic oxidation of methane, local production of methane is apparently prominent at the study site based on the occurrence of biomarkers of methanogens in the authigenic carbonate. This finding adds to the ongoing multidisciplinary effort to better constrain the environment at the MC118 observatory site and to determine the locally dominant biogeochemical processes.


      PubDate: 2014-08-17T21:04:42Z
       
  • An osmium-based method for assessing the source of dissolved rhenium and
           molybdenum to Archean seawater
    • Abstract: Publication date: 14 October 2014
      Source:Chemical Geology, Volume 385
      Author(s): Brian Kendall
      Authigenic enrichments of Re and Mo in 2.7–2.5Ga organic-rich mudrocks (ORM) indicate that dissolved Re and Mo were present in late Archean seawater. However, it can be difficult to determine if the dissolved Re and Mo were sourced from oxidative crustal weathering or magmatic/hydrothermal/extraterrestrial (MHE) inputs. Comparison of Re, Mo, and 192Os (non-radiogenic Os isotope) concentrations, and initial 187Os/188Os isotope compositions from Re–Os isochron regressions of ORM can shed insight on the source of Re and Mo to Archean seawater. Seawater 187Os/188Os reflects the relative contributions of radiogenic crustal versus unradiogenic MHE Os inputs to seawater. During the Phanerozoic, transient increases in magmatic/hydrothermal activity, marked by unradiogenic 187Os/188Os excursions and higher 192Os concentrations in ORM, are accompanied in some cases by higher Re concentrations. These unradiogenic Os sources could thus have played a greater role in the Archean marine Re (and possibly Mo) budget given that the riverine Re (and Mo) flux at that time was likely smaller than the Phanerozoic flux. Hence, a positive correlation between Re, Mo, and 192Os concentrations in Archean ORM that yields unradiogenic initial 187Os/188Os may reflect a MHE origin for dissolved Re and Mo in the overlying paleo-seawater. Local oxidative weathering in the presence of photosynthetic O2 of mafic-ultramafic rocks and crustal sulfide minerals with low Re/Os ratios is an alternative explanation, but the two scenarios cannot be readily distinguished on the basis of initial 187Os/188Os. By contrast, an initial 187Os/188Os from Archean ORM that is statistically higher than the MHE baseline of 0.10–0.11 points to oxidative mobilization of Re, Os, and Mo from crustal sulfide minerals. Rhenium and Mo concentrations that are negatively correlated with 192Os concentrations in Archean ORM yielding unradiogenic initial 187Os/188Os suggests that some Re and Mo were derived from oxidative weathering. Such negative correlations may reflect an increased marine Os sink (adsorption to oxide minerals) during a mild increase in local environmental O2 levels. These concepts may help trace the onset of photosynthetic O2 production and accumulation along Archean ocean margins.


      PubDate: 2014-08-17T21:04:42Z
       
  • The isotopic fingerprint of Fe cycling in an equatorial
           soil–plant–water system: The Nsimi watershed, South Cameroon
    • Abstract: Publication date: 14 October 2014
      Source:Chemical Geology, Volume 385
      Author(s): Alisson Akerman , Franck Poitrasson , Priscia Oliva , Stéphane Audry , Jonathan Prunier , Jean-Jacques Braun
      Following an initial study of a tropical lateritic hillside system showing little iron isotope fractionation despite a strong accumulation within the soil profile, the present work investigates iron isotope signatures within the organic matter rich swamp system that represents 20% of the studied watershed surface (from Nsimi, South Cameroon). This study considers the soil–plant–water continuum in order to better understand the Fe elemental and isotopic transfer out of the ecosystem. Within the swamp system, the iron isotope compositions of gleysol samples (δ57FeIRMM-14 ~+0.6‰) are significantly heavier than both the continental crust baseline and the reference lateritic soils from the hillslope (δ57FeIRMM-14 =+0.1‰). This enrichment towards heavy isotopes is attributed to a preferential removal of light iron isotopes during soil forming processes. Pedogenic transformations (i.e., gleyzation, organic complexation of metal and leaching) are responsible for the reductimorphic features observed in ferralitic horizons (i.e., incomplete degradation of organic matter in surface and soil whitening favored by good draining conditions). The organic carbon-rich waters of the swamp system are prone to redox processes and strong metal chelation. The dissolved iron (i.e., fraction <0.22μm) of the Mengong stream shows positive δ57Fe signatures, with a downstream enrichment in heavy isotopes, from +0.511±0.266‰ to +1.076±0.240‰. The binding of iron (FeIII) with organic matter can explain the observed enrichment in heavy isotopes in the dissolved fraction. On the contrary, plant leaves are significantly enriched in light Fe (δ57Fe of −0.665±0.035 and −1.119±0.080‰) relative to (i) the litter compartment (−0.166±0.078 to −0.262±0.013‰ for δ57Fe) and (ii) the most superficial soils. Iron isotopic compositions in plants and litter vary as a function of both plant species and season. Hence, the differences in Fe isotopic compositions between the various studied compartments suggest that Fe isotopes can be used (i) to study elemental transfers during soil pedogenesis in tropical environment and (ii) to better appraise and constrain iron biogeochemical cycle between surface horizon of soils, surface waters and the vegetation.


      PubDate: 2014-08-14T20:40:41Z
       
  • Using stable Mg isotopes to distinguish dolomite formation mechanisms: A
           case study from the Peru Margin
    • Abstract: Publication date: 14 October 2014
      Source:Chemical Geology, Volume 385
      Author(s): Vasileios Mavromatis , Patrick Meister , Eric H. Oelkers
      The magnesium isotope composition of diagenetic dolomites and their adjacent pore fluids were studied in a 250m thick sedimentary section drilled into the Peru Margin during Ocean Drilling Program (ODP) Leg 201 (Site 1230) and Leg 112 (Site 685). Previous studies revealed the presence of two types of dolomite: type I dolomite forms at ~6m below seafloor (mbsf) due to an increase in alkalinity associated with anaerobic methane oxidation, and type II dolomite forms at focused sites below ~230mbsf due to episodic inflow of deep-sourced fluids into an intense methanogenesis zone. The pore fluid δ26Mg composition becomes progressively enriched in 26Mg with depth from values similar to seawater (i.e. −0.8‰, relative to DSM3 Mg reference material) in the top few meters below seafloor (mbsf) to 0.8±0.2‰ within the sediments located below 100mbsf. Type I dolomites have a δ26Mg of −3.5‰, and exhibit apparent dolomite-pore fluid fractionation factors of about −2.6‰ consistent with previous studies of dolomite precipitation from seawater. In contrast, type II dolomites have δ26Mg values ranging from −2.5 to −3.0‰ and are up to −3.6‰ lighter than the modern pore fluid Mg isotope composition. The enrichment of pore fluids in 26Mg and depletion in total Mg concentration below ~200mbsf is likely the result of Mg isotope fractionation during dolomite formation, The 26Mg enrichment of pore fluids in the upper ~200mbsf of the sediment sequence can be attributed to desorption of Mg from clay mineral surfaces. The obtained results indicate that Mg isotopes recorded in the diagenetic carbonate record can distinguish near surface versus deep formed dolomite demonstrating their usefulness as a paleo-diagenetic proxy.


      PubDate: 2014-08-14T20:40:41Z
       
  • Gold concentrations in metamorphic fluids: A LA-ICPMS study of fluid
           inclusions from the Alpine orogenic belt
    • Abstract: Publication date: 14 October 2014
      Source:Chemical Geology, Volume 385
      Author(s): K. Rauchenstein-Martinek , T. Wagner , M. Wälle , C.A. Heinrich
      Vein and shear-zone hosted gold deposits in orogenic terrains of Archean to Phanerozoic age are formed from dominantly metamorphic fluids. It is debated, however, whether normal crustal source rocks are adequate to generate economic deposits, or whether selectively gold-enriched sources such as mafic to ultramafic lavas, black shales or an input of magmatic fluid make a decisive difference between the formation of high-grade deposits and barren vein systems. As an essential baseline study in this debate, we have analyzed the metal and sulfur content of fluid inclusions in barren veins across a prograde sequence from greenschist to amphibolite facies in the Central Alpine metamorphic belt. Comparison of the analyzed fluid compositions with thermodynamic solubility calculations shows that the fluids record metamorphic dehydration, decarbonation and desulfidation of the continental crust. However, gold with concentrations of 0.003 to 0.03ppm is increasingly undersaturated in the highest-temperature aqueous–carbonic fluids, which otherwise resemble those forming major gold deposits elsewhere. Our results show that the regional-metamorphic fluids could carry 10–1000 times higher gold concentrations, implying that pre-enriched source rocks or hydrous magmas may play an essential role in generating highly gold-endowed orogenic belts.


      PubDate: 2014-08-14T20:40:41Z
       
  • Measurements of HFC-134a and HCFC-22 in groundwater and unsaturated-zone
           air: Implications for HFCs and HCFCs as dating tracers
    • Abstract: Publication date: 14 October 2014
      Source:Chemical Geology, Volume 385
      Author(s): Karl B. Haase , Eurybiades Busenberg , L. Niel Plummer , Gerolamo Casile , Ward E. Sanford
      A new analytical method using gas chromatography with an atomic emission detector (GC–AED) was developed for measurement of ambient concentrations of hydrochlorofluorocarbons (HCFCs) and hydrofluorocarbons (HFCs) in soil, air, and groundwater, with the goal of determining their utility as groundwater age tracers. The analytical detection limits of HCFC-22 (difluorochloromethane, CHClF2) and HFC-134a (1,2,2,2-tetrafluoroethane, C2H2F4) in 1L groundwater samples are 4.3×10−1 and 2.1×10−1 pmolkg−1, respectively, corresponding to equilibrium gas-phase mixing ratios of approximately 5–6 parts per trillion by volume (pptv). Under optimal conditions, post-1960 (HCFC-22) and post-1995 (HFC-134a) recharge could be identified using these tracers in stable, unmixed groundwater samples. Ambient concentrations of HCFC-22 and HFC-134a were measured in 50 groundwater samples from 27 locations in northern and western parts of Virginia, Tennessee, and North Carolina (USA), and 3 unsaturated-zone profiles were collected in northern Virginia. Mixing ratios of both HCFC-22 and HFC-134a decrease with depth in unsaturated-zone gas profiles with an accompanying increase in CO2 and loss of O2. Apparently, ambient concentrations of HCFC-22 and HFC-134a are readily consumed by methanotrophic bacteria under aerobic conditions in the unsaturated zone. The results of this study indicate that soils are a sink for these two greenhouse gases. These observations contradict the previously reported results from microcosm experiments that found that degradation was limited above-ambient HFC-134a. The groundwater HFC and HCFC concentrations were compared with concentrations of chlorofluorocarbons (CFCs, CFC-11, CFC-12, CFC-113) and sulfur hexafluoride (SF6). Nearly all samples had measured HCFC-22 or HFC-134a that were below concentrations predicted by the CFCs and SF6, with many samples showing a complete loss of HCFC-22 and HFC-134a. This study indicates that HCFC-22 and HFC-134a are not conservative as environmental tracers and leaves in question the usefulness of other HCFCs and HFCs as candidate age tracers.


      PubDate: 2014-08-14T20:40:41Z
       
  • A new approach to Pb evaporation and condensation from zircon enabling
           sub-Ma age determination on Precambrian rocks
    • Abstract: Publication date: 14 October 2014
      Source:Chemical Geology, Volume 385
      Author(s): Abin Das , Donald W. Davis
      Sub-million year age precision on low to moderately metamict Precambrian zircon is often reproducible using the isotope dilution thermal ionization technique (ID-TIMS) exploiting U–Pb double spiking for precise mass fractionation correction. However, this requires total dissolution of sample and laborious chemical processing. On the other hand whole grain evaporation techniques of Pb extraction are simple and less time consuming but provide relatively inefficient ionization and are not amenable to double Pb isotope spiking for precise mass fractionation correction, which limits the precision of ages that they can provide (often restricted to ±few million years). A modified evaporation–condensation method is described that overcomes these limitations with the ability to use 202Pb–205Pb double spiking for precise mass fractionation correction in order to achieve sub-million year age precision on low to highly metamict Precambrian samples. Examples are shown of age determinations on previously dated zircon populations from the Marmion tonalite and Nevison Lake rhyolite, western Superior province (U<200ppm), which give 3001.8±0.5Ma and 2998.2±0.3Ma, respectively, and from the black norite and the felsic norite in the Sudbury igneous complex (U>1000ppm), which give 1848.9±0.2Ma and 1851.3±0.5Ma, respectively (all errors are 95% confidence).


      PubDate: 2014-08-10T20:30:57Z
       
  • A complete Os excursion across a terrestrial Cretaceous–Paleogene
           boundary at the West Bijou Site, Colorado, with evidence for recent open
           system behavior
    • Abstract: Publication date: 14 October 2014
      Source:Chemical Geology, Volume 385
      Author(s): Jessica Zaiss , Greg Ravizza , Steven Goderis , Justine Sauvage , Philippe Claeys , Kirk Johnson
      The few previously reported values of 187Os/188Os ratios from non-marine K–Pg boundary sections are distinctly higher than the range of 187Os/188Os ratios measured in chondrites and the range of ratios predicted by models of physical mixing between chondrites and upper crust. Here, Re–Os data from the West Bijou continental K–Pg boundary site, located within the Denver Basin, are used to better constrain the Os isotopic composition of fallout from the Chicxulub ejecta plume. For the first time, full vertical profiles of 187Os/188Os ratios and Re and Os concentrations across a continental K–Pg boundary section are reported. Within this section of lignite, the lowest measured 187Os/188Os ratio (0.182) coincides with the K–Pg boundary interval as previously determined by palynology and the distribution of shocked quartz in a nearby outcrop. However, sediments with elevated Os concentrations and measured 187Os/188Os below 0.23 extend over a 30cm interval, from ~5cm above the clay-rich boundary interval (~5cm thick) to ≈25cm below. Maximum Ir and Os concentrations occur 3cm and 10cm below the K–Pg boundary, respectively, demonstrating greater diagenetic mobility of Os relative to Ir. Low 187Os/188Os above the K–Pg boundary suggests that accumulation of impact derived Os at this site persisted after the impact event, perhaps due to redistribution of impact debris in the surrounding area. Importantly, calculated initial 187Os/188Os ratios throughout most of the section are impossibly low and require open system behavior, likely in the form of Re addition within the last 10 million years. Within the boundary interval, Re concentrations are among the lowest measured at Bowring Pit and calculated initial 187Os/188Os ratios are very similar to those previously reported from the western interior of the United States K–Pg boundary material (~0.14), suggesting super-chondritic 187Os/188Os is characteristic of the Chicxulub ejecta plume. Beyond implications for our understanding of Os as an impact tracer, this study emphasizes the dramatic effect diagenesis can have on the Re–Os system in organic-rich environments, especially through post-depositional addition of Re.


      PubDate: 2014-08-10T20:30:57Z
       
  • Implications of discordant U–Pb ages on Hf isotope studies of
           detrital zircons
    • Abstract: Publication date: 14 October 2014
      Source:Chemical Geology, Volume 385
      Author(s): Martin Guitreau , Janne Blichert-Toft
      Zircon is the ideal tool for unraveling Earth's history because of its refractory nature, being readily datable by the U–Pb isotope system, and owing to the fact that its Hf isotope composition can be precisely determined. However, most analyzed zircons have experienced Pb-loss to various degrees, resulting in present-day measured 207Pb/206Pb ages being younger than that of the time of crystallization, if the loss is ancient. This is of particular importance for ancient zircons from the Archean and the Hadean, notably the Jack Hills zircons. A zircon Lu–Hf and U–Pb isotope evolution model has been developed and shows that Pb-loss at 3700Ma affecting a simple zircon population that crystallized at 4350Ma can reproduce most of the broad ε Hf versus 207Pb/206Pb age trend observed for Jack Hills Hadean zircons. In addition, the model demonstrates that crystals having experienced widely different degrees of Pb-loss (2–80%) appear only slightly discordant (0.1–2.2%), while their apparent 207Pb/206Pb ages are different from that of crystallization by up to 466My. This is insignificant for calculating initial Hf isotope compositions but results in major shifts in initial ε Hf values by up to 11.3 units. This effect is particularly relevant to the global Hadean/Archean detrital zircon record for which several apparent ε Hf versus age trends, similar to that defined by the Jack Hills Hadean zircons, stand out and could be fully or partly artificial. To overcome, or at least significantly reduce, these issues, multiple U–Pb analyses should be undertaken for each detrital zircon. Simultaneous measurement of Hf and U–Pb isotopes by the split-stream technique is the ideal approach because it can provide information about the cause of discordance (Pb-loss or concurrent analysis of two unrelated growth zones) and thereby allow for the most robust and accurate Hf isotope data to be obtained.


      PubDate: 2014-08-10T20:30:57Z
       
  • High precision 87Sr/86Sr measurements by MC-ICP-MS, simultaneously solving
           for Kr interferences and mass-based fractionation
    • Abstract: Publication date: 14 October 2014
      Source:Chemical Geology, Volume 385
      Author(s): Jasper G. Konter , Lauren P. Storm
      Sr isotope measurements by multi-collector inductively coupled plasma mass spectrometer (MC-ICP-MS) are complicated by Kr interferences at atomic masses 84 and 86. Particularly the interference on 86Sr is problematic for 87Sr/86Sr measurements, since it is the normalizing stable isotope, and as such also used in the mass-based fractionation correction using 86Sr/88Sr. Although on-peak baseline measurements can subtract an average Kr contribution, signal variations and errors within-run are propagated into the final error, and signal-to-noise ratios are lower than combined Sr+Kr signals. Alternatively, intensities of interference-free 82Kr or 83Kr isotopes can be monitored to correct the interfering Kr masses, although the errors for these signals are amplified and propagated into the 86Kr and 84Kr correction. Instead, our approach depends on the use of the most abundant Kr isotope (84Kr) for correction, which can then be used to estimate interference by 86Kr on 86Sr. This is accomplished by solving how much Kr is needed to explain the difference between I84/I88 and I86/I88 versus 84Sr/88Sr and 86Sr/88Sr (I8×: signal intensity at mass 8×). A solution with the traditional isotope dilution equation would require off-line iteration to include an exponential mass fractionation correction. Instead, we formulate an online exponential correction of the Sr data without iterating, relying on a separate Kr correction that estimates Kr mass fractionation with either the linear law or a series expansion of the exponential law. Both methods advance MC-ICP-MS long-term 87Sr/86Sr precision for NIST SRM 987 to ±16∗10−6 or 22.5ppm 2σ, outperforming an 83Kr-based correction and an on-peak baseline correction applied to the same raw data. Both our methods find identical results for USGS rock standards that agree with published (TIMS) values.


      PubDate: 2014-08-10T20:30:57Z
       
  • Sr and Nd isotopic compositions of apatite reference materials used in
           U–Th–Pb geochronology
    • Abstract: Publication date: 14 October 2014
      Source:Chemical Geology, Volume 385
      Author(s): Yue-Heng Yang , Fu-Yuan Wu , Jin-Hui Yang , David M. Chew , Lie-Wen Xie , Zhu-Yin Chu , Yan-Bin Zhang , Chao Huang
      Apatite is an important common U- and Th-bearing accessory mineral in igneous, metamorphic and clastic sedimentary rocks. The advent of in situ U–Th–Pb apatite geochronology by the SIMS and LA-(MC)-ICP-MS methods has demonstrated the importance of having uniform and homogeneous reference materials. Recently, it has been shown that Sr and Nd isotopic data combined with U–Pb age and trace element concentration data can provide important constraints on apatite paragenesis because this phase usually exhibits high Sr and REE concentrations but has low Rb/Sr ratios which result in negligible corrections for the ingrowth of radiogenic Sr. However, as apatite can potentially have complex internal structures resulting from multiple thermal events, such as inherited cores and metamorphic overgrowths, requires that the Sr and Nd isotopic data should be measured with high spatial resolution. However isobaric interferences hamper the precise determination of Sr or Nd isotopic compositions in LA-MC-ICP-MS analysis. In this work we undertook in situ measurements of Sr and Nd isotopic compositions of eleven apatite reference materials (AP1, AP2, Durango, MAD, Otter Lake, NW-1, Slyudyanka, UWA-1, Mud Tank, McClure Mountain and SDG) commonly used in U–Th–Pb geochronology. Our obtained Sr and Sm–Nd isotopic compositions for these apatite samples are consistent with those values obtained by solution-based methods (isotope dilution and ion chromatography) using MC-ICP-MS or TIMS, which demonstrates the reliability and robustness of our analytical protocol.
      Graphical abstract image

      PubDate: 2014-08-10T20:30:57Z
       
  • Bank storage in karst aquifers: The impact of temporary intrusion of river
           water on carbonate dissolution and trace metal mobility
    • Abstract: Publication date: 14 October 2014
      Source:Chemical Geology, Volume 385
      Author(s): Amy L. Brown , Jonathan B. Martin , Elizabeth J. Screaton , John E. Ezell , Patricia Spellman , Jason Gulley
      Storms can trigger changes in river stage that alter the hydraulic gradients between rivers and adjacent aquifers. In eogenetic carbonate karst aquifer systems, storms enable river water to intrude >1km into the adjacent aquifer systems for days to weeks. This process is similar to bank storage of streams in siliciclastic sediments but can have longer temporal and larger spatial scales. River intrusion triggers changes in mineral saturation states and redox conditions in the aquifer due to the input of low pH, low specific conductivity (SpC), and high dissolved organic carbon (DOC) flood water. To assess the effects of river intrusion into karst aquifers, we measured SpC, temperature, pH, redox state, and concentrations of dissolved major and trace elements through an intrusion event at Madison Blue Spring in northern Florida, USA. River water displaced groundwater in the conduit at least 1km into the aquifer and flowed into the pores of the unconfined aquifer matrix. Distinct Cl− concentrations between river water and groundwater provide estimates for mixing fractions. The location and magnitude of oxidation of organic matter in the subsurface controlled trace metal concentration, redox state and saturation state of the water with respect to calcite (SIcal). Organic matter oxidation in the phreatic conduits was limited by the terminal electron acceptors (TEAs) present in the conduit water. Calcite dissolution and trace metal sorption were limited by the lower surface area to porosity ratios in the conduits than the matrix. Organic matter oxidation was enhanced in the matrix by mixing with matrix waters with available DO and NO3 −, resulting in greater CO2 production and calcite dissolution than in the conduit. After the intruded river water discharged, conditions remained more reducing in the aquifer than baseflow conditions due to the reduction of DO and NO3 − in the matrix water. The organic matter transported into the aquifer during river intrusion drives carbonate dissolution, alters redox state, and impacts trace metal mobility, impacting groundwater and surface water quality.


      PubDate: 2014-08-10T20:30:57Z
       
  • Metal scavenging by calcium carbonate at the Eyjafjallajökull
           volcano: A carbon capture and storage analogue
    • Abstract: Publication date: 25 September 2014
      Source:Chemical Geology, Volume 384
      Author(s): J. Olsson , S.L.S. Stipp , E. Makovicky , S.R. Gislason
      The reaction of CO2 and water with basaltic rock can release trace heavy metals, which pose a serious threat to the quality of surface waters. The pH of the carbonated water increases during dissolution of the host rock or dilution by pore fluids. This leads to precipitation of carbonate and other secondary minerals that often scavenge the released heavy metals. However, very little is known about uptake capacity of the precipitates in natural systems or how much divergence there could be, compared with behavior in laboratory experiments. The spring 2010 eruption of the Eyjafjallajökull volcano, Iceland, provides a unique opportunity to study the mobility of heavy metals that are released during CO2 injection into shallow basaltic aquifers and the ensuing precipitation of carbonate minerals. Following the Eyjafjallajökull eruption, rapid and constant travertine formation was discovered in the Icelandic river, Hvanná, in the vicinity of the volcano. The river water emerged from under the lava flow and was heavily charged with cations and dissolved CO2. The concentration of the major dissolved constituents was: dissolved inorganic carbon (DIC), 33.08mM; calcium, 6.17mM; magnesium, 4.27mM; sodium, 2.78mM and sulfur, 1.92mM. Carbon dioxide degassing of the river water increased pH from 6.6 to 8.5 and travertine precipitated for hundreds of meters downstream, rendering the stream bed white with calcite. Rapid crystallization rate produced dendritic structures or sometimes very porous material. Boxwork textures were observed within the porous calcite that probably originated from transformation of a metastable phase such as ikaite (CaCO3·6 H2O). A gradual decrease of conductivity from 1.8mS/cm at the river water outlet to 1.1mS/cm downstream and a clear drop in dissolved metal concentration strongly correlated with the precipitated calcite. Considering the complexity of the natural system, the estimated partition coefficients for Ba, Cd, Co, Cu, Mg, Mn, Na, Ni, Sr and Zn are in good agreement with the values derived from laboratory experiments under rather ideal conditions. Other elements were also scavenged from the river water, including Al, Fe, K, P, S, Si, Ti, V and the rare earth elements (REE). Our thermodynamic modeling suggests that, in addition to calcite and ikaite, silica, clay minerals, ferrihydrite, gibbsite and amorphous Ca, Mg carbonate minerals were supersaturated as the spring water degassed its CO2. Our results provide a valuable base for assessing the environmental impact of volcanic eruptions in basaltic terrain and carbon capture and storage (CCS) in basaltic rock.


      PubDate: 2014-08-05T20:11:21Z
       
  • Small-volume baddeleyite (ZrO2) U–Pb geochronology and Lu–Hf
           isotope geochemistry by LA-ICP-MS. Techniques and applications
    • Abstract: Publication date: 25 September 2014
      Source:Chemical Geology, Volume 384
      Author(s): Mauricio Ibanez-Mejia , George E. Gehrels , Joaquin Ruiz , Jeffrey D. Vervoort , Michael E. Eddy , Chen Li
      U–Pb geochronology of baddeleyite (ZrO2) is an increasingly used tool in the Earth and planetary sciences for determining the crystallization and emplacement ages of mafic igneous rocks. Additionally, baddeleyite has a strong affinity for hafnium and preferentially excludes the REE's, making it an important repository of Hf isotopic compositions, which can be used to provide constraints on the origin of these rocks. In this contribution we introduce a technique for U–Pb dating and Lu–Hf isotopic analysis of baddeleyite by LA-MC-ICP-MS. A systematic study of crystals with known ages demonstrates that our methodology is capable of producing 207Pb/206Pb results that are precise and accurate to within 1% of their TIMS values at a 2-sigma confidence level, while Phanerozoic crystals can generally be dated to within 1.5 to 3.0% accuracy using their 206Pb/238U compositions. These results are routinely reproducible with a variety of laser-spot sizes ranging from 30 to 10μm in diameter and with crater depths as shallow as ~3μm in depth. This represents a significant improvement in the sampled volume generally used for LA-ICP-MS geochronology and is a critical step for dating small baddeleyite crystals either as mineral separates or identified in situ within thin sections. No orientation-dependent biases on the measured 206Pb/238U values were identified from our data, suggesting that LA-ICP-MS dating of Phanerozoic crystals can be routinely performed without the biases previously reported for SIMS instruments. Our data show that initial 176Hf/177Hf ratios can be determined with an accuracy as good as 0.5 εHf units by taking the mean of 10 to 30 individual spot analyses acquired with a 40μm beam-diameter. These U–Pb and Hf results are comparable to what can be routinely achieved for zircon by LA-MC-ICP-MS. In addition to the methodological approach, we report a high-precision U–Pb TIMS age and four solution-MC-ICP-MS Hf isotopic results for new and/or important baddeleyite localities. These new data are used here to assess the adequacy of our fractionation, interference and mass-bias corrections and can be used as a basis for addressing the accuracy of LA-ICP-MS U–Pb and Lu–Hf data for future inter-laboratory calibration efforts.


      PubDate: 2014-08-01T19:39:28Z
       
  • Redox-driven dissolution of clay minerals by uranium under high pressure
           CO2 conditions
    • Abstract: Publication date: 15 September 2014
      Source:Chemical Geology, Volume 383
      Author(s): Yan Liu , Fubo Luan , William D. Burgos
      Geologic sequestration of supercritical CO2 is one technology proposed to mitigate global warming. Increased acidity of brine due to CO2 injection could lead to mineral dissolution of cap rock and well seals and mobilization of contaminants (e.g., U, Pb, As). In this study we examined the dissolution of nontronite NAu-2, an Fe(III)-rich clay mineral, and partially-reduced nontronite (R-NAu-2) in a synthetic brine (0.33M Na2SO4) under high pressure CO2 conditions (PT =9.66bar, PCO2 ≥8.66bar CO2, T=20°C) and in 1.40M H3PO4–0.50M H2SO4. Uranyl(VI) or biogenic uraninite(IV) was added as a redox-active contaminant and reaction kinetics were measured over a 15d period. Unaltered nontronite [3.4% Fe(II)] dissolved very little under high pressure CO2 conditions. However, chemically-reduced nontronite [48% Fe(II)] dissolved more rapidly (half-life of 78.4d under high pressure CO2 conditions, 17.8h in H3PO4–H2SO4). Structural Fe(II) in reduced nontronite [R-NAu-2 Fe(II)] was preferentially dissolved compared to structural Fe(III) in unaltered nontronite [NAu-2 Fe(III)]. No redox reactions were observed between R-NAu-2 Fe(II) and U(VI). In contrast, uraninite was oxidized by NAu-2 Fe(III) faster and to a greater extent under high pressure CO2 conditions as compared to ambient pressure conditions (PT =1.0bar, 95:5% N2:H2). Redox reactions between uraninite and NAu-2 Fe(III) enhanced the dissolution of both clay and U, indicative of potential risks associated with geologic carbon sequestration.


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


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


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


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

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


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

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


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




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


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


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




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


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


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

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


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


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


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


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


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


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


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


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


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

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


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


      PubDate: 2014-07-27T19:19:23Z
       
  • Geochemical response to pulsed sedimentation: Implications for the use of
           Mo as a paleo-proxy
    • Abstract: Publication date: 29 August 2014
      Source:Chemical Geology, Volume 382
      Author(s): J.L. McKay , T.F. Pedersen
      Deglacial sediments in piston core JT96-02 from the continental slope off western Canada contain numerous centimeter-thick, iron sulfide-rich layers. Major and minor element chemistry, as well as C and N isotopic data suggest that these layers represent background hemipelagic sedimentation. Intervening gray silty clay layers, each of which exhibits a fining-upward trend, are distal turbidites. The iron sulfides precipitated in response to the emplacement of each turbidite, which slowed oxygen diffusion into the sediment allowing anoxic conditions to develop in the underlying relatively organic-rich deposits. Resulting sulfate reduction and precipitation of Fe-sulfide fostered Mo enrichment (>2μg/g). It may also explain Ni, Cr, V, and Co enrichments, although differences in detrital mineralogy cannot be ruled out, particularly for Ni and Cr. Rhenium is also enriched, albeit slightly, within the sulfide layers, but low Re/Mo ratios (~0.78ng/g/μg/g) suggest that anoxic conditions developed very rapidly. Metals with high lithogenic concentrations (Cd and U) are not obviously enriched in the sulfide layers because there was insufficient time for noticeable authigenic accumulation before deposition of the next turbidite which severed diffusive communication with overlying seawater, the primary source of metals. Silver is also slightly higher in the sulfide layers, possibly linked to higher marine organic matter content. In summary, the pulsed character of sedimentation during the deglaciation led to episodic enrichment of Mo and occurred independently of changes in bottom water oxygen or organic carbon content. This observation potentially complicates the use of Mo as a paleo-proxy in some continental margin environments.


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


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


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


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


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


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


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


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