LATE PLEISTOCENE GLACIATION AND DEGLACIATION OF THE NORTHERN MEDICINE BOW MOUNTAINS, WYOMING: CHRONOLOGY, MODELING, AND PALEOCLIMATE IMPLICATIONS

At the Last Glacial Maximum (LGM) a small ice cap complex covered higher portions of the northern Medicine Bow Mountains in southern Wyoming. Surface-exposure dating of moraine boulders and glacially polished bedrock surfaces, combined with paleoglacier modeling employing a coupled energy/mass balance and flow model, clarifies the timing and climate of the local LGM, and the pacing and climatic and topographic forcing of subsequent deglaciation. Sixteen ¹⁰Be surface exposure ages from the Libby Creek drainage on the east side of the range indicate that following the ~21ka local LGM, glaciation in the drainage was reduced to about 60% its LGM extent at ~18ka, and to less than 10% at ~14ka. Modeling indicates that if temperature depression alone was responsible for LGM glaciation, a depression of ~6.0°C from present would have been necessary. Paired values of 8.0°C temperature depression and half modern precipitation or 3.2°C depression and twice modern precipitation could also have sustained the LGM glacier. The subsequent almost complete deglaciation by 14 ka could have resulted from a post-LGM temperature increase of only 1.8°C, involving a modest (~270m) rise in equilibrium line altitude (ELA). This sensitivity to small changes in ELA reflects range hypsometry – with large areas of low-relief upland surface only slightly above the altitude of the LGM ELA.

The initiation of deglaciation in the range prior to ~18 ka likely reflects forcing by rising northern hemisphere solar radiation prior to the start of global atmospheric CO₂ increase (~17 ka). The subsequent main deglaciation of the range between about 18 ka and 14 ka probably reflects additional forcing by increased atmospheric CO₂ and changing ice-sheet albedos. Changes in North Atlantic circulation and the consequent Bølling-Allerød warming may also have contributed, but only during the final phases of deglaciation.

Our glacier-model simulations suggest a considerably smaller LGM temperature depression in the Wyoming/Colorado region than do most climate models and some other glacier-based models. These differences are the focus of ongoing investigation.