4
Organic Matter: The Driving Force for Early Diagenesis
130
PaP = [15.9C·S·ρ(1-φ)] 0.66 ·S B-C
-0.71 ·D 0.32 (4.3) 1
where S B-C is the organic-carbon-free linear bulk
sedimentation rate. Given the complexity of the
sedimentation and burial processes of organic matter,
the wide range of chemical and physical properties of
organic matter from different organisms and the effects
of organic matter alteration during diagenesis after
incorporation in the sediment, the equations can only
be considered rough estimates. It can be expected that
the investigation of more oceanic sediment profiles in
the future will result in further modification of the
equations (cf. also Sarnthein et al. 1992; Stein and
Macdonald 2005 and references therein). In particular,
the extent of mixing of autochthonous marine organic
matter with allochthonous organic matter from the
continents (see 4.2.5) is difficult to estimate. Stein
(1986a) used data from Rock-Eval pyrolysis, calibrated
by organic petrographic data from microscopic analysis, to derive the marine organic matter fraction of a
sediment from bulk pyrolysis measurements, but the
correlation displays substantial scatter and can only
be regarded a crude approximation.
Müller and Suess (1979) applied their paleoproductivity relationship (Eq. 4.1) to sediments from
the deep ocean off Northwest Africa. They found
that the Pleistocene interglacial periods had about
the same productivity as that measured in the
present-day ocean. It was three times higher during
glacial periods, probably due to a higher nutrient
supply by more intense mixing of water masses or
stronger coastal upwelling. The more sophisticated
Equation 4.3 of Sarnthein et al. (1987, 1988) yielded
essentially the same results for the last 500,000 years
as those from Equation 4.1. Typical Pleistocene
productivities in the upwelling area off Northwest
Africa ranged between 150 and 300 g C org m
-2
yr
-1
,
whereas the values were 20 to 50 g C org m
-2
yr
-1
in the
central Atlantic Ocean (Stein et al., 1989).
The paleoproductivity equations above describe
the relationship between surface-water productivity
and organic carbon accumulation, specifically under
conditions of an oxic water column. A different relationship for anoxic depositional settings, suggested
by Bralower and Thierstein (1987), implies that at
least 2 % of the organic carbon in the gross photosynthetic production is preserved in the sediments:
PaP = 5C·S(ρ WB - 1.026φ/100)
(4.4)
Stein (1986a) used this equation to calculate
paleoproductivity in the Mesozoic Atlantic Ocean.
Interpretation was considered preliminary due to the
difficulty of obtaining precise age information, and thus
sedimentation rate data, for the older and more compacted Mesozoic sediments lean in microfossils. The
estimated productivity appeared to have been low off
Northwest Africa in the Jurassic, to have increased
during the Early Cretaceous and to have reached
maximum values similar to those today during AptianAlbian times (about 110 million years ago). Interestingly, low paleoproductivity was calculated for the
time of deposition of black shales at the CenomanianTuronian boundary (90 million years ago) indicating
that preservation may have played a more important
role for organic matter accumulation than productivity.
The empirical relationships for paleoproductivity
assessment illustrate how organic matter accumulation
is related to primary productivity through factors such
as organic carbon flux through the water column and
bulk sedimentation rate. In addition, there is evidence
that reduced oxygen concentrations in the water column
enhance organic matter preservation. Thus, organiccarbon-rich sediments and sedimentary rocks are likely
to be formed by the mutually enhancing effects of
oxygen depletion (static or dynamic; anoxia), and
productivity. In view of this, it appears to be too restrictive to assign a single controlling factor (Pederson and
Calvert 1990). For example, an anoxic water column in
the Holocene Black Sea is in itself apparently not
sufficient to lead to black shale formation, whereas the
enhanced primary productivity in equatorial upwelling
areas is not reflected in a high organic carbon content
of the underlying sediments due to oxidation of the
sinking organic matter in the deep oxic waters below
the oxygen-minimum zone.
4.2.3
Transport of Organic Matter Through
the Water Column
The extent of degradation of particulate organic
matter as it sinks through the water column is influenced by the residence time of organic matter particles
in the water column. A measure of vertical transport is
the sinking velocity (vs; m s
-1
), which for a spherical
particle follows Stokes’ law:
vs = [(ρ 2 - ρ 1 )·g·D 2 ]/18η
(4.5)
ρ 2 and ρ 1 are the densities (g cm
-3
) of the particle and
the water, respectively, g is the acceleration due to
1
Numerical values in this equation were rounded because the author of this chapter believes that the
number of decimals in the original publication suggests
more accuracy than is both justified by, and required
for, this empirical estimative approach.
Organic Matter: The Driving Force for Early Diagenesis
130
PaP = [15.9C·S·ρ(1-φ)] 0.66 ·S B-C
-0.71 ·D 0.32 (4.3) 1
where S B-C is the organic-carbon-free linear bulk
sedimentation rate. Given the complexity of the
sedimentation and burial processes of organic matter,
the wide range of chemical and physical properties of
organic matter from different organisms and the effects
of organic matter alteration during diagenesis after
incorporation in the sediment, the equations can only
be considered rough estimates. It can be expected that
the investigation of more oceanic sediment profiles in
the future will result in further modification of the
equations (cf. also Sarnthein et al. 1992; Stein and
Macdonald 2005 and references therein). In particular,
the extent of mixing of autochthonous marine organic
matter with allochthonous organic matter from the
continents (see 4.2.5) is difficult to estimate. Stein
(1986a) used data from Rock-Eval pyrolysis, calibrated
by organic petrographic data from microscopic analysis, to derive the marine organic matter fraction of a
sediment from bulk pyrolysis measurements, but the
correlation displays substantial scatter and can only
be regarded a crude approximation.
Müller and Suess (1979) applied their paleoproductivity relationship (Eq. 4.1) to sediments from
the deep ocean off Northwest Africa. They found
that the Pleistocene interglacial periods had about
the same productivity as that measured in the
present-day ocean. It was three times higher during
glacial periods, probably due to a higher nutrient
supply by more intense mixing of water masses or
stronger coastal upwelling. The more sophisticated
Equation 4.3 of Sarnthein et al. (1987, 1988) yielded
essentially the same results for the last 500,000 years
as those from Equation 4.1. Typical Pleistocene
productivities in the upwelling area off Northwest
Africa ranged between 150 and 300 g C org m
-2
yr
-1
,
whereas the values were 20 to 50 g C org m
-2
yr
-1
in the
central Atlantic Ocean (Stein et al., 1989).
The paleoproductivity equations above describe
the relationship between surface-water productivity
and organic carbon accumulation, specifically under
conditions of an oxic water column. A different relationship for anoxic depositional settings, suggested
by Bralower and Thierstein (1987), implies that at
least 2 % of the organic carbon in the gross photosynthetic production is preserved in the sediments:
PaP = 5C·S(ρ WB - 1.026φ/100)
(4.4)
Stein (1986a) used this equation to calculate
paleoproductivity in the Mesozoic Atlantic Ocean.
Interpretation was considered preliminary due to the
difficulty of obtaining precise age information, and thus
sedimentation rate data, for the older and more compacted Mesozoic sediments lean in microfossils. The
estimated productivity appeared to have been low off
Northwest Africa in the Jurassic, to have increased
during the Early Cretaceous and to have reached
maximum values similar to those today during AptianAlbian times (about 110 million years ago). Interestingly, low paleoproductivity was calculated for the
time of deposition of black shales at the CenomanianTuronian boundary (90 million years ago) indicating
that preservation may have played a more important
role for organic matter accumulation than productivity.
The empirical relationships for paleoproductivity
assessment illustrate how organic matter accumulation
is related to primary productivity through factors such
as organic carbon flux through the water column and
bulk sedimentation rate. In addition, there is evidence
that reduced oxygen concentrations in the water column
enhance organic matter preservation. Thus, organiccarbon-rich sediments and sedimentary rocks are likely
to be formed by the mutually enhancing effects of
oxygen depletion (static or dynamic; anoxia), and
productivity. In view of this, it appears to be too restrictive to assign a single controlling factor (Pederson and
Calvert 1990). For example, an anoxic water column in
the Holocene Black Sea is in itself apparently not
sufficient to lead to black shale formation, whereas the
enhanced primary productivity in equatorial upwelling
areas is not reflected in a high organic carbon content
of the underlying sediments due to oxidation of the
sinking organic matter in the deep oxic waters below
the oxygen-minimum zone.
4.2.3
Transport of Organic Matter Through
the Water Column
The extent of degradation of particulate organic
matter as it sinks through the water column is influenced by the residence time of organic matter particles
in the water column. A measure of vertical transport is
the sinking velocity (vs; m s
-1
), which for a spherical
particle follows Stokes’ law:
vs = [(ρ 2 - ρ 1 )·g·D 2 ]/18η
(4.5)
ρ 2 and ρ 1 are the densities (g cm
-3
) of the particle and
the water, respectively, g is the acceleration due to
1
Numerical values in this equation were rounded because the author of this chapter believes that the
number of decimals in the original publication suggests
more accuracy than is both justified by, and required
for, this empirical estimative approach.