The hypothesis of a km-thick ice shelf covering the entire Arctic Ocean during peak glacial conditions was proposed nearly half a century ago. Floating ice shelves preserve few direct traces after their disappearance, making reconstructions difficult. Seafloor imprints of ice shelves should, however, exist where ice grounded along their flow paths. Here we present new evidence of ice-shelf groundings on bathymetric highs in the central Arctic Ocean, resurrecting the concept of an ice shelf extending over the entire central Arctic Ocean during at least one previous ice age. New and previously mapped glacial landforms together reveal flow of a spatially coherent, in some regions >1-km thick, central Arctic Ocean ice shelf dated to marine isotope stage 6 (∼140 ka). Bathymetric highs were likely critical in the ice-shelf development by acting as pinning points where stabilizing ice rises formed, thereby providing sufficient back stress to allow ice shelf thickening.

The well-preserved Tvären crater is noteworthy for being one of a small number of Early and Middle Ordovician impact structures formed in a marine environment. It is demonstrated to be an impact structure by the presence of a breccia lens, consisting of crystalline basement rocks, and shocked quartz. The breccia lens formed under dry-hot conditions after expulsion of sea-water by the impact. Resurging sea-water thereupon deposited a positively graded, 60 m thick turbidite-like unit. This graded resurge deposit is a previously unknown feature, to be expected in open-sea impacts. Breccia in the lower part of this graded deposit contains fragments of a remarkably complete orthoceratite limestone succession that existed at the site of impact, resting on non-lithified sand of probably Early to earliest Middle Cambrian age. A sedimentary succession was deposited inside the crater at depths decreasing from more than 200 m in the initial stages to some 100 m at the time of deposition of the youngest preserved beds. The environment within the crater thus favoured deposition of an essentially complete stratigraphic succession with depth-controlled palaeoecology for a significant time interval after the impact. Whereas planktonic members, like graptolites and chitinozoa, are present throughout the post-impact succession, and asaphids, almost as persistent, became established at an early date, burrowers were somewhat reluctant to enter and remopleuridids and small strophomenids came in at a late stage. We suggest as a result of this study that structures formed by impact may offer unique information about the palaeogeology and palaeoenvironment of the region hit by the impact.

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M., 2015: Regional deglaciation and postglacial lake development as reflected in a 74 m sedimentary record from Lake Vättern, southern Sweden.Abstract: The withdrawal of the Late Weichselian ice sheet and rapid isostatic uplift in southern Scandinavia led to the entrainment of large volumes of melt water within the proglacial Baltic Ice Lake (BIL). The eventual western outpost of BIL, Lake Vättern, has been a focal point for studying the dynamic retreat history of the Late Weichselian ice sheet in south central Sweden. This part of the deglacial history is described from an abundance of terrestrial studies, but, to date, no complimentary long sediment cores from Lake Vättern have been available. Here, we present the results from a unique, 74 m borehole in southern Lake Vättern that recovered a Late Pleistocene to Holocene sedimentary sequence. Physical and chemical analyses of the sediment and pore water, together with geophysical mapping, reveal glacial as well as postglacial imprints implying an oscillating ice sheet margin, evidence for neotectonic activity and one or more marine incursions into the lake during deglaciation. We attribute the glaciotectonic deformation of the sediments at 54 m below the lake floor to an ice readvance that likely occurred at the same time or before the advance that formed the Levene moraine (,. After this event, potential readvances were likely restricted to a more northerly position in the basin. We identify the final drainage of the BIL, but find evidence for an earlier marine incursion into the Vättern basin (,13.0 cal. ka BP), indicating water exchange between the North Atlantic and the Baltic Ice Lake during the late Alleröd.

Lake Vättern 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 Vättern 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 Vättern, 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 Vättern 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 Visingsö 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 Vättern basin and its deep proglacial‐dammed lake are likely to have encouraged the dynamic behaviour of this major Fennoscandian outlet glacier.

After a short historical review of the correlation of Palaeozoic rocks between Estonia and Sweden, this paper focuses on the results of marine seismic studies, achieved during the cooperative Swedish-Estonian project since 1990. The most recent seismic correlation scheme of the Cambrian, Ordovician, and Silurian strata and their distribution at the seafloor across the northern Baltic Proper are presented. Thickness changes and trends, as well as the sedimentary structures, reef bodies, and erosional features of different seismic units, are treated in connection with structural and facies changes in the Palaeobaltic Basin. The immediate background of this project is outlined and the locations of the seismic lines shot during the joint expeditions to the Baltic Sea are shown.