7.5 Influence of Chemical - School of GeoSciences

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7.5 Influence of Chemical Weathering on Hillslope Forms
SM Mudd, University of Edinburgh, Edinburgh, UK
K Yoo, University of Minnesota, St. Paul, MN, USA
EJ Gabet, San Jose State University, San Jose, CA, USA
r 2013 Elsevier Inc. All rights reserved.
7.5.1
7.5.2
7.5.2.1
7.5.2.2
7.5.2.3
7.5.3
7.5.4
References
Introduction
A General Mass Balance Model of Hillslope Evolution Including Chemical Weathering
The Chemical Weathering Mass-Loss Term
The Soil Production Term and Chemical Weathering
The Sediment Transport Term and Chemical Weathering
Feedbacks between Chemical Weathering and Geomorphic Processes
Conclusions
Glossary
Bioturbation The physical disturbance of near-surface
materials via biological mechanisms.
Chemically altered zone (CAZ) That part of the Earth’s
near surface that is affected by chemical weathering but not
physically disturbed.
Denudation Removal of material via either chemical or
physical processes.
Physically disturbed zone (PDZ) That part of the
Earth’s near surface that is disturbed by physical
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56
57
58
59
61
63
63
processes. It may or may not be affected by chemical
processes.
Shrink-swell clays Clay minerals that expand when they
absorb water and contract when they lose water.
Soil production function A function that describes how
the rate parent material is converted to soil as a function of
landscape properties. Most frequently, soil production is
cast as a function of soil thickness.
Abstract
Chemical weathering affects hillslope form through dissolution and mineral transformations that lower the surface. In
addition, mineral transformations affect the rheology, hydrology, and nutrient cycling of soils, all of which alter the geomorphic processes sculpting landscapes. Soil rheology is altered by the weakening of rocks from chemical weathering and by
changes in soil cohesion from the formation of clays, a major product of weathering reactions. Clay formation can also
decrease a soil’s hydraulic conductivity and infiltration capacity, thereby altering the partitioning of surface and subsurface
flow and potentially increasing rates of hillslope erosion by overland flow. Changes in the hydrological characteristics and
nutrient cycling of a soil can affect the plants and animals that disturb and displace it. The complex interactions of these
processes suggest that chemical weathering can either accentuate or dampen landscape dissection and relief generation.
7.5.1
Introduction
The evolving shape of a landscape is determined by the rate
at which rock is converted to sediment by physical and
chemical weathering and by the processes that redistribute
these mobile particles. Chemical weathering can alter sediment’s susceptibility to transport, and it can alter its density
and mass through the dissolution of primary minerals and the
precipitation of secondary minerals that do not have the same
density as the primary minerals they replace. In many large
drainage basins, denudation from chemical weathering is
Mudd, S.M., Yoo, K., Gabet E.J., 2013. Influence of chemical weathering on
hillslope forms. In: Shroder, J. (Editor in Chief), Marston, R.A., Stoffel, M.
(Eds.), Treatise on Geomorphology. Academic Press, San Diego, CA, vol. 7,
Mountain and Hillslope Geomorphology, pp. 56–65.
56
equal to or exceeds denudation from physical processes;
examples include the Lena basin in Russia, the St. Lawrence
basin in Canada (Summerfield and Hulton, 1994), and the
Urucara basin in Brazil (Gaillardet et al., 1997). Although
chemical erosion is important in many landscapes, the effect
of chemical weathering on topographic form has been curiously neglected in geomorphic studies. Here, we examine the
influence of chemical weathering on hillslope form through
the prism of hillslope mass conservation.
7.5.2
A General Mass Balance Model of Hillslope
Evolution Including Chemical Weathering
The influence of chemical weathering on the evolution of
hillslopes can be conveniently assessed through considerations
of mass conservation. We define the physically disturbed zone
Treatise on Geomorphology, Volume 7
http://dx.doi.org/10.1016/B978-0-12-374739-6.00148-2
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Influence of Chemical Weathering on Hillslope Forms
(PDZ) as the surface and near-surface material that is mechanically disrupted and available for sediment transport. This
layer may or may not be chemically altered. The PDZ may be
underlain by a physically undisturbed but chemically altered
zone (CAZ). Underlying the CAZ is pristine bedrock (e.g., Yoo
and Mudd, 2008).
Together the PDZ and CAZ are the equivalent of the
weathering profile, although neither agrees precisely with traditional terms such as ‘soil’ or ‘regolith’. The terms ‘biomantle’ or
‘mobile regolith’ are perhaps most similar to PDZ and the
term CAZ is most like the ‘saprolite’; but because traditional
terms have somewhat fluid definitions (cf, Yoo and Mudd,
2008), we retain the terms PDZ and CAZ to differentiate
between the geomorphically active and inactive portions of the
weathering profile.
Mass balance equations for the PDZ and CAZ, which
explicitly include mass fluxes associated with chemical weathering (Yoo and Mudd, 2008), are
q ðrPDZ hPDZ Þ
¼
qt
hPDZ SPDZ þ rZ p þ rz d
r ðrPDZ QÞ
½1aŠ
and
q ðrCAZ hCAZ Þ
¼
qt
hCAZ SCAZ þ rr f
rZ p
½1bŠ
where rx, hx, and Sx are, respectively, the density, thickness,
and rate of mass change per unit volume from chemical
weathering [ML 3T 1] of layer x. The density terms rZ, rz, and
rr refer to the material at the ground surface, PDZ–CAZ
boundary, and unweathered bedrock, respectively. The rate
terms [LT 1], d, p, and f refer to deposition (or if d is negative,
erosion) of PDZ material at the surface, the conversion of CAZ
material into PDZ material, and the downward propagation of
the weathering front, respectively. Q is the flux of PDZ colluvial flux across a unit contour [L2T 1]. The overbars represent
depth-averaged quantities, for example,
S¼
1
h
Z
z
Sdz
½2Š
Z
where Z is the elevation of the PDZ-parent material boundary
and z the elevation of the surface.
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parent material (Ci,w and Ci,p, respectively) with
tj;w ¼
rw Cj;w
ðei;w þ 1Þ
rp Cj;o
1
½3Š
where ei,w is the strain of the weathered material, calculated with
ei;w ¼
rp Ci;o
rw Ci;w
1
½4Š
where Ci,w and Ci,p are concentrations of a chemically inert
species in chemically weathered material and its parent material,
respectively. The strain is positive when volumetric expansion
has occurred and negative in the case of volumetric collapse.
During soil formation on level landscapes, bioturbation
and the addition of organic matter result in volumetric
expansion, particularly in biologically active A horizons (i.e.,
the PDZ in this chapter; Chadwick et al., 1990; Brimhall et al.,
1992; Chadwick and Goldstein, 2004). Eventually, however,
mass losses from chemical weathering cause volumetric
collapse. Negative strain values were observed in basaltic
soils older than 0.1–1 million years in the Hawaiian Islands
(Chadwick and Goldstein, 2004) and a 240-Ky-old soil
formed from beach sand deposits in coastal California
(Chadwick et al., 1990). In the hyperhumid environment
of Hawaii, Chadwick and Goldstein (2004) estimated that
10 metres of basalt had been consumed to form the present
day 1-m-thick soil during the last 1.4 million years. Such
volumetric collapse can be even greater in karst landscapes.
For hilly landscapes, the morphologic evolution of hillslopes has often been modeled by considering physical erosion
alone (e.g., Culling, 1960; Fernandes and Dietrich, 1997). For
illustrative purposes, we present a similar model that includes
mass loss from chemical weathering. Consider a one-dimensional hillslope that has steady PDZ thickness and density, no
net erosion or deposition at the surface, and sediment transport that is linearly proportional to slope (i.e., Q ¼ Kq z/q x,
where K [L2T 1] describes the efficiency of sediment transport,
often called ‘diffusivity’). Note that the linear relationship
between sediment flux and slope is only one of many proposed relationships – it is adopted here for convenience. With
these simplifications eqn [1a] reduces to
q 2 z hPDZ SPDZ
¼
q x2
rPDZ K
rZ p
rPDZ K
½5Š
hPDZ SPDZ Þl2
2rPDZ K
½6Š
and the relief of the hillslope is
7.5.2.1
The Chemical Weathering Mass-Loss Term
Unlike physical soil erosion, which can be directly related to
the lowering of ground surface elevation by accounting for soil
bulk density, it is unclear how mass losses from chemical
weathering lead to changes in surface elevation. For level
landscapes with little soil erosion or deposition, Brimhall
et al. (1992) devised a method to describe the volumetric
changes that occur in the transformation of parent materials to
soils via chemical weathering. The fractional mass loss (t) of a
weatherable species, j, in the weathering profile (denoted by
subscript w) can be calculated by measuring the densities of
the weathered and parent material (rw and rp, respectively)
and the concentrations of species j in the weathered and
R¼
ðrZ p
where l is the hillslope length. Equation [6] shows that relief
is controlled by the rate of PDZ production minus the rate
of mass loss from chemical weathering. At steady state, all
material supplied to the PDZ must be removed through either
sediment transport or chemical mass loss (e.g., Mudd and
Furbish, 2004). Because physical transport processes are slope
dependent, a reduction in the mass that must be transported
physically results in a decrease of the hillslope’s gradient.
This reduction in gradient, in turn, reduces hillslope relief
(Figure 1). For example, if chemical weathering removes
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58
Influence of Chemical Weathering on Hillslope Forms
PDZ
produced
Mass lost to
weathering
PDZ
produced
Mass physically
transported
PDZ
Mass physically
transported
p
CAZ
Mass lost to
weathering
Distance downslope
Mass lost to PDZ
weathering
Distance downslope
Hillslope
gradient
Hillslope
gradient
Distance downslope
(a)
Mass lost to
weathering
p
CAZ
Distance downslope
(b)
Figure 1 Diagram showing how chemical weathering affects relief in a steady-state landscape. In (a), mass lost to chemical weathering is
relatively minor, and PDZ material that is produced must be removed via physical transport, requiring steep hillslope gradients. In (b), a greater
fraction of the PDZ produced is removed through chemical weathering, resulting in less physical transport and lower hillslope gradients.
50% of the mass supplied to the PDZ from the CAZ
(i.e., hPDZ SPDZ ¼ 0:5rZ p), then the relief would be half of what
it would be if chemical mass loss were negligible. This contribution of chemical weathering to total denudation appears
to be relatively common in temperate climates (Summerfield
and Hulton, 1994).
Equation [7] demonstrates how mass losses from chemical
weathering could reduce landscape relief if chemical weathering is spatially homogenous. Soil scientists, however, have
long noted that chemical weathering varies significantly in
space (e.g., Birkeland, 1999). Mudd and Furbish (2004)
demonstrated how spatially varying chemical weathering rates
could not only alter the relief of a landscape but also maintain
steady-state hillslopes that were convex near the hillcrest but
concave at their toe in a creep-dominated landscape. Such
convexo-concave slopes were previously thought to be diagnostic of either overland flow erosion or remnants of a
previously wetter climate (Rinaldo et al., 1995).
Bedrock chemistry can also influence hillslope form. In
karst terrain, the spatial variability of bedrock weathering
susceptibility determines the scale and morphology of karst
features (e.g., Bauer et al., 2005; Fleurant et al., 2008) and
determines if landscapes are karst or fluvially dominated
(e.g., Phillips et al., 2004). Variability in parent material can
have a strong impact in nonkarst terrain as well. Fletcher
and Brantley (2010) found that a model describing the
emergence of corestones based on the chemical weathering
of fracture-bounded blocks in granitic bedrock matched
the size distribution of corestones occurring at a field site
in Puerto Rico. Strudley and Murray (2007) found that in
arid landscapes susceptible to piedmont formation, alternating vertical layers of rock types with different weathering
susceptibilities could create large embayments of piedmont
between more resistant ridges. Horizontally bedded resistant
rock can also accentuate escarpment formation (Kooi and
Beaumont, 1994).
The weathering susceptibility of parent material has a
strong influence on the geomorphic response of landscapes to
climate change. Eppes and McFadden (2008) examined
Holocene fans in SE California and found that the fans were
made almost exclusively of granitic material even though the
drainage basins contributing to the fans were composed of
both granite and limestone bedrock. They hypothesized that
granite weathers more quickly during the periods of increased
precipitation in the Holocene, leading to pulses of granitederived sediment that formed the fans. In limestone terrain,
Eppes and McFadden (2008) found that Holocene climate
variations did not drive large enough changes in sediment
generation to dilute the fan sediments during periods of
increased granitic sediment production.
7.5.2.2
The Soil Production Term and Chemical
Weathering
Gilbert (1877) proposed that bedrock would weather faster
under a layer of soil than if left bare. He separated the near
surface into soil and bedrock but was referring to mobile
material when using the term soil (i.e., PDZ in this chapter).
Gilbert (1877) reasoned that a layer of soil would retard
runoff and increase the amount of time that water would
spend in contact with bedrock. The longer water–rock contact
times would result in greater chemical depletion of the bedrock, and experiments have confirmed his intuition (Gabet
et al., 2006). Carson and Kirkby (1972) extended Gilbert’s
idea to propose that the rate at which bedrock was converted
to soil peaked at an intermediate soil thickness. Field studies
have shown evidence for a peaked soil production function
(e.g., Small et al., 1999; Heimsath et al., 2001, 2009;
Wilkinson et al., 2005) but also for an exponential (Heimsath
et al., 1997, 2000) relationship in which soil production is at a
maximum on bare bedrock surfaces.
The peaked soil production function has been presented in
a number of forms. For example, Anderson (2002) suggested
the following:
p ¼ min½W0 þ bhPDZ ; W1 expð hPDZ =gފ
½7Š
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Influence of Chemical Weathering on Hillslope Forms
where ‘min’ means take the minimum value of the two terms
in brackets, W0 is the production rate if hPDZ ¼ 0, W1 scales the
production rate at intermediate PDZ thickness, and g is a
length scale that describes the decrease in production with
increasing PDZ thickness. The exponential soil production
function has the form (e.g., Heimsath et al., 1997)
½8Š
p ¼ W0 exp½ hPDZ =gŠ
Dietrich et al. (1995) posited that soil production is likely
to follow a peaked function in landscapes in which burrowing
animals disturb the PDZ–CAZ boundary because, for example,
burrowing animals cannot live on bare rock. Similarly, Gabet
et al. (2003) and Gabet and Mudd (2010) argued that, in
forested landscapes where root growth and tree toppling
produce PDZ material, the soil production function should be
peaked because tree density is low in thin soils. In addition,
nonbiotic weathering processes, such as frost cracking, may
also be most effective under a finite depth of mobile regolith
(Anderson, 2002). Chemical weathering can modify the efficacy of all of these soil production processes.
Burke et al. (2007) measured rates of soil production and
chemical weathering at the PDZ–CAZ boundary in southeastern Australia and found that the soil production rate was
positively correlated with the chemical weathering rate. Dixon
et al. (2009) reported a similar result from three field sites in
the Sierra Nevada of California. The extent of chemical
weathering at the PDZ–CAZ boundary, as demonstrated
by Burke et al. (2007) and Dixon et al. (2009), appears
to modulate the physical processes that produce soils. For
example, the ability of burrowing mammals and roots to
disrupt material at the CAZ–PDZ boundary is presumably
a function of CAZ strength. Indeed, Gupta and Rao (2000)
found that the unconfined compressive strength of pervasively
weathered specimens of quartzite, granite, and basalt
were 95.6%, 99.5%, and 93.8%, respectively, weaker than
unweathered specimens of the same parent material. Similarly,
Oguchi et al. (1999) found that basalt that had undergone
40 Ky of chemical alteration had a compressive strength that
was 74.6% lower than fresh basalt and a tensile strength that
was 90.9% lower. Oguchi et al. (1999) and Gupta and Rao
(2000) also found that the more weathered rocks were also
those that had weakened the most. We hypothesize that W
(which sets the rate of soil production for a given PDZ
thickness), and possibly g, depend on weathering extent, with
W being greater for more weathered CAZ material (Figure 2).
Chemical weathering can affect the entrainment of weathered material into the PDZ by altering the texture and
density of the PDZ materials. For example, the energetic costs
of burrowing are much higher in dense, clayey soils relative
to loose, sandy soils (Vleck, 1979). If burrowing becomes
more difficult g will decrease, whereas if burrowing becomes
easier g will increase (Figure 2). In addition, grain-size reductions from chemical weathering can lower the hydraulic
conductivity and infiltration capacity of the PDZ (e.g., Lohse
and Dietrich, 2005). The weathering extent in the CAZ
is controlled, in part, by the water–mineral contact time.
Laboratory experiments with a synthetic CAZ showed an increase in dissolved mass loss with decreasing hydraulic
conductivity (Gabet et al., 2006). The dissolution rate, however, decreased with decreasing hydraulic conductivity and
increased fluid residence time. Similarly, Maher (2010) showed
that, in natural soils, soil water concentrations can approach
equilibrium concentrations with increasing fluid residence
times, thereby slowing chemical weathering reactions.
7.5.2.3
The Sediment Transport Term and Chemical
Weathering
Geomorphologists have begun to estimate sediment fluxes
over the timescale of landscape adjustment (i.e., thousands to
tens of thousands of years) by integrating cosmogenic radionuclide (CRN)-derived soil production along hillslope transects (e.g., McKean et al., 1993; Heimsath et al., 2005). In
steadily eroding landscapes, the PDZ mass balance states that
sediment flux must be equal to the mass produced through
soil production minus the mass lost via chemical weathering
(e.g., Mudd and Furbish, 2004); so, in order to correctly
quantify sediment flux rates by integrating soil production,
PDZ
CAZ
s
er
ed
he
red
C
AZ
h
Figure 2 Diagram showing the effect of chemical weathering on biologically mediated soil production.
u
ro
,
po
ed P
er
ath ich
We clay-r
ath
we
t
ea
sw
p
Les
p
p
sh,
Fre
More
p
Weaker,
more-weathered CAZ
Soil production and
PDZ weathering
Soil production and
CAZ weathering
The soil production of one gopher
Stronger,
less-weathered CAZ
59
CA
Z
PD
Z
d
DZ ense
,
h
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60
Influence of Chemical Weathering on Hillslope Forms
one must also calculate chemical weathering rates. For
example, the estimate of sediment flux was reduced by half in
a granitic hillslope in SW Australia when the loss via chemical
weathering was considered (Yoo et al., 2007).
Chemical weathering, in addition to the direct contribution to the mass balances, can indirectly affect sediment
transport in a number of ways. To explore how chemical
weathering affects sediment transport, we briefly examine how
sediment transport is thought to operate on hillslopes. Culling
(1960), in constructing the first formal mass balance of hillslope sediment transport, proposed that sediment flux was
linearly proportional to slope. Since then, a number of sediment flux laws (Dietrich et al., 2003) have emerged from
theoretical and field studies. Andrews and Bucknam (1987)
and Roering et al. (1999) proposed that sediment flux is
determined by the balance of power, P, exerted on the volume
of colluvial soil per unit area (V/A) by various agents and the
net force resisting the transport (F):
Q¼
P V
F
A
½9Š
According to this expression, at a given slope gradient, sediment transport rate can be affected by chemical weathering as
it influences the intensity of bioturbation (as reflected in P). In
some ecosystems, mineral weathering supplies biologically
important elements such as phosphorus and calcium (e.g.,
Vitousek and Sanford, 1986; Likens and Bormann, 1995).
Sterner and Elser (2002) proposed that faster growing biota
will dominate sites with high nutrient availability, whereas
Moore et al. (2010) found that aboveground net primary
productivity decreases with soil age. Thus, old nutrient-poor
soils may have less-productive vegetation (e.g., less biomass
and root turnover), thus slowing sediment transport rates
if sediment transport is primarily driven by bioturbation (e.g.,
Gabet et al., 2003). By contrast, if vegetation inhibits erosion
from overland flow (e.g., Prosser et al., 1995) or rainsplash
(e.g., Gabet and Dunne, 2003; Dunne et al., 2010), a reduction
in plant productivity could enhance erosion (Figure 3).
Enhanced ecosystem
productivity
Initial state
Landscape
dominated
by
overland flow
Initial state
Landscape
dominated
by
bioturbation
Chemical weathering may also affect the soil’s resistance to
transport (F in eqn [9]). For example, the density and clay
content of PDZ material is positively correlated with its degree
of chemical weathering (e.g., Birkeland, 1999), and physiological studies have shown that a gopher’s energy expenditure
while burrowing increases nonlinearly in denser soils (Vleck,
1979). These observations suggest lower diffusivities for more
weathered PDZ material. Alternatively, coarse material in less
weathered PDZ may hinder sediment transport. Furbish et al.
(2009) attempted to clarify these relationships by relating K to
power input (in their case, stated as a particle activity) and to
the grain size and porosity of the PDZ.
However, not all slope-dependent sediment transport
processes fit into this physics-based analysis. Where sediment
transport is driven by the shrink–swell behavior of 2:1 clays,
identification of P in eqn [9] is difficult. Nevertheless, the role
of clays in modulating sediment transport rates is important.
In landscapes with shrink–swell clays, increasing clay content
can enhance sediment transport rates. Indeed, one of the
highest measured hillslope diffusivities was measured on a
hillslope rich with shrink–swell clays (McKean et al., 1993).
Note that most soils with clay contents high enough to
generate significant shrink–swell transport are developed from
clay-rich parent material, thus suggesting a link between
sediment flux and both parent material and chemical
weathering in the CAZ.
Clays can also inhibit sediment transport. Furbish et al.
(2009) proposed that sediment transport efficiency is linearly
proportional to a characteristic particle radius. Harris et al.
(2008) measured displacement by solifluction in experimental
PDZs with different clay contents and found that downslope
displacement was a function of depth: particles near the surface moved farther downslope than those at depth, mirroring
the results of Roering (2004). Harris et al. (2008) concluded
that in clay-rich soils the depth-integrated displacement was
less than in coarser soils. Although the clay-rich soils had
greater displacement deep in the soil profile, particles near the
surface of clay-poor soils moved much farther downslope,
thus leading to greater total downslope transport.
Accelerated
chemical
weathering;
more nutrients
Final state
Increased resistance
to overland flow
less sediment
transport
Enhanced ecosystem
productivity
Accelerated
chemical
weathering;
more nutrients
Final state
More bioturbation
more sediment
transport
Figure 3 Diagram showing two possible scenarios of increased nutrient supply on sediment transport.
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Influence of Chemical Weathering on Hillslope Forms
Clay content can also play a crucial role in determining a
landscape’s susceptibility to landslides and the mode of mass
movement after failure. Clay-rich, deeply weathered materials
will have higher cohesion (e.g., Selby, 1993) and, because
of reduced infiltration capacity (Lohse and Dietrich, 2005),
may be slower to build up the high pore pressures that
lead to shallow landslides (e.g., Dietrich et al., 1995). Iverson
(2000) found that whether a landslide accelerated rapidly
or only moved slowly at decadal timescales was due, in large
part, to the hydraulic conductivity of the landslide substrate,
a property closely related to clay content. Landscapes that
are deeply weathered with high clay contents are, therefore,
more likely to feature deep-seated, slow-moving landslides,
whereas rapidly eroding, relatively unweathered landscapes
are more likely to feature shallow landslides. In addition, failures in deeply weathered landscapes tend to be
rotational with arcuate failure planes, whereas failures in
less-weathered landscapes commonly have slope-parallel
failure planes (Selby, 1993). Clay content can also play a role
in determining if a failing landslide mass becomes a debris
flow. The low hydraulic conductivities of silt- and clay-rich
soils are able to sustain the high pore pressures needed
to liquefy a landslide mass (Iverson et al., 1997; Gabet and
Mudd, 2006).
Reductions in CAZ strength can reduce the probability that
weathered landscapes will form cliffs. Burnett et al. (2008)
investigated the causes of aspect-driven asymmetry on several
slopes in Arizona. On south-facing slopes cliffs accounted for
29% of the relief, whereas on north-facing slopes cliffs
accounted for only 2.5%. Burnett et al. (2008) also quantified
soil moisture and parent material strength. Despite being
underlain by the same geological formation, south-facing
slopes had stronger parent material. They also had greater
insolation and lower soil moisture. Based on these measurements, Burnett et al. (2008) hypothesized that wetter soils on
north-facing slopes led to greater parent material weathering,
reducing the likelihood of cliff formation.
Chemical weathering can influence the hydrological
response of a landscape that, in turn, influences geomorphic
processes. Many authors have noted a reduction in hydraulic
conductivity and infiltration capacity as PDZ and CAZ
material weathers (e.g., McAuliffe, 1994; Lohse and Dietrich,
2005; Dontsova et al., 2009), and chemical weathering
can create pores in otherwise resistant rock (White et al.,
2001; Rossi and Graham, 2010). Jefferson et al. (2010) examined the drainage densities of a landscape formed on basalt
lava flows with ages ranging from Holocene to Oligocene.
They found that drainage density was lowest on young lava
flows, with drainage density doubling over 1 million years.
Using hydrological data, Jefferson et al. (2010) concluded
that the young basalt flows were porous and rainfall infiltrated
only to emerge in springs, whereas storm runoff was enhanced
in the old lava flows from the reduced permeability brought
about by greater chemical weathering. Drainage density
strongly affects the relief of hillslopes; according to eqn [6],
a doubling of drainage density would decrease relief by a
factor of 4.
Changes in hydrology brought about by chemical weathering can also affect plant productivity (e.g., Burke et al.,
1998; Lohse et al., 2009). McAuliffe (1994) studied the density
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and maximum diameter of Larrea tridentat (a desert shrub) in
southern Arizona as a function of soil type and soil age. He
found that shrub diameters declined sharply as a function of
soil age and attributed this decline to a decrease in infiltration
capacity brought about by chemical weathering of the soils.
The size and productivity of plants can have a strong effect on
sediment transport rates. Based on geometric arguments, Gabet
et al. (2003) proposed that sediment transport by root growth
and decay should scale with root mass per unit area and the
root turnover rate. Reduced plant productivity should then be
reflected in reduced sediment transport. Corroborating this
finding, Hughes et al. (2009) showed a doubling in sediment
transport efficiency caused by vegetation change at the beginning of the Holocene at a site in New Zealand.
7.5.3
Feedbacks between Chemical Weathering and
Geomorphic Processes
As discussed in previous sections, chemical weathering affects
landscape evolution directly by removing mass or indirectly
by affecting soil production, hillslope hydrology, bioturbation, and sediment transport. To close the feedback loop,
we examine how the morphologic evolution of landscapes
amplifies or dampens chemical weathering processes.
The view that geomorphic processes control chemical
weathering is reflected in the concept that the rate at which
fresh minerals are supplied to the surface limits the rate of
chemical weathering (Hren et al., 2007; Ferrier and Kirchner,
2008; Gabet and Mudd, 2009; Hilley et al., 2010). This idea is
central to the hypothesis that major orogenic events may have
contributed to global cooling by accelerating chemical
weathering rates (Raymo and Ruddiman, 1992). Riebe et al.
(2001), by combining the geochemical mass balance model
with CRN-based total denudation rates, confirmed a link
between physical erosion and chemical weathering at geomorphically meaningful timescales (1–100 Ky). Furthermore,
West et al. (2005) compiled a global data set showing a
positive correlation between total denudation and chemical
weathering rates (as well as climate and precipitation). Riebe
et al.’s (2001) study focused on denudation, physical and
chemical, at the catchment scale. At the hillslope scale, soil on
a slope receives its minerals not only through in situ soil
production but also through sediment flux from upslope.
Acknowledging that minerals are supplied both locally and
from upslope, full equations for calculating the rate of
chemical weathering distributed across an eroding hillslope
were developed theoretically (Mudd and Furbish, 2006; Yoo
et al., 2007) and applied to field sites (Yoo et al., 2007, 2009).
With the advent of these new techniques, site-specific but
systematic relationships between topography and chemical
weathering rates are emerging (Green et al., 2006; Yoo et al.,
2007, 2009; Jin et al., 2010).
In addition to mineral supply rates, several other mechanisms have been hypothesized to drive spatial heterogeneity
in chemical weathering rates. White and Brantley (2003)
demonstrated that the chemical weathering rate of primary
minerals such as potassium feldspar, plagioclase, and biotite
declined sharply with time spent in the weathering zone.
Because the residence time of minerals in the weathering zone
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Influence of Chemical Weathering on Hillslope Forms
is inversely related to erosion rate (Mudd and Yoo, 2010),
minerals in rapidly eroding landscapes may experience higher
time-averaged dissolution rates (e.g., Gabet and Mudd, 2009).
In addition, chemical weathering rates may be determined by
fluid residence time rather than the minerals’ residence time in
the weathering zone (e.g., Maher, 2010). However, these two
hypotheses are likely linked. As regolith weathers, it accumulates secondary minerals (e.g., clays) that tend to reduce
hydraulic conductivity. All else equal, old regolith is more
likely to have greater fluid residence times and lower chemical
weathering rates. In addition, if climate and rainfall are equal,
rapidly eroding terrain will tend to have steeper slopes and
thinner soils than slowly eroding terrain, which will reduce
fluid residence time and increase chemical weathering rates.
Mineral residence time affects both the rate and relative
chemical depletion of minerals, so landscapes with greater
chemical weathering rates will tend to be less chemically
depleted (or fresher) than landscapes with lower chemical
weathering rates (e.g., Mudd and Yoo, 2010).
As an illustration of potential feedbacks between chemical weathering and hillslope form, we consider a scenario
(Figure 4) in which channel incision accelerates in a previously steady-state landscape (i.e., the denudation and uplift
rates are the same). Accelerated channel incision will cause a
steepening at the toe of the hillslope that will propagate
upslope (e.g., Furbish and Fagherazzi, 2001), thinning the
soils. The thinner soils will stimulate increased soil production
(eqn [8]), thus leading to younger mineral ages (e.g., Mudd
and Furbish, 2006), which will accelerate chemical weathering
rates (e.g., White and Brantley, 2003). In addition, accelerated
erosion may lead to enhanced CAZ weathering rates (e.g.,
Lebedeva et al., 2010); but because residence time is reduced,
material emerging from the CAZ will be less weathered (Yoo
and Mudd, 2008), such that PDZ material will be coarser and
more porous (e.g., Birkeland, 1999). If climate remains
constant, this coarse, porous material with higher hydraulic
conductivity will, combined with a steepened hillslope, lead
to shorter fluid residence times and, thus, higher chemical
weathering rates (e.g., Maher, 2010). The end result will be
enhanced mass loss from chemical weathering near the
channel, which will amplify landscape lowering on the hillslope initiated by accelerated channel incision (Figure 4).
Initial state: Hillslope equilibrated to the rate of channel incision. Hillslope properties are spatially homogenous.
PDZ
Soil
thickness
Mineral age
Soil production
Erosion rate
p
CAZ
Chemical mass loss rate
Fluid residence
time Hydraulic
conductivity
Particle size
Distance downslope
Distance downslope
Distance downslope
After the rate of channel incision accelerates
Channel incision accelerates.
Hillslope steepens near channel.
Physical erosion accelerates
near channel.
Ero
sio
PDZ
n fa
PDZ
ste
r
ne
thin
ner
ar
ch
a
ne
ar
c
p
CAZ
Pr o
duc
tion
Channel incision
accelerates
Erosion rate
nn
el
Distance downslope
Soil production
Mineral age
ha
nn
el
Soil thins near channel, leading
to increased PDZ production.
Soil
thickness Minerals are younger near
channel.
Lower mineral residence time
leads to coarser particles.
Particle size
Distance downslope
fas
te
rn
Fluid residence
time
Hydraulic
conductivity
e
ar
a
ch
Steeper, coarser hillslope near
channel leads to increased
hydraulic conductivity and
shorter fluid residence time,
leading to
e
nn
l
Distance downslope
Chemical mass loss rate
Enhanced chemical weathering near
channel. This amplifies landscape
lowering near channel.
Distance downslope
Figure 4 Diagram showing a hypothetical response of a hillslope to accelerated channel incision.
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Influence of Chemical Weathering on Hillslope Forms
Consider another scenario in which tectonic uplift slows in
a previously steady-state landscape. Initially, the gradient of
the streams and their neighboring hillslopes will decline,
leading to a drop in erosion rate. This will increase CAZ and
PDZ residence times, and the landscape will become more
chemically weathered. This may increase runoff, increasing the
landscape’s sediment transport efficiency and, thus, lowering
relief (once the landscape equilibrated to the new uplift rate).
But if the ecosystem on the landscape is limited by weatheringderived nutrients, biologic activity will slow, sediment transport efficiency will decline, and the relief in the landscape will
increase. This may seem counterintuitive, but examine eqn [6].
If total denudation is reduced by 50% (e.g., the tectonic uplift
rate is reduced by 50%), then to increase relief, the sediment
transport efficiency (K) would have to decline by 450%. This
is by no means unreasonable: diffusivities from different
vegetation types vary by at least a factor of 4 (Gabet et al.,
2003). Ultimately, a thorough understanding of the relative
sensitivity of hydraulic conductivity and ecosystem productivity to changes in chemical weathering is essential to predict
whether chemical weathering processes will damp or accentuate a landscape’s response to tectonic change.
7.5.4
Conclusions
Chemical weathering affects hillslope form by directly
removing mass from hillslopes. The transformation of primary
minerals to secondary minerals can also affect the hydrology,
biota, and stability of hillslopes, which in turn influence
sediment transport and landscape morphology. Erosion in
evolving landscapes can modulate and be modulated by
chemical weathering; for example, pulses of accelerated erosion can lower the residence time of hillslope materials,
thereby increasing their chemical weathering rates, which in
turn accelerate the incision signal. We have presented statements of mass conservation that allow quantification of the
important feedbacks between chemical weathering and hillslope form; however, many of the constitutive relationships
required to solve these equations are still lacking. For example,
we do not know if chemical weathering influences the efficiency of sediment transport or how it influences the rate
at which soil is produced at a given soil thickness. Studies
quantifying these relationships are required before we can
fully understand how chemical weathering influences hillslope form and the nature of the feedbacks between chemical
weathering and landscape evolution. These studies are necessary if we are to develop an integrative understanding of
dynamic feedbacks between climate, biogeochemical cycles,
and landscape evolution.
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Biographical Sketch
Dr. Simon M Mudd received his PhD in environmental engineering from Vanderbilt University in 2006. He then
worked as a research associate in Vanderbilt’s Department of Earth and Environmental Science until 2007, when
he took his current position as lecturer in Landscape Dynamics in the School of GeoSciences at the University of
Edinburgh. Dr. Mudd studies a wide range of Earth surface processes, such as flash flooding, sediment transport,
debris flows, the morphodynamics of salt marshes, and the topic of this chapter: the relationship between
hillslope evolution and chemical weathering.
After training in Korea as an atomic physicist (receiving an M.S. from Yonsei University in 1996), Dr. Kyungsoo
Yoo went on to gain a PhD in Ecosystem Sciences from the University of California, Berkeley in 2003. Following a
postdoc at U.C. Berkeley, Dr. Yoo joined the Department of Plant and Soil Sciences at the University of Delaware
in 2006 as an assistant professor before joining the Department of Soil, Water and Climate at the University of
Minnesota. Dr. Yoo examines both chemical weathering and the carbon cycle in soils, and has also examined how
biota affects hillslope sediment transport.
Dr. Manny Gabet received his PhD in Geological Sciences from the University of California, Santa Barbara in
2002. He continued as a postdoc at UCSB until 2003, examining erosion and chemical weathering in the
Nepalese Himalaya. Dr. Gabet went on to professorial positions in the University of Montana, Missoula, and the
University of California, Riverside before taking his current position as an associate professor in the Geology
Department at San José State University. Among Dr. Gabet’s interests are debris flows, sediment transport, the
effect of stochastic processes on landscape evolution, bioturbation, and the relationship between erosion and
chemical weathering.