Geoscience Reference
In-Depth Information
Seawater
Oxic zone: respiration and nitrification
CH
2
O
+
O
2
⇔
CO
2
+
H
2
O
+
-
NH
4
+
2O
2
⇔
NO
3
+
HO
+
2H
+
Denitrification
-
-
2CH
2
O
+
2NO
3
⇔
N
2
+
2HCO
3
+
H
2
O
-
CH
2
O
+
2MnO
2
+ H
2
O
⇔
2Mn
2+
+
HCO
3
+
3OH
-
(solid)
-
4Fe
2+
+
HCO
3
+
7OH
-
+
3H
2
O
CH
2
O
+
4Fe(OH)
3
⇔
(solid)
2-
H
2
S
+
2HCO
-
2CH
2
O
+
SO
4
⇔
Methanogenesis
2CH
2
O
⇔
CH
4
+
CO
2
v
Figure 8.9
The successive reactions of early diagenesis below the sediment-water interface. Interstitial water
from the upper layer is ventilated by burrowing animals. Bacteria use the dissimilatory reduction
of nitrate, solid MnO
2
,andFe(OH)
3
, then sulfate still present in interstitial water to oxidize dead
organic material, here represented by the molecule CH
2
O. The inception of methanogenesis
requires that the sediments were originally rich in organic material. The section represents only a
few tens of cm but the whole pile is moving downwards at the rate of sedimentation.
Denitrification is associated with progressive loss of N
2
, sulfate reduction with progres-
sive sulfide precipitation, and methanogenesis with methane seeping: Rayleigh distillation
amplifies isotopic fractionation between the reduced and the oxidized reservoirs of N,
S, and C with respect to equilibrium values and leads to major isotopic shifts in the
residue. Most of the methane produced by methanogene, however, is converted back into
organic matter by methanotrophic archæa and never reaches the sediment-water interface.
Methanotrophs are often associated with acetogenic bacteria which release acetate as an
end-product of fermentation. Such associations in which a species utilizes the poisonous
by-product of another are known as consortia.
Dead organisms and fecal pellets are highly unstable and prone to utilization as biologi-
cal fuel by other organisms. Hydrolysis of the phosphate groups and oxidation of the amino
group take place very rapidly during decay, while most carbohydrates such as cellulose
disappear during fermentation. At low temperature, the most resistent part of the origi-
nal organic components are (a) the long aliphatic chains of lipids, saturated (no double
