Radium Tracing Cross-Shelf Fluxes of Nutrients in the Northwest Pacific Ocean

Nutrients in surface waters of the northwest Pacific Ocean are known to be influenced significantly by anthropogenic inputs via the atmosphere. In this study, we evaluated the relative contribution of cross-shelf fluxes in the northwest Pacific Ocean. The shelf fluxes were estimated by multiplying the Ra-228 flux based on inverse modeling by the measured ratios of nutrients to Ra-228. The ratios were obtained from field observations of nutrients and Ra-228 in the Yellow Sea and the East China Sea in February 2017 when the water column was fully mixed vertically. The cross-shelf fluxes are estimated to be approximately 40% of the atmospheric depositional flux of nitrogen and 2 orders of magnitude greater than that of phosphorus. These results suggest that the cross-shelf fluxes of nutrients are an important, yet previously underappreciated, pathway of nutrients to the North Pacific Ocean, controlling carbon sequestration and biological production.