RAPID: Collaborative Research: Storm Surge Deposition on Salt Marshes: Impacts of Hurricane Irma from Florida to South Carolina

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Scientists continue to weigh the net impact of hurricanes on wetlands. These events may be considered beneficial because of net sedimentation that occurs during surge inundation, or detrimental due to erosion of the marsh edge and surface excavation by storm waves. This study will exploit the new sedimentologic record left by Hurricane Irma to examine these paradigms in the salt marshes of the South Atlantic Bight. Sites range from Florida to South Carolina along a latitudinal gradient of increasing spring tidal range (1.7-2.4 m), backbarrier marsh width (5?7 km), and flow-weighted average suspended sediment concentrations in nearby rivers (13?27 mg/l). They exhibit variations in physical characteristics, proximity to fluvial source, and sediment availability that will help constrain the relative role of these factors in contributing sediment to marshes during storms. Understanding marsh sedimentation is of interest to all stewards of low-lying vegetated coastal zones. Quantifying how coastal wetlands accrete in response to relative sea-level rise, and the contributions that storms may play, is paramount to modeling future wetland evolution and long-term resilience to storm impacts. This study has broad societal implications given the recognized importance of the ecosystem services provided by salt marshes, including reducing storm surge height and related socioeconomic costs, and providing nurseries for fish stocks. While Hurricane Irma tracked over the west coast of Florida, the wind patterns and size of the storm created strong onshore winds along the southeast coast, producing a large stormsurge up into South Carolina. Tide gauge data indicate that marshes from northern FL to central SC were flooded to a depth of 1 m, and remained so for ~7 to 17 hrs. Associated storm surge deposits are likely to be significant; their source may be local (marshes or lagoon floor) or from the inner shelf. The project will investigate the nature (lateral extent, thickness, and provenance) of Irma storm deposits (compared to ?background? sedimentation) through the analysis of (1) shallow trenches (~20 cm); (2) surface samples (of identified Irma deposits); and (3) double short cores (each 50-cm long, 8 cm in diameter) collected along two transects at each site from the backbarrier to mainland (5?6 cores/transect; ~45 sampling stations total). We will also map new erosional features, wrack lines, and vegetation damage or burial, related to Irma. Two cores from each transect containing a clear, representative Irma deposit will be sampled for short-lived radioisotope (210Pb, 137Cs) to provide multi-decadal accretion rates for comparison with event-sedimentation thicknesses. Surface (Irma) deposits and ?background? samples from depth in the two accretion cores, will also be used to examine event-sediment provenance signatures through analysis of bulk biogeochemical characteristics (%TOC, C:N, ä13CTOC, ä15NTN), and assemblages of diatoms and organic biomarkers adsorbed to/co-deposited with these sediments to determine source differences (marine, marsh/lagoon, fluvial) between storm and non-storm periods.