Reconstructing wildfire regime during the warm Paleocene-Eocene climate using molecular biomarkers from the Chuckanut Formation…

Wildfires are expected to increase in frequency and severity as climate changes in the Pacific Northwest of North America. To better understand the effect of warming climate on wildfire regimes, I present the first reconstruction of past wildfire frequency and severity during the Paleocene-Eocene transition in northwest Washington state, U.S.A. The Chuckanut Formation is a late Paleocene to Eocene sedimentary unit with a robust paleobotanical record showing the existence of subtropical forests in northwest Washington during the Paleocene, transitioning to a relatively cooler Eocene, dominated by temperate mixed forests. Geochemical records of environmental change coinciding with Paleocene-Eocene climatic change in this region are understudied. Polycyclic aromatic hydrocarbons (PAHs) are molecules produced during incomplete combustion of biomass that readily preserve in the sedimentary record and are used to reconstruct past wildfire frequency and severity. Analyses of PAH concentrations on Paleocene-age and Eocene-age members of the Chuckanut Formation indicate that wildfires were infrequent in the Paleocene, outside of two distinct stratigraphic intervals of increased fire activity, compared to more frequent wildfires in the Eocene. The aromatic structure of PAHs preserved in both members does not indicate a marked change in wildfire severity between the Paleocene and Eocene. I propose that a wetter hydroclimate during the Paleocene may explain the lower occurrence of wildfires compared to the Eocene, despite higher temperatures. Wildfire regime reconstructions combined with regional paleohydrology studies can provide insight into the environmental changes the Pacific Northwest may experience as a consequence of future climate change.