Important role of colloids in the cycling of 210Po and 210Pb in the ocean: Results from the East/Japan Sea

The activities of 210Pb and 210Po were measured for the truly dissolved (<10 kDa), colloidal (10 kDa–0.45 μm), and particulate (>0.45 μm) phases in the upper ocean (0–200 m) of the East/Japan Sea (EJS) in the summer of 2009. We report, for the first time, data on truly dissolved and colloidal 210Pb and 210Po in the ocean. The total 210Pb and 210Po activities in the EJS were in the ranges of 6.3–23 dpm/100L and 3.3–10 dpm/100L, respectively. In the upper ocean, the proportions of the truly dissolved, colloidal, and particulate phases were, respectively, 35 ± 3%, 48 ± 7%, and 17 ± 8% for 210Pb and 19 ± 2%, 36 ± 6%, and 45 ± 6% for 210Po. Using a net residence time model, which accounts for biological uptake and remineralization, the residence times of 210Po in the upper 100-m layer were calculated to be 92 ± 41, 63 ± 14, and 166 ± 45 days for the truly dissolved, colloidal, and particulate phases, respectively. The residence time of colloidal 210Po was several-fold longer than typical turnover times (<10 days) of high-molecular-weight dissolved organic carbon and colloidal residence times of short-lived 234Th in the surface water. This result suggests that 210Po turns over several times through the colloidal phase, perhaps together with other bio-reactive elements, before settling down from the upper ocean.