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Contrasted distributions of triterpene derivatives in the sediments of Lake Caçó reflect paleoenvironmental changes during the last 20,000 yrs in NE Brazil.

Abstract : Lipid fractions extracted from sediment layers deposited in a small Brazilian lake during the last 20,000 yrs were investigated by gas chromatography-mass pectrometry. Considerable differences in the distribution and the amount of triterpene derivatives in the aliphatic as well as in the aromatic fractions were observed all along the series. Although no precise identification of these compounds was undertaken, our interpretation of mass spectral data allowed us discriminating between des-A-triterpenes, mono- and triaromatic derivatives of pentacyclic triterpenes and a series of compounds tentatively identified as diaromatic derivatives of tetracyclic triterpenes. The largest compound diversity was found in the lowest levels of the series dated back to the end of the Last Glacial Maximum when good preservation of terrestrial plant debris was ensured by rapid burial in a semi-arid climate. Then, except for a period corresponding to the Younger Dryas, only des-A-lupane was detected in significant amounts. The high predominance of des-A-lupane is interpreted as resulting from the development of a belt of Eleocharis sp. (spike-rush) that filtered most of organic inputs from the catchment and was the site of des-A-lupane production and exportation towards the lake centre. During the Younger Dryas, a strong influx of des-Alupane is attributed to the destruction of the spike-rush belt consecutive to the lowering of the lake level under drier conditions. The temporary destruction of this barrier allowed other triterpene derivatives to reach the lake. The distinct dynamics of des-A and aromatic triterpenes derivatives under variable medium conditions led us to hypothesise that these two families of compounds derive from distinct pools that contributed differently to the sediment depending on environmental and climatic conditions. Des-A-lupane was most probably produced in sub-aquatic conditions within the belt of spike-rush. Reversely, aromatic derivatives of triterpenes could have resulted from the degradation of their biological precursors within reducing micro-environments in the catchment.
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Submitted on : Thursday, November 9, 2006 - 1:24:29 PM
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Jérémy Jacob, Jean-Robert Disnar, Mohammed Boussafir, Ana Luiza Spadano Albuquerque, Abdelfettah Sifeddine, et al.. Contrasted distributions of triterpene derivatives in the sediments of Lake Caçó reflect paleoenvironmental changes during the last 20,000 yrs in NE Brazil.. Organic Geochemistry, Elsevier, 2007, 38, pp.180-197. ⟨10.1016/j.orggeochem.2006.10.007⟩. ⟨hal-00022545v2⟩

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