EVALUATION OF STABLE ISOTOPIC MARSH ZONATION IN THE FOREST RIVER ESTUARY, SALEM, MASSACHUSETTS

Salt marshes are valuable ecosystems and the zonation of such systems can be useful in the interpretation of past environments and sea level. The relationship vegetation has on soil composition can help to define specific depositional sub-environments within a marsh. This study tests the hypothesis that differences in stable isotopic composition of organic matter can help identify these depositional sub-environments. A 68 meter long transect was measured in the Forest River Conservation Area in Salem, MA, where vegetation and soil samples were taken for isotopic analysis. Zones were defined by a combination of an elevation profile and identified vegetation which includes high (HM), middle (MM) and low marsh (LM) sub-environments. The vegetation and soil samples were analyzed using an element analyzer/ isotope ratio mass spectrometer (EA/IRMS), and t-tests were conducted in Excel. δ¹³C and δ³⁴S of sediments were not significantly different from vegetation along the transect (p = 0.888, 0.542, respectively). δ¹⁵N in sediments, however, was significantly enriched relative to vegetation (p = 0.0320), and ratios of organic carbon to nitrogen (OC/N) and sulfur (OC/N) were significantly lower for sediment compared to vegetation (p << 0.0001 for both ratios). Salt marsh zonation is apparent in the proxy data. HM sediments have significantly more depleted δ¹³C (p = 0.0088), more enriched δ³⁴S (p << 0.0001), lower OC/N (p = 0.0002), and higher OC/S values (p = 0.0003) as compared to the MM. MM sediments have significantly more depleted δ¹⁵N (p = 0.0332), more enriched δ³⁴S (p = 0.0001), and higher OC/S (p = 0.0430) values as compared to LM. HM sediments have significantly more depleted δ¹³C (p = 0.0143), more depleted δ¹⁵N (p = 0.0298), more enriched δ³⁴S (p = 0.0274), and higher OC/S values (p = 0.0002) as compared to the LM. Isotopic analysis shows signs of zonation from high, middle, and low marsh based on the values from primary experiments. The significance of the correlation between these data sets offers the potential to utilize sediment cores to make inferences about past environment and historic sea levels.