Derivatives of triterpenes formed during diagenesis in tropical lacustrine recent sediments (Lake Caçó, Brazil).
Résumé
Lipid fractions extracted from sediment layers deposited in a small Brazilian lake during the last 20,000 yrs, were investigated by gas chromatography-mass spectrometry. Considerable differences in the distribution and the amount of triterpene derivatives in the aliphatic as well as in the aromatic fraction were observed all along the series. The largest compound diversity was found in the lowest levels of the series where good preservation of terrestrial plant debris was ensured by rapid burial. There, the aliphatic fractions comprise des-A-triterpenes, onocerane I and hopanoids and the aromatics mono and triaromatic triterpenes. A third series of compounds was tentatively identified as diaromatic derivatives of tetracyclic triterpenes. 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 the environmental context. Des-A-lupane was most probably produced in sub-aquatic conditions within a belt of Eleocharis sp. that developed between 1 and 3m water depth around the lake. Reversely, aromatic derivatives of triterpenes could have resulted from the degradation of their biological precursors within reducing micro-environments in the catchment basin.
Domaines
Géochimie
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5-_Diagenetic_derivatives_-_Pub-OG_-_Jacob_et_al_2006.pdf (1.96 Mo)
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