226Ra in the Japan Sea and the residence time of the Japan Sea water

Abstract

The surface water of the Japan Sea contained 226Ra of 70 ± 4dpm m −3 which was nearly equal to that of the surface water in the North Pacific. The concentration of 226Ra in the Japan Sea deep water below 500 m was 151 ± 8dpm m −3 , showing a vertically and regionally small variation. This concentration of 226Ra in the deep water is unexpectedly high, because the Japan Sea deep water has a higher Δ 14 C value by about 50‰ than the Atlantic deep water containing the same 226Ra. One of the causes to be considered is larger contribution of 226Ra from biogenic particles dissolving in the Japan Sea deep water, but the Japan Sea is not so fertile in comparison to the Bering Sea. The other more plausible cause is the internal ventilation of the Japan Sea water, which means that the residence time of the Japan Sea Proper water is considerably long although the water is vertically mixed fairly well especially in winter. The ventilation may supply some amounts of radiocarbon and oxygen but does not change the inventory of 226Ra. The residence times of the Japan Sea deep water and of water within the Japan Sea are calculated by solving simultaneous equations for 226Ra and 14C with a three-box model to be 300–400 years and 700–1000 years, respectively.