Seafloor Weathering and the Middle to Late Ordovician Seawater 87Sr/86Sr Inflection Point Preserved in Conodont Apatite
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Seafloor weathering and the Middle to Late Ordovician seawater 87Sr/86Sr inflection point preserved in conodont apatite Thesis Presented in Partial Fulfillment of the Requirements for the Degree Master of Science in the Graduate School of the Ohio State University By Teresa Daniela Avila, B.S. Graduate Program in Earth Sciences The Ohio State University 2019 Thesis Committee Matthew Saltzman, Adviser Elizabeth Griffith John Olesik Copyright by Teresa Daniela Avila 2019 Abstract The strontium isotope ratio (87Sr/86Sr) of global seawater varies through geologic time and can serve as a proxy for silicate weathering patterns as well as rates of spreading in mid- ocean ridges. The 87Sr/86Sr value of seawater steadily decreases through the course of the Ordovician, with an increased rate of change during the Darriwilian to Sandbian (Middle to Late Ordovician). The precise age of this inflection point has been poorly constrained, making it difficult to ascertain its possible causes and effects. Here, conodont apatite from the Simpson Group of the Arbuckle Mountains, Oklahoma were analyzed in order to build a higher-resolution 87Sr/86Sr curve. The preparation of conodont samples via leaching in acetic acid is also investigated. In the case of Oklahoma section conodont elements with low thermal alteration (i.e., Color Alteration Index; (CAI) ≤ 1), leaching does appear to strip diagenetic Sr, but the overall effect on 87Sr/86Sr (7.47 x 10-6 ) is smaller than the external analytical error (8.22 x 10-6). To identify the inflection point in the new data set, a smoothing LOESS curve was used to produce a gradient curve, a process which has not yet been applied to the Middle to Late Ordovician. The 87Sr/86Sr inflection point falls in the transition from the Oil Creek to McLish Formations, within the holodentata conodont zone at 466.4 to 463.8 Mya. The shift in 87Sr/86Sr occurs at the Sauk-Tippecanoe sequence boundary and associated transgression, which may reflect increased spreading rates of mid-ocean ridges. Previous studies have linked the inflection point in 87Sr/86Sr to the Taconic Orogeny at c.a. 465 Mya, which may also play an important role in the shift of global 87Sr/86Sr but is unlikely to account for the transgression at the base of the McLish due to asynchronous timing of events. ii Dedication Dedicated to my family and their commitment to education iii Vita May 2010……………………Lafayette High School 2015…………………………B.S. Geological Sciences, University of Missouri 2015…………………………B.S. Science and Agricultural Journalism, University of Missouri 2015 to 2017………………...Laboratory Technician, Department of Earth and Planetary Sciences, Washington University in St. Louis 2017 to present………………Dean’s Graduate Enrichment Fellow, School of Earth Sciences, The Ohio State University Publications Warren, JW, Schiffbauer, JD, Avila, TD, Broce, JS. (2018). Ecophenotypy, temporal and spatial fidelity, functional morphology, and physiological trade-offs among intertidal bivalves. Paleobiology, 44(3), 530-545. Fields of Study Major Field: Earth Sciences iv Table of Contents Abstract……………………………………………………………………………………………ii Dedication………………………………………………………………………………………...iii Vita………………………………………………………………………………………………..iv List of Tables…………………………………………………………………………………..…vi List of Figures…………………………………………………………………………………....vii Introduction……………………………………………………………………………………......1 Background……………………………………………………………………………………..…5 Method…………………………………………………………………………………………...13 Results…………………………………………………………………………………………....16 Discussion………………………………………………………………………………………..23 Conclusion……………………………………………………………………………………….33 References………………………………………………………………………………………..34 Appendix A: Method Details………..…………………………………………………………...42 Appendix B: Statistics Details…..…..…………………………………………………………...46 Appendix C: Age Model…….…..…..…………………………………………………………...47 Appendix D: Non-Ordovician Samples……..…………………………………………………...48 v List of Tables Table 1. Sr concentration, 87Sr/86Sr, and associated errors of analyzed conodont samples…..16 Table 2. 87Sr/86Sr of leached samples, unleached samples, and leachates……………………18 Table 3. Sr abundance of leached samples and leachates…………………………………….19 vi List of Figures Figure 1. Ordovician conodont-based curve………………………………………………………3 Figure 2. Diagenetic alteration of conodont 87Sr/86Sr: A conodont element in vivo, B conodont element post mortem, C conodont element with low-temperature pore water alteration, D conodont element with thermal alteration, E conodont element leaching process…..9 Figure 3. Difference between leached samples and leachate in previous studies………………..11 Figure 4. Site location……………………………………………………………………………14 Figure 5. Difference in 87Sr/86Sr of leached samples, unleached samples, leachates…...…….....20 Figure 6. Leached samples vs. unleached samples vs. global LOWESS curve…………………24 Figure 7. Sr mass balance in leached conodont samples……………………………………...…25 Figure 8. SEM images of conodont samples before and after leaching………………………….26 Figure 9. Data plotted against depth……………………………………………………………..27 Figure 10. Data plotted against age, associated gradient curve, whole Ordovician…….……….28 Figure 11. Sr mass balance in leached Pennsylvanian-age conodont samples.………………….50 Figure 12. Difference in 87Sr/86Sr of Silurian leached samples, unleached samples, leachates…51 Figure 13. Data plotted against age, associated gradient curve.………………………………....52 Figure 14. Clear Springs data plotted against depth………….………………………………….53 Figure 15. Antelope Range data plotted against depth……….………………………………….54 Figure 16. Model demonstrating inflection point…………….………………………………….55 vii Introduction The Ordovician Period (485 to 444 Mya) is a valuable case study for the various, interconnected Earth systems that drive and respond to global climate change. The Ordovician climate is characterized by a roughly 20º C drop in average sea surface temperature—with smaller-scale changes superimposed—that encompassed the extreme warmth of the Early Ordovician (c.a. 42º C) to the end-Ordovician (Hirnantian) glaciation (c.a. 22º C; Trotter et al., 2008; Albanesi et al., 2019). This cooling to mild, modern-like sea surface temperatures potentially triggered the Great Ordovician Biodiversification Event (GOBE), but eventually may have led to the first of the “big five” extinctions: the end-Ordovician mass extinction (Sepkoski, 1996; Trotter et al., 2008). Multiple factors could have contributed to this cooling, including decreased volcanic degassing (McKenzie et al., 2016), the appearance of the first land plants in correlation with overall increased organic carbon burial (Lenton et al., 2012; Algeo et al., 2016), and increased weathering of calcium-bearing silicates (Swanson-Hysell and Macdonald, 2017; Saltzman, 2017; Macdonald et al., 2019). Understanding the interplay of these systems and their impact on the timing and magnitude of cooling steps throughout the Ordovician represents a longstanding problem. Previous studies have investigated the Ordovician cooling trend mostly in terms of how exposure of young, Ca- and Mg- bearing silicates (i.e., basalts) at low latitudes—such as the island-arc setting of the Ordovician Taconic orogeny in Laurentia—would have increased weathering rates and the drawdown of CO2 (Shields et al., 2003; Young et al., 2009; Swanson- Hysell and Macdonald, 2017; Macdonald et al., 2019). An important proxy for basaltic weathering in the geologic record is the trend in global marine 87Sr/86Sr values (McArthur et al., 1 2012). However, while the large-scale trend of decreasing 87Sr/86Sr in the Ordovician is fairly well established (Figure 1), uncertainty remains in the timing of a Middle to Late Ordovician (Darriwilian to Sandbian stages) inflection point in the curve and its correlation with the Ordovician paleotemperature curve (Trotter et al., 2008; Albanesi et al., 2019). The timing of the inflection point ranges from 458 Mya to 466 Mya, spanning several biostratigraphic zones (Figure 1; Shields et al., 2003; Young et al., 2009; McArthur et al., 2012; Saltzman et al., 2014; Swanson-Hysell and Macdonald, 2017). This timing appears problematic for the Taconic weathering hypothesis (Young et al., 2009; Saltzman, 2017), as the late Darriwilian to Sandbian appears to coincide with a slowing of cooling or even a slight warming trend superimposed on long-term cooling (Trotter et al., 2008; Albanesi et al., 2019). A role for Taconic weathering is evidenced by a similarly timed shift in neodymium (Nd) isotopes (Swanson-Hysell and Macdonald, 2017), but questions persist about the amount of basaltic versus more intermediate composition continental weathering that can be inferred using this proxy (Saltzman, 2017). Therefore, a more precise understanding of the timing of this 87Sr/86Sr inflection point is critical to evaluate other causal mechanisms that likely played a role distinct from Taconic weathering, namely seafloor spreading rates and associated eustatic changes (e.g., Shields et al., 2003; Saltzman et al., 2014). In order to better constrain the timing of the inflection point in the Ordovician 87Sr/86Sr curve, this study focuses on conodont microfossil apatite in the Arbuckle Mountains of Oklahoma, which contains one of the best-constrained conodont biostratigraphic data sets in the world for the Darriwilian to Sandbian (c.a. 465 to 455 Mya) study interval (Bauer, 1987; Bauer, 2 Figure 1. Ordovician conodont-based 87Sr/86Sr measurements with Locally Estimated Scatterplot Smoothing (LOESS) curve with global Locally Weighted Scatterplot Smoothing (LOWESS)