ABSTRACT

Dissolved yttrium and rare earth elements (YREEs) have been measured in the Antarctic Circumpolar Current Region, southwest of Australia. Their vertical features are fairly smooth, irrespective of the different water masses in the water column, suggesting that the biogeochemical processes including reversible scavenging reactions and sinking particulate matter are important control on their distribution. Comparison to the previous data at nearly the same latitude of ∼40°S in the Southern Ocean of the Atlantic sector (German et al., 1995) indicates that our concentrations are significantly depleted in the lighter REEs. The difference in their mean concentrations systematically decreases with increasing atomic number, i.e., 67% for La, 40% for Nd, 15% for Sm and less than 5% for the REEs heavier than Tb and Y. Gadolinium is an exception to this trend with a deficit that is anomalously high at ∼36% compared to those of its neighbors, Eu and Tb. Negative Gd-anomalies exceeding 30% are recorded for the subsurface waters, which contrast with positive Gd anomalies of up to ∼20% in the North Pacific Deep Water (Alibo and Nozaki, 1999). The relative deficit of 62% for Ce(IV), perhaps fortuitously, fits well between those of La and Nd, despite its vertical profile being different from those of strictly tri-valent YREE(III)s. These observations cannot be explained simply by the REE fractionation during scavenging of particulate matter and/or regional variation of scavenging intensity alone, and suggest that there must be a REE compositional difference in the external sources, e.g., from shallow water sediments and lithogenic materials sinking through the water column. The unique REE(III) patterns of the Circumpolar Deep Water (CDW) and the Antarctic Bottom Water (AABW) with a LREE depletion in the southeastern Indian Ocean appear to be originated from igneous rocks around the Indonesian Archipelago. This is in contrast to the REE(III) patterns of a LREE enrichment with a maximum at Nd-Sm, reflecting continental sedimentary rocks, in the CDW and AABW in the South Atlantic and the western Indian Ocean. The markedly different REE patterns in the eastern and western components of CDW and AABW indicate that they can serve as novel tracers in elucidating the deep water circulation of the Indian Ocean.