Variations in Oxygen and Carbon Isotopic Compositions and Microstructure of the Shell of Adamussium colbecki (Bivalvia)

Abstract

The oxygen and carbon isotopic variability within the calcitic shell of the Antarctic scallop Adamussium colbecki has been assessed by detailed sampling of the shell margin, an external transect along the line of maximum growth, and on the interior surface of the shell. The adult shell is largely calcitic, with aragonite restricted to myostracal deposits, an extremely thin layer of aragonitic homogeneous structure between the pallial line and the foliated inner layer of the right valve, and the fibrous ligament. No relationship between isotopic values and microstructure is apparent. Average {delta}{sup 18}O values are highest for the interior shell surface and lowest for the shell margin samples. The variability of {delta}{sup 18}O values is highest for the external shell surface. The relatively good agreement between {delta}{sup 18}O values of exterior surface samples along the growth transect of A. colbecki and estimated {delta}{sup 18}O values of calcite precipitated in equilibrium with ambient waters calculated indicate the precipitation of shell samples close to equilibrium. Because {delta}{sup 18}O values from A. colbecki reasonably well reflect ambient conditions, fossil forms would likely provide useful paleoenvironmental data. However, within shell variability in {delta}{sup 18}O values cautions against making interpretations based on a few microsamples. Carbonmore » isotopic values are highly variable throughout the shell and are likely influenced by vital effects.« less