T47 The Last Glacial–Interglacial Transition in Western North America: Paleoclimate Proxies, Environmental Responses, and Data-Model Comparisons

Sunday, 9 October 2022
1:30 PM - 5:30 PM
503

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

Paper number: T47-5
Program: Technical Sessions
Time: 2:25 PM – 2:40 PM

Authors

Abstract

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.

Geological Society of America Abstracts with Programs. Vol. 54, No. 6

doi: 10.1130/abs/2022AM-379494

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