Experimental investigation into partitioning of stable isotopes between scallop (Pecten maximus) shell calcite and sea water

Stable isotopic compositions of bivalve shells have often been used for the reconstruction of high-resolution records of palaeotemperature and palaeoproductivity cycles. A major assumption in such studies is that isotopic equilibrium between shell carbonate and sea water is maintained at the time of precipitation. This assumption was tested in the laboratory for scallops, Pecten maximus, cultured over the temperature range 10–17°C. At the low shell growth rates exhibited (<0.1 mm day−1), deviations of shell δ18O from equilibrium were +0.6‰ over the experimental temperature range, a temperature equivalency of approximately −3°C. This is hypothesised as reflecting possible differences in the solution carbonate chemistry at the site of mineralisation in the extrapallial fluid (EPF) as compared to that of the external sea water medium, from which the EPF is isolated. Measured depletions of shell δ13C (of the order of −2.0‰) are interpreted as resulting from introduction of 13C-depleted respiratory CO2 into the EPF and subsequent incorporation into the shell.