A PALEOENVIRONMENTAL HISTORY FOR A HOLOCENE MARSH IN THE CHESAPEAKE BAY WATER SHED: SEDIMENTOLOGICAL ANALYSIS OF ESTUARINE CORES FROM THE RAPPAHANNOCK RIVER, VIRGINIA

Sedimentological analysis of Holocene marsh cores collected from the Rappahannock estuary in Virginia (tributary to Chesapeake Bay) records major climate and anthropogenic changes in the mid-Atlantic region. Grain-size data demonstrates a fining upward trend from coarse silt to clay with a marked increase in grain-size between 2.4 and 1.4 m. A unit of coarse material marks the uppermost section from 0.4 to 0.0 m. Magnetic susceptibility measurements decrease upward with the exception of a strong peak at 0.4 m depth. Fourteen peaks in magnetic susceptibility characterize the interval from 5.5 to 2.5 m depth. Loss on ignition measurements show organic matter content is low (~10%) from the base to 2.5 m, high (~75%) from 2.3 to 2.0 m, then low (~5%) again from 1.4 to the top of the core. This physical and geochemical data indicate significant environmental change. Three environmental facies are recognized: (1) organic-matter poor olive-grey mud representative of the central estuary corresponding from 5.5 to 2.5 m depth, (2) organic-matter rich peat associated with fine-muds and low magnetic susceptibility representative of the low to high marsh corresponding to the interval from 2.5 to 1.4 m, and (3) a coarsening-upward sandy-silt with anomalously high magnetic susceptibility which probably records an anthropogenically induced tidal flat/sandbar above1.4 m depth. The facies successions identified in this core record increasing eutrophication upcore that corresponds to the middle-Holocene onset of peat accumulation. The uppermost sandy unit records an increased sedimentation rates in the Rappahannock River valley which is likely a response to post colonial deforestation and agricultural development in the Chesapeake Bay watershed.