Abstract Bacterial disproportionation of elemental sulfur (S 0 ) is a well-studied metabolism and is not previously reported to occur at pH values less than 4.5. In this study, a sediment core from an abandoned-coal-mine-waste deposit in Southwest Indiana revealed sulfur isotope fractionations between S 0 and pyrite (Δ 34 S es-py ) of up to −35‰, inferred to indicate intense recycling of S 0 via bacterial disproportionation and sulfide oxidation. Additionally, the chemistry of seasonally collected pore-water profiles were found to vary, with pore-water pH ranging from 2.2 to 3.8 and observed seasonal redox shifts expressed as abrupt transitions from Fe(III) to Fe(II) dominated conditions, often controlled by fluctuating water table depths. S 0 is a common product during the oxidation of pyrite, a process known to generate acidic waters during weathering and production of acid mine drainage. The H 2 S product of S 0 disproportionation, fractionated by up to −8.6‰, is rapidly oxidized to S 0 near redox gradients via reaction with Fe(III) allowing for the accumulation of isotopically light S 0 that can then become subject to further sulfur disproportionation. A mass-balance model for S 0 incorporating pyrite oxidation, S 0 disproportionation, and S 0 oxidation readily explains the range of observed Δ 34 S es-py and emphasizes the necessity of seasonally varying pyrite weathering and metabolic rates, as indicated by the pore water chemistry. The findings of this research suggest that S 0 disproportionation is potentially a common microbial process at a pH