PLANT COMMUNITY CHANGE AND THE MAGNITUDE OF THE CARBON ISOTOPE EXCURSION AT THE PALEOCENE-EOCENE THERMAL MAXIMUM

Leaf-wax n-alkanes from the Paleocene-Eocene Thermal Maximum (PETM) in the Bighorn Basin, Wyoming record a negative carbon isotope excursion (CIE) of 4-5‰, which is 1-2‰ larger than that observed in marine carbonate d13C records. Reconciling these records requires either that marine carbonates fail to record the full magnitude of the CIE or that the CIE in plants has been amplified. Amplification of the CIE has been proposed to result from an increase in available moisture that allowed terrestrial plants to increase 13C-discrimination during the PETM. Leaf physiognomy, paleopedology and hydrogen isotope ratios of leaf-wax lipids from the Bighorn Basin, however, all suggest that rather than a simple increase in available moisture, climate alternated between wet and dry during the PETM. We consider two other explanations and test them quantitatively with the carbon isotopic record of plant lipids. The “marine modification” hypothesis is that the marine carbonate record was modified by chemical changes at the PETM and that plant lipids record the true magnitude of the CIE. Using atmospheric CO2 d13C values estimated from the lipid record, and equilibrium fractionation between CO2 and carbonate, we estimate the expected CIE for planktonic foraminifera to be 6‰. Instead, the largest excursion observed is about 4‰. No mechanism for altering marine carbonate by 2‰ has been identified and we thus reject this explanation. The “plant community change” hypothesis is that the observed major changes in floral composition during the PETM amplified the CIE recorded in n-alkanes by 1-2‰ relative to the ocean-atmosphere CIE. This effect could have been caused by a rapid transition from a mixed angiosperm/conifer flora to a purely angiosperm flora. The plant community change hypothesis is consistent with both the observed magnitude and pattern of CIE amplification among the different n-alkanes, with longer chain-lengths demonstrating greater amplification. This hypothesis predicts that the magnitude and pattern of amplification of CIEs among different n-alkanes will vary regionally and systematically depending on the extent of the replacement of conifers by angiosperms during the PETM. Carbon isotope analyses of modern conifers and angiosperms will further refine quantitative predictions of the isotopic effects of plant community change.