Fluxes and processes responsible for elevated Ba in pore-water of the south-eastern Mediterranean shelf

Abstract

Diffusive chemical fluxes from shelf sediments, termed as benthic fluxes, play a key role in the oceanic inventory of chemical elements. However, there are still major gaps in our knowledge of such fluxes for some elements, which leads to increased uncertainty in their modern-day marine budget. Ba is an important tracer for various oceanic processes, but the role of benthic fluxes in its marine budget is poorly constrained. Here, we focus on understanding the processes responsible for elevated Ba in the pore-water of south-eastern Mediterranean shelf sediments; and quantifying the benthic fluxes in this region. We cored shallow sediments from two sites (SG1 and PC3), extracted pore-water samples along depth-profiles, and analysed their composition (Ba, Ca, Mg, Sr, Fe, Mn, Cl, SO₄ and Total Alkalinity (TA)). The concentration of Cl remains conservative in both profiles. At site SG1, dissolved Ca, Sr, and SO₄ concentrations decrease with depth, while those of Ba, Fe, and Mn are enriched relative to seawater. At site PC3, dissolved concentrations of Ca, Sr, and SO₄ show limited variations with depth relative to bottom seawater, but elevated concentrations of Ba, Fe, and Mn. At both sites, Mg shows removal and gain in the pore-water profile. Saturation index calculations indicate that pore-water supersaturated with respect to barite and calcite, while aragonite is undersaturated, suggesting that aragonite dissolution is a potential source of Ba in the pore-water. Additional potential source of Ba includes organic matter remineralization and dissolution of FeMn (oxyhydr)oxides. We use Fick's first law to calculate the benthic Ba fluxes at both sites SG1 and PC3 and estimate them at 693 μmol m⁻² yr⁻¹ and 792 μmol m⁻² yr⁻¹, respectively. Integrating the average of these fluxes over the entire basin of the eastern Mediterranean Sea, results in a benthic Ba flux of 2.4 ± 0.2 × 10⁸ mol yr⁻¹. Considering this result together with estimations of riverine, submarine groundwater discharge (SGD) and aeolian inputs to the eastern Mediterranean, we calculate a total Ba input to eastern Mediterranean of 3.9 × 10⁸ mol yr⁻¹. This calculated total flux is comparable to the Ba output to the western Mediterranean Sea estimated at the Sicily Strait (3.5 × 10⁸ ± 1.1 × 10⁸ mol yr⁻¹). While further investigations that include additional sites in the eastern Mediterranean basin are necessary to test and improve this assessment, within order of magnitude these results underscore the significance of the benthic Ba fluxes as key-player in regional and global budget of marine Ba.