273
8.2
Sulfate Reduction and the Degradation of Organic Matter
respiration, and possibly food particles, and
thereby extend the oxic zone (i.e. the zone containing O 2 ) deeper down into the sediment. Due to
this macrofaunal activity, an extended surface layer
of sediment remains oxidized (i.e. with positive
redox potential and with many chemical species
such as iron or manganese in their oxidized state).
The macrofauna thus has a critical influence on the
early diagenesis in sediments and affects both the
pathways and the rates of many processes.
8.2.1
Geochemical Zonation
Apart from the heterogeneous chemical structure
caused by faunal activity, marine sediments have
a distinct biogeochemical zonation of the main
aerobic and anaerobic mineralization processes.
Fig. 8.1 shows how the dominant oxidants for
mineralization change with depth, regulated partly
by their concentration in sea water and partly by
the energy yield of the process by which they are
consumed (cf. Chapter 5). Many microorganisms
gain energy from the oxidation of organic matter
with an external oxidant (electron acceptor).
Thermodynamically, oxygen is the most favorable
electron acceptor. The supply of oxygen from seawater to the sediment is, however, highly transportlimited. In coastal marine sediments or in ocean
areas of high productivity, e.g. upwelling regions,
a high organic matter flux or low oxygen content
of the bottom water reduces the thickness of the
oxic surface layer of the sediment to only a few
mm or cm. In deep sea sediments the depth of
oxygen penetration may be a dm or a m or even
more (Chapter 6).
Where oxygen has been consumed by aerobic
respiration, the sediment is anoxic, i.e. O 2 -free, and
microorganisms utilize other terminal electron
acceptors for the mineralization of organic matter.
Listed in an order of decreasing energy gain these
are: nitrate (NO 3
-
), manganese oxides (represented
by Mn(IV)), iron oxides (represented by Fe(III)),
and sulfate (SO 4
2) (Fig. 8.1). The sediment layer
where NO 3
-
, Mn(IV) and Fe(III) reduction
predominate has been termed the suboxic zone
(Froelich et al. 1979). Although these electron
acceptors are energetically more favorable than
sulfate, they are usually less important biogeochemically because of their limited supply to the
sediments. The processes of O 2 , NO 3
-
and Fe(III)
reduction are discussed in detail in Chapter 6 and 7.
Below the suboxic zone, sulfate reduction is
the main pathway of organic carbon oxidation and
it generally extends many meters down into the
sediment. The high concentration of sulfate in
seawater makes it a dominant electron acceptor
(Henrichs and Reeburgh 1987). With an average
concentration of about 29 mmol/l (Vairavamurthy
et al. 1995), sulfate concentrations in seawater are
more than two orders of magnitude higher than in
freshwater (about 0.1 mmol/l) (Bowen 1979). The
mineralization of organic material by sulfate in
marine sediments is, therefore, much more important than in freshwater.
In the even deeper subsurface sediment, where
also sulfate is exhausted, there is little net oxidation of the organic carbon but rather a
degradation to CH 4 and CO 2 . As a stable end
product of carbon degradation in the absence of
oxidants, methane tends to accumulate in deep
sub-surface sediments from where it slowly
diffuses up towards the sulfate zone. Upon entry
into the lower sulfate zone, however, also methane
becomes oxidized completely to CO 2 . The organic
material initially deposited on the sediment
surface thus undergoes an efficient progressing
degradation as it becomes buried deeper and
deeper down through this sequence of diagenetic
processes. Only a small fraction escapes mineralization and remains after thousands or millions of
years to contribute to the great pool of fossil organic carbon in marine sediments.
The main degradation of organic material takes
place as it gradually becomes buried down
through the oxic and suboxic zones. The fraction
that still remains once it reaches the sulfate
reduction zone therefore depends on how deep
oxygen, nitrate, manganese and iron reduction
predominate. In coastal sediments with high
organic sedimentation these oxidants are rapidly
depleted and the main sulfate reduction zone
starts already a few cm below the sediment surface. Although it is not so apparent from the
chemical zonation, sulfate reduction also occurs in
suboxic and even in oxic sediment where the
produced sulfide is rapidly reoxidized (Jørgensen
and Bak 1991). In the deep sea, the metal oxides
may be exhausted only several meters below the
surface and little organic material is available once
it becomes buried down into the sulfate reduction
zone. The overall sulfate reduction in sediments is
therefore very sensitive to the organic deposition
rate and is geographically strongly shifted
towards the continental margins and shelf sediments. In fact, a large part of the deep sea floor
lies under low-productivity ocean regions where
8.2
Sulfate Reduction and the Degradation of Organic Matter
respiration, and possibly food particles, and
thereby extend the oxic zone (i.e. the zone containing O 2 ) deeper down into the sediment. Due to
this macrofaunal activity, an extended surface layer
of sediment remains oxidized (i.e. with positive
redox potential and with many chemical species
such as iron or manganese in their oxidized state).
The macrofauna thus has a critical influence on the
early diagenesis in sediments and affects both the
pathways and the rates of many processes.
8.2.1
Geochemical Zonation
Apart from the heterogeneous chemical structure
caused by faunal activity, marine sediments have
a distinct biogeochemical zonation of the main
aerobic and anaerobic mineralization processes.
Fig. 8.1 shows how the dominant oxidants for
mineralization change with depth, regulated partly
by their concentration in sea water and partly by
the energy yield of the process by which they are
consumed (cf. Chapter 5). Many microorganisms
gain energy from the oxidation of organic matter
with an external oxidant (electron acceptor).
Thermodynamically, oxygen is the most favorable
electron acceptor. The supply of oxygen from seawater to the sediment is, however, highly transportlimited. In coastal marine sediments or in ocean
areas of high productivity, e.g. upwelling regions,
a high organic matter flux or low oxygen content
of the bottom water reduces the thickness of the
oxic surface layer of the sediment to only a few
mm or cm. In deep sea sediments the depth of
oxygen penetration may be a dm or a m or even
more (Chapter 6).
Where oxygen has been consumed by aerobic
respiration, the sediment is anoxic, i.e. O 2 -free, and
microorganisms utilize other terminal electron
acceptors for the mineralization of organic matter.
Listed in an order of decreasing energy gain these
are: nitrate (NO 3
-
), manganese oxides (represented
by Mn(IV)), iron oxides (represented by Fe(III)),
and sulfate (SO 4
2) (Fig. 8.1). The sediment layer
where NO 3
-
, Mn(IV) and Fe(III) reduction
predominate has been termed the suboxic zone
(Froelich et al. 1979). Although these electron
acceptors are energetically more favorable than
sulfate, they are usually less important biogeochemically because of their limited supply to the
sediments. The processes of O 2 , NO 3
-
and Fe(III)
reduction are discussed in detail in Chapter 6 and 7.
Below the suboxic zone, sulfate reduction is
the main pathway of organic carbon oxidation and
it generally extends many meters down into the
sediment. The high concentration of sulfate in
seawater makes it a dominant electron acceptor
(Henrichs and Reeburgh 1987). With an average
concentration of about 29 mmol/l (Vairavamurthy
et al. 1995), sulfate concentrations in seawater are
more than two orders of magnitude higher than in
freshwater (about 0.1 mmol/l) (Bowen 1979). The
mineralization of organic material by sulfate in
marine sediments is, therefore, much more important than in freshwater.
In the even deeper subsurface sediment, where
also sulfate is exhausted, there is little net oxidation of the organic carbon but rather a
degradation to CH 4 and CO 2 . As a stable end
product of carbon degradation in the absence of
oxidants, methane tends to accumulate in deep
sub-surface sediments from where it slowly
diffuses up towards the sulfate zone. Upon entry
into the lower sulfate zone, however, also methane
becomes oxidized completely to CO 2 . The organic
material initially deposited on the sediment
surface thus undergoes an efficient progressing
degradation as it becomes buried deeper and
deeper down through this sequence of diagenetic
processes. Only a small fraction escapes mineralization and remains after thousands or millions of
years to contribute to the great pool of fossil organic carbon in marine sediments.
The main degradation of organic material takes
place as it gradually becomes buried down
through the oxic and suboxic zones. The fraction
that still remains once it reaches the sulfate
reduction zone therefore depends on how deep
oxygen, nitrate, manganese and iron reduction
predominate. In coastal sediments with high
organic sedimentation these oxidants are rapidly
depleted and the main sulfate reduction zone
starts already a few cm below the sediment surface. Although it is not so apparent from the
chemical zonation, sulfate reduction also occurs in
suboxic and even in oxic sediment where the
produced sulfide is rapidly reoxidized (Jørgensen
and Bak 1991). In the deep sea, the metal oxides
may be exhausted only several meters below the
surface and little organic material is available once
it becomes buried down into the sulfate reduction
zone. The overall sulfate reduction in sediments is
therefore very sensitive to the organic deposition
rate and is geographically strongly shifted
towards the continental margins and shelf sediments. In fact, a large part of the deep sea floor
lies under low-productivity ocean regions where