Description: 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 in the incoming plate at ODP Site 1039/1253 vary between 13 and 29 mM. At ODP Site 1040/1254, 1.6 km arcward from the trench, [SO4]2- is depleted in the ~370 m of prism sediments as well as in the upper 30 m of the underthrust sediments. This suggests that, upon subduction, [SO4]2- diffusion from seawater into the underthrust sediment section ceases and the available pore fluid [SO4]2- at the top of the section is consumed by active microbial [SO4]2- reduction. Because the only remaining source of [SO4]2- is in the underthrust sediments, the depth of [SO4]2- depletion in the underthrust sediments must increase with distance from the trench. Dissolved Ba2+ concentrations in the uppermost underthrust sediments at Site 1040/1254 are several orders of magnitude greater than in the reference sediment section at Site 1039/1253, indicating intense barite dissolution coupled to [SO4]2- depletion. This is corroborated by a 50% decrease in the barite content within this unit. As a result of tectonic compaction, the dissolved Ba2+ released from barite dissolution is transported seaward and reprecipitated as barite when reaching [SO4]2- -rich fluids. As [SO4]2- depletion continues arcward, greater losses of sedimentary barite must occur in the subducting sediments. If all the barite is dissolved from the subducting sediment section, 60% of the incoming bulk sediment Ba will be distilled from the sediments in the shallow forearc. Balancing the Ba output flux with this lower input flux requires a much larger sediment component recycled to the volcanic arc than previously suggested. These results indicate that diagenetic mobilization of Ba from barite can have a profound impact on the chemical composition of sediments recycled to the arc and mantle, and should be considered in the global budget of subducted sediments.