ABSTRACT

Sixty-two mono- and 4 bicyclic alkanes, and 11 diamondoid hydrocarbons were detected in the Cretaceous-Tertiary (K/T) boundary sediments at Kawaruppu, Hokkaido, Japan. The monocyclic alkanes included series of n-alkylcyclohexanes, methyl-n-alkylcyclohexanes and n-alkylcyclopentanes, and other alkylcyclohexanes. The bicyclic alkanes included bicyclo[3.3.1]nonane, trans-decalin, and methyldecalins. The diamondoid hydrocarbons included adamantane, and methyl- and dimethyladamantanes, and diamantane and methyldiamantanes. These mono- and bicyclic alkanes were detected at a level of 0.001–0.1 nmol g^-1 and the diamondoid hydrocarbons were at the same or one order magnitude less than those of the cyclic alkanes. Concentrations of all cyclic alkanes within the K/T boundary claystone were markedly small compared to those in the sediments above and below the claystone. The molecular distributions of n-alkylcyclohexanes, methyl-n-alkylcyclohexanes and n-alkylcyclopentanes were relatively smooth within the claystone. On the other hand, those of n-alkylcyclohexanes and methyl-n-alkylcyclohexanes showed predominances from C₁₅ to C₁₈ and those of n-alkylcyclopentanes showed even carbon number predominances from C₁₆ to C₂₂ in the sediments above and below the boundary claystone. These characteristic features in concentration and molecular distribution within the boundary claystone are likely related to the large biomass extinction event at the end of the Cretaceous. Compositions of compounds among trimethylcyclohexanes, methylethylcyclohexanes, decalins, methyladamantanes, and methyldiamantanes were roughly constant over sedimentary sequence. This indicated that diagenetic effect to these compounds had been nearly constant over the sedimentary sequence.