Rare earth element (REE)–silicic acid complexes in seawater to explain the incorporation of REEs in opal and the “leftover” REEs in surface water: New interpretation of dissolved REE distribution profiles

Abstract

In light of a new finding that diatom opal carries rare earth elements (REEs) at an amount exceeding that which would explain the REEs/Si ratio in open ocean columns, the complex formation of REEs with silicic acid and subsequent incorporation of the complex into diatom opal has been explored. Reported complex formation constants of EuH3SiO42+ and an empirical linear relationship of the constants with hydrolysis constants were employed in the species calculation. The calculation reveals that REE–silicic acid complex is the dominant species of dissolved REEs especially in deep layers of the Pacific and the Atlantic Oceans. The degree of complex formation varies depending on dissolved silica concentration, pH, and pCO2. Assuming that only REEs in the form of REEH3SiO42+, which is diffused/advected from deep water to surface water together with dissolved silica, are incorporated into diatom opal, a REE incorporation theory has been developed. The theory links the intake of REEs and silica by diatoms in surface water and their concentrations in deep water and presents two distribution coefficients (Ds) of REEs in diatom opal, conventional D against surface water and columnar D against deep water as functions of pH and dissolved silica concentration. The theory successfully explains the significant REE concentration level (“leftover REEs”) in the surface water of the oceans and reproduces a concentration of REEs in diatom opal that is consistent with that observed in diatom opal in the North Pacific Ocean. The REE composition of oceanic deep water is largely explained by two fractionation processes: REE incorporation into opal and carbonate/oxide scavenging of REEs.