The presence of mass-independently fractionated sulphur isotopes (MIF-S) in many sedimentary rocks older than 2.4 billion years (Gyr), and the absence of MIF-S in younger rocks, has been considered the best evidence for a dramatic change from an anoxic to oxic atmosphere around 2.4 Gyr ago^{1, 2, 3, 4, 5, 6, 7, 8, 9}. This is because the only mechanism known to produce MIF-S has been ultraviolet photolysis of volcanic sulphur dioxide gas in an oxygen-poor atmosphere. Here we report the absence of MIF-S throughout 100-m sections of 2.76-Gyr-old lake sediments and 2.92-Gyr-old marine shales in the Pilbara Craton, Western Australia. We propose three possible interpretations of the MIF-S geologic record: (1) the level of atmospheric oxygen fluctuated greatly during the Archaean era; (2) the atmosphere has remained oxic since 3.8 Gyr ago, and MIF-S in sedimentary rocks represents times and regions of violent volcanic eruptions that ejected large volumes of sulphur dioxide into the stratosphere; or (3) MIF-S in rocks was mostly created by non-photochemical reactions during sediment diagenesis, and thus is not linked to atmospheric chemistry.

1. NASA Astrobiology Institute and Department of Geosciences, The Pennsylvania State University, University Park, Pennsylvania 16802, USA
2. Institute of Technology, Kyushu University, Fukuoka, 812-8581, Japan
3. Department of Geological Sciences and Engineering, University of Reno-Nevada, Reno, Nevada 89557, USA
4. Geological Survey of Canada, Ottawa, Ontario, K1A 0E8, Canada

Correspondence to: Hiroshi Ohmoto¹ Correspondence and requests for materials should be addressed to H.O. (Email: ohmoto@geosc.psu.edu).

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