Regional variability but global flux balance in the deep sulfur cycle

Sulfur is a key volatile that influences Earth's redox state, climate, and deep geochemical cycles. Subduction zones are the primary pathways carrying sulfur from the surface into the mantle, yet the global sulfur budget and recycling efficiency remain uncertain. Here, we compile a trench-by-trench inventory of subducting sulfur using sediment compositions from ocean drilling programs collected near major trenches, integrated with spatially resolved estimates of oceanic crustal and serpentinite thicknesses derived from seismic data. Our results reveal pronounced spatial heterogeneity in sulfur fluxes, driven by large variations in sedimentary sulfur contents and fundamental tectonic differences between erosive and accretionary margins. Slab-to-arc sulfur recycling efficiency averages 37% globally but varies markedly among individual subduction systems. Despite this heterogeneity, the global sulfur cycle appears balanced on the modern Earth: Sulfur input into the mantle via slab subduction (57 ± 3 Mt y^-1) is matched within uncertainty by mantle output (~60 ± 14 Mt y^-1) through mid-ocean ridges, volcanic arcs, and intraplate magmatism. This balance suggests that Earth's deep sulfur cycle operates in a steady state today. Sulfur isotopes reveal a systematic decoupling, with subducted sulfur carrying negative δ³⁴S values, whereas arc sulfur output is consistently positive. The strong spatial variability in sulfur inputs and recycling efficiency underscores the individuality of Earth's subduction zones, but the balanced input-output fluxes highlight its capacity for self-regulation. These findings have important implications for atmospheric chemistry, surface environments, and the long-term evolution of the deep Earth's sulfur cycle.