Speciation and isotopic composition of sedimentary sulfur in the Everglades, Florida, USA

We have studied the sulfur speciation and isotopic composition of two peat cores from Water Conservation Area 2A (WCA 2A) in the Florida Everglades. Core site E1 is affected by agricultural runoff from the Hillsboro Canal which drains the Everglades Agricultural Area; Core site U3 is distant from the canal and relatively unaffected by agricultural runoff. Depth profiles of the total sulfur content of both cores show fairly constant levels (??0.7 wt.%) below about 25-30 cm depth in Core E1 and below 40-45 cm in Core U3. Above these depths, total sulfur increases to as much as 1.52 wt.% in Core E1 and 1.74 wt.% in Core U3, suggesting that more sulfur has entered the sediments and/or that more sulfur is being retained in recent times at both sampling sites. These changes in total sulfur content with depth in Core E1 correlate with changes in total phosphorus that have been observed in other studies at core sites near the Hillsboro Canal. This correlation of total sulfur with phosphorus with depth is not seen in Core U3 located away from the canal, possibly because phosphorus is more effectively retained than sulfur in the organic sediment near the canal. Organic-sulfur (OS) concentrations are at least twice as high as the dusulfide-sulfur (DS) concentrations in the upper parts of both cores suggesting that iron is presently limiting the amount of dusulfide minerals formed in these sediments. The degree of pyritization (DOP) in the upper parts of the cores suggest that sulfide mineralization is limited by the availability of highly reactive iron during the earliest stages of diagenesis. Positive ??34S values for reduced sulfur forms in both cores indicate a relatively restricted sulfate reservoir, consistent with nearly complete reduction of the sulfate available in the sediment at any given time. Differences between the two core appear in the ??34S values for the near-surface sediments. The DS ??34S values in the upper 10.0 cm of sediment are more posotive at site E1, with a mean ??34S value of +12.9???, than at site U3, with a mean ??34S value of +2.9???. These reuslts may indicate that increased rates of organic deposition due to nutrient loading near the canal have increased the rate of sulfate reduction at the E1 site in recent times. Acid-volatile-sulfide (AVS) concentrations are lower than DS and OS concentrations by at least a factor of 10. Increasing ??34S values for AVS with increasing depth in both cores suggests ongoing reduction of a limited porewater sulfate reservoir after deposition. The disulfide and organic-sulfur ??34S values diverge from the ??34S values for AVS with depth, suggesting that most of the transformation of AVS into disulfide minerals or incorporation of sulfur into orgnaic matter occurs in the near-surface sediments. A comparison of organic-sulfur ??34S values in the dominant flora at the U3 site (sawgrass leaves and periphyton) with organic-sulfur ??34S values at the top of the U3 core indicates that there was early incorporation of an isotopically light sulfide species into the orgnaic matter.