Award: OCE-1232760

We participated in the U.S. GEOTRACES cruise to the Eastern Tropical South Pacific Ocean (ETSP) with the goal of studying the concentration and chemical form of mercury (Hg), a toxic metal present at low but still concerning levels in the ocean. Of particular importance on this cruise was the separate study of the various chemical forms of Hg that one can find in the ocean: total mercury (Hg) comprised mostly of inorganic Hg²⁺, monomethylmercury (CH₃Hg⁺), dimethylmercury ((CH₃)₂Hg), and elemental Hg (Hg⁰). It is important to study these forms separately because their fate and behavior in the ocean are quite 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 form of detoxification. On the hand, CH₃Hg⁺ is synthesized within the ocean, and this is the primary form of Hg that accumulates in seafood and threatens human and environmental health. Thus, in order to understand the threat to health from this toxic metal, we must gain an understanding of its chemical forms and how they interact with one another.

 

To do this, we analyzed the Hg concentration and chemical form in filtered water samples, suspended particles, as well as 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 of the fundamental questions regarding Hg cycling in the ocean including 1) how is methylated Hg synthesized in the ocean and 2) are hydrothermal systems important sources of total and methylated Hg?

 

Some highlights from our findings include: 

• As seen in the North Atlantic, Hg exhibited nurtrient-type profiles: low concentrations at the surface, increasing with depth. This is indicative of the bio-activity of Hg through it accumulation in plankton in the surface ocean (lowering the concentrations) and its release back into the water once those plankton die (or are consumed), sink and decompose.  
• Maxima in concentrations of methylated Hg species were observed near the sediment-water interface on the Peruvian margin and in the oxygen minimum zones in the water column. These are locations where we think there is net conversion of inorganic Hg into organic forms like CH₃Hg⁺ and (CH₃)₂Hg.
• As in some other results from the Pacific Ocean, we thought we might see higher concentrations of Hg⁰ associated with those locations where a lot of denitrification was taking place. Denitrification is the process in low oxygen waters that converts NO₃^- 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 strong as we hypothesized. The source of Hg⁰ in “dark” waters, away from sunlight, is still therefore a mystery.
• Even though there is evidence that hydrothermal fluids from the East Pacific Rise can be greatly enhanced in Hg, we saw no evidence of a Hg “plume” extending away from the EPR. This is similar to what we observed at the Mid-Atlantic Ridge, and suggests that vents do not contribute a significant amount of Hg to the global ocea...