Restricted utility of aryl isoprenoids as indicators of photic zone anoxia

Abstract

In a North Sea oil, the carotenoid derivatives -carotene, -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 13C values of -carotene and -isorenieratane are similar, whereas isorenieratane is ca. 15% heavier. This suggests that -isorenieratane is not derived from -isorenieratane 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 13C values, cannot be used to assess the presence of Chlorobiaceae, and, thus, photic zone anoxia in the depositional environment.