Molecular and carbon isotopic analysis of individual biological markers: Evidence for sources of organic matter and paleoenvironmental conditions in the Upper Ordovician Maquoketa Group, Illinois Basin, USA

Variations in the carbon isotopic composition (delta(13)C) of pristane, phytane, n-heptadecane (n-C-17), C-29 alpha alpha alpha 20R sterane, and aryl isoprenoids provide evidence for a diverse community of aIgal and bacterial organisms in organic matter of the Upper Ordovician Maquoketa Group of the Illinois Basin. Carbon isotopic compositions of pristane and phytane from the Maquoketa are positively covariant (r = 0.964), suggesting that these compounds were derived from a common source inferred to be primary producers (algae) from the oxygenated photic zone. A variation of 3 parts per thousand in delta(13)C values (-31 to -34 parts per thousand) for pristane and phytane indicates that primary producers utilized variable sources of inorganic carbon. Average isotopic compositions of n-C-17 (-32 parts per thousand) and C-29 alpha alpha alpha 20R sterane (-31 parts per thousand) are enriched in C-13 relative to pristane and phytane (-33 parts per thousand) suggesting that these compounds were derived from a subordinate group of primary producers, most likely eukaryotic algae. In addition, a substantial enrichment of C-13 in aryl isoprenoids (-14 to -18 parts per thousand) and the identification of tetramethylbenzene in pyrolytic products of Maquoketa kerogen indicate a contribution from photosynthetic green sulfur bacteria to the organic matter. The presence of anaerobic, photosynthetic green sulfur bacteria in organic matter of the Maquoketa indicates that anoxic conditions extended into the photic zone. The delta(13)C of n-alkanes and the identification of an unusual suite of straight-chain n-alkylarenes in the m/z 133 fragmentograms of Ordovician rocks rich in Gloeocapsomorpha prisca (G. prisca) indicate that G. prisca did not contribute to the organic matter of the Maquoketa Group. (C) 1997 Elsevier Science Ltd.