Geoscience Reference
In-Depth Information
waxes was already acquired in the Middle Devo-
nian. However, if we do not consider maturity
problems, there seems to be a temporal evolution
in the maximum of the n-alkane distribution which
could be related to plant evolution: around C
25
in
the Devonian to C
29
in the Carboniferous and
C
23
-C
25
during the Permian. Present-day higher
plants are characterized by a maximum in C
29
-C
31
.
Most coals older than the Devonian are
liptinite-rich coals, which may derive from spores
or from algae. Their n-alkane signature may there-
fore not be terrestrially derived. Due to the occur-
rence of odd-numbered n-alkanes in some algae,
the n-alkane distributions must be interpreted with
care. This is particularly true for samples older
than Devonian where terrestrial debris is scarce.
An odd-number n-alkane predominance has been
previously observed in several Botryococcus rich-
torbanite of Permian age and was mainly attributed
to a higher plant contribution (Araujo et al. 2003;
Dawson 2006). These odd-numbered n-alkanes
could also derive from the saturation of Botryococ-
cus lipids, however (for an overview on Botryococ-
cus lipids see Metzger et al. 2007).
Slight odd predominances in the range C
25
-C
31
were also observed in the extracts of a Cambrian
sediment from Tarim Basin (China) (Zhang et al.
2000) as well as in the extracts of several Protero-
zoic, Cambrian and Ordovician sediments from
different basins in East China (Li et al. 2002). In
the case of East China, this distribution was assigned
to a contribution from cyanobacteria of the Spirula
type (Li et al. 2002). Considering the stratigraphic
distribution of microscopic higher plant remains
(Fig. 1), a contribution from higher plant lipids is
somehow questionable in the Cambrian and Ordovi-
cian samples. Since land-plant derived n-alkanes are
(as far as C3 plants are considered) usually more
enriched in
12
C relative to
13
C than their algal
counterparts, comparison of the stable carbon isoto-
pic composition of these alkanes with the isotopic
composition of typical marine/aquatic biomarkers
may aid in the identification of a land-plant signal.
metabolism (CAM), where it may be an evolutionary
solution to severe drought stress (Boom et al. 2005).
Nuclear magnetic resonance (NMR) analysis
suggests that this aliphatic biopolymer also contains
aromatic units (Deshmukh et al. 2005). Cutin and
cutan should therefore be considered 'terrestrial
markers' of higher plants. The preservation potential
of cutin is, however, very low; the building blocks are
cross-linked with relatively weak esters while stron-
ger ether-bridges connect the cutan building blocks,
which is more resistant.
The production of aliphatic biopolymers was not
invented by land plants but evolved much earlier.
The analysis of the cell walls of a wide range
of algae shows that some algae produce a resistant
aliphatic biopolymer made of cross-linked long-
carbon chains called algaenan (see review by
Versteegh & Blokker 2004; Metzger et al. 2007).
Algaenans resist harsh chemical treatment and
they have a relatively high preservation potential
which may account for the long fossil record of
the Chlorophyta (Batten 1996; Batten & Grenfell
1996; Colbath 1996; Guy-Ohlson 1996; van Geel
& Grenfell 1996; Wicander et al. 1996). Even
although the exact biological role of algaenan
is not known, it has been suggested that the highly
aliphatic (plastic-like) algaenan may function as a
relatively waterproof layer; it is interesting to note
that apart from the marine eustigmatophyte Nanno-
chloropsis, the development of algaenan mostly
occurs in freshwater algae, notably Chlorophyta
(e.g. Tetraedron, Scenedesmus, Pediastrum) which
probably have the largest risk of desiccation.
The similarities in structure and function
between algaenan and cutin and cutan of land
plants (all aliphatic biopolymers, which are both
based on ether- or ester-linked long-chain fatty
acids; van Bergen et al. 2004) may imply that they
represent subsequent stages in the evolution of the
same biosynthetic pathway. This could constitute
an argument in favour of the freshwater algal origin
of terrestrial plants. In sediments, cutan is difficult
to discriminate from algeanan or from a highly ali-
phatic cutan-like geopolymer. The latter can be
formed from common lipids by oxidative polymer-
ization during early diagenesis discussed below
(Boom et al. 2005; Gupta et al. 2006a, 2007a;
de Leeuw 2007). This implies that fossil aliphatic
biopolymers do not provide evidence for a land-
plant (cuticular) origin of the organic matter per
se. In this case, stable carbon isotopic analysis of
the aliphatic constituents may also be needed to
obtain a conclusive answer.
Macromolecules. In addition to simple lipids, pro-
tection against desiccation could also be offered
by resistant biomacromolecules made of long-chain
lipids (see also reviews by van Bergen et al. 2004;
de Leeuw et al. 2006), although the exact role
of these macromolecules is not entirely clear.
Mono to tetra-functionalized long-chain alcohols
and acids form the building blocks of cutin, a bio-
polymer present in the cuticles of most land plants
(Kolattukudy 1981; Deshmukh et al. 2003). Pyrol-
ysis shows that aliphatic lipids are also major
building blocks of cutan (Tegelaar et al. 1989a;
Boom et al. 2005) a macromolecule which occurs
in the cuticles of plants with crassulacean acid
UV protection, radiation damage
Terrestrialization
also
involved
an
increased
exposure
to
harmful
radiation. Ultraviolet-B
