The influence of diet on the δ13C of shell carbon in the pulmonate snail Helix aspersa

The influence of diet and atmospheric CO2 on the carbon isotope composition of shell aragonite and shell-bound organic carbon in the pulmonate snail Helix aspersa raised in the laboratory was investigated. Three separate groups of snails were raised on romaine lettuce (C3 plant, delta(13)C = -25.8parts per thousand), corn (C4 plant, delta(13)C = -10.5parts per thousand), and sour orange (C-12-enriched C3 plant, delta(13)C=-39.1parts per thousand). The isotopic composition of body tissues closely tracked the isotopic composition of the snail diet as demonstrated previously. However, the isotopic composition of the acid insoluble organic matrix extracted from the aragonite shells does not track diet in all groups. In snails that were fed corn the isotopic composition of the organic matrix was more negative than the body by as much as 5parts per thousand whereas the matrix was approximately 1parts per thousand heavier than the body tissues in snails fed a diet of C3 plant material. These results indicate that isotopic composition of the organic matrix carbon cannot be used as an isotopic substrate for paleodietary reconstructions without first determining the source of the carbon and any associated fractionations. The isotopic composition of the shell aragonite is offset from the body tissues by 12.3parts per thousand in each of the culture groups. This offset was not influenced by the consumption of carbonate and is not attributable to the diffusion of atmospheric CO2 into the hemolymph. The carbon isotopic composition of shell aragonite is best explained in terms of equilibrium fractionations associated with exchange between metabolic CO2 and HCO3 in the hemolymph and the fractionation associated with carbonate precipitation. These results differ from previous studies. based primarily on samples collected in the field, that have suggested atmospheric carbon dioxide contributes significantly to the shell delta(13)C. The culture results indicate that the delta(13)C of aragonite is a good recorder of the isotopic composition of the snail body tissue, and therefore a better recorder of diet than is the insoluble shell organic carbon. Because the systematic fractionation of carbon isotopes within the snail is temperature dependent, the delta(13)C of the shell could provide an independent technique for estimating paleotemperature changes. (C) 2002 Elsevier Science B.V. All rights reserved.