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

Abstract

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 δ13CTOC, 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 δ13Clipid signature tracks δ13CTOC 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.