Sedimentary Dynamics of Florida Bay Mud Banks on a Decadal Time Scale

Topical Area: Sediment Chemistry/Sedimentology

Charles W. Holmes, Robert Halley and Marci Marot, U.S. Geological Survey, St. Petersburg, FL; John Robbins, Great Lakes Research Laboratory, Ann Arbor, MI; Michael Bothner and Marilyn Ten Brink, Wood's Hole Field Center-Quissett Campus, Woods Hole, MA

In order to manage a large ecosystem such as Florida Bay, it is necessary to quantify its rate of change through time. A general lack of historical records that can document an ecological transition requires the use of other methods to define rates and extent of change. One well known geochemical method is to use naturally occurring radioactive nuclides. In the South Florida program (Interagency Florida Bay Science Program), two natural radionuclides (²¹⁰Pb, ²²⁶Ra) were measured in sediments from several sites that exhibited a wide range of sedimentary characteristics. ²¹⁰Pb, with a half-life of 22.3 years, is one radioisotope in the ²³⁸U decay series that has become invaluable in recreating geochronological records. Sediment geochronologies are calculated by determining the relative decrease in unsupported ²¹⁰Pb activity with increasing sediment depth. The rate of accumulation at twenty sites was determined by the ²¹⁰Pb method and ranged from 0.33+- 0.05 to 3.56+0.30 cm/yr. The derived age determinations were independently confirmed by comparison of the distributions of atmospherically-derived anthropogenic lead in both the dated cores and in an annually banded coral. Results from this study indicate that the sediment in the western and northern fringe of Florida Bay are presently accumulating at ~0.3 cm/yr. In contrast, in the central part of the Bay, sediments accumulate at a slightly faster rate of 1.0 cm/yr. In the northeastern portion of the Bay, relatively rapid sediment accumulation rates were measured at Pass Key Bank (2.0 cm/yr). Based on this data, the most dynamic section of Florida Bay is the northern portion, with little sedimentological activity occurring in the western parts of the Bay. The moderate rates of accumulation in the northern and central portions of the Bay produced depositional banks that formed during the last 100 years. These sediments contain the geochemical data necessary to define ecological change within Florida Bay.