The Gaskiers glaciation (c. 580 Ma) has been classically traced along the Neoproterozoic Avalonian-Cadomian Terranes, which are now found scattered around the North Atlantic Ocean. Around 625 Ma these terranes were composed of volcanoes and arc-type basins. ‘Till-like’ diamictite horizons identified within these basins have been used as evidence for a ‘Snowball Earth-type’ glaciation at 580 Ma. However, others argue that these deposits are non-glacial debris flow deposits. To test the non-glacial interpretation of these deposits, a detailed sedimentological and basin analysis was conducted on the Neoproterozoic Squantum Member that occurs conformably with the volcanic-sedimentary rocks of the Boston Bay Group (eastern Massachusetts); this deposit is one of the most referenced ‘tillite’ deposits for the Gaskiers glaciation. This thesis shows that the ‘tillites’ of this succession are volcanically-influenced non-glacial debrites. Using the Lesser Antilles Arc and the adjacent Grenada Basin in the Caribbean Sea as a modern depositional analogue for the Avalonian-Cadomian Terranes, this study further reveals that debris flow facies types (diamicts) comparable to those of the Avalonian-Cadomian Terranes are produced at this modern arc and are recorded in the fill of the Grenada Basin. A similar study was conducted on the modern diamicts produced at the heavily glaciated Mount Rainier volcano (Washington, USA), revealing that despite the presence of local glaciers, debris flow is the dominant process depositing diamicts due to eruptions and flood events. The major thrust of this thesis is that it highlights the key role of tectonics and volcanism, not glaciation, in producing the diamictites of the Avalonian-Cadomian Terranes, and the importance of examining Neoproterozoic diamictite facies in their wider sedimentary, stratigraphic and tectonic context.

The Gaskiers glaciation (c. 580 Ma) has been classically traced along the Neoproterozoic Avalonian-Cadomian Terranes, which are now found scattered around the North Atlantic Ocean. Around 625 Ma these terranes were composed of volcanoes and arc-type basins. ‘Till-like’ diamictite horizons identified within these basins have been used as evidence for a ‘Snowball Earth-type’ glaciation at 580 Ma. However, others argue that these deposits are non-glacial debris flow deposits. To test the non-glacial interpretation of these deposits, a detailed sedimentological and basin analysis was conducted on the Neoproterozoic Squantum Member that occurs conformably with the volcanic-sedimentary rocks of the Boston Bay Group (eastern Massachusetts); this deposit is one of the most referenced ‘tillite’ deposits for the Gaskiers glaciation. This thesis shows that the ‘tillites’ of this succession are volcanically-influenced non-glacial debrites. Using the Lesser Antilles Arc and the adjacent Grenada Basin in the Caribbean Sea as a modern depositional analogue for the Avalonian-Cadomian Terranes, this study further reveals that debris flow facies types (diamicts) comparable to those of the Avalonian-Cadomian Terranes are produced at this modern arc and are recorded in the fill of the Grenada Basin. A similar study was conducted on the modern diamicts produced at the heavily glaciated Mount Rainier volcano (Washington, USA), revealing that despite the presence of local glaciers, debris flow is the dominant process depositing diamicts due to eruptions and flood events. The major thrust of this thesis is that it highlights the key role of tectonics and volcanism, not glaciation, in producing the diamictites of the Avalonian-Cadomian Terranes, and the importance of examining Neoproterozoic diamictite facies in their wider sedimentary, stratigraphic and tectonic context.

It has been commonly recognized that a series of global glaciation events occurred during the late Neoproterozoic Era (800 - 540 million years ago (Ma)). However, the extent of these glaciations continues to be hotly debated, namely whether the whole Earth was ice covered (ie. a “hard snowball”) or only the continents were fully ice covered but the oceans were not (“slushball/soft snowball”). Through a combination of climate modeling and carbon cycle modeling, I have investigated the plausibility of the “soft snowball” Earth hypothesis. It is demonstrated that the flow of land ice is critical to the formation of a “soft snowball”, such that low latitude land ice must be generated through ice transported from high latitudes. In order for a climate state of this kind to be realizable, continental fragments at low latitude must be well connected to those at high latitude, and the high latitude continents must be sufficiently extensive that a large ice sheet may initiate and subsequently flow to low latitude. It is found that these constraints are satisfied by the most accurate available continental reconstruction for both the initial Sturtian glaciation of the late Neoproterozoic and the subsequent Marinoan event. It is furthermore proposed that the alternative “hard snowball” hypothesis would have been prevented by a negative feedback due to the enhanced remineralization of dissolved organic carbon (DOC) in the ocean due to increased oxygen solubility in seawater at lower temperature. This process would release CO2 to the atmosphere, thus counteracting the initial climate cooling. I have also carried out detailed simulations in which an explicit model of the carbon cycle is coupled to the ice-sheet coupled climate model to investigate this feedback quantitatively. It is found that the remineralization of the DOC does indeed provide a strong negative feedback that counteracts climate cooling. The action of this feedback not only prevents the descent of the climate into a hard snowball state, but also enables the model to re-produce the δ13C carbon isotopic anomalies observed to accompany Neoproterozoic glacial events. The resistance of this carbon cycle coupled climate system to descent into a “hard snowball” state is further tested against stochastic perturbations, and shown to be robust in the presence of such influence.

It has been commonly recognized that a series of global glaciation events occurred during the late Neoproterozoic Era (800 - 540 million years ago (Ma)). However, the extent of these glaciations continues to be hotly debated, namely whether the whole Earth was ice covered (ie. a “hard snowball”) or only the continents were fully ice covered but the oceans were not (“slushball/soft snowball”). Through a combination of climate modeling and carbon cycle modeling, I have investigated the plausibility of the “soft snowball” Earth hypothesis. It is demonstrated that the flow of land ice is critical to the formation of a “soft snowball”, such that low latitude land ice must be generated through ice transported from high latitudes. In order for a climate state of this kind to be realizable, continental fragments at low latitude must be well connected to those at high latitude, and the high latitude continents must be sufficiently extensive that a large ice sheet may initiate and subsequently flow to low latitude. It is found that these constraints are satisfied by the most accurate available continental reconstruction for both the initial Sturtian glaciation of the late Neoproterozoic and the subsequent Marinoan event. It is furthermore proposed that the alternative “hard snowball” hypothesis would have been prevented by a negative feedback due to the enhanced remineralization of dissolved organic carbon (DOC) in the ocean due to increased oxygen solubility in seawater at lower temperature. This process would release CO2 to the atmosphere, thus counteracting the initial climate cooling. I have also carried out detailed simulations in which an explicit model of the carbon cycle is coupled to the ice-sheet coupled climate model to investigate this feedback quantitatively. It is found that the remineralization of the DOC does indeed provide a strong negative feedback that counteracts climate cooling. The action of this feedback not only prevents the descent of the climate into a hard snowball state, but also enables the model to re-produce the δ13C carbon isotopic anomalies observed to accompany Neoproterozoic glacial events. The resistance of this carbon cycle coupled climate system to descent into a “hard snowball” state is further tested against stochastic perturbations, and shown to be robust in the presence of such influence.

An interglacial mudstone sequence from the Port Askaig Tillite Formation on Islay was analysed using an Olympus XRF detector. The resulting geochemical log was compared with an XRF dataset acquired from a Quaternary sedimentary core from the Lomonosov Ridge in the Arctic Ocean. Chemical proxies representing climatic and environmental changes were analysed in an effort to specifically identify evidence of orbital forcing in the Cryogenian Period. The studied non-glacial rock-section from the Port Askaig Formation was interpreted as being deposited in a shallow marine setting at semitropical latitudes during an episode of global warming at some stage of the Sturtian glaciation (ca 717 – 660 Ma). The transport mechanism of glaciogenic material was by ice rafting. High hematite content was interpreted as an oxygenation event in a peritidal zone when isostatic rebound caused a sea level regression. Increasing amount of muscovite is interpreted to indicate increased weathering. Underlaying sequence of dolostone and overlaying sequence of sandstone were consistent with these interpretations. One interglacial phase is thus observed, which possibly could be attributed to Milankovitch orbital forcing. The interpretation of the paleoclimatic setting of the studied interglacial mudstone did not provide support for the Snowball Earth hypothesis in its “hard” version. Neither did other observations such as evidence of repeating glacial-interglacial cycles and banded iron formations (BIF) appearing also within the Sturtian glaciation.

This research is focused on strata deposited in northern Utah during the Cryogenian Period (850 – 635 Ma) of the Neoproterozoic Era, a period that derives its name from the widespread evidence for multiple, likely global, glacial events during this time, commonly referred to as “Snowball Earth” glaciations. This dissertation includes detailed studies of two Cryogenian successions in northern Utah that bracket potential “Snowball Earth” events: the upper part of the Uinta Mountain Group (deposited prior to the glaciations) and the dolomite member of the Kelly Canyon formation (hypothesized to have formed in the aftermath of a global glaciation that terminated at either 665 or 635 Ma). Both successions contain a lithostratigraphic, geochemical, and biotic record of the Earth’s oceans before and after the largest-magnitude glaciations in the history of our planet. The pre-glacial upper part of the Uinta Mountain Group in the area mapped for this study contains evidence of several (at least three) relatively short periods of ocean anoxia in which ferruginous conditions dominated and euxinia did not occur. There is no evidence that biota (organic-walled microfossil assemblages) were influenced by these brief anoxic events, but evidence from the composite Uinta Mountain Group stratigraphic record does suggest a gradual change in biota similar to that in the Chuar group. It is likely this biotic transition is related to nearshore eutrophication in the oceans, but additional redox geochemical information is needed to fully support this conclusion. The dolomite member of the Kelley Canyon Formation on Antelope Island (post-glacial component of this study) contains idiosyncratic lithologic features thought to be characteristic of 635 Ma deglacial strata, yet its C-isotope values do not lend unequivocal support to this global correlation, and regional correlations and U-Pb zircon ages suggest it is ~30 million years older. These results challenge the popular notion that Neoproterozoic post-glacial cap carbonates can be correlated based upon their lithologic “style,” and they also lend additional support to the possibility of a “Snowball Earth” event at ~665 Ma.

The climate of the Neoproterozoic Snowball Earth is tested in the UKMO Unified Model, specifically the HadCM3 climate model. The model is largely left unchanged, but the boundary conditions, both external and initial, are adjusted to create experiments based on the Snowball Earth hypothesis. The model can reproduce multiple equilibrium climates, as have been seen in energy balance models of the Earth's climate. The modelled present day and Neoproterozoic versions of Earth can both reproduce both ice capped and ice covered climate states. Neither can reproduce a climate which remains ice free throughout the year, even with an equilibrated ocean or elevated levels of C02. In all cases the ice free climate reverts toward the ice capped climate after the first polar winter. The modelled Neoproterozoic ice covered climate, that is the climate of Snowball Earth, has a climate very different from the present day. These changes are mostly driven by the lower thermal inertia, latitudinal temperature differences and the changed meridional circulation that results. The weather of the modelled Snowball Earth climate is also very different, dom- inated by a strong diurnal variation due to solar heating, as opposed to the more varied weather in the present day. The model responds well to the conditions of the Snowball Earth climate, with temperatures similar to those predicted by a simple physical model. The model responds less well to high levels of C02 in the Snowball Earth climate. The ice model also allows excessive heat and moisture to escape from the ocean into the atmosphere compared to that that would be predicted from solid ice coverage of the ocean. The exit from a Snowball Earth state was also tested within the model. Neither an decrease in albedo nor an increase in CO2 is unable to increase the temperature of the climate system sufficiently to exit the Snowball Earth state.

As capas carbonáticas neoproterozóicas revestem-se de grande importância, uma vez que se formaram no meio a mudanças paleoclimáticas e evolutivas singulares, cujas origens e influências na história subsequente do planeta e da vida ainda não foram devidamente esclarecidas. Este trabalho procurou compreender parte desta história através do estudo da sedimentação estromatolítica associada à capa carbonática representada pela Formação Mirassol DOeste (base do Grupo Araras), que se formou há cerca de 635 Ma, imediatemente após a glaciação Marinoana, representada pela Formação Puga. A pesquisa foi realizada na região de Mirassol DOeste, Mato Grosso, na borda sudoeste do Cráton Amazônico junto a Faixa Paraguai. Foram estudadas características meso e microscópicas dos estromatólitos, bem como das feições sedimentológicas associadas (estruturas tubulares, megamarcas onduladas, megapeloides), em afloramento, amostras cortadas e lâminas petrográficas. A dois metros da base da formação, inicia-se uma sucessão de 10 metros de espessura de boundstones microbianos, caracterizados, petrograficamente, por lâminas alternadamente delgadas e espessas, compostas de peloides (restos micritizados de colônias microbianas) com micrita subordinada e fenestras. Constituem estromatólitos lateralmente contínuos e de morfologia simples. Estromatólitos estratiformes ocorrem ao longo de toda a sucessão, com formas dômicas, de dimensões métricas irregularmente espalhadas lateral e verticalmente, até dois ou três metros do topo da sucessão. Estromatólitos muito irregularmente ondulados, comumente assimétricos, com dimensões decímetros predominam a parte superior e estes estão recobertos por grainstones-packstones peloidais dolomíticos, com megapeloides milimétricos, em estratos marcados por megamarcas onduladas formadas por ondas. A sedimentação microbiana cessou na Formação Mirassol DOeste quando o ambiente de plataforma de baixa energia onde se desenvolvia começou a ser assolado pela ação de ondas de hipertempestades, que penetraram a região com o aumento do nível do mar. Estruturas tubulares verticais, de comprimento até decimétrico e diâmetro estreito (<3 cm), preenchidos, via de regra, por doloesparito maciço, perpassam a laminação estromatolítica principalmente das porções mais altas dos domos. Sugere-se que gênese dessas estruturas tenha sido pela percolação de gases e/ou líquidos derivados da decomposição de matéria orgânica nas esteiras microbianas. / Neoproterozoic cap carbonates are of great importance because they formed during a period of singular paleoclimatic and evolutionary changes, whose origin and influences upon subsequent geological and evolutionary history have yet to be unraveled. This dissertation sought to comprehend part of this story through the study of stromatolitic sedimentation associated with the cap carbonate represented by the Mirassol DOeste Formation (base of the Araras Group), deposited about 635 Ma ago, immediately following the Marinoan glaciation, represented by the Puga Formation. This research was carried out at Mirassol DOeste, Mato Grosso, on the southwest border of the Amazon craton next to the Paraguai fold belt. Meso and macroscopic characteristics of stromatolites and associated sedimentological features (tubular structures, megaripples, megapeloids) were studied in outcrop, cut specimens and petrographic thin sections. Two meters above the base of the formation a 10 m-thick succession of dolomitic microbial boundstones begins, characterized throughout by alternating thin and thick laminae made up of peloids (interpreted as the micritized remains of colonial microorganisms), subordinate dolomicrite, and fenestrae. They make up laterally continuous and morphologically simple stromatolites. Stratiform stromatolites occur throughout the succession, with irregularly scattered meter-sized domes till about two to three meters from the top. Above this point, very irregularly undulated, commonly asymmetric, decimeter-sized stromatolites predominate, and these, in turn, are covered by megaripple-marked dolomitic peloidal grainstones-packstones with millimetric megapeloids. Stromatolitic sedimentation ceased in the previously calm platform environment of the Mirassol DOeste Formation when wave action began to rework bottom sediments as extremely intense storms reached the locale with the rise in sea level. Narrow (<3 cm), vertical tubular structures of decimetric length and filled by massive dolosparite cut stromatolitic sediments, principally in the central portions of domal forms. These structures appear to have formed by the percolation of gas and/or liquids derived from the decomposition of organic material in the microbial mats.

As capas carbonáticas neoproterozóicas revestem-se de grande importância, uma vez que se formaram no meio a mudanças paleoclimáticas e evolutivas singulares, cujas origens e influências na história subsequente do planeta e da vida ainda não foram devidamente esclarecidas. Este trabalho procurou compreender parte desta história através do estudo da sedimentação estromatolítica associada à capa carbonática representada pela Formação Mirassol DOeste (base do Grupo Araras), que se formou há cerca de 635 Ma, imediatemente após a glaciação Marinoana, representada pela Formação Puga. A pesquisa foi realizada na região de Mirassol DOeste, Mato Grosso, na borda sudoeste do Cráton Amazônico junto a Faixa Paraguai. Foram estudadas características meso e microscópicas dos estromatólitos, bem como das feições sedimentológicas associadas (estruturas tubulares, megamarcas onduladas, megapeloides), em afloramento, amostras cortadas e lâminas petrográficas. A dois metros da base da formação, inicia-se uma sucessão de 10 metros de espessura de boundstones microbianos, caracterizados, petrograficamente, por lâminas alternadamente delgadas e espessas, compostas de peloides (restos micritizados de colônias microbianas) com micrita subordinada e fenestras. Constituem estromatólitos lateralmente contínuos e de morfologia simples. Estromatólitos estratiformes ocorrem ao longo de toda a sucessão, com formas dômicas, de dimensões métricas irregularmente espalhadas lateral e verticalmente, até dois ou três metros do topo da sucessão. Estromatólitos muito irregularmente ondulados, comumente assimétricos, com dimensões decímetros predominam a parte superior e estes estão recobertos por grainstones-packstones peloidais dolomíticos, com megapeloides milimétricos, em estratos marcados por megamarcas onduladas formadas por ondas. A sedimentação microbiana cessou na Formação Mirassol DOeste quando o ambiente de plataforma de baixa energia onde se desenvolvia começou a ser assolado pela ação de ondas de hipertempestades, que penetraram a região com o aumento do nível do mar. Estruturas tubulares verticais, de comprimento até decimétrico e diâmetro estreito (<3 cm), preenchidos, via de regra, por doloesparito maciço, perpassam a laminação estromatolítica principalmente das porções mais altas dos domos. Sugere-se que gênese dessas estruturas tenha sido pela percolação de gases e/ou líquidos derivados da decomposição de matéria orgânica nas esteiras microbianas. / Neoproterozoic cap carbonates are of great importance because they formed during a period of singular paleoclimatic and evolutionary changes, whose origin and influences upon subsequent geological and evolutionary history have yet to be unraveled. This dissertation sought to comprehend part of this story through the study of stromatolitic sedimentation associated with the cap carbonate represented by the Mirassol DOeste Formation (base of the Araras Group), deposited about 635 Ma ago, immediately following the Marinoan glaciation, represented by the Puga Formation. This research was carried out at Mirassol DOeste, Mato Grosso, on the southwest border of the Amazon craton next to the Paraguai fold belt. Meso and macroscopic characteristics of stromatolites and associated sedimentological features (tubular structures, megaripples, megapeloids) were studied in outcrop, cut specimens and petrographic thin sections. Two meters above the base of the formation a 10 m-thick succession of dolomitic microbial boundstones begins, characterized throughout by alternating thin and thick laminae made up of peloids (interpreted as the micritized remains of colonial microorganisms), subordinate dolomicrite, and fenestrae. They make up laterally continuous and morphologically simple stromatolites. Stratiform stromatolites occur throughout the succession, with irregularly scattered meter-sized domes till about two to three meters from the top. Above this point, very irregularly undulated, commonly asymmetric, decimeter-sized stromatolites predominate, and these, in turn, are covered by megaripple-marked dolomitic peloidal grainstones-packstones with millimetric megapeloids. Stromatolitic sedimentation ceased in the previously calm platform environment of the Mirassol DOeste Formation when wave action began to rework bottom sediments as extremely intense storms reached the locale with the rise in sea level. Narrow (<3 cm), vertical tubular structures of decimetric length and filled by massive dolosparite cut stromatolitic sediments, principally in the central portions of domal forms. These structures appear to have formed by the percolation of gas and/or liquids derived from the decomposition of organic material in the microbial mats.

The composition of the atmosphere has changed substantially over Earth's history, with important implications for past climate. A number of case studies will be presented which employ coupled climate model simulations to assess the strength of these chemical feedbacks on the climate. The eruption of Mount Samalas in 1257 led to the largest stratospheric volcanic injection of aerosol precursor gases in the Common Era, however climate model simulations of the last millennium typically overestimate the resulting climatic cooling when compared with tree-ring proxy records. A novel configuration of the Met Office UM-UKCA climate model is presented which couples an atmosphere-ocean general circulation model to a rigorous treatment of the relevant atmospheric chemistry and microphysical aerosol processes. This permits the climate response to a particular stratospheric injection of reactive volatile gases to be quantified and for the first time to date applied to a historical volcanic eruption. This model configuration compares favourably to observational data for simulations of the 1991Mount Pinatubo eruption. Results from an ensemble of model simulations are presented, with different assumptions about the sulfur dioxide and halogen loadings based on a recent geochemical reconstruction. These show a muted climate response, in reasonable agreement with tree ring records. Emissions of halogenated compounds lead to an increase in the sulfur dioxide lifetime, widespread ozone depletion and a prolonged climatic cooling. Strong increases in incident ultraviolet radiation at Earth's surface also occur. Oxygen levels may have varied fromas little as 10% to as high as 35% in the Phanerozoic (541Ma - Present). An increase in atmospheric oxygen increases atmospheric mass which leads to a reduction in incident shortwave radiation at Earth's surface due to Rayleigh scattering. However, this is offset by an increase in the pressure broadening of greenhouse gas absorption lines. Dynamical feedbacks also lead to increased meridional heat transport, warming polar regions and cooling tropical regions. An increase in oxygen content using the HadCM3-BL and HadGEM3-AO climate models leads to a global mean surface air temperature increase for a pre-industrial Holocene base case, in agreement with idealised 1D and 2D modeling studies. Case studies from past climates are investigated using HadCM3-BL which show that in the warmest climates, increasing oxygen may lead to a temperature decrease, as the equilibrium climate sensitivity is lower. For the Maastrichtian (72.1 - 66.0Ma), increasing oxygen content leads to a better agreement with proxy reconstructions of surface temperature at that time irrespective of the carbon dioxide content. There is considerable uncertainty in the timing of the rise in atmospheric oxygen content from values around 1% in the Neoproterozoic (1000 Ma - 541 Ma) to the 10- 35% values inferred in the Phanerozoic with respect to two global glaciation episodes (717-635Ma). Results of simulations with HadCM3-BL which investigate the impact of oxygen content on the Neoproterozoic Snowball Earth glaciations are presented. These demonstrate that a smaller reduction in carbon dioxide content is required to initiate a Snowball Earth at low oxygen content. Geological evidence suggests the presence of a basaltic large igneous province before the Sturtian Snowball Earth episode. This could have caused episodes of paced explosive volcanism, injecting sulfate aerosol precursors into the stratosphere. Results of simulations to investigate the impact of different volcanic aerosol emission scenarios are presented. 500 Tg SO2 is investigated with a range of aerosol sizes. For aerosol size distributions consistent with the aerosol evolution in the aftermath of the Mount Pinatubo eruption, the Earth enters a Snowball Earth in between 30 and 80 years. Using a larger size of aerosols, consistent with a larger eruption, does not lead to a Snowball Earth. These simulations show that changes to the chemical composition of the atmosphere, whether reactive gases or bulk chemical composition may have played an important role in the past climate of Earth.