U–salinity relationships in the Mediterranean: Implications for Th:U particle flux studies

Knowledge of the ²³⁸U concentration in seawater is important for ²³⁴Th:²³⁸U disequilibrium studies of particle fluxes. However, these concentration data are normally obtained through a standard relationship between ²³⁸U and salinity, which has been determined for the open ocean. This study examines ²³⁸U data from both the open Mediterranean and the coastal Thermaikos Gulf, Greece, and compares it to the open ocean. No deviation from the open ocean ²³⁸U–salinity relationship was found for the Thermaikos Gulf, but some enhancement was noted close to Thessaloniki in the vicinity of a phosphate fertiliser plant. The open Mediterranean data showed a small enhancement relative to the open ocean. Although an analytical bias could not be ruled out, a review of ²³⁸U and salinity data in the literature shows that the standard relationship may not be as robust as is often assumed and the 1% uncertainty typically used is not justified at the present time. Nevertheless, salinity-based derivations continue to be the most appropriate means of determining ²³⁸U concentrations for routine applications. We propose a new relationship that accounts for the uncertainties observed, i.e. ²³⁸U (dpm l^− 1) = (0.0713 ± 0.0012) × salinity.