26
slight accumulation of particulate Mn at the surface sediment layer was identified at
Lauffen and Wieblingen. At Kochendorf, particulate Mn concentrations ranged from
360 to 760 mg/kg through the sediment core (Fig. 4.6).
The porewater and solid phase profiles of Mn are similar to those reported from
other sediments (Gendron et al. 1986; Johnson et al. 1992; Matsunaga et al. 1993).
These profiles demonstrate transport processes of Mn during the mineralizatign of
organic matter. Under anoxic conditions, Mn oxides are reduced and Mn z+ is
released into porewater:
(CH20) 113(NH3)t 5(H3PO4) + 248.5 MnO2 + 497 H +
248.5 Mn 2+ + 113 CO2 + 7.5 N2 + H3PO4 +384 H20
The release of Mn 2+ is limited by either the quantity of organic ~aatter or
abundance of available oxides. The high concentrations of dissolved Mn 2+ in the
Neckar River sediments at Wieblingen and in the Enz River sediments are probably
related to high cQntents of Mn in the solid phase (Fig. 4.6c, 4.7b).
9
2+
Dissolved Mn
in the porewat~r diffuses towards the surface sediment laver due
9
2+
.
.
.
.
.
".
9
to concentration gradients. As Mn
~s reoxldized as Mn oxides and ~mmobdlzed m
the surface oxic layer, an accumulation of particulate Mn in this layer should be
expected.
Mn 2+ + O2 ~ MnO2~"
The diagenetic enrichment of Mn in the surface layer has been reported from
marine sediments (Aller 1980b; Pedersen and Price 1982; Dahrnke et al. 1991).
However, only a slight accumulation of Mn was found in the sediments at Lauffen,
Wieblingen, and in the Enz River sediments, but no peak of Mn was measured at
Kochendorf and in the Elsenz River sediments. This might be the result of sediment
mixing processes by bioturbation and hydrodynamic forces.
2+.
The concentrations of Mn
m the porewater are influenced not only by redox
reactions, but also by a series of precipitation and dissolution of Mn minerals
(Matsunaga et al. 1993):
rhodochrosite:
Mn 2+ + CO32- ~ MnCO 3 ~,
alabandite "
Mn 2+ + HS" --~ MnS ,1, + H +
The Saturation Index (SI) of possible minerals was calculated by ion activity
products (IAP) in comparison to thermodynamic equilibrium constant. Corrections
were made for ionic strength. For instance, the SI of rhodochrosite (MnCO3) is
def'med as:
SI = Log (lAP/Ksp)
= Log(aMn aCO 3 /Ksp) _
= tog {([Mn
[co3
/ sp}
where LAP: Ion Activity Products,
Ksp: equilibrium constant for MnCO3,
2+
2aMn,aCo3,: activity of Mn and CO 3 in solution,
-#.
,
.
yZ~, ]re : acttvlty coefficient used to correct the concentration data to
activity,
f I2+
2[Mn2+], [CO32"]: the measured concentration o N n and CO 3
slight accumulation of particulate Mn at the surface sediment layer was identified at
Lauffen and Wieblingen. At Kochendorf, particulate Mn concentrations ranged from
360 to 760 mg/kg through the sediment core (Fig. 4.6).
The porewater and solid phase profiles of Mn are similar to those reported from
other sediments (Gendron et al. 1986; Johnson et al. 1992; Matsunaga et al. 1993).
These profiles demonstrate transport processes of Mn during the mineralizatign of
organic matter. Under anoxic conditions, Mn oxides are reduced and Mn z+ is
released into porewater:
(CH20) 113(NH3)t 5(H3PO4) + 248.5 MnO2 + 497 H +
248.5 Mn 2+ + 113 CO2 + 7.5 N2 + H3PO4 +384 H20
The release of Mn 2+ is limited by either the quantity of organic ~aatter or
abundance of available oxides. The high concentrations of dissolved Mn 2+ in the
Neckar River sediments at Wieblingen and in the Enz River sediments are probably
related to high cQntents of Mn in the solid phase (Fig. 4.6c, 4.7b).
9
2+
Dissolved Mn
in the porewat~r diffuses towards the surface sediment laver due
9
2+
.
.
.
.
.
".
9
to concentration gradients. As Mn
~s reoxldized as Mn oxides and ~mmobdlzed m
the surface oxic layer, an accumulation of particulate Mn in this layer should be
expected.
Mn 2+ + O2 ~ MnO2~"
The diagenetic enrichment of Mn in the surface layer has been reported from
marine sediments (Aller 1980b; Pedersen and Price 1982; Dahrnke et al. 1991).
However, only a slight accumulation of Mn was found in the sediments at Lauffen,
Wieblingen, and in the Enz River sediments, but no peak of Mn was measured at
Kochendorf and in the Elsenz River sediments. This might be the result of sediment
mixing processes by bioturbation and hydrodynamic forces.
2+.
The concentrations of Mn
m the porewater are influenced not only by redox
reactions, but also by a series of precipitation and dissolution of Mn minerals
(Matsunaga et al. 1993):
rhodochrosite:
Mn 2+ + CO32- ~ MnCO 3 ~,
alabandite "
Mn 2+ + HS" --~ MnS ,1, + H +
The Saturation Index (SI) of possible minerals was calculated by ion activity
products (IAP) in comparison to thermodynamic equilibrium constant. Corrections
were made for ionic strength. For instance, the SI of rhodochrosite (MnCO3) is
def'med as:
SI = Log (lAP/Ksp)
= Log(aMn aCO 3 /Ksp) _
= tog {([Mn
[co3
/ sp}
where LAP: Ion Activity Products,
Ksp: equilibrium constant for MnCO3,
2+
2aMn,aCo3,: activity of Mn and CO 3 in solution,
-#.
,
.
yZ~, ]re : acttvlty coefficient used to correct the concentration data to
activity,
f I2+
2[Mn2+], [CO32"]: the measured concentration o N n and CO 3