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Carbon, sulfur, and strontium isotope stratigraphy of the Lower-Middle Ordovician, Great Basin, USA: Implications for oxygenation and causes of global biodiversification

Edwards, Cole T

Abstract Details

2014, Doctor of Philosophy, Ohio State University, Geological Sciences.
This chemostratigraphic study uses new carbon (d13C) and sulfur (d34S) isotope data measured from Lower–Middle Ordovician carbonate rocks from the Great Basin region, USA. The Pogonip Group was sampled at meter-scale from Shingle Pass (east-central Nevada) and the Ibex area (western Utah) to integrate the stable isotope stratigraphy with a well-studied conodont biostratigraphic framework. The Pogonip Group is a succession of mixed carbonate and siliciclastic rocks that accumulated on a carbonate ramp under normal marine conditions during the Late Cambrian to Middle Ordovician. The d13C trend has four distinct characteristics recognized in both Great Basin sections: 1) a drop in d13C from +1‰ at the base of the Ordovician (Tremadocian) to -0.7‰, 2) a 1–2‰ positive d13C shift during the late Tremadocian, 3) a gradual d13C increase from -2‰ to ca. 0‰ during the end of the Early Ordovician (Floian), and 4) a steady d13C decrease from 0‰ to -4 to -5‰ during Middle Ordovician (Dapingian–Darriwilian). The d34S trend measured from carbonate-associated sulfate (CAS) at Shingle Pass has an overall decrease from +35‰ during the Tremadocian to +20 to +25‰ during the Floian. A 15‰ negative excursion is present near the Dapingian-Darriwilian boundary before d34S values increase up to +35‰ at the top of the section. Corresponding d34S measured from sedimentary pyrite shows an overall similar drop in the Lower Ordovician but pyrite d34S values are more variable. During the early Tremadocian Stage pyrite d34S varies between 0 to +20‰ but makes a major drop of 20‰ during the late Tremadocian to values between -10–0‰ throughout the Floian and Dapingian Stages. Pyrite d34S increases gradually near the Dapingian-Darriwilian boundary to values between +10 to +20‰ at the top of the section. The Lower–Middle Ordovician d13C and d34S trends reported here from the Great Basin are not consistent with a causal mechanism involving sea level change and subsequent migration of isotopically distinct water bodies. Instead, these isotope data most likely reflect primary seawater chemistry and changes in d13C and d34S on a global scale. This interpretation is supported by the excellent correlation of d13C and d34S to other isotopic trends reported from Argentina, Newfoundland, and global compilation data sets. These correlations, in conjunction with strontium isotope (87Sr/86Sr) data measured from conodont apatite and bulk carbonate, are consistent with published conodont biostratigraphic data and provide an integrated and high-resolution bio-chemostratigraphic framework for the Early–Middle Ordovician sedimentary record of the Laurentian margin. Long-term d13C and d34S trends indicate two pulses of oxygenation occurred during periods of increased burial rates of organic matter and pyrite. These oxygenation pulses are roughly coincident with the first major biodiversification pulses during the Middle Ordovician and suggest that increasing oxygen levels played an important role in global biodiversity. Continued research focused on resolving the precise timing of biodiversification globally or within individual basins will be able use chemostratigraphic correlation to compare with the d13C, d34S, or 87Sr/86Sr and isotopic data presented here.
Matthew Saltzman (Advisor)
William Ausich (Committee Member)
Stig Bergström (Committee Member)
Lawrence Krissek (Committee Member)
248 p.

Recommended Citations

Citations

  • Edwards, C. T. (2014). Carbon, sulfur, and strontium isotope stratigraphy of the Lower-Middle Ordovician, Great Basin, USA: Implications for oxygenation and causes of global biodiversification [Doctoral dissertation, Ohio State University]. OhioLINK Electronic Theses and Dissertations Center. http://rave.ohiolink.edu/etdc/view?acc_num=osu1405797089

    APA Style (7th edition)

  • Edwards, Cole. Carbon, sulfur, and strontium isotope stratigraphy of the Lower-Middle Ordovician, Great Basin, USA: Implications for oxygenation and causes of global biodiversification. 2014. Ohio State University, Doctoral dissertation. OhioLINK Electronic Theses and Dissertations Center, http://rave.ohiolink.edu/etdc/view?acc_num=osu1405797089.

    MLA Style (8th edition)

  • Edwards, Cole. "Carbon, sulfur, and strontium isotope stratigraphy of the Lower-Middle Ordovician, Great Basin, USA: Implications for oxygenation and causes of global biodiversification." Doctoral dissertation, Ohio State University, 2014. http://rave.ohiolink.edu/etdc/view?acc_num=osu1405797089

    Chicago Manual of Style (17th edition)