Geology Reference
In-Depth Information
All the modern leaves that we investigated contain abundant n -alkanes, n -alkanols
and n -alkanoic acids and, after saponifi cation, yield characteristic cutin constituents
(Fig. 3.6b ). However, our combined hydrolysis and py-GC/MS approach revealed no
evidence of cutan in any of the modern representatives of our fossil leaves. To test
our methodology, we analysed Agave americana and Cliviaminata , both known to
contain cutan (Nip et al. 1986 ), using methods identical to those that we used for the
other leaves. Pyrolysis of lipid-extracted Agave and Clivia leaves revealed the presence
of n -alkane/ n -alk-1-ene homologues ranging from C 8 -C 34 (including those >C 20 )
similar to those reported by Nip et al. ( 1986 ), this signal persisted after saponifi cation.
In contrast, in the ten leaves described here, n -alkanes and n -alkenes were absent after
hydrolysis. This might be an effect of preferential cleavage of more labile bonds
associated with lignin and cellulose during pyrolysis, swamping the aliphatic signal
and inhibiting detection of the aliphatic cutan. Although this seems unlikely, the cutan
signal was still detected in Agave and in Clivia after similar treatment.
We also analysed the extractable fatty acid and fatty acyl component of the modern
leaves. These fractions are dominated by the C 16 fatty acid, with lower abundances
of the C 18 homologue. Thus, the aliphatic content of the living leaves comprises C 16
and C 18 alkyl components present as FAs, PLFAs, di- and tri-glycerides or cutin,
with a contribution from long chain free fatty acids up to C 32 . Critically, aliphatic
moieties occur as part of a macromolecule not immune to base attack. Although
surprising when compared to the results of Tegelaar et al. ( 1991 ), which suggested
that cutan is widespread in modern leaves, the absence of cutan in our results is
consistent with more recent analyses of the leaves of gymnosperms including
Ginkgo biloba and 15 genera from 6 families of conifers (Collinson et al. 1998 )
which involved a more comprehensive procedure.
Composition of the Ardèche Fossil Leaves
All ten of the Ardèche fossil leaves have a chemical composition similar to that of
the representative examples discussed in the results section and illustrated in the in
spite of the variation in their internal organisation and the general absence (with
one exception) of recognisable leaf tissues. Chemical analyses, including 13 C-
NMR, FTIR, py-GC-MS and TMAH-py-GC-MS, indicate an aliphatic composi-
tion for each of the leaves. Aromatic components in the form of benzene and lignin
derivatives are also present in apparently subordinate abundances. However, as
mentioned before, Poirier et al. ( 2000 ) have shown that solid state 13 C NMR can
overestimate the contribution of aliphatic moieties in complex refractory material.
Similarly, TMAH assisted pyrolysis leads to an increased yield of the aliphatic
component when analysing the lipid signature in fossil samples with condensed
polymethylene networks (Almendros et al. 1998 ). Thus, the aromatic component
of the Ardèche fossil leaves may be more signifi cant than our results suggest; none
the less it is clear that all analysed leaves are comprised of a macromolecule with
a strong aliphatic component.
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