Quantifying the sources of REE in the deep Philippine Sea: Insights from numerical modeling and field observations

Despite the importance of rare earth elements (REE) as tools for studying ocean processes, the quantitative contributions of various sources to deep-ocean REE distributions remain poorly constrained. In this study, we combine numerical modeling and in-situ observations to investigate the sources and transport mechanisms of REE in the deep waters of the Philippine Sea. Our findings indicate that REE concentrations are relatively low in the upper water column, primarily due to lateral transport from North Pacific Tropical Water (NPTW). At intermediate depths (<1500 m), REE distributions are controlled by the remineralization of sinking organic particles, as evidenced by a strong positive correlation with apparent oxygen utilization (AOU), while in the Upper Circumpolar Deep Water (UCDW, 1500–3000 m), water mass mixing dominates, accounting for 80–100 %. In deeper waters (>3500 m), water mass mixing, porewater diffusion, and remineralization contribute approximately 70–80 %, 7 ± 5 %, and 19 ± 5 % to the neodymium (Nd) budget, respectively, though these contributions may vary regionally. Between 2500 and 3500 m, significant increases in REE concentrations were observed, with estimated input fluxes from lithogenic material sources ranging from 17 to 24 mmol Nd m⁻² yr⁻¹. These elevated concentrations are likely influenced by deep ocean currents, lateral transport, and seasonal variations, consequently affecting the spatial distribution and transport distance of REE.

Our results suggest that dissolved REE concentrations [dREE] in the deep ocean may be influenced by slope-derived materials rather than remaining constant. The bottom diffusion flux of Nd from the deepest layer is estimated at 0.6 ± 0.4pmol cm⁻² yr⁻¹. Model simulations further indicate that the porewater contributes up to 7 ± 5 % of the dissolved Nd, with its influence on ΔNd exceeding 10 % at depth beyond 4000 m. These findings suggest that the diffusion of porewater into the bottom seawater would play a highly significant role in advancing future research on deep-sea trace-metal biogeochemical cycling.