DIAGENETIC AND CATAGENETIC PRODUCTS OF ISORENIERATENE - MOLECULAR INDICATORS FOR PHOTIC ZONE ANOXIA

A wide range of novel diagenetic and catagenetic products of the diaro matic carotenoid isorenieratene, a pigment of the photosynthetic green sulphur bacteria Chlorobiaceae, has been identified in a number of se dimentary rocks ranging from Ordovician to Miocene. Compound identific ation is based on NMR, mass spectrometry, the presence of atropisomers , and stable carbon isotopes. Atropisomers contain an axially chiral c entre which, in combination with other chiral centres, results in two or more diastereomers that can be separated on a normal GC column. Chl orobiaceae use the reverse TCA cycle to fix carbon, so that their biom ass is enriched in C-13. High C-13 contents of isorenieratene derivati ves therefore support their inferred origins. Isorenieratene derivativ es include C-40, C-33, and C-32, diaryl isoprenoids and short-chain ar yl isoprenoids with additional aromatic and/or S-containing rings. C-3 3 and C-32 compounds are diagenetic products of C-33 and C-32 ''carote noids'' formed from isorenieratene during early diagenesis through exp ulsion of toluene and m-xylene, respectively. Cyclisation of the polye ne acyclic isoprenoid chain can proceed via an intramolecular Diels-Al der reaction, followed by aromatisation of the newly formed ring. Sulp hurisation is also an important process during early diagenesis, compe ting with expulsion and cyclisation. Sulphur-bound isorenieratane is r eleased during progressive diagenesis, due to cleavage of relatively w eak S-S and C-S bonds. Cleavage of C-C bonds during aromatisation of n ewly formed rings and during catagenesis yields short-chain compounds. The inherent presence of a conjugated double bond system in carotenoi ds 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 isoreni eratene or its diagenetic and catagenetic products in ancient sediment ary rocks and crude oils is therefore an excellent indication for phot ic zone anoxia in the depositional environment. Diagenetic and catagen etic products of isorenieratene are expected to find applications in r econstruction 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.