Contrasting diagenetic pathways of higher plant triterpenoids in buried wood as a function of tree species

Terpenoid biomarkers with high chemotaxonomic value, which can be used for the identification of an angiosperm input to sedimentary organic matter are desirable for studies devoted to the reconstitution of past environmental change. There is also a need, in the framework of archaeometric investigations, for reliable biomarkers to allow wood species determination from archaeological remains. In this context, we report the analysis of the triterpene content of buried wood and bark from oak, alder and birch, and comparison with that of the corresponding fresh material. Identification of new biomarkers led us to elucidate the diagenetic transformations undergone by triterpenoids in buried wood. A large set of triterpenoid biomarkers characterized by the presence of an oxygenated functionality in ring A at C-2 only could be exclusively evidenced in buried oak wood. An overall diagenetic pathway leading from the predominant polyfunctionalized triterpenoids in fresh oak wood (oxygenated functionalities at C-2/C-3/C-19/C-23/C-24/C-28) via numerous C-2 and C-2,C-3 functionalized intermediates to C-2 oxygenated aromatic triterpenoids is proposed. The evolution of the triterpenoid assemblage with respect to the level of diagenetic alteration could be followed. In contrast, the predominant triterpenoids in fresh alder and birch are common C-3 functionalized compounds. Their degradation follows the classical degradation/aromatization pathways reported for C-3 oxygenated triterpenes, ring A degradation appearing to represent only a minor process. In addition, the formation of phenolic triterpenoids, which have only been reported from brown coal, was observed.