Distribution of mercury and methylmercury in deep-sea surficial sediments of the Mediterranean Sea

Abstract

An investigation of the distribution, speciation and methylation of the mercury in deep-sea sediments of the Mediterranean Sea was performed. The samples were collected at different locations in the Western and Eastern Basins of the Mediterranean during the cruise of the Italian research vessel Urania in summer 2003 as part of MERCYMS project. Total mercury (Hg T) and methylmercury (MeHg) in pore water and sediments was determined down the sediment profile and coupled with other biogeochemical parameters. The diffusive fluxes of Hg T and MeHg were calculated to estimate the importance of sediment–water exchange as potential sources of Hg T and MeHg. In addition, isotopically enriched 199Hg was used to determine the methylation potentials. The concentrations of Hg T in sediments ranged between 0.06 and 2.23 nmol g − 1 and vary irregularly with depth, which may reflect changes or redistribution during diagenetic processes. No correlation between Hg T and organic carbon content was found, but a relatively high proportion of MeHg to Hg T of approx. 2.0% was observed. The integrated flux of Hg T was estimated to be 109 kmol year − 1 and agrees well with the mass balance calculation preformed for total underwater emissions of Hg T in the Mediterranean Sea of 80 kmol year − 1 by Rajar et al. [Rajar, R., Četina, M., Horvat, M., Žagar, D., 2007-this issue. Mass balance of mercury in the Mediterranean Sea. Mar. Chem. doi:10.1016/j.marchem.2006.10.001]. The emissions of MeHg were estimated to be 14 kmol year − 1 and indicate that deep-sea sediments could be an important source of the MeHg content in marine biota. It was found that accumulation of MeHg in surficial sediments of the Mediterranean is not directly related to the potential rates of bacterial Hg methylation. Much of the MeHg produced in the sediments are lost to the overlying water. Our results suggest that MeHg production depends on partitioning of Hg(II) influenced by the organic carbon content in the sediment. Reduction of organic carbon in sediment could increase pore water Hg(II) and enhance bacterial production of MeHg.