Chemotaxonomy of N-Alkanes in Epicuticular Leaf Waxes of Modern Angiosperms and Gymnosperms: Applications to the Paleocene-Eocene Thermal Maximum

The Paleocene-Eocene Thermal Maximum (PETM), ~55.8 Ma, was a period of rapid global warming, with a rise in temperature of 5-10°C in ~10 kyr returning to previous levels within 100-200 kyr. The PETM was associated with significant alteration of global carbon cycling, ocean temperatures, circulation, and chemistry, atmospheric temperatures, and terrestrial and marine ecosystems. A large carbon isotope excursion, ~5-6‰, is recorded in leaf wax lipids, and large migrations of both flora and fauna are observed in the fossil record. From PETM sites in Wyoming, documented in both megafossils and palynomorphs, there appears to be a rapid and complete turnover in floral composition: the plant community changes from a mix of conifers and angiosperms in the late Paleocene to an entirely different suite of angiosperms within the PETM and then back again to an early Eocene gymnosperm/angiosperm mix resembling the original. This study analyzed the compositional variation of odd-numbered, long-chain n-alkanes from epicuticular leaf waxes across a range of plant families and environmental variables including seasonality and shading. The species, including Metasequoia, Celtis, and Fabaceae representatives, were specifically chosen for comparison with relatively abundant or important PETM species known from sites in Wyoming. The variation observed between these modern angiosperm and gymnosperm species firstly provides a chemotaxonomic basis for distinguishing the different modern taxa. Secondly, it provides a basis for interpretation of fossil n-alkane distributions across the PETM. Our analysis of modern representatives of Paleogene plants demonstrates that dicot angiosperms appear to have longer chain lengths than gymnosperms. This could explain the increased n-alkane average chain length observed at PETM sites in the Bighorn Basin, Wyoming. Modern chemotaxonomic observations allow further evaluation of the plant community change hypothesis for the amplification of the PETM carbon isotope excursion in terrestrial reservoirs.