Restricted utility of aryl isoprenoids as indicators for photic zone anoxia

Abstract

Abstract--In a North Sea oil, the carotenoid derivatives/3-carotane,/3-isorenieratane, and isorenieratane were identified, together with a series of aryl isoprenoids with a 2,3,6-trimethyl substitution pattern for the aromatic ring. The 613C values of/~-carotane and/3-isorenieratane are similar, whereas isorenieratane is ca. 15%o heavier. This suggests that/3-isorenieratane is not derived from/~-isorenieratene biosynthesised by Chlorobiaceae, but from aromatisation of/~-carotene. This was confirmed by laboratory aromatisation of partially hydrogenated/~-carotene, which yielded/~-isorenieratane as the main product. The aryl isoprenoids, which can be formed by C-C bond cleavage of both isorenieratane and/~-isorenieratane, have a mixed isotopic signature in the oil. These results indicate that mere identification of aryl isoprenoids, without determination of their 6 ~3C values, cannot be used to assess the presence of Chlorobiaceae, and, thus, photic zone anoxia in the depositional environment.

A biomarker study of depositional paleoenvironments and source inputs for the massive formation of Upper Cretaceous lacustrine source rocks in the Songliao Basin, China

“…These suggest that the aryl isoprenoids in the samples from the K 2 n 1 member likely originated from both precursor compounds, while they may be solely derived from β-carotane in samples from the K 2 n 2 member. This difference implies that both photic zone anoxia and enhanced salinity stratification might have prevailed during the deposition of the K 2 n 1 member (Koopmans et al, 1996), which is consistent with the presence of gammacerane and the low Pr/Ph ratios. Furthermore, the low concentration of aryl isoprenoids may imply that under the lacustrine conditions the growth of green sulfur bacteria might have been restrained due to the likely low concentration of sulfate in the water.…”

“…8). These compounds are degraded molecules from both isorenieratane (XI) and β-carotane (XII), but only when they originate from isorenieratane are they considered as indicative of photic zone anoxia, as isorenieratane is biosynthesized by green sulfur bacteria which live in the water column where both H 2 S and light are essential for their growth (Koopmans et al, 1996;Grice et al, 1998). However, because neither isorenieratane or β-isorenieratene (XIII) were detected in the samples from the SK-1 borehole, and also because it is difficult to measure the δ 13 C composition of these aryl isoprenoids due to their low abundance, it is difficult to interpret their environmental significance.…”

Biomarkers and C and S Isotopes of the Permian to Triassic Solid Bitumen and Its Potential Source Rocks in NE Sichuan Basin

“…The occurrence of aryl isoprenoids in the Lower Silurian Longmaxi Fm and Upper Permian Dalong Fm alone is not enough to indicate the presence of green sulfur bacteria in the photic zone anoxic water column during the deposition [44]. However, elevated gammacerane/C 30 17 , 21 -hopane ratios (Table 2), which have been used to infer marine stratification during source rock deposition [46], and 34 S values as light as −34.5‰ in the Upper Permian Dalong Fm pyrite are consistent with the distribution of aryl isoprenoids, indicating that green and purple sulfur bacteria may have been thriving in stratified euxinic environment.…”

“…The aryl isoprenoids were considered to result from aromatic carotenoids of the green sulfur bacteria (Chlorobiaceae) [33-35, 42, 43]. However, a North Sea oil has been found to have aryl isoprenoids derived from mixtures of C-C bond cleavage of isorenieratanes and -isorenieratanes derived from aromatisation of -carotene; thus green sulfur bacteria Chlorobiaceae are not the unique source of the aryl isoprenoids [44]. No aryl isoprenoids were detected in the samples analyzed from the Cambrian and Upper Permian Longtan Fm.…”

“…These samples show similar maturity; thus, it is less likely for the OM maturity to control the occurrence of aryl isoprenoids. Instead, the distribution may result from euxinic environment or aromatisation of carotenoid -carotene [44]. It is hard to determine which one is the origin for the Lower Silurian aryl isoprenoids without analyses of carbon or sulfur isotopic composition.…”