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ORIGIN OF THE EAST-WEST ASYMMETRY OF PALEO-GLACIERS IN THE SIERRA NEVADAS

KESSLER, MARK A  UC Santa Cruz and INSTAAR, University of Colorado.
Anderson, Robert S  INSTAAR, University of Colorado.
Stock, Greg M  UC Santa Cruz.

Maps of paleo-glaciers in the Sierra Nevada range indicate a strong east-west asymmetry in glacier lengths . West-side glaciers extended as much as ten times further than those on the east side of the range. Two hypotheses to explain this asymmetry involve differences in topography and differences in meteorology. Topography: there is a very strong asymmetry in the slope of the eastern and western flanks of the range. Much lower slopes and consequently longer valleys exist on the west side. The lower-sloped west side would have had much larger snow accumulation areas and proportionately larger ablation areas and glacier lengths. Climate: if most of the precipitation in the Sierra Nevadas was derived orographically as westerly storms rose up the western flanks, then these air masses would have been depleted of water creating a precipitation shadow on the eastern side and smaller glaciers.

We present a two-dimensional numerical model of glacier flow in which a realistic mass balance and DEM-derived topography can be specified. Finite difference methods are used to time step the continuity equation for the change in ice thickness, given fluxes in two horizontal dimensions by depth averaged ice deformation and basal sliding. Ice is added or removed each time-step in accord with the mass balance pattern. In addition, discrete transport processes, such as avalanching, have been implemented using cellular/stocastic methods to redistribute ice on steep slopes.

The relative simplicity and speed of this numerical model permits an exploration of parameter space for the appropriate climate conditions to match both terminal moraine elevations and trimlines in each of many valleys simultaneously. We employ the model to determine a realistic climate scenario (mass balance pattern) for the south fork of Kings Canyon on the western side of the Sierra Nevadas at the last glacial maximum (LGM). Several climate scenarios were tested to determine how much east-west asymmetry in the climate is necessary to explain the difference between the lengths of east- and west-flowing paleo-glaciers at the LGM. Finally, we explore whether this climate scenario can be ascribed to the expected pattern of orographically-derived precipitation.


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