The extent of sulfidic (anoxic and H2S rich) seafloor, an important ocean redox indicator, can be constrained by the redox sensitive elements (RSE, e.g., Mo, U, V) contents in black shales. Although the sulfidic seafloor was at least sporadic and episodic and the ocean remained incompletely oxidized until the Paleozoic oxygenation event (POE) at 400-300 Ma, it is unclear whether the ocean was gradually oxidized or remained modest oxygenation followed by a stepwise oxidation in POE. In this study, we use sulfur (S) isotopes of evaporitic sulfate (δ34SEva) and carbonate associated sulfate (CAS, δ34SCAS) to the refine the ocean redox evolution in the past 600 Ma. The statistic difference between δ34SEva and δ34SCAS (ΔEva-CAS=δ34SEva-δ34SCAS) reflects a gradual shrinkage of sulfidic seafloor and weakening of VOSC cycle before POE, while the exclusive negative ΔEva-CAS values after 300 Ma indicates the thorough ocean oxygenation that muted the VOSC cycle.
Huang et al. (Thu,) studied this question.