The history of Earth’s sulfur cycle

Sulfur is a critical component of Earth’s surface evolution owing to its dynamic roles as a redox buffer and nutrient as well as forming the basis for some of Earth’s earliest metabolisms. In this Review, we provide a broad-scale reconstruction of the biogeochemical sulfur cycle and its evolution through time. Insights gleaned through microscale observations, diagenetic modelling and organic sulfur together with traditional bulk sulfur isotope measurements highlight that links between variations in the isotopic composition of reduced and oxidized sulfur species are more complex than previously assumed. The most up-to-date evidence suggests that marine sulfate concentrations have been variable, but generally characterized by background levels (μM to low mM) substantially lower than today’s oceans (28 mM), for most of Earth’s history. The shift towards relatively stable modern-like marine sulfate cycling conditions started in the Phanerozoic eon, with the exceptions of oceanic anoxic events (OAEs). Feedbacks between ocean deoxygenation, climate and sulfur, iron, carbon and nutrient cycles are in need of further study to quantify the importance of key processes that both drove OAEs as well as maintained the low sulfate conditions that characterized the majority of Earth’s past. Modern low-sulfate systems, such as certain lakes and marine basins, could be key to further understanding such feedbacks and therefore Earth’s early sulfur cycle. Future research further constraining Earth’s past sulfur cycle may offer critical insights into the potential impacts of modern anthropogenically driven ocean deoxygenation.