Implications for deglaciation chronology from new AMS age determinations in central west Greenland

New evidence has been obtained for the age of the Umivit/Keglen and Orkendalen moraine systems close to the present ice sheet margin in central West Greenland, The Umivit/Keglen moraine system is dated at 7504 to 6500 C-14 yr B.P., which is older than the previously assumed date of 7300 to 6000 C-14 yr B.P. The Orkendalen system is now dated at 6200 to 5600 C-14 yr B.P. against earlier estimates of 300 to 700 C-14 yr B.P. The new age is based on AMS radiocarbon-dated organic material within depressions between morainic ridges belonging to the Orkendalen system. A major implication of the new age is that ice margin positions prevailed for about 6000 years behind the present ice sheet margin. The retreat behind the present margin could be substantial, and in the light of deglaciation rates prior to the Orkendalen phase, may be ca. 10's of kilometers rather than kilometers, Circumstantial evidence is found for the retreat of the ice sheet margin behind its present position during the Holocene climatic optimum. The results, placed into a time frame of deglaciation since the last glacial maximum, enable comparison with Greenland ice sheet models and ice core records. (C) 1996 University of Washington.