Theses - Earth Sciences

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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.
Embargo
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.
Embargo
Generation of Buried Transient Landscapes and Residual Depth Anomalies
Conway-Jones, Benedict
During Cenozoic times, the Icelandic plume has played a dominant role in controlling periodic uplift of oceanographic gateways across the North Atlantic Ocean. Five exquisitely preserved Paleogene buried landscapes are mapped on three-dimensional seismic reflection surveys from the Faroe-Shetland basin. A regional biostratigraphic and lithostratigraphic framework shows that these transient landscapes recur at intervals of 2–4 million years. Dendritic drainage patterns recovered from these landscapes are disequilibrated and contain multiple knickzones and knickpoints that are systematically arranged within catchment areas. Applying the stream power law, longitudinal river profiles are inverted to calculate spatially and temporally varying uplift histories. These unique landscapes are attributed to laterally advecting pulses of hot material that travel away from the centre of the Icelandic plume. Kinematic modelling suggests that these pulses are 100±50 °C hotter than ambient plume material. There is a temporal relationship between timings of landscape exposure and transient volcanic activity. Globally, these landscapes are coeval with short-lived climatic hyperthermal events. A causal relationship is proposed, whereby pulses of basaltic volcanism serve as both sources of CO2 and triggers for methane release, thus generating hyperthermal aberrations. The present-day surface expression of the Icelandic plume is evaluated using a comprehensive database of residual depth measurements. This database suggests that the Icelandic plume head has a radius of ∼1500 km and a complex planform, consistent with free-air gravity and earthquake tomographic models. These observations cannot be reconciled with previous models of asthenospheric flow and lead to revised estimates of plume flux. An exhaustive database of residual depth measurements throughout the Atlantic Ocean is also presented. In particular, newly acquired seismic reflection surveys along the west coast of Africa reveal that dynamic topography varies spatially on shorter wavelengths than previously described. These observations are consistent with earthquake tomographic models and geochemical analysis of basaltic rocks.
Open Access
Radial Miscible Viscous Fingering of Icelandic Mantle Plume
Galbraith-Olive, Patricia
The Icelandic plume, a major convective upwelling, has had a considerable influence on the geological evolution of the North Atlantic region. Direct manifestations of this major convective upwelling include positive residual depth anomalies and long wavelength free-air gravity anomalies, both of which reach from Baffin Island to Norway and from Newfoundland to Svalbard. Signifi cant shear wave velocity anomalies, observed in full-waveform tomographic models between 100 km and 200 km depth, show the Icelandic plume has a complex, irregular planform. These anomalies suggest about fi ve horizontal fi ngers radiate away from the central plume conduit. The best imaged fingers lie beneath the British Isles, southern Scandinavia and Greenland, extending ~1,000 km from the Icelandic plume. It is proposed that these radial miscible fi ngers develop due to the Saffman-Taylor instability, a fluid dynamical phenomenon which occurs when a less viscous fluid is injected into a more viscous fluid. Mobility ratio (i.e. the ratio of fluid viscosities), Peclet number (i.e. the ratio of advective and diffusive transport rates) and thickness of the horizontal layer into which the fluid is injected, together control the presence of fi ngering due to the Saffman-Taylor instability. Estimates for the Icelandic plume suggest the mobility ratio is at least 15, the Peclet number is ~ 2 x 10⁴, and the asthenospheric channel thickness is 100 ± 50 km. Appropriately scaled laboratory experiments play a key role in developing a quantitative understanding of the spatial and temporal evolution of mantle plume planforms. My results prove that the presence or absence of radial miscible fi ngering due to the Saffman-Taylor instability is controlled by changes in mobility ratio, Peclet number and horizontal layer thickness. At large horizontal thicknesses, gravity has an increasingly important influence and acts to damp the production of radial viscous miscible fi ngers. Observed values from the Icelandic plume suggest the fluid dynamics may be more complex than the Saffman-Taylor instability alone. Additional processes, such as interaction with the base of the lithospheric plate, along with the Saffman-Taylor instability, may be the origin of the fi ngers.
Embargo
The Significance of Densely Sampled Observations of Mantle Dynamic Topography
Holdt, Megan; Holdt, Megan [0000-0002-6053-8043]
Dynamic topography is generated from mantle convection and can be isolated from isostatic topography using observational constraints. Quantifying observable dynamic topography yields valuable information on mantle convective processes. This study presents a revised and augmented dataset of 10,874 residual depth measurements from oceanic crust, which originate from 7,601 seismic experiments. The isostatic component of topography is identified and removed by correcting for sedimentary loading and crustal thickness variation. Methodological improvements are implemented for both sedimentary and crustal corrections. Residual depth measurements typically deviate by $\pm$1~km from the predicted plate cooling trend. Positive and negative anomalies correlate with independent geological and geophysical observations of uplift and subsidence. This oceanic dataset is combined with a continental dataset to generate globally continuous spherical harmonic representations out to degree 40. The resultant power spectrum demonstrates that most power occurs at degree 2 (i.e., wavelengths of $\sim$10$^4$~km), while considerable power exists out to and including degree 40 (i.e., wavelengths of $\sim$10$^3$~km). The power spectrum of observable dynamic topography is robust using different inverse methods. An analytical power spectrum is generated that is consistent with the observed power spectrum. Collating a global database of residual depth measurements has yielded a number of useful ancillary datasets. For example, interpreting the sediment-basement interface along 7,601 seismic profiles provides a benchmark for global maps of sedimentary thickness variation. In addition, a synthesis of crustal thickness measurements from 278 modern wide-angle experiments yields a revised mean oceanic crustal thickness of 6.38$\pm$1.12~km. Residual depth measurements also yield information on the plate cooling trend. Hence, a suite of new plate cooling models are developed by jointly inverting depth-to-basement observations (n = 10,874) and heat flow observations (n = 3,753). A numerical model, in which pressure and temperature vary with depth, yields a temperature, $T_p$, of 1326$^\circ$C, a lithospheric thickness, $z_p$, of 111~km, and a ridge depth, $z_r$, of 2.92~km. These recovered parameters are in close agreement with independent constraints of mantle potential temperature, lithospheric thickness and ridge depth. This study also demonstrates that improving the parameterization of an analytical model recovers values that are similar to a more complex numerical model. Passive margins provide an excellent natural laboratory for studying the influence of dynamic topography. Despite tectonic quiescence, topography along passive margins is highly variable. Elevated passive margins are often associated with temperature anomalies in the asthenosphere, inferred from slow shear-wave velocity anomalies, elevated residual depth measurements, Neogene-Quaternary volcanism, elevated marine stratigraphy, thin lithosphere and positive long wavelength gravity anomalies. These observations suggest that mantle dynamics play a fundamental role in the formation of elevated passive margins. Uplift caused by an asthenospheric density anomaly is modeled at several locations. Results suggest that low density anomalies create sufficient uplift to account for elevated topography. The presence of slow shear-wave velocity anomalies and Neogene-Quaternary volcanism suggests escarpment uplift may be youthful.
Open Access
Fluid Flow Processes at Mid-Ocean Ridge Hydrothermal Systems
(2000-09) Jupp, Timothy Edmund
The subseafloor structure and temporal variability of mid-ocean ridge hydrothermal systems are examined from a largely theoretical standpoint. The nature of tidal signals is considered in detail and there is a discussion of the mechanisms by which the tidal modulations observed at seafloor hydrothermal systems might be produced. A review of the known examples of tidal modulation at hydrothermal systems is presented, and a new procedure for the analysis of these tidally modulated time-series is proposed. Where possible, this new procedure is applied to datasets previously obtained at the seafloor and it is recommended for use in future analyses. It is shown that the nonlinear thermodynamic properties of pure water are sufficient to impose a structure consistent with the known constraints on subseafloor convection cells. In particular, it is demonstrated that the properties of water limit seafloor vent temperatures to ~400°C, even when the energy source driving the convection cell is much hotter. A scaling analysis is presented to reveal how the lengthscales and timescales associated with a subseafloor convection cell depend on the bulk crustal permeability. The equations of poroelasticity are reviewed to demonstrate how the nonlinear thermodynamic properties of water influence the response of a hydrothermal system to tidal loading at the seafloor. A selection of simple analytical solutions reveals the phase relationship of the effluent temperature and effluent velocity at the seafloor to the ocean tide. A numerical simulation illustrates the effect of tidal loading on a two-dimensional subseafloor convection cell incorporating the nonlinear properties of water.
Embargo
Dynamic Topography of the Borborema Province, South America
Santana, Ana Patricia
The Borborema Province is located in the northeast corner of the Brazilian Shield. It is characterized by a positive long-wavelength free-air gravity anomaly of ∼ +30 mGal, which is consistent with sub-crustal dynamic support. This inference is corroborated by the presence of positive residual depth anomalies of 100s of meters which are observed along the abutting continental margin. Onshore, regional uplift is manifest by Albian marine limestones, which crop out at 700 m within the Araripe basin, and by emergent Oligo-Miocene marine coastal terraces. It is inferred that sub-plate mantle convective processes play a significant role in generating and maintaining this elevated topography. Crustal velocity structure is constrained using a combination of legacy seismic wide-angle experiments and receiver function analyses. By exploiting a global compilation of rock physics measurements of velocity and density, residual topography anomalies are calculated for the onshore region. These anomalies have amplitudes of up to ∼ 1 km, consistent with offshore residual depth measurements. Finally, the temporal evolution of this domed region is investigated by analysing fluvial drainage patterns and legacy thermochronologic measurements. An inventory of more than 2, 200 longitudinal river profiles is assembled and inverted to recover uplift rate as a function of time and space, subject to independent calibration and validation. Results suggest that regional uplift commenced in Upper Cretaceous times and continued to the present day at an average rate of 0.005 mm yr−1. This history is consistent with inverse modeling of legacy apatite fission track samples. Rapid cooling events are recorded by samples located at the base of escarpments of the Borborema Plateau. Denudation of ∼ 1 km is estimated to have occurred during Miocene times. In conclusion, regional elevation of this region is youthful and maintained by a sub-lithospheric thermal anomaly that is manifest through a combination of slow shear-wave velocity anomalies, a lithospheric mantle shadow zone, and Neogene volcanism.
Embargo
Environmental change impacts on shell formation in the muricid Nucella lapillus
Mayk, Dennis; Mayk, Dennis [0000-0002-5017-1495]
Environmental change is a significant threat to marine ecosystems worldwide. Ocean acidification, global warming and long-term emissions of anthropogenic effluents are all negatively impacting aquatic life. Marine calcifying organisms, in particular, are expected to be severely affected by decreasing seawater pH, resulting in shell dissolution and retardations during the formation and repair of shells. Understanding the underlying biological and environmental factors driving species vulnerabilities to habitat alterations is thus crucial to our ability to faithfully predict impacts on marine ecosystems under an array of environmental change scenarios. So far, existing knowledge about organism responses mainly stems from short to medium term laboratory experiments of single species or over- simplified communities. Although these studies have provided important insights, results may not translate to organism responses in a complex natural system requiring a more holistic experimental approach. In this thesis, I investigated shell formation mechanisms and shape and elemental composition responses in the shell of the important intertidal predatory muricid Nucella lapillus both in situ and across heterogeneous environmental gradients. The aim was to identify potential coping mechanisms of N. lapillus to environmental change and provide a more coherent picture of shell formation responses along large ecological gradients in the spatial and temporal domain. To investigate shell formation mechanisms, I tested for the possibility of shell recycling as a function to reduce calcification costs during times of exceptional demand using a multi-treatment shell labelling experiment. Reports on calcification costs vary largely in the literature. Still, recent discoveries showed that costs might increase as a function of decreasing calcification substrate abundance, suggesting that shell formation becomes increasingly more costly under future environmental change scenarios. However, despite the anticipated costs, no evidence was found that would indicate the use of functional dissolution as a means to recycle shell material for a more cost-eﬀicient shell formation in N. lapillus. To investigate shell formation responses, I combined morphometric and shell thickness analyses with novel statistical methods to identify natural shape and thickness response of N. lapillus to large scale variability in temperature, salinity, wind speed and the carbonate system across a wide geographic range (from Portugal to Iceland) and through time (over 130 years). I found that along geographical gradients, the state of the carbonate system and, more specifically, the substrate inhibitor ratio ([HCO3−][H+]−1) (SIR) was the main predictor for shape variations in N. lapillus. Populations in regions with a lower SIR tend to form narrower shells with a higher spire to body whorl ratio. In contrast, populations in regions with a higher SIR form wider shells with a much lower spire to body whorl ratio. The results suggest a widespread phenotypic response of N. lapillus to continuing ocean acidification could be expected, affecting its phenotypic response patterns to predator or wave exposure regimes with profound implications for North Atlantic rocky shore communities. On the contrary, investigations of shell shape and thickness changes over the last 130 years from adjacent sampling regions on the Southern North Sea coast revealed that contrary to global predictions, N. lapillus built continuously thicker shells while maintaining a consistent shell shape throughout the last century. Systematic modelling efforts suggested that the observed shell thickening resulted from higher annual temperatures, longer yearly calcification windows, nearshore eutrophication, and enhanced prey abundance, which mitigated the impact of other climate change factors. An investigation into the trace elemental composition of common pollutant metals in the same archival N. lapillus specimens revealed that shell Cu/Ca and Zn/Ca concentration ratios remained remarkably constant throughout the last 130 years despite substantial shifts in the environmental concentration. However, Pb/Ca concentration ratios showed a definite trend closely aligned with leaded petrol emissions in Europe over the same period. Discussing physiological and environmental drivers for the observed shell bound heavy metal patterns, I argue that, unlike for Pb, constraints on environmental dissolved Cu species abundance and biologically mediated control on internal Zn levels were likely responsible for a decoupling of shell-bound to total ambient Cu and Zn concentrations. The results highlight the complexity of internal and external pathways that govern the uptake of heavy metals into the molluscan shell and suggest that the shell of N. lapillus could be a suitable archive for a targeted investigation of Pb pollution in the intertidal zone.
Open Access
Seismic structure beneath Southeast Asia from adjoint waveform tomography
Wehner, Deborah
Seismic tomography has played a crucial role in the illumination of deep Earth structure. Most existing tomographic methods are based on seismic ray theory and hence do not fully account for the true physics of wave propagation. Recent computational advances allow us to embrace the full complexity of seismic wave propagation by accurately solving the 3-D seismic wave equation numerically. This can account for effects such as wavefront healing, interference, scattering and (de)focusing, which are often ignored or not properly captured by other methods such as ray tracing. Thus, such methodologies are particularly suitable for strongly heterogeneous regions such as Southeast Asia, where large variations in elastic parameters are likely to be present. Here, an unprecedented dataset and access to sizeable computational resources allow their application to Southeast Asia for the first time. In the first part of this thesis, a continental-scale seismic model of the lithosphere and underlying mantle beneath Southeast Asia obtained from adjoint waveform tomography (often referred to as full-waveform inversion or FWI) is presented. FWI is a non-linear imaging method, where an initial model is updated in order to minimise the difference between observed and predicted waveforms. Based on > 3,000 h of analysed waveform data gathered from 13,000 unique source-receiver pairs and filtered at periods between 20 – 150 s, isotropic P-wave velocity, radially anisotropic S-wave velocity and density are imaged via an iterative non-linear inversion that begins from a 1-D reference model. At each iteration, the full 3-D wavefield is determined through an anelastic Earth, accommodating effects of topography, bathymetry and ocean load. SASSY21, the final model after 87 iterations, appears to be robust since it is able to explain true-amplitude data from events and receivers not included in the inversion. The new model reveals detailed anomalies down to the mantle transition zone, including multiple subduction zones. The most prominent feature is the (Indo-)Australian plate descending beneath Indonesia, which is imaged as one continuous slab along the 180 curvature of the Banda Arc. The tomography confirms the existence of a hole in the slab beneath Mount Tambora and locates a high S-wave velocity zone beneath northern Borneo that may be associated with subduction termination in the mid-late Miocene. A previously undiscovered feature beneath the east coast of Borneo is also revealed, which may be a signature of postsubduction processes, delamination or underthrusting from the formation of Sulawesi. In the second part of this thesis, SASSY21 is used as a starting model to obtain a more refined image of the eastern Indonesian region, using seismic data filtered at periods from 15 – 150 s. In this study, the fluid ocean is accounted for explicitly by solving a coupled system of the acoustic and elastic wave equation. This is computationally more expensive but allows seismic waves within the water layer to be simulated, which becomes important at shorter periods. The effects arising from surface topography, bathymetry and the fluid ocean on synthetic waveforms become pronounced at periods $\leq$ 20 s. In particular, surface elevation can result in a considerable phase advance and change in amplitude of the surface wave train, and has an effect on both horizontal and the vertical seismogram components for this simulation setup. The fluid ocean results in a phase delay as well as a change in amplitudes and duration of the surface wave train, and affects both the radial and vertical components. At periods $\leq$ 20 s, accounting for the fluid ocean explicitly can lead to more realistic lithospheric velocities and a more refined image compared to the commonly used ocean load approximation, even at greater depths. Furthermore, it allows for an improved waveform match for source-receiver paths passing partially or entirely through oceanic regions. The final model, SASSIER22, after 34 iterations reveals a convergent double-subduction along the southern segment of the Philippine Trench, which was not evident in the starting model and transitions to a divergent system in the Molucca Sea further south. A more detailed illumination of the slab beneath the North Sulawesi Trench subduction zone reveals a pronounced positive wavespeed anomaly down to 200 km depth, consistent with the maximum depth of seismicity, and a more diffuse but aseismic positive wavespeed anomaly that continues to the 410 km discontinuity.