SOURCE OF SEDIMENT AND RATE OF ACCUMULATION IN A COASTAL LAGOON DREDGED IN 1963; MARTHA’S VINEYARD, MA

The optimistically named Yacht Basin, a tidal lagoon on the north end of Martha’s Vineyard, MA, was originally intended to be a boat harbor, though the project was unsuccessful. However, tidal flow has been maintained since the initial 1963 dredging on behalf of the recreational and scenic value of the neighborhood. The lagoon is today surrounded by salt marsh. Considering sea level rise, marsh expansion, and sediment accumulation, a local homeowners’ association is concerned about the future of the area. We evaluated sediment physical properties and measured fallout radionuclides in sediment cores to quantify sediment accumulation in the lagoon since 1963. LOI analysis is ongoing to determine sources of sediment.

The fallout radionuclides (FRNs) ¹³⁷Cs and excess ²¹⁰Pb (²¹⁰Pb_ex) were gamma-detected on three sediment cores from the lagoon and several more from the adjacent marsh to determine sediment accumulation rates. ¹³⁷Cs can be used as a date marker for the 1950s-60s because of nuclear weapons testing in that period. The lagoon was dredged in 1963, which is the peak global fallout date of ¹³⁷Cs. The continually deposited, geogenic, excess Pb-210 is part of the U-238 radioactive decay series and can offer more refined estimates of sediment accumulation rates.

The results from the ¹³⁷Cs in all three lagoon cores suggest that 40-48 cm of sediment have accumulated since the 1963 dredging. Questions continue about the unequal ¹³⁷Cs profile found in all cores analyzed, which may suggest some disruption by physical or biological processes. Although near constant ²¹⁰Pb_ex concentrations in two lagoon cores suggest accelerating sediment accumulation rates, the rate is not fast enough to fill in significant portions of the lagoon before 2050; accounting for sea level rise pushes the infill date estimate even later. ²¹⁰Pb_ex analysis on an additional core from the lagoon is ongoing, but preliminary results corroborate the increasing rate of deposition since 1963. Given the relatively slow rate of accumulation, it is unlikely that dredging needs to happen in the next few years. We presume that the homeowners’ association’s concerns of the lagoon infilling actually reflect marsh expansion, an idea we will address with analysis of marsh cores.