Date of Award

December 2018

Degree Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Earth Sciences

Advisor(s)

Christopher K. Junium

Keywords

Carbon, Chemocline, Nitrogen, Pigments, Stable Istopes

Subject Categories

Physical Sciences and Mathematics

Abstract

This dissertation focuses on identifying and qualifying chemocline dynamics, namely depth and stability, in stratified aquatic systems through the use of sedimentary pigments and the stable carbon and nitrogen isotopes of organic matter. The central aim of the research comprising this volume is to identify how chemocline fluctuations are expressed in the pigment and stable isotope signatures of aquatic sediments, and how those fluctuations may have impacted nutrient cycling in past intervals of marine anoxia and mass extinction.

In order to help gauge how the depth of the chemocline may affect specific pigment signatures and concentrations in sediments, I first investigated how pigment distributions changed in microbialites over a shallow depth gradient. The immobile nature and relative stability of the environment surrounding the microbialites allowed for the investigation of how depth, and by extension changing light regime, can affect microbial community structure and the production of light harvesting vs. photoprotective pigments. It was found that the concentration of the photoprotective pigment scytonemin and its abundance relative to that of chlorophyll a decrease logarithmically with depth, consistent with the function of scytonemin as a UV screening pigment. As well, the increase in the concentration of chlorophyll a, b and the photosynthetic accessory carotenoids fucoxanthin and β-carotene with depth are consistent with lower irradiance at depth. The distribution and relative abundance of photosynthetic and light shielding pigments therefore, may provide a means for determining the relative water depth/incident radiation levels of ancient microbialites in which pigments or their derivatives are preserved. As well, it provides a modern proof of concept for utilizing changing pigment concentrations and ratios in an aquatic system to reconstruct past changes in light regime, or the depth of the locus of primary production.

The next phase of this research was then to investigate how pigment and stable isotope signatures varied in a modern stratified system during periods of know chemocline fluctuations. For this, I turned to the sediments of Lake Kivu, East Africa, a deep meromictic lake that has experienced large scale mixing events and chemocline destabilizations in the past. Within the studied core, sediments deposited coevally with mixing events exhibited distinct pigment, and carbon and nitrogen stable isotopic signatures (δ13Corg and δ15Nbulk respectively) compared to surrounding background sediments. The δ13Corg and δ15Nbulk values displayed sharp negative excursions at the base of the high TOC sapropel layers that are associated with the mixing events. These negative excursions provide evidence for the greater influence of 13C-depleted dissolved inorganic carbon and 15N-depleted ammonium derived from below the chemocline. Additionally, ratios of zeaxanthin:chlorophyllone (photoprotective and photosynthetic pigments respectively) display enormous fluctuations and spikes in the studied interval, with the sapropel layers hosting the highest values. This coupled with the presence of bacteriochlorophyll derivatives is further evidence for the breakdown of permanent stratification and shallowing of the chemocline during sapropel deposition. This study provides a mechanistic link to the strongly depleted δ15Nbulk values in the black shales of Mesozoic OAEs, and other anoxic basins of the past and can bolster predictions on the effects of future warming and deoxygenation on nutrient cycling in the modern ocean. Additionally, it provides a set of stable isotope and pigment signatures that can be used to characterize chemocline dynamics in ancient sedimentary sequences.

The final phase of this body of work characterizes chemocline dynamics in a more ancient sedimentary system, namely, the black shales from the Appalachian and Illinois basins, associated with the Late Devonian Frasnian-Famennian biotic crisis. The Frasnian-Famennian biotic crisis marked by two distinct intervals known as the Lower and Upper Kellwasser Events that in many locations are associated with deposition of organic-rich shales. Black shales from the Illinois and Appalachian basins, including the Kellwasser Events, are 15N-depleted and have significantly lower δ15Nbulk than interbedded grey shales, a trend consistent with many instances of black shale deposition in the Phanerozoic. Organic carbon isotopes exhibit the broad, positive excursions (~+3.5 ‰ from background) that are typical of the KWEs globally. Superimposed over these positive excursions in δ13Corg are sharp decreases within the black shale beds. The pattern of δ15Nbulk and δ13Corg values suggests that, similar to Lake Kivu, the depth/stability of the chemocline and the degree of water-column stratification exert a primary control on both δ15Nbulk and δ13Corg during black shale deposition. In the context of the Frasnian-Famennian biotic crisis, the oscillating redox state and changing temperatures would have likely placed extreme stress on organisms within the marine environment of the Appalachian and Illinois basins, and may potentially have been a contributing factor to diversity loss over this time period.

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Available for download on Sunday, July 14, 2019

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