A NEW HIGH-RESOLUTION CONTINENTAL RECORD OF THE PALEOCENE-EOCENE BOUNDARY CARBON ISOTOPE EXCURSION FROM MAMMALIAN TOOTH ENAMEL

Transient global warming of 5-10°C occurred during the Paleocene-Eocene thermal maximum (PETM). The PETM is marked by a negative carbon isotope excursion (CIE) resulting from a massive input of carbon to the atmosphere. Carbon isotope values decrease by 2 to 4‰ in marine carbonates, but by as much as 7‰ in terrestrial soil carbonates. Here we report a new detailed continental record of isotopic changes during the CIE in the Cabin Fork area of the southern Bighorn Basin, Wyoming, based on carbon isotopes in the carbonate component of mammalian tooth enamel. The CIE in this area has previously been recognized using δ¹³C values in leaf waxes (n-alkanes) and bulk organic matter. The CIE is also characterized by major faunal and floral turnover, and body size reduction in at least nine mammal groups.

Carbon isotope values in the tooth enamel ( δ¹³C_E) of mammalian herbivores reflect δ¹³C values of dietary vegetation, and can be used to track isotopic changes in local vegetation. Carbon isotope values in vegetation reflect primarily values of atmospheric carbon, although they are also influenced by environmental conditions. Thus, δ¹³C_E can be used to track broad isotopic changes during the CIE. We present a composite isotopic curve based on δ¹³C_E values from 214 mammal teeth representing 16 species classified in 10 genera. Teeth were collected over an area of several km² and tied to a composite stratigraphic section using marker beds. The base of the CIE (0 m level) is well defined by a large negative shift in δ¹³C_E of ~4.1‰. This is similar to a mean shift in δ¹³C of ~4.5‰ in n-alkanes from Cabin Fork, suggesting that both proxies track mean δ¹³C plant values. Mean δ¹³C_E values remain low (<-14.0‰) for the first ~32 m of section, with the most negative values occurring at ~13 m. A shift to more positive values occurs at ~36 m and the excursion appears to be largely over by ~40 m. Mean δ¹³C_E values remain more negative than late Paleocene values, however, even at 50 m. Decreases in mammalian body sizes occur in the CIE and a shift to larger body sizes occurs just after the 35 m level as the CIE wanes. This study demonstrates that carbon isotope values in mammalian tooth enamel are a useful tool for tracking isotopic changes in local vegetation, which reflect changes in the global carbon cycle.