The stability of the Greenland Ice Sheet through warm interglacial periods has long been under debate. Scientists turn to a number of different indicators including ice from the base of ice cores, sediment samples from Greenland southwestern lakes, 10Be exposure dates from glacial erratics, and more recently 10Be dates from rock under the base of the ice sheet to establish ice sheet extent and thickness through time. During our current interglacial, the Holocene, the biggest challenge has been determining the ice extent during the warm Thermal Maximum (~7,400 years ago), a time period that is high priority as the most recent analog to future warming and yet the evidence has been overprinted by an expanded ice sheet.
The Snow on Ice project is using continuous sediment sequences collected from the bottom of pro-glacial lakes, created by ice sheet meltwater and lying along the ice perimeter, and matched with new bed machine sub-elevation maps created from radar data, to hone in on the ice sheet margin during the Thermal Maximum.
Locked inside the lake sediment is a history of how much time the lake spent under ice cover versus ice-free. The lakes selected for coring have drainage basins that extend beneath the current ice sheet edge and are fed by the ice sheet meltwater (see top image). When the ice sheet covers the lake it collects mineral-rich glacial sediments from the movement of the ice over the rock. When the ice sheet retreats out of the lake basin the sediment transitions to organic-rich mud or gyttja. Collecting sediment cores and radiocarbon dating sections of the core where clear shifts in sediment lithology are evident is critical in providing a time series of ice cover for the area.
A careful bathymetric survey of the bottom of each selected lake is the preliminary step in sediment coring. If the area has too much accumulation the core will not be able to span the length of time needed for the Holocene. An area with too little accumulation will not provide the detailed history needed for the glacial expansion and retreat. A map is created to provide a blueprint for working in the lake.
Sediment cores are assessed in the field, and visually described, but the bulk of the analysis will occur back in the lab. Cores will be split, analyzed, and dated. The results are matched with sub-ice elevation maps created from radar data that provide the distance the lake drainage basins extend under the existing ice cover. It is the combination of the drainage basin maps and the lake sediment dates that provide a powerful tool for locating the ice sheet margin during the last Thermal Maximum.