Controls on the early Holocene collapse of the Bothnian Sea Ice Stream.

New high resolution multibeam data in the Gulf of Bothnia reveal for the first time the subglacial environment of a Bothnian Sea Ice Stream. The geomorphological record suggests that increased meltwater production may have been important in driving rapid retreat of Bothnian Sea ice during deglaciation. Here we apply a well-established one-dimensional flowline model to simulate ice flow through the Gulf of Bothnia and investigate controls on retreat of the ice stream during the post-Younger Dryas deglaciation of the Fennoscandian Ice Sheet. The relative influence of atmospheric and marine forcings are investigated, with the modelled ice stream exhibiting much greater sensitivity to surface melting, implemented through surface mass balance and hydrofracture-induced calving, than to submarine melting or relative sea level change. Such sensitivity is supported by the presence of extensive meltwater features in the geomorphological record. The modelled ice stream does not demonstrate significant sensitivity to changes in prescribed ice stream width or overall bed slope, but local variations in basal topography and ice stream width result in non-linear retreat of the grounding line, notably demonstrating points of short-lived retreat slowdown on reverse bed slopes. Retreat of the ice stream was most likely governed by increased ice surface meltwater production, with the modelled retreat rate less sensitive to marine forcings despite the marine setting