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 El 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 (similar to 0.7 wt.%) below about 25-30 cm depth in Core El and below 40-45 cm in Core U3. Above these depths, total sulfur increases to as much as 1.52 wt.% in Core El 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. The changes in total sulfur content with depth in Core El correlate with changes in total phosphorus that have been observed in other studies at con 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 disulfide-sulfur (DS) concentrations in the upper parts of both cores suggesting that iron is presently limiting the amount of disulfide 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 delta(34)S 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 cores appear in the delta(34)S values for the near-surface sediments. The DS delta(34)S values in the upper 10.0 cm of sediment are more positive at site El, with a mean delta(34)S value of + 12.9 per mil, than at site U3, with a mean delta(34)S value of + 2.9 per mil. These results may indicate that increased rates of organic deposition due to nutrient loading near the canal have increased the rate of sulfate reduction at the El site in recent times. Acid-volatile-sulfide (AVS) concentrations are lower than DS and OS concentrations by at least a factor of 10. increasing delta(34)S 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 delta(34)S values diverge from the delta(34)S values for AVS with depth, suggesting that most of the transformation of AVS into disulfide minerals or incorporation of sulfur into organic matter occurs in the near-surface sediments. A comparison of organic-sulfur delta(34)S values in the dominant flora at the U3 site (sawgrass leaves and periphyton) with organic-sulfur delta(34)S values at the top of the U3 core indicates that there was early incorporation of an isotopically light sulfide species into the organic matter. Published by Elsevier Science B.V.