Quantifying key drivers of marine pyrite burial flux and isotopic composition

Pyrite formation and burial in marine sediments decreases atmospheric CO2 and increases O2 levels. However, the sedimentological conditions that control pyrite burial remain poorly constrained, precluding quantitative reconstructions of sulphur cycle regulation on atmospheric compositions. Building on updated understanding of pyrite isotope dynamics, here we provide mechanistic insight by developing a non-dimensional diagenetic model that extracts the natural variables governing pyrite formation rates and sulphur isotopic compositions (δ34S values). Both properties are controlled by the local ratios of organic carbon content to sulphate concentration and organic carbon reactivity to sedimentation rate; formation rate is additionally sensitive to reactive iron delivery. Using only globally interpolated boundary values as inputs, our model accurately predicts signals recorded in a validation dataset of 216 sediment cores from diverse environmental settings across the modern ocean. Extrapolating this, we estimate a global pyrite burial flux of 7.0 × 10^12 mol S yr^-1 (sensitivity test range: 2.5 × 10^12 to 19.0 × 10^12 mol S yr^-1) with a weighted-average δ34S value of -4 ‰ VCDT (range: -8 to +3 ‰ VCDT). This flux is substantially larger than that of terrestrial pyrite oxidation (1.3 × 10^12 mol S yr^-1), indicating that the sulphur cycle is currently not in steady state, but rather described by net pyrite burial and thus atmospheric O2 accumulation. Finally, we utilise this model framework to invert the geologic pyrite δ34S record and assess changes in sedimentological properties and pyrite burial flux throughout the Phanerozoic Eon.