CLIMATE FORCING AND ANTHROPOGENIC INFLUENCE ON SEDIMENTATION FOR THE RAPPAHANNOCK ESTUARY IN THE CHESAPEAKE BAY WATERSHED

Sediment cores collected from Blandfield Point in the Rappahannock River record significant regional paleoenvironmental shifts for the tidal reaches of the Chesapeake Bay. The 1.4 m core of organic-rich, silty clay was analyzed for physical properties and micropaleontology with the goal to develop a record of paleosalinity spanning the past several hundred years. The sediments at Blandfield Point include two distinct units. The basal unit (101-142 cm) is characterized by light grey clay, low magnetic susceptibility values, and relatively abundant brackish ostracodes, including Cyprideis salebrosa and Cytheromorpha curta. Pore shape analysis of C. salebrosa demonstrates significant changes in shape from circular to highly irregular. The upper sedimentary unit comprises silty organic-rich clay characterized by variable magnetic susceptibility values and an ostracode population dominated by Darwinula stevensoni. Sedimentation rates calculated on the basis of ¹³⁷Cs are relatively high at Blandfield Point (5.4 mm/yr) and the up-stream locality, Hick’s Landing (7.1 mm/yr). The basal grey clay facies with relative abundance alternations between C. salebrosa and C. curta suggest variations in paleosalinity. The variable pore shapes in Cyprideis corroborate this hypothesis, which indicates that regional and/or global climate may have influenced the maximum landward extent of salt water in the Rappahannock Estuary prior to the Industrial Revolution. The abrupt transition to the freshwater ostracode D. stevensoni at 101 cm indicates increased fluvial influence in response to aggressive agricultural practices in the watershed at the onset of the Industrial Revolution.