Stable-isotopic composition of skeletal carbonates from living Antarctic marine invertebrates

The carbon and oxygen stable-isotopic composition of the shells of four invertebrate species from Signy island, Antarctica, has been determined. The two species of bivalve (Laternula elliptica, Yoldia eightsi), a brachiopod (Liothyrella uva) and a gastropod (Nacella concinna), inhabit slightly different shallow marine environments, have different feeding strategies, and represent both calcitic and aragonitic shells. Oxygen isotopic values range from +0.8 to +3.8 parts per thousand PDB and can vary by more than 2 parts per thousand within individual shells. There is 0.3-2 parts per thousand variation between the final increments of shells of the same species. The most positive values are compatible with equilibrium precipitation from ambient seawater. The wide range of measured values, however, is difficult to reconcile with equilibrium precipitation given the narrow annual range of environmental temperatures and measured water compositions, even assuming a bias toward shell precipitation during the austral summer. Vital effects cannot therefore be ruled out. Carbon isotopic values range from -0.2 parts per thousand to +2.2 parts per thousand PDB and show an overall pattern of covariance with the oxygen data. Most of the variation can be explained by likely seasonal changes in food supply and dissolved bicarbonate composition, although vital effects may also be involved. The range and variability of isotopic composition in samples collected from an environmentally stable site demonstrates the potential for inherent inhomogeneity in the isotopic record and provides a cautionary tale for those attempting to interpret similar data sets from the fossil record.