A GEOCHEMICAL APPROACH TO DETERMINING VARIATIONS IN LONG-TERM SEDIMENT SOURCE AND SUPPLY TO A BARRIER ISLAND SYSTEM IN NORTHERN MASSACHUSETTS

Climate change, both natural and anthropogenically induced, is a significant driver of morphologic change along coasts throughout the world. Human efforts to stabilize retreating shorelines in the face of sea-level rise can further alter and amplify sediment mass balances. The paraglacial Plum Island / Great Marsh barrier system in northern Massachusetts, has undergone both millennial- and centennial-scale changes in sediment supply over the past 8 000 years. The Merrimack River and nearshore late Pleistocene and early Holocene shelf deposits together serve as sediment sources to the barrier system, allowing it to maintain both long- (at least 3000 years) and short-term (decadal- to centennial-scale) stability, despite recent increases in the rate of relative sea-level rise. This study presents a preliminary 8000-year-long record of sediment supply to the Island through geochemical analysis of cores extracted from the interior of the barrier. This record encompasses both millennial- (natural climatic) and centennial-scale (anthropogenic) impacts on sediment source, supply, and overall dynamicity of the barrier system. A set of 10 direct-push sediment cores were collected along a ~ 13 km-long transect along the landward-side of Plum Island, with the deepest capturing a ca. 18-m thick sequence of backbarrier sediments and penetrating into underlying Pleistocene glaciomarine clay. Lateral correlations between lithologically-similar sediment layers observed in cores across the island indicate likely broad-scale (2-3 km) changes in sediment supply to the island through time. Bulk organic characteristics and stable isotopic compositions of sediments from within backbarrier (estuarine) deposits across the island and through time (down core) are used to differentiate changes in the relative contributions of sediment from each terrestrial (Merrimack River) and marine (offshore, shelf) sources. This approach provides insight into the spatial and temporal variability in sediment sources across the barrier system, with the ultimate goal of attaining a better understanding of potential sediment sources and supply rates to barrier-backbarrier systems in the face of climate change and accelerated sea-level rise.