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Abstract

A mineralogical, geochemical, and rock-magnetic investigation of sediments deposited during the last 300 years in Lake Greifen, a hard-water lake with moderate sulfate concentrations (<250 μmol/L) and seasonal anoxia, shows that both authigenic single-domain biogenic magnetite and multidomain detrital titanomagnetite were preserved within the bioturbated marls deposited prior to the onset of anthropogenically induced eutrophication. Subsequently, in response to a gradual change from oxic to dysaerobic to anoxic bottom-waters, the deposition of organic carbon-rich varved sediments occurred and the degree of magnetite preservation decreased as altered diagenetic conditions resulted in the rapid dissolution and sulfidization of the biogenic and detrital magnetite. The occurrence of both biogenically produced magnetite and detrital titano-magnetite within the upper 4 cm of sediment indicates that (1) biogenic magnetite may be produced within the near surface organic carbon-rich sediments, probably on an annual basis when the overlying waters are oxygenated, and (2) detrital magnetite is continuously deposited. Changes in magnetic properties below this zone of surface magnetite production and microscopic examination of corroded fine-grained biogenic magnetite extracted from this interval indicate the rapid destruction of the most recently produced (or deposited) magnetite. Our findings demonstrate that (1) lacustrine sedimentary magnetic properties may reflect redox conditions, which are in the case of Lake Greifen determined by productivity, and (2) rapid destruction and sulfidization of fine-grained and coarse-grained magnetite can occur in lacustrine systems that are characterized by high productivity, low available lake-water sulfate, low concentrations of dissolved sulfide, and rapid sediment accumulation rates. These findings differ from marine studies in which magnetite dissolution and sulfidization is postulated to occur in systems characterized by high productivity, high concentrations of dissolved sulfide, and low sediment accumulation rates. Based on our observations, we propose that microbially mediated processes are contributing, either directly or indirectly, not only to the authigenesis of magnetite in the Lake Greifen sediments but also to its destruction.

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