LATE PLEISTOCENE MOUNTAIN GLACIER EQUILIBRIUM LINE ALTITUDES IN THE GREAT BASIN AND NEIGHBORING SETTINGS IN THE SOUTHWESTERN USA

Much of the terrain in mountain regions across western North America was shaped by the conditions of Last Pleistocene glaciation. The Great Basin region, an area of closed drainage in southwestern North America, featured numerous mountain glaciers and lakes during the last glaciation including the large paleolake, Lake Bonneville. More than forty mountain ranges in the Great Basin region, feature abundant glacial deposits and landforms, such as lateral and terminal moraines, that enable reconstruction of Pleistocene glacier shapes and equilibrium line altitudes (ELAs) a quantitative measurement of paleoclimate. Additionally, previous work in the Great Basin has developed a cosmogenic ¹⁰Be chronology of moraines and glacially scoured bedrock, affording a temporal reconstruction of glacier changes through time. Here, we assess changes in paleo-glacier ELAs in mountains of the Great Basin and neighboring settings through the last glaciation based on reconstructed glacier shapes and cosmogenic exposure ages. Focusing on mountains near Lake Bonneville, cosmogenic-exposure ages are compiled in the informal cosmogenic nuclide exposure age database for alpine glacial features (ICE-D Alpine) and 3D glacier shapes are reconstructed using the GlaRe GIS tool. ELAs are estimated from the Area-Altitude Balance Ratio of the reconstructed glaciers. The cosmogenic chronology of Pleistocene moraines indicates that most glaciers occupied terminal moraines during the latter part of the global Last Glacial Maximum (26.5-19.0 ka) and many were near their maximum length during the overflowing phase of Lake Bonneville (18.0-15.5 ka). The latter observation has been interpreted to indicate that mountains downwind of Lake Bonneville maintained low glacier ELAs while the lake overflowed, possibly due to increased moisture availability from Lake Bonneville. Reconstructed ELAs support this idea, although glaciers throughout the region (not just downwind of Lake Bonneville) were near their maximum length while Lake Bonneville overflowed, and numerous other smaller lakes were at their maximum size. These observations suggest that regional climate favored the synchronous expansion of both glaciers and lakes in the Bonneville Basin during the early part of northern hemisphere deglaciation.