INVITED: THE USE OF BIOMARKERS FROM OIL INCLUSIONS TO CONSTRAIN PALEOPROTEROZOIC ECOSYSTEMS AND ENVIRONMENTS

The record of early life is sparse and based on often-equivocal microfossils, stromatolites, isotopic ratios and biomarkers. Oil-bearing fluid inclusions are closed systems, which are shielded from contamination by modern microbes and from destruction that normally affects porosity-bound hydrocarbons, thus overcoming problems of preservation and syngeneity of biomarkers. The biomarkers in oil inclusions can thus potentially provide relatively pristine and robust data on the Earth's earliest biota and environment.

Oil inclusions from the 2.45 Ga Matinenda Formation (Huronian) at Elliot Lake, Canada have been analysed using a rigorous cleaning system and sensitive gas chromatography-mass spectrometry. The inclusions contain multiple fluid phases including CO₂, CH₄, H₂O and fluorescing oil. They represent two episodes of oil migration; the first during diagenesis and the second during low-grade metamorphism at ~2.2 Ga. The extracted oil is mature, slightly biodegraded and rich in high molecular weight n-alkanes, dibenzothiophenes and phenanthrenes. Preliminary data suggest that biomarkers are present, including hopanes, 2a-methylhopanes, steranes and diasteranes.

The Matinenda Formation is a uraniferous arenite deposited in a reduced environment. The likely source rocks for its oils are argillites in the overlying McKim Formation, which in turn disconformably underlie the first major Paleoproterozoic glaciation. The first oxidized red-beds occur above the third glacial unit in the succession. Our biomarker data thus constrain redox evolution shortly before the “Great Oxidation Event”. 2a-Methylhopanes indicate the presence of cyanobacteria, all capable of oxygenic photosynthesis. Diverse steranes require some free oxygen during biosynthesis. Hence, biogenic oxygen production began before significant atmospheric accumulation, and some aquatic settings became oxygenated enough for eukaryotic sterol synthesis before the entire ocean.