Paper No. 10
Presentation Time: 11:15 AM


SCHACHTMAN, Nathan1, MACGREGOR, Kelly2, HENCIR, Nora Rose2, BRADTMILLER, Louisa3 and MYRBO, Amy4, (1)Geology, Macalester College, 1600 Grand Ave, St. Paul MN, MN 55105, (2)Geology, Macalester College, 1600 Grand Avenue, St. Paul, MN 55105, (3)Environmental Studies Department, Macalester College, 1600 Grand Avenue, Saint Paul, MN 55105, (4)LacCore/CSDCO, Department of Earth Sciences, University of Minnesota, 500 Pillsbury Dr. SE, Minneapolis, MN 55455,

It is important to understand how alpine landscapes have responded to rapid climate change in the past in order to predict and model future responses. The transition from the Last Glacial Maximum into the Holocene in western North America is typified by a gradual warming trend, punctuated by rapid cooling and subsequent warming during the Younger Dryas (YD) period. Few records in North America document the geomorphic and glaciologic response of the landscape to these climate transitions.

We investigated the late Pleistocene to Holocene glacial and geomorphic history of the northern U.S. Rocky Mountains using sediment cores from Swiftcurrent Lake in eastern Glacier National Park, MT. Swiftcurrent Lake is the eastern-most lake in a chain of four lakes sourced by Grinnell Glacier. The current age model shows the top ~6 m of core spans 13 ka; pending additional radiocarbon age control we anticipate the last ~3 m of the core dates to ~24 ka. We analyzed the core at 0.5 – 1 cm resolution for total carbon (%TC), total inorganic carbon (%TIC), carbon/nitrogen (C/N), grain size, and mineral content. Preliminary results show %TIC declines between ~24 and 13 ka. From 13 ka to 10.5 ka, %TIC values increase rapidly before decreasing into the Holocene. The inorganic carbon is sourced from the Helena Formation, a dolomite unit underlying Grinnell Glacier; previous work suggests the dolomite presence can be used as a proxy for glacier size. This implies Grinnell Glacier was sizeable but retreating during this time, except for the brief YD interval, where it was advancing. C/N values suggest primarily algal contributions to organic carbon sources in the lake, with an overall increase in C/N from 17 to 10.5 ka, indicating more terrestrial input to the lake. Mean grain size decreases between 20 and 15 ka from fine silt to clay, suggesting an increasingly distal source of sediment. Decreasing grain size and %TIC during the Late Pleistocene suggest a retreating Grinnell Glacier with additional lakes acting as sediment sinks between the glacier and Swiftcurrent Lake. During the YD an increase in %TIC and grain size values suggest a rapid resurgence and subsequent decline of the glacier. Additional radiocarbon and ash age controls will provide more accurate constraints on the timing of geomorphic responses to climate change in the northern U.S. Rocky Mountains.