U.S. GEOTRACES: DISTRIBUTION OF MERCURY SPECIES ACROSS A ZONAL SECTION OF THE NORTH ATLANTIC OCEAN

Mercury (Hg) speciation measurements were performed on board the R/V Knorr during the U.S. GEOTRACES North Atlantic Zonal section, a two-leg cruise from Portugal to Cape Verde (2010) and from Massachusetts to Cape Verde in 2011. All four principal forms of mercury (monomethylmercury, dimethylmercury, elemental and total Hg) were determined in high vertical and horizontal resolution through areas of upwelling, high atmospheric dust deposition, low-nutrient concentrations, and over a hydrothermal vent field. Preliminary results suggest that, in the northeast Atlantic, total Hg has both scavenged and nutrient-type distributions. Elemental Hg, in contrast, has strong nutrient-like profiles along the west coast of Africa with deep water concentrations as much as 50% of total Hg. Relative concentrations of elemental Hg decrease and vertical distributions appear to change with seaward distance from Africa, suggesting a different mode of in situ production. Monomethylmercury has pronounced mid-water maxima associated with the oxygen minimum zone, where it is likely to result from either in situ methylation or isopycnal transport from the margin. Vertical profiles suggest that deep-sea sediments are not an important source of monomethylmercury. High-quality speciation results from the international GEOTRACES program are important to understand the global distribution and cycling of Hg in the ocean.