Abstract
Vertical profiles of 228 Ra (half-life: 5.75 years) in the ocean provide valuable information on water mixing and ages of the upper ocean. However, its application is hampered by extremely low levels of 228 Ra in the deep ocean. In this study, we measured high-resolution 228 Ra/ 226 Ra ratio profiles (>21 depths) in the East Sea (Japan Sea) by mooring Mn-fiber. Using the measured 228 Ra profile from 228 Ra/ 226 Ra ratios and 226 Ra activities, together with other previously published data, we estimated the vertical eddy diffusivity (8.7–9.6 cm 2 s -1 ) in the permanent thermocline and water ages (10–15 years) in the upper 500–1000 m range. The estimated decomposition rate of organic carbon based on oxygen utilization rates using Ra-ages between 100 and 1000 m was 4.4 ± 0.8 mol C m -2 yr -1 . Our results show that ~50% of the upward nutrients through 100 m support export production, and that dissolved organic carbon accounts for ~20% of carbon export. This 228 Ra approach provides a holistic understanding of carbon and nutrient cycles in the ocean.