A Double-Edged Sword: The Role of Sulfate in Anoxic Marine Phosphorus Cycling Through Earth History

Modern anoxic marine sediments release phosphorus (P) to seawater, driving feedbacks at multiple timescales. On sub-Myr timescales, anoxic P regeneration amplifies ocean deoxygenation; on multi-Myr timescales, it stabilizes atmospheric O₂. Some authors have extended this thinking to the Precambrian: by analogy, widespread ocean anoxia would imply extensive P regeneration from sediments. However, this neglects the role of sulfate in P regeneration. While abundant in seawater today, sulfate was scarce in the Precambrian. Here a simple model is used to isolate the role of sulfate in anoxic P cycling. The model reproduces known feedbacks driven by anoxic P regeneration in a high-sulfate world, but shows muted P regeneration in low-sulfate, anoxic sediments. The transition between "limited" and "enhanced" P regeneration is thus controlled by sulfate. As a result, sulfate is a double-edged sword wielded by the anoxic P cycle: it can amplify oxygenation or deoxygenation depending on environmental context.