concept

Pyrite burial

The route by which sulfur is locked away from a planet's surface for good: bacteria in oxygen-free sediment reduce sulfate, the product meets iron and precipitates as pyrite — the brassy mineral old prospectors called fool's gold — and is buried. The surprise is that this is simultaneously one of the two long-term sources of atmospheric oxygen. Burying reduced material means not handing it back to oxygen to burn, so every pyrite crystal left in rock is a small quantity of oxygen permitted to exist at the surface. A planet's oxygen budget is, in the end, an accounting problem about what got buried.