Estimating Episodic Permafrost Development in Northern Germany during the Pleistocene

Abstract

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, MAGTestimates 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 ODP677 -reconstruction) or 170 m (MAGT ODP659 -reconstruction). We favour the MAGT ODP659 -reconstruction, since its results correlate well with available geological evidence for cold stages with indications of permafrost development during the Pleistocene.

Response of salt structures to ice-sheet loading: implications for ice-marginal and subglacial processes

“…The interaction between salt structures and ice sheets may be more complex due to the increased heat-flux above salt structures, which reduces the thickness of permafrost in the proximity of salt structures (Delisle et al, 2007;Grassmann et al, 2010) and thus accelerates permafrost decay beneath an advancing ice sheet. The more rapid decay of permafrost in the vicinity of salt structures may increase the drainage capacity of the substratum and thus increase the ice-bed coupling.…”

pT-effects of Pleistocene glacial periods on permafrost, gas hydrate stability zones and reservoir of the Mittelplate oil field, northern Germany

“…The most obvious effect of reduced sediment temperatures over prolonged periods of time is the freezing of pore water leading to the development of permafrost (Delisle et al, 2007). Permafrost blocks the pore space and reduces rock permeability.…”

“…Delisle et al (2007) applied 75 mW/m 2 whereas Grassmann et al (2005) used a calibrated basal heat input of 50 mW/m 2 as. However, both results appear to reflect well our perception of the sequence of cold stages in northern Germany for the last 1 million years.…”

“…Proxy data from pre-Eemian sub-aerial deposits tend to be too sparse to enable us to reconstruct a continuous record of the regional climate for the entire Pleistocene. Therefore, as an alternative approach, an estimate for an MAGT-curve for the Pleistocene of northern Germany based on the marine proxy record from the ODP-sites 659 was developed by Delisle et al (2007). In the basin model the MAGT-values were integrated as upper boundary condition in the calculation of the basin's heat budget, with a temporal resolution of 1000 years.…”

“…However, the reconstruction for the whole Pleistocene predicts prolonged sub-zero ground temperature episodes and phases of permafrost development. Investigations on the permafrost depth were based on two approaches: an established algorithm for the calculation of permafrost growth and decay (Delisle, 1998a(Delisle, ,b, 2003(Delisle, , 2007) was used to cross check the results of the integrated basin model.…”