Sulfides in sandy sediments: New insights on the reactions responsible for sedimentary pyrite formation

The formation of sedimentary iron sulfides was studied in sandy sediments of the Laguna Madre, TX, in order to better understand how this process operates in sediments where reactive iron is likely to be limiting for sulfide mineral formation. These sediments usually had reactive iron and total reduced sulfide concentrations one to two orders of magnitude less than in typical shallow water terrigenous muds, but organic-C concentrations typical of fine-grained sediments due to the extensive presence of seagrass beds. This resulted in moderate (0-150 mu m) dissolved H2S concentrations with maximum concentrations in the upper (3-5 cm) root zone. Based on citrate dithionite extractable-Fe the degree of sulfidization was usually 100% or greater. Acid volatile sulfides (AVS) typically comprised roughly 60% of total reduced sulfur and the proportion of AVS generally increased instead of decreasing with depth. The unusual proportion of TRS as AVS and persistence of AVS are attributed to exceptionally slow pyrite formation kinetics. The probable reasons for these slow reaction kinetics are the high (>7.8) pH of the sediments, which favors the slow polysulfide pathway for pyrite formation, high (typically about 2-4 mm) dissolved organic carbon concentrations that inhibit growth of pyrite and the low concentration of reactants which greatly increases the average transport distances necessary for diffusion controlled reactions.