13C NMR, micro-FTIR and fluorescence spectra, and pyrolysis-gas chromatograms of coalified foliage of late Carboniferous medullosan seed ferns, Nova Scotia, Canada: Implications for coalification and chemotaxonomy

Abstract

The cuticles and cuticle-free compressions of three Carboniferous medullosan seed-fern leaf species ( Macroneuropteris scheuchzeri, Neuropteris ovata var. simonii and Alethopteris lesquereuxii) were analyzed by elemental, 13C nuclear magnetic resonance (NMR), micro-FTIR (Fourier transform infrared) and coal petrographic techniques. The 13C NMR spectra of the cuticle-free compressions and the associated whole coal (high volatile A/B bituminous coal rank) are generally similar and consist of a large aromatic carbon peak, a smaller aliphatic carbon peak and a shoulder on the aromatic peak, representing phenolic carbons. In contrast, the 13C NMR spectra of the cuticles from the same leaves have a predominant peak for aliphatic carbons and a much smaller aromatic carbon peak. This difference in aromaticity between the cuticles and the cuticle-free compressions is also reflected in the higher atomic H C ratios of the cuticles. Micro-FTIR spectra of the cuticles show oxygenated functional groups (carboxyl and ketone) similar to those in modern cuticles but their most characteristic feature is very strong bands in the aliphatic stretching region. The cuticle-free compressions (mainly vitrinite), in turn, show the absence or significant reduction in oxygenated functional groups, reduction in aliphatic stretching bands and, usually, increased absorbance of aromatic out-of-plane deformation in the 700–900 cm −1 region. Fluorescence spectra for the cuticles from all three species show a great similarity with a λ max at 580–590 nm, probably reflecting a similardegree of coalification, which is consistent with the similar vitrinite reflectance ( R r) and H C and O C ratios of the cuticlefree compressions. These results indicate that leaf cuticle-free compressions, which were initially cellulose rich (∼ 90% cellulose and hemicellulose, < 10% lignin), can alter, during peatification and coalification, to a macromolecular structure similar to that of coalified wood (initially ∼50% cellulose and hemicellulose, ∼ 30%–50% lignin). Thus, a lignin-enriched structure is not a prerequisite for the formation of the macromolecular structure of vitrinite. In addition, the micro-FTIR spectra reveal the complexity of the molecular structure in coalified seed-fern leaves. The micro-FFIR data reveal some significant differences among the cuticles that may be of chemotaxonomic value. Clearly, a combination of macro- and micro-techniques offers a better basis for the interpretation of the molecular structure of pre-macerals and their alteration during peatification and coalification. Also, the data presented in this paper provide important new information that extends the data from morphological and cuticular taxonomic studies of some seed ferns. The data are encouraging preliminary advances in the chemotaxonomy of medullosan seed fern species. Pyrolysis-gas chromatography (PY-GC) data for the cuticles of three seed-fern leaves indicate distinct chemical signatures for the two neuropterid leaves as compared to the Alethopteris leaf. This perhaps indicates a chemotaxomic factor, or it could be related to the greater thickness of the cuticle of Alethopteris. Mass spectrometric data are needed to identify individual components in the PY-GC chromatograms.