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Hydrothermal circulation is a fundamental process in the formation and aging of the ocean crust, influencing its structure, physical and chemical properties, and the composition of the oceans and the mantle. The impact of hydrothermal circulation on mid-ocean ridge processes depends on the composition and volume of circulating hydrothermal fluids, and the extent of partitioning between high temperature axial- and low temperature ridge flank- systems, but these processes remain poorly constrained. This study uses whole rock and secondary mineral chemistries of altered ocean crust to (i) assess the extent of fluid-rock exchange during hydrothermal circulation, and (ii) determine the compositions of axial and ridge flank hydrothermal fluids. Sub Antarctic Macquarie Island is a unique sub-aerial exposure of a complete section of ocean crust in the ocean basin in which it formed. Sr and O isotope analyses from Macquarie Island, combined with stratigraphic reconstructions provide the first isotopic profiles through a complete section of normal ocean crust. Tracer transport mass balance calculations indicate that a timeintegrated fluid flux of 4 ± 1 x 106 kg/m2 is required to produce the observed shift in Sr-isotopic composition. This can be supported by the available mid-ocean ridge magmatic heat and is similar to estimates for sections of in situ ocean crust, but a factor of 10 lower than estimates for ophiolites indicating a fundamental difference between the hydrothermal cooling of mid-ocean ridge and supra-subduction zone ocean crust. Heat flow studies indicate that hydrothermal circulation persists for tens of millions on the ridge flanks, with approximately two-thirds of hydrothermal heat loss occurring off-axis at significantly lower-temperatures than in axial hydrothermal systems. Consequently a much larger volume of fluid is required and only small deviations in fluid compositions may result in significant contributions to ocean chemical budgets. Direct sampling of in situ basement fluids is extremely difficult, and can only be applied to active systems. Here, methods to calculate the compositions of ridge flank fluids from the compositions of secondary mineral precipitates are presented and applied to basalt-hosted calcium carbonate veins. Veins from the eastern flank of the Juan de Fuca Ridge record a temperature dependent fluid evolution, similar to that of near-basement pore fluids sampled by borehole studies. Carbonate veins from the Juan de Fuca Ridge and Ocean Drilling Program Site 1256 record a sufficient decrease in the fluid Sr-isotopic composition with temperature to balance the global ocean Sr budget, however, this result cannot be reconciled with the observation of Davis et al. (2003) that the studied ocean crust has exchanged insufficient Sr with the oceans to balance the global Sr budget. This suggests that these areas cannot be typical of the ocean crust as a whole.

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Effects of large-scale topographic features on the benthic boundary layer (BBL) flow characteristics and on sediment transport are analysed in a natural shallow coastal site and in a laboratory flume. The main signature of flow/bedform interaction processes consists of largescale flow structures, generated downstream of the bedforms; these are characterised by alternating modules of lower and higher streamwise speeds than the mean flow. These structures modify the suspended sediment transport, generating high turbidity levels within macro-scale features of low flow speed, and the bedload transport, generating more frequent and larger bedload movements during macro-scale features of high flow speeds. These particularities result from variations in the turbulent bursting process within the macroturbulence structures. Ejection events generate increases in suspended sediment concentration; these are more frequent and more intense within the macro-scale structures of low flow speed. Sweeps are shown to generate most of the bedload transport; these events are dominant and are magnified during modules of increasing horizontal velocity. Enhanced (negative) contribution of outward and inward interaction events to the Reynolds stress, downstream of large-scale bedforms, compared to those reported for uniform BBLs, are observed in the field and laboratory investigations. This induces 'abnormally' low Reynolds stress values, which are believed to invalidate the use of the Reynolds stress parameter to predict sediment transport. Other aspects of the turbulence are little affected by the presence of bedforms upstream, such as the coupling between the turbulent velocities and the Reynolds stress, or, at the field site, the turbulent energy dissipation rate.

The Lumwana Cu±Co deposits Malundwe and Chimiwungo are examples of pre-Katangan mineralized basement that are located in the Domes Region of the Lufilian Arc, an arcuate North neo-Proterozoic fold belt, which hosts the Zambian and Congolese deposits that make up the Central African Copperbelt. The Lumwana deposits are situated within the Mwombezhi Dome; a Mesoproterozoic basement inlier consisting of highly sheared amphibolite grade schist to gneiss units that host the Cu±Co mineralization. Kinematic indicators such as s-c fabrics and pressure shadows on porphyroblasts suggest a top to the North shear sense. Peak metamorphism of 750ºC ± 25ºC and 13 ± 1 Kb indicated by whiteschist assemblages occurred during the Lufilian Orogeny at ~530Ma, with burial depths of ~50km. A major decollement separates the high pressure mineral assemblages of the basement from the lower pressure mineral assemblages of the overlying Katangan Supergroup. The age range and lithologies of the pre-Katangan basement of the Domes Region is similar to the pre-Katangan basement of the Kafue Anticline, which underlies the neo-Proterozoic Zambian Copperbelt deposits situated 220km to the SW. The origin of the protolith to the mineralization is ambiguous at Lumwana with transitional contacts from unmineralized quartz-feldspar±phlogopite basement gneiss to Cu±Co mineralized quartz-phlogopite-muscovite-kyanite-sulphide Ore Schist. The transitional contacts and structural controls on mineralization has led to the hypothesis that these deposits represent metasomatically altered, mineralized and sheared basement, rather than mineralized neo-Proterozoic sediments with amphibolite grade metamorphism. This hypothesis is supported by petrographic analysis, stable isotope analysis (δ34S), whole rock geochemistry, and electron microprobe analysis of ore and host rock assemblages. The transitional contacts observed at Lumwana are due to an alteration event associated with mineralization that removed feldspar from ore horizons resulting in depleted Na and Ca and relatively higher Al components. Sulphides are deformed by the S1 fabric and overprinted by kyanite which formed at peak metamorphism. This indicates that copper was introduced to the basement either syn or pre-peak metamorphism. Post S1 metamorphism with associated quartz-muscovite alteration has remobilized sulphides into low strain zones and pressure shadows around porphyroblasts. δ34SSULPHIDES give values of +2.3 to +18.5‰ that fall within the range of values observed in the Copperbelt of -17 to +23‰. The mechanism of ore formation at Lumwana was dominated by thermochemical sulphate reduction (TSR), indicated by the relatively heavy δ34S values and the absence of the light bacteriogenic δ34S values observed in the Copperbelt. Electron microprobe data of muscovite, phlogopite and chlorite show little variation between early and late mineral phases indicating that metamorphic homogenization of silicate assemblages occurred. The Lumwana deposits are large mineralized shear zones within the pre-Katangan basement. Various styles of basement mineralization are also observed in the Kafue Anticline and the structural controls on mineralization and lithological similarities to the Lumwana deposits suggests that pre-Katangan basement is a viable source for the Cu-Co budget of the Central African Copperbelt and that basement structures had a key role in its formation.

The aim of this study is to better understand chemical recycling at destructive plate boundaries. Isotopic, major and trace element data of lavas sampled at varying distances from the trench in the Izu-Bonin arc provide constraints on the different types of flux that influence subduction zone magmas. Geochemical models are employed to quantify the exchange of elements between flux and mantle that lead to the creation of arc magmas. These calculations allow the changes in fluid flux from the slab and the variations of mantle composition to be correlated with the variations in across-arc geochemistry. All of the analysed lavas show arc-related geochemical signatures such as enriched LILEs, depleted HFSEs and fluid-modified strontium isotopes. However, a systematic variation in isotope and trace element ratios is recognised across the Izu-Bonin arc. Radiogenic 87Sr/86Sr and 143Nd/144Nd, low Nb/Y and high Ba/La in the volcanic front suggests melting of a depleted mantle source influenced by an aqueous fluid (AQF). Less radiogenic 87Sr/86Sr and 143Nd/144Nd and higher Th/Ce and Nb/Y in the rear and back arc suggest melting of a more fertile mantle metasomatised by supercritical fluids (SCF). Positive correlations between 87Sr/86Sr and 143Nd/144Nd are observed in lavas sampled across the width of arc. The correlation is also found within some volcanoes found behind the volcanic front (Taga and Hakone). This suggests that magmas affected by AQF and by SCF are tapped by single volcanoes and at least partially mixed prior to eruption. Analysis of chemostratigraphic variations in the Ashtaka, Taga, Hakone, Hachijojima, and Oshima volcanoes reveal that the fluxes are not in a steady state. Individual volcanoes can tap heterogeneous mantle sources and variable fluid input over tens of ka’s. High-precision double-spike Pb isotope measurements provide estimates of the amount of overprinting of the slab fluid isotope signature by the mantle wedge. The effect is greatest along the volcanic front where 70% of the Pb is derived from slab fluids compared to 34% in the back arc. Pb isotopes also highlight that the mantle source is heterogeneous along the arc with Δ8/4Pb and Δ7/4Pb increasing toward the north. The composition of the crust beneath the arc is estimated using the Tanzawa plutonic complex, and is found to be isotopically similar to the northern Izu-Bonin arc. This suggests that crustal contamination will only have a minor effect on the isotopic composition of the arc. The isotopic composition of Tanzawa complex indicates a source resembling the Indian Ocean mantle domain was present in the mantle wedge before 7Ma.

The Controlled Source ElectroMagnetic (CSEM) method has been developed over the past two decades, to provide information on oceanic crustal structure. The stark contrast between the low resistivity of rock structure saturated with salt water, compared to the high resistivity of oil, has meant that the method is increasingly being adopted by the hydrocarbon industry at the exploration stage. During initial assessment, and subsequent development of oil and gas fields, wells are routinely drilled for a variety of purposes. The boreholes provide a possible means of placing either EMsources or receivers within or beneath the target reservoir. This in turn presents the opportunity of applying CSEM methods, using a combination of seafloor and borehole sources and receivers to improve the characterisation and monitoring of the reservoir. In this thesis, forward modelling is used to test out the various survey configurations, in terms of the improvement Borehole CSEM (BCSEM) affords the appraisal and monitoring of hydrocarbon reserves. The results from 1D modelling have shown that the use of downhole instruments (either source or receivers) increased the amplitude anomalies associated with target layers when compared to conventional CSEM. The edge detection capabilities of BCSEM were tested and it was found that a single downhole receiver and a towed seafloor source produced a significant change in the amplitude of the electric field as the source moved over the edge of the modelled reservoir. The method also returned promising results for 3D bodies, showing sensitivity to small structures that are below the detection threshold for conventional CSEM. There is evidence to support that BCSEM could prove to be a useful 4D tool in monitoring reservoir changes during production. The modelling of a depleting anticline reservoir showed that the anomalies associated with the depletion are at potentially detectable levels.

This study examines the geochemistry of salt marshes and intertidal sediments from the Solent estuarine system, southern England. Micro-, meso- and macro-scale studies have been carried out to assess the geochemical processes operating in these intertidal sediments and the use of radionuclides as tracers of sedimentary processes in the Solent area. Measurement of 210Pbxs, 137Cs and 60Co provides valuable information on sedimentary processes occurring in the Solent estuarine system. Subsurface maxima in activity are observed for 137Cs and 60Co (after normalisation to Al) which can be related to historical input, providing dated sediment horizons. This has enabled determination of Recent sediment accumulation rates and reconstruction of historical pollution records. Sediment accumulation rates obtained using 210PbXS/ l37Cs and 60Co in salt marsh environments provide an indication of the rate of Recent sea level rise, estimated as 4 mm y-1. This is in broad agreement with tide gauge data and when compared with longer term archaeological and radiochronological data may indicate an increase in the rate of mean sea level rise over the last 1000 years. Salt marsh areas with accumulation rates lower than 4 mm y 1 (from 137Cs and 60Co dating) are degenerating (undergoing die-back) indicating that they are failing to keep pace with the rising sea level. While local (intra-estuary) effects influence rates of sediment accumulation on a particular salt marsh the main control of sediment accretion over the whole Solent area is a rise in mean sea level. Dating of salt marsh sediment cores has allowed assessment of the historical record of anthropogenic pollutant input to the Solent (in particular for Pb and Cu). Pb isotope studies of salt marsh sediments from Southampton Water indicate increasing importance of anthropogenic Pb (from Precambrian ores) since the beginning of this century. A very prominent transient spike due to an anthropogenic Pb input is found in salt marsh sediments from the relatively unpolluted Beaulieu estuary. This may be related to military marshalling in the Beaulieu river before the Allied D-Day landings in 1944. Anthropogenic Cu is present in the Solent region mainly due to discharge from the Esso refinery at Fawley, Southampton Water. Analysis of Cu fluxes has shown that discharges peaked in ca.1970 and have significantly reduced since 1971 (due to effluent clean-up by the refinery). The reliable use of radionuclides as indicators of sediment accumulation depends on their chemical immobility (lack of redox migration). 60Co shows evidence for early diagenetic remobilisation at two salt marsh sites, and 2iopb m a v a i s o be mobile in some salt marsh areas. Remobilisation of 210Pb is apparently localised and may be aided by flooding/drainage cycles in the marsh. The use of these radionuclides as tracers of sediment accumulation in areas where significant early diagenetic remobilisation occurs is problematic. Where possible a number of chemically different radionuclides should be used to eliminate bias in calculated sediment accumulation rates due to redox mobilisation or diffusive processes. Detailed geochemical studies can assist in determining whether redox movement of radionuclides is occurring. A critical examination of the geochemical behaviour of both the stable counterparts of radionuclides and redox-sensitive elements is required if realistic sediment accumulation rates are to be obtained.

This research presents a detailed study of the planktonic foraminiferal records of eleven sediment cores taken from a west-east transect in the Mediterranean Sea. Correlations are based on biostratigraphy, oxygen isotope stratigraphy and assisted by AMS 14C dating. This study assesses the potential to define a foraminiferal biostratigraphy of basin-wide validity. Principal Component Analysis (PCA) is used to determine variabilities in the planktonic foraminiferal records. The first principal component groups species on its positive and negative sides in such a way that this axis may be interpreted as an expression of SST variations. This interpretation is corroborated by its close similarity to oxygen isotope records. Mean PCA scores confirm previous observations that the temperature gradient in the eastern Mediterranean basin follows the same eastward increasing trend at glacial times as it does today. In contrast, the inferred sea surface temperatures (SST) from the western Mediterranean basin suggest a reversed gradient compared to the present. Preliminary results are discussed from the unprecedented high resolution marine records from two eastern Mediterranean cores. Based on PCA and planktonic foraminiferal ratios of warm, oligotrophic mixed layer species relative to cool, more eutrophic species, a series of cool episodes is observed throughout the Holocene. The cyclicity of these events is calculated at approximately 1300 years, a figure comparable with Holocene climate fluctuations recognised previously in the North Atlantic Ocean and the Greenland Ice Sheet. Micropalaeontological investigation of two western Mediterranean cores shows for the first time that abrupt cold spells, associated with Atlantic Heinrich-events, affected the Mediterranean Sea. Unusually high abundances of the subpolar species N. pachyderma (leftcoiling) in the Gulf of Lions reflect a thriving of a normally rare taxon in the western Mediterranean, in response to distinct, short term, episodes of favourable habitat development.

No description available.