LATE HOLOCENE EVOLUTION OF THE PHILLIPS BEACH WELDED BARRIER SYSTEM, SWAMPSCOTT, MA

Phillips Beach is part of a southwest-facing welded barrier system located along the mesotidal coast north of Boston, MA. The barrier is isolated, bound by bedrock headlands, and contains one continuous dune ridge with relief of ~6 meters. The back-barrier contains three small ponds and wetlands. We have investigated the evolution of this welded barrier system through a combined geomorphic, geophysical, and sediment stratigraphic approach.

The barrier is transgressive, as indicated by the exposure of lagoonal muds on the beachface after a 2010 winter storm erosional event. A woody twig was sampled in situ from the surface of the exposed mud and dated with AMS ¹⁴C at 2097 ± 60 cal BP. This date constrains the location of the dune ridge to >100 meters seaward of its present position at this time, yielding an average retreat rate of ~5 cm/year. Sediment cores (n=5) from the back-barrier ponds reveal up to 2.6 meters of organic-rich mud overlying basement. Muds contain interbeds of sand and a basal AMS ¹⁴C age of 2549 ± 172 cal BP provides a minimum age of barrier stabilization and lagoonal sedimentation.

GPR surveys run parallel to and landward of the dune crest reveal at least three paleochannels perpendicular to the barrier axis. The back-barrier sediment core proximal to the most pronounced paleochannel contains a complex ~70cm thick fine to coarse-grained sand unit with anomalously high bulk density and magnetic susceptibility and low organic matter as compared to the typical muds observed in the cores. This sandy unit is likely of flood tidal delta origin associated with the paleochannel observed in the GPR data.

A beach transect has been surveyed repeatedly from April 2010 to present in order to assess the geomorphic dynamics of the barrier. Profiles illustrate a well-defined dune with scarp on the seaward side. The profile shape and associated volume exhibits a fair-weather/storm beach cycle controlled by seasonal storm activity. Transect volume varies by ~19% over measured periods, with low volumes observed in the winter, and high volumes observed in the late summer.

The ongoing work at Phillips Beach is constraining the evolution of the welded barrier system, and will advance our understanding of similar systems in light of current sea level rise predictions.