228Ra flux in the northwestern Pacific marginal seas: Implications for disproportionally large submarine groundwater discharge

To determine the fluxes of 228Ra (half life: 5.75 years) to the northwestern Pacific marginal seas, 228Ra data were compiled for the East China Sea, Yellow Sea, South Sea, and East/Japan Sea. Most of the 228Ra data in the East Sea were newly obtained through the onboard surveys in July 2012, October 2012, and March 2013. A 228Ra mass balance model was established by considering the input fluxes from the Pacific Ocean, river, atmosphere, sediments, and submarine groundwater discharge (SGD) and the output fluxes by radioactive decay and to the Pacific Ocean and the deep East Sea through vertical mixing. Using this model, the 228Ra flux through SGD was estimated to be at least 2.3 × 1015 dpm yr−1 in the entire area of these marginal seas, which is about 20% of the total 228Ra input flux to this region. This SGD-driven 228Ra flux to this region accounts for about 2% of that to the global oceans. This result implies that the magnitude of SGD in this region could be disproportionally larger than those in general oceanic margins. We suggest that SGD may play a critical role in the delivery of terrestrial materials to these large marginal seas, perhaps due to large tidal pumping.