Letter

Nature 452, 456-459 (27 March 2008) | doi:10.1038/nature06811; Received 15 June 2007; Accepted 31 January 2008

Tracing the stepwise oxygenation of the Proterozoic ocean

C. Scott1, T. W. Lyons1, A. Bekker2,8, Y. Shen3, S. W. Poulton4, X. Chu5 & A. D. Anbar6,7

  1. Department of Earth Sciences, University of California, Riverside, California 92521, USA
  2. Geophysical Laboratory, Carnegie Institution of Washington, Washington DC 20015, USA
  3. Département des Sciences de la Terre et de l'Atmosphère, Université du Québec à Montréal, H3C 3P8, Canada
  4. School of Civil Engineering and Geosciences, Newcastle University, Newcastle upon Tyne, NE1 7RU, UK
  5. Key Laboratory of Mineral Resources, Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing 100029, China
  6. School of Earth & Space Exploration,
  7. Department of Chemistry and Biochemistry, Arizona State University, Tempe, Arizona 85287, USA
  8. Present address: Department of Geological Sciences, University of Manitoba, Winnipeg, Manitoba, R3T 2N2, Canada.

Correspondence to: C. Scott1 Correspondence and requests for materials should be addressed to C.S. (Email: cscot002@ucr.edu).

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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