Evidence for the in situ polymerisation of labile aliphatic organic compounds during the preservation of fossil leaves: Implications for organic matter preservation

Organic matter preservation is typically attributed to selective preservation of resistant biomolecules, random polymerisation of diagenetically degraded biomolecules (i.e. neogenesis) or in situ polymerisation of labile aliphatic components (in the case of fossil plants and insects). To evaluate these processes, we investigated the morphology and chemical structure of fossil leaves from the Ardèche diatomite (Late Miocene, southeast France) and compared them to their modern equivalents. Chemical analyses of the fossil leaves revealed the presence of a recalcitrant (non-hydrolysable) geopolymer comprised of benzene derivatives, lignin-derived components, pristenes and an aliphatic component; the latter consists partly of fatty acyl subunits ranging in carbon number from C8 to C32 with an abundance of C16 and C18 units. Chemical degradation of the modern plants failed to reveal the presence of the aliphatic biomacromolecule cutan, thereby precluding selective preservation of this compound as the source for the aliphatic component of the fossil leaves. In contrast, C16 and C18 fatty acyl units are predominant in the cutin and phospholipid fatty acid (PLFA) fractions of the modern leaves, while C10 to C32 acid units are characteristic of the free fatty acid (FA) fraction of epicuticular waxes. However, TEM and SEM investigations of the fossils revealed no evidence for cuticle preservation, and while a contribution from cutin cannot be excluded, the aliphatic component of the fossil polymer is possibly derived instead from the in situ polymerisation of labile cell membrane lipids and free fatty acids. A similar process involving lipid polymerisation has been observed previously in kerogen formation alongside selective preservation and, hence, may be important in organic matter preservation.