Biogeochemical signatures preserved in ancient sedimentary rocks provide clues to the nature and timing of the oxygenation of the Earth’s atmosphere. Geochemical data1,2,3,4,5,6 suggest that oxygenation proceeded in two broad steps near the beginning and end of the Proterozoic eon (2,500 to 542 million years ago). The oxidation state of the Proterozoic ocean between these two steps and the timing of deep-ocean oxygenation have important implications for the evolutionary course of life on Earth but remain poorly known. Here we present a new perspective on ocean oxygenation based on the authigenic accumulation of the redox-sensitive transition element molybdenum in sulphidic black shales. Accumulation of authigenic molybdenum from sea water is already seen in shales by 2,650 Myr ago; however, the small magnitudes of these enrichments reflect weak or transient7 sources of dissolved molybdenum before about 2,200 Myr ago, consistent with minimal oxidative weathering of the continents. Enrichments indicative of persistent and vigorous oxidative weathering appear in shales deposited at roughly 2,150 Myr ago, more than 200 million years after the initial rise in atmospheric oxygen1,2. Subsequent expansion of sulphidic conditions after about 1,800 Myr ago (refs 8, 9) maintained a mid-Proterozoic molybdenum reservoir below 20 per cent of the modern inventory, which in turn may have acted as a nutrient feedback limiting the spatiotemporal distribution of euxinic (sulphidic) bottom waters and perhaps the evolutionary and ecological expansion of eukaryotic organisms10. By 551 Myr ago, molybdenum contents reflect a greatly expanded oceanic reservoir due to oxygenation of the deep ocean and corresponding decrease in sulphidic conditions in the sediments and water column.
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This study was funded by the US NSF-EAR and NASA Astrobiology Institute. A.B. and Y.S. were funded by NSERC. S.W.P. acknowledges financial support from Danmarks Grundforskningsfond and a NERC Research Fellowship. X.C. was funded by NNFSC. The manuscript was improved by comments from J. Hayes and D. Vance. We thank B. Krapež, N. Beukes, F. Gauthier-Lafaye, P. Medvedev, D. Winston and the geological surveys of South Africa and Botswana for field support and access to sample collections. G. Arnold, G. Gordon and S. Severmann provided analytical support. B. Gill compiled Phanerozoic data.
He file contains Supplementary Discussion, Supplementary Tables 1-4 and additional references. The Supplementary Information includes a discussion of the sources and sinks involved in the modern Mo cycle. Tables 1-3 include an overall summary of the budget, a summary of published values on Mo burial rates and a comparison between seawater Mo concentrations and enrichment factors. Also included are descriptions of materials analyzed and a data table. (PDF 290 kb)
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Scott, C., Lyons, T., Bekker, A. et al. Tracing the stepwise oxygenation of the Proterozoic ocean. Nature 452, 456–459 (2008). https://doi.org/10.1038/nature06811
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