SULFUR GEOCHEMISTRY OF ORGANIC-RICH SEDIMENTS FROM MUD LAKE, FLORIDA,USA

Organic-rich sediment cores from Mud Lake, Florida, were analyzed for sulfur species and their isotopic compositions. The cores include the upper 4 m of sediment, which consist of four major horizons based on p etrographic analyses of the organic material: from 400- to similar to 300-cm depth, the sediment consists of a Cladium (sawgrass) peat; from 300- to 200-cm depth, the sediment consists mostly of Nymphaea (water -lily) peat; from 200- to 100-cm depth, the sapropel is more consolida ted and contains a high proportion of non-combustible material, mostly from sponge spicules; and the sediment consists of an amorphous sapro pel above 100-cm depth. The total-sulfur content of the sediment range s from similar to 1.53% to similar to 4.95% (3.35-10.7% on an ash-free and carbonate-free basis) and is dominated by disulfide (pyrite), wit h maxima for both total sulfur and disulfide-sulfur at 117 and 365 cm. Organic-sulfur contents are slightly lower than those of disulfides b ut have a similar depth profile. Systematic changes in the isotopic co mposition of sedimentary disulfide and organic sulfur coincide with va riations in the sulfur species and vegetation types. High disulfide co ntents and low disulfide isotopic ratio values (delta(34)S) characteri ze the saw-grass interval, indicating high sulfate availability during deposition or during later diagenesis. The water-lily interval is cha racterized by a change to higher delta(34)S-values for both disulfide- and organic sulfur, This change may be the result of higher organic m atter accumulation accompanied by a higher rate of sulfate reduction a nd/or a slower rate of sulfate supply to the sediment. The transition to sapropel is accompanied by a large negative shift in disulfide delt a(34)S-values, consistent with an increase in sulfate availability and a slower rate of sulfate reduction, Reactive iron availability does n ot appear to play a major role in limiting the amount of sulfide miner als in these sediments. Major shifts in delta(34)S-values at similar t o 117-350-cm depth coincide with maxima in the amount of non-combustib le material (mostly sponge spicules) in the sediment. These maxima may record refractory material which accumulated around the emergent vege tation at the margins of the wetland at the time of deposition. Altern atively, they may be records of dry periods when refractory material w as concentrated in the sediment as organic matter became oxidized expo sure to air.