Date of Award

Summer 7-1-2022

Degree Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Earth Sciences

Advisor(s)

Lu, Zunli

Keywords

Cap carbonates, Carbonate rocks, Devonian, Iodine, Neoproterozoic, Redox condition

Subject Categories

Earth Sciences | Geology | Physical Sciences and Mathematics

Abstract

Iodine is an important trace element in biogeochemical and redox reactions. It has a nearly consistent concentration of ~0.46 μmol/L in the seawater and the inorganic speciation (IO3− and I−) of iodine is primarily controlled by redox conditions in water columns. Iodate (IO3−), not iodide (I−), is the only species that can be incorporated into carbonate minerals. Therefore, I/Ca ratio from carbonate rocks can be used to constrain the redox condition in the water column over a range of time scale. This dissertation contains three chapters that utilizes I/Ca ratio from bulk carbonate rocks to reconstruct ocean redox changes during Devonian in Laurentia and during Neoproterozoic in South China, to investigate the relationship between ocean redox, paleoenvironmental changes and biotic events. Chapter 1 focuses on a Mid-Devonian core from Appalachian Basin. I/Ca ratios are generally stable and high in the Onondaga Formation but show large fluctuations in lower Marcellus strata. The pattern of redox changes is correlated to organic-rich shale deposition, sea-level changes and biotic transitions in Devonian marine systems. The range of I/Ca values of the core support the notion of a Devonian rise in atmospheric oxygen in the Paleozoic. Chapter 2 further investigates I/Ca data from well-dated Lower Devonian through Upper Devonian limestone sections from the Great Basin (western Laurussia) and the Illinois Basin (central Laurussia) and considers influences from carbonate lithology. Results indicate that lithologic changes do not exert a dominant control on bulk carbonate I/Ca trends. The overall reducing and variable redox conditions of local upper ocean during the Middle and Late Devonian of Great Basin coincide with evidence of increased global environmental volatility. And the distinct I/Ca ranges observed from three Devonian basins suggests a local redox control that potentially influenced by paleogeographic settings. Chapter 3 studies Late Neoproterozoic cap carbonates formed following the Cryogenian glaciations, from basal Doushantuo Formation, one of the most extensively studied Ediacaran units in South China. Among the four sections from a shallow-to-deep water transect, the slope section has the lowest I/(Ca+Mg) values, suggesting that it may have experienced more extensive fluid-buffered diagenesis and more severe loss of IO3–. In combination with available major, trace, rare earth element and isotope data, the temporal I/(Ca+Mg) variations support freshwater mixing and deep-water upwelling in proximal sites and transient oxidation during deposition of the Doushantuo cap carbonates.

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