Molecular isotopic heterogeneity of fossil organic matter: implications for delta(13) C-biomass and delta(13) C-palaeoatmosphere proxies

The degree of isotopic variation in fossil organic matter renders bulk delta(13)C signatures strongly influenced by molecular isotopic heterogeneity. For example, in fossil wood the relative abundance of less depleted C-13 moieties, i.e. preserved C-13 enriched polysaccharides versus the relatively C-13 depleted lignin moieties, can be seen to significantly bias delta(13)C(fossil wood) values. Moreover the variation in delta(13)C 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 delta(13)C values of fossil plant material as proxies (e.g., for delta(13)C(palaeoatmosphere), delta(13)C(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 delta(13)C value of the plant and might reveal atmospherically induced trends. (C) 2004 Elsevier Ltd. All rights reserved.