Patterns of deposition in subalpine lakes during the late Holocene and Anthropocene, Glacier National Park, Montana, USA

Lake sediment cores reflect changing climate conditions as well as the complexities of spatial sediment sourcing, transport and deposition. In alpine valleys, glacier advance and retreat is often the primary driver of sediment flux. The source region of transportable sediment within the basin (valley floor, hillslopes, glacier headwall, channels) also evolves with the glacier footprint and therefore with climate. Runoff and sediment transport is likely to be enhanced due to ice loss during glacial retreat. Sediment transport and deposition in proglacial lakes near human infrastructure may complicate interpretation of lacustrine records. A sequence of lakes within two major valleys in the Many Glacier region of Glacier National Park, Montana, USA have multiple sediment sources which include glacial erosion, hillslope processes, and fluvial environments between lakes. In addition, the arrival of Euro-Americans in the region and the creation of a National Park with its concomitant infrastructure and visitation has likely affected sedimentation. We focus on a transect of cores from Fishercap Lake in the Swiftcurrent Valley to better understand variability in deposition rates and sources in the lake during the late Holocene and into the present, and compare this to sedimentation in adjacent (upstream and downstream) lakes to better understand the role of basin hypsometry and human impacts on subalpine valleys.

Fishercap Lake is less than 0.25 square km, shallow (~0.8 m), with a dense gravel layer less than a meter below the sediment-water interface that is uniform across the lake. Ground penetrating radar shows the gravel layer is a complex braided channel, reflecting a period of lake desiccation. Radiocarbon ages at the gravel unit are between ~1300-1660 AD; the most upvalley reach of the lake is 1-m deep with a basal age of 4400 radiocarbon years. High resolution C/N analyses of this core show changing organic sources over the late Holocene in response to climate variability during this time. Lead-210 ages in Fishercap Lake and two adjacent lakes show sedimentation rates are significantly higher in the last two centuries in all three lakes compared to late Holocene rates. Differences in the depositional records likely reflect lake morphology, basin hydrology, glacier proximity and geomorphic sources of sediment, despite identical climate forcing during this time. These observations have implications for our interpretations of lake core records of climate change in alpine valleys.