Impacts of post-photosynthetic fractionation on the carbon isotopic composition of leaf wax n-alkanes under elevated CO2

The carbon isotopic composition of plant wax n-alkanes (δ¹³C_n-alkane) is a well-established proxy for bulk plant δ¹³C, which itself reflects plant community composition and palaeohydrology in the geologic record. Although the biosynthetic processes which form n-alkanes cause a depletion in ¹³C relative to bulk plant tissue, it is generally presumed that this depletion is constant. In particular, on geologic timescales bulk plant δ¹³C is invariant to changes in atmospheric CO₂, and it is therefore assumed that δ¹³C_n-alkane follows the same pattern. However, this assumption has not been tested, and it is possible that the biosynthetic fractionation during the formation of n-alkanes and other lipid biomarkers is affected by atmospheric CO₂ concentration independently of trends in bulk plant tissue. Here, I use the Birmingham Institute of Forest Research (BIFoR)’s Free Air Carbon Enrichment experiment (FACE) to investigate the impact of elevated CO₂ on both bulk and n-alkane δ¹³C in order to identify any such influence of elevated CO₂ on n-alkane isotopic composition_. If any such effects are detected, CO₂ levels should be accounted for in interpretations of deep-time δ¹³C_n-alkane records.