Interpreting the difference in magnitudes of PETM carbon isotope excursions in paleosol carbonate and paleosol organic matter : oxidation of methane in soils versus elevated soil respiration rates

The Paleocene-Eocene Thermal Maximum (PETM) was a rapid global warming event at ~56 Ma that was driven by a rapid release of carbon into the ocean-atmosphere system. The most recognizable feature marking the PETM in the rock record is a negative carbon isotope excursion (CIE) recorded in organic carbon and calcium carbonates deposited in both marine and terrestrial environments. Differences among excursion magnitudes (ΔCIE) exist between marine and terrestrial proxies, and between carbonates and organic carbon. We evaluated the plausibility of two hypothetical mechanisms behind the observed ~ 1.9‰ difference between the magnitude of the CIE as recorded by paleosol carbonate and paleosol organic matter (ΔCIEpc-som). 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 ΔCIE. A production-diffusion model used to simulate carbon isotope compositions of soil CO₂ and paleosol carbonates is coupled with a box model that constrains methane flux from hydrates into atmosphere. The box model simulates atmospheric CO₂ concentrations, the δ¹³C values of atmospheric CO₂ and of plants, and the methane flux into soils, which are all used in the production-diffusion model to simulate the δ¹³C value of paleosol carbonate. Given conservative prior distributions for model inputs grounded in previous empirical studies, model output demonstrates that oxidation of atmospheric methane in soil pore space is unlikely to cause the ΔCIEpc-som even for rapid methane release rates. However, increased respiration rates during the PETM could explain the observed ΔCIE, with a minimum approximate doubling of respiration rates required to reproduce a ΔCIEpc-som ≥ 2‰.