Discovery of the oldest known biomarkers provides evidence for phototrophic bacteria in the 1.73 Ga Wollogorang Formation, Australia

The discovery of mid-Proterozoic (1.8–0.8 billion years ago, Ga) indigenous biomarkers is a challenge, since biologically informative molecules of such antiquity are commonly destroyed by metamorphism or overprinted by drilling fluids and other anthropogenic petroleum products. Previously, the oldest clearly indigenous biomarkers were reported from the 1.64 Ga Barney Creek Formation in the northern Australian McArthur Basin. In this study, we present the discovery of biomarker molecules from carbonaceous shales of the 1.73 Ga Wollogorang Formation in the southern McArthur Basin, extending the biomarker record back in time by ~90 million years. The extracted hydrocarbons illustrate typical mid-Proterozoic signatures with a large unresolved complex mixture, high methyl alkane/n-alkane ratios and the absence of eukaryotic steranes. Acyclic isoprenoids, saturated carotenoid derivatives, bacterial hopanes and aromatic hopanoids and steroids also were below detection limits. However, continuous homologous series of low molecular weight C₁₄–C₁₉ 2,3,4- and 2,3,6-trimethyl aryl isoprenoids (AI) were identified, and C₂₀–C₂₂ AI homologues were tentatively identified. Based on elevated abundances relative to abiogenic isomers, we interpret the 2,3,6-AI isomer series as biogenic molecules and the 2,3,4-AI series as possibly biogenic. The biological sources for the 2,3,6-AI series include carotenoids of cyanobacteria and/or green sulphur bacteria (Chlorobiaceae). The lower concentrated 2,3,4-AI series may be derived from purple sulphur bacteria (Chromatiaceae). These degradation products of carotenoids are the oldest known clearly indigenous molecules of likely biogenic origin.