The potential role of redox reactions for the distribution of alkyl naphthalenes and their oxygenated analogues in terrestrial organic matter of Late Palaeozoic age

The occurrence and distribution of alkyl naphthalenes and alkyl phenanthrenes in 39 Late Palaeozoic coal and sediment samples, containing predominantly terrestrial organic matter, is related to thermal maturity and biological source input. This study investigates the relationship between aromatic hydrocarbons and the corresponding aldehydes, ketones, carboxylic acids and dicarboxylic acids based on the naphthalene skeleton. It is suggested that alkyl naphthalenes are oxygenated in the sediments. The behaviour of alkyl phenanthrenes and their oxygenated analogues is discussed in this context. Alkyl naphthalenes and phenanthrenes were abundant in all samples. The low polarity nitrogen, sulfur and oxygen compound fraction was characterised by the presence of alkyl naphthaldehydes and naphthyl ketones for the majority of the samples. The presence of naphthalene carboxylic acids and anthracene/phenanthrene carboxylic acids was restricted to samples of relatively enhanced maturity (vitrinite reflectance, Rr, 0.67–1.26%), with one exception. The correlations between the different classes of compounds based on the naphthalene skeleton led to the hypothesis that, within the maturity range investigated, aldehydes and ketones were readily formed from the corresponding aromatic hydrocarbons, while the carboxylic acids were formed at elevated maturity. For alkyl naphthalene dicarboxylic acids, two formation pathways are suggested, because of two different distribution patterns. A direct correlation of these dicarboxylic acids with the occurrence of other compounds having a naphthalene skeleton was not apparent.