Flash pyrolysis-gas chromatography/mass spectrometry (py-GC/MS) of the OM concentrate indicates a predominance of polyaromatic structures, often without alkylation. These chemical features are indicative of severe thermal alteration due to partial combustion and therefore consistent with the petrographic recognition of fossil charcoal. Among hydrocarbons (organic compounds containing only H and C), aromatics comprise 59% of the pyrolyzate, while aliphatics (long chain structures typically derived from fossil algae, bacteria and higher plant waxes) only account for 5%. Among heterocompounds (organic compounds which also contain O, N or S), which collectively account for 35% of the pyrolyzate, virtually all are aromatic.
Oxygenated compounds are abundant, comprising 24% of the pyrolyzate. Of these, about a third are phenolic. Phenols are characteristic pyrolysis products of vitrinite and are molecular markers for the structure inherited from the precursor lignin [5]. Thus, the phenols could be assumed to derive from the minor vitrinite in the Mimbral sample. However thermally-altered vitrinites (such as the Mimbral, which has a mean reflectance of 1.3%) have been shown to produce only minor amounts of phenols [6]. If, as suspected, the elevated reflectance of the Mimbral vitrinite is due to the brief, severe thermal pulse of the impact [4], chemical characteristics normally obliterated by slow, geothermal heating may still be present. The other major oxygenated compounds are the three-ring structures dibenzofuran and fluorene-9-one, together accounting for >7% of the pyrolyzate. The occurrence of both these compounds as major components of sedimentary OM, even one dominated by inertinite, is highly unusual. These may results from oxidation of highly aromatic material, such as inertinite, prior to deposition or during weathering at the outcrop. Alternatively, since the lignin of the original plant material was oxygen-rich, the oxygen in these compounds may be inherited directly from biomass.
Compounds containing both nitrogen and oxygen are surprisingly abundant, comprising >4% of the pyrolyzate. One compound, identified as an indole-dione, is similar to compounds produced during the flash pyrolysis of proteins [7] and thus could again represent direct inheritance from biomass. Another compound in this class, acridinone, has a larger, triaromatic structure, but is still likely derived from charred biomass. Dibenzothiophenes (triaromatic organosulfur compounds) are also abundant, accounting for 5% of the pyrolyzate. These likely result from the diagenetic incorporation of sulfur from sulfate in pore waters after deposition, rather than directly from biomass.
The unusually good preservation of botanical evidence, as observed by both petrographic and chemical methods, is consistent with rapid charring and deposition inferred from the scenario of impact-firestorm-megawave at the end of the Cretaceous Period.
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