Determination of the flow rate and mixing ratio of the Changjiang diluted water in surface seawater of the northwestern Pacific marginal seas using radium isotopes

Abstract

Ra isotopes (223Ra and 228Ra) in surface seawater of the northwestern Pacific marginal seas, including the East China Sea, southern sea of Korea, and the Yellow Sea, were measured to estimate the flow rate and to calculate the relative contribution of the Changjiang diluted water (CDW) during the summer season. On the basis of the horizontal distribution of 223Ra activities, current speed of the CDW from the river mouth to 450 km offshore northeast was estimated to be 16 ± 1 cm/s, which was similar to that determined in previous studies that used physical methods such as surface drifter buoys and numerical modeling. Moreover, the relative contributions of CDW at each sampling station were calculated using mass balance equations for the salinity and 228Ra activity. The relative contribution of CDW in surface water was more than 50% in most area of the northern East China Sea, the Yellow Sea, and even about 30% to the Korea/Tsushima Strait, which is approximately 800 km from the river mouth. These results suggest that CDW can be of high significance in the biogeochemistry of surface water of these northwestern Pacific marginal seas during the summer monsoon period.