6. Benthic Respiration in Aquatic Sediments
mineralization (Jorgensen 1978; Mackin and
Swider 1989). In recent experiments with coastal
Costa Rican, Danish, and Northeast Greenland sediments, we have noticed a tendency for higher
SO~ - reduction rates in the homogenized sediment
compared with whole-core incubations (Thamdrup
et al. 1996; Rysgaard et al. 1998; B. Thamdrup unpublished results). The source of these differences
is not yet clear. Homogenization could stimulate
mineralization by relieving metabolite inhibition of
bacterial metabolism around organic-rich microenvironments, but the observed differences could
also be due to an undersampling of active hot-spots
in the small cores (3 to 4 cm i.d.) used for wholecore SO~ - reduction measurements.
The internal consistency of the results from the
incubation technique has been confirmed by multicomponent diagenetic modeling (Wang and Van
Cappellen 1996). The incubation technique is very
work intensive, but it has provided quantitative evidence for the importance of Mn and, in particular,
Fe reduction for benthic C oxidation in a wide variety of continental margin sediments, and at the
moment it appears to be the only way to obtain
good estimates of the rates of these processes in
sediments where SO~ - reduction is of importance.
Sulfate Reduction
Benthic SO~- reduction has been extensively studied (J\ilrgensen 1982; Howarth 1984; Skyring 1987).
The process is exclusively catalyzed by SO~-reducing bacteria that couple it to the oxidation of
organic matter or H2, and produce H2S as the only
immediate extracellular sulfur product (Widdel and
Hansen 1991). Intense SO~- reduction can, however, occur without detectable accumulation ofH 2 S
due to rapid precipitation of iron sulfides and SO
(e.g., Canfield 1989; Thamdrupetal. 1994a). Sulfate
reduction typically contributes about 50% of the C
oxidation in shallow coastal sediments with oxic
bottom water (Jorgensen 1982), and the contribution
may be even higher in salt marsh and sheltered subtidal sediments (Howarth 1984; Mackin and Swider
1989), whereas in the deep sea it is of little importance (Jorgensen 1982; Canfield 1991; Canfield
1993). In freshwater sediments, the process is limited by the shallow penetration depth of SO~ - which
results from low SO~ - concentrations in freshwater,
95
though volume-specific rates can be as high as in
marine sediments (e.g., Capone and Kiene 1988;
Bak and Pfennig 1991).
The most widespread assay for SO~ - reduction
is the radiotracer technique (Sorokin 1962; Jorgensen 1978). Microliter volumes of carrier-free
35S0~ - tracer are injected into intact sediment
cores, and incubation times are typically from 1
hour to 1 day. Injections have been performed at
the seafloor using a benthic lander (Greeff et al.
1998). Since the discovery that significant amounts
of tracer may be incorporated into pyrite and SO
(Howarth 1979; Howarth and Jorgensen 1984), reduced radiolabeled S has been recovered by distillation with a hot acidic chromous solution that reduces these species to H 2 S (Zhabina and Volkov
1978; Fossing and Jorgensen 1989). A small
amount of radiolabel may be distilled from unreacted 35S0~- tracer, possibly due to slight contamination from other S species, and it is therefore
advisable to include a zero-time blank (Howarth
and Jorgensen 1984; Spratt et al. 1987; Roden and
Tuttle 1993b). In freshwater sediments, 10 to 40%
of total reduced tracer in SO~ - reduction rate incubations has been recovered as organic C-bonded
S, quantified as the residue after sequential distillation with chromium (Cr+) and hydroiodic acid
(Wieder and Lang 1988; Spratt and Morgan 1990;
Roden and Wetzel 1996).
The radiotracer technique has been validated by
comparison to SO~- depletion in long-term incubations (J\ilrgensen 1978; Westrich 1983; Crill and
Martens 1987; Canfield and Thamdrup 1996), to
sulfide accumulation or benthic H 2 S fluxes (Jorgensen 1978; Chanton et al. 1987; Roden and Tuttle
1993a), and to total C mineralization rates (Thamdrup and Canfield 1996). With the radiotracertechnique, SO~ - reduction rates can be measured in
oxidized and even oxic sediment where sulfide
reoxidation is possible (Canfield and Des Marais
1991; Jorgensen and Bak 1991). Determination of
SO~ - reduction rates is possible with concomitant
reoxidation when the reduced tracer is diluted into
an unlabeled pool of reduced S compounds, and the
incubation time is kept short relative to the turnover
time of this pool (Fossing et al. 1995). The dependence of the rate on incubation time can be used to
evaluate the effects of reoxidation on the measurements (Jorgensen 1978; Canfield and Des Marais
1993). Detailed time courses in oxidized surface
mineralization (Jorgensen 1978; Mackin and
Swider 1989). In recent experiments with coastal
Costa Rican, Danish, and Northeast Greenland sediments, we have noticed a tendency for higher
SO~ - reduction rates in the homogenized sediment
compared with whole-core incubations (Thamdrup
et al. 1996; Rysgaard et al. 1998; B. Thamdrup unpublished results). The source of these differences
is not yet clear. Homogenization could stimulate
mineralization by relieving metabolite inhibition of
bacterial metabolism around organic-rich microenvironments, but the observed differences could
also be due to an undersampling of active hot-spots
in the small cores (3 to 4 cm i.d.) used for wholecore SO~ - reduction measurements.
The internal consistency of the results from the
incubation technique has been confirmed by multicomponent diagenetic modeling (Wang and Van
Cappellen 1996). The incubation technique is very
work intensive, but it has provided quantitative evidence for the importance of Mn and, in particular,
Fe reduction for benthic C oxidation in a wide variety of continental margin sediments, and at the
moment it appears to be the only way to obtain
good estimates of the rates of these processes in
sediments where SO~ - reduction is of importance.
Sulfate Reduction
Benthic SO~- reduction has been extensively studied (J\ilrgensen 1982; Howarth 1984; Skyring 1987).
The process is exclusively catalyzed by SO~-reducing bacteria that couple it to the oxidation of
organic matter or H2, and produce H2S as the only
immediate extracellular sulfur product (Widdel and
Hansen 1991). Intense SO~- reduction can, however, occur without detectable accumulation ofH 2 S
due to rapid precipitation of iron sulfides and SO
(e.g., Canfield 1989; Thamdrupetal. 1994a). Sulfate
reduction typically contributes about 50% of the C
oxidation in shallow coastal sediments with oxic
bottom water (Jorgensen 1982), and the contribution
may be even higher in salt marsh and sheltered subtidal sediments (Howarth 1984; Mackin and Swider
1989), whereas in the deep sea it is of little importance (Jorgensen 1982; Canfield 1991; Canfield
1993). In freshwater sediments, the process is limited by the shallow penetration depth of SO~ - which
results from low SO~ - concentrations in freshwater,
95
though volume-specific rates can be as high as in
marine sediments (e.g., Capone and Kiene 1988;
Bak and Pfennig 1991).
The most widespread assay for SO~ - reduction
is the radiotracer technique (Sorokin 1962; Jorgensen 1978). Microliter volumes of carrier-free
35S0~ - tracer are injected into intact sediment
cores, and incubation times are typically from 1
hour to 1 day. Injections have been performed at
the seafloor using a benthic lander (Greeff et al.
1998). Since the discovery that significant amounts
of tracer may be incorporated into pyrite and SO
(Howarth 1979; Howarth and Jorgensen 1984), reduced radiolabeled S has been recovered by distillation with a hot acidic chromous solution that reduces these species to H 2 S (Zhabina and Volkov
1978; Fossing and Jorgensen 1989). A small
amount of radiolabel may be distilled from unreacted 35S0~- tracer, possibly due to slight contamination from other S species, and it is therefore
advisable to include a zero-time blank (Howarth
and Jorgensen 1984; Spratt et al. 1987; Roden and
Tuttle 1993b). In freshwater sediments, 10 to 40%
of total reduced tracer in SO~ - reduction rate incubations has been recovered as organic C-bonded
S, quantified as the residue after sequential distillation with chromium (Cr+) and hydroiodic acid
(Wieder and Lang 1988; Spratt and Morgan 1990;
Roden and Wetzel 1996).
The radiotracer technique has been validated by
comparison to SO~- depletion in long-term incubations (J\ilrgensen 1978; Westrich 1983; Crill and
Martens 1987; Canfield and Thamdrup 1996), to
sulfide accumulation or benthic H 2 S fluxes (Jorgensen 1978; Chanton et al. 1987; Roden and Tuttle
1993a), and to total C mineralization rates (Thamdrup and Canfield 1996). With the radiotracertechnique, SO~ - reduction rates can be measured in
oxidized and even oxic sediment where sulfide
reoxidation is possible (Canfield and Des Marais
1991; Jorgensen and Bak 1991). Determination of
SO~ - reduction rates is possible with concomitant
reoxidation when the reduced tracer is diluted into
an unlabeled pool of reduced S compounds, and the
incubation time is kept short relative to the turnover
time of this pool (Fossing et al. 1995). The dependence of the rate on incubation time can be used to
evaluate the effects of reoxidation on the measurements (Jorgensen 1978; Canfield and Des Marais
1993). Detailed time courses in oxidized surface