Together with a project from Veronica Krossa of the Marine Climate Research Group (CAU, Kiel), the outlined project here is funded within the framework of the DFG-SPP Project « Frühe Monumentalität und soziale Differenzierung » (http://www.monument.ufg.uni-kiel.de/). This project intends to reconstruct decadal-to-centennial scale climate variability between 6500 and 4000 yrs B.P. on a regional scale in Northern Germany and adjacent oceans. During this time interval Neolithic landscapes and societies in North Central Europe underwent profound changes, probably reflecting fast changes in economic and land use strategies. To what extent these changes were co-determined by climate and related environmental change is still an enigma.
Special emphasis is placed here on the cooling phase after the Holocene climate optimum. Although the timing for this termination appears to vary between regions with ages ranging from 6000 (e.g. Moros et al., 2004) to 4500 years B.P. (e.g. Seppä and Birks, 2001), in general one could argue that the transition from the Holocene thermal optimum (9.000–7.000 B.P.) to colder climate conditions went along with increasing “neolithisation” culminating after 6.500 B.P. in the Younger Neolithic. This is the time when human impact due to settlement, agriculture and animal husbandry can strongly be recognized not only in pollen records but also in lacustrine and fluvial sediments in Central Europe (Kalis et al. 2003), which makes it complicated to merely infer on natural climate variability.
A further complication influencing the interpretation of such records is their often regional nature, and the short-scale climate variability generated by atmosphere-ocean interactions e.g. the AMO (Atlantic Multidecadal Oscillation) or AO/NAO (Arctic/North Atlantic Oscillation). These factors are then superimposed on long-term trends but often cause opposite behaviour in different regions at historical (century scale), multi-decadal, or decadal scales (Hurrel 1995, Justino and Peltier 2005). The NAO describes fluctuations in the strength of geostrophic westerlies predominantly affecting the winter climate in the Baltic and Northern Germany. A positive NAO causes a “maritime mode” with strengthened westerlies transporting warm humid air masses eastward, thereby producing mild winters. The opposite situation (negative NAO: continental mode) is determined by a strengthened westward transport of cold and dry Siberian air towards Europe. This is accompanied by severe winters.
Consequently, to unravel changes in precipitation and surface temperatures in Northern Germany and adjacent oceans as potential impact factors on the development of human cultures (e.g., monumentality), economical, and societal structures, etc., we aim to reconstruct climate changes between 6.500 and 4.000 B.P. from high resolution marine sediment cores in the Skagerrak, which was probably not affected by human activities. Today, the hydrography of this deepest part of the North Sea is basically determined by the North Atlantic inflow and modulated by the Baltic Sea outflow and the Jutland Current. Since the establishment of the modern circulation system about 8300 - 7900 yrs B.P., related to the opening of the English Channel and the Danish Straits (Gyllencreutz et al., 2005), the influence of Atlantic surface and intermediate waters progressively increased in the Skagerrak. Major hydrographic shifts described by other authors occurred around 6300, 4700, and 4000 yrs B.P. (Gyllencreutz and Kissel, 2006) and are considered to reflect changes in the predominance of Baltic Sea and North Sea/North Atlantic influence. Deepwater renewal in the Skagerrak, as driven by surface water density, has been considered to be highly sensitive to NAO fluctuations over the last 1200 years (Brückner and Mackensen, 2006). Stable isotopes and geochemical analysis are used in order to reconstruct climate change via deep water temperatures and ventilation in the Skagerrak. Spectral analysis run on those high-resolution records contribute to decipher climate variability on a decadal to multi-centennial scale. Subsequently, these marine records are compared with regional ones from lake and river sediments or soil beds in Northern Germany. Comparison is improved with the identification of markers jointly contained in both terrestrial and marine records, such as tephra layers from Icelandic eruptions.
This approach may help to better separate the natural climate variability during the Middle to Late Holocene from the superimposed human impact on the continental climate archives and address three main questions:
- Did the Mid Holocene climate transition from a “climate optimum” at about 5000 yrs B.P. in Northern Germany and adjacent oceans lead to a phase of climate deterioration with increased climate instability or was there a change from Early Holocene warm, but highly variable climate behaviour to more stable, cooler conditions after 5000 yrs B.P.?
- Did fast changes in regional marine and terrestrial climate cause abrupt environmental change that in turn fostered adaptation in early Neolithic production techniques and land use strategies?
- Does a close link exist between abrupt regional climate changes and those in the open North Atlantic that probably originated from variable intensity of Atlantic meridional Overturning Circulation (AMOC) variations?