Sedimentary conditions drive modern pyrite burial flux to exceed oxidation

Pyrite (iron sulfide) formation and burial in sediments decreases atmospheric CO₂ and increases O₂ levels. However, the environmental and sedimentological conditions that regulate pyrite burial remain poorly constrained. Here we investigate such controlling mechanisms using a non-dimensional diagenetic model that extracts the natural variables governing pyrite formation rate and sulfur isotopic composition (<i>δ</i> ³⁴S). Both properties are controlled by the local ratios of organic carbon content to sulfate concentration and organic carbon reactivity to sedimentation rate; formation rate is additionally sensitive to reactive iron delivery. Using only globally interpolated boundary conditions, our model accurately predicts signals in 216 sediment cores distributed across the modern ocean. Extrapolating this, we estimate a global pyrite burial flux of 7.0 × 10¹² mol S yr^-1 (sensitivity test range: 2.5 × 10¹² to 19.0 × 10¹² mol S yr^-1) with a weighted-average <i>δ</i> ³⁴S value of -4‰ (range: -8 to +3‰). This flux is substantially larger than terrestrial pyrite oxidation, indicating that the sulfur cycle is currently not in steady state but is instead described by net pyrite burial and thus atmospheric O₂ accumulation. Finally, we interpret the geologic pyrite <i>δ</i> ³⁴S record within this model framework and identify flooded shelf area as the main control on pyrite burial throughout the Phanerozoic Eon.