The peninsula of Florida is separated from Florida Bay a coastal ridge called the Buttonwood Ridge by Craighead, (1964). The Buttonwood Embankment, averaging 0.5 m (1.5 ft) in height, was characterized as the "embankment that impounds the freshwater of the lower three counties of Florida" (Craighead, 1964). Presently, however, the creeks, lakes, and ponds adjacent to the embankment are brackish to marine. The change from fresh to brackish waters has been detrimental to the marsh. Studies have been launched to determine the best methods to return and sustain a more fresh/estuarine environment. To judiciously plan for this restoration two questions have been posed to clarify the history of the area. (1) What is the role of the "Coastal Levee" in the hydrologic (surface and groundwater) regime of southern Florida and Florida Bay? and (2) Have the environmental changes over the years resulted from rising sea-level, hydrologic management practices or both? To address these questions, we have analyzed the fauna, floral, and isotopic record in 50 cores: 8 on the embankment, 12 in the banks and basins of Florida Bay and 30 in the marsh/swamp. These data were merged with previously collected data (Davies, 1980: Cottrell, 1989). The timing of events was determined using short-lived isotopic chronological methods.

Long term development -- From these records, it is apparent that changes occurred on two time scales, 1 major environmental changes that can be tied to global climatic variations and 2. The significant marine encroachment of the past fifty years. Two climatic events, during the last 2000 years have left a record in the sediments of South Florida, (1) the Medieval Warm period (800 to 1300 AD) and (2) the Little Ice Age (1500 to 1800 AD) (Willard and Holmes, 1999). Concurrent with this climate variability, or because of it, is sea-level has changed. 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 older that this record. Many investigators present evidence of 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 (Wanless and others, 1995). Some evidence also suggests a higher stand of the sea (~ 0.5 meters) occurred between 600-1000 years BP (Fairbridge, 1974; Stapor and others, 1991), during the Medieval Warm Period. Our results are consistent with the latter interpretation because brackish invertebrate taxa occur in core landward of the embankment at about 750 Yr. BP (Willard and Holmes, 1997).

A conceptual model of bank formation was developed through examination of the embankment sediment record consisting of six phases, which are tied to sea level variability during the past 2,000 years. The region underlain by the present Buttonwood Ridge was a series of fresh-water lakes and ponds, preserved as fresh-water peat admixed with freshwater marl. These basal units are overlain by estuarine carbonate mud capped by a mangrove peat. ¹⁴C dates of this peat range in age from 1400 to 1700 BP. The mangrove layer is overlain by what Cottrell (1989) called a supratidal carbonate. On the Buttonwood Ridge, the supratidal sediment is very fine and devoid of any internal sedimentary structures. The scattered fossils within this layer are "terrestrial", living on leaf litter and on the underbrush. ¹⁴C dates of these fossils and organic material picked from the cores give ages between 1200 and 900 BP, corresponding to the Medieval Warm period. The process of deposition of this unit is unclear, but it is thought to resemble the processes, that are adding sediment to the area of Crocodile Point. On Crocodile Point the fringe mangrove forest filters sediment during very high water that invades a central low area. Dating of this accumulation indicates that it is accreting at the same rate as sea level is rising in the Bay.

If this model accurately describes this process, then sediments forming the Buttonwood Ridge are the result of rising sea level. 10 km inland from the coastline, the lower zone of a core contains pollen consistent with a brackish environment; the date of this zone places deposition at the end of the Medieval Warm period. This would be consistent with a higher sea level. During the Little Ice Age that followed, sea level may have dropped, exposing the supratidal mud and leading to erosion creating the Ridge as seen today.

Short-term development - During the past century, significant environmental changes have occurred in northeastern Florida Bay. Although minor vegetative changes occurred in the early part of the 20^th century, the most significant changes occurred during the 1950-1960 time frame. In the bay, the construction of the banks between Pass and Lake Keys and the extension of the bank south of Porjoe Key was initiated. Inland from the embankment, at least one pond began to close. Further inland, up Taylor Creek, the saw-grass plain was encroached on by mangrove forest. These events are attributed to the encroaching marine environment.

Groundwater/Surface Water Interface - In these studies, questions have been raised on the role of groundwater in the environmental changes. Fitterman (1998, personal communication) mapped the location of the fresh/salt water interface. Peat cores across this fresh/salt water boundary exhibit an increase in ²¹⁰Pb and ²²⁶Ra at the rock/peat boundary; the highest values (10 X ambient surface values) are found in the peats on the fresh water side of the boundary. The ¹³⁷Cs distribution within each core is highest in the surface layers decreasing to non-detectable at the peat/rock interface. The exception to this ¹³⁷Cs distribution occurs in those cores at the freshwater -saltwater groundwater boundary. These cores contain detectable ¹³⁷Cs at the rock/peat interface. This manmade isotope, added to the atmosphere in the late 1950's and early 1960's, is no longer being added to environmental systems. The presence of ¹³⁷Cs in cores on either side of the boundary and the knowledge that ¹³⁷Cs in mobile in peat environments, suggest that this isotope has been transported by some groundwater transport process (perhaps upwelling along the fresh/salt water interface?).

Cores taken along the northern fringe of the bay have ²¹⁰Pb and ²²⁶Ra subsurface distribution similar to those at the present fresh/salt water boundary inland. The highest subsurface concentrations occur in those cores taken in the thalweg of the submerged Taylor Slough. This increase is not present in other cores from the central part of the bay and is slightly recognizable in cores to the east. It is hypothesized that this increase represents "paleo-ground water" upwelling. ¹⁴C dates of organic material within the sediment at the rock/sediment interface confines the timing to less than 2000 years BP. Using the radium method of dating groundwater (Swarzenski and others, 1998), a ²²⁸ Ra/²²⁶Ra activity ratio (0.25) at peat/rock interface in the bottom core 5G (Whipray Basin) indicates a potential young age (<100 Years).

References

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

Cottrell, J.D., 1989, Holocene evolution of the coast and near shore islands, Northeast Florida Bay, Florida, Ph.D. Dissertation, University of Miami, Miami, Florida. 195 p.

Davies, T.J., 1980, Peat formation in Florida Bay and its significance in interpreting the recent vegetation history of the bay area, Ph.D. Dissertation, Penn. State University, College Station, PA.

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, Miami, Florida . pp. 223-231.

Robbin, D.M., 1984, A new Holocene sea-level curve for the upper Florida Keys and Florida Reef Tract, in Gleason, P.J., ed., Environments of south Florida, Present and Past, Miami Geological Society Memoir 2, Miami, Florida pp. 437-459.

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

Stapor, F.W., Mathews, T.M., and Linfors-Kearsn, F.E., 1991, Barrier-Island progradation and Holocene sea-level History in Southwest Florida, Journ. of Coastal Research, V. 7, pp. 815-838.

Swarzenski, P, Holmes, C.W., Shinn, E.A., and Moore, W., 1998, Tracing and mixing of groundwater into Florida Bay with naturally occurring radium isotopes, Unpublished Poster.

Willard, D.A., and Holmes, C.W., 1997, Pollen and Geochronological Data from South Florida: Taylor Creek Site 2, U.S.G.S. Open File Report 97-35.

Wanless, H. Tedesco, L.P., Risi, J.A., Bischof, B.G., and Gelsanliter, S., 1995, The role of storm processes on the growth and evolution of coastal and shallow marine sedimentary environments in South Florida, Pre-Congress Field Trip, The first SEPM Congress on Sedimentary Geology, SEPM, Tulsa, OK. 178 p.

(This abstract was taken from "Programs and Abstracts - 1999 Florida Bay and Adjacent Marine Systems Science Conference". (PDF, 1 MB))

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