Theses - Earth Sciences

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Open Access
The rheology and dynamics of the Precambrian lithosphere – insights from the British Isles
Miocevich, Sophie
This thesis combines a range of petrological, geochemical, and geophysical techniques to investigate the properties, behaviour, and evolution of the Precambrian lithosphere. In Chapter One, I summarise some of the key open questions regarding lithospheric evolution and the development of strength contrasts, and how they will be addressed in this thesis. Chapter Two investigates the feasibility of ‘sagduction’, which is a tectonic process that has been proposed to occur during Earth’s early history. I focus on a case-study from north-west Scotland, a region where Archean sagduction has been suggested. I integrate results from fieldwork, phase equilibria modelling, and numerical modelling to show that sagduction was unlikely to be feasible. Furthermore, I develop a method that allows the feasibility of sagduction to be assessed at other sites. I conclude that sagduction is unlikely to be an important Archean tectonic process. Chapters Three and Four focus on understanding how strength contrasts develop and persist in the lithosphere. I consider a region in England and Wales, known as the ‘Midlands Microcraton’. The microcraton formed in the late Precambrian and has behaved as a relatively strong block compared to its surroundings, displaying ‘craton-like’ properties. In Chapter Three, I integrate results from fieldwork, phase equilibria modelling, and geochronology. I demonstrate that the microcraton experienced a syn- to post- subduction metamorphic event. Using geotherm modelling, I infer that this event led to partial melting in the mid-to-lower crust, resulting in the development of the microcraton as a strong region. In Chapter Four, I compare the characteristics of the mantle on and off the microcraton using mantle xenoliths. I find that England and Wales are underlain by non-cratonic lithosphere that records a complex history of melting and metasomatic processes, confirming the untypical mode of formation of the strong crust proposed in Chapter Three. I then use the Phanerozoic deformation record to show that the microcraton has persisted as an (anisotropic) strong region throughout the Phanerozoic, and has exerted a strong influence on Britain’s geological evolution throughout this period. Although focused on Britain, the results in this thesis provide insights into the general concepts that control the rheology, deformation, and evolution of the lithosphere.
Embargo
Comparative Morphology and Macroevolution of the Avian Quadrate
Kuo, Pei-Chen
In birds, the quadrate acts as a hinge between the lower jaw and the skull, playing an important role in cranial kinesis. Therefore, the evolution of avian quadrate morphology presumably has been influenced by selective pressures related to feeding. Owing to its complex and variable morphology, the avian quadrate has been relatively overlooked with respect to other aspects of the crown bird skeleton, and previous researchers have frequently used different names to describe the same anatomical characters. Meanwhile, none have attempted to quantify evolutionary changes in quadrate shape to examine its geometric variation in light of ecological information. Here, we investigated the morphological variation of the quadrate across over 249 bird species covering all major lineages of extant birds to produce a clearly labelled anatomical atlas of the avian quadrate and describe the shape variance among different clades. Following this anatomical investigation, we used three-dimensional geometric morphometrics to quantify morphological variation of the quadrate to examine its relationship with an array of key ecological categories and perform ancestral shape reconstructions in the context of an up‐to‐date galloanseran phylogeny as a case study. We found non-ecological factors, such as allometry and phylogeny, exhibit stronger relationships with quadrate shape than ecological factors do. The avian quadrate evolved as an integrated unit and exhibits strong associations with the morphologies of neighbouring bones with which it articulates. Our results also illustrate the importance of incorporating fossil taxa into ancestral shape reconstructions and helps elucidate important aspects of the morphology and function of the avian feeding apparatus early in crown Galloanserae evolutionary history. Collectively, our results suggest a complex macroevolutionary scenario in which quadrate morphology evolved jointly with other elements of the avian kinetic system. With the knowledge of quadrate morphologies in crown birds, we plan to apply this explicitly quantitative framework to avian quadrates of stem group birds in future investigations.
Embargo
The Role of the Ocean in MIS 4 Climatic Changes and Atmospheric CO₂ Variability: a Geochemical Fingerprinting Approach
Radionovskaya, Svetlana
Since the industrial revolution, humans have caused profound climate changes, primarily by releasing geological carbon into the atmosphere and increasing atmospheric CO₂, with current levels reaching >400ppm, a concentration unprecedented in the last 800ka. This has led to far-reaching socioeconomic consequences for human society and risks for all levels of ecosystem. A better understanding of rapid climatic changes is desperately needed in order to inform mitigation and adaptation strategies for future climate change. The last glacial cycle experienced orbital and millennial scale climatic variability, as indicated by high latitude ice core records and many other high-resolution marine and terrestrial records. These climatic changes included, but were not limited to, changes in atmospheric CO₂, temperature, the hydroclimate, sea surface temperature (SST), ocean circulation and ocean biogeochemistry. The ocean is thought to have played a key role in controlling and modulating these changes through its impacts on both heat transport and the carbon cycle. High resolution marine sediment cores can be used to reconstruct these changes and may help to elucidate the mechanisms behind them. To date, most studies have focused on the deglaciation, and only sparse, low-resolution records exist for Marine Isotope Stage (MIS) 4, a key paleoclimatic interval for the last glacial inception. MIS 4 is characterised by a rapid CO₂ drop of ~40ppm, which is comparable in duration and magnitude to the first rapid increase seen during the last deglaciation. It also involved a large drop in temperature, as indicated by Greenland and Antarctic ice cores, a decrease in sea level, and a possible slowdown of Atlantic Meridional Overturning Circulation (AMOC) as reconstructed from various proxy records. Several millennial events occurred during MIS 4, including Heinrich Stadial 6 and Dansgaard-Oeschger (DO) events 16-19. MIS 4 is thus an ideal interval to study and disentangle, glacial-interglacial and millennial variability. It also provides a window into the mechanisms of rapid CO₂ change and their contribution to longer-term (orbital) climate change. Furthermore, the termination of MIS 4 allows for a comparison with the last deglaciation. In this thesis, I collect paleoceanographic data to improve coverage of this important interval from a suite of sediment cores retrieved from the Iberian Margin in the Northeast Atlantic, and a single core from the deep Sub-Antarctic Atlantic core site. This thesis ultimately aims to enhance the current understanding of the ocean’s role in and response to abrupt and orbital-scale climate changes during MIS 4 and to draw lessons on its wider implications for climate variability. Ultimately, this may contribute to our understanding of natural carbon cycle-climate feedbacks that will play a role in anthropogenic climate changes in the future. High resolution planktonic foraminifera Mg/Ca-based SST reconstructions from the Iberian Margin during MIS 4 show that certain aspects of the surface ocean response may not always track Greenland temperature and that Greenland ice core records do not serve as a universal template for climatic variability across the whole of the North Atlantic, likely due to the seasonal habitat biases associated with SST reconstructions. A strong hydroclimate signal is shown in planktic foraminifera δ¹⁸O from the Iberian Margin, whereby glacial (MIS 4) hydroclimate variability is coupled to a combination of the high-latitude North Atlantic changes and low-latitude tropical hydroclimate. Furthermore, for the first time, a high-resolution Mg/Ca-based SST record from the Iberian Margin, covering the last 85ka, demonstrates clear similarities between MIS 4 and MIS 2. This includes a similar decoupling of sub-tropical summer SST from Greenland temperatures recorded in ice core records during pre-HS 6 MIS 4 and the Last Glacial Maximum (LGM). The record also emphasises that the most severe (coldest and driest) conditions occurred in the midlatitude North Atlantic during Heinrich Stadials, rather than the ‘peak’ glacial conditions of MIS 4 or the LGM. The deep ocean likely played a key role in modulating CO₂ on millennial and astronomical timescales, for example through changes in its respired carbon inventory. Conservative parameters that are indicative of deep-water hydrography, and by extension circulation, are deep water temperature (T_dw) and associated δ¹⁸O_dw. Reconstructed T_dw changes from the Iberian Margin show a larger influence of southern sourced waters during MIS 4 and particularly during HS 6. Atlantic sector Southern Ocean (SO) T_dw closely follows Antarctic temperature, atmospheric CO₂ and the mean ocean temperature, implying that the deep SO contributed significantly to the global ocean energy budget on multi-millennial time scales across MIS 4, likely mediated by buoyancy forcing in the SO. This in turn was likely linked to sea-ice expansion at the MIS 5a/4 transition, aided by a parallel shoaling of North Atlantic Deep Water (NADW) as suggested by the North Atlantic Tdw record. Together with (arguably smaller) contributions from reduced air-sea gas exchange efficiency in the SO, these changes would have lowered atmospheric CO₂ during MIS 4, through their impact on the solubility- and soft tissue “pumps” (i.e. the ocean’s disequilibrium and respired carbon budgets). Because the amount of respired carbon in deep-water broadly scales with the dissolved oxygen concentration, bottom water O₂ reconstructions, [O₂]_bw, were investigated for a depth transect from the Iberian Margin and for the Atlantic sector of the Southern Ocean. Qualitative benthic foraminiferal assemblage records from a depth transect on the Iberian Margin show that shifts in oxygenated environments are primarily controlled by the quality and/or quantity of C_org reaching the sea floor, rather than [O₂]_bw. There are distinct shifts in assemblages associated with more periodic and/or degraded C_org flux during MIS 4 and an indication of low [O₂]_bw during HS 6 at the mid-depths, however no significant changes in the flux of C_org (i.e. ‘export production’) were found. Multi-proxy foraminiferal geochemical [O₂]_bw reconstructions from the Iberian Margin show a gradual decrease in [O₂]_bw at the mid-depth North Atlantic during MIS 4 with a minimum during HS 6, likely controlled by ventilation changes (i.e. changes in ocean circulation, including water mass sourcing combined with active but diminished transport, or altered preformed ‘end-member’ values). In the meantime, the [O₂]_bw record from the South Atlantic closely follows atmospheric CO₂, likely indicative of ocean ‘ventilation’ impacts on ocean-atmosphere carbon exchange. Indeed, the Southern Ocean appears to have represented a significant reservoir for sequestering CO₂ away from the atmosphere during MIS 4, as indicated by the respired- and equilibrium carbon inventory changes that are implied by the [O₂]_bw and T_dw reconstructions.
Embargo
Skeletal evolution and convergence in the hyperdiverse passerine radiation
Steell, Elizabeth; Steell, Elizabeth [0000-0002-3999-3500]
Among vertebrates, passerine birds are an exceptionally diverse group (>6,000 living species) encompassing more than half of living bird diversity. In this thesis I present primary descriptive anatomical work (Chapters 2 & 4), phylogenetic inference (Chapter 2), and novel quantitative approaches (Chapters 3 & 4) to shed light on aspects of passerine evolutionary history and diversification. Chapter 2 comprises a novel, comprehensive anatomical dataset of the passerine carpometacarpus, an anatomically variable bone in the wing. Using these data, I attempt to elucidate the phylogenetic affinities of several early passerine fossils from the Oligocene of Europe using Bayesian phylogenetic analyses of discrete carpometacarpus characters with a large-scale taxon sample encompassing >70% of passerine family-level clades. My results provide evidence that crown group suboscines (one of the two primary clades of passerine birds) were present in Europe by the early Oligocene, and that crown group oscines (the other primary clade of passerines) were present by the late Oligocene. Inadvertently, this project revealed evidence of pervasive convergent evolution, or homoplasy, in the passerine carpometacarpus. In Chapter 3, I present a new method of quantifying homoplasy in discrete character datasets and re-evaluate longstanding hypotheses about expected patterns of homoplasy considering changing variables in phylogenetic datasets. Through analyses of simulated and empirical datasets, I show that my new metric, the ‘relative homoplasy index’, outperforms the two most frequently used methods in the literature—the retention index and consistency index—in terms of accurately measuring homoplasy in discrete matrices. I also show that, contrary to expectations based on previous studies, relative homoplasy within phylogenetic matrices decreases as taxa are added to phylogenetic datasets. These results indicate that passerines exhibit high levels of homoplasy compared to other bird datasets. In Chapter 4, I investigate evolutionary constraints in the passerine wing and hindlimb skeleton. With my carpometacarpus dataset from Chapter 2 and an additional novel discrete anatomical dataset for the passerine tarsometatarsus (foot), I explore patterns of homoplasy and character state exhaustion across the two major sister clades of passerines, Tyranni (suboscines) and Passeri (oscines). I show that oscines, the more diverse subclade in terms of present-day species richness, exhibits substantially more homoplasy in the appendicular skeleton than suboscines. Simulated null distributions of character state exhaustion patterns reveal that oscines evolved under significant constraints to morphological diversification, whereas suboscines did not. Within discrete character morphospace for the carpometacarpus and tarsometatarsus, oscines show highly overlapping subclades, in contrast to pronounced separation among suboscine subclades; these results indicate differences in the mode of evolution for oscines and suboscines, which have culminated in greater levels of convergence among oscines. This thesis lays the foundation for future work investigating passerine macroevolution and constitutes an important step in broadening our understanding of how passerines became so spectacularly diverse.
Open Access
The Relationship Between Intraplate Earthquakes and Temperature
Emmerson, Brian
This dissertation uses a combination of techniques to investigate the rheological properties of the lithosphere in continental areas and subducting slabs. In these settings, the relationship between temperature and the distribution of seismicity, particularly in the mantle, is examined. The dissertation is divided into three studies, which are largely self-contained and deal with different geographical and seismotectonic settings. To begin, the properties of the lithosphere in the Lake Baikal region are examined in a multidisciplinary study incorporating observations of seismicity, gravity anomalies, topography, and seismic velocity and thermal structures. Teleseismic waveform-modelling is used to constrain the focal depths of 18 earthquakes, which are used along with published teleseismic and local-event studies to examine the seismogenic thickness (T_s) in the Baikal region. All sets of earthquake data show that the mantle in this region is not a significant source of seismicity. Estimates of elastic thickness (T_e) in the areas surrounding Lake Baikal are everywhere less than the seismogenic thickness, consistent with the simple interpretation that the long term strength of the lithosphere resides in its seismogenic layer, which in this region involves the whole crust but not, to any significant extent, the mantle. The apparent weakness of the mantle in the Baikal region is explained by its high temperature. The thermal conductivity of mantle rocks is strongly dependent on temperature, and has been shown to have a significant effect on the modelled thermal structure of cooling oceanic plates. The second study in this dissertation models temperatures in subducting slabs, taking into account the temperature-dependence of the relevant physical parameters, and investigates the maximum temperatures, potential temperatures, and homologous temperatures up to which intraslab earthquakes occur. An analysis of the world’s subduction zones reveals that intraslab seismicity is consistent with being limited to material having potential temperatures less than 600 °C. Apparent exceptions to this pattern occur in regions where the Nazca Plate subducts subhorizontally beneath South America, with which the final study in this dissertation is concerned. The unusual subduction geometry in the Peruvian and Pampean segments of the South American subduction zone keeps the overriding plate and the subhorizontal subducting slab in contact for several hundreds of km. Thermal modelling including this unusual geometry shows the subducting slab to be relatively cold in these regions, as compared with typical subduction zones where more immediate contact with the mantle wedge results in a hotter slab temperature structure. In the Peruvian and Pampean regions, intraslab seismicity is found to occur at potential temperatures less than 600 °C. Furthermore, the continental mantle above the slab is shown to be cold enough to generate earthquakes. Teleseismic waveform-modelling is used to constrain 39 earthquake focal depths, and confirms that the continental mantle of Peru is indeed seismogenic.
Open Access
The kinematics and dynamics of active mountain ranges
Knight, Elizabeth
This thesis explores the controls on tectonic deformation in and around active continental mountain belts, and comprises three interconnected studies. The first chapter investigates lateral variations in the foreland of the New Guinea Highlands, a young mountain range which sits on the northern edge of the Australian plate. I construct new earthquake source models across central and western New Guinea, and combine these data with thermal and elastic modelling. I find that pre-existing structural contrasts in the Australian lithosphere control along-strike variations in the temperature structure, seismogenic thickness, and strength of the New Guinea foreland. The underthrusting foreland supports the elevation of the overriding mountain belt, yet the elevation of the Highlands is not closely correlated with the foreland seismogenic thickness; this is likely due to the time taken to thicken the crust following changes in the rheology and strength of the foreland over time. Using force-balance arguments, I estimate the static coefficient of friction on foreland faults to be between 0.01–0.28. The second study expands in scope to examine the forces governing deformation along mountain range fronts, by analysing slip vectors from reverse-faulting earthquakes in 15 active mountain belts. I find that, along most range margins, slip vectors are better aligned with topographic gradients than with the direction of convergent plate motion. This implies that the gravitational buoyancy force acting between a mountain and its foreland exerts a strong control on the direction of slip. This effect is observed most clearly in high-elevation ranges like the Himalayas. By considering the force balance in these deformation belts, I suggest that, in most mountain ranges, the underthrusting foreland must act as a rigid base to the overlying range front, allowing it to act like a gravity current. Local factors, such as weak sediments in the foreland basin, can allow the gravitational force to be dominant even in low elevation ranges like the Zagros. The final chapter builds on these results by modelling a viscous flow over a rigid base, to examine the underlying dynamics. I explore how varying a range of geometric and rheological parameters can influence the instantaneous velocity field within the model mountain range. I find that the gravitational buoyancy force has a strong influence on the resulting velocities along the range front, supporting the observations made in the previous chapter. The velocity field is also significantly affected by the viscosity contrast between the upper and lower crust, and by variations in viscosity (within one order of magnitude) along-strike of the mountain range.
Open Access
Dynamics of deep-submarine explosive eruptions
Newland, Eric; Newland, Eric [0000-0003-3501-8115]
This thesis explores the dynamics of explosive volcanic eruption columns in the deep-ocean. Motivated by the observations of pyroclastic deposits on the sea floor at depths of up to 4 km, we investigate the initial mixing of erupted hot fragmented magma with the ambient seawater and explore the density and spatial evolution of the subsequent flow that forms. In chapter 2, we consider the initial mixing and rise of an explosive eruption in a quiescent ambient environment via a series of numerical integral models and find that a submarine eruption column tends to evolve as a turbulent particle-laden fountain. We investigate the controls on the density evolution of the flow, such as particle separation, and consider the particle dispersal mechanisms of these complex multi-phase flows. In chapter 3 we explore the dynamics of particle-laden fountains in a stratified environment, and identify two distinct flow regimes as a function of the average size of the particles, in which the flow is controlled by the ambient stratification or by the separation of particles. We build two numerical models for the rise and fall of particle-laden fountains in a linear density stratification. We contextualise the findings of this study for submarine explosive eruptions and estimate the dispersal distances of pyroclasts in the submarine environment. Next, in chapter 4, we consider the effect of a uniform lateral crossflow on the dynamics of particle-laden fountains. We perform a series of analogue experiments and examine the impact of particle separation on the structure of the flow. We develop the theory for single-phase fountains in a crossflow to identify the location of particle separation on the flow. Using field data on the dispersal of pyroclasts from deep-submarine eruptions, we show how the present work can be used to constrain eruption parameters, such as mass eruption rate. In chapter 5, we depart from the study of submarine volcanoes and consider a complimentary problem, motivated by the sediment plumes formed during the operations of deep-sea mining. We explore the dynamics of dense sediment plumes in a uniform crossflow and consider the evolution of the gravity currents that form when these plumes interact with a solid boundary. We investigate the effect of the crossflow speed on the morphology of the gravity currents formed and compare the propagation of the flows with the classical theory of turbulent gravity currents. Finally in chapter 6, we summarise the work of this thesis and build a framework to help interpret the dynamics and deposits of submarine eruptions using the analysis presented throughout.
Embargo
Observations of Dynamic Topography from South American Atlantic Margin
Siqueira, Leonardo
Topography of the Earth is mainly controlled by its crustal and lithospheric architecture. Variations in thickness and density of the crust and of the lithospheric mantle create the most noticeable topography. However, it is also widely recognized that mantle convective processes dynamically support topography at long wavelengths (i.e. longer than ∼ 1,000km). Observations of dynamic topography, varying in space and time, are more straightforward in the oceans since less complex lithosphere facilitates necessary corrections for topography arising from density and thickness variations of the crust, sediments and flexure. The aim of this dissertation is to investigate the spatial and temporal variation of dynamic topography and its expressions along the South American Atlantic margin. First, the spatial pattern of dynamic topography is examined through the study of residual depth anomalies on oceanic crust abutting this margin. An extensive seismic reflection and refraction database is compiled, and a revised and augmented residual bathymetric map is presented. This map defines the spatial distribution of dynamic topography along South American Atlantic margin. Results are compared to a range of independent geophysical and geologic observations. Secondly, the temporal evolution of dynamic topography is studied by focusing on the southern South Atlantic Ocean. A significant oceanic depression (∼ 2000km along its axis) occurs in the region known as Argentine Basin. This anomaly has a strong negative residual depth signal observed in different dynamic topography models. This study defines the amplitude and wavelength of the Argentine Basin depression and investigates its temporal evolution by looking into the stratigraphic record of sedimentary basins fringing this oceanic depression. To further investigate the temporal evolution of dynamic topography, Cenozoic epeirogeny of Argentine Patagonia is explored. Regional uplift is identified in this region which is contemporaneous with the evolution of the nearby Argentine Basin. Patagonia uplift is studied by inverse modeling of drainage networks, which provides spatial and temporal uplift patterns. The temporal evolution of this region is also analysed by geochemistry of intraplate magmatic rocks and earthquake tomography. Relationships between long-wavelength topography, spatial distribution of intraplate magmatism and uplifted marine terraces are discussed. Finally, constraints on the sub-lithospheric structure of Southern South America are assembled and an empirical conversion for shear-wave velocities to temperature is exploited with a view to investigating possible driving mechanisms for uplift and subsidence across Patagonia and the Argentine Basin.
Open Access
Magnetism of anthropogenic airborne particulate matter
Sheikh, Hassan Aftab
The global burden of disease associated with ambient particulate matter (PM_2.5) pollution is the leading threat to life expectancy according to the Air Quality Life Index (AQLI) 2023 report. The study of airborne PM is key to understanding both its source, and its impact on human health. The focus of this thesis is the study of Fe-bearing PM which is abundant in urban microenvironments. I start by exploring ways of monitoring and constraining the source of magnetic PM signatures in Lahore, Pakistan. I employ use of First Order Reversal Curves (FORCs) to unmix signals from ’passive biomonitors’— leaves. FORC signatures of leaf samples combine aspects of both exhaust residue and brake-pad end-members, suggesting that FORC fingerprints have the potential to identify and quantify the relative contributions from exhaust and non-exhaust (brake-wear) emissions. This thesis then examines into the indoor micro-environment of the London Underground (LU). I find that the LU is dominated by ultrafine (<100 nm) maghemite particles. The oxidised nature of the magnetic PM suggests that PM exposure in the LU is dominated by resuspension of aged dust particles relative to freshly abraded, metallic particles from the wheel-track-brake system. Therefore, I suggest that periodic removal of accumulated dust from underground tunnels might provide a cost-effective strategy for reducing exposure. I then apply magnetic modelling tools to real-world LU particles for a comparison to the experimental data. The thesis then looks at determining the efficacy of roadside green infrastructure (GI) in improving local air quality through the deposition and/or dispersion of airborne PM. I use a combination of magnetic measurements, electron microscopy, and fluid flow modelling to show that air quality downwind of a carefully selected and designed GI significantly improves through the deposition of vehicle-derived PM on leaves. I then demonstrate the application of a machine learning technique on PM hyperspectral imaging data sets. The automated method improves accuracy and reliability of chemical phase identification that is often limited by subjective human interpretation. Using magnetic and microscopy methods, I conclude that ultrafine magnetic particles are abundant and ubiquitous in urban microenvironments; and that their presence may be masked by larger particles in mass-specific traditional air quality monitoring methods.
Open Access
Differential Thermal Isotope Analysis: A Method for the Study of Past Climates
Walters, Gregory
Online measurements of δ¹⁸O and δ²H can be used to reveal more information about past climates than current offline methods. In this thesis I present work carried out developing the online Differential Thermal Isotope Analysis (DTIA) method, including demonstrations of the method on gypsum samples and clay samples, and the application of DTIA to the ongoing research into climate conditions during the Paleocene-Eocene Thermal Maximum (PETM). Measurements of gypsum and clay samples demonstrate the ability of DTIA to separate out different dehydration steps for individual measurement, both for minerals with multiple water environments, and for minerals with multiple-step dehydrations. The gypsum results are also used to examine the dehydration of gypsum to anhydrite, via the intermediate bassanite. I show that this dehydration reaction is highly sensitive to sample grain size and the partial pressure of water, and crucially, that the two-step dehydration of gypsum does not result from the presence of multiple water environments that are preferentially dehydrated at different temperatures, but rather from kinetic factors upon dehydration. DTIA is also applied to a series of clays buried in the North Sea Basin across the PETM. The results from the hydroxyl isotopic composition of the clays show a trend of slowly decreasing δ²H prior to the PETM, followed by abrupt decreases in δ²H at the onset of the PETM, indicating increased precipitation intensity and weathering, and implying an enhanced hydrologic cycle response to global warming, particularly at the early stages of the PETM. These results are consistent with other research indicating higher precipitation rates during the PETM. Our results are presented alongside consistent evidence from the measurements of clay composition and plankton species concentration undertaken by previous researchers at this section, demonstrating how DTIA can be used alongside other methods. This thesis shows the potential DTIA has to aid palaeoclimate reconstruction in a number of geological settings. DTIA can be applied both to isolated hydrated minerals and to assemblages of hydrated minerals to better understand the formation environments of these minerals, and thus gain insight into the palaeoclimate conditions under which geological deposits form.
Open Access
A spatial and temporal study of the carbon cycle; the role of carbonates in buffering Earth’s climate
Knapp, William
Canonically, changes in Earth’s atmospheric CO₂ concentrations have been attributed to an imbalance between volcanic degassing rates and silicate weathering. To force environmental changes in this way, such as the CO₂ decline observed during the Cenozoic Period (66 Myr - present), requires either a decrease in volcanic degassing, or an increase in silicate weathering and a change in the total amount of carbon in the combined ocean-atmosphere (OA) system, which is hard to square against proxy observations of invariant silicate weathering and volcanic degassing rates. Recent rethinking about carbon cycling suggests that rather than increasing sources or sinks of carbon, environmental change can be forced by redistributing carbon between the ocean and atmosphere, which satisfies the condition of maintaining a similar amount of carbon in the OA system. A proposed mechanism to achieve this redistribution is via increasing rates of carbonate weathering, which provides a renewed onus on investigating the impact carbonate rocks, and dissolution thereof, may have on buffering changes in Earth’s climate. The long term process of chemical weathering is of particular interest currently, as some suggest silicate mineral dissolution reactions provide a scalable escape route from anthropogenic greenhouse-gas emissions, and subsequent disruptive environmental perturbations. This seems to be at odds with decades of work prior, showing the timescales upon which silicate weathering operates are far too sluggish to be useful in the next 50-100 years. Indeed, the energy required to catalyse silicate weathering (i.e., mining, grinding) and transport minerals to field-sites has not been convincingly shown to outweigh potential CO₂ removal, yet. Furthermore, in watersheds it is difficult to prove additionality as a consequence of enhancing silicate mineral dissolution. In large, open systems it is hard to trace additional carbon removal from the atmosphere as a consequence of enhanced silicate dissolution, data are often very noisy and the fractal nature of river networks means signals are diluted very quickly. Seemingly, appreciation of the timescales upon which chemical weathering operates have become skewed. This thesis investigates the transport of carbon between the terrestrial and ocean realm, on both short and geological timescales. Carbonate rocks weather rapidly in comparison to silicates, and carbonate terrains can be very efficient at delivering alkalinity to the oceans. However, the efficiency of the carbonate weathering alkalinity pump is hampered by the solubility of CaCO₃, which commonly precipitates at Earth’s surface and in doing so out-gasses CO₂. These inputs of alkalinity to the global oceans are measurable, and dynamic enough to be observed on short timescales and are investigated. Given this, the chemistry of contemporary carbonate terrains is investigated using stable Mg isotopes to understand how their carbon transfer capacity can be improved. A global riverine carbonate chemistry model is presented to quantify the present day maximum global carbonate weathering flux of alkalinity to the oceans. A method for quantifying carbon removal rates using radiocarbon data from sites where the chemical weathering process has been expedited is also presented. The fate of alkalinity in the global oceans requires a much longer frame of reference, given the residence time of carbon and base cations in the ocean. Therefore, a Cenozoic palaeo-record of stable Mg isotopes in seawater is presented, and potential drivers of carbon redistribution between the OA-system are assessed.
Open Access
Inside-out diversity of rocky planets
Guimond, Claire; Guimond, Claire [0000-0003-1521-5461]
Exoplanets orbiting distant stars have revealed to us that no one could have predicted Earth. Many stars host planets which appear to be made of rock and iron like Earth, but sit at several times its mass—we have no visible analogues nearby to show us what these massive rocky planets look like, and how differ from our own. Further, measurements of stellar photospheres show a certain variability in their rock-forming element abundances, which implies a similar spread in the rocky building blocks of planets. Consequences of a planetary mass variability and compositional variability comprise the contents of this thesis. Because the surfaces of exoplanets are not directly detectable with present technology—their nature cannot be told by astronomy alone—questions about rocky planet diversity require a theoretical approach. Drawing on the wealth of geoscientific knowledge originally developed to understand Earth, I construct physical models of other possible worlds to see how ours fits in. The first consequence of variability I model is topography. Topography sets the size of the largest ocean that planets could contain below their highest point—the capacity of the continental bathtub. I calculate the bare-minimum, dynamic topography due directly to mantle convection, by finding scaling laws of dynamic topography with convective parameters. For increasingly massive planets, topography almost disappears. Smaller ocean basins might suggest flooded worlds, all else equal. Yet large portions of a planet's water can be buried in their mantles, predictable insofar as known mantle minerals have characteristic maximum water contents. Thus for a second consequence of variability, I leverage stellar abundances to constrain the mineral phase equilibria of exoplanet mantles. I assess whether various exoplanets would plausibly sequester water in their mantles, providing a key initial condition for planetary evolution. In another chapter, I then link these surface and interior reservoirs explicitly, employing a coupled 2D mantle convection and melting model to estimate rates of volcanic outgassing on the early Earth, before modern plate tectonics—early Earth provides an elucidative case study to ground our understanding of planet diversity. These estimates focus on the possible range of mantle oxygen fugacities, a measure of how oxidising the mantle is. I show how most scenarios produce significantly lower outgassing rates in the Archean, compared to what classical thermal history models would suggest. The last consequence of variability then turns to oxygen fugacity itself. I invoke a subtle but powerful phenomenon from the petrological modelling toolbox, and predict the minimum amount by which mantle oxygen fugacity should vary across rocky exoplanets, constrained again by host star element abundances and inferred mantle mineralogy. By the end of the thesis it should become clear that there is no good prototype for a ``terrestrial planet''. Nevertheless, through the late union of exoplanet astronomers and Earth scientists, we start to appreciate how understanding planets holistically is an obligate and confrontable challenge.
Open Access
Seismic Evidence for Layering at the Core-Mantle Boundary
Russell, Stuart; Russell, Stuart [0000-0002-9477-014X]
The core-mantle boundary (CMB) is the most extreme discontinuity in Earth's interior and plays an important role in regulating planetary scale processes, including convection in both the mantle and core. The CMB is usually treated as a sharp discontinuity with direct contact between the silicate mantle and iron core, a scenario that is rarely questioned. However this is likely an over-simplification. This thesis investigates, using seismology, the possibility that the CMB is a layered transition, with an intermediate kilometre-scale layer sandwiched between the core and mantle. This thesis uses seismic data from opposite ends of the frequency spectrum - high-frequency body waves and long-period normal modes. Before applying body waves to the CMB, this thesis first addresses the problem of ellipticity corrections for seismic phases such that they can be applied correctly to any seismic phase in any planetary model. Following this advancement, the effect of a thin layer at the CMB on P waves is examined, finding that a previously underutilised diffracted phase, PKKP_diff, is very sensitive to the inclusion of even very thin layers. A global dataset of over 12,500 PKKP_diff observations and 353 normal mode centre frequencies are then applied in turn to attempt to resolve whether a thin layer exists at the CMB. Despite being vastly different data types with differing frequency contents and sensitivities, the two studies come to the same conclusion - not only is a slow kilometre-scale layer at the CMB possible within the bounds of seismic data, but both data types are better-fitted if such a structure on the order of a kilometre thick exists at the CMB. The favoured seismic parameters are a density increase of tens of percent coupled with similar magnitude reductions in both P- and S- velocity, with S-velocity more reduced than P. Both studies have their own limitations that inhibit robust conclusions that the CMB is truly layered. Nevertheless the results of this thesis suggest that this is a plausible scenario that is permissible within the bounds of current seismic data, and should therefore be considered appropriately. This conclusion has wide reaching implications which are also briefly explored.
Open Access
Box models of thermohaline circulation
Li, Scott
This thesis investigates a number of buoyancy-driven flows in stratified environments that are motivated by the thermohaline circulation of the ocean. Simple models are presented which reduce the dynamics to a few leading order processes and we present an intuitive investigation with simple experiments or numerical calculations. We study the fluid dynamics of locally intensified mixing, referred to as `boundary mixing' within the ocean literature, and also the effect of mixing between the up and downwelling flows on the steady and transient circulation within an idealised filling-box flow. In chapters 2 and 3 we present an experimental and theoretical study of boundary mixing. We investigate the transport of buoyancy and tracer through a closed basin with buoyancy fluxes supplied at the top and bottom boundaries and, in both chapters, we compare the two cases in which the mixing either occurs uniformly across the tank or is locally confined to one portion. In chapter 2 we establish the simplest case in which the buoyancy fluxes supplied at the boundaries lead to no net flow in the system. By tracking the movement of dye we are able to visualise the flow patterns. An analytical model is developed that is consistent with the vertical and lateral flows in the experiment as well as with the evolution of the salinity stratification. We investigate the interaction of this effect with a vertical buoyancy flux and a net vertical flow with boundary mixing in chapter 3. We examine the stratification and the flow that develops in the steady state case. Next we investigate the transient adjustment of the buoyancy stratification due to changes in the supplied buoyancy fluxes. In chapters 4 and 5 we shift the focus to filling-box flows and investigate the interior stratification that develops from a local finite-mass source of destabilising buoyancy and a distributed stabilising buoyancy flux, both supplied to the surface of the domain with the addition of interior diffusive mixing. In chapter 4 we present a model showing that the system is controlled by two key non-dimensional parameters relating the source volume flux to the volume flux of entrained fluid and to the diffusive flux, resulting in four distinct regimes. If this system is perturbed, it exhibits inertia in adjusting to the new equilibrium when the perturbation timescale is short compared to the adjustment timescale and is no longer representative of the quasi-steady state. However, these profiles are still consistent with the simplified `Abyssal Recipes' profiles which are controlled only by mass conservation with constant upwelling and diffusivity. In chapter 5 we present a similar box model in which the entrainment increases linearly with depth and investigate the inverted values for the diffusivity and the upwelling rate. This leads to the recognition that it is difficult to distinguish more complicated models from the simplified models just from the data. This thesis finishes with a summary and discussion of the results in chapter 6.
Open Access
Biogeochemical investigations of methane-rich groundwaters in high Arctic glacial catchments
Kleber, Gabrielle
Permafrost and glaciers in the high Arctic form a near-impermeable ‘cryospheric cap’ that traps a potentially large reservoir of sub-surface methane and prevents it from reaching the atmosphere. The vulnerability of the cryosphere to climate warming is making releases of this methane possible, but uncertainty in the magnitude and timing of such releases makes predictions of Arctic greenhouse gas emissions difficult. In Svalbard, where air temperatures are rising more than twice as fast as the average for the Arctic, glaciers are retreating and leaving behind exposed forefields that enable rapid methane escape. Through an extensive spatial study of proglacial groundwater springs on Svalbard, groundwater systems within glaciated catchments are found to be bringing to the surface deep-seated methane gas that was previously trapped beneath glaciers and permafrost in the Arctic. In this thesis, I estimate the amount of methane being released by such springs and discuss its origin. Through a temporal study conducted at a single glacial catchment, Vallåkrabreen, I use biogeochemical data collected from groundwaters during two melt seasons to investigate the sources of groundwaters and the origin of the methane they transport to the surface. Waters collected from 123 groundwater springs in the forefields of 78 land-terminating glaciers are supersaturated with methane up to 600,000-times greater than atmospheric equilibration. The spatial sampling revealed a geologic control on the extent of methane supersaturation, with strong evidence of a thermogenic source. I estimate annual methane emissions from proglacial groundwaters could be up to 2.31 kt across the Svalbard archipelago. Further investigations into marine-terminating glaciers indicate emergent methane emissions as these glaciers transition into fully land-based glaciers. My findings within the Vallåkrabreen catchment demonstrate an interconnected hydrological system where shallow and deep groundwaters mix to moderate methane emissions. During summer, deep methane-rich groundwaters sourced from upper catchment snowmelt are diluted by shallow oxygenated groundwaters, leading to some methane oxidation prior to its emergence at the surface. Microbial activity is an important methane sink along this flow-path, removing up to 62% of methane before it is brought to the surface. During winter, deep groundwaters remain active while many shallow systems shut off, reducing subsurface methane oxidation and permitting greater emissions. Ratios of the differing groundwater sources will change markedly in years to come as aquifer capacities and recharge volumes change in a warming climate. My findings reveal that climate-driven glacial retreat facilitates widespread release of methane, a positive feedback loop that has the potential to contribute to enhanced greenhouse gas emissions in the Arctic. The findings are highly relevant for other regions of the Arctic that, due to their geology, are likely to experience similar methane emissions, either currently or with further glacial retreat.
Embargo
Application of Hydrogen and Oxygen Isotope Ratios of Water to the Study of Evaporation, Palaeoclimate, and Ancient Marine Conditions
Brady, Matthew; Brady, Matthew [0000-0003-2674-6946]
Triple-oxygen and hydrogen isotopes of water are critical to aid our understanding of the hydrological cycle on the modern and ancient Earth. As such, this thesis presents advancements in techniques and in the understanding of the isotopic behaviour of water that undergoes evaporation in different conditions. An application of these findings is made to palaeoclimate data, and an expansion of the use of water isotopes from hydrated minerals is applied to a novel mineral system. Of fundamental importance to the hydrological cycle is the process of evaporation. Yet, under- standing how water isotopes behave as they experience evaporation remains poorly quantified. One limitation is the ability to confirm theoretical models or palaoeclimatic data sets with accurate, precise, and controllable experiments. To overcome this, I developed a widely applicable experimental method that permits the measurement of the triple-oxygen and hydrogen isotope evolution of an evaporating fluid and its coincident vapour that is both highly accurate and with a precision and sampling rate previously unobtainable. Additionally, the methodology permits a wide range of variables to be controlled or manipulated, allowing future scholars to thoroughly explore the effect of differing climatic conditions on the full suite of water isotopes as they undergo evaporation. One such variable explored is the morphology of an evaporating basin. Whilst currently neglected in studies that examine lacustrine palaeoclimate records, I present experimental data for both idealised and real basin morphologies (generated by 3D printing) that highlights a significant effect on the rate of isotopic change as a function of basin morphology, despite constant environmental conditions. Experimental data suggest that unless the surface area to volume (SA:V) ratio of a lake is considered, palaeoclimatic data from two lakes with distinct morphologies but undergoing evaporation under identical climates could be misinterpreted. Similarly, the results from the real basin morphology experiments suggest that rapid changes in SA:V ratio could result in inflections in the isotopic record that are solely a response to morphology as opposed to changes in climate. Understanding from the evaporation experiments is then applied to gypsum hydration water (GHW) data obtained from Lake Petén Itzá, Guatemala. The lake is in the northern neo-tropics and provides a complete record of climate variability over the last 43 ka. Using the GHW isotopic data in combination with a Monte Carlo and combined hydrological-climate re-analysis models, I determine relative differences between the cooler, drier stadial periods that are recorded in the sediments. In addition, multiple proxies indicate that intra-stadial variability is recorded at Lake Petén Itzá and this mirrors variation observed in polar oxygen isotope records. This suggests a close climate teleconnection between the poles and the tropics during these periods. Finally, I explore the use of oxygen and hydrogen isotopes in sedimentary talc. This is achieved by experimentally determining the fractionation factors between amorphous Mg-silicate and the water from which it precipitates. This is followed by calculating the fractionation factors as these precipitates undergo synthetic metamorphism to true talc. Isotopic measurements from Neoproterozoic samples are measured and the isotopic composition of the ancient precipitating fluid is estimated using the uncertainty of these parameters and Monte Carlo modelling. This thesis provides a strong experimental backbone for future examination of evaporating water under a variety of palaeoclimatic conditions and contexts. It demonstrates how insights from evaporation experiments can be applied to interpreting palaeoclimate data and how hydrated minerals of various types, and from a range of geological settings and times, can be used to provide robust information about the climate and fluids from which they formed.
Open Access
Active tectonics and earthquake hazards in continental mountain ranges and foreland basins
O'Kane, Aisling; O'Kane, Aisling [0000-0001-7429-9103]
The regions adjacent to tectonically active mountain belts are exposed to significant earthquake hazard, since the range-bounding faults produce large earthquakes, and the underlying geological structure amplifies the resulting ground shaking. The aim of this dissertation is to investigate the regional-scale controls on earthquake ground motions and seismic hazard in these settings. The first part of this dissertation describes models of the seismic wavefield produced by thrust-faulting earthquakes on mountain range fronts. The earthquake source characteristics and foreland basin structure were varied within reasonable geological bounds, and the earthquake-induced ground shaking was calculated. The earthquake source parameters were determined to be the dominant control on the amount of near-source ground shaking. However, the foreland basin structure, in particular the basin depth relative to the dominant wavelength of the seismic waves, determines the importance of dispersion as the waves propagate through the basin. These results highlight the importance of accurately determining earthquake source characteristics (particularly depth), and the underlying geological structure, during hazard assessment. These principles were then applied to study the active tectonics and seismic hazard in the north-west Himalayas. Field, satellite, and seismological observations were used to determine the fault geometry beneath the NW Himalayas and investigate the relationship between thrust faulting and folding. These results were used to construct seismic-wavefield models, to determine earthquake ground motion estimates if the Main Himalayan Thrust in the region were to rupture. These models show that peak ground velocities are extremely sensitive to minor variations in the fault geometry. Finally, the earthquake-induced building damage in foreland basins was investigated. Using seismic-wavefield modelling, alongside fragility curves for generic building types, the relationships between earthquake location, characteristics, and building damage were investigated. The results quantify the previously poorly known trade-off between earthquake location and magnitude in determining damage distributions. Additionally, the results quantify the factors that can cause over- or under-estimates of the magnitudes of historical earthquakes based on reported damage distributions, with important implications for understanding the accumulated slip deficit in continental collision zones.
Open Access
Deciphering the thermal evolution of small planetary bodies.
Dodds, Kathryn
The parent bodies of meteorites were the first bodies to form in our solar system and the building blocks of the terrestrial planets, as well as the cores of the gas giants. They also played an important role in the delivery of volatiles to the inner solar system planets, including the Earth. These bodies also hosted a wide range of geological processes, from low temperature aqueous alteration, to explosive volcanism. However, determining certain properties of these asteroid-sized bodies such as their size and structure can be difficult. In this thesis, I use detailed models of planetesimal thermal evolution to constrain the accretionary histories and structures of a range of different meteorite parent bodies from a variety of observed meteorite properties. This model describes the thermochemical evolution of a planetesimal, from accretion and differentiation, through a period of early magma ocean convection and subsequent diffusive cooling, to solidification of its liquid iron core, for a wide range of accretionary scenarios that result in the proposed parent body structures. I then use this model for three different projects, which investigate: 1) the conditions for thermally-driven dynamo activity in planetesimal cores, 2) the accretionary histories of the magmatic iron parent bodies, and 3) the possible parent body structures of an unusual greenschist chondrite, Almahata Sitta stone AhS 202. Conditions for thermally-driven dynamo activity: The ability for a planetesimal to generate a thermally-driven field from 4 - 35 Myr after the start of the solar system is found to depend critically on its accretion rate and duration of core formation as this controls the depth and location of any thermal stratification that develops during core formation. This result allows us to constrain the accretion rate of these bodies for the first time as thermal dynamo generation requires accretion durations of > 100kyr. Additionally, the timings of the thermally-driven fields on the fully-differentiated angrite parent body and partially-differentiated, CV_Ox parent body require that both these bodies were > 420 km with > 200 km radius cores. The CV_Ox parent body also had a 7 - 12 km thick unmelted, chondritic lid at the surface, from which the CV_Ox chondrites originate. Accretionary histories of the magmatic iron parent bodies: The measured ¹⁸²W anomalies in magmatic iron meteorites, which originate from the cores of their parent bodies, are a product of the timing of core formation and differentiation. I use these anomalies to infer the accretion start times and durations of their parent bodies. I find that these parent bodies may have been either fully or partially differentiated, challenging the canonical assumption that they were fully differentiated. As a result, it is not possible to use the measured ¹⁸²W anomalies in iron meteorites to uniquely define the relative timings of planetesimal accretion in the inner and outer solar system, as done in many previous studies. Properties of the AhS 202 parent body: Almahata Sitta stone, AhS 202 is the only known meteorite that has experienced high pressure, greenschist-like metamorphism, requiring its parent body to be 300 - 900 km in radius. However, its association with the CR chondrites, the youngest meteorite group, means that its parent body did not accrete with sufficient 26Al, the dominant planetesimal heat source, to reach the temperatures required for this metamorphism. Instead, the heat for this metamorphism could have been provided by either internal heating by the decay of long-lived radioistopes in a > 550 km chondritic parent body or diffusive heating of a thick chondritic lid by an differentiated interior in a > 380 km partially-differentiated parent body. Finally, the crystallization of asteroid-sized cores is not well understood, which has made using the timings of compositionally-driven dynamo fields in planetesimal cores to constrain the properties of their parent bodies challenging. I have used thermodynamic calculations to show that due to their low pressures, these cores crystallized inwardly, requiring a different dynamo mechanism to the geodynamo. However, previous studies into dynamo generation in this regime have largely been restricted to numerical models. In this work, I have used novel analogue experiments to identify the key physics involved in inwards crystallization in asteroid cores. These experiments have allowed the identification of a new core crystallisation mechanism in which iron crystals form below the CMB and fall into the interior in crystal-rich downwelling plumes. However, whether this mechanism is capable of driving dynamo fields in cores of meteorite parent bodies is still uncertain due to difficulties in scaling our experimental results to the relevant core conditions. If future work shows that this is possible, this new mode of core crystallization will allow more accurate constraints to be placed on the size and structure of the parent bodies of meteorites that experienced a compositionally-driven field from 65 - 200 Myr after the start of the solar system.
Open Access
Crustal structure and tectonics of Borneo and Sulawesi: results from receiver function analysis and virtual deep seismic sounding
Linang, Harry Telajan
Southeast Asia is arguably the most tectonically active region on the planet, fuelled to a large extent by nearly 10,000 km of ongoing subduction on its western, southern and eastern flanks that accommodates the northward motion of the Indo-Australian plate and westward motion of the Philippine Sea plate. It has hosted one of the largest earthquakes ever recorded (Mw 9.2 Sumatra-Andaman earthquake in 2004) and perhaps the most famous volcanic eruption in history (Krakatoa eruption of 1883), which profoundly affected the Earth’s climate. While the western Pacific margin and Indonesian archipelago along the Sunda and Banda arcs have been well studied, the same is not true of the interior region of Southeast Asia, which includes Borneo and Sulawesi. Borneo is the 3rd largest island in the world and lies on the eastern margin of Sundaland, the continental core of Southeast Asia, but its intraplate setting means that it has no active volcanoes and little in the way of seismicity. By contrast, the adjacent island of Sulawesi features active subduction and a network of continental transform faults that give rise to high levels of earthquake activity. How this central region of southeast Asia was formed, and the tectonic relationship between Borneo and Sulawesi, is still poorly understood. The goal of this thesis is to exploit passive seismic data recorded by temporary and permanent seismic networks to image the first order crustal structure of both Borneo and Sulawesi. This will provide fresh insight into their deep structure, provenance and tectonic evolution, and how they have been impacted by recent events (e.g. opening of the South China Sea) that have clearly left their mark on surrounding regions. The passive seismic data used in this dissertation is in the form of teleseismic body wave arrivals, which interact with upper mantle and crustal structure before being recorded by stations on the surface. I utilise both receiver function analysis (RFA) and virtual deep seismic sounding (VDSS) to extract information from mode converted and reflected phases to constrain seismic properties including Moho depth, V_S and V_P 〖/V〗_S. In the case of RFA, H-k stacking, migration and inversion are separately applied, with the ensemble of results producing robust estimates of crustal thickness in particular. The primary strength of VDSS is its ability to constrain Moho depth using relative few sources, even in the presence of significant crustal complexity (e.g. thick sedimentary sequences); I therefore find this to be a particularly useful technique in some regions, such as northern Borneo, where RFA produces equivocal results due to strongly heterogeneous crustal structure. One of the main outcomes of this thesis is a new detailed map of Moho depth variations beneath northern Borneo from the application of VDSS to data from an array of 65 temporary and permanent broadband seismometers. This is the most recently active part of Borneo, having experienced at least two phases of subduction in the Miocene. Moho depths vary between ~45 km beneath the Crocker Range to less than 25 km beneath the central interior, consistent with the presence of a thick crustal root transitioning to an area of relatively thin crust that extends NE into the Sulu Sea. These results support a model of subduction polarity reversal (SPR) in Borneo, in which opening of the South China Sea led to SE-directed subduction of the proto-South China Sea beneath Northern Borneo, followed by continent-continent collision, which formed the Crocker Range, supported by a thicker crust. This was followed by subduction termination and the initiation of Celebes Sea subduction beneath northern Borneo of opposite polarity. Roll-back of the Celebes Sea slab resulted in the opening of the Sulu Sea, which extended into the interior of northern Borneo and caused localised crustal thinning, but did not proceed to rifting. In addition to the detailed study of northern Borneo, I also investigated Borneo in its entirety, albeit at a much lower resolution owing to the more limited data coverage at this scale. The primary tool in this case was RFA, which was used on data from 28 broadband stations distributed throughout Sarawak and Kalimantan, as well as a subset of stations from northern Borneo. VDSS was also applied to data recorded at a number of sites where RFA was not able to produce a well constrained Moho. The results show that on a broad scale, northern Borneo features the thickest crust, with the vast majority of Borneo having crust that is less than 35 km thick, despite the interior mountain range having an average elevation in excess of 1000 m. Some of the thinnest crust (~25 km) occurs beneath a Mesozoic accretionary complex in western Borneo, which is thought to mark the location of a previous subduction zone and crustal extension related to slab rollback. In Southwest Borneo, the Schwaner mountains are underlain by elevated V_P 〖/V〗_S, which are juxtaposed against lower Vp/Vs of the Kuching Zone. Thicker crust (~40 km) in southeast Borneo approximately overlays the Meratus Suture, and may be related to the docking of the East Java-West Sulawesi and Southwest Borneo blocks, which have East Gondwana provenance. The investigation of the crustal structure of Sulawesi made use of data from 23 permanent network stations and 18 temporary stations, with receiver function analysis applied to constrain the bulk crustal properties of Sulawesi. Crustal thickness ranges between about 20 km and 40 km, with the thinnest and thickest crust juxtaposed across the Palu-Koro fault, a > 500 km long sinistral strike slip fault in central Sulawesi. This fault - which produced an Mw 7.5 earthquake in 2015 that also unleashed a destructive tsunami - accommodates some 4 cm/yr of left-lateral motion along with clockwise motion of the North Arm of Sulawesi. Thinner crust along the western part of the North Arm of Sulawesi is likely related to roll-back along the North Sulawesi trench, while thicker crust to the east may be due to opposed subduction of the Celebes Sea and Molucca Sea straddling the land mass. The relatively simple crustal seismic models of Borneo and Sulawesi produced in this dissertation represent a solid first step towards understanding their tectonic assemblage and geological architecture. Future work might involve bringing in other datasets to aid with interpretation, such as gravity, magnetic, heat flow, geochronology, petrology etc., and using the new seismic models to help answer first order questions, such as the mismatch between predicted and observed dynamic topography in Borneo in particular. Incorporating data from a recent OBS deployment in the southern Celebes Sea and Makassar Strait may significantly improve our ability to connect the seismic structure of Borneo and Sulawesi, although it remains to be seen whether the data coverage and quality will be sufficient to substantially change the current picture.
Open Access
The Atmospheric Fingerprints of Volcanism: Simulating Volcanic Outgassing and Secondary Atmospheres on Rocky Planets
Liggins, Philippa Kate; Liggins, Philippa [0000-0003-2880-6711]
The study of the atmospheric composition and evolution of rocky planet atmospheres is key to understanding both the conditions required to develop a habitable planet, and to analyse the link between the deep interior and atmosphere of rocky bodies. This thesis uses volcanism as a chemical link between the mantle of a planet and its atmosphere, with the aim of analysing how a volcanically derived or supplemented atmosphere may appear, both under the end-member case where volcanism is the only factor affecting the atmosphere, and when changing surface temperatures and atmospheric escape is considered. Chapter 2 describes a newly developed model of volcanic degassing for COHSN elements, designed with the broad range of exoplanet geochemistry in mind. It also describes a model for simulating the evolution of a volcanic atmosphere through time, based on the initial volatile content of a planetary mantle, the surface temperature and a stipulation for the escape of hydrogen. Chapter 3 demonstrates that volcanic activity can sustain a fraction of hydrogen in planetary atmospheres undergoing hydrogen escape, which may have contributed to a cold, wet early Mars, and expands the liquid water habitable zone for exoplanets. Chapter 4 shows that on planets with Venus-like atmospheric temperatures, the mantle fO2 of a planet can be inferred from the chemistry and composition of a volcanic atmosphere as three distinct classes (defined by the presence/absence of certain indicator species) are formed. Specifically, Chapter 4 presents a set of volcanic atmospheres as an important base case for future research, exploring the effects of other processes on volcanic secondary atmospheres as produced by a range of geological conditions. Chapter 5 utilises chemical kinetics models to show that volcanic atmospheres must be at temperatures of 700K and above in order to be accurately modelled as in thermochemical equilibrium, with the reactions of key species (NH3, CO and CH4) being quenched over geological time below this point. Chapter 6 returns to the effect of hydrogen escape on volcanic atmospheres, exploring how escape modifies the atmospheric classes discussed in Chapter 4 and reduces or removes all indicators of mantle fO2 from the atmosphere. This thesis presents a new volcanic degassing model and a number of use-cases, demonstrating the wide range of chemical speciations which volcanically generated atmospheres can form.