8
Sulfur Cycling and Methane Oxidation
300
A multi-G model of Westrich and Berner (1984)
was applied to fit a smooth curve to these rate
data. The model assumes that the sediment
organic matter that feeds sulfate reduction consists of three pools, each of which is degraded
exponentially over depth and time, but each
having its own pool size and exponential decay
constant. Fig. 8.18 B and C show that this model
fits well with both the measured sulfate reduction
rates in the upper sediment layer and the sulfate
curve in the deeper sediment down to 330 cm
depth.
A point to observe in Fig. 8.18 C is that the
measured and modeled profiles of pore water
sulfate in the uppermost 0-20 cm hardly show any
curvature and, thus, indicate zero sulfate reduction in this layer. The radiotracer measurements in
Fig. 8.18 B, however, show that 66% of the sulfate
reduction in the entire sulfate zone takes place in
the top 15 cm where, in the deep sulfidic part of
the Black Sea, sulfate reduction is the predominant pathway of organic carbon mineralization
(Jørgensen et al. 2001). One possible reason for
this discrepancy is that modeling has low
sensitivity over narrow depth intervals because
diffusion is fast over short distances and thereby
reduces changes in concentration in spite of high
reaction rates. In contrast, over the entire sulfate
zone even a deep zone of relatively low sulfate
reduction rates may affect the entire sulfate
profile. This is why enhanced reduction rates in
the sulfate-methane transition tend to shape the
entire sulfate distribution in the sediment as seen
in Fig. 8.18 C. Thus, experimental measurements
are required to determine the sulfate reduction in
the near-surface sediment. Such a discrepancy
between measured and modeled rates is characteristic also of other sediments and is important to
consider in relation to the global SRR estimates
made in Fig. 8.3 and 8.4. Data from ocean margin
sediments were there mostly determined by the
radiotracer method (gross SRR) whereas those
from the deep sea were modeled (net SRR).
8.7
The Sulfur Cycle
The sea bed functions as a giant anaerobic reactor
where element cycles are coupled in a way that
differs fundamentally from the cycles in the oxic
ocean. The microbiological and geochemical processes of sulfur transformation thereby play key
functions that control the mineralization of
deposited organic matter and thereby the chemistry of the ocean. In the following we briefly summarize some main aspects of the sulfur cycle in
marine sediments and discuss its role for the
dynamics of sulfate in the ocean.
Rather than being a simple cycle, composed of
anaerobic bacterial reduction of sulfate to hydrogen sulfide and aerobic reoxidation of H 2 S to SO 4
2,
the transformations of sulfur in aquatic sediments
include a combination of intermediate cycles or
shunts and interactions with other element cycles
(e.g., Jørgensen 1990; Luther and Church 1991;
Thamdrup et al. 1993; van Cappellen and Wang
1996) schematically illustrated in Figure 8.16.
Within this complex cycle, sulfur compounds
occur in oxidation states ranging from -2 (H 2 S) to
+6 (SO 4
2)
By bacterial sulfate reduction H 2 S is produced
as the extracellular end-product (Widdel and
Hansen 1991). During the oxidation of H 2 S, oxic or
anoxic, chemical or biological, compounds such as
zero-valent sulfur (in elemental sulfur, polysulfides, or polythionates), thiosulfate (S 2 O 3
2), and
sulfite (SO 3
2) are produced (Cline and Richards
1969; Pyzik and Sommer 1981; Kelly 1988; Dos
Santos Afonso and Stumm 1992). These intermediates may then be further transformed by one
or several of the following processes:
• Respiratory bacterial reduction to H 2 S,
• bacterial or chemical oxidation,
• chemical precipitation (e.g. FeS formation), or
• bacterial disproportionation to H 2 S and SO 4
2.
By bacterial disproportionation H 2 S and SO 4
2are produced concurrently without participation
of an external electron acceptor or donor (Bak and
Pfennig 1987; Thamdrup et al. 1993). The biogeochemical transformations of sulfur in marine sediments are closely coupled to the cycles of iron
and manganese. Sulfate, iron oxides, and manganese oxides all serve as electron acceptors in the
respiratory degradation of organic matter. As there
are also non-enzymatic reactions between iron,
manganese and H 2 S within the sediment, the
quantification of dissimilatory, heterotrophic Fe
and Mn reduction is particularly difficult.
The precipitation of (authigenic) iron sulfides
resulting from the reaction between H 2 S and Fe
phases exerts an important control on the distribution of H 2 S in marine pore waters (Goldhaber
and Kaplan 1974; Canfield 1989; Canfield et al.
Sulfur Cycling and Methane Oxidation
300
A multi-G model of Westrich and Berner (1984)
was applied to fit a smooth curve to these rate
data. The model assumes that the sediment
organic matter that feeds sulfate reduction consists of three pools, each of which is degraded
exponentially over depth and time, but each
having its own pool size and exponential decay
constant. Fig. 8.18 B and C show that this model
fits well with both the measured sulfate reduction
rates in the upper sediment layer and the sulfate
curve in the deeper sediment down to 330 cm
depth.
A point to observe in Fig. 8.18 C is that the
measured and modeled profiles of pore water
sulfate in the uppermost 0-20 cm hardly show any
curvature and, thus, indicate zero sulfate reduction in this layer. The radiotracer measurements in
Fig. 8.18 B, however, show that 66% of the sulfate
reduction in the entire sulfate zone takes place in
the top 15 cm where, in the deep sulfidic part of
the Black Sea, sulfate reduction is the predominant pathway of organic carbon mineralization
(Jørgensen et al. 2001). One possible reason for
this discrepancy is that modeling has low
sensitivity over narrow depth intervals because
diffusion is fast over short distances and thereby
reduces changes in concentration in spite of high
reaction rates. In contrast, over the entire sulfate
zone even a deep zone of relatively low sulfate
reduction rates may affect the entire sulfate
profile. This is why enhanced reduction rates in
the sulfate-methane transition tend to shape the
entire sulfate distribution in the sediment as seen
in Fig. 8.18 C. Thus, experimental measurements
are required to determine the sulfate reduction in
the near-surface sediment. Such a discrepancy
between measured and modeled rates is characteristic also of other sediments and is important to
consider in relation to the global SRR estimates
made in Fig. 8.3 and 8.4. Data from ocean margin
sediments were there mostly determined by the
radiotracer method (gross SRR) whereas those
from the deep sea were modeled (net SRR).
8.7
The Sulfur Cycle
The sea bed functions as a giant anaerobic reactor
where element cycles are coupled in a way that
differs fundamentally from the cycles in the oxic
ocean. The microbiological and geochemical processes of sulfur transformation thereby play key
functions that control the mineralization of
deposited organic matter and thereby the chemistry of the ocean. In the following we briefly summarize some main aspects of the sulfur cycle in
marine sediments and discuss its role for the
dynamics of sulfate in the ocean.
Rather than being a simple cycle, composed of
anaerobic bacterial reduction of sulfate to hydrogen sulfide and aerobic reoxidation of H 2 S to SO 4
2,
the transformations of sulfur in aquatic sediments
include a combination of intermediate cycles or
shunts and interactions with other element cycles
(e.g., Jørgensen 1990; Luther and Church 1991;
Thamdrup et al. 1993; van Cappellen and Wang
1996) schematically illustrated in Figure 8.16.
Within this complex cycle, sulfur compounds
occur in oxidation states ranging from -2 (H 2 S) to
+6 (SO 4
2)
By bacterial sulfate reduction H 2 S is produced
as the extracellular end-product (Widdel and
Hansen 1991). During the oxidation of H 2 S, oxic or
anoxic, chemical or biological, compounds such as
zero-valent sulfur (in elemental sulfur, polysulfides, or polythionates), thiosulfate (S 2 O 3
2), and
sulfite (SO 3
2) are produced (Cline and Richards
1969; Pyzik and Sommer 1981; Kelly 1988; Dos
Santos Afonso and Stumm 1992). These intermediates may then be further transformed by one
or several of the following processes:
• Respiratory bacterial reduction to H 2 S,
• bacterial or chemical oxidation,
• chemical precipitation (e.g. FeS formation), or
• bacterial disproportionation to H 2 S and SO 4
2.
By bacterial disproportionation H 2 S and SO 4
2are produced concurrently without participation
of an external electron acceptor or donor (Bak and
Pfennig 1987; Thamdrup et al. 1993). The biogeochemical transformations of sulfur in marine sediments are closely coupled to the cycles of iron
and manganese. Sulfate, iron oxides, and manganese oxides all serve as electron acceptors in the
respiratory degradation of organic matter. As there
are also non-enzymatic reactions between iron,
manganese and H 2 S within the sediment, the
quantification of dissimilatory, heterotrophic Fe
and Mn reduction is particularly difficult.
The precipitation of (authigenic) iron sulfides
resulting from the reaction between H 2 S and Fe
phases exerts an important control on the distribution of H 2 S in marine pore waters (Goldhaber
and Kaplan 1974; Canfield 1989; Canfield et al.