STABLE-ISOTOPIC COMPOSITION OF SKELETAL CARBONATES FROM LIVING ANTARCTIC MARINE-INVERTEBRATES

The carbon and oxygen stable-isotopic composition of the shells of fou r invertebrate species from Signy island, Antarctica, has been determi ned. The two species of bivalve (Laternula elliptica, Yoldia eightsi), a brachiopod (Liothyrella uva) and a gastropod (Nacella concinna), in habit slightly different shallow marine environments, have different f eeding 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 shell s. There is 0.3-2 parts per thousand variation between the final incre ments of shells of the same species. The most positive values are comp atible with equilibrium precipitation from ambient seawater. The wide range of measured values, however, is difficult to reconcile with equi librium precipitation given the narrow annual range of environmental t emperatures and measured water compositions, even assuming a bias towa rd 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 patter n of covariance with the oxygen data. Most of the variation can be exp lained by likely seasonal changes in food supply and dissolved bicarbo nate composition, although vital effects may also be involved. The ran ge and variability of isotopic composition in samples collected from a n environmentally stable site demonstrates the potential for inherent inhomogeneity in the isotopic record and provides a cautionary tale fo r those attempting to interpret similar data sets from the fossil reco rd.