The behavior of rare earth elements in seawater: Precise determination of variations in the North Pacific water column

Abstract

We present new, highly precise and accurate data for ten rare earth elements (REEs) in two water column profiles and in bottom waters from three additional localities in the North Pacific. Our results presented here, and in an earlier publication, indicate that the water column in the western North Pacific is stratified with respect to the isotopic composition of Nd as well as Nd/La, Sm/Nd, and Er/La. Both the Nd isotopic and the REE data show that REE transport is strongly influenced by horizontal advection, even in the sluggish circulation of the Pacific. In general, REEs increase in concentration with depth. At mid-latitude, they reach a maximum concentration, at about 3000 m, and then decrease to the bottom. At high latitude, heavy REEs reach a maximum concentration near 3000 m, whereas light REEs continue to increase in concentration with depth. The deep water maximum and bottom water depletion is interpreted as an advective feature resulting from the flow of a lower concentration of REEs in Antarctic bottom water under a higher concentration of REEs in the North Pacific. Concentrations of REEs do not show a linear relationship with those of Si, indicating that the REE cycle is not directly coupled to that of the Si cycle. REE element ratios (i.e., Nd/La) correlate with hydrographie properties, which suggests that major features of the REE patterns may be conserved during lateral transport. Comparison of our REE data to those in previously published reports on Pacific seawater shows that the identification of REE distributions affected by advection is only made possible by the high-precision analysis we developed for this study. The Ce-concentration we observe is relatively constant over 200–4500 m depth at both sites where we have complete profiles. This unique constancy of Ce is in contrast to the contiguous REEs, La, and Nd, which show substantial concentration variations with depth.