Multiple re‐advances of a Lake Vättern outlet glacier during Fennoscandian Ice Sheet retreat, south‐central Sweden

Lake Vattern represents a critical region geographically and dynamically in the deglaciation of the Fennoscandian Ice Sheet. The outlet glacier that occupied the basin and its behaviour during ice-sheet retreat were key to the development and drainage of the Baltic Ice Lake, dammed just west of the basin, yet its geometry, extent, thickness, margin dynamics, timing and sensitivity to regional retreat forcing are rather poorly known. The submerged sediment archives of Lake Vattern represent a missing component of the regional Swedish deglaciation history. Newly collected geophysical data, including high-resolution multibeam bathymetry of the lake floor and seismic reflection profiles of southern Lake Vattern, are used here together with a unique 74-m sediment record recently acquired by drill coring, and with onshore LiDAR-based geomorphological analysis, to investigate the deglacial environments and dynamics in the basin and its terrestrial environs. Five stratigraphical units comprise a thick subglacial package attributed to the last glacial period (and probably earlier), and an overlying > 120-m deglacial sequence. Three distinct retreat-re-advance episodes occurred in southern Lake Vattern between the initial deglaciation and the Younger Dryas. In the most recent of these, ice overrode proglacial lake sediments and re-advanced from north of Visingso to the southern reaches of the lake, where ice up to 400 m thick encroached on land in a lobate fashion, moulding crag-and-tail lineations and depositing till above earlier glacifluvial sediments. This event precedes the Younger Dryas, which our data reveal was probably restricted to north-central sectors of the basin. These dynamics, and their position within the regional retreat chronology, indicate a highly active ice margin during deglaciation, with retreat rates on average 175 m a(-1). The pronounced topography of the Vattern basin and its deep proglacial-dammed lake are likely to have encouraged the dynamic behaviour of this major Fennoscandian outlet glacier.