TRANSITIONING FROM A REGRESSIVE TO TRANSGRESSIVE BARRIER: DISCUSSING THRESHOLD RESPONSE AND THE MECHANISMS OF CHANGE

In response to sea-level rise and storms, back-barrier shorelines of all barrier types erode, and without wash-over to replenish losses the island will narrow. The most seaward dune ridge on a regressive barrier is typically the highest in elevation and may prevent overwash for millennia, contributing to high rates of erosion along the back-barrier shoreline. Upon continued narrowing, regressive barriers may reach a critical width when wash-over occurs, causing the island to become transgressive. This conceptual model suggests that barrier transition from regression to transgression may have a threshold response to sediment supply, climate variation, underlying geologic framework, rate of sea-level rise, and anthropogenic influences.

Bogue Banks, North Carolina, is a previously regressive barrier with distinct beach-ridge plains preserved at both ends of the island. The central portion of the island is narrow and vibracores taken along the back-barrier show shoreface sand underlying lagoonal mud, separated by a bay-ravinement surface. This indicates preferential removal and reworking of sediment in the central portion of the back-barrier. Samples for optically stimulated luminescence dating and radiocarbon dating were taken to constrain the timing of barrier development and subsequent erosion of the central segment, respectively.

Bogue Banks was regressive from ~3000 cal yr. BP until ~1100 cal yr. BP. Rapid erosion of the back-barrier shoreline initiated ~1100 cal yr. BP, causing the island to narrow. This erosion was likely the result of higher storm frequency along the Atlantic coast during the Medieval Climate Anomaly. Relict inlet channels were discovered along the central portion of the island through a combination of high-resolution ‘chirp’ seismic data, ground penetrating radar, and historical maps. The channels are thought to have formed within the last 250 years, closing shortly after formation. These breaches along the central portion of the barrier indicate that Bogue Banks may be in transition from a regressive to transgressive barrier. This research indicates that large changes in barrier morphology occurred over a period when the rate of sea-level rise was relatively low, suggesting importance of sediment supply, climate change and geologic framework on barrier dynamics.