Paleogene plants fractionated carbon isotopes similar to modern plants

The carbon isotope composition (δ¹³C) of terrestrial plant biomarkers, such as leaf waxes and terpenoids, provides insights into past carbon cycling. The δ¹³C values of modern plant biomarkers are known to be sensitive to climate and vegetation type, both of which influence fractionation during lipid biosynthesis by altering plant carbon supply and its biochemical allocation. It is not known if fractionation observed in living plants can be used to interpret fossil lipids because plant biochemical characteristics may have evolved during the Cenozoic in response to changes in global climate and atmospheric CO₂. The goal of this study was to determine if fractionation during photosynthesis (δ_leaf) in the Paleogene was consistent with expectations based on living plants. To study plant fractionation during the Paleogene, we collected samples from eight stratigraphic beds in the Bighorn Basin (Wyoming, USA) that ranged in age from 63 to 53 Ma. For each sample, we measured the δ¹³C of angiosperm biomarkers (triterpenoids and n-alkanes) and, abundance permitting, conifer biomarkers (diterpenoids). Leaf δ¹³C values estimated from different angiosperms biomarkers were consistently 2‰ lower than leaf δ¹³C values for conifers calculated from diterpenoids. This difference is consistent with observations of living conifers and angiosperms and the consistency among different biomarkers suggests ancient ε_lipid values were similar to those in living plants. From these biomarker-based δ¹³C_leaf values and independent records of atmospheric δ¹³C values, we calculated δ_leaf. These calculated δ_leaf values were then compared to δ_leaf values modeled by applying the effects that precipitation and major taxonomic group in living plants have on δ_leaf values. Calculated and modeled δ_leaf values were offset by less than a permil. This similarity suggests that carbon fractionation in Paleogene plants changed with water availability and major taxonomic group to about the same degree it does today. Further, paleoproxy data suggest at least two of the stratigraphic beds were deposited at times when pCO₂ levels were higher than today. Biomarker data from these beds are not consistent with elevated δ_leaf values, possibly because plants adapted carbon uptake and assimilation characteristics to pCO₂ changes over long timescales.