ABSTRACT. Over the past decades, analysis of occluded carbon in phytoliths (opaline silica mineral bodies that form in and between plant cells) has become a workhorse of paleoclimate and archaeological studies. Since different plant types exhibit distinctive phytolith morphologies, their assemblages are used in identifying vegetation histories or food culture adaptations. A few direct radiocarbon AMS measurements of phytoliths have been carried out, but these measurements are difficult due to the low concentrations of phytoliths in some plant species, and the small amount of C per phytolith (<2%). In addition, no phytoliths samples of a known 14 C age are available to verify measurement accuracy and precision, and to check sample preparation protocols. Background corrections are also difficult to address due to the lack of suitable material. In this work, we designed a procedure to quantify a suitable blank using SiO 2 powder samples (close to the opal structure, and free of 14 C). The full phytolith extraction showed high carbon contamination components: a) ~3 g of modern C and ~2 g of dead C. We also performed accuracy tests on large phytolith-occluded carbon samples extracted from soils and harvested plants. The unexpected 14 C ages in some of the results triggered further investigations of possible sources of carbon contamination.
1 -Este artigo é um avanço no trabalho apresentado no VI Seminário Latino-americano de Geografia Física, Coimbra, 2010, e publicado em http://www.uc.pt/fluc/cegot/VISLAGF/actas/tema3/heloisa Resumo: Localizada na costa nordeste do Estado do Rio de Janeiro, Brasil, a região de Cabo Frio tem sido considerada um "enclave" fitogeográfico, com um clima mais seco que o restante do litoral fluminense, reduto de vegetação semelhante à caatinga, dominada por florestas xeromórficas, Cactaceae e Bromeliaceae. Objetivando contribuir para esta discussão de modo singular, este trabalho faz análises multivariadas de similaridade de variáveis do clima atual, comparando Cabo Frio com seu entorno mais úmido e com a caatinga do Nordeste brasileiro, e estuda possíveis variações da paleovegetação na região, utilizando como indicadores os fitólitos. Conclui-se que o clima de Cabo Frio se assemelha mais à caatinga que ao seu entorno, e que a vegetação, desde 13.000 anos cal AP, foi sempre de tipo pouco arbórea. Palavras-chave:Cabo Frio, clima, enclave fitogeográfico, ressurgência, análises multivariadas, fitólitos.Abstract: Located on the northeast coast of the State of Rio de Janeiro, Brazil, Cabo Frio region has been considered a phytogeographical enclave, with a drier climate than the rest of this coast, a redoubt for the "caatinga" like vegetation, dominated by xeromorphic forests, Cactaceae and Bromeliaceae. In order to contribute to this discussion in a singular way, this work makes multivariate analysis of similarity of variables of the current climate, comparing Cabo Frio with its more humid surroundings and the "caatinga" of Brazilian northeast. We also analyze possible changes in regional palaeovegetation, using phytoliths as proxy. We conclude that Cabo Frio has a climate more similar to the "caatinga" than to its surrounding, and that the vegetation, from 13,000 years cal BP, did not present major changes. A região de Cabo Frio apresenta peculiaridades geoecológicas, sendo considerada um enclave fitogeográfico no litoral do Rio de Janeiro. Este enclave seria explicado pelo clima semi-árido local de Cabo Frio, ligado, entre outros fatores, à presença de uma ressurgência costeira e ao regime de ventos da região, o qual pode ser perturbado por eventos El Niño. Algumas hipóteses foram propostas para explicar a dinâmica da vegetação da região durante o Quaternário. Ab' Saber (1977), baseado em estudos da flora e fauna atuais e em evidências geomorfológicas, acredita que, com as mudanças ambientais ocorridas na última glaciação, as caatingas se estenderam por grande parte do Brasil Tropical Atlântico, permanecendo, em alguns locais, desde
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