Constraining the Southern Part of the Greenland Ice Sheet since the Last Glacial Maximum from Relative Sea-Level Changes, Cosmogenic Dates and Glacial-Isostatic Adjustment Models

Sammanfattning: New results are presented from the investigation of relative sea-level changes in the Nanortalik and Qaqortoq-Narsarsuaq areas in SW Greenland from c. 11 000 cal. years BP to the present. Isolation and transgression sequences from seven lakes and seven tidal basins are identified using some of the traditional methods such as stratigraphic description, magnetic susceptibility, saturated induced remanent magnetisation (SIRM), loss on ignition, and macrofossil analyses. Macrofossils and bulk sediments have been dated by AMS radiocarbon dating. We also demonstrate that the use of the new XRF-scanning technique, combined with lithological description, pinpointed macrofossil analyses and radiocarbon dating, provide a quick and effective way of investigating isolation contacts/transgression sequences for the reconstruction of sea-level changes. In SW Greenland, the initial relative sea level fall was rapid and present-day level was reached at ~9000 cal. yr BP and continued falling until at least 8800 cal. yr BP. Between 8000 and 6000 cal. yr BP sea level reached its lowest level of around ~10 m below h.a.t. The late Holocene transgression was more gradual as it occurred over a longer time interval. Present sea-level was reached between ~2000-0 cal. yr BP. We have used glacial-isostatic adjustment models to determine the ice sheet evolution in southern Greenland from the LGM until the present. The sea-level observations from the Nanortalik and Qaqortoq areas are used to constrain the different ice-model scenarios tested. In situ produced cosmogenic 10Be and 26Al in bedrock and erratics give inference concerning the basal regime in the two areas. Our ice sheet history reconstruction shows an ice sheet extending to the shelf edge from 26,500 cal. yr BP until 22,000 cal. yr BP, followed by rapid retreat. By 12,000 cal. yr BP, the ice margin was inland of the present-day coast and by 10,500 it had reached the present margin. The ice sheet was smaller than at present from 10,500 cal. yr BP and reached a minimum of 30 km inland of the present-day margin at 9000 cal. yr BP. The neo-glacial re-advance started before 6500 cal. yr BP and the present-day margin was reached by 5500 cal. yr BP. The ice sheet was cold-based in the Nanortalik area, but warm-based and eroding in the Qaqortoq area during the Late-glacial.