Reappraisal of hydrocarbon biomarkers in Archean rocks

Hopanes and steranes found in Archean rocks have been presented as key evidence supporting the early rise of oxygenic photosynthesis and eukaryotes (e.g. 1), but it has been suggested that sample contamination could negate these results (e.g. 2). To resolve this debate, we performed a multilaboratory study of new cores from the Pilbara Craton, Australia, that were drilled and sampled using unprecedented hydrocarbon-clean protocols. We report that hopane and sterane concentrations measured in new ultraclean Archean drill cores from Australia are comparable to blank concentrations, yet their concentrations in the exteriors of conventionally collected cores of stratigraphic equivalence exceed blank concentrations by more than an order of magnitude due to surficial contamination. Polycyclic aromatic hydrocarbons and diamondoids in rock extracts and hydropyrolysates from the ultraclean cores exceeded blank concentrations and had high thermal maturity distribution patterns, comparable to those reported in previous Archean studies of conventionally collected samples. Together, these results demonstrate that previously studied Archean samples host mixtures of biomarker contaminants and indigenous overmature hydrocarbons. Therefore, existing lipid biomarker evidence no longer provides valid evidence for the emergence of oxygenic photosynthesis and eukaryotes by ∼2.7 billion years ago.