Sterane Distributions Record Eukaryotic Community Structure In the Proterozoic
Our results overwhelmingly support the classical interpretation of Proterozoic steranes as biomarkers for eukaryotes, most likely microalgae. In highly euxinic and restricted marine facies of the Barney Creek Formation, the abundance and diversity of eukaryotes appears to be low based on a dearth of regular 24-alkylsteranes. Nevertheless, microalgae are recorded by an unusual pattern of 4-methylsteranes, including dinosteranes. Bacterial biomarkers, such as hopanes, were far more abundant than total steranes. The dinosteranes record an adaptation of dinoflagellates to a hostile restricted mid-Proterozic marine environment with pervasive euxinia and shallow chemoclines.
In the post-Sturtian Cryogenian, basal animals appeared and marine demosponge inputs were first recorded by detection of significant amounts of 24-isopropylcholestanes in the Ghadir Manquil Formation of the Huqf Supergroup. The radiation of marine chlorophytes (green algae) also coincides with the first appearance of animals and is evidenced through a dominance of 24-ethylcholestanes over the other C27-C30 regular steranes. The Neoproterozoic dominance of chlorophytes as primary algal producers in South Oman, Siberia and Pakistan may reflect fairly iron-rich oceanic condition after the Sturtian and which were maintained into the early Cambrian. However, aquatic microbial communities, water chemistry and redox structure on the margins of Neoproterozic oceans were not uniform everywhere as evidenced by the contrast in sterane profiles recorded for the Doushantuo Formation in South China.