Geoscience Reference
In-Depth Information
Iron oxyhydroxide can react with water.
74
FeOOH
(S)
+
H
2
O
→
Fe(OH)
3
(S)
(4.16)
4.1.4.1.4.3 Manganese Oxidation
Mn
2+
compounds are more soluble than Mn
4+
compounds, and therefore exist in the
mg l
−1
range in sediment pore waters. As a consequence, Mn
2+
can diffuse to the
oxic layer of the sediment, and via the loss of two electrons, can be oxidized to
Mn
4+
by the oxygen present there.
74
Mn
2+
→
Mn
4+
+
2e
−
(4.17)
Half-reaction for oxygen is
O
2
+
4H
+
+
4e
−
→
2H
2
O
(4.18)
and the oxidation of manganese can be given as
1
2
Mn
2+
+
O
2
+
2H
+
→
Mn
4+
+
H
2
O
(4.19)
The Mn
4+
that is formed precipitates as manganese dioxide, and the overall redox
reaction is
1
2
Mn
2+
+
O
2
+
H
2
O
→
MnO
2(s)
+
2H
+
(4.20)
This reaction occurs under aerobic conditions.
74
4.1.4.1.5 Sediment Oxygen Demand
The decomposition of organic material and bioturbation activity of the macrofauna
result in the exertion of an oxygen demand at the sediment-water interface.
38
This
process can have profound effects on the dissolved oxygen concentration in the
overlying waters. Oxygen diffuses from the overlying water into the sediment;
therefore, organic matter decomposes aerobically in the upper sediment layers that
are in direct contact with water.
64
The aerobic surface layer usually has a thickness
of only a few millimeters. This is true even for sediment overlaid by oxygen-rich
water because of the slow diffusion rates of oxygen.
19,27
Oxygen uptake is regulated by the net deposition and degradability of organic
matter.
19
In eutrophic systems, phytoplankton settling can have a significant effect on
SOD levels.
64
Microbial benthic mineralization is a major recycling pathway of settled
particles in shallow marine environments. Increased organic loading due to sedimen-
tation increases oxygen consumption, which in turn leads to dissolved oxygen deple-
tion within the sediment.
28
This depletion of oxygen causes SOD to cease.
64
In Thau
Lagoon, France, the accumulation of organic matter in quantities that exceed the
mineralization capacity of the sediment, high temperatures, and the absence of wind
trigger anaerobic bottom conditions.
28,38
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