COMPARISON OF FOSSIL FERN KEROGEN FROM TWO EOCENE CHERTS

The processes of organic maturation that occur during the permineralization of fossils and the detailed chemistry of the resulting products are incompletely understood. Primary among such processes is the geochemical alteration of organic matter to produce kerogen, such as that which comprises the cell walls of the fossils here studied: essentially unmetamorphosed Eocene-age plant axes (specimens of the fossil fern Dennstaedtiopsis aerenchymata cellularly permineralized in cherts of the Clarno Formation of Oregon and the Allenby Formation of British Columbia). The composition and molecular structure of the kerogens that comprise the cell walls of such axes were analyzed by ultraviolet Raman spectroscopy (UV-Raman), solid-state ¹³C-nuclear magnetic resonance spectroscopy (NMR), and pyrolysis–gas chromatography–mass spectrometry (py–GC–MS).

Cellularly well preserved fern axes from both geologic units were analyzed. Specimens from the two units exhibit similar overall molecular structure, being composed primarily of networks of aromatic rings and polyene chains that, unlike more mature kerogens, lack large polycyclic aromatic hydrocarbon (PAH) constituents. The kerogenous cell walls of the Allenby Formation specimens, however, are geochemically less altered than those of the Clarno chert, exhibiting more prevalent oxygen-containing and alkyl functional groups and comprising a greater fraction of rock mass.

This study represents the first demonstration of the effectiveness (and limitations) of the combined use of UV-Raman, NMR, and py–GC–MS to analyze the kerogenous cell walls of chert-permineralized vascular plants.