Following a detailed review of the history and theory of sea ice modelling in the Arctic, a further review is made of previous analysis of the ice thickness distribution. In addition to this, ten new or partially cited upward looking sonar ice draft datasets derived from moored buoys and submarine profiles have been used to provide a detailed set of statistics for comparison with modelled ice thickness distribution functions in the Greenland Sea. The moored sonar buoys datasets are from the period 1991-1994 and the submarine datasets are from the spring seasons of 1987 and 1991. Suitable atmospheric and oceanic forcing fields for 1991-94 have been derived from UK Meteorological Office unified model simulations and analysis and verified for a sea ice model of the Arctic Ocean and Greenland Seas. The model represents the sea ice thickness distribution using 28 levels of ice after Flato and Hibler [1995]. The use of synoptic daily varying winds is shown to create coastal polynyas and the application of spatially and temporally varying precipitation fall generated a realistic snow cover. An extensive comparison of modelled and observed ice thickness distributions is presented for the Greenland Sea. Ice fluxes for Fram Strait have been produced using a variety of methods using data from moored upward looking sonar thickness distributions, SSM/I ice concentrations and International Arctic Buoy Programme derived drift data. Results show that it is important to include the variation in the thickness and velocity fields both spatially and temporally across Fram Strait in the calculation of ice fluxes. The modelled variation in the velocities, thickness distributions and the ice fluxes in Fram Strait compare well with the observations although the absolute value of the fluxes are too low due to excessive winter oceanic heat fluxes. When the observed fluxes were used in a boundary-forced Greenland Sea regional model, it was shown that the mechanisms that preferentially melt ridged ice types not present in the model but the decay of the mean thickness of the ice downstream can be represented. Overall, the model results emphasise the need for further work on dynamic-thermodynamic coupling in ice models to resolve these processes.

A series of theoretical models are developed in order to constrain the fundamental underlying controls on low-temperature hydrothermal circulation in seafloor venting systems. In terms of discharge, results of these models predict that (i) some diffuse flow will always exit permeable sulphide mounds or oceanic crust in the vicinity of black smoker vents, (ii) diffuse flow may also occur in the absence of seafloor black smoker venting and (iii) anticorrelated variations in diffuse flow temperature and vertical velocity previously recorded at a sulphide mound on the Juan de Fuca Ridge may have been caused by variations in either the isotropic or vertical permeability of the mound. In terms of recharge, model results predict that the volume flux of seawater entrained into seafloor hydrothermal systems and subsequently discharged as diffuse flow appears to primarily depend on the permeability of the system and on the circulation depth of the entrained seawater. Furthermore, the principal location of such entrainment along the surface of a sulphide mound is determined, to a large extent, by the permeability structure and aspect ratio of the mound. Hydrothermal flow models of subsurface mixing between black smoker fluid and seawater predict that diffuse flow evolves towards a white smoker type fluid as mineral precipitation reduces the permeability of the system. Conversely, low-temperature hydrothermal fluids exiting such systems are predicted to evolve towards a diffuse flow type fluid following permeability-enhancing events. Furthermore, results from sulphide mound models suggest that anhydrite precipitation resulting from low-temperature hydrothermal circulation may act as a self-promoting process.

Fluid flow through mid-ocean ridge <i>axes</i> has long been recognised by geophysical and geochemical methods as well as direct observations. However, hydrothermal circulation through mid-ocean ridge <i>flanks</i> is more difficult to study and quantify due to the lower temperatures and less extreme fluid chemistry. In this thesis, a region of low-temperature fluid flow (up to 65°C), on the eastern flank of the Juan de Fuca Ridge, N.E. Pacific Ocean, has been investigated using isotopic and geochemical tracers. Pore water samples contained within sediment cores were collected at outcrops and subcrops of igneous basement where over-pressurised basement fluid upwells through thin sediment cover into the overlying ocean. Strongly upwelling pore waters are representative of basement fluid and therefore may be geochemically and isotopically analysed to understand fluid flow within igneous basement. Initially, two methods were set up to analyse Li and its isotopes in seawater-derived fluids. Then, fluids from two areas of the Juan de Fuca Ridge flank (First and Second Ridge) were investigated using the methods developed and other standard techniques (geochemical analysis, radiocarbon dating, Sr isotopes and O isotopes). Geochemical profiles of Ca, Mg and for some sites, F, were modelled to constrain the vertical fluid flow at thinly sedimented basement outcrops and subcrops. This showed that upwelling of hydrothermal fluid from igneous basement is observed throughout the study area, while downwelling of seawater is less frequent and only occurs within Second Ridge Area. Radiocarbon age measurements show large variation across the study area suggesting that fluids sampled at different outcrops are not connected and may represent discrete systems.

No description available.

This thesis examines the flow of sea water in an embayment or coastal bay. The embayment is a relatively quiescent environment, separated from the external flow by a mixing or shear layer. The external flow is tidal and therefore flows back and forth across the embayment mouth. The exchange between the embayment and external flow is an important mechanism for the transport and dispersion of pollutants, and also substances such as nutrients and sediments. Entrapment, where pollutants are retained in the embayment for some time, is of particular interest. Understanding the flow patterns in and around an embayment may enable the prediction of substance transport and dispersion in its locality. The flow in an embayment was investigated in three ways. Fieldwork was undertaken in a coastal bay at Blackpool Sands (Devon, UK) in which measurements of the flow velocities were taken using an acoustic Doppler current profiler. Laboratory experiments were designed and conducted in a bi-directional flume tank where the effects of varying embayment and tidal parameters were investigated.  Finally, a numerical streamfunction-vorticity model with a second-order advection scheme was constructed to attempt to replicate the laboratory experiments. Measurements of the flow velocities in Blackpool Sands embayment indicated the presence of a gyre caused by the tidal streams which separate at the upstream headland. This gyre was present on both the ebb and flood tides and was found to be approximately two-dimensional. Laboratory experiments demonstrated the life-cycle of a gyre in a rectangular embayment with an oscillating freestream flow across the embayment mouth. The gyre was observed to be swept out of the bay when the tide changed direction and a dipole was formed as the gyre was swept past the downstream headland, where the flow separated. The numerical simulation of the laboratory experiments showed good agreement with the laboratory experiment and fieldwork measurements.

A variety of ice core and marine records have demonstrated regular climatic shifts in atmospheric and ocean temperature and nutrient content between glacial and interglacial periods. Reconstructions of Antarctic Intermediate Water (AAIW) have primarily focused upon the changes in the Atlantic, yet AAIW flows northward throughout the world’s oceans after forming in subantarctic waters in the Southern Ocean. The trace metal content of benthic foraminifera is commonly used to reconstruct changes in the nutrient content and temperature of waters. Trace metal incorporation into foraminiferal tests is usually controlled by environmental parameters. This proves not to be the case in the aragonitic foraminifera <i>Hoeglundina elegans. </i>Multi-element analysis demonstrates the covariance of unrelated proxies, suggesting there is an internal control upon trace metal incorporation. Calculating changes in the nutrient content and temperature of AAIW was accomplished by measuring calcitic foraminifera from core MD97-2120 located on Chatham Rise, East of New Zealand. AAIW is warmer and more nutrient rich during interglacial periods. Combining the nutrient data with previously published stable isotopic data from the same core allows estimation of changes in salinity and the average windspeed in the region of AAIW formation. During glacial times temperature decreased and salinity increased as a result of the incorporation of freshwater into continental ice caps. Nutrient levels decreased, as did the average wind speed. These changes were a result of the motion of the Westerlies away from the subantarctic and an increase in levels of export production in the region of AAIW formation. This reduced the preformed nutrient content of AAIW. AAIW also displays Heinrich Event signatures, which are resolvable as shifts related to the motion of the glacial Westerlies causing a change in the measured carbon isotopic signature.

This research focuses on the uncertainties associated with numerical modelling of extreme coastal flooding. It uses Canvey Island in the Thames estuary (UK) as a case study, where a linked storm surge and flood inundation model is developed and applied within an uncertainty framework. The case study is used to illustrate and evaluate the propagation of uncertainties from model inputs to model outputs as a function of the ‘boundary conditions’ encountered (input data, geography of the case study). The thesis is separated into six chapters, of which two provide an introduction to the research (<i>Chapters 1</i> and <i>2</i>), two focus on model development (<i>Chapters 3 </i>and <i>4</i>) and two address model application and evaluation (<i>Chapters 5 </i>and <i>6</i>). <i>Chapter 1</i> provides a theoretical context for the research and outlines the aims and objectives of the thesis. The research adopts a ‘critical realist’ perspective on the value of models as tools for probing understanding about uncertain coastal flood hazards. <i>Chapter 2</i> provides a description of the case study site and discusses its illustrative value from a flood risk perspective. <i>Chapter 3</i> describes the development of a storm surge model for predicting water levels at Canvey Island in response to meteorological and tidal forcing of the North Sea. The storm surge model is based on four nested models, which provide a consistent increase in spatial resolution towards the case study site. The predicted water levels are used to drive a flood inundation model of Canvey Island, which is described in <i>Chapter 4</i>. The flood model is based on a 2-D, shock capturing, numerical scheme, and resolves the terrain and buildings of the study area with high-resolution topographic data. <i>Chapter 5</i> describes the application of these models within an uncertainty framework where model sensitivities and uncertainties are evaluated for a range of storm forcing conditions over the North Sea and sea defence failures at Canvey Island.

The thesis aims to describe developments in the uses of planktonic foraminiferal calcite for oceanographic applications, specifically Mg/Ca ratios and foraminiferal shell weights as proxies for calcification temperature and paleo-<i>p</i>CO₂ respectively. Sample preparation for the analysis of Mg/Ca and Sr/Ca ratios in foraminiferal calcite is investigated with the aim of defining a method that will give reproducible results and minimise signal contaminations. Each step of the cleaning procedure is scrutinised in order to gauge which are most important and which may be omitted as unnecessary and potentially detrimental to the elemental measurements being made. Success in the application of Mg/Ca-thermometry depends on the ability to quantify and compensate for any alteration of the primary signal after deposition. Compositional variations in foraminiferal calcite may be associated with partial dissolution at the seaflow. Investigation is made into the excessive scatter observed within a latitudinal transect of core-top Mg/Ca and Sr/Ca ratios and attributed to partial dissolution. Possible means of correcting or minimising the effects of such alteration are investigated and assessed. Recent interest in planktonic foraminiferal shell weight loss as a proxy for dissolution, and as such a potential means of quantifying compositional variability, demands investigation into the possibility that initial shell weight may not be constant. It is demonstrated that considerable variability does occur in the shell weights of several species of planktonic foraminifera in the modern surface ocean. It is proposed that initial shell weight is a function of calcification rate and controlled ultimately by the carbonate ion concentration, [CO₃⁼], of ambient seawater during calcification. A first attempt is made to calibrate the effects of [CO₃⁼] on shell weight in the modern ocean. The potential effects of anthropogenic atmospheric CO₂ increases on marine calcification are then synthesised using the mineral relations between [CO₃⁼] and calcification rate.

This thesis is concerned primarily with the relationship between oxygen concentrations and benthic fauna on organic matter (OM) cycling at the Pakistan Margin (PM) in the Arabian Sea. The approach involved the analysis of lipids and stable carbon and nitrogen isotopes insediments and benthic fauna. Five sites (140, 300, 940, 1200 and 1850 m) were chosen; above, in the core and below the oxygen minimum zone (OMZ), which was defined by oxygen concentrations being < 0.5 mL Lot at the PM. Samples were collected on two pairs of cruises in order to assess temporal variability in OM and the response of the benthic fauna to the biogenic flux during the presumed low flux the inter-.monsoon period (IMP: MarchMay 2003) and the high flux late-monsoon period (LMP: August-October 2003). Lipid distributions of sediments were highly complex and predominantly marine in origin. Little temporal or spatial variability was noted, suggesting that oxygen is not the • primary controlling factor in lipid biogeochemistry and therefore in surficial sediment OM quality. In order to assess the nutritional quality of the sedimentary OM at the PM a food quality index (FQI) was derived using the lipid data set. The FQI of surficial sediments at the PM was compared to other previously published data sets for particulate OM (PaM). This led to the conclusion that PM surficial sediments had a relatively high nutritional value for the benthic fauna. There was little difference in the FQI of sinking paM collected from the Oman Margin at 500 and 950 m, suggesting that this reflected reworking ofOM in the surface waters of the Arabian Sea. Stable isotope data of surficial sediments supported lipid data, displaying little temporal or spatial variability. Stable isotopic compositions of benthic macrofauna displayed no temporal variability except in the case of one polychaete, Linopherus sp. Temporal variability was noted in 5 species of benthic megafauna suggesting changes in feeding mode and possibly selective ingestion/assimilation of certain components of OM. Benthic fauna generally became enriched in t3C and t5N downslope; application of the trophic level model demonstrated that feeding guilds changed downslope in response to oxygen concentrations, with more surface deposit feeding taxa in the OMZ and more predatory/scavenging taxa below the OMZ. Distributions of lipids in benthic fauna were complex, but provided insight into food sources for the benthic fauna. Strong temporal variability in lipid distributions of certain species e.g. Astropecten sp., Amphiura sp. and a natant decapod suggested that these fauna spawned in concurrence with the biogenic flux. Lipid distributions of the polychaete Linop/zerus sp. displayed spatial variability across the lower OMZ boundary (800 to 940 m). This may be a result of spawning, or a response to oxygen concentrations, as juveniles appeared to inhabit the more hypoxic regions in order to build up lipid reserves.

No description available.