EM - Escola de Minas
URI permanente desta comunidadehttp://www.hml.repositorio.ufop.br/handle/123456789/6
Notícias
A Escola de Minas de Ouro Preto foi fundada pelo cientista Claude Henri Gorceix e inaugurada em 12 de outubro de 1876.
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5 resultados
Resultados da Pesquisa
Item Tectonically-induced strontium isotope changes in ancient restricted seas : the case of the Ediacaran-Cambrian Bambuí foreland basin system, east Brazil.(2021) Guacaneme, Cristian; Babinski, Marly; Bedoya Rueda, Carolina; Santos, Gustavo Macedo de Paula; Caetano Filho, Sergio; Amaral, Matheus Henrique Kuchenbecker do; Reis, Humberto Luis Siqueira; Trindade, Ricardo Ivan Ferreira daThe Bambuí Group is a marine sedimentary record of an intracratonic foreland basin developed at the terminal Ediacaran and early Cambrian during the assembly of West Gondwana. Here we present a basin-scale high- resolution Sr isotope stratigraphy for the basal Bambuí Group, aiming to understand the spatial and temporal var- iations of the 87Sr/86Sr ratios and to explore the controls over the Sr isotope system in intracontinental marine environments. Assessment of the stratigraphic evolution of both Sr concentrations and Sr isotopes shows a major increase in Sr/Ca ratios (up to 0.004) and a decrease in the 87Sr/86Sr ratios from 0.7086 to 0.7076 in the high stand system tract of the basal 2nd-order sequence. These changes precede a large positive δ13C excursion typically found across the basin in the middle Bambuí Group. The high variability of both 87Sr/86Sr and Sr/Ca ra- tios was not caused by globally uniform changes in isotopic compositions of seawater, but rather likely reflect marine restriction and paleogeographic changes of the depositional environments at basin scale. This would re- sult from the tectonic uplift of Neoproterozoic orogenic belts around the São Francisco craton, which generated an isolated foreland marine basin. Compared to the global ocean, such a smaller intracontinental reservoir would be more sensitive to the Sr isotope composition from the different rock sources. We suggest that changes on the balance between carbonate production and accommodation associated with tectonically-related flexural subsi- dence progressively modified the continental drainage patterns, sedimentary sources and the chemical weathering regimes, altering the strontium influxes and isotopic compositions of the seawater in the early Bambuí basin cycle. Similar anomalies in the strontium isotope record are also recorded in coeval marine basins across West Gondwana and suggest that tectonics might have played an important role on seawater chemistry at the Neoproterozoic-Paleozoic transition.Item A large epeiric methanogenic Bambuí sea in the core of Gondwana supercontinent?(2021) Caetano Filho, Sergio; Sansjofre, Pierre; Ader, Magali; Santos, Gustavo Macedo de Paula; Guacaneme, Cristian; Babinski, Marly; Bedoya Rueda, Carolina; Amaral, Matheus Henrique Kuchenbecker do; Reis, Humberto Luis Siqueira; Trindade, Ricardo Ivan Ferreira daCarbon isotope compositions of both sedimentary carbonate and organic matter can be used as key proxies of the global carbon cycle and of its evolution through time, as long as they are acquired from waters where the dis- solved inorganic carbon (DIC) is in isotope equilibrium with the atmospheric CO2. However, in shallow water platforms and epeiric settings, the influence of local to regional parameters on carbon cycling may lead to DIC isotope variations unrelated to the global carbon cycle. This may be especially true for the terminal Neo- proterozoic, when Gondwana assembly isolated waters masses from the global ocean, and extreme positive and negative carbon isotope excursions are recorded, potentially decoupled from global signals. To improve our understanding on the type of information recorded by these excursions, we investigate the paired δ13Ccarb and δ13Corg evolution for an increasingly restricted late Ediacaran-Cambrian foreland system in the West Gondwana interior: the basal Bambuí Group. This succession represents a 1st-order sedimentary sequence and records two major δ13Ccarb excursions in its two lowermost lower-rank sequences. The basal cap carbonate interval at the base of the first sequence, deposited when the basin was connected to the ocean, hosts antithetical negative and positive excursions for δ13Ccarb and δ13Corg, respectively, resulting in Δ13C values lower than 25‰. From the top of the basal sequence upwards, an extremely positive δ13Ccarb excursion is coupled to δ13Corg, reaching values of þ14‰ and 14‰, respectively. This positive excursion represents a remarkable basin-wide carbon isotope feature of the Bambuí Group that occurs with only minor changes in Δ13C values, suggesting change in the DIC isotope composition. We argue that this regional isotopic excursion is related to a disconnection between the intrabasinal and the global carbon cycles. This extreme carbon isotope excursion may have been a product of a disequilibria between the basin DIC and atmospheric CO2 induced by an active methanogenesis, favored by the basin restriction. The drawdown of sulfate reservoir by microbial sulfate reduction in a poorly ventilated and dominantly anoxic basin would have triggered methanogenesis and ultimately methane escape to the atmosphere, resulting in a13C-enriched DIC influenced by methanogenic CO2. Isolated basins in the interior of the Gondwana supercontinent may have represented a significant source of methane inputs to the atmosphere, potentially affecting both the global carbon cycle and the climate.Item A large epeiric methanogenic Bambuí sea in the core of Gondwana supercontinent?(2021) Caetano Filho, Sergio; Sansjofre, Pierre; Ader, Magali; Santos, Gustavo Macedo de Paula; Guacaneme, Cristian; Babinski, Marly; Bedoya Rueda, Carolina; Amaral, Matheus Henrique Kuchenbecker do; Reis, Humberto Luis Siqueira; Trindade, Ricardo Ivan Ferreira daCarbon isotope compositions of both sedimentary carbonate and organic matter can be used as key proxies of the global carbon cycle and of its evolution through time, as long as they are acquired from waters where the dissolved inorganic carbon (DIC) is in isotope equilibrium with the atmospheric CO2. However, in shallow water platforms and epeiric settings, the influence of local to regional parameters on carbon cycling may lead to DIC isotope variations unrelated to the global carbon cycle. This may be especially true for the terminal Neoproterozoic, when Gondwana assembly isolated waters masses from the global ocean, and extreme positive and negative carbon isotope excursions are recorded, potentially decoupled from global signals. To improve our understanding on the type of information recorded by these excursions, we investigate the paired δ13Ccarb and δ13Corg evolution for an increasingly restricted late Ediacaran-Cambrian foreland system in the West Gondwana interior: the basal Bambuí Group. This succession represents a 1st-order sedimentary sequence and records two major δ13Ccarb excursions in its two lowermost lower-rank sequences. The basal cap carbonate interval at the base of the first sequence, deposited when the basin was connected to the ocean, hosts antithetical negative and positive excursions for δ13Ccarb and δ13Corg, respectively, resulting in Δ13C values lower than 25‰. From the top of the basal sequence upwards, an extremely positive δ13Ccarb excursion is coupled to δ13Corg, reaching values of þ14‰ and 14‰, respectively. This positive excursion represents a remarkable basin-wide carbon isotope feature of the Bambuí Group that occurs with only minor changes in Δ13C values, suggesting change in the DIC isotope composition. We argue that this regional isotopic excursion is related to a disconnection between the intrabasinal and the global carbon cycles. This extreme carbon isotope excursion may have been a product of a disequilibria between the basin DIC and atmospheric CO2 induced by an active methanogenesis, favored by the basin restriction. The drawdown of sulfate reservoir by microbial sulfate reduction in a poorly ventilated and dominantly anoxic basin would have triggered methanogenesis and ultimately methane escape to the atmosphere, resulting in a13C-enriched DIC influenced by methanogenic CO2. Isolated basins in the interior of the Gondwana supercontinent may have represented a significant source.Item Rare earth elements in the terminal Ediacaran Bambuí Group carbonate rocks (Brazil) : evidence for high seawater alkalinity during rise of early animals.(2020) Santos, Gustavo Macedo de Paula; Caetano Filho, Sergio; Enzweiler, Jacinta; Navarro, Margareth Sugano; Babinski, Marly; Guacaneme, Cristian; Amaral, Matheus Henrique Kuchenbecker do; Reis, Humberto Luis Siqueira; Trindade, Ricardo Ivan Ferreira daRare earth elements plus yttrium (REY) mass fractions of ancient carbonate rocks are used to track changes in chemistry of past seawater. Here we investigate REY patterns in two carbonate sections from the Ediacaran Bambuí Group, São Francisco Basin (Brazil), which comprise its two lowermost transgressive-regressive secondorder sedimentary sequences. Shale normalised distributions vary with the sequence stratigraphy framework. In the basal 2nd-order sequence, carbonate samples from the basal sequence transgressive systems tract display light REY (LREY) distributions slightly depleted to enriched that reflect input of freshwater, possibly in a post glacial episode. Upwards, carbonate rocks from the early highstand systems tract (EHST) yielded LREY enriched distributions, which progressively turns into LREY shale normalized depleted distributions on samples from the late highstand systems tract (LHST). This portion of the sequence also displays Y positive anomaly in some cases. Carbonate samples from the upper second-order sequence do not display coherent patterns. Ce/Ce* values > 1 in most samples throughout the two sections suggest permanent anoxia of seawater. The REY change from the EHST to LHST in the basal sequence marks an important paleoenvironmental overturn in the basin, with increasing alkalinity in seawater driving REY fractionation and LREY depletion. Confinement of the basin in the inner areas of West Gondwana due the uplift of marginal neoproterozoic orogens probably changed the weathering style of source areas to more congruent, thus delivering a higher ionic influx to a restricted setting, increasing alkalinity during LHST. Cloudina sp. fragments were reported in this stage of the Bambuí Group and in carbonate rocks with high Sr mass fractions in other West Gondwana basins, supporting the hypothesis that the high alkalinity of seawater during late Ediacaran may have driven the appearance of the first biomineralizing organisms.Item Sequence stratigraphy and chemostratigraphy of an Ediacaran-Cambrian foreland-related carbonate ramp (Bambuí Group, Brazil).(2019) Caetano Filho, Sergio; Santos, Gustavo Macedo de Paula; Guacaneme, Cristian; Babinski, Marly; Bedoya Rueda, Carolina; Peloso, Marília; Amorim, Kamilla Borges; Afonso, Jhon Willy Lopes; Amaral, Matheus Henrique Kuchenbecker do; Reis, Humberto Luis Siqueira; Trindade, Ricardo Ivan Ferreira daIn the terminal Neoproterozoic, drastic climate changes associated with biological innovations are coupled to isotope and elemental geochemical anomalies. However, lateral variability and local depositional controls may affect global geochemical signals, which can only be tracked through a proper stratigraphic/paleogeographic assessment. Here, we investigate the sequence stratigraphy and chemostratigraphy of the basal units of the Bambuí Group, central-east Brazil. This stratigraphic unit records a foreland basin system developed during the Ediacaran-Cambrian West Gondwana assembly and represents a 1st-order sequence, in which the two lowermost 2nd-order sequences record major geochemical disturbances. The first 2nd-order sequence started with the deposition of a transgresive systems tract, possibly in a postglacial scenario, which accompanies a negative-topositive δ13Ccarb excursion. The early highstand systems tract represents the establishment of a marine carbonate ramp throughout the basin. In terms of chemostratigraphy, it corresponds to a δ13Ccarb plateau close to 0‰ and Sr/Ca ratios around 0.001. The late highstand stage coincides with a remarkable increase in Sr content and Sr/Ca ratios at basinal scale. Occurrences of the Cloudina sp. late Ediacaran index fossil were reported in this stage. An erosional unconformity associated with a dolomitic interval, locally including subaerial exposure features, marks the top of the first 2nd-order sequence. This sequence boundary heralds an abrupt increase in δ13Ccarb values, up to +14‰. These extremely high δ13Ccarb values and high Sr/Ca ratios persist throughout the overlying sequence, as a result of progressive and enhanced restriction of the foreland basin system. Basin restriction at this stage has implications for the paleontological and chemostratigraphic record of epicontinental basins of the West Gondwana in the terminal Ediacaran. Late Ediacaran Sr-rich intervals in these basins show unusually nonradiogenic 87Sr/86Sr ratios, which may represent local depositional controls and deviations from the modern oceanographic models. Physiographic barriers and stressful conditions likely represented extreme environments for metazoan colonization.