EXCEPTIONAL PRESERVATION OF LIPID BIOMARKERS REVEALS NEW INSIGHTS INTO LATE EDIACARAN OLIGOTROPHIC SHALLOW-MARINE COMMUNITIES AND ENVIRONMENTS ON BALTICA

Middle to late Ediacaran (575-541 Ma) marine sedimentary rocks preserve the first unequivocal evidence for the appearance of macroscopic, multicellular body fossils in the geologic record, yet little is known about the environmental conditions and food sources that sustained this enigmatic fauna. Here we use detailed lipid biomarker and stable isotopic (δ¹⁵N_total and δ¹³C_TOC) analyses of exceptionally immature strata from multiple drill cores across Baltica to assess microbial community structure and environmental conditions that prevailed in oxic epicontinental seaways. Remarkably, hopane/sterane ratios (a broad measure of bacterial/eukaryotic source organism contribution) encompass an exceptionally wide range (1.6 to 119), but are mostly significantly higher than those found in previous Ediacaran studies and Phanerozoic averages for eutrophic basins (0.5 to 2.0). Elevated bacterial contributions to productivity along with low total organic carbon content (TOC < 1.1 wt%) suggests that these were oligotrophic (i.e., strongly nutrient-limited) settings. Bacteria outcompete eukaryotes in low nutrient waters due to a variety of adaptations as well as their smaller size and increased surface area. Hopane/sterane ratios are particularly high (>22) during the Redkinian time interval, when diversity and abundance of the Ediacaran fauna peaked all over Baltica. Our results from multiple drill cores from Russia and Ukraine depict oligotrophic, shallow-marine conditions, which extended hundreds of kilometers across Baltica, persisted for more than 10 million years, and controlled the availability of primary food sources for heterotrophic consumption by multicellular soft-bodied organisms.