Scientists from Wright State University and Woods Hole Oceanographic Institution will participate in the 2013 GEOTRACES cruise to the Eastern Tropical South Pacific (ETSP). Seawater samples collected during this transect from Peru to Tahiti will be analyzed for total mercury (Hg), monomethylmercury (CH3Hg+), dimethylmercury ((CH3)2Hg), and elemental Hg (Hg0) to construct high-resolution vertical and horizontal sections. In addition to filtered seawater samples, the scientists will also analyze suspended particles for total Hg and CH3Hg+, as well as rain and aerosol samples for Hg species and dissolved and particulate thiols, such as cysteine and glutathione.
The cruise track extends from the upwelling region off Peru, to an expansive oxygen minimum zone (OMZ), followed by the hydrothermal vent fields of the East Pacific Rise, and finally into the highly oligotrophic waters near Tahiti. These different oceanographic features will allow the researchers to determine their influence on the inputs and cycling of Hg in the ocean. Specifically, they plan to assess whether (1) continental margins off Peru are a significant source of Hg, CH3Hg+, and (CH3)2Hg; (2) the OMZ sustains the highest levels of methylated Hg in the ocean; (3) hydrothermal systems are important sources of total and methylated Hg?; and (4) the large gradient in productivity along this track impact the vertical distribution of this element, its bioavailability, and its speciation.
In terms of broader impacts, this research may have societal benefits in helping to understand the source of methylmercury in the ocean which bioaccumulates in fish and poses a hazard to those who eat seafood. In an effort to improve science education, the proponent from Wright State University and his graduate student plan to involve a class of 7th graders at Bellbrook Middle School in the Dayton area in their research experiences at sea via classroom visits, a blog, and interacting with them during the cruise. One graduate student and one undergraduate student from Wright State University would be supported and trained as part of this project.
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 ocean and that the Hg that is present in hot fluids likely precipitates out near the vents very quickly. We found that Hg0 evading from the Eastern Tropical Pacific Ocean increased the amount of Hg in the atmosphere downwind to a significant degree. Broader Impacts During this project we improved understanding of a serious human and ecosystem health threat in the form of methylated Hg. Also, this project supported dissertation research of a Ph.D. student at Wright State University, and this research will be a major component of her dissertation. This research also promoted science education at the grade-school level. Some of us also engaged a class of seventh-graders at a Dayton-area middle school with our oceanographic research. This was done through classroom visits by Dr. Hammerschmidt and his graduate student, as well as the seventh-graders following the graduate student’s web-log (aka, blog) and corresponding with her during the cruise. We are also currently involved in a series of Webinars, coordinated by the Centers for Ocean Sciences Education Excellence (COSEE), to highlight many of our findings on Hg during the cruise as well as in various ocean regions
