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 e xamined during January 1992 at 3 stations in the Ao Nam Bor mangrove, Phuket, Thailand. Patterns of sulfate reduction rates (0-53 cm) reflec ted differences in physical and biological conditions at the 3 station s, and highest rates were found at the vegetated site within the mangr ove (Rhizophora apiculata) forest. Due to extended oxidation of mangro ve sediments, a large portion of the added S-35-label was recovered in the chromium reducible pools (FeS2 and S0) (41-91% of the reduced sul fur). Pyrite was the most important inorganic sulfur component, attain ing 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)HC l, were generally low and Fe(III)HCl was only present in the upper sur face layers (0-5 cm). Maximum concentrations of dissolved Fe2+ 35-285 muM) occurred just about the depth where dissolved SIGMAH2S accumulate d. Furthermore Fe2+ and SIGMAH2S coexisted only where concentrations o f both were low. There was an accumulation of organic sulfur in the de ep sediment at 2 stations in the inner part of the mangrove. The reoxi dation of reduced sulfides was rapid, and storage of sulfur was minor in the upper sediment layers, where factors like bioturbation, the pre sence of roots, or tidal mixing enhance oxidation processes.