10:00 AM - 10:15 AM
[MZZ44-05] Systematic understanding of solid-liquid partitioning and isotope fractionation based on the elucidation of adsorbed species of barium on manganese oxides
Keywords:barium, isotope, adsorption
Despite being a non-essential element, barium (Ba) shows a vertical distribution in the ocean that correlates with nutrient elements due to its precipitation as barite (BaSO4) in organic matter particles. As barite precipitates, it selectively takes lighter isotopes, which induces the isotope ratio in the remaining seawater heavier (Von Allmen et al., 2010). This is because the isotope ratio of seawater varies with the amount of reservoirs, and it could be an important environmental indicator if there is any medium to record this information. Among various candidates, Ba is generally immobile in manganese oxides, which suggests that its isotope ratio may preserve the original value (Koschinsky et al., 2003; Horner et al., 2021). This is thought to result from Ba forming inner-sphere complexes on the surface of manganese oxides (Tanaka et al., 2024). The purpose of this study is to elucidate adsorption species and investigate the possibility of using Ba stable isotope ratios as a proxy for the paleo-environmental record.
To simulate the adsorption reaction of Ba in the ocean, we used synthesized manganese oxides (δ-MnO2) and hydrous ferric oxides (ferrihydrite). In addition, manganese nodules and crusts collected from the Pacific Ocean seafloor were analyzed and measured for the analysis of adsorption state and Ba stable isotope ratios. XAFS analysis was performed at synchrotron radiation facilities to elucidate the adsorption state. Ba isotope analysis was performed by the sample-standard bracketing method using a multi-collector inductively coupled plasma mass spectrometer (MC-ICP-MS), and Ba isotope ratios were corrected by external Ce correction. In this study, δ137/134Ba values (hereafter δ137Ba) are reported relative to the Ba standard NIST SRM3104a.
Ba K-edge EXAFS showed that Ba forms inner-sphere complexes on δ-MnO2. Ba LⅢ-edge HERFD-XANES showed that the major form of Ba in manganese nodules is the adsorbed species on δ-MnO2, rather than barite incorporated in the nodules widespread in marine sediments. These experiments suggest that the Ba stable isotope ratios are likely to be retained at the time of adsorption due to the formation of inner-sphere complexes on δ-MnO2.
Ba isotope analysis was performed on adsorbed samples and manganese crusts. In the adsorption reaction on δ-MnO2, δ137Ba was found to be heavier in the liquid phase and lighter in the solid phase, with an average fractionation of 0.20 ‰ between the two phases. Furthermore, this fractionation reaction was suggested to be an equilibrium process. The δ137Ba in the manganese crust was measured at five stratigraphically distinct points, and the formation ages of the corresponding parts were estimated to range from 11 to 2 Ma (calculated with reference to Usui et al. (2017)). An increasing trend in δ137Ba was observed toward deeper layers, varying between -0.02 ‰and 0.15 ‰. Using the observed fractionation value, the δ137Ba of seawater at the time of adsorption was estimated from the manganese crust data, yielding 0.21 ‰ at the point close to the surface. Notably, the present ocean's δ137Ba is reported as 0.20‰ at the sampling depth (Hsieh and Henderson, 2017), which agrees well with the estimates. Thus, the validity of estimating seawater δ137Ba from manganese crusts is supported.
The increasing trend of δ137Ba with depth may indicate a decline in bioproduction during the corresponding period. Cortese et al. (2004) reported a decreasing trend in marine biological production since about 15 Ma based on opal deposition in the oceans, suggesting that Ba stable isotope ratios in manganese crusts likely reflect past biological production. In this study, we elucidated the mechanism by which Ba stable isotope ratios are retained in manganese crusts by focusing on molecular-level processes, and demonstrated that these ratios can be used as a tool for environmental reconstruction.
To simulate the adsorption reaction of Ba in the ocean, we used synthesized manganese oxides (δ-MnO2) and hydrous ferric oxides (ferrihydrite). In addition, manganese nodules and crusts collected from the Pacific Ocean seafloor were analyzed and measured for the analysis of adsorption state and Ba stable isotope ratios. XAFS analysis was performed at synchrotron radiation facilities to elucidate the adsorption state. Ba isotope analysis was performed by the sample-standard bracketing method using a multi-collector inductively coupled plasma mass spectrometer (MC-ICP-MS), and Ba isotope ratios were corrected by external Ce correction. In this study, δ137/134Ba values (hereafter δ137Ba) are reported relative to the Ba standard NIST SRM3104a.
Ba K-edge EXAFS showed that Ba forms inner-sphere complexes on δ-MnO2. Ba LⅢ-edge HERFD-XANES showed that the major form of Ba in manganese nodules is the adsorbed species on δ-MnO2, rather than barite incorporated in the nodules widespread in marine sediments. These experiments suggest that the Ba stable isotope ratios are likely to be retained at the time of adsorption due to the formation of inner-sphere complexes on δ-MnO2.
Ba isotope analysis was performed on adsorbed samples and manganese crusts. In the adsorption reaction on δ-MnO2, δ137Ba was found to be heavier in the liquid phase and lighter in the solid phase, with an average fractionation of 0.20 ‰ between the two phases. Furthermore, this fractionation reaction was suggested to be an equilibrium process. The δ137Ba in the manganese crust was measured at five stratigraphically distinct points, and the formation ages of the corresponding parts were estimated to range from 11 to 2 Ma (calculated with reference to Usui et al. (2017)). An increasing trend in δ137Ba was observed toward deeper layers, varying between -0.02 ‰and 0.15 ‰. Using the observed fractionation value, the δ137Ba of seawater at the time of adsorption was estimated from the manganese crust data, yielding 0.21 ‰ at the point close to the surface. Notably, the present ocean's δ137Ba is reported as 0.20‰ at the sampling depth (Hsieh and Henderson, 2017), which agrees well with the estimates. Thus, the validity of estimating seawater δ137Ba from manganese crusts is supported.
The increasing trend of δ137Ba with depth may indicate a decline in bioproduction during the corresponding period. Cortese et al. (2004) reported a decreasing trend in marine biological production since about 15 Ma based on opal deposition in the oceans, suggesting that Ba stable isotope ratios in manganese crusts likely reflect past biological production. In this study, we elucidated the mechanism by which Ba stable isotope ratios are retained in manganese crusts by focusing on molecular-level processes, and demonstrated that these ratios can be used as a tool for environmental reconstruction.