MORPHOLOGICAL CHANGES WITHIN FLORIDA BAY AS A RESULT OF SEA LEVEL RISE

About 6000 BP, the rate of sea level rise slowed when the waters flooded the shallow shelves surrounding the reef platforms of the western Atlantic. The relative flat South Florida shelf, because of its relative slight tilt to the southwest, is an ideal local to assess the effects of this flooding. A morphological model of bank formation based on the sea level oscillations over the last 6000 years was constructed from analysis of 120 cores throughout Florida Bay.

The most substantial change was recorded in the mud banks, which formed a lattice like pattern dividing that bay into lakes and banks. Some of the changes were influenced by anthropogenic effects over the last half century. Around 1950, the northern fringe of the bay morphed from a fresh water environment to a marine environment. As a result, carbonate production enlarged the mud islands, extending tidal flats and closing passes between many of the islands, restricting circulation.

However, in the central bay, sediment accumulation was not affected by the change in hydrology but was controlled by variations in rate of progressive sea-level rise. The sea-level record as measured at Key West shows that sea level has been rising incrementally over the last century. Between 1931 and 1950, sea level rose at a rate of ~ 5 mm/yr. After 1950, it remained stable until 1971, when it again began to rise, but at a rate of 3 mm/yr. On the leeward side of mud banks, these variations in rate of rise resulted in shifts in sediment-accumulation rates, with accretion increasing during rising sea level and decreasing during stable periods. Between late 1970 and early 1972, a sharp jump in sea-level rise occurred that was approximately 10 cm higher than the preceding period. This jump, last for a period of two years, coincided with a strongly positive North Atlantic Oscillation (NAO), a la Niña (negative ENSO), and a negative Pacific Decadal Oscillation (PDO). Water driven northward into Florida Bay eroded banks along the northern coastline, increased sediment accumulation in the northern lakes, and increased accretion rates on the banks. This paradigm suggests, although anthropogenic hydrologic influences had an effect in northern Florida Bay, climate forcing has played a major role in changing the geographic structure throughout the bay.