46159-AC2
Structural, Molecular and Isotopic Composition of Organic Fossils and Their Relationship to Modern Counterparts: Role of Lipids in Kerogen Formation
Structural, molecular and isotopic composition of organic fossils and their relationship to modern counterparts Investigating the chemical changes that occur as the tissues of organisms become fossilized provides a key to understanding the molecular transformation of biomolecules to geomolecules on a geologic timescale, and their contribution to sedimentary organic matter and kerogen formation. In the project so far we have used arthropods and leaves decayed in experiments and natural conditions, respectively, to evaluate the fate of the constituent biopolymers and their alteration during early diagenesis. The results have been used to interpret the chemistry of fossil invertebrates and plant material, and their significance for our understanding of the formation of geomacromolecules. The importance of decay and timing the onset of diagenesis.- Diagenetic alteration is central to the preservation of organic fossils and to the formation of kerogen. This process takes place over millions of years, but it is not known how it is initiated. To investigate this, laboratory decay experiments were carried out on shrimp, scorpion and cockroach (arthropods that occur as fossils) to monitor changes in the chitin-protein and associated lipids of the cuticle. The cockroach and scorpion exoskeleton remained largely unaltered, but the shrimp experienced rapid decomposition within a month. Mass spectrometry and 13C NMR spectroscopy of the shrimp cuticle revealed the formation of an n-alkyl component with a chain length up to C24 indicating that labile lipids, such as fatty acids, were incorporated during the experiment. Such lipid incorporation was not detected in the scorpion and cockroach cuticle, indicating that decay is important in the transformation of macromolecular material. This experiment showed that lipids can become associated with macromolecules during the earliest stages of decay - the first stage of a polymerization process that results in the aliphatic rich composition ubiquitous in organic fossils and in kerogens. The role of cutin and the incorporation of longer chain waxes in leaf fossilization.- Chemical analysis of leaves of modern Metasequoia (the dawn redwood) showed that they are comprised of the structural polyester cutin, lignin and polysaccharides but no resistant aliphatic constituent such as cutan. Analysis of naturally decayed Metasequoia leaves (collected from ponds) revealed that the lignin units and cellulose were degraded relative to cutin. This is paralleled by the loss of internal tissue while the cuticular membrane survives in modern, decayed and fossil Metasequoia leaves. Analysis of Tertiary fossil Metasequoia from the Eocene of Republic (Washington State) showed a significant aliphatic component up to C23 while lignin and polysaccharides were absent. This suggests that cutin contributes to the ubiquitous presence of n-alkyl component in fossil leaves due to its resistant nature. Thermal maturation of cutin in laboratory experiments, however, showed that it generates an aliphatic polymer < C20; the longer chain component in fossil leaves may be a diagenetic product of leaf waxes. The nature diagenetic of cross linking.- Analysis of the jaws of Humboldt squid (Dosidicus gigas) and Nautilus belauensis using mass spectrometric techniques revealed a chitin-protein composition. In contrast, the jaws of fossil ammonites and a coleoid from Upper Cretaceous localities in North America and Japan showed no trace of the chitin-protein biopolymer complex. The fossils were composed of aromatic compounds, including alkyl benzene derivatives, alkyl phenols, naphthalenes and phenanthrene, as well as an n-alkyl component ranging in carbon chain length up to C24. Analysis of the fossil jaws using thermally assisted methylation (THM), which cleaves ester bonds, revealed carboxylic acids with chain lengths from C6 to C18. Methyl ketones, which indicate ether functional groups, were also detected with a significant n-alkyl (aliphatic) component. This aliphatic component in the fossil cephalopod jaws is likely derived from incorporation and polymerisation of labile lipid precursors (e.g. carboxylic acids), in part crosslinked by ether and ester bonds. This investigation is the first demonstration of ether linkages and of oxidative crosslinking in animal fossils, and suggests that their formation may be important in the fossilization of both animal and plant material. Future work.- Our results so far have advanced our understanding of the timing and nature of the diagenetic transformation of organic material in sediments. Collaboration with R.E. Summons (MIT) and G.D. Cody (Carnegie) has allowed the postdoc to gain considerable training in analytical and other research methods. The next stage of the project involves the use of spectroscopic and mass spectrometric methods to refine our understanding of the biomolecular transformation of plant and animal fossils from selected Tertiary localities. This includes an investigation of natural thermal maturation (resulting from a lava flow) and kerogen formation in Oligocene fossils from Enspel, Germany.
