Description: Based on swath bathymetry, sediment echosounding, seismic profiling and sediment coring we present results of the RV „Polarstern“ cruise ARK-XIII/3 (2008) and RV "Araon" cruise ARA02B (2012), which investigated an area between the Chukchi Borderland and the East Siberian Sea between 165°W and 170°E. At the southern end of the Mendeleev Ridge, close to the Chukchi and East Siberian shelves, evidence is found for the existence of Pleistocene ice sheets/ice shelves, which have grounded several times in up to 1200 m present water depth. We found mega-scale glacial lineations associated with deposition of glaciogenic wedges and debris-flow deposits indicative of sub-glacial erosion and deposition close to the former grounding lines. Glacially lineated areas are associated with large-scale erosion, accentuated by a conspicuous truncation of pre-glacial strata typically capped with mostly thin layers of diamicton draped by pelagic sediments. Our tentative age model suggests that the youngest and shallowest grounding event of an ice sheet should be within Marine Isotope Stage (MIS) 3. The oldest and deepest event predates MIS 6. According to our results, ice sheets of more than one km in thickness continued onto, and likely centered over, the East Siberian Shelf. They were possibly linked to previously suggested ice sheets on the Chukchi Borderland and the New Siberian Islands. We propose that the ice sheets extended northward as thick ice shelves, which grounded on the Mendeleev Ridge to an area up to 78°N within MIS 5 and/or earlier. These results have important implication for the former distribution of thick ice masses in the Arctic Ocean during the Pleistocene. They are relevant for global sea-level variations, albedo, ocean-atmosphere heat exchange, freshwater export from the Arctic Ocean at glacial terminations and the formation of submarine permafrost. The existence of km-thick Pleistocene ice sheets in the western Arctic Ocean during glacial times predating that of the Last Glacial Maximum (LGM) also implies significantly different atmospheric circulation patterns, in particular availability and distribution of moisture during pre-LGM glaciations.

Description: In the western Arctic Ocean glacial landforms are interpreted as a complex pattern of Pleistocene glaciations along the continental margin of the East Siberian Sea and the Chukchi borderland. These landforms include moraines, drumlinized features, glacigenic debris flows, till wedges, mega-scale glacial lineations (MSGL), and iceberg plough marks. Orientations of some of the landforms suggest the presence of former ice sheets on the Chukchi Borderland and the East Siberian shelf. In seismic and sub-bottom profiles as well as sediment cores, there is evidence that glaciations have occurred repeatedly. Typically, several generations of glacial wedges intercalate with well-stratified (interglacial) sediments in ice-distal locations. MSGL of former ice grounding in present water depths of more than 1200 m suggests that some ice sheets developed significant thickness and size. The extent of glacial features and deposits into the Arctic Ocean decreased with time. We interpret this as indication that ice sheets in the western Arctic Ocean were thicker and larger during earlier times of the Pleistocene and became restricted to the Chukchi Borderland during the most recent glaciation (Last Glacial Maximum, LGM). Finally, icebergs intensively ploughed the sediments along the Chukchi and East Siberian margin in a range from 350m to 80m present water depth. In water depth shallower than 80m, sub-bottom profiles in the East Siberian Sea exhibit acoustic facies more typical for submarine permafrost. Discontinuous (permafrost) reflectors mask sub-bottom strata beneath an unfrozen 10m thick top sediment layer. In places, unfrozen sediment-filled depressions (taliks) are visible to about 20m below the seafloor, which may be related to former thermokarst and/or channels. We suggest that only during the LGM permafrost formed in the exposed area of the entire East Siberian Sea, whereas some areas have been largely covered by ice sheets during previous glacial periods.