Molecular isotopic heterogeneity of fossil organic matter: implications for δCbiomass and δCpalaeoatmosphere proxies

The degree of isotopic variation in fossil organic matter renders bulk δ¹³C signatures strongly influenced by molecular isotopic heterogeneity. For example, in fossil wood the relative abundance of less depleted ¹³C moieties, i.e. preserved ¹³C enriched polysaccharides versus the relatively ¹³C depleted lignin moieties, can be seen to significantly bias δ¹³C_{fossil wood} values. Moreover the variation in δ¹³C values of specific compounds within fossil material are themselves highly variable and reflect the heterogeneity in isotopic values of different carbon atoms within individual compounds. For studies using δ¹³C values of fossil plant material as proxies (e.g., for δ¹³C_{palaeoatmosphere}, δ¹³C_biomass) it is recommended that the biases introduced through molecular heterogeneity, preservation type and taxonomic status of the fossil material are determined initially. Biases inherent in the bulk signature can then be reduced, rendering this value more robust. Alternatively, compound specific stable carbon isotope measurements of individual moieties preserved through geological time might prove to be an alternative proxy for monitoring changes in the bulk δ¹³C value of the plant and might reveal atmospherically induced trends.