Journal of Oceanography, Vol. 58 (No. 6), pp. 775-785, 2001
Noriko Nakayama*, Shuichi Watanabe and Shizuo Tsunogai
Marine and Atmospheric Geochemistry Lab., Graduate School of Environmental Earth Science, Hokkaido University, Sapporo 060-0810, Japan
(Received 14 August 2001; in revised form 18 December 2001; accepted 23 January 2002)
Abstract: Major gases dissolved in seawater were accurately determined with a shipboard gas chromatographic method. The standard deviations were 0.28, 0.34 and 0.36% for N2, O2 and Ar, respectively. The method was applied to water from the northwestern North Pacific Ocean collected in May to June 2000. We got 127 duplicate seawater samples from the surface 200 m layer at 11 stations. The O2 concentrations obtained by this method agreed with those given by the Winkler method. All the seawater samples from the surface 200 m, especially those from the upper 30 m, were supersaturated with respect to atmospheric N2 and Ar concentrations. In the topmost 30 m layer, the degrees of supersaturation in the inventory were 2.7-4.3% for N2 (N2) and 1.7-2.6% for Ar (Ar), and their ratios, N2/Ar, ranged from 1.53 to 1.81. This supersaturation seems to be chiefly due to air bubbles injected into the water and dissolved due to the water pressure, because the N2/Ar ratio of the air is around 2. The amounts of air bubbles dissolved in the upper 30 m water were relatively large, with mean value of 0.41 ml/kg or 18.4 mmol/kg. The N2, Ar and N2/Ar values were all positively well correlated with the wind velocities averaged for the last 24 hours prior to sampling, allowing the conclusion to be drawn that the weaker the wind velocity, the dissolved gas composition approaches in equilibrium with the air; while the stronger the wind velocity, it approaches in the air composition.