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 (delta(18)O and delta(13)C) 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 delta(13)C 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
delta(18)O 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 delta(18)O and delta(13)C 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.