2:15 PM - 2:30 PM
[BPT02-03] Boron incorporation into amorphous silica: an isotopic model for paleo-pH reconstruction
Keywords:pH proxy, boron isotopes, amorphous silica, synthesis
Amorphous silica is one of bio-minerals, produced by sea planktons such as diatoms and radiolarians. The siliceous shells preserved in marine sediments are potential archives recording paleo-environment. In this study, we focused on boron isotopic values (δ11B) as a paleo-pH proxy. Boron isotopes fractionate between dissolved boron species (boric acid and borate ion), whose proportion changes with the ambient pH. Respective to well-studied carbonate sediments, there are few studies on dissolved species incorporated into amorphous silica. We synthesized amorphous silica under controlled pH to analyze boron concentrations and isotopic values. We examined the reactions of boron species incorporated into amorphous silica that causes isotopic fractionation depending on the pH by applying model calculations to analyzed geochemical data.
Amorphous silica was synthesized by following traditional sol-gel method. Two different salinities of solvents were examined, 0 or 3.5% of sodium chloride. The pH of the solvent was controlled by the proportion of ammonium salts, 8.5-9.7 for the non-saline solvent and 7.9-9.2 for the 3.5% saline solvent. Boron concentrations and isotopic compositions in the amorphous silica and solvents were analyzed using a Q-ICP-MS and a MC-ICP-MS, respectively, at the Kochi Core Center.
The distribution factor of boron showed a convex upward curve with respect to pH. The total fractionation factor (αt) showed a convex downward curve in the range of 0.963-0.975, which was lower than those expected from borate ion (α = 0.974). Such low isotopic values may be achieved by 2 reactions: (1) boric acid (H3BO3) to four coordinate form (>Si-OB(OH)3) and (2) borate ion (B(OH)4-) to >Si-OB(OH)3. Isotopic fractionation factor for the reaction (1) was calculated to be maximized for the given isotopic values of amorphous silica, while that for the reaction (2) was assumed to be 1.00. We obtained similar fractionation factors for the reaction (1) for both non-saline and saline solvents. Using the fractionation factors, we evaluated the proportion of these two reactions at different pH. As a result, it was concluded that the reaction (2) occur at lower pH, and is replaced by the reaction (1) as pH increases. This model calculation well explains the complex fractionation of boron isotopes between water and amorphous silica, and improves the interpretation of pH proxy using bio-silica.
Amorphous silica was synthesized by following traditional sol-gel method. Two different salinities of solvents were examined, 0 or 3.5% of sodium chloride. The pH of the solvent was controlled by the proportion of ammonium salts, 8.5-9.7 for the non-saline solvent and 7.9-9.2 for the 3.5% saline solvent. Boron concentrations and isotopic compositions in the amorphous silica and solvents were analyzed using a Q-ICP-MS and a MC-ICP-MS, respectively, at the Kochi Core Center.
The distribution factor of boron showed a convex upward curve with respect to pH. The total fractionation factor (αt) showed a convex downward curve in the range of 0.963-0.975, which was lower than those expected from borate ion (α = 0.974). Such low isotopic values may be achieved by 2 reactions: (1) boric acid (H3BO3) to four coordinate form (>Si-OB(OH)3) and (2) borate ion (B(OH)4-) to >Si-OB(OH)3. Isotopic fractionation factor for the reaction (1) was calculated to be maximized for the given isotopic values of amorphous silica, while that for the reaction (2) was assumed to be 1.00. We obtained similar fractionation factors for the reaction (1) for both non-saline and saline solvents. Using the fractionation factors, we evaluated the proportion of these two reactions at different pH. As a result, it was concluded that the reaction (2) occur at lower pH, and is replaced by the reaction (1) as pH increases. This model calculation well explains the complex fractionation of boron isotopes between water and amorphous silica, and improves the interpretation of pH proxy using bio-silica.