A large metabolic carbon contribution to the delta C-13 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 (delta C-13) signal recorded in biogenic carbonates. This can obscure the record of delta C-13 of seawater dissolved inorganic carbon (delta C-13(DIC)) potentially archived in the shell carbonate. To assess the Cm contribution to Mercenaria mercenaria shells collected in North Carolina, USA, we sampled seawater delta C-13(DIC), tissue, hemolymph and shell delta C-13. All shells showed an ontogenic decrease in shell delta C-13, 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 delta C-13 values, therefore a change in the respired delta C-13 value cannot be the cause of this decrease. Hemolymph delta C-13, on the other hand, did exhibit a negative relationship with shell height indicating that respired CO2 does influence the delta C-13 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 (%C-M) 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%C-M. Interestingly, the hemolymph did contain less than 10%C-M, suggesting that complex fractionation might occur between hemolymph and calcifying fluids. Simple shell biometrics explained nearly 60% of the observed variability in %C-M, however, this is not robust enough to predict %C-M for fossil shells. Thus, the metabolic effect on shell delta C-13 cannot easily be accounted for to allow reliable delta C-13(DIC) 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)