Abstract
The vertical activity concentration distributions of the radium isotopes, 226 Ra and 228 Ra, 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 228 Ra present from the surface to 400 m depth by analyzing large seawater samples. Low 228 Ra and high 226 Ra activity concentrations in the surface layer in the WSAG were strongly influenced by upwelled deeper water with nutrients. The activity concentration distribution of 226 Ra 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. 228 Ra 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 228 Ra and nitrate according to 228 Ra decay indicated that the vertical transport by eddy diffusion was a minor process for the 228 Ra and nitrate fluxes, and lateral transport mainly affected 228 Ra 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 228 Ra in the intermediate depth.