LATE GLACIAL CLIMATE IN THE PIONEER MOUNTAINS, IDAHO, INFERRED FROM DEGREE-DAY MODELING

A degree-day model (DDM) was used to quantitatively reconstruct paleoclimate in the Pioneer Mountains during the Last Glacial Maximum (LGM; about 21-23,000 ka). The model simulates daily mean temperatures which are then used to determine the amount of snow or ice melt by means of a degree-day factor. Snow accumulation is based on the monthly distribution of precipitation, and the probability of snow for a given temperature. A specific net mass-balance curve is thus created by summing snow accumulation and melt at different elevations over a hydrologic year (October 1-October 1).

Validity of the model was tested using established values for model parameters (e.g., degree-day factors for snow and ice) and modern climatic variables recorded at local stations (e.g., mean annual temperatures, lapse rates, vertical precipitation gradients, and so forth). The model accurately reproduces modern snowpack evolution and yields modern snowline elevations consistent with observations.

Reconstruction of paleoglaciers suggests that LGM equilibrium-line altitudes (ELA) in the Pioneer Mountains were at approximately 2730 m. Assuming no significant changes in precipitation, the DDM requires mean annual (or alternatively, both mean January and mean July) temperatures 7.5 °C cooler than present for such ELA depression. A 50% reduction in annual precipitation requires only a slightly greater cooling of about 9 °C. The ELA appears to be more sensitive to changes in temperature than to those in precipitation. However, the hypsometries of reconstructed glaciers used in conjunction with specific net-balance curves yielded by the DDM suggest that zero net mass-balances can only be achieved with substantial reductions in annual precipitation. LGM climate in the Pioneer Mountains is thus inferred to have been both colder and much drier than today. This conclusion is consistent with other climate proxies and regional climate modeling.