In order to manage a changing ecosystem, it is necessary to ascertain its rate of change. The lack of historical records documenting ecological change in south Florida requires that other methods be used to define the rates. A common method is to use the decay of naturally occurring radioactive nuclides. The usefulness of any radioactive nuclide requires that 1) the chemistry of the nuclide (element) is known, 2) the half-life has a range which is long enough to span the time of interest, 3) the nuclide, once incorporated into the substrate, changes only by decay, and 4) the nuclide is relatively easy to measure. In south Florida, where the period of interest is the last 200 years, there are four nuclides which fit these criteria, 7Be, 14C, 137Cs, and 210Pb. Two of these are natural, 7Be and 210Pb; one has both a natural and anthropogenic contribution, 14C; one is solely anthropogenic, 137Cs.

In the South Florida Program, natural nuclides were measured at all study sites. 7Be, with a half-life of 53.3 days, is formed by cosmic ray spallation of nitrogen and oxygen within the earth's atmosphere. It is subsequently transferred via washout from the atmosphere to the marine and terrestrial environment. A highly reactive element, it is rapidly incorporated into a sedimentary substrate. This nuclide is very useful in determining if the sediment was in communication with the atmosphere within a year and if the surface sample was deposited within the past year.

210Pb, with a half-life of 22.8 years, is one of the isotopes in the 238U series. The disequilibrium between 210Pb and its distant relative is caused by the intermediate progenitor, 222Rn. Radon escapes into the atmosphere at a rate of about 42 atoms per minute per square centimeter of earth's surface. This isotope rapidly decays to form 210Pb. The newly formed lead, with a residence time in the atmosphere of about 10 days, is removed by rain or snow. Like 7Be, 210Pb is tightly bound to the sediment forming at the earth's surface. This flux produces a concentration of "unsupported" 210Pb. "Unsupported" lead is the 210Pb whose activity is greater than any activity that may be produced by ambient radon progenitors present within the sediment. "Dates" of the sediment are calculated by determining the decrease in "unsupported" 210Pb activity with depth. If the initial concentration is known, or is estimated using 7Be data, then the age of a horizon can be calculated. Ideally, a plot of 210Pb activity with a log scale versus depth will be a straight line. The slope of the line indicates the relative sedimentation rate. A steep slope represents a site of slow accumulation. The combination of 7Be and 210Pb produces good "dates" and very accurate rates of accumulation. But like all dating procedures, this method relies on reasonable assumptions for validity. To narrow the degree of uncertainty, concordance is sought with "ages" either by an independent "dating" method or by other independent measurements of time.

The most common nuclide used to verify the 210Pb dates is 137Cs. 137Cs, with a half-life of 30.3 years, is a thermonuclear by-product. Robbins (personnal commun., 1995) has plotted the thermonuclear fallout in South Florida based on NOAA data. This curve shows that the detection of fallout began in about 1954, peaking in 1960-61. The distribution of 137Cs, under ideal conditions, provides a chronometer for the last forty years. At most localities within Florida Bay and the Everglades, the 137Cs and 210Pb ages were synchronous and the "dates" at these sites are better than relying on one method alone. However, in the mangrove fringe of Florida Bay and at sites near the northern border of Water Conservation Area 2A, there was a significant discrepancy between the two methods. It has been demonstrated that in some cases the cesium will become mobile. In order to determine if this was happening, 14C measurements were made. 14C has two sources; the primary source is cosmogenic, similar to 7Be. The other source is from atmospheric testing of thermonuclear devices. Although cosmogenically-derived 14C makes up most of the total radiogenic carbon, the thermonuclear component is significant and readily measured. The importance of this component is that it provides a time-line in the carbon signal which is recognized in carbon-based sediment. The distribution of 14C was determined using AMS techniques, which measure the activity of radiocarbon on extremely small samples. The small samples were necessary to circumvent problems that arise with descending roots, and to isolate material that represents the time the sample was deposited. Results of these measurements confirm that cesium is migrating in the mangrove fringe sediment but not in the WCA sediments. The discrepancy in the latter is the result of physical disturbances around the sample locations.

Even with ages determined by multiple radiometric methods, there is still some reservation about the establishment of a chronology. To alleviate these concerns, confirmation of the chronology can be established based on independent observations. In Florida Bay, a comparison was made between the distribution of total atmospheric-derived lead within a coral, "dated" by counting the annual bands, and the total lead distribution within radiometrically dated sediment cores. Results of the comparison demonstrated concordance. In the emergent portion of the study area, radiometric "dates" were confirmed by comparison of the paleobotanical record to geographic changes recorded in areal photographs. These comparisons yield the best "dates", because they were established by three independent methods.

In summary, at sites where only naturally occurring radionuclide (210Pb and 7Be) data is available, rates of sediment accumulation can be accurately determined but "dates" are limited in accuracy because of the assumptions required. These sites are the good sites. At sites where there is additional radiometric information, such as 137Cs, for verification, the sediment "dates" are better. The best "dates", however, are those that are confirmed by other procedures in addition to radiometric dating. It is at these sites that the radiometric "dates" can fill in the gaps and detail changes in the ecology of the area over time.

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