Geology Reference
In-Depth Information
Finally, methane-producing organisms dominate in a sediment depleted
in oxygen, nitrate, and sulfate.
2CH
2
O
CH
4
þ
CO
2
DG
¼
23.5 kJ mol
1
e
Thus highly anoxic waters are commonly sources of hydrogen sulfide,
H
2
S, from sulfate reduction and of methane (marsh gas). The formation
of sulfide in sediments has led to precipitation of metal sulfides over
geological time, causing accumulations of sulfide minerals of many
elements, e.g. PbS, ZnS, HgS, etc.
-
7.4.1 Case Study 1: The Biogeochemical Cycle Of Nitrogen
Nitrogen has many valence states available and can exist in the envi-
ronment in a number of forms, depending upon the oxidising ability of
the environment. Figure 10 indicates the most important oxidation
states and the relative stability (in terms of free energy formation).
23
The oxides of nitrogen represent the most oxidised and least thermody-
namically stable forms. These exist only in the atmosphere. Ammonia
can exist in gaseous form in the atmosphere but rather rapidly returns to
the soil and waters as ammonium, NH
4
1
. Fixation of atmosphere N
2
by
leguminous plants leads to ammonia, NH
3
. In aerobic soils and aquatic
systems, NH
3
and NH
4
1
are progressively oxidised by micro-organisms
via nitrite to nitrate. The latter is taken up by some biota and used as a
nitrogen source in synthesising amino acids and proteins; the most
thermodynamically stable form of nitrogen. After the death of the
organism, microbiological processes will convert organic nitrogen to
ammonium (ammonification), which is then available for oxidation or
use by plants. Conversion of ammonia to nitrate is termed nitrification,
while denitrification involves conversion of nitrate to N
2
.
Figure 11 shows an idealised nitrogen cycle. The numbers in boxes
represent quantities of nitrogen in the various reservoirs, while the arrows
show fluxes.
23
It is interesting to note that substances involving relatively
small fluxes and burdens can have a major impact upon people. Thus
nitrogen oxides, NO, NO
2
,andN
2
O are very minor constituents relative
to N
2
but play major roles in photochemical air pollution (NO
2
), acid rain
(HNO
3
from NO
2
), and stratospheric ozone depletion (N
2
O).
Nitrate from fertilisers represents a very small flux but has major
implications in terms of eutrophication of surface waters.
7.4.2 Case Study 2: Aspects of Biogeochemical Cycle of Lead
Lead is a simpler case to study than nitrogen due to the small number of
available valence states. The major use of lead until recently was a
