Oxygenation of the Earth's atmosphere is thought to have proceeded in two broad steps near the beginning of the Proterozoic eon (2,500 million years ago) and its end (542 million years ago). The oxidation state of the Proterozoic ocean between its beginning and its end and the timing of deep-ocean oxygenation have important implications for the evolutionary course of life on Earth. A new perspective on ocean oxygenation based on the authigenic accumulation of molybdenum in sulfidic black shales is presented.
By 2,650 Myr ago accumulation of authigenic molybdenum from sea water is already seen in shales. The small magnitudes of these enrichments reflect weak or transient sources of dissolved molybdenum before about 2,200 Myr ago, consistent with minimal oxidative weathering of the continents.
At roughly 2,150 Myr ago, more than 200 million years after the initial rise in atmospheric oxygen, in deposited shales enrichments appear which are indicative of persistent and vigorous oxidative weathering.
After about 1,800 Myr ago expansion of sulfidic conditions maintained a mid- Proterozoic molybdenum reservoir at below 20 per cent of the modern concentration, which in turn may have acted as a nutrient feedback limiting the spatiotemporal distribution of euxinic ( sulfidic) bottom waters and perhaps the evolutionary and ecological expansion of eukaryotic organisms(10).
By 551 Myr ago, molybdenum contents reflect a greatly expanded oceanic reservoir due to oxygenation of the deep ocean and corresponding decrease in sulfidic conditions in the sediments and water column.
Scott, C., Lyons, T. W., Bekker, A., Shen, Y., Poulton, S. W., Chu, X., and Anbar, A. D., Tracing the, Nature, 2008, 452, 456-4U5.
Pearce, C. R., Cohen, A. S., Coe, A. L., and Burton, K. W., Molybdenum isotope evidence for global ocean anoxia coupled with perturbations to the carbon cycle during the early Jurassic, Geology, 2008, 36, 231-234.