3:30 PM - 3:45 PM
[ACG41-01] Vertical profile of inorganic iodine concentration in the western North Pacific surface layer during summer
Keywords:iodine cycle, western North Pacific, ion chromatography, tropospheric ozone, barrier layer
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 a part of the iodine in the surface layer exists as I-. Previous studies have reported that surface I- concentrations are higher in waters with high sea surface temperature (SST), but SST alone cannot explain the spatial distribution of I-. Tropospheric ozone, known as one of the greenhouse gases, is efficiently reduced by the reaction with I- at the surface oceans. From the perspective of future climate projections, it is important to understand the I- distributions, which may control tropospheric ozone concentrations. The purpose of this study is to investigate the vertical profile of inorganic iodine in the western North Pacific surface waters using an ion chromatography method originally developed, and to clarify the controlling factors of I- distributions.
Surface seawater samples were collected during R/V Hakuho Maru KH-23-3 and R/V MIRAI MR23-05 Leg 1 cruises which sailed the western North Pacific in June to July 2023. The water samples were collected by the Niskin bottles and transferred to 125-mL HDPE bottles. The bottles were stored at 4 ºC on board. An ion chromatography combining a dodecylammonium-coated ODS column with a UV detector was used for determination of five anionic species (IO3-, I-, NO2-, NO3- and Br-) in samples.
Surface I- concentrations were 0.050 μM in the subarctic (47oN) and 0.113 to 0.177 μM in the subtropical (20-35oN) and tropical regions (<20oN), and the tendency of higher I- concentrations in high SST waters was consistent with previous studies. In subtropical waters, I- concentration decreased with depth and could not be detected below 200 -500 m. On the other hand, in the tropical region, a "blocky" distribution with a constant and high I- concentration up to 100 m in the surface layer was observed. The “blocky” distribution coincided with the formation of layer with the strong stratification of salinity. Below the layer of maximum salinity, I- concentration decreased with depth and was not detected below 200 m. These results suggest that salinity and water mass structure play an important role in determining surface I- concentration in the ocean.
Surface seawater samples were collected during R/V Hakuho Maru KH-23-3 and R/V MIRAI MR23-05 Leg 1 cruises which sailed the western North Pacific in June to July 2023. The water samples were collected by the Niskin bottles and transferred to 125-mL HDPE bottles. The bottles were stored at 4 ºC on board. An ion chromatography combining a dodecylammonium-coated ODS column with a UV detector was used for determination of five anionic species (IO3-, I-, NO2-, NO3- and Br-) in samples.
Surface I- concentrations were 0.050 μM in the subarctic (47oN) and 0.113 to 0.177 μM in the subtropical (20-35oN) and tropical regions (<20oN), and the tendency of higher I- concentrations in high SST waters was consistent with previous studies. In subtropical waters, I- concentration decreased with depth and could not be detected below 200 -500 m. On the other hand, in the tropical region, a "blocky" distribution with a constant and high I- concentration up to 100 m in the surface layer was observed. The “blocky” distribution coincided with the formation of layer with the strong stratification of salinity. Below the layer of maximum salinity, I- concentration decreased with depth and was not detected below 200 m. These results suggest that salinity and water mass structure play an important role in determining surface I- concentration in the ocean.