A LOW GRADIENT OUTLET GLACIER OF THE PATAGONIAN ICE CAP AT LAGO BUENOS AIRES, ARGENTINA, DURING THE LAST GLACIAL MAXIMUM

Ice surface reconstructions of an outlet glacier of the Patagonian Ice Cap at Lago Buenos Aires (46°S, 70°W) indicate low surface slopes (gradients ~5-6 m/km) and low driving stresses (basal shear stresses 31-35 kPa) for the younger 3 of 6 LGM moraines. A "best estimate" of flow direction of the ice is reconstructed from measured striations and streamlined landforms, and ice surface contours are drawn perpendicular to these flowlines from positions along the moraine crests (elevation constrained with GPS and altimeter measurements).

We assume that streamlined landforms indicate ice flow direction to within ±10° of our best estimate and create two end member flow fields that lead to flatter and steeper profiles. Sensitivity tests show that driving stresses for these end member scenarios are not more than 25% from the best estimate. Another uncertainty in these reconstructions is the depth of the lake that occupies the valley. No bathymetry data are available so values of 50 m, 100 m, and 200 m are used, with 100 m being the best estimate. This uncertainty leads to a difference in basal shear stress of no more than 16% from the best estimate. Considering all of the above uncertainties, the range in driving stresses is 25-45, 29-45, and 28-47 kPa for each of the three moraines, with best estimates of 31, 33, and 35 kPa respectively. The ice surface slopes are similar for each moraine and range from 5.2 to 7.3 m/km. The lowering of the moraine surface due to melting of buried ice is a possibility, but is insignificant compared to the depth of the basin.

Whereas precise reconstructions of this outlet glacier are not possible, the results are consistently close to 30-35 kPa, much lower than the yield strength of ice. This indicates that the glacier was not frozen to its bed and some of the ice velocity was due to sliding and/or bed deformation. This is part of a broader effort to reconstruct the glacial and paleoclimatic history of this segment of the southern Andes.