A 9.2 KA PALEOECOLOGICAL HISTORY OF LOWER BIG BEAR LAKE, SAN BERNARDINO MOUNTAINS, CA, AND IMPLICATIONS FOR HOLOCENE CLIMATE CHANGE IN THE U.S. COASTAL SOUTHWEST

Big Bear Valley (elevation ~ 2060 m) in the San Bernardino Mountains of California spans the climatic and vegetation transition between Mediterranean and Mojave desert. The original Big Bear Lake basin has a continuous lacustrine record that includes much of the Holocene (9.2 – 0.5 ka), and here new paleoecological proxy data (e.g. charcoal, pollen, molluscan fauna and isotope data) are presented that demonstrate Big Bear Lake’s sensitivity to regional climate change. Evidence for a moist early Holocene from ~9 to 7.5 ka includes a vegetation assemblage dominated by aquatic plants and coniferous trees, variable charcoal accumulation, and near-absence of littoral-habitat mollusks, indicating a relatively high lake level. From 7.5 to 4.3 ka, arboreal taxa decline in favor of a shrubs and perennial groundcover, charcoal accumulation is quiescent, and near-shore mollusks flourish. These data suggest greater aridity both locally and regionally. Moist conditions return beginning 4.3 ka, though are not analogous to the Early Holocene, as they are interrupted by widespread, long-lasting drought events. Both pollen and charcoal accumulation are highest in the late Holocene (beginning c. 2 ka), a time of landscape reorganization. Prior sedimentological and geochemical work also indicates that the site was characterized by centennial-to-multidecadal pluvial episodes for the duration of the Holocene (Kirby et al., 2012), though a driving mechanism for episodic moisture delivery remains unclear. Hydroclimatic variability in the coastal southwest US during the Holocene remains a subject of debate, but it was likely due to some combination of eastern Pacific ocean-atmosphere processes, including dipping westerlies and enhanced atmospheric river activity.