Abrupt Increase in Washover Deposition Along a Transgressive Barrier Island During the Late Nineteenth Century Acceleration in Sea-Level Rise

Abstract

Determining the magnitude of barrier-island geomorphologic change and response time to an increase in the rate of sea-level rise is important because sea-level rise is accelerating, many barriers islands are urban centers, and barriers buffer mainland shorelines and estuaries from ocean processes, especially during storms. Here, we show that Onslow Beach, NC, a sediment-starved transgressive barrier island located along a cuspate shoreline, had an immediate increase in apparent frequency and landward extent of washover deposition during the late nineteenth century increase in the rate of relative sea-level rise. The evolution of the barrier at millennial to decadal time scales was reconstructed from sediment cores, radiocarbon dates, and remote sensing. Those data show that the oldest washover deposit preserved in the stratigraphy of the island is approximately AD 722 and at that time the island was seaward of its present location and an open-water lagoon separated it from the mainland. Barrier-island transgression progressed mainly through overwash processes and saltmarsh replaced the lagoon by AD 1500. During the nineteenth century, the number and landward extent of washover deposits increased abruptly along the island concurrently with a threefold increase in the rate of relative sea-level rise. This was not a period of increased storminess in the Atlantic. Rather, the increase in number and landward extent of washover deposits is interpreted to have been caused by an increase in the rate of island transgression. The increase in the rate of relative sea-level rise, and possibly other contemporaneous mechanisms such as changes in wave climate, likely caused island narrowing through landward movement of the shoreline, lowered the elevation of the island principally through erosion of the dunes and made 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 water level, which cause an abrupt decrease in resistance to overwash. The response of other barrier islands to accelerating sea-level rise may well be similarly rapid, but the degree of geomorphic change will vary depending on island morphology, rates of sediment supply, and physical processes.