Tracing the depth-dependent changes in organic carbon and nutrient fluxes using high-resolution <SUP>228</SUP>Ra profiles in the upper East Sea (Japan Sea)

초록

Vertical profiles of Ra-228 (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 Ra-228 in the deep ocean. In this study, we measured high-resolution Ra-228/Ra-226 ratio profiles (>21 depths) in the East Sea (Japan Sea) by mooring Mn-fiber. Using the measured Ra-228 profile from Ra-228/Ra-226 ratios and Ra-226 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 similar to 50% of the upward nutrients through 100 m support export production, and that dissolved organic carbon accounts for similar to 20% of carbon export. This Ra-228 approach provides a holistic understanding of carbon and nutrient cycles in the ocean.

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