Sedimentary geochemical response to human impact on Lake Nõmmejärv, Estonia (original) (raw)

2011, Estonian Journal of Ecology

In order to assess the geochemical and stable isotopic response of recent sediments to the changing anthropogenic influence on Lake Nõmmejärv, bulk geochemistry determined by XRF spectroscopy and carbon and oxygen stable isotopes in carbonates (δ 13 C carb and δ 18 O carb ) were analysed. A time scale was defined by the 210 Pb dating method. The most significant change in sediment composition was a steep transition from organic to mineral rich sediment at the beginning of the 1970s. Our data confirm that this shift was caused by an abrupt influx of allochthonous mineral matter (Si, Al, K, Rb, Ti, Zr) derived from mine waters that started to be drained from an oil shale mine into the lake through an artificial channel. Since then, the minerogenic sedimentation remains high but after the initial peak, the proportion of siliciclastic matter decreases in favour of CaCO 3 accumulation. During the last 40 years the carbonate content increased from 7% to 40%. Geochemical evidence supports the assumption that the key mechanism in the formation of carbonates was biologically mediated authigenic CaCO 3 precipitation. In addition, decrease in sediment organic matter due to high minerogenic sedimentation forms more favourable conditions for the preservation of the accumulated CaCO 3 . The shift in the δ 13 C carb values during the 1970s and 1980s indicates potential links between CaCO 3 precipitation and the increase in lake productivity.

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