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Erwerbs-Obstbau     Hybrid Journal  

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Journal Cover   Chemical Geology
  [SJR: 1.714]   [H-I: 111]   [12 followers]  Follow
   Hybrid Journal Hybrid journal (It can contain Open Access articles)
   ISSN (Print) 0009-2541
   Published by Elsevier Homepage  [2589 journals]
  • Single crystal U–Pb zircon age and Sr–Nd isotopic composition
           of impactites from the Bosumtwi impact structure, Ghana: Comparison with
           country rocks and Ivory Coast tektites
    • Abstract: Publication date: August 2010
      Source:Chemical Geology, Volume 275, Issues 3–4
      Author(s): Ludovic Ferrière , Christian Koeberl , Martin Thöni , Chen Liang
      The 1.07Myr old Bosumtwi impact structure (Ghana), excavated in 2.1–2.2Gyr old supracrustal rocks of the Birimian Supergroup, was drilled in 2004. Here, we present single crystal U–Pb zircon ages from a suevite and two meta-graywacke samples recovered from the central uplift (drill core LB-08A), which yield an upper Concordia intercept age of ca. 2145±82Ma, in very good agreement with previous geochronological data for the West African Craton rocks in Ghana. Whole rock Rb–Sr and Sm–Nd isotope data of six suevites (five from inside the crater and one from outside the northern crater rim), three meta-graywacke, and two phyllite samples from core LB-08A are also presented, providing further insights into the timing of the metamorphism and a possibly related isotopic redistribution of the Bosumtwi crater rocks. Our Rb–Sr and Sm–Nd data show also that the suevites are mixtures of meta-greywacke and phyllite (and possibly a very low amount of granite). A comparison of our new isotopic data with literature data for the Ivory Coast tektites allows to better constrain the parent material of the Ivory Coast tektites (i.e., distal impactites), which is thought to consist of a mixture of metasedimentary rocks (and possibly granite), but with a higher proportion of phyllite (and shale) than the suevites (i.e., proximal impactites). When plotted in a Rb/Sr isochron diagram, the sample data points (n =29, including literature data) scatter along a regression line, whose slope corresponds to an age of 1846±160Ma, with an initial Sr isotope ratio of 0.703±0.002. However, due to the extensive alteration of some of the investigated samples and the lithological diversity of the source material, this age, which is in close agreement with a possible “metamorphic age” of ∼1.8–1.9Ga tentatively derived from our U–Pb dating of zircons, is difficult to consider as a reliable metamorphic age. It may perhaps reflect a common ancient source whose Rb–Sr isotope systematics has not basically been reset on the whole rock scale during the Bosumtwi impact event, or even reflect another unknown geologic event.

      PubDate: 2015-02-24T15:47:02Z
  • Zircon U–Pb geochronology from the Paraná bimodal volcanic
           province support a brief eruptive cycle at ~135Ma
    • Abstract: Publication date: 2 February 2011
      Source:Chemical Geology, Volume 281, Issues 1–2
      Author(s): Viter Magalhães Pinto , Léo Afraneo Hartmann , João Orestes S. Santos , Neal Jesse McNaughton , Wilson Wildner
      Ion microprobe U–Pb isotopic data on zircons from the Paraná magmatic province are presented from one tholeiitic (high-Ti Pitanga type) and three felsic volcanic rocks from the low-Ti Palmas and high-Ti Chapecó types. Igneous zircons from the four volcanic rocks yield volcanism ages within error: i.e. 134.4±1.1Ma (basalt), 134.6±1.4Ma (rhyodacite), 134.8±1.4 (quartz latite) and 135.6±1.8Ma (quartz latite). The age of Paraná magmatism based on previous Ar–Ar geochronology has two divergent ranges: 1) 1 to 2million years with magmatic peak at 131–133Ma, and 2) over ~10m.y. between 137 and 127Ma. Our results show that the bimodal volcanics of the province, at least to the south of the Piquiri lineament, have very high effusion rates over a brief period with a main pulse at ~135Ma, about 2% older than proposal 1, more akin to a short period of magmatism. These results are most significant for the understanding of time relations in this large intraplate magmatic province.
      Highlights ►First zircon SHRIMP age determination of a basalt from the Paraná magmatic province. ►Short time interval (1m.y.) of major lava effusion. ►High effusion rate of lavas required.

      PubDate: 2015-02-24T15:47:02Z
  • Chemical weathering rates of silicate-dominated lithological classes and
           associated liberation rates of phosphorus on the Japanese
           Archipelago—Implications for global scale analysis
    • Abstract: Publication date: 21 August 2011
      Source:Chemical Geology, Volume 287, Issues 3–4
      Author(s): Jens Hartmann , Nils Moosdorf
      Lithology is an important characteristic of the terrestrial surface, whose properties influence chemical weathering rates. Specifically non-silicate minerals may contribute significantly to the weathering derived fluxes from silicate-dominated lithological classes. The Japanese Archipelago consists of predominantly silicate-dominated lithologies with a high proportion of volcanics. However, the spatially explicit representation of chemical weathering rates remains difficult for such a large region, because many controlling factors on chemical weathering rates are correlated with each other. Due to the spatial heterogeneity of lithology, a multi-lithological model approach to estimate spatially explicit chemical weathering rates for unmonitored areas is applied here. To achieve this, hydrochemical data of 381 catchments are used to train a set of models, recognizing the contribution of a variety of proposed factors influencing chemical silicate rock weathering rates (CSRWR: cations plus dissolved silica flux). The monitored catchments cover ~44% of the Japanese Archipelago. Cation chemical weathering rates (excluding Si) are linearly correlated with CSRWR and show outliers if basic volcanics or pyroclastic flows are present due to increased silica release rates. Lithology and runoff are identified as the strongest predictors for chemical weathering rates. Temperature and gradient of slope are of less relevance for the regional scale prediction while further proposed factors like soil properties or land cover are not identified as major predictors. Latter findings are partly attributed to geodata quality, low variability of parameter values as well as spatial correlations of proposed controlling factors with lithology or runoff. The calculated average CSRWR of the Archipelago is ~25tkm−2 a−1 and ranges from 5.9 to 107tkm−2 a−1 in monitored catchments. Weathering rates per lithological class as a function of runoff can be grouped into three classes: a) pyroclastic flows showing the highest chemical weathering rates; b) alluvial deposits, mixed sediments and basic to intermediate volcanics with medium rates; and c) metamorphics, siliciclastic sediments, acid volcanics, acid plutonics and unconsolidated sediments (other than alluvial deposits), showing the lowest rates. The recognition of lithogenic sulfur would add 9.7% to CSRWR of considered catchments. Results suggest that the lithological classes acid volcanics and unconsolidated sediments contribute above average to the sulfur fluxes. Possible biases of this observation are discussed. The contribution of Ca-fluxes from non-silicate calcic minerals (named Ca-excess, Ca-fluxes in addition to silicate Ca-fluxes) is about 10% of the CSRWR on average and is attributed by a wide value range. The calculated ratio “Ca-excess to total Ca-fluxes” from chemical weathering averages around 62%, 75%, 56%, 83% and 84% for the lithological classes acid plutonics, metamorphics, siliciclastic sediments, mixed sediments and acid volcanics, respectively. This suggests a major Ca-contribution from non-silicate calcic minerals for these lithological classes. Phosphorus release from rocks due to chemical weathering is estimated to be between 1kg Pkm−2 a−1 and 390kgPkm−2 a−1. The P-release patterns in dependence of runoff per lithological class are different from CSRWRs due to differences of applied P-content in rocks. The identified spatial P-release patterns suggest that the consideration of dynamic and spatially resolved P-release rates by chemical weathering might improve ecosystem studies. Later findings may be of importance for analysing the influence of P-release from rocks on the climate system via ecosystem functioning on geological time scales. A first application of the P-release model to the global scale suggests an annual release of 1.6Mt P (13.8kgPkm−1 a−1) by chemical weathering of silicate dominated lithological classes (excluding carbonate sedimentary rocks).
      Highlights ► Multi-lithological approach to estimate chemical weathering rates per lithological class. ► Estimated weathering rates depend on lithological information and environmental conditions. ► Estimation of the proportion of Ca-fluxes from non silicate minerals on total Ca-fluxes. ► Estimation of the contribution ...
      PubDate: 2015-02-24T15:47:02Z
  • Disturbance to the 40Ar/39Ar system in feldspars by electron and ion beam
    • Abstract: Publication date: 26 September 2013
      Source:Chemical Geology, Volume 355
      Author(s): S. Flude , S.C. Sherlock , M.R. Lee , S.P. Kelley
      The extent to which intracrystalline microtextures influence the diffusion of radiogenic Ar within alkali feldspars from slowly cooled igneous rocks is a long standing question in thermochronology. By combining high-resolution electron microscopy with in-situ UV-laser ablation microprobe 40Ar/39Ar analysis the interplay of microtextures with isotope ages can be studied directly, enabling some of the assumptions underlying 40Ar/39Ar thermochronological techniques to be tested and allowing deduction of thermochronological and geological histories. However, there are numerous potential mechanisms by which a sample can be damaged and its Ar-isotope system disturbed by such microscopy techniques. To test this hypothesis, perthitic alkali feldspars from the 270–280Ma Dartmoor Granite, UK, and gem-quality orthoclase from Itrongay, Madagascar (~470Ma) were polished with colloidal silica or etched with hydrofluoric acid and irradiated with electrons and Ga+ ions. The accelerating voltages and currents used were typical of those for electron beam imaging by scanning electron microscopy, X-ray analysis and mapping by electron probe, and for extraction of foils using the focused ion beam technique. No disturbance to the Ar-isotope system was observed for Ga+ ion irradiation, or for low-resolution SEM imaging, but electron irradiation of small areas for long durations, as occurs during extended high-magnification SEM imaging, was found to disturb the Ar-isotope system over hundreds of micrometre sized areas by the addition of K and atmospheric Ar, producing anomalously young apparent 40Ar/39Ar ages. The best explanation for this age disturbance is electromigration of K and implantation of atmospheric Ar during sample charging.

      PubDate: 2015-02-24T15:47:02Z
  • Geochemical and hydrological processes controlling groundwater salinity of
           a large inland wetland of northwest Australia
    • Abstract: Publication date: 24 October 2013
      Source:Chemical Geology, Volume 357
      Author(s): Grzegorz Skrzypek , Shawan Dogramaci , Pauline F. Grierson
      Understanding mechanisms of hydrochemical evolution of groundwater under saline and brine wetlands in arid and semiarid regions is necessary to assess how groundwater extraction or injection in large-scale basins may affect the natural interface between saline–fresh aquifers in those systems. We investigated the evolution of groundwater of the Fortescue Marsh, a large inland wetland of northwest Australia that is mainly supplied by floodwater from the upper Fortescue River catchment. The marsh is located in the Pilbara region, one of the world's major iron ore provinces, where mining activities often occur below water tables. Here, we quantified the stable isotope and chemical composition of groundwater, surface water and rainfall in and around the marsh to better understand how saline marshes and playa lakes function geochemically, hydrologically and ecologically. The deep groundwater (>50m depth) of the Fortescue Marsh is highly saline (>100gL−1), whilst shallow groundwater (~0–20m depth) and surface water are mainly fresh or brackish. Currently, the marsh is mainly recharged by occasional floodwater. Consequently, salt in the marsh is concentrated by evaporation of rainfall. We expected that the hydrochemical composition of surface and groundwater would therefore reflect the chemical composition of rainwater. We analysed 206 water samples for stable isotope composition (δ 2H, δ 18O) and water chemistry, including: pH, dissolved oxygen, electrical conductivity (EC), and total dissolved solids (TDS), as well as Na, Ca, Mg, K, Si, Fe, HCO3, SO4, Cl, Sr and Br. We then developed geochemical models and a salt inventory to estimate the geological time of salt accumulation and to decouple geochemical characteristics of salt from modern groundwater. We found that groundwater associated with the marsh can be divided into two distinct groups that are characterised by their stable isotope compositions; i) fresh and brackish groundwater (TDS <10gL−1; δ 18O −8.0±0.9‰) and ii) saline and brine groundwater (TDS >10gL−1, δ 18O varies from +2.5 to −7.2‰). Fresh groundwater was evaporated by <20% compared to rainwater. Brackish water mainly reflects modern recharge whilst saline and brine groundwater reflects mixing between modern rainfall, brackish water and relatively old groundwater. The Cl load in the groundwater originates from rainfall and accumulates over time as it is recycled due to precipitation of evaporates and re-dissolution on the marsh during subsequent flooding events. The stable isotope composition of the deeper brine groundwater also suggests a complex evolution, which cannot be explained by evaporation under current conditions from modern rainfall. We thus conclude that the deeper brine groundwater under the Fortescue Marsh developed under a different climatic regime and that the current salt in the marsh has accumulated over at least 40,000years, but could have been as long as 700,000years.

      PubDate: 2015-02-24T15:47:02Z
  • A new purge and trap headspace technique to analyze low volatile compounds
           from fluid inclusions of rocks and minerals
    • Abstract: Publication date: 4 November 2013
      Source:Chemical Geology, Volume 358
      Author(s): Ines Mulder , Stefan G. Huber , Torsten Krause , Cornelius Zetzsch , Karsten Kotte , Stefan Dultz , Heinz F. Schöler
      A new method for the analysis of trace gases from fluid inclusions of minerals has been developed. The purge and trap GC–MS system is based on the system described by Nolting et al. (1988) and was optimized for the analyses of halogenated volatile organic compounds (VOCs) having boiling points as low as −128°C (carbon tetrafluoride). The sample preconcentration cold trap consists of a U-shaped glass lined steel tube (GLT™), that is immersed into a small liquid nitrogen Dewar vessel for cooling. A rapid desorption step heats up the preconcentration tube in <30s from −196°C to 200°C. The process is carried out by using a pressurized air stream to dissipate the liquid nitrogen followed by resistive heating of the trap. The design of the cold trap and the direct transfer of desorbed analytes onto the GC column via a deactivated capillary column retention gap made sample refocusing within the GC oven unnecessary. Furthermore, a special air-tight grinding device was developed in which samples ranging from soft halite (hardness 2, Mohs scale) to hard quartz (hardness 7) are effectively ground to average diameters of 1000nm or below, thereby releasing gases from fluid inclusions of minerals. The gases are then purged from the grinding chamber with a He carrier gas flow. The detection and quantitative determination of gases, such as SF6 and CF4 released from fluorites and CH3Cl from halite samples is demonstrated.

      PubDate: 2015-02-24T15:47:02Z
  • Response of the U–Pb chronometer and trace elements in zircon to
           ultrahigh-temperature metamorphism: The Kadavur anorthosite complex,
           southern India
    • Abstract: Publication date: 24 November 2011
      Source:Chemical Geology, Volume 290, Issues 3–4
      Author(s): Ellen Kooijman , Dewashish Upadhyay , Klaus Mezger , Michael M. Raith , Jasper Berndt , C. Srikantappa
      Zircon from quartzites in the contact aureole and wider environs of the Kadavur anorthosite complex, SE India, was studied by laser ablation ICP-MS to assess the response of the U–Pb isotope system and trace element concentrations to ultrahigh-temperature (UHT) contact metamorphism (≥1000°C). Combined cathodoluminescence imaging and LA-ICP-MS analyses show that zircon grains contain detrital cores, which yield ages between 3.4Ga and 1.8Ga and exhibit a large spread in REE concentrations. These cores are associated with one or two rims that provide concordant age populations at 955±16Ma and 810±7Ma (2σ) and relatively uniform REE patterns. The older ages of ca. 955Ma record the imprints of regional early Neoproterozoic metamorphism related to Rodinia assembly. The younger age is interpreted to date anorthosite emplacement and its associated contact metamorphism during a second regional metamorphic episode, coeval with the intrusion of A-type granites in the area. Zircon grains from the country rocks away from the anorthosite show additional rims yielding ages from 590 to 490Ma corresponding to regional Pan-African tectonometamorphism. The zircon rims are either newly grown domains or represent recrystallized and re-equilibrated parts of precursor zircon. Discordance is only observed in 25% of the detrital cores. These cores have relatively high U concentrations (>400ppm), which strongly suggests that Pb loss occurred under (U)HT conditions by partial recrystallization (annealing) of zircon that had become metamict. The preservation of old concordant ages and source REE characteristics by the low-U (i.e., weakly to non metamict) detrital cores shows that pristine zircon is extremely robust to thermal disturbance and that its U–Pb systematics can remain unaffected despite multiple (U)HT metamorphic episodes. These observations imply that U–Pb ages in zircon cannot be reset by volume diffusion under any crustal conditions. Disturbance of the U–Pb system in individual zircon crystals or parts thereof can only be achieved by recrystallization of radiation damaged zircon.
      Highlights ► U–Pb system and REE abundances in zircon are robust even during (U)HT metamorphism. ► Metamictization of zircon controls recrystallization during metamorphism. ► Regional metamorphism in Kadavur (S India) at ca. 955Ma, 810Ma and 590–490Ma. ► Metamorphism at ca. 810Ma was associated with anorthosite intrusion.

      PubDate: 2015-02-24T15:47:02Z
  • Strontium sorption and precipitation behaviour during bioreduction in
           nitrate impacted sediments
    • Abstract: Publication date: 4 May 2012
      Source:Chemical Geology, Volumes 306–307
      Author(s): Clare L. Thorpe , Jonathan R. Lloyd , Gareth T.W. Law , Ian T. Burke , Samuel Shaw , Nicholas D. Bryan , Katherine Morris
      The behaviour of strontium (Sr2+) during microbial reduction in nitrate impacted sediments was investigated in sediment microcosm experiments relevant to nuclear sites. Although Sr2+ is not expected to be influenced directly by redox state, bioreduction of nitrate caused reduced Sr2+ solubility due to an increase in pH during bioreduction and denitrification. Sr2+ removal was greatest in systems with the highest initial nitrate loading and consequently more alkaline conditions at the end of denitrification. After denitrification, a limited re-release of Sr2+ back into solution occurred coincident with the onset of metal (Mn(IV) and Fe(III)) reduction which caused minor pH changes in all microcosms with the exception of the bicarbonate buffered system with initial nitrate of 100mM and final pH>9. In this system ~95% of Sr2+ remained associated with the sediment throughout the progression of bioreduction. Analysis of this pH 9 system using X-ray absorption spectroscopy (XAS) and electron microscopy coupled to thermodynamic modelling showed that Sr2+ became partially incorporated within carbonate phases which were formed at higher pH. This is in contrast to all other systems where final pH was <9, here XAS analysis showed that outer sphere Sr2+ sorption predominated. These results provide novel insight into the likely environmental fate of the significant radioactive contaminant, 90Sr, during changes in sediment biogeochemistry induced by bioreduction in nitrate impacted nuclear contaminated environments.
      Highlights ►Stronium biogeochemistry is affected by sediment bioreduction processes. ►Indigenous sediment microorganisms can mediate bioreduction across a range of nitrate concentrations. ►High nitrate leads to high pH in sediment microcosms and to partial incorporation of Sr into carbonate minerals.

      PubDate: 2015-02-24T15:47:02Z
  • Anthropogenic impact records of nature for past hundred years extracted
           from stalagmites in caves found in the Nanatsugama Sandstone Formation,
           Saikai, Southwestern Japan
    • Abstract: Publication date: 6 June 2013
      Source:Chemical Geology, Volume 347
      Author(s): Shota Uchida , Kousuke Kurisaki , Yoshiro Ishihara , Satoshi Haraguchi , Toshiro Yamanaka , Masami Noto , Kazuhisa Yoshimura
      In the Nanatsugama area, Saikai City, Nagasaki Prefecture, Japan, covered by Paleogene calcareous sandstone, the environmental change information for the past hundred years was extracted from growing stalagmites in two limestone caves. Their annual microbanding information was used for dating. From the Shimizu-do Cave stalagmites, the vegetation change from forest to grassland during 1500 to 1700 could be read using the carbon isotope and Mg/Ca ratios of the stalagmites. Before 1500, the stable carbon isotope ratios ranged from −9 to −10‰, which are characteristic of forest vegetation. From 1600 to 1700, the stable carbon isotope ratio increased (δ13C=−2‰), suggesting a drastic change to grassland vegetation probably for the purpose of collecting grasses for agricultural use. The increase in Mg/Ca due to the reduction in the biomass from 1500 to 1700 also showed the vegetation change from forest to grassland. A Ryuo-do Cave stalagmite recorded the change in the SO4 2− concentration several times during the period from 1600 to 1900. In addition to the sulfate of sea salt origin, the higher concentration and smaller δ34S of sulfate in the Ryuo-do Cave drip water compared to those in the Shimizu-do Cave drip water may be due to the oxidation of biogenic pyrite in the marine Kamashikiyama Tuff Formation, which covers the calcareous sandstone of the Nanatsugama Sandstone Formation. The oxidation of pyrite is promoted by the biological activity of sulfur-oxidizing bacteria under oxic conditions, indicating that the surface cover was repeatedly changed into non-paddy fields. The change in the SO4 2− concentration could then be related to the changes in the amount of the Nagasaki Prefecture coal production in the early 20th century and the China fossil fuel consumption in the late 20th century transported a long-distance by a monsoon from the China continent. Thus, the combination of annual microbanding information and the Mg2+ and SO4 2− concentrations and C and S stable isotope ratios of the stalagmites made it possible to extract local and/or global anthropogenic environmental changes in nature. The records extracted in this way were in good agreement with those partially extracted from ancient documents, ancient picture maps and topographical maps.
      Graphical abstract image

      PubDate: 2015-02-24T15:47:02Z
  • Nitrogen isotopes in intra-crystal coralline aragonites
    • Abstract: Publication date: 2 August 2013
      Source:Chemical Geology, Volume 351
      Author(s): Atsuko Yamazaki , Tsuyoshi Watanabe , Naoto Takahata , Yuji Sano , Urumu Tsunogai
      To assess the preservation of the nitrogen isotope composition in reef corals, nitrogen isotopes in a well-preserved Pliocene fossil coral (located in the Tartaro formation on Luzon Island, Philippines (14°N, 121°E)) and in a modern coral (Kochi, Japan (32°N, 132°E)) were analysed using stepwise heating methods. The thermal decomposition of aragonite triggered the largest release of nitrogen at 700°C for the modern coral and 550°C for the Pliocene coral. The highest rate of nitrogen gas emission occurred at the aragonite collapse temperature, indicating that organic nitrogen was bound within the intra-crystals of coralline aragonites in both corals. After the aragonite collapsed in both corals, the nitrogen isotope ratios increased due to fractionation and then decreased to values similar to those observed in bulk samples of the modern (+10.1%) and Pliocene (+4.4%) corals. These results suggested that fresh organic nitrogen was released due to the decomposition of the internal skeletal structure at higher temperatures (900–1000°C). Nitrogen isotopes in coral skeletons were preserved in intra-crystal aragonite, even in a Pliocene fossil, and stepwise heating methods were shown to be useful for determining the preservation of coralline nitrogen isotopes.

      PubDate: 2015-02-24T15:47:02Z
  • Revision of Fontes &amp; Garnier's model for the initial 14C content
           of dissolved inorganic carbon used in groundwater dating
    • Abstract: Publication date: 2 August 2013
      Source:Chemical Geology, Volume 351
      Author(s): Liang-Feng Han , L. Niel Plummer
      The widely applied model for groundwater dating using 14C proposed by Fontes and Garnier (F&G) (Fontes and Garnier, 1979) estimates the initial 14C content in waters from carbonate-rock aquifers affected by isotopic exchange. Usually, the model of F&G is applied in one of two ways: (1) using a single 13C fractionation factor of gaseous CO2 with respect to a solid carbonate mineral, εg/s, regardless of whether the carbon isotopic exchange is controlled by soil CO2 in the unsaturated zone, or by solid carbonate mineral in the saturated zone; or (2) using different fractionation factors if the exchange process is dominated by soil CO2 gas as opposed to solid carbonate mineral (typically calcite). An analysis of the F&G model shows an inadequate conceptualization, resulting in underestimation of the initial 14C values (14C0) for groundwater systems that have undergone isotopic exchange. The degree to which the 14C0 is underestimated increases with the extent of isotopic exchange. Examples show that in extreme cases, the error in calculated adjusted initial 14C values can be more than 20% modern carbon (pmc). A model is derived that revises the mass balance method of F&G by using a modified model conceptualization. The derivation yields a “global” model both for carbon isotopic exchange dominated by gaseous CO2 in the unsaturated zone, and for carbon isotopic exchange dominated by solid carbonate mineral in the saturated zone. However, the revised model requires different parameters for exchange dominated by gaseous CO2 as opposed to exchange dominated by solid carbonate minerals. The revised model for exchange dominated by gaseous CO2 is shown to be identical to the model of Mook (Mook, 1976). For groundwater systems where exchange occurs both in the unsaturated zone and saturated zone, the revised model can still be used; however, 14C0 will be slightly underestimated. Finally, in carbonate systems undergoing complex geochemical reactions, such as oxidation of organic carbon, radiocarbon ages are best estimated by inverse geochemical modeling techniques.

      PubDate: 2015-02-24T15:47:02Z
  • The rise of oxygen and the hydrogen hourglass
    • Abstract: Publication date: 20 December 2013
      Source:Chemical Geology, Volume 362
      Author(s): Kevin J. Zahnle , David C. Catling , Mark W. Claire
      Oxygenic photosynthesis appears to be necessary for an oxygen-rich atmosphere like Earth's. But available geological and geochemical evidence suggest that at least 200Myr, and possibly more than 700Myr, elapsed between the advent of oxygenic photosynthesis and the establishment of an oxygen atmosphere. The interregnum implies that at least one other necessary condition for O2 needed to be met. Here we argue that the second condition was the oxidation of the surface and crust to the point where O2 became more stable than competing reduced gases such as CH4. The cause of Earth's surface oxidation would be the same cause as it is for other planets with oxidized surfaces: hydrogen escape to space. The duration of the interregnum would have been determined by the rate of hydrogen escape and by the size of the reduced reservoir that needed to be oxidized before O2 became favored. We suggest that continental growth has been influenced by hydrogen escape, and we speculate that, if there must be an external bias to biological evolution, hydrogen escape can be that bias.

      PubDate: 2015-02-24T15:47:02Z
  • The zinc isotopic composition of siliceous marine sponges: Investigating
           nature's sediment traps
    • Abstract: Publication date: 16 September 2013
      Source:Chemical Geology, Volume 354
      Author(s): Katharine R. Hendry , Morten B. Andersen
      The zinc (Zn) content and isotopic composition of marine biogenic opal have the potential to yield information about the nutrient availability, utilization and export of particulate organic matter from surface to deep waters. Here, we report the first measurements of the Zn isotopic composition of deep-sea marine sponge skeletal elements – spicules – collected in the Southern Ocean. Our results highlight different Zn uptake and isotopic fractionation behavior between the two major siliceous sponge clades (hexactinellids and demosponges), which is most likely linked to sponge feeding strategy. Hexactinellid spicule Zn isotopic compositions are not fractionated with respect to seawater, most likely due to Zn transport via the open internal structure of the sponges. In contrast, demosponge spicules exhibit a wide range of Zn isotopic compositions that are related to the opal Zn concentration, most likely reflecting variable Zn isotope compositions in the organic matter particles on which they feed, and internal fractionation processes.

      PubDate: 2015-02-24T15:47:02Z
  • Global chemical weathering and associated P-release — The role of
           lithology, temperature and soil properties
    • Abstract: Publication date: 10 January 2014
      Source:Chemical Geology, Volume 363
      Author(s): Jens Hartmann , Nils Moosdorf , Ronny Lauerwald , Matthias Hinderer , A. Joshua West
      Because there remains a lack of knowledge about the spatially explicit distribution of chemical weathering rates at the global scale, a model that considers prominent first-order factors is compiled step by step and the implied spatial variability in weathering is explored. The goal is to fuel the discussion about the development of an “Earth System” weathering function. We use as a starting point an established model of the dependence of chemical weathering on lithology and runoff, calibrated for an island arc setting, which features very high chemical weathering rates and a strong dependence on lithology and runoff. The model is enhanced stepwise with further factors accounting for soil shielding and temperature, and the observed variation of fluxes is discussed in context of observed data from large rivers globally. Results suggest that the global soil shielding reduces chemical weathering (CW) fluxes by about 44%, compared to an Earth surface with no deeply weathered soils but relatively young rock surfaces (e.g. as in volcanic arc and other tectonically active areas). About 70% of the weathering fluxes globally derive from 10% of the land area, with Southeast Asia being a primary “hot spot” of chemical weathering. In contrast, only 50% of runoff is attributed to 10% of the land area; thus the global chemical weathering curve is to some extent disconnected from the global runoff curve due to the spatially heterogeneous climate as well as rock and soil properties. The analysis of carbonate dissolution reveals that about half of the flux is not delivered from labeled carbonate sedimentary rocks, but from trace carbonates in igneous rocks as well as from siliciclastic sediment areas containing matrix carbonate. In addition to total chemical weathering fluxes, the release of P, a nutrient that controls biological productivity at large spatial scales, is affected by the spatial correlation between runoff, lithology, temperature and soil properties. The areal abundance of deeply weathered soils in Earth's past may have influenced weathering fluxes and P-fuelled biological productivity significantly, specifically in the case of larger climate shifts when high runoff fields shift to areas with thinner soils or areas with more weatherable rocks and relatively increased P-content. This observation may be particularly important for spatially resolved Earth system models targeting geological time scales. The model is discussed against current process knowledge and geodata with focus on improving future global chemical weathering model attempts. Identified key processes and geodata demanding further research are a) the representation of flowpaths to distinguish surface runoff, interflow and baseflow contributions to CW-fluxes, b) freeze-thaw effects on chemical weathering, specifically for the northern latitudes, c) a more detailed analysis to identify to what extent the spatially heterogeneous distribution of Earth surface properties causes a decoupling of the Earth system rating functions between CW-fluxes and global runoff, as well as d) an improved understanding of where and to what extent trace or matrix carbonates in silicate-dominated rocks and sediments contribute to carbonate weathering. The latter demands e) an improved representation of carbonate content in lithological classes in the lithological representation of the Earth surface. Further improvement of the lithological database is needed for f) the age of rocks and g) the geochemistry of sediments with focus on unconsolidated sediments in the large basins. And clearly h) an improved global soil database is needed for future improvements with reliable soil depth, mineralogical composition as well as physical properties.

      PubDate: 2015-02-24T15:47:02Z
  • The biogeochemistry and bioremediation of uranium and other priority
    • Abstract: Publication date: 10 January 2014
      Source:Chemical Geology, Volume 363
      Author(s): Laura Newsome , Katherine Morris , Jonathan R. Lloyd
      Microbial metabolism has the potential to alter the solubility of a broad range of priority radionuclides, including uranium, other actinides and fission products. Of notable interest has been the biostimulation of anaerobic microbial communities to remove redox-sensitive radionuclides such as uranium U(VI) from contaminated groundwaters at nuclear sites. Particularly promising are bioreduction processes, whereby bacteria enzymatically reduce aqueous U(VI) to insoluble U(IV) coupled to oxidation of an organic electron donor; and uranium phosphate biomineralisation, in which bacterial phosphatase activity cleaves organophosphates, liberating inorganic phosphate that precipitates with aqueous U(VI) as uranyl phosphate minerals. Here we review the mechanisms of uranium bioreduction and phosphate biomineralisation and their suitability to facilitate long-term precipitation of uranium from groundwater, with particular focus on in situ trials at the US Department of Energy field sites. Redox interactions of other priority radionuclides (technetium, neptunium, plutonium, americium, iodine, strontium and caesium) are also reviewed.

      PubDate: 2015-02-24T15:47:02Z
  • Trace element budgets and (re-)distribution during subduction-zone
           ultrahigh pressure metamorphism: Evidence from Western Tianshan, China
    • Abstract: Publication date: 4 February 2014
      Source:Chemical Geology, Volume 365
      Author(s): Yuanyuan Xiao , Yaoling Niu , Huaikun Li , Huichu Wang , Xiaoming Liu , Jon Davidson
      We have conducted an LA-ICP-MS in situ trace element study of garnet, epidote group minerals, phengitic muscovite and paragonite in rocks of basaltic and sedimentary protolith from an ultrahigh pressure metamorphic belt along Western Tianshan, China. The data are used to evaluate the capacity of these minerals for hosting incompatible elements in response to subduction-zone metamorphism (SZM). The results confirm existing studies in that the presence and stability of these minerals largely control the geochemical behaviors of elements during SZM. We found that redistribution of rare earth elements (REEs), Th and U into newly-formed minerals during progressive SZM precludes the release of these elements from the down-going ocean crust, which contradicts the common perception in models of slab-dehydration and flux-melting. This suggests that additional processes, such as the involvement of supercritical fluids or hydrous melts formed at depth are required to supply these elements to the mantle wedge for arc magmatism. In addition, the ready release of large ion lithophile elements (LILEs) by different minerals, and the high immobility of REEs in rocks of basaltic protolith indicate that the contribution of altered ocean crust after SZM may not be responsible for the correlated Sr–Nd (Hf) isotope systematics observed in oceanic basalts. That is, subducted ocean crust that has gone through SZM cannot be the major source material for ocean island basalts.

      PubDate: 2015-02-24T15:47:02Z
  • Fluid flow and CO2–fluid–mineral interactions during
           CO2-storage in sedimentary basins
    • Abstract: Publication date: 13 March 2014
      Source:Chemical Geology, Volume 369
      Author(s): Niko Kampman , Mike Bickle , Max Wigley , Benoit Dubacq
      Modelling the progress of geochemical processes in CO2 storage sites is frustrated by uncertainties in the rates of CO2 flow and dissolution, and in the rates and controlling mechanisms of fluid–mineral reactions that stabilise the CO2 in geological reservoirs. Dissolution of CO2 must be controlled by the complexities of 2-phase flow of CO2 and formation brines and the smaller-scale heterogeneities in the permeability in the reservoirs which increase the fluid contact areas. The subsequent fluid mineral reactions may increase storage security by precipitating CO2 in carbonate minerals but the consequences of fluid–mineral reactions on caprock rocks or potential leakage pathways up fault zones are less certain as the CO2-charged brines may either corrode minerals or decrease permeabilities by precipitating carbonates. Observations from CO2-injection experiments and natural analogues provide important constraints on the rates of CO2 and brine flow and on the progress of CO2 dissolution and mineral–fluid reactions. In these experiments brines in contact with the propagating plume appear to rapidly saturate with CO2. Dissolution of the CO2 drives the dissolution of oxide and carbonate minerals, on times scales of days to weeks. These reactions buffer fluid pH and produce alkalinity such that carbonate dissolution moves to carbonate precipitation over time-scales of weeks to months. The dissolution of Fe-oxide grain coatings and the release of Fe to solution is important in stabilising insoluble Fe–Mg–Ca carbonate minerals but the rate limiting step for carbonate mineral precipitation is the transport of CO2-charged brines and silicate mineral dissolution rates. Observations from CO2-EOR experiments and natural analogues suggest that the silicate mineral dissolution reactions are initially fast in the low pH fluids surrounding the CO2 plume but that reaction progress over months to years drives minerals towards thermodynamic equilibrium and dissolution rates slow over 2–5 orders of magnitude as equilibrium is approached. The sluggish dissolution of silicate minerals is likely to preside over the long-term fate of the CO2 in geological reservoirs. Observations from injection experiments and natural analogues suggest that the potentially harmful trace elements mobilised by the drop in pH are immobilised as adsorbed and precipitated phases as fluid pH is buffered across mineral reaction fronts. There are very few observations of caprock exposed to CO2-rich brines. Preliminary examination of core recently recovered from scientific drilling of a natural CO2 accumulation in Utah suggests that the diffusion of CO2 into reservoir caprocks drives dissolution of Fe-oxides but subsequent precipitation of carbonate minerals likely retards the diffusion distance of the CO2. At this site thin siltstone layers are shown to be effective seals to the CO2-charged fluids, which has significant implications for the long term security of CO2 in geological reservoirs.

      PubDate: 2015-02-24T15:47:02Z
  • Isotopic evidence for reduction of anthropogenic hexavalent chromium in
           Los Alamos National Laboratory groundwater
    • Abstract: Publication date: 12 May 2014
      Source:Chemical Geology, Volume 373
      Author(s): Jeffrey M. Heikoop , Thomas M. Johnson , Kay H. Birdsell , Patrick Longmire , Donald D. Hickmott , Elaine P. Jacobs , David E. Broxton , Danny Katzman , Velimir V. Vesselinov , Mei Ding , David T. Vaniman , Steven L. Reneau , Tim J. Goering , Justin Glessner , Anirban Basu
      Reduction of toxic Cr(VI) to less toxic Cr(III) is an important process for attenuating Cr(VI) transport in groundwater. This process results in immobilization of chromium as Cr(III) and effectively decreases the overall mobility of the chromium inventory. During both abiotic and biotic reduction of Cr(VI) to Cr(III), a kinetic isotope effect occurs in which the lighter isotope, 52Cr, reacts preferentially, leaving the remaining dissolved Cr(VI) enriched in the heavier isotope, 53Cr. Cr isotopes have proven to be a useful tool for estimating the magnitude of Cr(VI) reduction and for determining where in a hydrologic system reduction is occurring. In this paper, we discuss patterns of reduction in perched-intermediate and regional aquifer systems contaminated with Cr(VI) related to historical use of potassium dichromate as an anticorrosion agent in cooling towers at a power plant at the Los Alamos National Laboratory in northern New Mexico. We utilize Cr isotopes to assess the relative effects of mixing and reduction on measured δ53Cr in groundwater, with an emphasis on where in the system reduction occurs. Chromium isotope measurements provide strong evidence for reduction of Cr(VI) in vadose zone basalts.

      PubDate: 2015-02-24T15:47:02Z
  • U–Pb dating of cements in Mesozoic ammonites
    • Abstract: Publication date: 29 May 2014
      Source:Chemical Geology, Volume 376
      Author(s): Q. Li , R.R. Parrish , M.S.A. Horstwood , J.M. McArthur
      Dating sedimentary carbonates using the U-Pb method can help improve the Phanerozoic timescale. Using a novel combination of laser-ablation, multi-collector, inductively-coupled-plasma, mass-spectrometry (LA–MC–ICP–MS) and thermal ionization multi-collector mass spectrometry (TIMS), U-Pb numerical ages were obtained on early-diagenetic calcite cements in Jurassic ammonites in which concentrations of U range from 0.47 to 5.3ppm. The calcite cements of two ammonites, IS1 and IS2, from the uppermost Bifrons Zone of the Toarcian (179–180Ma) of the UK, gave TIMS-normalized LA U–Pb dates of 164.9±5.3Ma and 166.7±4.8Ma respectively. Normalizing LA–ICP–MC–MS data to an in-house calcite standard gave a more precise date of 165.5±3.3Ma for IS1 cement. An unzoned ammonite, SS2, of Bajocian age (168–170Ma) yield a TIMS-normalized LA U–Pb age of 158.8±4.3Ma for its early-diagenetic cement. Both the combined LA–MC–ICP–MS and TIMS approach, and the use of a calcite laser ablation standard can result in accurate ages of cements with uncertainties of 2–3% (2σ). The later, however, is more efficient and precise. These U-Pb dates of cements are 10 to 20Myr younger than the numerical ages of the biostratigraphic intervals from which the ammonites derive. The U-Pb dates are taken to represent the time at which the aragonite shell of the ammonite inverted to calcite and released its U to precipitate in a late-diagenetic alteration of early-diagenetic fringing cements. Concentrations of U and Pb in a range of other pristine biogenic carbonates were found too low (U<0.01ppm) for meaningful dating using laser ablation method.

      PubDate: 2015-02-24T15:47:02Z
  • Late season mobilization of trace metals in two small Alaskan arctic
           watersheds as a proxy for landscape scale permafrost active layer dynamics
    • Abstract: Publication date: 14 August 2014
      Source:Chemical Geology, Volume 381
      Author(s): Amanda J. Barker , T.A. Douglas , A.D. Jacobson , J.W. McClelland , A.G. Ilgen , M.S. Khosh , G.O. Lehn , T.P. Trainor
      Increasing air temperatures in the Arctic have the potential to degrade permafrost and promote the downward migration of the seasonally thawed active layer into previously frozen material. This may expose frozen soils to mineral weathering that could affect the geochemical composition of surface waters. Determining watershed system responses to drivers such as a changing climate relies heavily on understanding seasonal controls on freshwater processes. The majority of studies on elemental concentrations in Arctic river systems have focused on sampling only from spring snowmelt to the summer season. Consequently, there remains a limited understanding of surface water geochemistry, particularly with respect to trace metals, during late fall and early winter. To examine the variability of metal concentrations as a function of seasonality, we measured trace metal concentrations from spring melt to fall freeze-up in 2010 in two high Arctic watersheds: Imnavait Creek, North Slope, Alaska and Roche Mountanee Creek, Brooks Range, Alaska. We focused on aluminum (Al), barium (Ba), iron (Fe), manganese (Mn), nickel (Ni) and zinc (Zn). Concentrations of ‘dissolved’ (<0.45μm) Al, Ba, Fe, and Mn in Imnavait Creek waters and Ba in Roche Mountanee waters were highest in late fall/early winter. To link observed surface water concentrations at Imnavait Creek to parent soil material we analyzed the elemental composition of a soil core from the watershed and tracked the soil temperatures as a function of time and depth. The results from this study show a distinct seasonal signature of trace metal concentrations in late fall that correlates with the depth of the thawed active layer.

      PubDate: 2015-02-24T15:47:02Z
  • 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: 2015-02-24T15:47:02Z
  • 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: 2015-02-24T15:47:02Z
  • An optimized method for stable isotope analysis of tree rings by
           extracting cellulose directly from cross-sectional laths
    • Abstract: Publication date: 30 January 2015
      Source:Chemical Geology, Volumes 393–394
      Author(s): Akira Kagawa , Masaki Sano , Takeshi Nakatsuka , Tsutomu Ikeda , Satoshi Kubo
      Stable isotopes in tree-ring α-cellulose are valued as environmental proxies and their use is steadily increasing; however, preparation of α-cellulose is a bottleneck in isotope analysis. Recent methodological breakthrough for extracting tree-ring α-cellulose directly from tree-ring cross-sectional laths drastically increased the throughput of tree-ring isotope data. In this paper, we evaluate our recently designed “cross-section” method. This method employs polytetrafluoroethylene (PTFE) cases, enabling direct extraction of α-cellulose from 1-mm thick tree-ring laths, in combination with fixation sheets to prevent disintegration of freeze-dried α-cellulose laths. Perforated PTFE cases are easily producible at an affordable cost. They are made of commonly available lab consumables in catalogs and do not require specially made PTFE parts. Freeze-dried α-cellulose laths preserved distinct anatomical structure, enabling precise separation at the tree-ring boundaries. Once separated from a lath, tree-ring α-cellulose can be weighed directly into silver or tin capsules for analysis. We checked chemical purity of α-cellulose prepared by the cross-section method from five tree species (larch, pine, spruce, beech, and oak). Residual lignin and hemicellulose contents were quantitatively assessed by Fourier transform infrared spectrometry and gas chromatography. The average chemical purity of α-cellulose laths from the five species was 94.5%, similar to the chemical purity of α-cellulose prepared with the standard Jayme-Wise method. Both oxygen and carbon isotope values of α-cellulose prepared by the cross-section method also closely matched those prepared by the standard method. We conclude that, by overhauling the method of α-cellulose preparation for tree-ring isotope analysis, we increased throughput of tree-ring oxygen and carbon isotope data without sacrificing sample purity.

      PubDate: 2015-02-24T15:47:02Z
  • 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: 2015-02-24T15:47:02Z
  • Determination of in situ dissolved inorganic carbon concentration and
           alkalinity for marine sedimentary porewater
    • Abstract: Publication date: 10 November 2014
      Source:Chemical Geology, Volume 387
      Author(s): Justine Sauvage , Arthur J. Spivack , Richard W. Murray , Steven D'Hondt
      Dissolved inorganic carbon (DIC) concentration and total alkalinity in marine sediment vary with biological activity, mineral diagenesis and past bottom ocean water composition. Reliable interpretation of this data is often compromised due to precipitation of calcium carbonate (CaCO3) during sediment recovery, processing and sample storage. Here we present and test a method that corrects for this precipitation and consequently allows quantification of in situ carbonate system chemistry. Our method relies on the over-determination of the dissolved carbonate system by (i) measuring DIC, alkalinity and calcium, and (ii) explicitly assuming CaCO3 saturation in the sediment. We experimentally tested this method using data from Integrated Ocean Drilling Program (IODP) Site U1368 in the South Pacific Gyre. Our results show that we can accurately reproduce in situ aqueous carbonate system chemistry if DIC, alkalinity and calcium concentration are measured simultaneously. At Site U1368, the correction for sampling associated precipitation is equivalent to 4.5 and 8.9% of the measured DIC and alkalinity, respectively. The method is well suited for any sediment porewater that is saturated with respect to calcium carbonate; consequently, it is applicable for approximately 50% of the global oceanic seafloor.

      PubDate: 2015-02-24T15:47:02Z
  • Characterization of cores from an in-situ recovery mined uranium deposit
           in Wyoming: Implications for post-mining restoration
    • Abstract: Publication date: 18 December 2014
      Source:Chemical Geology, Volume 390
      Author(s): G. WoldeGabriel , H. Boukhalfa , S.D. Ware , M. Cheshire , P. Reimus , J. Heikoop , S.D. Conradson , O. Batuk , G. Havrilla , B. House , A. Simmons , J. Clay , A. Basu , J.N. Christensen , S.T. Brown , D.J. DePaolo
      In-situ recovery (ISR) of uranium (U) from sandstone-type roll-front deposits is a technology that involves the injection of solutions that consist of ground water fortified with oxygen and carbonate to promote the oxidative dissolution of U, which is pumped to recovery facilities located at the surface that capture the dissolved U and recycle the treated water. The ISR process alters the geochemical conditions in the subsurface creating conditions that are more favorable to the migration of uranium and other metals associated with the uranium deposit. There is a lack of clear understanding of the impact of ISR mining on the aquifer and host rocks of the post-mined site and the fate of residual U and other metals within the mined ore zone. We performed detailed petrographic, mineralogical, and geochemical analyses of several samples taken from about 7m of core of the formerly the ISR-mined Smith Ranch–Highland uranium deposit in Wyoming. We show that previously mined cores contain significant residual uranium (U) present as coatings on pyrite and carbonaceous fragments. Coffinite was identified in three samples. Core samples with higher organic (>1wt.%) and clay (>6–17wt.%) contents yielded higher 234U/238U activity ratios (1.0–1.48) than those with lower organic and clay fractions. The ISR mining was inefficient in mobilizing U from the carbonaceous materials, which retained considerable U concentrations (374–11,534ppm). This is in contrast with the deeper part of the ore zone, which was highly depleted in U and had very low 234U/238U activity ratios. This probably is due to greater contact with the lixiviant (leaching solution) during ISR mining. EXAFS analyses performed on grains with the highest U and Fe concentrations reveal that Fe is present in a reduced form as pyrite and U occurs mostly as U(IV) complexed by organic matter or as U(IV) phases of carbonate complexes. Moreover, U–O distances of ~2.05Å were noted, indicating the potential formation of other poorly defined U(IV/VI) species. We also noted a small contribution from UO at 1.79Å, which indicates that U is partially oxidized. There is no apparent U–S or U–Fe interaction in any of the U spectra analyzed. However, SEM analysis of thin sections prepared from the same core material reveals surficial U associated with pyrite which is probably a minor fraction of the total U present as thin coatings on the surface of pyrite. Our data show the presence of different structurally variable uranium forms associated with the mined cores. U associated with carbonaceous materials is probably from the original U mobilization that accumulated in the organic matter-rich areas under reducing conditions during shallow burial diagenesis. U associated with pyrite represents a small fraction of the total U and was likely deposited as a result of chemical reduction by pyrite. Our data suggest that areas rich in carbonaceous materials had limited exposure to the lixiviant solution, continue to be reducing, and still hold significant U resources. Because of their limited access to fluid flow, these areas might not contribute significantly to post-mining U release or attenuation. Areas with pyrite that are accessible to fluids seem to be more reactive and could act as reductants and facilitate U reduction and accumulation, limiting its migration.

      PubDate: 2015-02-24T15:47:02Z
  • Simultaneous determination of δ11B and B/Ca ratio in marine biogenic
           carbonates at nanogram level
    • Abstract: Publication date: 21 January 2015
      Source:Chemical Geology, Volume 392
      Author(s): Karina Kaczmarek , Ingo Horn , Gernot Nehrke , Jelle Bijma
      In this study we introduce a new in situ technique which allows the determination of the boron isotopic composition and B/Ca ratios simultaneously at the nanogram level using a combination of optical emission spectroscopy and multiple ion counting MC ICP-MS with laser ablation. This technique offers a new application in the paleo-field of oceanography and climatology since small samples like e.g. single foraminiferal shells can be analyzed. The simultaneous determination of the boron isotopic composition and B/Ca ratios provides two independent proxies which allow the reconstruction of the full carbonate system. To test the new technique we performed measurements on the cultured, benthic foraminifer Amphistegina lessonii. Our results yielded an average boron isotopic composition δ11B=18.0±0.83‰ (SD) with an average internal precision of 0.52‰ (RSE). The boron concentration was 53±7μg/g (SD). These results agree with the range reported in the literature. The reconstructed mean pH value is in excellent agreement with the measured pH of the seawater in which the foraminifers grew. The analysis of a foraminifer consumed approximately 1200ng calcium carbonate containing ca. 0.06ng boron. Compared to bulk analytical methods, this new technique requires less material and reduces the time for sample preparation.

      PubDate: 2015-02-24T15:47:02Z
  • Chemical and isotopic evidence for accelerated bacterial sulphate
           reduction in acid mining lakes after addition of organic carbon:
           laboratory batch experiments
    • Abstract: Publication date: 15 April 2004
      Source:Chemical Geology, Volume 204, Issues 3–4
      Author(s): Andrea Fauville , Bernhard Mayer , René Frömmichen , Kurt Friese , Jan Veizer
      Acid mine drainage is a widespread environmental problem in Lusatia in the eastern part of Germany, where extensive brown coal and lignite mining was carried out during the last century. As a result of oxidation of pyrite in strip mine dumps of this region, numerous mine drainage lakes have pH values <3 and are characterized by high concentrations of dissolved sulphate and ferrous iron. Using chemical and isotopic techniques, we tested in laboratory bottle experiments the extent to which the addition of degradable organic carbon to acid lake water and sediments could stimulate bacterial (dissimilatory) sulphate reduction. The overall goal was to evaluate whether bacterial sulphate reduction, and subsequent formation of sedimentary sulphide minerals, could generate enough alkalinity to potentially neutralize the acid lakes in Lusatia. Pyruvate, glucose, succinic acid, lactate, ethanol, acetate, and various industrial by-products were added in various concentrations to original lake water and sediments in glass bottles and incubated between 4 and 12 weeks in darkness. The extent of bacterial sulphate reduction was determined by analyzing the concentrations and the isotopic compositions of sulphate in the water column, and the contents and δ 34S values of various sedimentary sulphur compounds at the beginning and the end of the experiments. Bacterial sulphate reduction was significantly enhanced after addition of pure substances (lactate, pyruvate, acetate, and ethanol) resulting in elevated pH values (4.5 to 6.0) in the bottle water at the end of the experiments. Cheap industrial by-products (whey, molasses, Pfezi-granules, and Carbokalk) as organic carbon source were also shown to be effective in enhancing bacterial sulphate reduction, thereby increasing the pH value of the bottle water. In general, the addition of selected organic carbon compounds enhanced sufficiently bacterial sulphate reduction and the formation of sedimentary sulphur minerals, generating enough alkalinity for raising the pH of the bottle water to near-neutral values. Further research in enclosures and under field conditions is needed to determine whether this technique constitutes a feasible remediation strategy for the acid lakes in Lusatia.

      PubDate: 2015-02-24T15:47:02Z
  • Origin of fluids and anhydrite precipitation in the sediment-hosted
           Grimsey hydrothermal field north of Iceland
    • Abstract: Publication date: 15 December 2003
      Source:Chemical Geology, Volume 202, Issues 1–2
      Author(s): T Kuhn , P.M Herzig , M.D Hannington , D Garbe-Schönberg , P Stoffers
      The sediment-hosted Grimsey hydrothermal field is situated in the Tjörnes fracture zone (TFZ) which represents the transition from northern Iceland to the southern Kolbeinsey Ridge. The TFZ is characterized by a ridge jump of 75 km causing widespread extension of the oceanic crust in this area. Hydrothermal activity occurs in the Grimsey field in a 300 m×1000 m large area at a water depth of 400 m. Active and inactive anhydrite chimneys up to 3 meters high and hydrothermal anhydrite mounds are typical for this field. Clear, metal-depleted, up to 250 °C hydrothermal fluids are venting from the active chimneys. Anhydrite samples collected from the Grimsey field average 21.6 wt.% Ca, 1475 ppm Sr and 3.47 wt.% Mg. The average molar Sr/Ca ratio is 3.3×10−3. Sulfur isotopes of anhydrite have typical seawater values of 22±0.7‰ δ34S, indicating a seawater source for SO4 2−. Strontium isotopic ratios average 0.70662±0.00033, suggesting the precipitation of anhydrite from a hydrothermal fluid–seawater mixture. The endmember of the venting hydrothermal fluids calculated on a Mg-zero basis contains 59.8 μmol/kg Sr, 13.2 mmol/kg Ca and a 87Sr/86Sr ratio of 0.70634. The average Sr/Ca partition coefficient between the hydrothermal fluids and anhydrite of about 0.67 implies precipitation from a non-evolved fluid. A model for fluid evolution in the Grimsey hydrothermal field suggests mixing of upwelling hydrothermal fluids with shallowly circulating seawater. Before and during mixing, seawater is heated to 200–250 °C which causes anhydrite precipitation and probably the formation of an anhydrite-rich zone beneath the seafloor.

      PubDate: 2015-02-24T15:47:02Z
  • 87Sr/86Sr, δ13C and δ18O evolution of Phanerozoic seawater
    • Abstract: Publication date: 30 September 1999
      Source:Chemical Geology, Volume 161, Issues 1–3
      Author(s): Ján Veizer , Davin Ala , Karem Azmy , Peter Bruckschen , Dieter Buhl , Frank Bruhn , Giles A.F. Carden , Andreas Diener , Stefan Ebneth , Yves Godderis , Torsten Jasper , Christoph Korte , Frank Pawellek , Olaf G. Podlaha , Harald Strauss
      A total of 2128 calcitic and phosphatic shells, mainly brachiopods with some conodonts and belemnites, were measured for their δ18 O , δ13 C and 87 Sr/86 Sr values. The dataset covers the Cambrian to Cretaceous time interval. Where possible, these samples were collected at high temporal resolution, up to 0.7 Ma (one biozone), from the stratotype sections of all continents but Antarctica and from many sedimentary basins. Paleogeographically, the samples are mostly from paleotropical domains. The scanning electron microscopy (SEM), petrography, cathodoluminescence and trace element results of the studied calcitic shells and the conodont alteration index (CAI) data of the phosphatic shells are consistent with an excellent preservation of the ultrastructure of the analyzed material. These datasets are complemented by extensive literature compilations of Phanerozoic low-Mg calcitic, aragonitic and phosphatic isotope data for analogous skeletons. The oxygen isotope signal exhibits a long-term increase of δ18 O from a mean value of about −8‰ (PDB) in the Cambrian to a present mean value of about 0‰ (PDB). Superimposed on the general trend are shorter-term oscillations with their apexes coincident with cold episodes and glaciations. The carbon isotope signal shows a similar climb during the Paleozoic, an inflexion in the Permian, followed by an abrupt drop and subsequent fluctuations around the modern value. The 87 Sr/86 Sr ratios differ from the earlier published curves in their greater detail and in less dispersion of the data. The means of the observed isotope signals for 87 Sr/86 Sr , δ18 O , δ13 C and the less complete δ34 S (sulfate) are strongly interrelated at any geologically reasonable (1 to 40 Ma) time resolution. All correlations are valid at the 95% level of confidence, with the most valid at the 99% level. Factor analysis indicates that the 87 Sr/86 Sr , δ18 O , δ13 C and δ34 S isotope systems are driven by three factors. The first factor links oxygen and strontium isotopic evolution, the second 87 Sr/86 Sr and δ34 S , and the third one the δ13 C and δ34 S . These three factors explain up to 79% of the total variance. We tentatively identify the first two factors as tectonic, and the third one as a (biologically mediated) redox linkage of the sulfur and carbon cycles. On geological timescales (≥1 Ma), we are therefore dealing with a unified exogenic (litho-, hydro-, atmo-, biosphere) system driven by tectonics via its control of (bio)geochemical cycles.

      PubDate: 2015-02-24T15:47:02Z
  • The Mg/Ca–temperature relationship in brachiopod shells: Calibrating
           a potential palaeoseasonality proxy
    • Abstract: Publication date: 18 March 2015
      Source:Chemical Geology, Volume 397
      Author(s): Scott Butler , Trevor R. Bailey , Caroline H. Lear , Gordon B. Curry , Lesley Cherns , Iain McDonald
      Brachiopods are long-lived, long-ranging, extant organisms, of which some groups precipitate a relatively diagenetically stable low magnesium calcite shell. Previous work has suggested that the incorporation of Mg into brachiopod calcite may be controlled by temperature (Brand et al., 2013). Here we build upon this work by using laser ablation sampling to define the intra-shell variations in two modern brachiopod species, Terebratulina retusa (Linnaeus, 1758) and Liothyrella neozelanica (Thomson, 1918). We studied three T. retusa shells collected live from the Firth of Lorne, Scotland, which witnessed annual temperature variations on the order of 7°C, in addition to four L. neozelanica shells, which were dredged from a water depth transect (168–1488m) off the north coast of New Zealand. The comparison of intra-shell Mg/Ca profiles with shell δ18O confirms a temperature control on brachiopod Mg/Ca and supports the use of brachiopod Mg/Ca as a palaeoseasonality indicator. Our preliminary temperature calibrations are Mg/Ca=1.76±0.27 e(0.16±0.03)T, R 2 =0.75, for T. retusa and Mg/Ca=0.49±1.27 e(0.2±0.11)T, R 2 =0.32, for L. neozelanica (errors are 95% confidence intervals).

      PubDate: 2015-02-07T18:04:02Z
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