Aryl isoprenoids in severe biodegradation oils from the Miaoxi Depression, Bohai Bay Basin

Abstract

Aryl isoprenoids (AIs), a series of aromatic carotenoid derivatives, have served as significant biomarkers for paleoecology reconstructions. However, the effect of biodegradation on AIs remains unclear to date. Here, a confirmed family of progressively biodegraded oils from the Miaoxi Depression, Bohai Bay Basin was analyzed to investigate the fate of these compounds during severe biodegradation under natural conditions. Two pseudohomologous series with carbon numbers ranging from 13 to 31 and 40 were identified as 2,3,6- and 2,3,4-AIs in the reference oil. The concentrations of total AIs showed a gradual decline with increasing biodegradation but AIs survive in the most severely biodegraded oil (with a PM rank of 8 and an M-MN2 of 983) in this study. The differential proximity of adjacent methyl groups on the benzene ring may result in 2,3,4-AIs being more resistant to biodegradation compared to 2,3,6-AIs. No preferential biodegradation of individual AIs is proceeded systematically by carbon number, which may result from competition of two biodegradation pathways and favor of specific microbial communities. Moreover, the AIs-bearing proxies, including aryl isoprenoid ratio (AIR) and concentrations of total AIs, 2,3,6-AIs and 2,3,4-AIs, exhibit a significant decrease with increasing microbial alteration. Therefore, these proxies must be employed with caution to reconstruct photic-zone euxinia (PZE) for crude oils with potential microbial degradation.