USING COSMOGENIC ISOTOPES TO RECONCILE TWO CONTRASTING HYPOTHESES FOR THE EXTENT OF THE GREENLAND ICE SHEET DURING THE LAST GLACIATION

In the East Greenland fiord zone, interfiord uplands and coastal lowlands are commonly strongly weathered, whereas the fiord and valley troughs are characterised by fresh-looking surfaces of glacial erosion. Erratic boulders exist on both glacially scoured and unscoured terrain. In spite of the fact that the surface morphology of the unscoured terrain shows no sign of glacial overriding, the erratics reveal that ice has indeed been there. Based on the degree of weathering and on the lack of identified glacial deposits, these areas have been suggested as ice free at least during the last glacial maximum (LGM) and with the erratic boulders being transported to their present location during an earlier all-inundating advance of the Greenland Ice Sheet (GIS). However, the concept of a restricted LGM glaciation has recently been challenged by both marine and land-based field data, indicating that the GIS reached the continental shelf break at this time. Our research is aimed at applying cosmogenic isotopes to weathering zone research in East Greenland to reconcile the two contrasting hypotheses for ice extent and dynamic behaviour of the GIS during the LGM. Samples have been collected from boulders and bedrock in glacially scoured and unscoured terrain and thus far we have preliminary ¹⁰Be results from two sites. In Jameson Land (70°N), ages of three upland erratics (c. 800 m asl) lie between 17 and 48 ka. On Store Koldewey Island (76°N) one upland boulder (617 m asl) has a ¹⁰Be age of 19 ka, whereas a nearby tor is >130 ka. The preliminary five ¹⁰Be results so far suggest that uplands in the East Greenland fiord zone were covered by cold-based ice; however, it is still inconclusive whether this ice was dynamically connected to the GIS or not during the LGM.