Journal Article

Tracing the stepwise oxygenation of the Proterozoic ocean

Shen, Y., Chu, X., Anbar, A. D., Poulton, S. W., Bekker, A., Lyons, T. W., Scott, C.

2008

Overview

Oxygen steps in the ocean The oxygenation of the Earth's atmosphere is thought to have occurred in two steps near the beginning and the end of the Proterozoic eon, around 2,500 to 550 million years ago. The oxidation state of the ocean between these two steps and the timing of deep ocean oxygenation, however, remain poorly known. Scott et al . now use molybdenum and total organic carbon data from black shales to track the redox state of the ocean at this time. Molybdenum is an essential participant in nutrient cycling, and its availability is highly sensitive to Earth's redox state. The results provide a new narrative for the historical texture of Earth's oxygenation, and will be of relevance for the study of the events that presaged the appearance of animals on Earth. Molybdenum and total organic carbon data from black shales is used to gain insights into the redox state of the ocean. The data suggests mild oxidative weathering of the continents before ∼2,200 Myr ago, but weathering becomes more persistent and vigorous at ∼2,150 Myr ago, 200 million years after the initial rise in atmospheric oxygen. Limited availability of molybdenum after 1,800 Myr ago may have acted as a negative nutrient feedback limiting the spatial and temporal extent of sulphidic conditions. Biogeochemical signatures preserved in ancient sedimentary rocks provide clues to the nature and timing of the oxygenation of the Earth’s atmosphere. Geochemical data 1 , 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 transient 7 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 oxygen 1 , 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 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 sulphidic conditions in the sediments and water column.

• Publisher: Nature Publishing Group UK,Nature Publishing,Nature Publishing Group
• Subject: Accumulation / Atmosphere / Atmosphere - chemistry / Atmospheric chemistry / Biochemistry / Biogeochemistry / Continents / Earth / Earth sciences / Earth, ocean, space / Exact sciences and technology / Geochemistry / Geologic Sediments - chemistry / History, Ancient / Humanities and Social Sciences / letter / Molybdenum / Molybdenum - analysis / multidisciplinary / Nutrients / Oceans and Seas / Oxidation / Oxygen - analysis / Oxygen - chemistry / Oxygenation / Paleoecology / Reservoirs / Science / Science (multidisciplinary) / Seawater / Seawater - chemistry / Sedimentary rocks / Sedimentation & deposition / Shales / Stratigraphy / Sulfides - chemistry / Time Factors / Water column / Weathering