LAST GLACIAL MAXIMUM CLIMATE IN THE SOUTHERNMOST ROCKY MOUNTAINS, USA

Both climate models and low-altitude lacustrine and speleothem records suggest that at the time of the last glacial maximum (LGM), ca 21±2 ka., conditions in the US Southwest were cool and substantially more moist than today. In this study, we assess LGM climate at high altitude in the southern Rocky Mountains of New Mexico, where climate models also suggest increased LGM moisture. This work represents the first modeling of LGM paleoclimate based on evidence from the small paleoglaciers of the southernmost Rocky Mountains. Reconstruction of LGM glacial ice extents focused on two glacial valleys in the southern Sangre de Cristo Mountains, near Santa Fe. A 2-D numerical model of glacier energy/mass balance and flow (Plummer and Phillips, 2003) was used to evaluate LGM climate. The model yields combinations of changes in precipitation and temperature from present conditions that could have sustained glaciers at their LGM extents. We found that any temperature depression of less than 8.4°C, would have required greater-than-modern precipitation to sustain LGM glaciers. Temperature depressions of 6°C, 7°C, and 8°C would need to have been associated with 2.8x, 1.9x, and 1.3x modern precipitation conditions, respectively. Results also show that temperature depressions of 9° and 10°C would require climate drying, with 0.8x and 0.4x modern precipitation, respectively. Comparison of these results with those of modeling using the same approach in the Rockies further north indicates either that LGM temperature depression was greater in the southernmost Rockies than further north or that precipitation was enhanced in the south. Global and regional climate models suggest that (1) LGM temperature depression in the southern Rockies was no greater, and likely less, than that further north and (2) it is unlikely that in the southern Rockies temperature depression was as great as 8.4°C. Thus, we conclude that an increase in precipitation, relative to modern conditions, was necessary to support the glaciers we modeled in the southernmost Rocky Mountains. Findings from this study encourage future use of the numerical model to investigate LGM climate conditions throughout the U.S. Rocky Mountains.