474
10.8), because the benthic degradation loss, BDL, is
much lower under anaerobic conditions than under
aerobic ones. Therefore, a decrease due to dilution
rather than an increase in Ce with growing inorganic
input, F], is expected. This is in agreement with actual data shown in Fig. 10.8.
10.5.4 Fluxes and Burial of OC in Model Basins
In this section, the vertical organic carbon flux from
the site of primary production to the location of ultimate OC burial in the sediment is demonstrated by a
number of semi-quantitative models (Fig. 10.9). Each
of these models displays two scenarios, one with low
and another one with high primary production. The
flux rates in the water column are defined as Fe
(=AR e ) and in the sediment as ARe or AR] (inorganic accumulation rate). These rates are plotted as
boxes below each diagram, together with the concentrations of organic carbon, Ce, ultimate1y buried in
the sediment. The models neglect the different types
of organic matter discussed in Sect. 14.1 and the
changing proportions of bioclastic and terrigenous
material in the inorganic flux. The models address
the following environmental conditions:
- Deep-sea oxic environments (Fig. 10.9a) with either low or high primary and new production, very
low or low to medium inorganic accumulation rate,
AR], and relatively high to medium benthic degradation loss, BDL. (The absolute values of BDL are low
becimse oflimited OC input.) As a result, the organic
carbon flux, ARe, into the deeper sediment is very
low to low; Ce is consequently very low or medium,
respective1y.
- Large or smaller deep basins with stratified water
masses and an oxygen minimum zone (Fig. 10.9b);
primary and new production are either low or high.
Then, low AR] enables high to very high values of Ce
in both scenarios. However, BDL by bacterial activity (e.g. sulfate reduction) must not be neglected.
High AR] (clastic dilution) leads to low to medium
Ce values although BDL is limited. The OC preservation factor (see below), however, is higher with
high AR].
- Shallow seas (e.g., shelves) with oxic conditions,
low or high primary and new production, and medium to very high inorganic accumulation rate, AR]
(Fig. 10.9c). Here, organic carbon input, Fe and thus
ARe, into the benthic zone may become very high,
but the carbon concentration values, Ce. turn out to
be low to medium due to clastic dilution.
- Shelves, shallow epicontinental basins, or specific
basins within these settings, characterized by anoxic
or dysoxic bottom waters (Fig. 1O.9d). The oxygen
minimum zone results from restricted water circulation or coastal upwelling; primary and new producChapter 10 Sedimentation Rates and Organic Matter
tion are either low or high. BDL is limited due to the
lack or scarcity of benthic macro-organisms, but bacterial activity may be substantial. The resulting low
to high values of Ce are mainly controlled by Fe and
AR].
One of the results of this exercise is that sediments
with high to very high Ce values are formed in both
shallow- and deep-water environments if they are
anoxic or dysoxic. To achieve high Ce values, primary productivities can be fairly low, but then the
inorganic accumulation rates must also be low or, at
the most, intermediate (in shallow water). Under
such conditions, organic carbon content does not increase with the sedimentation rate (Stein 1991).
These models may be modified by additional factors, such
as lateral input of organic carbon, including plant material,
by bottom currents and gravity mass movements. The uppennost sediment layers can be reworked, and organic matter already buried may again be redeposited in the zone of
intense benthic degradation.
The identification of the different types of depositional
environments preserving organic matter (cf. Fig. 10.11),
including black shales (see below) is often difficult in ancient rocks. One has to keep in mind that paleo-oceanographic conditions in the past may have been entire1y different from the modem world, at least during certain time
intervals.
During greenhouse times of the globe, the oxygen
content of sea water was most likely lower and water
circulation more sluggish than today. As a result, the
remineralization of organic matter and thus the recycling of nutrients was hampered. In addition, an increased loss of nitrate through denitrification in oxygen deficient waters may have limited fertility and
caused "starved ocean basins" (Berger et al. 1989).
Hence, oxygen minimum zones of wider extent could
repeatedly form both in deep and shallow seas (cf.
Sects. 5.2 and 7.8) in spite of reduced organic production. However, the general processes outlined
above must also have operated in the geological past.
Specific planktonic microfaunas of low diversity and high
amounts of biogenic opal may indicate upwelling conditions (e.g., Thiede and Suess 1983; Kuhnt et al. 1986, cf.
Sect. 5.3.5). High concentrations of trace metals trapped
from sea water by hydrogen sulfide andlor organic matter
are probably best explained by a stagnant anoxic basin and
a low sedimentation rate (e.g., Brumsack 1986b).
10.5.5 Organic Carbon Preservation Factor
Finally, we may ask the question: which proportion
of the primary production, PP, in overlying surface
waters is preserved in the sediments be10w the zone
of benthic activity? The organic carbon accumulation
rate, ARe, in the sediment, expressed as a percentage
10.8), because the benthic degradation loss, BDL, is
much lower under anaerobic conditions than under
aerobic ones. Therefore, a decrease due to dilution
rather than an increase in Ce with growing inorganic
input, F], is expected. This is in agreement with actual data shown in Fig. 10.8.
10.5.4 Fluxes and Burial of OC in Model Basins
In this section, the vertical organic carbon flux from
the site of primary production to the location of ultimate OC burial in the sediment is demonstrated by a
number of semi-quantitative models (Fig. 10.9). Each
of these models displays two scenarios, one with low
and another one with high primary production. The
flux rates in the water column are defined as Fe
(=AR e ) and in the sediment as ARe or AR] (inorganic accumulation rate). These rates are plotted as
boxes below each diagram, together with the concentrations of organic carbon, Ce, ultimate1y buried in
the sediment. The models neglect the different types
of organic matter discussed in Sect. 14.1 and the
changing proportions of bioclastic and terrigenous
material in the inorganic flux. The models address
the following environmental conditions:
- Deep-sea oxic environments (Fig. 10.9a) with either low or high primary and new production, very
low or low to medium inorganic accumulation rate,
AR], and relatively high to medium benthic degradation loss, BDL. (The absolute values of BDL are low
becimse oflimited OC input.) As a result, the organic
carbon flux, ARe, into the deeper sediment is very
low to low; Ce is consequently very low or medium,
respective1y.
- Large or smaller deep basins with stratified water
masses and an oxygen minimum zone (Fig. 10.9b);
primary and new production are either low or high.
Then, low AR] enables high to very high values of Ce
in both scenarios. However, BDL by bacterial activity (e.g. sulfate reduction) must not be neglected.
High AR] (clastic dilution) leads to low to medium
Ce values although BDL is limited. The OC preservation factor (see below), however, is higher with
high AR].
- Shallow seas (e.g., shelves) with oxic conditions,
low or high primary and new production, and medium to very high inorganic accumulation rate, AR]
(Fig. 10.9c). Here, organic carbon input, Fe and thus
ARe, into the benthic zone may become very high,
but the carbon concentration values, Ce. turn out to
be low to medium due to clastic dilution.
- Shelves, shallow epicontinental basins, or specific
basins within these settings, characterized by anoxic
or dysoxic bottom waters (Fig. 1O.9d). The oxygen
minimum zone results from restricted water circulation or coastal upwelling; primary and new producChapter 10 Sedimentation Rates and Organic Matter
tion are either low or high. BDL is limited due to the
lack or scarcity of benthic macro-organisms, but bacterial activity may be substantial. The resulting low
to high values of Ce are mainly controlled by Fe and
AR].
One of the results of this exercise is that sediments
with high to very high Ce values are formed in both
shallow- and deep-water environments if they are
anoxic or dysoxic. To achieve high Ce values, primary productivities can be fairly low, but then the
inorganic accumulation rates must also be low or, at
the most, intermediate (in shallow water). Under
such conditions, organic carbon content does not increase with the sedimentation rate (Stein 1991).
These models may be modified by additional factors, such
as lateral input of organic carbon, including plant material,
by bottom currents and gravity mass movements. The uppennost sediment layers can be reworked, and organic matter already buried may again be redeposited in the zone of
intense benthic degradation.
The identification of the different types of depositional
environments preserving organic matter (cf. Fig. 10.11),
including black shales (see below) is often difficult in ancient rocks. One has to keep in mind that paleo-oceanographic conditions in the past may have been entire1y different from the modem world, at least during certain time
intervals.
During greenhouse times of the globe, the oxygen
content of sea water was most likely lower and water
circulation more sluggish than today. As a result, the
remineralization of organic matter and thus the recycling of nutrients was hampered. In addition, an increased loss of nitrate through denitrification in oxygen deficient waters may have limited fertility and
caused "starved ocean basins" (Berger et al. 1989).
Hence, oxygen minimum zones of wider extent could
repeatedly form both in deep and shallow seas (cf.
Sects. 5.2 and 7.8) in spite of reduced organic production. However, the general processes outlined
above must also have operated in the geological past.
Specific planktonic microfaunas of low diversity and high
amounts of biogenic opal may indicate upwelling conditions (e.g., Thiede and Suess 1983; Kuhnt et al. 1986, cf.
Sect. 5.3.5). High concentrations of trace metals trapped
from sea water by hydrogen sulfide andlor organic matter
are probably best explained by a stagnant anoxic basin and
a low sedimentation rate (e.g., Brumsack 1986b).
10.5.5 Organic Carbon Preservation Factor
Finally, we may ask the question: which proportion
of the primary production, PP, in overlying surface
waters is preserved in the sediments be10w the zone
of benthic activity? The organic carbon accumulation
rate, ARe, in the sediment, expressed as a percentage