TURNING FERNS INTO FOSSILS: BIOGEOCHEMICAL ALCHEMY

The main thrust of paleontology has always been the study of morphologically intact fossils. Additionally, the study of fossil organic matter has long been a prime interest of those in the energy industry and, more recently, of paleobiologists concerned with what organic biomarkers can tell us about the history of life. The research here described adds to the growing number of studies that combine such morphological and chemical approaches by using microchemical techniques to study, in situ, the organic chemistry of individual micro- and macro-fossils to infer the processes of fossilization. Understanding these processes is vital to the interpretation of fossils themselves, especially of geochemical fossils. It is for this reason that new techniques and approaches to organic chemical characterization of morphological fossils and geochemical biosignatures must be developed.

Laser-Raman spectroscopy, ¹³C nuclear magnetic resonance spectroscopy, and pyrolysis-gas chromatography/mass spectrometry are three such techniques. This study uses these three techniques, along with other organic geochemical methods, to analyze the structural chemistry of an Eocene-aged, three dimensionally permineralized fossil fern (Dennstaedtiopsis aerenchymata) and its modern analogue (Dennstaedtia cicutaria), as well as representative individual structural biochemicals of fern cell walls. Specimens of the modern fern and aliquots of the biochemicals were sequentially thermally altered to simulate the degradation that occurs during fossilization, and the chemical changes thus induced were tracked over the sequence to a stage of geochemical maturity comparable to that exhibited by the fossil fern.

The analyses show that there is a wealth of chemical information preserved in the fossil organic matter that is of sufficient quality to be informative about its original composition. Also, the heating experiments show that the organic matter of the modern fern can be converted into carbonaceous matter that closely resembles that of the fossil, providing new insight into the processes and products of geochemical maturation. Moreover, this study demonstrates that laser-Raman spectroscopy is a useful technique for non-destructive characterization of the structural chemical composition of modern and fossil organic matter.