Advance and retreat of Grinnell Glacier during the last glacial maximum, Younger Dryas, and early Holocene as recorded in Lake Josephine sediments, Glacier National Park, Montana

Glacier National Park in Montana is known for its spectacular landscape, carved by valley glaciers. Understanding the complex pattern of glacial advance and retreat may provide important insights into the relationship between climate and glacial cycles in alpine environments, and may help us to better predict the future of such landscapes. In the Grinnell Glacier valley, the remaining glacier sits atop a carbonate formation of the Belt Supergroup. Previous studies of lake sediment cores within the drainage system (MacGregor et al., 2011; Schachtman et al., in press) have found higher concentrations of carbonate sediments in parts of the core inferred to be deposited during times of glacial advance, suggesting that the concentration of carbonate in sediments may be used as a proxy for periods when the Grinnell Glacier advanced. In this study, we tested this hypothesis by measuring carbonate concentrations in sediments deposited during the Pleistocene and Holocene (a period of over 6,000 years) from a glacially fed lake closer to the Grinnell Glacier. Carbonate concentrations in sediments are higher during inferred periods of cooling, such as the Younger Dryas, when the Grinnell Glacier is expected to have been advancing. Conversely, carbonate concentrations decrease during warming periods, such as the Bølling-Allerød and early Holocene. This supports our hypothesis that carbonate concentration may be used as proxy for glacial extent within the Grinnell Glacier valley.