A high resolution and quasi zonal transect of dissolved Ba in the Mediterranean Sea

The dissolved barium (D_Ba) data set for the Mediterranean Sea was expanded with data from a large-scale transect sampled in April 2011 (M84/3 cruise) at high resolution. Over the basin the D_Ba content ranges from 38 to 85 nmol kg^-1 with local deep D_Ba maxima reaching up to 172 nmol kg^-1. Deep D_Ba maxima were associated with near bottom waters influenced by benthic processes and brine waters. The water column is largely undersaturated with respect to barite (BaSO₄, the main phase of particulate biogenic barium P_Ba), with water column barite saturation state ranging between 0.2 and 0.6 over the basin. This new D_Ba dataset shows that the general zonal distribution of D_Ba is impacted by the large-scale Mediterranean circulation, as evidenced by the Levantine Intermediate Water zonal and meridional progression as well as by the eastward flow of surface Atlantic Water. However biogeochemical processes are also at play, as suggested by an elevated D_Ba content of deep waters and by local lower D_Ba contents in intermediate waters. These features could be attributed to active cycling between the particulate and dissolved Ba phases. Since P_Ba barite has been recognized in previous studies as a proxy for particulate organic carbon remineralization at intermediate depths, the significance of local changes in the water column D_Ba patterns may be key to better constrain the Ba and carbon dynamics in the Mediterranean Sea. In order (i) to resolve the question of the relative importance of mixing vs. biogeochemical processing in controlling the D_Ba signal, and (ii) to determine the solid-solute interaction of the Ba phases, we applied a parametric optimum multi-parameter (POMP) analysis on the D_Ba dataset. POMP is a robust refinement of the classical end-member mixing analysis, supplying estimates of the mixing contributions of predefined source water masses and associated uncertainties. Using these mixing patterns, the conservative fraction of the D_Ba distribution was reconstructed. By comparing these calculated values with observed D_Ba contents, we identified and differentiated waters that were depleted or enriched with Ba. The potential of D_Ba to determine the strata where D_Ba subtraction occurs to form barite, and to trace changes in local processes of particulate organic matter degradation, is discussed.