No abstract

The widely distributed aromatic di-and triterpenoids from higher plants occuring in sediments are formed by diagenetic microbial processes affecting their precursor plant lipids, and these compounds and their stable isotopic composition ( 13 C and  2 H) have the potential to be used for palaeoenvironmental and palaeoclimatic studies. In the present study, the isotopic composition of di-and triterpenoids aromatized to a different extent has been measured to examine the isotopic effects associated with the aromatization reaction. To overcome the possibility of multiple higher plant sources as is generally the case with sedimentary lipids, the  13 C and  2 H values of aromatized di-and triterpenoids from lipid extracts recovered from conifer and angiosperm buried wood have been determined, allowing an unambiguous genetic precursor/product relationship to be insured. The results show that the  13 C compositions of both di-and triterpenes do not seem to be significantly affected by Version non révisée 2 progressive aromatization, whereas the situation is more contrasted with  H values. In the case of diterpenoids related to abietic acid, a significant increase of the  2 H values by up to ‰ with ongoing aromatization was measured. This is in contrast to what is expected for dehydrogenated compounds which should be globally more 2 H-depleted than their precursor molecules (e.g., biosynthesized unsaturated fatty acids vs. their saturated precursors). This 2 H enrichment of aromatized diterpenoids could indicate that they represent only minor residual intermediates, the majority of these intermediates having been further degraded by processes favoring the degradation of the 1 H-containing substrates and having a moderately pronounced 12 C/ 13 C selectivity. With triterpenoids, a preservation of the  2 H values was observed whatever the nature and extent of the aromatization process considered, which may be related to enzymatic reactions showing a limited carbon and hydrogen isotopic effect upon aromatization.