Abstract
AbstractModern 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.
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