QUANTIFYING THE SLOPE OF ANTECEDENT TOPOGRAPHY UNDERLYING NJ AND MD BARRIER ISLANDS

The slope and composition of the antecedent substrate surface upon which barrier islands migrate play central roles in long-term island response to storms and sea-level rise. For example, barrier islands will tend to migrate landward more rapidly over shallower topography. Additionally, the erodibility of underlying sediments can affect shoreface erosion rates and influence barrier-island composition. Here, textural analysis of thirteen sediment cores (6 – 25 m) from the barrier-backbarrier systems of Brigantine (NJ, USA) and Assateague (MD, USA) islands is used to quantify the antecedent slopes underlying, and document potential substrate sediment sources to, these islands. At the mixed-energy Brigantine Island, the slope of the pre-Holocene sedimentary surface is steep (8 m / 1 km; 0.8% grade) proximal to the mainland, extending to a depth of 18 m under the modern barrier island ca. 10 km offshore. This is in direct contrast to the wave-dominated Assateague Island, where the pre-Holocene slope is shallower (4 m / 1 km; 0.4% grade), and the island is located nearer to the mainland (ca. 8.5 km). The Pleistocene substrate at Brigantine Island ranges from a stiff blue-green clay, to interbedded clay and fine sand, to coarse sand and pebbles, with most units capped by organic-rich marsh deposited during the middle to late Holocene transgression. At Assateague, the antecedent substrate primarily consists of muddy, fine to coarse sand. The subaerially exposed “South Point Neck”, located 3.5 km landward of the modern Assateague shoreline, may function as a future anchoring or stabilization point for the landward-migrating island, much in the same manner that a similar sandy Pleistocene ridge “pinned” several of the Virginia barrier islands ca. 1000 years ago, initiating a period of stability and growth during the late Holocene. Together, these data demonstrate that antecedent substrate may control backbarrier accommodation during landward island migration, and act as a sediment source during periods of island breaching and inlet formation. Though sea-level rise and storm surge are primary drivers which shape and modify barrier islands, antecedent substrate slope and texture provide additional controls on migration rates and patterns, and thus future island stability.