Northeastern Section (45th Annual) and Southeastern Section (59th Annual) Joint Meeting (13-16 March 2010)

Paper No. 11
Presentation Time: 8:00 AM-12:05 PM

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


WALKER, Lindsay J., Earth and Environmental Sciences, University of Mary Washington, UMW Box 1492, 1701 College Avenue, Fredericksburg, VA 22401, TIBERT, Neil E., Department of Earth and Environmental Sciences, University of Mary Washington, Jepson Science Center, 1301 College Avenue, Fredericksburg, VA 22401, KISILA, Ben Odhiambo, Earth and Environmental Sciences, University of Mary Washington, Jepson Science Center, 1301 College Ave, Fredericksburg, VA 22401, COOPER, Olivia R., Earth and Environmental Sciences, University of Mary Washington, UMW Box 2354, 1701 College Avenue, Fredericksburg, VA 22401 and BALIWAG, Filla S.C., Earth and Environmental Sciences, University of Mary Washington, UMW Box 2307, 1701 College Avenue, Fredericksburg, VA 22401, lwalker@mail.umw.edu

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.