Hydrocarbon biomarkers preserved in carbonate veins of potentially Paleoproterozoic age, and implications for the early biosphere.

Research on the early rise of oxygenic photosynthesis and eukaryotes has recently encountered a major pitfall, as some hopane and sterane biomarkers reported in Archaean rocks are the results of contamination. Following an extensive petrological framework in the Pilbara Craton, Western Australia, oil-bearing fluid inclusions and solid bitumens were identified in replacement and hydrothermal carbonate veins cross-cutting Archaean metasedimentary rocks. The 2.55-2.63 billion years old metasedimentary rocks were found to be depleted of indigenous biomarkers. Here we show novel biomarker results from the solvent extraction of the carbonate veins. Volcanic rock blanks, outside rinses, and instrumental blanks showed no biomarkers, and the surrounding rocks were metamorphosed to a sufficiently high extent to not yield any biomarkers, but the biomarkers found in the veins are most likely indigenous. Biomarkers detected include C21-22 ααα- and αββ-steranes (pregnanes), C27-29 αββ-steranes, C19-26 tricyclic terpanes, C29-30,34 αβ-hopanes, C30 17α-diahopane, and trisnorhopanes, which are in the range 2-180 pg/g. The extracted organic matter is highly mature, based on the biomarker configurations and calculated vitrinite reflectance that ranges from 2.4-3.0 (methylphenanthrene index), 1.4-1.9 (methyladamantane index), and 1.4-2.3 (methyldiamantane index). As the biomarkers are highly mature and the biomarker assemblages have a distinctive pattern to each vein type the likelihood of sample contamination by recent, less mature, biomarkers from a different assemblage is unlikely. The detection of steranes suggests that molecular oxygen was available when the veins were formed, possibly between 2.2 and 1.8 billion years ago, but no evidence for oxygenic photosynthesis in the form of cyanobacterial biomarkers has been found. Carbonate minerals that seem to better preserve biomarkers, such as concretions or veins, show the growing importance of new and exciting opportunities to seek biomarkers in the early Earth rock record, and potentially on other planets. Our results demonstrate for that first time that biomarkers can be found in veins cutting through highly metamorphosed Archaean rocks, and gives an insight into ancient environments.