Experimental data of CO2, CO3, Omega calcite saturation, pH values, alpha and epsilon fractionation factors, and Dδ¹⁸O calctite-water

The oxygen isotopic composition (d18O) of calcium carbonate of planktonic calcifying organisms is a key tool for reconstructing both past seawater temperature and salinity. The calibration of paloeceanographic proxies relies in general on empirical relationships derived from field experiments on extant species. Laboratory experiments have more often than not revealed that variables other than the target parameter influence the proxy signal, which makes proxy calibration a challenging task. Understanding these secondary or \"vital\" effects is crucial for increasing proxy accuracy. We present data from laboratory experiments showing that oxygen isotope fractionation during calcification in the coccolithophore Calcidiscus leptoporus and the calcareous dinoflagellate Thoracosphaera heimii is dependent on carbonate chemistry of seawater in addition to its dependence on temperature. A similar result has previously been reported for planktonic foraminifera, supporting the idea that the [CO3]2- effect on d18O is universal for unicellular calcifying planktonic organisms. The slopes of the d18O/[CO3]2- relationships range between -0.0243 per mil/(µmol/kg) (calcareous dinoflagellate T. heimii) and the previously published -0.0022 per mil/(µmol/kg) (non-symbiotic planktonic foramifera Orbulina universa), while C. leptoporus has a slope of -0.0048 per mil/(µmol/kg). We present a simple conceptual model, based on the contribution of d18O-enriched [HCO3]- to the [CO3]2- pool in the calcifying vesicle, which can explain the [CO3]2- effect on d18O for the different unicellular calcifiers. This approach provides a new insight into biological fractionation in calcifying organisms. The large range in d18O/[CO3]2- slopes should possibly be explored as a means for paleoreconstruction of surface [CO3]2-, particularly through comparison of the response in ecologically similar planktonic organisms.

浮游钙化生物（planktonic calcifying organisms）碳酸钙的氧同位素组成（d¹⁸O），是重建古海水温度与盐度的核心工具。古海洋代用指标（paleoceanographic proxy）的校准，通常依赖于从现生物种野外实验中推导得出的经验关系。实验室实验往往证实，除目标参数外的其他变量同样会影响代用指标信号，这使得代用指标校准成为一项极具挑战性的工作。厘清这些次生效应，即所谓的“生命效应（vital effect）”，对于提升代用指标的精度至关重要。本研究呈现的实验室实验数据显示：颗石藻（coccolithophore）Calcidiscus leptoporus与钙质甲藻（calcareous dinoflagellate）Thoracosphaera heimii在钙化过程中的氧同位素分馏，除受温度调控外，还与海水碳酸盐化学状态存在显著关联。此前针对浮游有孔虫（planktonic foraminifera）的研究曾报道过类似结果，这支持了“碳酸根离子（[CO₃]²⁻）对d¹⁸O的影响，对于单细胞浮游钙化生物具有普适性”这一学术观点。d¹⁸O与[CO₃]²⁻的关系斜率区间为：钙质甲藻T. heimii的斜率为-0.0243 千分比/(微摩尔/千克)，已发表的非共生浮游有孔虫Orbulina universa的斜率为-0.0022 千分比/(微摩尔/千克)，而C. leptoporus的斜率为-0.0048 千分比/(微摩尔/千克)。本研究基于钙化囊泡（calcifying vesicle）内，富集d¹⁸O的碳酸氢根离子（[HCO₃]⁻）对碳酸根离子库的贡献，提出了一个简洁的概念模型，该模型可解释不同单细胞钙化生物中[CO₃]²⁻对d¹⁸O的影响效应。该研究思路为解析钙化生物的生物分馏机制提供了全新视角。d¹⁸O/[CO₃]²⁻斜率的较大变化区间，或可被开发为表层海水[CO₃]²⁻的古重建手段，尤其可通过对比生态相似的浮游生物的响应特征来实现这一目标。