The amount of sulfate in the early ocean was tied directly to oxygen levels in the atmosphere and the deep ocean. These concentrations and other environmentally diagnostic biogeochemical pathways of the sulfur cycle can be expressed through isotope fractionation between sulfate and pyrite. The balance between rising oxygen and sulfate concentrations and varying hydrothermal iron inputs led to a pattern of iron, oxygen, and sulfide domination that varied in time and space in the early deep ocean and was more complex than previously recognized. Through all this change, no element played a bigger role than sulfur as a recorder of early oxygenation of the biosphere and the coevolution of life.

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