EVIDENCE FOR RAPID COASTAL CHANGES DURING THE HOLOCENE: THE ROLE OF CLIMATE, SEA LEVEL AND THRESHOLDS

The Outer Banks barrier islands and adjacent estuarine system of North Carolina contain a geologic record which demonstrates that rapid coastal reorganization has occurred in the past. Rapid reorganization is likely the result of a combination of changing sea level, variations in storm patterns, and threshold-crossing accompanied by positive feedbacks. Time slice reconstructions of coastal environments, based upon paleontological, sedimentological, geophysical, geomorphic and geochronologic data reveal at least three episodes of coastal reorganization along the NC coast during the last 4000 years, which were marked by significant changes in barrier continuity and dynamics. These episodes occurred at ca. 4000-3000 yBP, 1100 yBP (the Medieval Warm Period - MWP) and ca. 500 yBP (the Little Ice Age - LIA). Geophysical and geomorphic data indicate shoreface ravinement several kilometers landward of the modern shoreline between 4-3 ka. Accumulation rates within the estuaries increase by a factor of four to six at this time, suggesting a rapid sea-level rise, or a greatly modified tidal regime. Regressive systems above the shoreface ravinement surface suggest an ensuing sea-level fall, or a major increase in sediment flux. Comparison to climate proxy data indicates that this event occurred during a period of rapid climate change and a possible sea-level oscillation. Barrier and estuarine facies associated with the MWP and the LIA have been imaged using geophysical techniques, and cored and dated using radiocarbon and optically stimulated luminescence techniques (OSL). OSL data indicate the occurrence of numerous inlets and wide-spread flood-tide delta formation during both time intervals, suggesting elevated storm conditions (relative to today). Comparison to proxy climate and storm data suggests that inlet activity during the MWP responded to intensified hurricane impacts, while elevated inlet activity during the LIA was likely in response to increased subtropical or extratropical storm activity. A general decrease in storminess at mid-latitudes in the North Atlantic over the last 300 years has allowed the system to evolve into a more continuous barrier with few inlets; however, projections of sea-level change and hurricane impacts suggest that the current geomorphic condition is likely to change rapidly.