New geophysical evidence for a revised maximum position of part of the NE sector of the Greenland ice sheet during the last glacial maximum

Abstract

The extent and dynamics of the north-eastern sector of the Greenland ice sheet (GIS) during the last glacial still remain very uncertain, and maximum ice extents to inner-shelf, mid-shelf and outer-shelf positions have been suggested. Here we argue that the margin of the GIS reached the shelf break along part of its NE sector during the last glacial maximum. Swath-bathymetry and sub-bottom profiler data from the outermost part of the Store Koldewey glacial trough reveal the first documentation of moraine ridges/grounding-zone wedges in this area, supporting ice expansion to the shelf break at ~76°N. A complex pattern of retreat moraines in the outer trough reflects a stepwise early deglaciation. It is suggested that this behavior is not in line with a deglaciation triggered by an abrupt sea level rise. According to established glacial geomorphological models this should have resulted in ice lift-off and a sea floor dominated by landforms formed during full-glacial conditions (streamlined bedforms) and ice disintegrations (iceberg plough marks). Instead, formation of the recessional moraines must have involved other mechanisms resulting in a step-wise ice retreat. This could possibly be related to melting of the grounded ice controlled by a temperature increase of the ocean as inferred for other parts of the Greenland ice sheet. An ice front position at the shelf break instead of at the inner or mid-shelf also implies a larger ice volume for this sector of the GIS during the LGM. It also implies a larger contribution of meltwater from GIS during the deglaciation.