OXYGEN AND CARBON ISOTOPIC COMPOSITION AND SHELL MICROSTRUCTURE OF THE BIVALVE LATERNULA-ELLIPTICA FROM ANTARCTICA

Oxygen and carbon isotopic measurements were obtained from microsample s of an adult Laternula elliptica, an infaunal, aragonitic, Antarctic bivalve. The deltaO-18 values of samples from the exterior surface are , on average (approximately 4.5 parts per thousand), similar to calcul ated values inferred to represent precipitation in equilibrium with am bient environmental conditions. This indicates that bulk samples as we ll as many microsamples from the exterior surface would provide reliab le isotopic estimates of paleotemperatures and paleosalinities. Nevert heless, both oxygen and carbon isotopic values from discrete shell are as may be influenced by vital effects. There is a statistically signif icant difference in deltaO-18 values of samples along the shell margin (deposited approximately synchronously) from the posterior and ventra l region of the shell, and in deltaC-13 values of samples from the ant erior, posterior and ventral regions. In addition, deltaC-13 and delta O-18 values of shell margin samples are positively and linearly relate d to sample distance measured from the shell umbo. This relation is op posite to that observed in studies where aragonite inorganically preci pitated at a slow rate has higher deltaC-13 values than that precipita ted at high rates. Comparison of isotopic compositions of aragonitic L . elliptica with the calcitic bivalve Adamussium colbecki collected at the same location and time demonstrates an O-18 enrichment of biogeni c aragonite relative to calcite at temperatures below O-degrees-C. Iso topic values from the adult portion of the shell of A. colbecki were f ound to be reliable indicators of typical environmental conditions. Th ese results demonstrate the potential usefulness of isotopic evidence from fossil L. elliptica and A. colbecki in reconstructing ancient Ant arctic seawater temperature/salinity and environments.