Geology Reference
In-Depth Information
Atmosphere
5
3,4
Land
3,4
5
1
2
Oceans
Freshwater
7
6
Marine sediment/
rock
Figure 9 Schematic diagram of the major fluxes and compartments in a biogeochemical
cycle: (1) runoff; (2) streamflow; (3) degassing; (4) particle suspension; (5)
wet and dry deposition; (6) sedimentation and (7) remobilisation
reduction of elements such as iron and manganese, which has implica-
tions for their mobility and bioavailability.
Biological reduction processes in sediments may be viewed as the
oxidation of carbohydrate (in its simplest form CH
2
O) with accompa-
nying reduction of an oxygen carrier. In the first instance, dissolved
molecular oxygen is used. The reaction is thermodynamically favoured,
as reflected by the strongly negative DG.
CO
2
þ
H
2
O DG
¼
125.5 kJ mol
1
e
When all of the dissolved oxygen is consumed, anaerobic organisms take
over. Initially, nitrate-reducing bacteria are favoured
CH
2
O
þ
O
2
-
2CO
2
þ
H
2
O
þ
NH
4
DG
¼
82.2 kJ mol
1
e
Once the nitrate is utilised, sulfate reduction takes over
2CH
2
O
þ
NO
3
þ
2H
1
-
SO
2
4
þ
H
1
þ
2CH
2
O
HS
þ
2H
2
O
þ
2CO
2
DG
¼
25.6 kJ mol
1
e
-
