Controls on the early Holocene collapse of the Bothnian Sea Ice Stream 1 2

35 36 New high resolution multibeam data in the Gulf of Bothnia reveal for the first time the 37 subglacial environment of a Bothnian Sea Ice Stream. The geomorphological record suggests 38 that increased meltwater production may have been important in driving rapid retreat of 39 Bothnian Sea ice during deglaciation. Here we apply a well-established one-dimensional 40 flowline model to simulate ice flow through the Gulf of Bothnia and investigate controls on 41 retreat of the ice stream during the post-Younger Dryas deglaciation of the Fennoscandian Ice 42 Sheet. The relative influence of atmospheric and marine forcings are investigated, with the 43 modelled ice stream exhibiting much greater sensitivity to surface melting, implemented 44 through surface mass balance and hydrofracture-induced calving, than to submarine melting 45 or relative sea level change. Such sensitivity is supported by the presence of extensive 46 meltwater features in the geomorphological record. The modelled ice stream does not 47 demonstrate significant sensitivity to changes in prescribed ice stream width or overall bed 48 slope, but local variations in basal topography and ice stream width result in non-linear retreat 49 of the grounding line, notably demonstrating points of short-lived retreat slowdown on reverse 50 bed slopes. Retreat of the ice stream was most likely governed by increased ice surface 51 meltwater production, with the modelled retreat rate less sensitive to marine forcings despite 52 the marine setting. 53 54 55