Navegando por Autor "SANTOS, Renan Fernandes dos"

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Acesso aberto (Open Access)
A capa carbonática marinoana do Sul do Cráton Amazônico: multiproxies aplicados na reconstituição paleoceanográfica e geobiológica do início do Ediacarano.
(Universidade Federal do Pará, 2024-08-30) SANTOS, Renan Fernandes dos; SANSJOFRE, Pierre; NOGUEIRA, Afonso César Rodrigues; http://lattes.cnpq.br/8867836268820998
The post-Marinoan glaciation (~650-635 Ma) was the most severe event from the late Cryogenian period of the planet. The snowball Earth conditions induced a climate anomaly, triggering major changes in the paleoenvironmental and ocean chemistry recorded in the carbonate cap sequences. The dramatic effects on the global sea level were caused by glacial-isostatic adjustment (GIA) and ice gravity on the coastal zones associated with the ocean thermal expansion under greenhouse conditions. The fast input of meltwaters contributed to a stable density ocean stratification formed by hypersaline deep waters and meltwater surface layers. The ocean destratification occurred in a timescale ranging from tens of thousands to thousands of years. The Puga cap carbonate (~635 Ma), the basal deposits of the Araras-Alto Paraguai basin from the southern Amazon Craton, is revisited in the classical sections from the Tangará da Serra and Mirassol d'Oeste, Mato Grosso State. This succession is one of the best records for evaluating the supersaturation events under GIA and transgression conditions that controlled the accommodation space in the southern Amazon Craton. Sedimentological and stratigraphic data were integrated with new paleoceanographic, and paleo-redox data combined with diagenetic, crystallographic, geochemical (primarily rare earth elements and trace metals), and isotopic (whole rock 87Sr/86Sr, εNd(t), δ 13C, δ 18 O, Sm/Nd) results providing further insights to understand the post-Marinoan conditions. The Puga cap carbonate sequence spans approximately 90 meters, with the first 10 meters composed of glaciomarine deposits, diamictites, and dropstones from the Puga Formation. The basal contact with diamictites is plastically deformed, indicating rapid carbonate precipitation. The cap dolostone consists of stratiform doloboundstones with gypsum pseudomorphs and domal doloboundstones with tubestone, which were deposited in a shallow platform with intense microbial activity. Peloidal dolomudstone/dolopackstone with laminations parallel to bedding planes and peloidal dolograinstones/dolomudstone with quasi-planar laminations and low-angle truncation were formed in a wave-influenced shallow platform. The cap limestone conformably overlies the cap dolostone deposits, marked by dolomitic marlstone with calcite crystal fans (aragonite pseudomorphs) interbedded with mega-rippled limestone. The facies association of the cap limestone indicates moderately deep-water conditions dominated by waves and storms transitioning to a deep platform supersaturated with CaCO3. The main diagenetic process is dolomitization during syn-depositional and shallow burial stages. Rare earth element + yttrium patterns have been analyzed in these deposits. Low Y/Ho ratios (<36) in the cap dolostone suggest a mixture of meltwater and seawater, while the base records superchondritic Y/Ho values up to 70 and high Eu/Eu* values up to 3, indicating upwelling of hypersaline seawater with hydrothermal fluid interaction, suggesting dolomite precipitation during ocean destratification. The radiogenic isotopic compositions of Nd, combined with other proxies such as δ 13C and 87Sr/86Sr, indicate the influence of continental and marine contributions. The Nd isotopic system, less susceptible to diagenetic exchanges, revealed distinct signatures of water masses and enhanced weathering of the Amazon Craton during deglaciation. This process is indicated by geochemical trends (e.g., Y/Ho) and 87Sr/86Sr, εNd(t), δ 13C values. 87Sr/86Sr data in the cap dolostone range from 0.7264 to 0.7084, higher than pre- and postglacial seawater values. More radiogenic 87Sr/86Sr values associated with less radiogenic εNd(t) values, similar to those found in diamictites, reinforce coastal weathering contribution to meltwaters. This multiproxies approach is a reconciliation with the previous rapid cap carbonate precipitation model following the short-term timescale for ocean destratification. Redox-sensitive trace metal data, U, Mo, V, Ni, Cu, P, and δ13C isotopes, indicated paleo-redox states and paleoproductivity during the post-glacial transgression. The cap dolostone precipitated under oxygenated conditions with extensive microbial community contributions, transitioning to predominantly dysoxic conditions with wave action in the last deposition phase. The abrupt sea-level rise altered the biogeochemical cycle, indicating a direct relationship between oxygen production and rapid microbial community colonization. The rapid sea-level rise and continental weathering reduced seawater Mg/Ca ratios with substantial Ca2+ input, also demonstrated by εNd(t) values, causing the change of dolomitic platform to CaCO3 -supersaturated seas in the Amazon Craton margin. Additionally, low siliciclastic content in the cap dolostone is consistent with the siliciclastic starvation model, and the abrupt increase caused the decline of microbial communities coincident with the predominance of dysoxic conditions and longterm transgression. During specific Cryogenian-Ediacaran deglaciation scenarios, anomalous sedimentary and geochemical processes generated one of the most complex paleoenvironmental disturbances in the biogeochemical cycle. They strongly influenced the rapid primary productivity, directly impacting microbial life. The analysis of Precambrian scenarios in the Amazon Craton unraveling the extreme climates sheds critical light on extremophile life proliferation and has strong implications for understanding other planetary surfaces.
Acesso aberto (Open Access)
Paleoambiente e evolução dos estromatólitos gigantes da capa carbonática marinoana do Cráton Amazônico, Tangará da Serra-MT
(Universidade Federal do Pará, 2020-03-10) SANTOS, Renan Fernandes dos; NOGUEIRA, Afonso César Rodrigues; http://lattes.cnpq.br/8867836268820998
Giant domical stromatolites, associated with tubstone structures, are recorded here for the first time the sequence of the Puga cap carbonate the southeast of the Amazonian Craton region of Tangará da Serra, Brazil. The well-preserved giant mounds reach 12 m in diameter and up to 10 m in height, forming a 200 m laterally continuous biostrom, with individualized domes, which display an expressive synoptic, metric to decametric relief. This work aimed to de- scribe and interpret the first occurrence of giant stromatolites and attest to their biogenicity, considering their macro, meso and microscopic aspects. It also determined the stratigraphic biogenic facies relationship, formed by organomineralization processes, whether bioinduced or bioinfluenced together with the other inorganic facies, originated by precipitation and/or sedimentary (mechanical) processes of the Puga carbonate sequence and finally, proposed an evolutionary model for giant stromatolites with the other facies, providing new inferences for the hydrodynamics of the epicontinental sea, of the SE of the Amazonian craton, with pale- oenvironmental and bioevolutionary implications for the beginning of Ediacaran. The begin- ning of Ediacaran marked by epicontinental, shallow and stratified seas, formed by dense and hypersaline waters, contrasting with the melting waters, light and rich in nutrients. The origin of the giant stromatolites is the product of a sum of anomalous paleoenvironmental and biotic factors, which occurred after the Marinoana glaciation (635Ma). The diamicton substrate was colonized by extremophiles, halophilic microbial communities in hypersaline waters with little or no direct influence of hydrodynamic processes, forming stratiform stromatolites with gypsum pseudomorph cement. With the advancement of greenhouse conditions, there was a continuous generation of accommodation space, caused by rising sea levels, influenced by syn-deglacial transgression and by the glacio-isostatic adjustment (GIA) that caused the uplift of the coastal zone and the mixing of the waters. The mixture of the waters that occurred at the end of the glaciation was essential for the development of microbial mats, since the cool water was, rich in nutrients. The microbial communities developed initially in balance with the constant migration of the photic zone, caused by the gradual rise in sea level, with the end of the GIA's influence. The accommodation space is controlled only by post-glacial transgres- sion, resulting in a more significant impact on hydrodynamic processes, registered in the ex- pressive synoptic relief. The sum of these conditions would have propitiated the development of microbial communities that would become giant stromatolitic mounds, with metric to dec- ametric synoptic relief. The constant increase in energy would result in the reworking of mi- crobial mats, generating macropeloids, agglomerates of micropeloids. The record demon- 1 strates an alternation of macropeloids and micropeloids blades, which suggests that in times of stabilization, there was an attempt to colonize microbial communities, that were reworked continuously by the wave. The decline of giant stromatolites, in the southern of the Amazoni- an craton, would be related to the height of greenhouse conditions, with an sudden increase in the influence of post-glacial transgression, which would condition a massive siliciclastic en- try, promoting the burial of microbial communities. Thus, the stromatolitic strata were suc- ceeded by a limestone factory, induced mainly by inorganic processes, in a sea saturated with CaCO3. Giant stromatolites are considered here as an essential record for understanding post- glacial conditions, from the beginning of Ediacaran.