Proxy of paleo-seawater pH reconstruction based on boron isotopes of marine carbonates and its applications

Boron (B) is widely distributed in nature. The fractionation of its stable isotopes ¹⁰B and ¹¹B establishes boron as a significant tool for reconstructing paleoceanographic and paleoclimate conditions. The sensitivity of the δ¹¹B values to seawater pH makes it possible to establish a δ¹¹B-paleo-seawater pH proxy. This proxy enables the inversion of paleo-seawater pH using the boron isotope values of marine carbonate minerals, which in turn facilitates the analysis of the atmospheric CO₂ concentration changes. Although the δ¹¹B-paleo-seawater pH proxy has great theoretical potential, its accuracy remains a challenge. By analyzing the boron cycling in the ocean and methods for boron isotope determination, we investigate the underlying mechanisms and application potential of the δ¹¹B-paleo-seawater pH proxy. The results indicate that major factors affecting the proxy accuracy include boron deviation caused by biological effects and inaccuracies in the fractionation coefficient between boron isotopes, insufficient understanding of the exchange processes between B(OH)₃ and B(OH)₄^- in the seawater, and the inability of lab simulations to fully replicate the fractionation of boron isotopes during carbonate precipitation in the natural environment. To enhance the accuracy of the proxy, it is recommended to incorporate additional proxies such as the B/Ca ratio and to select carbonate samples less affected by biological effects. Furthermore, it is necessary to enhance the precision of boron isotope fractionation coefficient calculations, establishing a more scientific empirical method for correcting the biological effects. These advancements will contribute to more scientific and broader applications of boron isotopes in reconstructing paleoceanographic and paleoclimate conditions.