HYDROCLIMATE RESPONSE OF SPRING ECOSYSTEMS TO A TWO-STAGE YOUNGER DRYAS EVENT IN WESTERN NORTH AMERICA

The Younger Dryas (YD) climate event is the preeminent example of abrupt climate change in the recent geologic past. Climate conditions during the YD were spatially complex, and high-resolution sediment cores in the North Atlantic, western Europe, and East Asia have revealed it unfolded in two distinct stages, including an initial stable climatic period between ~12.9 and 12.2 ka associated with weakening of the Atlantic Meridional Overturning Circulation (AMOC) followed by unstable conditions until 11.7 ka as the AMOC recovered. Despite decades of investigations, unequivocal evidence of this stepped phenomenon has not been identified in western North America. Here we present new hydroclimate data from paleospring deposits from the Rogers Basin in northern Death Valley National Park (California, USA) that document high groundwater levels persisted in this part of the valley during the entirety of the YD event and demonstrate that two-stage partitioning of the YD event occurred in temporal synchroneity with proxy records elsewhere in the Northern Hemisphere. Stable and long-lived spring ecosystems prevailed in the Rogers Basin between ~13.0 and 12.2 ka, and were immediately replaced by alternating wet and dry environments until ~11.8 ka. This hydrologic record provides robust evidence of the two-stage nature of the YD event, firmly establishes that this intra-YD climate transition extended into western North America, and shows that even short-lived changes in the AMOC can have profound consequences for ecosystems worldwide. Further, these results provide valuable data for climate modeling and other studies aimed at reconstructing hydroclimate conditions in western North America during the late Pleistocene.