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Selected Time Windows
Three time windows will be especially emphasised in the project:
1) The d18OP records for the last millennium, with an intended precision of a varve-based and radiometrically calibrated chronology of about a decade, will be used to define the prevailing modes of decadal to centennial scale natural climate variability in Europe. In particular these records will enable us to study the relation of climate variability to external forcing mechanisms such as changes in solar activity and/or to internal climate modes such as variations in the strength of the North-Altlantic thermo-haline circulation or to NAO-like decadal variability. The central objective of this part of the proposal is, however, to provide a spatially distributed set of a quantitative climate parameter for significant parts of Europe which is directly linked to atmospheric processes and not biased by human activity or modified through and biological response.
2) For the Mid-Holocene period the proposed high-resolution study is focussed on a prominent climate perturbation (the so-called 8.2-ky-event) for which the triggering mechanism is relatively well known, and thus provides an ideal testing scenario for fully coupled ocean-atmosphere general circulation models. The Mid-Holocene records of d18OP which will be produced within DecLakes will serve as quantitative model-independent data set to test the model performance in representing the respective regional expression of the event. Ideally, the data-to-model-comparison is performed with high resolution GCMs equipped with isotope analytics (isoGCMs), with the advantage that those models imply the major physical controls on d18OP. In addition to a simplified data-to-model comparison the isoGCMs runs will therefore allow to analyse the specific regional sensitivity of d18OP during this period. They will accordingly provide an evaluation of the stability of d18OP-climate relation compared to the last centuries, for which it can be derived from the comparison of ostracod-based reconstructed d18OP and directly observed climate. A second reason for selecting this period is that its younger part overlaps with one of the periods chosen by MOTIF, a paleo-model-intercomparison project to test the capacity of coupled ocean-atmosphere GCMs to reproduce climate variability under boundary conditions differing from the modern ones. A third point for our interest is the indication of generally cooler and more humid conditions in the Mediterranean region during this period.
3) The third proposed high-resolution window aims at answering open questions arising from the quantitative comparison of the existing d18OP -records from Ammersee and Central Greenland: both records show intriguingly similar decadal-scale to centennial variations, including the fast high-amplitude transitions at the onset of the 'Bölling/Alleröd' interstadial and the beginning and the end of the Younger Dryas stadial. However, there is still ongoing discussion on the strict synchronism between specifically those both records and the climate development in Greenland and Central Europe in general (Lowe, 2001). In addition, the lack of similarly highly resolved and quantitative d18OP records in Europe justifies questioning it's over-regional significance. Our new records will provide a strong, regional distributed data set allowing to investigate both the European-wide synchronism and the quantitative regional isotopic expression of the centennial cold phases within the 'Bölling/Alleröd' interstadial. Eventually, systematic patterns in the differential responses of d18OP during well defined events might provide means to develop a better understanding of their causes (which, as for the 8.2 k-event, are most probably related to fresh-water discharge events during the regression of the Scandinavian and the Laurentide ice sheets).
A second point of controversial discussion is the nature of the Younger Dryas cold reversal. Whereas the majority of the north-American palaeoclimate community still favours the hypothesis of a large freshwater discharge event (from Lake Agassiz) as the trigger of the Younger Dryas, modelling experiments have large problems to maintain the northern hemispheric climate in the cold state for more than 1000 years, if the freshwater perturbation is instantaneous. However, the succession of events from the warming at the onset of the Boe/All to the end of the YD in the Greenland ice cores shares almost all features of the frequently recurring DOEs during marine isotope stages 3 and 2. Parts of the European palaeoclimatologists therefore consider the YD as the cold part of the last Dansgaard-Oeschger-Event (DOE), generated as the earlier ones, by a still not completely understood ocean-ice-atmosphere feedback loop working on times-scales of several millennia. The existing d18OP record from Ammersee provides evidence for significant Greenland/European differences in the overall trend during the Boe/All, the warm phase of this presumed last DOE. Whereas d18OP in all Central Greenland ice cores significantly decreases, the European records remain at the same mean level during Boe/All, probably indicating a melt-water induced gradual built-up of a cold 'East-Greenland current' and permanent sea-ice cover along the East-Greenland coast. However, the long-term plateau-like form of the Ammersee record could also result from a 'Greenland-like' cooling trend counterbalanced by a local warming due to the increasing distance to the Scandinavian Ice Sheet, melting back during this period. We therefore need quantitative d18OP -records from geographical distributed sites for this period in order quantify those inner-European gradients and to provide the 'European-wide' Boe/All climate response.
