Boron isotopic fractionation and trace element incorporation in various species of modern corals in Sanya Bay, South China Sea

Abstract

The boron isotope paleo-pH proxy has been extensively studied due to its potential for understanding past climate change, and further calibrations were considered for accurate applications of the proxy because of significant variability related to biocarbonate microstructure. In this work, we studied the boron isotopic fractionation between modern marine corals and their coexisting seawater collected along shallow area in Sanya Bay, South China Sea. The apparent partition coefficient of boron (K_D) ranged from 0.83×10⁻³ to 1.69×10⁻³, which are in good agreement with previous studies. As the analyzed coral skeleton (∼5 g) spanned the growth time period of 1–2 years, we discussed the boron isotopic fractionation between pristine corals and modern seawater using the annual mean seawater pH of 8.12 in this sea area. Without taking the vital effect into account, (¹¹B/¹⁰B)_coral values of all living corals spread over the curves of (¹¹B/¹⁰B)_borate vs. (¹¹B/¹⁰B)_sw with the α ₄₋₃ values ranging from 0.974 to 0.982. After calibrating the biological effect on the calcifying fluid pH, the field-based calcification on calcifying fluid pH (i.e., Δ(pH_biol-pH_sw)) for coral species of Acropora, Pavona, Pocillopora, Faviidae, and others including Proites are 0.42, 0.33, 0.36, 0.19, respectively, and it is necessary to be validated by coral culturing experiment in the future. Correlations in B/Ca vs. Sr/Ca and B/Ca vs. pH_biol approve temperature and calcifying fluid pH influence on skeletal B/Ca. Fundamental understanding of the thermodynamic basis of the boron isotopes in marine carbonates and seawater will strengthen the confidence in the use of paleo-pH proxy as a powerful tool to monitor atmospheric CO₂ variations in the past.

Keywords

boron isotope / paleo-pH proxy / boron isotopic fractionation / trace element / modern coral

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