Red to Green: A Lipid Biomarker Record of Tonian-Ediacaran Algae in Siberia.

Multiple lines of evidence point to a major diversification of Eukarya in the late Tonian period, ca. 800-720 Ma ago. Regular steranes, lipid biomarkers primarily sourced from eukaryotes, are first unambiguously reported from multiple sites. Here, we present lipid biomarker stratigraphic records from two drill-cores from the Lena-Anabar Basin in northern Siberia, in addition to new biomarker data from the Walcott Member of the Chuar Group, USA. The Siberian drill-cores span the late Tonian (ca. 800-750 Ma) and late Ediacaran-early Cambrian periods (ca. 565-539 Ma), with Cryogenian strata absent. Compositional differences in sedimentary organic matter were assessed using a suite of biomarker hydrocarbon parameters, with seawater redox and phosphorus availability determined using iron speciation, trace metals, and phosphorus phase partitioning. Steranes were consistently detected in Tonian strata in the Siberian drill-cores, with sterane/hopane (S/H) values being uniformly low (< 0.05), consistent with a low-productivity, marine environment in which bacteria outcompeted eukaryotes for limited nutrients. This was confirmed by phosphorus phase partitioning data that suggest that phosphorus was a key limiter of late Tonian marine eukaryote abundance. Unlike Siberia, late Tonian samples from the Chuar Group showed consistently higher S/H values (up to 0.38) and elevated total organic carbon content (1.3-5.6 wt.%) indicative of higher algal productivity in a eutrophic marine setting. Tonian strata primarily yielded cholestane (C