Caracterização e cronologia das tufas calcárias da fazenda Aurora, Ourolândia – BA
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2016-10-24
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Universidade Estadual Paulista (Unesp)
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Este trabalho caracterizou e buscou a compor o quadro paleoambiental em que se formou o depósito paludal de tufas calcárias num pequeno paleovale (72.000 m²) na Fazenda Aurora, em Ourolândia, centro-norte do Estado da Bahia, Brasil. Neste intuito, optou-se por uma abordagem multi-indicadores (multy-proxy), baseada na descrição e análise integrada da petrografia e ultraestruturas das tufas, além de dados de geoquímica isotópica de carbono e oxigênio e cronologia a partir de datações pelo método LOE (Luminescência Opticamente Estimulada). O depósito de Ourolândia é composto por tufas microdetríticas, com idade de 29.293±4.347 anos, desenvolvidas nas margens norte e sul, e tufas fitoermais do tipo boundstone e framestone, com idade de 16.631±4.319 anos, formadas sobre surgências alinhadas ao longo do fundo plano do paleovale. As tufas estudadas contêm moldes de plantas aquáticas, semiaquáticas e terrestres, além de abundantes moldes de cianobactérias das espécies cf. Phormidium incrustatum e cf. Phormidium foveolarum, que indicam evolução do sistema em condições de água rasa, doce e à temperatura ambiente. A excelente preservação de algumas tufas, de acordo com imagens de Microscópio Eletrônico de Varredura, permitiu reconhecer Extracellular Polymeric Substances e a ultraestrutura dos carbonatos (desde nanocristais até microcolunas). Os valores isotópicos de δ13C indicam fonte predominante de carbono derivado de matéria orgânica vegetal, com contribuição de plantas C3 ligeiramente maior que de plantas C4 no segundo evento deposicional (-8.52 e -10.40 ‰ VPDB, respectivamente). Já os valores isotópicos de δ18O sugerem águas de procedência meteórica e maior incidência de chuvas no segundo evento deposicional (-4,41 e -6,35‰ VPDB, para cada evento). As possíveis condições favoráveis à formação das tufas em ~30.000 e ~16.000 anos provavelmente se deram em resposta às mudanças climáticas ocorridas no nordeste brasileiro, em decorrência dos eventos cíclicos milenares no Hemisfério Norte Heinrich Stadial (HS) 3 e 1, respectivamente, que geraram aumento significativo da umidade da região, interrompendo longos períodos predominantemente semi-áridos, com clara correlação entre a formação das tufas de Ourolândia - BA e o aumento da taxa de crescimento e diminuição dos valores isotópicos de δ18O de espeleotemas em cavernas próximas ao depósito.
This research characterizes and reconstructs paleoenvironments of paludal tufas in a small paleovalley (72,000 m2) at Ourolândia, State of Bahia, Northeast Brazil, applying a multiproxy approach based on petrography and ultrastructure analyses, in addition to carbon and oxygen isotope geochemistry and tufa geochronology by Optically Stimulated Luminescence -OSL. The deposit consists of 29,293±4,347 microdetritic tufas, formed on the northern and southern margins, and 16,631±4,319 boundstone and framestone tufas, edified on aligned springs along the flat bottom of the paleovalley. The tufas contain aquatic, semi-aquatic and terrestrial plant molds, as well as abundant cyanobacterial molds of cf. Phormidium incrustatum and cf. Phormidium foveolarum, indicating the evolution of the system under shallow, fresh water conditions at ambient temperature. The preservation of some tufas on Scanning Electronic Microscope images enabled clear recognition of Extracellular Polymeric Substances and the ultrastructure of carbonates (from nanocrystals to microcolumns). The isotopic values of δ13C indicate a predominant source of carbon derived from organic matter, with a contribution of C3 plants possibly slightly higher than C4 plants in the second depositional event (-8.52 and -10.40 ‰ VPDB, respectively). The δ18O isotopic values suggest that the water in the paleoenvironment was mainly of meteoric origin and the pluviometric index was higher in the second depositional period (-4.41 and -6.35 ‰ VPDB, for each event). The favourable conditions for the tufa formation around ~30,000 and ~16,000 years were probably a response to exceptionally humid phases in Northeast Brazil that occurred as brief interruptions of long predominantly semiarid periods. These humid phases were also recognized in speleothems of caves near the tufa deposit, as enhance of growth rate and negative incursion of δ18O during Heinrich Stadial (HS) 3 and 1 millenary cyclical events in the Northern Hemisphere.
This research characterizes and reconstructs paleoenvironments of paludal tufas in a small paleovalley (72,000 m2) at Ourolândia, State of Bahia, Northeast Brazil, applying a multiproxy approach based on petrography and ultrastructure analyses, in addition to carbon and oxygen isotope geochemistry and tufa geochronology by Optically Stimulated Luminescence -OSL. The deposit consists of 29,293±4,347 microdetritic tufas, formed on the northern and southern margins, and 16,631±4,319 boundstone and framestone tufas, edified on aligned springs along the flat bottom of the paleovalley. The tufas contain aquatic, semi-aquatic and terrestrial plant molds, as well as abundant cyanobacterial molds of cf. Phormidium incrustatum and cf. Phormidium foveolarum, indicating the evolution of the system under shallow, fresh water conditions at ambient temperature. The preservation of some tufas on Scanning Electronic Microscope images enabled clear recognition of Extracellular Polymeric Substances and the ultrastructure of carbonates (from nanocrystals to microcolumns). The isotopic values of δ13C indicate a predominant source of carbon derived from organic matter, with a contribution of C3 plants possibly slightly higher than C4 plants in the second depositional event (-8.52 and -10.40 ‰ VPDB, respectively). The δ18O isotopic values suggest that the water in the paleoenvironment was mainly of meteoric origin and the pluviometric index was higher in the second depositional period (-4.41 and -6.35 ‰ VPDB, for each event). The favourable conditions for the tufa formation around ~30,000 and ~16,000 years were probably a response to exceptionally humid phases in Northeast Brazil that occurred as brief interruptions of long predominantly semiarid periods. These humid phases were also recognized in speleothems of caves near the tufa deposit, as enhance of growth rate and negative incursion of δ18O during Heinrich Stadial (HS) 3 and 1 millenary cyclical events in the Northern Hemisphere.
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Tufas calcárias paludais, Quaternário, LOE, Heinrich Stadial (HS) 3 e 1, Isótopos de oxigênio e carbono, Ultraestruturas de calcita, Paludal calcareous tufa, Quaternary, OSL, Heinrich Stadial (HS) events 3 and 1, Carbon and oxygen stable isotopes, Carbonate ultrastructures