Evaluation of the role of the Baltic depression during deglaciation of the last Scandinavian Ice Sheet; a landform-driven investigation

Abstract. Landforms left behind by the last Scandinavian Ice Sheet (SIS) offer an opportunity to investigate controls governing ice sheet dynamics. Terrestrial sectors of the ice sheet have received considerable attention by landform and stratigraphic investigations, but much less so for marine areas such as in the Baltic Sea. In contrast, despite its geographical importance, the Baltic Sea remain poorly constrained due to limitations in bathymetric data. The Baltic depression hosted the extensive Baltic Ice Lake, which likely exerted a considerable control on ice dynamics, providing an aqueous calving front that might have resulted in rapid collapse of this ice sheet sector. Both ice sheet scale investigations and regional studies at the southern periphery of the SIS have considered the Baltic depression as a preferential conduit for ice flux towards the southern ice margin throughout the last glaciation. Here we test this hypothesis using newly available bathymetric data and peripheral topographic data. For the first time, these data reveal an extensive landform suite stretching from Denmark in the west to Estonia in the east and from the southern European coast to the Aland Sea, comprising an area of 0.3 million km². We use these landforms to reconstruct the ice dynamic history of the Baltic sector of the SIS. Landform evidence indicates a complex retreat pattern that changes from lobate ice margins with splaying lineations to parallel MSGL in the deeper depressions of the Baltic Basin. Ice margin still-stands on underlying geological structures indicate the likely importance of pinning points during deglaciation resulting in a stepped retreat signal. Over the length of the study area we identify broad changes in ice flow geometry, ranging from SE-NW to N-S and then to NW-SE. Mega-scale glacial lineations reveal distinct corridors of fast ice flow (ice streams) with widths of 30 up to 95 km, rather that the often-interpreted Baltic-wide (300 km) accelerated ice flow zone. These smaller ice streams are interpreted to have operated during late stages of deglaciation. Where previous ice sheet-scale investigations inferred a single ice source, our mapping identifies flow and ice marginal geometries from both Swedish and north Bothnian sources. We anticipate our landform mapping and interpretations may be used as a framework for more detailed empirical studies by identifying targets to acquire high resolution bathymetry and sediment cores and also for comparison with numerical ice sheet modelling.