Sources, Transport, and accumulation of Mercury in the northwestern Mediterranean margin sediments during the Industrial Era and influence of turbiditic events

Abstract

Sources and pathways of the Hg accumulated in the sediments of the Gulf of Lion (GoL) and its adjacent marine areas (Northwestern Mediterranean) have been explored using sediment grab samples, sediment cores, and sediment trap samples. The main source of Hg along this margin is the Rhône River, whose suspended sediments settle mainly in the prodelta area but also along the mid-shelf, then reaching the continental rise via wave resuspension and cascading of dense shelf waters. Seaward, these riverine particles are mixed with carbonated ooze conveyed to the bottom by the biological pump. The Hg is enriched in surface sediments of the GoL with decreasing concentrations westward and seaward from the Rhône prodelta to the continental rise. Dated cores from the Rhône prodelta give access to riverine sediments deposited over the last 400 years and show that the Hg concentration time trend resembles the evolution of coal consumption in France, peaking during in the 1960s. Similar trends were observed in sediment cores collected along the GoL slope and submarine canyons along with the preservation of traces of erosion and sediment instability events. Seaward, on the continental rise, the Hg concentration distribution suggests the deposition of fine planktonic-derived material and particles episodically advected from the shelf during deep cascading pulses. Anthropogenic Hg accumulated in GoL and continental rise sediments during the Industrialized Era is ∼ 150 Mg (tons), two-thirds of which are buried in the Rhône prodelta area. Significant correlations are found between Hg and organic matter in GoL sediments, but the relationships differ between areas and are disrupted by the inputs of anthropogenic Hg and by the Hg availability in the Rhône River watershed during the pre-industrial period. The HgT availability in waters appears to be the limiting factor to the Hg enrichment of the particulate organic matter. Monomethyl mercury (MMHg), which represents on average 0.3 % of the total Hg, was positively correlated to total Hg. Their distributions suggest in situ MMHg formation and a more effective net Hg methylation for surface sediment or particles collected in traps compared with particles buried in the sedimentary column.