The distribution and geologic history of nitrogen on Earth is poorly known. Traditionally thought to be an inert gas, with only a small but important biologic cycle, geochemical investigation highlights that it can also be present in rocks and minerals. Even at low concentrations, the great mass of the solid Earth allows for the possibility of substantial N mass and cycling in the geosphere over Earth history. Thus, the assumption that N on the surface of the Earth has remained in steady state over Earth history can be questioned. The research goals of this thesis are to investigate the Earth System N cycle using both large- and small-scale approaches. I present a comprehensive literature compilation to ascertain the N budget of Earth. Determining the total abundance of N in all reservoirs of the Earth, including the atmosphere, oceans, crust, mantle, and core is crucial to a discussion of its cycling in the past. This budget study suggests that the majority of planetary N is likely in the core, with the Bulk Silicate Earth a more massive reservoir than the atmosphere. I also present experimental data and data from lunar samples as added context. As quantification of geologic N is difficult, I present research detailing the adaptation of a fluorometric technique common in aquatic geochemistry for use on geologic samples. I compare fluorometry analysis of geochemical standards to several other techniques: colourimetry, elemental analyzer mass spectrometry, and neutron activation analysis. Fluorometry generally behaves well for crystalline samples, and is a relatively quick and easy alternative to more expensive or intensive techniques. As a preliminary application, I have determined a N budget estimate for the continental crust based on analysis of crystalline crustal rocks and glacial tills from North America. This budget is consistent with published work, suggesting about 2 × 1018 kg N, or half a present atmospheric mass of N, is in the continental crust. I also present a geochemical study measuring N-isotopes and redox sensitive trace elements from a syn-glacial unit deposited during the the Marinoan Snowball Earth. Snowball Earth events were the most extreme glaciations in Earth history. The measurements presented herein are the first to quantify biologic activity via N-isotopes as well as the redox state of the atmosphere and ocean using trace elements from this intriguing time period in Earth history. The data suggests that there was active N- fixing in the biosphere, persistent but limited O2, nitrification, and nearly quantitative denitrification during the glaciation. After the glacial interval, O2 levels increased and denitrification levels dropped, indicated by near-modern δ15N values. The combined use of N-isotope with redox sensitive trace elements provides a more nuanced and comprehensive view in reconstructing past ocean and biologic conditions. Lastly, I present an Earth-system N cycle model with nominal results. Previous modelling efforts have agreed with the traditional notion that atmospheric N-levels have remained constant over geologic time. This is in contrast with modern geochemical evidence suggesting net transport of N from the surface into the mantle. The aim, in turn, of this model is to model N cycling over Earth history by explicitly incorporating both biologic and geologic fluxes. The model is driven by a mantle cooling history and calculated plate tectonic speed, as well as a prescribed atmospheric O2 evolution history. This approach is the first of its kind, to my knowledge, and produces stable model runs over Earth history. While tuning and sensitivity studies may be required for publishable results, nominal runs are compelling. In model output, atmospheric N varies by an factor of 2 − 3 over Earth history, and the availability of nutrients (i.e., PO4) exerts a strong control on biologic activity and movement of N throughout the Earth system. Such a planetary perspective on N serves as an entry point into discussions of planetary evolution as a whole. With the great increase in the number of discovered exoplanets, the scientific community is charged with developing models of planetary evolution and factors that promote habitability. Comparison of Earth to its solar system neighbours and future data on exoplanets will allow a system of evolution pathways to be explored, with the role of N expected to be prominent in discussions of habitability and planetary evolution. / Graduate / 0996 / 0425 / bwjohnso@uvic.ca

Water is a primary driver of the physical, geochemical and biological evolution of the Earth. The near-surface hydrosphere (exosphere) includes the atmosphere, cryosphere (glacial and polar ice), the biosphere, surface water, groundwater, and the oceans. The amounts of water in these various reservoirs of the hydrologic cycle have likely varied significantly over the past 700 Ma, with the cryosphere and continental biosphere reservoirs likely showing the most dramatic variations relative to the modern. For example, 700 Ma, during snowball-Earth conditions, the planet may have been almost entirely enveloped in ice, whereas throughout much of the Phanerozoic, greenhouse conditions predominately prevailed and the Earth had a much smaller cryosphere. Similarly, before about 444 Ma and the proliferation of land plants, the continental biosphere reservoir would have effectively non-existent. However, today, plants play a critical role in storage and transfer of water within the hydrologic cycle. Because the amount of water in the exosphere is thought to have remained relatively constant during the past 700 Ma, variations in the amounts of water held by the in the various exogenic reservoirs exert concomitant effects on other reservoirs in the exosphere. We present a conceptual and numerical model that examines variations in the amount of water in the various reservoirs of the near-surface hydrologic cycle (exosphere) during the past 700 Ma and quantify variations in the rates of exchange of water between these reservoirs in deep time. Variations in the sizes of major reservoirs are primarily controlled by changes in global average temperature, and the movement of water between the atmosphere, surface water, and ocean reservoirs varies in concert with the waxing and waning of the cryosphere. We find that variations in the sizes of major reservoirs are primarily controlled by changes in global average temperature, and the flux of water between the atmosphere, surface water, and ocean reservoirs varies in concert with the waxing and waning of the cryosphere, with some fluxes decreasing to 0.0 kg/yr during snowball-Earth conditions. We find that the amount of water precipitated from the atmosphere to the cryosphere increases from greenhouse conditions to -10.5°C and decreases from -10.5°C to snowball-earth conditions, highlighting "tipping-point" behavior due to changes in temperature and cryosphere surface area. The amount of surface runoff to the oceans varies in proportion to the amount of water removed from the surface water reservoir and transferred into the continental biosphere. Variations in the movement of water between near-surface reservoirs that are driven by the waxing and waning of the cryosphere and emergence and growth of plant life thus have significant implications for the transfer of weathering products to the oceans and could contribute to short-term (<1 Ma) variations in seawater composition and isotopic signatures. / Ph. D.

Dans le Supergroupe Ouest-Congolais du bassin du Niari-Nyanga (Afrique centrale), les marqueurs des Glaciations Globales Sturtienne et Marinoenne sont représentés par les formations des «Diamictite inférieure» et «Diamictite supérieure».Ce travail de thèse présente une étude à haute résolution du Cap Carbonate associé à la Diamictite supérieure (6 coupes dans le bassin et 2 coupes dans la zone externe de la Chaîne du Mayombe). Les études pétrographiques révèlent la préservation des structures sédimentaires primaires et permettent de définir six microfaciès (MF0 à MF5) caractéristiques des paléoenvironnements de types inter- à supratidaux ou subtidaux. Dans le Cap Carbonate étudié, les indices de Kübler montrent une évolution croissante depuis la diagenèse profonde dans le bassin jusqu'à l'épimétamorphisme dans la zone externe de la Chaîne du Mayombe. Le signal isotopique ([delta]13C et [delta]18O) des Cap Carbonate échantillonnés est généralement reproductible dans le synclinal comme dans la chaîne, avec les valeurs du [delta]13C montrant une excursion négative variant de -2.6 [pour mille] à -5.6 [pour mille]. Les valeurs de d18O oscillent entre -6 [pour mille] et -12 [pour mille]. Par ailleurs, la confrontation des données minéralogiques, chimiques et isotopiques indique une influence négligeable des transformations post-sédimentaires sur la signature isotopique du Cap Carbonate indiquant la préservation des valeurs du [delta]13C de l'océan néoprotérozoïque. Enfin, l'ensemble des données de cette thèse et les résultats préliminaires des « Projets GLANEC» replacés dans un contexte régional permettent de définir le Membre SCIa du synclinal du Niari-Nyanga comme un Cap Carbonate lié à la Glaciation Marinoenne. / Within West-Congolian Supergroup of Niari-Nyanga Basin (Central Africa), the markers of Sturtian and Marinoan Global Glaciations are documented by the so-called «Lower Diamictite» and «Upper Diamictite». This work is based on 6 and 2 lithological sections from the basin and the external zone of the Mayombe fold belt. It corresponds to a high-resolution study of the Cap Carbonate lying unconformably on the Upper Diamictite. Petrographic analyses show preservation of primary sedimentary structures and lead to define six microfacies (MF0 to MF5). These microfacies caracterize paleoenvironments such as inter- to supratidal or subtidal types. Kübler index values of the studied Cap Carbonate display an increasing evolution from East to West, ie from a deep diagenesis in the basin to an epimetamorphism in the Mayombe external zone. Stable isotope signature ([delta]13C, [delta]18O) of the various components of this Cap Carbonate is reproducible throughout the basin as in the Mayombe external zone, with [delta]13C values displaying a negative excursion, decreasing from -2.6 ? to -5.6 ?. [delta]18O values of these components vary between -6 ? and -12 ?. Moreover, comparison between mineralogic, chemical and isotopic data indicates that post-sedimentary transformations had a very negligible influence on the isotopic signature of the Cap Carbonate, indicating preservation of d13C values of the Neoproterozoic ocean. Finally, when they are compared with other regional studies, all data of the present work and the preliminary results of the GLANEC Projects lead to the conclusion that the SCIa Member of the Niari-Nyanga Basin must be considered as a Cap Carbonate related to the Marinoan Global Glaciation.

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