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

Oxygen and carbon isotopic measurements were obtained from microsamples 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 calculated values inferred to represent precipitation in equilibrium with ambient environmental conditions. This indicates that bulk samples as well as many microsamples from the exterior surface would provide reliable isotopic estimates of paleotemperatures and paleosalinities. Nevertheless, both oxygen and carbon isotopic values from discrete shell areas may be influenced by vital effects. There is a statistically significant difference in deltaO-18 values of samples along the shell margin (deposited approximately synchronously) from the posterior and ventral region of the shell, and in deltaC-13 values of samples from the anterior, posterior and ventral regions. In addition, deltaC-13 and deltaO-18 values of shell margin samples are positively and linearly related to sample distance measured from the shell umbo. This relation is opposite to that observed in studies where aragonite inorganically precipitated at a slow rate has higher deltaC-13 values than that precipitated 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 biogenic aragonite relative to calcite at temperatures below O-degrees-C. Isotopic values from the adult portion of the shell of A. colbecki were found to be reliable indicators of typical environmental conditions. These results demonstrate the potential usefulness of isotopic evidence from fossil L. elliptica and A. colbecki in reconstructing ancient Antarctic seawater temperature/salinity and environments.