Boron isotopic compositions in growing corals from the South China Sea

In order to determine incorporation of boron species, boron isotopic fractionation, and influence of trace elements on isotopic compositions of boron in corals (δ¹¹B_coral), concentrations of Mg, Sr, Na, B and δ¹¹B_coral in growing corals from the South China Sea were measured. Relative to seawater, Sr enriched while Mg depleted in corals in the South China Sea. Although the δ¹¹B_coral values were different from various species and were not closely correlated with the element concentrations in corals in the South China Sea, Mg(OH)₂ existed in corals can result in high δ¹¹B_coral. Thus, it is necessary to examine the existence of Mg(OH)₂ and to choose the same species when δ¹¹B_coral is used in the δ¹¹B-pH proxy. Based on the measured δ¹¹B values of corals and coexisting seawater as well as the seawater pH in the South China Sea, a new isotopic fractionation factor a_4-3 between B(OH)4- and B(OH)₃ was determined to be 0.979. Besides B(OH)4- into corals, our results showed that B(OH)₃ may also be incorporated into corals with variable proportions. The incorporation of B(OH)₃ into corals may challenge the hypothesis of δ¹¹B_coral=δ¹¹B₄, resulting in increasing uncertainty to the calculated seawater pH values to the δ¹¹B-pH proxy. We suggested that a best-fit empirical equation between δ¹¹B of bio-carbonates and seawater pH needs to be established by the precipitation experiments of inorganic carbonates or culture experiments of corals or foraminifera.