Varve and radiocarbon dating support the rapid advance of Jakobshavn Isbræ during the Little Ice Age

Large outlet glaciers draining the Greenland Ice Sheet significantly influence overall ice sheet mass balance. Considerable short term (years to decades) retreat and fluctuations in velocity of Jakobshavn Isbræ, western Greenland, illustrate the complex nature by which large outlet glaciers respond to climate change, making predictions of future ice sheet change challenging. To provide a longer-term view (centuries), we investigate the geological record of Jakobshavn Isbræ change. We use continuous sediment records from lakes that were influenced by the recent advance of Jakobshavn Isbræ, which took place during the Little Ice Age. In particular, we explore the use of annually laminated lake sediments (varves) to precisely constrain the advance of the ice margin as it approached its late Holocene maximum extent. We find that the ice margin advanced recently, at least after ～1650 to ～1700 AD, and more likely ～1800 AD. We suggest that during this period Jakobshavn Isbræ advanced at a rate that was similar to its historically documented average retreat since ～1850 AD. Our results indicate that Jakobshavn Isbræ, and presumably other large marine calving glaciers, have the ability to advance quickly in response to climate forcing.