Deep time biomarkers: a study of organic matter and fluid inclusions in Precambrian rocks

Hydrocarbon biomarkers are organic molecules with a specific biosynthetic source that can be preserved over billions of years (Ga). They are being searched for in Archean (>2.5 Ga) rocks to investigate (1) the origin of eukaryotic life, (2) the onset of oxygenic photosynthesis by cyanobacteria, and (3) the role of organisms during the Great Oxidation Event. Unfortunately, the detection of biomarkers in Archean rocks struggles with contamination issues, unspecific biomarkers and the lack of suitable sedimentary rocks due to extensive thermal overprints. In 2012 the Jeerinah Formation (2.63 Ga) and Carawine Dolomite (2.55 Ga) from the Pilbara Craton (Western Australia) were drilled under ultra-clean conditions by the Agouron Institute. These sedimentary rocks were solvent extracted with two techniques (bitumen I, bitumen II). Organic matter obtained by the bitumen I extraction (free kerogen) indicate high thermal maturity and biomarkers, such as hopanes and steranes, indicative of bacteria and eukaryotes,respectively, are absent. These findings rebutted previous biomarker evidence from mineral exploration boreholes that were used to explain the rise of eukaryotes and early oxygenic photosynthesis. Organic matter obtained by the bitumen II extraction (mineral-bound kerogen), contained some biomarkers, but these are likely the result of sample contamination and/or the simultaneous extraction of later carbonate veins. The later veins were petrologically assessed and it was found that they formed between2.2-1.a Ga under relatively mild conditions (180-200°C), while the sedimentary rocks revealed temperatures of ~400°C, which is too high for any biomarkers to have been preserved. Solid bitumens and oil-bearing fluid inclusions captured inside the later veins were analysed using bitumen I extraction and hopanes and steranes were successfully detected. This thesis demonstrates the complexity in investigating Archean biomarkers and the dynamics of oil-bearing fluid inclusions to better understand the early biosphere.