THE PRESERVATION OF SEDIMENTARY ORGANIC MATTER BY IN SITU POLYMERISATION

The preservation of organic matter is a critical process in the carbon cycle, the formation of kerogen and in the preservation of fossils. The mechanism of organic matter preservation is a matter of controversy: it is usually attributed either to selective preservation of decay-resistant biomolecules or to random polymerisation of a range of molecular components in sediment. In an attempt to resolve this debate we analysed leaves from the Ardèche diatomite (Miocene, southeast France), both chemically and morphologically, and compared them to their modern equivalents. The fossil leaves consist predominantly of a recalcitrant aliphatic biopolymer even though there is no resistant aliphatic precursor (e.g. cutan) in the leaves of the modern plants. This precludes selective preservation as an explanation for the composition of the fossils. The aliphatic polymer in the fossil leaves consists of C₁₆ and C₁₈ carboxylic acid components which also occur in cutin, an important component of the cuticle, suggesting that this might be the source in the fossils. However, electron microscopy revealed that the Miocene leaves do not preserve any trace of the cuticle. Thus the composition of the fossils must be derived from other parts of the leaf: the highly labile cell membrane lipids. These analyses shows that preservation of the leaves is the result of in situ polymerisation of organic components that are decay prone rather than recalcitrant. This mechanism likely accounts for the aliphatic composition of much sedimentary organic material.