Slow retreat of a land based sector of the West Greenland Ice Sheet during the Holocene Thermal Maximum: evidence from threshold lakes at Paakitsoq

Records from two connected proglacial threshold lakes at Paakitsoq, west Greenland have been analyzed in order to investigate the response of a land-based ice sheet margin to Holocene climate change. The results are used to test whether or not the land terminating margin at Paakitsoq behaved synchronously with the nearby marine terminating Jakobshavn Isbræ during the Holocene. The radiocarbon dated lake sediment cores indicate that the ice margin retreated to its present position ~6.5ka ago and thereafter maintained a relatively stable configuration similar to the present for >1000 years, despite summer temperatures >2°C higher than today. The lakes became non-glacial after 5.4cal.kaBP, when the ice margin retreated behind a drainage divide situated ~1.5km inland of the present margin. By this time, Jakobshavn Isbræ had already reached its minimum configuration. The Paakitsoq ice margin remained >1.5km inland of its present margin until after 240±20cal.yrBP; after this time, both Paakitsoq ice margin and Jakobshavn Isbræ reached their Holocene maxima during the later stage of the Little Ice Age. Our results suggest that the present ice margin position at Paakitsoq is relatively stable in a warming climate but after a total retreat of ~1.5km behind the present margin position it may become marine based and more unstable due to marine melting and calving processes.