Vertical Profiles of Ra-226 and Ra-228 Activity Concentrations in the Western Subarctic Gyre of the Pacific Ocean

• Published in: Frontiers in Marine Science
• Main Authors: Tazoe, Hirofumi, Obata, Hajime, Hara, Takuya, Inoue, Mutsuo, Tanaka, Takahiro, Nishioka, Jun
• Format: Article in Journal/Newspaper
• Language: English
• Published: Frontiers Media
• Subjects: radium-228; radium-226; Subarctic Gyre of the Pacific Ocean; vertical flux; lateral transport; 452; Pacific; Subarctic
• Online Access: http://hdl.handle.net/2115/86324; https://doi.org/10.3389/fmars.2022.824862

The vertical activity concentration distributions of the radium isotopes, Ra-226 and Ra-228, which are useful as radiotracers, were obtained for the first time in the western Subarctic Pacific Gyre (WSAG). It was possible to detect short-lived Ra-228 present from the surface to 400 m depth by analyzing large seawater samples. Low Ra-228 and high Ra-226 activity concentrations in the surface layer in the WSAG were strongly influenced by upwelled deeper water with nutrients. The activity concentration distribution of Ra-226 especially was in good agreement with the silicate concentration distribution, which was consistent with previous reported findings. These distributions were uniform from the surface to 100 m of the dichothermal layer due to vertical mixing in winter. Ra-228 activity concentration decreased with water depth below the pycnocline and reached the undetectable level at 600 m which was within the oxygen minimum layer. Estimations of vertical fluxes of Ra-228 and nitrate according to Ra-228 decay indicated that the vertical transport by eddy diffusion was a minor process for the Ra-228 and nitrate fluxes, and lateral transport mainly affected Ra-228 in the intermediate warm water, that is, the mesothermal layer below the main pycnocline. Vertical mixing or submarine underground water and lateral transport to the WSAG could yield this Ra-228 in the intermediate depth.