Investigation of mercury speciation in plume waters and sediments from the hydrothermal vent field of Tonga Arc, South Pacific

Abstract

Hydrothermal vent systems play a significant role on energy and material transfer from the earth’s crust to the oceans. In order to understand the behaviors and mass fluxes of mercury (Hg) in the hydrothermal vent field of Tonga Arc, South Pacific, we determined levels of total Hg (THg), methylmercury (MeHg), and chromium-reducible sulfide (CRS) from plume water, seawater, and sediments samples. The vent field fractions were determined from Mg concentrations of the plume water. As a result, higher Hg and CRS concentrations were found in the plume water containing higher vent fluid fractions. Interestingly, there was a positive relationship between the dissolved CRS and dissolved THg in plume samples collected from the single vent, indicating that the nano-particulate pyrite might be a potential HG carrier for long-range transportation from hydrothermal vents to the open ocean. Regarding Hg speciation in sediment, THg and CRS were higher in the chimney sediment than nearby controls. However, MeHg levels were similar between two samples. It indicates that Hg(II) metylations might be more active in the nearby control sediments. Using the global vent fluid discharge rate (1-4x10 g/yr) and the concentration of dissolved Hg in the vent fluid end-member (106 pM), global Hg flux from the hydrothermal vent was calculated to be 1.1-4.2 Mmol/year, which is approximately 7-25% of river and atmospheric deposition flux. These preliminary results suggest that hydrothermal vent may serve as an important source of mercury in ocean water.