Isorenieratene: Biomarker for Photic Layer Anoxia?

The use of biomarkers is a relatively new and growing field and while these novel proxies hold the potential to revolutionise our understanding of past environments it is important that we fully understand the assumptions that underlie their use and the systems in which they operate. The use of the biomarker, isorenieratene to identify photic layer anoxia, particularly in marine systems is increasing. However recent papers have identified this carotenoid in settings that also show evidence of deep ventilation. This synthesis will assess the history and evidence for the use of this biomarker, focusing on the diagenesis of isorenieratene and the habitat of the Chlorobiaceae species that are its main producers. While it is clear this biomarker can be produced under euxinic condition in the photic layer I highlight evidence that: 1) The most common occurrence of these Chlorobiaceae species is photolithotropic and that even in marine systems it preferentially occurs at the sediment water interface rather than in pelagic conditions; 2) That these species are not the only source of this pigment and its products; and 3) These species can also operate in common micro - niches within more oxic environments.