Boron isotope fractionation during brucite deposition from artificial seawater

Abstract

Experiments involving boron incorporation into brucite (Mg(OH)₂) from magnesium-free artificial seawater with pH values ranging from 9.5 to 13.0 were carried out to better understand the incorporation behavior of boron into brucite and the influence of it on Mg/Ca-SST proxy and &delta;¹¹B-pH proxy. The results show that both the concentration of boron in deposited brucite ([B]_d) and its boron partition coefficient (<i>K</i>_d) between deposited brucite and final seawater are controlled by the pH of the solution. The incorporation capacity of boron into brucite is almost the same as that into corals, but much stronger than that into oxides and clay minerals. The isotopic compositions of boron in deposited brucite (&delta;¹¹B_d) are higher than those in the associated artificial seawater (&delta;¹¹B_isw) with fractionation factors ranging between 1.0177 and 1.0569, resulting from the preferential incorporation of B(OH)₃ into brucite. Both boron adsorptions onto brucite and the precipitation reaction of H₃BO₃ with brucite exist during deposition of brucite from artificial seawater. The simultaneous occurrence of both processes determines the boron concentration and isotopic fractionation of brucite. The isotopic fractionation behaviors and mechanisms of boron incorporated into brucite are different from those into corals. The existence of brucite in corals can affect the &delta;¹¹B and Mg/Ca in corals and influences the Mg/Ca-SST proxy and &delta;¹¹B-pH proxy negatively. The relationship between &delta;¹¹B and Mg/Ca in corals can be used to judge the existence of brucite in corals, which should provide a reliable method for better use of &delta;¹¹B and Mg/Ca in corals to reconstruct paleo-marine environment.