Mercury in surface waters of the open ocean

Abstract

Mercury was determined in samples of surface seawater and rainfall from coastal and open ocean areas of the northwest Atlantic and Pacific Oceans. Mercury concentrations in surface seawater and open ocean rainfall ranged from 0.5 to 11 pM and 6 to 130 pM, respectively. The pluvial flux of Hg to the open ocean was estimated at 1.7 × 109 g/yr, using mean concentrations of Hg in rainfall and annual rainfall volume estimates for the major ocean basins. Fluvial input, while difficult to reliably assess, was a less important (about 0.18 × 109 g/yr) source of Hg to the ocean. The mean residence time of Hg in the surface mixed layer (to 75 m) was calculated at 4–7 years, which is similar to other very reactive elements such as Al, Pb and Bi. Surface seawater Hg distributions are markedly influenced by atmospheric sources. Due to an elevated supply of Hg to the northwest Atlantic by rain, higher Hg values were observed in surface seawater in the northwest Atlantic Ocean (4.0 ± 0.7 pM) than at low latitudes in the central North Pacific Ocean (2.1 ± 0.4 pM). Surface seawater Hg concentrations between coastal New England and the Sargasso Sea do not show the large offshore concentration gradient typical of elements which have relatively large fluvial or coastal sources. Rather, the Hg distribution in this region was similar to that obtained for the atmospherically derived constituents Pb and 210Pb. Surface seawater Hg measurements in the central Pacific Ocean along 160°W between 20°N and 20°S showed a depression (>60%) in the equatorial upwelling area which coincided with the transect region exhibiting low 234Th/238U activity ratios. This relationship implies that Hg has enhanced scavenging and removal from surface seawater in biologically productive oceanic zones. In contrast to the surface distribution in the northwest Atlantic, a pronounced on‐shore gradient in sea surface Hg concentration was observed in the Tasman Sea. This may have resulted from the upwelling of water high in Hg content into the near coastal zone.