Southern California Vegetation, Wildfire, and Erosion Had Nonlinear Responses to Climatic Forcing During Marine Isotope Stages 5–2 (120–15 ka)

A multiproxy record from Baldwin Lake, San Bernardino Mountains, allowed us to examine variation and relationships between erosion, wildfire, vegetation, and climate in subalpine Southern California from 120 to 15 ka. Bulk organics, biogenic silica, and molar C:N data were generally antiphased with magnetic and trace element data and displayed long-term (10⁵ year) shifts between autochthonous and allocthonous deposition. This was most pronounced during Marine Isotope Stage (MIS) 5, and we hypothesize that local summer insolation was the primary driver for Baldwin Lake's productive and unproductive lake state alternations. Wildfire history was inferred from charcoal concentrations and vegetation change from pollen. Relationships between these ecological processes, basin deposition, and summer insolation were often nonlinear. Sagebrush expansion, wildfire, and weak basin weathering characterized MIS 4, while during MIS 2, the basin was highly erosive, rarely burned, and the forest was impacted by shifts in Southern Californian hydroclimate. Despite coniferous forest cover throughout MIS 3, submillennial oscillations in charcoal, pollen, and bulk organic content occurred, consistent with pollen records from Eurasia's Mediterranean biome that span multiple glacial-interglacial cycles. Highly resolved global CO₂ records and sea surface temperatures in key regions of the Pacific show no apparent relationship to these landscape conditions, and we suggest submillennial hydroclimatic variability as a potential driver. Highly resolved long pollen records from Southern California are an urgent research need to better understand the finer-scale (≤10³ year) interactions between past vegetation, wildfire, and erosion, given the current natural disaster risks that 21st century climate change poses to both human and ecological communities.