Ruthenium tetroxide oxidation of immature sulfur-rich kerogens from the Nِrdlinger Ries (southern Germany)

Ruthenium tetroxide oxidation was applied to sulfur-rich kerogens from the Nördlinger Ries, southern Germany, to provide complementary structural information about the nature of alkyl substituents attached to aromatic moieties. The oxidation products contained series of straight-chain monocarboxylic acids (C8–C29), α,ω-dicarboxylic acids (C7–C26), isoprenoid acids (C14–C21, except C18), non-isoprenoid branched acids (mainly iso and anteiso C11–C18 acids), 3β-carboxysteroids (C28–C31) and steroid ketones (C27–C30). Furthermore, 4-hydroxy-4,8,12,16-tetramethylheptadecanoic acid γ-lactone, 6,10,14-trimethylpentadecan-2-one, and 10-oxo-n-octadecanoic acid were identified. The oxidation products are discussed in terms of their original contribution to the macromolecular structure. The results support previous suggestions that alkyl–aryl ether groups are present in the kerogen, probably as a result of O-alkylation between phenols and aliphatic precursors. Of particular interest, however, is the identification of a predominant homologous series of 2-methyl n-alkanoic acid methyl esters (C9–C28) in one of the samples. The release of this scarcely reported series of branched acids points toward the specificity of RuO4 oxidation revealing structural information on the alkyl chains substituting the aromatic moieties in the kerogen structure, and implies that some of the bound lipids were contributed by sulfate-reducing bacteria.