Seasonal variations of coastal sedimentary trace metals cycling: insight on the effect of manganese and iron (oxy)hydroxides, sulphide and organic matter.

The combination of analysis, multivariate treatment (PCA) and chemical speciation calculation confirmed the control of Fe, Mn, sulphide and organic matter on metals dynamics in coastal sediments (0-5 cm surface sediments and sediments cores) of Toulon Bay (NW Mediterranean). The temporal monitoring of the physic-chemical parameters as well as the dissolved/particulate minor (Fe/Mn) and trace elements (i.e. Ag, Cd, Co, Cu, Ni, Pb, Zn, …) concentrations in porewaters and sediments were assessed. Multivariate treatment revealed different behaviours for marine elements, terrestrial ones and contaminants. Seasonal variations of metals mobilization in porewater were observed, related to diagenesis activity. Element mobility was studied by selective extractions (ascorbate, acid and alkaline) on sediments. Thermodynamic simulation (PHREEQC) was performed to calculate the elemental dissolved speciation, the mineral saturation index and then to simulate the solid/liquid interaction through precipitation processes, studying the contrasted influence of dissolved organic matter and sulphide.