Evaluation of fossil geomacromolecular sources by the pyrolysis of density fractions of kerogens from Cretaceous and Miocene organic-rich marine shales

Abstract

Kerogen density fractions separated by the sequential density centrifugation of kerogens from Cretaceous Leenhardt and Miocene Onnagawa shales were analyzed using pyrolysis-GC/MS. The pyrolysates from both kerogens mainly comprised aliphatic compounds, such as n-alkane/n-alkene doublets and acyclic isoprenoid hydrocarbons, along with aromatic compounds including alkylbenzenes, alkylnaphthalenes, and alkylphenanthrenes. The sporomorph-concentrate obtained from the Onnagawa kerogen was mainly composed of sporopollenin geomacromolecules, consistent with the dominances of long-chain (C₂₂–C₂₅) n-alkane/n-alkene doublets. In the algae/sporomorph-abundant fraction from the Leenhardt kerogen, mid-chain-length (C₁₆–C₂₀) n-alkane/n-alkene dominances suggested a larger contribution of dinoflagellate-derived algaenan. Pyrolysis of the weakly fluorescent amorphous kerogen (WFA)-containing fractions of the Onnagawa kerogen generated C₁₃–C₂₀ acyclic isoprenoid hydrocarbons (except C₁₇), as well as alkylbenzenes including 1,2,3,5- and 1,2,3,4-tetramethylbenzenes. The isoprenoid hydrocarbons and the alkylbenzenes were components of fossil geomacromolecules likely derived from marine microalgae (diatoms and cyanobacteria). Non-fluorescent amorphous kerogen (NFA), the major kerogen of the Leenhardt shale, contained terrigenous suberin geomacromolecules, consistent with the dominance of terrigenous organic matter in the Leenhardt kerogen and the abundance of long-chain even carbon-numbered (C₂₂ and C₂₄) n-alkanes. The distributions of the pyrolytic isoprenoid alkanes (C₁₃, C₁₆, and C₁₈–C₁₉) from the Leenhardt kerogen suggested their distinctive fossil geomacromolecular sources, such as freshwater/coastal microalgae and cyanobacteria.