432
9.2.4 Chemical Denudation Rates in Different
Morpho-Climatic Zones (Overview)
Specific Transport Rates and
Mean Denudation Rates (Definitions)
If the annual average concentrations of all dissolved
species, the total dissolved solids, TDS (mg/l or
g/m 3 ), are multiplied by the average annual runoff
(m 3 /year) one obtains the average total solute load
(e.g. in metric tons/year, tla) of the river system (cf.
Fig. 9.5). Seasonal and short-term variations have to
be taken into account for this calculation. The longterm annual mean of runoff can be expressed either
as specific runoff per unit area of the drainage basin
(e.g. m 3 1km 2 /a) or height of water in mmla (as annual
precipitation).
Measurements of element concentrations and runoff (volurne of water per time unit) should be carried out at short
intervals for several years in order to evaluate seasonal or
short-term changes in water chemistry. Moreover, it should
be ascertained that no substantial quantity of groundwater
is lost by subsurface flow from the area studied.
The product of TDS and specific runoff is the specific transport rate or specific yield (load) of dissolved constituents, SY eh (e.g. in tlkm 2 /a). SY eh can
be converted into a mean (total) chemical denudation
rate, DReh,! (mm/ka or m/Ma). To obtain the
"denudational" chemical denudation rate, DReh' the
atmospheric components of the solute load have to
be subtracted. The most important ones of these are
chloride and hydrogen carbonate. The latter is
needed to dissolve silicate minerals and, in addition,
to trans form carbonates into hydrogen carbonates.
F or the weight to volume conversion, an average
rock density has to be assumed (here 2.5 tlm 3 ).
Examples from River Catchments and
Various Morpho-Climatic Zones
Values of the specific yield, SY eh , or transport rates
of dissolved constituents and chemical denudation
rates, DReh' as defined above, frequently range between 5 and 80 tlkm 2 /a or between about 2 and 32
mmlka, respectively. Large river systems of mixed
relief, rock types and elimate display a narrower
range of denudation rates than smaller catchments in
which specific rock types play a major role. In most
cases, a elose positive relationship between runoff
and DReh can be found.
This rule even applies to regions where TDS concentrations are fairly low (e.g., Iceland or most of the tropical
river basins, Figs. 9.8 and 9.9b and c). Under conditions of
low specific runoff (arid to semi-arid climates), salt concentrations in river water must be extremely high in order
Chapter 9 Denudation and Mass Transport
to produce high values of SY eh and DReh' Examples of this
situation are catchments of the Aral Sea, the Caspian Sea,
and the Shatt el Arab. Due to medium concentrations and
high specific runoffs, many mountain ranges have high to
very high DReh values (e.g., the Alps, partially drained by
the Danube River, or the rivers Indus, Ganges, and
Brahmaputra originating in the Himalayas).
Climate control of the chemical denudation rates,
D~h' becomes most obvious if specific transport
rates or yields of dissolved matter, SY eh , and DReh are
correlated with various morpho-elimatic zones (Fig.
9.10). The drainage areas used for this correlation
should be composed either of similar rocks or a variety of different rock types to eliminate rocks as an
important factor. This approach elearly shows two
maxima of solute transport and chemical denudation:
(1) the temperate zone with high rainfall and (2) the
humid mountainous zone in the tropics. Minima of
chemical denudation are found in arid zones as weIl
as in tundra and taiga regions with low specific runoffs and moderate solute concentrations. The maximum and minimum DReh values of the morpho-elimatic zones differ by a factor of 20 to 30.
9.2.5 Global Effects of Chemical Denudation
GlobaIly, the mean (discharge-weighted) transport
rate of dissolved solids is about 50 x 10 9 t/a or ~ 34
tlkm 2 /a (e.g. Degens et al. 1991). After subtraction of
the atmospheric components (e.g. part of RC0 3 and
Cl, i.e. ~25% ofTDS) we get ab out 25 tlkm 2 /a, corresponding to a chemical denudation rate of 10 mmlka.
This rate would produce a "salt" layer of roughly 4
mmIka in the oceans with the assumption that the
salts are distributed evenly. Of these, roughly 2
mmlka would be carbonate, 1.5 mmlka other salts,
and 0.5 mmlka silica. This estimate is based on the
average chemical composition of the world rivers (cf.
Fig.9.8).
In the geological past, the global average of the
chemical denudation rate may have been different.
Righer rates can be assumed for the greenhouse state
of the Earth (e.g. the Cretaceous and the Paleocene)
when precipitation and runoff were enhanced.
This is also postulated for the Eocene, but Si0 2 flux to the
ocean seems to have been lower than today, because the
area of exposed silicate rocks was smaller (Sloan et al.
1997) than it is at present.
9.2.6 Geomorphological Consequences of
Differential Chemical Denudation
Differences in the chemical denudation rates of various rock types control the landscape development to
some extent. This topic is addressed here only briefly
in a semi-quantitative way.
9.2.4 Chemical Denudation Rates in Different
Morpho-Climatic Zones (Overview)
Specific Transport Rates and
Mean Denudation Rates (Definitions)
If the annual average concentrations of all dissolved
species, the total dissolved solids, TDS (mg/l or
g/m 3 ), are multiplied by the average annual runoff
(m 3 /year) one obtains the average total solute load
(e.g. in metric tons/year, tla) of the river system (cf.
Fig. 9.5). Seasonal and short-term variations have to
be taken into account for this calculation. The longterm annual mean of runoff can be expressed either
as specific runoff per unit area of the drainage basin
(e.g. m 3 1km 2 /a) or height of water in mmla (as annual
precipitation).
Measurements of element concentrations and runoff (volurne of water per time unit) should be carried out at short
intervals for several years in order to evaluate seasonal or
short-term changes in water chemistry. Moreover, it should
be ascertained that no substantial quantity of groundwater
is lost by subsurface flow from the area studied.
The product of TDS and specific runoff is the specific transport rate or specific yield (load) of dissolved constituents, SY eh (e.g. in tlkm 2 /a). SY eh can
be converted into a mean (total) chemical denudation
rate, DReh,! (mm/ka or m/Ma). To obtain the
"denudational" chemical denudation rate, DReh' the
atmospheric components of the solute load have to
be subtracted. The most important ones of these are
chloride and hydrogen carbonate. The latter is
needed to dissolve silicate minerals and, in addition,
to trans form carbonates into hydrogen carbonates.
F or the weight to volume conversion, an average
rock density has to be assumed (here 2.5 tlm 3 ).
Examples from River Catchments and
Various Morpho-Climatic Zones
Values of the specific yield, SY eh , or transport rates
of dissolved constituents and chemical denudation
rates, DReh' as defined above, frequently range between 5 and 80 tlkm 2 /a or between about 2 and 32
mmlka, respectively. Large river systems of mixed
relief, rock types and elimate display a narrower
range of denudation rates than smaller catchments in
which specific rock types play a major role. In most
cases, a elose positive relationship between runoff
and DReh can be found.
This rule even applies to regions where TDS concentrations are fairly low (e.g., Iceland or most of the tropical
river basins, Figs. 9.8 and 9.9b and c). Under conditions of
low specific runoff (arid to semi-arid climates), salt concentrations in river water must be extremely high in order
Chapter 9 Denudation and Mass Transport
to produce high values of SY eh and DReh' Examples of this
situation are catchments of the Aral Sea, the Caspian Sea,
and the Shatt el Arab. Due to medium concentrations and
high specific runoffs, many mountain ranges have high to
very high DReh values (e.g., the Alps, partially drained by
the Danube River, or the rivers Indus, Ganges, and
Brahmaputra originating in the Himalayas).
Climate control of the chemical denudation rates,
D~h' becomes most obvious if specific transport
rates or yields of dissolved matter, SY eh , and DReh are
correlated with various morpho-elimatic zones (Fig.
9.10). The drainage areas used for this correlation
should be composed either of similar rocks or a variety of different rock types to eliminate rocks as an
important factor. This approach elearly shows two
maxima of solute transport and chemical denudation:
(1) the temperate zone with high rainfall and (2) the
humid mountainous zone in the tropics. Minima of
chemical denudation are found in arid zones as weIl
as in tundra and taiga regions with low specific runoffs and moderate solute concentrations. The maximum and minimum DReh values of the morpho-elimatic zones differ by a factor of 20 to 30.
9.2.5 Global Effects of Chemical Denudation
GlobaIly, the mean (discharge-weighted) transport
rate of dissolved solids is about 50 x 10 9 t/a or ~ 34
tlkm 2 /a (e.g. Degens et al. 1991). After subtraction of
the atmospheric components (e.g. part of RC0 3 and
Cl, i.e. ~25% ofTDS) we get ab out 25 tlkm 2 /a, corresponding to a chemical denudation rate of 10 mmlka.
This rate would produce a "salt" layer of roughly 4
mmIka in the oceans with the assumption that the
salts are distributed evenly. Of these, roughly 2
mmlka would be carbonate, 1.5 mmlka other salts,
and 0.5 mmlka silica. This estimate is based on the
average chemical composition of the world rivers (cf.
Fig.9.8).
In the geological past, the global average of the
chemical denudation rate may have been different.
Righer rates can be assumed for the greenhouse state
of the Earth (e.g. the Cretaceous and the Paleocene)
when precipitation and runoff were enhanced.
This is also postulated for the Eocene, but Si0 2 flux to the
ocean seems to have been lower than today, because the
area of exposed silicate rocks was smaller (Sloan et al.
1997) than it is at present.
9.2.6 Geomorphological Consequences of
Differential Chemical Denudation
Differences in the chemical denudation rates of various rock types control the landscape development to
some extent. This topic is addressed here only briefly
in a semi-quantitative way.