The Buttonwood Embankment, a coastal levee averaging 1.5 ft in height, was first described as the "embankment that impounds the freshwater of the lower three counties of Florida" and separates the peninsula of Florida from Florida Bay (Craighead, 1964). Of the many questions concerning environmental change in Florida Bay two are: (1) What part does this "Coastal Levee" play in the hydrologic regime of southern Florida? and (2) Are rising sea-level or hydrologic management practices responsible for the environmental changes over the years? These questions were addressed in a recent study sponsored by the Everglades National Park in cooperation with the U.S. Geological Survey. A total of 32 cores was obtained: 6 on the embankment (a traverse along Taylor Creek and a traverse near Crocodile Point), 8 in the banks and basins of Florida Bay and 18 in the marsh/swamp. The chronology of events was established using short-lived isotopic chronological methods and the paleontological analysis.

Results of the chronological and paleontological analyses show that there have been long-term fluctuations related to sea-level changes during the last 2,000 years. There have also been significant changes noted during the last 50 years that can be attributed to hydrologic management of the water system of southern Florida.

Long term development: The nature of the long-term changes is best explained by climate variations that have taken place during the past 2,000 years. Two climatic events occurred within this timeframe, (1) the Medieval Warm period (800-1300 AD) and (2) the Little Ice age (1500-1800 AD). Each of these has left their signature on the vegetation of this part of South Florida (Willard and Holmes, 1999).

Concurrent with climate variability is sea-level change. Sea level in the Florida Bay area has been rising at ~ 3 mm per year for the past 150 years, as measured at Key West and Miami. There is a dichotomy of opinion on the nature of sea-level rise along the Florida Coast. Many investigators invoke a slow and continuous rise in sea-level (Scholl and others, 1966; Robbin, 1984), while others present evidence of step-type changes in sea level. The latter suggest that a higher sea level than present (~ 0.5 meters) occurred between 600-1000 years BP (Fairbridge, 1974; Stapor and others, 1991), the time of the Medieval warm period.

A conceptual model of bank formation consisting of six phases was constructed and tied to sea level variation over the past 2,000 years In the initial phase, about 2,000 years ago, the region that is presently occupied by the embankment was a coastal freshwater pond. Peat and freshwater marl were deposited on the floor of the pond, which was separated from the marine environment by a coastal ridge and maintained a freshwater hydraulic head similar to the pre-1900 Everglades uplands. During the second phase, from about 2,000 to 1500 years ago, the coastal ridge was breached, and estuarine carbonate sediments were deposited in the pond. During the winter months, weather fronts stirred up the sediment in the adjacent bay. Some of the resultant turbid water was transported into the coastal ponds by tides. The coastal pond became a mixing zone of freshwater runoff from precipitation associated with the front and marine turbid tidal water, causing a shoaling of the pond. Similar to development of Lake One near Taylor Creek, this situation provided an opportunity for mangrove encroachment and resulted in mangrove peat being deposited over estuarine marl. Phase 4 was a situation similar to the present one at Crocodile Point. The central ephemeral pond catches sediment filtered through the mangrove fringe. The trapped sediment was very fine, but contained terrestrial fauna that live on the short interior vegetation. Phase 5 was a continuation of this process that resulted in a relatively thick terrestrial deposit as evidenced by the meter-thick section containing terrestrial snails on the Taylor Creek traverse. This deposit has a date of approximately 1,000 years, which corresponds to the Medieval Warm period and the high stand as determined by Stapor and others (1991). The final phase included the lowering of sea level during the Little Ice age which exposed the fringe to erosion and elevated the region relative to sea level creating the embankment obvious today along the shoreline of the bay.

Short-term development: Through the past century there have been changes to the embankment caused by the "replumbing" of the South Florida drainage. Early changes are hinted at in apparent change in vegetative patterns, but the most significant changes recorded in the ecosystem took place during 1950-60. Because the bank was emergent during this time, there is no depositional record on it, but there were some significant alterations both land ward and seaward. In the bay, the building of the bank began between Pass Key and Lake Key. This infilling appears to have been completed in twenty years. Inland, in the vicinity of Taylor Creek, a lake was filled and much of the freshwater vegetation disappeared becoming a mangrove marsh.

Comparison of the long-term and short-term changes indicates significant differences. The sealevel rise during the Medieval Warm period and Little Ice Age, with the potential increase in saltwater influx, produced only slight changes to the vegetative patterns and there were no recognizable changes within Florida Bay. Even though the changes that have occurred since 1950 have taken place during rising sea level, these changes are far more extensive than those during previous high stand. This points to the hydrological management practices associated with the urban development are the major factors in these recent changes.

REFERENCES

Craighead, F.C., Jr., 1964, Trees of South Florida, volume 1, The natural environments and their succession: Coral Gables, University of Miami Press, 212 p.

Fairbridge, R.W., 1984, The Holocene sea-level record in South Florida, in Gleason, P.J., ed., Environments of South Florida, Present and Past: Miami Geological Society, Memoir 2, p. 223-231.

Robbin, D.M., 1984, A new Holocene sea-level curve for the upper Florida Keys and Florida reef in tract, Gleason, P.J., ed., Environments of South Florida, Present and Past: Miami Geological Society, Memoir 2, p. 437-459.

Schoell, D.W., Craighead, F.C., and Stuiver, M., 1966, Florida submergence curve, revised: its relation to coastal sedimentation rates: Science, v. 163, p. 562-564.

Stapor, F.W., Mathews, T.M., and Linfors-Kearsin, F.E., 1991, Barrier-island progradation and Holocene sea-level history in southwest Florida: Journal of Coastal Research, v.7, p. 815-838.

Willard, D.A., and Holmes, C.W., 1997, Pollen and geochronology data from south Florida Taylor Creek Site 2: U.S. Geological Survey Open-File Report 97-35, 28 p.

(This abstract was taken from the Proceedings of the South Florida Restoration Science Forum Open File Report)