4
Organic Matter: The Driving Force for Early Diagenesis
140
Insoluble, Nonhydrolyzable Highly Aliphatic
Biopolymers
Insoluble, nonhydrolyzable aliphatic biopolymers were
discovered in algae and higher plant cell walls as well
as in their fossil remnants in sediments (see de Leeuw
and Largeau 1993; van Bergen et al. 2004 for overviews).
These substances are called algaenan, cutan or suberan
according to their origin or co-occurrence with cutin
and suberin in extant organisms. They consist of
aliphatic polyester chains cross-linked with ether
bridges (Blokker et al. 1998, 2000) which render them
very stable toward degradation. Pyrolysis and other
rigorous methods are needed to decompose these
highly aliphatic biopolymers. This explains why they
are preferentially preserved in sediments.
Monomeric lipids
Biologically produced compounds that are insoluble
in water but soluble in organic solvents such as
chloroform, ether or acetone are called lipids. In a wider
sense, these also include membrane components and
certain pigments. They are common in naturally occurring fats, waxes, resins and essential oils. The low water
solubility of the lipids derives from their hydrocarbonlike structures which are responsible for their higher
survival rates during sedimentation compared to other
biogenic compound classes like amino acids or sugars.
Various saturated and unsaturated fatty acids are
the lipid components bound to glycerol in the
triglyceride esters of fats (see Fig. 4.8 for examples of
chemical structures of lipid molecules). Cell membranes
consist to a large extent of fatty acid diglycerides with
the third hydroxyl group of glycerol bound to phosphate or another hydrophilic group. In waxes, fatty
acids are esterified with long-chain alcohols instead of
glycerol. Plant waxes contain unbranched, long-chain
saturated hydrocarbons (n-alkanes) with a
predominance of odd carbon numbers (e.g. C 27 , C 29 ,
C 31 ) in contrast to the acids and alcohols which show
an even-carbon-number predominance.
Isoprene (2-methylbuta-1,3-diene), a branched
diunsaturated C 5 hydrocarbon, is the building block of
a large family of open-chain and cyclic isoprenoids
and terpenoids (Fig. 4.8). Essential oils of higher plants
are enriched in monoterpenes (C 10 ) with two isoprene
units. Farnesol, an unsaturated C 15 alcohol, is an
example of a sesquiterpene with three isoprene units.
The acyclic diterpene phytol is probably the most
abundant isoprenoid on Earth. It occurs esterified to
chlorophyll a and some bacteriochlorophylls and is,
thus, widely distributed in the green pigments of aquatic
and subaerial plants. Sesterterpenes (C 25 ) are of
relatively minor importance except in some methanogenic bacteria (cf. Volkman and Maxwell 1986).
Cyclization of squalene (or its epoxide) is the biochemical pathway to the formation of a variety of
pentacyclic triterpenes (C 30 ) consisting of six isoprene
units. Triterpenoids of the oleanane, ursane, lupane
and other less common types are restricted to higher
plants, and in exceptional cases may dominate the
extractable organic constituents of deep-sea sediments
like in Baffin Bay (ten Haven et al. 1992). The geochemically most important and widespread triterpenes
are from the hopane series, like diploptene which
occurs in ferns, cyanobacteria and other eubacteria.
The predominant source of hopanoids are bacterial cell
membranes, however, which contain bacteriohopanetetrol (and closely related molecular species) as
rigidifiers. This C 35 compound has a sugar moiety
attached to the triterpane skeleton via a carbon-carbon
bond (Fig. 4.8). The widespread distribution of bacteria
on Earth through time makes the hopanoids ubiquitous
constituents of all organic-matter assemblages (Rohmer
et al. 1992).
Steroids are tetracyclic compounds that are also
biochemically derived from squalene epoxide
cyclization, but have lost, in most cases, up to three
methyl groups. Cholesterol (C 27 ) is the most important
sterol of animals and occurs in some plants as well.
Higher plants frequently contain C 29 sterols (e.g.
sitosterol) as the most abundant compound of this
group. Steroids together with terpenoids are typical
examples of biological markers (chemical fossils)
because they contain a high degree of structural
information that is retained in the carbon skeleton after
sedimentation (e.g. Poynter and Eglinton 1991; Peters
et al. 2005) and often provides a chemotaxonomic link
between the sedimentary organic matter and the
precursor organisms in the biosphere.
Carotenoids, red and yellow pigments of algae and
land plants, are the most important representatives of
the tetraterpenes (C 40 ). Due to their extended chain of
conjugated double bonds (e.g. β-carotene; Fig. 4.8)
they are labile in most depositional environments and
are found widespread but in low concentrations in
marine surface sediments. Aromatization probably is
one of the dominating diagenetic pathways in the
alteration of the original structure of carotenoids in the
sediment. Diagenetic intermolecular cross-linking by
sulfur bridges may preserve the carotenoid carbon
skeletons to a certain extent.
A second pigment type of geochemical significance
are the chlorophylls and their derivatives that during
Organic Matter: The Driving Force for Early Diagenesis
140
Insoluble, Nonhydrolyzable Highly Aliphatic
Biopolymers
Insoluble, nonhydrolyzable aliphatic biopolymers were
discovered in algae and higher plant cell walls as well
as in their fossil remnants in sediments (see de Leeuw
and Largeau 1993; van Bergen et al. 2004 for overviews).
These substances are called algaenan, cutan or suberan
according to their origin or co-occurrence with cutin
and suberin in extant organisms. They consist of
aliphatic polyester chains cross-linked with ether
bridges (Blokker et al. 1998, 2000) which render them
very stable toward degradation. Pyrolysis and other
rigorous methods are needed to decompose these
highly aliphatic biopolymers. This explains why they
are preferentially preserved in sediments.
Monomeric lipids
Biologically produced compounds that are insoluble
in water but soluble in organic solvents such as
chloroform, ether or acetone are called lipids. In a wider
sense, these also include membrane components and
certain pigments. They are common in naturally occurring fats, waxes, resins and essential oils. The low water
solubility of the lipids derives from their hydrocarbonlike structures which are responsible for their higher
survival rates during sedimentation compared to other
biogenic compound classes like amino acids or sugars.
Various saturated and unsaturated fatty acids are
the lipid components bound to glycerol in the
triglyceride esters of fats (see Fig. 4.8 for examples of
chemical structures of lipid molecules). Cell membranes
consist to a large extent of fatty acid diglycerides with
the third hydroxyl group of glycerol bound to phosphate or another hydrophilic group. In waxes, fatty
acids are esterified with long-chain alcohols instead of
glycerol. Plant waxes contain unbranched, long-chain
saturated hydrocarbons (n-alkanes) with a
predominance of odd carbon numbers (e.g. C 27 , C 29 ,
C 31 ) in contrast to the acids and alcohols which show
an even-carbon-number predominance.
Isoprene (2-methylbuta-1,3-diene), a branched
diunsaturated C 5 hydrocarbon, is the building block of
a large family of open-chain and cyclic isoprenoids
and terpenoids (Fig. 4.8). Essential oils of higher plants
are enriched in monoterpenes (C 10 ) with two isoprene
units. Farnesol, an unsaturated C 15 alcohol, is an
example of a sesquiterpene with three isoprene units.
The acyclic diterpene phytol is probably the most
abundant isoprenoid on Earth. It occurs esterified to
chlorophyll a and some bacteriochlorophylls and is,
thus, widely distributed in the green pigments of aquatic
and subaerial plants. Sesterterpenes (C 25 ) are of
relatively minor importance except in some methanogenic bacteria (cf. Volkman and Maxwell 1986).
Cyclization of squalene (or its epoxide) is the biochemical pathway to the formation of a variety of
pentacyclic triterpenes (C 30 ) consisting of six isoprene
units. Triterpenoids of the oleanane, ursane, lupane
and other less common types are restricted to higher
plants, and in exceptional cases may dominate the
extractable organic constituents of deep-sea sediments
like in Baffin Bay (ten Haven et al. 1992). The geochemically most important and widespread triterpenes
are from the hopane series, like diploptene which
occurs in ferns, cyanobacteria and other eubacteria.
The predominant source of hopanoids are bacterial cell
membranes, however, which contain bacteriohopanetetrol (and closely related molecular species) as
rigidifiers. This C 35 compound has a sugar moiety
attached to the triterpane skeleton via a carbon-carbon
bond (Fig. 4.8). The widespread distribution of bacteria
on Earth through time makes the hopanoids ubiquitous
constituents of all organic-matter assemblages (Rohmer
et al. 1992).
Steroids are tetracyclic compounds that are also
biochemically derived from squalene epoxide
cyclization, but have lost, in most cases, up to three
methyl groups. Cholesterol (C 27 ) is the most important
sterol of animals and occurs in some plants as well.
Higher plants frequently contain C 29 sterols (e.g.
sitosterol) as the most abundant compound of this
group. Steroids together with terpenoids are typical
examples of biological markers (chemical fossils)
because they contain a high degree of structural
information that is retained in the carbon skeleton after
sedimentation (e.g. Poynter and Eglinton 1991; Peters
et al. 2005) and often provides a chemotaxonomic link
between the sedimentary organic matter and the
precursor organisms in the biosphere.
Carotenoids, red and yellow pigments of algae and
land plants, are the most important representatives of
the tetraterpenes (C 40 ). Due to their extended chain of
conjugated double bonds (e.g. β-carotene; Fig. 4.8)
they are labile in most depositional environments and
are found widespread but in low concentrations in
marine surface sediments. Aromatization probably is
one of the dominating diagenetic pathways in the
alteration of the original structure of carotenoids in the
sediment. Diagenetic intermolecular cross-linking by
sulfur bridges may preserve the carotenoid carbon
skeletons to a certain extent.
A second pigment type of geochemical significance
are the chlorophylls and their derivatives that during