Investigation of hydrocarbon biomarkers preserved in the Fortescue Group in the Pilbara Craton, Western Australia

Biomarker studies on the 2.78-2.63 Ga Fortescue Group, Pilbara Craton, Western Australia, have gained tremendous attention for understanding Neoarchean ecosystems. However, there has been a serious debate about the indigeneity and syngeneity of biomarkers contained in the Group. This work focuses on the development of clean processing methods of Archean rock samples free from contamination, and the interpretation of potential Archean biomarkers to reveal information about the ancient biosphere preserved in stromatolite fossils and shales. The examined rock samples were from outcrops and drill cores, and were collected from the Kylena, Tumbiana, Maddina and Jeerinah formations in the Fortescue Group. There are plenty of stromatolite outcrops with various morphologies in the northeast part of the Pilbara region. The three drill cores recovered across the Pilbara region were also examined. Rock samples were sliced into several pieces from the outside towards the inside, so as to separately analyse the hydrocarbon content. This procedure enables recent contamination to be distinguished from indigenous hydrocarbon signatures preserved deep inside the rocks. Most samples including all drill core samples only contain trace amounts of aliphatic and aromatic hydrocarbons, at levels almost comparable but in some cases somewhat above laboratory contamination levels. The hydrocarbon distributions are characteristic of a high thermal maturity, consistent with the prehnite-pumpellyite metamorphic facies that the Fortescue Group experienced. However, one outcrop sample from the Tumbiana Formation preserves various hopanes and steranes, as well as mid-chain branched monomethylalkanes deep inside the rock,which is interpreted as indicating the presence of cyanobacteria and eukaryotes at 2.7 Ga. The finding further supports the presence of oxygen and the emergence of oxygenic photosynthesis before the Great Oxidation Event at 2.4 Ga. However, the samples which contain potential Archean biomarkers are limited, and further extensive studies will be necessary to decisively constrain the timing of evolution of cyanobacteria. In addition to the ancient cyanobacteria, recent cyanobacterial habitation on rock surfaces is indicated by the high concentration of mid-chain branched monomethylalkanes within the surface layers of the investigated rocks.