This PhD thesis is about understanding some aspects of the dynamics of the ocean surface mixed layer by means of satellite Sea Surface Temperature (SST) measurements. The focus is on surface density fronts which are used as a measure of the dynamical activity at the meso- and submesoscale. A review of the current knowledge on the non-linear physical mechanisms that occur in the vicinity of fronts is presented in Chapter 2. Chapter 3 characterizes the ability of two algorithms for detecting fronts of various sizes and strengths that are embedded in a complex turbulent flow and sampled on noisy two-dimensional images. Chapter 4 explores and compares the performances of several new multi-sensor Level-4 SST products in resolving the small scale gradients. These Group for High Resolution Sea Surface Temperature (GHRSST) data sets offer an unprecedented spatial and temporal SST coverage. Their production however involves a variable and partially unknown level of smoothing which “hides” some of the small scale variability. In Chapter 5, a robust, flexible, automatic and optimized Matlab-based methodology for detecting fronts on SST images and calculating a frontal index is presented. A basic frontal index capturing the frontal length and strength is exploited to quantify the spatial scales present in the various Level-4 SST products. More advanced frontal indexes based on physical oceanography results by others are constructed in order to estimate vertical exchanges occurring at fronts from their signature on the SST. These new frontal indexes, which characterize fronts according to their dynamical significance, allow the quantification of the upwelling, subduction and restratification associated with frontal submesoscale processes. Finally, the spatial and temporal variability of ocean fronts is explored in order to determine their sensitivity to climatic signals.

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This thesis develops and applies novel methods to understand water mass variability and change in the global ocean. A water mass framework is developed that determines the diathermal and diahaline transformations of water between water mass classes from the temporal variations in their volumetric distribution. Water masses are defined in terms of their temperature and salinity. This reveals the roles of air--sea fluxes, mixing and transport changes. The second chapter explores the drivers of interannual variability in the overturning circulation in the North Atlantic subtropical gyre using the water mass framework. Variations in the volumetric distribution of water masses reveal that transport anomalies at the gyre boundaries predominantly set the volume and heat budget and that these transport anomalies are governed by Ekman pumping over the gyre. In the third and fourth chapters of this thesis the water mass framework is applied to observations of temperature and salinity in the Southern Ocean. Seasonal variations in the distribution of water masses reveal the imprint of the Southern Ocean overturning. This highlights the importance of seasonally varying air-sea fluxes in the formation of intermediate water at the expense of deep water, winter water and surface water. This reveals a diabatic pathway for the upwelling and conversion of deep water into intermediate water. Deep water is first cooled and freshened during the winter by mixing with overlying winter water triggered by a cabbeling instability. Sea ice-melt and surface heating then warm and freshen this seasonally formed water mass to create intermediate water during the summer months. These results suggest that the process of cabbeling could be a rate determining step in the global overturning circulation and the upwelling of deep waters. The fifth chapter of this thesis explores an alternative method to determine a volumetric distribution using individual Argo profiles. The volumetric distribution determined using this profile based estimate is compared to the distribution calculated using a geographically interpolated dataset. This comparison reveals that the interpolation scheme used to geographically grid Argo appears to artificially mix water masses toward the centre of the distribution.

La présence des perturbateurs endocriniens dans le milieu aquatique a suscité des préoccupations croissantes en raison de leurs effets néfastes potentiels sur les écosystèmes et les humains. Celles-ci sont généralement présents à l’état de trace (ng/L), dans des matrices aqueuses complexes, telles que les eaux de surfaces ce qui rend l’analyse difficile. Dans ce travail, on a mis au point une méthode analytique pour la détermination simultanée de 13 substances perturbatrices endocriniennes (PEs) dans l’eau, dont 5 estrogènes stéroïdes, 1 progestérone, 1 androgène et 6 composées phénoliques, , par la chromatographie gazeuse couplée à la spectrométrie de masse (GC-MS). La méthode a été validée. Les limites de quantification étaient respectivement de 1 ng/L et de 5 à 50 ng/L pour les composés phénoliques et les hormones. La méthode mise au point a été appliquée pour évaluer le niveau de contamination des PEs ciblées dans de nombreux sites du Nord de la France et de la Tunisie. La majorité des composés considérés ont été détectés dans les sites d'échantillonnage. La dernière partie de ce travail est consacrée à l'étude de faisabilité d'élimination du PEs par des techniques d'adsorption sur les résidus d’une brique modifiée et la bio-rémédiation par l’algue bleu-vert (Spiruline). / The occurrence of endocrine-disrupting compounds in the aquatic environment has brought increasing concern due to their potential adverse impacts on ecosystems and humans. These compounds are generally present in complex water matrices, such as surface waters at trace levels (ng/L) making their analysis difficult. In this work, an analytical method for the simultaneous determination of 13 endocrine disrupting chemicals (EDCs), including 5 steroid estrogens, 1 progestogen, 1 androgen and 6 endocrine-disrupting phenols in water was developed using solid phase extraction (SPE), derivatization and gas chromatography-mass spectrometry (GC-MS).The method was validated by spiking the 13 EDCs to the interest matrix. The limits of quantification were 1 ng/L and 5-50 ng/L for phenolic compounds and hormones respectively. The validated method was applied to assess the contamination level of the targeted EDCs in many sites in Northern France and in Tunisia. The majority of the considered compounds were detected in the different sampling sites. The last part of this work is the feasibility study for the elimination of PE by adsorption techniques on modified brick residues and bio-remediation by blue-green algae (SPIRULINA).

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The Southern Ocean is a region of the world's ocean which is fundamental to the generation of cold deep ocean water which drives the global thermo-haline circulation. Previous investigations of deep-sea sediment cores south of the Polar Front have been significantly constrained by the lack of a suitable correlation and dating technique. In this study, deep-sea sediment cores from the Bellingshausen, Scotia and Weddell seas have been investigated for the presence of tephra layers. The major oxide and trace element composition of glass shards have been used to correlate tephra isochrones over distances in excess of 600 km. The source volcanoes for individual tephra layers have been identified. Atmospheric transport distances greater than 1500 km for >32 μm shards are reported. One megascopic tephra is identified and correlated across 7 sediment drifts on the continental rise in the Bellingshausen Sea. Its occurrence in a sedimentary unit that has been biostratigraphically dated to δ^18 0 substage 5e identifies it as a key regional marker horizon for that stage. An unusual bimodal megascopic ash layer erupted from Deception Island, South Shetland Islands, has been correlated between 6 sediment cores which form a 600 km NW-SW transect from the central Scotia Sea to Jane Basin. The megascopic ash layer has been ^14C dated at 10,670 years BP. It represents the last significant input of tephra into the Scotia Sea or Jane Basin from that volcano and forms an important early Holocene marker horizon for the region. Five disseminated tephras can be correlated to varying extents across the central Scotia Sea cores. Together with the megascopic tephra they form a tephrostratigraphic framework that will greatly aid palaeoclimatic, palaeoenvironmental and palaeoceanographic investigations in the region.

Coordenação de Aperfeiçoamento de Pessoal de Nível Superior / In this study, a new modulator that is simple, robust and presents low operation costs was developed. This modulator uses compressed air to cool two small portions of the second chromatographic column of a GC×GC system. To evaluate the modulation process, solutions of alkanes C7 to C29, and pesticides (bifenthrin, cyfluthrin, cypermethrin, esfenvalerate, fenitrotion, fipronil, trifloxystrobin and trifluralin) were analyzed by GC×GC-FID. The results showed small variations of the peaks areas, between 0.67 and 2.80% for alkanes and between 0.24 and 5.34% for pesticides. The standard deviation (SD) for the retention time in the first column (1tR) was around 0.05 min, for alkanes and pesticides. For retention times in the second column (2tR), the SD was 0.04 s for the majority of the compounds, except for cyfluthrin and permethrin (0.08 s). The linear range was between 0.1 and 5.0 mg L-1 with a coefficient of determination (r2) greater than 0.9945 for pesticides analyzed by GC×GC-FID. The GC×GC system proposed was applied in the determination of pyrethroid pesticides (bifenthrin, cypermethrin, deltamethrin, fenvalerate and esfenvalerate and permethrin cis and trans) in grape samples Italy cultivar spiked at 3 levels (0.02, 0.05 and 0.5 mg kg- 1). Samples were extracted by the mini-Luke modified method (acetone method) and pesticides were quantified by GC×GC-μECD. The parameters evaluated in the validation of the method were: analytical curve, linearity, limit of detection (LOD), limit of quantification (LOQ), precision (in terms of repetitivity and intermediate precision) and accuracy (recovery), as well as the matrix effect on the chromatographic response. All analytical curves showed a linear range between 0.02 and 0.5 mg L-1 with r2 greater than 0.9956 and 0.9994 for the curves prepared in solvent (ethyl acetate) and matrix extract (grape), respectively. From the results obtained by calculation of the matrix effect percentage it can be concluded that the pyrethroid pesticides showed a considerable negative matrix effect. The values of method LOQ were 0.01 to 0.02 mg kg-1 for all pyrethroids. The values of recovery for the 3 spiked levels were between 94.3 and 115.2%, with good precision (RSD<18.4%), demonstrating that the performance of the mini-Luke modified method employed for the extractions is satisfactory. This study also showed that the GC×GC-μECD system using a modulator with a double jet of compressed air has potential for application in the analysis of pyrethroid pesticide residues in grapes, since it supplies low values of LOD and LOQ, and good accuracy in the analytical response. / Neste trabalho foi desenvolvido um novo modulador, simples, robusto e de baixo custo de operação. Este modulador utiliza ar comprimido para resfriar duas pequenas porções da segunda coluna cromatográfica de um sistema GC×GC. Para avaliar o processo de modulação utilizaram-se soluções de alcanos C7 a C29 e pesticidas (bifentrina, ciflutrina, cipermetrina, esfenvalerato, fenitrotiona, fipronil, trifloxistrobina e trifluralina) e análise por GC×GC-FID. Os resultados mostraram pequenas variações das áreas dos picos, entre 0,67 e 2,80% para alcanos e entre 0,24 e 5,34% para pesticidas. O desvio padrão (SD) para o tempo de retenção na primeira coluna (1tR) de alcanos e pesticidas foi em torno de 0,05 min. Para o tempo de retenção na segunda coluna (2tR), o SD foi de 0,04 s para a maioria dos compostos, exceto para ciflutrina e permetrina (0,08 s). A faixa linear situou-se entre 0,1 e 5,0 mg L-1 com coeficiente de determinação (r2) maior que 0,9945 para os pesticidas analisados por GC×GC-FID. O sistema GC×GC proposto foi aplicado na determinação de pesticidas piretróides (bifentrina, cipermetrina, deltametrina, fenvalerato e esfenvalerato, e permetrina cis e trans) em amostras de uva cultivar Itália fortificadas em 3 níveis (0,02; 0,05 e 0,5 mg kg-1). As amostras foram extraídas pelo método de mini-Luke modificado (método da acetona) e os pesticidas quantificados por GC×GC-μECD. Os parâmetros avaliados de validação do método foram: curva analítica, linearidade, limite de detecção (LOD), limite de quantificação (LOQ), precisão (em termos de repetitividade e precisão intermediária) e exatidão (recuperação), bem como o efeito matriz na resposta cromatográfica. Todas as curvas analíticas apresentaram faixa linear entre 0,02 e 0,5 mg L-1 com valores de r2 maiores que 0,9956 e 0,9994 para as curvas preparadas em solvente (acetato de etila) e extrato da matriz (uva), respectivamente. A partir dos resultados obtidos através do cálculo do efeito matriz percentual concluiu-se que os pesticidas piretróides avaliados apresentaram considerável efeito matriz negativo. Os valores de LOQ do método foram de 0,01 a 0,02 mg kg-1 para todos os piretróides. A recuperação para os 3 níveis de fortificação ficou entre 94,3 e 115,2%, com boa precisão (RSD<18,4%), o que mostrou que o desempenho do método mini-Luke modificado empregado para as extrações é satisfatório. Este estudo mostrou também, que o sistema GC×GC-μECD com modulador de duplo jato de ar comprimido tem potencial para aplicação nas análises de resíduos de pesticidas piretróides em uva, uma vez que fornece baixos valores de LOD e LOQ, e boa precisão da resposta analítica.

Cardiff Bay is an artificial freshwater lake created by the impoundment of 100 ha of estuarine intertidal mudflats in the early 2000s. This dramatic environmental change is reflected in the sedimentary record as a sharp transition between lacustrine gyttja and estuarine clay sediments. This study utilises a combination of geochemical, molecular genetic and novel cultivation based approaches to explore how the methanogen community of Cardiff Bay sediments has responded to the transition from brackish to freshwater conditions caused by impoundment, and how they might respond to future climate change. Microbial methanogenesis is active in newly deposited Cardiff Bay lacustrine gyttja sediments. Porewater methane concentrations regularly exceed saturation limits, suggesting that ebullition may be a direct pathway for atmospheric methane emissions. Sediment slurry incubations suggest that methane production in Cardiff Bay sediments would be further enhanced by both increased substrate availability and climate relevant temperature increases. Hydrogenotrophic methanogenesis mediated by the genus Methanobacterium is the major pathway for methane production in Cardiff Bay, which is atypical for either estuarine or freshwater environments, and reflects the importance of recalcitrant terrestrial carbon inputs. In fact, cultivation based experiments provide tentative evidence that syntrophic acetate and methylamine utilizing consortia involving hydrogenotrophic methanogens were able to outcompete acetotrophic and methylotrophic methanogens under certain environmental conditions. The temperature and salinity ranges and characteristics, and substrate types, of Cardiff Bay methanogenic communities varied little between sediments deposited pre- and post-impoundment. Differences between the methanogen community composition of lacustrine gyttja and estuarine clay sediments were driven predominantly by a reduction in the quantity and availability of organic matter with increasing sediment depth. A phylogenetically and physiologically diverse range of methanogens were enriched from Cardiff Bay sediments, including thermophilic methanogens and selected marine methanogens. Methanogen strains isolated from Cardiff Bay were closely related to cultivated strains, yet displayed novel physiological characteristics. These included a strain of the thermophilic genus Methanothermobacter able to produce methane at 25 °C, and a strain of the marine genus Methanolobus which grew under freshwater conditions (0.01 M NaCl).

Deep water formation in the Labrador Sea and the Gulf of Lion, for example, results from convection. A cyclonic circulation leads to a doming of the isopycnals at its centre, where stratification is then completely eroded by high surface winter buoyancy loss. This thesis assesses the causes of the interannual variability of deep convection. We first aim to quantify the relative importance of preconditioning, defined as the temperature and salinity structures and contents of the water column before the onset of convection, and of the buoyancy forcing (averaged over one winter) on the final convective mixed layer depth and on the temperature and salinity of the water mass formed. This study focuses on the Mediterranean and uses data from the Medar/Medatlas and Dyfamed data sets. The heat fluxes are studied and characterised. It is shown the the preconditioning is as important as the winter buoyancy fluxes in setting the final depth of convection. At the Dyfamed site (Corsica Strait), the seasonal cycle shows that the stratification frequency reaches a maximum in the intermediate layer in winter. This winter maximum is thought to be of critical importance. The second (and main) part focuses on the effect of the short-term (O(day)) variability of the surface forcing on convection, using an idealised model. The MIT model is integrated over a square box of size 64km x 64 km x 2km initialised with homogeneous salinity and a linear vertical temperature gradient. The configuration of the model is described and validated. A time-periodic cooling is then applied over a disc of radius 20km at the centre of the surface of the box. It is shown that the final mixed layer depth depends little on this short-term time variability because the lateral buoyancy fluxes are very responsive to the surface ones. Our results are compared with traditional parameterisation of the lateral buoyancy fluxes. General characteristics of the patch are also looked at, such as the rim current, the location of the angular momentum surfaces, the potential vorticity and the residual stratification in the mixed layer. The characteristics of the final water mass in each experiment are studied, showing that the short-term time variability of the forcing has an impact on the characteristics of the water mass formed. The last part compares the modelling study to gliders data for the Labrador Sea obtained by Peter Rhines and Charlie Eriksen of the University of Washington, WA, USA, in winter 2004-05. In that part of the real ocean, the variability of the boundary current seems more important than the variability in the surface forcing.