DEPOSITIONAL RECORD OF WASHOVER REVEALS RAPID RESPONSE OF A TRANSGRESSIVE BARRIER TO ACCELERATED SEA-LEVEL RISE

An increase in the rate of sea-level rise will increase the rate of beach erosion along many barrier islands, but few field studies have resolved the magnitude of geomorphologic change and response time to past accelerations in sea-level rise. The evolution of Onslow Beach, NC at millennial to decadal time scales, including the late 19^th century increase in the rate of relative sea-level rise, is presented here. Onslow Beach is a sediment-starved transgressive barrier island located along a cuspate shoreline. We sampled and radiocarbon dated multiple washover fans along the island as fining-upward beds of gravelly coarse sand to medium-grained sand with heavy-mineral laminae overlaying carbonaceous sand and mud, interpreted as back-barrier saltmarsh. Those data show that the oldest washover fan preserved in the stratigraphy of the island is approximately AD 722 and at that time the island was seaward of its present location with an open-water lagoon separating it from the mainland. Barrier-island transgression progressed mainly through overwash processes and back-barrier saltmarsh replaced the lagoon by AD 1500. During the 19^th century, the number and landward extent of washover fans increased abruptly along the island concurrently with a three-fold increase in the rate of relative sea-level rise. This was not a period of increased storminess, rather, the increase in number and landward extent of washover fans is interpreted to be the result of an increase in the rate of island transgression. The increase in the rate of relative sea-level rise likely narrowed the island through landward movement of the shoreline and lowered the elevation of the island principally through erosion of the dunes, making the island more vulnerable to overwash. These data suggest that transgressive barriers, especially those at the center of a coastal embayment, are extremely sensitive to increases in the rate of sea-level rise, which is manifested as an immediate decrease in resistance to overwash.