The China Marginal Seas (CMS) are key land–ocean interfaces where riverine materials are transformed before entering the Pacific, yet the behaviour of dissolved rare earth elements (REEs) across these systems remains poorly understood. Based on a high-resolution, multi-seasonal dataset (2016–2018) from the Bohai Sea, Yellow Sea, Changjiang Estuary, and East China Sea, this study reveals three major findings.

First, the Changjiang Estuary acts as a seasonally oscillating biogeochemical reactor: during the high-discharge wet season, internal estuarine processes release dissolved REEs faster than salt-induced coagulation, reversing dilution and making the estuary a net REE source; during the low-discharge dry season, it becomes a net sink. Second, benthic diffusive fluxes from shelf sediments sustain non-conservative enrichment of light and middle REEs across the CMS, with fluxes (+5.0 to +8.0 pmol cm⁻² yr⁻¹) 3–13 times higher than abyssal values, confirming continental margins as primary benthic REE sources. Porewater profiles show exceptionally high Nd remobilisation (~+45.0 pmol cm⁻² yr⁻¹) in the Yellow Sea mud depocenter, identifying such depocenters as localized hot spots. Third, a severe anthropogenic gadolinium (Gd) overprint originates from the estuary (Gd anomalies up to 4.94, anthropogenic Gd >30 pmol kg⁻¹). Unlike particle-reactive geogenic REEs, synthetic Gd chelates remain truly dissolved, bypassing estuarine removal and travelling conservatively with the Changjiang Diluted Water across the shelf—confirming that the CMS exports medical microcontaminants to the Northwest Pacific.

Collectively, these results demonstrate that the CMS actively transforms riverine REE signatures rather than passively transporting them. The strong benthic–pelagic coupling and the conservative anthropogenic gadolinium anomalies illustrate that marginal seas serve two concurrent yet contrasting functions: they regulate natural biogeochemical cycling while simultaneously facilitating the offshore export of persistent anthropogenic contaminants, with measurable implications for regional water quality and marine ecosystem health.

Keywords: Rare earth elements · Anthropogenic gadolinium · China Marginal Seas · Estuarine filter · Benthic flux · Changjiang Estuary