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Abstract

Because the chemical composition of organic matter (OM) varies among different organisms, the biological source of OM can exert an important control on its preservation and influence the formation of petroleum source rocks. Molecular compounds derived from specific organisms, biomarkers, are a useful tool in the evaluation of the biological sources of organic matter to ancient sediments. Previous work has shown that organic matter in the Kimmeridge Clay Formation derives from a range of organisms, represented in the biomarker record by isotopically distinct n-alkanes, branched alkanes, and isoprenoids. Here we report the abundances of these compounds and other select biomarkers in the extractable-organic-matter fraction and the distribution of n-alkanes and isoprenoids in the kerogen; these data are used to determine the stratigraphic variation in the occurrence of different organic-matter assemblages through the lower hudlestoni biozone in a sediment core from Dorset, southern England.

The TOC-normalized abundances of n-alkanes and specific methyl and isopropyl branched alkanes in the extract are relatively constant through the studied interval, suggesting that their contribution to sedimentary organic matter did not vary. In contrast, the occurrence of isoprenoid-rich kerogen and bitumen is largely restricted to discrete horizons, three of which were clearly identified in this study. Because changes in the abundance of extractable isoprenoids correlate with changes in the relative abundances of kerogen-bound isoprenoids, the former does reflect a change in the predominant source of organic matter during these intervals. However, the proportional abundances of isoprenoids in both the bitumen and kerogen correlate poorly with both TOC contents and TOC contents calculated on a carbonate-free basis, indicating that such variation in organic-matter source did not influence organic-matter preservation. Instead, a strong correlation between TOC contents and the relative abundance of unresolved complex mixtures released during pyrolysis and inferred to consist of S-bound organic matter suggests that the availability of sulfide was the main control on OM accumulation regardless of the predominant sources of organic matter.

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