EVIDENCE OF CHANGES IN GLACIAL THERMAL REGIME: A TEST CASE FROM NEVADO SAJAMA, BOLIVIA

Basal thermal regime is a primary control on glacial erosion and deposition. For example, glacial erosion rates can vary by up to two orders of magnitude depending rather the glacier bed is frozen or not. However, insights to the temporal evolution of basal regime is lacking for small glacier systems. Recent field investigation on Nevado Sajama in the Western Cordillera, Bolivia provides geomorphic evidence of wet based glacial activity in an area that currently contains only cold based ice. Thus, glaciers have changed thermal regime at sometime in the past.

A numerical model has been used to estimate past equilibrium line altitudes (ELA) and glacial extents in Aychuta Valley on the west side of Sajama. Model inputs include paleo-precipitation data from the Sajama ice-core, tropical paleo-temperature data derived from noble gas concentrations in ground water, and precipitation and temperature lapse rates derived from local meteorological observations.

Preliminary results indicate that environmental conditions during the past 25 ka (the length of the ice-core) have not been simultaneously warm and wet enough for our model to predict a glacial margin extending below the 0 oC isotherm. Thus, we hypothesize that wet based ice has not existed on the mountain during this time period implying that the outermost moraines which formed under wet based conditions are older than 25 ka. A field campaign has been undertaken to test the modeling results by dating glacial moraines on Nevado Sajama.