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

Abstract

Lake Neuchatel 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 (δ18O and δ13C) from both bulk carbonate and ostracode calcite in an 81 cm long, radiocarbon-dated sediment core represent the last 1500 years of Lake Neuchatel'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 δ13C 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 δ18O 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 δ18O and δ13C 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.