Terpenoid biomarkers of ambers from Miocene tropical paleoenvironments in Borneo and of their potential extant plant sources

The chemical composition of ambers is highly diverse, characterized by the occurrence of a variety of terpenoids including: mono-, sesqui-, di-, and triterpenoids. The direct analyses of their chemical constituents in total extracts using polar solvents permits the elucidation of unaltered natural products, which are characteristic of the source plants or paleobiome. At this time, knowledge is limited about the plant origins of fossil resins that formed in tropical climatic conditions. Here, we present the complex chemical characteristics of Miocene fossil resins (resinites, termed here as ambers) from the tropics of Kalimantan (Borneo, Indonesia). Extant plant resins from the same geoclimatic region were also analyzed to identify the potential botanical sources of the ambers. Gas chromatography-mass spectrometry (GC-MS) analyses of total extracts (silylated and methylated) of natural and amber samples were carried out and compared with standard compounds. The main producers of resins forming these Miocene ambers were angiosperms - probably resins of Shorea and less likely Hopea, but not Dipterocarpus species. The key chemotaxonomic marker, present in the ambers and extant Shorea species, was asiatic acid. All samples were composed of sesquiterpenoids and triterpenoids in various proportions, without diterpenoids, characteristic for flowering plants. The sesquiterpenoids in the resins of both ambers and extant plants were primarily natural products with the cadinane skeleton. The triterpenoids of the extant resins of the Dipterocarpaceae and Miocene ambers were characterized by a prevalence of ursane over oleanane types. Polymerization of cadinoids in resins from Shorea species and in the ambers was not extensive. Based on the amber compositions we conclude that the molecular alteration of the Miocene deposits from Kalimantan is rather low, but differs depending on their location.