Inputs of rare earth elements (REEs) and iron from submarine groundwater discharge (SGD) and glacier melting to the ocean

Abstract

To evaluate the role of submarine groundwater discharge (SGD) from permeable volcanic island as a source of rare earth elements (REEs) and dissolved Fe (DFe) in the coastal ocean, I 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 middle REEs (MREE) enrichments 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 both REE and DFe 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 both Nd and DFe flux from the entire Jeju Island during this summer was 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 and DFe fluxes to the ocean. Furthermore, I estimated the SGD associated discharge of REEs into two semi-enclosed coastal bays in the southern coast of Korean Peninsula. The mass balances of REEs demonstrated that the REE fluxes through SGD were also predominant sources of REEs in both bays. The Nd inputs through the total SGD from the two small coastal bays, Gamak Bay (148 km2) and Hampyeong Bay (85 km2), were estimated to be 0.7×104 – 1.3×104 mol y-1, which is 0.06 – 0.3% of the total Nd fluxes from global rivers. In this region, coastal seawater was observed to have a substantially higher middle REEs (MREE), which appears to be due to a large discharge of groundwater that is highly enriched with MREE. High MREE bearing waters were observed over the entire shore of the southern coast of Korea, implying that such high SGD-driven REE fluxes are common in coastal area. These results suggest that the SGD-driven REE fluxes may contribute considerably to the global budget of REEs in the ocean. In order to evaluate the terrestrial Fe and REE inputs from modern glacier melting to the Southern Ocean, I measured the dissolved Fe (DFe) and REEs in ice, snow, and coastal seawater of Marian Cove in northernmost Antarctica (62°S). The concentrations of total DFe and all REEs increased from bay-mouth to the glacier valleys and decreased from the shoreline to the center of the bay due to the significant inputs of terrigeneous material enriched-glacier melt to bay waters. The ice and snow samples showed elevated MREE which seems to be originated from Patagonian origins. The enrichment of both MREE and HREE was also identified in bay waters, resulting from mixing between ice melt and open ocean water. The glacier-borne DFe concentration (27±2 nM) in seawater was estimated on the basis of the relationship between salinity and DFe in coastal seawater of Marian Cove. When this value is applied to the entire amount of annual glacier melting, the DFe input from glacier melt water/particles (GMWP) in Antarctica is estimated to be approximately 4.5×107 - 5.6×107 mol Fe yr-1, which is an order of magnitude higher than that of atmospheric dust deposition to the Southern Ocean (>65°S). Thus, this result suggests that increased modern glacier melting in Antarctica related to global warming may cause significant addition of new DFe inputs to the Southern Ocean, thereby resulting in enhanced CO2 drawdown.