SIMULATING THE DYNAMIC BEHAVIOR OF A SIERRA NEVADA GLACIER IN RESPONSE TO THE FORCING IMPLIED BY A NEW HIGH-RESOLUTION RECORD OF YOUNGER DRYAS TEMPERATURE VARIATIONS

The late-Quaternary glacial chronology of the Sierra Nevada, California includes a glacial advance described by the Recess Peak type moraines, located in the high cirques of the range, not far distant from the few remaining active glaciers. Based on estimated paleoglacier shapes, and assuming an accumulation area ratio (AAR) similar to that of many modern glaciers, these moraines typically evidence an approximately 200-m depression in glacial ELA. In previous studies, age constraints on the Recess Peak deposits have been obtained through radiocarbon dates on moraine-dammed lake sediments and from boulder age determinations via in-situ accumulation of cosmogenic nuclides. These ages are in good agreement, and the former, more precise, dates suggest that the advance slightly predated the Younger-Dryas period (YD) of the North Atlantic region. The climatic change that led to development of the Recess Peak moraines is thus of considerable interest as a clue to the global changes occurring at that time. Recent studies of chironomid taxa in modern lakes and in sediment cores from Sierra Nevadan lakes have resulted in the first relatively high-temporal-resolution reconstruction of climate variations during an interval encompassing the YD. Using preliminary data from those studies – a time-series of temperature variations during the latest Pleistocene - and a 2-D snow/energy balance model, we calculated the implied changes in net annual snow accumulation that would have occurred in an eastern Sierra basin, where the Recess Peak advance is well mapped. Using those changes as input to a transient, 2-D, in-the-plane, glacier flow model, we simulated the advance and retreat of glaciers that would result from that forcing. The simulations demonstrate both the general magnitude of the resulting glacial advance, as well as the accelerations and decelerations of the glacier snout that control the development of terminal and lateral moraines. Results suggest that the chironomid-based record of temperature variations would have resulted in glacier advances very similar to those described by Recess Peak moraines, but at a slightly later date than is currently accepted for those deposits.