Hydrochemical mercury distribution and air-sea exchange over the submarine hydrothermal vents off-shore Panarea Island (Aeolian arc, Tyrrhenian Sea)

Abstract

There is a growing concern about the mercury (Hg) vented from submarine hydrothermal fluids to the marine surrounding and exchange of dissolved gaseous mercury (DGM) between the sea surface and the atmosphere. A geochemical survey of thermal waters collected from submarine vents at Panarea Island (Aeolian Islands, southern Italy) was carried out in 2015 (15–17th June and 17–18th November), in order to investigate the concentration of Hg species in hydrothermal fluids and the vertical distribution in the overlying water column close to the submarine exhalative area. Specific sampling methods were employed by Scuba divers at five submarine vents located along the main regional tectonic lines. The analysis of the hydrothermal fluids indicates a site-to-site variation, with filtered total mercury (FTHg) concentrations ranging from 1072 to 4711 pM, as a consequence of the gas bubbles partial dissolution. These results are three orders of magnitude higher than the FTHg concentrations found in the overlying seawater column (ranging from 5.3 to 6.3 pM in the mid waters), where the efficient currents and vertical mixing result in more dilution, and potentially rapid transfer of the dissolved gaseous Hg to the atmosphere. Dissolved gaseous mercury (DGM) and gaseous elemental mercury (GEM) were simultaneously measured and combined in a gas-exchange model to calculate the sea-air Hg0 evasional flux. Based on the data of DGM (range: 0.05–0.22 pM) and atmospheric GEM (range: 1.7 ± 0.35–6.4 ± 2.6 ng m−3), we argue that the surface seawater off Panarea is mostly supersaturated in dissolved elemental gaseous mercury compared to the atmosphere, with a sea-air Hg0 net flux ranging from 0.7 to 9.1 ng m−2 h−1 (average: ~4.5 ± 3.5 ng m−2 h−1). Since the empirical gas-exchange model does not include the contribution of Hg0 released as gas bubbles rising from the vents toward sea-surface, the calculated Hg0 evasional flux for this location is most likely larger.