1. Department of Applied Geology, Curtin University of Technology, Kent Street, Bentley, Western Australia 6102, Australia
2. The Research School of Earth Sciences,
3. Centre for Macroevolution and Macroecology, The Australian National University, Canberra, Australian Capital Territory 0200, Australia
4. Centre for Microscopy, Characterisation and Analysis, The University of Western Australia, 35 Stirling Highway, Crawley, Western Australia 6009, Australia

Correspondence to: Birger Rasmussen¹ Correspondence and requests for materials should be addressed to B.R. (Email: B.Rasmussen@curtin.edu.au).

Abstract

The evolution of oxygenic photosynthesis had a profound impact on the Earth's surface chemistry, leading to a sharp rise in atmospheric oxygen between 2.45 and 2.32 billion years (Gyr) ago^{1, 2} and the onset of extreme ice ages³. The oldest widely accepted evidence for oxygenic photosynthesis has come from hydrocarbons extracted from 2.7-Gyr-old shales in the Pilbara Craton, Australia, which contain traces of biomarkers (molecular fossils) indicative of eukaryotes and suggestive of oxygen-producing cyanobacteria^{4, 5, 6, 7}. The soluble hydrocarbons were interpreted to be indigenous and syngenetic despite metamorphic alteration and extreme enrichment (10–20) of ¹³C relative to bulk sedimentary organic matter^{5, 8}. Here we present micrometre-scale, in situ ¹³C/¹²C measurements of pyrobitumen (thermally altered petroleum) and kerogen from these metamorphosed shales, including samples that originally yielded biomarkers. Our results show that both kerogen and pyrobitumen are strongly depleted in ¹³C, indicating that indigenous petroleum is 10–20 lighter than the extracted hydrocarbons⁵. These results are inconsistent with an indigenous origin for the biomarkers. Whatever their origin, the biomarkers must have entered the rock after peak metamorphism 2.2 Gyr ago⁹ and thus do not provide evidence for the existence of eukaryotes and cyanobacteria in the Archaean eon. The oldest fossil evidence for eukaryotes and cyanobacteria therefore reverts to 1.78–1.68 Gyr ago and 2.15 Gyr ago^{10, 11}, respectively. Our results eliminate the evidence for oxygenic photosynthesis 2.7 Gyr ago and exclude previous biomarker evidence for a long delay (300 million years) between the appearance of oxygen-producing cyanobacteria and the rise in atmospheric oxygen 2.45–2.32 Gyr ago¹.

1. Department of Applied Geology, Curtin University of Technology, Kent Street, Bentley, Western Australia 6102, Australia
2. The Research School of Earth Sciences,
3. Centre for Macroevolution and Macroecology, The Australian National University, Canberra, Australian Capital Territory 0200, Australia
4. Centre for Microscopy, Characterisation and Analysis, The University of Western Australia, 35 Stirling Highway, Crawley, Western Australia 6009, Australia

Correspondence to: Birger Rasmussen¹ Correspondence and requests for materials should be addressed to B.R. (Email: B.Rasmussen@curtin.edu.au).

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