BACKBARRIER DYNAMICS IN A GLACIATED ENVIRONMENT: THE EVOLUTION OF A SMALL SALTMARSH SYSTEM IN BOSTON HARBOR

Saltmarsh evolution is closely linked to sea-level rise (SLR) and sediment supply, and in a regime of accelerating SLR, their survival depends the ability of the marsh platform to grow vertically through organic and inorganic accumulation. In glaciated settings the formation and maintenance of saltmarsh is complicated due to widespread steep upland boundaries and low sediment availability. Boston Harbor contains over 30 small islands formed through the flooding and gradual reworking of a drumlin field, yet within this system more than six small barrier marsh systems have developed. Thompson Island, situated in the inner harbor, in sheltered waters close to the mainland, possesses the largest saltmarsh system (40 acres) within the Boston Harbor system.

A series of auger and vibracores along with an extensive ground-penetrating radar (GPR) survey of the upland regions provide a means for reconstructing both long- and short-term evolution of the marsh. Pervasive steeply dipping clinoforms in GPR profiles suggest that the marsh developed in a topographic low that formed between prograding delta lobes. Initial radiocarbon dating of basal peats demonstrates that the marsh is more than 3,000 years old and that these data might be used to construct a late-Holocene sea level curve for Boston Harbor. Stratigraphy indicates a change in marsh morphology from high to low marsh during the 20th century, and long-term accretion rates determined by Pb-210 will shed light on the ability of the marsh to maintain elevation with forecasted accelerated SLR. Sediment cores, aerial photographs and GPR data provide evidence of changes in the number and position of channels and tidal inlets linking the marsh to the open harbor. The recent changes in dominant marsh vegetation are likely to be a response to anthropogenic alterations, however it offers an insight into the ability of these small systems to respond to future acceleration in SLR.