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

Abstract

It is well known that the incorporation of isotopically light metabolic carbon (C M ) significantly affects the stable carbon isotope (d 13 C) signal recorded in biogenic carbonates. This can obscure the record of d 13 C of seawater dissolved inorganic carbon (d 13 C 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 d 13 C DIC , tissue, hemolymph and shell d 13 C. All shells showed an ontogenic decrease in shell d 13 C, 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 d 13 C values, therefore a change in the respired d 13 C value cannot be the cause of this decrease. Hemolymph d 13 C, on the other hand, did exhibit a negative relationship with shell height indicating that respired CO 2 does influence the d 13 C value of internal fluids and that the amount of respired CO 2 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 d 13 C cannot easily be accounted for to allow reliable d 13 C 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).

Size-dependent pH effect on calcification in post-larval hard clam Mercenaria spp.

“…It is important to note that these predictions are only applicable to range of sizes and at the salinity/alkalinity of the current experiments. Additionally, had this relationship been determined for the Florida hatchery Mercenaria hybrids (based on the limited data we have), a smaller size at which dissolution pressure could be overcome would have been found in hard clams indicate that larger/older clams utilize more respired CO 2 for calcification versus smaller/ younger clams (Elliot et al 2003, Lorrain et al 2004, Gillikin et al 2007). Although to date little is known about the isotope ratios in early juvenile shells, the increase in respired C in shell material deposited with age suggests the potential importance of feeding physiology on calcification.…”

Ecogeochemistry potential in deep time biodiversity illustrated using a modern deep-water case study

“…Variations in the isotopic composition of respired and environmental carbon offer an alternative proxy for metabolic rate, which is recorded in the isotopic composition of biomineral carbonate and thus applicable to deep time studies. Ecogeochemical proxies for organism field metabolic rate revolve around the proportion of respired carbon present in blood fluids during deposition of biominerals [56][57][58]. In fishes, carbon in the blood is derived from dissolved inorganic carbon ingested via the gills and stomach and from carbon released by the cellular respiration of ingested food nutrients.…”

Stable isotope composition of the modern freshwater bivalve Corbicula fluminea

“…Some studies have argued that δ 13 C ar is controlled by the δ 13 C DIC in water, (e.g., Mook, 1971;Fritz and Poplawski, 1974), many others have suggested that both kinetic and metabolic effects play an important role in determing δ 13 C ar (Keith et al, 1964;Tanaka et al, 1986;Klein et al, 1996;McConnaughey et al, 1997;Dettman et al, 1999;Lorrain et al, 2004;Gillikin et al, 2006Gillikin et al, , 2007Gillikin et al, , 2009. Kinetic isotope effects refer to the simultaneous depletion of 18 O and 13 C associated with fractionation during CO 2 hydration and hydroxylation (McConnaughey, 1989).…”

“…Metabolic controls on the incorporation of various sources of carbon, such as from respiration or food, into shell carbonate can produce such isotope effects. Metabolic carbon represented a small portion (~10%) of the carbon deposited in molluscan shell carbonate (McConnaughey et al, 1997;Kennedy et al, 2001;Lorrain et al, 2004;Gillikin et al, 2005) and more recently up to 37% (Gillikin et al, 2007). Studies show that contribution of metabolic carbon can dampen the δ 13 C DIC signal recorded in shells and explain δ 13 C ar disequilibrium in bivalves (Krantz et al, 1987;Klein et al, 1996;Keller et al, 2002;Lorrain et al, 2004;Gillikin et al, 2007).…”

“…On the other hand, shell carbonates were often found not to reflect the predicted equilibrium fractionation, being in general less enriched than predicted in 13 C (e.g., Klein et al, 1996;McConnaughey et al, 1997;McConnaughey, 2003;Kaandorp et al, 2003 and many others). Most authors explain this offset by a contribution of metabolic effects (Tanaka et al, 1986;Klein et al, 1996;Veinott and Cornett, 1998;Dettman et al, 1999;Gillikin et al, 2007Gillikin et al, , 2009) and kinetic effects (McConnaughey et al, 1997;McConnaughey, 2003;Lorrain et al, 2005).…”