A large metabolic carbon contribution to the δ13C record in marine aragonitic bivalve shells

It is well known that the incorporation of isotopically light metabolic carbon (CM) significantly affects the stable carbon isotope (δ13C) signal recorded in biogenic carbonates. This can obscure the record of δ13C of seawater dissolved inorganic carbon (δ13CDIC) potentially archived in the shell carbonate. To assess the CM contribution to Mercenaria mercenaria shells collected in North Carolina, USA, we sampled seawater δ13CDIC, tissue, hemolymph and shell δ13C. All shells showed an ontogenic decrease in shell δ13C, with as much as a 4‰ decrease over the lifespan of the clam. There was no apparent ontogenic change in food source indicated by soft tissue δ13C values, therefore a change in the respired δ13C value cannot be the cause of this decrease. Hemolymph δ13C, on the other hand, did exhibit a negative relationship with shell height indicating that respired CO2 does influence the δ13C value of internal fluids and that the amount of respired CO2 is related to the size or age of the bivalve. The percent metabolic C incorporated into the shell (%CM) was significantly higher (up to 37%, with a range from 5% to 37%) than has been found in other bivalve shells, which usually contain less than 10%CM. Interestingly, the hemolymph did contain less than 10%CM, suggesting that complex fractionation might occur between hemolymph and calcifying fluids. Simple shell biometrics explained nearly 60% of the observed variability in %CM, however, this is not robust enough to predict %CM for fossil shells. Thus, the metabolic effect on shell δ13C cannot easily be accounted for to allow reliable δ13CDIC reconstructions. However, there does seem to be a common effect of size, as all sites had indistinguishable slopes between the %CM and shell height (+0.19% per mm of shell height).