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

The Ordovician Period (489 to 443 Ma) is marked by a transition from a warm, greenhouse climate to an icehouse by the Late Ordovician, making it a valuable case study for the question of how various, interconnected Earth systems drive and respond to global climate change. In service of this larger question, this study focuses on the role of one such system: silicate weathering. Silicate weathering draws down and sequesters atmospheric carbon, and younger basalts draw down CO₂ at a higher rate than older granites. An increased rate of basaltic weathering in the geologic past can be traced by a correlating decrease in the value of a well-established proxy: strontium isotope ratios (⁸⁷Sr/⁸⁶Sr). In the case of the Ordovician, global ⁸⁷Sr/⁸⁶Sr decreases gradually over the course of ~20-30 My of the Middle to Late Ordovician, before the rate of change increases and ⁸⁷Sr/⁸⁶Sr begins to drop sharply in the early part of the Late Ordovician (Sandbian). The precise timing of this inflection point has not been well constrained, making it difficult to ascertain its potential causes and effects. This study represents a renewed effort to increase the resolution of ⁸⁷Sr/⁸⁶Sr data derived from conodont apatite.

⁸⁷Sr/⁸⁶Sr measurements of conodont samples from strata of the Simpson Group in the Arbuckle Mountains, Oklahoma build on previously published measurements from this locality (Saltzman et al., 2014, GSA Bulletin, v. 126). Use of a new ThermoFisher Triton Plus TIMS at Ohio State provides a lower external error (2SD=8.22×10^-6, n=96) than previous work. In addition, we investigate the impact of samples’ pre-treatment by leaching conodonts in acetic acid, which some previous authors indicate removes diagenetic Sr.

Results place the point of greatest ⁸⁷Sr/⁸⁶Sr change in the lower Bromide Formation and ~458 Mya. This age is consistent with the interpretation that the inflection point represents a shift in the nature of the Taconic orogeny occurring on the eastern margin of Laurentia. Additional ⁸⁷Sr/⁸⁶Sr data from other sites will be collected to confirm this age of the inflection point.