THE DISTRIBUTION AND CYCLING OF MERCURY SPECIES IN THE WESTERN MEDITERRANEAN

Total mercury concentrations were determined in water samples from ten vertical profiles in the western Mediterranean. Most Hg concentration s ranged from 0.5 to 4 pM, with a geometric mean of 2.26 pM. Such conc entrations are in the same range as those measured in the North Atlant ic and equatorial Pacific waters. There is no indication that the pres ence of geothermal activity or cinnabar deposits around the Mediterran ean basin induces higher mercury concentrations in waters. Vertical co ncentration profiles were characterized by a maximum just below the th ermocline, which is mainly developed in regions with relatively high p rimary production in the overlying waters. These observations support the remobilization model according to which, the mercury, associated w ith degradable organic matter, is solubilized from the particles accum ulated in the thermocline layer. Additional measurements of certain me rcury species, including reactive mercury (Hg-R) and gaseous species [ elemental mercury (Hg degrees) and dimethylmercury (DMHg)], were perfo rmed on three profiles in the Alboran Sea and the Strait of Gibraltar. While 50% of the total mercury consisted of unidentified organic asso ciation, a maximum of the three determined species were observed below the thermocline: up to 0.71 pM, 0.43 pM and 0.30 pM for Hg-R, Hg degr ees and DMHg, respectively. DMHg and Hg degrees appear to be formed in the low oxygen zone. A specific methylation rate of 3 x 10(-9) s(-1) can be estimated, which is six times higher than the values proposed f or the North Atlantic waters (Mason et al., 1995a, Water, Air and Soil Pollution, 80, 665-677). The mass balance budget in the western Medit erranean shows that, while the total mercury exchanges at the straits are not unbalanced, mercury enters the Mediterranean as inorganic merc ury and is exported to the Atlantic Ocean partially as methylated spec ies. Riverine input is mainly composed by particulate mercury which is readily accumulated in coastal sediments. The dissolved mercury flux from non-marine sources is largely dominated by atmospheric deposition , which is one order of magnitude higher than riverine input. The merc ury accumulation rate in the sediments is quantitatively equivalent to the atmospheric inputs, but mainly constituted by terrestrial materia l deposited nearshore. On the basis of box-core analyses and the globa l model of Mason et al. (1994a, Geochimica et Cosmochimica Acta, 58, 3 191-3198) and present results the pre-industrial Hg riverine, atmosphe ric and sedimentation fluxes are estimated to have been about one-thir d the current one. (C) 1997 Elsevier Science Ltd.