96
sediments have shown decreasing SO~ - reduction
rates over the initial 0.25 to 1 hour of incubation
after which rates became constant (Moeslund et al.
1994; Fossing 1995; Isaksen and Finster 1996).
This was interpreted as a result of reoxidation of
tracer from a small (undetectable) H2S pool that
rapidly became saturated with tracer, after which
the measured rate represented the rate of net incorporation of S into the solid reduced pools. The results were not conclusive, however, and further investigations are needed to constrain to what extent,
and under which conditions, the radiotracer technique may underestimate rates of SO~ - reduction
in oxidized sediments.
When the technique is used with freshwater sediments, special attention must be paid to an accurate
determination of SO~ - concentrations due to low
levels and steep gradients, and incubation times
must be kept short due to the brief turnover time of
the SO~- pool (e.g., Bak and Pfennig 1991; Roden
and Tuttle 1993b; Urban et al. 1994). High SO~reduction rates are in some cases measured at what
appears to be threshold level SO~ - concentrations
based on the concentration profile. Rapid recycling
of SO~ - has been suggested as an explanation, but
recently, such high rates have been found to significantly exceed total C mineralization rates corrected
for the contribution of methanogenesis, which indicated that the reduction rates were overestimated
(Roden and Wetzel 1996). It has been suggested
that the threshold concentrations represent a nonreducible SO~ - pool of unknown nature (Bak and
Pfennig 1991; Roden and Tuttle 1993b). A more
extensive comparison of total mineralization rates
measured as LC02 accumulation to the sum of individual degradative pathways, similar to the approach of Roden and Wetzel (Roden and Wetzel
1996), could serve as control on SO~ - reduction
rate measurements in freshwater sediments.
Sulfate reduction rates can also be determined
from SO~ - depletion in the incubation of sediment
in discrete depth intervals (e.g., Goldhaber et al.
1977; Swider and Mackin 1989; Bak and Pfennig
1991). This approach yields only net reduction
rates, however, it has far lower sensitivity than the
tracer technique, and it requires longer incubation
times. In the deep sea where SO~ - reduction is restricted to below the zone of bioturbation and irrigation, rates are more easily quantified from
SO~ - concentration profiles (Bender and Heggie
Bo Thamdrup and Donald E. Canfield
1984; Canfield 1991; but see also Parkes et al.
1995).
Methanogenesis
In the absence of inorganic electron acceptors, the
mineralization of organic matter will proceed by
fermentative processes with methanogenesis as the
terminal step. Due to competitive inhibition,
methanogenesis is insignificant when inorganic
electron acceptors are present (Lovley and Klug
1986; Capone and Kiene 1988). In marine sediments, therefore, the onset of methanogenesis generally lies far below the sediment surface where
little reactive organic matter is left, and decomposition through this pathway is estimated to be only
10% of the C oxidation through SO~- reduction
(Canfield 1993). Higher contributions are found in
coastal sediments with extreme deposition rates and
SO~ - depletion only a few decimeters below the
surface (Martens and Klump 1984). In freshwater
sediments, methanogenesis accounts for 20 to 90%
of organic decomposition (Capone and Kiene
1988).
In marine sediments where the onset of methanogenesis is below the zone of bioturbation, CH4 production rates can be obtained by modeling concentration profiles provided that loss through bubble
ebullition, either in situ or as a result of decompression after sediment collection, can be excluded
(Reeburgh 1976,1983). Under these conditions and
the assumption of steady state, areal rates can also
be estimated from rates of CH 4 oxidation determined directly using radiolabeled 14CH4 (Reeburgh
1980). Acetate and H2 + CO 2 are generally found
to be the most important substrates for methanogenesis (Capone and Kiene 1988), and rates can
therefore be determined directly by monitoring the
incorporation of 14C into CH 4 from 14C-labeled acetate or CO 2 (Crill and Martens 1987; Kuivila et al.
1989). Methanogenesis co-occurring with SO~ - reduction is to a large extend attributed to "noncompetitive" substrates, mainly C 1 compounds such as
methyl amines, and can be quantified in incubations
with radio labeled versions of these compounds
(King et al. 1983; Ferdelman et al. 1997).
sediments have shown decreasing SO~ - reduction
rates over the initial 0.25 to 1 hour of incubation
after which rates became constant (Moeslund et al.
1994; Fossing 1995; Isaksen and Finster 1996).
This was interpreted as a result of reoxidation of
tracer from a small (undetectable) H2S pool that
rapidly became saturated with tracer, after which
the measured rate represented the rate of net incorporation of S into the solid reduced pools. The results were not conclusive, however, and further investigations are needed to constrain to what extent,
and under which conditions, the radiotracer technique may underestimate rates of SO~ - reduction
in oxidized sediments.
When the technique is used with freshwater sediments, special attention must be paid to an accurate
determination of SO~ - concentrations due to low
levels and steep gradients, and incubation times
must be kept short due to the brief turnover time of
the SO~- pool (e.g., Bak and Pfennig 1991; Roden
and Tuttle 1993b; Urban et al. 1994). High SO~reduction rates are in some cases measured at what
appears to be threshold level SO~ - concentrations
based on the concentration profile. Rapid recycling
of SO~ - has been suggested as an explanation, but
recently, such high rates have been found to significantly exceed total C mineralization rates corrected
for the contribution of methanogenesis, which indicated that the reduction rates were overestimated
(Roden and Wetzel 1996). It has been suggested
that the threshold concentrations represent a nonreducible SO~ - pool of unknown nature (Bak and
Pfennig 1991; Roden and Tuttle 1993b). A more
extensive comparison of total mineralization rates
measured as LC02 accumulation to the sum of individual degradative pathways, similar to the approach of Roden and Wetzel (Roden and Wetzel
1996), could serve as control on SO~ - reduction
rate measurements in freshwater sediments.
Sulfate reduction rates can also be determined
from SO~ - depletion in the incubation of sediment
in discrete depth intervals (e.g., Goldhaber et al.
1977; Swider and Mackin 1989; Bak and Pfennig
1991). This approach yields only net reduction
rates, however, it has far lower sensitivity than the
tracer technique, and it requires longer incubation
times. In the deep sea where SO~ - reduction is restricted to below the zone of bioturbation and irrigation, rates are more easily quantified from
SO~ - concentration profiles (Bender and Heggie
Bo Thamdrup and Donald E. Canfield
1984; Canfield 1991; but see also Parkes et al.
1995).
Methanogenesis
In the absence of inorganic electron acceptors, the
mineralization of organic matter will proceed by
fermentative processes with methanogenesis as the
terminal step. Due to competitive inhibition,
methanogenesis is insignificant when inorganic
electron acceptors are present (Lovley and Klug
1986; Capone and Kiene 1988). In marine sediments, therefore, the onset of methanogenesis generally lies far below the sediment surface where
little reactive organic matter is left, and decomposition through this pathway is estimated to be only
10% of the C oxidation through SO~- reduction
(Canfield 1993). Higher contributions are found in
coastal sediments with extreme deposition rates and
SO~ - depletion only a few decimeters below the
surface (Martens and Klump 1984). In freshwater
sediments, methanogenesis accounts for 20 to 90%
of organic decomposition (Capone and Kiene
1988).
In marine sediments where the onset of methanogenesis is below the zone of bioturbation, CH4 production rates can be obtained by modeling concentration profiles provided that loss through bubble
ebullition, either in situ or as a result of decompression after sediment collection, can be excluded
(Reeburgh 1976,1983). Under these conditions and
the assumption of steady state, areal rates can also
be estimated from rates of CH 4 oxidation determined directly using radiolabeled 14CH4 (Reeburgh
1980). Acetate and H2 + CO 2 are generally found
to be the most important substrates for methanogenesis (Capone and Kiene 1988), and rates can
therefore be determined directly by monitoring the
incorporation of 14C into CH 4 from 14C-labeled acetate or CO 2 (Crill and Martens 1987; Kuivila et al.
1989). Methanogenesis co-occurring with SO~ - reduction is to a large extend attributed to "noncompetitive" substrates, mainly C 1 compounds such as
methyl amines, and can be quantified in incubations
with radio labeled versions of these compounds
(King et al. 1983; Ferdelman et al. 1997).