Theses - Earth Sciences

Permanent URI for this collection

https://www.repository.cam.ac.uk/handle/1810/219098

Browse

Now showing 1 - 20 of 133

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.
Embargo
Data-driven approaches towards understanding trace metal and magmatic processes in arc volcanoes, with applications to Java, Indonesia
Barber, Nicholas; Barber, Nicholas [0000-0003-4513-2421]
Expanding human populations combined with the challenges of climate change, have resulted in higher degrees of human risk exposure to natural disasters like volcanoes. Such increased volcanic risk is especially pronounced along tectonic plate margins like subduction zones (or "arcs"), where explosive volcanism is more common. The aforementioned social and economic pressures will necessitate a move towards "greener" economies, invariably leading to increased competition for natural mineral resources. While much progress has been made in the past 40 years understanding what processes shape ore development and volcanism in subduction zones, it is only in recent years that the enormous amounts of data produced can be synthesized to develop data-rich models to elucidate a more refined understanding of subduction zone magmatism and igneous geochemistry. Here, I present a data-driven approach to both global and regional scale magmatic processes in subduction zones using a combination of global datasets, novel computational methods, and microanalytical petrology. Beginning with a global perspective, I identify key metamorphic processes that are controlling geochemical patterns in subduction zone magmas, using careful compilations and statistical analyses of large datasets. These investigations address longstanding questions in igneous petrology, such as: which specific mineral reactions are driving fluid mobile element enrichment in arc magmas and how does this vary from arc to arc? A later chapter asks: what petrological processes favor the enrichment of copper and the optimization of ore-forming potential in a magma? These global insights are complemented by a detailed petrological study of Mount Slamet volcano, Java, Indonesia, infused with computational advances in machine-learning. Java has 45 active volcanoes, which showcase a variety of hazardous behaviors, and the island hosts one of the largest and most productive porphyry copper deposits in the world. To understand Java’s volcanism, I combine my data-driven petrological methods with microanalytical techniques applied to whole rock powders, individual crystals, and melt inclusions (small pockets of magma trapped inside a crystal). At Slamet volcano, I show the transcrustal complexity of polygenetic-monogenetic volcanic systems, and highlight the role of scoria cones in probing deeply-stored magmas. Together, these three threads comprise a holistic picture of the physical and chemical processes shaping magmatism in subduction zones.
Open Access
New ice core records from West Antarctica and their spatial context: from 1000 to 100,000 years
Rowell, Isobel
The West Antarctic Ice Sheet (WAIS) is vulnerable to warming as a result of anthropogenic climate change, with the potential to contribute several metres to global sea level rise over the coming centuries. The Thwaites and Pine Island glaciers are already undergoing acceleration due to climate change, threatening the stability of the WAIS. There is a need to understand the stability of the WAIS during warm periods, such as the Last Interglacial (LIG) when the WAIS is proposed to have retreated, and the Holocene prior to anthropogenic warming. Ice core records can be used to reconstruct climatic changes and infer past ice sheet configurations. However, existing long-term ice records are sparse. Two drilling campaigns were carried out under the WACSWAIN project, aiming to contribute two new ice records to refine the uncertainties of WAIS stability in warm periods. One drilled an ice core to bedrock (651m) on Skytrain Ice Rise, adjacent to the Ronne Ice Shelf; a second used a novel drilling technology (Rapid Access Isotope Drill, RAID), on Sherman Island (SI), obtaining ice chippings to a depth of 323 m. The RAID was previously deployed at Little Dome C in the East Antarctic, obtaining samples to 460 m depth. The records from RAID ice chippings and their continental spatial context are the focus of this thesis. Chemical and water isotope data from RAID-drilled ice samples are presented for the first time and are comparable to those of conventionally-drilled ice cores. The dataset from SI extends to over 1000 years before present, more than doubling the length of existing records from the coastal WAIS. SI shows little overall change in stable water isotope values over the last millennium, and does not demonstrate the increased accumulation rate in recent decades apparent in comparative cores. The RAID record of stable water isotopes from Little Dome C is compared with nearby EPICA Dome C to investigate the limits of common centennial scale variability. An Antarctic-wide array of water isotope records extending through the last glacial period is synchronised. Continental stacks of water isotope records for the Holocene and Last Glacial periods are presented using all available data for the first time, including the new RAID records, placing them into their continental scale spatial context on timescales from 1000 to 20,000 years. This compilation enables an investigation into the spatial variability of the timing and amplitude of major events throughout the last glacial cycle, focusing on the glacial to Holocene transition and an Antarctic Isotopic Maximum (AIM12) event. The timing of onset of events varies by up to 2000 years, with no obvious regional consistency. Results from this thesis highlight the ongoing need for accurate dating and synchronisation of ice cores and the benefit of greater numbers of records. The RAID is a valuable new tool that can rapidly obtain several hundreds of metres of ice samples, producing quality data, comparable to conventional ice cores, to meet this need. The new SI dataset provides a wealth of new data for the coastal Amundsen-Bellingshausen Sea sectors over the last millennium. Comparing the multi-centennial scale variability of a large number of deep water isotope records reveals important spatial differences in Antarctic climate variability.
Embargo
Geological constraints on Neoproterozoic glacial episodes
Tindal, Benjamin
Neoproterozoic glacial episodes are amongst the most intense glaciations that the Earth has experienced, and are associated with major changes in the Earth System, such as the breakup of a supercontinent, the evolution of complex multicellular life and extreme geochemical perturbations. A Neoproterozoic stratigraphic scheme has been developed over the last thirty years that uses these major glacial episodes for correlation, yet Neoproterozoic tillites remain stratigraphically under-constrained. This thesis investigates stratigraphic and sedimentological constraints on Neoproterozoic glacial episodes through field based case studies on all Neoproterozoic purported tillites in Britain and on the namesake formations of the three classic Neoproterozoic glacial episodes. This is accompanied by an analysis of a database of all pre-Neogene purported tillites constructed from a literature survey. Together these strands of research offer insights into the constraints on Neoproterozoic glacial episodes provided by different stratigraphic techniques, and the application of those techniques to reconstruct Neoproterozoic glacial episodes. Results show that 1) age constraints on Neoproterozoic purported tillites are on average ten times less precise than on Paleozoic ones; 2) 190 tillites have age constraints that are compatible with an Ediacaran age; 3) cap carbonate stratigraphy is not applicable to 63% of Neoproterozoic tillites; 4) the presence of Neoproterozoic cap carbonates is not conclusive evidence of a Hard Snowball Earth; and 5) a single purported tillite in Great Britain has strong evidence for ice. This thesis demonstrates that glacial episodes do not provide a reliable basis for the correlation of Neoproterozoic successions.
Embargo
Sediment chemistry as an archive of provenance and weathering in the Mekong River Basin
Feng, Linshu
The chemical weathering of rocks, in particular silicate weathering, has been proposed to be an important natural carbon-removal process that regulates the Earth’s climate over millions of years. Tectonically active regions, such as the sub-Himalayan region, are considered a hotspot of silicate weathering and key sites for understanding weathering processes. Rivers, as a carrier of eroded and weathered materials, provide a primary archive for the study of silicate weathering. This study focuses on one of the largest rivers draining from the Himalayan-Tibetan-Plateau region, the Mekong River. A major new data set is presented of the elemental abundance (major and trace) and isotopic composition (lithium-strontium-neodymium) for the suspended and bank/bedload particulates collected from depth profiles along the Mekong main channel and spot samples from tributaries. This includes the residues after leaching and a suite of sequential extractions targeting the exchangeable cation pool and Fe-Mn oxyhydroxide pool. In conjunction with the river sediments, new data on major elements and lithium isotopes in the dissolved load and from a sediment core of Tonle Sap, a lake connected to the Mekong, are also presented for comparison. The major conclusions from these data are: 1) The exchangeable and iron-manganese oxyhydroxide pools are too small to account for lithium fractionation between water and solids in the Mekong. 2) Chemical variations of suspended sediments along depths reflect a strong influence from hydrodynamic sorting and the downstream evolution of average sediment composition is attributable either to progressive chemical weathering or a change in the source material. 3) Lithium isotopes in the residues of Mekong sediments display no systematic trends with depth or grain size. If not an artefact from dams or agriculture, it might reflect a supply source different to the Mackenzie and Amazon, potentially dominated by more recent marine sediments, with a higher fraction of products from the modern weathering cycle. 4) Despite receiving sediments mainly from the Mekong River, the lake deposits are distinct from modern river sediments, which raises concerns about reconstructing weathering history from sediment cores.
Open Access
A heavy stable isotope approach to tracing mantle source and process
Soderman, Caroline; Soderman, Caroline [0000-0001-5586-1586]
The geochemistry of global mantle melts suggests that both mid-ocean ridge basalts (MORB) and ocean island basalts (OIB) sample lithological heterogeneities originating in both the upper and lower mantle, with recycled crustal material accounting for a significant part of this variability. Recently, heavy stable isotopes have been suggested as a new tool to complement existing tracers of mantle heterogeneity and lithology (e.g., major and trace elements, radiogenic isotopes), because mineral- and redox-specific equilibrium stable isotope fractionation effects can link the stable isotope ratios of melts to their source mineralogy and melting degree. In this thesis, I present a unique `bottom-up' approach to understanding how mantle lithology, such as recycled crust (pyroxenite), could be reflected in the stable isotope composition of the erupted melts, and the insights that heavy stable isotope data from basalts could provide into mantle source and process. Throughout this thesis, I investigate five stable isotope systems (Mg-Ca-Fe-V-Cr) that have shown promise in models or natural samples as tracers of mantle lithology. I develop a quantitative model, combining thermodynamically self-consistent mantle melting and equilibrium isotope fractionation models, to explore the behaviour of the stable isotope ratios of these elements during melting of three mantle lithologies (peridotite, and silica-excess and silica-deficient pyroxenites). I also present new Fe isotope data for Samoan shield and Azores volcanoes, and for a suite of samples from 90 million years of evolution of the Galápagos mantle plume system. These OIB allow me to study the role of recycled mantle components in generating Fe isotope variability in melts, to compare to my mantle melting and isotope fractionation model. I find that single-stage melting of a MORB-like eclogitic pyroxenite cannot generate the high δ⁵⁷Fe seen in some OIB, notably Pitcairn, the Azores and rejuvenated Samoan lavas. Instead, the generation of high δ⁵⁷Fe melts in OIB requires: (1) processes that make subducted eclogite isotopically heavier than its pristine precursor MORB (e.g., hydrothermal alteration, metamorphism, sediment input); (2) lithospheric processing, such as remobilisation of previously frozen small-degree melts, or a contribution from lithospheric material metasomatised by silicate melts; and/or (3) melting conditions that limit the dilution of melts with high δ⁵⁷Fe by ambient lower δ⁵⁷Fe materials. Therefore, it cannot be assumed that a pyroxenite lithology derived from recycled crustal material is the sole producer of high δ⁵⁷Fe melts in OIB, as has sometimes been assumed in the literature. Instead, the observation of high δ⁵⁷Fe OIB melts cannot be ascribed to a unique source or process. This ambiguity reflects the multitude of processes operating from the generation of recycled lithologies through to their mantle melting: from MORB generation, its low temperature alteration, through mantle heterogeneity development and lithospheric processing, to eruption at ocean islands. I also find that, given current analytical precision, the five stable isotope systems examined here are not predicted to be sensitive to mantle potential temperature variations through equilibrium isotope fractionation processes, for the melting of peridotite. By contrast, source lithological heterogeneity is predicted to be detectable in some cases in the stable isotope ratios of erupted basalts, although generally only at proportions of > 10% MORB-like pyroxenite in the mantle source, given current analytical precision. However, even when considering analytical uncertainty on natural sample measurements, the range in stable isotope compositions seen across the global MORB and OIB datasets suggests that kinetic isotope fractionation, or processes modifying the isotopic composition of recycled crustal material such that it is distinct from MORB, may be required to explain all the natural data. Finally, I combine the insight into and modelling of Fe stable isotope behaviour presented throughout the thesis to highlight the potential of heavy stable isotopes to constrain mantle dynamics, in the Galápagos plume system. I show that although the proportion of pyroxenite-derived melt has increased through time as the plume has cooled by 400°C over its lifetime, these results are consistent with a cooling plume containing a small and approximately constant proportion of pyroxenite. This result is consistent with geodynamic models of entrainment of dense material, such as from a lower mantle low velocity superstructure underlying the plume. The small proportion of pyroxenite throughout plume evolution also suggests that geochemical signatures of primordial mantle may be diluted approximately uniformly by recycled components throughout plume evolution and therefore could be identified in early plume localities. From my combined mantle melting and isotope fractionation model, and comparison to natural datasets, I conclude that the five stable isotope systems considered in this thesis have potential to be powerful tracers of the source lithology of erupted basalts, complementary to other geochemical tools. However, continued improvements in analytical precision in conjunction with experimental and theoretical predictions of isotopic fractionation between mantle minerals and melts are required before these heavy stable isotopes can be unambiguously used to understand source heterogeneity in erupted basalts.
Open Access
Capillary Pressure Effects in the Geological Storage of Carbon Dioxide
Mortimer, Patrick
Carbon capture and storage has been identified as a key technology in the global effort to reduce carbon emissions and mitigate the adverse effects of climate change. In this thesis we present a series of theoretical and experimental studies pertaining to surface tension effects in the geological sequestration of carbon dioxide. The aim of the thesis is to further our understanding of the role of surface tension, or capillarity, in geological flows. We consider a number of important problems related to carbon storage and present simplified models which consider the key dynamical controls in each case. In chapters 2 and 3, we explore injection of CO2 into layers of permeable storage rock, where the layers are separated by thin low permeability mudstones and the formation takes the shape of an anticline. We assume that the mudstone layers are continuous and have a capillary entry pressure. We show that discrete pools of CO2 may form below the mudstone horizons and that the pool depth is a function of both the capillary entry pressure and the flux of CO2 into a given layer. We assume that the maximum pool depth before CO2 spills into the neighbouring aquifer is equal to the vertical deformation of the anticline. We then develop a dynamic model for rate of filling in each layer and show that spilling may occur from the system both during and after injection. The model is then applied to a two layered anticline and we show that there is a critical injection flux as a function of capillary pressure, above which spilling will occur before the capacity of the anticline is reached. We then extend our model to a system with more than two layers, and consider injection into several of the storage layers. We then discuss with an example how the choice of injection strategy may depend on the uncertainty in properties of the mudstone. Motivated by CO2 migration through fractures and micro-fractures, in chapters 4 and 5 we consider capillary driven flows in non-uniform channels. In chapter 4, we develop a model for the capillary driven exchange of two immiscible fluids in a v-shaped wedge. We present a suite of analytical and numerical solutions, which consider capillary flow with and without gravitational forces. Our solutions demonstrate that unconfined flows in non-uniform channels may transition from a gravity current at early times to a capillary current at later times. We also identify the analogue between our exact model for fluid saturation as a function of capillary pressure and the classical empirical models. In chapter 5, we present a general model for capillary driven flow in a channel of arbitrary cross-sectional shape, as well as a series of capillary flow experiments in a v-shaped channel. We conclude with a summary of our findings and identify some areas for further investigation in chapter 6.
Open Access
The X-discontinuity: Seismic signatures of a heterogeneous upper mantle
Pugh, Stephen; Pugh, Stephen [0000-0001-5997-9004]
The structure and composition of the Earth’s interior provokes great debate within the Earth Science community. While the bulk composition and density of the Earth are largely known, many discrepancies exist between different probes of the Earth, and little is understood of the distribution of compositional heterogeneity within the Earth. Seismic discontinuities arise from jumps in velocity and density, with global discontinuities manifesting from mineral phase transitions and large-scale changes in composition like the core-mantle boundary. Widely distributed previous seismic observations of localised reflectors in the upper mantle have been termed the X-discontinuity (X; 230-350 km depth) and allude to the widespread presence of chemical heterogeneity, but the exact cause of the X is yet to be determined. Receiver function (RF) techniques using P-to-S converted waves are capitalised upon to study the X at higher spatial resolution than previously attainable. Stacking RFs in the depth and time-slowness domains provides identification of the X beneath 15 hotspots. The X appears more prevalent at hotspots compared to six reference locations. Amplitude information from recorded and synthetically modelled RFs constrains the causal mechanism of the X in mantle plume settings to the coesite-stishovite phase transition and/or the presence of carbonated silicate melt, shedding insight into the nature of mantle plumes and their role in mantle convection. I study the presence of the X beneath Africa, using continuous observations and a vote map style procedure to understand the length scales of the X, its short wavelength variation, and the presence of topography across the discontinuity. The X continues to be co-located with regions of mantle upwelling and observations appear to show a strong dependence on back azimuthal distribution, suggesting that the X may possess short wavelength topography. Finally, with the potential to add further constraints on the X and increase data coverage, I determine the feasibility of using S-to-P converted RFs for studying the X. While it is possible to record signals from the upper mantle using these RFs, the presence of interfering phases greatly limits the epicentral distance range available for study and requires careful windowing of the source for iterative deconvolution. I do not recommend this potential complimentary probe.
Open Access
The nature and co-behaviour of volatile and non-volatile elements in the sub-continental lithospheric mantle
Crosby, James
The lithospheric mantle represents a critical interface in global volatile cycles because it separates Earth’s convecting mantle from the rigid crust. Nevertheless, the source, abundance and flux of volatiles stored in this reservoir are poorly constrained. In this work, the concentrations of volatiles stored both within the crystal lattice (H, Li, B, F, P and Cl) and fluid-inclusions (He and C) in mantle phases, together with their isotope compositions (3He/4H and 13C/12C), have been determined in 55 well-characterised peridotites and pyroxenites predominantly from off-craton mantle. The peridotites include spinel- and garnet-bearing lherzolites to harzburgites and are representative of the suites found at Kilbourne Hole, Colorado Plateau (SW USA), Tariat (Mongolia), West Eifel (Germany), Antarctic Peninsula, S. Patagonia, N.Tanzania and Ichinomegata (Japan). Their major and trace elements record a wide range in melt extraction (from 5 to 40%) but most xenoliths have undergone subsequent enrichment, via fluid or melt-related metasomatism. Fluid-inclusions are isotopically highly variable, with 3He/4He = 5.3 to 8.4 Ra and δ13C = -25.0 to 5.6 ‰ (V-PDB). 3He/4He-major-trace-element systematics shows that the subcontinental lithospheric mantle dominantly reflects formation from a temporally evolving upper-mantle that has been overprinted by radiogenic processes. Furthermore, the influence of metasomatism is enhanced by 3He/4He-13C/13C systematics which show the dominant role of small-fraction volatile-rich upper-mantle melts with variable inputs from melts/fluids derived from recycled oceanic lithosphere containing organic material and carbonates. Coupled links between volatile and non-volatile elements suggest that the CO2 hosted in fluid inclusions in the off-craton lithospheric mantle is approximately 41 ppm equating to an influx of 2.5 x 1019 g Myr-1. The estimated concentration for CO2 in off-craton mantle is lower than H2O (85 to 100 ppmw), similar to P (40 ppm) add higher than for the halogen (F = 15 to 27 ppm; Cl = 3.4 to 5.4 ppm), B (0.09 to 0.22 ppm) and Li (1.77 ppm). In comparison with the depleted mantle, the off-craton mantle is more concentrated in Li, B and F has similar CO2, P and Cl and lower amounts of H2O. Finally, this study expands the importance of pyroxenites in the off-craton lithospheric mantle as important hosts for volatiles to including 3He, CO2, Li and P. Overall, this study provides the first, fully internally-consistent appraisal of the petrography, mineral and calculated whole-rock major, trace and volatile element chemistry, and helium and carbon compositions of fluid-inclusions to advance the understanding of volatiles in the sub-continental lithospheric mantle on a global scale.
Open Access
Phosphorus Pathways In Deep Time
Walton, Craig; Walton, Craig [0000-0003-2659-644X]
Some of the most fundamental questions in natural science ask about the nature of early Earth. The conditions under which Earth formed and life emerged on its surface are especially uncertain. However, we are left with precious little evidence to study: most sufficiently ancient terrestrial rocks have long since been destroyed, and our sampling of the wider Solar System remains largely incomplete. This deficit may be reduced by combining insights from planetary science, geochemistry, and biology. The element phosphorus (P) is limiting for life in many environments on the modern Earth. Changes in global P availability may have played a large role in shaping biogeochemical evolution. Moreover, the baseline availability of P in planetary crusts is determined by processes of accretion, core formation, and late bombardment. Phosphorus is therefore of biological, cosmochemical, and astrophysical interest, providing a focal point from which to explore these diverse yet inter-related topics. Most of the P in our Solar System is stored in the form of minerals. Phosphorus-bearing minerals preserve information on pressures and temperatures experienced both during their initial formation and across the subsequent reaches of geological time. These minerals act as useful tools for probing the geological history of rocky objects, including the collisional processes through which asteroids and planets may be assembled, or indeed destroyed. However, the mechanisms by which P-bearing minerals form and by which they record collisions are uncertain, compromising interpretation of shocked meteorites as a record of Solar System history. The highly shocked Chelyabinsk meteorite exemplifies this point, containing a suite of variably deformed phosphate minerals of uncertain origin that have been used to infer several mutually exclusive scenarios for the collision history of the parental asteroid. Chelyabinsk preserves three lithologies: light (host rock), dark (containing a higher proportion of melted phases), and shock-melt (fully melted and quench crystallised material). Here, a comprehensive analysis of P mineral distribution and associated microtextures in each lithology is presented. I observe continuously strained as well as recrystallized strain-free merrillite populations. Grains with strain-free subdomains are present only in the more intensely shocked dark lithology, indicating that phosphate growth predates the development of primary shock-metamorphic features. Complete melting of portions of the meteorite is recorded by the shock-melt lithology, which contains a population of phosphorus-rich olivine grains. The response of phosphorus-bearing minerals to shock is therefore hugely variable throughout this monomict impact breccia. I propose a paragenetic history for P-bearing phases in Chelyabinsk involving initial phosphate growth via P-rich olivine replacement, followed by phosphate deformation during an early impact event. This event was also responsible for the local development of shock melt that lacks phosphate grains and instead contains P-enriched olivine. I generalise these findings to propose a new classification scheme for Phosphorus-Olivine-Assemblages (Type I-III POAs). I highlight how POAs can be used to trace radiogenic metamorphism and shock metamorphic events that together span the entire geological history of primitive asteroids. Whilst phosphate mineral microtextures help to determine a relative series of geological events in the history of an asteroid, absolute dating methods allow a temporal sequence to be more exactly defined. Such information is crucial for gaining confidence in our understanding of how primitive asteroids may record the long term collisional evolution of the Solar System. For example, at 4.5-4.4 billion years ago, the final orbital architecture of our Solar System was established by the migration of Giant Planets and the Earth-Moon forming giant impact event. An invaluable record of this period may be written in the phosphate minerals of asteroids, which should have experienced enhanced collisional activity during such events. However, there is long-standing uncertainty in the interpretation of phosphate mineral ages which, via meteorites, can otherwise be used to trace ancient asteroid collisions. Here, again studying the Chelyabinsk meteorite, it is shown that phosphate U-Pb systematics should be filtered by observed mineral textural features. Damaged phosphate domains record a recent minor collision, which liberated Chelyabinsk from its parent asteroid. Pristine phosphate domains record an early high-energy collision at the expected time of Earth-Moon formation and Solar System reorganisation. Phosphorus-bearing minerals are not just useful tools for tracing ancient events in our Solar System. Phosphorus is a key ingredient for the chemistry that likely gave rise to life on Earth. Lacking a major gas phase at ambient conditions, the concentration of P in early aqueous environments will have been governed by the mineral sources of P present at Earth's surface. A knowledge of early Earth P mineralogy and prevailing global and local environmental conditions is therefore needed to understand which scenarios for prebiotic chemistry are most plausible. Here, I reassess the diversity of P-bearing phases at Earth’s surface during the emergence of life. I consider phases that were delivered by meteorites (exogenous phases), as well as those that developed solely as a result of Earth system processes (endogenous phases). I take into account the known formation conditions of individual phases, as well as the observed temporal distributions of P-bearing minerals found at Earth’s surface today. Our approach allows us to leverage what is known about changes in the Earth system in order to rule out the prebiotic relevance of many P-bearing phases. Meanwhile, I highlight a small number of phases that are of possible prebiotic relevance; specifically, exogenous schreibersite, merrillite, and apatite, and endogenous apatite, olivine, and glass. Prebiotic mineral-chemical scenarios can be formulated for each phase, with distinct requirements for the environmental and tectonic state of early Earth. We can therefore relate the plausibility of mineral-chemical scenarios to the nature of early Earth, bridging the fields of geoscience and prebiotic chemistry. If P is considered limiting for life, then the possible total mass of a P-dependent biosphere will be set by the composition of crustal rocks. However, the chemical composition and relative abundance of rock types within Earth’s crust over time remains uncertain. Here, Macrostrat – a database of rock age, volume, and chemistry – is used to reconstruct the evolution of Earth’s weatherable continental crust. I identify a long-term increase in the relative abundance of sedimentary rock, which reshaped crustal nutrient inventories whilst leaving the bulk composition largely unchanged. Rapid compositional change occurred across the Neoproterozoic-Phanerozoic boundary (600-400 Ma) as elevated erosion replaced Precambrian rocks with young, nutrient-rich sediments. Plate tectonics may have acted to increase global nutrient supply coincident with the rise of animal life. Focusing on P offers one particular perspective on mechanisms that may have in part governed the emergence and evolution of life on Earth. However, the chemical and geological origins of life currently remain a mystery. This is no small part owing to the lack of accepted tests that a plausible scenario for prebiotic chemistry must pass. Here, a conceptual framework is presented that allows for the formulation and application of one such test: interference chemistry. In interference chemistry, a prebiotic reaction, or reaction system, is placed into a geochemical context (environment), creating a prebiotic scenario. The interaction between reaction efficacy and environmental conditions may be neutral, or alternatively result in constructive or destructive interferences with the pathway. Systematically exploring environmental interference chemistry for given reactions provides a common language with which to evaluate the plausibility of different scenarios for the origin of life: a test of environmental resilience which goes beyond asking whether the minimum conditions for a pathway are reached. Instead, interference chemistry provides a means to identify where on the early Earth prebiotic pathways may have been most favoured. A truly interdisciplinary approach to interference chemistry would incorporate constraints on early Earth environments from the study of astrophysics, meteorites, and preserved crustal rocks.