Geology Reference
In-Depth Information
seasonal influences,
modes of element introduction and transport,
element associations in the water column,
depositional and redistributional processes and rates (e.g. via
radionuclide and stable isotope studies) in the sediments,
concentration profiles and speciation in the overlying water, solid
sediment, and pore water,
element/solid phase associations (e.g. via sequential chemical
extraction procedures), and
authigenic mineral identification in the sediments.
While opinions may differ as to whether particular sediment columns
have been disturbed by mixing or the extent to which certain elements are
vertically mobile, it is accepted that post-depositional redox-controlled
cycling can affect the vertical profiles of manganese and iron.
111,112
The
driving force is the microbiological decomposition of organic matter
(represented as CH
2
O) through bacterial utilization of O
2
and inorganic
oxidizing agents in thermodynamically favoured sequence.
113
DG
0
(kJ mol
1
CH
2
O)
(3.99)
CH
2
O
þ
O
2(g)
" CO
2(g)
þ
H
2
O
475
(3.100)
5CH
2
O
þ
4NO
3
" 2N
2(g)
þ
4HCO
3
þ
CO
2(g)
þ
3H
2
O
448
CH
2
O
þ
3CO
2(g)
þ
H
2
O
þ
2MnO
2(s)
" 2Mn
21
þ
4HCO
3
(3.101)
349
CH
2
O
þ
7CO
2(g)
þ
4Fe(OH)
3(s)
" 4Fe
21
þ
8HCO
3
þ
3H
2
O
(3.102)
114
2CH
2
O
þ
SO
2
4
"
H
2
S
(g)
þ
2HCO
3
(3.103)
77
(3.104)
2CH
2
O
"
CH
4(g)
þ
CO
2(g)
58
Upon dissolution of oxides and (hydr)oxides of manganese and iron
under reducing conditions at depth, divalent cations of these elements
can diffuse upwards through the pore waters to be oxidized and
precipitated in near-surface oxic layers, leading to the characteristic
near-surface enrichment of manganese and iron in the sediments of
many well oxygenated lakes (see Section 3.2.4.3). Phosphorus and
