Distribution of Anthropogenic <sup>129</sup>I in the Western South China Sea and Its Application for Tracing the Sources and Movement of Pollution
Mengting Zhang, Xiaolin Hou, Luyuan Zhang, Jixin Qiao, Ruiqin Gao, Qi Liu
Abstract
Anthropogenic 129 I has been dispersed all over the world and could be utilized as an oceanographic tracer based on its conservative nature in the ocean. The first datasets of 129 I and 127 I were obtained by analysis of seawater of 36 water columns collected in the western South China Sea during August–September 2018. The measured 129 I concentrations decreased with depth from (0.93–1.61) × 10 7 atoms/L in the upper 200 m to (0.04–0.14) × 10 7 atoms/L at 1500 m, indicating a clear anthropogenic source in the upper layer, mainly originated from the global fallout. The riverine input of the deposited 129 I on the catchment area of the Mekong River is an important source besides the direct deposition in the seas. The water mass with high 129 I from the Mekong River water moves to the east at 11°N by the North Nansha Current in the surface layer (2–25 m). The exponentially decreasing 129 I level with depth indicates that the vertical dispersion of 129 I from the upper to the lower layer was mainly through slow diffusion, and the deep water at more than 1500 m was not significantly contaminated by the upper layer water at least in the past 70 years.