Sulfur cycles through Earth's reservoirs — oceans, sediments, volcanoes, mantle — in a loop that geology textbooks describe as geochemical. Rianço-Silva et al. built a dynamical box model tracking sulfur fluxes across surface and deep Earth reservoirs without biological activity. An Earth-like planet, same volcanism, same weathering, same oceans, but no life. The sulfur distribution that results is unrecognizable.
Marine sediment sulfate: two orders of magnitude higher than present-day Earth. Marine sediment sulfide: four orders of magnitude lower. Without microbial sulfur metabolism, sulfate accumulates because nothing reduces it. Sulfide vanishes because nothing produces it at biologically relevant rates. The familiar ratio of sulfate to sulfide in Earth's sediments — the one we treat as a geochemical baseline — is a biological artifact.
The mechanism is specific: microbial sulfate reduction converts sulfate to sulfide, and microbial sulfur oxidation runs the reaction the other way. These metabolic pathways are ancient — they may predate oxygenic photosynthesis. Remove them, and sulfur sits in whatever oxidation state inorganic chemistry produces. Volcanic outgassing emits SO₂; weathering generates sulfate; without biology, the sulfate just accumulates. The sulfide that fills the geological record — the pyrite that records ocean chemistry, the hydrogen sulfide that drives chemosynthetic ecosystems — is all biogenic.
This inverts the usual framing. We think of life as running on Earth's geochemistry, adapting to the chemical environment it finds. But for sulfur, life IS the geochemistry. The environment it finds is the one it built. Remove life and you don't get pristine geology — you get a planet-sized chemical stockpile that nothing knows how to process.
The sulfur cycle isn't a geological process that life participates in. It's a biological process that geology provides the raw materials for.