Bacteria and fungi, which are found in almost all
soil types, are active in breaking down minerals.
Animals also contribute to weathering, and certain
marine organisms such as mussels are able to bore
into solid rock (see Chap. 8). Microbiology has
become a key area of research in the quest to understand how minerals are dissolved and precipitated.
3.6.3 Chemical Weathering
There is no sharp demarcation between biological and
chemical weathering, because we find biological
activity in almost all soils and rocks near the surface.
The chemical environment in water at the surface of
the Earth is very much affected by local biological
activity, and in most cases it is biological processes
that cause weathering to continue after rainwater has
been neutralised through reaction with minerals. We
will therefore use the term “weathering” here for both
chemical and biological processes.
3.6.4 Weathering Profiles (Soil Profiles)
Both chemical and biological weathering are to a large
extent controlled by climate. The crucial factor is the
ratio between precipitation and evaporation in an area.
In areas where precipitation far exceeds evaporation,
podsol profiles develop in which there is a net transport of ions down through the soil profile as minerals
are dissolved. In other words, we get weathering due to
the fact that rainwater is slightly acidic (on account of
its CO 2 and H 2 SO 4 content) and contains oxygen.
Rainwater is initially undersaturated with respect to
all minerals. Some minerals are only very slightly
soluble, others more soluble in this slightly acidic,
oxidising water. Dissolved ions are transported down
to the water table, but ferrous iron liberated from ironbearing minerals will be oxidised and precipitated as
ferric iron (Fe(OH) 3 ). Vegetation at the top of the soil
profile produces CO 2 from roots and organic
compounds, particularly humic acid, which will
increase the solubility of silicate minerals. Similarly,
aluminium derived from the solution of feldspar and
mica, for example, precipitates as Al(OH) 3 but is less
noticeable because aluminium hydroxide is white. The
uppermost part of the soil profile, where dissolution
due to undersaturated rainwater and organic acids
dominates, is called the A-horizon. Some of the
dissolved salts and particularly iron hydroxide is
precipitated in the B-horizon below (Fig. 3.10).
These may develop into a layer of solid rock (hardpan) cemented with iron and aluminium oxides and
hydroxides.
Where precipitation is approximately equal to
evaporation, there is less leaching within the soil profile. At a certain depth (about 0.5–1 m) carbonate will
be precipitated and form an indurated layer (calcrete)
which may be eroded and form conglomerates.
The organic content is greater in the B-horizon
which is brown due to less oxidation of organic matter,
hence the term brown-earth profiles. If evaporation is
greater than precipitation there will be a net upward
transport of porewater, causing dissolved salts from
the groundwater to be precipitated high in the soil
profile.
3.6.5 What Factors Control Weathering
Rate and Products?
Because weathering is the most important sedimentproducing process, we are interested in understanding
how the rate of weathering is related to rock type,
precipitation, temperature, vegetation, relief etc. We
also try to establish correlations between weathering
products, particularly clay minerals, and these factors.
By studying sediments from older geological periods,
we can learn something about weathering conditions
at those times. Weathering products will also bear the
stamp of the rocks undergoing weathering. The stability of a mineral during weathering is largely a function
of the strength of the bonds holding the cations in the
crystal lattice. Potassium (K
+ ) in mica is held by weak
bonds because the univalent charge is co-ordinated
with 12 Oxygen or OH molecules in the crystal lattice
of mica responsible for the pronounced cleavage. In
biotite, the Mg
++ and Fe
++ in the octahedral layer will
also be weakly bonded. During weathering, cations
like K
+
, Na
þ
; Ca
þþ
; Mg
þþ and Fe
++
can be attacked
by protons (H
+
) which will replace them and send
them into solution. Chain silicates like hornblendes
and pyroxenes will also be relatively unstable and
rapidly weather. In feldspars the alkali ions are
dissolved so that the whole mineral disintegrates. Stability is lowest in calcium-rich plagioclase, while pure
3 Sedimentary Geochemistry
105
soil types, are active in breaking down minerals.
Animals also contribute to weathering, and certain
marine organisms such as mussels are able to bore
into solid rock (see Chap. 8). Microbiology has
become a key area of research in the quest to understand how minerals are dissolved and precipitated.
3.6.3 Chemical Weathering
There is no sharp demarcation between biological and
chemical weathering, because we find biological
activity in almost all soils and rocks near the surface.
The chemical environment in water at the surface of
the Earth is very much affected by local biological
activity, and in most cases it is biological processes
that cause weathering to continue after rainwater has
been neutralised through reaction with minerals. We
will therefore use the term “weathering” here for both
chemical and biological processes.
3.6.4 Weathering Profiles (Soil Profiles)
Both chemical and biological weathering are to a large
extent controlled by climate. The crucial factor is the
ratio between precipitation and evaporation in an area.
In areas where precipitation far exceeds evaporation,
podsol profiles develop in which there is a net transport of ions down through the soil profile as minerals
are dissolved. In other words, we get weathering due to
the fact that rainwater is slightly acidic (on account of
its CO 2 and H 2 SO 4 content) and contains oxygen.
Rainwater is initially undersaturated with respect to
all minerals. Some minerals are only very slightly
soluble, others more soluble in this slightly acidic,
oxidising water. Dissolved ions are transported down
to the water table, but ferrous iron liberated from ironbearing minerals will be oxidised and precipitated as
ferric iron (Fe(OH) 3 ). Vegetation at the top of the soil
profile produces CO 2 from roots and organic
compounds, particularly humic acid, which will
increase the solubility of silicate minerals. Similarly,
aluminium derived from the solution of feldspar and
mica, for example, precipitates as Al(OH) 3 but is less
noticeable because aluminium hydroxide is white. The
uppermost part of the soil profile, where dissolution
due to undersaturated rainwater and organic acids
dominates, is called the A-horizon. Some of the
dissolved salts and particularly iron hydroxide is
precipitated in the B-horizon below (Fig. 3.10).
These may develop into a layer of solid rock (hardpan) cemented with iron and aluminium oxides and
hydroxides.
Where precipitation is approximately equal to
evaporation, there is less leaching within the soil profile. At a certain depth (about 0.5–1 m) carbonate will
be precipitated and form an indurated layer (calcrete)
which may be eroded and form conglomerates.
The organic content is greater in the B-horizon
which is brown due to less oxidation of organic matter,
hence the term brown-earth profiles. If evaporation is
greater than precipitation there will be a net upward
transport of porewater, causing dissolved salts from
the groundwater to be precipitated high in the soil
profile.
3.6.5 What Factors Control Weathering
Rate and Products?
Because weathering is the most important sedimentproducing process, we are interested in understanding
how the rate of weathering is related to rock type,
precipitation, temperature, vegetation, relief etc. We
also try to establish correlations between weathering
products, particularly clay minerals, and these factors.
By studying sediments from older geological periods,
we can learn something about weathering conditions
at those times. Weathering products will also bear the
stamp of the rocks undergoing weathering. The stability of a mineral during weathering is largely a function
of the strength of the bonds holding the cations in the
crystal lattice. Potassium (K
+ ) in mica is held by weak
bonds because the univalent charge is co-ordinated
with 12 Oxygen or OH molecules in the crystal lattice
of mica responsible for the pronounced cleavage. In
biotite, the Mg
++ and Fe
++ in the octahedral layer will
also be weakly bonded. During weathering, cations
like K
+
, Na
þ
; Ca
þþ
; Mg
þþ and Fe
++
can be attacked
by protons (H
+
) which will replace them and send
them into solution. Chain silicates like hornblendes
and pyroxenes will also be relatively unstable and
rapidly weather. In feldspars the alkali ions are
dissolved so that the whole mineral disintegrates. Stability is lowest in calcium-rich plagioclase, while pure
3 Sedimentary Geochemistry
105