Biogeochemical signatures preserved in ancient sedimentary rocks provide clues to the nature and timing of the oxygenation of the Earth's atmosphere. Geochemical data 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 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. 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. 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.

Recent geochemical data from Oman, Newfoundland, and the western United States suggest that long-term oxidation of Ediacaran oceans resulted in progressive depletion of a large dissolved organic carbon (DOC) reservoir and potentially triggered the radiation of acanthomorphic acritarchs, algae, macroscopic Ediacara organisms, and, subsequently, motile bilaterian animals. However, the hypothesized coupling between ocean oxidation and evolution is contingent on the reliability of continuous geochemical and paleontological data in individual sections and of intercontinental correlations. Here we report high-resolution geochemical data from the fossil- acritarchs ͉ isotopes ͉ redox ͉ Neoproterozoic ͉ early animals T he Ediacaran (635-542 Ma) Earth witnessed profound changes in the aftermath of widespread and potentially global ice ages, including the evolution and radiation of complex megascopic life and major perturbations of the global carbon cycle that accompanied oxygenation of the deep ocean (1-9). These biological and environmental events have been speculatively linked, yet their temporal relationships have not been accurately documented in relatively continuous and fossil-rich sections that span a range of well-documented depositional settings. For example, geochemical data from siliciclasticdominated Ediacaran successions in Newfoundland (5) and the western United States (7) are incomplete, and those from the early Ediacaran interval in Oman (4) are of low stratigraphic resolution. Furthermore, paleontological data from these successions are limited to macroscopic Ediacara fossils and the biomineralizing animal Cloudina (10).To further test the proposed linkages between redox changes and biological evolution (4, 5), we carried out a high-resolution chemostratigraphic and biostratigraphic investigation of the fossiliferous Doushantuo Formation in the Yangtze Gorges area, South China. Our data reveal pulsed oxidation events that coincide with the origination and diversification of acanthomorphic acritarchs and other multicellular life forms in the basin. In combination with available data from other Ediacaran successions, our results indicate that oxidation of terminal Proterozoic oceans may have been episodic (4), with the final and permanent oxidation occurring Ϸ551 Ma. Sedimentological, Paleontological, and Geochemical DataThe Doushantuo Formation in the Yangtze Gorges area, constrained between 635.2 Ϯ 0.6 and 551.1 Ϯ 0.7 Ma (11), is divided into four lithostratigraphic members (Fig. 1). At the Jiulongwan section [supporting information (SI) Fig. 3], member I represents an Ϸ5-m-thick cap dolostone overlying the Nantuo glacial diamictite and contains a suite of enigmatic sedimentary structures and textures (12, 13). Member II is characterized by Ϸ70 m of alternating organic-rich shale and dolostone beds with abundant pea-sized chert nodules. Member III is Ϸ70 m thick and is composed of dolostone and bedded chert in the lower part that passes up-section into alternating limestone-dolostone ''ribbon rocks.'' Me...

Materials and methods Sample preparation All outcrop samples were collected on a field trip to South China in March 2007. Large blocks (>200 g) of the freshest exposures were deliberately targeted for sampling. The large rock blocks were subsequently trimmed by a water-cooled rock saw or by hammer in the laboratory to remove the potentially weathered surfaces and then broken into small pieces. The pieces were further crushed into powders using a SPEX 8515 Shatterbox with an alumina (ceramic) puck. Rock pieces that contained readily visible pyrite nodules or bands were discarded prior to crushing. Total organic carbon (TOC) and total inorganic carbon (TIC) TOC was determined as the difference between total carbon (TC) and total inorganic carbon (TIC) measured using a CS-500 carbon/sulfur analyzer with a hightemperature furnace and acidification module (Eltra, Germany). For TC, ~100 mg of sample powder were weighed into a ceramic boat and combusted in pure (99.95%) O 2 at 1350 °C for ~3 min. The total carbon liberated was then measured by infrared spectral absorption of the evolved CO 2. For TIC, ~100 mg of sample powder were reacted with 20% HCl, heated at 50 °C and stirred. TIC was also quantified by infrared absorption detection of the CO 2 generated. Analytical errors for TOC and TIC are ±0.1 wt% based on analysis of carbonate standard AR1034 (Alpha, USA). Pyrite sulfur isotopes (δ 34 S py) and concentrations Disseminated pyrite concentrations and isotopic compositions were analyzed by the chromium reduction method (S1). Pyrite extraction was performed under N 2 by the addition of 20 ml of concentrated HCl and 40 ml of 1M chromous chloride solution. The reaction mixture was heated for 2 h, with the liberated sulfide collected either as silver sulfide after bubbling through 30 ml of 3 wt% silver nitrate solution with 10% NH 4 OH by volume (for isotopic analysis) or as zinc sulfide after bubbling through 30 ml of 3 wt% zinc acetate with 10% NH 4 OH by volume for pyrite-S concentration. Mean recovery of parallel replicate pure pyrite standards was 105.6%. Filtered, rinsed and dried Ag 2 S precipitates were combined with an excess of V 2 O 5 and analyzed for S-isotope composition following online combustion using a Thermo Instruments Delta V Plus isotope ratio mass spectrometer coupled with a Costech elemental analyzer at the University of California, Riverside. Sulfur isotope compositions are expressed as δ 34 S = (R sample /R standard-1) × 1000, where R is the ratio of 34 S/ 32 S, reported as permil (‰)