Sulfur isotopes in rivers: Insights into global weathering budgets, pyrite oxidation, and the modern sulfur cycle

The biogeochemical sulfur cycle is intimately linked to the cycles of carbon, iron, and oxygen, and plays an important role in global climate via weathering reactions and aerosols. However, many aspects of the modern budget of the global sulfur cycle are not fully understood. We present new δ³⁴S measurements on sulfate from more than 160 river samples from different geographical and climatic regions—more than 46% of the world's freshwater flux to the ocean is accounted for in this estimate of the global riverine sulfur isotope budget. These measurements include major rivers and their tributaries, as well as time series, and are combined with previously published data to estimate the modern flux-weighted global riverine δ³⁴S as 4.4 ± 4.5‰ (V-CDT), and 4.8 ± 4.9‰ when the most polluted rivers are excluded. The sulfur isotope data, when combined with major anion and cation concentrations, allow us to tease apart the relative contributions of different processes to the modern riverine sulfur budget, resulting in new estimates of the flux of riverine sulfate due to the oxidative weathering of pyrites (1.3 ± 0.2 Tmol S/y) and the weathering of sedimentary sulfate minerals (1.5 ± 0.2 Tmol S/y). These data indicate that previous estimates of the global oxidative weathering of pyrite have been too low by a factor of two. As pyrite oxidation coupled to carbonate weathering can act as a source of CO₂ to the atmosphere, this global pyrite weathering budget implies that the global CO₂ weathering sink is overestimated. Furthermore, the large range of sulfur isotope ratios in modern rivers indicates that secular changes in the lithologies exposed to weathering through time could play a major role in driving past variations in the δ³⁴S value of seawater.