Document Type

Honors Project - Open Access

Abstract

Glaciers are a key driver of bedrock erosion and sediment transport in alpine settings, and subsequent sediment deposition in proglacial lakes records glacial retreat, subglacial erosion rates, and other evidence of environmental change, like wildfires. Sedimentation in these proglacial lakes can reflect a variety of processes and characteristics such as glacier and lake size, proximity to the glacier, depositional patterns, other sources of sediment in the basin, and summer melt rates. Accurate dating of lake sediments in glacial systems can be challenging, particularly on sub-millennial timescales due to limited terrestrial organic matter in these environments. Atmospheric fallout of cesium-137 from aboveground nuclear weapon detonation has been used as a tracer for modern sediment age and deposition rates in northern hemisphere terrestrial landscapes; fewer studies have been done in the southern hemisphere, especially farther south than 50° S, and results are likely complicated by lower atmospheric concentrations recorded there. Peak cesium-137 activity in the cores is an approximate equivalent for peak atmospheric radionuclide fallout from 1962-1965 (Garcia Agudo et al., 1998). Here I analyze a downvalley transect of cores from two alpine lakes, the 100 km long ice-contact Lago Argentino in Argentina and from 2 km long Lake Josephine in Glacier National Park, Montana, the third in a series of paternoster lakes, to assess downvalley variability in sedimentation rates in two endmember glacial lake types.

Gamma-ray spectroscopy was used to measure cesium-137 decays at 2 mm to 1 cm intervals, which was converted to specific activity based on dried sample weight. In Lago Argentino, analyzed cores showed average post-1963 sedimentation rates from 23 mm/yr 20 km from the ice front, to 2.3 mm/yr 80 km downvalley in the main basin (after Van Wyk de Vries et al., 2022). Preliminary results suggest high and episodic sedimentation in ice proximal settings may complicate cesium-137 interpretation. At Lake Josephine, sedimentation rates varied from 0.4-0.8 mm/yr on the upvalley side from cesium-137 and lead-210 dating (Diener, 2015) to 0.4-0.5 mm/yr on the downvalley end using radiocarbon ages (Wydeven, 2009). While it is unlikely sediment production and delivery rates are consistent during this period of climatic variability, these rates are similar to average sedimentation rates found for this lake during the Younger Dryas and early Holocene based on bracketing volcanic ash ages. As expected, cesium-137 concentrations were higher in the North American glacial lake, but southern hemisphere levels were high enough to detect using standard techniques, and coupled with other tools such as varve counting (Van Wyk de Vries et al., 2022) provide a useful dating tool in glacial lakes.

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