Atmospheric circulation over Europe during the Younger Dryas

Large scale and regional demographic responses to climatic changes in Europe during the Final Palaeolithic

The European Final Palaeolithic witnessed marked changes in almost all societal domains. Despite a rich body of evidence, our knowledge of human palaeodemographic processes and regional population dynamics still needs to be improved. In this study, we present regionally differentiated population estimates for the Greenland Interstadial 1d-a (GI-1d-a; 14-12.7 ka cal BP) and the Greenland Stadial 1 (GS-1; 12.7-11.6 ka cal BP) for Southern, Western, Northern and Central Europe. The data were obtained by applying the Cologne Protocol, a geostatistical approach for estimating prehistoric population size and density, to a newly compiled dataset of Final Palaeolithic sites. On a large spatio-temporal scale and compared to preceding Upper Palaeolithic phases, areas north of the Alps become the dominant demographic growth area for the first time since the dispersal of anatomically modern humans into Europe. At smaller scales, we observe divergent regional trends, with a conspicuous lack of archaeological evidence appearing in previously occupied areas of central France and Germany. Our study also shows that during the Final Palaeolithic, the climatic cooling of GS-1 coincides with a pronounced population decline in most parts of the study area. An apparent increase in population density occurs only in north-eastern Central Europe and north-eastern Italy. Our estimates suggest that the total population was reduced by half. Similar results, with a relationship between decreasing temperatures and decreasing populations, have already been observed for the late phase of the Gravettian, when populations were reduced to only one third of those estimated for the early phase. Yet, in contrast to the collapse of local populations during the late Gravettian, the increase in population densities in Central Europe during GS-1 indicates population movements eastwards, possibly in response to deteriorating climatic conditions, particularly in western regions during the Younger Dryas.

Late Glacial summer paleohydrology across Central Europe

It is generally accepted that a weakening of the North Atlantic thermohaline circulation caused the Younger Dryas cooling. Although the role of seasonality was emphasized previously, this aspect is rarely considered yet, and it remains elusive how this impacted hydroclimate during winters and summers across Central Europe. Here, we coupled biomarker-based δ18O and δ2H from Bergsee in southern Germany to reconstruct deuterium excess as a proxy for evaporation history from the Bølling-Allerød to the Preboreal. We compared this dataset with other biomarker isotope records in Central Europe. They are all lacking a strong isotopic depletion during the Younger Dryas, which is best explained by the summer sensitivity of the biomarker proxies: As Younger Dryas summers were relatively warm, there is an absence of the strong winter cooling signals recorded in annual water isotope records like Greenland or Lake Steißlingen. Lake evaporation at Bergsee together with other paleohydrological reconstructions draw a coherent picture of the Late Glacial hydroclimate, with strong evidence for warm and dry Younger Dryas summers. Rather than a southward shift of the Westerlies during winter, we suggest that a recently proposed feedback mechanism between North Atlantic sea ice extend, strong winter cooling and summer atmospheric blocking serves as a suitable explanation for summer dryness. Additional confidence to the robustness of these biomarker records is provided by the overall agreement of paleohydrological fluctuations during the Preboreal.