MULTI-PROXY RECORDS OF ANTHROPOGENIC IMPACTS ON FLUVIAL SEDIMENT SUPPLY FROM THE JOPPA FLATS TIDAL FLATS, NORTHERN MA
Tidal flat deposits from this site are dominantly composed of clay-rich mud and muddy fine sand. The three most river-proximal cores reveal a 40–50-cm thick peat deposit (organic content: 11–13 %; bulk density: 0.5–0.9 g/cc) at approximately 70 cm below the sediment surface. This deposit, > 400 m in lateral extent, presents a pre-historic record of marsh formation, accretion, and loss. The two most shore-proximal cores contain 20-40 cm thick fining-upward sequences (from poorly-sorted muddy medium-to-coarse sand to muddy fine sand) interpreted as tidal-channel fill. Together, these two sets of deposits document the highly dynamic nature of long-term change within this system.
In the shorter term, accretion rates derived from radioisotope dating indicate an accretion rate of 1.6-1.9 mm/year for the last 50 years. Centimeter-scale down-core inorganic geochemical data indicate an accretion rate of 1.3-2.1 mm/year for the last 150 years. Radiocarbon dating of deeper sections will shed light on changes in sediment deposition rates over time and better constrain our understanding of the effects of extensive 17th century deforestation and 19th century industrial dam-building within the Merrimack River basin on sediment supply to the Plum Island barrier-backbarrier system.