Molecular Transformation of Plant Biopolymers in High P-T Conditions - Biopolymers: A Molecular Paleontology Approach - page 102

This composition differs from that of unmatured cutin which yielded vinyl phenol
and C 16 (di) unsaturated and hydroxy fatty acids as the main moieties without any
alkane/alkene homologues (Tegelaar et al. 1989b ).
An alternative or additional source for the low molecular weight n -alkyl compo-
nents are labile lipids, including membrane lipids and di- and triacylglycerides. To
test this, we matured and analysed tomato mesocarp (cutin-free) tissue. This also
yielded a distinct aliphatic component comprising n -alkane/alkene homologues
ranging from n -C 8 to n -C 26 and n -alkanoic acids ranging from n -C 9 to n -C 16
(Fig. 5.4b ); the unmatured mesocarp consisted of polysaccrarides, phenolic com-
pounds and fatty acyl moieties.
We also matured and analysed pure lignin and cellulose and the cuticle of Agave ,
the last of which is comprised largely of cutan and provides a control (Table 5.1 ).
Matured lignin and cellulose yielded aromatic and phenolic moieties, but neither
fatty acids nor n -alkanes were detected. Pyrolysates of Agave matured without any
pre-treatment contained n -alkane/alk-1-ene homologues ranging from n -C 8 to n -C 35
(Fig. 5.5a ). Benzene derivatives, phenols and fatty acyl moieties were also observed
as in the other plant tissues. The n -alkane/alk-1ene homologues (including those
with carbon number > n -C 20 ) were also present in Agave that had been pre-extracted
(Fig. 5.5b ), or pre-extracted and saponifi ed (Fig. 5.5c ), prior to maturation, in con-
trast to all other pre-treated leaf tissues. Thus, cutan survives exposure to these ele-
vated temperatures and pressures.
The high pressure and temperature conditions used in the experiments have
been used before to understand the origin of the aliphatic component in kero-
gen using modern scorpion cuticle (Stankiewicz et al. 2000 ). It is critical to
acknowledge that these conditions do not simulate organic diagenesis and
represent only an indirect simulation (due to compressed timescales) of cata-
genesis. Consequently, while we could speculate that a range of reaction path-
ways, particularly free radical reactions, are likely important during our
experiment, it remains unclear how important such reactions are during natural
kerogen formation. Nonetheless, the experiment does provide a direct test of
the possible role of different plant molecular constituents in contributing to the
formation of an aliphatic polymer.
All previously reported pyrolysates of pre-Tertiary fossil leaves and cuticles
(Briggs et al. 2000 ), irrespective of age, plant type or enclosing lithology, contain
aliphatic components (Nip et al. 1986 ; Tegelaar et al. 1991 ; Mösle et al. 1998 ;
Collinson et al. 1998 ; Gupta et al. 2007 ). The experiments described here provide a
closed system that precludes the incorporation of compounds from an external
source. They indicate that, in the absence of cutan, other components, including
cutin, higher plant waxes and internal lipids, have served as the sources for high-
and low-molecular-weight aliphatic components of a generated macromolecule(s).
As such, the incorporation of lipids into macromolecules or even the formation of
macromolecules from lipids could play a major role in the fossilisation of plant
constituents and the formation of aliphatic moieties commonly observed in kerogen
(especially those of terrestrial origin) and sedimentary organic matter (Gillaizeau
et al. 1996 ) .