Lacustrine oxygen isotope record of 20(th)-century climate change in central Europe: evaluation of climatic controls on oxygen isotopes in precipitation

We report oxygen isotope data from a 108-yr (1885-1993) sequence with annua l laminae of bio-induced authigenic calcite in a frozen core from Baldegger see, a small lake in Central Switzerland. These isotope results provide pro xy data on the isotopic composition of past precipitation in the Baldeggers ee catchment region and are quantitatively compared with instrumental clima te data (i.e. mean annual air temperature and atmospheric circulation patte rn indices) to evaluate climatic controls on oxygen isotopes in precipitati on. Monitoring the isotope hydrology of Baldeggersee demonstrates that the oxyg en isotopic composition of the lake water is controlled by the isotopic com position of local atmospheric precipitation (delta O-18(p)) and that the is otopic signal of precipitation is preserved, albeit damped, in the lake cal cite oxygen isotope record (delta O-18(c)). Comparison of the calcite oxyge n isotope proxy for delta O-18(p) in the catchment with historical mean ann ual air temperature measurements from Bern, Switzerland confirms that authi genic calcite reliably records past annual air temperature in the region. T his delta O-18(c)/temperature relationship is calculated to be 0.39 parts p er thousand/degreesC for the period 1900-1960, based on an isotope mass-bal ance model for Baldeggersee. An exception is a 0.8 parts per thousand anoma lous negative shift in calcite delta O-18 values since the 1960s. Possible explanations for this recent delta O-18(c) shift, as it is not related to m ean annual air temperature, include changes in delta O-18(p) due to synopti c circulation patterns. In particular, the 0.8 parts per thousand negative shift coincides with a trend towards a more dominant North Atlantic Oscilla tion (NAO) index. This circulation pattern would tend to bring more isotopi cally more negative winter precipitation to the region and could account fo r the 0.8 parts per thousand offset in delta O-18(c) data.