Dissolved rare earth elements in the eastern Indian Ocean: chemical tracers of the water masses

摘要

New data on vertical profiles of dissolved rare earth elements (REEs) in the southeastern Indian Ocean are presented. The compositions of dissolved REEs are uniquely characterized by strong depletion in the light REEs; the extent of depletion decreasing systematically from La to Sm, as compared to those in the western Indian Ocean and the South Pacific. Dissolved Gd is also anomalously depleted in many of the water masses occupying this region. The LREE depleted patterns with a Gd deficit appear to be derived by input from volcanic sources around the Indonesian Archipelago, although the exact location and mechanism are not yet well understood. Nevertheless, the unique REE patterns can be used to trace the pathways of the water masses in the eastern Indian Ocean. The salinity minimum centered at ～1100 m at 40°S in the Antarctic Circumpolar region, southwest of Australia, can be considered as a boundary between the upper and the deeper layers, since the REE patterns around that depth are clearly different from those of the Antarctic Intermediate Water (AAIW) at 30°S on the Perth Basin and can be produced largely by mixing of the two waters. The unique REE patterns of AAIW with no significant Gd deficit between 497 and 2474 m in the Perth Basin indicate that the waters must be derived from somewhere else by lateral transport. The upper portion of the intermediate waters may be derived from a near-surface region in the Subantarctic zone, including the southernmost station in this study (40°S), and the deeper part may be contributed from the South Pacific reaching the area from south of Tasmania. The northward flow of the intermediate waters can be traced by the characteristic REE patterns at least to 10°S in the southeastern Indian Ocean, but does not reach the Bay of Bengal. The surface waters in the North Australia Basin have REE patterns similar to those of the surface waters in the western equatorial Pacific, suggesting a strong influence of Pacific waters to the region north of Australia. The light REE and Gd-deficient patterns are also found in the deep and bottom waters of the southeastern basins as well as in the surface waters from subtropical to temperate offshore regions, and they are presumably due to the influence of the Indonesian Archipelago. The unique pattern is clearly traced to the station at 40°S, 110°E, in the South Australia Basin, suggesting a southward flow of deep and bottom waters along the eastern edge of the Indian Ocean.