A HISTORICAL RECORD OF CLIMATE CHANGE AND HUMAN IMPACT IN GLACIER NATIONAL PARK, MONTANA, USA: USING LACUSTRINE SEDIMENTS TO UNDERSTAND GEOMORPHIC, CLIMATIC, AND ANTHROPOGENIC INFLUENCES ON A GLACIATED ALPINE VALLEY

Geochemical analysis of lacustrine sediment cores from a chain of three lakes in Glacier National Park, MT show the region's response to increasing human activity, climate change, and glacial retreat within the last 200 years. These­­­­ lakes span ~8 km along the length of a valley just east of the continental divide from their source, the rapidly retreating Grinnell Glacier, to the base of the drainage basin where cabins, campgrounds, well-traveled roads, and the largest hotel in the park line the lakeshore.

We analyzed cores spanning 1790 A.D. to the present from the three lakes at 5 to 10 year resolution for carbon/nitrogen (C/N), carbon and nitrogen isotopes (δ¹³C and δ¹⁵N), mass accumulation rate (MAR), and percent total organic and inorganic carbon (%TOC and %TIC). MAR increases markedly throughout the 20^th century, especially in the two downvalley lakes where sedimentation rates in the last 30 years are up to three times higher than pre-1900 accumulation. All three lakes display decreasing C/N and δ¹³C values toward the present, indicating increasingly algal sources of organic material. This shift is especially prominent in the most upvalley lake, where at ~1930 C/N values decrease abruptly. In the lake most proximal to roads and buildings, δ¹³C values decrease steadily toward the present, from -25 ‰ to -28.5 ‰ beginning in 1910, the year Glacier National Park was established. Data for the midvalley lake extends back to ~600 A.D. The initiation and termination of the Little Ice Age is clear, characterized by elevated %TIC, C/N and δ¹³C records, however, geochemical trends observed in the midvalley lake in the last 200 years are subtler than in the other two lakes. This suggests that this record may primarily reflect regional climate, rather than localized human activity downstream or glacial and hillslope responses upstream. Within the last two centuries, the general trends in all data sets are similar across the valley, but the timing and magnitude of these changes differ. Preliminary interpretations suggest that all three lakes have been impacted by warming climate and increased human activity, but vary depending on their proximity to more local controls such as hillslope activity, Grinnell Glacier, and human influences.