DECODING THE RECORD OF SEDIMENT AVAILABILITY FROM BARRIER ISLAND GEOMORPHOLOGY AT FIRE ISLAND, NEW YORK

Modern coastal barrier systems are experiencing historically unprecedented changes in sediment availability and rates of sea-level rise due to human interventions. To predict their future morphological development, it is necessary first to understand how these barrier systems have responded to past changes in climatic and geologic forcing, as well as consider the effects of inherited morphology on system evolution. In this investigation, we combine observational, field, and lab analyses to infer the centennial-scale evolution of Fire Island, the largest of the barrier islands fronting the Atlantic coast of Long Island, New York. Specifically, we reconstruct past changes in the landscape of the island using historical shorelines, lidar-derived elevation models, ground-penetrating radar scans, and optically-stimulated luminescence age-controlled sediment cores. Our results demonstrate that Fire Island has undergone major shifts in morphologic state over the last millennium, with the modern system partly comprising an amalgamation of at least three rotational barrier remnants. These rotational remnants have left successions of relict dune ridges behind the modern foredune, and in many places restrict the flow of overwash sediments to the backbarrier. As a result, Fire Island has undergone narrowing from the lagoon side as fringing marshes and barrier flats drown and erode, rendering the system vulnerable to future retrogradation. Similar morphologies and associated histories of long-term morphologic change can be found at other locations along the U.S. Atlantic and Gulf coasts, which suggests this vulnerability is intrinsic to many modern barriers. Therefore, we hypothesize that the future evolution of these barrier systems will be strongly dependent on inherited morphology, in some cases setting up the conditions for rapid morphologic state shift triggered by enhanced sea-level rise, storm impacts, and anthropogenic modification of sediment supply.