ORGANIC MATTER ISOTOPE ANALYSIS OF ROSIER CREEK, A TRIBUTARY TO THE POTOMAC RIVER, VA

Chesapeake Bay has experienced changing environmental conditions over the late Holocene as a result of both natural and anthropogenic forcings. There are few studies, however, that have quantified such variability in the tidal tributaries to the Chesapeake Bay. In this study a 3.64m Kullenberg core was recovered from the confluence of Rosier Creek and the Potomac River, VA. Using volume magnetic susceptibility (MS), bulk density, lithology, and δ¹³C data, paleoenvironmental conditions were reconstructed. Core stratigraphy was interpreted in relation to sub-bottom CHIRP stratagraphic data. Physical stratigraphy was studied using core descriptions based on color, grain size and bedding, wet bulk, dry bulk and GRAPE density, as well as MS. Two distinct lithofacies were identified in the sediment core. The lower lithofacies is a grey silty clay with very fine sand. From 32-38cm unevenly spaced black laminations are observed that gradually transition into the upper lithofacies. The upper lithofacies is a tan-grey clay unit with very fine sand. Over two hundred subsamples were analyzed for stable isotope composition of carbon, nitrogen, and sulfur, as well as elemental concentrations, using an EA-IRMS (Elemental Analyzer-Isotopic Ratio Mass Spectrometer). In the lower lithofacies, δ¹³C values have a mean of -27.9±0.41‰, whereas the upper lithofacies has a mean value of -28.1±0.34‰. GRAPE density data in the lower lithofacies is between 1.4-1.8 g/cm³ and the upper lithofacies ranges between 1.2-1.6 g/cm³. Elevated MS values in the upper 50 cm of the core are likely associated with fossil fuel emissions. In the lower lihtofacies there is little change in MS until approximately 300cm where there is a slight increase to the base of the core. δ¹³C and C/N values are indicative of a dominant estuarine phytoplankton source of organic matter with variable terrigenous input. δ¹³C data in conjunction with dry bulk density shows that above 300 cm there is a shift from a dominant marine phytoplankton source to an increase in terrigenous matter. A trend back toward higher δ¹³C values in the upper ~100 cm suggests a eutrophication signal predating the industrial MS increase. Analyses of nitrogen and sulfur will provide further insight into the depositional history and anthropogenic impacts on tributaries of Chesapeake Bay.