LATE HOLOCENE HISTORY OF SANIBEL ISLAND: IMPLICATIONS FOR SOUTHWEST FLORIDA’S COASTAL RESILIENCE IN THE CONTEXT OF CLIMATE CHANGE

Barrier Islands provide ecosystem and societal services to the Florida Gulf Coast by protecting mainland areas against storms and by creating a low energy, brackish water habitat for estuarine ecology. The effects of climate change such as sea-level rise (SLR) and more intense hurricanes are making barrier islands more dynamic and vulnerable. Barrier island development and progradation is sensitive to changes in SLR rate and sediment supply. In this study, Sanibel Island serves as a sentinel for Southwest Florida’s barrier island response to climate change. The timing of the island’s formation, the generation of its back-barrier estuarine paleoecology, and its SLR regime and tempestologic history were investigated. Ten sediment cores were taken in back-barrier settings at multiple locations along the island’s length using hand coring techniques. Standard sedimentologic and stratigraphic methods, and radiocarbon dating were employed to interpret late Holocene history. Additionally, paleotempestites, storm layers, were identified and described to infer the influence of major hurricanes on Sanibel Island. The timing of transition from open shelf to back-barrier mangrove forest (i.e., peats forming on the landward side of the island) is discernable in multiple cores and occurs between 4221-3130 cal yBP (for well-preserved shells from the top of the open shelf facies). Basal mangrove peats, from the back-barrier island facies, date between 2734-2242 cal yBP (for in situ root fibers). Estuarine sandy muds are located above this transition and harbor a brackish water molluscan fauna. Paleotempestites were identified within the subtidal estuarine muds but only in the 6 eastern-most cores; each of these cores reveals at least 1 or 2 intense storm events (category 3 or higher hurricanes). The geochronologic data from the Sanibel cores further inform and are consistent with the regional SL curve. Complementary research investigating the nature of strandplain development utilizing ground penetrating radar and LiDAR topography is underway for Sanibel and neighboring barrier islands. This work will further define late Holocene history. Collectively, these studies will provide insights for coastal resiliency planning.