Structure and distribution of branched aliphatic alkanes with quaternary carbon atoms in Cenomanian and Turonian black shales of Pasquia Hills (Saskatchewan, Canada)

Twelve pseudohomologous series of branched alkanes with one or two quaternary carbon atoms (BAQCs; C14-C33 range) are present in the extractable organic matter of Cenomanian and Turonian black shales of Pasquia Hills (Saskatchewan, Canada). Five of these pseudohomologous series are also observed in a Holocene sample from the Russian Arctic. Three of the pseudohomologous series were identified by Kenig et al. [PNAS 100 (2003) 12554-12558] as 3,3- and 5,5-diethylalkanes (DEAs) and 2,2-dimethylalkanes (DMAs). The nine other pseudohomologous series of BAQCs are tentatively identified on the basis of their characteristic mass spectral fragmentation and retention indices as 3,3-dimethylalkanes (DMAs), 3-ethyl-3-methyl- and 5-ethyl-5-methylalkanes, 9,9-, 7,7-, and 6,6-diethylalkanes (DEAs), 3,3,ω3,ω3- and 5,5,ω5,ω5-tetraethylalkanes, and 5-butyl-5-ethylalkanes. Pseudohomologous series of branched alkanes with tertiary carbon atoms, such as monomethyl- and monoethylalkanes are also present in the same carbon-number range. All these pseudohomologous families share the particularity of including isomers with exclusively odd or exclusively even carbon numbers, clearly indicating a biosynthetic origin. BAQCs such as 5,5-DEAs, 3,3-DEAs and 3-ethyl-3-methylalkanes seem to be widespread in sediments and sedimentary rocks (ranging from modern to ∼2.1 billion years). A xenobiotic source for BAQCs can be excluded at this point. BAQCs seem to have a very low biodegradability, in part explaining their widespread occurrence in ancient sediments.