Award: OCE-1232979

We participated in the U.S. GEOTRACES cruise to the eastern tropical South Pacific Ocean with the goal of studying the concentration and chemical form of mercury (Hg), a toxic metal present at low but concerning concentrations. Of particular importance on this cruise was the study of various chemical forms of Hg that are present in the ocean: total mercury (Hg) composed mostly of inorganic Hg²⁺, monomethylmercury (CH₃Hg⁺), dimethylmercury ((CH₃)₂Hg), and elemental Hg (Hg⁰). It is important to study these forms of Hg separately because their fate and behavior in the ocean are different from one another. For example, Hg⁰ is a dissolved gas that builds up in surface ocean water enough to degas from the ocean and enter the atmosphere, representing a natural process of detoxification of Hg from the ocean. In contrast, CH₃Hg⁺ is synthesized within the ocean and is the form of Hg that accumulates in seafood and poses a risk to human and environmental health. Thus, in order to understand the potential health risks of CH₃Hg⁺ in seafood, we must gain an understanding of the chemical forms of Hg in seawater and how they interact with one another.

 

We measured concentrations of different chemical forms of Hg in filtered water, suspended particles, and rain and aerosols. These represent the sources and sinks of Hg to the ocean as well as the primary physical and chemical forms in the ocean. This particular cruise track, which extended west from the upwelling region of Peru, over the hydrothermal vent fields of the East Pacific Rise submarine mountain range and into low productivity waters near Tahiti, allowed us to test some fundamental questions regarding Hg cycling in the ocean including 1) how are methylated forms of Hg synthesized in the ocean and 2) are hydrothermal systems important sources of Hg to the ocean?

 

Some highlights from our findings include: 

• Mercury exhibited nutrient-type vertical profiles: low concentrations at the surface that increase with depth. Such distributions are indicative of the bio-activity of Hg through its accumulation in plankton in the surface ocean (lowering the concentrations) and its release back into the water at depth as the plankton sink and decompose.  
• The greatest concentrations of methylated Hg forms were observed near the seafloor on the Peruvian continental margin and in oxygen minimum zones in the water column. These are locations where we think there is net conversion of forms of Hg²⁺ into CH₃Hg⁺ and (CH₃)₂Hg.
• Based on other studies in the Pacific Ocean, we expected to see higher concentrations of Hg⁰ associated with locations where a lot of denitrification was occurring. Denitrification is the process in oxygen deficient waters that converts nitrate to N₂ gas, leading to a loss of fixed nitrogen (nutrient N) from an aquatic system. In oceanographic terms, evidence for denitrification is often demonstrated with the N* metric, with negative N* values indicative of denitrification. During the cruise, we saw some weak evidence for a coupling of production of Hg⁰ from Hg²⁺ when N* was negative, but not as strongly as we had hypothesized. The source of Hg⁰ in “dark” waters, away from sunlight, is still therefore a mystery.
• Although there is evidence that hydrothermal fluids from the East Pacific Rise can contain greatly increased concentrations of Hg, we saw no evidence of increased levels of Hg in the hydrothermal plume extending away from the East Pacific Rise. This is similar to what we observed at a hydrothermally active site at the Mid-Atlantic Ridge, and suggests that vents do not contribute a substa...