Abstract
The extractable lipids of two outcrop samples of Messel oil shale have been analysed by GC and GC-MS. Both samples contain as major lipids similar distributions of 4α-methylsteroids, hopanoids and long-chain (C 28-C 36) alkanediols and 1-hydroxyalkanones. Compared with other reported distributions of sedimentary alkanediols and 1-hydroxyalkanones, those reported herein extend to higher carbon numbers, greater variety of mid-chain positional isomers and are the oldest. The major sterol has been shown to be 4α-methyl, 24-ethyl- 5α(H)-cholestan-3β-ol, whereas 4α, 23,24-trimethyl- 5α(H)-cholest- 22-en- 3β-ol (dinosterol) is more abundant than the 24-ethyl structural isomer. Two novel C 31 4α-methylsterols have been recognised and are proposed to have a 23-methyl, 24-ethyl-Δ 28 side-chain. Desmethylsteroids are of low abundance compared with 4α-methylsteroids. The two outcrop samples differ principally in their distributions of hydrocarbons; whereas sample 1 contains negligible steroidal hydrocarbons, sample 2 exhibits abundant desmethylsterenes and 4-methylsterenes. Such differences illustrate the inhomogeneous nature of the Messel deposit in regard to its content of biological marker compounds. Alkaline hydrolysis of the kerogen of sample 1 releases mainly hydroxy-compounds similar in composition to the corresponding extractable lipids, together with trace amounts of ketones and hydrocarbons. Similarly, lithium aluminium hydride treatment of the kerogen of sample 1 yields hydroxy-compounds, but with a different composition to the alkaline hydrolysis products. Overall, the content of biological marker compounds suggests that the major sources of the sedimentary organic matter were dinoflagellates and bacteria, especially cyanobacteria. The algal assemblage was probably dominated by dinoflagellates of low species diversity, with other algal classes in relatively low abundance. Deposition probably took place into an anoxic hypolimnion supporting a significant population of Chlorobium-type bacteria.
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