Navegando por Assunto "Holoceno tardio"

Agora exibindo 1 - 4 de 4

Acesso aberto (Open Access)
Caracterização mineralógica e química dos regolitos de uma área de transição savana-floresta em Roraima: uma análise da evolução da paisagem
(2012-12) MENESES, Maria Ecilene Nunes da Silva; COSTA, Marcondes Lima da
The present study was carried out in an area of transition savanna-forest of the north of Roraima state. The main objective was to characterize the landscape regoliths and evaluate the evolution of the landscape during the late Holocene and even the present-day. Thus, four topossequences representative of the geomorphological, pedological and botanical patterns along the regional landscape were selected to sampling of soils and sediments. These samples were analyzed as for their granulometrical, mineralogical, chemical and chronological characteristics. The results revealed a landscape dominated by sandy to silty soils constituted essentially of quartz and kaolinit, besides muscovite, goethite, sillimanite and albite in low amounts. The high values of SiO2 confirm the quartzic character of these regoliths. The mineralogical and chemical compositions of these materials indicate provenience of metamorphic rocks and of laterites of the area, which due to the dominant warm and humid climatic conditions during the last 1550 years before the present, has been suffering intense chemical weathering and leaching. The landscape evolution is controlled by hidrological erosion of the hillsides and the filling of the valleys of 'veredas' causing the leveling of the landscape and the development of extensive sandy plains from the interfluves soils.
Acesso aberto (Open Access)
Dinâmica dos manguezais no litoral norte do Espírito Santo durante o Holoceno tardio
(Universidade Federal do Pará, 2018-07-01) SILVA, Fernando Augusto Borges da; FRANÇA, Marlon Carlos; http://lattes.cnpq.br/8225311897488790
This work aims to identify the mangrove dynamics in the North coast of Espírito Santo State, close to São Mateus (ES), during the Late-Holocene, using the depositional identification of paleoenvironments and the paleoflora, further a characterization of the sources of sedimentary organic matter and chronological determination of the events in the study area. In order to perform an integrated analysis of the palynological, C-14 data, sedimentological and geochemical data (δ13C, δ15N, C:N and C:S ratio), two sediment cores (MBN and LI-34) were sampled with a peat core sampler. The sedimentary profiles were extracted from mangroves close to the mouth of São Mateus and Barra Seca rivers. The deposits age was around ~2662 cal yr BP, characterized by sand cross laminations (facies Sc), lenticular heterolithic deposits (facies Hl), sand with parallel lamination (facies Sp), wavy heterolytic deposits (facies Hw), flaser heterolithic deposits (facies Hf and Sf), massive sand (facies Sm), sand parallel laminations (facies Smh facies), and mud with parallel laminations (facies Ml), with shells and vegetal fragments. The first facies association (A) consists of an estuarine point bar (~2662 and ~2215 cal yr BP), the second (B) consists of a tidal flat with mangrove/herbs vegetation, since at least ~2215 cal yr BP, and finally the third (C) such as an inlet, a typical distal estuarine channel (~1337 cal yr BP). The geochemical results indicate a mixture of C3 and C4 plants, near the basal portions, with a subsequent predominance of C3 plants towards the top of the samples, besides the presence of organic matter of marine/estuarine. The C:N ratio indicates an oscillation between the aquatic and terrestrial influences, corroborating the values of the C:S ratio (0.02-5.18), which also revealed a mixture of organic matter with marine/aquatic and terrestrial influence. The pollen results showed that the mangrove in this region has been present for at least 2662 cal yr BP, however, changes in its biodiversity have been observed, since currently this ecosystem is colonized mainly by the genus Laguncularia in the region of the mouth of the Barra Seca river, different from previous periods, where there was full development of Rhizophora and Avicennia trees. However, on the mouth of São Mateus river, the genus Rhizophora beginning the mangrove and after Avicennia. Thus, through of this study, it is possible to infer that there were no significant climatic oscillations in this region, however, the stabilization of the relative sea-level during the late-Holocene, as well as the sedimentary dynamics may be controlling the expansion/contraction process of these mangroves.
Acesso aberto (Open Access)
A dinâmica dos manguezais subtropicais no litoral norte de Santa Catarina durante o Holoceno tardio.
(Universidade Federal do Pará, 2019-06-28) PINHEIRO, Vanessa da Conceição; FRANÇA, Marlon Carlos; http://lattes.cnpq.br/8225311897488790
The goal of this work is identifying the establishment and expansion of mangroves on the northern coast of Santa Catarina during the Late Holocene. For this work, pollen data, 14C dating, organic geochemical (δ13C, C:N, TN and TOC) and results of sedimentary analyzes were integrated. These data were obtained from the analyses carried out from two sedimentary cores (SF7 and SF8) collected using a Russian sampler in the Babitonga Bay, northern coast of Santa Catarina. The data reveals a typical tidal channel and tidal plain deposits throughout the records. The tidal channel deposit was accumulated between > 1692 cal yr BP to ~ 667 cal yr BP, close to the bottom of the cores. This deposit is formed by fine-to-medium sand with parallel laminated sand (Sp facies), cross lamination (facies Sc), low angle planar lamination (Sb facies) and massive sand (Sm facies). The sedimentary deposit is characterized as a tidal plain around ~ 1223 cal yr BP, constituted by facies flaser heterolithic bedding (Hf), wavy heterolithic bedding (Hw), lenticular heterolithic bedding (H1) and parallel mud (Mp). The pollen content preserved along the tidal channel deposits reveals predominance of trees, shrubs, herbs and palms surroundings of the channel and on topographically higher regions. Only SF8 core was found mangroves pollen while the isotopic data revealed δ13C (-24.4 to -21.47 ‰), C/N ratio (4.77 to 20.81) with a strong contribution of marine organic matter and C3 terrestrial plants. The tidal channel was clogged and allowed the beginning of the deposition of the tidal plain. The deposit of the tidal plain has a large amount of plants fragments and pollen content characterized by predominance of herbs, trees, shrubs, palms and mangroves. The results of δ13C (-22.48 to -21.18 ‰) and C:N ratio (11.49 to 19.89) indicated the contribution of C3 terrestrial plants as well as contribution of marine organic matter. In addition, the mangroves beginning from approximately ~1692 cal yr BP with the genus Laguncularia and Avicennia, still at the edge of the tidal channel. Around ~586 cal yr BP was observed the mangrove development with a Rhizophora genus. The Laguncularia and Avicennia were initially established in sandy substrates and then also occupied in silty sandy soil. In the tidal plain environment, the genus Rhizophora was established in mud substrate. The mangroves of this region settled in the topographically higher regions and later expanded to the lower and closest regions of the Babitonga bay, possibly due to the decrease of the relative sea level recorded during the late Holocene as well as the migration and filling of the tidal channels. The occurrence of Rhizophora pollen close to the surface probably was a result of the temperature rise during the Late Holocene.
Acesso aberto (Open Access)
A Evolução da paisagem de transição savana-floresta em Roraima durante o Holoceno tardio: base mineralógica, geoquímica e palinológica
(Universidade Federal do Pará, 2011-01-10) MENESES, Maria Ecilene Nunes da Silva; COSTA, Marcondes Lima da; http://lattes.cnpq.br/1639498384851302; https://orcid.org/0000-0002-0134-0432
The Amazonia includes several vegetation physiognomies, besides the large and well-known tropical forest. The savannas, for instance, are present in several areas as discontinuous patches or covering extensive areas as it is the case of the savannas found in the north and northeast portions of the Roraima state. These savannas belong to the so-called Rio Branco-Rupununi Complex considered the largest continuous block of that vegetation type in the Brazilian Amazonian which in the state of Roraima covers about 41.000 km2 of a total of 53.000 km2 being limited to the south and west by tropical forests. In order to understand the vegetational dynamic and the evolution of the whole landscape in face of the possible climatic changes occurred along the geological time, the present study was carried out. The chosen area for this work represents a zone of transition between savannas and forests in the western portion of the savanna block, where four topossequences (FC, FH, TIA and RU) representative of the relief, vegetation and pedological cover patterns were selected for sampling. Then, samples of the regolith cover were collected starting at the base of the topossequences (veredas) until their top parts following the visible texture and color variations of the materials. Additionally, other two veredas (AM and MB) were also cored. These regoliths samples were submitted to analyses such as granulometrical by wet sieving; mineralogical by XRD and chemical (major and trace elements) by ICP-MS and XRF methods. The sediments of the veredas were still dated by radiocarbon method using the AMS (Accelerator Mass Spectrometry) technique, and also submitted to palynological analyses including the counting of charred particles. The regoliths of the studied area vary of sandy to silty sand textures being composed by quartz and kaolinite, and in minor amounts by muscovite, sillimanite, goethite, microcline and albite. The high values of SiO2 confirm the quartzose character of these regoliths, while the values of Al2O3, more expressive in the saprolite and in the sediments from veredas reflect the higher participation of the kaolinite, which is the single clay mineral identified in these samples. The mineralogical and chemical compositions of these materials indicate provenience of metamorphic rocks and laterites that faced with warm and wet climatic conditions have been suffered strong chemical weathering and leaching. The formation and wide development of vereda of Mauritia flexuosa since about 1550 years BP as showed by the pollen analyses corroborate the humidity increase in the area, which also favored the expansion of forests. In fact, trees of Virola, Alchornea, Melastomataceae and Moraceae among other were frequent suggesting that gallery forests as well as patches of secondary forests indicated by Didymopanax, Cecropia and Attalea occurred in this region in the most of the recorded period. In spite of those humid conditions, there are records of reduction of the forests about 1400-1100 years ( FC), 900-200 years (AM and FC) and between 700 and 300 years (site TIA) in favor of the expansion of the savannas. It is probable that the reduction of forests during those periods has been provoked by the increase in the intensity of fires, inferred by the highest concentration of charred particles in the sediments, prior and during the forest cover decrease. These fires are still common in the area, being more frequent in the neighborhoods of human establishments (indigenous) and cattle farms where they possibly work as a barrier to forest expansion. Although, alternate hydromorphic and hydrological stress conditions also contribute to prevent a forest expansion. The granulometrical, mineralogical and geochemical characteristics of the regolith cover combined to the sediments pollen and ages records allowed to interpret that the studied savanna-forest landscape, which is currently marked by a complex mosaic of grassy and woody savanna crossed by veredas of Mauritia flexuosa, gallery forest corridors and islands of forest covering sandy, quartzose and kaolinitic regoliths agree with the modern warm and wet climate conditions prevailing in this region since the Late Holocene. In compensation, features such as stone lines composed by fragments of quartz and crusts denote erosive events occurred probably in the Late Pleistocene and even in the Medium Holocene when dry to arid climates dominated the region. Then, the regarded landscape has been a stage of intense ecological and geomorphological transformations fomented mainly by the climatic changes imposed to the area, although the prehistoric and the modern man has also contributed, in the sense of delaying the expansion of the forests on the savannas.