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Climate change and associated sea level rise are of a large concern, and studying the environment’s history will allow for better predictions on the future. Salt marshes provide an excellent site for the study of changes that have occurred, as they accrete at a rate synonymous to sea level rise. Tidal salt marshes accrete upwards and landwards as diurnal tides flood the marsh, bringing in sediment. Major storm events also are known to provide sediment for marsh accretion. Lower rates of sea level rise allow marsh plants to keep in pace with incoming sediments (Niering and Warren, 1980). Sediment cores at a tidal salt marsh on Peddocks Island in Boston Harbor, Hull, MA were analyzed in order to determine the post-glacial history of marsh and island formation. Marsh stratigraphies were compared with other findings and fit to a relative sea level curve to develop the history and provide evidence for post-glacial conditions in Boston Harbor.
Three main units in Peddocks Marsh stratigraphy display evidence of major post-glacial events in Boston Harbor. The bottom unit was found the be the Boston Blue Clay, a glaciomarine mud deposited 13,500 years BP as meltwater from the retreating ice sheet inundated Boston Harbor. The middle unit was a dark brown silty mud, with no roots or rhizomes evident in the bottom but an increasing amount towards the top. Increases in % organic carbon content up the core quantify this, as peat layers contained an average 18.1% organic matter, and bottom clay layers 3.0%. This correlates to tidal mud flats that acted as a platform in which marsh could begin to colonize. Marsh likely colonized the flats between 4000 and 2000 years BP, when rates of sea level rise decreased from 2.5 mm/yr to 1 mm/yr (Warren and Niering, 1993). The top unit was a lighter brown silty peat, with roots and rhizomes all throughout. This correlates to a time of full marsh colonization, evident in present day.
A radiocarbon date of a wood chip found at 48 cm in Peddocks Core 14-1-1 was obtained, yielding an age of 300 14C BP, or 1559 AD with calibration. This wood chip was found right at the contact of a 5 cm thick gravel deposit. This gravel deposit was interpreted as overwash sediment associated with the Great Colonial Hurricane of 1635. Evidence of less significant storm events may exist in peaks of larger grain sizes throughout the core.
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Bachelor of Science
Enos, Sean W., "Reconstruction of the Post-Glacial Formation of the Peddocks Island Salt Marsh, Hull, MA" (2015). Standard Theses. 23.
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