Volcanic eruptions release a large amount of sulphur dioxide (SO2) into the atmosphere1,2. SO2 is oxidized to sulphate and can subsequently form sulphate aerosol3, which can affect the Earth's radiation balance, biologic productivity and high-altitude ozone concentrations, as is evident from recent volcanic eruptions4. SO2 oxidation can occur via several different pathways that depend on its flux and the atmospheric conditions3. An investigation into how SO2 is oxidized to sulphate—the oxidation product preserved in the rock record—can therefore shed light on past volcanic eruptions and atmospheric conditions. Here we use sulphur and triple oxygen isotope measurements of atmospheric sulphate extracted from tuffaceous deposits to investigate the specific oxidation pathways from which the sulphate was formed. We find that seven eruption-related sulphate aerosol deposition events have occurred during the mid-Cenozoic era (34 to 7 million years ago) in the northern High Plains, North America. Two extensively sampled ash beds display a similar sulphate mixing pattern that has two distinct atmospheric secondary sulphates. A three-dimensional atmospheric sulphur chemistry and transport model study reveals that the observed, isotopically discrete sulphates in sediments can be produced only in initially alkaline cloudwater that favours an ozone-dominated SO2 oxidation pathway in the troposphere. Our finding suggests that, in contrast to the weakly acidic conditions today5, cloudwater in the northern High Plains may frequently have been alkaline during the mid-Cenozoic era. We propose that atmospheric secondary sulphate preserved in continental deposits represents an unexploited geological archive for atmospheric SO2 oxidation chemistry linked to volcanism and atmospheric conditions in the past.
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H.B. thanks D. Loope, H. LaGarry and J. Swinehart for guidance in the field, M. Khachaturyan, K. Jenkins, K. Howell, I. Kohl and A. J. Kaufman for technical assistance, B. Li for statistical treatment, and the National Park Service for sampling permission at Scotts Bluff National Monument (permit numbers SCBL-2000-SCI-000, BADL-2004-SCI-0012 and SCBL-2004-SCI-0005). Financial support was provided by the NSF (EAR-0408986 to H.B.). S.Y. thanks S. T. Rao, D. Mobley, K. Schere, R. Mathur, J. Pleim, J. Godowitch and S. Roselle for comments. The United States Environmental Protection Agency through its Office of Research and Development funded and managed the part of the research that is related to sulphur chemistry modelling. It has been subjected to the Agency’s administrative review and approved for publication. D.Q.T. is grateful to S. Fine, D. Byun and R. Artz for discussion and acknowledges constructive internal Air Resources Laboratory comments.
The authors declare no competing financial interests.
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Bao, H., Yu, S. & Tong, D. Massive volcanic SO2 oxidation and sulphate aerosol deposition in Cenozoic North America. Nature 465, 909–912 (2010). https://doi.org/10.1038/nature09100
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