EVIDENCE OF HURRICANE IRMA IN A SOUTHWEST FLORIDA BACK-BARRIER LAGOONAL SITE

On September 10, 2017 Category 3 Hurricane Irma made landfall along the Southwest Florida coastline between Cape Sable and Cape Romano. Geologic evidence of this storm is apparent in a back-barrier lagoon behind the Big Hickory Barrier Island, which is located approximately 64 km north of Irma’s landfall point and is positioned 43-65 m east of the Gulf of Mexico. Beach profiling indicates a modern dune height of approximately 0.83-0.88 m. Irma’s storm surge was recorded between 0.9-1.5 m at several nearby locations, leading to inundation over the barrier island dune and subsequent sediment deposition in the Big Hickory Island Lagoon. Geologic evidence is likely only found at this location due to the site’s proximity to the Gulf of Mexico and the shallow barrier itself. Several cores were taken and analyzed for percent moisture, percent inorganics, grain size, and foraminiferal assemblages. The presence of a hurricane signature or “tempestite” is evident in the uppermost sediments of each core and includes a fining upward signature of medium sand to clay against a background of organics and fine-grained sediments. Tempestite layers were thicker behind narrower sections of the beach, indicating easier back-barrier deposition behind narrow beach sections. Several different foraminiferal species within the tempestite sediments further indicate a marine sediment origin. In addition to the core results, historic satellite imagery demonstrates that the Big Hickory Island Barrier is very susceptible to geomorphological change through time, especially due to storm impacts. This research demonstrates the usefulness of back-barrier coring in understanding hurricane history and barrier island vulnerability.