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

Abstract

The Paleocene-Eocene Thermal Maximum was a rapid global warming event at~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~−2‰ difference between the magnitudes of the CIE recorded by paleosol carbonate and paleosol organic matter (ΔCIE pc-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 ΔCIE pc-om . A production-diffusion model was used to simulate the carbon isotope compositions of soil CO 2 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 ΔCIE pc-om . When reasonable initial respiration rates and changes in atmospheric CO 2 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 ΔCIE pc-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.

Soil pore water evaporation and temperature influences on clay mineral paleothermometry

“…In addition to the complication that depth trends in pore water δ 18 O and δ 2 H composition imposes, the isotopic composition of authigenic soil minerals (i.e., minerals that form as a result of in situ processes within the soil environment) records the soil temperature at the time in which those minerals precipitate, not the instantaneous or annual average air temperature. An analogous example includes recent studies focused on carbonate clumped isotope signatures (Δ 47 ) of soil carbonates, which have noted the potential bias in soil carbonate formation temperatures whereby soil temperatures recorded by carbonates may be warmer than annual or seasonal air temperatures due to radiative heating effects [60][61][62][63] . Nonetheless, clay minerals in weathering profiles and soils are likely less seasonally biased than carbonates, a fact that has been previously leveraged when measured together with carbonates in the same profile and/or stratigraphic sections and time intervals 30,31,34 .…”