DEEP PLEISTOCENE ICEBERG PLOWMARKS ON THE YERMAK PLATEAU - SIDESCAN AND 3.5 KHZ EVIDENCE FOR THICK CALVING ICE FRONTS AND A POSSIBLE MARINEICE-SHEET IN THE ARCTIC-OCEAN

The southern Yermak Plateau (northwest of Spitsbergen) was investigate d with 11-12 kHz sidescan sonar and 3.5 kHz profiler to test whether t hick Pleistocene ice streams ever calved into the Arctic Ocean and/or whether a continuous Arctic ice sheet existed. In either case, the ice would have grounded on the plateau crest on its way out of the Arctic . Several distinct sets of relict iceberg plowmarks were discovered at present water depths from less than 450 m to at least 850 m, perhaps recording the deepest iceberg keels known. The plateau crest (510-530 m) is nearly bare of plowmarks, perhaps because of beveling by an ice rise formed of grounding shelf ice. This implies continuous, 400-600-m -thick grounded ice, consistent with but not proving the postulated ex istence of an Arctic ice sheet. Most plowmarks are probably late Pleis tocene products of the calving of Barents-Kara ice sheets into the Arc tic Ocean. Plowmarks in 450-850-m-deep water require unusually deep ic e-sheet grounding lines (e.g., Svyataya Anna Trough) and/or draft-incr easing capsizing events, even if sea levels were 100-150 m lower than today. At water depths of 800 to 1800 m, the sidescan texture and topo graphy suggest bottom currents descending the northeast slope of the p lateau.