Barium during the GEOTRACES GA-04S MedSeA cruise: The Mediterranean Sea Ba budget revisited
Résumé
The distribution of barium (Ba) in seawater was determined in the main basins and straits of the Mediterranean Sea during the GEOTRACES GA-04S MedSeA cruise. In addition, the concentrations of Ba and radium (Ra) isotopes (228Ra) were determined in groundwaters discharging in the Mediterranean Sea. The dissolved Ba concentration increases from the inflowing Atlantic surface water (Ba ~ 45 nmol/kg) to the intermediate (Ba ~ 70 nmol/kg) and deep (Ba ~ 70–75 nmol/kg) Mediterranean Sea waters in general agreement with previously published profiles. We use these data to build a Ba budget of the Mediterranean Sea and to evaluate the role of different sources of Ba (dusts, rivers, sediments, submarine groundwater discharges, Messinian evaporites). Evaporation alone cannot account for the Ba concentration increase because salinity increases only by 8% between the Gibraltar Strait and the Eastern Basin, whereas the Ba concentration increases up to 75% between the Atlantic Water and the deep waters. Thus, the particulate Ba flux that maintains the Ba concentration gradient between the surface and the deep waters must be 10–100 times stronger than previously estimated and the net Ba loss at the Gibraltar Strait or out of the Western Basin must be balanced by external inputs to the Mediterranean Sea. Newly available data from Mediterranean rivers confirm that rivers are insufficient Ba sources to balance this budget. Using recent estimates of Saharan dust fluxes, we show that dust deposition is also a negligible Ba source, whereas it was previously considered as the main external source. Using Ba-228Ra data, we show for the first time that submarine groundwater discharges can significantly contribute to the Ba budget, but large uncertainties exist due to the high variability of the chemical composition of these groundwaters. The inputs from Messinian evaporites require to be further investigated. Ba transfer from the surface water to the intermediate and deep waters occurs through active particulate Ba cycling and/or Ba dissolution in the sediment. Our study suggests that the mean particulate Ba cycling is more intense than previously estimated.