The Antarctic Subpolar Seas are a poorly observed and understood region with a disproportionate importance for global ocean-climate processes. The region has been subject, over recent decades, to many dramatic climatic forcing perturbations. In particular, it has seen a substantial increase in surface wind stress, and an increase in freshwater forcing from the Antarctic Ice sheet. The impact of increased freshwater forcing is investigated here using austral summer satellite altimetry measurements. Between 1992 and 2011, an anomalous circumpolar rise in sea level is observed across the Antarctic Subpolar Seas of 1–5 mm yr-1 above the global mean. Several lines of evidence suggest that the observed Subpolar Sea SSH anomaly is a steric response to a recent decadal increase in glacial runoff. An ocean GCM is used to simulate the response of the Antarctic Subpolar Seas to a realistic increase in glacial run-off. The resulting response agrees well with observations and provides insight into the ocean’s adjustment. In particular, the model suggests that approximately half of the steric change in sea level is driven directly by freshening in the upper 800 meters, and that deep ocean warming drives the remainder. The response of the Subpolar Seas to an increase in surface wind forcing is then investigated using an idealised regional configuration of the MITgcm model, referred to as the gyre model. The gyre model is a simplified, Bousinesq, primitive equation model, designed to explore the dynamics of a southern subpolar gyre’s adjustment to wind forcing perturbations. The gyre model has four sub-configurations of ranging complexity. Each configuration is examined for its response to a 20% step-increase in surface wind forcing. The gyre model’s response varies between sub-configurations, however some common features are found in all runs. The density surfaces of the southern gyre are shown to dome in all sub-configurations following adjustment. Waves and advective features that move cold anomalies northwards dominate the western boundary adjustment. Two distinct types of boundary propagations occur in the gyre model: comparatively fast numerical short boundary waves (NSBW), which move at around 0.05 to 0.5 m s-1; and slower moving advective features, which propagate at between 0.01 and 0.05 m s-1. Analysis suggests that viscous parameters play an important role in the NSBW balance of terms. Further, the pathlines traced by neutrally buoyant particles seeded into the western boundary flow field suggest that the advective features are by the advection of temperature anomalies. The depth, magnitude and propagation velocity of the boundary features vary considerably following minor adjustments in model parameters, such as viscosity, diffusivity and background GM diffusivity. Further work is required to understand the role of boundary waves and boundary propagations, both in observations and models. The research presented in this thesis highlights that the recent climatic perturbations in the forcing of the Antarctic subpolar Seas are changing the regions circulation in significant and far reaching ways.

This thesis examines the change in ocean model utility with changing horizontal resolution. Oceans are a crucial part of the climate system, with numerical models offering important insights into our mechanistic understanding. We use a 30 year integration (1978 to 2007) of the NEMO model at 1o, 1/4o and 1/12o to investigate the impact of modelling choices associated with horizontal resolution changes. Changes in degrees of freedom associated with the increasing resolution allow alternative energy dissipation pathways, with potential impact on model accuracy. We develop a measure of utility based on an estimate of the accuracy, as well as a penalisation which scales with resolution. Overall, accuracy is thought to increase with resolution, and we examine the associated change in utility on a range of model fields. The exploration of the NEMO model assesses the surface mixed layer, deep (>2000m) to surface (<2000m) communication through the ocean interior and the changes in the meridional overturning with topographic interactions. Assessing these areas, we illustrate potential changes in the energy pathways in the system. We investigate the surface in terms of the mixed layer depth globally, but also investigating a case study in the Southern Ocean. We find that the mixed layer does not change significantly with resolution, and that NEMO compares well with observations. Minor changes with resolution are attributed to increased numbers of fronts with increasing resolution. When the mixed layer is assessed, we see no significant change with resolution, and so find that 1o has the highest utility. For our case study, we investigate the zonally asymmetric deepening of the mixed layer in the Southern Ocean. We find that the stratification set by the advection is key, and confirm this using the 1D Price-Weller-Pinkel model. The communication between the surface and the deep ocean is assessed by looking at the steric height variability, and specifically its covariance between the surface and the deep. We find that there are large changes with resolution, and attribute these to the higher resolutions' ability to include eddy effects. This suggests that the Gent-McWilliams scheme that is active at low resolution fails to capture this. We look at the low and high frequency parts of the variance, finding that strongly eddying regions dominate the high frequency steric height covariance, confirming the importance of eddies. The ratio between the surface and the deep steric height shows poor utility in both ORCA1 and ORCA025, while we find seasonal leakage obscuring our accuracy measure for the steric height. The overturning is assessed in density space, and we notice a strengthening of the anti-clockwise component in the Southern Ocean. Decomposing the transport into its baroclinic and barotropic components, we find that changes in the baroclinic overturning can account for this. The lack of western boundaries in the Southern Ocean suggests that eddies, as well as interaction with topography, are especially important here, and we investigate the change in the balance of forcing in terms of the associated vortex stretching. We assess this in terms of the bottom pressure torque, but find the major changes in the baroclinic component of the bottom pressure torque. We find that increasing the resolution still leads to increased utility, particularly in the barotropic and baroclinic density space overturning case. The major implications of our results are that low resolution is appropriate for fields such as the mixed layer depth, but increasing the resolution is seen to improve the mean overturning through allowing eddy activity.

This work addresses the complex interactions between bed?generated coherent turbulence structures and suspended non-cohesive sediment in wave-dominated environments and under combined wave-current flows. Coherent structures are intermittent, connected fluid masses with a defined spatial extent and life-span and characterised by vorticity; and define entrainment and momentum exchanges in turbulent flows. Prototype-scale experimental observations of flow, bedforms and suspensions are presented; focusing on the structural, spatial and temporal dynamics of these motions and their role in sediment entrainment. Two scenarios are considered: (a) shoaling (and breaking) erosive and accretive waves in the nearshore of a sandy barrier beach (spilling and plunging breakers); and (b) collinear steady currents aligned with, and opposing the direction of wave propagation over a rippled, sandy bed. Measurements collected in the nearshore of a prototype-scale sandy barrier beach show intermittent momentum exchanges characterised by large wave induced coherent structures within the benthic boundary layer, corresponding to the mean flow properties. These structures can be described by a 3D bursting sequence which plays a significant role in moving and maintaining sediment in suspension. The temporal variability of these events dictates the net onshore and offshore transport. Periods associated with a succession of powerful turbulent events cause powerful suspension clouds across multiple frequency scales. The bulk of suspension is attributed to wave-induced fluctuations of low frequencies. These modulate smaller, rapidly decaying high-frequency turbulence extending outside the boundary layer. As larger structures persist for a considerable amount of time, suspensions near the bed are amplified before decaying as the supply of momentum ceases. Outside the boundary layer, momentum transfer via turbulent fluctuations maintains the suspensions until their energy is depleted. In combined wave-current flows, the current plays a significant role in dictating the prevalence of specific turbulent motions within a bursting sequence. The current-aligned structures contribute significantly to the stress, and display characteristics of wall-attached eddies formed by the pairing of counter-rotating vortices. In aligned wave-current flows, the flow is characterised by a local balance between turbulent production and dissipation, and displays the ‘universal’ inertial cascade of energy in the outer flow, while just outside the combined boundary layer a superposition of eddies is observed, often linked to intermittent coherent turbulent structures at an intermediate range between energy production and dissipation. When the current opposes the direction of wave propagation, stress-bearing coherent structures break rapidly as they are ejected higher in the water column. Suspended clouds through vortex shedding thus cease rapidly in opposing flows, compared to more continuous sediment transport in aligned flow. Both studies indicate that bed-induced coherent turbulence structures play a significant role in the entrainment and transport of suspended sediment in flows typically encountered in the coastal environment. Suspension clouds induced be vortex shedding are maintained by the continuous supply of momentum through vortex clusters; and transport is described by the advection of these particles through net currents. A complex feedback is then imparted by the suspended particles on these structures. This merits a reevaluation of coastal sediment transport models, and a shift towards a stochastic description of the problem.

The warming of the oceans and consequent enhanced stratification will have significant consequences for ecosystem functioning and carbon sequestration. Nutrient supply will reduce as a result of a strengthening in the stratification, with consequences for microbial ecosystems. Oligotrophic ocean regions are therefore predicted to increase in size as a consequence of global warming. This strengthens the requirement for analytical techniques with low limits of detection for nitrate and phosphate as conventional methods are unable to detect the nanomolar nutrient concentrations in surface waters in these regions. In recent years, sensitive techniques have been developed for shipboard nutrient analysis at nanomolar level with a high sample throughput. An analyser coupled with liquid waveguide capillary cells was deployed in the Atlantic. These techniques are however not suitable for autonomous deployment in oceans for long-term observations. Therefore, I have been working on the miniaturisation of nanomolar nutrient techniques using novel Lab-on-a-chip devices. The aim is to develop systems that are small, low-cost and low-power, and can be used autonomously and remotely to provide in situ real-time data on processes with high temporal and spatial resolution. Microfluidic technology is being used as it enables minimization of reagent and power consumption for in situ deployment of wet-chemical methods which provide accurate results with low limits of detection and high spatial and temporal resolution. In the 3rd chapter, we describe the development of an autonomous analyser for the determination of dissolved reactive phosphorus based on the vanadomolybdate method which allows long-term deployments thanks to the stability of the reagents. It has been deployed off the coast in Plymouth and during D361 (Atlantic). Then in the 4th chapter, a microfluidic platform to measure phosphate with the molybdenum blue method is characterised with optimised parameters and applied to marine waters.

This thesis examines the nature of trace metal cycling in hydrothermal plumes, which have only recently been recognized as a significant source of Fe to the oceans. To study the influence of hydrothermal vents and their plumes on global trace metal cycles, two “black smoker” type vents and a previously unrecognized type of off-axis venting are examined. The trace metal chemistry of previously uncharacterized vents from the Von Damm vent field (VDVF) and the Beebe vent field (BVF) on the Mid-Cayman Rise (MCR) are described along with the processes of colloid formation in plumes over the Mid-Cayman Rise. Also Fe isotope analyses of a hydrothermal plume in the Southern Ocean reveals distinct isotope signatures to deep-waters dependent on plume chemistry. The role of soluble, colloidal and particulate partitioning of trace metals is understood to mediate the entire inventory of hydrothermal trace metals entering the ocean. In plumes over the MCR colloids are found to dominate dissolved iron (dFe) (48 to 87 % at Beebe and 14 to 81 % at Von Damm) in hydrothermal plumes. At Beebe soluble Fe (sFe) remains stable throughout plume dispersion, while particulate Fe is enriched (~25 %) by aggregating colloids. In the Von Damm plume colloidal Fe (cFe) and sFe maxima appear in the most dispersed regions of the plume where particulate Fe (pFe) is low. Plume processing of cFe and pFe will control the flux of dFe to the deep ocean from hydrothermal systems. This study shows that in order to accurately model the flux of dFe from vents, the behaviour of cFe needs to be incorporated into models of hydrothermal vent dFe fluxes, which at present do not consider these processes. Iron isotopes provide a means to measure the impact of hydrothermal venting on the oceanic Fe inventory, but no studies have examined the mechanism producing hydrothermal dFe isotope compositions. This study demonstrates that ?56Fe values of dFe (?56dFe) within the hydrothermal plume change dramatically during plume dispersal, ranging from -2.39 ± 0.05 ‰ to -0.13 ± 0.06 ‰ (2 SD). The isotopic composition of total dissolvable Fe (?56TDFe) was consistently heavier than dFe consistent with Fe oxyhydroxide precipitation as the plume ages. It is estimated that stable dFe exported from the plume will have a ?56dFe of -0.28 ‰, and this provides the first highly resolved constraint on hydrothermal plumes as a source of dFe isotopes to the ocean interior. This suggests that this distinctive isotope signature can be used to trace plume dFe inputs to the deep ocean. This will help constrain the impact of hydrothermal Fe on ocean biogeochemistry.

The rise of atmospheric carbon dioxide (CO2), due to decades of burning of fossil fuels, is a key driver of anthropogenic climate change. Carbon Capture and Storage (CCS) is one of the most promising mitigation strategies for long-term sequestration of CO2. Unlike most conventional CCS investigations targeting deep saline aquifers, this thesis focuses on the potential of the uppermost oceanic crust, inspired by the strong evidence that basaltic seafloor has acted, in the past, as a major sink for CO2. The study of temperature, pressure, and density of CO2 and seawater at the sediment-basement interface for the whole seafloor highlights the influence of water depth, sediment thickness, and oceanic crustal age on the relative gravitational stability of CO2. Consequently, 8% of the entire oceanic crust is recognised as suitable for gravitational and physical trapping of CO2 injected into the basement. Five potential targets are proposed, and even the smallest of these provides sufficient carbon dioxide sequestration capacity for the next centuries. Batch experiments on the mineral dissolution of submarine mafic rocks and ophiolitic gabbro, in CO2-rich solutions, contribute to improve the fundamental understanding of geochemical reactions at mid-ocean ridge flank temperatures (40 ?C). Concentrations of silicon and calcium in solution, and particle size are identified as the key factors to quantify the rock reactivity. Ca dissolution rates suggest calcite, plagioclase and amphibole are the principal sources of calcium at pH ~5. The attempted estimation of costs related to the transport and storage of 20 Mt/yr of CO2 in deep-sea basalts, as a function of distance from the shore, injection rate, and water depth, shows the economic feasibility of potential offshore CCS projects. Overall, the expenditures are dominated by the number of ships and wells required to deliver large volumes of CO2 to reservoirs located far from the coast, rather than by the water depth. These financial considerations could potentially improve if the CCS strategies conquered a significant place in the global market.

Without small oceanic organisms atmospheric CO2 levels would be about 200 ppm higher than they are today; phytoplankton convert dissolved inorganic carbon (DIC) to particulate organic carbon (POC) during photosynthesis, influencing the air-sea exchange of CO2. Eventually some of this POC is exported out of the upper ocean, often as either phytodetrital aggregates or zooplankton faecal pellets. Because of the complexity of this biological carbon pump (BCP), the fate of the exported POC in the mesopelagic zone is difficult to predict. To make things more complex all of these processes vary temporally and spatially. Marine snow catchers (MSCs) were used to analyse fast and slow sinking particles separately, which is a unique approach as slow sinking POC fluxes are not often quantified. To investigate what controls the fate of particles in the upper mesopelagic zone (50 - 500 m) particles were collected from three contrasting oceanic regions: the Southern Ocean (SO), Equatorial Tropical North Pacific (ETNP) oxygen minimum zone (OMZ) and the temperate North Atlantic. In all sampling areas the slow sinking POC flux was as large if not larger than the fast sinking POC flux. This emphasises the importance of slow sinking particles in the upper mesopelagic zone. The main outcome from this thesis is the importance of the role of zooplankton in BCP processes. For instance the efficiency which particles were exported from the mixed layer varied inversely with primary production in the SO, and was likely due to the zooplankton grazing down the phytoplankton. When extending the data to include the ETNP and the North Atlantic this relationship still held, conflicting the long-standing theory that as primary production increases export efficiency increases. In the ETNP oxygen minimum zone a high proportion of exported POC sank through the mesopelagic zone. Microbial oxygen uptake incubations showed for the first time that fast sinking particles are turned over significantly slower than slow sinking particles (0.13 d?1 and 5 d?1 respectively). Microbial degradation of POC could explain most of the fast sinking POC attenuation with depth, with the remainder lost due to abiotic fragmentation. Therefore it is likely that zooplankton degradation of particles is reduced in OMZs as their abundance and metabolism are lowered. This reduces the overall remineralistion of POC, hence a higher fraction of POC is transferred to depth in OMZs. Phytoplankton lipid biomarkers dominated lipid particle composition throughout the upper mesopelagic zone in the ETNP, further emphasising the minor role of zooplankton in OMZs. Comparing the observations with an ecosystem model output at all three oceanic sites further emphasised the importance of zooplankton in the BCP. The model poorly parameterises zooplankton processing of particles and thus the observations and model matched best in the ETNP, where zooplankton processing of particles is naturally low. Changes in climate will effect the abundance and distribution of these small organisms. Further understanding of how zooplankton community structure and metabolism may change in the future will be important to predict how atmosphericCO2 levels may change.

Marine hydrates, which lock-up vast quantities of methane, are considered to be a prospective alternative energy source, a slow tipping point in the global carbon cycle and a probable trigger for submarine failures. In this thesis marine hydrate systems offshore of Mauritania and associated structural and sedimentary features are investigated by utilising two surveys of high-quality three-dimensional (3-D) seismic data. Interpreting them provides new insights into marine hydrate systems and how they respond to changes in ambient conditions. In one region of one of the 3-D seismic surveys, a shear zone covering 50 km2 is identified immediately above the hydrate bottom simulating reflector (BSR). It is considered to be the initial stages of a failure that did not result in widescale downslope transport of the succession. Due to this failure not going to completion, some free gas remains trapped at the level of the BSR. At this level the presence of free gases is supported by the continuous high-amplitude reflections. It is proposed that buoyancy built up by the inter-connected gas accumulation increases the pore pressure of the overlying hydrate-bearing to the level such that its base was critically stressed. In this research there is no seismic evidence for failures triggered by hydrate dissociation but the role of free gas in priming submarine failures is examined. Whether marine hydrates can release significant amounts of methane into the atmosphere is inconclusive. In this research a proposed model indicates that methane was re-captured in the hydrate stability zone after being liberated. Ocean warming since the last glacial maximum (LGM) gave rise to the shoaling of the base of the hydrate stability zone (HSZ). Gases released from hydrate accumulating at the base entered the HSZ, driven by buoyancy built up in the gas accumulation. The hydrate seal was breached and this is manifested by 15 gas chimneys in seismic data. Hydrates then re-formed at a specific level within the HSZ. This study implies that not all of methane would enter the ocean after released from hydrates and therefore the contribution of marine hydrates to the atmospheric methane budget may be not that much as it was predicted before. Gas venting is an effective way to transport methane at depth vertically to the ocean and an example of it is found in the feather edge of marine hydrate. This venting was possible due to the presence of faults above a salt diapir and is manifested by a series of pockmarks and mounds at the seabed. The BSR at this site is convex upwards and hence formed a trapping geometry for underlying free gases. Numerical model shows that this up-convex geometry is caused by the salt diapir having a higher thermal conductivity. Permeable migration conduits along the faults and excess pore pressure at the top of the trap allow for the happening of the venting. Compared with the neighbouring area where the BSR can be well observed, the region affected by diapirism has a limited scale of the observable BSR. This absence is proposed to result from the formed trap intercepting methane-rich pore fluid that would migrate landwards along the level of the base of the HSZ.

Human activities, including intensive agriculture and fertiliser use, are altering the radiative forcing on the planet by increasing the amount of the climatically active gases, nitrous oxide (N2O) and methane (CH4), in the troposphere. There are currently uncertainties in quantifying the natural sources and sinks of N2O and CH4 in the marine environment. This thesis presents high-­‐resolution surface-­‐ocean and atmospheric data for N2O, CH4, CO2 and CO, collected using Integrated Cavity Output Spectrometry (ICOS, Los Gatos) during the D366 (NW European shelf seas, June/ July, 2011) and Atlantic Meridional Transect 22 (Atlantic Ocean, October/November, 2012) cruises. Interpretation of the N2O and CH4 datasets revealed that shelf seas produced a greater atmospheric source of N2O and CH4 than the open ocean. Shelf sea surface waters were slightly supersaturated in N2O at 102 %, but undersaturated in the open ocean at 97 %, acting as slight atmospheric sink. The cause of undersaturation in surface waters is unclear, but may be attributed to seasonal cooling and a potential microbial N2O consumption pathway. The dominant pathway for N2O formation in the open ocean is nitrification. Both nitrification and denitrification may be important in the production of N2O in shelf seas. Vertical mixing processes and upwelling influence the surface concentration of N2O. Surface waters in the shelf seas and the Atlantic Ocean were supersaturated in CH4 at 118 % and 107 %. Methanogenesis, which occurs within anoxic sediments of shelf seas, rivers and estuaries, was the dominant source of CH4 during D366. Freshwater inputs that were supersaturated in CH4, and a fully mixed water column vertically mixing CH4 from marine sediments, influenced the surface concentration of CH4 in shelf seas. In the open ocean, methanogenesis occurs in-­‐situ in the surface waters. Seasons influenced the surface concentration and saturation of N2O and CH4, with sea surface temperatures, seasonal effects, and stratification affecting these gases.

Submarine canyons are common bathymetric features incising the shelf edge and are known to trap and focus internal waves leading to high levels of turbulent mixing. Whittard Canyon, located at the Celtic Sea shelf edge, is a dendritic canyon where little is known about the internal tide, yet where it is postulated to have a huge impact on biology within the canyon and also play a role in the generation of nepheloid layers. High-resolution simulations of the M2 tide in Whittard Canyon using a modified version of the Princeton Ocean Model are used to determine the generation, propogation, spatial structure and dissipation of the internal tide within the canyon. Shamrock canyon and Brenot Spur are identified as key remote sources of internal tide generation, which modulate local generation in a flux-conversion feedback mechanism which causes the observed assymmetry in barotropic-tobaroclinic conversion within the canyon limbs. Depth-integrated baroclinic energy flux within the canyon is elevated, but variably so in different limbs, with values reaching >8 kW m¡1. The eastern limb of the canyon is notable for being particularly energetic. Enhancement of near-bottom baroclinic tidal currents are seen within the canyon with velocities reaching 0.4 m s¡1. The three-dimensional structure exhibits bottom intensification due to topographic focusing by the steep canyon walls, and the dominantly supercritical limb heads. Within the upper canyon the internal tide exhibits a typical mode-1 structure. Cores of baroclinic energy flux, in a dominantly up-canyon direction, form over the depth range of 1000-2500 m and are correlated with potential source regions for nepheloid layers. The sensitivity of the model to bathymetric resolution is tested and it is found that using 500 m resolution bathymetry results in domain-averaged conversion rates higher than for the smoothed bathymetries tested, highlighting the need for high-quality, high-resolution bathymetric datasets.