Geology Reference
In-Depth Information
Although not previously found in ancient fossil leaves, protein moieties have been
detected in kerogen samples 140 million years old (Mongenot et al.
2001
), refl ect-
ing the potential for protein preservation. Encapsulation within a resistant sediment
or macromolecule (Knicker et al.
2001
; Mongenot et al.
2001
; Riboulleau et al.
2001
) could protect otherwise labile molecules (including proteins) enhancing their
diagenetic survival; thus, it is possible that rapid formation of a resistant aliphatic
geomacromolecule (Gupta et al.
2007
) from cutin or lipids could facilitate protein
preservation. Additionally, tissue type (Opsahl and Benner
1995
) and depositional
setting (Stankiewicz et al.
1997a
; van Bergen et al.
1997
,
1998
) are important con-
trols on molecular preservation.
Comparison Between Sediment and Fossil
The differences in the aliphatic component of fossil and associated sediment pyroly-
sates and TMAH-pyrolysates are highlighted in Table
4.1
. Py-GC-MS of the fossil
conifer generates
n-
alkane/
n
-alk-1-ene homologues ranging from C
9
to C
33
with a
bimodal distribution of
n
-alkanes (maxima at C
13
and C
29
) and a predominance of
relatively short-chain
n
-alkenes. In some respects, pyrolysates of the enclosing sedi-
ment are similar, with
n-
alkane/
n
-alk-1-ene homologues ranging from C
9
to C
30
and
a similar predominance of short-chain
n
-alkenes. However, the
n
-alkane distribution
differs considerably, with a broad distribution of abundant
n
-alkanes and a maxi-
mum at C
21-23
.
Thermochemolysis of the conifer generates abundant fatty acid methyl esters,
presumably derived from macromolecular fatty acyl moieties, with a broad carbon
number range and a clear predominance of even-carbon-number C
14-18
components
and lower abundances of
n
-alkane/
n-
alk-1-ene doublets. This distribution is similar
to fossil leaves from the Ardèche diatomite, where the sediment is relatively organic-
lean and much less likely to have been a source of fossil leaf organic matter (Gupta
et al.
2007
). The distribution differs from the TMAH pyrolysate of the associated
sediment, which contains relatively higher abundances of
n
-alkane/
n
-alk-1-ene
homologues and higher-molecular-weight FAME (>C
20
). This suggests that the fos-
sil conifer has a different aliphatic composition than the surrounding sediment, both
with respect to the distribution of
n
-alkyl chain lengths and the chemical bonds that
link the alkyl units together. In particular, it seems that ester linkages are relatively
more important in the leaf, refl ected by the high FAME to
n
-alkane ratios in the
TMAH pyrolysate. Thus, at least part of the fossil leaf aliphatic material, and par-
ticularly the fatty acyl component, cannot derive from the associated sediments.
Pyrolysis of the fossil weevil releases
n
-alkanes/
n
-alkenes ranging in carbon
number from C
9
to C
33
. The most abundant
n
-alkanes are the C
13,15,21
components
and the most abundant
n
-alkenes are the C
15,13, 22
components. This distribution is
similar to that of
n
-alkanes and
n
-alkenes generated by pyrolysis of the sediment;
n
-alkyl components range in carbon number from C
9
to C
30
, and the C
15, 21, 13
and C
16,
21, 22
homologues are the most abundant
n
-alkanes and
n
-alkenes, respectively.
Search WWH ::
Custom Search