Of Sea and Smoke: Evidence of Marine Dissolved Organic Matter Deposition from 2020 Western United States Wildfires

The record-setting wildfires that ravaged the western United States throughout 2020 released high concentrations of organic carbon (C) into the environment, including the adjacent Pacific Ocean. Yet little is known about the fate of marine wildfire-derived C, solubilized as dissolved organic matter (DOM), despite growing observations of ash deposition in such systems. We sought to quantify and characterize DOM inputs to Pacific surface waters spanning the California coastline from August 1 to October 31, 2020. Across over 290 field samples, dissolved organic C concentrations peaked 2-to 4-fold higher after the eruption of fire systems than immediate pre-wildfire levels. C concentrations were well correlated with atmospheric pyrogenic proxies PM2.5 and ozone, supporting pyrogenic sourcing. Molecular characterization of DOM by ultrahigh-resolution FTICR-MS revealed both a diversity of formulas, supporting a growing consensus of pyrogenic heterogeneity, and temporal shifts conserved across sites. An initial increase in highly aromatic, oxygen-containing compounds aligned with PM2.5 concentrations, burn extent, and C deposition. Over time, transformation to increasingly aliphatic DOM occurred. The latter is hypothesized to be a result of complex interplay between biotic and abiotic processes, warranting further study. Our observations suggest that wildfires are a substantial yet dynamic source of marine surface organic C.