Rare earth elements in seawater: particle association, shale-normalization, and Ce oxidation

Abstract

Dissolved (<0.04 μm, not <0.4 μm) and total acid-soluble concentrations of rare earth elements (REEs) and yttrium were measured by using ICP mass spectrometry in the seawaters obtained from various depths in the western North Pacific near Japan. The difference, i.e., acid-soluble particulate fraction, was found to be small, 2–5% for all tri-valent light and middle REEs and less than 1% for heavy REEs and yttrium. The high particulate fraction of 31% for Ce is consistent with its predicted oxidation state of tetra-valence. Elevated particulate fraction of all REEs was found within ∼80 m above the bottom due to contribution of flocculated resuspended particles. The vertical profiles of REE(III)s show smoothly increasing convex curves with depth similar to those reported previously. Dissolved Ce concentration decreases from ∼6 pmol/kg near the surface to a minimum at 2.5 pmol/kg around 400 m where the North Pacific Intermediate Water penetrates, and then approaches to nearly constant value of ∼4 pmol/kg below 800 m. Particulate Ce concentration significantly increases from the surface to 200 m depth suggesting oxidation of Ce(III) to Ce(IV) and subsequent scavenging in the upper water column. However, there is no evidence in our data showing that Ce oxidation is continuously taking place even in the deep sea. Shale-normalized patterns of dissolved REEs were examined in detail, based on three datasets of composite shales available in the literature. Distinctively positive La and only slightly positive Gd anomalies were identified together with well-documented negative Ce-anomaly as common features of seawater. These anomalies systematically change with depth. Rapid changes occur in the upper several hundred meters suggesting that their distributions are largely governed by ocean circulation and biogeochemical cycling.