Paleocene-Eocene carbon isotope excursion in organic carbon and pedogenic carbonate: Direct comparison in a continental stratigraphic section

Abstract

The negative carbon isotope excursion at the Paleocene-Eocene boundary is a chemo stratigraphic marker widely used for correlation of marine and continental stratigraphic sections. It is linked to massive dissociation of sedimentary methane hydrates and helps to explain an important greenhouse thermal event (Paleocene–Eocene thermal maximum), marine extinctions, and mammalian faunal change on three continents. We show that the carbon isotope excursion recorded in dispersed organic carbon (DOC) from fine-grained terrestrial sedimentary rocks at Polecat Bench, Wyoming, is very similar to that described in two high-resolution studies of pedogenic soil-nodule carbonate from the same section. All show a rapid onset, an ∼40 m series of excursion values, and a slower recovery. However, the carbon isotope excursion in soil-nodule carbonate starts and ends ∼3–5 m lower stratigraphically than that in DOC. We hypothesize that enhanced diffusion of atmospheric CO2 and subsurface diagenesis in an environment of good drainage and elevated temperature and p CO2 may explain this offset. The reliability of δ13C in DOC is attributed to mixing and averaging of isotopic signals from different organic compounds and tissues.