INFLUENCE OF ENVIRONMENTAL AND ANTHROPOGENIC FORCINGS ON THE LATE HOLOCENE STRATIGRAPHY OF THE POTOMAC RIVER ESTUARY OFF WESTMORELAND STATE PARK, VA

The Potomac River is a major tidal tributary to Chesapeake Bay. The stratigraphy of Holocene Potomac River sediments off Westmoreland State Park, VA were characterized and interpreted for a larger project to constrain Late Holocene sea level rise. The seismic stratigraphy was quantitatively defined previously by Wondolowski et al. (2015). Here, we used sediment cores to interpret the history reflected in those strata through analysis of physical stratigraphy and organic matter stable isotope analyses of carbon, nitrogen, and sulfur. Core KC1’s base is a sub-unit between Facies 2 (F2) and 3 (F3), a gray clayey sand with Crassostrea virginica shells and shell hash. It has a mean density of 1.75g/cm³, and is at least 2m thick in sub-bottom lines. The magnetic susceptibility (MS) and organic carbon (OC) are low (4x10^-5 SI, 0.34±0.08%, respectively). The δ¹³C value is -26.07±0.59‰ with a low C/N ratio, suggesting a sandy, shallow environment with organic matter sourced primarily from phytoplankton, formed from transgressive sands deposited on pre-Holocene F3, and provided a habitat for oysters. F2 is a gray sandy clay, with C. virginica shells. It has a mean density of 1.45g/cm³, and a thickness in sub-bottom lines > 1.4m and low MS (15±5x10^-5 SI). A sedimentation rate of 0.06 cm/yr was calculated from two C. virginica ¹⁴C dates (3451±78 cal BP) at F2’s base and adjacent core’s Reflector BC. F2 has higher OC (1.4%), and lower δ¹³C values (-27.55±0.17‰) than the sub-unit, suggesting a primary source of organic matter from phytoplankton, with an increasing contribution from terrigenous vegetation. F2 is interpreted to represent deeper estuarine conditions than F3 as sea level rose in the Holocene. Facies 1 (F1) is a fining upward brown mottled clay with minor very fine sand, its mean density is 1.37g/cm³, with a thickness in sub-bottom lines of 0.7 - 1.2m. The MS of F1 contains a peak at 60cm of 60.4 x10^-5 SI. A sedimentation rate of 0.22cm/yr was calculated from a C. virginica ¹⁴C date (447±21 cal BP) at F1’s base. F1’s OC content is 1.54% (bottom) to 2.69% (top). F1 is an anthropogenically influenced estuarine unit. Decreased density, with increasing OC and δ¹³C suggest increased productivity during this time. These units show transgression through the Holocene to the present as well as recent anthropogenic effects on sedimentation.