RECONSTRUCTING HOLOCENE ICE SHEET MARGIN HISTORY NEAR JAKOBSHAVN GLACIER, SOUTHWEST GREENLAND, USING RADIOCARBON DATED PROGLACIAL-THRESHOLD LAKE SEDIMENTS

Throughout Earth’s climate history, there are periods of warming and cooling. The past 10,000 years have experienced warming, both due to natural variability and human impact. To understand the impact of warming on the Greenland Ice Sheet, scientists study sedimentological records of past ice sheet change, such as those preserved in proglacial lakes. Proglacial lakes experience influxes of glacial sediment via meltwater when ice sheets are more extensive, and an absence of glacial silt during small ice sheet configurations. One ice margin that is experiencing rapid retreat is Jakobshavn Isbrae (Ilulissat Glacier) of the Greenland Ice Sheet in western Greenland. To document this glacier’s response to the Holocene climate change, we studied sediment from proglacial lakes fronting the ice sheet. Proglacial lakes Infinity Lake and Ted Lake (informal names) were studied using an array of techniques including radiocarbon dating, Itrax XRF elemental geochemistry methods, and magnetic susceptibility. The focus of this presentation is on Ted Lake, a proglacial lake currently not receiving meltwater. The core provides information spanning to ~7300 cal yr BP based on radiocarbon analysis. The bottom 15 cm is inorganic silt, characterized by elevated magnetic susceptibility (MS) and titanium (indicators of mineral-rich sediment), and low chlorin content (an indicator of organic matter). The middle 80 cm is organic-rich sediment with low MS, and titanium, as well as high iron and chlorin values. The uppermost 15 cm transitions back to inorganic silt, characterized by similar values from the bottom 15 cm. During the last deglaciation, the ice margin retreated out of Ted Lake’s catchment at 7290±120 cal yr BP. The lake catchment then remained ice free until 420±80 cal yr BP, at which time the Greenland Ice Sheet readvanced into it. Ted Lake is now a non-glacial lake due to the current recession of the ice margin, and does not receive any meltwater from neighboring lakes. This new information adds to the knowledge of Holocene ice changes and can be used for future projections of ice sheet modeling in response to climate change.