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Kathleen McFadden, Virginia Tech (United States)
Shuhai Xiao, Virginia Tech (United States)
Ganqing Jiang, University of Nevada, Las Vegas (United States)
Alan J. Kaufman, University of Maryland (United States)
Jing Huang, Institute of Geology and Geophysics (China)
Xuelei Chu, Institute of Geology and Geophysics (China)
Chuanming Zhou, Nanjing Institute of Geology and Palaeontology (China)
Xunlai Yuan, Nanjing Institute of Geology and Palaeontology (China)
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In the aftermath of the Cryogenian glaciations, the Earth system witnessed profound environmental and biological events, including oxygenation of the deep oceans and evolution of modern biological systems, which may have been causally coupled although the timing and duration for the rise of pO2 remain poorly constrained.
The Ediacaran oceans likely represented a non-steady state carbon system involving a large DOC reservoir below a chemocline and a relatively small DIC reservoir in the surface waters. High-resolution C and S isotopic data from the Doushantuo Formation (635-551 Ma) in the Yangtze Gorges area of South China provide evidence for an anoxic deep ocean. Two prominent negative excursions in the middle and upper Doushantuo (EN2 and EN3) likely represent pulsed oxidation of the DOC reservoir driven by enhanced bacterial sulfate reduction during regression. Increased sulfate supply from terrestrial oxidative weathering of sulfides suppressed the chemocline, oxidized the DOC, and drove δ13Ccarb values negative. The large DOC remained buffered during EN2, but underwent a dramatic reduction during EN3.
While the DOC reservoir was affected by episodic oxidation events, anoxia in the deepest ocean may have persisted well into the Cambrian Period. Crossplots of δ13Ccarb vs. Δδ13C for geochemical data from Oman and South China suggest that the oceans maintained long-term decoupling of the DIC and DOC reservoirs during the Ediacaran Period, with broad δ13Ccarb spreads and invariant δ13Corg values. EN2 and EN3 negative δ13Ccarb excursions coincide with trends indicating the oxidation of the DOC reservoir by sulfate reduction bacteria, but oceanic stratification returned soon after the perturbations. However, by the early Cambrian, the isotopic spread for both δ13Ccarb and Δδ13C narrow to typical Phanerozoic ranges, suggesting that the DIC became increasingly buffered through time at the expense of a diminishing DOC reservoir.
Both EN2 and EN3 oxidation events coincide with increased diversity of fossil assemblages in the Doushantuo basin. Acanthomorphic acritarchs and possible animal embryos diversified after the first oxidation event EN2. Radiation of macroscopic algae (and possible macroscopic metazoans) occurred at the end of EN3, which is followed by the appearance of motile bilaterians and biomineralizing animals. Overall increase in taxonomic diversity as seen in the Doushantuo Formation emphasizes an important coupling between oxidation events and biological evolution. Comparison with other Ediacaran basins suggests the geographic distribution of oxygen-requiring organisms may have tracked local redox conditions until after EN3 (ca. 551 Ma) when Ediacaran oceans appear more pervasively oxidized such that late Ediacaran taxa reached global distribution. This interpretation has important implications for inter-basinal biostratigraphic correlation of Ediacaran successions.
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