Organic Preservation of Biopolymers in Fossil Leaves - Biopolymers: A Molecular Paleontology Approach - page 68

In addition, 13 C NMR reveals the importance of alkoxy functional groups
(56 ppm) indicating the presence of lignin. Further, Micro-FTIR reveals the presence
of ether bonds (those associated with aromatic, saturated and vinyl linkages) in peaks
between 1,020 and 1,280 cm −1 (Fig. 3.11c , Table 3.2 ). Thus, the aliphatic macromole-
cule is likely bonded by a combination of ester and ether linkages.
Possible Origin of the Aliphatic Macromolecule
in the Ardèche Fossil Leaves
An unexpected aspect of this study is the absence of cutan in modern representatives
of the fossils, precluding it as a source of the aliphatic macromolecule in the fossil
leaves. Thus, the aliphatic character of the fossil leaves cannot be explained by
selective preservation of cutan (Tegelaar et al. 1989a , b , 1991 ; de Leeuw and
Largeau 1993 ). The selective preservation process emphasizes the importance of a
non hydrolysable polymer to survive diagenesis and eventually enter the fossil
record. In the absence of cutan, alternative biogeochemical components must have
served as the source of the aliphatic component.
Allochthonous organic matter is also an unlikely source given its paucity in the
matrix; although bacteria could have been more abundant in microenvironments
within and near the leaves than in the organic-lean diatomite, the lack of diagnostic
bacterial compounds (e.g. hopanoids) in any of the analyzed fractions suggests that
such organisms did not contribute to the preserved organic matter. Even if some
surface material is derived from an external source this would have contributed a
minute proportion of the total fossil leaf organic matter (for detailed discussion on
the effect on matrix in fossils, see Chap. 4 for effect of sediment on organic con-
tamination in fossils). All leaves show well-preserved gross morphology and vena-
tion pattern, one shows internal cellular structure (albeit slightly modifi ed) and the
others with open texture have comparable preserved thicknesses. These facts make
it unreasonable to argue that the organic matter constituting the leaves was derived
from anywhere except the leaves themselves.
Cutin monomers are normally hydrolysable under basic conditions (as observed
here in recent leaves); however Schmidt and Schonherr ( 1982 ) indicated that inter-
molecular reactions between epoxy and alcohol groups in the cutin monomers may
lead to the formation of ether linkages rendering them resistant to acid and base
catalysed reactions and their eventual diagenetic survival (Tegelaar et al. 1991 ).
This would lead normally to hydrocarbon fragments with chain lengths less than the
original (typically C 16 and C 18 ) upon pyrolysis (Hartgers et al. 1995 ). Hydrocarbon
fragments greater than C 30 may be produced as a result of di/polymerisation of cutin
constituents by carbon-carbon bonds with contribution from extra cuticular waxes
(Mösle et al. 1998 ).
The living relatives of the Ardèche leaves contain cutin, phospholipid fatty acids,
di and triglycerides, and free fatty acids, all dominated by the C 16 and C 18 fatty acid
homologues. Cutin is more stable diagenetically than phospholipids and fatty