Formation of pyrite in freshwater sediments: Implications for C/S ratios

The vertical distribution of pyrite, acid volatile sulphide (AVS), carbon, and total S (St) were determined directly in the sediments of three lakes of different trophic status. The results showed that freshwater pyrite formation reflects the redox status of the sediment or overlying waters. It appears to form chiefly in reducing sediments which are subject to oxidizing influences, by either a low turnover of organic carbon or periodic incursions of oxygen. Although there are high concentrations of AVS in the near-surface sediments of productive lakes, very little is diagenetically converted to pyrite.The feasibility of using sulphur ratios to diagnose whether rocks were formed in marine or freshwater environments is assessed. New values for FeS₂/FeS of 0.5-5 show that this ratio does not provide a reliable test. Values of C/Sp, where Sp represents pyrite sulphur, lie within the range of 160–700 and are much higher than previously measured ratios of C/S_t of 1–50. These new determinations show that, if pyrite sulphur is unequivocally measured, C/S ratios may be a more sensitive indicator of salinity than had been previously thought.