UNLV Theses, Dissertations, Professional Papers, and Capstones 5-1-2017 Environmental Changes across the Early Mississippian Carbon Isotope Excursion Dev Krishna Maharjan University of Nevada, Las Vegas, [email protected] Follow this and additional works at: https://digitalscholarship.unlv.edu/thesesdissertations Part of the Climate Commons, Geology Commons, and the Sedimentology Commons Repository Citation Maharjan, Dev Krishna, "Environmental Changes across the Early Mississippian Carbon Isotope Excursion" (2017). UNLV Theses, Dissertations, Professional Papers, and Capstones. 3005. https://digitalscholarship.unlv.edu/thesesdissertations/3005 This Dissertation is brought to you for free and open access by Digital Scholarship@UNLV. It has been accepted for inclusion in UNLV Theses, Dissertations, Professional Papers, and Capstones by an authorized administrator of Digital Scholarship@UNLV. For more information, please contact [email protected]. ENVIRONMENTAL CHANGES ACROSS THE EARLY MISSISSIPPIAN CARBON ISOTOPE EXCURSION By Dev Krishna Maharjan Bachelor of Science – Geology Trichandra Campus, Nepal 1999 Master of Science – Geology Tribhuvan University, Nepal 2001 Master of Science – Geology University of Louisiana Lafayette 2010 A dissertation submitted in partial fulfillment of the requirements for the Doctor of Philosophy – Geoscience Department of Geoscience College of Science The Graduate College University of Nevada, Las Vegas May 2017 Copyright by Dev Maharjan 2017 All Rights Reserved Dissertation Approval The Graduate College The University of Nevada, Las Vegas May 10, 2017 This dissertation prepared by Dev Krishna Maharjan entitled Environmental Changes across the Early Mississippian Carbon Isotope Excursion is approved in partial fulfillment of the requirements for the degree of Doctor of Philosophy – Geoscience Department of Geoscience Ganqing Jiang, Ph.D. Kathryn Hausbeck Korgan, Ph.D. Examination Committee Chair Graduate College Interim Dean Stephen M. Rowland, Ph.D. Examination Committee Member Elisabeth M. Hausrath, Ph.D. Examination Committee Member Rosemarie Came, Ph.D. Examination Committee Member Paul Schulte, Ph.D. Graduate College Faculty Representative ii Abstract Environmental changes across the Early Mississippian carbon isotope excursion By Dev Krishna Maharjan Dr. Ganqing Jiang, Examination Committee Chair Professor of Geology University of Nevada, Las Vegas The Early Mississippian K–O (Kinderhookian-Osagean) δ13C excursion or TICE (mid- Tournaisian carbon isotope excursion) is characterized by an anomalous carbonate carbon isotope value of ≥5‰ that has been documented from numerous stratigraphic sections across the globe. This δ13C excursion coincides with global cooling and sea-level fall and predicts a significant change in seawater chemistry. However, sulfate sulfur isotope data across the K-O δ13C excursion reported in previous studies seem not to be responsive to the carbon cycle. Likewise, a recent study has documented a unidirectional increase in nitrogen isotopes across this excursion, which is not anticipated considering the amount of organic carbon burial required to form the prominent positive δ13C excursion and its resultant oxygen increase and global cooling. This study aims to understand the coupling between carbon, nitrogen, sulfur and oxygen cycles at this critical transition. This research uses Early Mississippian carbonate successions from Star Range (SR), Mountain Home Range (MH), and Pahranagat Range (PR) that indicate peritidal, shallow subidal to deep subtidal depositional environment in the Great Basin of western USA. Samples from these sections are analyzed for carbon isotopes (δ13C), sulfur isotopes (δ34S), nitrogen isotopes (δ15N) and oxygen isotopes (δ18O). Additionally, SR samples are analyzed for fluid inclusions, iii REEs, major and trace element concentrations. Stratigraphic records from PR and MH sections indicate that the outer shelf section (PR) may be more representative of seawater isotope record. 34 13 The δ SCAS records a ≥7‰ positive anomaly near the peak of the K-O δ C excursion. Numerical modeling suggests that pyrite burial rates 5–10 times higher than that of the modern 34 ocean are required to produce the observed δ SCAS anomaly in a sulfate-rich Early Mississippian ocean. The aerial and volumetric expansion of sulfate reduction and pyrite burial was likely fused by abundantly available organic matter at the peak of the K-O δ13C excursion when oxygen minimum zone (OMZ) in the ocean has substantially expanded. At the falling limb of the K-O 13 34 18 δ C excursion, coupled negative shifts in δ SCAS and δ OCAS imply increase of sulfide reoxidation in the ocean and pyrite-derived riverine sulfate input, in response to global cooling, sea-level fall, and oxygenation resulted from enhanced organic carbon and pyrite burial, resulting in gradually decreasing the volume of OMZ. Such a change in ocean redox is reflected in the carbon and nitrogen cycles. Stratigraphic records indicate that the deeper-water outer shelf 13 13 section (PR) show double spikes in both δ Ccarb and δ Corg with a magnitude up to 7‰ and a negative shift down to 4‰ between the peaks which is more representative of the seawater isotope record. However, considering the sensitivity of δ15N to redox conditions of depositional 13 15 environments, the coupled δ Ccarb – δ N pattern from the MH section may better record the – 13 15 isotope signature of the oceanic nitrate (NO3 ) reservoir, while the decoupled δ Ccarb – δ N in the PR section reflects involvement of local N-biochemical cycling in periodically developed suboxic-euxinic environments. The increase of δ15N toward the peak of the K-O δ13C excursion may record the expansion of oxygen minimum zone (OMZ) in the ocean that promotes water- 15 – column denitrification and N enrichment in the marine nitrate (NO3 ) reservoir. The decrease of δ15N at the falling limb of the K-O δ13C excursion reflects the shrink of the OMZ and reduces the iv water-column denitrification in response to more oxygenated and cooler oceans resulting from enhanced organic carbon burial. The significant amount of organic carbon burial during Early Mississippian results global cooling and possibly glaciation which is partly supported by the presence of highly depleted δ18O values, down to –34‰, in SR samples. Block samples from SR and PR are studied for carbon and oxygen isotopes, fluid inclusion, REEs, major and trace elements to investigate the origin and processes involved during carbonate diagenesis. These geochemical data suggest that the carbonate diagenesis took place from a diagenetic fluid originated form freshwater. Importantly, SR samples are from close to or below the unconformity surfaces which may have been altered by glacial melt water with δ18O values of ≤ –15‰. Then, it could have been further modified by a burial diagenesis with geothermal temperature < 200°C to produce the highly depleted δ18O values down to –34‰. Collectively, carbon, nitrogen, sulfur and oxygen isotope records from Early Mississippian carbonates show a coupled variation in response to the environmental change during this time. v Acknowledgements Accomplishment of this work has been only possible with the help of numerous honorable people. First, I would never forget to mention my own academic advisor, Dr. Ganqing Jiang, who always emphasizes me to explore a workable scientific problem for future goals. I heartily appreciate him that he allow me a significant amount of time to work on my dissertation with a continuous financial and moral support until the final phase of my dissertation. I also acknowledge my research committee members, Dr. Stephen Rowland, Dr. Elisabeth Hausrath, Dr. Rosemarie Came and a graduate college representative, Dr. Paul Schulte. I would like to thank Dr. Jean Cline for allowing me to work in her laboratory and providing guidance to understand and collect the data from fluid inclusions from calcite. I also acknowledge Dr. Minghua Ren and Dr. Arya Udry for their help to work in their laboratory. Importantly, my gratitude goes to Dr. Yongbo Peng, Assistant Professor of Research at Louisiana State University, for his valuable time and assistance in sample analyses. The never lasting memories in my dissertation field work had been accompanied by the ―Carbonate Group‖ and I really appreciate Swapan Sahoo, Leon Taufani, Angel Ventrelli, and Andrew Miller for their valuable time and good moment during field work. Swapan and Leon were the companion during the search of good Mississippian carbonate sections across Utah and Nevada — without their effort the field work would not have been completed on time. I would also like to thank Jeevan Jayakody, Shaimaa Haleem and Zhoujun Xie for their good times during my study. I acknowledge Maria, Liz, Cynthia and Maribel in the Geoscience department office at UNLV for their assistance and help in processing paper works during my study. vi Finally, I am so lucky to have my parents who nurture my life with care and love, always supporting me to take the right way of success. Whatever gratitude I express in words to them is just a tiny. On the other hand, I am grateful to have my beloved wife, Kalpana, who understood my different feelings and situations during the study and gave me a continuous support by any means although she has been taking care of our children Denik and Keden. vii Table of contents Abstract………………………………………………………………………………………… iii Acknowledgements……………………………………………………………………………...vi Table Contents………………………………………………………………………………....viii