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Chemical geology
Elsevier
Chemical geology

Elsevier

0009-2541

Chemical geology/Journal Chemical geologySCIISTPAHCIEI
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    Long-term elemental trends in drip waters from monitoring Bunker Cave: New insights for past precipitation variability

    Riechelmann, Dana Felicitas ChristineRiechelmann, SylviaSchroeder-Ritzrau, Andrea
    15页
    查看更多>>摘要：A seven-year (2006-2013) monitoring was conducted in Bunker Cave, northwest Germany. The monitoring programme incorporates cave air temperature, the amount of precipitation, drip rates at five drip sites (TS 1, 2, 3, 5, 8) in the cave, and the analyses of cation (Ca2+, Mg2+, S-r2+, Na+, K+) and anion (HCO3-, NO3-, SO42-, Cl-) concentrations of rain, soil, and drip waters with a monthly (2006-2011) and bimonthly (2012-2013) interval. Further, the Ca, Mg, and Sr concentrations of the two host rock components, limestone and dolomite, were analysed. The analysis of rain, soil, and drip waters allowed to distinguish between geogenic and anthropogenic influences on the hydrochemistry of the cave. The limestone is the source of Ca2+, Mg2+, Sr2+, and HCO3- in all drip waters, as well as SO42- from pyrite dissolution from the limestone. Chloride and NO3- were inserted by anthropogenic sources such as aerosols from mad salt, fertilisers, and industry. Drip site TS 1 shows a strong seasonal drip rate, which is simultaneous with a seasonal variation in ion concentrations (Ca2+, Mg2+, Sr2+, HCO3-, SO42-, induced by younger water transported by fracture flow during episodes of fast drip rates. Drip water at drip site TS 5 is recharged by water which probably dissolves prior calcite precipitation (PCP), resulting in low and constant Mg2+ and Sr2+ concentrations. Drip water at drip sites TS 2, 3, and 8 are mainly influenced by PCP, whereas drip waters at TS 2 and 8 are additionally influenced by incongruent dolomite dissolution indicated by their increasing Mg2+ and stable Sr2+ concentrations. Furthermore, Cl- and SO42- concentrations show increasing and decreasing long-term trends for these three PCP-influenced drip waters at sites TS 2, 3, and 8, respectively. The decreasing pattern in infiltration/precipitation from 2000 to 2013 is not very pronounced; however, the continuous draining of the aquifer is visible in the decreasing drip rates over the seven-year monitoring period. This is reflected in the long-term increasing trends of Mg/Ca, Sr/Ca ratios induced by PCP, and Cl- concentrations, as well as in the decreasing long-term trend of the SO42- concentrations in drip waters of drip sites TS 2, 3, and 8. For a better understanding of the cave system with its complex processes, a multi-element-proxy long-term cave monitoring of rain, soil, and drip waters appears to be a reasonable step to be followed by the interpretation of the element proxies of speleothems. This monitoring emphasizes Mg/Ca and Sr/Ca ratios, as well as Cl- and in particular SO42- concentrations as potential past infiltration/precipitation proxies. Mg/Ca and Sr/Ca ratios are the strongest proxies to reconstruct infiltration/precipitation of measured elements in stalagmites, in cases Sr/Ca is not overprinted by growth rate influences. In addition, the sulphur concentration in speleothems may be an infiltration/precipitation proxy in the case of a geogenic source from the host rock.
      原文链接:
    • NSTL
    • Elsevier

    Uranium isotope cycling on the highly productive Peruvian margin

    Bruggmann, S.Gilleaudeau, G. J.Romaniello, S. J.Severmann, S....
    12页
    查看更多>>摘要：Uranium isotopes (delta U-238 values) in ancient sedimentary rocks (shales, carbonate rocks) are widely used as a tool to reconstruct paleo-redox conditions, but the behaviour of U isotopes under modern non-sulfidic anoxic vs. oxic conditions remains poorly constrained. We present U concentration and isotope data for modern sediments from the Peruvian margin, a highly productive open ocean environment with a range of redox conditions. To investigate U in different host fractions of the sediment (reactive, silicate, and HNO3-soluble fraction), we conducted a series of sequential extractions. Detrital-corrected authigenic U isotope compositions (delta U-238(auth)) in sediments deposited beneath an oxic water column show little deviation from the dissolved seawater U source, while anoxically deposited sediments have delta U-238(auth) values that are up to 0.4 parts per thousand heavier compared to seawater delta U-238. Under anoxic, non-euxinic conditions, the U isotope offset between sediment and seawater is larger compared with oxic, but significantly smaller when compared with euxinic conditions from the literature. The results from sequential extractions show that the reactive sediment fraction records more pronounced differences in delta U-238(reactive) than delta U-238(auth) values depending on the oxidation state of the overlying water column. Furthermore, we found a strong correlation between total organic carbon (TOC) and both U concentrations (U-auth) and delta U-238(auth) values (R-2 = 0.70 and 0.94, respectively) at the persistently anoxic site that we examined. These correlations can be caused by several processes including U isotope fractionation during microbially-mediated U reduction at the sediment-water interface (diffusive U input), during sorption onto and/or incorporation into organic matter in the water column (particulate U input) and diagenetic redistribution of U, or a combination of these processes. Our data show that several factors can influence delta U-238 values including oxidation state of U, the presence or absence of hydrogen sulfide and organic matter. These findings add new constraints to the degree of U isotope fractionation associated with U incorporation into sediments in different low-oxygen environments, thus aiding in interpretation of ancient paleo-redox conditions from U isotope data.
      原文链接:
    • NSTL
    • Elsevier