Baltic Sea water exchange is primarily governed by atmospheric forcing of the inflow of saline waters by strong westerly winds prevailing over the central North Atlantic and north-western Europe. Our sediment core study uses geochemical element records indicative of phytoplankton and cyanobacterial blooming as well as continent-derived mineral input for reconstructing hydrographic changes in the deeper Baltic Sea basins around AD 1200. An alkenone-based Sea Surface Temperature (SST) reconstruction for the relevant time span, AD 500-1500, is presented for another sediment core obtained from the shallow Isefjord located at the southern coast of the Kattegat at the entrance of the Baltic. At the termination of the Medieval Climate Anomaly at approximately AD 1200, the basin sediment facies and the geochemical records reveal an environmental change indicative of a marked decrease of inflow activity and marine productivity. This change coincides with a SST decrease and recently reported general fall in Kattegat sea level. A comparison with palaeo-climate data from the wider North Atlantic region demonstrates that this regime shift in Baltic Sea water exchange is linked to a large-scale change in ocean and atmosphere circulation from a dominating, positive North Atlantic Oscillation (NAO+) mode to more negative NAO conditions.
- Programområde 5: Natur og klima