HARRISON GLACIER THROUGH THE HOLOCENE: A MULTI-PROXY LAKE SEDIMENT STUDY IN GLACIER NATIONAL PARK

Contemporary glacial retreat is strongly linked to anthropogenic climate change, making historical glacial behavior an important area of study. Harrison Glacier is located in Glacier National Park (GNP), Montana, at a mean elevation of 2541 m and an area of 1.4 km². Because it faces south-southeast, it is theoretically quite susceptible to melting. However, this glacier is the largest remaining and second-slowest retreating glacier in GNP. To better understand the history of the Harrison Glacier before the historical Little Ice Age advance, a 620-cm long sediment core was retrieved from Harrison Lake ~8 km below the glacier in July 2010. A depth-age model based on six AMS radiocarbon dates indicates that the core extends to 4 ka BP, with a resolution of ~6 years/cm. This high-resolution core was analyzed in the lab for multiple proxies including magnetic susceptibility, X-radiography, grain size analysis, bulk density, color spectrophotometry, loss on ignition, biogenic silica content, and carbon-nitrogen ratio. Organic content and biogenic silica content exhibit synchronous centennial-scale oscillations. Mean grain size, organic content, and biogenic silica were notably low during the Little Ice Age (~A.D. 1400-1850) while the abundance of fine silt was high, suggesting downstream effects of rock flour derived from the glacier. Older minima of mean grain size, organic content, and biogenic silica ca. 1500, 2400, and 3200 BP may represent other episodes of glacier expansion. The biogenic silica content of the modern sediment is near the lowest value reached in the record, while organic content is near a record high, suggesting that the current combination of biologic productivity in the watershed and meltwater delivery from the Harrison Glacier is unprecedented in the past 4000-years.