The widespread occurrence and wide structural diversity of sulfurized diterpenoids in ambers

• A molecular investigation showed widespread organic sulfur compounds in amber. • The relative proportion of organic sulfur compounds in amber is variable. • Organic sulfur compounds in amber present a wide structural diversity. • Organic sulfur compounds in amber are related to tricyclic and labdanoid diterpenoids. • Organic sulfur compounds in amber are markers for specific paleoenvironments. Although the presence of sulfur in amber has been previously reported, its origins and nature are not precisely known. The recent identification by NMR of sulfides related to tricyclic resin acids in a Cretaceous amber sample unambiguously showed that sulfur occurs in amber as organic sulfur compounds (OSC), at least to some extent. Molecular investigation of a large set of ambers from various botanical/geological origins and ages using GC–MS revealed that OSC are relatively widespread in ambers which was previously unknown. Their proportions relative to non-sulfurized terpenoids are variable among amber samples, and some samples proved to be devoid of OSC. In addition, it has been shown that OSC in ambers present a wide structural diversity, and comprise compounds related to tricyclic resin acids and labdanoids. Several structures could be proposed notably based on comparison with sulfurized labdanoids previously identified in sulfur-rich sediments. Laboratory sulfurization experiments involving functionalized diterpenoids from the resins of Cedrus atlantica and Araucaria columnaris and polysulfides successfully resulted in the formation of OSC identical or analogous to the natural compounds occurring in amber. This validates the predominant proposed pathways for the formation of OSCs from amber. The early diagenetic sulfurization of diterpenoids in resin thus appears to be a key geochemical process influencing amber composition and contributing to the molecular diversity observed in fossil resins. The diverse OSC occurring in several amber types could potentially be used as geochemical markers of specific palaeoenvironments where sulfate is available and where the conditions of sedimentation promote, at least locally or in microenvironments, the establishment of anaerobic conditions favourable for the development of sulfate-reducing microorganisms.