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.