2015 GSA Annual Meeting in Baltimore, Maryland, USA (1-4 November 2015)

Paper No. 121-3
Presentation Time: 9:00 AM-6:30 PM


SHAWLER, Justin L., Department of Geology, College of William & Mary, Williamsburg, VA 23187 and HEIN, Christopher J., Department of Physical Sciences, Virginia Institute of Marine Science, College of William and Mary, 1375 Greate Road, Gloucester Point, VA 23062, jlshawler@email.wm.edu

Barrier islands and their associated backbarrier marshes, tidal channels, and tidal flats not only provide essential ecosystem services, but also serve as a line of defense for inland communities against storms. Sufficient sediment input is a crucial factor determining the ability of these barrier systems to maintain elevation and stability in the face of accelerated sea-level rise. The supply of sediment to the coastal zone is greatly impacted by human activities such as the damming of rivers, quarrying, and deforestation. This study presents multi-proxy records from a kilometer-long transect of five piston cores collected across a dynamic estuarine tidal flat at the mouth of the Merrimack River (northern MA, USA). These are used to elucidate anthropogenic impacts on coastal sediment supply in the last 400 years and understand the dynamics of fine-grained sediment supply to the coastal zone over a longer (1000+ year) timescale.

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.