AmericanChemicalSociety.com

We have used analyses of modern materials, Tertiary fossil samples, and the results of maturation experiments to investigate the nature and rate of chemical changes that occur as the tissues of organisms become fossilized.  Such analyses provide a key to understanding the molecular transformation of biomolecules to geomolecules, and the formation of kerogen, because they involve materials of known starting composition.  A complementary approach to understanding such processes is to explore the effect of volcanic activity on the thermal maturation of Tertiary fossils.

Thermal alteration of organic matter in the Enspel Formation, Germany. - The Oligocene lake deposit of Enspel, Germany, is one of the most famous maar-lake settings located in the Tertiary Central European volcanic belt.  During a period of intense volcanic activity the lake system was covered by a basalt flow.  This flow filled up the basin and capped the lake succession. 

Previous work. - Geochemical studies of samples from the Enspel site investigated both animal and plant fossils resulting in the discovery of the oldest trace of chitin (in a fossil beetle) and the presence of biopolymeric lignin in leaves. Additionally, the fossils were shown to contain a significant aliphatic component that is neither the result of selective preservation of an original precursor nor of migration from the aliphatic-rich host sediment. The fossilised soft tissues of a tadpole and an associated coprolitic structure from Enspel were shown to consist of fossilised bacterial cells with associated extracellular polymeric substances. Rock eval pyrolysis was used to analyse n-alkanes, alcohols and fatty acids in the sediments in order to reconstruct the ecosystem and paleoclimate.  The organic matter is characterized by molar C/N ratios between 21.5 to 53. The distribution of free aliphatic hydrocarbons dominated by long chain n-alkanes with an odd over even predominance indicates that this organic matter originated from terrestrial plant waxes. This was confirmed by the presence of macroscopically identifiable leaf remnants. A lack of streams capable of transporting larger woody materials into the Enspel lake resulted in this predominance of lipid-rich leaf waxes and cuticles.

Our investigation. - We investigated the thermal maturation of the organic materials at Enspel caused by the overlying basalt flow.  We collected seven oil shales samples from the Enspel site.  In our study several independent criteria such as pollen color index, vitrinite reflectance, and aromaticity content were determined, along with multiple biomarker maturity parameters, in order to assess the maturity trend.

Oil shales from Enspel levels 2, 4, 6, 8, 10, 12 and 14 (2 being immediately adjacent to the basalt flow and 14 farthest away at a distance of 65 cm) were collected and analysed. The TOC varies from 3.5% in the layers closest to the basalt to 24% in levels 6 and 8. Elemental C/N ratio values range from 30 to 37 indicating a possible land plant contribution, as supported by the long chain n-alkanes with odd over even predominance reported previously. Aromaticity values of organic carbon in the samples, revealed by NMR, ranged from 0.487 in level 2 closest to the basalt (where the aromatic signal in the region 100-150 ppm is much higher) to 0.267 in level 14. This maturity trend is supported by pollen color that changes from orange brown in level 14 to black in level 2 corresponding to vitrinite values of 0.25 % and 1.07 % respectively.  The biologically stable ββ isomer is absent in levels closest to the basalt. The moretane to hopane ratio (ββ/ββ+αβ+ βα ratio for the C₃₀ isomer) is 0.46 and 0.54 in level 14 and 12 respectively and drops to 0.2 in level 10 and to 0.18 in level 8, indicating thermal stress. The isomers βα and αβ are present in all the 7 levels sampled, and βα/βα+αβ shows a clear decrease in value from 0.85 in level 14 to 0.41 in levels 2 and 4. Calculation of Ts/Ts+Tm (Tm is C₂₇ 17α-22,29,30-trisnorhopane; Ts is C₂₇ 18α-22,29,30-trisnorhopane) revealed that the thermally stable isomer is absent in levels 12 and 14, those farthest away from the effect of thermal stress. The ratio stays constant at 0.05 in levels 10, 8 and 6 and increases to 0.11 and 0.08 in levels 4 and 2 respectively. The ratio 22S/22S+22R (the biologically produced 17α hopane precursor carries a 22R configuration that is converted gradually to a mixture of 22R and 22 S diastereomers; ratios are typically calculated for C₃₁ or C₃₂ homologs) remains at 0.13 in levels 8 and 10 and decreases to 0.37 and 0.23 in levels 6 and 4 respectively; the thermally stable isomer (22S) is absent in levels 12 and 14.  The parameter C₂₇ βα /βα +Tm was computed to consider the ratio of these compounds, which are present in samples from all the levels. It ranges from 0.96 to 1.39 in levels 14 and 12 respectively to a minimum of 0.36 in level 4. Previous studies of organic contact metamorphism have focused on the generation of biomarkers from the macromolecular phase.  Our investigation of the Enspel sequence complements such studies by providing independent measures of maturity trends as well as data from biomarkers.

Additional investigations as part of the Summer Research Fellowship. -

1. Py-GC-MS analysis of Metasequoia, both modern and fossil, from the Miocene Clarkia deposit as part of an investigation of the diagenesis of plant biopolymers in this taxon - the dawn redwood (Gupta et al. 2009). 

2. Analysis of hydrogen isotopes in exceptionally preserved Tertiary Metasequoia material as part of a paleoenvironmental investigation of the High Arctic during Paleogene warming (Yang et al. 2009). 

3. Investigation of labile biomolecules in other exceptionally preserved Tertiary plants from the Canadian Arctic and from Alaska.

4. Preliminary analysis of fruits from a 7,000 year old archeological site which yields the earliest record of domesticated rice in China, in order to assess the molecular preservation of plant material in this setting.