Academic literature on the topic 'Geochemistry : Carbonate rocks'

The subject of BIF genesis, particularly their environmental conditions and ocean chemistry at the time of deposition and their evolution through time, has been a subject of much contentiousness, generating a wealth of proposed genetic models and constant refinements thereof over the years. The prevailing paradigm within the various schools of thought, is the widespread and generally agreed upon depositional and diagenetic model(s) which advocate for BIF deposition under anoxic marine conditions. According to the prevailing models, the primary depositional environment would have involved a seawater column whereby soluble Fe²⁺ expelled by hydrothermal activity mixed with free O₂ from the shallow photic zone produced by eukaryotes, forming a high valence iron oxy-hydroxide precursor such as FeOOH or Fe(OH)₃. An alternative biological mechanism producing similar ferric precursors would have been in the form of photo-ferrotrophy, whereby oxidation of ferrous iron to the ferric form took place in the absence of biological O₂ production. Irrespective of the exact mode of primary iron precipitation (which remains contentious to date), the precipitated ferric oxy-hydroxide precursor would have reacted with co-precipitated organic matter, thus acting as a suitable electron acceptor for organic carbon remineralisation through Dissimilatory Iron Reduction (DIR), as also observed in many modern anoxic diagenetic environments. DIR-dominated diagenetic models imply a predominantly diagenetic influence in BIF mineralogy and genesis, and use as key evidence the low δ¹³C values relative to the seawater bicarbonate value of ~0 ‰, which is also thought to have been the dissolved bicarbonate isotope composition in the early Precambrian oceans. The carbon for diagenetic carbonate formation would thus have been sourced through a combination of two end-member sources: pore-fluid bicarbonate at ~0 ‰ and particulate organic carbon at circa -28 ‰, resulting in the intermediate δ¹³C values observed in BIFs today. This study targets 65 drillcore samples of the upper Kuruman and Griquatown BIF from the lower Transvaal Supergroup in the Hotazel area, Northern Cape, South Africa, and sets out to explore key aspects in BIF carbonate petrography and geochemistry that are pertinent to current debates surrounding their interpretation with regard to primary versus diagenetic processes. The focus here rests on applications of carbonate (mainly siderite and ankerite) petrography, mineral chemistry, bulk and mineral-specific carbon isotopes and speciation analyses, with a view to obtaining valuable new insights into BIF carbonates as potential records of ocean chemistry for their bulk carbonate-carbon isotope signature. Evaluation of the present results is done in light of pre-existing, widely accepted diagenetic models against a proposed water-column model for the origin of the carbonate species in BIF. The latter utilises a combination of geochemical attributes of the studied carbonates, including the conspicuous Mn enrichment and stratigraphic variability in Mn/Fe ratio of the Griquatown BIF recorded solely in the carbonate fraction of the rocks. Additionally, the carbon isotope signatures of the Griquatown BIF samples are brought into the discussion and provide insights into the potential causes and mechanisms that may have controlled these signatures in a diagenetic versus primary sedimentary environment. Ultimately, implications of the combined observations, findings and arguments presented in this thesis are presented and discussed with particular respect to the redox evolution and carbon cycle of the ocean system prior to the Great Oxidation Event (GOE). A crucial conclusion reached is that, by contrast to previously-proposed models, diagenesis cannot singularly be the major contributing factor in BIF genesis at least with respect to the carbonate fraction in BIF, as it does not readily explain the carbon isotope and mineral-chemical signatures of carbonates in the Griquatown and uppermost Kuruman BIFs. It is proposed instead that these signatures may well record water-column processes of carbon, manganese and iron cycling, and that carbonate formation in the water column and its subsequent transfer to the precursor BIF sediment constitutes a faithful record of such processes. Corollary to that interpretation is the suggestion that the evidently increasing Mn abundance in the carbonate fraction of the Griquatown BIF up-section would point to a chemically evolving depositional basin with time, from being mainly ferruginous as expressed by Mn-poor BIFs in the lower stratigraphic sections (i.e. Kuruman BF) to more manganiferous as recorded in the upper Griquatown BIF, culminating in the deposition of the abnormally enriched in Mn Hotazel BIF at the stratigraphic top of the Transvaal Supergroup. The Paleoproterozoic ocean must therefore have been characterised by long-term active cycling of organic carbon in the water column in the form of an ancient biological pump, albeit with Fe(III) and subsequently Mn(III,IV) oxy-hydroxides being the key electron acceptors within the water column. The highly reproducible stratigraphic isotope profiles for bulk δ¹³C from similar sections further afield over distances up to 20 km, further corroborate unabatedly that bulk carbonate carbon isotope signatures record water column carbon cycling processes rather than widely-proposed anaerobic diagenetic processes.

The Penedes half-graben represents a natural field laboratory for the study of the link between tectonics and palaeofluids because it exposes numerous outcrops that allow a global and complete diagenetic study of the basin from Mesozoic to present times. The Penedes half-graben is located in the central part of the Catalan Coastal Ranges (CCR) and results from the superposition of three main tectonic events: (1) the Mesozoic extensional phase which is divided into two Mesozoic rift episodes: the first, Late Permian to Triassic in age and the second, latest Oxfordian to Aptian in age; (2) the Paleocene to middle Oligocene compressional phase which includes the emplacement of ENE-to-NE-trending thick-skinned thrust sheets bounded by SE dipping thrusts with a limited left-lateral strike-slip motion; (3) the late Oligocene?- Neogene extensional phase which split the CCR into a set of ENE-WSW blocks mainly tilted toward the NW, constituting the actual horst-and-graben systems now present at the northwestern Mediterranean. Samples were taken in 19 different outcrop areas located within three main structural domains: the Gaia-Montmell domain, which represents the footwall block of the SE-dipping major normal faults that bound the north-western margin of the basin, the Central Penedes domain, which comprises the central Miocene syn- and post-rift deposits and the Garraf domain, which comprises a group of small syn-depositional tectonic horsts and half-grabens developed in the Garraf horst during its Neogene evolution. Based on the macro and microstructural analysis combined with geochemical results from host rocks, fault rocks and fracture cements, the following diagenetic events have been identified: (1) Very early stages characterized by micritization, early irregular micro-fractures resulting from opening in poorly-lithified sediments and early calcite cement precipitations; (2) Progressive burial stages characterized by brecciation, stylolization and dolomitization; (3) Fracturing and cementations characterized by ninth major deformation stages with their related cements, breccias and stylolites, and (4) four karstification events with associated collapse breccias, sediments and cements filling the fracture, vug and cavern porosities. A depositional control of the δ(18)O values of the syn-rift Mesozoic sediments (Valanginian, Barremian and Aptian) related to the position of the different outcrops with respect to the Mesozoic normal faults is inferred from the values reported in this study. The isotopic values of the Miocene marine facies, depleted in δ(18)O and δ(13)C respect to the expected values for the Miocene seawater, together with the chalky appearance of these limestones, indicate that the Miocene marine limestones were re-balanced under the meteoric diagenetic environment. The meteoric fluid precipitating the calcite cement in the conglomerates of the lower continental complexes was responsible for diagenetically altering the marine host limestone. A different meteoric fluid, more influenced by soil-derived CO2, precipitated the calcite cement present within the upper continental complexes. The fluids circulating through the fractures attributed to the second stage of the Mesozoic rifting were precipitated from formation waters during the progressive burial of the sediment, in a closed palaeohydrological system. From the Paleocene to the mid-Oligocene the fluids circulating through the compressional fractures had a meteoric origin. Due to the Paleogene compression, Mesozoic rocks were uplifted, subaerially exposed and extensively karstified. Different types of sediments and cements were deposited filling the karstic cavities under the meteoric diagenetic environment. Related to the syn- and early post-rift stage, the fractures were sealed by meteoric fluids under both, phreatic and vadose zones. The normal faults attributed to the late post-rift stage favoured the upflowing of marine fluids expelled from the compaction of the late Burdigalian to the early Serravallian marine sediments producing the dolomitization of the host rocks and the precipitation of dolomite cements within the fractures. During the late post-rift and related to latest tensional fractures occurrence different types of meteoric fluids circulated through the fractures. These fluids were precipitated from phreatic to vadose cements, agreeing with the uplift of the entire basin and/or with the falling-down of the meteoric water table related to a generalized sea level fall in the Mediterranean area during the Messinian.<br>La formació de la conca del Penedès està associada a un període extensiu d’edat neògena que provocà l’obertura del Solc de València. El marge nord-oest del Solc de València està constituït per una sèrie de grabens (Penedès, Vallès, Barcelona ...) i horsts (Garraf, Gaià-Montmell, Montnegre ...), el conjunt dels quals formen la Serralada Costanera Catalana. Aquesta serralada resulta de la superposició de tres esdeveniments tectònics principals: (1) l’extensió Mesozoica, compresa entre el Pèrmic i el Cretàcic inferior, (2) la compressió Paleògena, la qual produí la inversió de les principals conques extensives Mesozoiques i (3) l’extensió neògena, compresa entre l’Oligocè tardà i el Miocè mig, la qual generà l’actual sistema de rift de la Mediterrània occidental. S’han estudiat 19 afloraments localitzats tant en els alts estructurals, Garraf i Gaià-Montmell, com en el sector central de la conca del Penedès. A partir de les dades macro I microestructurals, juntament amb els resultats geoquímics de les roques encaixants, roques de falla i els ciments que reomplen les fractures, s’han identificat els següents estadis diagenètics: i. un primer estadi diagenètic temprà caracteritzat per la formació de microfractures de morfologies irregulars, formades en un sediment poc litificat i per la precipitació d’un ciment de calcita poc interaccionat amb la roca de caixa. ii. un segon estadi d’enterrament caracteritzat per la bretxificació i dolomitització de la roca encaixant i la generació d’estilòlits sub-paral•lels a l’estratificació. iii. nou etapes de deformació amb diferents tipus de rebliments associats a les fractures. iv. quatre estadis de carstificació caracteritzats per diferents tipus de bretxes de col•lapse, sediments i ciments que reomplen les porositats tipus fractura i vug, generades a partir de la dissolució. Els fluids relacionats amb l’extensió Mesozoica són característics d’aigües de formació, en canvi, els fluids que circularen al llarg de les fractures compressives paleògenes, són coherents amb fluids d’origen meteòric altament interaccionats amb la roca de caixa. L’extensió Neògena es caracteritza per una circulació predominant de fluids meteòrics no interaccionats amb la roca de caixa. És en l’estadi de post-rift on s’ha definit una dolomitització parcial de l’encaixant produïda per la barreja d’aigües marines i meteòriques. Durant l’estadi de post-rift tardà tingué lloc un esdeveniment de dissolució càrstica molt extens, el qual es relaciona amb la baixada del nivell del mar durant el Messinià.

Made available in DSpace on 2014-10-09T12:37:32Z (GMT). No. of bitstreams: 0<br>Made available in DSpace on 2014-10-09T14:03:17Z (GMT). No. of bitstreams: 1 05174.pdf: 4868507 bytes, checksum: ef4d7e8312562d1a1e608bf1fd65e9ec (MD5)<br>Tese (Doutoramento)<br>IPEN/T<br>Instituto de Pesquisas Energeticas e Nucleares, Sao Paulo

The decline of tree islands in the freshwater-Everglades wetland because of hydrologic manipulation, has compromised valuable ecosystem services. Although the role of tree islands in maintaining freshwater quality stems largely from evapotranspiration processes, fundamental questions remain about the effects of different geologic materials on their hydrogeochemical functioning. To reduce this uncertainty, the lithological composition of a set of man-made tree islands was investigated coupled with long-term hydrologic and hydrochemical data. Key results indicate that limestone substrates and peat substrates with elevated proportions of sand, facilitated surface water-groundwater interactions and mineral dissolution. However, limestone-based islands were more effective in lowering the water table and concentrating solutes in response to evapotranspiration during low surface water stages. Additionally, the peat substrate of an island with low sand content favored the thermodynamic conditions for calcite accumulation in the phreatic zone, while phosphorus concentrations in the groundwater were associated with the breakdown of organic matter.

Le bassin de Jaca (Pyrénées espagnoles) est un exemple classique de bassins d’avant pays, où les grandes lignes du remplissage sédimentaire, ainsi que la chronologie des failles ont été très étudiées. Il reste toutefois à mieux comprendre la paléo-hydrologie et l’histoire thermique du bassin, de manière à proposer un modèle de circulation des fluides pendant sa mise en place et sa déformation (Paléocène-Oligocène). Pour ce faire, ce travail propose d’analyser la répartition de la fracturation, d’étudier les conditions de formation des veines syn-tectoniques et de caractériser la maturité de la matière organique sur l’ensemble du paléobassin d’avant-pays de Jaca, des zones internes vers les zones externes.L’analyse pétrographique, géochimique et microthermométrique des veines montre que la grande majorité des fluides minéralisateurs sont à l’équilibre isotopique et thermique avec l’encaissant. Dans le détail, nous avons identifié 2 événements principaux de formation de veines dans la zone interne du bassin (Sierras Interiores), que nous proposons d’associer au fonctionnement des failles majeures dans le socle. Nous suggérons que les fluides circulent le long des niveaux de décollements et sont expulsés sur de courtes distances (&lt; 10 km), au travers des réseaux de fractures, vers le bassin d’avant-pays. Le reste du bassin enregistre principalement des fluides locaux, parfois associés à l’infiltration d’eau météorique. L’analyse des températures d’enfouissement (50°C à 250°C), qui inclut des données de Δ47, montre une organisation N-S relativement homogène depuis les Sierras Interiores (fenêtre à gaz) jusqu’aux Sierras Exteriores (immature), avec des anomalies longitudinales marquées. Les modélisations thermiques 1D sur 9 puits virtuels suggèrent que les températures maximales vers les Sierras Interiores peuvent résulter d’un enfouissement sédimentaire, dont une grande partie est érodée actuellement. Nous proposons que ces parties érodées correspondent à des dépôts tardi-orogéniques conglomératiques déposés à proximité de la zone axiale. Les données suggèrent une répartition non homogène de ces dépôts selon un axe E-W, impliquant des transferts sédimentaires plus complexes qu’habituellement discutés. Au vu de nos résultats et des précédentes études, le modèle paléohydrologique et thermique du bassin de Jaca, et à plus grande échelle, de la chaîne plissée sud-pyrénéenne, est compartimenté à la fois dans l’espace et dans le temps, en lien avec à la propagation latérale et frontale de la déformation, qui contrôle l’ouverture du système. Le modèle paléohydrologique et thermique de la chaîne plissée sud-pyrénéenne constitue donc un potentiel analogue aux chaînes plissées dont le raccourcissement résulte d’une convergence oblique<br>The Jaca basin (Spanish Pyrenees) is a classical example of a foreland basin, where the sedimentary filling and the calendar of thrust activation have been extensively studied. It remains to understand the paleohydrology and the thermal history of the basin, so as to provide a fluid flow model related to its formation and deformation (Paleoecene-Oligocene). To do this, this work proposes to analyze the distribution of fracturing, to study the conditions of formation of syn-tectonic veins and to characterize the maturity of organic matter throughout the Jaca foreland basin, from hinterland to external areas.Petrographical, geochemical and microthermometric analysis of veins show that the vast majority of mineralizing fluids are at the isotopic and thermal equilibrium with the host-rock. In detail, we identified two main events of vein precipitation in the inner part of the basin (Sierras Interiores), probably related to major basement thrust activations. We suggest that fluids flow along decollement levels and are expelled over short distances (&lt;10 km), through fracture networks towards the foreland basin. The other part of the basin mainly record local fluids, sometimes associated with the infiltration of meteoric water. Analysis of burial temperatures (50 °C to 250 °C), which includes Δ47 data, shows a relatively homogeneous N-S organization from the Sierras Interiores (gas window) to Sierras Exteriores (immature), with strong longitudinal anomalies. Thermal 1D modelling of 9 virtual wells suggest that the maximum temperatures of Sierras Interiores result from sedimentary accumulation, whose a large amount is now eroded. We propose that this eroded thickness corresponds to late-orogenic conglomeratic deposits near the axial zone. The data suggest an inhomogeneous distribution of the deposits along an E-W axis, involving more complex sedimentary transfers than usually discussed. Given our results and previous studies, the paleohydrological and thermal model of the Jaca basin, and on a larger scale, of the South Pyrenean fold and thrust belt, is compartmentalized both in space and in time, in response to the propagation of and oblique deformational front, which controls the opening of the system. The paleohydrological and thermal model of the South Pyrenean fold and thrust belt is therefore a potential analogue to fold and thrust belt including shortening due to an oblique convergence

M.Sc.<br>Certain carbonate bearing formations in the Paleoproterozoic Pretoria Group and its Griqualand West equivalent exhibit remarkable geochemical and stable isotopic signatures. The 8'3Ccarb isotopic signatures from the Duitschland and Silverton Formations exhibit large positive excursions, which seemingly coincide with a significant increase in atmospheric oxygen between 2.4 and 2.0 Ga. The Duitschland Formation with its distinctive basal unconformity is composed primarily of limestone and dolomite units, interbedded with two compositionally different shale units and quartzite. Toward the base of the formation there is a distinct conglomeratic quartzite which forms a sequence boundary above which isotopic and geochemical signatures change dramatically. Normal marine isotopic signatures characterize the lower portion of the succession while above the sequence boundari, the carbonates are enriched in "C. This enrichment, however, appears to be the result of local processes occurring within a closed basin. Furthermore it is apparent that the Duitschland Formation (with its three distinct marker beds) is the equivalent of the Rooihoogte Formation and therefore constitutes the base of the Pretoria Group. The Mooidraai Dolomite Formation which outcrops only locally in the Northern Cape Province, is characterized by fenestral and microbially laminated dolomite. The geochemical properties are relatively homogeneous with increases in the FeO and MnO concentrations, resulting from post depositional diagenesis. The stable isotope signatures of these dolomites represent normal marine signatures. There is, however, a depletion in the 813C and 8180 signatures in the ankeritic and sideritic lithofacies, which suggests that this succession was deposited from a stratified water column with respect to the total dissolved CO2. The positive 6 13C excursion present in the carbonates of the Lucknow Formation in Griqualand West, traditionally grouped with the Olifantshoek Group can be correlated with carbonates near the top of the Silverton Formation in the Transvaal area. The latter also displays distinctly positive 6 43C values. One possibility is that if these successions were deposited in closed anoxic basins and that the isotopic anomalies are the result of local processes such as fermentive diagenesis and methanogenesis. However, the close association of the carbonates with shallow marine orthoquartzites suggests that these were deposited in an open marine system and that the positive 8 !3C values reflect a shift in the composition of the ocean water at the time of deposition of the carbonates at 2.2 Ga. Other carbonates present in the Pretoria Group, namely from the Vermont and Houtenbek Formations, display normal open marine 8' 3C values of close to zero. A systematic stratigraphic compilation of all 6 43C values available from the Transvaal Supergroup indicates that two clear-cut positive 5' 3C excursions are present. These excursions were apparently short-lived and well defined and did not occur over an extended period of time as suggested by earlier studies based on global compilations with large uncertainties in radiometric ages of deposits.