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Climatic and anthropogenic influence on the stable isotope record from bulk carbonates and ostracodes in Lake Neuchatel, Switzerland, during the last two millennia.

Filippi, M.L., Lambert, P., Hunziker, J., Kubler, B. and Bernasconi, S. 1999; Journal of Paleolimnology 21: 19-34


Lake Neuchåtel is a medium sized, hard-water lake, lacking varved sediments, situated in the western Swiss Lowlands at the foot of the Jura Mountains. Stable isotope data (delta18O and delta13C) from both bulk carbonate and ostracode calcite in an 81 cm long, radiocarbon-dated sediment core represent the last 1500 years of Lake Neuchåtel''s environmental history. Comparison between this isotopic and other palaeolimnologic data (mineralogical, geochemical, palynological, etc.) helps to differentiate between anthropogenic and natural factors most recently affecting the lake. An increase in lacustrine productivity (450–650AD ca), inferred from the positive trend in delta13C values of bulk carbonate, is related to medieval forest clearances and the associated nutrient budget changes. A negative trend in both the bulk carbonate and ostracode calcite delta18O values between approximately 1300 and 1500AD, is tentatively interpreted as due to a cooling in mean air temperature at the transition from the Medieval Warm Period to the Little Ice Age. Negative trends in bulk carbonate delta18O and delta13C values through the uppermost sediments, which have no equivalent in ostracode calcite isotopic values, are concomitant with the recent onset of eutrophication in the lake. Isotopic disequilibrium during calcite precipitation, probably due to kinetic factors in periods of high productivity is postulated as the mechanism to explain the associated negative isotopic trends, although the effect of a shift of the calcite precipitation towards the warmer months cannot be excluded.