Impact of elevated CO2 on the δ13C of n-alkane biomarkers

The stable carbon isotopic composition of n -alkane biomarkers is a commonly used proxy for studying plant community change and palaeohydrology in the geologic record. However, interpretations of n -alkane δ 13 C tend to assume that all variation is due to changes in bulk plant δ 13 C, reflecting a signal of fractionation due to photosynthesis, and that the biosynthetic fractionations which occur after photosynthesis (ε n -alkane ) are constant. This untested assumption may overlook environmental influences on ε n -alkane which can offer alternative explanations for n -alkane δ 13 C trends, thereby allowing for the reconstruction of plant biochemistry in geological records. In this study, we use the Free Air CO 2 Enrichment experiment at the Birmingham Institute of Forest Research to investigate the impact of elevated CO 2 concentrations on the response of ε n -alkane . We measured the δ 13 C levels of bulk and n -alkanes in two plant species, Acer pseudoplatanus (European sycamore) and Corylus avellana (common hazel), over four years. Our findings indicate that the ε n -alkane levels in both species increased under elevated CO 2 compared to ambient levels. However, the seasonal variation in ε n -alkane suggests that this increase is due to separate mechanisms. In A. pseudoplatanus , this increase occurs consistently throughout the growth season, suggesting that changes in carbon allocation to different tissue types within the plant is responsible. Conversely, in C. avellana , the effect becomes more pronounced as the growth season progresses, indicating that altered timing of n -alkane synthesis under elevated CO 2 may influence ε n -alkane . Collectively, this suggests that ε n- alkane should not be assumed constant in the geologic record, and highlights that biosynthetic processes should be considered when interpreting compound-specific isotopes of carbon.