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

It is well known that the incorporation of isotopically light metabolic carbon (C_M) significantly affects the stable carbon isotope (δ¹³C) signal recorded in biogenic carbonates. This can obscure the record of δ¹³C of seawater dissolved inorganic carbon (δ¹³C_DIC) potentially archived in the shell carbonate. To assess the C_M contribution to Mercenaria mercenaria shells collected in North Carolina, USA, we sampled seawater δ¹³C_DIC, 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 (%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 δ¹³C cannot easily be accounted for to allow reliable δ¹³C_DIC reconstructions. However, there does seem to be a common effect of size, as all sites had indistinguishable slopes between the %C_M and shell height (+0.19% per mm of shell height).