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Under the highly stagnant condition at the Cariaco Trench, the lanthanide tetrad effect takes place abruptly within an extremely narrow depth range. The lanthanide tetrad effect maximum at the Cariaco Trench is observed at 256 m coinciding with its oxic/anoxic interface at 280 m. It is also interesting that the maximum value of the lanthanide tetrad effect at this trench is substantially the same as that in ordinary open ocean.

The depth profiles for degree of the lanthanide tetrad effect are presented for three seawater columns from an area in the western Indian Ocean off Madagascar. The features of the depth profiles for the upper layer of this area show a developed local variation. With the exception of this locality, the profiles as a whole exhibit the middle-valley patterns, in common with those in the North Pacific. The depth corresponding to the middle valley and the maximum value for the lanthanide tetrad effect observed in the column have been estimated for the first time from the curve drawn for the lanthanide tetrad effect profile. A comparison is made between Indian and North Pacific Oceans. There are clear differences between these two oceans in both the water depth of the middle valley and the maximum aberration due to the tetrad effect observed in the column. These characteristics are consistent with a picture presented thus far by oceanography researchers. That is, the difference between the two oceans is considered to reflect a distinction between southern hemisphere and northern hemisphere.

Vertical profiles of three separate lanthanide tetrad effects, ABEXEL, ABEX-3 and ABEX-4 in two seawater columns representing the subtropical and subpolar areas of the North West Pacific have been studied in connection with the hydrographical data. The depth profiles of ABEX-4 show remarkable difference between the subtropical and subpolar columns. The change of ABEX-4 between 640 and 1000 m at the subtropical site appears to be related to the discontinuous change in salinity between the corresponding depths.Besides, in general the gross features of the depth profile of ABEX-4 appears different from those of ABEXEL and ABEX-3.Key words : Lanthanide tetrad effect; seawater column; fourth lanthanide tetrad peculiarity; salinocline; stagnancy; ABEX-4.In 1979, Masuda and Ikeuchil~ found for the first time the lanthanide tetrad effect in seawater and marine samples. Further, Masuda et al. 2) (1987) recognized the presence of two conjugate types of the lanthanide tetrad phenomenon. But this effect was not quantified until Masuda et al.3~ developed mathematical expressions involving the application of a parabolic function to a set of observed values. The lanthanide tetrad phenomenon allows lanthanides to be divided into four subgroups each composed of four elements, i.e., La-Nd, Pm-Gd, Gd-Ho and Er-Lu, resulting in four cusps4),5~ between Nd and Pm, at Gd and between Ho and Er.For reasons of mathematical limitations and the limited number of lanthanides whose abundances can be precisely determined by stable isotope dilution, the first and second subgroups of the light rare-earth elements (REE) are assumed to have the same extent of the lanthanide tetrad effect.3~ The extent of this effect is exhibited as that of the intratetrad fractionation within the subgroup concerned. The degree of this fractionation can be evaluated as an extremum of aberration (ABEX) from a mathematically derived linear line or a quadratic coefficient of the parabolic function. Theoretically, there are four ABEX values,6~ ABEX-1, ABEX-2, ABEX-3 and ABEX-4. The foregoing assumption, however, implies ABEX-1= ABEX-2, which in general is denoted by ABEXEL.Our recent studies7~ based on the precise data obtained by Piepgras and Jacobsen8~ for water columns at two sites, 271-1 and 39-1, of the North West Pacific have revealed that the depth profile of ABEXEL can be potentially a new tool for oceanographic researches in combination with the isotopic composition of Nd.7~'9~ The site 271-1 (24° 17.2'N, 150° 28.2'E) represents the subtropical location, while the site 39-1(47° 00.0'N, 161° 08.2'E) marks the subpolar location.The purpose of this communication is to present the depth profiles of ABEX-3 and ABEX-4 resulting from the data presented by the same authors8~ for the same water columns. In the calculation using TRIMY method developed by Masuda and his coworkers,3>,5) the precise abundance data on La, Nd, Sm, Gd, Dy, Er, Yb and Lu are requisite. To our regret, the concentrations of Lu for the depths of 381 and 1046 m at site 271-1 are missing...

A depth profile of lanthanide tetrad effect is presented for a water column collected in the southeastern Atlantic Ocean. The ABEXEL value to quantitatively indicate the lanthanide tetrad effect is plotted against the water depth. The features of the ABEXEL-depth profile provide understanding for the unique marine conditions of the Antarctic Ocean, and are quite different from those observed in the eastern Indian Ocean and the eastern North Pacific Ocean. The water column can be divided into six layers based on the features of the ABEXEL-depth profile. The development of a unique structure in the ABEXEL-depth profile and its implications are discussed together with the potential use of the lanthanide tetrad effect in elucidation of our understanding of the formation of deep seawaters within the oceans.