WHEN ARE BRACHIOPODS OR CONODONTS MOST USEFUL FOR SR ISOTOPE STRATIGRAPHY OF PALEOZOIC SUCCESSIONS?

Strontium isotope stratigraphy is one of several geochemical techniques used to correlate Paleozoic stratigraphic successions, particularly when certain fossils are insufficient for biostratigraphy or if chemostratigraphic data, such as δ¹³C, are compromised. Strontium has a long oceanic residence time (>2 myr), so well-preserved rocks and fossils should yield a global Sr isotopic (⁸⁷Sr/⁸⁶Sr) signature that is correlative to sections across the globe. Brachiopods are considered an ideal fossil material for this technique because the preservation of shell microstructure can be verified using scanning electron or cathodoluminescence microscopy. However, well-preserved brachiopods are not always abundant for high-resolution analysis, so other fossils must be used. Conodont apatite is one such fossil that can be abundant in many Paleozoic limestones, and the phosphatic element is enriched in Sr and resistant to isotopic exchange with porewater. But, conodonts have drawbacks, particularly in some intervals (i.e., Permian–Triassic strata) where conodont Sr concentration is fairly low (<2000 ppm) and ⁸⁷Sr/⁸⁶Sr values are more radiogenic than brachiopods from the same section.

In this study we test the hypothesis that there are certain times or conditions that make it ideal to measure conodonts for Sr isotope stratigraphy. Here we measured the ⁸⁷Sr/⁸⁶Sr value of 59 conodonts from the Appalachian Basin, Great Basin, and Baltica regions, ranging from Ordovician to Triassic in age, and compare these values to the literature. We then compare brachiopod (n = 1682) and conodont (n = 1164) ⁸⁷Sr/⁸⁶Sr values to the global ⁸⁷Sr/⁸⁶Sr value. We find that high Sr concentrations ([Sr]) are correlated with fossils that are closer to the global ⁸⁷Sr/⁸⁶Sr value. Reported brachiopod [Sr] values are relatively constant throughout the Paleozoic (~1000 ppm), whereas conodont [Sr] values were very high during the Ordovician (up to 30,000 ppm) but dropped to <2000 ppm Sr by the Permian. Conodonts with high [Sr] values are closer to the global ⁸⁷Sr/⁸⁶Sr curve, which is true for Ordovician to Devonian conodonts and corresponds Calcite Sea vs. the Aragonite Seas of the late Paleozoic. This research suggests that conodonts can yield ⁸⁷Sr/⁸⁶Sr values close to the global value, but should only be used if they have [Sr] values >2500 ppm Sr.