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

Defining the late Pleistocene and Holocene fluctuations of the Greenland Ice Sheet (GrIS) margin provides a longer-term perspective on present ice margin fluctuations and informs how the GrIS may respond to future climate conditions. We focus on mapping and dating past GrIS extents in the Nunatarssuaq region of northwestern Greenland. In summer 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. ¹⁰Be ages of boulders deposited during deglaciation of the GrIS subsequent to the Last Glacial Maximum range from ~15-80 ka. The apparently older ages of these samples indicate the presence of ¹⁰Be inherited from prior periods of exposure. We suggest that the youngest ages from this range provide a maximum-limiting constraint on deglaciation at ~15 ka. Subfossil plants exposed at the GrIS margin on shear planes date to ~ 4.6-4.8 cal. ka BP and indicate less extensive ice during middle Holocene time. Radiocarbon ages from in situ subfossil plants on a nunatak date to ~3.0-3.2 cal. ka BP. Down valley from the nunatak, a suite of ¹⁰Be ages from a glacial drift limit range from 2.1-6.0 ka. The ¹⁴C and ¹⁰Be ages provide evidence for a neoglacial advance of the GrIS margin that began by ~3 ka. Geomorphic mapping of glacial landforms near Nordsø, a large proglacial lake, documents the existence of a higher lake level. We suggest that the ~3 ka advance of the GrIS blocked the modern drainage pathway of Nordsø and dammed the lake at a higher level. A fresh drift limit 30-50 m distal to the modern GrIS margin, characterized by lichen-free clasts, is estimated to be late Holocene in age. However, ¹⁰Be ages of boulders within the drift range from 550 yr to 2.2 ka, which may indicate that some boulders contain inherited ¹⁰Be. In situ subfossil plants exposed by the recent retreat of the GrIS margin suggest that the GrIS was less extensive between ~1760-1890 AD. The combined mapping and new chronological data indicate that the GrIS margin in northwestern Greenland responded sensitively to Holocene climate changes.