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 c. 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.