Long-term, low temperature simulation of early diagenetic alterations of organic matter from conifers: Aliphatic hydrocarbons

Early diagenetic alterations of organic matter from conifers were simulated at low temperature of 80°C for five and a half years.Their diagenetic products of aliphatic hydrocarbons were identified and quantified.The results reveal that in the early sedimentation of organic matter from conifers, n-alkanes have developed; natural terpenoids have transformed into diagenetic terpenoids, in which some seldom reported evolutionary intermediates-two cadinatetraenes, three norabietatetraenes and one abietatetraene-were detected and identified.The diagenetic pathways for cadalane-type sesquiterpenoids and abietane-type diterpenoids are proposed.The experimental data also show that the diagenesis of aliphatic hydrocarbons is influenced by different inorganic environments.The influences can be concluded as follows: high salinity is propitious to generate and expulse aliphatic hydrocarbons; iron may promote the increase of alkanes, but interrupt the alterations of terpenoids; gypsum, when not coexisting with metal, may slow the degradation from long chain alkanes to short ones; and heavy metal probably delays the early diagenesis of both alkanes and terpenoids.These influences observed under low temperature may enrich studies on early diagenesis of organic matter from higher plants.Keywords: coniferous organic matter, early diagenetic alternations, alphatic hydrocarbons, inorganic environment, low temperature simulation terpenoids, after undergoing complex geochemical and geological transformations, wildly exist in the geosphere and their diagenetic pathways are also the interest of some researchers.Simoneit et al. (1986) reported that retene may originate from abietic acid and abietadienes; Otto et al. (1997) suggested retene may originate from other functionalized abietane-type diterpenoids such as taxodone, dehydrosugiol and ferruginol.The varied diagenetic pathways of terpenoids are reported to be related to diverse sedimentary environments.For instance, anoxic environments appear to be associated with formation and preservation of aromatic terpenoids (Simoneit and Mazurek, 1982).Oxygenated environments appear to be associated with hydrogenated analogues (Simoneit et al., 1986).Later investigations (Martin et al., 1999) also demonstrated that, through the anaerobic degradation by undefined microorganisms, aromatized and decarboxylated diterpenoids (such as retene) could be generated from resin acids (tricyclic diterpenoids).However, because of the complexity of biogeochemical processes in the geosphere, investigating the origin and diagenetic pathways of organic matter from higher plants and assessing the influences of sedimentary environments on their diagenesis are difficult.Therefore, on the hypothesis that short-term heating at high temperature can result in similar changes of organic