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

Abstract

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 δ¹⁵N 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 ~100 to 3,200 years old. Left valves were sampled by milling the hinge area and the resulting powder was analyzed for %N and δ¹⁵N values. Comparison of δ¹⁵N values between C. virginica shells shows relatively constant values from ~1250 BC to ~1800 AD. After ~1800 AD, there are rapid increases in ¹⁵N enrichment in the shells, which continue to increase in value up to the modern shell values. The increase in δ¹⁵N 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 δ¹⁵N values as a useful environmental proxy.

Figure 1. Location of study area near Edgewater, MD, USA.

Sample locations are represented by their corresponding archaeological time periods. The site labeled “Dock” denotes the location of modern oyster sampling. Map was created in ArcMap version 10.3 and modified in Adobe Illustrator version CC (http://desktop.arcgis.com/en/arcmap/10.3/main/get-started/whats-new-in-arcgis.htm and http://www.adobe.com/products/illustrator.html, respectively).

Figure 2. Plot of δ¹⁵N (‰ AIR) values for modern and archaeological C. virginica shells.

Precision (1σ) was better than 0.09‰ based on analysis of multiple standards over a range of isotopic values.

Figure 3. Plot of percent nitrogen values for modern and archaeological C. virginica shells.

Figure 4

(A) Image of disarticulated valves of C. virginica. (B) Cross section of C. virginica through the resilifer. The red line represents the micro-drilled transect.