Deglaciation of the Sierra Nevada (USA) during Heinrich Event 1

Abstract. A polar jet stream (PJS) split by the Laurentide Ice Sheet (LIS) is a well-established feature of Ice-Age atmospheric circulation. California’s central Sierra Nevada Mountains (37–38° N) lie near the reconstructed position of the PJS’s southern branch. Previous studies concluded that rapid deglaciation began here at ca. 16–15 ka after millennia of relatively stability at ~60 % LGM length. However, this conclusion is largely based on the behavior of glaciers in a single valley, Bishop Creek Canyon. We report 31 new ¹⁰Be samples from two new locations – Lyell Canyon and Mono Creek Canyon – and 26 recalculated ³⁶Cl dates from Bishop Creek Canyon (n = 57). These dates indicate rapid deglaciation began at 16.4 ± 0.8 ka and lasted for ca. 1.0 kyr. Placing two previously published paleoenvironmental reconstructions (Swamp Lake and McLean’s Cave) with centennial-or-better-scale resolution on new age-depth models that provide age-uncertainty estimates, we find evidence for warming in the central Sierra Nevada at 16.4 ± 0.4 ka and drying at 16.20 ± 0.13 ka. Collectively, we interpret that rapid deglaciation began at 16.20 ± 0.13 ka. This timing is indistinguishable from that of Heinrich Event 1 (HE1), which occurred between 16.22 ± 0.04 ka and 16.04 ± 0.04 ka. We hypothesize that the Sierra Nevada’s deglaciation was driven by a northward repositioning and focusing of the winter-storm track over western North America in response to PJS reunification, bringing warmer and drier weather to the central Sierra Nevada, and that PJS reunification occurred in response HE1 thinning the LIS.