Mercury distributions in the North Pacific Ocean—20 years of observations

Vertical mercury (Hg) distributions for the North and Central Pacific Ocean are reported here for three different cruises over a time period of 20 years: N. Pac (1980), VERTEX (1986–87) and IOC (2002). The vertical distribution was not controlled solely by the hydrographic characteristics or by internal biogeochemical “nutrient type” recycling and mixing processes. Rather, Hg distribution appeared to be regulated by the local magnitude of external sources and the intensity of water column processes. During the 2002 IOC cruise, the total mercury (Hg TOT) concentrations averaged 1.15±0.86 pM with the highest concentrations found within the Japanese coastal waters. The overall upper-water Hg concentration average, calculated for the IOC study (0.64±0.26 pM), was similar to the earlier VERTEX cruise (0.58±0.37 pM) but was lower compared to the N. Pac cruise (1.40±0.34). Variance in Hg concentrations in the upper water, generally within or close to the main thermocline, was observed among several stations and for both VERTEX and IOC campaigns. Horizontal advection of water along isopycnals, vertical mixing, ventilation, the presence of a thermocline associated with remobilization of mercury as a result of remineralization of settling particles, the influence of diagenetic processes from continental margin sediments and seasonal stratification of the euphotic zone all appeared to be factors that can account for the distribution of Hg in the upper water. The deep and bottom waters of the North Pacific Ocean averaged 1.10±0.31 pM and were characterized by comparable Hg concentrations among the different cruises, which reinforced the homogeneous characteristics of this particular water mass. In addition, the comparison between the Atlantic and the Pacific deep water showed the presence of inter-ocean Hg fractionation resulting in Hg concentrations in the deep North Pacific Ocean being three- to sixfold lower than in the deep Atlantic.