SALT MARSH STRATIGRAPHY AND ENVIRONMENTAL HISTORY: FOREST RIVER ESTUARY, SALEM, MASSACHUSETTS

Salt marshes are valuable ecosystems and the zonation of such systems can be used in the interpretation of past environments and sea level. In this study we hypothesize that differences in stable isotopic (δ¹³C, δ¹⁵N, δ³⁴S) composition and elemental ratios of sedimentary organic matter can be utilized to identify salt-marsh sub-environments in core samples due to the relationship between marsh vegetation and sediment organic matter. Isotopic analysis of core sediments should demonstrate the transgression of C₄ plants (i.e. S. alterniflora), which may be attributed to rising sea levels over the depositional history of the estuary. Vegetation, surface sediment, and 8 Livingstone cores were collected along a 68 m transect in the Forest River Conservation Area, Salem, MA. Core sections were stratigraphically correlated while one core (FR14LC2) was processed for MS, XRF and isotopic analysis. Three modern marsh zones were defined by a combination of an elevation profile and identified vegetation. Vegetation, surface sediment, and core sediments were analyzed using an element analyzer stable isotope mass spectrometer (EA/IRMS).

Analysis shows varying lithological shifts down core. The topmost unit consists of a maximum of 3.2 m of organic rich peat with numerous and regular intervals of light grey clay units measuring 2-4 cm each. This upper peat unit represents late Holocene estuarine/marsh sedimentation. The upper unit was deposited unconformably upon the basal layer, which contains no less than 24 cm of dense blue-grey clay. We interpret this unit as late Pleistocene-aged glaciomarine clays (i.e. Presumpscot Formation). Preliminary XRF data indicates possible human influences above approximately 28 cm down core where both lead and arsenic levels become elevated with respect to background levels. These results coupled with pending ¹⁴C dates will aid in chronologic reconstruction. Isotopic analysis of modern marsh sediments and vegetation shows signs of zonation from high, middle, and low marsh based on the values from previous experiments. The significance of this isotopic zonation on the modern marsh surface offers the potential to utilize stable isotopic proxies as proxies for marsh zonation over this ecosystems depositional history and may offer perspective on sea level change over the late Holocene epoch.