Empirical relationship between pH and the boron isotopic composition of Globigerinoides sacculifer: Implications for the boron isotope paleo‐pH proxy

Abstract

Culture experiments were carried out with the planktonic foraminifera Globigerinoides sacculifer at three different pH values (7.6±0.05, 8.20±0.05, and 8.60±0.05) in order to establish the pH dependence of the boron isotopic fractionation between the calcitic tests of G. sacculifer and seawater. Additional objectives were to compare the pH dependences of boron isotopic composition of G. sacculifer with that of Orbulina universa and inorganic calcite in order to evaluate the robustness of the boron isotope paleo‐pH proxy. The relationship between the boron isotopic composition of G. sacculifer and the culture water pH shows heavier boron isotopic values at higher pH. This empirical relationship is similar but offset (by ∼ 3‰) from that of O. universa, indicating the presence of biogenic influence (“vital effect”) on the boron isotopic composition of foraminifera. However, comparison of the boron isotopic composition of O. universa and G. sacculifer with that of inorganic calcite grown over a common pH range shows that inorganic processes are the dominant control on the boron isotopic composition of foraminifera and the “vital effects” play a small but not insignificant role. The results imply that in spite of a small biogenic influence the variation in boron isotopic composition of foraminiferal shells with pH (at least for O. universa and G. sacculifer) is consistent with that for inorganic calcite, demonstrating the potential of this isotopic signature in foraminifera as a reliable paleo pH proxy.