δ 13 C of fluvial mollusk shells (Rhône River): A proxy for dissolved inorganic carbon?

The relationship between the δ 13 C 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 δ 13 C 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 δ 13 C DIC varied from —11.5‰ to —7.5‰, and the δ 13 C of particulate inorganic matter (POM) varied from —31.7‰ to —25.4‰. At a given site, the δ 13 C of all species except the pulmonate were within 1‰ of each other. Whole‐shell δ 13 C correlated positively with δ 13 C 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 δ 13 C 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 δ 13 C 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 δ 13 C DIC because metabolic and seasonal variations in δ 13 C DIC produced similar isotopic fluctuations (≤2.5‰).