Brachiopod shells are ubiquitous since the Early Cambrian up to now. As they secrete a shell made of low-magnesium calcite, more resistant to diagenesis than biocarbonates richer in Mg, their geochemical signatures are generally considered a powerful tool for paleo-environmental and paleo-climatic reconstructions. However, gaps in knowledge still remain on the underlying controls of the shell chemistry, in particular at a high spatial resolution. In this study, in situ oxygen and carbon isotope measurements by SIMS (Secondary Ion Mass Spectrometry) were performed in brachiopod shells of the cold-temperate water species constituted of a primary and a secondary layer. The individual specimens studied here grew under controlled conditions mimicking the natural environment and in experiments under low-pH (high CO) and high-temperature conditions. Transversal carbon and oxygen profiles showed a “brachiopod pattern” typical of extant two-layered brachiopods, with the primary layer depleted in O and C relative to equilibrium and the secondary layer showing a gradual increasing trend until reaching a near-equilibrium plateau. Overall, shells cultured at low pH were found to have δO and δC values closer to equilibrium when compared to shells from the control experiment. These near-equilibrium values may reflect a decrease in shell precipitation rate, leading to less kinetic effects, and/or a more rapid kinetics for the equilibration between DIC species and water. By close pairing of seawater δO and δC to that of shell microstructure, our study enables us to derive layer-specific C and O enrichment factors, which show the extent of pH and temperature effects superimposed on the seawater δO and DIC δC signal inherited. Finally, we show that during brachiopod shell growth, newly precipitated calcite is added to the calcite already existing, thus empirically validating the conceptual accretionary growth model proposed by .

中文翻译：

---

培养腕足动物的高分辨率碳和氧同位素组成：pH、温度和生长的影响


自早寒武世至今，腕足动物的贝壳已无处不在。由于它们分泌的壳由低镁方解石制成，比富含镁的生物碳酸盐更能抵抗成岩作用，因此它们的地球化学特征通常被认为是古环境和古气候重建的有力工具。然而，在壳化学的基本控制方面仍然存在知识空白，特别是在高空间分辨率下。在这项研究中，通过 SIMS（二次离子质谱）对由初级层和次级层构成的冷温带水生物种的腕足动物壳进行了原位氧和碳同位素测量。这里研究的单个样本在模仿自然环境的受控条件下生长，并在低 pH（高 CO）和高温条件下进行实验。横向碳和氧剖面显示出现存两层腕足动物典型的“腕足动物模式”，第一层的 O 和 C 相对于平衡状态有所减少，而第二层则显示出逐渐增加的趋势，直到达到接近平衡的平台。总体而言，与对照实验中的贝壳相比，低 pH 条件下培养的贝壳的 δ18O 和 δ13C 值更接近平衡。这些接近平衡的值可能反映了壳沉淀速率的降低，导致动力学效应较小，和/或 DIC 物质和水之间的平衡动力学更快。通过将海水 δ18O 和 δ13C 与壳微结构紧密配对，我们的研究使我们能够推导出特定层的 C 和 O 富集因子，这些因子显示了叠加在继承的海水 δ18O 和 DIC δ13C 信号上的 pH 和温度影响的程度。 最后，我们表明，在腕足动物壳生长过程中，新沉淀的方解石被添加到已经存在的方解石中，从而从经验上验证了提出的概念性增生生长模型。