A study of oxygen isotopic fractionation during bio-induced calcite precipitation in eutrophic Baldeggersee,Switzerland |
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| Institution: | 1. Instituto de Investigaciones en Biodiversidad y Medioambiente (INIBIOMA, CONICET-UNCo), Av. de los Pioneros 2350 (8400), S.C. de Bariloche, Río Negro, Argentina;2. Sede Alto Valle y Valle Medio, Universidad Nacional de Río Negro, Isidro Lobo 516, Gral. Roca, Río Negro, Argentina;3. Museo Argentino de Ciencias Naturales “Bernardino Rivadavia”, CONICET. Av. Ángel Gallardo 470 (C1405DJR), Buenos Aires, Argentina;4. Área de Paleontología, Departamento de Geología, Universidad de Buenos Aires, Pabellón 2, Ciudad Universitaria (C1428EGA), Buenos Aires, Argentina;5. Consejo Nacional de Investigaciones Científicas y Técnicas (CONICET), Argentina;1. Institute of Geochemistry and Petrology, ETH Zurich, 8092 Zurich, Switzerland;2. Institut für Mineralogie, Universität Münster, 48149 Münster, Germany;3. Institute of Geological Sciences, University of Bern, 3012 Bern, Switzerland;4. Laboratoire de Géologie de Lyon, ENS Lyon and UCBL, UMR 5276, CNRS, France |
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| Abstract: | In order to better understand environmental factors controlling oxygen isotope shifts in autochthonous lacustrine carbonate sequences, we undertook an extensive one-year study (March, 1995 to February, 1996) of water-column chemistry and daily sediment trap material from a small lake in Central Switzerland. Comparisons between calculated equilibrium isotope values, using the fractionation equation of Friedman and O’Neil, (1977) and measured oxygen isotope ratios of calcite in the sediment-traps reveal that oxygen isotopic values of autochthonous calcite (δ18O) are in isotopic equilibrium with ambient water during most of the spring and summer, when the majority of the calcite precipitates. In contrast, small amounts of calcite precipitated in early-spring and again in late-autumn are isotopically depleted in 18O relative to the calculated equilibrium values, by as much as 0.8‰. This seasonally occurring apparent isotopic nonequilibrium is associated with times of high phosphorous concentrations, elevated pH (～8.6) and increased CO32?] (～50 μmol/l) in the surface waters. The resulting weighted average δ18O value for the studied period is ?9.6‰, compared with a calculated equilibrium δ18O value of ?9.4‰. These data convincingly demonstrate that δ18O of calcite are, for the most part, a very reliable proxy for temperature and δ18O of the water. |
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