Benthic Ba fluxes in the central Equatorial Pacific, implications for the oceanic Ba cycle

High resolution pore-water dissolved Ba concentration-depth profiles were determined at seven sites across an Equatorial Pacific productivity gradient from 12 degrees S to 9 degrees N, at 140 degrees W. These data are important for understanding the physical, chemical, and biological controls on Pa recycling in the ocean, and for evaluating the paleo-oceanographic significance of Pa content in central Equatorial Pacific sediments. Pore-water Pa concentrations at all sites are higher than in the overlying bottom water, leading to a diffusive flux of Pa into the ocean. A pronounced subsurface concentration maximum exceeding barite solubility characterizes the dissolved Ba pore-water profiles, suggesting that the Ba regenerated in the upper few millimeters of sediment is not controlled by barite solubility. A few centimeters down-core Pa concentrations reach a relatively constant value of approximately barite saturation. The benthic Pa flux shows a clear zonal trend, with a maximum between 2 degrees S and 2 degrees N, most probably due to higher productivity at the equatorial divergence zone, and with lowest values at the southern and northern extremes of the transect. The dissolved Ba flux between 2 degrees S and 2 degrees N is similar to 30 nmol cm(-2) yr(-1) and drops to 6 nmol cm(-2) yr(-1) at 12 degrees S. Even the lowest fluxes are significantly higher than those previously reported for the open ocean. In the Equatorial Pacific the calculated Ba recycling efficiency is about 70%. Thus, similar to 30% of the particulate Ba flux to the deep ocean is preserved in the sediments, compared with less than 1% for organic carbon and similar to 5% for biogenic silica, Mass balance calculation of the oceanic Ba cycle, using a two-box model, implies benthic Ba fluxes similar to those reported here for a steady-state ocean.