Managing an ecosystem ideally requires a full knowledge of the environmental dynamics. If no historical environmental records exist, the employment of other methods is necessary to obtain this information. A well-known geochemical procedure that supplies time information is the use of shortlived isotope chronological methods. Of the many naturally occurring nuclides, ²¹⁰Pb was found to be the best suited for gauging the timing of environmental changes in Florida Bay and in the Everglades of South Florida. The age-depth relationships in cores were calculated using the ²¹⁰Pb method at 35 sites within Florida Bay and at 58 sites in the Everglades. In the Florida Bay system, ages were independently confirmed by comparing the distribution of the known concentrations of atmospherically anthropogenic total stable lead recorded in dated cores to similar data in an annually banded coral. In the Everglades system the ages were confirmed by photographic evidence that bracketed changes measured in the cores.

Cores from three sites in the central part of Florida Bay were selected for further analysis. X-radiographs revealed laminae over most of their length that indicated negligible sediment disturbance. The ²¹⁰Pb and ²²⁶Ra were measured at 2-cm increments throughout these cores. Maximum activities of total ²¹⁰Pb were not much higher than ²²⁶Ra activity. Thus, high-frequency measurements of radium were needed to construct accurate chronologies from excess ²¹⁰Pb (total ²¹⁰Pb minus ²²⁶Ra). The ²¹⁰Pb profiles were nearly exponential, indicated evidence of little sediment mixing, and were consistent with a constant rate of delivery of excess ²¹⁰Pb and sediment mass. Mean accumulation rates ranging from 0.17 ± 0.02 to 0.92 ± 0.04 g/sq cm/yr (ca. 0.42 to 1.22 cm/yr) provided age-depth assignments to about 100 years BP.

Because ²¹⁰Pb dating is usually based on several plausible but untested assumptions, age-depth assignments generally must be validated by independent means. Fallout ¹³⁷Cs and stable lead profiles in the cores were compared with time records of atmospheric deposition at Miami (1964 maximum) and continental atmospheric lead concentrations (1972 maximum), respectively. For lead, profiles were also compared with lead/calcium ratios (1978 maximum) in annual coral bands from a specimen (Montastrea annularis) located on the Atlantic side of the Florida Keys (Shen and Boyle, 1987). Agedepth assignments in the cores were confirmed by the correlation of the ¹³⁷Cs peak as well as the nearly perfect match with the lead distribution in the coral (Robbins and others, 1998). The analysis of the cesium distribution also indicated that there is a time averaging of the distribution of metals in the bay of approximately 16 years. ^239+240Pu measurements confirmed this time averaging model (Robbins and others, 1998). This means that the system requires approximately 16 years to remove any metal that is introduced into it. Two cores were analyzed for mercury, barium, and uranium in addition to the lead. The barium and uranium concentrations vary, whereas a mercury maximum occurs in the cores around 1960.

Three cores were selected for metal analysis in the Everglades system. These cores are on a north-south traverse transecting an impacted area. High phosphorous concentration and the high growth rate of cattails distinguish the impacted area. Because of the high growth rate of the cattails and consequent increased production of peat at the surface, the concentration ²¹⁰Pb became diluted, resulting in declining profiles. However, at all sites the convergence of the ²¹⁰Pb and ²²⁶Ra activities at depth demonstrate that an equilibrium level is clearly defined. This level is estimated to be the 100±25 year horizon. The determination of the metal inventory above this horizon indicated that most metals were deposited at a similar rate. One core taken outside of the impacted zone was dated. The distribution of metals in this core has a unique separation between normally coherent metals. Mercury, aluminum, and titanium are very coherently peaking around 1960 and around 1920, whereas copper, arsenic, cadmium and manganese show only a continuous increase through time. Lead peaks around the 1972 horizon, confirming the date as determined by the ²¹⁰Pb. The conclusion of this study is that short-lived isotopic chronologic methods are valuable in determining the history of metal deposition.

REFERENCES

Robbins, J.A., Holmes, C.W., Halley, R.B, Bothner, M., Shinn, E., Graney, J., Keeler, G., tenBrink, M., Orlandini, K.A., Rudnick, D., 1998, First-order time average fluxes of ¹³⁷Cs, Pb and ^239+240Pu to ²¹⁰Pb dated sediments in Florida Bay. (Submitted to Journal of Geophysical Research)

Shen, G.T., and Boyle, E.A., 1988, Determination of lead, cadmium, and other trace metals on annually banded corals: Chemical Geology, v. 67, p. 47-62.

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