Determination of oxygen isotope fractionation between water and phosphate from living lingulids: potential application to palaeoenvironmental studies

Lingulids are inarticulate brachiopods, with phosphatic shells, which live exclusively in marine waters. Their distribution is generally restricted to the continental shelf, within the 40° belt from temperate to equatorial areas. They show a range of morphological, physiological, and behavioral features that have remained remarkably constant since the Cambrian. The oxygen isotope fractionation between water and phosphate from living lingulids was determined as a tool to investigate the temperature and/or salinity of past coastal seawater. Oxygen isotope measurements were performed according to the silver phosphate method on a collection of lingulids coming from well-known and varied marine environments. A global δ18O variation of 3.5‰ was found for these lingulids which proves their good sensitivity to environmental factors namely the oxygen isotope composition and the temperature of seawater. Oceanographic data provided average temperature and salinity values in the living sites of the studied specimens. Least squares treatment of the data gave the following fractionation equation: T(°C) = 112.2 ± 15.3 − 4.20 ± 0.71[δ18O(PO4) − δ18O(H2O)]; which is similar to the equations determined by Longinelli and Nuti (1973a) and Kolodny et al. (1983) for other phosphate-bearing invertebrates and fishes. All these results suggest that a unique equation describes the phosphate-water fractionation of many marine organisms except for mammals.