Sedimentological evidence for stacked paleo-thaw (thermokarst) lake successions in Western Europe

Sedimentary successions of Last Glacial age in Europe show evidence of episodic presence of permafrost, by a range of fossil permafrost phenomena such as ice wedge pseudomorphs and cryoturbation. Also lake deposits have been found that may have originated from permafrost thaw, in particular in basins and river valleys of the northwestern European plains along the North Sea and the Baltic. These deposits contain valuable information on the reaction of permafrost to past rapid changes in climate. We present new data on a lacustrine succession near Hengelo (Netherlands), which has been investigated with cores and exposures. In this succession, evidence for at least four, and possibly five stacked thaw lake units has been found. The lacustrine sediment units are separated from each other by sands, loessic silts and peat with ice wedge pseudomorphs and large cryoturbations, indicating subaerial deposition. The lake sediments vary from almost purely clastic sediment (laminated silts) to organic sediment (gyttja). The underlying ice wedge pseudomorphs may have a partial infilling of lake sediment (see figure). Sandy lake shore benches with delta-like foresets indicate the repeated occurrence of periods of enhanced sediment supply to the lakes, from lake banks or from fluvial input. We conclude that thaw lakes did occur repeatedly, in particular as a result of permafrost thaw during Marine Isotope Stage (MIS) 3 or Middle Weichselian. By contrast, the permafrost thaw during the Last Glacial Termination (MIS 2) in the same area did not cause widespread formation of thaw lakes; permafrost thaw occurred probably under rather dry conditions, evidenced by ubiquitous deflation horizons and eolian sand sheet and dune deposits of this age. The sedimentological evidence at Hengelo is compared with other paleo-thaw lake deposits in northwestern Europe, and with thaw lake successions observed in Eastern Siberia, Alaska and Canada, to determine criteria for distinguishing thaw lake sediments in the geological record. This confirms a large variability of deposits, also seen in the paleorecord. The interpretation of paleo-thaw lakes as indicators of climate change is not straightforward. The evidence on present-day thaw lake expansion is mixed, despite pronounced climate warming in the Arctic, and shows stability, net contraction or expansion of lake area in various regions. Widespread lake formation in present-day Arctic thaw lake regions did occur mainly during the Last Glacial Termination and early Holocene. By contrast, in Western Europe evidence of thaw lake formation as a result of permafrost thaw during the Last Glacial Termination is limited. The paleo-thaw lake deposits in Western Europe appear to be associated with climate warming during the interstadials of the Middle Weichselian. We infer that thaw lake formation may have contributed to higher CH4 emission during these interstadials, indicated by high CH4 concentration in ice cores of this age.

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