Effects of chemical preparation protocols on δ13C values of plant fossil samples

For over a half-century, studies of the stable carbon isotope compositions of plant fossils have provided noteworthy conclusions about the evolution of Earth climate. However, different preparation protocols were used prior to carbon isotope analysis of plant fossils and the accuracy of measured 13C/12C ratios may strongly depend on them. We have applied the most frequently used chemical preparation protocols to Frenelopsis leaves from four Cretaceous localities of Western Europe. Our results indicate that procedures to extract plant fossils from sediments are not adequate to retrieve the original plant δ13C values. The use of Schulze's reagent to bleach cuticles in paleobotany allows misleading δ13C values of specimens from a same locality with positive shifts of up to 13.6‰ and increases intra-specimen and inter-specimen variances. Sample treatment involving HCl or HCl + HF gives consistent δ13C mean values for a same leaf and for leaves from a same sedimentary bed. However, although HF does not provide any advantage in removing potential contamination sources, it alters plant fossil tissues, slightly increasing the intra- and inter-leaf variances of δ13C values. Our results indicate that processing plant fossils with HCl only is the most accurate and conservative way to retrieve original δ13C values.