86
3
Quantification of Early Diagenesis: Dissolved Constituents in Marine Pore Water
wards away from the zone of its release. This time,
however, the substance flux does not reach up to
the bottom water above the sediment, instead,
very low manganese concentrations are measured
at a depth which is only few centimeters below the
sediment surface. A flat positive gradient leads a
smaller fraction of the released manganese to
greater depth where it is withdrawn from the pore
water at about 14 to 15 m below the sediment surface.
Here as well, similar to the previously discussed example, both fluxes can be calculated.
The upwardly directed flux from the manganese release zone (J manganese, up ) is obtained from the gradient (-0.5 mol m -3 m -1 ), where in the upper zone the
sediment possesses an assumed porosity of 0.80.
Considering the values in Table 3.1 and 3.2, as
well as the Equation 3.5, a sedimentary diffusion
coefficient of D sed = 2.6·10 -10 m 2 s -1 yields the following manganese flux:
J manganese,up = - 0.80 · 2.6·10 -10 · (-0.5)
= 1.0·10 -10 [mol m -2 s -1 ]
(3.21)
or
J manganese,up = 1.0·10 -10 · 31,536,000
= 3.2·10 -3 [mol m -2 a -1 ]
(3.22)
Likewise, the downward-directed manganese flux
is calculated, however, taking a porosity degree of
φ = 0.60 into account and an accordingly calculated D sed = 1.9 E-10 m 2 s -1 :
J manganese,down
= - 0.60 · 1.9·10 -10 · 0.0084
= - 9.6·10 -13 [mol 1 m -2 s -1 ]
(3.23)
or
J manganese,down
= - 9.6·10 -13 · 31,536,000
= - 3.0·10 -5 [mol m -2 a -1 ]
(3.24)
Again, both fluxes added together constitute the
total release of dissolved manganese from the
sediment into the pore water. Since the downwarddirected flux is in this case almost two orders of
magnitude lower than the upward-directed flux, it
may be neglected considering the possible errors
occurring in the determination of the upward
stream. The release of manganese is equivalent to
the conversion of oxidized tetravalent manganese
into the soluble divalent manganese. Which substance is the electron donor in this reaction cannot be concluded from the manganese profile. It
could be organic matter as proposed by Froelich
et al. (1979). In this case we should not overlook
the fact that the converted substance amounts are
one order of magnitude lower than they are, for instance, in sulfate fluxes (Fig. 3.6), and more than
two orders of magnitude lower than they are in the
flux of oxygen. (Fig. 3.5). It should also be noted
that upon reducing one mole of Mn(IV) to Mn(II)
only two moles of electrons are exchanged,
whereas it amounts to 4 moles of electrons per
mole oxygen, and even 8 moles of electrons per
mole sulfate. Even the ‘impressive’ gradient of the
manganese profile shown in Figure 3.8 does not
represent an essential fraction of the overall diagenetic processes in the sediment, involved in
the oxidation of organic matter.
The upward-directed manganese flux does not
reach into the bottom water, instead, the Mn 2+ is
re-oxidized in a depth of only few centimeters below the sediment surface. Generally, one would exFig. 3.8 A manganese profile in pore water of sediments
off the Congo River estuary, in a water depth of approx.
4000 m. The profile is, in principle, quite similar to the
profile of nitrate previously shown in Figure 3.7. Here,
manganese is released into the pore water at a specific
depth below the sediment surface. A gradient with a high
negative slope leads most of the Mn 2+ upwards; another
gradient, positive and more level, conveys manganese
into a precipitation zone, which was just included in the
lowest core meter (between 14 and 15 m).
0
5
10
15
0
5 0
1 0 0
1 5 0
depth [m]
Mn [µmol/l]
GeoB 1401
g r a d = 0 .0 0 8 4 m o l/ m 4
grad = - 0.5 mol/m 3 m
3
Quantification of Early Diagenesis: Dissolved Constituents in Marine Pore Water
wards away from the zone of its release. This time,
however, the substance flux does not reach up to
the bottom water above the sediment, instead,
very low manganese concentrations are measured
at a depth which is only few centimeters below the
sediment surface. A flat positive gradient leads a
smaller fraction of the released manganese to
greater depth where it is withdrawn from the pore
water at about 14 to 15 m below the sediment surface.
Here as well, similar to the previously discussed example, both fluxes can be calculated.
The upwardly directed flux from the manganese release zone (J manganese, up ) is obtained from the gradient (-0.5 mol m -3 m -1 ), where in the upper zone the
sediment possesses an assumed porosity of 0.80.
Considering the values in Table 3.1 and 3.2, as
well as the Equation 3.5, a sedimentary diffusion
coefficient of D sed = 2.6·10 -10 m 2 s -1 yields the following manganese flux:
J manganese,up = - 0.80 · 2.6·10 -10 · (-0.5)
= 1.0·10 -10 [mol m -2 s -1 ]
(3.21)
or
J manganese,up = 1.0·10 -10 · 31,536,000
= 3.2·10 -3 [mol m -2 a -1 ]
(3.22)
Likewise, the downward-directed manganese flux
is calculated, however, taking a porosity degree of
φ = 0.60 into account and an accordingly calculated D sed = 1.9 E-10 m 2 s -1 :
J manganese,down
= - 0.60 · 1.9·10 -10 · 0.0084
= - 9.6·10 -13 [mol 1 m -2 s -1 ]
(3.23)
or
J manganese,down
= - 9.6·10 -13 · 31,536,000
= - 3.0·10 -5 [mol m -2 a -1 ]
(3.24)
Again, both fluxes added together constitute the
total release of dissolved manganese from the
sediment into the pore water. Since the downwarddirected flux is in this case almost two orders of
magnitude lower than the upward-directed flux, it
may be neglected considering the possible errors
occurring in the determination of the upward
stream. The release of manganese is equivalent to
the conversion of oxidized tetravalent manganese
into the soluble divalent manganese. Which substance is the electron donor in this reaction cannot be concluded from the manganese profile. It
could be organic matter as proposed by Froelich
et al. (1979). In this case we should not overlook
the fact that the converted substance amounts are
one order of magnitude lower than they are, for instance, in sulfate fluxes (Fig. 3.6), and more than
two orders of magnitude lower than they are in the
flux of oxygen. (Fig. 3.5). It should also be noted
that upon reducing one mole of Mn(IV) to Mn(II)
only two moles of electrons are exchanged,
whereas it amounts to 4 moles of electrons per
mole oxygen, and even 8 moles of electrons per
mole sulfate. Even the ‘impressive’ gradient of the
manganese profile shown in Figure 3.8 does not
represent an essential fraction of the overall diagenetic processes in the sediment, involved in
the oxidation of organic matter.
The upward-directed manganese flux does not
reach into the bottom water, instead, the Mn 2+ is
re-oxidized in a depth of only few centimeters below the sediment surface. Generally, one would exFig. 3.8 A manganese profile in pore water of sediments
off the Congo River estuary, in a water depth of approx.
4000 m. The profile is, in principle, quite similar to the
profile of nitrate previously shown in Figure 3.7. Here,
manganese is released into the pore water at a specific
depth below the sediment surface. A gradient with a high
negative slope leads most of the Mn 2+ upwards; another
gradient, positive and more level, conveys manganese
into a precipitation zone, which was just included in the
lowest core meter (between 14 and 15 m).
0
5
10
15
0
5 0
1 0 0
1 5 0
depth [m]
Mn [µmol/l]
GeoB 1401
g r a d = 0 .0 0 8 4 m o l/ m 4
grad = - 0.5 mol/m 3 m