Decoupled carbonate chemistry controls on the incorporation of boron into Orbulina universa

In order to fully constrain paleo-carbonate systems, proxies for two out of seven parameters, plus temperature and salinity are required. The boron isotopic composition (delta 11B) of planktonic foraminifera shells is a powerful tool to reconstruct changes in past surface ocean pH. As [B(OH)]4- is substituted into the biogenic calcite lattice in place of [CO3]2-, it has been suggested that B/Ca ratios in biogenic calcite are a possible proxy for [CO3]2-. However, differentiating between the effects of pH and [CO3]2- is problematic, as they co-vary closely in natural systems, and so the major control on boron incorporation remains unclear. To deconvolve the effects of pH and [CO3]2- on the B/Ca ratio and to test whether d11B remains constant at constant pH, but under changing [CO3]2- (pH 8.05 with 238, 285 and 532 µmol/kg [CO3]2-) and vice versa, we decoupled pH and [CO3]2- (276 +/- 19.5 µmol/kg [CO3]2- with pH 7.7, 7.9 and 8.05) and grew the planktonic foraminifer Orbulina universa in these manipulated culture media. Measurements of the isotope composition of boron and the B/Ca ratio were performed simultaneously using a femtosecond laser ablation system coupled to an MC ICP-MS. Results show that delta11B is controlled by pH and does not respond to changes in [CO3]2-. On the other hand, the B/Ca ratio is driven by [HCO3]- independently of pH. This suggests that B/Ca ratios in foraminiferal calcite may be used as a second, independent, proxy for paleo-carbonate system reconstructions.为精准约束古碳酸盐系统，需获取7项参数中2项的代用指标，同时配套温度与盐度数据。浮游有孔虫壳体的硼同位素组成（δ¹¹B）是重建过去表层海水pH变化的有效手段。由于[B(OH)₄]⁻可替代[CO₃]²⁻进入生物成因方解石晶格，有研究提出生物成因方解石中的B/Ca比值可作为[CO₃]²⁻的潜在代用指标。然而，区分pH与[CO₃]²⁻的影响存在较大挑战：二者在自然系统中紧密共变，目前硼元素掺入方解石的主要控制机制仍不明确。为解耦pH与[CO₃]²⁻对B/Ca比值的影响，并验证在恒定pH但[CO₃]²⁻改变（pH为8.05，[CO₃]²⁻分别为238、285和532 μmol/kg）以及反之的条件下，δ¹¹B是否保持恒定，我们将pH与[CO₃]²⁻进行解耦调控（[CO₃]²⁻为276±19.5 μmol/kg，pH分别为7.7、7.9和8.05），并在这些人工配置的培养基中培养浮游有孔虫圆球抱球虫（Orbulina universa）。采用飞秒激光剥蚀系统耦合多接收电感耦合等离子体质谱仪（MC ICP-MS），同步测定了硼同位素组成与B/Ca比值。结果显示，δ¹¹B仅受pH调控，不受[CO₃]²⁻变化的影响；而B/Ca比值则由[HCO₃]⁻主导，与pH无关。这表明有孔虫方解石中的B/Ca比值可作为独立的第二代理指标，用于古碳酸盐系统的重建。