The geologic record of Hurricane Irma in a Southwest Florida back-barrier lagoon

Abstract

On 10 September 2017, Category 3 Hurricane Irma made landfall along the Southwest Florida coastline between Cape Sable and Cape Romano. Geologic evidence of this storm is preserved in a back-barrier lagoon behind the Big Hickory Barrier Island, which is located ~64 km north of the landfall point and is positioned 43–65 m east of the Gulf of Mexico. Modern dune height is ~0.83–0.88 m, which was exceeded by the storm surge (recorded height 0.9–1.5 m) allowing for sediment deposition in the Big Hickory Island Lagoon. Geologic evidence is likely found at this location due to proximity to the Gulf of Mexico and the shallow barrier itself. Three cores were analyzed for moisture, inorganic content, grain size, and foraminiferal assemblages. The presence of a hurricane signature (tempestite) is evident in the uppermost horizon of all the cores and includes a fining upward trend 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 preferential back-barrier deposition behind narrow barrier sections. Several different foraminiferal species within the tempestite sediments corroborate a marine sediment origin. In addition, historic satellite imagery shows that the Big Hickory Barrier Island is very susceptible to geomorphological change through time, especially due to storm impacts. This research demonstrates the utility of back-barrier sediment cores in understanding hurricane history and barrier island vulnerability.