LAUSR

Abstract Barrier islands and their associated backbarrier ecosystems front much of the U.S. Atlantic and Gulf coasts, yet threshold conditions associated with their relative stability (i.e., state changes between progradation, erosion, and landward migration) in the face of sea-level rise remain poorly understood. The barrier islands along Virginia's Eastern Shore are among the largest undeveloped barrier systems in the U.S., providing an ideal natural laboratory to explore the sensitivity of barrier islands to environmental change. Details about the developmental history of Parramore Island, one of the longest (12 km) and widest (1.0–1.9 km) of these islands, provide insight into the timescales and processes of barrier-island formation and evolution along this mixed-energy coast. Synthesis of new stratigraphic (vibra-, auger, and direct-push cores), geospatial (historical maps, aerial imagery, t-sheets, LiDAR), and chronologic (optically stimulated luminescence, radiocarbon) analyses reveals that Parramore has alternated between periods of landward migration/erosion and seaward progradation during the past several thousand years. New chronology from backbarrier and barrier-island facies reveals that Parramore Island has existed in some form for nearly 5000 years. Following a period of rapid overwash-driven retrogradation, and coinciding with a period of slow relative sea-level rise (~1 mm/ yr), Parramore stabilized ~1000 years ago in partial response to pinning by and sediment delivery from erosion of a Pleistocene-aged antecedent high. Following pinning, Parramore built seaward through development of successive progradational beach and dune ridges. Morphological and historical evidence suggests that these processes were interrupted by inlet formation—possibly associated with an interval of enhanced storminess—at least three times during this period. Following inlet closure in the early 1800s, island progradation was rapid, with Parramore Island reaching its maximum width ca. 150 years ago. It has since switched states again, undergoing accelerating erosion (~12 m/ yr since 1980). The relative youth of Parramore Island is in contrast to many East Coast barrier islands, which generally reached their present positions about 3500–2000 years ago. Moreover, these results demonstrate that the apparent robustness and stability of Parramore are ephemeral features of an island that has undergone multiple state changes within the last 1000 years. Finally, they refine current knowledge of the roles of antecedent topography, sediment delivery rates, storms, and sea-level rise in barrier-island stability and resilience to future climate change.