Seawater Sr-87/Sr-86 ratios along continental margins: Patterns and processes in open and restricted shelf domains

To better constrain the Sr isotope budget in marginal domains without any fluvial inputs, we analyzed the chemical composition and Sr-87/Sr-86 ratio of waters and shells from four locations: two coastal lagoons, one hemipelagic platform and one open marine shelf. Our results highlight homogeneous Sr-87/Sr-86 ratios typical of oligotrophic oceanic waters (OOW) (i.e., 0.709172 +/- 0.000023) in the Pacific Tatakoto atoll and along a Mediterranean shore to offshore transect (similar to 25 km off Banyuls-sur-Mer, BSM). This attests that oceanic inputs from oligotrophic areas remain the main Sr source in open shelf areas compared with submarine groundwater discharges (SGD) or particulate dissolution influences. In BSM, only foreshore data are more radiogenic, possibly due to rainwater mixing, local groundwater springs or more efficient particle dissolution in the intertidal zone. In restricted areas, we report variable Sr-87/Sr-86 ratios between the Salses-Leucate (France) and Oualidia (Morocco) lagoons. The first one has homogeneous Sr-87/Sr-86 ratio typical of OOW except close to SGD. In Oualidia, Sr-87/Sr-86 ratios decrease by 1.2 x 10(-3) from OOW values close to the Atlantic inlet to progressively less radiogenic ones upstream within the interior of the lagoon. These differences depend on several factors including the leaky, restricted or choked morphology of lagoons modulating the oceanic Sr inputs, but also SGD fluxes whose Sr-87/Sr-86 ratios and Sr concentrations are highly variable according to the nature of rocks leached in karstic aquifer. In Oualidia, the low Sr-87/Sr-86 ratios correspond to high Sr concentrations (up to 150 mu mol.l(-1)) issued from the dissolution of Mesozoic evaporites, leading to SGD fluxes accounting for 60% of the local Sr budget. Through data compilation, we show that similar Sr-87/Sr-86 gradients and processes prevail at the whole Mediterranean scale. Finally, we postulate that high coastal water retention times can also account for anomalous coastal Sr-87/Sr-86 ratios and that the combination of water mass restriction, SGD, bioadsorption and early diagenetic processes could decrease seawater Sr concentrations in some marginal areas.