10.5 Organic Matter Preservation in Marine Sediments
A situation similar to that of the Santa Catalina basin
has been studied in the zone of coastal upwelling off Peru
(Suess et al. 1987). In the southem part ofthis region, average primary organic carbon production is on the order of
300 g/m 2 /a, and the carbon flux arriving on the sea floor
(water depth ;<;500 m) is 88 g/m 2 /a. Ultimately, 24 g/m 2 /a
are buried, i.e., about 8% of the primary production. This
results in an organic carbon content of <:5% in the sediment which signifies that about 70% of the organic carbon
fallout are used up by benthic organisms on the sea floor
and by processes of early diagenesis.
Oxygen consumption by benthic respiration, for example, is substantial in ocean regions with high organic carbon fallout to the sea floor. It contributes
significantly to the strong oxygen minimum in many
shallow-marine regions (Berger et a1. 1989).
CO 2 released by aerobic decomposition of organic
carbon can lead to the dissolution of calcium carbonate. In the upwelling zone off Peru, most of the
biogenic carbonate reaching the sea floor (on the order of 150 gim 2 /a) is dissolved. This process operates
according to the following rule: In order to dissolve a
CaC0 3 flux of 100 g/m 2 /a, an organic carbon flux of
24 gC/m 2 /a is required. This quantity of OC is transformed into CO 2 and HC0 3 - which move into the
pore water and overlying sea water thereby increasing alkalinity.
Another OC loss in the benthic boundary layer
and the uppermost sediment is accomplished by bacterial sulfate reduction under anoxic conditions (e.g.
Kempe 1990). To reduce 80 g S04-· present in sea
water or pore water, 24 g OC are consumed. The
HC0 3 - released during this process enhances the alkalinity which is sometimes referred to a "alkalinity
pump". The presence of substantial quantities of iron
sulfide in many marine black shales testifies to the
fact that a considerable proportion of OM has been
consumed within the sediment.
10.5.3 Inorganic Sediment Accumulation Rates
and OC Burial
Flux Rates and OC Concentrations
Direct measurements of carbon flux rates close to the
water-sediment interface as mentioned above are still
rare, and their results cannot be applied direcdy to
other regions. However, a number of studies compare
organic carbon concentrations or accumulation rates
in young marine sediments with present-day production rates in the overlying water body (e.g., Müller
and Suess 1979; Johnson Ibach 1982; Bralower and
Thierstein 1984; Emerson 1985; Stein 1991; and references mentioned earlier). These data can be used to
gain an overview of the complicate interrelationship
between the varies factors controlling organic carbon
preservation.
471
In addition to the processes discussed in the previous section, organic carbon preservation is also favored by an increasing overall sedimentation rate, i.e.
mainly by the inorganic rate of sedimentation or accumulation. OC preservation is therefore mainly controlled by the following four factors:
(1) Primary organic production and terrestrial OC
influx.
(2) Flux of organic matter to the sea floor (new production and export production).
(3) Carbon loss through benthic life activity in the
uppermost sediment,
(4) The inorganic sedimentation (accumulation) rate.
The results of a simplified empirical approach to this
problem are shown in Fig. 10.7. Here, points 1 and 4
of the above list are considered. Instead of (decreasing) OC fluxes through the water column and uppermost sediment layer, the concentrations of organic
carbon, C org , in the sediment below the zone of benthic degradation are plotted. For oxic environments,
this diagram clearly shows that the C org contents of
the sediments increase with growing sedimentation
rates. In the case of anoxic environments, the benthic
degradation losses and thus also the influence of the
sedimentation rates are lirnited. Under these conditions, even zones of low to intermediate productivity
can lead to relatively high C or concentrations.
Considering, in addition, flux rates of organic carbon, Fe, and inorganic material, F" accumulation
rates for organic carbon, ARe, and in organic material, AR" as weIl as benthic degradation los ses, BDL,
the relationship between the factors mentioned above
become more complicate (Fig. 10.8).
These parameters are further defined as folIows:
- Flux rates of organie carbon, Fe, and inorganic material,
F" refer to fluxes from the water body into the benthic
zone.
- Flux rates or accumulation rates of organie carbon, AR e,
and inorganic material, AR" describe processes in the sediment below the zone ofbenthic life activity.
- Concentrations of organic carbon, C org or Ce, are given in
weight percent of the total sediment.
If erosional processes and dissolution on the sea floor
can be exduded, AR, is equal to F" but ARe becomes less
than Fe due to benthic degradation. When describing these
relationships, one should not confuse accumulation rates,
e.g. in g/m 2 /a, linear sedimentation rates, e.g. in rnIMa, and
concentrations in percent by dry weight.
The relationship between the inorganic accumulation
rate, AR" and the total sedimentation rate, SR (induding
biogenie skeletal material), is shown in Fig. 10.8 at the horizontal scales for three different porosities (25, 50, and
75%). The concentration of organie carbon buried in
deeper sediments is Ce=ARc/(AR,+ARc).
A simple relationship exists between F c and F, (=
AR,) and thus between these parameters and Cc. If F c
A situation similar to that of the Santa Catalina basin
has been studied in the zone of coastal upwelling off Peru
(Suess et al. 1987). In the southem part ofthis region, average primary organic carbon production is on the order of
300 g/m 2 /a, and the carbon flux arriving on the sea floor
(water depth ;<;500 m) is 88 g/m 2 /a. Ultimately, 24 g/m 2 /a
are buried, i.e., about 8% of the primary production. This
results in an organic carbon content of <:5% in the sediment which signifies that about 70% of the organic carbon
fallout are used up by benthic organisms on the sea floor
and by processes of early diagenesis.
Oxygen consumption by benthic respiration, for example, is substantial in ocean regions with high organic carbon fallout to the sea floor. It contributes
significantly to the strong oxygen minimum in many
shallow-marine regions (Berger et a1. 1989).
CO 2 released by aerobic decomposition of organic
carbon can lead to the dissolution of calcium carbonate. In the upwelling zone off Peru, most of the
biogenic carbonate reaching the sea floor (on the order of 150 gim 2 /a) is dissolved. This process operates
according to the following rule: In order to dissolve a
CaC0 3 flux of 100 g/m 2 /a, an organic carbon flux of
24 gC/m 2 /a is required. This quantity of OC is transformed into CO 2 and HC0 3 - which move into the
pore water and overlying sea water thereby increasing alkalinity.
Another OC loss in the benthic boundary layer
and the uppermost sediment is accomplished by bacterial sulfate reduction under anoxic conditions (e.g.
Kempe 1990). To reduce 80 g S04-· present in sea
water or pore water, 24 g OC are consumed. The
HC0 3 - released during this process enhances the alkalinity which is sometimes referred to a "alkalinity
pump". The presence of substantial quantities of iron
sulfide in many marine black shales testifies to the
fact that a considerable proportion of OM has been
consumed within the sediment.
10.5.3 Inorganic Sediment Accumulation Rates
and OC Burial
Flux Rates and OC Concentrations
Direct measurements of carbon flux rates close to the
water-sediment interface as mentioned above are still
rare, and their results cannot be applied direcdy to
other regions. However, a number of studies compare
organic carbon concentrations or accumulation rates
in young marine sediments with present-day production rates in the overlying water body (e.g., Müller
and Suess 1979; Johnson Ibach 1982; Bralower and
Thierstein 1984; Emerson 1985; Stein 1991; and references mentioned earlier). These data can be used to
gain an overview of the complicate interrelationship
between the varies factors controlling organic carbon
preservation.
471
In addition to the processes discussed in the previous section, organic carbon preservation is also favored by an increasing overall sedimentation rate, i.e.
mainly by the inorganic rate of sedimentation or accumulation. OC preservation is therefore mainly controlled by the following four factors:
(1) Primary organic production and terrestrial OC
influx.
(2) Flux of organic matter to the sea floor (new production and export production).
(3) Carbon loss through benthic life activity in the
uppermost sediment,
(4) The inorganic sedimentation (accumulation) rate.
The results of a simplified empirical approach to this
problem are shown in Fig. 10.7. Here, points 1 and 4
of the above list are considered. Instead of (decreasing) OC fluxes through the water column and uppermost sediment layer, the concentrations of organic
carbon, C org , in the sediment below the zone of benthic degradation are plotted. For oxic environments,
this diagram clearly shows that the C org contents of
the sediments increase with growing sedimentation
rates. In the case of anoxic environments, the benthic
degradation losses and thus also the influence of the
sedimentation rates are lirnited. Under these conditions, even zones of low to intermediate productivity
can lead to relatively high C or concentrations.
Considering, in addition, flux rates of organic carbon, Fe, and inorganic material, F" accumulation
rates for organic carbon, ARe, and in organic material, AR" as weIl as benthic degradation los ses, BDL,
the relationship between the factors mentioned above
become more complicate (Fig. 10.8).
These parameters are further defined as folIows:
- Flux rates of organie carbon, Fe, and inorganic material,
F" refer to fluxes from the water body into the benthic
zone.
- Flux rates or accumulation rates of organie carbon, AR e,
and inorganic material, AR" describe processes in the sediment below the zone ofbenthic life activity.
- Concentrations of organic carbon, C org or Ce, are given in
weight percent of the total sediment.
If erosional processes and dissolution on the sea floor
can be exduded, AR, is equal to F" but ARe becomes less
than Fe due to benthic degradation. When describing these
relationships, one should not confuse accumulation rates,
e.g. in g/m 2 /a, linear sedimentation rates, e.g. in rnIMa, and
concentrations in percent by dry weight.
The relationship between the inorganic accumulation
rate, AR" and the total sedimentation rate, SR (induding
biogenie skeletal material), is shown in Fig. 10.8 at the horizontal scales for three different porosities (25, 50, and
75%). The concentration of organie carbon buried in
deeper sediments is Ce=ARc/(AR,+ARc).
A simple relationship exists between F c and F, (=
AR,) and thus between these parameters and Cc. If F c