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Stratigraphic Record of Washover Deposition Shows Rapid Response of Barrier Islands to Sea-Level Rise


Short-term flooding of a barrier island from increased ocean-water levels (overwash) transports sand from the beach across the barrier where it is deposited as washover terraces and fans. Washover deposits are commonly interpreted as event beds, representing discrete storm events; however, here we show that washover deposition can be a continual process, especially when initiated across narrow transgressive barrier islands. In addition, washover deposition at the back-barrier shoreline is the primary mechanism for landward island migration with sea-level rise and washover deposition can be used as a proxy for island migration. At Onslow Beach, a transgressive barrier island, a recent washover deposit formed as an amalgamation of four depositional events, each recognized as a fining-upward bed with sandy gravel at the base and medium fine-grained sand with heavy-mineral laminae at the top. The beds formed during major storms that impacted the island since AD 1996, resulting in a large washover above carbonaceous back-barrier marsh sediment. In AD 2011, Hurricane Irene overwashed the island and formed a new washover terrace. Over the subsequent four-year period, we recorded 86 days when the washover fan experienced overwash. Most of the overwash events resulted from spring tides and small-scale sea-level anomalies imposed on low-elevation island topography. Based on 17 high-resolution DEMs, the washover terrace increased an order of magnitude in volume and areal extent to 24,100 m3 and 37,300 m2, respectively, as back-barrier accommodation was filled. When compared to the geologic record of washover deposition on the island, the size and frequency of recent washovers is unprecedented. The oldest washover deposit preserved is approximately AD 722 and during the 19th century, the number and landward extent of washover deposits increased abruptly, concurrently with a three-fold increase in the rate of sea-level rise. The change in washover deposition was likely caused by an increase in the rate of island transgression associated with island narrowing through landward movement of the ocean shoreline and decreased elevation of the island through dune erosion. These data suggest that transgressive barriers are extremely sensitive to increases in the rate of sea-level rise and respond suddenly by increasing their rate of transgression and deposition of washover, a depositional environment that does not necessarily form during a single storm event.