COSMOGENIC 10BE EXPOSURE AGES OF TERMINAL MORAINES IN THE WESTERN U.S.: RECONSIDERING PRODUCTION RATES AND CLIMATE CHANGES ACCOMPANYING THE LAST DEGLACIATION

Cosmogenic ¹⁰Be exposure dating has been widely applied to glacial features in the western U.S., and has become an essential tool for developing the chronology of the last Pleistocene glaciation. Observed spatial patterns in the timing of the last Pleistocene glaciation and deglaciation have been used to infer the timing and magnitude of local and regional-scale climate changes relative to global-scale climate changes. In many instances, however, comparison of ¹⁰Be exposure ages of glacial features with climate events assumes that the production rate of in situ cosmogenic ¹⁰Be is accurately and precisely known. The most precisely limited production rates are derived from measurements of ¹⁰Be concentrations of independently dated surface materials. Recent calibrations of in situ ¹⁰Be production rates yield values substantially lower than those used in the late 1990s and early 2000s, and about 13% less than the globally averaged production rate used by numerous authors to compute exposure ages of glacial features in the western U.S.

Here, a great number of cosmogenic ¹⁰Be exposure ages of late Pleistocene moraines in the Great Basin and Rocky Mountains are recalculated and reconsidered in the context of regional and global-scale climate changes. In these and other settings of the western U.S., published cosmogenic ¹⁰Be exposure ages of terminal moraines indicate an onset of overall deglaciation at 17-15 ka, coinciding approximately with the start of Bølling warming. These published ages become older by >2 ka when recalculated using recently calibrated lower ¹⁰Be production rates. An earlier start of deglaciation is more closely aligned with warming observed in a variety of records from northern middle latitudes and by the onset of rising CO₂. Regionally, synchroneity of moraine deposition and highstands of paleolakes in the Great Basin at ca. 17 ka is still observed, although with a substantially shorter duration of overlap between moraine deposition and the time when Lake Bonneville was overflowing. In the Northern Rocky Mountains (Wyoming and Montana), the start of deglaciation is still somewhat later than in many settings in the Middle and Southern Rocky Mountains, which is consistent with the hypothesis that precipitation changes were an important control on mountain glacier extent.