d¹³C of fluvial mollusk shells (Rhône River): A proxy for dissolved inorganic carbon?
Aucour, Anne-Marie, Simon M. F. Sheppard, and Régine Savoye
Limnol. Oceanogr., 48(6), 2003, 2186–2193

The relationship between the d¹³C of dissolved inorganic carbon (DIC) and modern mollusk aragonite from rivers was calibrated for the purpose of reconstructing DIC paleochemistry from the shell record. The d¹³C values of aragonitic bivalves (Dreissena polymorpha, Corbicula fluminea), prosobranch gastropods (Bithynia tentaculata, Theodoxus fluviatilis, Viviparus viviparus), and an air-breathing pulmonate gastropod (Limnea auricularia) were analyzed from several locations on the Rhône River (-13.7‰ to -6.0‰) and its major tributary, the Saône River (-11.4‰ to -10.2‰). The d¹³C_DIC varied from -11.5‰ to -7.5‰, and the d¹³C of particulate inorganic matter (POM) varied from -31.7‰ to -25.4‰. At a given site, the d¹³C of all species except the pulmonate were within 1‰ of each other. Whole-shell d¹³C correlated positively with d¹³C_DIC, with a slope close to unity. Bioaragonite–DIC fractionations were 0–1.5‰ for bivalves and 0–2.7‰ for gastropods (excluding the pulmonates). Applying these fractionations, bivalves that live in open water are a reliable proxy, monitoring the average d¹³C_DIC value to within its natural ~2‰ temporal variation within the growth period. For the suspension feeders (bivalves) using POM as a food source, the d¹³C of whole shells and bulk POM indicated that the incorporation of carbon derived from respiratory sources lay in the range 10–30%. Fine-scale analyses of growth increments of C. fluminea could not be related simply to d¹³C_DIC because metabolic and seasonal variations in d¹³C_DIC produced similar isotopic fluctuations (2.5‰).