Introduction

In order to manage an ecosystem, it is imperative to define the rate at which ecologic, physical and chemical changes which have occurred. The lack of historical records documenting ecological changes dictates that other methods are used to measure the rate of change. A common method of "dating" change is to measure the decay of naturally occurring radioactive nuclides . The use of radioactive isotopes is founded on the known physical property of radioactive material, the half-life. A half-life of an isotope is the amount of time it takes for half of a given number of atoms to "decay" to another element. The age of objects that contain radioactive isotopes with

known half-lives can be calculated by determining the percent of the remaining radioactive material. To use this method successfully certain other prerequisites must be met. These are: 1. the chemistry of the nuclide (element) is known; 2. once the nuclide is incorporated into the substrate the only change is radioactive decay, and 3. in order to be useful, it is relatively easy to measure.

In south Florida, there are two elements which fit these criteria, ⁷Be, and ²¹⁰Pb. ⁷Be, with a half-life of 53.3 days, is a naturally produced radionuclide formed by cosmic ray bombardment of atmospheric nitrogen and oxygen. Once formed ⁷Be is removed from the atmosphere and is incorporated into forming sedimentary material. The very short half-life, its rapid removal from the atmosphere, and the strength of attachment to the substrate makes this nuclide very useful in determining if the substrate was formed within the past year. This allows the calibration of other methods and establishes "time zero (T₀)" ²¹⁰Pb, with a half-life of 22.8 years, is an isotope in the ²³⁸U series. ²¹⁰Pb is formed by the decay of radon which is diffusing into the atmosphere at about 42 atoms per minute per square centimeter of earth's surface. This ²¹⁰Pb has a residence time in the

Figure 2 -- Ideal ²¹⁰Pb curve with depth (time).

atmosphere of approximately 10 days. It is removed by rain or snow and is rapidly adsorbed to or incorporated within sediment forming at the earth surface (figure 3). In south Florida, ²¹⁰Pb is incorporated in the organic peat deposits.

The activity of the unsupported ²¹⁰Pb decreases as a function of time determined by its half-life. The "age" of a horizon is calculated by the following formula:

T_age = 1/ ln(A ²¹⁰Pb₀/ A ²¹⁰Pb_h )

substituting the constants,

where A²¹⁰Pb₀ is the unsupported lead activity in disintegrations per minute at time zero (the present) and A ²¹⁰Pbh is the activity in disintegrations per minute at depth h. In an ideal situation the plot of ²¹⁰ Pb activity will show a logarithmic decrease with depth (Figure 4).

Sampling

In order to provide a regional picture for south Florida, 17 cores were taken. The sites were co-located with other investigators so that the results could be correlated to changes in chemistry and ecology. The cores were taken with a modified piston core 10.16 cm (4 inches) in diameter which was capable of taking a 1 meter core.

Radioelement measurements

Along with the measured of ²¹⁰ Pb activity of each sample, the activity of ⁷Be and ²²⁶Ra was also determined. ⁷Be and ²²⁶Ra were determined by measuring the activity of the 0.578Mev energy. ²²⁶ Ra is determined by the measuring the activity of ²¹⁴Bi. These analyses were made with a GeLi detector couple to a multichannel analyzer. The activity of ²¹⁰Pb was determined by measuring its granddaughter, ²¹⁰Po. ²¹⁰Po decays solely by -radiation which is extremely easy to measure.

Data Reduction and Analysis

In order to determine the distribution of ²¹⁰Pb within the sedimentary column, the raw values for each segment were converted to the natural log. A standard linear best fit calculation was made on these data. If the original curve was logarithmic, the best fit would yield a straight line. To be acceptable the Best Fit must have a correlation coefficient (R²) value of greater than 0.9. The rate of sedimentation was then calculated using the calculated values of ²¹⁰Pb versus depth and the formula shown above. Figure 4 is an example of this procedure using data from conservation area site F4. This data demonstrates the value of the procedure.

Summary

The data produced during the first year of this program indicated that sediment forming in the South Florida Ecosystem can be "dated." There are, however, precautions in retrieving cores and subsequent analysis. In the Water Conservations areas, there is good evidence that many cores contained surface disturbed zones.

In the cores retrieved from the areas with thin peat, such as the northern part of Everglades National Park and Big Cypress, cores were taken in "solution" pits. There is good evidence that ground water is contaminating the sediment in these pits. This prevents "date" and rates of sedimentation from being calculated at this time. In a core from the Big Cypress Preserve, there was sufficient disequilibrium that a rate of sedimentation was calculated. In the Taylor Creek region, the ²¹⁰Pb profiles and other data indicate a good area for age measurements.

Collaboration and Partnerships

Data collected within this project is dissiminated to all scientists with the south Florida ecosystem study. It is an important piece of the puzzle in the determination of accumulation rates of nutrients, and other chemicals, in the determination of changes in ecolocial history, and in determining accretion rates necessary for construction of hydrologic models. This data is also shared with members of the South Florida Water Management District, the Everglades National Park, Big Cypress Wildlife Refuge, Loxahatchee Wildlife Refuge, and with local governmental agenies that require information on the timing of ecological changes.

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For more information contact:

Charles W. Holmes
Center for Coastal Geology
U.S. Geological Survey
600 Fourth St. South
St. Petersburg, FL 33701
cholmes@usgs.gov

Related information:

SOFIA Project: Geochronology in the South Florida Ecosystem and Associated Ecosystem Programs