Southern California climate, hydrology and vegetation over the past ~96 ka from Baldwin Lake, San Bernardino Mountains, California

Abstract:

Continuous paleoclimate records are scarce from terrestrial sites in Southern California beyond the Last Glacial Period (i.e. Marine Isotope Stage 2, MIS 2). Baldwin Lake in the Big Bear Valley, San Bernardino Mountains (SBM), is a playa lake in the ecotone between desert and Mediterranean climate and vegetation. We recovered a 27 m core from the site in 2012, which spans ~96 – 10 ka, based upon radiocarbon dating, infrared stimulated luminescence dating, and orbital tuning. Total organic content, total carbonate content, density, magnetic susceptibility, x-ray fluorescence, and grain size data show a lake system that responded in tandem with Marine Isotope State transitions. After the basin closed during MIS 5b, Baldwin Lake was productive for MIS 5a, then cycled through an inorganic phase to a highly organic lowstand by the end of MIS 4. A stratified lake of rapidly-deposited organic silt prevailed throughout MIS 3, then shifted to an inorganic, slow sedimentation regime during MIS 2. Paleoecological data (charcoal and fossil pollen) suggest that the Valley was most prone to wildfire during climate transitions (e.g. the end of the Last Glacial Maximum, ~21 ka). Forest cover was dominated by pine for much of the basin’s history, save for the dry period at the onset of MIS 2, and a greater presence of oak woodland at the beginning of MIS 3. The reduced pine cover and increased sagebrush steppe in early MIS 2 suggests a more arid landscape of sagebrush steppe c. 29 – 25 ka, before reverting to wet conditions by the LGM. Throughout MIS 5a – 2, lake organic content fluctuates in tandem with solar radiation values; a possible link between lake productivity and insolation is currently being explored with biogenic silica (BiSi) analysis. The lake was desiccated by ~10 ka, perhaps driven by increasing insolation rates at the onset of MIS 1.