Contribution of metabolic carbon to mollusc and barnacle shell carbonate

Abstract

Carbon in marine calcareous tests is not necessarily derived only from dissolved inorganic carbon (DIG) in ambient sea water; metabolic carbon can also be incorporated into carbonate tests, as shown by experiments using ¹⁴C-labelled food during the incubation of sea urchin embryos and coral^1,2. ¹⁴C-rich organic matter (relative to seawater DIC) from terrestrial vegetation as well as marine organic matter (reflecting seawater DIC) is used as food by marine organisms in near-shore environments. This use provides the basis for a natural experiment on the systematics of metabolic carbon incorporation into carbonate tests. Here we have combined ¹⁴C/¹²C and ¹³C/¹²C ratio measurements on both the calcareous and the organic parts of marine organisms and on DIC, plankton and other carbon-bearing materials collected in and around New Haven Harbor (Connecticut, USA) in Long Island Sound, where the various sources of carbon can be identified, and we deduce that a large percentage of the carbon in calcareous tests is metabolic carbon. Thus, it is at best difficult to use the δ¹³C values of ancient biogenic carbonate from molluscs to predict the ancient δ¹³C values of seawater DIC