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.
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We thank members of the Organic Geochemistry Lab at The University of Colorado Boulder N. Dildar, J. van Oosten, T. Bond, S. Gandhi-Besbes, J. Straight, S. Tostanoski, A. Mcquade, H. Nguyen and J. Lopez for laboratory assistance, B. Davidheiser-Kroll and K. Snell for help with the organic carbon analysis, members of the NSF-funded Collaborative Research: Perturbation of the Marine Food Web and Extinction During the Oceanic Anoxic Event at the Cenomanian/Turonian B. Sageman, M. Jones, T. Bralower, R. L. Oakes, M. Leckie and A. L. Parker for field activities, sample collection and fruitful discussions, A. Titus (Bureau of Land Management, Grand Staircase-Escalante National Monument) for prospecting coring sites and obtaining collecting permits, J. Spencer (US National Park Service Glen Canyon Recreational Area) for accessing outcrop sections, J. Parlett, S. Crawford, the western US Geological Survey drilling crew, C. Lowery, S. Karduck, Q. Li, M. Wnuk and L. Victoria for sample collection during fieldwork, and J. van Dijk, T. Marchitto and F. D. Boudinot for helpful comments on the manuscript. This study was funded by the NSF Division of Earth Sciences, Earth-Life Transitions (ELT) programme grant number 1338318, and by the American Chemical Society Petroleum Research Fund (ACS-PRF) – Doctoral New Investigator Award number 58815-DNI2. F.G.B. acknowledges support from the Department of Geological Sciences at the University of Colorado Boulder.
The authors declare no competing interests.
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Extended Data Fig. 1 SH#1 core pyroPAH concentrations that indicate enhanced forest fire frequency during the Plenus interval of OAE2.
Shading scheme is the same as in Fig.2.
Extended Data Fig. 2 Other pyroPAH concentrations in the SH#1 core that do not increase during the Plenus compared to pre-and post-Plenus intervals.
Shading scheme is the same as in Fig.2.
Extended Data Fig. 3 The effect of different atmospheric δ13CCO2 values on forest fire area estimates.
Axes and blue shading same as in Fig. 3 main text. Line types indicate different δ13CCO2 values (solid = 2‰, dashed = 4‰) used to test sensitivity of carbon mass balance equation to starting CO2 carbon isotopic composition. Minimum estimate (900 ppm CO2, δ13CCO2 = 4‰, total forest size of 1000 Gt C) is 28%, maximum estimate (1100 ppm CO2, δ13CCO2 = 2‰, total forest size of 900 Gt C) is 41%.
Extended Data Fig. 4 The effect of different terrestrial biomass δ13C values on forest fire area estimates.
Axes and blue shading same as in Fig. 3 main text. Line types indicate different forest biomass δ13C values (solid = −28‰, dashed = -24‰) used to test sensitivity of carbon mass balance equation to starting forest biomass carbon isotopic composition. Minimum estimate (900 ppm CO2, δ13Cforest = −28, total forest size of 1000 Gt C) is 26%, maximum estimate (1100 ppm CO2, δ13Cforest = −24, total forest size of 900 Gt C) is 40%.
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Boudinot, F.G., Sepúlveda, J. Marine organic carbon burial increased forest fire frequency during Oceanic Anoxic Event 2. Nat. Geosci. 13, 693–698 (2020). https://doi.org/10.1038/s41561-020-0633-y