Up in smoke: A role for organic carbon feedbacks in Paleogene hyperthermals

Sedimentary archives from the worldʹs oceans and continents indicate that as the world warmed from the mild climate conditions of the mid-Paleocene to the extreme global warmth of the Early Eocene, a series of abrupt perturbations shifted the carbon isotope budget of the ocean/atmosphere/biosphere (exogenic) system. Consideration of the rates and magnitude of carbon isotope change, along with independent evidence for ocean acidification, dictates that these “hyperthermal” events involved the transfer of thousands of petagrams of reduced carbon to the actively cycling exogenic system. Careful study of stratigraphically resolved carbon isotope records spanning the hyperthermals, in particular the first and most prominent of them, the Paleocene–Eocene Thermal Maximum (PETM), has informed our understanding of carbon cycle perturbation during these events. Several important features of these records, however, remain difficult to explain with conventional ocean/atmosphere carbon cycle models, including divergence of carbon isotope records from marine and terrestrial systems, a prolonged interval of low δ13C values during the ‘body’ of the PETM carbon isotope excursion (CIE), and rapid recovery of δ13C values at the CIE termination. Here I use data from well-resolved, independently dated marine and terrestrial PETM carbon isotope records to characterize these distinctive and challenging features of the records and discuss their implications. I then propose a simple set of mechanisms, involving climatically-mediated increases in organic carbon respiration rates and CO2-driven changes in photosynthetic 13C discrimination triggered by an initial release of carbon from geological reservoirs, which produce patterns, magnitudes, and rates of carbon isotope change consistent with records from the event. If the proposed scenario is correct, it suggests that the land plants and soils may have first released, and then taken up, several thousand Pg of carbon during the early and late stages of the event, with concordant changes in rates of organic carbon burial in sediments. Full elaboration and rigorous evaluation of the proposed scenario will require additional work, but the initial results suggest that organic carbon feedbacks, similar in nature to those being explored in work on modern and future Earth systems, may have played a significant role in shaping global changes at the Paleocene–Eocene boundary.