海洋におけるホウ素同位体とpH

抄録

The composition of two stable isotopes ¹⁰B (〜20%) and ¹¹B (〜80%) can vary significantly on the earth and provides important information on the global geochemical cycle. Art Spivack has proposed a simple and elegant idea called "palaeo-pH", that boron isotopes in foraminifera might indicate past oceanic pH values. The mean oceanic residence time of boron is calculated at 1.6×10⁷ years. Therefore, the boron concentration (4.5ppm) and isotopic composition (δ¹¹B=39〜40‰) in seawater are quite uniform throughout the world's oceans (Tablel). The dominant boron species in seawater is B(OH)₃ and B(OH)₄^-. There is about 2% difference between the isotopic composition of B (OH)₃ and B(OH)₄^-, as a result of the isotope fractionation. The ¹¹B isotope is concentrated into B(OH)₃ in seawater. The relative proportion of the two species is controlled by pH. Boron isotopic composition of B(OH)₃ and B(OH)₄^- in seawater is shown as a function of pH. If one of each species is incorporated into marine carbonates, the boron isotopes should reflect seawater records of pH in which they have grown. Using the boron isotope "palaeo-pH" method,the Lamont group attempted to estimate surface oceanic pH differences between the last glacial period and the Holocene. Studies on boron isotopes from coral samples clarify some issues on the "palaeo-pH" in aquatic environments. More detailed studies will make progress on the boron isotope pH-meter.