THE EFFECT OF SEA LEVEL RISE ON THE MORPHOLOGY OF SOUTH FLORIDA AS MEASURED BY SHORT-LIVED ISOTOPIC CHRONOLOGY

The geography of South Florida is ideally suited to studying morphologic changes due to sea-level fluctuations because the slopes of the seafloor and land surface are low and minor changes in sea level result in large-scale morphological adjustments. Florida Bay is a shallow subtropical bay located between the Everglades and the Florida Keys. The bay is dissected by a labyrinth of carbonate mud banks with lower relief areas between banks termed “lakes”. Hydrologic inputs from both man-made canals and decreased runoff from the Everglades, has modified the northern sections of Florida Bay, but both the northern sections and the entire bay have also made adjustments to sea-level rise. Using short-lived isotopes (⁷Be, ²¹⁰Pb, and ¹³⁷Cs) to constrain temporally the morphological changes, the depositional history was determined throughout Florida Bay These changes were compared to South Florida sea-level rise record. The most substantial change was habitat transformation in the lakes and banks along the northern boundary of the bay. Prior to ~1950, the lake floors were hardbottom habitat and the salinity conditions of the lakes was oligohaline. After 1950, concurrent with a decrease in freshwater flow from the north, the environment changed to mesohaline and has since become normal marine. With this shift, carbonate production increased and enlarged banks, islands, and extended tidal flats. As a result, passes between islands were closed, further restricting circulation. The central bay sediment accumulation record shows this area is more responsive to sea-level rise. Minor variations in the rates of sea-level rise and times of static sea level (as measured at Key West) have resulted in shifts in sediment accumulation rates on the leeward side of mud banks; increasing during rising sea level and decreasing during static periods. In addition, in the early 1970s, a strongly positive North Atlantic Oscillation, a La Niña, and a negative Pacific Decadal Oscillation, drove water northward into Florida Bay and resulted in accelerated erosion and accretion in several areas of Florida Bay.