Analysis of δ15N values in mollusk shell organic matrix by elemental analysis/isotope ratio mass spectrometry without acidification: an evaluation of long-term preservation

Nitrogen stable isotope ratios have successfully been applied in the study of trophic linkages, as well as of human impacts in aquatic ecosystems. Anthropogenic wastewater input typically elevates 15N values in dissolved inorganic nitrogen and this 15N enrichment subsequently propagates throughout the food chain. Bivalve mollusks are of interest for studies of this human influence since they are primary consumers and are known to trace environmental 15N variability. The 15N values of their soft tissues have, for example, been found to correlate with the fraction of residential development in watersheds around lakes and salt marshes.To determine the undisturbed ?15N values in an ecosystem, before anthropogenic nitrogen input, 15N records need to be extended into the past. Bivalve shells can be useful for this, since they are often abundant in archaeological deposits as well as historic museum collections. A predictable relationship has been demonstrated between the 15N values of shell organic matter and soft tissues and 15N values of this organic matrix indeed trace anthropogenic influences.