Geoscience Reference
In-Depth Information
the extent to which the biomolecules survive tapho-
nomic processes. Knowledge of present degradative
pathways also provides a key to the past, enabling
the sedimentary organic molecules to be linked to
their biological sources. Through this, the evolution
of past life and environment can be reconstructed.
Biological tissues are made of different types of
molecules which can be broadly classified accord-
ing to their chemical functions into carbohydrates
(simple sugars and polymers among which cellu-
lose), lipids, peptides (simple amino acids and
their polymers, the proteins) and lignin, the princi-
pal component of wood. Although different from
peptides, nucleic acids (which are the building
blocks of DNA) have a fate which is similar to
that of peptides during burial, and therefore can be
assimilated to peptides.
After organism death and during burial in sedi-
ments, the organic matter is bio- or chemically
degraded. This results in an important loss of the
organic material, but also to chemical modifications
of the biomolecules. After this stage, the original
material can be either totally unrecognizable
or recognizable to a certain degree. During this
transformation of biomolecules to geomolecules
(the diagenesis), the fate of the different classes of
compounds is very different: simple sugars and pep-
tides are generally rapidly mineralized while lipids,
complex sugars, sporopollenin and lignin are less
easily degraded and therefore have a higher
chance of being buried in sediments.
The sedimentary environment is clearly of prime
importance. Burial rates, primary productivity,
oxygen availability, water depth, organic matter
concentration and mineral composition all influence
organic matter preservation (Tyson 2001; Burdige
2007; Rothman & Forney 2007). In addition, the
initial chemistry of the organic matter is also impor-
tant (Middelburg 1989; Sinninghe Damst´ et al.
2002; Versteegh & Zonneveld 2002; Prahl et al.
2003).
From the study of the organic matter deriving
from a wide variety of sedimentary and diagenetic
environments, a series of preservation pathways
has been proposed (de Leeuw & Largeau 1993; de
Leeuw et al. 2006; de Leeuw 2007). The degra-
dation-recondensation pathway (Tissot & Welte
1984) is based on the formation of macromolecular
organic matter by random, post-mortem polymeriz-
ation reactions of degradation residues. Because the
organic matter involved in this pathway is generally
highly degraded, the deduction of the biological
affinities of the fossil organic matter preserved
along this pathway is somehow complicated.
In contrast to this, the selective preservation
pathway (Philp & Calvin 1976; Tegelaar et al.
1989b) assumes that the biomolecules preserve as
they have been synthesized. This pathway concerns
many lipids and a few specific biomacromolecules
including lignin. Selective preservation of macro-
molecules is generally associated with the preser-
vation of the morphology (Largeau et al. 1986)
although the opposite, excellent morphological
preservation, does not imply excellent chemical
preservation (see review of de Leeuw et al. 2006;
de Leeuw 2007; Gupta et al. 2007b). Biomolecules
preserved through this pathway are highly recogniz-
able, even after millions of years of burial (Derenne
et al. 1988).
The natural sulphurization (Sinninghe Damst´ &
de Leeuw 1990) and oxidative polymerization path-
ways (Harvey et al. 1983; Versteegh et al. 2004; de
Leeuw 2007; Gupta et al. 2007b) stress that free
sulphur species and oxidizing agents cause conden-
sation and crosslinking, respectively, of both lipids
and macromolecules. This reduces the bioavail-
ability of the material so that labile compounds
that otherwise would have been mineralized may
escape into the fossil record (Kok et al. 2000). Mol-
ecules preserved through this pathway can retain
most of their original specificity, even after long
periods of time (Koopmans et al. 1997; Versteegh
et al. 2007). Clearly, due to the much higher avail-
ability of oxygen in air and much longer oxygen
exposure times, the oxidative polymerization
pathway is particularly likely to happen on land.
Burial to important depth, or for long periods of
time will also lead to the thermal modification
of the organic matter (OM). This process, termed
cracking, is the base of petroleum and natural gas
formation. The particular organic matter becomes
increasingly aromatic and cyclic by selective
removal of the aliphatic components and by aroma-
tization and cyclization of the residue. The more
the compound is thermally degraded, the less will
its original structure will recognizable.
Although maturation of organic matter may play
an important role in relatively young sediments
(provided temperature is sufficiently high), this is
a clear issue on Palaeozoic and older material
(Roberts et al. 1995; Yule et al. 2000) where slow
transformation at mild temperature conditions
is compensated for by long periods of time. The
same is true for changes in the composition of
stable carbon and hydrogen in the organic matter.
This results in a
13
C depletion of the released
compounds and a
13
C enrichment of the kerogen
(Schimmelmann et al. 2001). For hydrogen,
changes are larger and depend on the compounds
considered. In particular, hydrogen on tertiary
carbons (e.g. in isoprenoids) is subject to exchange
with the surrounding water (Pedentchouk et al.
2006). Nevertheless, shifts are minor compared to
the natural variations in the distributions of these
stable isotopes in organic matter (Schimmelmann
et al. 2001; Pedentchouk et al. 2006).
