Pergamon Geochimicaet Cosmochimica Acta, Vol. 60, No. 22, pp. 4467-4496, 1996 Copyright© 1996 ElsevierScienceLtd Printed in the USA.All rights reserved 0016-7037/96 $15.00 + .00 PII S0016-7037(96)00238-4 Diagenetic and catagenetic products of isorenieratene: Molecular indicators for photic zone anoxia MARTIN P. KOOPMANS,JURGEN KOSTER,* HEIDY M. E. VAN KAAM-PETERS, FABIEN KENIG, t STEFAN SCHOUTEN, WALTER m. HARTGERS, ~ JAN W. DE LEEUW, and JAAP S. SINNINGHEDAMSTI~ Netherlands Institute for Sea Research (NIOZ), Department of Marine Biogeochemistry and Toxicology, P.O. Box 59, 1790 AB Den Burg, The Netherlands (Received May 17, 1995; accepted in revised form July 2, 1996) Abstract--A wide range of novel diagenetic and catagenetic products of the diaromatic carotenoid isorenieratene, a pigment of the photosynthetic green sulphur bacteria Chlorobiaceae, has been identified in a number of sedimentary rocks ranging from Ordovician to Miocene. Compound identification is based on NMR, mass spectrometry, the presence of atropisomers, and stable carbon isotopes. Atropisom- ers contain an axially chiral centre which, in combination with other chiral centres, results in two or more diastereomers that can be separated on a normal GC column. Chlorobiaceae use the reverse TCA cycle to fix carbon, so that their biomass is enriched in 13C. High ~3C contents of isorenieratene derivatives therefore support their inferred origins. Isorenieratene derivatives include C4o, C33, and C32, diaryl isoprenoids and short-chain aryl isoprenoids with additional aromatic and/or S-containing rings. C33 and C32 compounds are diagenetic products of C33 and C32 "carotenoids" formed from isorenieratene during early diagenesis through expulsion of toluene and m-xylene, respectively. Cyclisation of the polyene acyclic isoprenoid chain can proceed via an intramolecular Diels-Alder reaction, followed by aromatisation of the newly formed ring. Sulphurisa- tion is also an important process during early diagenesis, competing with expulsion and cyclisation. Sulphur-bound isorenieratane is released during progressive diagenesis, due to cleavage of relatively weak S-S and C-S bonds. Cleavage of C-C bonds during aromatisation of newly formed rings and during catagenesis yields short-chain compounds. The inherent presence of a conjugated double bond system in carotenoids implies that similar diagenetic and catagenetic reactions can occur with all carotenoids. Chlorobiaceae live at or below the oxic/anoxic boundary layer and require both light and H2S. The presence of isorenieratene or its diagenetic and catagenetic products in ancient sedimentary rocks and crude oils is therefore an excellent indication for photic zone anoxia in the depositional environment. Diagenetic and catagenetic products of isorenieratene are expected to find applications in reconstruction of palaeoenvironments and in oil-oil and oil-source rock correlation studies. Their presence in several petroleum source rocks suggests that anoxia is an important environmental parameter for the preservation of organic matter. 1. INTRODUCTION It is generally accepted that anoxic depositional environ- ments favour the preservation of organic matter (Demaison and Moore, 1980), albeit that Pedersen and Calvert (1990) have recently challenged this. Anoxic conditions in the ma- rine environment occur when the supply of oxygen in bottom waters is limited by restricted water circulation arising from density or temperature stratification, or by contact of an oxy- gen-minimum layer with the seafloor (e.g., on continental shelves and slopes). Since anoxia and thus preservation of organic matter can lead to formation of petroleum source rocks, it is important to be able to recognise (ancient) anoxic basins. Hence, recognition of biomarkers in sedimentary rocks and oils that are unambiguously related to anoxic depo- * Present address: Department of Petroleum Geology, Technical University of Clausthal, Institute for Geology and Palaeontology, D-38678 Clausthal-Zellerfeld, Germany. t Present address: Department of Geological Sciences, University of Illinois, Chicago, Chicago, IL 60607-7059, USA. * Present address: Departamento de Qulmica Ambiental, CID- CSIC, Jordi Girona 18-26, 08034 Barcelona, Spain. sitional conditions is a challenge for petroleum geochemistry and organic geochemistry alike. Green sulphur bacteria (Chlorobiaceae) are photoautotro- phic organisms that are strictly anaerobic and require both light and H2S. Their presence attests to an anoxic water layer reaching into the photic zone, and euxinic conditions. Chlorobiaceae produce two types of characteristic pigments (bacteriochlorophylls and isorenieratene) that can be used to trace their distribution in the geological record. The bac- teriochlorophylls c, d, and e are unique compared to other (bacterio)chlorophylls in that their tetrapyrrole nucleus can contain as many as thirty-eight carbon atoms (Scheer, 1991 ). The diaromatic carotenoid isorenieratene (I; see Appendix A) comprises an irregular (tail-to-tail) isoprenoid chain, and a specific 1-alkyl-2,3,6-trimethyl substitution pattern for both its aromatic rings (Liaaen-Jensen, 1978a,b). The high ~3C content of isorenieratene and the bacteriochlorophylls c, d, and e determined, respectively, from octadecahydro- isorenieratene (or isorenieratane; II) (Kohnen et al., 1992; Sinninghe Damst6 et al., 1993b; Hartgers et al., 1994c) and the breakdown products of the extended porphyrins (Grice et al., 1995), can be attributed to carbon fixation via the 4467