Carbon and oxygen isotopic compositions have been determined on modern pelecypod shells and ancient pelecypod shells whose ages had been measured by the radiocarbon method. The modern shells collected along the Brazilian coastline in order to identify possible isotopic composition trends in the shells of the pelecypods living along this coastline. The ancient pelecypod shells were collected mostly from the Cananéia-Iguape coastal plain (State of Sao Paulo). The isotopic composition ratios of these samples can be correlated with sea-level fluctuations during the Holocene, whose transgressive and regressive phases left their own typical isotopic compositions imprinted on these shells. Some samples of freshwater pelecypod shells were also analysed to establish the limites of δC13 and δO18 in fluviatile and lacustrine environments. The analytical results showed that isotopic composition can be used to distinguish between pelecypod shells in freshwater environments from those in saltwater environments, as previously has been verified elsewhere. This fact demonstrates that biological control of the isotopic fractionation mechanisms may be overriden by environmental controls.
