oa Quantifying the Heterotrophic Contribution to Steranes and Hopanes in the Sedimentary Record

Steranes and hopanes are diagenetic products of eukaryotic sterols and bacterial hopanols that can remain stable over geological timescales. The relative proportions of steranes and hopanes have been used to track the taxonomic composition of primary producers throughout Earth’s history. However, these biomarker parameters are also influenced by aerobic heterotrophic reworking of biomass: as molecules from primary producers are consumed and degraded, both bacterial and eukaryotic heterotrophs contribute their biomarkers to the molecular pool, leading to a loss of primary ecological information. If overlooked, this effect can significantly alter reconstructions of ancient ecosystems.

Analyzing covariations between various biomarker parameters, especially when combined with elemental distribution data, can help estimate the impact of heterotrophic reworking. However, when this effect is strong, such methods often fail to reconstruct the original biomarker distributions that reflect the taxonomic composition of primary producers. Here we show that incorporating stable carbon isotope data from individual steranes and hopanes can help quantify the heterotrophic contribution. Our findings indicate that nearly all hopanes found in Ediacaran rocks from Siberia and the East European platform likely originate from heterotrophic bacteria growing on primary biomass of eukaryotic green algae.