Geology Reference
In-Depth Information
Chemistry of Fossil Leaves
Fossil leaves were extracted and the residues (residue 4, Fig.
3.1
) analysed by pyrolysis- and
TMAH-pyrolysis-GC-MS. Pyrolysis of fossil leaf residues released a homologous
series of
n-
alkanes and
n-
alk-1-enes ranging in carbon number from C
8
to C
32
(Fig.
3.8
, Table
3.1
). Typically, hydrocarbons ranging in carbon number from C
26
to
C
31
are dominant (Figs.
3.8c
,
3.9a, b
) often maximising at
n-
C
29
(Figs.
3.8a, b,
,
3.9a, b c
,
Table
3.1
). In addition, the pyrolysates of
A. pseudocampestre
,
Q. hispanica
(Fig.
3.12
),
Q. palaeocerris
,
Q. suber
,
V. teutonica
,
C. vesca
(Fig.
3.8a
) and
Pinus
(Fig.
3.8b
) all contain C
16
and, at lower abundances, C
18
n
-alkanoic acids.
C. vesca
and
Pinus
reveal the presence of C
14
n
-alkanoic acid as well. Other major components
released by pyrolysis of fossil leaves are the isoprenoids, prist-1-ene (from all the
fossil leaves) and prist-2-ene (from all but
Pinus
and
Vitis)
. These isoprenoids most
likely are related to the pyrolysis of tocopherol (Goosens et al.
1984
; Logan et al.
1993
; Hold et al.
2001
). In addition to aliphatic components, pyrolysis also released
lignin components from all fossil leaves except
Populusalba
1fossils (Fig.
3.8c
,
Table
3.1
). The internal organisation of
Populusalba
1 is also unique in having an
open layered structure least resembling modern leaf tissues. Specifi c summed mass
chromatograms were used to evaluate the presence of guaiacyl
(m/z
124 + 138 + 150
+ 152 + 164 + 166) and syringyl units (
m/z
154 + 168 + 180 + 182 + 194 + 196). Guaiacyl-
related units include 2-methoxyphenol (guaiacol), 4-methyl-2-methoxyphenol,
4-ethenyl-2-methoxyphenol, 3-allyl-6-methoxyphenol (Figs.
3.8a, b
,
3.9a, b
). Syringyl-
related units, 2,6-dimethoxyphenol and 4-vinyl-2,6-dimethoxyphenol, were detected
in
Quercushispanica
(Fig.
3.9a, b
)
, Q. suber, Q.
sp.,
Vitis
and
Pinus
(Fig.
3.8b
)
.
Phenol and its alkyl derivatives were observed in all but
Populus
specimen 1, very
likely a product of biogegraded lignin (van Bergen et al.
1995
; Almendros et al.
1999
). Benzene and its alkyl derivatives (revealed by
m/z
78 + 91 + 92 + 105 + 106 +
119+120+133+134 mass chromatogram) include toluene, ethyl benzene,
m
,
p
,
and
o
xylene, trimethyl benzenes and 1,2,3,4-tetramethyl benzene (see Hartgers
et al.
1992
for detailed discussion). Napthalene (
m/z
155 + 170) was detected in all
but
Populus
specimen 1. In general, the distribution of lignin components is similar
in all analysed fossil leaves; however, the
Pinus
needle (gymnosperm) pyrolysate
differs from those of all other leaves in that released aromatic, lignin-derived
components are more abundant than aliphatic components (Fig.
3.8b
). Additionally,
Pinus
also showed the
n
-alkane/
n
-alk-1-ene homologue maximising at C
31
(Fig.
3.8b
)
unlike any of the other fossil leaves.
Pinus
leaves have a lower surface area to volume
ratio than dictotyledon leaves and contain a high proportion of thick-walled cells.
A higher lignin content would be expected based on living representatives and is
not inconsistent with the internal organisation (mixed open and closed texture)
of the fossil.
The extracted samples of the fossil leaves were also subjected to TMAH-assisted
thermochemolysis (Fig.
3.10
, Table
3.1
) in order to evaluate further the aliphatic
macromolecular composition. This process involves simultaneous pyrolysis and
methylation, and assists structural analysis of polar functionalities such as esters,
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