Transition of a regressive to a transgressive barrier island as a function of back-barrier erosion, climate change, and low sediment supply, Bogue Banks, North Carolina, USA

Although back-barrier erosion is a prevalent process of island narrowing, it is often overlooked in barrier-island evolution modeling. In many wave-dominated barrier island settings, the absence of overwash precludes the expansion of the back-barrier and hence the island as a whole from sustaining its width. Typically, regressive barriers are wide and exhibit high elevations with the most seaward dune ridge possessing the highest elevation. This morphology may prevent overwash from reaching the back-barrier shoreline for millennia and hence contribute to high rates of back-barrier erosion. Upon continued narrowing and lowering of the island, regressive barriers may reach a critical state, making overwash imminent and transitioning the island to a transgressive state. The modern day morphologic variability along the 40-km long island of Bogue Banks, North Carolina includes both regressive and transgressive segments, making this setting ideal for examining whether or not the transition between these barrier island types is gradual or threshold-driven. Bogue Banks consists of two discrete compartments characterized by high-elevation beach ridges, large island widths, and stratigraphy consistent with regressive barrier islands. These regressive island segments are separated by a broad, narrow section of the island devoid of any washover fans or other transgressive elements. The analyses of seismic data from the inner continental shelf reveal paleo-channels intersect the wider sections of the island, while the narrow central part of the island occupies an inter-fluvial area. Reworking of fluvial sediment from paleo-channels was an important sediment source for the barrier during regression. Optically Stimulated Luminescence (OSL) dates from the most landward beach ridges constrained initiation of island regression at ~3000 cal yr. BP as the rate of relative sea-level rise slowed to ~0.8 mm/yr. Transects of cores, seismic data, ground-penetrating radar data, and radiocarbon and OSL dates show that prior to ~1500 cal yr. BP the central narrow section of the island was wide and regressive similar to adjacent areas. Back-barrier erosion of the central part of the barrier primarily caused island narrowing as a result of increased storminess, which occurred around the Medieval Warm Period (~1100 cal yr. BP). This part of the island was more vulnerable to erosion than adjacent areas due to increased bay ravinement (Bogue Sound is widest there) and its lower elevation (further away from paleo-channel/sediment source). Relict inlet channels exist along the central portion of the island, formed within the last 250 years, and likely closed shortly after formation. The presence of historical inlets along the narrow central section of the island indicates Bogue Banks may be nearing a critical width threshold and will subsequently transition to a transgressive barrier. Since the change in barrier morphology associated with back-barrier erosion occurred over a period of time when the rate of sea-level rise was relatively low, low sediment supply and climate change (resulting in increased storm frequency) are the main forcing mechanisms of island narrowing. These impacts, in addition to a predicted increase in sea-level rise rates and human modifications (e.g. maintenance of a high-elevation fore-dune, closing of inlets that artificially prevent island overwash and associated sediment supply to the back-barrier shoreline) will likely promote rapid transition of regressive barrier islands to those dominated by transgressive processes