Dissertations / Theses on the topic 'Isotopic model'

Tendo como referencial teórico abordagens focadas na Arqueologia Marítima (MUCKELROY, 1978; ADAMS, 1998, 2002), na Antropologia Marítima (MALINOWSKI, 1986 [1922]; DIEGUES, 1998) e em estudos de Percepção Ambiental (INGOLD, 2000), buscou-se desenvolver e testar a hipótese de que, embora houvesse uma unidade cultural que permitisse o estabelecimento de uma ampla sociedade sambaquieira, os povos dos sambaquis estariam organizados em comunidades costeiras regionais, as quais teriam se desenvolvido e se organizado em diferentes comunidades marítimas, costeiras e fluviais, em conseqüência dos diferentes modos com que eles se relacionariam (dialeticamente) com o ambiente. Para sustentar tal hipótese foi elaborado um modelo de predição de sítios submersos e realizadas análises da composição isotópica do estrôncio (HÖLZL, 1997; PRICE et al., 2000; BENTLEY et al, 2003; HODELL, 2004), do carbono e do oxigênio (KEITH, 1964; COSTA, 2000; MIZUTA, 2007) presentes (em amostras de conchas e ossos humanos) em sambaquis localizados ao longo do médio e baixo Vale do Ribeira, bem como em sítios do litoral central (Baixada Santista e Bertioga) e norte do estado de São Paulo (Ubatuba). Essas evidências foram correlacionadas à abordagem teórica através de uma proposta elaborada a partir do estudo dos processos de formação do registro arqueológico desenvolvidos por Schiffer (1972). Com base nessas análises, além de diferenciar os povos do médio Ribeira dos sambaquieiros do litoral, foi possível, entre outras, propor uma fronteira cultural entre os conjuntos de sítios do litoral sul/centro do estado de São Paulo e os sambaquis do litoral norte de São Paulo/sul do Rio de Janeiro. Especificamente em relação aos sambaquis de Cananéia, a análise isotópica das conchas evidenciou, ainda, que os locais e os propósitos da coleta estariam mais relacionados a aspectos culturais do que à simples exploração dos recursos mais abundantes.<br>Using as theoretical reference approaches focused on the Maritime Archaeology (MUCKELROY, 1978; ADAMS, 1998, 2002), on the Maritime Anthropology (MALINOWSKI, 1986 [1922]; DIEGUES, 1998) and on studies of Environmental Perception (INGOLD, 2000), we tried to develop and test the hypothesis that, notwithstanding the evidence of a cultural unity that allowed for the establishment of an ample shellmound society, people of the shellmounds were organized in regional coastal communities. These would have developed and organized in different maritime communities, both coastal and fluvial, in consequence of the different ways in which they (dialectically) related with the environment. To support such hypothesis a model was elaborated predicting underwater sites, an analyses was realized of the isotopic composition of strontium (HÖLZL, 1997; PRICE et al., 2000; BENTLEY et al, 2003; HODELL, 2004), of carbon and of oxygen (KEITH, 1964; COSTA, 2000; MIZUTA, 2007) present (in samples of shells and human bones) in shellmounds located along the medium and low Vale do Ribeira, as well as in sites at the central coast (Baixada Santista and Bertioga) and north (Ubatuba) of the state of São Paulo. The evidences were correlated to the theoretical approach through a proposal elaborated from the study of the formation processes of the archaeological record developed by Schiffer (1972). Based on these analyses, beyond differentiating the people from the medium Ribeira from the coastal shellmound people, it became possible, among other things, to propose a cultural borderline between the ensembles of sites of the south/center coast of São Paulo and the shellmounds of northern São Paulo and southern Rio de Janeiro. Specifically with regard to the shellmounds of Cananéia, the isotopic analysis of shells evinced, yet, that the locals and goals of collecting were more related to cultural aspects than to the simple exploration of the more abundant resources.

An injection technique, in which undisturbed soil cores are labelled with ³²P to study dissolution of phosphate fertilisers in the soil, was evaluated in field and glasshouse trials. When ³²P was injected between 0-150 mm depths of the undisturbed soil columns and fertilisers applied at the surface, the amounts of fertiliser P dissolved, as measured by the increases in the exchangeable P pools, were overestimated. Three possible reasons were suggested: (i) the interaction between surface-applied fertiliser, ³²P injected through the whole soil column, and the vertical decline in root density, (ii) the decline of specific activity in the exchangeable P pool due to losses of ³²P to nonexchangeable P pools and continuous addition of P from fertiliser dissolution, and (iii) non-uniform distribution of ³²P vis-a-vis ³¹P phosphate. The injection technique may be employed to assess the effectiveness of phosphate fertilisers by introducing a concept, the fertiliser equivalent (FE). The FE is a measure of the amounts of soil exchangeable P that the fertilisers are equivalent to in supplying P to plants, when applied at the specific location. Soluble single superphosphate (SSP) applied at the surface of undisturbed grassland soil cores (Tekapo fine sandy loam), was much more effective than surface-applied unground North Carolina phosphate rock (NCPR) and 30% acidulated NCPR with phosphoric acid (NCPAPR) within the 56 day period of plant growth. An isotopic dilution method, based on tracer kinetic theory, was developed to study the rates of dissolution (F in) and retention (F out) of phosphate fertilisers in the soil in growth chamber experiments. The estimation of F in and F out required labelling of the soils with carrier-free ³²P and determination of the corresponding values of the specific activities of the exchangeable P pools, SA₁ and SA₂, and the sizes of the exchangeable P pools, Q₁ and Q₂, at times t₁ and t₂. Most of the phosphate in the monocalcium phosphate (MCP) solution entered the exchangeable P pool immediately after addition to the soils (Tekapo fine sandy loam and Craigieburn silt loam), and there was little further phosphate input. With increasing periods of incubation, the phosphate was quickly transformed to less rapidly exchangeable forms. In the soils treated with ground North Carolina phosphate rock (<150 µm, NCPR) or partially acidulated (30%) NCPR with phosphoric acid (NCPAPR), the initial exchangeable P pools were not as large as those in the soils treated with MCP, but were maintained at relatively stable concentrations for extended periods, due to the continuous dissolution of PR materials and to lower rates of pretention. An increase in P-retention caused a slight rise in the rate of PR dissolution, but also a rise in the rate of P-retention by the soil. The rate of dissolution was higher at a lower application rate in relative terms, but smaller in absolute terms. The trends in the changes of plant-available P in the soils, measured by the water extractable P, Bray I P and Olsen P, correspond to those predicted by the F in and F out values. The average rates of dissolution between 1-50 and 50-111 days estimated by the F in, however, were higher than those estimated by extractions with 0.5 M NaOH followed by 1 M HCl, and with 0.5 M BaCl₂/TEA. This is partly because the Fin values reflect a plant growth effect on PR dissolution. The relative agronomic effectiveness of NCPR and NCPAPR with respect to MCP was higher after 50 and 111 days of incubation than after 1 day. The F in values were included in all the two-variable models constructed by stepwise regression to describe the relationship between plant P uptake and soil measurements. The amounts of variation in plant P uptake accounted for by the regression model was significantly improved by including F in in the model. This indicates the importance of fertiliser dissolution rates in affecting soil P supply, when phosphate fertilisers differing in solubility are applied.

This study developed an understanding of hydrological processes within the Cressbrook Creek catchment of the upper Brisbane River, in particular for the alluvial aquifers. Those aquifers within the lower catchment are used for intensive irrigation, and have been impacted by long-term drought followed by flooding. The study utilised water chemistry, isotopic characters and hydraulic measurements to determine factors such as recharge, links between creeks and groundwater, and variations in water quality. The catchment-wide study will enable improved management of the local water resources.

Delineating spatial patterns of precipitation isotopes (“isoscapes”) is important for studies including the hydrology of terrestrial systems, present and past interpretations of climate, and tracer-aided hydrological modelling, among others. However, the extent to which precipitation isoscapes can be predicted across Canada has not been fully articulated. This thesis combines isotopes in precipitation (δ18Oppt) observations from two regional and one global network to create long term and time series precipitation isoscapes for Canada and the northern United States. Multi-linear regressions of a small suite of geographic and climate variables generate the best performing long-term and seasonal models of δ18Oppt. These models are used to develop long term isoscapes for Canada, which capture the general spatial and seasonal trends in δ18Oppt, showing an improvement upon results from previous studies using global models. Building upon long-term δ18Oppt prediction, δ18Oppt observations alongside climatological and geographic predictors are used to create empirical time series prediction models. Five regionalization approaches are used to separate the study domain into isotope zones to explore the effect of spatial grouping on simulations. Generally, the models capture the timing and magnitude of intra-annual (seasonal) δ18Oppt cycles across the study domain while simulating moderate inter-annual variation; however often fail to capture the anomalies in observed δ18Oppt. Uncertainty in predictions is quantified spatially and temporally, and the Köppen-Geiger (Kpn) regionalization is selected as the preferred regionalization scheme for future applications due to adequate model performance and lack of border issues at regional boundaries. Finally, estimates of monthly δ18Oppt from Kpn models, long term annual averages, and daily REMOiso output are used to force an isotope-enabled hydrological model, isoWATFLOOD, in the Fort Simpson Basin, NWT, Canada. Results show streamflow simulations are not significantly impacted by choice of δ18Oppt input; however, oxygen-18 in streamflow and the internal apportionment of water (and model parameterizations) are impacted, particularly during large precipitation and snowmelt events. This work shows how isoWATFLOOD can be used in regions with limited δ18Oppt observations, and that the model can be of value in such regions. This study reinforces that a tracer-aided modelling approach works towards diagnosing issues surrounding model equifinality.<br>February 2017

La sureté du stockage géologique des déchets nucléaires de haute activité est un sujet au cœur des grands challenges scientifique et technologique de notre siècle. Ces déchets, issus des combustibles nucléaires, sont vitrifiés et entreposés sur les sites de production (La Hague, Marcoule) en attendant l’ouverture d’un site de stockage définitif. Leur durabilité chimique fait l’objet de nombreux travaux pour évaluer le devenir des radionucléides dans l’environnement. Cette thèse s’inscrit dans cette thématique et repose sur deux axes d'étude. Le premier se focalise sur l’effet du désordre structural sur la dissolution des silicates à 90°C, pH 1.5 et 3. Historiquement, deux mécanismes d’altération sont proposés : le mécanisme de dissolution – reprécipitation, attribué aux cristaux, et le mécanisme d’hydrolyse – recondensation, attribué aux verres. Cependant, des études récentes ont montré le besoin de revisiter cette dichotomie. Pour cela, nous avons étudié la dissolution d’un silicate amorphe, le verre albitique, et son équivalent cristallin, l’albite, au contact d’une solution marquée en 29Si. A travers l’incorporation de cet isotope dans la couche altérée, la contribution de la solution à la formation de la couche altérée a pu être quantifiée pour discuter du poids de chacun des mécanismes. De plus, un effet antagoniste de l’Al a été mis en évidence en fonction du désordre structural. En outre, la limitation des propriétés de transport au sein du gel était le phénomène clé contrôlant l’altération tout au long de l’expérience. Le deuxième axe concerne la dissolution d'un verre modèle (International Simple Glass – ISG) dans un environnement riche en Ca et Si. Parmi les scenarios à l’étude du site de stockage géologique, il en est un où une solution plus ou moins chargée en Si et Ca entrerait en contact avec la matrice silicatée. Afin de simuler cette matrice, le verre modèle ISG a été altéré dans une solution à 90°C, pH &gt; 10. A travers l’évolution de la vitesse de dissolution, deux observations ont été faites : i) la vitesse d’altération est fortement corrélée à l’état de saturation de la solution par rapport à la silice amorphe et ii) la présence de phases secondaires, en particulier de silicate de calcium hydratée (CSH), n’a qu’un effet secondaire sur la vitesse d’altération à court terme (&lt; 20 jours). Pour les expériences d’une durée plus longue, une reprise d’altération du verre a été observée entre 19 jours et 76 jours, correspondant à une période de croissance des CSH. Après cette période, une compétition entre la formation des CSH et celle des carbonates de calcium limite l’altération du verre.Enfin, l’effet du Ca et les propriétés diffusives du gel formé au cours des 20 premiers jours d’altération, a été étudié plus en détail. L’ajout de Ca dans la solution diminue la vitesse d’altération. Deux effets principaux sont discutés : le premier consiste en un effet catalytique du Ca sur la recondensation du gel et le second sur la formation d’une couche riche en Si et Ca qui passive le gel. Les coefficients de diffusions du gel estimés sur la base d’une expérience de traçage étaient systématiquement inférieurs de plusieurs ordres de grandeur à ceux basés sur l’épaisseur totale du gel. Si cette différence s’explique en partie par une différence de température, d’autres hypothèses telles que i) l’évolution des propriétés de diffusivité de la couche au cours de l’expérience, ii) une différence de diffusivité entre la pénétration et le relargage d’éléments au sein de cette couche ou encore iii) une restructuration post-mortem du gel, sont discutés.En conclusion, ces travaux apportent plusieurs contraintes pour la compréhension du continuum mécanistique existant entre la dissolution des silicates cristallins et vitreux, et sur la dissolution des verres dans des environnements chimiques pertinents pour le stockage géologique à long terme<br>The safety of high-level nuclear waste geological storage is a major scientific and technological challenges of our century. These wastes, inherited from nuclear fuels, are vitrified and stored on production sites (La Hague, Marcoule), waiting on the opening of a storage facility. Their chemical durability is the object of several studies to assess the impact of the release of radionuclides in the environment. Based on this background, this thesis relies on two axes.The first one focuses on the effect of structural disorder on the dissolution of silicates at 90°C, pH 1.5 and 3. Historically, two alteration mechanisms have been identified to explain silicate dissolution: the dissolution–reprecipitation mechanism, generally attributed to crystals, and the hydrolysis–recondensation mechanism, generally attributed to glasses. However, recent studies have shown the need to revisit this dichotomy. To address this point, the dissolution of an amorphous silicate, albite glass, and its crystalline equivalent, albite feldspar, in contact with a solution marked with 29Si was studied. Through the incorporation of this isotope into the altered layer, the contribution of the solution to the formation of the altered layer was quantified to discuss the significance of each mechanism. An antagonistic effect of Al was highlighted as a function of the amorphous and crystalline structure. Moreover, we suggested that the limitation of transport properties within the gel was the key phenomenon controlling the alteration throughout the experiment.The second axis concerns the dissolution of a model glass (International Simple Glass – ISG) in an environment rich in Ca and Si. One of the most likely scenarios for the geological storage site suggests that environmental fluids more or less concentrated in Si and Ca would come into contact with the glass. A 6-oxides glass simulating the nuclear glass was altered in a solution at 90°C, pH &gt; 10. Literature shows that the presence of Ca can have antagonistic effects on the material's dissolution depending on the environment. Among these effects, calcium silicate hydrates (CSH) can precipitate and promote glass alteration.Through the evolution of the dissolution rate, two observations were made: i) the alteration rate is strongly correlated with the solution's saturation state with respect to amorphous silica and ii) the presence of secondary phases, especially CSH, has only a secondary effect on the short-term alteration rate (&lt;20 days). For longer duration experiments, a resumption of glass alteration was observed between 19 days and 76 days, corresponding to a period of CSH growth. After this period, a competition between the formation of CSH and calcium carbonate limited the glass alteration. Finally, the effect of Ca and the transport properties of the gel formed during the first 20 days of alteration were studied in more detail. The addition of calcium to the solution reduces the alteration. Two main effects are discussed: the first consists of a catalytic effect on the recondensation of the gel, and the second on the formation of a (Si, Ca)-rich layer that passivates the gel. It was found that the gel diffusion coefficients based on the tracing experiment were systematically several orders of magnitude lower than those based on the total thickness of the gel. If this difference is partly explained by a variation in temperature, other hypotheses such as i) the evolution of the transport properties of the gel layer during the experiment, ii) a difference in diffusivity between the penetration and release of glass elements within this layer, or iii) a post-mortem restructuration of the gel, are discussed. In conclusion, this work brings several critical information to the understanding of the mechanistic continuum existing between the dissolution of crystalline and vitreous silicates, and of the glass dissolution in chemically relevant environments for long-term geological storage

This dissertation is concerned with the development of mathematical aquifer models that combine hydrological, hydrochemical and isotopic data. One prerequisite for the construction of such models is that prior information about the variables and parameters be quantified in space and time by appropriate statistical methods. Various techniques using multivariate statistical data analyses and geostatistical methods are examined in this context. The available geostatistical methods are extended to deal with the problem at hand. In particular, a three-dimensional interactive geostatistical package has been developed for the estimation of intrinsic and nonintrinsic variables. This package is especially designed for groundwater applications and incorporates a maximum likelihood cross-validation method for estimating the parameters of the covariance function. Unique features of this maximum likelihood cross-validation method include: the use of an adjoint state method to compute the gradient of the likelihood function, the computation of the covariance of the parameter estimates and the use of identification criteria for the selection of a covariance model. In addition, it can be applied to data containing measurement errors, data regularized over variable lengths, and to nonintrinsic variables. The above methods of analysis are applied to synthetic data as well as hydrochemical and isotopic data from the Tucson aquifer in Arizona and the Madrid Basin in Spain. The dissertation also includes a discussion of the processes affecting the transport of dissolved constituents in groundwater, the mathematical formulation of the inverse solute transport problem and a proposed numerical method for its solution.

High nitrogen discharge from mining sites has been an environmental issue that has been closely studied in the recent years. The environmental effects of high nitrogen discharge are mainly eutrophication, but can also lead to changed species composition and algae blooms. Nitrogen is a highly abundant element and is the most abundant element in the atmosphere, where 78% by volume is present as dinitrogen (N2 ). Nitrogen is present in reduced form in all organic life as ammonium (NH4 + ). Nitrogen is also present in reduced form as nitrate (NO3 - ) or nitrite (NO2 - ) in most aquatic systems.  Both nitrate and ammonium is contributing to eutrophication problems worldwide and ammonia (NH3 ) is direct toxic in high concentration to certain sensitive aquatic species. Nitrate in high concentration is also direct toxic, both to humans but also to aquatic biota.  To trace and quantify different nitrogen transforming processes, their sources and their sinks is called tracing nitrogen cycling and is important due to the environmental effect of nitrogen. Nitrogen is available in many different species and oxidation states which all have their respective geochemistry. This thesis focuses on tracing the complex nitrogen cycle in two different cold climate mining systems in northern Sweden using two different methods. The two studied systems are: The LKAB underground iron ore mine in Kiruna Boliden Minerals AB open pit copper ore mine Aitik outside Gällivare  Two different approaches were used to trace the nitrogen cycling. The LKAB Kiruna mine was investigated using stable nitrogen isotopes. The isotope analysis showed high capability to trace nitrogen cycling, both quantative and qualitative. We also showed the origin of the isotope signals which gives indication to the different sources of nitrogen in the mine. The presented study shows presence of nitrification, ammonium volatilization and ammonium adsorption to waste rock to occur in the water transport system.  The nitrogen cycling in the Boliden Aitik mine was investigated using a nitrogen model which we developed as part of this thesis. The model is based on Yakushevs Redox Layer model (ROLM). The model contains the state variables ammonium, nitrate, nitrite, plankton, phosphate, dead organic material (both particulate and dissolved) as well as oxygen. The nitrogen concentrations in the Boliden Aitik mine was modeled for the clarification pond and showed, in general, low biological activity. The biological mediated reactions such as nitrification, denitrification, phytoplankton growth and grazing were low in relation to natural lake systems

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Thesis (Ph. D.)--University of Maryland, College Park, 2005.<br>Thesis research directed by: Chemistry. Title from t.p. of PDF. Includes bibliographical references. Published by UMI Dissertation Services, Ann Arbor, Mich. Also available in paper.

The influence of climate and antecedent moisture conditions on hydrological and biogeochemical fluxes was studied and contrasted in three nested, high-elevation, snowmelt-dominated catchments in the Sierra Nevada, California and one basin-floor, semi-arid catchment in southeastern Arizona. Investigations were completed within a different two-year period at each site, with the second year being climatically different (typically drier) than the first. Spring snowmelt, widespread winter frontal precipitation, and episodic summer rains induce surface water flow in these catchments, though the timing and magnitude of nutrient redistribution among soil and stream compartments varies in each. Surface water flow from spring snowmelt in high-elevation catchments travels through the subsurface or across the surface as direct runoff A more typical process producing surface water flow in semi-arid catchments is flooding during episodic or widespread rainfall. Hydrograph separations at Emerald Lake, Topaz Lake and Marble Fork catchments in Sequoia National Park, California, revealed that the majority of snowmelt flowed through soil before entering the stream in both average and highsnow years. The Emerald Lake watershed had a higher fraction of old water in its outflow in the average accumulation year because of the previous year's high accumulation and longer melt season. A mixing model analysis performed of the upper San Pedro River, Arizona, for wet and dry years showed that summer flood hydrographs were composed mainly of precipitation and surface runoff in both years, though a higher soil-water input occurred in the wetter year and in early season floods in the dry year. Stream and soil water nitrate concentrations were higher during floods in the dry year. Early season floods in the dry year exhibited more variability in stream water nitrate and sulfate, whereas late season flood concentrations reflected a well-mixed system and therefore less variation of these species during flood hydrographs. These data showed that periods of below average precipitation preceding major runoff periods result both in less soil water and solute export during summer floods in basin-floor catchments and less direct snowmelt in high-elevation catchments. Hydrologic and solute export in each catchment, despite their differing geographical locations, responds in similar ways to climate variability.

L'océan Pacifique représente environ 50% du volume global des océans, ce qui en fait un acteur essentiel des cycles biogéochimiques globaux et de leur réponse aux perturbations. En particulier, l'océan Pacifique profond abrite des réservoirs majeurs de carbone et de nutriments, dont la taille et les variations sont largement contrôlées par les transports physiques de traceurs. Dans cette thèse, nous visons à mieux comprendre et contraindre le transport de traceurs dans l'océan Pacifique profond à travers les états climatiques. Pour cela, nous générons de nouvelles et utilisons d'anciennes observations de traceurs conservatifs, notamment les isotopes de l'oxygène, ainsi que des modèles numériques de circulation océanique. Nous montrons que le mélange isopycnale a un contrôle essentiel sur la ventilation des profondeurs moyennes du Pacifique. Un retour des eaux abyssales vers la surface est identifié dans le Pacifique subarctique moderne. Cette voie de remontée semble avoir diminué pendant le dernier maximum glaciaire, il y a environ 20 000 ans, et le Pacifique Nord profond pourrait avoir été plus fortement stratifié. Cependant, les preuves de changements nécessaires dans le sud du bassin pour expliquer cette stratification profonde restent rares. Les modèles de pointe de circulation océanique peinent à représenter les trajectoires des traceurs et les vitesses de ventilation dans le Pacifique Nord moderne, déduites des observations. Cependant, ces trajets de traceurs restent insuffisamment contraints et la dynamique sous-jacente est mal comprise. L'analyse des mesures in situ du rapport isotopique de l'oxygène 18 (18O) de l'eau de mer constitue un moyen efficace pour mieux contraindre les origines et les itinéraires des traceurs dans l'océan profond : les observations actuelles de 18O dans les océans Austral, Indien et Pacifique permettent de mieux comprendre ces itinéraires. Nous suggérons que des mesures supplémentaires de 18O à partir d'échantillons d'eau de mer modernes et de coquilles de calcite dans les carottes de sédiments fourniraient des contraintes précieuses sur les réservoirs et les flux de traceurs actuels et passés dans l'océan profond<br>The Pacific Ocean represents about 50% of the global ocean volume, making it an essential player in global biogeochemical cycles and their response to external perturbations. In particular, the deep Pacific Ocean hosts major reservoirs of carbon and nutrients, whose size and variations are largely controlled by physical tracer transports. In this thesis, we aim to better understand and constrain the transport of tracers in the deep Pacific Ocean across climate states. We use historical and new observations of conservative tracers, notably oxygen isotopes, combined with numerical models of ocean circulation. We show that isopycnal mixing is an essential control of the ventilation of Pacific mid-depths. A return of abyssal waters to the surface is identified in the modern subarctic Pacific. This upwelling pathway may have been weaker during the Last Glacial Maximum about 20,000 years ago, and the deep North Pacific may have been more strongly layered; however, evidence for the required end member changes in the south of the basin to explain this deep layering remains sparse. State-of-the-art prognostic models of global ocean circulation struggle to represent observationally inferred tracer pathways and turn-over times in the modern North Pacific. However, these tracer pathways remain insufficiently constrained and the underlying dynamics are poorly understood. Analysis of in-situ measurements of the oxygen-18 (18O) isotopic ratio of seawater provides an efficient means to better constrain the origins and routes of tracers in the deep ocean: insights on these routes are derived from existing 18O observations in the Southern, Indian, and Pacific Oceans. We suggest that additional measurements of 18O from modern ocean water samples, and from calcite shells in sediment cores, would provide valuable constraints on present-day and past tracer reservoirs and fluxes in the deep ocean

Thesis (S.M.)--Massachusetts Institute of Technology, Dept. of Materials Science and Engineering, 2012.<br>Cataloged from PDF version of thesis.<br>Includes bibliographical references (p. 41-46).<br>Carbon related systems have attracted a large amount of attention of the science and technology community during the last few decades. In particular, graphene and carbon nanotubes have remarkable properties that have inspired applications in several fields of science and engineering. Despite these properties, creating structurally perfect samples is a difficult objective to achieve. Defects are usually seen as imperfections that degrade the properties of materials. However, defects can also be exploited to create novel materials and devices. The main topic of this thesis is studying the effect of isotope doping on the phonon properties of graphene. The advantage of the isotope enrichment technique is that only phonon frequencies or thermal properties can be modified without changing the electrical or chemical properties. We calculated the values of the phonon lifetimes due to isotope impurity scattering for all values of isotopic fractions, isotopic masses and for all wave-vectors using second order perturbation theory. We found that for natural concentrations of 13C, the contribution of isotopic scattering of optical modes is negligible when compared to the contribution from the electron-phonon interaction. Nevertheless, for atomic concentrations of 13C as high as [rho] = 0.5 both the isotopic and electron-phonon contributions become comparable. Our results are compared with recent experimental results and we find good agreement both in the 13C atomic density dependence of the lifetime as well as in the calculated spectral width of the G-band. Due to phonon scattering by 13C isotopes, some graphene phonon wave-functions become localized in real space. Numerical calculations show that phonon localized states exist in the high-energy optical phonon modes and in regions of flat phonon dispersion. In particular, for the case of in-plane optical phonon modes, a typical localization length is on the order of 3 nm for 13C atomic concentrations of [rho] ~~ 0.5. Optical excitation of phonon modes may provide a way to experimentally observe localization effects for phonons in graphene.<br>by Joaquin F. Rodriguez-Nieva.<br>S.M.

Les conditions oxydantes de la surface terrestre actuelle sont dues à la teneur élevée en dioxygène de l’atmosphère. Au début de l’histoire de la Terre il y a 4.54 milliards d'années (Ga), l’oxygène n’était pas stable dans l’atmosphère. Il a fallu deux épisodes d’augmentation brutale de ce gaz atmosphérique pour qu’il atteigne son niveau actuel : l’un vers 2.4 Ga, nommé le Grand Evènement Oxydant (GOE) qui fait l’objet de ce projet, l’autre 2 milliards d’années plus tard, nommé l’Evènement Oxydant Néo-protérozoïque (NOE). Le GOE est vraisemblablement le résultat de l’émersion généralisée de larges continents dont l’érosion libère le phosphate dans l’océan, un nutriment nécessaire à la production biologique, qui a donc permis l’explosion de la photosynthèse oxygénée. Ces deux hausses d’oxygène atmosphérique coïncident avec deux évolutions majeures dans l’histoire de la vie : (i) peu après le GOE, les eucaryotes sont apparus, alors que (ii) le NOE correspond à l’apparition des métazoaires et à l’explosion cambrienne. L’étude de ces phénomènes atmosphériques primitifs peut avoir d’importantes répercussions sur notre compréhension de l’origine et de l’évolution de la vie, qu’on estime principalement marine à cet âge. Les seules archives de ces temps primitifs sur Terre sont les roches sédimentaires. Pour savoir comment l’oxygénation de l’atmosphère a pu être reliée à cette vie marine, il faut tout d’abord comprendre comment l’océan a interagi avec l’atmosphère lors de cet évènement d’oxygénation. Cette question est au coeur de ce projet : comment le GOE a-t-il affecté les cycles biogéochimiques océaniques dont la vie est dépendante ? Nous nous sommes intéressés aux formations ferrifères litées ou BIFs (Banded Iron Formations). La chimie de ces roches marines fait écho à celle de l’océan contemporain à leur formation. Déterminer quantitativement la composition de l’océan à partir de celles des sédiments, même chimiques, est un défi quasiment impossible à relever y compris dans l’océan moderne. C’est pourquoi nous avons proposé de déterminer le temps de résidence d’éléments sensibles aux conditions redox de la surface, le soufre, le fer et le cuivre dans l’océan pré-GOE. Nous avons obtenu, par des séries temporelles, le spectre des fluctuations isotopiques de ces éléments enregistrées dans des carottes de formations ferrifères litées. La limite inférieure du spectre donne le temps de résidence de ces éléments dans l’eau de mer et fournit donc une indication solide sur la teneur de ces éléments dans l’océan à cette période. Nous avons analysé des échantillons protérozoïques proches de la limite Archéen-Protérozoïque du Transvaal (Afrique du Sud) et d’Hamersley (Australie). Des échantillons eoarchéens de Nuvvuagittuq (Canada) ont été récoltés mais n'ont pas pu être analysés faute de temps<br>The present-day oxidizing conditions at Earth's surface are due to the high oxygen content of the atmosphere. However, oxygen was not always stable in the terrestrial atmosphere. Two distinct periods during which oxygen increased in a step-like manner were required to reach the current atmospheric oxygen level. The first, at about 2.4 Ga, is known as the Great Oxidation Event (GOE) and is at the core of this Ph.D. thesis. The other, occurring almost two billion years later, is called the Neo-Proterozoic Oxidation Event (NOE). The GOE likely is the result of the beginning widespread emergence of large continental expanses whose subsequent erosion gradually released phosphate into the ocean. Phosphate, a nutrient essential to organic production, in turn allowed the explosion of oxygenated photosynthesis. The GOE and NOE coincide with two major changes in the history of life. Shortly after the GOE, eukaryotes appeared, while the NOE corresponds to the appearance of metazoans and the Cambrian explosion. A better grasp of the GOE hence may have important implications for the understanding of the origin and evolution of life, which is thought to have been mainly marine at this stage in Earth history. The only records of the oxygen level during these ancient times are found in terrestrial sedimentary rocks. To understand how oxygenation of the atmosphere relates to marine life, we must first understand how the ocean was connected to the atmosphere during the GOE and how the GOE affected life-dependent ocean biogeochemical cycles. To this end we focused on banded iron formations (BIF). The chemistry of these sedimentary marine rocks directly reflects the chemistry of the contemporary ocean. Deriving quantitatively the composition of the ocean from a hydrogenous sediment is a challenge almost impossible to meet, even for the modern ocean. This is why we instead determined the residence time of redox-sensitive elements (in this case sulfur, iron, and copper) in the pre-GOE ocean. We specifically targeted the periods of isotopic fluctuations in these elements as recorded in BIF cores. The lower limit of the spectrum provides the residence time of these elements in seawater, hence giving a robust indication of their contents in the pre-GOE ocean. We sampled early Proterozoic BIF near the Archean-Proterozoic boundary in Transvaal (South Africa) and Hamersley (Australia), as well as Archean BIF from Nuvvuagittuq (Canada), though the latter were not analyzed during this thesis due to shortage of time

La comprensione dei processi di ricarica, di movimento e di immagazzinamento delle acque sotterranee negli acquiferi carbonatici rappresenta una grande sfida per l’idrogeologia. Infatti, questi acquiferi sono ospitati in formazioni geologiche caratterizzate da forti eterogeneità e anisotropie che si possono ripercuotere nelle caratteristiche idrodinamiche del sistema. Questa comprensione diventa tanto più complicata quando si trattano acquiferi ricadenti in aree montuose, ove la complessità geologico-strutturale si unisce a una grande eterogeneità topografica. Quest’ultima può influenzare il regime pluviometrico in maniera significativa, ed inoltre le quote elevate sono spesso caratterizzate dalla presenza del manto nevoso che apporta un contributo non trascurabile ai processi di ricarica degli acquiferi. In questo contesto, è stato dimostrato che i terremoti ad elevata magnitudo possono cambiare il comportamento idrodinamico delle sorgenti mostrando variazioni transitorie o di lunga durata, rendendo la gestione della risorsa idrica molto impegnativa. Questo è il caso di una ampia zona Appenninica dell’Italia centrale, quella interessata da una intensa sequenza sismica tra agosto 2016 e gennaio 2017, caratterizzata da nove eventi di Mw ≥ 5.0 di cui il più forte di Mw = 6.5 con epicentro ubicato all’interno dell’area di studio. Questa ricerca incrementa notevolmente le conoscenze idrogeologiche ed idrodinamiche relative alla circolazione delle acque sotterranee negli acquiferi carbonatici, approfondendo i possibili cambiamenti delle caratteristiche qualitative e quantitative del flusso idrico sotterraneo, durante e dopo una serie di forti eventi sismici. È stato utilizzato un approccio olistico combinato coinvolgente diverse discipline e tecniche, a partire da un'indagine idrogeologica più tradizionale basata sull'applicazione del bilancio idrologico e sull'analisi degli idrogrammi delle portate sorgentizie, validata mediante i risultati di numerosi test con traccianti artificiali, e analisi geochimiche ed isotopiche. Durante le indagini è stato effettuato un rilevamento idrogeostrutturale di dettaglio, integrato con un'analisi basata su sistemi geografici informativi, finalizzato sia alla identificazione delle strutture geologiche principali responsabili del cambiamento del flusso idrico sotterraneo, sia alla caratterizzazione delle varie idrostrutture presenti nell'area in studio e dei rapporti tra esse. I risultati confermano la complessità geologico-strutturale di questa area e la ripercussione sull'assetto idrogeologico, evidenziando in particolare il ruolo che le faglie, ad estensione sia regionale che locale, giocano nelle modificazioni del flusso di acque sotterranee. Le evidenze ottenute da ciascuna tecnica di indagine sono state confrontate tra loro per delineare un modello idrogeologico concettuale indispensabile per la corretta e sostenibile gestione della risorsa idrica sotterranea.<br>Understanding the groundwater flow, recharge, and storage processes in carbonate aquifers represents a challenging aspect in hydrogeology. These aquifers are hosted in geological formations characterised by strong heterogeneities and anisotropies affecting the hydrodynamic characteristics of the systems. This comprehension becomes even more complicated when dealing with mountainous aquifers, where the geological and the geo-structural complexity is combined with high topographical heterogeneity. The topographical heterogeneity can significantly influence the rainfall regime, and the higher altitudes, often characterised by the presence of snow cover, make a non-negligible contribution to the aquifers' recharge processes. In this context, it has been demonstrated that high-magnitude earthquakes can change the hydrodynamic behaviour of springs and rivers showing transient or long-lasting hydrogeological modifications, making the water management a real hard task. This is the case of a wide Apennines area located in central Italy, the one affected by an intense seismic sequence between August 2016 and January 2017, characterized by nine events of Mw ≥ 5.0 (Mmax = 6.5) whose epicentre is located within the study area. This research aims to increase the hydrogeological and hydrodynamic knowledge related to groundwater circulation in carbonate aquifers by investigating possible changes in the qualitative and quantitative characteristics of groundwater flow, during and after a series of strong seismic events. A combined holistic approach, involving different disciplines and techniques, was used. The investigation starts with a more traditional hydrogeological approach, based on the application of the hydrological balance and the analysis of springs hydrographs, then validated through the results of several artificial tracer tests, hydrochemical and isotopic analyses. During the study, a detailed hydro-geo-structural survey was carried out, integrated with GIS-supported analysis, aimed both at identifying the main geological structures responsible for the groundwater flow modifications, and at characterising the hydrogeological connections between the hydrostructures recognised in the study area. The results of this research confirm the geo-structural complexity of the domain and its influence on the hydrogeological features, highlighting the role of faults, having a regional or local extension, played in modifying the groundwater flow. The outcomes obtained from each investigation technique had compared each other in order to outline a detailed conceptual hydrogeological model which has been updated step by step following the post-seismic phases to provide to the public and private drinking companies operating in the area, a tool for the correct and sustainable groundwater management operations.

Understanding the spatial and temporal dynamics occurring in headwater catchments enhances our ability to effectively manage the natural and unnatural inputs from the landscape to the stream. DOC is particularly important in northern peat dominated catchments, where concentrations in rivers have been increasing over recent decades. Due to the significant downstream impacts this increase has on the functioning of aquatic ecosystems and the quality of drinking water, it is vital to understand the tight coupling between the landscape and the stream. This study set out to explore the use of integrating high-frequency DOC data, stable isotopes and modelling as a novel way to increase our understanding of the hydrological and biogeochemical processes that control spatial and temporal DOC dynamics. By deploying in-situ FDOM sensors, across nested catchments, we captured 15 minute DOC dynamics. This yielded insights into seasonal, event and diel temporal variability, along with spatial variability. Results showed the utility of linking these DOC dynamics with stable isotopes and water ages, extracted from a tracer-aided runoff model. This allowed the main runoff generating processes, that transport the DOC from the sources to the stream, to be assessed, and showed the effects of hydrologic connectivity and antecedent conditions on DOC delivery. Incorporating modelling allowed the non-stationary hydrological processes influencing runoff generation, which cannot be easily measured by field techniques, to be evaluated. Overall, the findings of this thesis underline the utility of integrating highfrequency DOC data, stable isotopes and modelling to extract a highly informative dataset that helps produce a more complete symphony of the highly variable dynamics occurring in upland catchments. Such knowledge is crucial in order to effectively evaluate the influence of climate change on the water resources that both nature and humans so heavily depend on.

The 14Be nucleus is a good candidate for having a halo structure. When a three- body model is used to calculate the properties of this nucleus, it relies on a knowledge of the potentials involved and hence on the structure of the underlying two-body subsystem 13Be. Previuosly published calculations showed that, in order to describe 13Be and 14Be simultaneously, 13Be had to be either bound, or have a p-shell ground state, which is not consistent with the experimental data. In this thesis 13Be and 14Be are described as one or two neutrons outside a deformed 12Be core. The idea of the method is that deformation of the core couples the neutron motion with core excitations. The core is treated as a rigid rotor here, and for the neutron-core interaction we used a deformed Woods-Saxon potential. We explore the potential parameters compatible with the known properties of 12Be, 13Be and 14Be. The three-body model for 14Be used a hyperspherical expansion including core degrees of freedom. Compared to the previous works, we find that both 14Be and 13Be are described simultaneously if the 12Be core has large positive quadrupole deformation. The resulting three-body model wave function was used in calculations of reaction observables. The reaction cross section of 14Be on a carbon target at 850 MeV/A was calculated in a four-body Glauber model, after the formalism was extended to include core degrees of freedom. The calculated reaction cross section agrees with the experiment. One-neutron knockout reactions of the Borromean nuclei 6He, 11Li and 14Be are discussed. The integrated cross sections for stripping and diffraction processes are calculated in the four-body Glauber model including core excited components in the wave function for 14Be. The neutron-core relative energy distributions within 5He, 10Li and 13Be following one-neutron removal, are calculated by a spectator model in the eikonal limit. The integrated cross sections and energy distributions for 6He are in agreement with the experiment. The results for 11Li and 14Be breakup demonstrated that the further investigation of the reaction model is needed.

This dissertation investigated the physical and biogeochemical processes affecting the source, fate, and transport of sediment, carbon, and nitrogen within a highly-coupled fluviokarst system. Elemental and isotopic datasets were collected at surface and subsurface locations for both dissolved and particulate contaminant phases, new methodology regarding data collection was presented to the karst research community, an in-cave sediment transport model coupling physical transport with elemental and isotopic mass balances of carbon and nitrogen was formulated, pathway and process control on nitrate leaching from agricultural karst watersheds was assessed, and nitrate mobilization and fractionation were modeled using high frequency storm sampling and long-term low-flow sampling. Data and modeling results indicate that phreatic karst conduits are transport-limited during hydrologic events and experience subsurface deposition of labile, storm-injected sediment which is subsequently decomposed by heterotrophic bacteria. An estimated 30% of the organic carbon associated with sediment is decomposed during transport in the subsurface karst. Concentrations of nitrate in subsurface waters are consistently 50% greater than surface inputs suggesting an additional source of subsurface nitrate. Further modeling of nitrate leaching indicates that quick-flow water sources dilute nitrate concentrations and slow-flow (epikarst and phreatic) sources account for approximately 90% of downstream nitrate delivery. Field sampling of extreme events highlights the physical transport and delayed release of high nitrate concentrations by intermediate karst pathways, which is likely associated with a transition from epikarst to soil drainage during storm recession. Modeling of sediment carbon and nitrogen within the karst SFGL supports the idea that the cave sediment bed experiences hot spots and hot moments of biogeochemical activity. Sediment nitrogen tracing data show a significant increase in δ15NSed at the spring outlet relative to karst inputs indicating the potential for isotope fractionation effects during dissolved N uptake by cave biota. Dissolved nitrogen stable isotopic composition shows a significant downstream decrease in δ15NNO3 within the conduit, likely associated with nitrification. Data and modeling results of sediment, carbon, and nitrogen emphasize the role of multiple pathways, turbulent transport, and in-conduit transformations in controlling contaminant flux from karst watersheds.

Le déphasage entre augmentation de température et augmentation de gaz à effet de serre (GES) dans l'atmosphère lors des grandes transitions climatiques passées est estimé grâce aux mesures effectuées dans les carottes de glace polaires dans la phase glace pour la température et dans la phase gaz (bulles d’air piégées) pour la concentration en GES. Ce déphasage est encore mal contraint et, pour résoudre ce problème, il est nécessaire de mieux comprendre le processus mécanique de transformation de neige en glace près de la surface de la calotte (centaine de mètres supérieurs, le névé). En l'absence de fusion, la transformation de la neige (matériau à porosité ouverte en contact avec l'atmosphère) en glace (matériau contenant des bulles d'air isolées) s'effectue progressivement sous l'action des gradients de température près de la surface, puis sous l'action du poids des couches de neige situées au-dessus. Selon les conditions de température et précipitation, ce processus peut prendre quelques décennies à plusieurs millénaires et s'étend sur une centaine de mètres de profondeur. Il contrôle la différence d'âge entre la glace et les gaz qu'elle renferme. La prédiction de la profondeur de piégeage des gaz représente un enjeu majeur pour la paléoclimatologie en particulier sur la séquence des changements relatifs de température et de concentration en gaz à effet de serre.Un modèle thermo-mécanique de densification de la neige a été conçu et développé au LGGE en intégrant la formulation des processus mécaniques, des propriétés thermiques, et des critères de piégeage des gaz. Les performances de ce modèle peuvent être testées et améliorées grâce à des études de structure de névés actuels (densité, rapport porosité ouverte/fermée, …). Pour les périodes plus anciennes, des mesures d’isotopes des gaz inertes d15N et d40Ar) dans l’air piégé dans les carottes de glace polaire permettent d’obtenir des informations directes sur les variations passées de la structure du névé (e.g. épaisseur de la zone diffusive). Les larges divergences observées en Antarctique entre les sorties de modèle de densification et les mesures isotopiques de gaz piégé dans la glace génèrent une grande incertitude sur les reconstructions climatiques passées et comprendre ce désaccord est un défi majeur de la paleoclimatologie actuelle.Dans le cadre de cette thèse, nous avons pris en compte les influences de la dépendance à la température des énergies d'activation et des impuretés (poussières) sur la vitesse de densification. Cela a permis de concilier les données et le modèle. Les résultats du modèle modifié sont globalement cohérents avec les profils de densité mesurés pour des névés actuels et les données d'isotopes de gaz inertes pour les déglaciations (aussi appelées terminaisons). Nous avons également présenté de nouvelles mesures à haute résolution de d15N et d40Ar pour les terminaisons 2 (129-138ka) et 3 (243-251ka) des carottes antarctiques de Dôme C et Vostok. Nous avons montré que les différentes évolutions de d15N entre les différents sites et différentes déglaciations s’expliquaient largement par les différences de taux d’accumulation qui contrôlent la profondeur de transition neige-glace. Nous avons aussi montré que l’utilisation des isotopes de l’air était un complément important à l’utilisation des isotopes de l’eau pour contraindre la dynamique climatique locale en Antarctique de l’Est lors des déglaciations<br>The phasing between increases in temperature and greenhouse gas concentrations during large climatic variations in the past is classically estimated using analyses in polar ice cores, in the ice phase for the temperature and in the gas phase (trapped air bubbles) for the concentration of greenhouse gases. This phasing is still insufficiently constrained and solving this problem requires a better understanding of the mechanical process of snow to ice metamorphism near to the top of the ice sheet (i.e. the firn, about 100 m deep). In the absence of melting, the transformation of snow (a material with open porosity in contact with the atmosphere) into ice (a material containing isolated bubbles) occurs progressively as a response to temperature gradients near the surface, and the weight of overlying snow in deeper layers. Depending on temperature and precipitation conditions, this process occurs in a few decades to several millennia and a ~100 meters depth range. It controls the age difference between the ice and the entrapped gases. Predicting the gas trapping depth is a major issue in paleoclimatology, especially in order to understand the phasing between temperature changes and changes in greenhouse gas concentrations.A thermo-mechanical model of snow densification has been developed at LGGE, it includes the main mechanical processes, the thermal properties of ice, and gas trapping criteria. The model performances can be tested and improved using experimental studies of modern firns (density, open/closed porosity ratio, etc). For firnification under ancient climates, measurements of isotopes of inert gases (d15N et d40Ar) in the air trapped in ice cores provide direct informations about past variations of firn structure (e.g. diffusive zone thickness). Large differences between firn densification model outputs and gas isotopic data are obtained in Antarctica, and imply a large uncertainty on past climatic reconstructions. Understanding this discrepancy is a major issue in paleoclimatology.As part of this thesis work, we took into account the effects of the temperature dependence of activation energies and impurities (dust) on the firn densification speed. It allowed to reconcile the model results with available data. The modified model results show an overall consistency with measured density profiles of present-day polar firns and isotopes of inert gases over deglaciations (also called terminations). We also analyze new high resolution measurements of d15N and d40Ar over Terminations 2 (129-138ka) and 3 (243-251ka) on the Dome C and Vostok ice cores. We have shown that the different evolutions of d15N between different sites and different deglaciations are largely explained by differences in accumulation rates that control the snow/ice transition depth. We also showed that the use of air isotopes was an important complement to the use of water isotopes to constrain local climatic dynamics in eastern Antarctica during deglaciations

As ever-increasing levels of carbon dioxide alter the chemistry of the Earth’s atmosphere, understanding the global carbon cycle becomes increasingly important. A particularly important component is the riverine carbon cycle, as rivers are the primary conduits for dissolved inorganic carbon from terrestrial watersheds to ocean basins. Stable carbon isotopes (13C/12C) were collected weekly and input into the mixing model IsoSource to delineate seasonal carbon sourcing along two nested basins in the upper Green River System, Kentucky. In the more siliciclastic upstream catchment, dissolved inorganic carbon (DIC) was primarily derived from soil respiration (34%). Groundwater dissolving carbonate bedrock and carbonate dissolution/precipitation reactions contributed 31% and 11%, respectively. The more carbonate-dominated downstream catchment also was influenced greatly by soil respiration (35%). Due to the more pronounced levels of carbonate bedrock, carbonate reactions contributed double that of the upstream catchment (20%), with groundwater contributing 22%. Seasonally, the upstream basin gathered most DIC from soil respiration from late spring to winter. Early spring precipitation and still limited photosynthesis caused the primary carbon sourcing to shift to groundwater. Downstream, the primary source throughout the entire study period was soil respiration. Collectively, this study provides insight into the carbon cycling process in a mid-latitude, karstic river using carbon isotope sourcing to aid in the quantification of global carbon flux in the critical zone.

In order to achieve a more accurate input value for groundwater age determining models, δ13CVPDB values for soil gas were evaluated at 50 cm depths in locations throughout Utah in order to define correlations between δ13C and environmental parameters. 16 sites were chosen that exploited large changes in elevation and latitude which provided variations in climate, precipitation, plant community, etc. Gas samples were collected over 1-1½ years, and soil samples were collected at depth during installations. Field and laboratory studies were also used to evaluate CO2 and δ13C change with depth. It was discovered that in mountainous recharge areas, the mean δ13C value was −21.8 ± 2.8‰ (2σ). Use of this value reduced the uncertainty in groundwater age models by close to half relative to the current assumed range of -13‰ to -28‰. One arid and one semi-arid site were located at Pilot Valley (mean -11.0‰ ± 2.0, 2σ) and the mouth of Cedar Canyon (mean -17.5‰ ± 0.8, 2σ). The high values at these locations may be due to low root respiration and low microbial activity rates caused by high temperatures, low precipitation, and low vegetation density. Correlations among environmental parameters and δ13C values were discovered with ANOVA, but the differences were small enough that for practical purposes they are insignificant.

Malgré leur distance de la pression industrielle, les milieux marins austraux et antarctiques sont contaminés par des polluants globaux, tels que le mercure (Hg), par le transport atmosphérique et les courants océaniques. Jusqu'à présent, les voies de contamination du Hg dans l'Océan Austral restent peu connues, en particulier dans le secteur Indien, et de nouvelles études sont nécessaires pour élucider son devenir et son impact dans ces régions. Les oiseaux marins, en tant que prédateurs supérieurs des chaînes alimentaires marines, sont exposés à des concentrations élevées de méthylmercure (MeHg) via leur régime alimentaire. De plus, ils présentent des habitats de prédation contrastés dans les différents compartiments marins (en termes spatiaux et de profondeur). Par conséquent, ils sont considérés comme des bioindicateurs efficaces de la contamination par Hg et la bonne connaissance de leurs caractéristiques écologiques permet leur application comme traceurs du Hg dans des environnements éloignés, qui sont autrement difficiles à échantillonner. L'objectif principal de ce travail de doctorat est la caractérisation des voies d'exposition de MeHg accumulé par les oiseaux marins et l'identification des processus impliqués dans le cycle biogéochimique du Hg dans l'Océan Austral (des eaux antarctiques aux eaux subtropicales). L'approche méthodologique proposée consiste à combiner la composition isotopique du Hg et la spéciation du Hg dans les tissus d’espèces d’oiseaux marins de l'Océan Austral choisis précisément. Dans une première étape, une évaluation des signatures isotopiques de Hg spécifique de chaque tissu a été réalisée, notamment dans le sang et les plumes, car ceux-ci peuvent être échantillonnés de manière non létale. Chez les poussins, les deux tissus peuvent être utilisés efficacement et indifféremment comme bioindicateurs de la contamination locale en utilisant des isotopes du Hg, alors que chez les adultes, chaque tissu donne accès à une exposition temporelle différente : le sang à l'échelle récente (c'est-à-dire l'exposition pendant la période de reproduction) et les plumes à l'échelle annuelle, fournissant ainsi des informations isotopiques complémentaires aux différentes étapes du cycle annuel des oiseaux. Une deuxième partie a été axée sur l'exploration des sources de MeHg dans quatre espèces de manchots d’une même zone subantarctique, les îles de Crozet. Les populations ont été discriminées par leurs signatures isotopiques du Hg en fonction de leurs habitats de prédation et leurs mouvements latitudinaux, qui se trouvent être les facteurs principaux déterminant leur exposition à différentes sources de MeHg. Dans une troisième partie, les isotopes du Hg ont été étudiés dans deux modèles d'oiseaux (poussins de labbes et manchots adultes) sur une grande échelle latitudinale de l'Antarctique à la zone subtropicale. Les variations latitudinales des valeurs isotopiques du Hg (δ202Hg, Δ199Hg) semblent être influencées par une différente amplitude des processus photochimiques et d'autres voies biogéochimiques telles que la réduction du Hg, et les processus de méthylation / déméthylation, ainsi que les processus trophiques ou métaboliques<br>Despite their distance from industrial pressure, marine southern and Antarctic environments are contaminated by worldwide distributed pollutants, such as mercury (Hg), through atmospheric transport and oceanic currents. So far, Hg contamination pathways in the Southern Ocean remains poorly understood, particularly in the Indian sector, and new studies are required to elucidate its fate and impact in these regions. Seabirds, as top predators of marine food webs, are exposed to elevated concentrations of biomagnified methylmercury (MeHg) via dietary intake and moreover, they forage in the different marine compartments both in spatial and depth terms. Therefore, they are considered as effective bioindicators of Hg environmental contamination and the good knowledge of their ecological characteristics permits their application for tracing Hg in such remote environments otherwise of difficult access. The main objective of this doctoral work is the characterization of the exposure pathways of the MeHg accumulated in model seabirds and the identification of the processes involved in the Hg biogeochemical cycle in the Southern Ocean (from Antarctic to subtropical waters). The proposed methodological approach consisted on the combination of Hg isotopic composition and Hg speciation in tissues of a precise selection of seabirds of the Southern Ocean. In a first step, the evaluation of tissue-specific Hg isotopic signatures was accomplished notably in blood and feathers, as they can be non-lethally sampled. In chicks, both tissues can be effectively and indifferently used for biomonitoring of local contamination using Hg isotopes, whereas in adults each tissue provides access to different temporal exposure : blood at recent scale (i.e. exposure during the breeding period) and feathers at annual scale, thus providing complementary isotopic information at the different stages of seabird annual cycle. A second part was focused on the exploration of MeHg sources in four penguin species within a same subantarctic location, the Crozet Islands. Hg isotopes effectively discriminated the four populations and species-specific foraging habitats and latitudinal movements were found the main factors determining their exposure to distinct environmental MeHg sources. In a third part, Hg isotopes were investigated in two ubiquitous seabird models (skua chicks and penguins) over a large a latitudinal scale from Antarctica to the subtropics. Latitudinal variations of Hg isotopic values (δ202Hg, Δ199Hg) appeared to be influenced by different extent of photochemical processes and other biogeochemical pathways such as Hg reduction, and methylation/demethylation processes, as well as trophic or metabolic processes

Climate change is expected to have significant and uncertain impacts on methane (CH4) emissions from northern peatlands. Biogeochemical models can extrapolate site-specificCH(4) measurements to larger scales and predict responses of CH4 emissions to environmental changes. However, these models include considerable uncertainties and limitations in representing CH4 production, consumption, and transport processes. To improve predictions of CH4 transformations, we incorporated acetate and stable carbon (C) isotopic dynamics associated with CH4 cycling into a biogeochemistry model, DNDC. By including these new features, DNDC explicitly simulates acetate dynamics and the relative contribution of acetotrophic and hydro-genotrophic methanogenesis (AM and HM) to CH4 production, and predicts the C isotopic signature (delta C-13) in soil C pools and emitted gases. When tested against biogeochemical and microbial community observations at two sites in a zone of thawing permafrost in a subarctic peatland in Sweden, the new formulation substantially improved agreement with CH4 production pathways and delta C-13 in emitted CH4 (delta C-13-CH4), a measure of the integrated effects of microbial production and consumption, and of physical transport. We also investigated the sensitivity of simulated delta C-13-CH4 to C isotopic composition of substrates and, to fractionation factors for CH4 production (alpha(AM) and alpha(HM)), CH4 oxidation (alpha(MO)), and plant-mediated CH4 transport (alpha(TP)). The sensitivity analysis indicated that the delta C-13-CH4 is highly sensitive to the factors associated with microbial metabolism (alpha(AM), alpha(HM), and alpha(MO)). The model framework simulating stable C isotopic dynamics provides a robust basis for better constraining and testing microbial mechanisms in predicting CH4 cycling in peatlands.

Green sea turtles in Bermuda are overgrazing the seagrasses on which later life stages are thought to specialize. I hypothesized that larger green turtles in Bermuda would display individual diet specializations during seagrass scarcity. Stable isotope methods were used to determine the diet composition of green sea turtles from the Bermuda Platform as a function of size class and in turtles captured in successive years. Individual turtles had a wide range of diets, however, the variation in diets was driven by differences among size class rather than within the size classes of larger turtles, indicating that green turtles undergo a dietary ontogenetic shift during their residency on the Bermuda Platform and no clear specialization of diets among late-stage individuals. The apparent lack of dietary specialization of larger turtles indicates that older turtles are not diversifying their diets in response to the drastic reductions in seagrass in Bermuda.

The Daya Bay reactor neutrino experiment is a high sensitivity experiment designed to determine the last unknown neutrino mixing angle $theta_{13}$ by measuring disappearance of reactor antineutrinos emitted from six 2.9 $GW_{th}$ reactors at the Daya Bay Nuclear Power Station. There are eight identical Gd-loaded liquid scintillator detectors deployed in two near (flux-weighted baseline 512 $m$ and 561 $m$) and one far (1579 $m$) underground experimental halls to detect the inverse beta decay interaction. This dissertation describes the Daya Bay Experiment and individual contributions to this experiment. Chapter 1 reviews the history of the neutrino and the neutrino oscillation phenomena. The reactor based neutrino experiments in different times are described in this chapter in detail. It presents the motivation of the Daya Bay Experiment. In Chapter 2, the neutrino detection method and the $theta_{13}$ relative measurement method are introduced. This chapter focuses on the design of the Daya Bay Experiment, including antineutrino detector, calibration system, muon veto system and muon tagging system. Chapter 3 shows the design, development, construction, and assembly of Muon Pool PMT calibration system, and presents an algorithm of calculating the muon pool PMT timing offset values. Chapter 4 focuses on the manufacture, installation and commissioning of RPC HV system. Chapter 5 presents the analyses of the radioactive isotopes induced by comic muons. The Daya Bay detector energy response model is also described in detail. The relative rate analysis results exclude a zero value from $sin^22theta_{13}$ with a significance of 7.7 standard deviation using 139 days of data, 28909 (205308) antineutrino candidates which were recorded at the far hall (near halls) and shows $sin^22theta_{13} = 0.089pm0.011$ in a three-neutrino framework. A combined analysis of the $overline nu_e$ rates and energy spectra based on the detector energy response model improved measurement of the mixing angle $sin^22theta_{13} = 0.090^{+0.008}_{-0.009}$ by using 217 days of data, 41589 (203809 and 92912) antineutrino candidates were detected in the far hall (near halls). Also the first direct measurement of the $overline nu_e$ mass-squared difference $|Delta m^2_{ee}|= (2.59^{+0.19}_{-0.20})times10^{-3}$ $eV^2$. It is consistent with $|Delta m^2_{mumu}|$ measured by muon neutrino disappearance, supporting the three-flavor oscillation model.<br>Ph. D.

The stable isotopes 2H and 18O, combined with noble gases and radioisotopes (e.g., 3H, 14C, 36Cl), are used to infer groundwater age and climate during recharge. Flow regimes within low-velocity flowpaths and long residence times could allow an aquitard-aquifer diffusive flux to alter isotope abundance. Consequently, the diffusion of isotopes (e.g., 14C, 2H and 18O) between aquifers and confining layers needs to be considered in such conditions. In this study, COMSOL Multiphysics was used to determine if diffusion of 18O (and 2H by proxy) from a bounding aquitard could explain observed downgradient enrichment of 2H and 18O within a regional aquifer. Using the geologic and hydraulic properties of the lower Wilcox aquifer of the Mississippi Embayment aquifer system in Missouri and Arkansas, the advection-dispersion equation was solved along a 1-D groundwater flow domain, coupled with a cross-contact aquitard-aquifer diffusive flux. Although the observed signal within the lower Wilcox was not matched, a sensitivity analysis indicated the importance of the isotope composition gradient between the aquifer and aquitard. Furthermore, groundwater velocity was suggested as a controlling influence on aquitard-aquifer exchange that could alter aquifer isotope composition.

The present dissertation seeks to deepen our understanding of hydrogen transfers and especially C-H bond activations in enzymes. Hydrogen transfers are ubiquitous in chemistry and biology and a thorough understanding of how they occur and what factors influence them will facilitate developments in biomimetic catalysis, rational drug design, and other fields. A particular difficulty with H-transfers is the importance of nuclear quantum effects to the reaction, particularly tunneling. The overall scope of the work here aims to examine how experimental kinetic isotope effects (KIEs) can be interpreted with a particular type of tunneling model, referred to as Marcus-like models, to yield a semi-quantitative picture of the physical mechanisms of H-transfers. Previous work had used this kind of model to qualitatively interpret experimental data using a combination of intuition and generalized theories. The work here examines these theories in quantitative detail, testing and calibrating our intuition in the context of several experimental systems. The first chapter of research (ch. II) focusses on the temperature dependence of primary KIEs and how these experiments can be quantitatively interpreted as a probe for certain kinds of enzyme or solvent dynamics. The subsequent chapters (ch. III-VI) focus on the use of secondary KIEs to determine the detailed structures of tunneling ready states (TRSs) and how the dynamics of H-tunneling affect those structures. These chapters focus primarily on the TRS of the enzyme alcohol dehydrogenase, but by examining an uncatalyzed analogue to that reaction (ch. VI), the work gains some insight about similarities and differences between catalyzed and uncatalyzed reactions. In summary, the work uncovers some principles of catalysis, not just the mechanism of a catalyzed reaction. The mechanism of C-H activation presented here provides an elegant solution to problems that have been vexing to accommodate within traditional models. This work constitutes some initial steps in making Marcus-like models quantitatively useful as a supplement or even replacement for traditional models of reactivity.

The present thesis describes the results of an experiment which was performed at the GSI laboratory, Darmstadt, Germany in September 2009, aiming at studying neutron-rich nuclei in the region of lead nuclei. In the introduction the main modern research lines on nuclear structure are briefly described. A particular attention is devoted to the study of exotic nuclei, i.e. of nuclei far from the valley of stability. They are one of the most important themes of research in nuclear physics since their study may point out details of the nuclear interaction not easy to disentangle investigating only stable nuclei. Moreover, significant modifications of the nuclear structure are expected, as the disappearance of the known magic numbers and the appearance of new ones. The study of neutron-rich nuclei is also fundamental to quantitatively understand the nucleosynthesis of heavy elements via the rapid process thought to happen in supernovae. However, the study of exotic nuclei is hampered by many difficulties, concerning both their production and their measurement. The region of neutron-rich lead nuclei, which is the object of the present thesis, has proved quite complicate to study due to the fact that these isotopes cannot be populated via fusion-evaporation or deep-inelastic reactions using stable beams. Therefore, other production mechanisms must be exploited, as the fragmentation reactions. The drawbacks of this method are the low production cross sections and the necessity of using high-energy uranium beams (1 GeV A) in order to facilitate the separation in mass and atomic number. As a consequence, the neutron-rich isotopes around doubly-magic 208Pb have been less investigated with respect to regions with lower mass, as for example those around doubly-magic 132Sn and 78Ni. Nevertheless, these isotopes are very important. On the one hand, they are fundamental for evaluating the evolution of the nuclear shell structure beyond N = 126. On the other hand, the measurement of the beta half lives of these nuclei is of paramount importance to better parameterize the models from which the half lives of the nuclei directly involved the rapid process stellar nucleosynthesis have to be extrapolated. The exotic nuclei close to lead were produced by the fragmentation of a relativistic uranium beam at an energy of 1 GeV A. The available energy and beam current, around 10^9 particles per second (pps), allowed to cope with the aforementioned mass separation issue and to have a sufficient production yield, despite the low cross sections. The reaction products were separated with the mass spectrometer FRagment Separator (FRS), which allows one to select the channels of interest and to identify, event by event, the produced ions in mass and atomic number. The identified fragments were then slowed down in a degrader and implanted in double-sided silicon-strip detector, which allows one to measure the time and the position of the implantation. This array also detects the beta decay of the implanted isotopes, by revealing the produced electrons. This implantation setup is surrounded by the high-efficiency array RISING, composed of 105 germanium crystals. Its purpose is to measure energy and time of the gamma rays emitted in the isomer or beta decay of the implanted ions. Employing the described setup, new isomers in the even-even lead isotopes 212;214;216Pb were identified. The excitation spectrum of such nuclei, observed for the first time, follows the seniority scheme as expected for semi-magic nuclei outside a double shell closure. From the measurement of the isomer lifetimes the electromagnetic transition matrix elements are deduced. They are a sensitive probe of the nuclear wave function and hence their comparison with theoretical predictions indicates to which extent the nuclear models employed are able to reproduce the nuclear structure. The observed significant experimental discrepancies with the shell-model calculations (derived from realistic nuclear hamiltonian with state-of-the-art shell-model codes) require the introduction of state-dependent effective charges. It is shown that this need is actually the result of neglecting of effective three-body forces. These forces appear when an interaction is adapted to a certain valence space, but are almost always not considered as they are thought to produce negligible effects. In the present thesis it is proved that, on the contrary, these forces can produce significant changes on the final results of the calculations. Isomers were also observed in 217Bi, 213Pb, 211;213Tl and 208;210Hg. The assignment of spin and parity to the levels in 217Bi and 208Hg was straightforward, whereas in the other isotopes it was more challenging. In conclusion, the present work reports results on several new isomers in the neutron-rich nuclei in the lead region. The technical difficulties of the experiment and the devices exploited to overcome them are also highlighted. The physical cases studied appear very interesting, as they point out previously neglected aspects of the nuclear structure. The future investigations that will be possible with the new accelerators and detection setups under construction will shed more light on these issues<br>La presente tesi di dottorato descrive la preparazione ed i risultati di un esperimento svoltosi presso il laboratorio del GSI, Darmstadt, Germania nel settembre 2009. Nella parte introduttiva si descrivono brevemente le principali linee della moderna ricerca sulla struttura nucleare. Particolare risalto viene dato alla tematica dello studio dei nuclei esotici, cioè lontani dalla valle di stabilità. Essi costituiscono uno dei più importanti obbiettivi di ricerca della fisica nucleare, poiché il loro studio metterà in evidenza aspetti dell'interazione nucleare altrimenti difficili da comprendere. Inoltre, ci si aspettano significativi cambiamenti della struttura del nucleo, come la scomparsa degli usuali numeri magici e l'apparizione di nuovi. Lo studio dei nuclei ricchi di neutroni è anche fondamentale per comprendere quantitativamente la nucleosintesi degli elementi pesanti nel processo rapido, che si pensa accada nelle supernovae. Lo studio dei nuclei esotici presenta tuttavia numerose difficoltà, legate sia alla loro produzione che alla loro misura. La regione dei nuclei di piombo ricchi di neutroni si è infatti rivelata particolarmente complicata da studiare poiché questi isotopi non possono essere popolati con reazioni di fusione-evaporazione o di trasferimento di nucleoni usando fasci stabili. Quindi devono essere sfruttati altri meccanismi di produzione, come le reazioni di frammentazione. Questo metodo ha però degli svantaggi, come le basse sezioni d'urto di produzione e la necessità di disporre di fasci di uranio ad alta energia (1 GeV A) al fine di agevolare la separazione in massa e in numero atomico. Conseguenza di ciò è il fatto che gli isotopi ricchi di neutroni nella zona del nucleo doppio magico 208Pb, siano stati assai meno studiati rispetto a regioni di massa inferiore, come per esempio quelle attorno ai nuclei doppio magici 132Sn e 78Ni. Pur tuttavia questi isotopi sono estremamente interessanti. Da un lato essi sono fondamentali per valutare l'evoluzione della struttura a shell nucleare allontanandosi dal 208Pb, cercando di capire se le chiusure di shell Z = 82 e N = 126 permangano tali. D'altra parte la misura della vita media per decadimento beta di questi nuclei è di vitale importanza per parametrizzare meglio i modelli dai quali poi si devono estrapolare le vite medie degli isotopi direttamente coinvolti nel processo rapido di nucleosintesi stellare. I nuclei esotici vicini al piombo sono stati prodotti mediante frammentazione di un fascio di uranio relativistico ad una energia di 1 GeV A. L'energia e la corrente di fascio disponibili, circa 10^9 particelle per secondo, hanno permesso di affrontare i problemi appena menzionati riguardanti la separazione in massa e di avere una produzione sufficiente dei nuclei di interesse, nonostante le basse sezioni d'urto. I prodotti di reazione sono stati separati con lo spettrometro di massa FRS, che permette di selezionare i canali di interesse e identificare gli ioni prodotti in massa e in numero atomico, evento per evento. I frammenti così identificati sono quindi stati rallentati in un degrader e poi impiantati in un rivelatore a strip di silicio, che permette di misurare tempo e posizione dell'impianto. Questo rivelatore permette inoltre di studiare il decadimento beta degli isotopi in esso fermati, misurando gli elettroni prodotti. Questo rivelatore è circondato da un apparato ad alta efficienza per la misura di fotoni , RISING, composto da 105 cristalli di germanio. Il compito di RISING è quello di misurare energia e tempo dei raggi gamma emessi nel decadimento degli isomeri o nel decadimento beta degli ioni impiantati. Utilizzando gli apparati descritti, si sono studiati gli isomeri negli isotopi pari-pari del piombo 212;214;216Pb. Il loro spettro di eccitazione, studiato per la prima volta, segue molto bene lo schema di seniorità previsto in questi casi. La misura delle vite medie degli isomeri ha inoltre permesso di ricavare gli elementi di matrice di transizione elettromagnetica. Essi rappresentano una sonda molto accurata della funzione d'onda nucleare e quindi la loro comparazione con le stime teoriche consente di stabilire se i modelli nucleari impiegati sono veramente in grado di riprodurre la struttura del nucleo. Le discrepanze che si sono osservate rispetto calcoli di modello a shell (effettuati con hamiltoniani realistici e i più moderni codici di calcolo), richiederebbero l'introduzione di cariche efficaci dipendenti dallo stato per essere risolte. Si è dimostrato che questo è dovuto al fatto che le forze efficaci a tre corpi sono state trascurate. Queste forze, che compaiono ogniqualvolta un hamiltoniano è adattato ad uno spazio di valenza, sono quasi sempre ignorate perché si ritiene producano effetti molto piccoli. In questa tesi si dimostra invece che questa approssimazione può portare a importanti variazioni nei risultati finali dei calcoli. Sono stati osservati isomeri anche negli isotopi 217Bi, 213Pb, 211;213Tl e 208;210Hg. Attribuire spin e parità ai livelli nel 217Bi e nel 208Hg si è rivelato semplice, mentre neglialtri nuclei è stato più difficile. In conclusione, la tesi riporta risultati su diversi nuovi isomeri nei nuclei ricchi di neutroni vicino al piombo. Si sono anche messe in luce le difficoltà tecniche legate all'esperimento e gli accorgimenti adottati per risolverle. I casi fisici trovati appaiono molto interessanti, poiché mettono in evidenza aspetti prima ignorati della struttura nucleare. Le future indagini che saranno possibili con gli acceleratori e gli apparati oggi in costruzione forniranno molte più informazioni su queste tematiche

PANcreatic DERived factor (PANDER, FAM3B) is a member of a superfamily of FAM3 proteins that are uniquely structured and strongly expressed from the endocrine pancreas and co-secreted with insulin. Unique animal models available to our lab have indicated that PANDER can induce a selective hepatic insulin resistant (SHIR) phenotype whereby insulin signaling is blunted yet lipogenesis is increased. The complexity of the biological networks involved with this process warranted the logical approach of employing quantitative mass spectrometry based proteomic analysis using stable isotope labeling of amino acids in cell culture (SILAC) to identify the global proteome differences between the PANDER transgenic (TG) overexpressing murine model to matched wild-type mice under three metabolic states (fasted, fed and insulin stimulated). Additionally, this technique was used to compare the hepatic proteome of mice on a high fat diet to elucidate early and late mechanisms of disease progression. The “spike-in” process was employed by equal addition of lysate obtained from livers of heavy L-Lysine (13C6, 97%) fed mice to the mice liver protein lysate (PANTG and WT) for relative quantitative analysis. Upon acquisition of the dataset by use of liquid chromatography tandem mass spectrometry (LC-MS/MS, LTQ Orbitrap), geometric means and Uniprot Protein identification numbers were uploaded to Ingenuity Pathway Analysis (IPA) to reveal the effect of PANDER on hepatic signaling. IPA identified lipid metabolism and fatty acid synthesis as top cellular functions differentially altered in all metabolic states. Several molecules with a role in lipid metabolism were identified and include FASN, ApoA1, ApoA4, SCD1, CD36, CYP7A1 and ACC. Furthermore, central to the differentially expressed proteins was the revealed activation of the liver X receptor (LXR) pathway. In summary, our SILAC proteomic approach has elucidated numerous previously unidentified PANDER induced molecules and pathways resulting in increased hepatic lipogenesis. In addition, we have demonstrated strong utility of this approach in comprehensively phenotyping animal models of hepatic insulin resistance. PANDER may potentially propagate pro-hepatic lipogenic effects by LXR activation in contrast to increased LXRα expression. This can be evaluated through the use of LXR agonists (T0901317) antagonists (GSK 2033). LXR activity can be measured by luciferase assays using an LXRE response plasmid. Our central hypothesis is that PANDER induces activation of LXR and is measured and predicted in our line of experiments. In general, PANDER induced LXR activation will be enhanced by T0901317 and diminish effects of GSK 2033 along with direct correlation of downstream metabolic effects such as increased hepatic lipogenesis and fatty acid metabolism. Taken together, PANDER strongly impacts hepatic lipid metabolism and may induce a SHIR phenotype via the LXR pathway. Additionally, phosphoproteomic analysis uncovered large-scale differences in protein phosphorylation states as PANDER impacts insulin signaling. A notable finding was the increased phosphorylation of glycogen synthase (GSK), possibly responsible for the decreased hepatic glycogen content in the PANTG mouse. In an effort to map out critical molecules involved in non-alcoholic fatty liver disease (NAFLD) pathogenesis, the same proteomic approach was carried out, providing a unique dataset of differentially expressed hepatic proteins due to a high at diet.

Thesis (Ph. D.)--University of Missouri-Columbia, 2007.<br>The entire dissertation/thesis text is included in the research.pdf file; the official abstract appears in the short.pdf file (which also appears in the research.pdf); a non-technical general description, or public abstract, appears in the public.pdf file. Title from title screen of research.pdf file (viewed on September 27, 2007) Vita. Includes bibliographical references.

Le Crétacé est décrit comme la période la plus chaude des derniers 300 millions d’années. La circulation océanique et l’origine des eaux profondes alimentant les bassins restent mal connues pour le Crétacé, alors qu’elles sont capitales dans la compréhension du rôle de l’océan dans l’évolution du climat à cette époque. Les isotopes du néodyme (Nd) permettent de tracer la circulation océanique et les échanges entre les masses d’eau, et ont été utilisés pour explorer la circulation océanique globale des océans actuels et dans le passé. La composition isotopique en Nd (εNd) des océans dérive de celle des continents qui les entourent. Les courants océaniques exportent cette signature, et les eaux profondes de chaque bassin océanique ont une composition en Nd qui leur est propre. L’interprétation du signal du Nd des eaux océaniques au Crétacé est rendue difficiles du fait de l'insuffisance de la couverture spatiale et temporelle des données existantes. L’objectif de cette thèse est l’acquisition la signature en Nd sur les marges continentales et dans les régions dépourvues de données pour le Crétacé, en se focalisant prioritairement sur les zones potentielles de production d’eau profonde. L’εNd est analysé à partir d’échantillons de dents de poissons, d’oxydes encroûtant les tests de foraminifères et de fraction détritique extraits des sédiments. Les résultats sont comparés aux données de la littérature afin d’identifier les sources des eaux profondes et leur évolution au cours du Crétacé. Les liens entre les changements océanographiques, paléogéographiques et climatiques sont explorés avec un modèle climatique couplé océan-atmosphère<br>The Cretaceous is depicted as the warmest period of the last 300 Ma. The oceanic circulation and location of the source zones of deep-waters are essential to understand the role of oceans in the evolution of the climate during the Cretaceous, yet they remain unclear for this period. The neodymium (Nd) isotopes are used to track oceanic circulation and exchanges between water masses, in both past and modern oceans. The Nd isotope composition (εNd) in the ocean is related to the nature of the surrounding continental landmasses. The oceanic currents transport this isotopic signature, thus every oceanic basin acquires a singular εNd. Unequivocal interpretations of the Cretaceous seawater εNd values stem from the insufficient spatial and temporal cover of available data. This PhD thesis intents to collect the Nd signature of continental margins and in regions without data for the Cretaceous, with special attention given to the potential source zones of deep-water production. Fossil fish teeth, coatings on foraminifera tests and detrital fraction from Cretaceous sediments are analyzed for their εNd. The results are compared to published data sets, in order to identify deep-waters source zones and their evolution throughout the Cretaceous. The links connecting oceanographic, palaeogeographic and climatic changes are investigated with a coupled ocean-atmosphere circulation model

My research focuses on reconstructing past climate in southern Arizona using cave deposits called speleothems. However, this necessitates a broader perspective than simply a geochemical time series, and therefore, I also investigate modern cave systems using a combination of modeling and observational datasets. Finally, cave deposits are fundamentally non-renewable resources, and sampling for past climate reconstruction can be destructive, unlike other cave uses. My last investigation is focused on developing possible best practice recommendations for paleoclimate scientists and other cave stakeholders moving forward. We developed two new stalagmite records of past climate variability in southern Arizona over the past 7000 years. Past climate reconstruction from two caves (Cave of the Bells and Fort Huachuca Cave) highlights insolation control of southern Arizona hydroclimate from 7000-2000 years before present. Additionally, comparison between two stalagmites with different seasonal sensitivities uncovers a few eras of multi-decade long droughts in southern Arizona, which align with other regional reconstructions of past climates and elucidate forcings on Southwest paleoclimate as emergent from both external (insolation) and internal climate variability in the Pacific and Atlantic Ocean basins. Although the oxygen isotopic signal of cave calcite in speleothems is complex, agreement with these other records indicates that the speleothem records from these caves primarily record a climate signal.Modeling and monitoring of modern caves both helps us interpret paleoclimate records and enhances our understanding of cave systems in their own right. Modeling of Cave of the Bells dripwaters demonstrates the effect of storage and mixing on the dripwater oxygen isotope signal; non-climate processes can imprint on dripwater variability on multidecadal timescales. Monitoring shows that on very small spatial scales, every cave is different, and even sites within the same cave respond uniquely to surface climate. Most notably, calcite oxygen isotopic composition, used to reconstruct past climate, shows seasonal variability unrelated to dripwater and surface rainfall oxygen isotope variability. Substantial oxygen isotope disequilibrium is identified at numerous caves sites in southern Arizona, and this understanding aligns with a growing number of cave studies that demonstrate the long-held assumption of isotopic equilibrium in cave systems may not always be valid or that the way in which we define isotopic equilibrium insufficiently captures the variety of processes controlling the oxygen isotopic composition of speleothems. Overall, however, monitoring can identify stalagmites that are more sensitive to surface climate and less sensitive to these in-cave processes by identifying sites with dripwater variability responses to surface rainfall variability and sites that precipitate close to oxygen isotopic equilibrium. Finally, a major missing component in speleothem research is the fact that speleothems take thousands and sometimes hundreds of thousands of years to form. They are non-renewable resources on human timescales, and habitat for myriad microbes that have yet to be identified. Removal of speleothems for paleoclimate research is one of the only destructive uses of these deposits. With that in mind, I also analyze current methods of collecting speleothems and develop a framework based on two surveys of scientists and stakeholders to assist scientists and managers when evaluating potential methods of incorporating cave conservation into the speleothem sampling process. Thus, I approach caves from a variety of angles and timescales, from the past through the present to the future, illuminating caves as complex scientific and social systems.

This thesis presents the development of a model of the Medical Isotope Production Reactor (MIPR): a conceptual Aqueous Homogeneous Reactor (AHR). The model is a point kinetics model with zero and one-dimensional thermal hydraulic feedbacks. Three versions of the model of increasing complexity are presented and a number of scenarios are modelled with each version. The results of these simulations shows the stability of the reactor against reactivity insertions caused by the strong negative reactivity feedbacks inherent to AHRs. The first version of the model is modified using intrusive polynomial chaos in order to simulate the effects of uncertainty in key parameters. This allows a novel study into which physical parameters and processes are important at each stage of a transient and in determining steady state conditions. The final version of the model is used to model the CRAC-43 experiment and, after modification to include the delay of radiolytic gas production which accompanies the start up of an AHR, good agreement was found between model and experiment. The development of the equations, correlations and parameters used in the model is approached from the point of view of the governing physics. This approach to model development enables a comprehensive exploration of the physical processes underpinning the behaviour of AHRs. As well as being one of the most complete and fundamentally based models of an AHR presented within the literature, the final model is also extremely versatile and general. Given the appropriate input neutronic and thermal-hydraulic data the model presented in this thesis should be able to simulate a very wide range of AHR behaviour.