Stable carbon and oxygen isotope compositions of mollusc shells from Britain and New Zealand

Abstract

Stable carbon and oxygen isotope analysis of the carbonate shells of two fossil mollusc species from the Plio-Pleistocene of New Zealand and two Recent species from the west coast of Scotland has revealed an unexpectedly high degree of variability for each species and a positive correlation between δ 13C and δ 18O in each case. This phenomenon may be due to kinetic isotope effects which are inherent in fastgrowing shells or areas of shell. If this is the case then equilibrium isotope partitioning may not have had time to occur before the completion of calcite precipitation. Kinetic effects would tend to favour the lighter isotopes of both carbon and oxygen; it is therefore the highest δ 13C and δ 18O values that are most likely to reflect equilibrium with the environment. Very small carbonate particles often give particularly low δ 13C and δ 18O values. These results have implications for the use of isotopic data in environmental reconstruction based on fast growing shells and may also be applicable to other carbonate precipitating organisms.