1231 / 2024-09-20 18:07:48

Dissolved rare earth element distributions in the Yellow River estuary: role of submarine groundwater discharge and particle dissolution

Dissolved rare earth element,particle dissolution,Yellow River estuary

Rare earth elements (REEs) serve as valuable tracers for tracking oceanic processes. However, the source of approximately 90% of REEs (such as Nd) in the ocean remain unknown. Research indicates that the submarine groundwater discharge (SGD) and the dissolution of particulate matter could account for critical sources, yet our understanding on these sources and quantitative evaluation is still limited. To quantify the various sources of REEs in the estuary, we collected dissolved REEs in the Yellow River estuary, which is one of the world's most turbid estuaries, as well as in the Yellow River and surround groundwater during the wet season (August 2022) and the dry season (April 2023). The Yellow River estuary exhibited a typical REEs distribution, namely an initial decline followed by a gradual increase with increasing salinity in both seasons. The initial reduction is likely due to the removal of colloidal REEs by salt-induced flocculation. The subsequent increase could be contributed by SGD and particle release. The average concentration of REEs in the groundwater is higher than that in the Yellow River estuary (Nd:231 pmol/kg vs 72 pmol/kg), suggesting a potential source of REEs for the estuary. We further estimated the SGD-derived Nd flux to be up to 27 mol/d using radium isotopes mass balance, which is higher than riverine flux of 7 mol/d in these two seasons. To quantify the effect of particles, we conducted laboratory cultivation experiments on particles, suggesting that the release of particulate matter is a significant source of REEs in the Yellow River estuary, with a flux comparable to that from river. The flux of dissolved Nd from particulate matter is about 8 mol/d, while the flux from the Yellow River during the dry season is about 5 mol/d and increases to 8 mol/d during the wet season. In conclusion, this study provides important insights into the sources and fluxes of REEs in the Yellow River estuary, highlighting the significance of SGD and particulate matter dissolution in shaping the REEs distribution and budget. Future research should focus on exploring these sources in other estuaries to understand their global significance.