Applied Geochemistry 26 (2011) S3–S5
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Applied Geochemistry
journal homepage: www.elsevier.com/locate/apgeochem
Exploring weathering and regolith transport controls on Critical Zone
development with models and natural experiments
Suzanne P. Anderson a,b,⇑, Robert S. Anderson a,c, Eve-Lyn S. Hinckley a, Patrick Kelly a,b, Alex Blum d
a
Institute of Arctic and Alpine Research, University of Colorado, Boulder, CO 80309, USA
Department of Geography, University of Colorado, Boulder, CO 80309, USA
c
Department of Geological Sciences, University of Colorado, Boulder, CO 80309, USA
d
United States Geological Survey, Boulder, CO 80309
b
a r t i c l e
i n f o
Article history:
Available online 22 March 2011
a b s t r a c t
The architecture of the Critical Zone, including mobile regolith thickness and depth to the weathering
front, is first order controlled by advance of a weathering front at depth and transport of sediment at
the surface. Differences in conditions imposed by slope aspect in the Gordon Gulch catchment of the
Boulder Creek Critical Zone Observatory present a natural experiment to explore these interactions.
The weathering front is deeper and saprolite more decayed on north-facing than on south-facing slopes.
Simple numerical models of weathering front advance, mobile regolith production, and regolith transport
are used to test how weathering and erosion rates interact in the evolution of weathered profiles. As the
processes which attempt are being made to mimic are directly tied to climate variables such as mean
annual temperature, the role of Quaternary climate variation in governing the evolution of Critical Zone
architecture can be explored with greater confidence.
Ó 2011 Elsevier Ltd. All rights reserved.
1. Introduction
Understanding geomorphic and weathering controls on Critical
Zone architecture, the thickness and character of mobile regolith,
saprolite, and weathered rock layers, are major aims of the
Boulder Creek Critical Zone Observatory. The Gordon Gulch subcatchment in Boulder Creek offers a useful landscape in which
to explore these controls. Gordon Gulch is a 3.7 km2 catchment
at 2600 m elevation in the Colorado Front Range (Fig. 1). It lies
within the upper montane forest zone of Marr (Birkeland et al.,
2003), within the Rocky Mountain surface, an area of relatively
low relief despite its altitude, and is underlain by Paleoproterozoic biotite gneiss (Cole and Braddock, 2009). The Rocky Mountain
surface in the Colorado Front Range lies outside the limits of
Pleistocene glaciers, and above canyons incised during the Quaternary by rivers draining eastward toward the Great Plains. This
region has evolved without substantial tectonic perturbation
since the end of the Laramide orogeny at approximately 50 Ma
(Anderson et al., 2006). The lack of rapid erosional perturbations
and presence of some areas of hydrothermal alteration produce a
landscape in which the depth to the weathering front averages
8 m, and reaches up to 30 m, as determined from water well
drilling records and sparse road-cuts (Dethier and Lazarus,
⇑ Corresponding author at: Institute of Arctic and Alpine Research, University of
Colorado, Boulder, CO 80309, USA. Tel.: +1 303 492 7071.
E-mail address: [email protected] (S.P. Anderson).
0883-2927/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved.
doi:10.1016/j.apgeochem.2011.03.014
2006). The Gordon Gulch catchment lies only 5 km east of the
limits of maximum Pleistocene glacial extent (Madole, 1986;
Madole et al., 1999). It was, therefore, presumably periglacial
during much of the Quaternary. Current mean annual surface soil
temperatures of about 4 °C (at 1–3 cm depth, based on 1 year of
observations), would have shifted to subfreezing mean annual
temperatures that are likely to have supported development of
permafrost.
Gordon Gulch is aligned east–west, and, therefore has northfacing and south-facing slopes with differing energy balances
(Anderson et al., 2010), vegetation (Peet, 1981), and depth to
fresh rock (Befus, 2010). A snowpack develops on north-facing
slopes each winter, but winter snowcover is patchy in both time
and space on south-facing slopes. The north-facing slopes are
dominated by Pinus contorta (lodgepole pine), which grows in
dense stands with little to no understory, while south-facing
slopes are dominated by Pinus ponderosa (ponderosa pine) woodlands, with widely spaced trees and grass-herbaceous understory.
Tors mark the topography on both slopes, although a greater
number occur on south-facing slopes (Trotta, 2010). In a series
of nine soil pits dug through the mobile regolith and into the
underlying saprolite, the average depth of mobile regolith was
greater on sites on north-facing slopes (50–75 cm vs. 20–
70 cm). The hand excavations of saprolite extended to greater
depths on north-facing slopes as well (30–65 cm vs. 20–45 cm),
owing to material that was easier to dig through on north-facing
slopes, giving a qualitative sense of more weathered saprolite on
S4
S.P. Anderson et al. / Applied Geochemistry 26 (2011) S3–S5
Fig. 1. The topographic setting of the Boulder Creek Critical Zone Observatory
(BcCZO). Top: newly acquired lidar – based bare-earth DEM of the Gordon Gulch
catchment depicted in the rectangular box in the inset showing the entire Boulder
Creek catchment. The topographic relief varies with position, and ranges from
50 m in the upper basin to 150 m in the lower basin. Topography is lumpy, with
scattered bedrock outcrops interrupting both the convex hillcrests and the straight
slopes. Maximum slopes are 14° in the upper basin, and 23–27° in the lower basin,
as shown in the profiles in the lower plot.
north-facing slopes. Shallow seismic refraction surveys reinforce this impression: depth to seismic velocities of >3500 m/s
characteristic of fresh crystalline rock were on average 15 m on
north-facing slopes and 5–10 m on south-facing slopes in catchment-crossing transects (Befus, 2010). Understanding the origins
of the difference in degree and depth of weathering in the
bedrock is the target of this research.
2. Modeling Critical Zone evolution
The goal is to develop a model to track the advance of two
interfaces: the deep weathering front between weathered and
unweathered rock, and the interface between mobile regolith
and saprolite (immobile regolith). In addition, an understanding
is sought of what controls the degree of weathering in the saprolite. In particular, the roles of erosion, which could be viewed as
refreshing the weathered profile by sweeping away weathered
products, and the rates at which rock breaks down by weathering
are explored. One hypothesis is that rock is less weathered on
south-facing slopes in Gordon Gulch owing to more vigorous sediment transport on these slopes. Alternatively, north-facing slopes
may be more weathered due to wetter conditions and possibly a
greater influence of vegetation enhancing weathering rates. To test
these ideas, a simple hillslope model was constructed that captures
both weathering and regolith transport.
The model has 3 components: mobile regolith transport, bedrock weathering, and mobile regolith production. Regolith transport in the mobile regolith layer is slope dependent up to a
critical slope at which rapid mass movements are expected to
dominate (31°), and is linearly dependent on mobile regolith
thickness up to a depth at which regolith discharge ‘‘saturates’’
(e.g., Riggins et al., 2011). At small slopes and for thick soils, this
reduces to Q = kSH, where Q is sediment discharge, k is the efficiency of slope-dependent transport, S is slope, and H is mobile
regolith thickness. Bedrock weathering is envisioned as the
ensemble of processes that break down rock, including chemical
alteration and processes such as frost cracking that fracture rock,
ultimately releasing it for entrainment in mobile regolith. In the
present model the process of frost cracking is mimicked by
tracking the time the rock spends in a specified sub-zero window (e.g. Walder and Hallet, 1985, 1986; Anderson, 1998). This
may be calculated for any year given the mean annual surface
temperature, the amplitude of the temperature swing at the
surface, and the thermal diffusivity of the regolith and rock.
Damage is accumulated through time in all parcels of rock as
the surface lowers. The susceptibility to entrainment into the
mobile regolith of the rock at the regolith interface is taken to
be proportional to this accumulated damage. The rate of mobile
regolith production is also taken to depend on the thickness of
the overlying mobile regolith to acknowledge the role of surface
processes such as tree root growth in releasing the rock from the
regolith interface.
The efficiency of regolith transport (captured in k) and the rate
of rock weathering with slope aspect are varied to test the
hypotheses. The simulations start with two hillslopes separated
by a divide, with identical boundary conditions at the base of
the slopes, governed by the prescribed incision rates of the adjacent channels. These hillslopes are intended to represent the
north-facing and south-facing slopes in Gordon Gulch, although
the geometry here is across a divide, rather than across a channel,
for ease of both computation and visualization. In each simulation, the two slopes have contrasting values of the parameters
that control regolith transport and weathering rate. In the present
simulations, regolith production depends upon aspect because the
mean annual temperature is allowed to vary with aspect: S-facing
slopes are taken to be warmer than N-facing slopes, meaning that
the subsurface on N-facing slopes is more likely to experience
frostcracking, and to greater depths. Transport efficiency, k, is
taken to be lower on the N-facing slopes.
3. Results
The resulting simulations allow exploration of the relative
roles of rock damage and of regolith transport in governing the
shapes of the hillslopes, the thickness of the regolith, and the
character of the underlying saprolite. The simulation summarized
in Fig. 2 captures several features that are observed in Gordon
Gulch. The slope angles on the N-facing slopes can be made to
be slightly higher than on S-facing slopes if the transport rate efficiency (k) is lower on N-facing slopes. In a steady landscape, this
results in migration of the hillcrest toward the low-k hillslope, so
that its hillslope angles can be steeper to compensate for the lower efficiency. The rocks immediately subjacent to the mobile regolith interface on the N-facing slopes (saprolite), have sustained
much higher frost-cracking damage en route to the surface than
their counterparts on S-facing slopes. Finally, the highest regolith
thickness is found near the base of the N-facing slope.
S.P. Anderson et al. / Applied Geochemistry 26 (2011) S3–S5
S5
Fig. 2. Results of 1 million year simulation with initial 100 m 20° slopes to either side of a peak centered at x = 0. Top: topographic profile (black) and mobile regolith interface
(gray), underlain by contours of equal rock damage due to frost cracking (50% of maximum in dashed, 10% of maximum dotted). Bottom: mobile regolith thickness at the end
of the simulation, showing slight increase in thickness on the N-facing slope. Top inset: the field of rock damage at the end of the simulation, as a function of both depth into
the subsurface, and distance across the slope. Damage reaches to greater depths on the N-facing slope, and reaches its maximum at the base of the N-facing slope, where
mean annual temperatures are lowest.
4. Conclusions
Models of regolith transport, mobile regolith production, and
rock ‘‘damage’’ by weathering processes show that variations in
environmental parameters can explain differences in the character
of the Critical Zone seen on different slope aspects in Gordon
Gulch. While there are no doubt other processes operating on the
hillslopes in Gordon Gulch that have not been incorporated in
the models, it is encouraging that explicit recognition of several
specific surface and subsurface processes can capture the essence
of the observed character of the Critical Zone architecture. That
these processes are all directly tied to climate variables, such as
mean annual temperature, allows more robust exploration of the
role of climate change in governing the evolution of Critical Zone
architecture.
Acknowledgements
This work was supported by NSF-0724960. We thank S. Banwart and R. Fuge for their reviews.
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