Source: Dissertation Abstracts International, Volume: 30-02, Section: B, page: 0713. / Thesis (Ph.D.)--The Florida State University, 1968.

Two Middle-Late Ordovician sections from the Appalachian Basin of eastern North America were analyzed for chemostratigraphic and sequence stratigraphic correlations to elucidate possible causal mechanisms facilitating the Great Ordovician Biodiversification Event (GOBE). Paired stable isotope (δ13C and δ34S) as well as sequential iron extractions and total digest analysis were used in both carbonate-dominated and shale-dominated localities from the Appalachian Basin to reconstruct the marine redox states during integral periods of biodiversification, while sequence stratigraphic analyses were utilized to reconstruct fluctuations both local and eustatic sea level. Geochemical investigations of the shale-dominated sequence will bring potential insights into the redox environment of the deeper portions of the Appalachian Basin while this carbonate-dominated sequence is one of the most expanded Sandbian Stage (Late Ordovician) sequences known from North America, and brings new high-resolution reconstructions of long-term global carbon and sulfur cycles fluctuations and eustatic sea level changes. Integrating paleoredox reconstructions from relatively shallow and deep water sequences within the same basin can help to understand any contraction or expansion of oxidative or reducing conditions during the Middle–Late Ordovician within this portion of the Appalachian Basin that may be linked to global paleoredox dynamics. We find that although this part of the Appalachian Basin may have been slightly restricted from open marine settings during the latest Darriwilian, trends in stable isotopes and reconstructed sea level (sequence stratigraphy) can be correlated to global trends utilizing previous conodont and graptolite biostratigraphy along with conodont 87Sr/86Sr values. Here we identify several intervals of decoupled δ13Ccarb and δ34SCAS trends: as δ13C increases or remains constant, δ34S trends drop, signaling a discontinuity between carbon and sulfur cycles. However, these decoupled stratigraphic intervals are followed by coupled trends δ13Ccarb and δ34SCAS (parallel positive shifts in both). Causal mechanisms for the observed δ13C and δ34S trends may be linked to a reduction of pyrite burial due to increased ventilation in Middle-Late Ordovician marine environments, while coupled trends likely represent increased organic carbon and pyrite burial associated with expansion of reducing oceanic conditions. These alternations from more oxic to anoxic/euxinic conditions throughout the Middle-Late Ordovician may in part explain the pulsed nature of marine faunal diversity trends previously observed throughout the GOBE. The results presented here demonstrate the unfolding complex evolution of the long-term sulfur and carbon cycles, marine paleoredox conditions, continental weathering, and climate during the Early Paleozoic world. / A Thesis submitted to the Department of Earth, Ocean and Atmospheric Science in partial fulfillment of the requirements for the degree of Master of Science. / Summer Semester 2017. / June 29, 2017. / Carbon isotopes, Great Orodovician Biodiversification Event, Oxygenation, Pyrite Burail, Sulfur Isotopes / Includes bibliographical references. / Seth A. Young, Professor Directing Thesis; Jeremy D. Owens, Committee Member; Angela N. Knapp, Committee Member.

The Ireviken extinction event occurred within the early-middle Silurian, spanning the Llandovery-Wenlock boundary, ~431 million years ago and has been proposed to have been initiated at least partly through the transition from late Ordovician- early Silurian icehouse conditions to middle Silurian greenhouse conditions. This extinction, like many of the biotic crises throughout the Paleozoic, coincides with the rising limb of a subsequent carbon isotope excursion and the magnitude of the Ireviken carbon isotope excursion (CIE) has recorded δ13C values of ≥+4‰ worldwide. Epicontinental seaways have long been thought to have been the most likely place for high enough organic carbon burial to have the capability of producing this positive isotope excursion. However, the Ireviken extinction event and its subsequent carbon isotope excursion had both occurred during times of increased carbonate production, rather than organic carbon burial, in these shallow epeiric seas. This led to the development of a two steady-state ocean climate model, known as primo (P) and secondo (S) states, that could best explain these lithostratigraphic and biostratigraphic trends. This model demonstrates how alternating between climate states can affect ocean circulation via changing the site of deep water formation and ultimately induce anoxia throughout portions of the deep ocean. It assumes of a globally deoxygenated deep ocean with δ13C alone and lacks any direct evidence for pervasive deep ocean anoxia. It is for that reason that this study has conducted the first paired δ34S-carbonate associated sulfate (CAS) and δ13C study along with a carbonate paleoredox proxy, I/(Ca+Mg) ratio analysis, to bring new insights on the paleoredox conditions of the late Llandovery-early Wenlock oceans during the Ireviken extinction and CIE. Covarying positive shifts in δ13CCarb and δ34SCAS are consistent with the onset of sea-level rise following the end of Late Ordovician-Early Silurian icehouse conditions and represents an overall increase in the fraction of anoxic waters in the global ocean. Trends in δ13COrg show overall low oxygen trends and downwelling of nutrient poor water masses resulting from expansion of epeiric seas. I/(Ca+Mg) ratios during this time also indicate local and pervasive low oxygen conditions. Shallow oxic surface waters were shown to be in direct exchange and within proximity to anoxic water masses suggesting a shallow chemocline prior to the Ireviken CIE and an expanded one during. The net effect of organic carbon and pyrite burial results in an overall increase in atmospheric O2, eventually cooling climate, reestablishing thermohaline circulation, and, therefore, ending the Ireviken CIE. / A Thesis submitted to the Department of Earth, Ocean, and Atmospheric Science in partial fulfillment of the Master in Science. / Summer Semester 2017. / June 29, 2017. / Carbon, Ireviken, ocean anoxia, Paleoredox, Sulfur / Includes bibliographical references. / Seth Young, Professor Directing Thesis; Jeremy Owens, Committee Member; Angie Knapp, Committee Member.

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