This thesis records the results of one of the studies currently in progress on various aspects of the geology of the Coastal Andean Batholith of Peru. In much the same manner as its counterparts in the Patagonian Andes and the North American Rockies (fig 1), the Coastal Andean Batholith is a complex of many spatially related but temporally distinct intrusive episodes. A suite of rock types is present which ranges from ultrabasic to adamellitic in character. A study of the mafic plutonic rocks was chosen because in other batholithic associations equivalent rock types to those found in Peru have received scant attention and are improperly understood. This possibly reflects the comparatively small volumes that these rocks . occuply relative to the intermediate and acid rocks in batholithic environments. Too often, it seems, the basic representatives are dismissed as potential basalt which somehow became trapped during its upward migration whereupon it crystallised early in the batholithic sequence. Moreoever, where reference is made to the presence of basic rocks it is often as part of a regiqnal study. As a consequence. the general emphasis in such accounts inclines towards the descriptive rather than the interpretive.

This thesis describes the development of two numerical models for the study of (1) incompressible multiphase flow and (2) compressible multiphase flow. Both models employ a state-of-the-art adaptive unstructured mesh-based approach which allows the mesh, upon which the model equations are discretised, to be optimised in order to focus numerical resolution in areas important to the dynamics and decrease it where it is not needed as a simulation progresses. The implementation of the models takes place within a computational fluid dynamics code called Fluidity. The application of the models concerns the multi-scale simulation of volcanic ash transport in aqueous solutions and in the atmosphere. Simulations of ash settling in a water tank, which mimic published laboratory experiments, are performed primarily in two dimensions. The results demonstrate that ash particles can either settle slowly and individually, or rapidly and collectively as an ash-laden cloud, referred to as a plume. Two quantities used to measure the tendency for plumes to form are then evaluated with a parameter study. Particles settling collectively are slowed by inertial drag, rather than viscous drag, and it is shown that such quantities must account for this. An improvement to the measures is proposed, along with an alternative measure which uses a more accurate expression for the collective settling timescale. Finally, a two-dimensional kilometre-scale volcanic eruption of hot gas and ash into the atmosphere is simulated. The results are compared with those from MFIX, a leading multiphase flow code. Both Fluidity and MFIX are able to successfully capture the key characteristics of an eruption event. The benefits of the adaptive unstructured mesh-based approach are highlighted throughout the thesis by demonstrating that it maintains solution accuracy whilst providing a substantial reduction in computational requirements when compared to the same simulation performed using a fixed mesh of uniform resolution.

Peatlands are important terrestrial carbon stores, both in the United Kingdom and globally. The cool and wet climate of the UK allows blanket bog peatlands to form in upland regions, with peat deposits covering the landscape across entire hillslopes. Blanket bogs are important sinks and sources of CO2 and dissolved organic carbon (DOC). Many factors affect the carbon cycle of peatlands, including climate, hydrology, vegetation, land management and topography. Although hillslope position can influence the hydrology of peatlands, the effect it has on the production and transport of different carbon species is poorly understood. This thesis investigates the impact hillslope position has upon the hydrology and carbon release pathways of blanket bogs in upland regions. Hydrology, CO2 fluxes and DOC concentrations were studied at two hillslopes in the Peak District, Derbyshire, across four hillslope positions: top-slope, upper mid-slope, lower mid-slope and bottom-slope. Results show that slope position was the dominant control affecting water table variation. Although slope position did influence variation in CO2 fluxes, its impact was small compared to other factors, including small-scale heterogeneity and microtopographic variation. Slope position was an important control influencing variation in DOC concentrations. Dissolved organic carbon concentrations decreased down-slope as high water tables and water movement flushed DOC from the peat subsurface. Model results indicate that slope position is an important factor that should be included in carbon budget models but further work is required to further improve understanding of hillslope processes.

The continental crustal growth has been a great interest to geoscientists and its importance is self-evident. Any models trying to reveal how it works must be able to explain 1) how the continental crust gains its andesitic bulk composition with juvenile isotope signatures; 2) how the volume of the crust increases episodically. Granitoids with the juvenile isotopic signatures in the collision zones provide the special insights into the nature of the continental crust and a unique opportunity to investigate the mechanism of crust growth. This thesis reports results from major and trace element data, whole rock Sr-Nd-Hf-Pb isotopic data, zircon geochronology and in situ Hf isotopes of granitoids as well as, if any, the enclosed mafic magmatic enclaves (MMEs) from the three ancient collision zones crossing the Northern Tibetan Plateau. I-type granitoids and their MMEs from the East Kunlun Orogenic Belt (EK) in the Northern Tibetan Plateau are dated as 250 Ma years old. They are cal-alkaline in nature with compositions resembling the bulk continental crust (BCC). Whole rock Sr-Nd-Pb-Hf isotopes reveal that they are products of partial melting of the ‘trapped’ subducted oceanic crust at the onset of the collision and the MMEs are the aggregated early cumulates in the parental magma rather than the mafic endmember involved in the magma mixing as previously suggested. I- and S-type granitoids from the Qilian Block (QB) further north in the Great Plateau are dated as 450 Ma. Their lithological and geochemical heterogeneity and isotopic changes with time are inferred to correspond to the collision. The abundant inherited zircons with ages as old as Archean reveal the presence of the Archean basement underneath the Qilian Block and indicate that the Qilian Block may have been a micro continent during its drift in the ancient ocean. Granitoids and their enclosed cumulates from Kekeli Batholith further north in the plateau are 500 Ma years old. They have decoupled whole rock Nd and Hf isotopes and discrepancy between whole rock Hf and zircon in situ Hf isotopes. These inconsistences are understood to result from different mineral crystallization timings during mixing between endmembers with distinct isotopes. This highlights the need for detailed whole-rock or non-zircon phases Hf isotopic investigation in order to develop a comprehensive understanding of the granitoids of hybrid origin.

Through geodynamical modelling, two hypotheses about the craton stability and evolution were revisited and an important process of cratonization is investigated. Unlike most previous, related numerical studies, non-Newtonian rheology with composition dependence was used in these studies, and the rheological parameters are thus directly comparable with laboratory experiment of mantle. The first hypothesis, that the cratonic lithosphere is “isopycnic”, is found to be not strictly necessary for craton stability and longevity. The high viscosity of the cratonic litho- sphere due to compositional effects on the mantle rheology is found to be essential to maintain a thickness difference between cratonic and non-cratonic lithosphere for over billions of years and it allows a modest negative buoyancy of the cratonic root, depending on the strengthening factor due to the compositional effects. The second hypothesis to be tested is that mantle plume im- pingements cause rapid, significant removal of subcontinental lithosphere. The results presented in this thesis show that the erosion caused by a plume impact on a continent that is strong enough to have survived billions of years of Earth’s history is rather limited. A special weaken- ing mechanism of such highly viscous and buoyant roots is required to reactivate this cratonic lithosphere and thus cause significant thinning within 10s of Myrs. The fluid/melt-rock interac- tion during mantle metasomatism is probably the most likely mechanism to modify and weaken depleted cratonic lithosphere. Therefore, metasomatic weakening is essential for the significant thinning of subcontinental lithosphere observed, e.g.at North China Craton and Namibia, south- ern African, no matter whether caused by a plume impact or another tectonic event. Using the reasonable compositional effects on the buoyancy and rheology of mantle rocks from the above studies, numerical experiments are performed to study the formation of thick cratonic lithosphere from a layered, depleted mantle material. In this scenario, substantial tec- tonic shortening and thickening of previously depleted material seems to be an essential ingre- dient to initiate the cratonization process. Afterwards, gravitational self-thickening will cause further thickening. Compositional buoyancy resists Rayleigh-Taylor instability collapse and stabilizes the thick cratonic root, while the secular cooling also has a stabilizing effect on the cratonic root by reducing the thermal buoyancy contrast between lithosphere and asthenosphere and increasing mantle viscosity. The presented numerical results are consistent with the vertical movement of cratonic peridotite as suggested on petrological grounds.

CO2 capture and storage (CCS) has been proposed as a climate change mitigation strategy. The basic principle is to prevent CO2 which would normally be emitted from large point sources, such as power stations, from entering the atmosphere. This is achieved by capturing the CO2 at source and storing it in a location where it will be trapped and unable to enter the atmosphere. This work looks specifically at geological storage of CO2 in deep saline formations. Dynamic simulations can be used to investigate the fundamental physical and chemical processes which occur when CO2 is injected into geological formations. They can also be used to determine the suitability of a particular site for CO2 storage. The scale of the processes being simulated is important when building a dynamic model. Here dynamic simulations have been used to explore three different aspects of geological CO2 storage in deep saline formations. The first model investigates large scale CO2 migration and pressure build up at a potential CCS site. The second model concentrates on the small scale processes of CO2 dissolution and convection. The third model attempts to accurately model both the large scale processes of CO2 injection and migration and the small scale processes of CO2 dissolution and convection. Dynamic simulations have been used to model storage capacity, CO2 migration and pressure buildup at a potential CO2 storage site in the UK North Sea. There are large uncertainties in the input data so various models have been run using a range of parameters. The primary control on pressure buildup at the site is the permeability of the unit directly beneath it. The plume diameter is primarily controlled by the porosity and permeability of the reservoir unit. Despite uncertainties in the input data, the use of a full three-dimensional (3D) numerical simulation has been extremely useful for identifying and prioritizing factors that need further investigation. Dissolution of CO2 into existing formation waters (brine) leads to an increase in brine density proportional to the amount of dissolved CO2. This can lead to gravitational instabilities and the formation of convection currents. Convection currents, in turn, will increase CO2 dissolution rates by removing CO2 saturated brine from the CO2-brine interface. The dissolution and subsequent convection of CO2 which has leaked through a fracture is investigated using dynamic simulations. The instigation of convection currents due to flow through a fracture increases dissolution rates. Comparison of our results with fracture flow rates shows that for typical fracture apertures dissolution from a fracture is small relative to the amount of CO2 flowing through the fracture. Two phase flow effects and the currents caused by an advancing plume of injected CO2 can affect patterns of CO2 dissolution and convection within a reservoir. Most existing models of CO2 dissolution and convection use a static boundary layer or do not involve two phase flow effects. A radial, two phase, two component model has been built to model the injection process along with convection enhanced dissolution. The model performs well compared to analytical solutions in terms of the large scale processes of CO2 migration and pressure buildup but modelled convection is highly dependent on grid resolution. Numerical instabilities are also present. Further work is needed to increase the accuracy of the model in order to allow higher resolution modelling to be carried out and modelling of the smaller scale processes to be improved.

This work examines the tectonic evolution of the Eastern Venezuela foreland basin by analysis of gravity anomalies and geodynamical modelling. Gravity data (8ºN-12ºN and 60ºW-66ºW) were processed to produce gravity anomaly maps, the most prominent feature being the minimum of -200 mGal isostatic, Bouguer and free-air anomalies associated with the basin. Positive gravity values characterize the northern terranes. Backstripping analysis of the sedimentary successions of four boreholes penetrating the Eastern Venezuela basin was applied to evaluate the history of subsidence. This demonstrated that an early passive margin phase (Cretaceous to early Oligocene) was followed by Oligocene to Recent tectonic subsidence of the foreland basin linked to the northern coastal compressional tectonic belt. Geohistorical analysis shows a major contribution to subsidence from early Oligocene, renewed during mid-Miocene times, in response to the loading of the South American plate on its northern margin. This tectonic loading pattern is younger from north to south. The observed gravity anomaly paired in eastern Venezuela is adequately reproduced by crustal models along profiles OO´ and II´ showing the isostatic negative anomaly over the foreland basin, primarily caused by the 10-13 km of sediments, the downwarping of dense lower crust and Moho down to 48 km depth. The positive gravity anomalies to the North are associated with southward thrusting of metamorphic and magmatic terranes, and dense Caribbean lithosphere. Dense subducting mantle may also be contributing to the northern positive anomaly belt as deep seismicity suggests. Two mechanisms were applied to explain the formation of the Eastern Venezuela foreland basin during a collisional regime. First, the “hidden load” approach explains the deflection but includes an intracrustal load whose magnitude is around a third of the supracrustal loads computed by an iterative process. This gives rise to a major misfit between the gravity anomaly computed for the structural model which takes into account all the geological and geophysical constraints when the contribution of the “hidden load” is included in the calculations. Second, since the hidden load hypothesis fails to explain the gravity profiles, a remaining viable explanation is release of compressional strain energy involving N-S crustal shortening involving faulting. I therefore explored a fault-based hypothesis which does not depend on hidden gravitational loads, but takes into account the clear relationship between the subsidence and the complementary uplift. In conclusion the prime cause of the evolution of the system depends on the forces on fault planes as they move, where the tractions on a thrust fault develop when a frictionless fault occurs in response to horizontal deviatoric compression of an elastic layer. As compression acts at the edges of the plate, the stresses are re-orientated producing the movement of the two plates along dipping fault planes (including the El Pilar fault). This led to the formation of the Eastern Venezuela foreland basin and linked uplift of the Eastern Serranía del Interior by spasmodic release of elastic strain energy.

Peridotite xenoliths transported to the surface in basaltic magma from the upper mantle wedge beneath the West Bismarck island arc, Papua New Guinea, present a rare opportunity to assess the nature of the mantle wedge in an active intra-oceanic island arc. This thesis reports comprehensive new geochemical and isotopic data for harzburgites, pyroxenites and dunites, from the island of Ritter, in order to understand how partial melting and hydrous metasomatism generate chemically, isotopically and physically distinct mantle. The highly depleted major and moderately incompatible trace element composition of residual phases and the radiogenic strontium isotope composition of texturally well-equilibrated harzburgites are best explained by hydrous partial melting and metasomatism associated with a previous period of subduction. Harzburgites that record textural evidence for melt-rock reaction, meanwhile, have elevated equilibration temperatures, oxidised spinel compositions, elevated olivine water contents and strontium isotope compositions identical to regionally erupted basalts. These features reflect interaction between ambient mantle and primary hydrous, oxidised basaltic melts in the upper mantle wedge. Modelling of trace element diffusion profiles in olivine constrains this event to approximately one year before exhumation. The low water contents of both coarse-grained olivine and orthopyroxene are consistent with equilibrium in chemically depleted upper mantle. The absence of hydrated silicon vacancies in olivine despite overall increases in water content during melt-rock reaction indicates that the mantle wedge may not change significantly in mechanical strength during hydrous melting and metasomatism. Chemical and radiogenic signatures of subduction are thus more likely to survive convective homogenisation. The whole-rock budget of highly siderophile elements (HSE) is contained within heterogeneously distributed trace sulphides and inferred alloy phases, and is controlled by both partial melting and metasomatism. An absence of any correlation of Sr isotopes with either HSE or Os isotopes indicates these elements may be immobile in slab-derived fluids. Elevated concentrations of Pt and Pd in pyroxenite are mirrored by depletions in dunite, demonstrating that melt-rock reaction is instead responsible for enrichments in these elements in arc mantle. A correlation between whole-rock 187Os/188Os and phosphorous in olivine offers clues to ancient processes unrelated to active subduction, not recorded by any other chemical or isotopic system.

The northern peatlands cover only 3% of the Earth, and store between 20 and 30% of the terrestrial carbon pool. In the UK, 15% of the land is covered in peatland, which is estimated to store 2.3 Pg of carbon. Recently, a trend of increasing DOC concentrations in surface waters has been observed in the northern hemisphere, and the in-stream processing and degradation of DOC to CO2 could represent a major and increasing source of greenhouse gas to the atmosphere. This thesis measured net DOC loss in unfiltered river water samples across different catchment scales, ranging from 0.005 to 1086 km2, with the peat content varying from 0 to 100%, and over time scales from 30 hours to 10 days. Experiments were carried out monthly for three years, and considered total loss, photo and aphotic degradation and the rate of each process. The composition of DOC and various source materials was analysed. There was a clear diurnal cycle in the degradation of DOC, with the rates of decline being much higher during the day and lower over night. The initial rates of DOC degradation were higher in source waters than from large downstream sites. Adding nutrients to the water decreased the initial rate, whereas exposing the water to light increased the rate, compared with water kept in the dark. The apparent quantum yield and activation energy of the degradation were calculated. The initial rate of DOC degradation was found to be related to the oxidation state of the material, with samples that were more reduced being degraded faster. The total DOC loss was estimated to be 76%, which equates to a loss of up to 14678 Gg CO2eq/yr from UK peat-covered catchments, which is 2.5% of the UK total GHG emissions, or 0.7% of the global CO2 emissions from inland waters.

The Asian Summer Monsoon generates intense seasonal precipitation across India, China and Indochina, comprising Earth’s largest monsoonal climate regime, and this vital component of the global energy and water cycles directly impacts the world’s most populous regions. Accurate palaeomonsoon reconstructions are required to investigate natural climate variability beyond the coverage of instrumental records and inform predictions of future monsoon trends. Stable oxygen isotope ratios (δ18O) are an important proxy for hydroclimate variability and stalagmite δ18O is widely used to investigate East Asian palaeoclimate, typically interpreted as a semi-quantitative measure of precipitation amount. However, recent studies suggest δ18O instead reflects multiple hydroclimatic processes, warranting a detailed understanding of precipitation and proxy δ18O systematics. This thesis (i) presents a quantitative Lagrangian study of atmospheric moisture transport and precipitation across central and eastern China, a continental region affected by the East Asian Summer Monsoon (EASM); (ii) investigates the hydroclimatic drivers of precipitation δ18O variability at Wanxiang Cave, an important site for palaeomonsoon reconstruction located near to the northerly EASM limit; (iii) evaluates the extent to which seasonal- to decadal-scale proxy δ18O variability reflects terrestrial moisture fluxes by constructing a pseudoproxy record for Wanxiang. Present-day precipitation across monsoonal China is primarily derived from the northern Indian Ocean and recycled intensely over the East Asian continent; Pacific Ocean moisture export peaks during winter. A geographically variable isotopic source effect is apparent in present-day mean monthly precipitation δ18O data. Wanxiang Cave precipitation δ18O variability during AD 2001-2002 is successfully reproduced by a recently-developed Lagrangian model which simulates air masses’ isotopic evolution along three-dimensional atmospheric trajectories, though winter values are ~2 ‰ over-depleted. Contributions of Tibetan Plateau- and free troposphere-derived moisture are identified as synoptic-scale isotopic depletion processes. Overall, land-derived moisture causes isotopic enrichment during summer, thus moderating depletion due to strengthened EASM circulation or increased precipitation amount. As such, periods of elevated summer surface temperatures may attenuate the seasonal EASM signal in precipitation and proxy δ18O. An idealised Rayleigh-type isotope model is unable to capture these effects accurately, emphasising the importance of regional moisture fluxes. A pseudoproxy record, driven by terrestrial moisture fluxes, replicates seasonal- to decadal-scale stalagmite δ18O variability at Wanxiang. The work presented in this thesis offers new insights into the influence of atmospheric moisture transport dynamics on precipitation and proxy δ18O variability across central and eastern China. This constitutes an important advancement in our ability to use δ18O to reconstruct past climate variability quantitatively in the EASM region.