This 210Pb has a residence time in the atmosphere of 10 days. It is removed by rain or snow and is rapidly adsorbed to or incorporated within sediment forming at the earth surface. 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: Tage = 1/[Image] ln(A 210Pb0/ A 210Pbh ) substituting the constants, Tage = (ln(A 210Pb0/ A 210Pbh))/0.03114 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. 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 measurement 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 measuring the activity of 214Bi. These analyses were made with a GeLi detector coupled to a multichannel analyzer. The activity of 210Pb was determined by measuring its granddaughter, 210Po. 210Po decays solely by alpha-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.
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.