Award: OCE-1132515

We measured the concentration of mercury (Hg) in seawater of the North Atlantic Ocean between Portugal and the United States at 35 locations and 24 depths as part of the US GEOTRACES program. As with many of the trace elements and isotopes measured during this project, this represents an unprecedentedly detailed examination of this toxic trace metal in the ocean. In addition to the total amount of Hg present in seawater, we also determined the amount of three specific forms of Hg, elemental Hg (Hg^o), monomethylHg (CH₃Hg⁺) and dimethylHg ((CH₃)₂Hg). These distinctions are important to make as the chemical form of Hg determines how much of this metal will bioaccumulate in marine foodwebs; the CH₃Hg⁺ is the form which biomagnifies, while the others do not. The biogeochemical processes which result in the formation of this CH₃Hg⁺ form in seawater is a major research theme for marine Hg scientists, and a detailed examination of the concentration of all Hg forms in the ocean will shed light on that important process.

Hg is present at very low concentrations in the open ocean (femtomolar to picomolar). Furthermore, some of the Hg forms (Hg^o and (CH₃)₂Hg) are volatile and have no “shelf life.” Thus, we sent two scientists on the expedition to analyze Hg forms on board immediately following collection.

 

Our findings included: 

• Total and Hg^o exhibited nutrient-like vertical distributions in most locations (lower concentrations below the surface due to uptake and particle scavenging and a subsurface maximum associated with particle regeneration).
• Maxima in methylated Hg species concentrations were found in the oxygen minimum zone of the water column. This is likely a very important location for methylated Hg production in the ocean.
• There is much less Hg in the ocean than our earlier analyses suggested there was. This is likely the result in a general improvement in our ability to accurately measure this element at the extremely low levels found in the open ocean.
• Our data have allowed us to make an estimate for the amount of pollution Hg in the ocean, and it represents about half of all Hg found in water shallower than about 1000 m depth. This is significant as these are the waters that interact with marine foodwebs on annual to decadal timescales.

 

 

The broader impacts of this work include a greater understanding of a serious human and ecosystem health threat in the form of methylated Hg accumulation in fish. Also two graduate students were supported and trained during this project, and this work comprises significant portions of their doctoral dissertations.

 

 

Last Modified: 04/01/2013
Modified by: Carl H Lamborg