Systematic understanding of solid-liquid partitioning and isotope fractionation based on the elucidation of adsorbed species of barium on manganese oxides

Abstract

Although barium (Ba) is not an essential biological element, it exhibits a nutrient-like vertical distribution in the ocean due to the precipitation and subsequent re-dissolution of barite (BaSO₄) in association with biological productivity. During barite precipitation, lighter isotopes are preferentially incorporated into the solid phase, resulting in an enrichment of heavier isotopes in seawater (i.e., higher delta-¹³⁷Ba). Thus, seawater delta-¹³⁷Ba is considered to reflect variations in marine biological productivity. Barite has commonly been used to track changes in seawater delta-¹³⁷Ba, but because it causes the isotopic fractionation, there is a need for other materials that can record the seawater delta-¹³⁷Ba after the fractionation has occured. Recently, manganese oxides have been highlighted as promising candidates, as Ba is thought to form inner-sphere complexes on their surfaces, potentially preserving its isotopic signature. In this study, we conducted sorption experiments using synthesized manganese oxides (delta-MnO₂) and analyzed marine ferromanganese crusts collected from the Pacific Ocean. Through characterization of Ba sorption species and isotopic measurements, we demonstrate that delta-¹³⁷Ba values in ferromanganese crusts can reflect past variations in seawater Ba isotopic composition.