Barium concentrations were measured in pore fluids and sediments from the shallow forearc of the Costa Rica subduction zone to investigate the impact of progressive barite dissolution coupled to [SO4]2- depletion on the residual sediment Ba flux to the volcanic arc and mantle. At the Costa Rica subduction zone, the entire sediment section entering the trench is underthrust beneath the prism sediments of the overriding plate. Dissolved [SO4]2- concentrations measured in the reference sediment section...