RARE-EARTH ELEMENTS OF SINKING PARTICULATE MATTER IN THE JAPAN TRENCH

Settling particles were collected with time-series sediment traps at f our 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 Marc h 5, 1991 to March 2, 1992. The composition of rare earth elements (RE Es) in the sinking particles selected for four seasons were measured t o investigate the REE fractionation during scavenging and settling tra nsport through the water column. The REE-particle associations were al so investigated by sequential digestion with three chemical treatments (acetic acid, mild HCl/HNO3, and HF/HNO3/HClO4). The measured vertica l particulate flux increased with depth suggesting that there is a sig nificant addition of particles, at least, to the deeper traps from hor izontal sources.No systematic variation was observed in the REE concen tration in the particles with respect to depth and season. The shale-n ormalized REE data show either hat or slightly heavy REE-enriched patt erns with a middle REE enrichment in which the maximum is usually cent red 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 midd le REE-enriched pattern with a maximum at Eu and distinctively negativ e Ce anomaly. The mild HCl/HNO3 leach had a middle REE-enriched patter n 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 RE E enrichment with a maximum at Gd. These results are compared with tho se reported previously for sinking and suspended particles in other pa rts of the oceans. It is suggested that REEs adsorbed on the suspended particles art: 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 copre cipitate the REEs attracted to the particle surfaces. (C) 1998 Elsevie r Science B.V. All rights reserved.