Orientador: Fabiano Tomazini da Conceição / Resumo: O processo de denudação continental é controlado por fatores exógenos como intemperismo químico e erosão dos solos. Nesse processo, os sistemas fluviais desempenham um papel fundamental ao transportar os materiais sólidos e dissolvidos de seu local origem até atingir os oceanos. Visando investigar o balanço de denudação no Planalto Alcalino de Poços de Caldas – MG, adotou-se o método de balanço de massa geoquímico aplicado em escala de bacia hidrográfica, cuja análise quantitativa foi baseada nos fluxos de entrada e saída de elementos/compostos químicos em duas pequenas bacias hidrográficas situadas na borda noroeste do planalto. A média da descarga fluvial de sólidos dissolvidos foi estimada em 32,5 t km-2 ano-1 sendo que cerca de 40 % desse fluxo foi atribuído à deposição atmosférica (14,0 t km-2 ano-1) enquanto o restante foi associado ao intemperismo químico das rochas (18,5 t km2 ano-1), enquanto a descarga sólida, estimada em 5,4 t km-2 ano-1, demostra um sistema fluvial de baixo fluxo de sedimento. A taxa de intemperismo químico de fonólitos, tinguaítos e nefelina sienito de Poços de Caldas (2,1 m Ma) foi inferior à taxa de remoção dos solos do planalto (4,0 m Ma), revelando uma tendência de exumação das rochas do maciço a uma velocidade de 1,9 m Ma. A taxa de denudação do Planalto de Poços de Caldas foi reduzida com o passar do tempo geológico e sob o clima atual o relevo de Poços de Caldas tende a se manter na paisagem com pouca alteração em sua forma em um lento pro... (Resumo completo, clicar acesso eletrônico abaixo) / Abstract: The continental denudation process is controlled by exogenous factors such as chemical weathering and soil erosion. In this process, river systems play a key role in transporting solid and dissolved materials from their source site to the oceans. In order to investigate the denudation balance in the Poços de Caldas Alkaline Plateau - MG, the geochemical mass balance method applied in a watershed scale was adopted, whose quantitative analysis was based on the input and output flows of chemical elements / compounds in two small watersheds situated on the northwest edge of the plateau. The average fluvial discharge of dissolved solids was estimated at 32.5 t km-2 year-1 and about 40% of this flow was attributed to atmospheric deposition (14.0 t km-2 year-1) while the remainder was associated with the chemical weathering of the rocks (18.5 t km-2 year-1), while the solid discharge estimated at 5.4 t km-2 year-1 demonstrates a low sediment flow river system. The chemical weathering rate of phonolites, tinguaites and nepheline syenite from Poços de Caldas (2.1 m Ma) was lower than the removal rate from the plateau soils (4.0 m Ma), revealing a tendency for rock mass exhumation at a speed of 1.9 m Ma. The denudation rate of the Poços de Caldas Plateau has been reduced over time and under current weather the relief of Poços de Caldas tends to remain in the landscape with little change in its form in a slow process of exhumation of the rocks. / Doutor

L’objectif de cette thèse est de déterminer une reconstruction de l’altération continentale dans le bassin du Nil depuis 100.000 ans, afin de mieux comprendre l’impact des variations climatiques sur les sols, les apports à la Méditerranée et le cycle du carbone. Ce travail repose sur une étude géochimique fine des argiles extraites d’archives sédimentaires du delta du Nil sur une échelle de temps de 100.000 ans. Il repose sur une approche inédite du couplage d’un traceur de source (isotopes du néodyme) et de traceurs d’altérations (isotopes du lithium). Les résultats de cette étude mettent en évidence une réponse rapide de l’altération continentale aux variations hydro-climatique en Afrique du Nord. De plus, les changements climatiques en Atlantique Nord et du ralentissement de l’AMOC ont eu une influence importante sur la diminution de l’intensité d’altération continentale dans le bassin du Nil. A l’actuel, les taux d’altération, et la consommation de CO2 associée, des trapps d’Ethiopie sont relativement faibles par rapport aux autres régions basaltiques. Nos résultats montrent cependant que durant l’African Humid Period, la consommation de CO2 dans cette région était 2 à 3 fois plus importante qu’aujourd’hui. Cela indique que les trapps d’Ethiopie ont pu jouer un rôle non négligeable dans le cycle du CO2, et en particulier lors des périodes de fort runoff. Enfin, des développements analytiques ont été réalisés afin de pouvoir exploiter les compositions isotopiques en lithium des carbonates biogéniques marins, comme nouveaux traceurs des apports en eau douce du Nil. Les résultats obtenus suggèrent une influence des effets dits « vitaux » et des processus de diagénèse. / This thesis presents a reconstruction of the chemical weathering in the Nile basin since 100.000 years. His objective is to better understand the impact of climatic variations on chemical weathering, to better. This work is based on the geochemistry study on marine core recorded in the Nile delta, on a time scale of 100.000 years. It use a novel approach with the coupling of à source tracer (εNd) and chemical weathering tracers (δ7Li and elementary ratios) on the fine fraction (< 2µm). In addition, it was analyzed samples of each actual Nil sources and a core sample from Tana Lack (Ethiopie). The results of this studies have enabled to reconstruct the chemical weathering variations in the Nile basin since 100.000 years. It show a rapid respond of chemical weathering to climate variations. This was also observed by important impact of North Atlantic climate variations on the decrease of chemical weathering in the Nile basin. At present, the chemical weathering flux and the CO2 consumption of the Ethiopian traps are low comparted to the other basaltic regions, as the Deccan. However, our results show that during the most humid periods (African Humid Period) the CO2 consumption was 2 to 3 time higher than today. This indicates that the Ethiopian traps could be have played a non-negligible role in the global CO2 cycle during the Cenozoic, and principally during the most humid periods in North Africa. Finally, analytical developments have been realized to be able to exploit the lithium isotopes on biogenic carbonates as a new tracer of Nile water flood. The results suggest an influence of “vital” effect and diagenetic process.

Indiana University-Purdue University Indianapolis (IUPUI) / Subglacial chemical processes in Antarctica are potentially significant contributors to global geochemical cycles, but current understanding of their scale and nature is limited. A sequential chemical extraction procedure was developed and tested to investigate the utility of meteoric 10Be as a tracer for chemical weathering processes beneath the East Antarctic Ice Sheet. Subglacial meltwater is widely available under the Antarctic Ice Sheet and chemical constituents within it have the potential to drive geochemical weathering processes in the subglacial environment. Meteoric 10Be is a cosmogenic nuclide with a half-life of 1.39×106 years that is incorporated into glacier ice, therefore its abundance in the subglacial environment in Antarctica is meltwater dependent. It is known to adsorb to fine-grained particles in aqueous solution, precipitate with amorphous oxides, and/or be incorporated into authigenic clay structures during chemical weathering. The presence of 10Be in weathering products derived from beneath the ice therefore indicates chemical weathering processes in the subglacial environment. Freshly emerging subglacial sediments from the Mt. Achernar blue ice moraine were subject to chemical extractions where these weathering phases were isolated and 10Be concentrations therein quantified. Optimization of the phase isolation was developed by examining the effects of each extraction on the sample mineralogy and chemical composition. Experiments on 10Be desorption revealed that pH 3.2-3.5 was optimal for the extraction of adsorbed 10Be. Vigorous disaggregation of the samples before grain size separations and acid extractions is crucial due to the preferential fractionation of the nuclide with clay-sized particles. 10Be concentrations of 2-22×107 atoms g-1 measured in oxides and clay minerals in freshly emerging sediments strongly indicate subglacial chemical weathering in the catchment of the Mt. Achernar moraine. Sediment-meltwater contact in the system was calculated to be on the order of thousands of years, based on total 10Be sample concentrations, local basal melt rates, and 10Be ice concentrations. Strong correlation (R = 0.96) between 10Be and smectite abundance in the sediments indicate authigenic clay formation in the subglacial environment. This study shows that meteoric 10Be is a useful tool to characterize subglacial geochemical weathering processes under the Antarctic Ice Sheet.

<p>Chemical weathering of highly reactive mafic and ultramafic igneous rocks may be a key sink in the global carbon cycle. Understanding how uplift of these rocks during arc-arc and arc-continent collisions through earth history has affected the evolution of global climate, including the onset of icehouse periods, requires improved constraints on the relative sensitivity of their weathering rates to physical erosion vs. climate. If weathering rates depend chiefly on erosion, then tectonic uplift of mafic and ultramafic rocks may have a strongly destabilizing effect on global climate. Conversely, if weathering rates are limited primarily by temperature or runoff, then a negative feedback mechanism between weathering and climate may attenuate the effects of rock uplift. This work characterizes the relationship between chemical weathering rates, physical erosion rates, and climate in tropical, montane watersheds in Puerto Rico that are underlain by volcanic arc rocks and associated ophiolitic serpentinite. Key to this analysis are new constraints on long-term erosion rates on these rocks from cosmogenic Cl-36 produced <em>in situ</em> in magnetite. These cosmogenic erosion rates are paired with classical measurements of stream solute fluxes and sediment geochemistry across runoff gradients to quantify the limits to volcanic arc rock and serpentinite weathering rates. </p> <p><br></p> <p>This work is divided into three chapters. Chapter 2 constrains the altitude scaling behavior of Cl-36 production in magnetite. This allows erosion rates to be determined more accurately in watersheds near sea level in Puerto Rico. Chapter 3 demonstrates that volcanic arc rock weathering rates in the humid tropics are more strongly limited by physical erosion than by climatic factors. However, a positive correlation between erosion and runoff observed in this landscape may enhance the coupling between climate and weathering rates. Chapter 4 finds that, in contrast to volcanic arc rocks, serpentinite weathering is strongly limited by runoff and weakly limited by erosion. These results are presented as empirical power-law relationships that can be readily applied in global carbon cycle modeling.  </p>

Scope and contents: An investigation into some arctic soils located on Southwest Devon Island. Some aspects of chemical weathering in the area are considered and detailed studies of patterned ground are undertaken. A terrain map was compiled to locate the major areas of soils and mineral and organic covers and to show this relationship to relief units. / No abstract provided. / Thesis / Master of Science (MSc)

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Silicate minerals are the most common mineral group in the earth's crust so it is not surprising that their weathering reactions dominate the chemistry of many earth surface processes. This project used forsterite as a model system to identify the important factors that affect silicate mineral dissolution rates and grain lifetimes in the weathering environment. I determined an empirical rate law for forsterite dissolution of forsterite in oxalic acid solutions: based on a series of 124 semi-batch reactor experiments over a pH range of 0 to 7 and total oxalate concentrations between 0 and 0.35 m at 25°C. These experiments show that oxalate-promoted dissolution rates depend upon both oxalate concentration and pH. I propose a reaction mechanism in which a hydrogen ion and an oxalate ion are simultaneously present in the activated complex for the reaction that releases H4SiO4 into solution. By analogy, I propose that water acts as a ligand in the absence of oxalate. I also ran 82 batch reactor experiments in magnesium and sodium sulfate and magnesium and potassium nitrate solutions. These experiments show that ionic strength up to 12 m, log mMg up to 4 m, and log mSO4 up to 3 m have no effect on forsterite dissolution rates. However, decreasing aH2O slows forsterite dissolution rates. The effect of decreasing dissolution rates with decreasing aH2O is consistent with the idea that water acts as a ligand that participates in the dissolution process.Forsterite dissolution rate data from previously published studies were combined with results from my experiments and regressed to produce rate laws at low and high pH. For pH < 5.05 or and for pH > 5.05 or I then developed a diagram that shows the effect rate-determining variables on the lifetime of olivine grains in weathering environments using these rate laws. / Ph. D.

The Toarcian Oceanic Anoxic Event (T-OAE; ~183 million years ago) represents an interval during the Mesozoic when the emplacement of the Karoo-Ferrar Large Igneous Province (LIP) is thought to have resulted in significant environmental change. Associated with this interval was the widespread deposition of organic-rich sediments, carbon cycle and seawater chemistry changes, global warming, the development of marine anoxia, and major extinction events. The majority of studies of this event that have documented these responses have come from the Boreal and Tethyan regions of Europe, thus casting some doubt to the regional versus global significance of the event. Thus my dissertation has sought to reconstruct biogeochemical and paleoenvironmental changes across the T-OAE from a sedimentary succession that was deposited on the margins of a different ocean basin away from the well-studied European successions. Specifically, I have studied the chemostratigraphy of the Fernie Formation of the Western Canada Sedimentary Basin (WCSB), which was deposited on the eastern margin of the Panthalassa Ocean. The Toarcian carbon isotope excursions (CIEs) in the WCSB confirm that these features are global phenomena. I have suggested a new driver for small-scale CIEs observed during the event: the release of wetland-derived methane during progressive global warming. The osmium isotope record and numerical modeling of the osmium cycle suggests that continental weathering rates increased during the T-OAE by 230 – 540%. Rhenium abundance data also suggests that the increased geographic extent of marine anoxia during the T-OAE caused a global drawdown in the seawater rhenium inventory. Iron speciation data are used to reconstruct redox conditions within the WCSB, which suggest ferruginous conditions developed in the more distal locations at the onset of the T-OAE before returning to euxinic (anoxic and sulfidic) conditions. This is likely related to enhanced pyrite burial on a global scale, which caused the drawdown of the seawater sulfate inventory, thus limiting pyrite formation in the distal locations. The proximal setting remained euxinic across the T-OAE, and in all locations the iron speciation data suggest anoxic conditions persistent well after the interval that has been traditionally called the end of the T-OAE. / Ph. D.

O papel da litologia e da organização tectônica Proterozóica para entendimento da gênese do relevo, em termos de erosão diferencial, tem sido relativamente negligenciado nos últimos decênios em favor de linhas de explicação baseadas nas mudanças paleoclimáticas, tectônica moderna, e geoquímica de superfície. Fato que tem permitido a formação de modelos explicativos fechados sobre a gênese e dinâmica do relevo. Entretanto, em áreas de embasamento cristalino a organização tectônica e a diversidade composicional químico-mineralógica das rochas auxilia fortemente o entendimento do papel da erosão diferencial, na explicação da diversidade de compartimentos geomórficos. A organização tectônica das rochas, através de sistemas de falhas e dobras de fundo (estudados na década de 1950 por Francis Ruellan), tem papel fundamental no entendimento dos processos de erosão diferencial responsável pela diversidade de compartimentos geomórficos. Neste sentido, este trabalho procura provar, através do método da Associação e Indeterminação Geomorfológica de Leopold e Langbein (1970), que a Região Serrana do Espírito Santo, e, em especial, a da bacia hidrográfica do Rio Benevente é resultante da erosão diferencial sobre rochas de rica diversidade químico-mineralógica, organizada por meio de dobras de fundo e sistemas de falhas transcorrentes conjugadas. Para realização desta investigação foi elaborado uma proposta de cartografia hidrogeomorfológica para pesquisa de bacias hidrográficas, estudando-se a evolução dos perfis fluviais longitudinais, perfis morfogeológicos, estruturas tectônicas e composição químico-mineralógica das unidades litológicas associado a compartimentação morfológica do relevo. Tudo fundamentado no método escolhido. O que certamente parece ser uma coisa óbvia, no entanto, evidencia a atuação de um verdadeiro sistema de dissecação fluvial, associado ao rebaixamento de níveis de base (knickzonas) que evoluem conforme o mergulho da foliação metamórfica dos flancos das dobras e da zona de dano associado às falhas. A conclusão que se chega é a de que o relevo da bacia do Rio Benevente, e por sua vez da Região Serrana do Estado, configura-se como compartimentos geomórficos residuais resultantes da evolução de knickzonas fluviais associadas à organização tectônica das dobras de fundo responsáveis pela gênese de níveis diferenciais de resistência litológica e de distribuição espacial das falhas e fraturas. A composição das unidades litológicas também influencia, fortemente, a compartimentação do relevo, estando o nível de convexização dos relevos e a dissecação na forte dependência do grau de participação dos minerais do grupo dos plagioclásios, anfibólios e piroxênios na constituição litológica das litoestruturas / The role of lithology and Proterozoic tectonic organization for understanding the genesis of relief in terms of differential erosion, has been relatively neglected in recent decades in boon of lines of explanation based on paleoclimatic changes, modern tectonics, and geochemistry. This fact has allowed the formation of \"closed\" on the genesis and dynamics of relief explanatory models. However, in sites of crystalline basement tectonics organization and compositional diversity chemical-mineralogical Rocks strongly supports the understanding of the role of differential erosion, in explaining the diversity of geomorphic compartments. The tectonic organization of the rocks through system failure and bottom folds (studied in the 1950s by Francis Ruellan) plays a fundamental role in understanding the processes of differential erosion responsible for the diversity of geomorphic compartments. In this sense, this work seeks to prove, by the method of the Association and Indeterminacy Geomorphological Leopold and Langbein (1970), the mountainous region of the Espírito Santo, and in special the river basin Benevente is the result of differential erosion of rocks rich chemical-mineralogical diversity, organized by folds background and strike-slip fault systems combined. To carry out this research hydrogeomorphological a proposed mapping is designed to survey watershed studying the evolution of longitudinal river profiles, profiles morphogeologicals, tectonic structures and chemical-mineralogical composition of rock units associated with morphological partitioning of relief. All based on the method chosen. This certainly seems like an obvious thing, however, highlights the work of a true system of river dissection, associated with the lowering of baseline levels (knickzones) that evolve according to the metamorphic foliation dipping flanks of the folds and the damage zone associated with the fault. The conclusion reached is that the relief Benevente River basin, and in turn the mountainous region of the state, appears as residual geomorphic compartments resulting from the evolution of river knickzonas associated with the organization of tectonic folds background responsible for the genesis of differential levels of lithologic resistance and spatial distribution of faults and fractures. The composition of the lithological units also influences strongly the partitioning of relief, being the level of reliefs and round dissecting the strong dependence of the degree of participation of the group of the minerals plagioclase amphibole and pyroxene in the lithological constitution of lithostructurals