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 petrographic 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 consolidated and contains a high proportion of non-combustible material, mostly from sponge spicules; and the sediment consists of an amorphous sapropel above 100-cm depth. The total-sulfur content of the sediment ranges 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), with maxima for both total sulfur and disulfide-sulfur at 117 and 365 cm. Organic-sulfur contents are slightly lower than those of disulfides but have a similar depth profile. Systematic changes in the isotopic composition of sedimentary disulfide and organic sulfur coincide with variations in the sulfur species and vegetation types. High disulfide contents and low disulfide isotopic ratio values (delta(34)S) characterize the saw-grass interval, indicating high sulfate availability during deposition or during later diagenesis. The water-lily interval is characterized by a change to higher delta(34)S-values for both disulfide- and organic sulfur, This change may be the result of higher organic matter accumulation accompanied by a higher rate of sulfate reduction and/or a slower rate of sulfate supply to the sediment. The transition to sapropel is accompanied by a large negative shift in disulfide delta(34)S-values, consistent with an increase in sulfate availability and a slower rate of sulfate reduction, Reactive iron availability does not appear to play a major role in limiting the amount of sulfide minerals in these sediments. Major shifts in delta(34)S-values at similar to 117-350-cm depth coincide with maxima in the amount of non-combustible material (mostly sponge spicules) in the sediment. These maxima may record refractory material which accumulated around the emergent vegetation at the margins of the wetland at the time of deposition. Alternatively, they may be records of dry periods when refractory material was concentrated in the sediment as organic matter became oxidized exposure to air.