Paleoceanographers use boron proxies (i.e., δ¹¹B, B/Ca ratio) to reconstruct past seawater carbonate parameters and atmospheric pCO₂. However, the mechanism for the boron incorporation into foraminifera is not fully understood. Here, we construct a model of boron isotope composition (δ¹¹B) in calcifying fluid for foraminifera to explore the selective incorporation of boric acid (BOH)₃, B3) and borate (B(OH)₄^-,B4) into foraminiferal calcite. PB3 and PB4 are defined as the membrane permeability coefficients for B3 and B4. The model results indicate that when the PB3/PB4 ratio varies among 0~0.01 and 0.01~+∞, the B(OH)₄^- δ¹¹B in calcifying fluid is close to the value in seawater. Furthermore, the relationships between boron partitioning coefficient (λ_B) and carbonate saturation (Ω) in inorganic and foraminiferal calcite show a consistent pattern, i.e., λ_B increases with Ω, when PB3/PB4 ratio falls within 0 to 0.01. Thus, a PB3/PB4 ratio between 0 and 0.01 can account for both δ¹¹B and λ_B values measured in the benthic foraminifera from the global ocean. This research firstly reports the modeled δ¹¹B in the calcifying fluid and reveals the proportion of B3, B4 incorporated by foraminifera.

seawater and calcifying fluid chemistry, inorganic calcite, benthic foraminifera, vital effect, boron partition coefficient (λB), δ11B