____________________________________________
DecLakes Strategy
Here we propose to concentrate reasonable resources on a European-wide effort to exploit this proxy, d18OP as inferred from deep-lake ostracods, for gaining the characteristics of decadal to centennial climate variability of past climate in Europe. Based on the experience from Ammersee, we have identified five additional lakes, potentially providing d18OP records for the last 15000 years in a very similar quality. All lakes are characterised by
- large drainage basins relative to their surface area, warranting a relatively small contribution of evaporation in the water balance and, in consequence, a very small, almost constant offset between regional d18OP and the isotopic composition of the lake water (d18OL)
- expected short theoretical water residence times smaller than five years resulting in a fast reaction of d18OL to changing d18OP
- maximal water depths larger than 60 m, i.e. warranting near to constant bottom water temperatures close to the density maximum of fresh water (4°C) which nearly excludes water temperature dependent bias on the d18OL reconstruction from benthic ostracod calcite
- dimictic or monomictic mixing behaviour and relatively high calcium concentrations in both lake and interstitial water, thus ideal settings for the development and preservation of in-situ deep-lake ostracod fauna.
- High sediment accumulation rates allowing decadal resolution in the time intervals to be studied.
The same rigorous observation of the modern lake and hydrological settings, and the same sensitivity modelling approach will be applied for all these sites in order to provide an adequate quality control for the d18OP reconstructions. The most recent parts of the records will be evaluated against 1) the direct observations of d18O in meteoric water from the respectively nearest GNIP (WMO/IAEA) station, 2) the longest regionally available homogenized climate record, and 3) the output of an regionalised climate model modified for isotope diagnostics (Sturm et al., submitted). Combining these evaluations will provide evidence of the climatic significance of the regional and local d18OP records. Highly, at least decadal resolved d18OP reconstructions will be produced for the last millennium, for the 1000-year period bracketing the 8.2-ky-event and for selected parts of the Bölling/Alleröd. Embedded in the background of centennial variations described by roughly 50-year-spaced discontinuous sampling over the remaining sections of the 15000-years period, those records will provide a spatial view of d18OP with a high temporal resolution on a East-West transect along the northern edge of the European Alps, provide a first trans-alpine d18OP-comparison over those periods, and, by adding a record from North-East Poland, address the possible variation of continentality over time. For all three periods, we aim in particular separating NAO-like decadal variability, which is most probably expressed by moderate inner-European differences and a general Greenland-European anti-correlation, from northern-hemisphere-wide variability due to the north-Atlantic heat-flux changes, which should cause unidirectional d18OP signals in all European and Greenland sites.
The objective of DecLakes is to provide a unique set of a physical climate proxy, which is virtually independent of direct human activity and, other than most biotic indicators, will not be weakened from tolerance ranges and their plasticity due to eventual adaptation. A further advantage of the expected decadal records is that archive specific trends (such as, for instance, growth curves in tree ring characteristics or the increasing, down hole dampening of the higher frequency changes in reconstructions from borehole temperatures) are absent. Moreover, the reconstructed parameter is identical not only for all the 6 sites in Europe, but also is the prominent climate proxy from the Greenland ice core studies.
