As lower microwave frequency bands become saturated with users, there is a motivation for the research of applications that utilize higher frequencies, especially the 60 GHz band. This band is plagued with high atmospheric absorption due to atmospheric oxygen, but has a lot of bandwidth, which makes it desirable for multi-media applications. Recently, research of wideband digital links within the 60 GHz band gained the interest of the wireless communication industry when the FCC announced that a license is not required for a wideband digital signal in this band. Previous research on 60 GHz signals focused on how much attenuation due to atmospheric oxygen exists in the link. But a look at the physical properties of atmospheric oxygen reveals both the reason why atmospheric oxygen absorbs electromagnetic waves and how pressure affects atmospheric oxygen. Atmospheric oxygen resonates at 60 GHz due to transitions between its three closely spaced rotational states. These transitions, combined with the magnetic dipole moment of atmospheric oxygen, cause attenuation and phase dispersion in electromagnetic waves. At lower pressures, the individual resonance lines of atmospheric oxygen appear in the attenuation and the phase dispersion plots. As pressure increases, the resonance lines broaden and contribute to neighboring resonant lines. The effect of attenuation and phase dispersion in a wideband signal becomes greater at lower atmospheric pressures, which results in signal distortion. The signal distortion leads to more bit errors and results in the presence of inter-symbol interference (ISI) in the received signal. This thesis aims to analyze the effects of atmospheric oxygen on a wideband digital link, especially at lower pressures and higher data rates. In order to simulate the effects of atmospheric oxygen in the atmosphere, an empirical atmospheric model was used, which characterizes the behavior of oxygen under various atmospheric pressures. A wideband communication system was simulated with the absorption and dispersion due to atmospheric oxygen represented as a transfer function and placed in the link part of the system. Eye diagrams were used to view the impact of the atmospheric oxygen attenuation and phase dispersion in the signal. Also bit error rate plots were computed in order to determine the extra margin needed. / Master of Science

Fire is a natural process that has existed on our planet for more than ~350 million years, and is a process that continues to influence our everyday lives. On Earth, a relationship exists between the process of combustion and the natural functioning of the Earth system. Here, the process of combustion has been implicated in playing an essential role for life on Earth, where natural Earth system processes have been shown to influence ignition probability, fire spread and fire behaviour, and where fire can provide a variety of feedbacks, to the Earth system over different timescales. Over medium timescales of decades to hundreds of thousands of years, the likelihood and behaviour of fires are controlled by regional climate changes and vegetation type, whilst the occurrence of fire can play a crucial role in influencing biome persistence and development. Over long timescales (hundreds of thousands to multi-million year), the components influencing the probability of fire and fire behaviour not only involve processes occurring over local and regional spatial scales, and over short and medium timescales, but also long term processes occurring globally, such as changes in atmospheric oxygen concentration and the evolution of vegetation. Across these timescales in Earth’s past, combustion has been shown to impact global ecosystems, climate and the carbon cycle by generating feedbacks that influence Earth’s biogeochemical cycles. However, it is clear that our understanding of the role that fire plays in the Earth system, although improving is still developing. This thesis provides an analysis of these Earth system - fire relationships and feedbacks across medium and long timescales in deep time, in order to understand the role that fire may have played and what the record of fire can tell us about the functioning and re-equilibrating of the Earth system during and after significant carbon-cycle perturbation events occurring in Earth’s deep past. The results presented in this thesis contribute what is believed to be the first fossil evidence that rising atmospheric oxygen and fire feedbacks may have aided in the termination of a significant carbon-cycle perturbation event, termed the ‘Toarcian oceanic anoxic event’ that occurred ~183 million years ago during the Jurassic period, and the return of the Earth system towards ‘background functioning’. This thesis also provides an analysis of the record of wildfire in the form of fossil charcoal across the initiation of an anoxic event that occurred ~93 million years ago, during the Cretaceous period. The results illustrate that CO2 - climate driven changes in wildfire activity can be observed across medium timescales even during times of significant carbon-cycle perturbations, and modelled high atmospheric oxygen concentrations. These results illustrate how hypothesized changes in the hydrological cycle, and likely moisture content of fuel, appear to be the dominant control on wildfire activity during this period. Finally, this thesis provides an analysis of charcoal abundance variations occurring across natural, orbitally forced cycles, termed the Milankovitch cycles. The results presented illustrate that natural variations in charcoal abundance are possible over intermediate timescales within the geological record. This thesis therefore illustrates a need to take into consideration and incorporate ‘natural background’ fluctuations in fire activity occurring over medium timescales, when analysing and predicting past and future climate change patterns.

Important geochemical and tectonic events in the Paleoproterozoic Era lay the foundation for the status and operation of the modern Earth, including the initial rise of atmospheric oxygen paving the path for animal evolution, and the emergence of modern plate tectonic processes leading to the amalgamation of the Canadian Shield (Laurentia). Rudimentary geological and geochronological documentation of Paleoproterozoic sedimentary basins is the foundation from which we can ask larger questions about geochemical changes or plate tectonic events on the evolving Earth, since those questions are largely answered by analyzing the sedimentary record. This thesis outlines the stratigraphy, detrital zircon U-Pb geochronology, elemental and isotopic geochemistry, and basin evolution of the Paleoproterozoic Penrhyn and Piling basins on the Rae craton in Arctic Canada, which record important tectonic and geochemical events on both a regional and global scale. The concentration of the redox-sensitive trace element, U, in seawater has not been constant throughout geologic time and is linked to changes in oceanic and atmospheric oxygen content. Secular variations in the record of U contents of shales and iron formations indicate that the redox state of the atmosphere-ocean system after the Great Oxidation Event (GOE) was more dynamic than previously thought. Trends towards lower oxygen content recorded after ~2.05 Ga in the middle Proterozoic suggest that oxygen level decreased. This is contrary to traditional models assuming unidirectional atmospheric oxygen rise throughout the Proterozoic. The data demonstrate the earliest signal of oxidative U cycling, manifested in 2.47 - 2.43 Ga iron formations, and show that oxygenation was a protracted process initiated shortly after the end of the Archean. It has been proposed that a global and long-lived magmatic and tectonic shutdown event from ~2.45 to 2.22 Ga played a causal role in the GOE, since it overlaps the time interval in which atmospheric oxygen initially rose on Earth. Coupled U-Pb, Hf, and O isotope data on magmatic and detrital zircon determine that plate tectonic processes continued to operate during this interval. It is argued instead that plate tectonic processes are necessary to promote conditions favorable for atmospheric oxygen to rise.

Le climat du Quaternaire est défini par une succession de périodes glaciaires et interglaciaires enregistrées dans les archives climatiques à différentes latitudes. La carotte de glace d’EPICA Dome C fournit un enregistrement haute résolution sur les derniers 800 ka du δ18Oatm (i.e. δ18O de la molécule d’oxygène de l’air) qui combine les variations passées du cycle hydrologique des basses latitudes et de la productivité de la biosphère. En l’absence du comptage des couches annuelles, ce proxy peut être utilisé comme méthode de datation orbitale des carottes de glace, en lien avec l’insolation au 21 juin à 65°N. Cependant, un décalage de 6 ka entre le δ18Oatm et l’insolation, généralement observé lors des terminaisons glaciaires-interglaciaires, est appliqué sur l’ensemble de l’enregistrement lors de la construction de l’échelle d’âge. Ce décalage et la complexité du signal du δ18Oatm expliquent l’incertitude élevée de 6 ka des carottes de glace, ce qui limite leur interprétation en termes de variations climatiques et environnementales conjointement à d’autres archives. J’ai donc développé une nouvelle chronologie pour les carottes de glace, basée sur le lien entre le δ18Oatm et le δ18Ocalcite des spéléothèmes est-asiatiques, à partir de nouvelles mesures isotopiques permettant d’avoir pour la première fois un enregistrement complet sur les derniers 800 ka à Dome C. Cette nouvelle chronologie permet de réduire les incertitudes par rapport à la chronologie actuelle et d’avoir une meilleure séquence des évènements entre les hautes et basses latitudes. J’ai ensuite développé un modèle simulant la composition isotopique de l’oxygène atmosphérique afin de répondre au manque d’interprétations quantitatives de ce proxy ainsi que pour vérifier son lien avec le δ18Ocalcite sur plusieurs cycles climatiques. Pour modéliser le δ18Oatm nous avons dû coupler le modèle climatique de complexité intermédiaire iLOVECLIM avec le modèle de végétation CARAIB. Le δ18Oatm simulé par le modèle couplé sur plusieurs dizaines de milliers d’années confirme que ses variations sont en phase avec celles de l’insolation de l’hémisphère Nord (hormis lors d’évènements de Heinrich) et avec celles du δ18Ocalcite via des modifications du cycle hydrologique des basses latitudes, impactant la composition isotopique de l’eau de pluie utilisée par la biosphère terrestre lors de la photosynthèse. / Quaternary glacial-interglacial cycles are recorded in various climatic archives from high to low latitudes. The EPICA Dome C ice core provides a high-resolution record over the last 800 ka of δ18Oatm (i.e. δ18O of atmospheric O2) which combines past variations of the low latitude water cycle and of the biosphere productivity. In absence of annual layer counting, this proxy can be used for orbital dating in association with the June 21st insolation at 65°N to build an ice core chronology. However a lag of 6 ka between the δ18Oatm and the insolation, classically observed during glacial-interglacial terminations, is applied to the entire record during the chronology construction. This lag and the complexity of the δ18Oatm signal are the main reasons why the ice core chronology presents a high 6 ka uncertainty which limits their interpretation, jointly with other paleoclimate archives, in terms of past climate and environmental variations. To solve this issue I have developed a new ice core chronology based on the relation between the δ18Oatm and the δ18Ocalcite of east-asian speleothems, using new isotope measurements allowing for the first time a complete record over the last 800 ka at Dome C. This new chronology reduces the uncertainties compared to the actual ice core chronology strongly based on δ18Oatm and shows a better sequence of events between the high and low latitudes records. Then, I have developed a model to reproduce the isotopic composition of atmospheric O2 to address the lack of quantitative interpretations of this proxy and to check our assumption of synchronicity with the δ18Ocalcite over several climatic cycles. To reproduce the variations of the δ18Oatm, it was necessary to couple the intermediate complexity climate model iLOVECLIM and the vegetation model CARAIB. Finally, the δ18Oatm variations simulated with the new coupled model over several thousand years are in phase with the insolation of the Northern hemisphere (except during Heinrich events) and with low latitudes δ18Ocalcite variations. This can be explained by changes in the low latitude water cycle related to changes in the isotopic composition of meteoric water used by the terrestrial biosphere during photosynthesis.