The effects of diagenetic aromatization on the carbon and hydrogen isotopic composition of higher plant di- and triterpenoids: Evidence from buried wood

The widely distributed aromatic di- and triterpenoids from higher plants occurring in sediments are formed by diagenetic microbial processes affecting their precursor plant lipids. These compounds and their stable isotopic composition (delta C-13 and delta H-2) have the potential to be used for palaeoenvironmental and palaeoclimatic studies. In the present study, the isotopic composition of di- and triterpenoids aromatized to different extents has been measured to specifically 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 delta C-13 and delta H-2 values of aromatized di- and triterpenoids from lipid extracts recovered from conifer and angiosperm buried wood have been determined, allowing an unambiguous substrate/product relationship along the aromatization pathway to be assured. The results show that the delta C-13 compositions of both di- and triterpenes do not seem to be significantly affected by progressive aromatization, whereas the situation is different for delta H-2 values. In the case of diterpenoids related to abietic acid, a significant increase of the delta H-2 values by up to 86%, with ongoing aromatization was measured. This is in contrast to what is expected for dehydrogenated compounds which should be globally more H-2-depleted than their precursor molecules (e.g., biosynthesized unsaturated fatty acids vs. their saturated precursors). This H-2 enrichment of aromatized diterpenoids could indicate that they represent only minor residual intermediates, the majority having been further degraded by processes favoring the degradation of the H-1-containing substrates and having a moderately pronounced C-12/C-13 selectivity. With triterpenoids, preservation of the delta H-2 values was observed whatever the nature and extent of the aromatization process considered, which may be related to enzymatic reactions showing a limited influence on the hydrogen stable isotopic composition of the aromatized compounds. (C) 2019 Elsevier Ltd. All rights reserved.