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Evaluation of two genera of benthic foraminifera for down-core paleotemperature studies in the western south atlantic

Abstracts

In this study we have compared the oxygen isotopic composition of two genera of benthic foraminifera (Uvigerina and Cibicidoides) from core-top samples with modern oxygen isotopic composition of seawater (delta18O). Based on a new relationship between delta18O and salinity for the mid-latitude western South Atlantic, we estimated the isotopic composition of equilibrium calcite (delta18Oeq) using two different equations: (1) O'Neil et al. (1969), modified by McCorkle et al. (1997) and (2) Kim & O'Neil (1997). When using (1), the small difference between delta18Oeq and delta18O of Uvigerina suggests that this genus precipitates its shell close to equilibrium with ambient seawater. The delta18O Cibicidoides data are 0.82 ‰ lower than the predicted (equilibrium) oxygen isotopic composition. Conversely, using (2) the Cibicidoides delta18O data show excellent agreement with the oxygen isotopic composition predicted from delta18O and water temperature while Uvigerina delta18O data are 0.69 ‰ higher than predicted oxygen isotope equilibrium values. Based on the evidences presented here and on the results from previous studies we suggest using the genus Cibicidoides and applying Kim & O'Neil's (1997) equation for down-core paleotemperature investigations. In the absence of enough Cibicidoides specimens we suggest using Uvigerina delta18O data and applying a correction factor of -0.69 ‰.

benthic foraminifera; oxygen isotopes; palaeoceanography; paleothermometry


Neste estudo, compara-se a composição de isótopos de oxigênio de dois gêneros de foraminíferos bentônicos (Uvigerina e Cibicidoides) de amostras de topo de testemunho com a composição isotópica moderna da água do mar (delta18O). Baseados em uma nova relação entre delta18O e salinidade para a latitude média do Atlântico Sul ocidental, estimou-se a composição isotópica da calcita em equilíbrio (delta18Oeq) a partir de duas equações diferentes: (1) O'Neil et al. (1969), modificada por McCorkle et al. (1997) e (2) Kim & O'Neil (1997). Utilizando (1), a pequena diferença entre delta18Oeq e delta18O de Uvigerina sugere que este gênero precipita as suas testas próximo ao equilíbrio com a água. Já os dados de delta18O de Cibicidoides são 0,82 ‰ menores que a composição isotópica prevista. Ao contrário, utilizando (2) os dados de delta18O de Cibicidoides mostram uma concordância excelente com a composição isotópica esperada, enquanto que os dados de delta18O de Uvigerina são 0,69 ‰ maiores que os valores de equilíbrio previstos. A partir das evidências apresentadas neste trabalho e em estudos prévios sugerimos a utilização do gênero Cibicidoides e a aplicação da equação de Kim & O'Neil (1997) para pesquisas de paleotemperatura. Na ausência de suficientes espécimens de Cibicidoides sugerimos a utilização de delta18O de Uvigerina aplicando um fator de correção de -0,69 ‰.

Foraminíferos bentônicos; Isótopos de oxigênio; Paleoceanografia; Paleotermometria


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Publication Dates

  • Publication in this collection
    24 Oct 2007
  • Date of issue
    Mar 2006

History

  • Accepted
    24 Feb 2006
  • Reviewed
    03 Feb 2006
  • Received
    23 Aug 2005
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