Calibration of a conodont apatite-based Ordovician ⁸⁷Sr/⁸⁶Sr curve to biostratigraphy and geochronology: Implications for stratigraphic resolution

The Ordovician ⁸⁷Sr/⁸⁶Sr isotope seawater curve is well established and shows a decreasing trend until the mid-Katian. However, uncertainties in calibration of this curve to biostratigraphy and geochronology have made it difficult to determine how the rates of ⁸⁷Sr/⁸⁶Sr decrease may have varied, which has implications for both the stratigraphic resolution possible using Sr isotope stratigraphy and efforts to model the effects of Ordovician geologic events. We measured ⁸⁷Sr/⁸⁶Sr in conodont apatite in North American Ordovician sections that are well studied for conodont biostratigraphy, primarily in Nevada, Oklahoma, the Appalachian region, and Ohio Valley. Our results indicate that conodont apatite may provide an accurate medium for Sr isotope stratigraphy and strengthen previous reports that point toward a significant increase in the rate of fall in seawater ⁸⁷Sr/⁸⁶Sr during the Middle Ordovician Darriwilian Stage. Our ⁸⁷Sr/⁸⁶Sr results suggest that Sr isotope stratigraphy will be most useful as a high-resolution tool for global correlation in the mid-Darriwilian to mid-Sandbian, when the maximum rate of fall in ⁸⁷Sr/⁸⁶Sr is estimated at ∼5.0–10.0 × 10^–5 per m.y. Variable preservation of conodont elements limits the precision for individual stratigraphic horizons. Replicate conodont analyses from the same sample differ by an average of ∼4.0 × 10^–5 (the 2σ standard deviation is 6.2 × 10^–5), which in the best case scenario allows for subdivision of Ordovician time intervals characterized by the highest rates of fall in ⁸⁷Sr/⁸⁶Sr at a maximum resolution of ∼0.5–1.0 m.y. Links between the increased rate of fall in ⁸⁷Sr/⁸⁶Sr beginning in the mid-late Darriwilian (Phragmodus polonicus to Pygodus serra conodont zones) and geologic events continue to be investigated, but the coincidence with a long-term rise in sea level (Sauk-Tippecanoe megasequence boundary) and tectonic events (Taconic orogeny) in North America provides a plausible explanation for the changing magnitude and⁸⁷Sr/⁸⁶Sr of the riverine Sr flux to the oceans.