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.