EVOLUTION OF BOSTON HARBOR’S EMBAYED SALTMARSHES: A REFINED HOLOCENE SEA-LEVEL CURVE FOR MASSACHUSETTS

Saltmarsh evolution is closely linked to sea-level rise (SLR) and sediment supply and, as SLR accelerates, the persistence of marshes depends on the ability of the platform to grow vertically through organic and inorganic accumulation. In glaciated settings, the formation and maintenance of a saltmarsh is complicated by steep upland boundaries and low sediment availability. In addition, many northern marshes have experienced anthropogenic alteration, such as ditching or tidal restriction. Boston Harbor contains over 30 small islands formed through the flooding and reworking of a drumlin field. Despite minimal sediment input from nearby fluvial systems, several of the islands exhibit small saltmarsh systems. While many studies have considered the development of larger saltmarshes in Massachusetts (e.g. the Great Marsh), little is known about the Harbor’s isolated systems and their evolution throughout the Holocene. A series of auger and vibracores within two embayed saltmarshes (on Thompson and Peddocks Islands) provide a means for reconstructing both long- and short-term geomorphology. Radiocarbon dating of basal peats reveal that these marshes developed 2-4 ka when sea-level rise rates decelerated from ~ 3 mm/yr to 1mm/yr. Using these data, we refine the Massachusetts sea-level curve, identifying a period of sea-level deceleration between 3.3 and 3.8 ka. More recent evolution is examined using vertical accretion rates (Pb²¹⁰) and surface elevation tables, which indicate Boston Harbor saltmarshes are presently keeping pace with sea-level rise (accretion of 3-6 mm/yr). Historically, however, much higher rates of accretion are observed on the Thompson marsh (~12 mm/yr), in response to the reopening of a dike. While numerical modeling suggests that sediment sourced from the nearby eroding drumlins may reach the marsh platforms, drumlin erosion has been reduced in recent years through shoreline protection. We suggest that the internal cannibalization of the marsh platform may explain these higher rates of platform accretion observed, despite the sediment-starved conditions.