Pyrite formation and the measurement of sulfate reduction in salt marsh sediments1

A new method was used to study the formation of pyrite plus elemental sulfur during 35S0,2reduction experiments in salt marsh sediments: the reduction with chromium(I1) of pyrite and elemental sulfur to hydrogen sulfide. It is both more specific and more sensitive than our previous method, the oxidation of pyrite and elemental sulfur to sulfate by aqua regia, which measures the formation of refractory organic sulfur compounds as well as pyrite and elemental sulfur. A direct comparison of the two methods in salt marsh sediments in Georgia and in Massachusetts showed that they compare very well. The chromium(I1) reduction method, combined with our previous results, conclusively shows that pyrite is the major product of 35S0,2- reduction measurements in these sediments. The agreement between the two methods indicates that the formation of 35Slabeled, refractory organic matter is a minor process if it occurs at all. Our results suggest that both the aqua regia oxidation method and the chromium(I1) reduction method, if used with care, are suitable for measuring incorporation of 3sS into pyrite and elemental sulfur during 35S0,2- reduction measurements in salt marsh sediments. Failure to explicitly measure the formation of 35S-labcled pyrite can result in the rate of sulfate reduction being underestimated by 2-fold to lo-fold or more.