Depth dependency of the Paleocene-Eocene carbon isotope excursion: Paired benthic and terrestrial biomarker records (Ocean Drilling Program Leg 208, Walvis Ridge)

The Paleocene Eocene Thermal Maximum (PETM) coincided with a negative carbon isotope excursion (CIE) in carbonate and organic matter, in terrestrial and marine records. Consequently the PETM global warming of 5-6 degrees C has been attributed to the rapid emission of a large amount of isotopically light carbon into the ocean-atmosphere system, and the magnitude of the CIE has been used to estimate the amount of carbon greenhouse gas emitted. However, there are large discrepancies between the magnitude of the CIE in marine and in terrestrial material, in different types of marine carbonate records (bulk carbonate, planktic and benthic foraminiferal carbonate), and in marine carbonate records from different locations. The marine carbonate record of the CIE may be incomplete in most deep-sea (bathyal-abyssal) locations because of ocean acidification and widespread dissolution of seafloor carbonates. We demonstrate that the CIE in benthic foraminiferal stable isotope records is relatively abrupt and not as gradual as portrayed in bulk carbonate records along the Walvis Ridge depth transect (SE Atlantic). The benthic CIE is about -1.5 parts per thousand at 3600 m paleodepth, in contrast to about -3.5 parts per thousand at 1500 m paleodepth, consistent with depth-dependent truncation by carbonate dissolution. Strong covariance between the benthic record and the stable carbon isotope values of terrestrial n-alkanes isolated from the same core (i.e., Site 1263) supports a relatively rapid excursion of at least -3.5 parts per thousand but no more than -5.0 parts per thousand.