Oxygen and Carbon Isotopes in Modern and Historic Mussels from the Snake River, Idaho, May Show Modern Rise of Corn Production

Stable carbon and oxygen isotope compositions were analyzed from local mussels ranging in age from 0 to 1000 years before the present to identify any trends and seasonal variability in the oxygen and carbon (d6ff;18O and d6ff;13C values), compare trends and averages in d6ff;18O and d6ff;13C across different time periods and species of mussel, and finally use d6ff;18O to gain insight into how climate conditions might have changed in the past 1000 years. The Western Ridged Mussel (Gonidea angulate) and Western Pearlshell Mussel (Margaritifera falcata) are two species of freshwater mussels found in the Snake River in southern Idaho. Both species seasonally produce a calcium carbonate shell outward as they age. Previous research has shown mussels typically form their shells in isotopic equilibrium with the surrounding water and therefore can be a reliable indicator of environmental conditions such as temperature and seasonality. Outer growth bands were sampled sequentially from mussels dated ~1200 years before present, as well as from modern shells. The powdered samples were analyzed using an isotope ratio mass spectrometer in the Department of Geosciences, Boise State University, to obtain δ18O and δ13C values. Modern shells were found to have an average δ18O value of -16.6‰ (VPDB) and an average δ13C value of -8.8‰ (VPDB). δ18O was nearly constant across the shell, but δ13C increased as the mussel grew. Historic shells had homogeneous δ18O values of -16.4‰ and homogeneous δ13C values of -11.6‰. δ18O values of the two time periods are very similar, suggesting no resolvable changes to climatic conditions using this proxy. Increased δ13C values in modern mussels relative to historic mussels suggest a potentially significant increase of C4 plant contribution to the Snake River. We interpret this enrichment to be due to modern production of corn (a C4 plant) along the Snake River, especially since ~2000 CE.