Organic Preservation of Biopolymers in Fossil Leaves - Biopolymers: A Molecular Paleontology Approach - page 43

a non-hydrolysable, organically insoluble macropolymer (Larter and Horsefi eld
1993 ) while Type III kerogens are more carbon rich. However, the source of the
aliphatic character of kerogens, as well as other sedimentary organic matter such as
organic macrofossils, remains a subject of debate.
Kerogen formation has been traditionally attributed either to selective preservation
of resistant biomolecules or random polymerisation of labile biomolecules, i.e.
neogenesis. According to the neogenesis hypothesis (Tissot and Welte 1984 ), kero-
gen is formed by random intermolecular polymerisation and polycondensation of
degraded biopolymers (e.g. amino acids, sugars, lipids, lignin, etc). However, it is
unclear how such mechanisms could result in the predominantly aliphatic character
of many kerogens (e.g. Type I and II) and associated bitumens. The selective preservation
model proposes that kerogen is derived predominantly from resistant biopolymers
in living organisms that survive decay more readily than other biomolecules (Tegelaar
et al. 1989a , b ; see de Leeuw and Largeau 1993 for review; Love et al. 1998 ). As
highly aliphatic resistant biopolymers are shown to be present in living counterparts
of algae (algaenan) (Gillaizeau et al. 1996 ; Gelin et al. 1999 ; Blokker et al. 2000 ),
in plant cuticles (cutan) (Tegelaar et al. 1991 ; van Bergen et al. 1994 ), and in suber-
inised plant tissue (suberan) (Tegelaar et al. 1995 ), and can be tracked in their fossil
counterparts, the aliphatic composition of kerogen was attributed to the selective
preservation of these polymers. In addition, natural vulcanisation, which involves
the reaction between reduced sulfur and various functional groups in organic com-
pounds, resulting in the formation of a S-rich macromolecule, is also recognised as
an important pathway (Kok et al. 2000 ). The widespread importance of selective
preservation is questionable because such resistant biopolymers are not components
of many of the living organisms that are present in the fossil record (Mösle et al.
1997 , 1998 ; Collinson et al. 1998 ; Stankiewicz et al. 1998a , 2000 ). Such concerns
with proposed models for the formation of aliphatic kerogen have prompted
dubious, albeit apparently more thermodynamically robust, alternative mechanisms
for petroleum n -alkane formation, such as the application of mantle-level pressures
(Kenney et al. 2002 ).
An alternative process was invoked recently for fossil organic matter preservation
(Briggs 1999 ; Collinson et al. 1998 ; Stankiewicz et al. 1998a , 2000 ). It posits
that the aliphatic component of sedimentary organic matter may be formed by the
condensation and polymerisation of labile lipids comprised of n -alkyl moieties
already present in the living counterpart. As this process was recognized within
individual organically preserved insect fossils it has been termed in situ polymerisa-
tion (Briggs et al. 1998 ; Briggs 1999 ; Stankiewicz et al. 2000 ) or 'within cuticle
diagenetic stabilisation' in the case of plants (Collinson et al. 1998 , 2000 ). This
process has been invoked to explain the aliphatic character of fossil arthropods
(Stankiewicz et al. 2000 ) and fossil leaves (Mösle et al. 1997 , 1998 ; Collinson et al.
1998 , by the stabilisation of aliphatic chains, including contributions from entrained
and surface waxes). However, previous characterisations of macrofossils were
incomplete. Stankiewicz et al. ( 1997b , 1998a ) did not identify explicitly a source of
the aliphatic moieties in arthropods and Mösle et al. ( 1997 , 1998 ) performed only
pyrolysis and in some cases FTIR.