Theses - Earth SciencesNo Descriptionhttps://www.repository.cam.ac.uk/handle/1810/2190982024-03-28T16:24:34Z2024-03-28T16:24:34Z1411Comparative Morphology and Macroevolution of the Avian QuadrateKuo, Pei-Chenhttps://www.repository.cam.ac.uk/handle/1810/3651172024-03-15T01:44:06Zdc.title: Comparative Morphology and Macroevolution of the Avian Quadrate
dc.contributor.author: Kuo, Pei-Chen
dc.description.abstract: 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.
The Role of the Ocean in MIS 4 Climatic Changes and Atmospheric CO₂ Variability: a Geochemical Fingerprinting ApproachRadionovskaya, Svetlanahttps://www.repository.cam.ac.uk/handle/1810/3641572024-03-14T01:43:54Zdc.title: The Role of the Ocean in MIS 4 Climatic Changes and Atmospheric CO₂ Variability: a Geochemical Fingerprinting Approach
dc.contributor.author: Radionovskaya, Svetlana
dc.description.abstract: Since the industrial revolution, humans have caused profound climate changes, primarily by releasing geological carbon into the atmosphere and increasing atmospheric CO<sub>2</sub>, 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 CO<sub>2</sub>, 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 CO<sub>2</sub> 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 CO<sub>2</sub> 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 δ<sup>18</sup>O 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 CO<sub>2</sub> 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<sub>dw</sub>) and associated δ<sup>18</sup>O<sub>dw</sub>. Reconstructed T<sub>dw</sub> 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<sub>dw</sub> closely follows Antarctic temperature, atmospheric CO<sub>2</sub> 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 CO<sub>2</sub> 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 O<sub>2</sub> reconstructions, [O<sub>2</sub>]<sub>bw</sub>, 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<sub>org</sub> reaching the sea floor, rather than [O<sub>2</sub>]<sub>bw</sub>. There are distinct shifts in assemblages associated with more periodic and/or degraded C<sub>org</sub> flux during MIS 4 and an indication of low [O<sub>2</sub>]<sub>bw</sub> during HS 6 at the mid-depths, however no significant changes in the flux of C<sub>org</sub> (i.e. ‘export production’) were found. Multi-proxy foraminiferal geochemical [O<sub>2</sub>]<sub>bw</sub> reconstructions from the Iberian Margin show a gradual decrease in [O<sub>2</sub>]<sub>bw</sub> 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 [O<sub>2</sub>]<sub>bw</sub> record from the South Atlantic closely follows atmospheric CO<sub>2</sub>, likely indicative of ocean ‘ventilation’ impacts on ocean-atmosphere carbon exchange. Indeed, the Southern Ocean appears to have represented a significant reservoir for sequestering CO<sub>2</sub> away from the atmosphere during MIS 4, as indicated by the respired- and equilibrium carbon inventory changes that are implied by the [O<sub>2</sub>]<sub>bw</sub> and T<sub>dw</sub> reconstructions.
Skeletal evolution and convergence in the hyperdiverse passerine radiationSteell, Elizabethhttps://www.repository.cam.ac.uk/handle/1810/3646842024-02-21T01:41:32Zdc.title: Skeletal evolution and convergence in the hyperdiverse passerine radiation
dc.contributor.author: Steell, Elizabeth
dc.description.abstract: 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.
The Relationship Between Intraplate Earthquakes and TemperatureEmmerson, Brianhttps://www.repository.cam.ac.uk/handle/1810/3644022024-02-10T01:43:11Zdc.title: The Relationship Between Intraplate Earthquakes and Temperature
dc.contributor.author: Emmerson, Brian
dc.description.abstract: 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<sub>s</sub>) 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<sub>e</sub>) 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.
The kinematics and dynamics of active mountain rangesKnight, Elizabethhttps://www.repository.cam.ac.uk/handle/1810/3641592024-02-07T01:42:14Zdc.title: The kinematics and dynamics of active mountain ranges
dc.contributor.author: Knight, Elizabeth
dc.description.abstract: 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.
Dynamics of deep-submarine explosive eruptionsNewland, Erichttps://www.repository.cam.ac.uk/handle/1810/3631332024-01-10T02:15:26Zdc.title: Dynamics of deep-submarine explosive eruptions
dc.contributor.author: Newland, Eric
dc.description.abstract: 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.
Observations of Dynamic Topography from South American Atlantic MarginSiqueira, Leonardohttps://www.repository.cam.ac.uk/handle/1810/3622952023-12-22T15:00:43Zdc.title: Observations of Dynamic Topography from South American Atlantic Margin
dc.contributor.author: Siqueira, Leonardo
dc.description.abstract: 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.
Magnetism of anthropogenic airborne particulate matterSheikh, Hassan Aftabhttps://www.repository.cam.ac.uk/handle/1810/3620342023-12-22T14:51:43Zdc.title: Magnetism of anthropogenic airborne particulate matter
dc.contributor.author: Sheikh, Hassan Aftab
dc.description.abstract: The global burden of disease associated with ambient particulate matter (PM<sub>2.5</sub>) 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.
Differential Thermal Isotope Analysis: A Method for the Study of Past ClimatesWalters, Gregoryhttps://www.repository.cam.ac.uk/handle/1810/3607622023-12-22T14:27:02Zdc.title: Differential Thermal Isotope Analysis: A Method for the Study of Past Climates
dc.contributor.author: Walters, Gregory
dc.description.abstract: 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.
A spatial and temporal study of the carbon cycle; the role of carbonates in buffering Earth’s climateKnapp, Williamhttps://www.repository.cam.ac.uk/handle/1810/3585992023-12-22T14:21:16Zdc.title: A spatial and temporal study of the carbon cycle; the role of carbonates in buffering Earth’s climate
dc.contributor.author: Knapp, William
dc.description.abstract: Canonically, changes in Earth’s atmospheric CO<sub>2</sub> concentrations have been attributed to an imbalance between volcanic degassing rates and silicate weathering. To force environmental changes in this way, such as the CO<sub>2</sub> 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 CO<sub>2</sub> 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 CaCO<sub>3</sub>, which commonly precipitates at Earth’s surface and in doing so out-gasses CO<sub>2</sub>. 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.