High-Resolution Coral Records of Rare Earth Elements and Yttrium in Seawater Driven by Submarine Groundwater Discharge in a Basalt Island: A Case Study in the Northern South China Sea

Abstract

Rare earth elements and yttrium (REY) are widely employed as tracers for oceanic geochemical processes, which require a thorough understanding of their sources, sinks, and drivers of variability in the marine environment. However, significant uncertainties exist in the marine REY cycle, the so-called “missing Nd flux,” particularly regarding the contribution of submarine groundwater discharge (SGD) and basalt weathering in volcanic islands. Here, we present a 10-year record of monthly Porites coral REY parameters from Weizhou Island, a volcanic island built up underwater from basalt eruptions during the Quaternary, to investigate the sources and seasonal characteristics of surface seawater REY. Results demonstrate a robust seasonal cycle in the coral Y/Ho ratios, exhibiting a strong correlation with the rainfall-controlled SGD on monthly timescales and East Asian Summer Monsoon on interannual timescales, both of which are associated with basalt weathering. Combined with multiple climatic and environmental data, we find that coral Nd_N/Yb_N ratios may be mainly controlled by precipitation associated with SGD and the adsorption-desorption processes of marine biogenic particles, whereas coral REY/Ca ratios are influenced by the remobilization of sediment driven by winter monsoon. Our research suggests that the high coral Y/Ho ratios may be primarily influenced by basalt weathering during the wet season, when SGD from the island is the chief source of REY to the coastal waters. This study provides new insights into the sources and characteristics of marine REY in volcanic islands, highlighting the potential for SGD-driven REY fluxes from basaltic islands.