Controlling factors of the δ 11B-pH proxy and its research direction

Abstract

Significant boron isotope fractionation occurs in nature (−70 ‰ to +75 ‰) due to the high geochemical reactivity of boron and the large relative mass difference between 10B and 11B. Since the 1990s, reconstruction of ancient seawater pH using the isotopic composition of boron in bio-carbonates (δ11Bcarb), and then calculation of the past pCO2 have become important issues for the international isotope geochemistry community, and are called the δ11B-pH proxy. Although many achievements have been made by this proxy, various aspects of boron systematics require rigorous evaluation. Based on the previous researches, mechanism of boron isotope fractionation, variation of boron isotope (δ11B) in nature (especially in bio-carbonates) and controlling factors of the δ11B-pH proxy, such as the dissociation constant of B(OH)3 in seawater (pKa), the δ11B of seawater (δ11BSW), the boron isotopic fractionation factor between B(OH)4− and B(OH)3 (α4–3), and the incorporated species of boron into bio-carbonates, are reviewed in detail and the research directions of this proxy are proposed. Generally, the controversy about pKa, δ11Bsw, and α4–3 is relatively less, but whether boron incorporated into bio-carbonates only in the form of B(OH)4− remains doubtful. In the future, it is required that the physicochemical processes that control boron incorporation into carbonates be rigorously characterized and that the related chemical and isotopic fractionation be quantified. It is also necessary and important to establish a “best-fit empirically equation” between δ11Bcarb and pH of seawater based on the precipitation experiments of inorganic or culture experiments of corals or foraminifera. In addition, extended application of the δ11B-pH proxy to the earlier part of the Phanerozoic relying on the Brachiopods is worthy of studying. Like other geochemical indicators, there are limiting factors of δ11B; however, it remains a very powerful tool in the reconstruction of past seawater pH at present.