LAUSR

Volcanic-driven nutrient flux to the oceans stimulated marine productivity and organic matter burial during Oceanic Anoxic Event 2 (OAE2; ~94 million years ago). While the preferential burial of 13C-depleted organic matter led to a general 13C enrichment of sediments during the event, a 2‰ 13C depletion punctuated the first half of the event (known as the Plenus), raising questions about carbon cycle feedbacks during OAE2. Here we present organic geochemical evidence (for example, pyrogenic polycyclic aromatic hydrocarbons) from the Western Interior Seaway that indicates increased forest fire frequency in the western United States during the Plenus. Carbon mass balance equations, which account for the amount and carbon isotopic composition of atmospheric CO2 and forest biomass during OAE2, potentiate fires in the western United States as part of a widespread increase in forest fires that could have alone caused the global 2‰ 13C depletion during the Plenus. Plant biomarkers suggest that local precipitation and plant type did not change significantly, indicating that elevated atmospheric oxygen levels from widespread organic carbon burial increased the frequency of fires in wet forest ecosystems that were extensive during OAE2. Plant biomarkers also indicate that forest fires amplified the flux of terrestrial organic matter and nutrients to the oceans, which may have enhanced marine productivity, organic carbon burial and the return to 13C-enriched sediments at the end of the Plenus. The extent that this feedback impacted global biogeochemistry during the Plenus and the rest of OAE2, as well as other events in Earth history, warrants further investigation. A global carbon cycle perturbation during Oceanic Anoxic Event 2 was probably due to elevated oxygen levels leading to a transient increase in wildfire activity, according to a record of plant biomarkers tracking fire frequency in western North America.