Behavior and fluxes of Pt in the macrotidal Gironde Estuary (SW France)

Dissolved and particulate platinum (PtD and PtP, respectively) have been measured along the salinity gradient in surface water of the Gironde Estuary during two contrasting hydrological regimes; namely, moderate discharge (November 2012) and high discharge (March 2013). During both sets of conditions, PtD concentrations displayed maxima but at different locations along the salinity gradient. These observations are attributed to the addition of Pt from suspended particles traversing the estuary, an effect that is kinetically constrained and sensitive to discharge conditions. A minimum in PtD concentration in the low salinity range observed under high discharge conditions also suggests that Pt removal may occur in the maximum turbidity zone (MTZ). Particulate Pt also exhibited mid-estuarine maxima, an effect attributed to the presence of anthropogenic Pt in this region, but no clear trend for PtP–PtD distribution coefficients was evident during either sampling. ted daily gross dissolved (PtD) and particulate Pt (PtP) fluxes at La Réole, the main fluvial entry to the Gironde Estuary, were 0.018–0.11 mol day− 1 (3.6–22 g day− 1) and 0.001–0.18 mol day− 1 (0.21–35 g day− 1) in November and March, respectively. Using Boyleʹs method, the daily net PtD fluxes from the Gironde Estuary to the coastal ocean were estimated at 0.03 mol day− 1 (6.5 g day− 1) for moderate discharge and 0.17 mol day− 1 (34 g day− 1) during high discharge, reflecting the regime-dependent magnitude of Pt remobilization along the salinity gradient. The findings of the study suggest that estuarine reactivity is of major importance for both the distributions of Pt in estuaries and its export to the global ocean.