1. Department of Geology and Geophysics, E235 Howe-Russell Geoscience Complex, Louisiana State University, Baton Rouge, Louisiana 70803, USA
2. Institute of Geophysics and Planetary Physics, Department of Earth and Space Sciences, University of California, Los Angeles, California 90095, USA
3. State Key Laboratory of Palaeobiology and Stratigraphy, Nanjing Institute of Geology and Palaeontology, Chinese Academy of Sciences, 39 East Beijing Road, Nanjing 210008, China

Correspondence to: Huiming Bao¹ Correspondence and requests for materials should be addressed to H.B. (Email: bao@lsu.edu).

Understanding the composition of the atmosphere over geological time is critical to understanding the history of the Earth system, as the atmosphere is closely linked to the lithosphere, hydrosphere and biosphere. Although much of the history of the lithosphere and hydrosphere is contained in rock and mineral records, corresponding information about the atmosphere is scarce and elusive owing to the lack of direct records. Geologists have used sedimentary minerals, fossils and geochemical models to place constraints on the concentrations of carbon dioxide, oxygen or methane in the past^{1, 2, 3, 4}. Here we show that the triple oxygen isotope composition of sulphate from ancient evaporites and barites shows variable negative oxygen-17 isotope anomalies over the past 750 million years. We propose that these anomalies track those of atmospheric oxygen and in turn reflect the partial pressure of carbon dioxide () in the past through a photochemical reaction network linking stratospheric ozone to carbon dioxide and to oxygen^{5, 6}. Our results suggest that was much higher in the early Cambrian than in younger eras, agreeing with previous modelling results². We also find that the ¹⁷O isotope anomalies of barites from Marinoan (635 million years ago) cap carbonates display a distinct negative spike (around -0.70), suggesting that by the time barite was precipitating in the immediate aftermath of a Neoproterozoic global glaciation, the was at its highest level in the past 750 million years. Our finding is consistent with the 'snowball Earth' hypothesis^{7, 8} and/or a massive methane release⁹ after the Marinoan glaciation.

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