Positive feedback drives carbon release from soils to atmosphere during Paleocene/Eocene warming

Abstract

The Paleocene-Eocene Thermal Maximum (PETM) is the most rapid climatic warming event in the Cenozoic and informs us how the Earth system responds to large-scale changes to the carbon cycle. Warming was triggered by a massive release of ¹³C depleted carbon to the atmosphere, evidenced by negative carbon isotope excursions (CIE) in nearly every carbon pool on Earth. Differences in these CIEs can give insight into the response of different ecosystems to perturbations in the carbon cycle. Here we present records of δ¹³C_cc of pedogenic carbonates and δ¹³C_org from preserved soil organic matter in corresponding paleosols to understand changes to soil carbon during the PETM. CIEs during the event are larger in pedogenic carbonates than preserved organic matter for corresponding paleosols at three sites across two continents. The difference in the CIEs within soil carbon pools can be explained by increased respiration and carbon turnover rates of near-surface labile soil carbon. Increased rates of labile carbon cycling combined with decreases in the amount of preserved organic carbon in soils during the PETM suggests a decrease in the size of the soil carbon pool, resulting in a potential increase in atmospheric pCO₂ and a positive feedback with warming. The PETM is a model for how the earth system responds to warming, and this mechanism would suggest that soils might serve as a large source for atmospheric CO₂ during warming events.