Proglacial lake records of recent environmental change in Glacier National Park, Montana, USA

Abstract We examined trends in sediment deposition, organic carbon sourcing, and carbon and nitrogen isotopes in a transect of four lake sediment cores from eastern Glacier National Park (GNP), Montana, USA to understand how a connected chain of subalpine lakes downvalley from a retreating Grinnell Glacier have responded to environmental change over the last two centuries. Based on 210 Pb ages, all three lakes showed a two- to five-fold increase in mass accumulation rate (MAR), with increased MAR beginning at most sites just prior to when GNP was established in 1910 CE, and again at the start of “The Great Acceleration.” Changes in MAR as a result of glacier retreat occurred at the most upvalley site, complicated by shifts in lake size and hydrology. A decrease in C:N ratios and slightly enriched δ 13 C values since ∼1850s CE reflect a shift toward decreasing terrestrial organic contributions and increased lake productivity. Concurrently, δ 15 N values were increasingly depleted across all sites over time. The most downvalley site captured spikes in MAR, C:N, and δ 13 C coincident with recorded flood events. This work demonstrates how organic geochemical and isotopic proxies together capture evolving connectivity between glaciers, catchments, lakes, and human activity under a warming climate.