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Deposition of sulfate aerosols with positive Δ³³S in the Neoarchean

Paris, Guillaume and Fischer, Woodward W. and Johnson, Jena E. and Webb, Samuel M. and Present, Theodore M. and Sessions, Alex L. and Adkins, Jess F. (2020) Deposition of sulfate aerosols with positive Δ³³S in the Neoarchean. Geochimica et Cosmochimica Acta, 285 . pp. 1-20. ISSN 0016-7037. doi:10.1016/j.gca.2020.06.028.

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Anomalous sulfur isotope compositions present in Archean rocks have been intensely scrutinized over the last 20 years because they record key aspects of Earth's atmospheric composition prior to the appearance of free molecular oxygen ca. 2.3 billion years ago. These isotopic compositions can be described as mass anomalous fractionations (MAF) and are produced in the atmosphere as UV light interacts with SO₂ molecules. Most interpretations suggest that atmospheric processes generate a reduced S-phase with a positive (³³S-enriched) MAF signature, as measured in pyrites, and an oxidized S-phase with a negative anomaly, as measured in bedded barite deposits. However, recent data for carbonate-associated sulfate (CAS) — a direct proxy for the isotopic composition of sulfur from seawater sulfate — in Neoarchean rocks showed no such negative values, but rather the opposite. To understand if the positive MAF anomalies we measured in Neoarchean CAS reflect secondary processes (diagenetic, metamorphic, handling) instead of original signals of Archean seawater sulfate, we collected additional sample suites with various degrees of preservation and metamorphic alteration across the Campbellrand-Malmani platform in South Africa. Results illustrate that within this comprehensive suite, less-altered samples all contain positive MAF values while secondary processes tend to either remove CAS from the sample and/or decrease the ³³S-enrichment. This positive MAF signal in sulfate is therefore reasonably interpreted as a primary depositional origin, and implies that the assumption that sulfate always carries a negative MAF anomaly throughout the Archean rock record needs to be reconsidered. Our CAS observations suggest that future experiments and calculations should also consider atmospheric and/or sulfur cycling processes that can produce oxidized sulfur with a positive MAF signature.

Item Type:Article
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URLURL TypeDescription
Paris, Guillaume0000-0001-8368-1224
Fischer, Woodward W.0000-0002-8836-3054
Present, Theodore M.0000-0002-4747-2174
Sessions, Alex L.0000-0001-6120-2763
Adkins, Jess F.0000-0002-3174-5190
Additional Information:© 2020 Published by Elsevier Ltd. Received 21 February 2020, Revised 23 June 2020, Accepted 24 June 2020, Available online 3 July 2020. We thank Dr. LaFlamme, Dr. Marin-Carbonne and one anonymous reviewer for their helpful reviews and Prof. Ono for handling the editorial process. Altogether, they helped us improve this article. We thank Prof. Beukes for his help in accessing the cores and for feedback on the geological context. Use of the Stanford Synchrotron Radiation Lightsource, SLAC National Accelerator Laboratory, is supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences under Contract No. DE-AC02-76SF00515. Financial support was received from the NSF (grant EAR-1349858 attributed to WWF and JFA). WWF acknowledges support from the Simons Foundation Collaboration on the Origins of Life. The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Funding AgencyGrant Number
Department of Energy (DOE)DE-AC02-76SF00515
Simons FoundationUNSPECIFIED
Record Number:CaltechAUTHORS:20200706-145806917
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Official Citation:Guillaume Paris, Woodward W. Fischer, Jena E. Johnson, Samuel M. Webb, Theodore M. Present, Alex L. Sessions, Jess F. Adkins, Deposition of sulfate aerosols with positive Δ33S in the Neoarchean, Geochimica et Cosmochimica Acta, Volume 285, 2020, Pages 1-20, ISSN 0016-7037, (
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:104232
Deposited By: Tony Diaz
Deposited On:06 Jul 2020 22:07
Last Modified:16 Nov 2021 18:29

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