THE YOUNGER DRYAS RECORDED IN LACUSTRINE SEDIMENTS OF EASTERN GLACIER NATIONAL PARK, MONTANA, USA

Alpine environments are extremely sensitive to changes in climate. It is therefore critical to understand how they react to rapid climate change in order to predict future responses. The Younger Dryas (YD) global cooling event was particularly intense throughout Europe, with some evidence of cooling in North America. However, the response of cirque glaciers in North America to this cooling was not uniform, and few records document the glaciologic or geomorphic response to the YD in the western U.S.

Here we characterize the transition from the late Pleistocene to the early Holocene, encompassing the YD period, in the northern US Rocky Mountains using sediment cores from Swiftcurrent Lake in eastern Glacier National Park, MT. Grinnell Glacier sits in a cirque basin at an elevation of ~2100 m, sourcing a chain of three meltwater lakes before Swiftcurrent Lake. Using an existing age model from ash and radiocarbon ages, we focus on a ~60 cm section of the core representing ~13.2 to 10.4 ka. We sampled the core at 0.5-1 cm intervals, and analyzed percent total carbon (%TC), grain size, carbon/nitrogen (C/N), and mineralogy. Preliminary results show that %TC varies between 0.5 and 3.5%. Total carbon is low until 12.7 ka, increasing abruptly and staying at ~2% until decreasing again at ~11.7 ka. After 11.7 ka, %TC increases steadily into the early Holocene. C/N values suggest primarily algal contributions to organic carbon deposition in the lake, ranging from 2 to 10 with an overall increase in C/N from 13 to 10.5 ka. Mean grain size generally increases from clay to fine silt between 13 and 10.5 ka. Dolomite presence, likely sourced from the subglacial and englacial Helena Formation at Grinnell Glacier, is more frequent between 13 and 11.3 ka than in the period after the YD. Our preliminary results suggest the YD began abruptly in the region at ~12.7 ka, and began to wane ~11.5 ka. Increased %TC, C/N and grain size during the YD suggest the period may have been cool and dry, with increased geomorphic activity on hillslopes contributing sediment and terrestrial organic material to the lake. However, our data suggests that conditions may have been variable during the YD. Additional radiocarbon and ash age controls, along with %TOC data will allow us to more accurately constrain the timing and impacts of the YD in northern US Rocky Mountains.