Depth Profiles of the Lanthanide Tetrad Effect in Seawater Columns, Centered on a Study of Water Column at the Cariaco Trench off the Venezuelan Coast

Abstract

In 1979, Masuda and Ikeuchi found the occurrence of the lanthanide tetrad effect in the marine environments. It is observed ideally as four cyclic slight curves spanning the atomic number range of lanthanides. However, this effect was not quantified until recently. Masuda e t al. (1994) devised a mathematical method to extract the extent of the lanthanide tetrad effect and gave a numerical expression to it, which is based on an application of a parabolic function to the slightly curved plotting. This value is expressed as an extremum of aberration from an rectilinear linear line assumed for a case where there is no affection of the lanthanide tetrad effect. The aberration extremum (ABEX) for light rare earth elements is designated as ABEX-1, or ABEXEL. However note that, in order to get virtually significant values for ABEXEL, La, Nd, Sm and Gd should be determined with precisions better than 1-2%. For example, if 10% error is involved for any of them, the resultant ABEXEL value is subject to serious uncertainty, potentially leading us to erroneous assessment. According to our observation, the ABEXEL values for seawaters generally fall between ~0.0 and -0.22. Needless to say, the greater absolute values refer to the higher lanthanide tetrad effect, while ~0.0 to little tetrad effect. Following the interpretation by Masuda (1996), the lanthanide tetrad effect in seawater is related with the internal electron condition within the lanthanide atom and the outer physico-chemical circumstance surrounding the atom concerned: the internal atomistic factor for 'free' lanthanide ion is that the atoms of lanthanides have cyclic change in steric symmetry offelectronns as well as stability about 4f electron configuration with atomic number and , effectively, in resultant inter-electronic repulsion between 4f electrons, while the outer strong biased electric field factor results from hydration of of free ions in water which gives rise to the high polarizing field due to the high polarizability of water molecules. A combination of these factors can lead to an internal slight cyclic fractionation within four adjacent lanthanides constituting a tetrad subgroup. Factors other than mentioned above can somewhat affect the effect under consideration. Virtually, the effect in question reflects the cycles of variation in steric f electron symmetry responsible for slight cycles of internal difference in chemical bonding between the lanthanide ions or atoms and ligand. Apart from the physico-chemical arguments about the effect in question, it is phenomenologically evident that the lanthanide tetrad phenomenon takes place in presence of water, which means its occurrence in seawater as well as rocks or minerals exposed to hydrothermal solution. In this report, our attention is directed to the depth profile of the lanthanide tetrad effect in seawater column. If very interesting new implications are found in the depth profile of the lanthanide tetrad effect, it would not only erase a dubiosity conceived by traditionalists but also provide us with a new promising tool for oceanographic investigations. We have studied the depth profiles of the lanthanide tetrad effect at several sites in oceans, and proved successfully its invaluable significance. Our current presentation is centered on the depth profile of the lanthanide tetrad effect at the Cariaco Trench off the Venezuelan coast, which intrigues us since this trench represents an oceanographically simple closed water column. De Baar et al. (1988) carefully studied this highly stagnant trench for the rare earth distribution and the marine chemistry of Mn, Fe, phosphate and silicate. They describe that the vertical mixing is restricted by the strong pycnocline at 150-300 m depth resulting from the sharp temperature gradient.The sill depth between the trench and the Caribbean Sea is 146 m, the slope of the trench dips steeply between 500 and 1400 m, and the trench site is surrounded by islets. The depth profile of the lanthanide tetrad effect evaluated by us (Masuda et al. , 1998) shows that the