Paired bedrock and boulder Be-10 concentrations resulting from early Holocene ice retreat near Jakobshavn Isfjord, western Greenland

We measuredin situcosmogenic10Be in 16 bedrock and 14 boulder samples collected along a 40-kmtransect outside of and normal to the modern ice margin near Sikuijuitsoq Fjord in central-westGreenland (69N). We use these data to understand better the efficiency of glacial erosion and to inferthe timing, pattern, and rate of ice loss after the last glaciation. In general, the ages of paired bedrock andboulder samples are in close agreement (r2¼0.72). Eleven of the fourteen paired bedrock and bouldersamples are indistinguishable at 1s; this concordance indicates that subglacial erosion rates are sufficientto remove most or all10Be accumulated during previous periods of exposure, and that few, if any,nuclides are inherited from pre-Holocene interglaciations. The new data agree well with previously-published landscape chronologies from this area, and suggest that two chronologically-distinct landsurfaces exist: one outside the Fjord Stade moraine complex (w10.30.4 ka;n¼7) and another inside(w8.00.7 ka;n¼21). Six10Be ages from directly outside the historic (Little Ice Age) moraine show thatthe ice marginfirst reached its present-day positionw7.60.4 ka. Early Holocene ice margin retreatrates after the deposition of the Fjord Stade moraine complex werew100e110 m yr1. Sikuijuitsoq Fjordis a tributary to the much larger Jakobshavn Isfjord and the deglaciation chronologies of these two fjordsare similar. This synchronicity suggests that the ice stream in Jakobshavn Isfjord set the timing and paceof early Holocene deglaciation of the surrounding ice margin.