28 Quaternary Geology/Geomorphology (Posters)

RECENT EFFECTS OF SEA-LEVEL RISE ON COASTAL WETLANDS IN SOUTHWEST FLORIDA

Paper number: 28-4
Program: Technical Sessions
Time: 1:00 PM – 5:00 PM

Authors

Abstract

Rate of sea-level rise (SLR) has increased abruptly since the industrial revolution. Along the Southwest Florida coast the rate has increased an order of magnitude from values around 6 cm / 100 yrs beginning approximately 3500 ybp to present rates as high as 30 cm / 100 yrs. Southwest Florida's coastal geomorphology is particularly sensitive to SLR because of its low relief and mangrove-dominated vegetation. Fringe mangrove forests have prograded over the last 3500 years to generate our estuarine geomorphology, because rates of sedimentation have exceeded the rate of SLR. Early manifestations of geomorphologic degradation should be evident in light of the recent SLR rate acceleration. This research attempts to recognize early signs of system change by corroborating the following hypotheses: (1) A landward shift of the brackish water ecotone has occurred within coastal wetlands. (2) That transgression occurs as a patchwork mosaic, rather than as a simple linear shift of vegetative habitats. And (3) barrier island salterns, herein defined as tidal pools with only ground water connections to the sea, have recently increased in size. Hypotheses were tested through: standard stratigraphic, sedimentologic, and paleontologic practices; vegetative mapping; and comparison with historic photography. A saltern was studied on Keewaydin Island; tidal ponds and the history of ecotone shift were investigated in the Ten Thousand Islands Wildlife Refuge.

Sediment cores taken from the tidal ponds exhibit a transgression of the brackish ecotone; facies change upsection from marly soils containing freshwater mollusks to estuarine muds. Cores taken across a mangrove forest to tidal-pond transect show a thinning of the buried freshwater prairie as the edge of the pond is approached, indicating the transgression is non-linear. A comparison of the 1940 and 1995 vegetative habitat maps further supports the patchwork transgression and ecotone shift. Sedimentological changes within the saltern cores and floral changes on the surface indicate that this structure has increased in size. The abundance of halophylic plants and signs of halophobic plant decay increase progressively as the interior of the saltern is approached.