Guebuem Kim

We measured seepage rates of coastal groundwater from a volcanic island, standing in the South Sea of Korea. The seepage rates measured along the sandy coast were in the range 50–300 m/yr, which are much higher than those reported from typical continental coast. The ¹⁸O tracer in potential seeping waters signifies that they are mixtures of basal groundwater and pristine seawaters. On the eastern coast of Jeju, almost all groundwater discharge is attributed to recirculating seawater, while fresh groundwater contributes about 20% of the total submarine groundwater discharge (SGD) on the western coast of Jeju. Thus, SGD appears to be important for the management of water resources in western Jeju. The measured radionuclides (²²⁸Ra, ²²⁶Ra, and ²²²Rn) and nutrients in the groundwater suggest that the discharge of both fresh and recirculated seawater will have a significant influence on the budget of coastal nutrients and other chemical constituents in this region.

During a 3‐year record of ²²⁸Ra/²²⁶Ra ratio profiles (from nine cruises) in the Sargasso Sea, two profiles in the Spring (April 1997 and 1998) show distinct subsurface maxima. Such maxima in the radium isotope ratio record the subsurface advection of surface waters, such as subtropical mode waters. The April 1997 maximum occurred below the euphotic zone (between 300–400 m) and was also coincident with that of POC and dissolved oxygen, supporting the subsurface advection of surface waters. This single event represents a large isopycnal injection (23 g m⁻²) of organic carbon into the subsurface ocean. Although we cannot quantify carbon export by this process on a basin scale based on these single‐site measurements, it may be an important pathway for short‐term oceanic carbon export.