Carbon Isotope and Stomata! Data of Late Pliocene Betulaceae Leaves from SW China: Implications for Palaeoatmospheric CO2-levels

The cuticular d13C values and stomatal parameters (stomatal density and stomatal index: SD and SI) of two Betulaceae species, Betula mioluminifera Hu et Chaney and Carpinus miofangiana Nathorst, from a suite of superposed horizons in West Yunnan, southwestern China, were measured in order to recover Late Pliocene CO2 levels. Correlations are given for d13C, SD, epidermal cell density (ECD), and SI. d13C reveals a positive trend with the SD and SI in the two species, and such a positive correlation can also be observed between the d13C and ECD in C. miofangiana. However, d13C has a slightly negative correlation with the ECD in B. mioluminifera (R2= 0.06), possibly influenced by their different genotypes. Reflecting the changes through time, the d13C values of B. mioluminifera and C. miofangiana significantly increase with high determination coefficients (R2= 0.67 and R2= 0.65, respectively), as do SD (R2= 0.66 and R2= 0.51, respectively) and SI (R2= 0.50 and R2= 0.79, respectively). Research on extant B. luminifera and C. fangiana shows that the SD and especially SI, exhibit a prominent negative correlation with CO2 concentration. Pliocene CO2 levels are reconstructed as 381.5-439.4 ppmv and 377.8-472.3 ppmv, respectively, based on comparisons of the two fossil species with their nearest living equivalent (NLE) species. The significant positive trends of the d13C, SD and SI with ascending position of the fossils in the section indicate that the atmospheric CO2 levels declined in the Late Pliocene (3.30-2.83 Ma). Furthermore, the calculated CO2 levels are higher than in other studies and probably demonstrate that local CO2 enrichment can be caused by frequent volcanic eruptions over a long time scale