DATING THE DEGLACIATION OF TUOLUMNE MEADOWS AND LYELL CANYON IN YOSEMITE NATIONAL PARK WITH 10BE

Given the importance of glaciers in shaping Yosemite National Park’s landscape and the long history of glacial research in the park, surprisingly little is known about the timing of glacial retreat since the Last Glacial Maximum (LGM). Dühnforth et al. (2010) previously dated 28 bedrock surfaces in a transect along the Tuolumne River from the LGM margin to Mount Lyell. Many of their bedrock samples, however, contained inherited ¹⁰Be and, instead of documenting the rate of ice recession, their dates confirmed the importance of bedrock fractures to the rate and style of glacial erosion. To minimize the problem of inheritance, we targeted boulders—dating 18 with ¹⁰Be in Tuolumne Meadows and Lyell Canyon. Other than one anomalously old age (21.5 ka), all our dates range between 13.0 and 15.5 ka. (These ages were calculated with the CRONUS ¹⁰Be calculator hosted at ku.edu and used “Lal/Stone” time independent scaling.) The inverse-variance weighted mean age of these 17 boulders is 14.1±0.4 ka (95% confidence, analytical uncertainty only). Assuming a uniform 15% snow correction (as Dühnforth et al. did) shifts the weighted mean age to 16.6±0.5 ka. Because we sampled boulders, however, and they would have been above at least a portion—perhaps all—of the winter snowfall, this is the largest plausible snow correction for our samples. The deglaciation of Tuolumne Meadows and Lyell Canyon is therefore likely no later than 14.1 ka and no earlier than 16.6 ka. Glaciers in the Sierra Nevada were at their maximum extent between 17 and 20 ka. Therefore, our data indicates that the Tuolumne Glacier lost ~90% of its length and ~95% of its elevation range within a few thousand years. This is consistent with data from elsewhere in the Sierra Nevada Mountains and coincides with meltwater pulse 1A and rapid warming in Greenland.

 

Reference: Dühnforth, M., R. S. Anderson, D. Ward, and G. M. Stock. 2010. Bedrock fracture control of glacial erosion processes and rates: Geology. v. 38: 423-426. 10.1130/G30576.1.