151
matter has the same effect on some bulk parameters as
an admixture of terrigenous organic matter, because
the latter is commonly enriched in oxygen through
biosynthesis. It is, therefore, advisable to rely on more
than one parameter, and to obtain complementary
information.
C/N Ratio
Carbon/nitrogen (C/N) ratios of phytoplankton and
zooplankton are around 6, freshly deposited marine
organic matter ranges around 10, whereas terrigenous
organic matter has C/N ratios of 20 and above (e.g.
Meyers 1994, 1997 and references therein). This difference can be ascribed to the absence of cellulose in
algae and its abundance in vascular plants and to the
fact that algae are instead rich in proteins. Both weight
and atomic ratios are used by various authors, but due
to the small difference in atomic mass of carbon and
nitrogen, absolute numbers of ratios do not deviate
greatly.
Selective degradation of organic matter components
during early diagenesis has the tendency to modify
(usually increase) C/N values already in the water column.
Still, these ratios are sometimes sufficiently well preserved in shallow-marine sediments to allow a rough
assessment of terrigenous organic matter contribution
(e.g. Jasper and Gagosian 1990; Prahl et al. 1994). A
different trend exists in deep oceanic sediments with
low organic carbon contents. Inorganic nitrogen
(ammonia) is released during organic matter decomposition and adsorbed to the mineral matrix (particularly clays)
where it adds significantly to the total nitrogen. The C/
N ratio is then changed to values below those of normal
marine/terrigenous organic matter proportions (Müller
1977; Meyers 1994). This effect should be small in
sediments containing more than 0.3 % organic carbon.
On the other hand, many sapropels from the eastern
Mediterranean Sea and organic-matter-rich sediments
underlying upwelling areas have conspicuously high
C/N ratios (>15), i.e. well in the range of land plants
despite a dominance of marine organic matter, for reasons
yet to be determined (see Bouloubassi et al. 1999 for an
overview). Because such “atypical” C/N ratios were
determined in quite a number of sediments more recently
it is advised not to place too much emphasis on the
significance of this bulk parameter.
Hydrogen and Oxygen Indices
Hydrogen Index (HI) values from Rock-Eval pyrolysis
(see Sect. 4.5.2) below about 150 mg HC/g TOC are
typical of terrigenous organic matter, whereas HI values
of 300 to 800 mg HC/g TOC are typical of marine organic
matter. Deep-sea sediments rich in organic matter
usually show values of only 200-400 mg HC/g TOC,
even if marine organic matter strongly dominates.
Oxidation has lowered the hydrogen content of the
organic matter in this case. It should also be mentioned
that Rock-Eval pyrolysis was developed as a screening
method for rapidly determining the hydrocarbon
generation potential of petroleum source rocks
(Espitalié et al. 1985) and that a range of complications
may occur with sediments buried only to shallow depth.
For example, unstable carbonates may decompose
below the shut-off temperature of 390 °C (cf. Sect. 4.5.2)
which increases the Oxygen Index and falsely indicates
a high oxygen content of the organic matter.
Furthermore, Rock-Eval pyrolysis cannot be used for
sediments with TOC < 0.3 % because of the so-called
mineral matrix effect. If sediments with low organic
carbon contents are pyrolyzed, a significant amount of
the products may be adsorbed to the sediment minerals
and are not recorded by the flame ionization detector,
thus lowering the Hydrogen Index (Espitalié et al. 1977).
Maceral Composition
If the morphological structure of organic matter is well
preserved in sediments, organic petrographic
investigation under the microscope is probably the
most informative method to distinguish marine and
terrestrial organic matter contributions to marine
sediments by the relative amounts of macerals
(organoclasts) derived from marine biomass and land
plants (see Sect. 4.5.3). Many marine sediments,
however, contain an abundance of non-structured
organic matter (e.g. in upwelling areas; Lückge et al.
1996) which cannot easily be assigned to one source
or the other. Furthermore, comprehensive microscopic
studies are time-consuming. In his paleoproductivity
assessments (see Sect. 4.2), Stein (1986a) calibrated
Hydrogen Indices from Rock-Eval pyrolysis of marine
sediments with microscopic data and suggested to use
the more readily available pyrolysis data as a proxy for
marine/terrigenous organic matter proportions.
Stable Carbon and Hydrogen Isotope Ratios
Carbon isotope ratios are principally useful to
distinguish between marine and terrestrial organic
matter sources in sediments and to identify organic
matter from different types of land plants. The stable
carbon isotopic composition of organic matter reflects
the isotopic composition of the carbon source as well
as the discrimination (fractionation) between
12
C and
4.4
Organic Geochemical Proxies
matter has the same effect on some bulk parameters as
an admixture of terrigenous organic matter, because
the latter is commonly enriched in oxygen through
biosynthesis. It is, therefore, advisable to rely on more
than one parameter, and to obtain complementary
information.
C/N Ratio
Carbon/nitrogen (C/N) ratios of phytoplankton and
zooplankton are around 6, freshly deposited marine
organic matter ranges around 10, whereas terrigenous
organic matter has C/N ratios of 20 and above (e.g.
Meyers 1994, 1997 and references therein). This difference can be ascribed to the absence of cellulose in
algae and its abundance in vascular plants and to the
fact that algae are instead rich in proteins. Both weight
and atomic ratios are used by various authors, but due
to the small difference in atomic mass of carbon and
nitrogen, absolute numbers of ratios do not deviate
greatly.
Selective degradation of organic matter components
during early diagenesis has the tendency to modify
(usually increase) C/N values already in the water column.
Still, these ratios are sometimes sufficiently well preserved in shallow-marine sediments to allow a rough
assessment of terrigenous organic matter contribution
(e.g. Jasper and Gagosian 1990; Prahl et al. 1994). A
different trend exists in deep oceanic sediments with
low organic carbon contents. Inorganic nitrogen
(ammonia) is released during organic matter decomposition and adsorbed to the mineral matrix (particularly clays)
where it adds significantly to the total nitrogen. The C/
N ratio is then changed to values below those of normal
marine/terrigenous organic matter proportions (Müller
1977; Meyers 1994). This effect should be small in
sediments containing more than 0.3 % organic carbon.
On the other hand, many sapropels from the eastern
Mediterranean Sea and organic-matter-rich sediments
underlying upwelling areas have conspicuously high
C/N ratios (>15), i.e. well in the range of land plants
despite a dominance of marine organic matter, for reasons
yet to be determined (see Bouloubassi et al. 1999 for an
overview). Because such “atypical” C/N ratios were
determined in quite a number of sediments more recently
it is advised not to place too much emphasis on the
significance of this bulk parameter.
Hydrogen and Oxygen Indices
Hydrogen Index (HI) values from Rock-Eval pyrolysis
(see Sect. 4.5.2) below about 150 mg HC/g TOC are
typical of terrigenous organic matter, whereas HI values
of 300 to 800 mg HC/g TOC are typical of marine organic
matter. Deep-sea sediments rich in organic matter
usually show values of only 200-400 mg HC/g TOC,
even if marine organic matter strongly dominates.
Oxidation has lowered the hydrogen content of the
organic matter in this case. It should also be mentioned
that Rock-Eval pyrolysis was developed as a screening
method for rapidly determining the hydrocarbon
generation potential of petroleum source rocks
(Espitalié et al. 1985) and that a range of complications
may occur with sediments buried only to shallow depth.
For example, unstable carbonates may decompose
below the shut-off temperature of 390 °C (cf. Sect. 4.5.2)
which increases the Oxygen Index and falsely indicates
a high oxygen content of the organic matter.
Furthermore, Rock-Eval pyrolysis cannot be used for
sediments with TOC < 0.3 % because of the so-called
mineral matrix effect. If sediments with low organic
carbon contents are pyrolyzed, a significant amount of
the products may be adsorbed to the sediment minerals
and are not recorded by the flame ionization detector,
thus lowering the Hydrogen Index (Espitalié et al. 1977).
Maceral Composition
If the morphological structure of organic matter is well
preserved in sediments, organic petrographic
investigation under the microscope is probably the
most informative method to distinguish marine and
terrestrial organic matter contributions to marine
sediments by the relative amounts of macerals
(organoclasts) derived from marine biomass and land
plants (see Sect. 4.5.3). Many marine sediments,
however, contain an abundance of non-structured
organic matter (e.g. in upwelling areas; Lückge et al.
1996) which cannot easily be assigned to one source
or the other. Furthermore, comprehensive microscopic
studies are time-consuming. In his paleoproductivity
assessments (see Sect. 4.2), Stein (1986a) calibrated
Hydrogen Indices from Rock-Eval pyrolysis of marine
sediments with microscopic data and suggested to use
the more readily available pyrolysis data as a proxy for
marine/terrigenous organic matter proportions.
Stable Carbon and Hydrogen Isotope Ratios
Carbon isotope ratios are principally useful to
distinguish between marine and terrestrial organic
matter sources in sediments and to identify organic
matter from different types of land plants. The stable
carbon isotopic composition of organic matter reflects
the isotopic composition of the carbon source as well
as the discrimination (fractionation) between
12
C and
4.4
Organic Geochemical Proxies