North Pacific Intermediate Water Studied Chiefly with Radiocarbon

The importance of the North Pacific Intermediate Water as a sink for the anthropogenic carbon dioxide has been examined by mapping chemical and radiochemical properties at two isopycnal surfaces of σθ of 26.6 ad 27.2 obtained in 1970’s. Its radiocarbon contents in 1980’s were determined for comparison. The isopleths of depth and salinity at the two isopycnal surfaces obviously show that the intermediate layer of the entire mid-latitudes of the North Pacific is occupied by a similar water mass. The distributions of dissolved oxygen contents and Si/N ratios in the intermediate water indicate its source in the northwestern North Pacific and its sink in the eastern Pacific. The ∆14C values clearly designate the intrusion of the artificial radiocarbon of mostly 1960’s origin into the upper intermediate water of the western North Pacific having its maximum in the subarctic zone of 40–45°N and 160–180°E in 1973. The maximum region for tritium is much broader extending to the north. These suggest that the subboreal region is active in the gas exchange and/or the warm water residing for a long time at the surface and flowing into the region across the subarctic front sinks quickly in winter. At the lower isopycnal surface, the increase in ∆14C value for 14 ± 4 years was around 27‰, which is smaller than that expected from the total carbonate increase, indicating an active isopycnal mixing. 1