LATE HOLOCENE FLUCTUATIONS OF THE MAMMOTH GLACIER, WIND RIVER RANGE, WYOMING

Multiple sediment cores from Pleistocene moraine dammed lakes (Upper and Lower Green River Lakes) in the Wind River Range, Wyoming, preserve a detailed record of late Holocene glaciation. The primary source of glacier outwash to the lakes is from Mammoth Glacier, the largest (~1.9 km²) glacier on the western slope of the Wind River Range. As a result, these lakes trap the majority of the rock flour produced by the glacier, preserving a proxy record of fluctuations in size and activity of the glacier. Our cores include a long core (3.5 m) from Upper Green River Lake and numerous shorter, higher resolution cores from both lakes. The age model for the cores in the upper lake is based on 6 AMS radiocarbon analyses. The youngest part of this age model will be refined by ²¹⁰Pb analyses that are in process.

Rock flour preserved in the deep cores from the upper lake (recorded in the visual stratigraphy – VS, organic content – OC, and magnetic susceptibility – MS) began to increase shortly after ~1000 cal yr B.P., increasing rapidly after ~500 cal yr B.P. This increase is capped by a dramatic surge in clastic sediment recorded in VS, OC, and MS and defines the maximum rock flour flux to the lake. The age model indicates that this event occurred at ~280 cal yr B.P., shortly before the maximum for the Little Ice Age (LIA) in the region (~165 cal yr BP: Schuster, 2000). Similar sedimentation in the lower lake is muted but consistent with this sequence. Sediments predating the last millennium in the upper lake maintain relatively high MS values and low OC values, with a modest peak in MS centered at ~3900 cal yr BP.

The increase in rock flour after 1000 cal yr B.P. likely records the growth of the Mammoth Glacier at the onset of the LIA in the range. The large surge in clastic sedimentation at 280 cal yr B.P. appears to record an outburst flood from a small lake (Scott Lake) immediately below the glacier. The coincidence of the outburst with the maximum rock flour flux suggests a causal link between the flood and the LIA maximum of Mammoth Glacier. The relatively constant high MS and low OC values in sediments deposited between ~4500-1000 cal yr B.P. suggests that the Mammoth Glacier was active then but significantly smaller than the LIA maximum.