85
The maximum at a specific depth below sea-level
is the result of oxidation of organic material by the
oxygen that diffuses into the sediment zone from
above. Here, the nitrogen of the organic material is
converted into nitrate. Mostly, a more pronounced
gradient transports the major proportion of nitrate
upwards into the bottom water. The smaller
proportion travels downwards along a shallow
gradient where it is finally consumed as an
electron acceptor in the oxidation of other substances.
All these processes can be derived directly
and quantitatively from Figure 3.7. Since they are
bound to a sedimentary zone which lies very close
to the surface, a high degree of porosity can be
assumed. According to Table 3.2, the porosity φ =
0.80 corresponds to a tortuosity (θ 2 ) of 1.45. Table
3.1 shows that the diffusion coefficient for nitrate
in free sea-water at 5 °C amounts to D sw =
1.08·10 -9 m 2 s -1 . Applying Equation 3.5 yields a
sedimentary diffusion coefficient of D sed =
7.4·10 -10 m 2 s -1 . Hence, the diffusive nitrate flux
from the sediment to the bottom water compartment is calculated as:
J nitrate,up = - 0.80 · 7.4·10 -10 · (-8.4)
= 5.0·10 -9 [mol m -2 s -1 ]
(3.14)
or as
J nitrate,up = 5.0·10 -9 · 31,536,000
= 0.16 [mol m -2 a -1 ]
(3.15)
The nitrate flux in a downward direction is calculated accordingly:
J nitrate,down
= - 0.80 · 7.4·10 -10 · 2.5
= - 1.5·10 -9 [mol m -2 s -1 ]
(3.16)
or as
J nitrate,down
= - 1.5·10 -9 · 31,536,000
= - 0.047 [mol m -2 a -1 ]
(3.17)
The sum of both fluxes yields a minimal estimate value of the total nitrate concentration released from organic matter due to its reaction with
oxygen. However, the real value for the total
amount of released nitrate must be higher than the
sum of both calculated fluxes. The gradient of the
downward directed flux may be quite reliably calculated from numerous points, however, the more
pronounced gradient of the flux leading upward
into the bottom water consists only of two points,
one of which merely represents the concentration
in the bottom water whereas the other represents
the pore water of the uppermost 0.5 cm of sediment. It is probably correct to assume that the gradient is more pronounced in closer proximity to
the sediment surface, and hence the flux should
also prove to be more enhanced. A more accurate
statement would only be possible under in-situ
conditions with a depth resolution similar to the
oxygen profile shown in Figure 3.5. As for measurements performed under ex-situ conditions, a
better depth resolution than shown in Figure 3.7 is
very hard to obtain.
If we assume that the released nitrate exclusively originates from the oxidation of organic
matter, and that the organic matter is oxidized in a
manner in which the C:N ratio corresponds to the
Redfield-ratio of 106:16 (cf. Sect. 3.2.5), then we
can also calculate the conversion rate of organic
matter on the basis of the nitrate profile:
R ox,Corg = [abs(J nitrate,up )
+ abs(J nitrate,down )] · (106/16)
(3.18)
or by employing the values of the above example:
R ox,Corg = ( 0.16 + 0.047) · (106/16)
= 1.37 [mol m -2 a -1 ]
(3.19)
or
R ox,Corg = 16.5 [gC m -2 a -1 ]
(3.20)
At any rate, such a value might serve only as a
rough estimation, since apart from the aforementioned error, the calculation procedure implicitly
contains some specific assumptions. It has been
already mentioned that the C:N ratio of the oxidized material is supposed to be (106/16 = 6.625).
Publications of Hensen et al. (1997), however,
indicate that especially in sediments with a rich
abundance of organic matter a distinctly lower
ratio of almost 3 is imaginable (cf. Chap. 6). It
must also be considered that the measured
profiles are not just influenced by diffusion in
the surface zones, but also by the processes of
bioturbation and bioirrigation (cf. Sect. 3.6.2).
In principle, the shape of the manganese profile shown in Figure 3.8 is not dissimilar to the nitrate profile of Figure 3.7. Here, we again identify
the zones of maximum concentrations – in this
particular case about 0.5 m below the sea-floor
level – as the site of dissolved manganese release
into the pore water. Again, a pronounced negative
gradient transports the dissolved manganese up3.2
Calculation of Diffusive Fluxes and Diagenetic Reaction Rates
The maximum at a specific depth below sea-level
is the result of oxidation of organic material by the
oxygen that diffuses into the sediment zone from
above. Here, the nitrogen of the organic material is
converted into nitrate. Mostly, a more pronounced
gradient transports the major proportion of nitrate
upwards into the bottom water. The smaller
proportion travels downwards along a shallow
gradient where it is finally consumed as an
electron acceptor in the oxidation of other substances.
All these processes can be derived directly
and quantitatively from Figure 3.7. Since they are
bound to a sedimentary zone which lies very close
to the surface, a high degree of porosity can be
assumed. According to Table 3.2, the porosity φ =
0.80 corresponds to a tortuosity (θ 2 ) of 1.45. Table
3.1 shows that the diffusion coefficient for nitrate
in free sea-water at 5 °C amounts to D sw =
1.08·10 -9 m 2 s -1 . Applying Equation 3.5 yields a
sedimentary diffusion coefficient of D sed =
7.4·10 -10 m 2 s -1 . Hence, the diffusive nitrate flux
from the sediment to the bottom water compartment is calculated as:
J nitrate,up = - 0.80 · 7.4·10 -10 · (-8.4)
= 5.0·10 -9 [mol m -2 s -1 ]
(3.14)
or as
J nitrate,up = 5.0·10 -9 · 31,536,000
= 0.16 [mol m -2 a -1 ]
(3.15)
The nitrate flux in a downward direction is calculated accordingly:
J nitrate,down
= - 0.80 · 7.4·10 -10 · 2.5
= - 1.5·10 -9 [mol m -2 s -1 ]
(3.16)
or as
J nitrate,down
= - 1.5·10 -9 · 31,536,000
= - 0.047 [mol m -2 a -1 ]
(3.17)
The sum of both fluxes yields a minimal estimate value of the total nitrate concentration released from organic matter due to its reaction with
oxygen. However, the real value for the total
amount of released nitrate must be higher than the
sum of both calculated fluxes. The gradient of the
downward directed flux may be quite reliably calculated from numerous points, however, the more
pronounced gradient of the flux leading upward
into the bottom water consists only of two points,
one of which merely represents the concentration
in the bottom water whereas the other represents
the pore water of the uppermost 0.5 cm of sediment. It is probably correct to assume that the gradient is more pronounced in closer proximity to
the sediment surface, and hence the flux should
also prove to be more enhanced. A more accurate
statement would only be possible under in-situ
conditions with a depth resolution similar to the
oxygen profile shown in Figure 3.5. As for measurements performed under ex-situ conditions, a
better depth resolution than shown in Figure 3.7 is
very hard to obtain.
If we assume that the released nitrate exclusively originates from the oxidation of organic
matter, and that the organic matter is oxidized in a
manner in which the C:N ratio corresponds to the
Redfield-ratio of 106:16 (cf. Sect. 3.2.5), then we
can also calculate the conversion rate of organic
matter on the basis of the nitrate profile:
R ox,Corg = [abs(J nitrate,up )
+ abs(J nitrate,down )] · (106/16)
(3.18)
or by employing the values of the above example:
R ox,Corg = ( 0.16 + 0.047) · (106/16)
= 1.37 [mol m -2 a -1 ]
(3.19)
or
R ox,Corg = 16.5 [gC m -2 a -1 ]
(3.20)
At any rate, such a value might serve only as a
rough estimation, since apart from the aforementioned error, the calculation procedure implicitly
contains some specific assumptions. It has been
already mentioned that the C:N ratio of the oxidized material is supposed to be (106/16 = 6.625).
Publications of Hensen et al. (1997), however,
indicate that especially in sediments with a rich
abundance of organic matter a distinctly lower
ratio of almost 3 is imaginable (cf. Chap. 6). It
must also be considered that the measured
profiles are not just influenced by diffusion in
the surface zones, but also by the processes of
bioturbation and bioirrigation (cf. Sect. 3.6.2).
In principle, the shape of the manganese profile shown in Figure 3.8 is not dissimilar to the nitrate profile of Figure 3.7. Here, we again identify
the zones of maximum concentrations – in this
particular case about 0.5 m below the sea-floor
level – as the site of dissolved manganese release
into the pore water. Again, a pronounced negative
gradient transports the dissolved manganese up3.2
Calculation of Diffusive Fluxes and Diagenetic Reaction Rates