The characteristics and preservation potential of Hurricane Irma's overwash deposit in southern Florida, USA

Overwash deposits from tropical cyclone-induced storm surges are commonly used as modern analogues for paleo-storm studies. However, the evolution of these deposits between their time of deposition and their incorporation into the geologic record is poorly understood. To understand how the characteristics of an overwash deposit can change over time, we analyzed overwash deposits from four mangrove islands in southern Florida two to three months and twenty-two months after Hurricane Irma's landfall in the region on 10 September 2017. We analyzed the stratigraphy, mean grain size, organic and carbonate contents, stable carbon isotopic signatures, and microfossil (foraminifera and diatom) assemblages of pre-Irma and Irma overwash sediments. Hurricane Irma's storm surge deposited light gray carbonate muds and sands up to 11 cm thick over red organic-rich mangrove peats throughout mangrove islands in southern Florida. Stratigraphy, grain size, loss-on-ignition, and foraminifera analyses provided the strongest evidence for differentiating Irma's overwash deposit from underlying mangrove peats and, if preserved, are expected to identify Hurricane Irma's overwash event within the geologic record. Mean grain size showed the overwash deposit (5.0 ± 0.8 ɸ) was coarser than underlying mangrove peats (6.7 ± 0.7 ɸ), and loss-on-ignition showed the overwash deposit had a lower organic content (19.8 ± 9.1%) and a higher carbonate content (67.8 ± 20.7%) than the underlying peats (59.4 ± 14.6% and 33.7 ± 11.0%, respectively). The overwash deposit was dominated by a diverse, abundant assemblage of sub-tidal benthic calcareous foraminifera compared to a uniform, sparse assemblage of agglutinated foraminifera in the pre-Irma mangrove peats. Geochemical indicators were not able to provide evidence of an overwash event by differentiating organic δ13C or C/N of the overwash deposit from those of the mangrove peats. The complex relationship between diatoms and local environmental factors prevented diatom assemblages from providing a statistically clear distinction between Irma's overwash sediments and underlying mangrove peats. By visiting Hurricane Irma's overwash deposit immediately following landfall and nearly two years post-storm, we were able to document how the overwash deposit's characteristics changed over time. Continued monitoring on the scale of five to ten years would provide further insights into the preservation of overwash deposits for paleo-storm studies.