Diagenetic and catagenetic products of isorenieratene: Molecular indicators for photic zone anoxia

Abstract

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. Atropisomers 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. Sulphurisation 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.

Restricted utility of aryl isoprenoids as indicators for photic zone anoxia

“…16%o. This is in agreement with values obtained from a suite of carbonaceous rocks throughout the Phanerozoic (Koopmans et al, 1996). However, the 6~3C value of /3-isorenieratane, which was expected to be similar to that of isorenieratane because their unsaturated counterparts/3-isorenieratene and isorenieratene are biosynthesised by Chlorobiaceae, is close to that of/3-carotane.…”

“…The occurrence of aryl isoprenoids with a 2,3,6-tfimethyl substitution pattern for the aromatic ring and a tail-to-tail isoprenoid chain (I; see Appendix) in sedimentary rocks and crude oils is often taken as an indication for the presence of brown-coloured photosynthetic green sulphur bacteria (Chlorobiaceae) and photic zone anoxia in the depositional environment Powell, 1986, 1987;Clark and Philp, 1989;Yu Xinke et al, 1990;Fowler, 1992;Requejo et al, 1992;Hartgers et al, 1994a,b;Koopmans et al, 1996). The aryl isoprenoids are thought to form from C-C bond cleavage of the isoprenoid chain of isorenieratene (II) and /3-isorenieratene (III), both biosynthesised by Chlorobiaceae (Liaaen-Jensen, 1978a,b;de Wit and Caumette, 1995), that are bound in high-molecular-weight organic matter fractions (e.g., kerogen) of the sediment (Requejo et al, 1992;Hartgers et al, 1994b).…”

Evidence for photic‐zone euxinia in the Early Albian Santana Formation (Araripe Basin, NE Brazil)

“…Our findings attest the occurrence of intensified carbon burial Koopmans et al (1996). Black circles indicate n-alkanes, numbers refer to carbon atom numbers.…”

“…5). The carbon isotopic composition of isorenieratane, as determined in two samples using GC-IRMS, is relatively heavy ()17.9& ± 0.3&) compared to typical algal lipids, clearly demonstrating an origin from Chlorobiaceae (Koopmans et al, 1996). The first occurrence of isorenieratane, although in low concentration (6.2 lg g )1 TOC), is recorded in the ostracod-rich limestone located just below the black shale (Fig.…”

“…Information on the redox state of the water column during OC accumulation can be obtained from the presence of a highly specific biomarker, isorenieratene and its diagenetic derivatives (Summons and Powell, 1986;Sinninghe Damste´et al, 1993;Koopmans et al, 1996). The aromatic carotenoid isorenieratene is exclusively synthesized by the browncoloured strain of green sulphur bacteria (Chlorobiaceae).…”