Spatiotemporal variation of dissolved rare earth elements in the North Pacific Subtropical Gyre: Influence of biogeochemical cycling and application in tracing deep water

The North Pacific Subtropical Gyre (NPSG), the largest continuous marine ecosystem, significantly influences the cycling of trace elements through biological and seawater interface processes. Understanding these processes, particularly their seasonal impacts, is crucial for tracing oceanic dynamics, yet remains underexplored. In this context, rare earth elements (REEs) in seawater serve as valuable tracers for studying these processes. This study presents the spatiotemporal distribution of dissolved REE concentrations based on two GEOTRACES-CHINA process study cruises (GPpr15) conducted in summer and winter, along with published results from a GEOTRACES-CHINA cruise (GP09) during spring. Above the depth of chlorophyll maxima (DCM), REE levels were lowest in winter compared to summer, reflecting enhanced scavenging of REEs by particulate matter, primarily driven by increased chlorophyll-α during winter. In subsurface to intermediate waters (150–1000 m), release efficiencies (∼0.04 pmol Nd/μmol apparent oxygen utilization) exhibited no seasonal variations in the NPSG. These efficiencies were consistent with those in the North Atlantic Gyre at similar latitudes, but differed from those at higher latitudes (∼0.15), which may be attributed to variations in the plankton community structure across regions. Furthermore, inputs of slope sediments to intermediate waters (500–1000 m) off the Philippine Islands were identified using Ce anomalies and Yb/Nd ratios. These inputs peaked during winter, with Nd contribution from slope sediment accounting for 15–43 % of the total Nd concentration. In deep waters (>4500 m) of the Philippine Basin (stations K2/K2b, K3, K13/13a, and K14), elevated REE concentrations indicated extra inputs from the seafloor and lateral transport from the Philippine Islands, contributing 17 ± 6 % of Nd. The contributions of Nd from settled particles and water mass mixing were estimated at 10 ± 5 % and 73 ± 3 %, respectively. Additionally, Yb reliably traced the distribution of lower circumpolar deep water in the Philippine Basin. In summary, these findings highlight the significant influence of biogeochemical processes on seasonal variations of REEs above the DCM and underscore the potential of REE in tracking deep water transport.