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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.

Cryogenian Snowball Earth glaciations are some of the most extreme changes in Earth’s climate through its history. Decades of work have established the global ubiquity of Snowball Earth sedimentary deposits and their synchroneity in time, yet key questions remain concerning the potential causes for global glaciations and their drastically different durations and expressions in the rock record. This dissertation addresses these questions and proposes explanations for the onset of global glaciation and how the abrupt termination of a Snowball event could explain the differences in chemical sediments seen between the two Cryogenian Snowball events. In Chapter 1, I evaluate the emplacement of a large igneous province as a potential trigger for the Sturtian Snowball Earth (717-661 Ma). This work was done in collaboration with Francis A. Macdonald, Mark D. Schmitz, Robert H. Rainbird, Wouter Bleeker, Barra A. Peak, Rebecca M. Flowers, Paul F. Hoffman, Matthew Rioux, and Michael A. Hamilton. Previous geochronology has suggested a rough coincidence of glacial onset with one of the largest magmatic episodes in the geological record, the Franklin large igneous province. I show that chemical abrasion-isotope dilution-thermal ionization mass spectrometry (CA-ID-TIMS) U-Pb geochronology on zircon and baddeleyite from sills associated with the paleo-equatorial Franklin large igneous province in Arctic Canada record rapid emplacement between 719.86 ± 0.21 and 718.61 ± 0.30 Ma ago, 0.9 to 1.6 Ma before the onset of widespread glaciation. Geologic observations and (U-Th)/He dates on Franklin sills are compatible with major post–Franklin exhumation, possibly due to development of mafic volcanic highlands on windward equatorial Laurentia and increased global weatherability. After a transient magmatic CO2 flux, long-term carbon sequestration associated with increased weatherability could have nudged Earth over the threshold for runaway ice-albedo feedback.In Chapter 2, I address whether there is evidence for glaciations in the 50 Myr prior to the Sturtian glaciation by examining the proposed ca. 750 Ma Kaigas glaciation. I evaluate this hypothesis at the eponymous location with detailed stratigraphy and geochronology through the Kaigas, Rosh Pinah, and Numees formations on the western margin of the Kalahari craton in southern Namibia. This work was done in collaboration with Francis A. Macdonald, Emily F. Smith, Jahandar Ramezani, and Nicholas Swanson-Hysell. We find that glacial deposits previously assigned to the Kaigas Formation are instead ca. 717-661 Ma diamictites of the Sturtian Numees Formation. Pre-Numees strata, including the Kaigas Formation, host facies associations diagnostic of fan delta deposition along an active normal fault. Interbedded volcanic rocks in the Rosh Pinah Formation overlying the Kaigas Formation were dated with U-Pb CA-ID-TIMS on zircon at ca. 752 Ma. These Tonian deposits are interpreted as being deposited in an active rift basin without evidence for glaciation. Rosh Pinah magmatism could be correlative with the Mount Rogers Complex in Virginia, USA, consistent with a scenario of the Kalahari craton actively rifting from Laurentia and associated terranes within 20º of the equator at the time. We conclude that, at least in marine settings, evidence for low-latitude glaciation is limited to the 717-635 Ma Cryogenian Period. In Chapter 3, I propose an explanation for why the Sturtian and Marinoan glaciations differ. This work was done in collaboration with Mark D. Schmitz and Francis A. Macdonald. I present new geological mapping, measured stratigraphic sections, and U-Pb zircon geochronology and geochemistry on Cryogenian successions exposed in the Naukluft Nappes of Namibia. Stratigraphic sections measured in the context of geological mapping document glacial deposition on a slope setting below the ice grounding line. Deglaciation is marked by an abrupt coarsening, boulder-sized dropstones, and the appearance of volcaniclastic deposits. Large lithic fragments within the volcaniclastics and detrital zircon provenance using laser ablation split stream mass spectrometry suggest a local source. CA-ID-TIMS on these units yielded a weighted mean 206Pb/238U date of 635.84 ± 0.22/0.29/0.71 Ma, which is interpreted as a depositional age. We suggest that glacio-isostatic unloading reactivated the formerly rifted passive margin. This age overlaps with dates near the top of Marinoan glacial deposits on the Swakop terrane, Australia, and South China. We suggest that all of these dates record deglaciation and that deglacial volcanism associated with isostatic unloading provided a positive feedback for both albedo and CO2 that shortened the Marinoan glaciation.

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 (

The Neoproterozoic Era, from around 1000 to 570 million years ago, witnessed the widespread deposition of Iron Formations (IFs) in close association with global glaciations. These IFs record distinct iron isotope and rare earth element patterns that allow us to reconstruct their depositional environment. The Fulu Formation of South China is a regional example of a global phenomenon. The Fulu was deposited immediately following the breakup of the Supercontinent Rodinia, which provided a restricted rift basin and contributed to an ample supply of iron. Specifically, iron isotopes and rare earth element profiles point to hydrothermally sourced iron in an oxygen-limited environment as the major driver of deposition for the Fulu Formation.

The Neoproterozoic Era was one of major biotic change against a background marked by fluctuations in oceanic and atmospheric chemistry, formation and rifting of the supercontinent Rodinia, and at least two global glaciations (Snowball Earth events). Presented here are three studies of differing aspects of those biological changes. The first is a systematic study of the diverse and well-preserved, organic-walled microfossil assemblage of the Alinya Formation of eastern Officer Basin, Australia. The use of scanning electron microscopy (SEM) revealed an unexpected level of morphological detail not visible in transmitted light microscopy and led to the recognition of new species and emendation of existing species as well as establishment of degradational sequences. In total, thirty-three taxa are described here including nineteen previously named forms, five newly described species and two new combinations. The second study describes the organic-walled microfossil assemblages from five successions that span the first (Sturtian) glaciation (~717 Ma) and interglacial interval (>635 Ma), and integrates those data with a critical evaluation of primary paleontological literature of units deposited from ~850 to 650 Ma. The described successions from Australia and Svalbard record low species richness throughout this interval and when placed in context of all available body fossil data from the mid-Neoproterozoic, indicate global species richness may have decreased much earlier than previously realized. This finding of temporal decoupling between loss of richness and glacial onset suggests the extinctions previously associated with the Snowball Earth glacial events may not have been glacially driven. The last of these three studies provides a broader view of the early to middle Neoproterozoic biosphere (1 Ga to 635 Ma) and describes application of the CONOP correlation and seriation algorithm to a new database of paleontological, geochemical and radiometric data. Paleobiological (first and last species appearances), geochemical and age events were placed into an ordinal sequence and calibrated to the geological time-scale to reveal a high-resolution species richness record for the first 80% of the Neoproterozoic Era. Major features of this record include an increase in species richness ~805 Ma, sustained high richness levels until a decrease ~770 Ma and a short-lived increase ~760 Ma before a steep decline ~750 Ma. The findings of the two studies described above can be placed within the context of this broader synthesis: the diverse assemblage of the Alinya Formation is representative of the richness peak between ~805 and 775 Ma and the successions recording the Sturtian glacial and interglacial assemblages in Australia and Svalbard are indicative of the extended nadir that began ~750 Ma with a recovery in species richness delayed until after the termination of the second (Marinoan) glacial event.These three studies, together, describe a broad view of the early to middle Neoproterozoic Era and detail important vignettes within that story. From a more detailed and temporally constrained record of the Neoproterozoic biosphere, relationships between biotic and abiotic events during this transformative time can become better understood.

The geological and paleomagnetic record indicate that around 750 million and 580 millions years ago glaciers grew near the equator, though as of yet we do not fully understand the nature of these glaciations. The well-known Snowball Earth Hypothesis states that the Earth was covered entirely by glaciers. However, it is hard for this hypothesis to account for certain aspects of the biological evidence such as the survival of photosynthetic eukaryotes. Thus the Jormungand Hypothesis was developed as an alternative to the Snowball Earth Hypothesis. In this paper we investigate previous models of the Jormungand state and look at the dynamics of the Hadley cells to develop a new model to represent the Jormungand Hypothesis. We end by solving for an analytical approximation to the model using a finite Legendre expansion and geometric singular perturbation theory. The resultant model gives a stable equilibrium point near the equator with strong hysteresis that satisfies the Jormungand Hypothesis.