δ15N Values in Crassostrea virginica Shells Provides Early Direct Evidence for Nitrogen Loading to Chesapeake Bay

Crassostrea virginica is one of the most common estuarine bivalves in the United States' east coast and is frequently found in archaeological sites and sub-fossil deposits. Although there have been several sclerochronological studies on stable carbon and oxygen isotopes in the shells of this species, less is known about delta N-15 values within their shells, which could be a useful paleoenvironmental proxy to assess estuarine nitrogen dynamics. Modern C. virginica samples were collected in Chesapeake Bay for comparison with archaeological shells from nearby sites ranging in age from similar to 100 to 3,200 years old. Left valves were sampled by milling the hinge area and the resulting powder was analyzed for % N and delta N-15 values. Comparison of delta N-15 values between C. virginica shells shows relatively constant values from similar to 1250 BC to similar to 1800 AD. After similar to 1800 AD, there are rapid increases in N-15 enrichment in the shells, which continue to increase in value up to the modern shell values. The increase in delta N-15 values is evidence of early anthropogenic impact in Chesapeake Bay. These results corroborate the observation that coastal nitrogen pollution occurred earlier than the 19th century and support the use of oyster shell delta N-15 values as a useful environmental proxy.