STRATIGRAPHIC SIGNIFICANCE OF EASTERN OYSTERS IN THE POTOMAC RIVER, A MAJOR TRIBUTARY TO THE CHESAPEAKE BAY: IMPLICATIONS FOR SEA LEVEL AND ANTHROPOGENIC CHANGE

Eastern oyster populations have declined in Chesapeake Bay since the late 1800’s, but their stratigraphic record can constrain Holocene transgression and environmental changes. We constrained the timing of inundation and anthropogenic changes which may have aided in declines in oyster populations in the Potomac River; using sub bottom CHIRP data groundtruthed with dated sediment cores. Sediment cores were studied through analyses of frequency dependent susceptibility (X_FD), magnetic susceptibility and bulk stable isotopes of C, N and S. A principle component analysis distilled proxy data to 2 components: PC1 and PC2, indicative of estuarine productivity and anthropogenic forcing respectively.

Holocene stratigraphy of the Westmoreland region of the Potomac River includes a downcut gray clayey sand basement unit with abundant oyster shells (F3). An oyster radiocarbon date of 5003±132 cal BP constrains timing of inundation to the area. A 0.6m minimum oyster habitat depth places mean sea level at this time to 8.55m below present. A gray clay unit (F2) was deposited in the F3’s downcut regions with horizontal internal reflectors exhibiting onlap features indicative of transgression. Occasional oysters in F2 were likely transported from sandy F3 environments not yet covered by estuarine muds. Proxy data shows that estuarine productivity was higher in F2 than F3. Neither unit has an anthropogenic signature. A mottled brown clay lacking oyster shells (F1) which onlaps and drapes F2 and F3 was deposited 4 times faster than previous deposits. Rapid sedimentation of silt and clay can cover hard substrates, required for oyster survival. Proxy data suggests F1 was deposited post European contact, a radiocarbon date constrains its age as more recent than 1500±21 C.E. High X_FD values suggest that the large flux of sediments consisted of anthropogenically cultivated soils. PC1 increases in F1, likely from eutrophication and reduced consumption of phytoplankton, congruent with the decline in oyster populations. Our results constrain inundation timing near Westmoreland to a minimum age of 5003±132 cal BP, and suggest that anthropogenic increases in sedimentation likely led to oyster habitat loss. Continued work linking estuarine stratigraphy to local salt marsh records will further constrain sea level history for the region.