USING STABLE ISOTOPES OF ORGANIC MATTER TO EXAMINE THE LATE HOLOCENE ENVIRONMENTAL HISTORY OF ROSIER CREEK SALT MARSH, NORTHERN NECK OF VIRGINIA

Salt marshes are ecosystems sensitive to sea level and environmental changes, and physical and microfossil stratigraphy have been used to examine environmental histories. The purpose of this study is to use CNS stable isotopes to help constrain depositional history of the Potomac River. In 2014, a ~7 meter long Livingstone core was recovered from Rosier Creek salt marsh, a tidal tributary of the Potomac River. The core consists of two lithofacies; an upper peat and organic rich clay, and a lower grey clay, with the transition at 340cm (Clevenger et al, 2015). A radiocarbon age of 3361 ± 45 cal BP was obtained at 568cm, near the base of the core. Samples were taken at 2cm resolution for this study and wet and dry bulk densities (WBD, DBD) were measured. Once freeze dried, samples were analyzed for stable isotopic values and elemental concentrations of organic carbon, nitrogen, and sulfur using an Elemental Analyzer/Isotopic Ratio Mass Spectrometer (EA/IRMS). An anthropogenic zone extends to the upper 140cm, with lower percent of organic carbon (%OC) and higher DBD than the rest of the lithofacies, indicating post-colonial land clearance. A radiocarbon date for 146cm is 304 ± 19 cal BP, and two dates in the upper 78cm are younger than 1950 CE. %OC shows a consistent pattern with loss on ignition performed by Clevenger et al (2015). A significant (t test P: <0.0001) change in densities occurs at 340 cm depth, associated with the lithofacies shift. DBD changes from an average of 0.33 g/cm³ to 0.56 g/cm³, while WBD changes from an average of 1.04 g/cm³ to 1.21 g/cm³. Preliminary isotope data suggest that the carbon signature of preserved organic matter is sensitive to the environment of deposition. In the upper peat unit, δ¹³C and C/N data are consistent with a mixed source of marsh grass and phytoplankton. In the lower clay facies, lower δ¹³C indicates more phytoplankton influence. Pending nitrogen and sulfur analyses will complement the existing proxy data. We find that as the Rosier Creek environment has shifted from estuarine to marsh deposition, that the density of the sediment has decreased and organic matter has increased. These results provide promise in furthering our understanding of the evolving environment in VA tidal creeks during the late Holocene transgression through the addition of isotopic and density analysis.