Sterane Distributions Record Eukaryotic Community Structure In the Proterozoic

Detailed analyses of hydrocarbon biomarkers have been performed on Proterozoic sediment extracts from the Barney Creek Fm (1.64 Ga) of the McArthur Group, the Chuar Group (770-740 Myr), the Doushantuo Fm. of South China (635-550 Myr), as well as the Huqf Supergroup from the South Oman Salt Basin (713-540 Myr) using metastable reaction monitoring GC-MS analyses. Parallel investigations of kerogen-bound biomarkers, released by catalytic hydropyrolysis (HyPy), confirmed that the extractable biomarkers are indigenous and syngenetic with the host matrix and not artifacts of contamination.

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.