Organic Preservation of Biopolymers in Fossil Leaves - Biopolymers: A Molecular Paleontology Approach - page 61

in the aliphatic component of the pyrolysate (Fig. 3.10a ) with comparative reduction
in the alkane/alkene homologues seen in normal pyrolysis. Although the C 16 and C 18
FAMEs typically are predominant, other FAMEs released during thermochemolysis
(Fig. 3.10b ) include C 14 , C 15 and C 18:1 , which are relatively abundant in all analysed
samples. Lesser abundances of C 10 and C 12 FAMEs, as well as higher-molecular-
weight FAMEs ranging in carbon number up to C 32, are present (Table 3.1 ).
Methylated derivatives of the lignin components and aromatics observed during
routine pyrolysis are also present in the TMAH-pyrolysates, as expected (Fig. 3.10a ).
Thus, TMAH assisted py-GC-MS, like py-GC-MS, suggests that the fossil leaves
consist of aliphatic and aromatic, and lignin derived components. No carbohydrate
moieties were detected, confi rming their absence.
Spectroscopic Studies
Further compositional information was obtained using 13 C solid state NMR and
global and micro-FTIR of the fossil Quercushispanica specimen 1, Castaneavesca
specimen 1 and Populusalba specimen 1 leaves.
Solid state 13 C NMR has rarely been used for studies of fossil plant cuticles
primarily because of the small sample generally obtained from pure fossil cuticles
combined with the impure nature of many fossil cuticles. This may account for
slightly low signal to noise ratio observed in fossil spectra (Lyons et al. 1992 ).
Notable previous studies making use of this technique in plant fossil cuticles include
van Bergen et al. ( 1994 ) on Upper Carboniferous pteridosperm cuticles and Lyons
et al. ( 1992 ) on Carboniferous seed fern cuticles. The NMR spectra of fossil leaves
obtained here are in agreement with the pyrolysis data, as also seen in the previous
investigations of cuticles. The spectra (Fig. 3.11 ) show the presence of a number of
relatively broad resonances in the range of approximately 0-150 ppm. In the spectra
the peak of maximum intensity is centred at ca 30 ppm, ascribed to methylene
moieties and the aliphatic component of the leaf. A similar shift was observed in the
analysis of Karinopteris cuticle (van Bergen et al. 1994 ). The shoulder at 15 ppm
can be ascribed to terminal methyl groups. In addition, the spectra show a relatively
narrow resonance at 56 ppm and are likely related to carbon atoms at ether linkages
for lignin (aryl alkyl linkages-OCH 3 ). The peaks from 100 to 150 ppm are due largely
to aromatic carbons with varying substituents and may account for the aromatic
compounds recognised in the pyrolysate, as noted by van Bergen et al. ( 1994 ).
The NMR spectra reveals the predominance of aliphatic moieties relative to aromatic
moieties. However, contrary to previous results based on solid state 13 C NMR
spectroscopy and py-GC-MS, which pointed to the highly aliphatic nature of organic
matter in forest soils, Poirier et al. ( 2000 ) used quantitative methods to show that
aliphatic moieties with long alkyl chains are only minor components. Thus, observations
via solid state 13 C NMR spectroscopy and pyrolysis experiments can lead to overes-
timation of aliphatic moieties in heterogeneous materials when applied without quan-
titative constraints derived from elemental analysis and/or quantitative pyrolysis.