Organic Stable Isotopes in Ancient Oyster Shell Trace Pre-colonial Nitrogen Sources

Oysters (Crassostrea virginica) were an important food resource for native peoples of the northern Gulf of Mexico, who harvested oysters and deposited waste shell and other artifacts in middens. Shell δ¹⁵N is a proxy for oyster tissue δ¹⁵N that reflects nitrogen (N) in food sources of bivalves. We tested the use of shell δ¹⁵N as a paleo proxy of ancient N sources, which to our knowledge has not been previously done for archeological bivalve specimens. To determine δ¹⁵N of the very low-N and high-carbonate ancient specimens, we tested established and modified acidification techniques developed for modern clams and oysters to decalcify organic shell matrix and extract sufficient N for analyses. Centrifugation following acidification better concentrated N from ancient shells for stable isotope analysis. Careful screening was required to detect effects of diagenesis, incomplete acidification, and sample contamination. Modern oyster shells did not require acidification and bulk shell material was directly analyzed for δ¹⁵N using an EA-IRMS coupled to a CO₂ trap. δ¹⁵N values in ancient oyster shells did not differ from modern oyster shells from the same sites, but %N and % organic carbon (C) were lower in ancient than in modern shells. Organic δ¹³C in ancient shells had a significant negative relationship with shell age, possibly due to an effect of sea level rise increasing marine suspended particulate matter (SPM) sources to oysters. In modern oysters, δ¹⁵N had a significant relationship with soft tissue δ¹⁵N, and predicted by SPM δ¹⁵N, water column nitrate, and water column dissolved organic nitrogen (DON) concentrations, demonstrating the effectiveness of oyster shell δ¹⁵N to identify N sources to bivalves such as oysters. Our study has demonstrated the usefulness of δ¹⁵N from midden oyster shells as a proxy for N sources in an estuary that has undergone relatively light impacts from human land-use change through the past 2000 years.