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  • Thumbnail for Numerical modeling of late-Pleistocene glaciers  in the Front Range, Colorado :  insights into LGM paleoclimate and  post-LGM rates of climate change
    Numerical modeling of late-Pleistocene glaciers in the Front Range, Colorado : insights into LGM paleoclimate and post-LGM rates of climate change by Gall, Ryan D.

    A 2D numerical model developed by Plummer and Phillips (2003) was employed to reconstruct the Middle Boulder Creek (MBC) and North St. Vrain (NSV) paleoglaciers, of the Colorado Front Range. The model was used to investigate two climatic aspects of the late-Pleistocene Pinedale Glaciation (~30,000-12,000 ka): 1. The specific combinations of temperature and precipitation present in the Front Range that may have sustained the MBC and NSV glaciers during the Last Glacial Maximum (LGM, ~21 ka), and 2. The magnitudes and rates of climate change that drove the Front Range ice recession following the LGM. The ArcMap 3.3-based model has two components, an energy/mass balance component that calculates an annual mass balance grid from input climate data and valley topography, and an ice flow component that utilizes the mass balance grid to calculate the glacial flow according to ice flow laws and topography. After several hundred model years, the glacier reaches a steady state geometry that is in mass balance equilibrium with the input climate. Determining the input climate parameters that sustain a modeled glacier at field-mapped extents therefore provides quantified insight into the regional climate. Modeling of the NSV system did not provide reliable results, likely due to issues concerning input modern climate data collected from secondary sources for use in this study. However, results from modeling the MBC glacier to its LGM mapped-extent suggest the glacier may have been sustained with temperature depressions of 5.0˚C, 6.6˚C, and 8.6˚C, respectively coupled with modern precipitation factors of 1.5x, 1.0x, and 0.5x, and is agreement with results of earlier studies. The rate of climate change was determined by modeling the MBC glacier to multiple CRN-dated ice margin locations associated with the post-LGM deglaciation (from Ward et al., 2009), and by assessing the temperature changes between intervals assuming a constant precipitation equal to today’s. This modeling suggests an initial warming of ~0.1˚C/ky from 21 – ~18 ka, a slight cooling period (~0.2˚C) and glacial stillstand from ~18 – ~16 ka, followed by rapid warming of ~0.7˚C/ky from ~16 – 13.5 ka. This first ever attempt to quantify Pinedale deglaciation rate of temperature changes in the Rocky Mountains has error associated the selected CRN dates (up to +/- 2.7 ky), and does not account for potential changes in precipitation. Despite the potential error, the quantified results provide appreciable generalized insight into the natural rate of climate change associated with post-LGM deglaciation.