424
late Quaternary, for example, a high kaolinite/chlorite ratio in sediment sampies taken in the
South Atlantic correlates with warm interglacials
(Diekmann and Kuhn 1997). During these times
North Atlantic deep water masses transported more
kaolinite from low-Iatitude warm and humid regions
to the south and southeast than during glacial periods. The latter were characterized by enhanced formation of cold Antarctic bottom water reducing the
influence of currents from the north and carrying
high amounts of chlorite and coarser silt-size sediment from south to north.
Reworked weathering products (e.g. clay minerals,
laterite, ferricrete, bauxite, silcrete) and the presence
of iron oolites and phosphorites in fluvial, lacustrine,
and marine basins of northeast Africa reflect the
paleoclimate ofthe African continent (Gennann et al.
1994).
9.2 Chemical Denudation
(Quantitative Aspects)
9.2.1 Introduction
Chemical denudation is the lowering of the land surface by total or partial dissolution of soils and rocks
in water at or below the surface. The dissolved constituents are carried away by running surface water or
groundwater.
The chemistry of natural surface waters and
groundwaters is the result of interaction between infiltrating rainwater and the solid matter of soils and
rocks (water-rock interaction). Carbonates and
evaporites are relatively easily dissolved after a short
contact time. The concentrations of many dissolved
species, however, are lower than those possible at the
thermodynamic equilibrium between water and the
corresponding solid phases. This applies in particular
to silicate minerals in low-temperature systems when
the water-mineral contact is relatively short (for more
information on dissolution kinetics see, e.g., Lasaga
et al. 1994). As a result, short-term overland flow
during storm events as weIl as near-surface groundwater flow (interflow) do not reach the same concentrations for silicate-derived species as deeper and
longer circulating groundwater.
Nevertheless, the chernistry of river water during low and
intennediate runoff (between peak floods) is often elose to
the chemistry of groundwaters of the same region. This is
so, because the groundwater component and long-tenn
interflow make up 60 to 80% ofthe total discharge ofmany
river systems. In other words, the composition of river water largely reflects the major soil and rock types present in
the catchment area rather than other environmental conditions (e.g., elimate and relief). In addition, the presence or
absence of vegetation cover may play a significant role,
Chapter 9 Denudation and Mass Transport
particularly in carbonate karst terrains (see below). However, streams soleJy draining areas of high relief, polar and
subpolar regions, or ephemeral streams resulting from
stonn events in arid zones, mostly show low concentrations
of dissolved species as a result of dilution (if no highly soluble salts are piesent at the surface ofthe drainage area).
Apart from rock type, chemical denudation is of
course also controlled by temperature (enhancing, for
example, the dissolution rate of silica) and the effective rainfall (creating runoff, i.e., precipitation minus
evapotranspiration). High runoff can bring about
substantial chemical denudation even in regions of
cold climate and under nonequilibrium conditions of
water-soil-rock interaction. For these reasons, chemical denudation rates of large regions (up to the size
of continents) do not differ as much as do mechanical denudation rates (see below).
Chemical denudation in a particular region may
also vary considerably with the nature and thickness
of the soil profile. According to several authors (e.g.,
Carson and Kirkby 1 ?72; Stallard 1985) there is presurnably an optimum soil thickness which maximizes
the rate of chemical rock weathering. For less than
optimum soil thickness, the soil cannot retain the water supplied by precipitation long enough to interact
sufficiently with the altered or fresh subsurface bedrock. Then, chemical denudation decreases. If soil
thickness exceeds the optimum, more water is stored
in the soil and cvapotranspiration can reach its maximum. As a result, less water is available for the
weathering of bedrock and export of dissolved material. In addition, the formation of thick, low-permeable soil layers can further reduce the weathering
rate. Thus, the denudation rate for a certain area may
decrease through time, for example after 50 to 500 ka
of soil formation, even if the climatic conditions remain constant.
The method of determining both chemical and
mechanical denudation from the dissolved and solid
river loads is demonstrated in Fig. 9.5 and further
explained in Section 9.2.4. It can also be applied to
the denudational effects of rock dissolution by
groundwater.
Besides the river-Ioad method, chernical denudation rates
can also be inferred from chemical residues of the parent
rocks enriched in the soil profile (e.g. clay minerals on
carbonate rocks). In this case, the degree of dissolution
which the parent rock has undergone since it was exposed
to weathering is analyzed. This method can be applied only
under favorable conditions when, for example, the age of
the original soil or a former fresh surface can be ascertained and subsequent mechanical denudation can be mied
out. Fresh surfaces are produced by datable Quaternary
deposits or by erosion, e.g., by glacier abrasion. Thus far,
only sparse data have been obtained by this method (e.g.,
Colman and Dethier 1986; April et al. 1986; also see Chap.
9.5.1). The following discussion is therefore based mainly
on groundwater chemistry and the "river-discharge
method".
late Quaternary, for example, a high kaolinite/chlorite ratio in sediment sampies taken in the
South Atlantic correlates with warm interglacials
(Diekmann and Kuhn 1997). During these times
North Atlantic deep water masses transported more
kaolinite from low-Iatitude warm and humid regions
to the south and southeast than during glacial periods. The latter were characterized by enhanced formation of cold Antarctic bottom water reducing the
influence of currents from the north and carrying
high amounts of chlorite and coarser silt-size sediment from south to north.
Reworked weathering products (e.g. clay minerals,
laterite, ferricrete, bauxite, silcrete) and the presence
of iron oolites and phosphorites in fluvial, lacustrine,
and marine basins of northeast Africa reflect the
paleoclimate ofthe African continent (Gennann et al.
1994).
9.2 Chemical Denudation
(Quantitative Aspects)
9.2.1 Introduction
Chemical denudation is the lowering of the land surface by total or partial dissolution of soils and rocks
in water at or below the surface. The dissolved constituents are carried away by running surface water or
groundwater.
The chemistry of natural surface waters and
groundwaters is the result of interaction between infiltrating rainwater and the solid matter of soils and
rocks (water-rock interaction). Carbonates and
evaporites are relatively easily dissolved after a short
contact time. The concentrations of many dissolved
species, however, are lower than those possible at the
thermodynamic equilibrium between water and the
corresponding solid phases. This applies in particular
to silicate minerals in low-temperature systems when
the water-mineral contact is relatively short (for more
information on dissolution kinetics see, e.g., Lasaga
et al. 1994). As a result, short-term overland flow
during storm events as weIl as near-surface groundwater flow (interflow) do not reach the same concentrations for silicate-derived species as deeper and
longer circulating groundwater.
Nevertheless, the chernistry of river water during low and
intennediate runoff (between peak floods) is often elose to
the chemistry of groundwaters of the same region. This is
so, because the groundwater component and long-tenn
interflow make up 60 to 80% ofthe total discharge ofmany
river systems. In other words, the composition of river water largely reflects the major soil and rock types present in
the catchment area rather than other environmental conditions (e.g., elimate and relief). In addition, the presence or
absence of vegetation cover may play a significant role,
Chapter 9 Denudation and Mass Transport
particularly in carbonate karst terrains (see below). However, streams soleJy draining areas of high relief, polar and
subpolar regions, or ephemeral streams resulting from
stonn events in arid zones, mostly show low concentrations
of dissolved species as a result of dilution (if no highly soluble salts are piesent at the surface ofthe drainage area).
Apart from rock type, chemical denudation is of
course also controlled by temperature (enhancing, for
example, the dissolution rate of silica) and the effective rainfall (creating runoff, i.e., precipitation minus
evapotranspiration). High runoff can bring about
substantial chemical denudation even in regions of
cold climate and under nonequilibrium conditions of
water-soil-rock interaction. For these reasons, chemical denudation rates of large regions (up to the size
of continents) do not differ as much as do mechanical denudation rates (see below).
Chemical denudation in a particular region may
also vary considerably with the nature and thickness
of the soil profile. According to several authors (e.g.,
Carson and Kirkby 1 ?72; Stallard 1985) there is presurnably an optimum soil thickness which maximizes
the rate of chemical rock weathering. For less than
optimum soil thickness, the soil cannot retain the water supplied by precipitation long enough to interact
sufficiently with the altered or fresh subsurface bedrock. Then, chemical denudation decreases. If soil
thickness exceeds the optimum, more water is stored
in the soil and cvapotranspiration can reach its maximum. As a result, less water is available for the
weathering of bedrock and export of dissolved material. In addition, the formation of thick, low-permeable soil layers can further reduce the weathering
rate. Thus, the denudation rate for a certain area may
decrease through time, for example after 50 to 500 ka
of soil formation, even if the climatic conditions remain constant.
The method of determining both chemical and
mechanical denudation from the dissolved and solid
river loads is demonstrated in Fig. 9.5 and further
explained in Section 9.2.4. It can also be applied to
the denudational effects of rock dissolution by
groundwater.
Besides the river-Ioad method, chernical denudation rates
can also be inferred from chemical residues of the parent
rocks enriched in the soil profile (e.g. clay minerals on
carbonate rocks). In this case, the degree of dissolution
which the parent rock has undergone since it was exposed
to weathering is analyzed. This method can be applied only
under favorable conditions when, for example, the age of
the original soil or a former fresh surface can be ascertained and subsequent mechanical denudation can be mied
out. Fresh surfaces are produced by datable Quaternary
deposits or by erosion, e.g., by glacier abrasion. Thus far,
only sparse data have been obtained by this method (e.g.,
Colman and Dethier 1986; April et al. 1986; also see Chap.
9.5.1). The following discussion is therefore based mainly
on groundwater chemistry and the "river-discharge
method".