Carbon isotope biogeochemistry of plant resins and derived hydrocarbons

Hydrocarbons derived from plant resins are major components of some terrigenous oils and bitumens. These compounds are structurally distinct and this makes them useful biomarkers applicable in petroleum exploration as well as sources of biogeochemical information about palaeoenvironment and palaeobotany. Although recent studies have elucidated the molecular structure of resinites, very little information has been available for the carbon isotope composition of resinites and no studies of resin-derived compounds in oils had been performed prior to the present study. Hence, carbon stable isotope analyses were carried out on a suite of modern and fossil resins of diverse origins, including compound specific isotope analysis of individual hydrocarbons produced during resin pyrolysis. Oils derived from resinitic source organic matter were also analysed. The results showed that 'Class I' resinites derived from gymnosperms were enriched in the heavy carbon isotope compared with those from angiosperms ('Class II' resinites). Furthermore, both fossil resinites themselves and individual hydrocarbons derived from them were isotopically heavy compared with modern plant resins. The isotopic signatures of diterpanes and triterpanes in various early Tertiary oils from Australasia and Southeast Asia reflect their origins from gymnosperms and angiosperms, respectively.