Biogeochemical Cycling of Sulfur and Iron in Sediments of a South-East Asian Mangrove, Phuket Island, Thailand

Benthic sulfate reduction and sediment pools of sulfur and iron were examined during January 1992 at 3 stations in the Ao Nam Bor mangrove, Phuket, Thailand. Patterns of sulfate reduction rates (0-53 cm) reflected differences in physical and biological conditions at the 3 stations, and highest rates were found at the vegetated site within the mangrove (Rhizophora apiculata) forest. Due to extended oxidation of mangrove sediments, a large portion of the added 35S-label was recovered in the chromium reducible pools (FeS2 and S0) (41-91% of the reduced sulfur). Pyrite was the most important inorganic sulfur component, attaining pool sizes 50-100 times higher than acid volatile pools (FeS). HCl-extractable (0.5 M HCl) iron pools, including Fe(II)HCl and Fe(III)Hcl, were generally low and Fe(III)HCl was only present in the upper surface layers (0-5 cm). Maximum concentrations of dissolved Fe2+ (35-285 μM) occurred just about the depth where dissolved Σ H2S accumulated. Furthermore Fe2+ and Σ H2S coexisted only where concentrations of both were low. There was an accumulation of organic sulfur in the deep sediment at 2 stations in the inner part of the mangrove. The reoxidation of reduced sulfides was rapid, and storage of sulfur was minor in the upper sediment layers, where factors like bioturbation, the presence of roots, or tidal mixing enhance oxidation processes.