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Scientific background
The stable oxygen isotopic composition of past atmospheric precipitation (δ18OP) is an excellent quantitative climatic proxy, which since several decades is used to reconstruct climate variability in polar regions. Over the last decades, important progress has been made to identify reliable archives of this proxy in temperate climate regions, where direct preservation of snow fall in form of glacier ice is limited to high altitude and relatively short periods. Oxygen isotope records from specific benthic ostracods in carefully selected deep lakes have been successfully used for a quantitative reconstruction of δ18OP in Europe over the last 15000 years. For those records, secondary isotopic effects, arising from residual temperature variations, hydrological peculiarities, and physiological discrimination could be quantified by direct observation under the modern settings, and, by sensitivity modelling, proven to vary only within a very narrow range even under extreme past climatic conditions (Danis et al., 2003). The δ18OP record from Ammersee, a lake in southern Germany, was successfully validated against the 200 year long observational record from the Hohenpeissenberg meteorological station, situated in the catchment area of the lake (von Grafenstein et al., 1996). At least for this specific setting and period, δ18OP as reconstructed from the lake archive showed up to be a reliable, quantitative proxy for decadal mean atmospheric temperature, in contrast to almost all continental biological indicators, which are more sensitive to variations of summer temperatures. The record also provided the first central European evidence for the strongest Holocene event at about 8200 years BP known from the Greenland ice core record (von Grafenstein et al., 1998). This result helped to confirm the over-regional significance of this climate event, to identify its forcing by the Hudson Bay ice dome collapse (Barber et al., 1999; de Vernal et al., 1997), and to focus the search of it in other European climate proxy archives (McDermott et al., 2001; Wick and Tinner, 1997 and many others). The late-glacial to early Holocene decadal δ18OP record from Ammersee convincingly demonstrates very strong climatic synchronicity of climate in Greenland and Europe with numerous identical centennial scale cooling events, but significant quantitative differences with respect to millennial scale trends, a finding potentially useful in the search for the mechanism provoking the Younger Dryas interstadial and, more generally, the recurring Dansgaard-Oeschger events (von Grafenstein et al., 1999).
