[A-CG46] Biogeochemical linkages between the surface ocean and atmosphere

[ACG46-P01] Spatial distribution of chemical-state-specific iodine species in the tropical western North Pacific surface water

*Yoko Iwamoto¹, Kazuhiko Takeda¹, Hisahiro Takashima³, Fumikazu Taketani², Yugo Kanaya² (1.Graduate School of Integrated Sciences for Life, Hiroshima University, 2.The Japan Agency for Marine-Earth Science and Technology, 3.Faculty of Science, Fukuoka University)

Keywords:iodate, iodide, The tropical Pacific, Ion chromatography

Iodine in seawater exists mainly as iodate (IO₃^-) and iodide (I^-) ions, with a total reported to be approximately 0.4 μM. Iodine is present mostly as IO₃^- below 200 m, and approximately half of the iodine in the surface layer exists as I^-, as reduced by organisms. It is well known that iodine in the atmosphere is oceanic origin and involved in various chemical reaction in the atmosphere. Tropospheric ozone, known as one of the greenhouse gases, is reported to be efficiently depleted by the reaction with iodine species over the tropical oceans, where SST reaches nearly 30 ℃. However, the mechanism of the reaction is not fully understood. To understand the iodine cycle in the oceans and evaluate its impact on the atmospheric environment, it is essential to use methods for quantifying iodine in seawater according to its chemical state. The objective of this study is to obtain spatial distributions of iodine in the western North Pacific with high SST, using an originally developed ion chromatography method.
Surface seawater samples were collected during R/V Mirai MR21-03 cruise which sailed the tropical western North Pacific in early summer. The water samples were collected from the faucet for research use on board. All the seawater samples were collected in 125-mL HDPE bottles. The bottles were stored at ∼4 ℃ on board. An ion chromatography combing a high ion exchange capacity column and a UV detector were used for determination of five anionic species (IO₃^-, I^-, NO₂^-, NO₃^- and Br^-) in samples.
Previous studies have indicated that the I^- concentration in surface water is related to SST. In this study, however, although oceanic region with SST of 30 ℃ or higher were continuously sampled, the concentration of I^- varied relatively largely and SST alone could not explain the concentration fluctuation. The relationship with other dissolved substances and organic iodine will also be discussed in the presentation.