Marine sediments as rare earth elements pollution archives: A review of trends, sources, and distribution

Rare earth elements (REEs), including the lanthanides, scandium, and yttrium, are emerging contaminants of increasing concern due to their widespread use in high-tech, green energy, and biomedical applications. This review synthesizes 31 years (1995-2025) of data on REE accumulation in marine sediments across 31 countries and different marine environments, focusing on temporal trends, spatial distribution, sources, and ecological implications. Total REE concentrations (ΣREE+ScY) ranged from 50.47 to 5445,723 ng g^-1, with estuaries identified as major anthropogenic sinks. Five pollution phases were recognized, culminating in a major anomaly period (2012-2016) marked by an exceptionally high gadolinium (Gd) concentration recorded in 2015, when Gd reached 3,982,885 ng g^-1and accounted for 71.7% of the total REE mass - an anomaly attributed to the discharge of gadolinium-based contrast agents (GBCAs) from medical imaging facilities into coastal environments. A subsequent acceleration phase (2017-2025) was associated with heavy REE enrichment from electronic waste and high-tech manufacturing. Major contamination hotspots were identified in China, Brazil, and India. Reported bioaccumulation in bivalves and macroalgae highlights the ecological relevance of sediment-bound REEs. These findings indicate increasing global REE contamination and support the need for standardized monitoring, stricter discharge controls, and improved wastewater treatment to protect marine ecosystems.