Carbon, Oxygen, and Nitrogen Isotopic Analysis of Mollusca: Unionidae of the Ohio River System during the Anthropocene

Abstract

Isotopic compositions of Obovaria olivaria, Leptodea fragilis, and Quadrula quadrula of the Wabash River contain clues to the anthropogenic modifications they have undergone during the time of the shell formation. In this study we analyzed the isotopic composition of the carbon (δ13C), oxygen (δ18O), and nitrogen (δ15N) in the carbonate sampled from growth bands of the shells. These results are compared to published data, and with accurately documented historical river data to better understand the human impact on river systems in the Ohio River Valley in the last 250 years. Mollusks construct their shells using CO2 and nitrogen that is dissolved in the water. Since mollusks are sedentary animals, their isotopic signatures will reflect the chemistry of the water in which they grow. Comparing the locality in which the mollusks lived to the chemical changes recorded in their shells over time will provide temporal-spatial constraints on the natural variability of the river as well as the influence of anthropogenic inputs. Specifically, the δ13C and δ18O stored in the shells of the mollusks record changes in the environment such as air and river temperature, acidity of the river, changes in the river chemical composition, and may indicate a change in the source of the water. The isotopic analysis of δ15N recovered from the shell can help track changes in nutrient delivery during the time of shell formation. Through this geochemical study we hope to provide a much needed record of the timing and extent of human impact on river systems.