THE LITTLE ICE AGE IN THE SIERRA NEVADA AND CASCADE MOUNTAINS: THE STORY FROM CIRQUE GLACIERS

Alpine moraine and lake-sediment records in the Sierra Nevada and Cascades provide a consistent story of glacier advance during the Little Ice Age (LIA), indicating that most cirque and small valley glaciers throughout these mountains were much smaller or absent immediately before the LIA, reached their Holocene maxima late in the LIA, and have retreated rapidly since then. Our work further suggests that sediment cores from lakes below small glaciers in crystalline bedrock may provide highly sensitive proxy records of glacier extent and health in such basins. Detailed field mapping, relative weathering constraints, and dendrochronological dating on moraines provide direct evidence that the Holocene maximum advance throughout the Sierra and much of the Cascades occurred during the last millennium, and, where constrained, during the last 200 years. This finding is consistent with historic observations and photographs showing glaciers in the region at or near their LIA maxima late in the 19th Century. Glacial rock flour deposited in lakes below modern glaciers provides a less direct but more complete record of glacier activity. Increases in fine clastic rock flour began shortly after ~600 cal yr. B.P., and culminated in the last 200 yr. In the case of the Palisade Glacier in the central Sierra Nevada, the rock flour record peaked ~A.D. 1800. This record is particularly noteworthy because the decline in rock flour after this peak resulted from thinning of the glacier rather than actual retreat from its moraine, which did not commence until the early 20th Century. This finding suggests that rock-flour production in such sediment-starved basins relies on sliding velocity as well as glacierized area, making such sites highly sensitive to glacier health (as reflected by ice-flow rate) as well as areal extent. Future studies focusing on close-sampling should therefore be able to take advantage of this relationship to develop higher-resolution records of late-Holocene glacier fluctuations. It's notable that records of Holocene glacier advances appear to be more complex on the major Cascade volcanoes and in the Coast Ranges of British Columbia, where the LIA advances may have been preceded by more extensive Neoglacial advances.