COUPLED BARRIER-BACKBARRIER DYNAMICS AND THE FORMATION AND STABILITY OF BARRIER-ISLAND SYSTEMS

Backbarrier marshes and tidal flats host diverse organisms, provide ecosystem services, and lessen storm impacts on mainland communities. Additionally, through complex inlet and barrier dynamics, backbarriers affect the stability of fronting barrier islands over decadal to centennial timescales. Backbarrier systems that have restricted inorganic sediment inputs are highly vulnerable to conversion to intertidal or open-water areas because they must rely solely on organic production to keep pace with sea-level rise. A loss in wetlands will increase tidal prism, leading to enlarged inlet systems and sand sequestered in offshore reservoirs. This will in turn lead to secondary impacts including shortening and narrowing of adjacent barriers, making them more susceptible to erosion and overwash. These coupled barrier-backbarrier dynamics are investigated in this study through the development of chronostratigraphic reconstructions of the backbarrier of Plum Island, MA. Geophysical, sedimentologic and geochronologic data indicate that the island formed in its modern location by ca. 4 ka from sediments derived from the nearby Merrimack River and the erosion and reworking of glacial and paraglacial deposits on the proximal shelf. Discovery of a 3600-year-old paleo-inlet sequence in central Plum Island indicates that the tidal prism was initially larger than at present, likely due to incomplete infilling of the backbarrier. Differential backstripping based on stratigraphic facies, coupled with preliminary hydrodynamic modeling, demonstrate that sediment influx to the open-water backbarrier lagoon over several millennia led to bay sedimentation, formation of tidal flats and marshes, and a vast reduction in the bay tidal prism. The backbarrier tidal prism decreased significantly from 3.6 ka to present despite a ca. 30% increase in tidal range over that same period. This reduced tidal fluxes through the paleo-inlet, resulting in inlet closure through channel shoaling and infilling. Backbarrier marshes and tidal flats have continued to expand in recent millennia, while at the same time the barrier has stabilized and prograded seaward, thus demonstrating the critical coupled response of barrier-backbarrier systems to sea-level rise.