Large fluxes of rare earth elements through submarine groundwater discharge (SGD) from a volcanic island, Jeju, Korea

To evaluate the role of submarine groundwater discharge (SGD) as a source of rare earth elements (REEs) in the coastal ocean, we estimated the SGD associated discharge of REEs into two semi-enclosed coastal bays off a volcanic island, Jeju, Korea. The coastal brackish groundwater showed pronounced enrichments of middle REEs (MREE) relative to light REEs (LREE) and heavy REEs (HREE) when normalized against the upper continental crust (UCC), whereas seawater samples outside the bays showed a HREE enrichment pattern. The enrichment of both MREE and HREE was clearly identified in bay waters, resulting from mixing between groundwater and offshore seawater. The mass balances of REEs demonstrated that the REE fluxes through SGD were two to three orders of magnitude higher than those that occurred through the other sources, such as diffusion from bottom sediments and atmospheric dust fallout. The SGD-driven Nd flux from the entire Jeju Island during this summer was approximately 120 ± 60 mol d− 1, which is comparable to the Nd fluxes from major rivers (i.e., Mississippi River). Our results imply that highly permeable oceanic islands are particularly important for REE fluxes to the ocean.