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

Culture experiments were carried out with the planktonic foraminifera Globi gerinoides 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 seaw ater. Additional objectives were to compare the pH dependences of boron iso topic composition of G. sacculifer with that of Orbulina universa and inorg anic calcite in order to evaluate the robustness of the boron isotope paleo -pH proxy. The relationship between the boron isotopic composition of G. sa cculifer and the culture water pH shows heavier boron isotopic values at hi gher pH. This empirical relationship is similar but offset (by similar to 3 parts per thousand) from that of O. universa, indicating the presence of b iogenic. influence ("vital effect") on the boron isotopic composition of fo raminifera. However, comparison of the boron isotopic composition of O. uni versa and G. sacculifer with that of inorganic calcite grown over a common pH range shows that inorganic processes are the dominant control on the bor on isotopic composition of foraminifera and the "vital effects" play a smal l but not insignificant role. The results imply that in spite of a small bi ogenic influence the variation in boron isotopic composition of foraminifer al shells with pH (at least for O. universa and G. sacculifer) is consisten t with that for inorganic calcite, demonstrating the potential of this isot opic signature in foraminifera as a reliable paleo pH proxy.