Element Accumulation Rates in Sediments

Tables 11 and 12 present the element accumulation rates (EAR, expressed both as g m^-2 yr^-1 and µg cm^-2 yr^-1) for selected major and trace elements in five sediment cores extracted from Taylor Slough. These same data (tables 11 and 12) are plotted in figures 8, 9, and 10.

Figures 8, 9 and 10 are down-core plots of major element ( Al, Ca, Fe, K, Mg, Na, and P), trace element (Cr, Cu, Ni, Pb, and Zn), and Hg accumulation rates, respectively, for core sites TS1, TS7, TS7E, TS9, and TS15 (see fig. 2). Unlike the concentration plots of figures 4, 5, and 6 above, accumulation plots are less vulnerable to variations in sedimentation throughout the core profile that can cause dilution of the target analyte. Accumulation rates are normalized to time and minimize the problem of covariance among different sedimentary components (Rood and others, 1995). Nevertheless, a comparison of an element’s concentration plot with its accumulation rate plot shows that they are very similar indeed. This is due to the uniform bulk density and sedimentation rate measurements throughout the length of most of the cores (tables 11 and 12).

Many of the down-core patterns discussed above for both the major and trace elements are much less obvious in the examination of the accumulation rate plots. For example, the generally higher concentration of Cu and Zn near the surface of cores TS2 and TS15 is not at all obvious in the accumulation plots. Chromium, however, continues to show a general increase in accumulation with depth that parallels its concentration trends. Not surprisingly, because of the higher sedimentation rates observed at the core site TS1 (fig. 2), most elements express their highest accumulation rates at the head of the Slough. For example, the EAR values for Pb in TS1 are two orders of magnitude greater than for any of the other cores.

Patterns for Pb (fig. 9) typically show a small spike between 8-12 cm in depth. Studies such as those by Weiss and others (1999) attribute this spike to the global industrial atmospheric input of Pb that resulted in a significant Pb enrichment in peat during two major periods--from about 1880-1920 and from 1960-1980. Our data are not as precisely delimited as those presented by Weiss and others; however, table 10 shows that this spike in our data represents a period of from 20 years ago (core TS1) to 60-180 years ago (cores TS7, TS9, TS7E, and TS15). Certainly cores TS9 and TS15 (fig. 9) correspond most closely to typical patterns of Pb accumulation trends reported in the literature. More work at the sites where these cores were taken may be of interest in future attempts to further define Pb trends in Everglades peat.

Figure 10 shows total mercury accumulation rates as well as the approximate geochronology for five of the Taylor Slough cores (TS1, TS7, TS7E, TS9, and TS15). Because of the inability to successfully collect the upper 0-2 cm in many of these cores, and a sedimentation rate of about 0.1 cm yr^-1 (except core TS1), the calculated ages of the upper-most core samples was highly variable. Except for core TS1, the total Hg accumulation rates are within the range of those reported for the Water Conservation Areas by Delfino and others (1993) and Rood and others (1995). The high EAR values for TS1 directly reflect high sediment accumulation rates and not high total Hg concentrations. These EAR values are several-times greater (~200 to ~320 µg m^-2 yr^-1) than those found in post-1985 core material from WCA (23-141 µg m^-2 yr^-1)(Delfino and others, 1993). In addition, our data do not show greater total mercury accumulation rates near the surface (past 90+ years) as do these authors. This uniformity in Hg accumulation rate patterns in our data is the result of uniform Hg concentrations (not highly variable down-core) and of the very low calculated accumulation rates. Mercury accumulation rate data varied as follows: core TS1, 200-320 µg m^-2 yr^-1; core TS7, 6.8-63 µg m^-2 yr^-1; core TS7E, 19-62 µg m^-2 yr^-1; core TS9, 12-71 µg m^-2 yr^-1; core TS15, 2.2-10 µg m^-2 yr^-1.

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