Analysing conodont δ18O by SIMS

Oxygen isotope compositions of phosphatic conodont microfossils are a more robust indicator of early Paleozoic sea surface temperatures than the O compositions of calcitic brachiopods. SIMS microanalysis allows conodont O compositions to be determined on an ~30 µm scale, so the isotopic homogeneity of individual conodont elements and of multiple elements from a single rock sample can be assessed. Analyses can be obtained on very small and/or rare specimens, and targeted on those parts of conodonts that are best preserved. Differences in composition between co-existing species from different biofacies can be measured. SIMS analyses, however, are less precise and more sensitive to differences in the chemical composition of the sample than conventional IRMS analyses. The latter is important because the conodont crown can consist of two tissue types: albid—large (>100 µm) crystals containing mostly PO4, and hyaline—microcrystals with PO4 and some CO3, and a higher trace element content. Some also contain poorly mineralised basal tissue that potentially is isotopically contaminated. In addition, conodont crystal structure can be modified at temperatures above ~100˚C. No conodont sample is available that is both isotopically uniform and abundant enough to be distributed between SIMS and IRMS labs as an O isotope standard. SIMS measurements are currently referenced to a mineral apatite standard (Durango 3 at ANU), and potential systematic bias is assessed by comparing SIMS and IRMS analyses of large bioapatite samples (e.g. shark enameloid), and of conodonts from the same or closely equivalent rock samples. As part of our study of sea surface temperatures at critical periods of biotic crisis and environmental change in the Paleozoic and Early Mesozoic using conodont O isotopes, we have addressed issues of bias, inter- and intra-conodont heterogeneity, and thermal alteration (as indicated by Colour Alteration Index). In some samples we find O variation within and between elements, but more commonly there is inter- and intra conodont uniformity between genera and species in a single rock sample within the sub-permil precision of the measurements. The rare exceptions have both biological and geological implications.