Rare earth elements of sinking particulate matter in the Japan Trench

Abstract

Settling particles were collected with time-series sediment traps at four depths (1174 m, 3680 m, 5687 m, and 8688 m) moored at the deepest site (water depth 9200 m) of the Japan Trench for the period from March 5, 1991 to March 2, 1992. The composition of rare earth elements (REEs) in the sinking particles selected for four seasons were measured to investigate the REE fractionation during scavenging and settling transport through the water column. The REE-particle associations were also investigated by sequential digestion with three chemical treatments (acetic acid, mild HCl/HNO 3, and HF/HNO 3/HClO 4). The measured vertical particulate flux increased with depth suggesting that there is a significant addition of particles, at least, to the deeper traps from horizontal sources. No systematic variation was observed in the REE concentration in the particles with respect to depth and season. The shale-normalized REE data show either flat or slightly heavy REE-enriched patterns with a middle REE enrichment in which the maximum is usually centred at Gd, but sometimes at Eu and a significantly positive Ce anomaly. The chemical digestion experiments indicated characteristic patterns for shale-normalized REEs. The acetic acid leach showed a marked middle REE-enriched pattern with a maximum at Eu and distinctively negative Ce anomaly. The mild HCl/HNO 3 leach had a middle REE-enriched pattern with a maximum at Gd but roughly no Ce anomaly. The residue showed a rather flat REE pattern with a marked positive Ce and often middle REE enrichment with a maximum at Gd. These results are compared with those reported previously for sinking and suspended particles in other parts of the oceans. It is suggested that REEs adsorbed on the suspended particles are altered into some refractory phases prior to or during transport by settling. The possibility exists that authigenic barite formed in the microenvironment around aggregates may effectively coprecipitate the REEs attracted to the particle surfaces.