GRAIN SIZE AND ELEMENTAL ANALYSIS OF HURRICANE IRMA SEDIMENT DEPOSITS ON FLORIDA BAY ISLANDS, EVERGLADES NATIONAL PARK, USA

As part of a project investigating Holocene history of Florida Bay islands, we are examining the role that storms play in island development. Resource managers are concerned about island resiliency because the islands serve as habitat and nursery sites for many species and as a buffer from storm surges to mainland Florida. On September 10, 2017 Hurricane Irma made landfall in the lower Florida Keys approximately 88 km southwest from the center of Florida Bay as a category 4 storm. Fieldwork was conducted in January 2018 on four previously sampled (2014) Florida Bay islands in order to capture Hurricane Irma’s storm signature. Short cores (50 cm) and surface sediment overwash deposits were collected from all four islands for analyses. We collected high resolution images, magnetic susceptibility, color spectrophotometry, and x-ray fluorescence (XRF) data from the sediment cores and overwash samples before subsampling at 1 cm intervals for laser particle grain size analysis, mollusks and other faunal and floral remains, and other proxies. Here we focus on XRF and grain size data in order to examine the sedimentary and geochemical signatures of the storm. Of the islands sampled, Jim Foot Key is the western-most and was closest to the eye of Hurricane Irma, therefore, experiencing the highest winds and storm surge. Sediment overwash deposits from Hurricane Irma on the southeast corner of Jim Foot Key exhibited the following general structure: 0.5-2 cm algal crust and matted leaf and seagrass layer overtop what appears to be a single event storm deposit measuring 5.0-6.5 cm in thickness with another thin algal layer and estuarine mud underneath. Within the storm deposit layer, finer grained sediment appears to be overlying coarser sediment that potentially settled out as water drained from the area after the storm surge. Identifying storm signatures from grain size analysis and elemental composition in known hurricane deposits will aid in our ability to recognize storm deposits in the sediment record of these islands. Storm history and island development are key to understanding the resiliency of these islands considering future predicted sea-level rise and increased storm frequency and/or intensity scenarios over the next century.