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

Abstract

A new method was used to study the formation of pyrite plus elemental sulfur during 3 5 SO 4 2‒ reduction experiments in salt marsh sediments: the reduction with chromium(II) 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(II) reduction method, combined with our previous results, conclusively shows that pyrite is the major product of 35 SO 4 2‒ reduction measurements in these sediments. The agreement between the two methods indicates that the formation of 35 S‐labeled, 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(II) reduction method, if used with care, are suitable for measuring incorporation of 3 5 S into pyrite and elemental sulfur during 35 SO 4 2‒ reduction measurements in salt marsh sediments. Failure to explicitly measure the formation of 3 5 S‐labeled pyrite can result in the rate of sulfate reduction being underestimated by 2‐fold to 10‐fold or more.