HYDROCLIMATIC MODULATION OF CONTINENTAL CARBON BURIAL THROUGH THE PETM

Climatic wetness is a primary driver of short- and long-term variations in terrestrial carbon storage today, yet the response of the continental and global carbon cycles to feedbacks involving the hydroclimate are poorly constrained for the future and practically unknown in Earth’s past. We present new records of paleosol wetness and organic and authigenic inorganic C storage from a series of cores spanning the Paleocene-Eocene thermal maximum at Polecat Bench (Bighorn Basin, Wyoming, USA). Our qualitative reconstructions, based on paleosol carbonate color and morphology, suggest generally drier conditions within the body of the PETM, consistent with previously reported outcrop evidence. Our data also show a ~2x decline in the total organic C content of Polecat Bench sediments during the PETM, similar to changes we observe elsewhere in the Bighorn Basin and those previously reported from other parts of the US western interior. Changes in paleosol carbonate characteristics and organic C lagged the onset of the PETM carbon isotope excursion and carbon cycle change by 5 - 7 meters (ca. 10 - 15 kyr), defining a discrete zone early in the PETM characterized by low C isotope ratios, limited hydroclimatological change, and higher-than-average paleosol organic C content. During the rest of the PETM organic content collapsed but preservation of authigenic soil carbonate increased by a factor of 2, such that the total C content of Polecat Bench sediments did not change substantially. The new records suggest that the response of terrestrial C storage and burial to PETM carbon cycle and climatic perturbation was complex, with an initial pulse of organic C sequestration, perhaps in response to stimulation of productivity by warming and elevated CO₂, followed by a hydroclimatically-driven switch from reduced to oxidized C burial. If the Polecat Bench case can be used as a model, widespread changes in the ratio of organic to inorganic C burial in terrestrial sediments may be reflected in the anomalous, prolonged body of the PETM C isotope excursion. These results indicate that authigenic soil carbonates are an important and climatically sensitive mode of continental C burial that should be considered in assessments of the response of continental biogeochemical cycling to global hydroclimatic change.