Controls on the stable isotope composition of seasonal growth bands in aragonitic fresh-water bivalves (unionidae)

Water temperature, oxygen isotope composition and the delta(13)C of dissolved inorganic carbon were measured in 2 southern Michigan rivers, the Huron River and Fleming Creek, between late September 1990 and June 1992. The final full year of shell growth in 3 unionids collected in 1992 from these rivers was sampled for stable isotope analysis with a resolution of 30 mu m The delta(18)O of both shell nacre and the prismatic layer is accurately predicted by a fractionation relationship developed for biogenic aragonite. High resolution sampling of 3 species and bulk sampling of 3 other species suggest that all unionids adhere to this oxygen isotope fractionation relationship. This relationship is used to show that shell growth ceases below approximately 12 degrees C. In these 2 settings the average delta(18)O value of shell (PDB scale) is within 0.5 parts per thousand of the average delta(18)O of river water (SMOW scale). Unionids can therefore be used in oxygen-isotope-based paleoclimatic and paleohydrologic reconstructions. In contrast, the carbon isotope ratio of shell is not accurately predicted by published fractionation factors between D.I.C. and carbonate. Shell delta(13)C is more negative than predicted values and the offset is highly variable suggesting a significant and variable incorporation of metabolic carbon into the shell carbonate. Copyright (C) 1999 Elsevier Science Ltd.