ASSESSING THE UTILITY OF ION MICROPROBE ANALYSES OF CONODONTS FOR ANCIENT OCEAN PALAEOTHERMOMETRY

Conodonts have the potential to elucidate the intricacies of the Palaeozoic climate system, and this is particularly true if isotopic analyses of single tooth-like ‘elements’ can be used to differentiate evolving sea surface palaeotemperatures and oceanic water masses. They have a diagenetically inert apatitic composition, an ecological distribution as both pelagic and nektobenthic organisms, a well-established biostratigraphy, and are abundant in Cambrian–Triassic rocks. However, ion microprobe analyses of δ¹⁸O in conodont elements have proceeded directly to palaeotemperature interpretation without full appreciation of inter- and intra-element variability or post-mortem artifacts. Our ion microprobe analyses of Ordovician–Silurian elements establishes that: 1. δ¹⁸O within elements can vary by up to ~4‰ in crown tissue, but typically does so by ~1‰; 2. δ¹⁸O varies systematically across some elements suggesting an underlying microstructural control; 3. δ¹⁸O varies between same taxon representatives by up to ~2‰; 4. δ¹⁸O of same sample candidates of pelagic and nektobenthic taxa are distinguished by a 2–3‰ offset; 5. elements processed with a combination of formic acid and bromoform have the most variable δ¹⁸O; 6. thermal alteration of elements does affect the isotope signal. Single element ion microprobe δ¹⁸O values can be closely comparable to those derived from multiple element silver phosphate precipitates, but the magnitude of δ¹⁸O variation observed in ion microprobe analyses (up to 4‰) is equivalent to that interpreted as representing cooling oceans over the duration of the Ordovician. Previously, bulk chemical and ionprobe analyses may have masked such variability.