Effect of thermal maturation on plant-derived terpenoids and leaf wax n-alkyl components

Plant biomarkers, such as terpenoids and leaf wax components (n-alkanes, n-alkanoic acids and n-alkanols), are frequently found in sediments and can be used, often in association with stable carbon (and hydrogen) isotope measurements, as paleovegetation and paleoclimate proxies. However, few controlled studies have monitored plant biomarker alteration to determine if certain plant biomarkers are preferentially lost relative to more recalcitrant forms. To investigate the role of selective alteration and degradation of plant biomarkers, hydrous pyrolysis was used to artificially mature leaves from four plant species, including the deciduous angiosperms Acer rubrum and Platanus occidentalis, the deciduous conifer Taxodium distichum and the evergreen conifer Pinus sylvestris. Leaves were artificially matured at temperatures ranging from 150 to 330 °C for 72 h to simulate maturation. With increasing temperature, functionalized di- and triterpenoid yields decreased, with a greater loss of triterpenoids at lower temperature. Both diterpene and triterpene yield increased during maturation up to 310-320 °C. A greater amount of diterpenes and triterpenes was generated for P. sylvestris and A. rubrum, respectively, and might be related to differences in terpenoid starting composition. Terpenols were preferentially converted to terpenes over terpenoic acids. Taken together, hydrous pyrolysis of plant biomarkers indicates that paleovegetation reconstruction from terpenoids can be informative, but may only be a qualitative vegetation proxy under many conditions. The n-alkane yield largely increased up to 320 °C, whereas the n-alkanol yield mainly decreased with increased maturity. The n-alkanoic acids initially increased, but then decreased. The stable carbon isotopic composition (δ13C) of the n-alkanes was generally, though not universally, constant up to 200 °C. Above this, the δ13C values of individual chain length hydrocarbons, for some species, changed by ca. 2‰. This suggests that n-alkane δ13C values should be unaltered in immature rocks, but can vary in the catagenic stage of maturation (oil window).