Dataset belonging to "Constraining sulfur incorporation in calcite using controlled growth experiments"

The sulfur over calcium ratio (S/Ca) in foraminiferal shells was recently proposed as a new and independent proxy for reconstructing marine inorganic carbon chemistry. This new approach assumes that sulfur is incorporated into CaCO3 predominantly in the form of sulfate (SO42-) through lattice substitution for carbonate ions (CO32-), and that S/Ca thus reflects seawater [CO32-]. Although foraminiferal growth experiments validated this approach, field studies showed controversial results suggesting that the potential impact of [CO32-] may be overwritten by one or more parameters. Hence, to better understand the inorganic processes involved, we here investigate S/Ca values in inorganically precipitated CaCO3 (S/Ca(cc)) grown in solutions of CaCl2 - Na2CO3 - Na2SO4 - B(OH)3 - MgCl2. Experimental results indicate the dependence of sulfate partitioning in CaCO3 on the carbon chemistry via changing pH and suggest that faster precipitation rates increase the partition coefficient for sulfur. Based on the positive correlation found between S/Ca(cc) and the concentration of only one aqueous sulfate species, [CaSO40](aq), we here suggest a critical evaluation of this potential proxy. As sulfate complexation seems to control sulfate uptake in inorganic calcite, application as a proxy using foraminiferal calcite may be limited to periods for which seawater chemistry is well-constrained. As foraminiferal calcite growth is modulated by inward Ca2+ flow to the site of calcification coupled to outward H+ pumping, sulfate incorporation as CaSO40 ion-pair in the foraminifer?s shell also provides a mechanistic link for the observed relationship between S/Ca(cc) and [CO32-].

有孔虫（foraminifera）壳体中的硫钙比（S/Ca）近日被提出作为一种全新的独立代用指标（proxy），用于重建海洋无机碳化学特征。该新方法假定，硫主要以硫酸根（SO₄²⁻）的形式通过晶格替代碳酸根离子（CO₃²⁻）的方式掺入碳酸钙（CaCO₃）中，因此硫钙比可反映海水的碳酸根离子浓度([CO₃²⁻])。尽管有孔虫培养实验验证了该方法，但野外研究却得出了存在争议的结果，表明碳酸根离子浓度对硫钙比的潜在影响可能被一个或多个参数所掩盖。因此，为更好地理解所涉及的无机过程，本研究对CaCl₂-Na₂CO₃-Na₂SO₄-B(OH)₃-MgCl₂溶液中无机沉淀的碳酸钙（S/Ca(cc)）的硫钙比值展开了探究。实验结果表明，碳酸钙中的硫酸根分配行为可通过改变pH值响应碳酸水化学特征，且更快的沉淀速率会提升硫的分配系数。基于本研究发现S/Ca(cc)仅与单一水溶液硫酸物种——[CaSO₄⁰](aq)的浓度呈正相关，我们建议对这一潜在代用指标开展批判性评估。由于硫酸根络合作用似乎调控着无机方解石中的硫酸根吸收过程，因此基于有孔虫方解石的该代用指标的应用，可能仅能局限于海水化学特征得到精准限定的地质时期。由于有孔虫方解石的生长受钙化位点的Ca²⁺内流与H⁺外排耦合过程调控，因此作为CaSO₄⁰离子对掺入有孔虫壳体的硫酸根，也为S/Ca(cc)与碳酸根离子浓度之间的观测关系提供了机制层面的解释。