127
4.2
Organic Matter Accumulation in Sediments
rich rocks. A few examples are the Jurassic Posidonia
Shales or Kimmeridge Clays in northwestern Europe,
the Cretaceous black shales of the Atlantic Ocean and
other oceanic areas of the world, and the Pliocene to
Holocene sapropels of the Mediterranean Sea.
Stagnant oceanic bottom waters with a low
concentration or absence of oxygen (anoxia) for a long
time were considered the main prerequisite for the
accumulation of high amounts of organic matter in
sediments (Demaison and Moore 1980). More recently,
a controversy developed about two contrasting models
to explain the deposition of organic-matter-rich
sediments in the marine realm, either (1) by preservation
under anoxic conditions in a static situation or (2) by
high primary productivity in a dynamic system (Fig.
4.2; Pedersen and Calvert 1990, 1991; Demaison 1991).
The relative importance of these two dominant
controlling parameters is still being heavily debated,
although Stein (1986a) already conceived that either
one of these parameters could play a decisive role in
different oceanographic situations. Another parameter
brought into discussion more recently is the protective
role of organic matter adsorption on mineral surfaces
and its influence on organic matter accumulation in
marine sediments (Keil et al. 1994a, b; Mayer 1994, 1999;
2005; Ransom et al. 1998).
4.2.1
Productivity Versus Preservation
Recognition of the sensitivity of organic matter
toward oxidative destruction led to the idea that
the concentration of free oxygen in the water
column and particularly at the sediment/water
interface is the most important factor determining
the amount of organic matter that is incorporated
into sediments (e.g. Demaison and Moore 1980).
The stagnant basin or Black Sea model (Fig. 4.2A),
developed from this idea, is based on the observation that lack of replenishment of oxygen by
restricted circulation in the bottom part of larger
water bodies can lead to longer-term oxygendepleted (anoxic, suboxic; see Table 4.1 for
definition) conditions in the water column and at
the sediment/water interface. In the Black Sea
(exceeding 2000 m water depth in the center), this is
caused by the development of a very stable halocline (preventing vertical mixing) at about 100 m to
150 m water depth. The surface layer is fed by
relatively light riverine freshwater from the continent, and denser saline deep water is flowing in at a
low rate from the Mediterranean Sea over the shallow Bosporus sill. Over time, oxidation of sinking
remnants of decayed organisms consumed all of
the free oxygen in the deeper water, which was not
effectively replenished by Mediterranean water.
Instead, the deep water in the Black Sea (like in
Lake Tanganyika, an analogous contempora-neous
example of a large stratified lake; Huc 1988)
contains hydrogen sulfide restricting life to anaerobic microorganisms that are commonly thought to
degrade organic matter less rapidly than aerobic
bacteria, although there are also opposing views
(see, e.g., discussion by Kristensen et al. 1995;
Hulthe et al. 1998). Lack of intense organic matter
degradation under anoxic conditions would then
not necessarily require high surface water bioproductivity for high organic carbon concentrations to
occur in the sediment.
The proponents of primary productivity as the
decisive factor controlling organic matter accumulation
(e.g. Calvert 1987; Pedersen and Calvert 1990; Bailey
1991) suggested that changes in primary productivity
with time in different areas of the world, induced by
climatic and related oceanographic changes, explain
the distribution of Cretaceous black shales and more
recent (Quaternary) organic-matter-rich sediments
better than the occurrence of anoxic conditions in
oceanic bottom waters. Reduced oxygen concentrations
in the water column, according to these authors, are a
consequence of large amounts of decaying biomass
Table 4.1
Terminology for regimes of low oxygen concentrations and the resulting biofacies according to Tyson and
Pearson (1991)
Oxygen (ml/l)
Environments
Biofacies
Physiological regime
8.0-2.0
Oxic
Aerobic
Normoxic
2.0-0.2
Dysoxic
Dysaerobic
Hypoxic
2.0-1.0
Moderate
1.0-0.5
Severe
0.5-0.2
Extreme
0.2-0.0
Suboxic
Quasi-anaerobic
0.0 (H 2 S)
Anoxic
Anaerobic
Anoxic
4.2
Organic Matter Accumulation in Sediments
rich rocks. A few examples are the Jurassic Posidonia
Shales or Kimmeridge Clays in northwestern Europe,
the Cretaceous black shales of the Atlantic Ocean and
other oceanic areas of the world, and the Pliocene to
Holocene sapropels of the Mediterranean Sea.
Stagnant oceanic bottom waters with a low
concentration or absence of oxygen (anoxia) for a long
time were considered the main prerequisite for the
accumulation of high amounts of organic matter in
sediments (Demaison and Moore 1980). More recently,
a controversy developed about two contrasting models
to explain the deposition of organic-matter-rich
sediments in the marine realm, either (1) by preservation
under anoxic conditions in a static situation or (2) by
high primary productivity in a dynamic system (Fig.
4.2; Pedersen and Calvert 1990, 1991; Demaison 1991).
The relative importance of these two dominant
controlling parameters is still being heavily debated,
although Stein (1986a) already conceived that either
one of these parameters could play a decisive role in
different oceanographic situations. Another parameter
brought into discussion more recently is the protective
role of organic matter adsorption on mineral surfaces
and its influence on organic matter accumulation in
marine sediments (Keil et al. 1994a, b; Mayer 1994, 1999;
2005; Ransom et al. 1998).
4.2.1
Productivity Versus Preservation
Recognition of the sensitivity of organic matter
toward oxidative destruction led to the idea that
the concentration of free oxygen in the water
column and particularly at the sediment/water
interface is the most important factor determining
the amount of organic matter that is incorporated
into sediments (e.g. Demaison and Moore 1980).
The stagnant basin or Black Sea model (Fig. 4.2A),
developed from this idea, is based on the observation that lack of replenishment of oxygen by
restricted circulation in the bottom part of larger
water bodies can lead to longer-term oxygendepleted (anoxic, suboxic; see Table 4.1 for
definition) conditions in the water column and at
the sediment/water interface. In the Black Sea
(exceeding 2000 m water depth in the center), this is
caused by the development of a very stable halocline (preventing vertical mixing) at about 100 m to
150 m water depth. The surface layer is fed by
relatively light riverine freshwater from the continent, and denser saline deep water is flowing in at a
low rate from the Mediterranean Sea over the shallow Bosporus sill. Over time, oxidation of sinking
remnants of decayed organisms consumed all of
the free oxygen in the deeper water, which was not
effectively replenished by Mediterranean water.
Instead, the deep water in the Black Sea (like in
Lake Tanganyika, an analogous contempora-neous
example of a large stratified lake; Huc 1988)
contains hydrogen sulfide restricting life to anaerobic microorganisms that are commonly thought to
degrade organic matter less rapidly than aerobic
bacteria, although there are also opposing views
(see, e.g., discussion by Kristensen et al. 1995;
Hulthe et al. 1998). Lack of intense organic matter
degradation under anoxic conditions would then
not necessarily require high surface water bioproductivity for high organic carbon concentrations to
occur in the sediment.
The proponents of primary productivity as the
decisive factor controlling organic matter accumulation
(e.g. Calvert 1987; Pedersen and Calvert 1990; Bailey
1991) suggested that changes in primary productivity
with time in different areas of the world, induced by
climatic and related oceanographic changes, explain
the distribution of Cretaceous black shales and more
recent (Quaternary) organic-matter-rich sediments
better than the occurrence of anoxic conditions in
oceanic bottom waters. Reduced oxygen concentrations
in the water column, according to these authors, are a
consequence of large amounts of decaying biomass
Table 4.1
Terminology for regimes of low oxygen concentrations and the resulting biofacies according to Tyson and
Pearson (1991)
Oxygen (ml/l)
Environments
Biofacies
Physiological regime
8.0-2.0
Oxic
Aerobic
Normoxic
2.0-0.2
Dysoxic
Dysaerobic
Hypoxic
2.0-1.0
Moderate
1.0-0.5
Severe
0.5-0.2
Extreme
0.2-0.0
Suboxic
Quasi-anaerobic
0.0 (H 2 S)
Anoxic
Anaerobic
Anoxic