Southern California Climate and Vegetation Over the Past 125,000 Years from Lake Sequences in the San Bernardino Mountains

Long sediment records from offshore and terrestrial basins in California show a history of vegetation and climatic change since the last interglacial (130,000 years BP). Vegetation sensitive to temperature and hydroclimatic change tended to be basin-specific, though the expansion of shrubs and herbs universally signalled arid conditions, and landscpe conversion to steppe. Multi-proxy analyses were conducted on two cores from the Big Bear Valley in the San Bernardino Mountains to reconstruct a 125,000-year history for alpine southern California, at the transition between mediterranean alpine forest and Mojave desert. Age control was based upon radiocarbon and luminescence dating. Loss-on-ignition, magnetic susceptibility, grain size, x-ray fluorescence, pollen, biogenic silica, and charcoal analyses showed that the paleoclimate of the San Bernardino Mountains was highly subject to globally pervasive forcing mechanisms that register in northern hemispheric oceans. Primary productivity in Baldwin Lake during most of its history showed a strong correlation to historic fluctuations in local summer solar radiation values. Rapid organic perturbations in the Baldwin Lake core were coeval with Dansgaard-Oeschger (D-O) events from the North Atlantic, which were evident in records from the Santa Barbara Basin. The predominant vegetation signal at Baldwin Lake was one of temperate conifer forest expansion during moist conditions, and contraction during dry conditions. This expansion and contraction was paced with summer insolation fluctuations during Marine Isotope Stage 5 (110,000 – 71,000 years BP), before a regime change towards more rapid, shorter-lived hydroclimate extremes. Wildfire is an important agent of landscape change throughout the Valley’s history, with two hiatuses during cold and moist conditions from 25,000 – 14,000 years BP, and arid conditions 7,000 – 3,000 years BP. Taken together, this multi-proxy dataset suggests that paleoclimatic changes in alpine southern California have been highly sensitive to three climate drivers: 1) shifts in local summer insolation, 2) rapid warming in the North Atlantic, and 3) changes in the strength of the California Current. The record also showed a wide range of possible moisture states in southern California’s past that vary from present conditions, including multi-millennial states of long-term aridity and pre-glacial deep lake conditions