DEFINING GEOLOGIC VARIABILITY IN CONODONT 87SR/86SR AND CHRONOSTRATIGRAPHIC IMPLICATIONS

Conodont microfossils record seawater strontium isotope values (⁸⁷Sr/⁸⁶Sr), permitting chemostratigraphic correlation for tectonic and climatic reconstructions of the Paleozoic and early Mesozoic (541–201 Ma). Advances in laser ablation multiple collection inductively coupled plasma mass spectrometry (LA MC-ICP-MS) enable rapid, high resolution ⁸⁷Sr/⁸⁶Sr analyses of conodont bioapatite, but require validation by comparison with solution analysis. We compare solution and LA MC-ICP-MS ⁸⁷Sr/⁸⁶Sr analyses of Carboniferous-age conodonts. The two analytical methods generally agree for conodonts from the same stratigraphic level. In addition, we find that individual conodonts from the same stratigraphic level exhibit ⁸⁷Sr/⁸⁶Sr variability beyond the precision of reference materials. This finding suggests that solution ⁸⁷Sr/⁸⁶Sr values determined by dissolving multiple conodonts are homogenizing distinct ⁸⁷Sr/⁸⁶Sr signatures of individual conodonts. As such, the precision of these solution measurements does not capture the geologic variability of conodont ⁸⁷Sr/⁸⁶Sr within a stratigraphic level. We explore possible reasons for this geologic variability including time averaging, spatial variability in in seawater ⁸⁷Sr/⁸⁶Sr, and differential diagenesis. We further calculate what fraction of the geologic variability in conodont strontium isotope values contributes to the uncertainty in age estimations based on correlations to the seawater strontium isotope curve. Conodont ⁸⁷Sr/⁸⁶Sr measurements that do not account for geologic variability are at risk of false calibrations with the paleo-seawater ⁸⁷Sr/⁸⁶Sr curve, which has implications for the timing of geologic events and reconstructions of paleo-environmental changes.