Craighead (1964) defines the Buttonwood Ridge or embankment as the greatest inland extension of the marine shoreline. This feature stretches approximately 60 kilometers (km) across the southern tip of Florida, averages 1 km in width, and lies a few centimenters (cm) above the present sea level. It appears to form a barrier between the terrestrial environment (Everglades) to the north and the marine environment (Florida Bay) to the south. This study was undertaken to define the historical environmental changes that have occurred within this critical ecological zone along the south Florida shoreline and to determine the influence of the Buttonwood Ridge in these changes.

Borings and cores through the Buttonwood Ridge along Taylor Creek and across Crocodile Point show that the ridge is dominated by fine carbonate mud inter-layered with 5 to 10-cm peat layers. Along Taylor Creek, a three-core transect revealed a unique developmental stratigraphy. The cores were taken at 10, 500, and 1000 m from the shoreline. The basal sediment in each of the three cores is a cream colored marl overlain by a grey carbonate mud. The faunal record indicates that the basal marl is freshwater in origin. It yields a carbon date of 1920 ± 150 years BP. The basal marl is overlain by a carbonate mud containing a diverse faunal assemblage suggestive of an upper estuarine environment. This zone is overlain by peat, which pollen and petrographic analyses indicate is a mangrove peat. At the site closest to the shoreline, the peat is 100 cm below present sea level but no ¹⁴C age is available from this core. At the center of the ridge, the peat layer is 100 cm below sea level and has a ¹⁴C age of 1430 ± 70 BP. At the inland-most site, the peat layer is 80 to 93 cm below sea level and has a ¹⁴C age of 1750 ± 70 BP.

In the three-core transect normal to the shoreline across Crocodile Point, the two cores closest to Florida Bay, one 10 m from the shoreline and the other 50 m from the shoreline, contain two distinct peat layers. One peat layer is 30 to 40 cm below sea level and the other is 140 to 150 cm below sea level. Below the lower peat layer is a thin marl with a basal organic peat. The carbon dates are 280 ± 70 BP for the upper peat layer and 370 ± 80 BP for the lower peat layer. Both peats are much younger than the peat in the Taylor Creek transect. Sediment between these peat layers is a shelly marl and differs from the fine carbonate mud found along the Taylor Creek transect. Both peat layers across Crocodile Point have a very heavy ¹³C signature ( -24 ‰), as compared to -27 ‰ for the mangrove peat in the Taylor Creek transect. This heavy signature is indicative of a subaqueous plant such as sea grass. The third core, 100 m from the Florida Bay shoreline, is similar to the Taylor Creek cores. The stratigraphic relationship at Crocodile Point indicates that a significant amount of material recently was accreted onto an older ridge in this area. The geometry of this feature may be a key to the process of bank formation and could explain the difference in carbon ages between the peats along the Taylor Creek traverse.

In addition to the two vibracore traverses, four cores in the marsh landward of Buttonwood Ridge were analyzed. Sediment in the lower sections of the two inland marsh cores nearest Florida Bay is dominated by saw grass pollen, whereas sediment in the upper parts of these cores is dominated by mangrove pollen. On the marine to non-marine traverse up Taylor Creek into the saw grass plain, ²¹⁰Pb profiles demonstrate two episodes of sediment accumulation. Based upon ²¹⁰Pb "dating," the transition from saw grass to mangrove began around 1950. Comparison of areal photographs of the area in 1940, 1950, 1970 and 1990, confirms that the inland marsh site nearest Florida Bay changed from a lake to a mangrove swamp around 1950. This transition suggests an incursion of seawater landward and/or a reduction in freshwater input into the lake over the past 50 years.

Offshore, the "dating" of cores situated at the mouths of Little Madeira Bay and Joe Bay, and on Pass Key Bank, indicate an increase in sediment accumulation over the past few decades. At Pass Key, it appears that the banks have closed up, which could have an affect on circulation in this portion of Florida Bay. The rapid sediment accumulation at the mouth of the fringing bays is most likely the result of decreased circulation in the bays and/or a decrease in freshwater input from the mainland.

REFERENCE

Craighead, F.C., 1964, Land, mangroves, and hurricanes: Bulletin of the Fairchild Tropical Garden, v. 19, p. 5-32.

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