THE DEVELOPMENT OF ESTUARINE SYSTEMS IN SOUTHWEST FLORIDA: A PERSPECTIVE FROM THE LATE HOLOCENE HISTORY OF OYSTER REEF DEVELOPMENT

The coastal geomorphology of Southwest Florida is driven largely by reef-building vermetiform gastropods (Vermetus nigricans and Vermicularia knorri) and American oysters (Crassostrea virginica), all of which provide a foundation for the estuarine islands along the coast, and which rely on relatively slow rates of sea-level rise (SLR). This study investigates the timing of reef formation and the alteration of paleoenvironmental conditions by considering stratigraphic, paleontologic, taphonomic, and isotopic evidence from late Holocene oyster and vermetiform gastropod reefs in Estero Bay and the Ten Thousand Islands.

Two sediment packages were described: (1) a transgressive systems tract consisting of a transgressive basal peat dated at ~ 6,000 ybp, to a subtidal marine facies; (2) a regressive highstand systems tract progressing from a vermetiform gastropod reef facies at ~ 2700 ybp, to an oyster-dominated reef facies at ~ 500 ybp. The shift in dominant reef-building organisms from stenohaline gastropods to euryhaline oysters was supported by a concomitant shift in the associated molluscan fauna. A trend toward more brackish water conditions over the period of reef development was further demonstrated by a shift to more negative mean δ¹³C values up-core for oysters from both locations and more negative mean δ¹⁸O values up-core in Estero Bay. Increased seasonal variation was also apparent for Estero Bay. Sedimentary evidence for Estero Bay illustrates a trend toward fining upward grain size and poorer sorting demonstrating that reef development was coupled with a transition to more variable energy conditions. Results suggest that autogenic succession occurred within the reefs from both locations. However, radiocarbon dates acquired from a barrier island bordering Estero Bay to the west suggest that these structures likely played a significant role in the transition to estuarine conditions at this location, providing an allogenic mechanism for environmental change. This combination of approaches provides compelling evidence that reef builders are critical players in the successful development of coastal estuarine environments in Southwest Florida where stabilization in the face of accelerated rates SLR is crucial.