Oxygen isotope variability in conodonts: implications for reconstructing Palaeozoic palaeoclimates and palaeoceanography

Conodonts have the potential to elucidate the intricacies of Palaeozoic climates, especially if δ¹⁸O values of single apatitic tooth-like ‘elements’ can be used to map evolving sea surface temperatures and differentiate oceanic water masses. Their ecological distribution as pelagic and nektobenthic organisms, high-resolution biostratigraphy, and abundance in Cambrian–Triassic rocks qualifies them as potentially robust climate archives. Previous ion microprobe conodont δ¹⁸O studies have proceeded directly to palaeotemperature interpretation without appreciation of inter- and intra-element variability or post-mortem artefacts. Here, ion microprobe analyses of Ordovician and Silurian conodonts establishes that: intra-element crown tissue δ¹⁸O typically varies by ≤1‰ (53% of conodonts analysed), is normally ≤2‰ (92% of analyses), and rarely varies by 2–4‰; δ¹⁸O can vary across elements, suggesting a microstructural and/or diagenetic control; δ¹⁸O can vary between species representatives by <em>c</em>. 3‰; δ¹⁸O of pelagic and nektobenthic taxa can be offset by 2–3‰; elements processed with formic acid have highly variable δ¹⁸O; and thermal alteration does affect δ¹⁸O. Conodont ion microprobe δ¹⁸O values are comparable with those of bulk methods, but utilization of material with no consideration of geological context or processing history may introduce significant artefacts. A protocol for future conodont oxygen isotope ion microprobe studies is proposed.

牙形石（Conodonts）具备揭示古生代（Palaeozoic）气候复杂机理的巨大潜力，尤其当单颗磷灰石质齿状‘构件’的氧同位素δ¹⁸O值可用于绘制演化中的海表温度图谱、区分不同海洋水团时。作为浮游及底栖游泳生物（nektobenthic），其生态分布特征、高分辨率生物地层学（biostratigraphy）价值，以及在寒武纪-三叠纪（Cambrian–Triassic）岩层中的丰度，使其成为极具潜力的可靠气候档案载体。

此前的离子微探针（ion microprobe）牙形石δ¹⁸O研究均直接开展古温度（palaeotemperature）解释，却未关注构件间与构件内的变异，以及死后蚀变假象（post-mortem artefacts）。本研究通过对奥陶纪（Ordovician）与志留纪（Silurian）牙形石开展离子微探针分析，证实如下结论：
1. 构件内齿冠组织（crown tissue）的δ¹⁸O值通常变化幅度≤1‰（占分析样本的53%），多数情况下≤2‰（占分析数据的92%），仅极少数样本存在2‰~4‰的波动；
2. 不同牙形石构件间的δ¹⁸O值存在差异，这暗示其受微观结构（microstructural）和/或成岩作用（diagenesis）调控；
3. 不同物种类群代表的δ¹⁸O值差异约为3‰；
4. 浮游类群与底栖游泳类群的δ¹⁸O值可存在2‰~3‰的偏移；
5. 经甲酸（formic acid）处理的构件的δ¹⁸O值存在显著波动；
6. 热蚀变（thermal alteration）确实会对δ¹⁸O值产生影响。

牙形石离子微探针δ¹⁸O值与整体分析法（bulk methods）所得结果具有可比性，但未考虑地质背景或样品处理历史就直接使用实验材料，可能会引入显著的蚀变假象。本研究最后提出了未来牙形石氧同位素离子微探针研究的标准化实验流程（protocol）。