MOUNTAIN GLACIER RETREAT RATES DURING THE LAST PLEISTOCENE GLACIATION IN THE WESTERN UNITED STATES

Cosmogenic ¹⁰Be exposure ages of terminal moraines deposited by mountain glaciers during the last glaciation are well established and provide useful limits on the time of glacier maxima. Age limits on recessional moraines and other glacial features representing ice retreat are relatively fewer in number in the western U.S., but can provide insights about how climate changed during the last deglaciation interval of the Pleistocene. Retreat rates of glaciers in several mountains of the Great Basin and Rocky Mountains were determined by recalculating cosmogenic ¹⁰Be exposure ages of terminal and recessional moraines and other glacial features using newer models of in situ production of ¹⁰Be in quartz. Although exposure ages of terminal moraines and retreat rates vary among glacial valleys, results of our calculations reveal some consistency in the regional pattern of ice retreat. Retreat rates from terminal moraines were at a maximum during the early part of deglaciation at 19-17 ka, ranging from 2-7 m/yr. After glacier lengths were substantially reduced by 17 ka, retreat rates decreased dramatically to 1 m/yr or less until ca. 15 ka. After 15 ka, retreat rates increased to ca. 2-5 m/yr. A relatively rapid retreat from 19-17 ka reflects the maximum sensitivity of glacier lengths to warming induced by increasing solar radiation and atmospheric CO₂ concentrations. Reduced retreat rates at 17-15 ka likely reflect changes in ice dynamics, thickness and shape, but may also indicate a response to regional cooling associated with the Oldest Dryas/Heinrich Stadial 1. The interval 15-13 ka, corresponding to the Bølling-Allerød interval, regional warming is the most likely cause of the increased retreat rates. These findings are further considered in the context of the results of glacier modeling experiments and regional compilations of paleoclimate records.