HOLOCENE HISTORY OF THE GREENLAND ICE SHEET MARGIN IN NUNATARSSUAQ, NORTHWEST GREENLAND

Defining the Holocene fluctuations of the Greenland Ice Sheet (GIS) margins, in particular during periods that were as warm as or warmer than present, provides a longer-term perspective on present-day marginal changes. We are investigating the past fluctuations of the GIS in the Nunatarssuaq region in Northwest Greenland. In summer 2014, we conducted geomorphic mapping, obtained sediment cores from ice-proximal lakes and collected samples for surface exposure dating. We also searched for and obtained subfossil plants that have been exposed by recent ice retreat. Here we present preliminary results from locations near the modern GIS margin allowing for the investigation of past ice-margin fluctuations.

Our geomorphic mapping and preliminary surface exposure dating suggest that much of the Nunatarssuaq region was covered by ice until approximately 15 ka. More proximal to the modern GIS margin, we mapped glacial landforms including grounding lines, deltas, and paleo-shorelines. Together, these features suggest that the region now occupied by Nordsø, a large proglacial lake, was filled to a higher level. The present-day outflow of Nordsø is a narrow river that flows south, through a large deposit of unconsolidated sediments. The previously higher lake level likely resulted from damming by a more extensive GIS margin. We are currently processing surface exposure samples to help determine when the high lake level occurred. In addition, basal radiocarbon ages from sediment cores collected from a modern lake will provide constraints on the high lake level. We mapped a drift limit approximately 30-50 m distal to the modern GIS margin. This drift is characterized by fresh unweathered and commonly glacially molded and polished clasts. The age of this drift will also be constrained by surface exposure samples. Preliminary radiocarbon ages of subfossil plants suggest that the GIS margin was near or behind its present location at ~1650-1800 A.D. and ~4,600-4,850 yr BP. The results thus far indicate that the Northwest GIS margin responded sensitively to Holocene climate changes.