Effects of weathering on organic matter: Part II: Fossil wood weathering and implications for organic geochemical and petrographic studies

Bulk geochemical, petrographical, mineralogical and molecular compositions of unweathered, weathered and transitional zones of a Middle Jurassic fossil wood were analyzed to trace changes caused by oxidative weathering of the immature terrestrial organic matter (OM). The occurrence of such zones was confirmed by the mineral composition, showing replacement of siderite and pyrite by goethite. Vitrinite reflectance analysis of weathered and unweathered fossil wood samples revealed that weathering elevated the vitrinite reflectance values by ca. 0.1%, which should be taken into account during modeling of low maturity terrestrial OM. In the weathered part of the wood, most of the biomarkers and biomolecules were totally removed or the concentration decreased significantly. The concentration of most of the polynuclear aromatic hydrocarbons (PAHs) decreased by 50–80%, being >90% for the more reactive and less stable benzo[a]pyrene and perylene. On the other hand, several aromatic compounds, like phenanthrene and its methyl derivatives, phenyl naphthalenes, fluoranthene and oxygen-containing aromatic compounds increased in concentration in the weathered zone. This results from processes such as formation of phenyl derivatives of PAHs and their cyclization, as well as aromatization of diterpenoids and incorporation of oxygen into aromatic structures. Weathering should always be considered in studies of fossilized terrestrial OM, especially in the case of thermal maturation modeling, because it significantly decreases the OM content and total sulfur content, changes vitrinite reflectance values and alters the extract composition as a result of organic compound degradation.