A watery ice sheet demise: Formation and drainage of ice-dammed lakes in Southern Norway during the Early Holocene

The final demise of the Scandinavian Ice Sheet in southern Norway was associated with partial glacial retreat in ice-dammed lakes, but how the subaqueous retreat affected glacial dynamics is largely unknown. Here, we present results from comprehensive mapping that demonstrate the existence and final catastrophic drainage of multiple ice-dammed lakes in southern Norway during the Early Holocene, as well as key landforms that detail the course of glacial retreat. We have identified several previously unknown ice-dammed lakes and described three hitherto unknown 15–80 km³-large glacial lake outburst floods. The floods formed mega dunes, large flood bars, canyons and other distinctive features in the impacted areas downstream. We present a relative chronology for the ice-dammed lakes and drainage events that, coupled with existing chronological constraints, allow detailed palaeoglaciology reconstructions in a period of rapid deglaciation. The ice-dammed lake record documents that the mountainous areas became ice-free first, whereas the large inland valleys in southern Norway were deglaciated last. The very last remnants of the Scandinavian Ice Sheet in southern Norway were probably located in the valleys around Jotunheimen. The ice-dammed lakes induced dynamic glacial retreat and probably caused a significant increase in ice-sheet drawdown and retreat rates compared to where glacial retreat was land-based, e.g. as indicated by a near tenfold increase in retreat rates when the Store Dølasjø ice-dammed lake formed, and the glacier became water-terminating. The study highlights the value of ice-dammed lakes for reconstructing palaeo-ice sheets, and the data serve as a relevant analogue for future ice mass loss in glacierized regions worldwide.