Evidence for ancient halophiles? Testing biomarker syngeneity of evaporites from Neoproterozoic and Cambrian strata

Molecular fossils from evaporites can provide significant insights into the biotic composition of ancient hypersaline ecosystems. However, hydrocarbon contamination can mask the true nature of this composition and potentially result in misleading interpretations. In this study, drill core samples of Neoproterozoic- and Cambrian-age dolomite, anhydrite and halite from the Amadeus Basin in central Australia contain distinctive and varied hydrocarbon signatures. The results are consistent with previous studies of ancient hypersaline settings and include elevated concentrations of heptadecane (n-C₁₇), docosane (n-C₂₂), a diverse range of acyclic isoprenoids, hopanes and steranes, including high concentrations of pregnanes. Furthermore, several samples yielded hydrocarbon signatures with high concentrations of mono- and dimethylalkanes relative to n-alkanes, which is characteristic of many Cambrian and Precambrian bitumens. By quantifying the exterior/interior concentration differences of biomarkers in these evaporites, considerable insight into the syngeneity of these molecules was obtained. In most samples, the majority of biomarkers were only detected in the exterior, demonstrating that they are contaminants and not indigenous to the evaporites. These include biomarkers typically assigned to hypersaline environments, demonstrating, in this case, that their presence is not necessarily an indicator of syngeneity. We show that even in combination, these molecules can have a wide variety of sources that are not necessarily indigenous to the host rocks. Nevertheless, some Neoproterozoic evaporites, while also marred by contamination, contain demonstrably indigenous biomarkers with equal concentrations on the rock exterior and interior. We show that the ability to discern biomarker syngeneity in a complex mix of contaminated and indigenous hydrocarbons requires the need for each compound in every sample to be separately evaluated. This study provides an example of how to unravel such a complex series of contamination and obtain more accurate insights into biomarkers of ancient evaporites. The approach outlined herein can be readily applied to biomarker studies of other ancient settings.