GEOMORPHIC AND CLIMATIC CONTROLS ON HOLOCENE SEDIMENTATION IN TWO PROGLACIAL LAKES: SWIFTCURRENT LAKE AND LAKE JOSEPHINE, GLACIER NATIONAL PARK, MONTANA

Glaciers are important geomorphic agents in alpine landscapes. Quantifying rates of glacial erosion is challenging; sediment cores collected from downstream lakes may yield important information on rates of sub- and supraglacial erosion in catchments. In summer 2005 we collected sediment cores from Swiftcurrent Lake and Lake Josephine, located near Many Glacier on the eastern side of Glacier National Park, Montana. Swiftcurrent Lake is the most distal of four lakes currently receiving sediment from Grinnell Glacier, a small cirque glacier located at the continental divide. We present preliminary results from XRD, grain size, and coulometry conducted on three cores, each ~6 m in length.

We attempt to characterize the erosive history of Grinnell Glacier during recent glacial retreat through sedimentological and mineralogical analyses of lake sediments. Age models based on tephrachronological fingerprinting of a Mazama Ash layer (~7500 ybp) suggest the longest Swiftcurrent Lake core spans ~10,000 years, and radiocarbon analysis on the three longest cores shows the average rate of deposition ranges from 0.5 to 1.0 mm/yr. The major minerals present in all cores include quartz, illite and clinochlore, likely reflecting erosion of the argillites and siltstones downvalley of the current glacier terminus. A greater abundance of dolomite is found in the entire Lake Josephine core, closer to Grinnell Glacier. We are interested in the origin of these dolomite packages and what their presence in the absence of other carbonate minerals can tell us about the lag time in deposition of eroded material into distal glacial lakes. Preliminary grain size analysis suggests variable lake levels over the period of record, and variability in the total organic carbon records correlate well with temperature records. We consider models for using mineralogical composition, grain size, and sedimentation rate found in sediment sequences to track glacial retreat through the varying lithologies of the Grinnell Glacier/Many Glacier valley during the Holocene.