The interactions of vermetid gastropod reef development and sea-level fluctuations on late Holocene coastal geomorphology and environmental evolution of the Ten Thousand Islands, Florida

The carbonate-producing coastline of Southwest Florida attributes its geomorphology to oyster and vermetid reef construction, which in turn becomes substrate for mangrove forest progradation. Understanding the unique development process of the mangrove-forested islands is crucial to the management of this ecosystem. This study aims to understand the constraints placed on Holocene vermetid reef development by the physical environment and the living ecology. The historic distribution of oyster reefs landward and vermetid reefs seaward may be a result of a well-defined salinity gradient. Reef history extends 3,850 yBP into the Holocene, providing potential for sea-level interpretations. Twelve sediment cores were taken with this study. Altogether, they illustrate a transgressive/regressive sequence. Sedimentology and elevation data suggest vermetid reef growth closely mimics the patterns of mid- to late-Holocene sea-level rise and seaward progradation of the shoreline. Vermetus nigricans is the vermetid gastropod responsible for this wave-resistant framework, which often includes incorporation of Crassostrea virginica and barnacles. A geomorphologic study shows that relict vermetid reefs play a unique role in shaping the mangrove islands when compared to the same function of relict oyster reefs as frame-builders. Morphologies of vermetid reef construction through time and space vary as a response to environmental stimuli. Salinity and sedimentology are significant controls on distinct changes in vermetid reef morphology. V. nigricans is a low-intertidal sea-level indicator, and radiocarbon dating of the vermetids reveals no evidence of a local sea-level highstand. However, evidence is presented for an unsmooth sea-level curve, with a possible drawdown event occurring at approximately 2,500 yBP.