EQUILIBRIUM-LINE ALTITUDE RECONSTRUCTION IN THE SUBTROPICAL ANDES DURING THE LAST GLACIAL MAXIMUM

The arid subtropical Andes are currently unglaciated between 18°30’S and 27°S because there is not enough moisture to sustain glaciers in the region today. Previous studies suggest this area was glaciated during the Last Glacial Maximum (LGM; 21 ka); however, little is known about the extent of the glaciation or the climate required to sustain it. Glaciers respond in part to changes in temperature. Temperatures during the LGM in the subtropical Andes were colder, with estimates from proxy records and modeling results ranging from 4°C to 12°C colder than modern. Previous studies have also found that glaciers in the subtropical Andes responded to changes in humidity. Analyzed global climate model (GCM) output data, acquired from the third Paleoclimate Model Intercomparison Project (PMIP3), suggests that colder temperatures in combination with an increase in humidity during the LGM would have been capable of sustaining glaciers in the subtropical Andes.

Despite these climatic conditions that would have supported glaciation during the LGM, there is limited observational evidence. Changes in the equilibrium-line altitude (ELA) can be used to characterize the history of glaciation, and changes in ELA between the LGM and modern in the subtropical Andes can be used to further our understanding of the regional climate during the LGM. Using a glacial mass-balance model in conjunction with GCM output, we calculate the change in ELA between modern and LGM climates in the subtropical Andes. The model uses positive degree-days, the number of days in a year in which the mean annual air temperature is greater than 0°C, to calculate ablation. Ablation is then assumed to be proportional to temperature in order to calculate the change in ELA. Using output from eight GCM simulations, we compare the change in ELA between LGM and modern climates across the different models for the subtropical Andes. These simulations suggest that the changes in climate resulted in a sufficient lowering of ELAs to support glaciation in the subtropical Andes during the LGM.