PLEISTOCENE GLACIAL RECONSTRUCTION OF THE FISH LAKE HIGHTOP PLATEAU: IMPLICATIONS FOR CLIMATE AT THE LAST GLACIAL MAXIMUM

The Fish Lake Hightop Plateau (2700 – 3500 m) in south-central Utah experienced at least two episodes of Pleistocene glaciation. The geographic extent of eight, late Pleistocene paleoglaciers (Last Glacial Maximum- LGM) were determined through field mapping of moraines and topographic map and aerial photography interpretation. Paleoglaciers were reconstructed with a spreadsheet model that generated glacier longitudinal profiles constrained by bedrock morphology, moraine crest elevations, basal shear stress and shape factor (the ratio between the “hydraulic radius” and centerline ice thickness) for each glacier. Equilibrium line altitudes (ELAs) were determined for reconstructed glaciers with accumulation-area ratio, toe-to-headwall-area ratio, median altitude of lateral moraines, and cirque headwall altitude methods. ELAs respond directly to variations in climate, and the difference between modern and Pleistocene ELAs is used as a proxy for climate change. No glaciers presently exist in southern Utah, thus direct measurement of modern ELAs is not possible. Regression to the 0ºC isotherm from summer climate data (Loa, Utah) with an assumed atmospheric lapse rate of 6ºC/km indicates a modern ELA of 4900 m. ELAs for reconstructed Fish Lake glaciers range from 2950 to 3250 m, a depression of 1650 to 1950 m from the current equilibrium line altitude.

General circulation models suggest a colder, drier climate in the western U.S. at the last glacial maximum. If precipitation is held constant over the last 21 thousand years, and ELA depression is attributed solely to summer temperature depression, the magnitude of the measured ELA depression in this study corresponds to a 10 to 12ºC lowering of summer season temperatures at the LGM. The temperature depression is likely overestimated, because little is known about the possible lake enhancement of pluvial Lake Bonneville on increased precipitation in the High Plateaus or the overall precipitation regime of south-central Utah at the LGM. Regardless of the uncertainties due to the dynamic relationship between temperature, precipitation, and their combined effect on glacier mass balance, the ELA depression in southern Utah measured in this study suggests LGM summer temperatures were around 10ºC cooler than modern values on the Fish Lake Hightop plateau.