The multiple sulphur isotope fingerprint of a sub-seafloor oxidative sulphur cycle driven by iron

Oxidative sulphur cycling is pervasive in marine sediments, replenishing the oxidised sulphur reservoir via re-oxidation of sulphide. An active, yet cryptic, sulphur cycle has been proposed to operate at depth beneath the sulphate-methane transition (SMT), fuelled by simultaneous sulphide oxidation and sulphate reduction under low-sulphate conditions. The existence of a cryptic sulphur cycle, however, is centred on porewater and genetic data that have little, to no, preservation potential, and thus are rarely accessible from the geological record. The absence of a suitable archive has hindered our ability to reconstruct the operation and importance of the cryptic sulphur cycle through space and time. To overcome this obstacle, and to develop a better understanding of the oxidative sulphur cycle in the deep biosphere, we have determined the abundance and triple sulphur isotope composition (Δ³³S and δ³⁴S) of both elemental sulphur and pyrite extracted from sediments recovered from the methane prone Taixinan Basin, South China Sea. Here, multiple sulphur isotope systematics of pyrite clearly reveal a tiering, with organoclastic sulphate reduction succumbing to sulphate-driven anaerobic oxidation of methane at depth. Importantly, a negative Δ³³S-δ³⁴S correlation was found at the periphery of the SMT that requires repeated and sustained iron-driven sulphide oxidation with concomitant disproportionation of the elemental sulphur product. We conclude that minor sulphur isotopes may provide a unique lens to resolve the cryptic sulphur cycle, allowing the importance of the deep biosphere to be evaluated over geological timescales. In turn, a better understanding of the cryptic sulphur cycle remains central to testing hypotheses linking major elemental cycles and diverse microbial activities that persist under the energy-limited conditions that typify the deep biosphere.