Lacustrine oxygen isotope record of 20th-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 annual laminae of bio-induced authigenic calcite in a frozen core from Baldeggersee, a small lake in Central Switzerland. These isotope results provide proxy data on the isotopic composition of past precipitation in the Baldeggersee catchment region and are quantitatively compared with instrumental climate data (i.e. mean annual air temperature and atmospheric circulation pattern indices) to evaluate climatic controls on oxygen isotopes in precipitation. Monitoring the isotope hydrology of Baldeggersee demonstrates that the oxygen isotopic composition of the lake water is controlled by the isotopic composition of local atmospheric precipitation (delta O-18(p)) and that the isotopic signal of precipitation is preserved, albeit damped, in the lake calcite oxygen isotope record (delta O-18(c)). Comparison of the calcite oxygen isotope proxy for delta O-18(p) in the catchment with historical mean annual air temperature measurements from Bern, Switzerland confirms that authigenic calcite reliably records past annual air temperature in the region. This delta O-18(c)/temperature relationship is calculated to be 0.39 parts per thousand/degreesC for the period 1900-1960, based on an isotope mass-balance model for Baldeggersee. An exception is a 0.8 parts per thousand anomalous 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 mean annual air temperature, include changes in delta O-18(p) due to synoptic circulation patterns. In particular, the 0.8 parts per thousand negative shift coincides with a trend towards a more dominant North Atlantic Oscillation (NAO) index. This circulation pattern would tend to bring more isotopically more negative winter precipitation to the region and could account for the 0.8 parts per thousand offset in delta O-18(c) data.