Molecular and carbon isotope composition of hydrocarbons from ambers of the Eocene Shenbei coalfield (Liaoning Province, NE China)

The molecular and (bulk and compound-specific) stable carbon isotope composition of ambers from the Shenbei coalfield, northeastern China, were analysed to determine their origin and palaeobotanical affinitions. The applicability of the delta C-13 data of saturated diterpenoid hydrocarbons for palaeoenvironmental reconstruction was addressed by comparison with published carbon isotope ratios of diterpenoids from adjacent areas. The detected sesquiterpenoids are common biomarkers for the gymnosperm families of Pinaceae and Cupressaceae. The diterpenoids dominate the saturated hydrocarbon fraction with high abundances of kaurane-type compounds. Abietane-type, cadalene-type and alkylated compounds are the main biomarkers in the aromatic hydrocarbons in the ambers. Proposed typical biomarkers for amber (e.g., amberene and homoamberene) have also been detected and probably derived from the diagenesis of the abietane-type terpenoids in the studied ambers. The biomarker composition suggest that the ambers probably originated from gymnosperms such as Taxodium and Pinus. The delta C-13 values of bulk amber and individual compounds fall within the range of ancient ambers, but the delta C-13 values of bulk amber are isotopically more enriched than modern ambers. The delta C-13 values of compounds in hydrocarbon fractions vary in broad ranges. In the saturated hydrocarbon fraction, the delta C-13 value of sesquiterpenoids (-28.0 parts per thousand to -24.5 parts per thousand ) fluctuates more than that of diterpenoids (-22.5 parts per thousand to -21.8 parts per thousand ), and the average delta C-13 value of the latter is approximately 4 parts per thousand higher than the former. Aromatic hydrocarbons are on average enriched in C-13 by around 2 parts per thousand compared to the delta C-13 of non-aromatic compounds, the result is influenced by isotopically light sesquiterpenoids in the saturated fraction. The similar carbon isotope compositions of aromatic abietane-type compounds and abietane confirm that progressive aromatization does not have a significant impact on the delta C-13 value of compounds. The obtained higher delta C-13 values of saturated diterpenoids in contrast to that of lower Eocene lignite from the nearby regions provide evidence for growth of the precursor plants of the amber in dryer habitats or differences in precipitation distribution between different locations and ages.

Automated summary

Analysis of amber from the Shenbei coalfield in northeastern China provides insights into its origin and palaeobotanical associations, predominantly from gymnosperms. Carbon isotope composition comparisons reveal information about the ancient environment in which the amber formed. Variations in carbon isotope values of compounds suggest differences in amber sources.