PULSED RETREAT OF THE SIERRA NEVADA ICE CAP AFTER THE LAST GLACIAL MAXIMUM IN HOPE VALLEY, SIERRA NEVADA, CALIFORNIA

During the Last Glacial Maximum (LGM) the Sierra Nevada Mountains in California were covered by a large ice cap that spanned ~20,000 km². Glacial features from this region, such as moraines and striated bedrock, are ideally suited for ¹⁰Be cosmogenic exposure dating and can provide the timing of glacial occupation of the region. This study combines geological mapping with ¹⁰Be cosmogenic exposure dating to document the retreat of the Sierra Nevada Ice Cap at the end of the LGM in Hope Valley, ~17 km south of Lake Tahoe, CA. Sixteen ¹⁰Be exposure ages were obtained from three different moraines and two striated bedrock surfaces. Snow cover corrections were applied to our bedrock samples using historical snow water equivalent data, resulting in an ~11-12% increase to the apparent age. The oldest moraines in the northern part of Hope Valley have apparent boulder ages between 27 ± 1.5 and 103 ± 5.0 ka (n = 5), pre-dating the LGM. The Willow Creek moraine, three km south of the oldest moraines, provides a median boulder age for the maximum extent of the LGM at 20.0 ± 0.8 ka (n = 4). Two bedrock samples located 8 and 12 km south of the Willow Creek moraine have snow corrected ages of 17.7 ± 0.9 ka and 14.8 ± 0.9 ka, respectively. The Elephants Back moraine, in southern Hope Valley, provides a median age of 15.5 ± 1.3 ka (n = 5). This data suggests that between 20-15.5 ka, the ice cap retreat rate was ~2.5 m/yr. After 15.5 ka, the glacier stabilized for some time (1000-2000 years), before rapidly retreating and disappearing from the valley. The presence of moraines at ~20 and 15.5 ka suggests a time of glacial stability, whereas the intervening period was a time of glacier shrinkage. Overall, our glacial chronology is consistent with regional climate records that suggest warm and dry conditions from 20-16 ka when the ice cap would have shrunk, followed by a brief cold/wet period from ~16-14.5 ka, when the ice cap stabilized for a time and formed the Elephant’s Back moraines. After 14.5 ka, regional climate records suggest that warmer temperatures and lower winter precipitation caused the rapid retreat and disappearance of the Hope Valley glacier. Our results highlight the sensitivity of ice caps to subtle changes in temperature and precipitation and suggest the few existing glaciers in the Sierra Nevada have a bleak future in our warming climate.