REFINING THE TIMING OF THE MIDDLE-LATE ORDOVICIAN INFLECTION POINT IN SEAWATER 87SR/86SR USING CONODONT APATITE

Strontium isotope (⁸⁷Sr/⁸⁶Sr) stratigraphy is a useful method for tracking changes in the Earth’s geologic history. Due to its residence time, changes in the marine ⁸⁷Sr/⁸⁶Sr value are assumed to reflect global changes in the Earth’s silicate weathering patterns.

Seawater ⁸⁷Sr/⁸⁶Sr decreases at a steady rate through the Early Ordovician and most of the Middle Ordovician. The rate of decline increases—and ⁸⁷Sr/⁸⁶Sr drops sharply—during an inflection point in the late Darriwilian and early Sandbian. This increased rate of change has been interpreted as weathering of volcanic rocks from island-arc settings in the Taconic orogeny in eastern Laurentia. This would act as a sink for atmospheric pCO₂ and initiate the icehouse transition observed in the Katian and Hirnantian. However, the timing of this shift does not fit a clear cause-and-effect relationship with δ¹⁸O, i.e. seawater temperature. Improved resolution of changes in ⁸⁷Sr/⁸⁶Sr and δ¹⁸O can help address whether a shift in weathering patterns influenced temperature or vice-versa. In addition, lower sedimentation rate could create the illusion of a sharper decrease in ⁸⁷Sr/⁸⁶Sr in the measured sections. In order to rule out a role for reduced sedimentation rate, data must be collected at high resolution from multiple well-dated stratigraphic sections.

The Middle to Late Ordovician inflection point is apparent in the ⁸⁷Sr/⁸⁶Sr values of conodont elements, the bioapatitic remains of a now-extinct marine chordate. Our samples come from the I-35 section of the Simpson Group in Arbuckle Mts, OK; the inflection point appears to occur within the upper Tulip Creek and lower Bromide formations. This study seeks to better constrain the ⁸⁷Sr/⁸⁶Sr shift by increasing resolution within the ~458 to 463 Mya period represented by part of the Simpson Group. Furthermore, this study differs from previous work in the Simpson Group (Saltzman et al., 2014, GSA Bulletin, v. 126) in that the conodont elements are processed via leaching in acetic acid and analysed for ⁸⁷Sr/⁸⁶Sr on a new generation TIMS instrument at Ohio State with higher external reproducibility (SRM 987 yields 2SD = 1.2x10^-5; n=52). Additional data will be collected from the Antelope Range, NV in an effort to rule out the possibility that the observed inflection point is an artefact of changing sedimentation rates.