Elemental sulfur dynamics in acid sulfate soil landscapes

The precipitation and dissolution of iron and sulfur minerals has a primary influence on water quality of acid sulfate affected landscapes (Fanning et al. 2002; Nordstrom 2000). The two key processes involved are the microbially mediated reduction and oxidation of sulfate and iron (Peine et al. 2000), which drive the geochemical store and flux of acidity. Iron sulfide minerals are the dominant store of acidity in sediments subjected to relatively stable reducing conditions (Fanning et al. 2002). However, we have found abundant elemental sulfur in the near-surface sediments of acid sulfate affected drain sediments that are subject to seasonally fluctuating water tables and redox conditions. In these drain sediments, elemental sulfur occurs as sub-micron crystals in association with contemporary monosulfidic oozes. Elemental sulfur is generally most abundant in the upper-most sediment layer (i.e. 10-20mm beneath the surface) at the sediment-water boundary. Its concentration diminishes dramatically with sediment depth, indicating that it is not stable under reducing conditions and is forming at the slightly oxic sediment-water interface. Elemental sulfur is known to have a potentially important role in the digenic transformation of precursor iron monosulfides to pyrite (Berner 1984). It and related polysulfide intermediates are known to enable the rapid formation of pyrite, including complex framboidal morphologies, and it is a key component for pyrite formation in sub-oxic environments such as sub-tidal sediments (Berner 1984). Lesser known for elemental sulfur is its role in the contemporary sulfur-cycle of acid sulfate soil landscapes involving iron monosulfide formation and acidifying oxidation processes. We present new data on the occurrence and dynamics of elemental sulfur from coastal acid sulfate soil landscapes and laboratory oxidative resuspensions and reductive incubations. Our results show the formation of elemental sulfur is a primary and short-lived product of iron monosulfide oxidation. The kinetics of its formation and further oxidation to sulfate (accompanied by the liberation of acidity) or reduction to sulfide are presented. These observations illustrate that elemental sulfur has a fundamental role in contemporary sulfur cycling in acid sulfate soil landscapes. The implications of elemental sulfur to water quality in coastal landscapes are discussed.