A MULTI-PROXY INVESTIGATION OF THE WHITECROW GLACIER USING A SEDIMENT CORE FROM COSLEY LAKE IN GLACIER NATIONAL PARK, MONTANA

The Whitecrow Glacier in Glacier National Park lost nearly 50% of its area between 1966 and 2005, and studies have projected that most glaciers in the Park will disappear by 2030. In order to better understand the history of this glacier, a 503-cm long sediment core was retrieved from Cosley Lake about 5.5 km downstream from the Whitecrow Glacier. A dense diamicton interpreted as till was encountered at the base of the core, indicating that the record extends back to the latest Pleistocene deglaciation. That constraint, combined with 4 AMS radiocarbon analyses and a tephra layer assumed to be from the Mazama eruption, was utilized to develop a depth-age model for the core. Multiple sediment properties were investigated in the laboratory at 1- or 2-cm intervals, corresponding to an average resolution of 30-60 years/sample. Analyses included biogenic silica, organic matter, carbonate content, color spectrophotometry, magnetic susceptibility, bulk density, and grain size distribution. Low levels of biogenic silica indicate that the lake was not productive before about 8600 BP. Sediment near the base of the core has high values of mean grain size, redness (a*), and carbonate content. These values decrease steadily until 8600 BP after which they level out. This major shift at 8600 BP indicates a profound change in the style of sediment being delivered to Cosley Lake at that time, perhaps reflecting final progradation across the valley of the alluvial fan that now separates Cosley from the next lake upstream. In more recent sections of the core, the amount of organic matter and carbonate in the sediment fluctuate at a significant ~80-year period that may be driven by behavior of the Whitecrow Glacier. Oscillations in the abundance of fine silt also exhibit significant fluctuations of ~130 years. Together, these proxy time-series should support development of an environmental history of the Cosley Lake-Whitecrow Glacier system spanning from the latest Pleistocene to the present.