Geology Reference
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acids and might be expected to serve as the primary source of C 16 and C 18 moieties
and the aliphatic component in the fossil leaves. However, microscopic analyses
indicate that none of the fossils possess a recognizable cuticle. It could be argued
that small fragments of cuticle are preserved in pieces of leaf not subjected to
electron microscopy or that cuticle is present but has been modifi ed physically so
that it is no longer recognisable. Even in such situations, cuticle likely represents only
a small part of the total fossil leaf organic matter given the total leaf thicknesses and
the internal organic matter proportions observed in TEM sections of all leaves. The
wide variation seen in the preservation of internal leaf organisation shows that the
aliphatic macromolecule cannot be linked to preservation of any specifi c leaf tis-
sue or cell types and suggests that its precursors were probably widespread within
the leaves. Alternative sources of the C 16 and C 18 moieties of the aliphatic components
include membrane lipids may have been preserved by in situ polymerisation
with contribution from higher molecular weight plant waxes (Walton 1990 ), and
contributed to the formation of an aliphatic macromolecule. Cutin-derived acids
could also have been incorporated into the forming macromolecule. Thus, the
aliphatic component could be a product of the polymerisation of labile (extractable
and hydrolysable) precursors that condense into a recalcitrant non-hydrolysable aliphatic
macromolecule. A similar process was suggested by Stankiewicz et al. ( 1998a , b , 2000 )
in the case of arthropods, which likewise do not have a resistant aliphatic precursor.
Implications
The results of this investigation demonstrate that C 16 and C 18 carboxylic acid
together with other fatty acid components ranging from C 8 to C 32 are important
constituents in the aliphatic composition of the Ardèche fossil leaves. Cutan is not
the source of this aliphatic material; C 16 , C 18 and higher-molecular-weight fatty acyl
components could be derived from cutin, free fatty acids (e.g. leaf waxes) or mem-
brane PLFAs. The fossil macromolecule is immune to base hydrolysis, suggesting
that the fatty acyl components in the geopolymer could be sterically protected by
n -alkyl chains. In addition ether bonds are also present. None of the fossil leaves
analysed from the Ardèche locality preserves the cuticle; the fossils consist of a
substantial thickness of internal tissue, albeit strongly modifi ed. Thus, membrane
lipids together with free fatty acids could be the source of components in aliphatic
macromolecule. Lipid polymerisation has been observed to contribute signifi cantly
to kerogen formation, where the role of selective preservation was limited (Riboulleau
et al. 2001 ), and also in early stages of the formation of coorongite (Gatellier et al.
1993 ). In such cases the organic matter revealed an amorphous composition to some
extent comparable to the loss of recognisable leaf tissues in most fossil leaves
here. This reveals that in situ polymerisation of labile lipid components could be of
widespread importance. All previously reported pyrolysates of 'older' fossil leaves
and cuticles, irrespective of age, plant type or enclosing lithology, show the presence
of aliphatic components (Nip et al. 1986 ; Tegelaar et al. 1991 ; Collinson et al. 1998 ;
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