Sediment Geochemistry

The concentration of selected major and trace elements in the sediment core material from six cores taken from Taylor Slough is plotted in figures 4 and 5. These are the same data as those presented in table 6. Table 6 also presents the arithmetic mean and standard deviation for the concentration of elements in core material. These summary statistics were calculated by combining all sampling intervals for those elements without censored values (all values reported as above the LLD). These data can be compared in a general way to regional data from the literature. For example, a very recent study by Chen and others (1999) compare element concentrations in Florida soils to those in various soils world wide. Comparing our data, for organic-rich sediments, to their data from more mineralized soils from throughout Florida, shows that our Hg concentrations are two-to three-times greater. In addition, most of the other environmentally important metals (Cr, Cu, Ni, Pb, and Zn) that we report are two- to eight-times greater.

Based on similar data from cores we collected in WCA (L.P. Gough, unpublished data) there appears to be very little variation in element concentration among cores for the major elements (fig. 4). There is, however, considerable variation in the down-core concentration trends. For example, Ca in core sites TS7, TS9, and TS7E shows a pronounced increase in concentration near the surface. This reflects variations in the biogenically derived marl content of the core material. Except for Al and Fe in core TS9, most of the major elements do not show a pronounced increase in concentration with depth. Even core TS15, the southern-most site located only a few kilometers from the saline waters of Florida Bay and collected well within the red mangrove ecotone, does not show an increase in Na. Core chemistry uniformity with depth is important because it means that, for the major elements, very little difference should be expected in sediment chemistry north-to-south within the Slough.

The concentration of selected trace elements in sediment core material (table 6), plotted in figure 5, show much greater regional and down-core variability than the major element patterns. Concentrations of Cu, Pb, and Zn are commonly enriched in ombrotrophic surface peat layers because of significant anthropogenic atmospheric inputs (Shotyk and others, 1992). Our data, in general, do not show strong near-surface trace element enrichment. Lead and Zn do show enrichment in the top 10-15 cm for cores TS2, TS7E, TS9, and TS15; however, toward the bottom of the core their concentrations also occasionally increase. All five elements show increases with depth in one or more of the cores--especially in the lower half (>30 cm). For example, the pattern for Cr does show large concentration differences especially in the deeper part of the cores (e.g., TS9 and TS15). However, the quality control (precision) for Cr analyses in sediment was poor (see Methods section) and caution is needed in the interpretation of these data. These two southern-most cores show a general pattern down-core of high-low-high for the concentration of most trace elements.

Sulfur plays an important role in the sequestration of metals in sedimentary systems. Under anoxic conditions, microbial sulfate reduction reduces sulfate to sulfide. Sulfide is a highly reactive chemical species, especially with metal ions, and will quickly form highly insoluble metal sulfides (Boulegue and others, 1982; DeLaune and Smith, 1985; Huerta-Diaz and Morse, 1992). Metal sulfide species once formed are generally stable under the anoxic and circumneutral conditions found in most wetland sediments. Freshwater, peat-forming wetlands like the Everglades, however, typically have low levels of sulfate, limiting sulfate reduction and the formation of metal sulfides. Under these conditions, other processes such as complexation of metal ions by the organic matrix or the admixing of mineral phases with the peat may be more important in influencing the distribution of metals in sediments.

The environment of Taylor Slough ranges from a freshwater, low sulfate environment in the north, to brackish water, high sulfate conditions in the south. Sulfur concentrations and speciation were determined in the sediment and pore water and are reported elsewhere (Orem and others, 1999; W.H. Orem, unpublished data). This was done in order to examine the possible influence of sulfur on metal distributions. Total sulfur contents of sediments ranged from < 0.5% (dry wt.) in the freshwater areas to 8% in the brackish water areas. Most of the sulfur is present as organic sulfur in the freshwater areas, although disulfides can also be important. Disulfides appear to become somewhat more important in the brackish water zone. Vertical profiles of total sulfur were nonsystematic in the freshwater zone, but were observed to generally increase with depth in the brackish water areas. Metals showed generally poor correlations with total sulfur content in the freshwater areas, with the best fits for Pb, Mn, and Zn (correlation coefficients of only 0.4 to 0.6). This suggests that in the freshwater areas sulfur is not a major control on metal distributions in the sediments. A better case can be made for sulfur control of metal distributions in the brackish water areas. Several metals from the brackish water zone show reasonably strong correlations with sulfur in the sediments: Ni, Cr, Fe, and Cu, all with correlation coefficients of 0.8 or higher. The strength of these correlations is primarily driven by increases in both metal and sulfur concentrations in the lower half of the cores. In this area, the brackish water from Florida Bay comes up along the base of the peat, with a fresher layer of water, due to runoff, at the surface. Thus, the major sulfate source at these sites is from the base of the peat. Correlation, of course, is not proof of a mechanism, but does provide an indication that sulfur may play a control on metal distributions in sediments from the brackish water zone in Taylor Slough.

Figure 6 presents the spatial trends for the concentration of total Hg in five of the Taylor Slough cores. The plots also contain the bulk density and geochronology measurements for the same samples as well as the simple correlation coefficient (R²) for the relation between bulk density and Hg concentration. Except for core TS15, there is no strong pattern of higher Hg concentrations near the sediment surface. This generalization is in contrast with other studies that have shown greater Hg concentrations in the past 90+ years as compared to sediment material deposited before about 1900 (Delfino and others, 1993; Rood and others, 1995). Except for core TS1 (furthest north, at the head of the Slough), the concentration of total Hg in sediment samples is an order of magnitude below the highest values reported by Delfino and others (1993). There appears to be no consistent spatial pattern for the concentration of Hg among sites (north/south within the Slough) or down-core. Also, the relation between bulk density and Hg concentrations is always positive but weak.

Figure 7. Ternary diagram showing the relation of Hg concentrations in core material to bulk density and the concentration of organic carbon. Click on image to open larger version.

Figure 7 presents a ternary plot of Hg concentrations related to bulk density and organic carbon concentrations in the four cores (TS7, TS9, TS7E, and TS15) for which we have complete data for all three parameters. Core segregation is evident with TS7 and TS15 showing the strongest association with organic carbon. There does not appear to be a north/south trend. Like figure 6, figure 7 also demonstrates the positive but weak association of Hg concentrations with bulk density as well as organic carbon. The dramatic segregation of core TS9 reflects the marly, non-organic nature of much of the core material. This site was in the south-central region of the slough.

Next: Results and Discussion continued - Element Accumulation Rates in Sediments

Return to Publications Page