Characterization of the geochemical alteration of permineralized fossil plants based on macromolecular structure and composition

The processes and products of organic maturation that occur during permineralization of fossils are incompletely understood. Primary among such processes is the thermal alteration of organic cell walls to produce the kerogen that comprises such fossils, here studied by comparison of Eocene and experimentally heated fern axes. Cellularly permineralized specimens of a fossil fern ( Dennstaedtiopsis aerenchymata) from cherts of the Clarno and Allenby Formations (of Oregon and British Columbia, respectively) were compared with those of a modern analogue (Dennstaedtia cicutaria) and to woody axes of the Clarno Formation. The composition and molecular structure of such samples were analyzed by ultraviolet resonance Raman spectroscopy (UV Raman), solid-state 13C nuclear magnetic resonance spectroscopy (NMR), and pyrolysis-gas chromatography-mass spectrometry (py-GC-MS). The fossil fern studied here is cellularly well-preserved in both geologic units. The kerogen comprising the Allenby specimens is geochemically less altered than that of the Clarno, exhibiting more prevalent oxygen-containing functional groups and present as a greater fraction of rock mass. Kerogens comprising the fossil ferns and woody axes of the Clarno differ somewhat in composition, as measured by NMR and py-GC-MS, evidently due to differences of preservation. However, data obtained by UV Raman show the kerogens of the two types of fossils to be nearly identical in molecular structure, composed of networks of aromatic rings and polyene chains and, unlike more mature kerogens, are not composed of large polycyclic aromatic hydrocarbons. Results of two sets of heating experiments (at 200° C and 250° C over periods of 2 to 1000 hours) of the modern fern axes show that temperature is of greater importance than the duration of heating in the alteration of organic matter (viz., the cellulose and lignin of which plant cells are composed). Degradation of lignin began at a lower temperature than that of cellulose, but because of its higher peak pyrolysis temperature, lignin was altered more slowly over a broader temperature range. Overall, these heating experiments appear to have mimicked natural thermal alteration rather well, producing a proto-kerogen similar in composition and molecular structure to that of the fossil ferns, namely a network of aromatic rings and polyene chains having diverse oxygen-containing functional groups.