Navegando por Assunto "Carbono - Isótopos"

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Acesso aberto (Open Access)
Dinâmica dos manguezais do litoral de Natal-RN de acordo com as mudanças no clima e nível do mar desde o Holoceno médio
(Universidade Federal do Pará, 2017-08-05) RIBEIRO, Samuel Rodrigues; COHEN, Marcelo Cancela Lisboa; http://lattes.cnpq.br/8809787145146228
The factors behind the Holocene mangrove dynamics have long been a theme of great interest for research. Among all hypotheses, the establishment, expansion, and contraction of mangrove areas have been mainly attributed to sea level and climate changes. However, other driving forces of limited occurrence in time may be significantly affecting this system. Along the valley of the Ceará Mirim river, near the Natal city, Rio Grande do Norte littoral, northeastern Brazilian the integrate geomorphological, sedimentological, and palynological data with radiocarbon dating, as well as δ13C, δ15N and C/N from sedimentary organic matter from four sediment cores reveal an estuarine influence with mangrove development since 6950 cal yr BP after the post-glacial sea level rise, and it has been stable during the middle and late Holocene. The mangrove expansion along this fluvial valley since the middle Holocene was caused by the sea-level stabilization. The multi-proxies indicate a wetland dynamic was mainly controlled by autogenic factors, relating to tidal channel migrations on the muddy tidal flats formed after the channel abandonment provide suitable conditions for expansion of vegetation surrounding, instead of allogenic process, associated to sea level and/or climate change. Probably, some influence of sea-level and climate changes on mangrove dynamics along in this valley of the Ceará Mirim river have been weakened by the intense tidal channels activities since middle Holoceno.
Acesso aberto (Open Access)
Paleoambiente e quimioestratigrafia da porção superior do Grupo Araras, neoproterozoico da faixa Paraguai Norte, estado do Mato Grosso.
(Universidade Federal do Pará, 2015-06-22) RUDNITZKI, Isaac Daniel; ADER, Magali; NOGUEIRA, Afonso César Rodrigues; http://lattes.cnpq.br/8867836268820998; 8867836268820998
In the central portion of the South-America Platform occurs the carbonate succession of the Araras Group, that register an extensive carbonate platforms of the Neoproterozoic, developed along a passive margin at S-SW portion of the Amazon Craton. These sedimentary basins were submitted to a compressional collisional event at Ediacaran-Cambrian transition, resulting in the origin of the Paraguay Belt. While the lower Araras Group are recognized as a post-glacial record of the Marinoan event (~635 Ma), the final paleoceanographic and sedimentary events that led the end of carbonate deposition are discussed, for the first time, in this thesis. The upper Araras Group, represented by Serra do Quilombo and Nobres formations, are the last stage of Araras carbonate platform sedimentation. These units are covered by siliciclastic deposits of Alto Paraguay Group, Middle Ediacaran, which the basal deposits are related to the glacial Gaskiers event (~580 Ma), reported only on the eastern portion of the Belt. Stratigraphy, sedimentology and carbon isotope studies were performed on carbonate rocks exposition in Cáceres and Nobres regions, respectively the western and eastern portion of the belt, Mato Grosso state, Brazil. The Serra do Quilombo and Nobres formations are a unique carbonate ramp in a homoclinal system composed by: i) deep ramp with massive thin dolostone of offshore. ii) Middle ramp with massive thin dolostone, intraclastic and sandy dolostone with hummocky and swaley cross stratification, planar and wave lamination, oncolitic dolostone with wavy bedding and massive oolitic dolostone, from shoreface, oolitic bar complex, and foreshore/shallow subtidal zones; and iii) inner ramp comprising in massive thin dolostone, intraclastic dolostone and dolomitic sandstone with megaripple bedding, microbialites, chert, evaporite molds, laminated sandstone mudstone, deposited in tidal flats. These deposits were developed during highstand conditions after the Marinoan transgressive event , and preceding the implantation of siliciclastic platform from Alto Paraguay Group during the Ediacaran-Cambrian transition. The palinomorphs assemblages from Nobres Formation are represented by spheroidal forms, such as Leiospharidia, rare filamentous and acanthomorphous acritarchs represented mostly by Tanarium correlated to biozone ECAP (Ediacaran Complex Acantomorph Palynoflora) that suggest an age between 600-550 Ma. The High frequency stratigraphy appoint to orbital force as the principal mechanism responsible to accommodation space generation and peritidal cycles composition, and secondarily influenced by tectonic. The patters of carbon isotopes from carbonate (δ13Ccarb), organic matter (δ13Corg) and the coupling of carbon isotope composition (Δ13Ccarb-org = δ13Ccarb - δ13Corg) associated with low TOC suggests an oxidant ocean in equilibrium with atmosphere, with minimal redox variation on sediment/water interface, that includes: i) anoxic layer with δ13Ccarb around ~0‰ and δ13Corg <-28‰, restrict to the distal zone of deep ramp with early organic diagenesis conduct by chemosynthetic organism; ii) oxic layer with δ13Ccarb around ~0‰ and δ13Corg between -28 a -25‰ in open sea system of deep and middle ramp, with primary photosynthetic activities; iii) oxic layer with low pCO2 associates to positives signal of δ13Ccarb (+3‰) and δ13Corg (>-25‰) in shallow middle ramp associated with high ration of inorganic carbonate precipitation; and iv) oxic layer restrict features by negative δ13Ccarb (-2‰) and δ13Corg between -31 to -25‰, exclusive to inner ramp, related to primary photosynthetic production and effective organic early diagenesis drive by sea level changes. These data define the upper Araras Group as the record of a carbonate ramp with restrict coastal environments, on an oxidant ocean, tropical zones with warm waters, high evaporation rate and photosynthetic biomass, during the Middle Ediacaran. The pre-glacial influence indicated by carbon cycles anomalies or glacioeustasy they were not documented to the upper Araras Group. The final sedimentation stage of Araras carbonate platform was influenced by initial continental uplift source areas responsible for the siliciclastic input on carbonate coastal zone, preceding a progradation of continental-coast systems from Alto Paraguay Group at Middle Ediacaran.
Acesso aberto (Open Access)
Paleoambiente, paleogeografia e isótopos de carbono e oxigênio de depósitos carbonáticos miocenos da Plataforma Bragantina, Nordeste do estado do Pará, Brasil
(Universidade Federal do Pará, 2016-09-16) AMORIM, Kamilla Borges; NOGUEIRA, Afonso César Rodrigues; http://lattes.cnpq.br/8867836268820998
The onset of the Neogene is market by the Oligocene-Miocene transition characterized by sea level global variations that triggered one of the major marine transgressions in the Earth. In Brazil, deposits related to this event are recorded in north equatorial coast with meaningful exposures in eastern Bragantina Platform, north of Pará State. These are composed by carbonate and siliciclastic deposits of the Pirabas Formation corresponding to onshore portion of a shallow carbonate platform. Stratigraphic studies allowed the Pirabas Platform division in inner platform and inner/middle platform. The inner platform is composed by tidal flats and lagoon (shallow and deep) deposits. The tidal flats are characterized by terrigenous dolomudstone, peloidal dolomudstone, boundstone with microbial mats, bioturbated rhythmites, and massive argillite. Shallow lagoon deposits are composed by laminated wackestone/packstone and bioturbated calcimudstone and the deep lagoon are constituted by dolowackstone, massive floatstone with bryozoan and massive wackestone with equinoderms. The inner/middle platform is composed by tidal inlets and bioclastic/front shoal barriers. Tidal flats deposits are constituted by wackestone/packstone with bryozoan, packstone with bryozoan and grainstone with foraminifers and red algae that display low-angle cross stratification. Bioclastic/front shoal barriers are constituted by bafflestone with bryozoan, wackestone/ packstone with Marginopora sp. and terrigenous, packstone/grainstone with foraminifers, and rudstone with bivalves. The platform displays a rich fossiliferous content composed by bryozoan, equinoderms, bivalves, gastropods, benthic and planktonic foraminifers, green and red algae, ostracods, coral fragments fossils; Gyrolithes, Thalassinoids, Sinusichnus trace fossils, this last one made by decapods crustaceous. In the inner platform the faunistic diversity is smaller dominated by bryozoan, planktonic foraminifers, ostracods, and trace fossils, while in the inner/middle platform zone this diversity is higher widely constituted by benthonic foraminifers fossils, bryozoans, bivalves and gastropods. The platform shows variations in the mineralogical content, where the calcite amount is directly related to exposition periods of the inner/middle platform with great carbonate precipitation. On the other hand, the dolomite, quartz, gypsum and pirite are related to progadation periods in the inner platform, with higher evaporation rates and continental influx. Faciological, fossiliferous and mineralogical variations displays that the Pirabas Formation was closely related to sea level variations leading to changes in shoreline recorded in high frequency shallow-upward cycles, with the cycles in the base of succession predominantly retrograditional while in the top are progradational. The chemostratigraphic framework from Pirabas Formation was made by carbon (δ13Ccarb) and oxygen (δ18Ocarb) isotopes, rare earth elements (ETR) and traces. Carbon isotopic ratios reflect a primary isotopic signature with variations of values related to each depositional environment. Oxygen isotopic ratios demonstrate a dispersive pattern related to diagenetic influence. The ETR’s show a homogeneous pattern with enriched concentrations in light ETR’s and heavy ETR’s depletion. Trace elements concentration (Fe, Sr and Mn) is within expected values to carbonate rocks with little influence of diagenesis in the geochemical content. The trend and δ13Ccarb excursion curve coincide with the variations observed in shallow-upward depositional cycles from Pirabas Formation. Intervals related to the sea level rise are marked by the δ13Ccarb ratios close to 0‰ while the intervals of the cycles with negative δ13Ccarb anomalies are linked to sea level falls. Correlations among δ13Ccarb curves from Pirabas Formation and global do not show close covariance, however we suggested that the δ13C purchased reflect, even minimally, the global isotopic excursions that marks the Eomiocene-Mesomiocene interglacial period. The sea level curve variation of Pirabas Formation display intervals similar to the short-term global eustatic curve. However, the bigger frequency of this sea level variations observed in Pirabas Formation probably indicates local tectonic factors interference in the sedimentation. Previous works suggested that the carbonate platform collapse in the Bragantina Platform region was influenced by the siliciclastic influx from Proto-cone of Amazonas River during the Mesomiocene. The comparative analysis of stratigraphic dates from basins and platforms along the coastal eastern portion in Amazon coastal zone suggests that the progressive increase of siliciclastic sedimentation, noted in the upper Pirabas Formation is related to the Barreira Formation progradation, as an answer to the transpressive/transtensive tectonic in Eo/ Mesomiocene due faults reactivations generated in the last thermal subsidence event in the Brazilian coast during the south Atlantic ocean formation.
Acesso aberto (Open Access)
Processos autocíclicos e alocíclicos afetando os registros da paleoflora da foz do rio Jucuruçu, litoral sul da Bahia, durante os últimos 1000 anos
(Universidade Federal do Pará, 2016-06-07) MORAES, Caio Alves de; COHEN, Marcelo Cancela Lisboa; http://lattes.cnpq.br/8809787145146228
The vegetation holocenic history along the southern Bahia littoral is characterized by establishment, expansion and contraction phases of mangroves. This vegetation dynamics is mainly associated to climate change and to relative sea level oscillations. However, punctually and in a smaller time scales, for example, during the last 1000 years, other processes inherent to the sedimentary dynamics of depositional environment, termed autocyclic processes, are controlling the pollen assembly along stratigraphic profiles formed by active channels followed by its abandonment. Based on sedimentary features, pollen grains, isotopes (δ13C, δ15N C/N) and radiocarbon dating of sedimentary organic matter from two cores (PR-11 and PR-12) sampled from an abandoned meander and a tidal flat in the mouth of the Jucuruçu river, is proposed a model for evolution of a tidal channel surrounded by wetlands. The core PR-11, with 1.48 meters depth, was sampled from a mangrove zone, with a maximum age of 678 cal yr BP. The PR-12 was collected from a várzea vegetation, approximately 2.7 km from the current shoreline with 1.92 meters depth and, maximum age of 680 cal yr BP. The data present two facies associations: (A) Tidal channel, represented by massive sand deposits (facies Sm); cross-stratified sand (Scs) and facies flaser heterolithic bedding (Hf) and (B) Tidal plain, represented by facies wavy heterolithic bedding (Hw), lenticular heterolithic bedding (Hl), massive mud (Mm) and massive sand (Sm). The pollen data reveals that the facies association (A) presents absence of pollen grains of mangroves. It may have been caused be an intense activity of sediments reworking of channel margins and sediment deposition with pollen grains sourced from vegetation units not necessarily from the study site. In the top of the studied succession, in association facies (B), is possible to identify the establishment and expansion of mangroves in the newly formed tidal flat (PR-11) or lake (PR-12). Regarding the core PR-12, it is marked by channel abandonment. The reduced interaction between fluvial discharge and tidal waves have caused the infilling of mud sediments, and a highest potential for organic matter preservation. It provided conditions for the mangrove development and pollen preservation. In the case of PR-11, the tidal channel migration have caused the development of a tidal flat, which have favored the mangrove expansion. These depositional environments, favorable to the formation of mangroves, can be partially or completely modified by the natural dynamics of tidal channels and estuarine channels, which are under the influence of changes of sedimentary fluvial contribution on the shore and littoral drift processes along the coast associated with tides, waves and currents. These results were compared with some data from one core sampled 23 km upstream of Jucuruçu River that also indicated the presence of mangroves on tidal flat with estuarine organic matter during the early and middle Holocene, followed by herbaceous vegetation on a fluvial plain with organic matter sourced from freshwater during the late Holocene (Fontes, 2015). In this case, the sea level fluctuations and climate change were the main driving forces controlling the wetlands dynamics at the mouth of this river during the Holocene, characterizing an allocyclic process. However, considering the stratigraphic sequences of the analyzed cores in this master's work (PR-11 and PR-12), such sedimentary sequences associated with changes in vegetation and sources of organic matter are related to the infilling processes of coastal depressions, mainly marked by active channels, abandoned channels and tidal flats. Therefore, in a small time scale of coastal vegetation changes are not necessarily directly associated with sea-level and climate change (allocyclic processes). On the other hand, processes associated to the sedimentary dynamics of the depositional environment (autocyclic processes) must have controlled pollen assembly along the studied stratigraphic profiles.