Interpreting the Difference in Magnitudes of PETM Carbon Isotope Excursions in Paleosol Carbonate and Organic Matter: Oxidation of Methane in Soils Versus Elevated Soil Respiration Rates

The Paleocene-Eocene Thermal Maximum was a rapid global warming event at similar to 56 Ma that is associated with a negative carbon isotope excursion (CIE) and was driven by a geologically rapid release of carbon into the ocean-atmosphere system. We evaluated the plausibility of two hypothetical mechanisms behind the observed similar to-2 parts per thousand difference between the magnitudes of the CIE recorded by paleosol carbonate and paleosol organic matter (Delta CIEpc-om). Specifically, we test whether (1) oxidation within soils of isotopically light methane or (2) increases in soil respiration rates are plausible explanations for the observed Delta CIEpc-om. A production-diffusion model was used to simulate the carbon isotope compositions of soil CO2 and paleosol carbonates. Model output demonstrates that soil respiration rates would have needed to, at minimum, double during the Paleocene-Eocene Thermal Maximum in order explain the magnitude Delta CIEpc-om. When reasonable initial respiration rates and changes in atmospheric CO2 are considered, an increase by 4-5 times is required. Such large increases in soil respiration rates are difficult to sustain, but a shift in the seasonality of soil carbonate formation could help explain these observations. The oxidation of atmospheric methane in soil pore space is unlikely to have caused the Delta CIEpc-om, even for rapid methane release rates. However, methane oxidation in soils might explain the preonset excursion observed in Polecat Bench core samples. The absence of this excursion in other P/E boundary records and the rapid recovery from this excursion are otherwise difficult to explain.