Using proglacial-threshold lakes to constrain fluctuations of the Jakobshavn Isbrae ice margin, western Greenland, during the Holocene

The future response of the Greenland Ice Sheet (GIS) and its potential contribution to sea level rise are uncertain. Rapid changes of Greenland's outlet glaciers over the past decade have made it difficult to extrapolate ice sheet change into the future. This significant short-term variability highlights the need for longer-term, geologic (e.g., Holocene) records of ice margin fluctuations. However, a major challenge with reconstructing the GIS during the Holocene stems from it having been smaller than it is at present, thus traditional glacial geologic approaches are not suitable. We use radiocarbon-dated sediment sequences from seven proglacial-threshold lakes spanning similar to 50 km of the western GIS margin near Jakobshavn Isbrae to constrain the timing of early Holocene deglaciation, the duration that this sector of the western GIS was smaller than its present configuration, and the timing of its advance during Neoglaciation. Our reconstructions suggest deglaciation similar to 7300 cal yr BP, minimum ice extent similar to 6000-5000 cal yr BP and smaller-than-present ice configuration until at least similar to 2300 cal yr BP for the ice margin south of Jakobshavn Isbrae, and until similar to 400 cal yr BP for the ice margin north of Jakobshavn One relatively large proglacial lake that became briefly ice-free during the middle Holocene lies in a catchment that likely extends 10s of km inland beneath the GIS, suggesting significant middle Holocene retreat of this portion of the GIS. The overall pattern of ice sheet change is inconsistent with existing ice sheet model reconstructions for this region, but is consistent with numerous paleoclimate proxy and relative sea level data. These continuous lacustrine records corroborate, but provide closer age control than, existing non-continuous records of radiocarbon-dated reworked bivalves from historical moraines in the region. Reconstructing ice margin change from proglacial-threshold lakes is one of few approaches with the potential to constrain smaller-than-present ice sheet extent. (C) 2010 Elsevier Ltd. All rights reserved.