Leaf carbon isotope composition of fossil and extant oaks grown under differing atmospheric CO2 levels

The aim of this study is to investigate the effects of long-term variations in atmospheric CO2 levels (pCO2) on stable carbon isotope ratios in plants. Isotope measurements were undertaken on two kinds of oak leaves: fossil leaves originating from two Miocene levels of well-documented and differing pCO2 (i.e., 280 and 360 ppmv), and extant leaves of the same species coming from plants experimentally grown under contrasted pCO2. Measurements were performed separately on sun and shade leaves to avoid bias linked to leaf morphotype, and on bulk leaves and individual n-alkanes extracted from the leaves, to detect possible diagenetic alteration of the isotope signal. Although extant oaks exhibit no significant trend in isotope discrimination with varying pCO2, fossil oaks from the high pCO2 level tend to exhibit slightly smaller isotope discrimination than those from the low pCO2 level. However, the difference in fossil discrimination is only of 0.5‰ and its origin cannot be unequivocally assessed since other environmental parameters could be responsible for it. Therefore, it appears that if plant carbon isotope composition is sensitive to long-term pCO2 variations, its response is rather minimal.