Dead in the water: the final demise of the Scandinavian Ice Sheet

In the late stages of the last deglaciation, the southern and eastern margins of the Scandinavian Ice Sheet were largely water-terminating, a consequence of glacio-isostatic depression and of episodic damming of the Baltic basins. Similarly, along the western ice margin, glacial lakes were dammed between the easterly retreating ice margin and the water divide within the mountains to the west.

The geomorphological imprint of the ice-dammed lakes is of particular importance in inland Scandinavia, as other geological evidence pertaining unequivocally to the final ice sheet decay is sparse. The interpretation of other glacial landforms has also proven complicated since the ice sheet is thought to have mainly been cold-based during final decay, inhibiting sliding at the ice-bed interface and limiting the construction (or destruction) of landforms indicative of the changing shape and flow of the ice sheet. Furthermore, dated sediment sequences marking the onset of ice-free conditions have been woefully few. Likewise, with the high age uncertainty and inadequate geographical cover of available cosmogenic nuclide exposure dates, the timing and location of final ice sheet decay have remained elusive.

In this keynote, we show that the ice-dammed lakes along the mountain range were an intricate part of the deglacial dynamics and demonstrate how mapping and dating them offer a solution to these problems. Even with a frozen ice-bed interface, surface melting and meltwater drainage creates landforms unequivocally associated with ice sheet decay: e.g. drainage channels, lake shorelines, and deltas. By reconstructing series of connected and evolving ice-dammed lakes along the mountain range we can now trace the retreating ice-margin in great detail, while radiocarbon dating of cored sediments that capture the glacial- to nonglacial lake transitions offers a chronology.

Our results clearly supports a long-standing curiosity of the decay of the Scandinavian Ice Sheet – demise to lower, rather than to higher elevation – that questions fundamental relationships between glacier mass balance, climate and topography. Often depicted as one coherent ice mass, we now see evidence of recurring unzipping and partitioning of the ice sheet along the mountain range. Coupled to these ice-sheet partitioning events, we find independent, geomorphological, sedimentological and stratigraphical, evidence for catastrophic lake drainages, arising from a complex interplay between the rapidly retreating ice sheet and sub- and pro-glacial topography. Most notably, we have identified and dated key lake drainages responsible for depositing marker beds in the varve-based “Swedish Time Scale” along the Swedish east coast, including the so-called “Zero Varve” of the time scale. By doing so, we have now anchored the Swedish Time Scale in absolute time and expanded the chronology from Sweden to Norway and Finland at a sub-century scale with revised ages of key deglacial and climatic events of the last glacial-interglacial transition. Not only does this work provide a revised, and chronologically more constrained, reconstruction of the ice recession chronology but it opens the door for precise investigation of the interactions between the dynamics of the Scandinavian Ice Sheet, climate, lake level history, geography, and glacial isostatic adjustment.