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

摘要

The Ordovician ~(87)Sr/~(86)Sr isotope sea-water 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 ~(87)Sr/~(86)Sr decrease may have varied, which has implications for both the strati-graphic resolution possible using Sr isotope stratigraphy and efforts to model the effects of Ordovician geologic events. We measured ~(87)Sr/~(86)Sr 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 ~(87)Sr/~(86)Sr during the Middle Ordovician Darriwilian Stage. Our ~(87)Sr/~(86)Sr 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 ~(87)Sr/~(86)Sr is estimated at ～5.0-10.0 x 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 x 10~(-5) (the 2σ standard deviation is 6.2 x 10~(-5)), which in the best case scenario allows for subdivision of Ordovician time intervals characterized by the highest rates of fall in ~(87)Sr/~(86)Sr at a maximum resolution of ～0.5-1.0 m.y. Links between the increased rate of fall in ~(87)Sr/~(86)Sr beginning in the mid-late Darriwilian (Phragmodus po-lonicus 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 megaseqiience boundary) and tectonic events (Taconic orogeny) in North America provides a plausible explanation for the changing magnitude and ~(87)Sr/~(86)Sr of the riverine Sr flux to the oceans.