Landesamt fuer Bergbau, Energie und Geologie, Federal Republic of Germany

Climate variations in central Europe during the Weichselian can be retraced with reasonably good confidence on the basis of proxy data such as botanical macrofossils and pollen content in Weichselian sediments, for which good age control is available. The availability of proxy data from pre-Eemian subaerial deposits tends to be too erratic in space and time to enable us to reconstruct from them, with confidence, a continuous record of northern Germany's regional climate for the whole Pleistocene, in particular prior to the times of major glacial advances from the north. The likely duration and maximum depth of the occurrence of permafrost in northern Germany throughout the Pleistocene, however, can be estimated. This assumes that the Pleistocene climate trend archived in oxygen isotope records from marine sites (such as e.g. ODP-sites 659 and 677) can be considered in broad terms to reflect the global climatic variations. Here we discuss the approach of using a previously presented reconstruction of the mean annual ground temperatures (MAGT) of the last 120 ky for northern Germany as a calibration tool to reconstruct, from marine proxy data, MAGT-estimates for northern Germany during the Pleistocene. These MAGT-data are the basis of calculations to estimate permafrost depth fluctuations in northern Germany. Calculations for the growth and decay of permafrost are presented for a vertical sediment column and a 75 km long profile modelled after a seismic line that crosses in an east-west direction several salt domes in the subsurface of the German North Sea sector. The latter model intends to demonstrate the interplay between variations in terrestrial heat flow caused by the presence of salt structures and changing climatic conditions over time on the development and decay of permafrost. Depending on the applied climate curve, the maximum vertical extent of permafrost during the Pleistocene is estimated to be c. 130 m (MAGT (sub ODP677) -reconstruction) or 170 m (MAGT (sub ODP659) -reconstruction). We favour the MAGT (sub ODP659) -reconstruction, since its results correlate well with available geological evidence for cold stages with indications of permafrost development during the Pleistocene.