<p>The tropics are a significant source of heat and moisture export, which drive global circulation patterns, thus it is vital to systematically understand the land, ocean and sedimentological interactions within the tropics. The Brazilian continental margin is an ideal region to characterize the tropics due to its unique local oceanography and proximity to the atmospheric engine that is the Amazon Basin. A combination of: 1) terrestrial organics and hydrology; 2) oceanographic temperature, isotopic composition, and salinity and 3) early diagenesis and geochemistry of sedimentary interstitial water and methane hydrate, provide a detailed understanding of the primary constituents that influence the South American tropics.</p><p>Sedimentological, organic and paleoceanographic reconstructions of the Amazon Basin, Brazilian Nordeste, and western equatorial Atlantic have been undertaken on two sediment cores located on the Brazilian continental slope representing 30 and 110 ka, respectively. High-resolution XRF analyses of Fe, Ti, K and Ca are used to define the sedimentological history of the Amazon Basin and northern Nordeste. Here we present elemental ratios of Ti/Ca and Fe/K, in addition to magnetic susceptibility, to determine variability in Amazon Basin and Nordeste hydrology. Bulk organic proxies d13C and d15N of sedimentary organic carbon are used to define the organic history of the Amazon Basin. Peaks in Ti/Ca and Fe/K ratios largely correlate in both the Amazon Basin and in the Nordeste Record. These excursions correlate with commonly modeled global slowdown of the Atlantic meridional overturning circulation (AMOC) during increased northern hemisphere glaciation. Differences in the Fe/K and Ti/Ca records suggest periods of increased chemical weathering independent of precipitation driven sediment discharge in the Brazilian Nordeste. Bulk organic geochemistry indicates the vegetative history of the Amazon Basin has been relatively stable during the late Quaternary.</p><p>High-­resolution stable oxygen isotopic analysis and Mg/Ca paleothermometry undertaken on the near-­surface-dwelling planktic foraminiferal species Globierinoides ruber provide a picture of paleoceanographic forcings in the western equatorial Atlantic. The Nordeste core exhibits a rapid warming of ~3.5ºC between the last glacial maximum and the early Holocene. Furthermore, in almost all cases during the last glacial stage, there was a 0.5 to 2ºC warming of the western equatorial Atlantic during the periods of high Ti/Ca ratios that correlate with slowdown of AMOC. Thus, as observed in some previous studies, the western equatorial Atlantic was warm and the adjacent southern tropical continent was wet coincident with increased glaciation in the high latitude northern hemisphere.</p><p>Interstitial pore waters were analyzed from the Amazon Fan and Brazilian continental slope to determine early diagenesis, methane hydrate potential and its geographic variability. Interstitial waters were measured for total Mg, Ca, SO4, alkalinity, Cl, and d18O, combined with seismic bottom simulating reflectors (BSRs) to investigate early diagenesis. Interpolated maps of sulfate reduction, sedimentation rate and maximum alkalinity were produced to examine geographic variability in early diagenesis and methane hydrates. Inorganic precipitation of calcium and magnesium, likely via dolomite and siderite, correlates with a decrease in alkalinity through inorganic carbonate precipitation and methanogenesis, but alternatively increases through redox pathways, specifically sulfate reduction.</p><p>Through multiple lines of evidence it is likely that there is extensive methane hydrate occurrence on the Amazon continental shelf. A combination of: 1) A rapid linear decline in sulfate at an unusually shallow depth; 2) Uncommonly high sedimentation rates and terrestrial organic carbon input; 3) Significant variability in what should be conservative chloride concentrations; 4) Large variability in interstitial oxygen isotopes; 5) Widespread occurrence of BSRs provide substantial support for the presence of methane hydrates. Sulfate reduction rates are lowest along the main Amazon channel, with highest values distal of the main channel. Sedimentation rates are relatively low on the continental shelf and surrounding the main channel and highest toward the distal end of the main channel. Sulfate reduction rates provide key insights to the potential geographic variability of methanogenesis and methane hydrate formation. Given the tremendous influx of sediment from the Amazon River, this region is prone to massive sediment failures, subsequent release of methane hydrates, as well as significant potential for natural gas hydrates.</p> / Dissertation

The Guinea Plateau is a shallow-marine, flat-lying bathymetric province situated along the equatorial West African margin, offshore Republic of Guinea. The Guinea Plateau and the conjugate Demerara Plateau hold particular geologic significance, as they represent the final point of separation between the African and South American continents during Gondwana break-up. Recent interpretation of both 2-D and 3-D seismic surveys along the Guinean margin have illuminated subsurface features related to Early Cretaceous crustal extension. Seismic structural investigations on these datasets suggest that the majority of extension is accommodated along large-scale listric normal faults located on a relatively narrow (<50 km) continental slope (up to ~39% extension). Minimal faulting reveals that little upper-crustal extension has occurred on the Guinea Plateau. Additionally, multiple 2-D seismic profiles image the transition from continental crust on the plateau and slope, to oceanic crust in the deeper marine basin. This continent-ocean boundary is the most representative boundary when testing the accuracy of plate reconstructions. Mapping of both the continent-ocean boundary and fracture zones across the equatorial Atlantic suggests that the Demerara Plateau and the South American plate are too far south in previous pre-rift reconstructions. A revised model introduces 20 km of Early Cretaceous NNW-oriented contraction across the Amazon Basin; an area of relative weakness where both geologic and geophysical data support such accommodation. Sea-surface flow models, which used this revised reconstruction and interpreted paleo-bathymetric data, predict upwelling throughout the newly formed equatorial seaway, and later along the West African margin during periods of regional organic-rich black shale deposition. With reduced decomposition of organic matter strongly correlated to upwelling, being able to predict these zones is of particular significance to petroleum companies, who have recently started exploring both the equatorial South American and West African coastlines.

O ciclo diurno do fluxo de CO2 no oceano Atlântico equatorial para o mês de agosto é estimado utilizando um algoritmo de transferência de gases. O algoritmo é baseado na teoria de similaridade de Monin-Obukhov para fluxos turbulentos na interface ar-mar e na física da transferência de CO2 na camada molecular oceânica. O ciclo diurno do fluxo de CO2, obtido na região, caracteriza o oceano Atlântico equatorial, durante o período, como fonte de CO2 para a atmosfera e seus valores estão entre 0,71 e 0,85 mol CO2 m^-2 ano^-1. / The diurnal cycle of CO2 is estimated for the month of August on the Atlantic Ocean using a gas transfer algorithm. The algorithm is based on the Monin-Obukhov similarity theory for turbulent transfer at the air-sea interface and the physics of the CO2 transfer at the oceanic molecular layer.

Coordenação de Aperfeiçoamento de Pessoal de Nível Superior / Os fatores que explicam a distribuição observada em plantas e animais é uma pergunta que intriga naturalistas, biogeógrafos e ecólogos há mais de um século. Ainda nos primórdios da disciplina de ecologia, as tolerâncias ambientais já haviam sido apontadas como as grandes responsáveis pelo padrão observado da distribuição dos seres vivos, o que mais tarde levou à concepção de nicho ecológico das espécies. Nos últimos anos, o estudo das distribuições dos organismos ganhou grande impulso e destaque na literatura. O motivo foi a maior disponibilidade de catálogos de presença de espécies, o desenvolvimento de bancos de variáveis ambientais de todo o planeta e de ferramentas computacionais capazes de projetar mapas de distribuição potencial de um dado organismo. Estes instrumentos, coletivamente chamados de Modelos de Distribuição de Espécies (MDEs) têm sido desde então amplamente utilizados em estudos de diferentes escopos. Um deles é a avaliação de potenciais áreas suscetíveis à invasão de organismos exóticos. Este estudo tem, portanto, o objetivo de compreender, através de MDEs, os fatores subjacentes à distribuição de duas espécies de corais escleractíneos invasores nativos do Oceano Pacífico e ambas invasoras bem sucedidas de diversas partes do Oceano Atlântico, destacadamente o litoral fluminense. Os resultados mostraram que os modelos preditivos da espécie Tubastraea coccinea (LESSON, 1829), cosmopolita amplamente difundida na sua região nativa pelo Indo- Pacífico demonstraram de maneira satisfatória suas áreas de distribuição nas áreas invadidas do Atlântico. Sua distribuição está basicamente associada a regiões com alta disponibilidade de calcita e baixa produtividade fitoplanctônica. Por outro lado, a aplicação de MDEs foi incapaz de predizer a distribuição de T. tagusensis (WELLS,1982) no Atlântico. Essta espécie, ao contrário de sua congênere, tem distribuição bastante restrita em sua região nativa, o arquipélago de Galápagos. Através de análises posteriores foi possível constatar a mudança no nicho observado durante o processo de invasão. Finalmente, o sucesso preditivo para T. coccinea e o fracasso dos modelos para T. tagusensis levantam importantes questões sobre quais os aspectos ecológicos das espécies são mais favoráveis à aplicação de MDEs. Adicionalmente, lança importantes ressalvas na utilização recentemente tão difundida destas ferramentas como forma de previsão de invasões biológicas e em estudos de efeitos de alterações climáticas sobre a distribuição das espécies. / The factors underpinning the observed distribution of plants and animals across time and space are a central question in ecology and has intrigued scientists for over a century. But even back on those early times, the role of climatic tolerances of the species were recognized as one of the main explanations for such distributional patterns. Later, these assumptions gave rise to the concept of niche which triggered several advances in the study of natural history. Recently, these studies were addressed in the light of novel computational techniques capable of providing potential distributional maps for a given species, generically called Species Distribution Models (SDMs). This coupled with the broader availability of species occurrence records and of environmental data from international databases made studies with SDMs very popular and ubiquitous in the literature. One of the main uses of the SDMs approach is the assessment of potentially susceptible areas of invasion by non- indigenous species. Therefore, here we used SDMs to better understand the major factors related to the current distribution of two well established invasive scleractinian coral species in the Atlantic, both from the Pacific Ocean. The results showed that the models were successful in predicting the potentially invaded sites by the cosmopolitan Tubastraea coccinea (LESSON, 1829), broadly distributed throughout the Pacific. This species distribution was basically associated with increasing concentrations of calcite and lower levels of phytoplankton activity. However, the models were incapable of predicting the survival and establishment of T. tagusensis (WELLS, 1982) in the Atlantic. This species, unlike its congener, has a very restricted distribution in its native regions, the Galapagos Islands. A posterior analyzes indeed showed a niche shift during the invasion event of T. tagusensis in the Atlantic. Finally, the good modelling results for T. coccinea contrasted with the failure of modelling T. tagusensis invasion highlight important explanations on methodological procedures in SDMs. It also helps to better understand which ecological aspects of the species are favourable toward good modelling performance. In addition to that, these results calls for precaution when analyzing SDMs results, particularly in invasion and climate change scenarios studies.

Coordenação de Aperfeiçoamento de Pessoal de Nível Superior / Os fatores que explicam a distribuição observada em plantas e animais é uma pergunta que intriga naturalistas, biogeógrafos e ecólogos há mais de um século. Ainda nos primórdios da disciplina de ecologia, as tolerâncias ambientais já haviam sido apontadas como as grandes responsáveis pelo padrão observado da distribuição dos seres vivos, o que mais tarde levou à concepção de nicho ecológico das espécies. Nos últimos anos, o estudo das distribuições dos organismos ganhou grande impulso e destaque na literatura. O motivo foi a maior disponibilidade de catálogos de presença de espécies, o desenvolvimento de bancos de variáveis ambientais de todo o planeta e de ferramentas computacionais capazes de projetar mapas de distribuição potencial de um dado organismo. Estes instrumentos, coletivamente chamados de Modelos de Distribuição de Espécies (MDEs) têm sido desde então amplamente utilizados em estudos de diferentes escopos. Um deles é a avaliação de potenciais áreas suscetíveis à invasão de organismos exóticos. Este estudo tem, portanto, o objetivo de compreender, através de MDEs, os fatores subjacentes à distribuição de duas espécies de corais escleractíneos invasores nativos do Oceano Pacífico e ambas invasoras bem sucedidas de diversas partes do Oceano Atlântico, destacadamente o litoral fluminense. Os resultados mostraram que os modelos preditivos da espécie Tubastraea coccinea (LESSON, 1829), cosmopolita amplamente difundida na sua região nativa pelo Indo- Pacífico demonstraram de maneira satisfatória suas áreas de distribuição nas áreas invadidas do Atlântico. Sua distribuição está basicamente associada a regiões com alta disponibilidade de calcita e baixa produtividade fitoplanctônica. Por outro lado, a aplicação de MDEs foi incapaz de predizer a distribuição de T. tagusensis (WELLS,1982) no Atlântico. Essta espécie, ao contrário de sua congênere, tem distribuição bastante restrita em sua região nativa, o arquipélago de Galápagos. Através de análises posteriores foi possível constatar a mudança no nicho observado durante o processo de invasão. Finalmente, o sucesso preditivo para T. coccinea e o fracasso dos modelos para T. tagusensis levantam importantes questões sobre quais os aspectos ecológicos das espécies são mais favoráveis à aplicação de MDEs. Adicionalmente, lança importantes ressalvas na utilização recentemente tão difundida destas ferramentas como forma de previsão de invasões biológicas e em estudos de efeitos de alterações climáticas sobre a distribuição das espécies. / The factors underpinning the observed distribution of plants and animals across time and space are a central question in ecology and has intrigued scientists for over a century. But even back on those early times, the role of climatic tolerances of the species were recognized as one of the main explanations for such distributional patterns. Later, these assumptions gave rise to the concept of niche which triggered several advances in the study of natural history. Recently, these studies were addressed in the light of novel computational techniques capable of providing potential distributional maps for a given species, generically called Species Distribution Models (SDMs). This coupled with the broader availability of species occurrence records and of environmental data from international databases made studies with SDMs very popular and ubiquitous in the literature. One of the main uses of the SDMs approach is the assessment of potentially susceptible areas of invasion by non- indigenous species. Therefore, here we used SDMs to better understand the major factors related to the current distribution of two well established invasive scleractinian coral species in the Atlantic, both from the Pacific Ocean. The results showed that the models were successful in predicting the potentially invaded sites by the cosmopolitan Tubastraea coccinea (LESSON, 1829), broadly distributed throughout the Pacific. This species distribution was basically associated with increasing concentrations of calcite and lower levels of phytoplankton activity. However, the models were incapable of predicting the survival and establishment of T. tagusensis (WELLS, 1982) in the Atlantic. This species, unlike its congener, has a very restricted distribution in its native regions, the Galapagos Islands. A posterior analyzes indeed showed a niche shift during the invasion event of T. tagusensis in the Atlantic. Finally, the good modelling results for T. coccinea contrasted with the failure of modelling T. tagusensis invasion highlight important explanations on methodological procedures in SDMs. It also helps to better understand which ecological aspects of the species are favourable toward good modelling performance. In addition to that, these results calls for precaution when analyzing SDMs results, particularly in invasion and climate change scenarios studies.

O ciclo diurno do fluxo de CO2 no oceano Atlântico equatorial para o mês de agosto é estimado utilizando um algoritmo de transferência de gases. O algoritmo é baseado na teoria de similaridade de Monin-Obukhov para fluxos turbulentos na interface ar-mar e na física da transferência de CO2 na camada molecular oceânica. O ciclo diurno do fluxo de CO2, obtido na região, caracteriza o oceano Atlântico equatorial, durante o período, como fonte de CO2 para a atmosfera e seus valores estão entre 0,71 e 0,85 mol CO2 m^-2 ano^-1. / The diurnal cycle of CO2 is estimated for the month of August on the Atlantic Ocean using a gas transfer algorithm. The algorithm is based on the Monin-Obukhov similarity theory for turbulent transfer at the air-sea interface and the physics of the CO2 transfer at the oceanic molecular layer.

La forte hétérogénéité de l'écosystème marin se traduit par une production inégale des ressources sur un large éventail d'échelles spatio-temporelles, qui conditionne par conséquent les déplacements des tortues marines. Considéré comme l’un des plus dynamiques au monde, le plateau des Guyanes est une région très complexe d'un point de vue océanographique et qui héberge trois des sept espèces de tortues marines présentes dans le monde (la tortue olivâtre, la tortue verte et la tortue luth). L'objectif de cette thèse était de comprendre comment les contraintes océanographiques peuvent-elles influencer les déplacements en mer de ces trois espèces. Le déploiement de 55 balises satellites sur des femelles reproductrices en Guyane française a permis de fournir des informations sur leurs trajectoires, sur leur comportement de plongée et sur les caractéristiques de leur environnement. Notre étude a mis en évidence une forte plasticité comportementale interspécifique qui semble être principalement dictée par les traits reproducteurs, les stratégies de reproduction, les conditions locales de l'habitat ou encore les adaptations physiologiques. / The strong heterogeneity of the marine ecosystem leads to a patchy distribution of the resources in time and space, shaping therefore the movements of sea turtles. Considered as the most dynamic ecosystem in the world, the Guiana shield is a highly dynamic system which hosts three of the seven sea turtle species in the world (the olive ridley, the green turtle and the leatherback turtle). The aim of this thesis was to understand how the oceanographic constraints can influence the at-sea movements of these three species during the reproduction and the migration phases. The deployment of 55 satellite tags on adult females in French Guiana provided information on their trajectories, their diving behavior and on the environment encountered. Our study shows a strong behavioral plasticity between species, which seems to be mainly dictated by the reproductive traits and strategies, the local conditions of the habitat and the physiological adaptations.