4:15 PM - 4:30 PM
[ACG50-08] Distribution of Inorganic Iodine in Shallow Waters off Boso, Japan
Keywords:Iodide, Iodite, Southern Kanto gas field, Iodine-rich brine, Tropospheric ozone
The Kujukuri area of the Boso Peninsula is located on the eastern edge of the Southern Kanto gas field, with 99% of its natural gas consisting of methane. Moreover, the formation water containing methane is brine rich in iodine. Some iodine compounds in seawater, after migrating to the sea surface, may react with atmospheric ozone, a greenhouse gas, and contribute to ozone depletion in the troposphere. This study aimed to investigate the spatial distribution of inorganic iodine in shallow waters off Boso.
During the Shinsei Maru KS-24-12 cruise from July 10 to 14, 2024, we conducted a methane bubble survey using a multibeam echosounder, a precision depth recorder, and a quantitative echo sounder, identifying several methane venting sites. Water samples were collected using Niskin bottles and transferred to HDPE bottles. Ion chromatography, combining a high ion exchange capacity column and a UV detector, was used to determine iodine species in the samples.
The total inorganic iodine (TII = IO3- + I-) concentration ranged from 0.35 to 0.47 mM, with an average of 0.41 mM at the venting sites. These values were consistent with the global range of 0.4 to 0.45 mM reported in the world's oceans. The TII concentrations remained relatively constant in the vertical profile, indicating that TII is a conservative component even at venting sites. The mean concentration of I- was 0.038 mM, which was lower than the values observed in the open sea at the same season and latitude zone. The concentration of I- in surface water is known to depend on sea surface temperature (SST). The SST in the study area ranged from 18 to 20 ºC, which was lower than that in the surrounding sea area. This temperature difference may explain the observed lower I- concentration. In the future, the spatial distribution of iodine off Boso will be further clarified by comparing iodine concentrations obtained in a control plot, and the effects of methane venting will be discussed.
During the Shinsei Maru KS-24-12 cruise from July 10 to 14, 2024, we conducted a methane bubble survey using a multibeam echosounder, a precision depth recorder, and a quantitative echo sounder, identifying several methane venting sites. Water samples were collected using Niskin bottles and transferred to HDPE bottles. Ion chromatography, combining a high ion exchange capacity column and a UV detector, was used to determine iodine species in the samples.
The total inorganic iodine (TII = IO3- + I-) concentration ranged from 0.35 to 0.47 mM, with an average of 0.41 mM at the venting sites. These values were consistent with the global range of 0.4 to 0.45 mM reported in the world's oceans. The TII concentrations remained relatively constant in the vertical profile, indicating that TII is a conservative component even at venting sites. The mean concentration of I- was 0.038 mM, which was lower than the values observed in the open sea at the same season and latitude zone. The concentration of I- in surface water is known to depend on sea surface temperature (SST). The SST in the study area ranged from 18 to 20 ºC, which was lower than that in the surrounding sea area. This temperature difference may explain the observed lower I- concentration. In the future, the spatial distribution of iodine off Boso will be further clarified by comparing iodine concentrations obtained in a control plot, and the effects of methane venting will be discussed.