PLANT TERPENOIDS AND ISOTOPE TOOLS TO RECONSTRUCT THE TERRESTRIAL CARBON CYCLE AND PALEOECOLOGY: RESULTS FROM MODERN AND ANCIENT STUDIES

Plant-specific biomarkers and compound-specific δ¹³C approaches have enhanced terrestrial carbon cycle records and paleocological studies. However, one of the challenges with using δ¹³C values of plants, and especially biomarkers, is the need to constrain differences in isotope fractionation during photosynthesis, as a result of climate and plant community effects and fractionation during lipid synthesis. Without these corrections, it is difficult to compare carbon isotope records temporally and spatially when differences in climate and plant community exist. Plant terpenoids provide taxonomic specificity where tricyclic diterpenoids and pentacyclic triterpenoids are nearly exclusively produced by gymnosperms and angiosperms, respectively. Even though terpenoids have long been recognized as plant biomarkers, their potential use as phylogenetic δ¹³C proxies or as proxies for taxon abundance remains largely unexplored. We present terpenoid abundance and isotopic data for 44 tree species in 21 families, representing both angiosperms and gymnosperms, and both deciduous and evergreen leaf habits. Di- and triterpenoid abundances are significantly higher in evergreen leaf habits compared to deciduous species, reflecting differences in growth strategies and increased chemical investment in longer-lived leaves. δ¹³C values of terpenoid lipids are similar to leaf tissues, indicating biosynthetic isotope effects are small during di- and triterpenoid synthesis. To determine if modern terpenoid data is a representative of ancient plants when climate and atmospheric CO₂ conditions were drastically different than today, we test our modern results in context of climatic changes and high pCO₂ from the late Paleocene to the early Eocene in sediments from the Bighorn Basin, WY. We find that carbon isotope fractionation during photosynthesis (Δ_leaf), as measured from angiosperm and conifer derived terpenoids and corrected for lipid fractionation are consistent with predicted values. As for modern plants, precipitation (sensitivity to water availability) appears to be the primary control on Δ_leaf. Our data suggest temperature/atmospheric CO₂ changes are not important factors controlling Δ_leaf in Paleocene-Eocene plants.