Iodine cycling in the subarctic Pacific Ocean: Insights from 129I

As the largest iodine reservoir on the Earth's surface, the ocean plays a vital role in global iodine cycling. Here, we present results on concentrations of stable-and radio-iodine (127I and 129I) of total iodine (TI), iodide (I-), and iodate (IO3-) in seawater samples collected in the top 800 m of the subarctic Pacific Ocean. The speciation distributions of both 127I and 129I showed large variations in depth and geographical loca-tion. The T127I concentrations ranged from 321 to 484 nM, were lower at the surface, and increased with depth. In all depth profiles of T129I, the maximum concentrations occurred in the subsurface layer, below which, the T129I concentrations decreased with depth and ranged from 0.2 to 2.1 x 107 atoms/L. Besides, the 129I/127I atom ratios in iodide were significantly higher than those in iodate. It can be inferred from existing research that anthropogenic 129I from atmospheric deposition is the overwhelming source of 129I in the subarctic Pacific Ocean. On this basis, we successfully simulated the 129I vertical dispersion using a one-dimensional vertical diffusion model. The unbalanced distribution of 129I among different iodine species provided useful information for demonstrating the inevitability of the oxidation process of iodide in nature, although whether this oxidation occurred in the ocean remains to be further explored, and the indispensable contribution of iodate to the iodine removal processes in the upper ocean. Our study suggests that anthropogenic 129I can be used as an effective intrinsic tracer for the study of iodine cycling.(c) 2023 Elsevier Ltd. All rights reserved.

Automated summary

This study investigated the concentrations of stable and radioiodine in seawater samples collected in the subarctic Pacific Ocean. The results showed large variations in the distributions of iodide and iodate concentrations with depth and geographical location. The study also revealed that anthropogenic 129I from atmospheric deposition is the dominant source of 129I in the subarctic Pacific Ocean.