LATE GLACIAL AND HOLOCENE HISTORY OF THE GREENLAND ICE SHEET MARGIN, NUNATARSSUAQ, NORTHWESTERN GREENLAND

Delineating the late glacial and Holocene fluctuations of the Greenland Ice Sheet (GrIS), particularly during periods that were as warm as or warmer than present, provides a longer-term perspective on present ice margin fluctuations and can inform how the ice sheet may respond to future climate conditions. We focus on mapping and dating past GrIS extents in the Nunatarssuaq region of northwestern Greenland. In the summer of 2014, we conducted geomorphic mapping and collected rock samples for ¹⁰Be surface exposure dating and subfossil plant samples for ¹⁴C dating. We also obtained sediment cores from an ice-proximal lake. Preliminary ¹⁰Be ages of boulders deposited during deglaciation of the GrIS subsequent to the Last Glacial Maximum (LGM) range from ~37-15 ka. The apparently older ages indicate the presence of ¹⁰Be inherited from prior periods of exposure. The youngest age is interpreted as an approximate age of ice recession from the LGM, however further data are needed to test this hypothesis. Glacial landforms mapped near Nordsø, a large proglacial lake, including grounding lines, moraines, paleo-shorelines, and deltas, indicate the existence of a higher lake level that resulted from a more extensive GrIS margin likely during Holocene time. ¹⁰Be samples from these geomorphic features are being processed and analyzed. Subfossil plants exposed at the GrIS margin on shear planes date to ~4.6-4.8 cal. ka BP and in situ subfossil plants on a nunatak date to ~3.1 cal. ka BP. These ages indicate less extensive ice during middle Holocene time. Approximately 30-50 m outboard of the modern GrIS margin is a fresh drift limit characterized by unweathered, lichen-free clasts and is estimated to be late Holocene in age. Additional radiocarbon ages of subfossil plants exposed by recent retreat of the GrIS margin suggest that the GrIS was at or behind its present location at AD ~1650-1800 and ~1820-1890. Results thus far indicate that the GrIS margin in northwestern Greenland has responded sensitively to Holocene climate changes. Ongoing research will improve the chronological constraints on these fluctuations.