INCREASED CARBON ISOTOPE DISCRIMINATION BY PLANTS DURING THE PETM: RECONCILING TERRESTRIAL AND MARINE CARBON ISOTOPE EXCURSIONS

The magnitude of the negative carbon isotope excursion (CIE) observed at the Paleocene/Eocene boundary is twice as large in paleosol carbonates (~6 ‰) as it is in the marine carbonates (~3 ‰). Soil organic matter carbon from paleosols from the Bighorn Basin, WY, demonstrates an excursion of intermediate magnitude (~3-4 ‰) (This study and Magioncalda et al., 2004). One recent hypothesis for the greater magnitude of excursion in terrestrial reservoirs calls for increased carbon isotope discrimination in plants resulting from a ~20% increase in available moisture (Bowen et al., 2004). We test the hypothesis that discrimination increased during the PETM by measuring compound-specific carbon isotope signatures of plant lipids. The compound-specific approach has a distinct advantage over bulk measurements that are subject to preservational biases. In the Bighorn Basin, WY, we have observed that the quantity of organic matter in paleosols is inversely correlated with its isotopic composition. This correlation suggests removal of ¹²C through microbial degradation of soil organic matter. In contrast, the d¹³C values of plant lipid biomarkers directly record changes in plant isotopic signatures. Leaf wax n-alkanes from the Bighorn Basin, Wy, demonstrate a magnified carbon isotope excursion relative to the marine carbonate CIE. We suggest that the magnification of the CIE in terrestrial reservoirs results from increased discrimination by plants. We use the hydrogen isotope ratio (dD) of these same n-alkanes to test for significant changes in the hydrologic cycle that may have occurred during the PETM.