Landward migration of barrier island sands under stable sea level conditions; Plum Island, Massachusetts

The origin and development of barrier islands systems has received much attention during the last decade as marine scientists have become more concerned with coastal environments. Plum Island, part of a barrier island system located off the northeastern coast of Massachusetts where sea level is believed to have remained stable for the last three thousand years, was selected to test the hypothesis that dune migration and longshore sediment transport patterns are causing its landward (west) development and migration, rather than the commonly accepted "rising sea level hypothesis." Ninety-four surface sand samples taken from the top 2 to 3 inches (5-7 cm) and 94 subsurface sand samples from a 3-ft depth (1 m) were collected for statistical size analysis along six east-west traverses at one mile intervals perpendicular to the island. Trend surface analysis was then performed with the graphic mean size values from the surface and subsurface samples to determine any consistent stratigraphic and geographic textural shifts between the surfaces. Comparison of the third-order surfaces indicates that the graphic mean grain size generally coarsens from the subsurface to surface. This shift appears to indicate that there is a general landward (west) shift through time in the graphic mean size distributions from the older subsurface to the younger surface. This shift is interpreted to be a direct response of the dune field to the predominant northeasterly storm conditions modified by longshore currents in an area of stable sea level. It is therefore concluded that a rising sea level is not necessary for barrier island migration landward.