Compound-specific carbon isotope measurements of individual lipid biomarkers from immature Ediacaran rocks of Baltica

Compound-specific carbon isotope ratios (CSIA) were measured for a suite of lipid biomarker compounds extracted from immature, late Ediacaran sedimentary rocks from drill cores sampled across Baltica. Using a newly developed picomolar-scale CSIA (pico-CSIA) method, we measured carbon isotope compositions of the abundant n-alkanes and hopanes, as well as C₂₉ sterane, pristane, and phytane. Total organic carbon (TOC) of the Kotlin Regional Horizon in Baltica (Saint Petersburg area, Utkina Zavod drill core), from a low-salinity coastal environment, is consistently enriched in ¹³C by up to 10‰, compared to that for Redkino and Kotlin marine rocks from other locations in Baltica. This ¹³C enrichment is also recorded by the n-alkanes, hopanes, phytane, and C₂₉ sterane. In all locations, the δ¹³C values of the C₂₉ sterane are within 2‰ of the bacterial hopane δ¹³C values and within 0.7‰ of δ¹³C_TOC, suggesting that the abundant hopanes within these sediments could be derived from RuBisCO Calvin-Benson-Bassham pathway-utilizing organisms, as well as from bacterial heterotrophs. Since δ¹³C_lipid signature tracks δ¹³C_TOC values for the Kotlin Regional Horizon samples from Utkina Zavod location, the significant ¹³C enrichments in this interval reflect either the δ¹³C composition of DIC used for autotrophy or a muted magnitude of carbon isotope fractionation during lipid biosynthesis, but are not due to enhanced preservation of organic compounds and geopolymers derived from ¹³C-enriched biochemicals. Pico-CSIA and biomarker data provide evidence for both regional environmental heterogeneity and secular changes in carbon cycling during deposition of sediments of the Kotlin and Redkino Regional Horizon intervals.