Organic molecular records of fire–hydroclimate–vegetation dynamics through the last deglaciation archived in a California stalagmite

The volatility of modern California wildfires emphasizes the importance of understanding long-term connections between fire, hydroclimate, and vegetation in the western United States. We use the abundance and distributions of pyrogenic polycyclic aromatic hydrocarbons (PAHs) entombed within a stalagmite as a novel proxy for past fire dynamics through the last deglaciation in the central Sierra Nevada (California). PAH flux at multi-centennial resolution reveals two periods of significantly increased wildfire activity (ca. 17.7−17.6 kyr B.P. and 15.4−14.9 kyr B.P.) within Heinrich Stadial 1 (HS1), a time associated with global cooling and major hydroclimatic shifts across the western United States. A third, weaker, increase in wildfire activity occurred at the Allerød−Younger Dryas transition (AL-YD; ca. 13.0−12.7 kyr B.P.). PAH distributions and pollen records suggest vegetation composition provided an underlying control on wildfire: peak fire activity during HS1 is characterized by lower combustion temperatures coincidental with higher regional proportions of arid herbs and shrubs, while the AL-YD exhibits unusually high fire activity characterized by higher-temperature burning and low proportions of these species. Changes in stalagmite δ13C, fluid-inclusion−derived deuterium excess, and phosphorus concentrations indicate that centennial- to millennial-scale periods of reduced effective moisture provided hydroclimatic conditions conducive to elevated wildfire activity within a moisture-limited fire regime. Comparison with regional charcoal records highlights the utility of PAHs to provide a more complete record of regional fire that is less biased by fuel type.