A new pathway for pyrite formation in low-sulfate sediments driven by mineralization of reduced organic sulfur

Although pyrite is the main sedimentary form of sulfur, an ample mechanistic comprehension of its formation in low-sulfate environments is lacking. Applying high depth-resolution multigeochemical and stable sulfur isotope composition (δ³⁴S) analysis of a sediment core recovered from a large shallow freshwater lake (Baiyangdian) in north China, we show that the pyrite forms dominantly in the top 4 cm layer and the participating sulfide stems primarily from mineralization of reduced organic sulfur in biomass. This mechanism was further verified by the formation of pyrite in anoxic incubation of biomass (Ceratophyllum demersum L. or Spirulina) with hematite in the absence of external sulfate. This finding reveals an alternative pathway other than microbial sulfate reduction (MSR) for producing sulfide to form pyrite in low-sulfate sediments.