NUMERICAL MODELING OF THE MIDDLE BOULDER CREEK PALEOGLACIER IN THE COLORADO FRONT RANGE: INSIGHTS INTO LGM PALEOCLIMATE AND POST-LGM RATES OF CLIMATE CHANGE

Advances in numerical modeling of alpine glaciers, combined with mapped Quaternary paleoglacier extents and deglaciation chronologies, have provided a new means to investigate the rate of climate change during the deglaciation period following the Last Glacial Maximum (LGM, ~21 ka). A 2D coupled energy/mass balance and flow model developed by Plummer and Phillips (2003) allowed multiple reconstructions of the Middle Boulder Creek paleoglacier (MBCG) of the Colorado Front Range, providing insight into the regional climate that could of sustained the glacier at its maximum LGM stand, and into the magnitudes and rates of climate change that drove ice recession following the LGM. The LGM climate assessment focuses on the temperature depression and/or precipitation change, relative to modern values, that could have sustained the MBCG at its field-mapped LGM extent. Results indicate that the MBCG LGM extent could have been sustained with temperature depressions of 5.0˚C, 6.6˚C, and 8.6˚C, respectively coupled with 150%, 100%, and 50% of modern precipitation, in agreement with results of similar Front Range LGM studies. A second aspect of this study is an attempt to quantify the rate of climate change during the deglaciation period following the LGM (~21 – ~12 ka). The rate of climate change was determined by modeling the MBCG to multiple CRN-dated ice margin locations associated with the post-LGM deglaciation (Ward et al., 2009), and by assessing the temperature changes between intervals assuming a constant precipitation equal to today’s. The modeling suggests an initial warming of 0.2˚C/ky from 21 – 18 ka, a cooling period of 0.1˚C/ky from 18 – 16 ka, and a greatly increased warming rate of 0.7˚C/ky from 16 – 13.5 ka. Despite uncertainty associated with CRN-dates (up to +/- 2.7 ky) and potential fluctuations in precipitation during the post-LGM deglaciation of the Middle Boulder Creek drainage, these results provide a significant quantitative step towards understanding the magnitude of the natural rates of climate change in the Colorado Front Range.