The distribution and speciation of mercury in the South and equatorial Atlantic

Mercury concentration and speciation were measured in surface and deep ocea n waters of the South and equatorial Atlantic. In the surface waters, total Hg was 2.9 +/- 1.7 pM on average, with a significant fraction present as r eactive Hg (1.7 +/- 1.2 pM). The reactive Hg fraction consisted of elementa l Hg (Hg degrees) as the dominant species (1.2 +/- 0.8 pM). Measurements in surface waters also showed that Hg partitioned to the "colloidal" phase (0 .33 +/- 0.28 pM) and was associated with particulate matter (0.1 +/- 0.05 p M), No dimethylmercury (DMHg < 0.01 pM) or monomethylmercury (MMHg; < 0.05 pM) was detected in mixed layer samples. The highest DMHg concentrations we re found in recently formed deep waters - Antarctic Intermediate Water and Antarctic Bottom Water and in the equatorial subthermocline region, Higher concentrations of DMHg coincided with higher values of apparent oxygen util ization, indicative of a link between microbial activity and methylated Hg production, The lowest-deep water DMHg concentrations were found in the cor e of the North Atlantic Deep Water. Incubation experiments on-board demonst rated that light enhanced the decomposition of DMHg, with MMHg as the major product. In deep waters, Hg degrees was still an important constituent and is likely formed as a decomposition product from MMHg. These results sugge st that methylated Hg production occurs primarily in regions of high biolog ical activity, and that ionic Hg is strongly complexed to organic matter (c olloidal material) in open ocean surface waters. (C) 1999 Elsevier Science Ltd. All rights reserved.