94
been determined in sediment incubations to calculate manganese reduction rates (Canfield 1993b).
The available techniques are, however, only applicable to very Mn-rich sediments.
The only experimental approach to determine in
situ rates of organotrophic Mn and Fe reduction in
sediments presented so far is based on a comparison
of rates of total C oxidation and of SO~ - reduction
in anoxic incubations of homogenized sediment
sectioned into discrete depth intervals (Fig. 6.2;
Canfield et al. 1993a,b; Aller 1994; Thamdrup and
Canfield 1996). Total C oxidation rates are determined from the accumulation of LC02 (see previous section on other total mineralization assays),
and SO~ - reduction rates are measured in subsampIes by the 35S0~- -technique (see following section). The C oxidation rate corresponding to the
measured SO~ - reduction is calculated and subtracted from the total C oxidation rate. Based on
the measured depth distributions of O2 and NO; ,
a zone where only Mn and Fe reduction can account
for the excess C oxidation may be distinguished.
With further support from the depth distributions of
reactive Mn and Fe oxides and from the accumulation of Mn 2 + and Fe 2 + during the incubation, the
contributions of Mn and Fe reduction to C oxidation can be partitioned. The sediment is incubated
with no head-space in "Wtirgler" bags of laminated
gas-tight plastic (Kruse 1993). In this way, all samples are taken from the same batch of sediment,
which can be mixed intermittently. This minimizes
Bo Thamdrup and Donald E. Canfield
the variability among samples considerably and
thereby increases the total precision, which allows
for brief incubation times of one to a few days with
coastal sediments. Short incubation times preclude
the depletion of the reactive Mn and Fe pools during the incubation and minimize other effects of the
inhibition of vertical transport caused by discrete
interval sediment sampling.
One limitation of this method is that Mn and Fe
reduction in, and often a little below, the 021N0;
zone cannot be quantified due to a limited spatial
resolution of 0.5 to 1 cm. The potential underestimation is probably larger for Mn since Mn cycling
is often focused very close to the oxic-anoxic interface (e.g., Canfield et al. 1993b; Thamdrup et al.
1994a). A further concern is associated with the
stoichiometry of C oxidation by SO~ - reduction,
but considering the crudeness of the method in general this potential error is relatively small.
The greatest consideration about the accuracy of
the method concerns the effect of sediment homogenization on C oxidation rates. In our study off
Chile (Thamdrup and Canfield 1996), there was a
general agreement between C oxidation rates and
in situ LC02 fluxes where both parameters were
measured, and also between SO~- reduction rates
in incubations of homogenized sediment and intact
sediment cores (Ferdelman et al. 1997). Close
matches have previously been reported between intact cores and homogenized (but not slurried) sediment for rates of both SO~ - reduction and total
Carbon oxidation rate, nmol em.:! d-1
Nitrate, JiM
FIGURE 6.2. Results from anoxic
bag incubation experiments with
sediment from a station at 2000-m
depth off Chile (data from Thamdrup
and Canfield 1996). Left: Total C oxidation rates (measured as CO2 production), and C oxidation rates due
to SO~- reduction (measured with
35S0~-). Right: Depth distribution
of oxidants. Near the surface, SO~reduction contributes little to carbon
oxidation but at 9-cm depth, all carbon oxidation is explained by SO~reduction. Below 1.5 cm, Fe(III) is
the only significant oxidant other
than SO~-, and the excess C oxidation below this depth is attributed to
Fe reduction.
0
50
100
150
200 0
20
40
60
0
2
t
Oxie
E 4
<.l
.&:."
C. II) 6
0
•
N0 3 '
a
0
Total
o Mn
• Bysullate
• Fe(l ll)
10
0.0 0.1
0.2 0.3 0.4 0.5
Reactive Mn, pmol cm· 3
0
25
50
75 100 125
Oxalate extractable Fa(ill), pmol cm.:!
been determined in sediment incubations to calculate manganese reduction rates (Canfield 1993b).
The available techniques are, however, only applicable to very Mn-rich sediments.
The only experimental approach to determine in
situ rates of organotrophic Mn and Fe reduction in
sediments presented so far is based on a comparison
of rates of total C oxidation and of SO~ - reduction
in anoxic incubations of homogenized sediment
sectioned into discrete depth intervals (Fig. 6.2;
Canfield et al. 1993a,b; Aller 1994; Thamdrup and
Canfield 1996). Total C oxidation rates are determined from the accumulation of LC02 (see previous section on other total mineralization assays),
and SO~ - reduction rates are measured in subsampIes by the 35S0~- -technique (see following section). The C oxidation rate corresponding to the
measured SO~ - reduction is calculated and subtracted from the total C oxidation rate. Based on
the measured depth distributions of O2 and NO; ,
a zone where only Mn and Fe reduction can account
for the excess C oxidation may be distinguished.
With further support from the depth distributions of
reactive Mn and Fe oxides and from the accumulation of Mn 2 + and Fe 2 + during the incubation, the
contributions of Mn and Fe reduction to C oxidation can be partitioned. The sediment is incubated
with no head-space in "Wtirgler" bags of laminated
gas-tight plastic (Kruse 1993). In this way, all samples are taken from the same batch of sediment,
which can be mixed intermittently. This minimizes
Bo Thamdrup and Donald E. Canfield
the variability among samples considerably and
thereby increases the total precision, which allows
for brief incubation times of one to a few days with
coastal sediments. Short incubation times preclude
the depletion of the reactive Mn and Fe pools during the incubation and minimize other effects of the
inhibition of vertical transport caused by discrete
interval sediment sampling.
One limitation of this method is that Mn and Fe
reduction in, and often a little below, the 021N0;
zone cannot be quantified due to a limited spatial
resolution of 0.5 to 1 cm. The potential underestimation is probably larger for Mn since Mn cycling
is often focused very close to the oxic-anoxic interface (e.g., Canfield et al. 1993b; Thamdrup et al.
1994a). A further concern is associated with the
stoichiometry of C oxidation by SO~ - reduction,
but considering the crudeness of the method in general this potential error is relatively small.
The greatest consideration about the accuracy of
the method concerns the effect of sediment homogenization on C oxidation rates. In our study off
Chile (Thamdrup and Canfield 1996), there was a
general agreement between C oxidation rates and
in situ LC02 fluxes where both parameters were
measured, and also between SO~- reduction rates
in incubations of homogenized sediment and intact
sediment cores (Ferdelman et al. 1997). Close
matches have previously been reported between intact cores and homogenized (but not slurried) sediment for rates of both SO~ - reduction and total
Carbon oxidation rate, nmol em.:! d-1
Nitrate, JiM
FIGURE 6.2. Results from anoxic
bag incubation experiments with
sediment from a station at 2000-m
depth off Chile (data from Thamdrup
and Canfield 1996). Left: Total C oxidation rates (measured as CO2 production), and C oxidation rates due
to SO~- reduction (measured with
35S0~-). Right: Depth distribution
of oxidants. Near the surface, SO~reduction contributes little to carbon
oxidation but at 9-cm depth, all carbon oxidation is explained by SO~reduction. Below 1.5 cm, Fe(III) is
the only significant oxidant other
than SO~-, and the excess C oxidation below this depth is attributed to
Fe reduction.
0
50
100
150
200 0
20
40
60
0
2
t
Oxie
E 4
<.l
.&:."
C. II) 6
0
•
N0 3 '
a
0
Total
o Mn
• Bysullate
• Fe(l ll)
10
0.0 0.1
0.2 0.3 0.4 0.5
Reactive Mn, pmol cm· 3
0
25
50
75 100 125
Oxalate extractable Fa(ill), pmol cm.:!