Earth Surface Processes and Landforms
Fluvial
bedrock
incision
in Taiwan
Earth Surf.
Process.
Landforms
30, 955–971 (2005)
Published online in Wiley InterScience (www.interscience.wiley.com). DOI: 10.1002/esp.1256
955
Fluvial bedrock incision in the active mountain belt
of Taiwan from in situ-produced cosmogenic nuclides
M. Schaller,1* N. Hovius,1 S. D. Willett,2 S. Ivy-Ochs,3 H.-A. Synal4 and M.-C. Chen5
1
Department of Earth Sciences, University of Cambridge, Downing Street, Cambridge, CB2 3EQ, UK
Department of Earth and Space Sciences, University of Washington, Seattle, Washington 98195, USA
3
Institute of Particle Physics, ETH Hönggerberg, CH-8093 Zürich, and Department of Geography University of Zürich – Irchel,
CH-8057 Zürich, Switzerland
4
Paul Scherrer Institut, c/o Institute of Particle Physics, ETH Hönggerberg, CH-8093 Zürich, Switzerland
5
Taroko National Park Headquarters, Fusu Village, Hualien, Taiwan R.O.C.
2
*Correspondence to: M. Schaller,
Department of Geological
Sciences, University of Michigan,
2534 C.C. Little Building, 1100
N, University Ave., Ann Arbor,
MI 48109-1005, USA.
E-mail: [email protected]
Received 1 September 2004;
Revised 28 February 2005;
Accepted 17 April 2005
Abstract
The concentration of cosmogenic nuclides in rocks exposed at the Earth’s surface is proportional to the total duration of their exposure. This is the basis for bedrock surface exposure
dating and has been used to constrain valley lowering rates in the Taroko gorge, eastern
Central Range, Taiwan. Taroko gorge contains a uniquely complete geomorphic record of
fluvial valley lowering: continuous, fluvially sculpted surfaces are present in the lower 200 m
of this marble gorge. Assuming no post-fluvial erosion of the gorge wall, the concentration of
in situ-produced cosmogenic 36Cl measured in gorge wall marbles reveals exposure ages from
0·2 ka in the active channel to 6·5 ka at 165 m above the present river. These ages imply
an average fluvial incision rate of 26 ± 3 mm a−1 throughout the middle and late Holocene.
Taking into account lateral gorge wall retreat after initial thalweg lowering would give rise
to calculated older exposure ages. Without considering gorge wall retreat, our estimates
therefore represent maximum incision rates. Estimated maximum Holocene incision rates
are higher than the long-term exhumation rates derived from fission track dating. The longterm gorge development governed by tectonic uplift is superimposed by short-term variations in incision rates caused by climatic or regional tectonic changes. Copyright © 2005
John Wiley & Sons, Ltd.
Keywords: fluvial incision; cosmogenic nuclides; tectonics; climate; Taiwan
Introduction
Where hillslopes and valley floors are effectively coupled, landscape lowering is driven by fluvial incision into
uplifting rock mass, and hillslopes follow. This is the case in most active mountain belts. Independent assessments of
fluvial incision and its controls (Tinkler and Wohl, 1998) are fundamental to the understanding of erosional landscape
evolution (Howard and Kerby, 1983), crustal deformation (e.g. Beaumont et al., 1992; Koons, 1989; Willett, 1999),
basin fill (e.g. Clift and Gaedicke, 2002), and ocean chemistry and atmospheric composition and circulation (FranceLanord and Derry, 1997; Raymo and Ruddiman, 1992). Rates of fluvial bedrock incision are necessarily influenced by
climatic and tectonic processes (Whipple et al., 1999). Models of physical processes of fluvial incision are contentious, but many arguments have been made that incision is proportional to water discharge or discharge variability
(Snyder et al., 2003; Tucker, 2004; Tucker and Bras, 2000) and may be enhanced by temperature-dependent weathering (Gaillardet et al., 1999), thus linking incision to climatic processes. In addition, tectonic processes perturb river
channel slopes (Snyder et al., 2000) and/or cross-sections (Lavé and Avouac, 2000) and rock mass properties, thus
setting the bed shear stress of a given flow, and the erodibility of its substrate.
The effective evaluation of climatic and tectonic controls on river incision requires quantitative constraints on
erosion rates and patterns on timescales of climate change and tectonic forcing. Dated strath terraces are commonly
used for this purpose (e.g. Amorosi et al., 1996; Burbank et al., 1996). Straths are planar remnants of old bedrock
channel floors, isolated above an incising river. They are separated by erosional steps and do not normally permit the
Copyright © 2005 John Wiley & Sons, Ltd.
Earth Surf. Process. Landforms 30, 955–971 (2005)
956
M. Schaller et al.
reconstruction of an incision history at high temporal resolution (Pratt et al., 2002). Taroko gorge in east Taiwan
contains a rare, continuous record of river incision, consisting of extensive, fluvially sculpted sections of the marble
gorge wall. Using in situ-produced cosmogenic 36Cl in marble samples collected from a 200 m high cliff section, we
have dated fluvially sculpted features in the gorge wall, and reconstructed the Holocene incision history of the Liwu
River. Existing erosion data for the Liwu catchment include annual measurements of fluvial bedrock wear at selected
sites (Hartshorn et al., 2002), decadal suspended sediment transport estimates from hydrometric measurements (Water
Resources Agency, 1970–2003; Dadson et al., 2003), millennial valley lowering estimates from dated river terraces
(Liew, 1988), and fission track estimates of rock exhumation over a million-year timescale (Liu et al., 2001; Willett
et al., 2003), making this an optimal location to study (fluvial) erosion and its controls. Placed in the context of
these other estimates of incision and erosion rates, our data reveal that Holocene incision by the Liwu River has
been considerably faster than the Quaternary average, and that incision rates may have varied within this interval.
Study Area
Taiwan
Taiwan is possibly the best documented collision orogen dominated by fluvial processes. The formation of the orogen
results from collision of the Luzon Arc, on the Philippine Sea plate, and the Asian continental margin (Teng, 1990;
Figure 1), and links opposite-dipping subduction systems at the Manila and Ryukyu trenches. Obliquity between the
Manila trench and the Asian continental margin has led to southward propagation of the collision over the past
c. 5 Ma. This propagation manifests as a progression from submarine accretionary prism building at the Manila trench
south of Taiwan, through full-scale arc–continent collision represented by the subaerial mountain belt, to orogen
Figure 1. The catchment of the Liwu River in eastern Taiwan. The Liwu River flanks the Central Range from the main divide
(3500 m a.s.l.) to the Pacific Ocean and drains approximately 600 km2 of steep terrain. Where the Liwu River crosses a 6 km
thick sequence of marbles and gneisses, the river forms the deep Taroko gorge. Erosion and incision rates from different studies
and the approximate study locations are indicated.
Copyright © 2005 John Wiley & Sons, Ltd.
Earth Surf. Process. Landforms 30, 955–971 (2005)
Fluvial bedrock incision in Taiwan
957
destruction in response to Ryukyu back-arc extension in the far north of Taiwan (Teng, 1996). Across central Taiwan,
metamorphic grade increases from poorly consolidated, Late Tertiary sediments in the Western Foothills thrust belt,
through slates in the Hsuehshan and western Central Ranges, to greenschist-grade pre-Tertiary metasediments in the
eastern Central Range. The current rate of convergence between the Philippine Sea plate and Asia is 80 mm a−1 (Yu
et al., 1997), and rock uplift rates of 5–7 mm a−1 have been calculated from Holocene coastal platforms (Bonilla,
1977; Liew et al., 1990). Rapid rock uplift has resulted in the construction of up to 4 km of subaerial relief, but due to
its low latitudinal position, Taiwan has never experienced extensive glaciation (Ho, 1988). The main drainage divide
of the mountain belt runs parallel to, and c. 25 km west of, the range-bounding Longitudinal Valley fault. Regularly
spaced transverse rivers drain the mountain belt to the east, cross-cutting the structural grain. Westward drainage is by
larger rivers that follow structural trends. Straight slopes, mostly around 35° (less in weak sedimentary rocks in the
Western Foothills) and with thin (<1 m), discontinuous regolith cover, flank the montane valleys (Hovius et al., 2000).
Valley floors are in bedrock, mantled by discontinuous, coarse-grained lag deposits. In these cascading channels little
material is available for fluvial transport unless provided by hillslope mass wasting (Montgomery and Buffington, 1997).
Taiwan has a subtropical climate with an average of four typhoons per year and mean annual precipitation of
2500 mm (Wu and Kuo, 1999). Precipitation is orographically enhanced at high elevation, but, otherwise, is symmetrically distributed across the mountain belt. Runoff in the main streams draining the mountain belt reflects the seasonality
of precipitation. The bulk of the water discharge occurs between June and October. During typhoon passage, daily
rainfall rates in the region can top 400 mm, causing peak discharges of >100 times the annual average. Runoff
variability is greatest along the east flank of the Central Range, which is exposed to direct impact of typhoons moving
off the Pacific Ocean, and in the southwest of Taiwan. The pollen record of Sun Moon Lake, central Taiwan, indicates
that the climate during the Last Glacial Maximum was substantially colder and drier (Kuo and Liew, 2000) (Figure 2).
Liwu catchment
The Liwu catchment is situated towards the northern end of the compressional Taiwan orogen (Figure 1). The
catchment flanks the Central Range from the main divide (3500 m a.s.l.) to the Pacific Ocean and drains approximately 600 km2 of steep terrain underlain by metasediments, mainly schists, gneisses and marbles (Figure 3). Mean
annual precipitation is around 2200 mm throughout the catchment and precipitation rates have reached up to 600 mm/
day during typhoon passage. A palaeoclimate record is not available for the catchment, but it is believed that general
climate trends are shared with the Sun Moon Lake location (Kuo and Liew, 2000) across the main divide.
Long-term exhumation rates estimated from fission track dating of zircon and apatite are around 4 mm a−1 (Liu
et al., 2001; Willett et al., 2003), similar to fluvial incision rates measured directly between February 2000 and May
2002 (Hartshorn et al., 2002). Catchment-wide erosion rates derived from river load gauging over the last 30 years in
the Liwu River are 12·5 mm a−1 (Dadson et al., 2003). Radiocarbon-dated strath terraces along the main stream imply
higher average incision rates of ≥6 mm a−1 and ≥11 mm a−1 over the last 2·5 ka (Liew, 1988).
The Liwu River crosses a 6 km thick sequence of marbles and gneisses (Figure 3). These rocks have a compressional
strength of 45–70 MPa (unpublished industry data: H. Chen, pers. comm. 2004) and their unjointed nature has
permitted the development of a 2 km deep bedrock gorge with very steep (>60°) walls (Figure 4). The flanks of
Taroko gorge contain suites of flutes. These elongate, smooth depressions and ribs are up to >10 m long, and dip
gently and flare out in a downstream direction. They are thought to have formed in the active river channel due to
impact abrasion by suspended sediment in turbulent stream eddies which were locked in place by existing channel bed
roughness. Evolving flutes are found in outcrops of unjointed rock in the present river channel. They are particularly
well developed on the steep sides of marble reaches where they may be preserved where thalweg lowering has
Figure 2. Precipitation and temperature variations in Taiwan over the last 15 ka. The unpublished data were supplied by P.-M. Liew
of the National Taiwan University and based on Liew and Huang (1994) and Kuo and Liew (2000).
Copyright © 2005 John Wiley & Sons, Ltd.
Earth Surf. Process. Landforms 30, 955–971 (2005)
958
M. Schaller et al.
Figure 3. Map showing a geological overview of the Taroko gorge, Taiwan, and its lithologies summarized from Chen et al. (2000).
The star represents the locality of the wall section sampled for in situ-produced cosmogenic 36Cl analysis.
brought them above the maximum flood level. In the absence of mass wasting, long vertical suites of such flutes may
form due to progressive fluvial incision of uplifting bedrock. In Taroko gorge we have found fluted bedrock surfaces
up to 300 m above the active river channel (a.a.r.c.), and continuous flute sequences up to 200 m a.a.r.c. The active
channel is defined as the lowest part of the bedrock valley which is episodically flooded. Flood marks are present in
Taroko gorge up to 15 m above the channel bed.
Vertical suites of fluvial sculptures contain an integral record of fluvial incision, and provide an opportunity to study
incision rates over a timescale not otherwise preserved. We have dated fluvially sculpted facets in the Taroko gorge in
order to obtain a record of incision of the Liwu River. In the following sections we give an account of this work.
Subsequently, we evaluate the principal results in the context of other estimates of (fluvial) erosion in the Liwu
catchment.
Method
Sample collection and processing
We collected 12 rock samples at regular intervals along a vertical line from a 205 m high buttress at the upstream end
of the Taroko marble gorge (Figures 4 and 5). Sample sites were located on shallowly dipping, fluvially sculpted
surfaces with preserved boulder impact divots that formed in the palaeochannel. Above 165 m a.a.r.c. no fluvially
sculpted surfaces could be found with impact divots. Instead, these surfaces show signs of carbonate dissolution (e.g.
hackly surfaces and dissolution pits) and the flutes are less clearly visible. At each sample location, a slab of marble 2
to 6 cm thick (weighing up to 2 kg) was collected from the surface with hammer and chisel. Sample locations were
surveyed with a total station, and present sky-exposure geometry was constrained with a compass-clinometer.
For exposure dating of the fluvially sculpted surfaces in calcareous rocks, whole-rock samples were crushed and
sieved. Approximately 100 g of the grain size fraction 0·125–0·25 mm or 0·125–0·5 mm were cleaned with weak nitric
acid (c. 0·2 M). Samples were spiked with 4 mg of 35Cl before dissolution. The spike method allows measurement of
the total Cl concentration with high precision on the same sample material as the 36Cl concentration (Elmore et al.,
1997). Chlorine was then extracted using the method described in Stone et al. (1996). The 36Cl/35Cl as well as 37Cl/35Cl
ratios were measured at the accelerator mass spectrometry facility of the Paul-Scherrer Institute and the ETH of
Zürich, Switzerland. Details of the extraction and measurement procedures as well as the standards used are given by
Ivy-Ochs et al. (2004). Major and trace elements were determined by XRAL, Canada. Results are given in Table I.
Calculation of surface exposure ages
The principles of surface exposure dating with cosmogenic nuclides were described by Phillips et al. (1986), Lal
(1991), Zreda et al. (1991), and Cerling and Craig (1994). Recent summaries of the technique and its applications are
given by Zreda and Phillips (2000) and Gosse and Phillips (2001), and are described only briefly here.
Copyright © 2005 John Wiley & Sons, Ltd.
Earth Surf. Process. Landforms 30, 955–971 (2005)
Fluvial bedrock incision in Taiwan
959
Figure 4. Overview of the gorge wall selected for sample collection. This gorge section is the uppermost marble cliff of the
Taroko gorge. (a) Upper section of the gorge wall. Samples were collected from below and above the overhanging section. The
highest sample (C10D) is from an altitude of 205 m above the active river channel. (b) Fluvially sculpted marble surfaces in
the lower section of the gorge wall. Sample C6D was collected from an altitude of 3 m above the active river channel. (c) Welldeveloped flutes in marble in the Liwu catchment, Taiwan. Such surfaces were collected for cosmogneic nuclide analysis and
exposure dating.
Figure 5. Diagram giving an overview of the sample locations. Six samples from below and six from above the overhanging wall
were successfully analysed. Samples from higher than 165 m above the active river channel show dissolution marks. The highest
sample (C10D) was collected from the flat cliff top.
The Earth’s surface is continuously bombarded by cosmic rays such as neutrons and muons. The interaction of these
cosmic rays with 40Ca, 39K and 35Cl in minerals near the Earth’s surface results in the generation of in situ-produced
36
Cl. The exposure age of a rock that has been completely shielded until its continuous exposure at the Earth’s surface
without further surface erosion is given by:
t=
Copyright © 2005 John Wiley & Sons, Ltd.
−1 
λ C( R − R0 ) 
ln 1 −



P
λ
(1)
Earth Surf. Process. Landforms 30, 955–971 (2005)
960
M. Schaller et al.
Table I. Chemical composition of Taroko gorge samples
Sample
CaO
(wt%)
MgO
(wt%)
Sm
(ppm)
B
(ppm)
Gd
(ppm)
Cl rock
(ppm)
C6
C7D
C7
C31D
C32C
C33D
C36C
C20CD
C19D
C16D
C14D
C12D
C10DT
C10DB
54·89
55·03
55·28
55·86
41·50
52·34
52·35
50·03
55·29
56·00
55·70
55·30
55·82
55·84
1·16
0·71
0·74
0·62
12·67
2·46
2·32
4·63
0·94
0·17
0·36
0·47
0·58
0·56
0·05
0·05
0·20
0·05
0·05
0·20
0·50
0·05
0·05
0·05
0·05
0·05
0·05
0·05
0·25
0·25
0·25
1·50
1·50
0·25
1·50
0·25
1·50
1·50
1·50
0·25
1·50
1·50
0·25
0·25
0·25
0·50
0·50
0·25
0·50
0·25
0·50
0·50
0·50
0·25
0·50
0·50
2·72 ± 0·02
0·78 ± 0·01
0·81 ± 0·81
0·63 ± 0·01
2·05 ± 0·02
2·46 ± 0·02
1·69 ± 0·02
11·47 ± 0·11
1·85 ± 0·02
0·36 ± 0·01
0·71 ± 0·01
1·18 ± 0·01
0·57 ± 0·01
0·76 ± 0·01
36
f35
Th
(ppm)
U
(ppm)
ClCosm.*
(10 atoms (g(rock))−1)
5·77E-04
1·65E-04
1·72E-04
1·30E-04
5·16E-04
5·28E-04
3·35E-04
2·54E-03
3·86E-04
7·30E-05
1·46E-04
2·50E-04
1·18E-04
1·58E-04
0·2
0·2
0·3
0·05
0·05
0·1
0·4
0·2
0·05
0·05
0·05
0·1
0·05
0·05
0·95
2·09
2·35
2·38
1·19
2·19
2·87
2·41
1·52
1·39
5
1·34
2·52
2·66
2·4 ± 1·0
13·3 ± 1·2
16·2 ± 1·6
21·5 ± 4·0
25·5 ± 2·7
23·9 ± 2·2
21·6 ± 3·2
51·1 ± 2·2
60·1 ± 4·6
106·7 ± 4·1
80·0 ± 5·2
19·7 ± 2·3
30·5 ± 2·7
29·1 ± 2·8
3
* In situ-produced cosmogenic 36Cl concentration corrected for blank and production of 36Cl induced by U and Th.
where t is the exposure age [a], λ is the decay constant for 36Cl [2·30 × 10−6 a−1], C is the Cl content in the rock [atoms
g(rock)−1], P is the total cosmogenic production rate of 36Cl [atoms g(rock)−1 a−1], R is the measured 36Cl/Cl ratio corrected
for blank measurements, and R0 is the 36Cl/Cl ratio supported by radiogenic production from U and Th. The total
cosmogenic production rate of 36Cl must be adjusted for the altitude and latitude of the sample, the dip of the sampled
surface, shielding by the surrounding topography, the depth of the sample below the surface, changes of the
palaeomagnetic field variation over time, and the concentration of the target element and the assumed elemental
production rate. The fact that 36Cl is not only a spallogenic and muonic product, but also produced by thermal and
epithermal production out of Cl, complicates the interpretation of the exposure ages. However, in our case of low Cl
concentration, the production of Cl is mainly spallogenic and muonic and the exposure age reflects a minimum age
(see figure 20 of Gosse and Phillips, 2001).
The altitude and latitude of our samples have been accounted for using Dunai’s procedure (Dunai, 2000), expanded
for reduced intensity of stopped and fast muons following Allkofer (1975). Given that our samples were collected
from a gorge wall, the cosmic ray flux is reduced by the steep inclination of the surface and by topographic shielding
(shielding factor <1). The correction of the production rate for surface dip and topographic shielding uses the equation
of Nishiizumi et al. (1989). However, the shielding geometry of a given sample site has potentially changed as the
valley deepened. In the absence of precise constraints on the palaeovalley geometry, we assume that the valley
geometry has remained constant over time, and that the sampled surface has moved upward with respect to the valley
floor and into progressively less shielded positions. For each sample site, the present-day shielding factor was calculated. Exposure ages of the sampled surfaces were then calculated using the average of the shielding factors at and
below the sample site. Changes of cosmogenic nuclide production rate due to palaeomagnetic field variations over the
last 10 ka were within 1 per cent of present-day values (Masarik et al., 2001), and no correction has been made for
this effect. Moreover, irradiation of the sample before exposure at the surface is assumed to have been negligibly
small compared to post-exposure irradiation.
For the determination of the production of 36Cl, we considered the production by spallation, fast and stopped muons
from 40Ca, the thermal production from 35Cl as well as background production of 36Cl by thermal neutrons resulting
from U and Th reactions. The epithermal production has been neglected, following Mitchell et al. (2001). For the
production parameters of 36Cl from 40Ca by spallation, stopped and fast muon capture we used the values of 18·6 ± 0·6
atoms g(CaCO3)−1 a−1, 2·59 ± 0·29 atoms g(CaCO3)−1 a−1, and 0·296 ± 0·063 atoms g(CaCO3)−1 a−1, respectively (Heisinger
et al., 2002a, b). These production rates have been corrected for sample thickness using the depth-dependences of
Schaller et al. (2002). The production rates of 36Cl by thermal neutron capture were determined with the equations and
parameters as provided in Mitchell et al. (equation A5 in Mitchell et al., 2001). However, for the depth correction of
the thermal production, we used the attenuation length of neutrons of Schaller et al. (2002) in the first term of equation
A5 in Mitchell et al. (2001) and a characteristic length for neutrons in pure calcite of 33 g cm−2 (Liu et al., 1994) in
the second term. Background production of 36Cl by thermal neutrons resulting from U and Th reactions was calculated
Copyright © 2005 John Wiley & Sons, Ltd.
Earth Surf. Process. Landforms 30, 955–971 (2005)
Fluvial bedrock incision in Taiwan
961
Table II. Locations, surface exposure ages and incision rates of Taroko gorge samples
Surface exposure ages
36
Sample(1)
C6D
C7D
C7
C31D
C32C
C33D
C36C
C20CD
C19D
C16D
C14D
C12D
C10DT
C10DB
Altitude
m
3
33
33
52
55
58
81
150
159
165
171
181
205
205
(2)
Shielding(3)
0·744
0·783
0·783
0·744
0·731
0·715
0·724
0·745
0·516
0·768
0·763
0·765
0·795
0·795
ClCosm·(4)
103 atoms
(g(rock))−1
2·4
13·3
16·2
21·5
25·5
23·9
21·6
51·1
60·1
106·7
80·0
19·7
30·5
29·1
±
±
±
±
±
±
±
±
±
±
±
±
±
±
1·0
1·2
1·6
4·0
2·7
2·2
3·2
2·2
4·6
4·1
5·2
2·3
2·7
2·8
Zero
erosion(5)
ka
0·17
0·88
1·07
1·46
2·36
1·79
1·55
3·53
5·55
6·48
4·88
1·19
1·75
1·67
(5a)
(5b)
Error
ka
Error
ka
0·07
0·08
0·10
0·27
0·25
0·16
0·23
0·15
0·42
0·25
0·32
0·14
0·15
0·16
0·09
0·10
0·13
0·29
0·31
0·22
0·26
0·31
0·61
0·57
0·48
0·17
0·21
0·20
0·05 mm a−1
erosion(6)
ka
Incremental
incision rate(7)
mm a−1
±
±
±
±
±
±
±
±
±
±
±
±
±
±
17 ± 14
0·17
0·89
1·09
1·51
2·52
1·87
1·61
3·85
6·77
8·27
5·79
1·21
1·81
1·76
0·09
0·11
0·14
0·31
0·36
0·24
0·29
0·37
0·94
0·96
0·70
0·18
0·23
0·22
38 ± 11
25 ± 16
34 ± 8
5± 4
7 ± 10
Average
incision rate(8)
mm a−1
17
38
31
36
23
32
52
42
29
25
35
152
117
123
±
±
±
±
±
±
±
±
±
±
±
±
±
±
14
7
6
8
4
5
9
4
3
2
4
22
14
15
(1)
(2)
The latitude and longitude of the sample site is 23°59′ and 121°31′, respectively.
The indicated altitude is the altitude above the active river channel, which is 354 m a.s.l., The estimated absolute error in altitude is 5 m, except for
C6D where it is 2 m.
(3) The shielding factor for one sample is an average of shielding factors derived from the shielding geometry of all samples situated below the
addressed sample. The correction for shielding by the sample dip and the surrounding gorge walls are based on equations by Nishiizumi et al.
(1989).
(4) Cosmogenic 36Cl concentration after subtraction of radiogenic 36Cl. Corrections are <4 per cent. Uncertainties are ± 1σ including blank corrections,
10 per cent error on the radiogenic correction, Cl concentration and AMS errors.
(5) Surface exposure ages assuming zero erosion of the surface.
(5a) The error includes anlytical and blank uncertainties.
(5b) The error includes analytical and blank uncertainties, uncertainties in scaling factors for altitude and latitude (5%), uncertainties in the altitude of the
sample, the uncertainties of the production rate resulting from different sample chemistry and the uncertainty in production rate at sea level (error
for production by spalllation, fast and stopped muons is taken from Heisinger et al., 2002a, b).
(6) Surface exposure ages assuming an erosion rate of 0·05 mm a−1 of the surface. The error is based on the same uncertainties as stated in note (5b).
(7) Incremental incision rate is based on the altitude and age differences between the sample in the row and the next sample lower in altitude.
Exceptions are C7 and C7D where the average age and C31D, C32C, and C33D where the average altitude and age have been used. Errors are
based on uncertainties in surface exposure ages as reported in note (5b) and altitude above the active river channel.
(8) Average incision rate is based on the altitude over the active river channel of the sample and the age of the sample. Errors are based on
uncertainties in surface exposure ages as reported in note (5b) and altitude above the active river channel.
using the method of Fabryka-Martin (1988). As the Cl content of the investigated rocks is generally low, the amount
of 36Cl produced from U and Th reaction is less than 4 per cent of the total 36Cl concentration.
Analytical uncertainties arise from geochemical parameters such as the concentrations of Cl and Ca and the 36Cl/Cl
ratio. The estimate of the production rate of cosmogenic nuclides at sea level and high latitude is a source of
uncertainty in exposure age determination. Our error estimates are based on the uncertainties as given in Heisinger
et al. (2002a, b), The topographic shielding correction on production rate, and particularly its change in time are
poorly constrained and the source of an unquantified error. Our estimate of the total error in exposure ages includes
analytical and blank uncertainties (1σ), uncertainties in scaling factors for altitude and latitude (5%), uncertainties
in the altitude of the sample, the uncertainties of the production rate due to sample chemistry, and uncertainties in
the production rate at sea level and high latitude. Errors on individual exposure ages are between 0·09 ka and 0·61 ka
(see footnote 5b in Table II). For inter-sample comparison, uncertainties in scaling factors for altitude and latitude,
altitude of the sample, and the production rate at sea level can be neglected as they are the same for all samples. This
reduces the range of errors to 0·07 to 0·42 ka (see footnote 5a in Table II). It should be noted that this method
of exposure dating assumes that the dated feature has not been eroded after its formation. In our case of low Cl
concentration in the rock, any assumed erosion of fluvial sculptures on the gorge wall after their emergence from the
active channel would result in an increase of the calculated apparent surface age. Thus, the calculated surface ages are
minimum ages for the fluvial features targeted by us.
Copyright © 2005 John Wiley & Sons, Ltd.
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M. Schaller et al.
Figure 6. In situ-produced cosmogenic 36Cl concentration for samples from different altitudes above the active river channel.
Black squares represent the samples prepared with the standard one-step leaching techniques. Two samples, one from below and
one from above the overhanging wall, were dissolved in three steps and analysed separately (open squares). The dissolution steps
of the sample from 52 m above the active river channel are within error and not distinguishable. The first dissolution step of the
sample from 181 m above the active river channel is higher than the following two dissolution steps.
Cosmogenic nuclide concentrations in a surface sample can also be used for the direct calculation of maximum
erosion rates (Lal, 1991), assuming steady state of production and loss of nuclides. In this method, the same assumptions and corrections apply as described above for surface exposure dating.
Results of the Cosmogenic Nuclide Study
Nuclide concentrations and dissolution experiments
Measured Cl concentrations in 12 samples from different altitudes in the Taroko gorge are between 0·4 and
11·5 ppm. The blank corrected concentrations of in situ-produced 36Cl nuclide are very low, 0·24 × 104 to 10·7 ×
104 atoms g(rock)−1 (Table I and Figure 6). For the lowermost sample (C6D) the influence of the blank correction is
dominant and the reliability of the measured 36Cl nuclide concentration low. Reliability is better in higher samples, and
there is good agreement between nuclide concentrations measured independently in separate samples collected from
the second lowest site (C7 and C7D). A general increase in nuclide concentration is observed from 3 to 165 m above
the active river channel (a.a.r.c.), but concentrations are constant within error between 50 and 80 m a.a.r.c. In samples
collected above 165 m a.a.r.c. measured nuclide concentrations are lower than the directly underlying samples at 150
to 165 m a.a.r.c.
Rock surfaces with low nuclide concentrations, above 165 m a.a.r.c., are characterized by (partial) dissolution of
fluvially sculpted features such as impact divots. Is post-fluvial erosion the cause of low nuclide concentrations in
these samples or is the rock an open system for Cl nuclides? As flutes are still visible, erosion cannot have been more
than a few centimetres and is therefore thought not to have affected the nuclide concentration significantly (this is
confirmed by our calculations in the next section). Open system behaviour could be the more important factor.
Incorporation of meteoric Cl (35Cl, 36Cl and 37Cl) in near-surface rocks would cause the calculated 36Cl concentration
to diverge from the in situ-produced cosmogenic 36Cl concentration (Kubik et al., 1984; Stone et al., 1996). If the
meteoric Cl were rich in 36Cl, an exposure age based on the total concentration of 36Cl would be erroneously high.
Incorporation of meteoric Cl rich in 35Cl and 37Cl would have an opposite effect on the calculated age. Incorporation
of meteoric Cl would elevate the total Cl content of the rock and this would lead to an overestimate of 36Cl produced
by radiogenic neutron capture (e.g. from U and Th). In our calculations, the in situ-produced cosmogenic 36Cl
concentration would then be overcorrected, resulting in an underestimate of the exposure age. An open system could
Copyright © 2005 John Wiley & Sons, Ltd.
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963
Table III. Stepwise dissolution of two samples from Taroko gorge
Sample
Sample 52 m above active river channel
C31C-3 * Dissolution 1
C31C-2 * Dissolution 2
C31C-1
Dissolution 3
Composite of dissolution steps
C31D
Standard technique
36
Weight
loss (g)
Cumulative
fraction
dissolved (%)
Ca
(g g−1 in rock)
Cl
(ppm in rock)
45·35
72·37
99·67
21
54
100
not measured
on individual
fraction
5·44
0·94
0·60
1·72
0·63
not measured
on individual
fraction
3·66
1·32
2·87
2·70
1·18
not measured
on individual
fraction
0·401 ± 0·013
118·33
Sample 181 m above active river channel
C12D-3
Dissolution 1
C12D-2
Dissolution 2
C12D-1
Dissolution 3
Composite of dissolution steps
C12D
Standard technique
51·89
44·33
66·99
76·92
32
59
100
not measured
on individual
fraction
0·397 ± 0·013
f35
1·30E-04
2·50E-04
ClCsom.
(103 atoms
(g(rock))−1)
23·8 ± 6·0
20·5 ± 4·6
22·8 ± 3·1
22·3 ± 4·2
21·5 ± 4·0
35·7 ± 6·1
15·3 ± 4·1
19·5 ± 3·0
23·5 ± 4·5
19·7 ± 2·3
* The measured 36Cl/35Cl ratio for these samples is an upper limit. For the calculation of the 36Cl concentration this upper limit and an error of 15 per
cent have been assumed.
also result in loss of in situ-produced cosmogenic 36Cl by preferential leaching (Kubik et al., 1984; Stone et al., 1996),
causing reduction of the exposure age estimate. The weathered nature of samples collected above 165 m a.a.r.c.
indicates that samples have likely been affected by one of these processes, but it is not clear by which process and to
what extent. These samples are suspect and we assign no importance to their apparent ages.
Next, we need to establish if samples collected from apparently unweathered surfaces below 165 m a.a.r.c. show open
system behaviour. In order to investigate possible open system behaviour, we have performed a sequential dissolution
of two samples, one from below and one from above 165 m a.a.r.c. Uniform Cl concentrations for all dissolution
steps would imply a closed system for Cl. Differences in concentrations between dissolution steps would indicate
incorporation or loss of Cl. Stone et al. (1996) in a six-stage dissolution experiment observed that only the first step
had a lower 36Cl concentration. They concluded that the slight depletion in the first step is due to natural dissolution
of grain surfaces, but that their standard sample preparation procedure guarantees the integrity of the calcite analysed
for age determination. For our dissolution experiment, we have dissolved samples from 52 m a.a.r.c. (C31D) and
181 m a.a.r.c. (C12D) in three steps (Table III). For sample C31D, the 36Cl concentration of the three dissolution steps
is within error the same as the concentration derived from the same sample material treated according to the standard
preparation technique which involves a single dissolution step after cleaning of the sample (called standard one-step
dissolution technique). In contrast, the first dissolution step of sample C12D has a significantly higher 36Cl concentration than steps 2 and 3, which are comparable to the concentration measured in the sample prepared with the
standard one-step dissolution technique. The resulting weight-weighted average of the 36Cl concentration is higher
than the concentration determined from the sample prepared with the standard one-step dissolution technique.
These observations have important implications. (1) The standard one-step dissolution technique effectively removes any possible contaminating Cl nuclides from the sample. (2) Sample C31D (52 m a.a.c.r.) has not been affected
by incorporation or dissolution of Cl. We conclude that samples without dissolution marks did not suffer from open
system behaviour, and can be used to determine surface exposure ages. (3) Sample C12D (181 m a.a.r.c.) was not a
closed system and has been affected by incorporation or loss of Cl. The high cosmogenic 36Cl and total Cl concentration measured in the first step may be due to incorporation of meteoric Cl. This would primarily result in dissolution
and recrystallization of calcite along grain boundaries. The Cl composition of the newly formed mineral would be
unrelated to that of the replaced material, and might dominate the first dissolution step. If 36Cl nuclides were contributed by a meteoric source and successfully removed during the standard one-step dissolution technique, exposure ages
calculated from samples prepared with the standard one-step dissolution technique would reflect the true age of the
surface. Alternatively, the high cosmogenic 36Cl and total Cl concentration in the first dissolution step could be due to
chemical weathering compromising the calcite crystal structure. Chemical weathering could be enhanced due to the
high abundance of pyrite in the marbles of Taroko gorge, and sulphate in surface runoff and groundwater (Yoshimura
et al., 2001). Sulphuric acid produced by pyrite dissociation promotes the dissolution of carbonate, and may facilitate
selective leaching of Cl. Exploiting this same effect, we have used nitric acid to remove meteoric Cl from our
Copyright © 2005 John Wiley & Sons, Ltd.
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M. Schaller et al.
Figure 7. Exposure ages for the fluvially sculpted surfaces collected from different altitudes above the active river channel. A
general increase in the exposure age is observed from 3 m to 165 m above the active river channel (filled squares). Surfaces of
samples collected from above 165 m reveal decreasing exposure ages (open squares). Linear regression through samples collected
from below 165 m above the active river channel indicate an incision rate of 26 ± 3 mm a−1.
samples, and to dissolve the rocks. Natural sulphuric acid could mobilize weakly bound Cl from the calcite crystal
lattice in marble, thus lowering the 36Cl and total Cl concentration in the rock and the associated apparent surface
exposure age. At this stage, we cannot attribute the differences in 36Cl and Cl concentrations between the initial and
subsequent dissolution steps of sample C12D to one or the other process. We conclude that in our case, samples with
dissolution marks should not be used for surface exposure dating, but samples showing no dissolution marks are safe
to use.
Exposure ages and incision rates for progressive incision
Measured cosmogenic 36Cl concentrations have been used to calculate surface exposure ages and incision rates based
on the assumption that the Liwu River has cut the surfaces that we subsequently sampled (Figure 7). We also assume
that these surfaces have never been covered after their formation in the river channel, but may have undergone later
(post-fluvial) erosion. Due to the low Cl concentration in our rocks and assuming no post-fluvial erosion as well as
instantaneous abandonment of the active river channel, the calculated exposure age is a minimum age and hence the
incision rate is a maximum rate. Two scenarios have been explored in detail: one without post-fluvial erosion and
another with post-fluvial surface retreat at a rate of 0·05 mm a−1 (Table II). This rate is considered to be a high rate of
erosion on steep gorge walls. It would lead to the obliteration of most fluvially sculpted gorge wall relief within 10 ka,
and was chosen to establish the extent to which surface exposure ages may be affected by post-fluvial erosion. The
calculations have been done for all samples regardless of the observation of open system behaviour.
For the case without post-fluvial erosion, calculated exposure ages range between 0·2 ka and 6·5 ka (Figure 7).
Exposure ages increase to an altitude of 55 m a.a.r.c., but are slightly younger for the next higher samples (58 m and
81 m a.a.r.c.). Above the overhanging gorge wall section, where sample collection was not possible, exposure ages
increase up to 165 m a.a.r.c. Samples collected above 165 m a.a.r.c. reveal young exposure ages. Exposure ages
calculated for a post-fluvial erosion rate of 0·05 mm a−1 are higher, between 0·9 ka and 8·3 ka, and the difference
between the two age estimates increases with the age of the surface to >30 per cent for sample C16D. We consider
these amended surface ages to be at or above the upper limit for samples collected from clearly fluted surfaces with
impact divots (all samples below or at 165 m a.a.r.c.). The correction for post-fluvial erosion is more appropriate for
sample locations above 165 m a.a.r.c. (C14D, C12D and C10D), but it does not increase the ages of these surfaces
beyond 6 ka. Given that the minimum exposure age of the next lower sample is 6·5 ka, progressive, post-fluvial
surface lowering at a rate of up to 0·05 mm a−1 cannot be the sole cause of low 36Cl concentrations in gorge wall
samples collected above 165 m a.a.r.c. Only sample C10D, which was collected from the karstic top of the buttress,
Copyright © 2005 John Wiley & Sons, Ltd.
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965
may have been affected by faster post-fluvial erosion. Assuming steady-state conditions, a mean post-fluvial erosion
rate of 0·9 mm a−1 is implied by the cosmogenic nuclide concentration measured in this sample. Low nuclide concentrations in samples C12D and C14D are likely due to a combination of post-fluvial surface retreat and open system
behaviour for Cl.
Assuming steady-state erosion in the active river channel, maximum surface lowering at site C6D has proceeded at
9 ± 5 mm a−1 in the last 130 years. Above the active channel, maximum average incision rates were calculated using
the sample altitude with respect to the river channel and the local surface exposure age (Table II). Incremental incision
rates were derived from the difference in altitude and exposure age of two successive samples (Table II). In these
calculations we have used surface exposure ages without corrections for post-fluvial erosion; the incision rates are thus
maximum values. According to our calculations, average fluvial incision rates were 26 ± 3 mm a−1 over the last 6·5 ka.
During this interval, rates have varied between 17 mm a−1 and 52 mm a−1. In the last millennium incision occurred at
38 ± 11 mm a−1. Between 2 ka and 1 ka (samples C31D, C32C and C33D) the incremental incision rate was
25 ± 16 mm a−1. Using samples from directly below and above the overhanging wall (C36C and C20CD, respectively),
the incision rate was 34 ± 8 mm a−1 between 3·5 ka and 1·5 ka. However, using the average altitudes and exposure
ages of all four samples collected just below the overhang (C31D, C32C, C33D and C36C) yields a higher incision
rate of 51 ± 15 mm a−1. Samples above the overhang have yielded incremental incision rates of 5–7 mm a−1 for the
period 6·5 ka to 3·5 ka. Post-fluvial erosion of 0·05 mm a−1 reduces the average fluvial incision rate to 20 ± 3 mm a−1:
it does not have a major effect on average incision rates. Taking into account that the gorge walls were not abandoned
instantaneously, but instead emerged slowly from the active river channel does not affect the average incision rate.
Lacking constraints on palaeoflow depths, we have to assume that all surfaces were abandoned at the same altitude
above the floor of the active river channel and at the same time after the first surface exposure. This assumption affects
all exposure ages and estimates of incision in the same way. Our incision rate estimates are subject to further
uncertainty associated with the history of gorge formation which will be explored in the next section.
Effects of aggradation and re-incision
The lowering of Taroko gorge is likely to have been punctuated by phases of flooding and aggradation associated with
massive gorge wall failure and damming of the river (cf. Pratt-Sitaula et al., 2004). During subsequent re-incision, the
river could cut through the landslide dam and fully remove the associated valley fill; it could relocate to the bedrock–
fill interface, causing lateral gorge wall retreat during removal of the blockage; or it could shift into a nearby
depression and cut a new bedrock channel. Here we consider cases of valley fill and re-incision with and without
lateral gorge wall retreat. Remnants of massive valley fills have been observed in the Liwu catchment (Hsieh et al.,
2003; Liew, 1988). One such remnant is located upstream of our sample section. Its base is 40 m a.a.r.c., its top above
the buttress from which we have collected samples, and an age of 2·5 ka has been proposed (M. L. Hsieh, pers.
comm.; Hsieh et al., 2003; Liew, 1988). We have used these observations to constrain some alternative, hypothetical
incision scenarios. These scenarios are presented here to illustrate how valley fill and re-incision may have affected
apparent exposure ages and incision rates calculated from measured cosmogenic nuclide concentrations.
For simplicity we have only explored scenarios with one phase of aggradation, preceded and followed by progressive fluvial incision. The following assumptions have been made: (1) prior to aggradation, the valley floor was at 40 m
above the present-day channel floor; (2) the history of topographic shielding before aggradation was identical to the
one used in age and incision rate calculations in the previous section; (3) instantaneous aggradation occurred at 2·5 ka;
(4) the valley fill covered the entire buttress and had a density of 2·0 g cm−3; (5) the fill was instantaneously removed
at 1·25 ka; (6) during removal of the valley fill, the gorge wall retreated by a given amount (here between 0 and
100 cm); (7) subsequent to removal of the valley fill, the river has incised the remaining 40 m to its present level;
(8) during this phase of incision the topographic shielding was as it is today.
Measured nuclide concentrations have been taken to reflect pre-aggradation accumulation, shielding during valley
fill, possible removal of rock from the gorge wall during re-incision, and post-aggradation accumulation, and sample
ages and incision rates have been recalculated (Figure 8). The scenario without lateral gorge wall retreat yields only
slightly higher exposure ages and an average rate of valley lowering for all samples collected at or below 165 m a.a.r.c.
of 23 ± 3 mm a−1, compared with 26 ± 3 mm a−1 for the case without temporary valley fill. Lateral gorge wall retreat
gives rise to older apparent ages of the original surfaces. For the case of 100 cm of lateral retreat, initial surface ages
are up to three times greater than for the case without lateral retreat, and the corresponding average valley lowering
rate is 7 ± 1 mm a−1. Removal of 25 cm of rock from the gorge wall during re-incision would result in an average
valley lowering rate of 17 ± 2 mm a−1 over the last 9·9 ka. We conclude that taking temporary valley fill into account
is unlikely to have a major effect on the calculated apparent exposure ages of the sampled surfaces, but that assuming
lateral gorge wall retreat during removal of the fill may cause a significant increase of these calculated exposure ages.
Copyright © 2005 John Wiley & Sons, Ltd.
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M. Schaller et al.
Figure 8. The assumption of different incision histories for the gorge formation results in variable incision rates. One phase
of aggradation starting at 2·5 ka and lasting for 1·25 ka is assumed. While removing the sediment, the river may refresh the
fluvially sculpted surfaces. The amount of gorge wall retreat, e.g. 0 cm (grey squares), 25 cm (open squares), 50 cm (grey triangles),
and 100 cm (open triangles), influences strongly the calculated ages and hence the incision rates (see text for further explanation
and discussion).
Several kilometre-scale landslide scars with steep, rough surfaces are present in the Taroko gorge. Some scars reach
down to within a few tens of metres from the present-day channel bed, but the Liwu River is currently not obstructed
by landslide debris anywhere in the gorge. We infer that the river may have been blocked within the past few
millennia, and that removal of the landslide dam(s) has occurred relatively quickly. Small blockages, with a fill depth
of a few tens of metres, may have occurred more frequently. This leaves room for more complex gorge incision
histories, with vertically non-uniform gorge wall retreat. Localized lateral wear of several metres during re-incision
episodes cannot be excluded, and it is also possible that nearby surfaces remained untouched due to alluvial cover.
This mechanism would offer a convenient explanation for the anomalously young surface exposure ages found in
samples C33D and C36C, but in the absence of firm, independent constraints on the aggradation and re-incision
history of Taroko gorge such interpretations remain speculative. In extremis, it is conceivable that the different surface
exposure ages of our samples are the result of differential gorge wall retreat during re-incision superimposed on
constant bedrock channel lowering. However, we do not believe that this is likely: progressive younging of surface
exposures down the gorge, with the aforementioned exceptions, suggests that lateral gorge wall retreat during reincision has played a secondary role in the formation of Taroko gorge, although it may have suppressed some
exposure ages. In the following discussion we take the cosmogenic rates calculated for simple scenarios of gorge
evolution at face value, mindful of the possible complications introduced by lateral river cutting during re-incision of
temporary valley fills.
Discussion
Incision and erosion rates have been determined in the Liwu catchment by different methods, including direct measurement of fluvial wear, suspended sediment gauging, terrace dating and surveying, fission track thermochronometry
and cosmogenic nuclide analysis (Figure 9). We briefly review the key results of these approaches in order of
increasing time coverage, noting that the rates are location- and process-specific.
Direct measurements of river wear. At a hydrometric station 3 km upstream of our sample location, Hartshorn et al.
(2002) measured incremental changes of channel bed topography along traverses across the active river channel with
a precision of about 0·5 mm. Between February 2000 and December 2001, the spatially averaged wear rate in schists,
the dominant lithology along this reach, was 3·4 mm a−1, with maximal local erosion of 69 mm. A wide range of
discharges was recorded during this interval, including a 25-year return time flood. Unpublished fluvial bedrock
incision measurements from a location at the base of the sampled marble buttress show similar wear rates.
Copyright © 2005 John Wiley & Sons, Ltd.
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967
Figure 9. Compilation of incision and erosion rates over different timescales from the Taroko gorge area. Cosmogenic nuclidederived rates (this study), rates from dated fill terraces (Liew, 1988), and bedrock lowering rates (Hartshorn et al., 2002) are
incision rates. Rates derived from river load gauging (Dadson et al., 2003) and rates from fission track dating (Liu et al., 2001;
Willett et al., 2003) reflect erosion rates. The Holocene incision rates are higher than the long-term erosion rate determined
by fission tack dating.
Erosion rates derived from river load gauging. At the same hydrometric station, water discharge has been measured daily and suspended sediment concentrations at an average frequency of 24 samples per year since 1970 (Water
Resources Agency, 1970–2003). Using a bias-corrected average of the reported measurements, Dadson et al. (2003)
obtained an average annual suspended sediment load of 14·4 Mt derived from an area of 435 km2 upstream of the
gauging station. This is equivalent to a catchment-wide average surface lowering rate of 12·5 mm a−1 (using the
density of quartz). A 25 per cent uncertainty applies to this estimate. Between 1970 and 2000, about 70 per cent of all
suspended sediment was transported by floods with a recurrence interval of one year or more, and 50 per cent of the
suspended load was carried by floods with a frequency of five years or more.
Incision rates derived from terraces. Liew (1988) reported on river terraces and perched palaeochannels along the
Liwu River, upstream of Taroko gorge. The ages of two terrace deposits have been dated with 14C to 2·4 ka and 2·5 ka,
respectively (Liew, 1988). Using the height of the bedrock straths of these features above the current channel, Liew
(1988) calculated river incision rates of 6 mm a−1 and 11 mm a−1. These rates are considered to be minimum incision
rates because in some cases the river has shifted course after aggradation and incised a new bedrock channel adjacent
to the filled passage. Accordingly, the thickness of the fill should be added to incision rate calculations.
Cosmogenic nuclide-derived incision rates. Our cosmogenic nuclide work has shown that channel lowering at the
upstream end of Taroko gorge has occurred at a maximum average rate of 9 mm a−1 over the last 130 years. Assuming
progressive channel lowering without temporary aggradation, we have calculated a maximum average rate of river
incision of 26 ± 3 mm a−1 since 6·5 ka. Incision rates were higher in the last 2·5 to 3·5 ka, and only 5–7 mm a−1 during
the preceding interval. However, these estimates are sensitive to post-fluvial erosion and lateral gorge wall retreat
during re-incision of temporary valley fills. Rates may have been lower as a result, but we believe that the mid–late
Holocene acceleration of river incision is a robust feature of our data.
Fission track exhumation rates. Apatite and zircon fission track ages provide cooling rates from temperatures of
about 100 °C and 240 °C, respectively. Assuming that this cooling rate reflects conductive cooling of samples as they
are exhumed to the surface by erosion, ages translate directly into erosion rates. Liu et al. (2001) reported a zircon
fission track age of 2·3 ± 1·6 Ma from a gneiss in the downstream reach of the Taroko gorge. Apatite from the same
unit yields a fission track age of 0·9 ± 0·2 Ma (Willett et al., 2003). Interpreted through a one-dimensional model for
cooling and exhumation, these ages provide an internally consistent estimate of erosion rate of 3 to 5 mm a−1 over the
last 2 Ma.
The following points emerge from this summary. (1) Average Holocene river incision rates in Taroko gorge and
nearby terrace locations have been higher than the Quaternary exhumation rate of the Liwu catchment by at least a
factor two and possibly much more. (2) Millennial incision rates may have varied through the Holocene, and late
Holocene river incision was fast compared to the preceding interval. (3) Modern fluvial incision rates at and upstream
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of the top end of Taroko gorge are lower than the Holocene average incision rate, and similar to the Quaternary
exhumation rate. (4) Decadal catchment-wide erosion rates upstream of Taroko gorge are faster than modern river
incision rates and long-term exhumation rates.
Temporal variability of fluvial incision and catchment lowering may have been caused by trends in rock uplift, sea
level and/or climate. Few independent constraints exist on the rock uplift history of east Taiwan, but given that
regional plate motion vectors have been relatively constant during the Quaternary (Seno et al., 1993), we expect that
average rock uplift rates have not changed importantly in this interval. The uniformity of zircon and apatite fission
track exhumation rates in the Liwu catchment supports this interpretation. On shorter timescales, very large earthquakes (>Mw7·5) may have caused rapid rock and surface uplift and sediment production. Measured Holocene incision
would offset the cumulative effect of many earthquake cycles, but it is conceivable that routing of coseismically
produced sediment (Dadson et al., 2004) has promoted rapid fluvial incision. Temporal clustering of very large
earthquakes might explain elevated Holocene river incision and catchment erosion rates. Lack of independent
evidence bars the validation of this hypothesis.
More information is available for sea-level and climate change. If sea-level rise since the Last Glacial Maximum
has had an effect on the incision of Taroko gorge and catchment lowering further upstream (cf. Merritts et al., 1994),
it should have been to suppress these processes. All available erosion data point in the opposite direction, ruling out
glacial–interglacial sea-level change and smaller Holocene fluctuations (Chen and Liu, 1996) as important controls on
the incision of the Liwu River and erosion of its catchment.
A more likely cause of high Holocene erosion rates is climate forcing. Pollen records from Sun Moon Lake, central
Taiwan, indicate that conditions changed from cool and relatively dry to warm and wet at c. 11 ka when the southeast
monsoon reached Taiwan (Kuo and Liew, 2000). Although the general shift to wetter, warmer conditions during the
Holocene is likely to have enhanced fluvial incision and catchment erosion, the special role of typhoons is evident, for
example, from the disproportionate importance of infrequent, large floods in transporting sediment in the Liwu
catchment and elsewhere in Taiwan (Dadson et al., 2003). Taiwan occupies a central position within the northern west
Pacific typhoon belt. Currently this belt spans the Pacific Rim from the south Philippines to central Japan, with highest
typhoon frequencies in the north Philippines (Joint Typhoon Warning Center, 1972–2001). Latitudinal shifts of the
typhoon belt would cause changes of typhoon incidence frequency and strength in Taiwan. Such changes may have
occurred during the Holocene, and certainly since the Last Glacial Maximum. We attribute elevated Holocene rates of
river incision and landscape lowering in the Liwu catchment, measured by three independent methods, to a combination of warm, wet average conditions and frequent, extreme precipitation and runoff during typhoon impact. Observations of elevated Holocene erosion rates in the Liwu catchment match a two- to three-fold increase of post-glacial
sedimentation rates in Sun Moon Lake since the Last Glacial Maximum.
If high Holocene erosion rates are representative of most Quaternary interglacials then it follows that erosion during
glacial intervals has been, on average, slower than the Quaternary average exhumation rate of 4 mm a−1 estimated
from fission track data. We note that the specific case of Taiwan is in conflict with Molnar’s hypothesis (Molnar, 2001)
that high Quaternary erosion rates are due mainly to increased runoff variability during cold stages. Other upland
regions exposed to frequent typhoon impact may have a similar emphasis on interglacial, storm-driven erosion.
The case for variability of erosion rates during the Holocene is less clear. Terrace-derived incision rates, recent
channel lowering estimated from cosmogenic nuclide concentrations, and decadal suspended sediment erosion rates
are similar at around 10 mm a−1. Relatively low annual incision rates measured by Hartshorn et al. (2002) may not
reflect the full range of water discharge and sediment transport in the Liwu River, making it difficult to extrapolate
these measurements to longer timescales. Our cosmogenic nuclide work may show an acceleration of river incision
rates from 5–7 mm a−1 prior to 3·5 ka to much higher rates since then. Variations of temperature and precipitation have
occurred in Taiwan during the Holocene (Liew and Huang, 1994). The pollen record shows warm, wet conditions
between 11 ka and 5 ka, with increasing strength of the summer monsoon during this interval, and cooler, dryer
conditions since then (Figure 2). Sedimentation rates in Sun Moon Lake track with these climate changes, peaking
during the mid-Holocene climate optimum. Thus, there is a discrepancy between the timing and sense of change of
erosion rates in Taroko gorge and the Sun Moon hinterland, suggesting that climate change may not have been the
principal cause of accelerated late Holocene erosion in Taroko gorge. An additional possible mechanism which we
mention, but cannot easily test, is the possibility of temporal clustering of very large earthquakes.
Finally, we return briefly to the coupling between hillslopes and river channels mentioned at the outset. In contrast
to river incision rates, erosion rates derived from river load gauging reflect the lowering of an entire catchment. If
rivers and hillslopes are perfectly and directly coupled, then the two rates should be the same. Differences between
river incision rates and catchment erosion rates represent a disequilibrium in the landscape. It has been proposed that
maximum hillslope erosion is likely to lag behind maximum river incision (Hartshorn et al., 2002; Pratt-Sitaula et al.,
2004). Although the decadal, catchment-wide erosion rate upstream of Taroko gorge is higher than estimates of recent
Copyright © 2005 John Wiley & Sons, Ltd.
Earth Surf. Process. Landforms 30, 955–971 (2005)
Fluvial bedrock incision in Taiwan
969
fluvial incision in and upstream of the gorge, we have no firm, quantitative evidence to corroborate or reject this
hypothesis for the case of the Liwu River. However, the presence of extensive, fluvially sculpted surfaces along the
very narrow and steep inner section of the Taroko gorge suggests that the marble valley sides do not in general
respond immediately to river incision, and that a time lag of up to 104 years is characteristic for this landscape.
Conclusions
Taroko gorge in the active Taiwan orogen contains a rare, continuous record of Holocene river incision. Measured
concentrations of in situ-produced 36Cl in fluvially sculpted facets of the marble gorge wall imply progressive incision
of the gorge at a maximum average rate of 26 ± 3 mm a−1 since 6·5 ka. Incorporation of meteoric Cl and/or preferential
leaching of 36Cl from near-surface rocks, probably enhanced by the abundance of pyrite in the rock mass, has
compromised the cosmogenic nuclide record of river incision at higher levels within the gorge, and limited our
window of observation to the Holocene. Moreover, temporary fill of the gorge due to landslide damming, and lateral
gorge wall retreat during re-incision may have shielded the fluvial facets from cosmic ray impact and permitted
removal of irradiated rock. This may have created an out-of-sequence pattern in the apparent surface ages, and may
result in a decrease of calculated river incision rates. However, when combined with other estimates of erosion in the
Liwu catchment, our data are firm evidence for fast Holocene incision with respect to the Quaternary average exhumation rate. The warm, wet climate of Taiwan, which is punctuated by frequent impacts of large typhoons, is the likely
cause of elevated interglacial incision rates. We have also found indications of incision rate changes within the
Holocene, but cannot explain them in the context of available climate data.
Acknowledgements
We thank the Taroko National Park authority for permission to sample in the park and for generous logistic support, Hsieh MengLong for sharing his knowledge of river terraces with us, and Lin Jiun-Chuan and Hongey Chen for providing a base in Taipei. We
are indebted to Liew Ping-Mei for making available unpublished data on the Late Quaternary climate of Taiwan. The manuscript has
been improved by the comments of N. P. Snyder and W. B. Dade. Support of the Zürich AMS crew is greatly appreciated. The
Zürich AMS facility is jointly operated by the Swiss Federal Institute of Technology, Zürich, and Paul Scherrer Institute, Villigen,
Switzerland. This study was supported by a grant of the Schweizerische Nationalfonds to M.S., and grants from the Newton Trust
and the Leverhulme Trust to N.H.
References
Allkofer OC. 1975. Introduction to Cosmic Rays. München: Verlag Karl Thiemig.
Amorosi A, Farina M, Severi P, Preti D, Caporale L, Di Dio G. 1996. Genetically related alluvial deposits across active fault zones: an
example of alluvial fan-terrace correlation from the upper Quaternary of the southern Po Basin, Italy. Sedimentary Geology 102: 275–295.
Beaumont C, Fullsack P, Hamilton J. 1992. Erosional control of active compressional orogens. In Thrust Tectonics, McClay KR (ed.).
Chapman and Hall: London.
Bonilla MG. 1977. Summary of Quaternary faulting and elevation changes in Taiwan. Geological Society of China Memoir 2: 43–55.
Burbank DW, Leland J, Fielding E, Anderson RS, Brozovic N, et al. 1996. Bedrock incision, rock uplift and threshold hillslopes in the
northwestern Himalayas. Nature 379: 505–510.
Cerling TE, Craig H. 1994. Geomorphology and in-situ cosmogenic isotopes. Annual Review of Earth Planetary Science 22: 273–317.
Chen Y-G, Liu T-K. 1996. Sea level changes in the last several thousand years, Penghu Islands, Taiwan Strait. Quaternary Research 45:
254–262.
Chen CH, Ho H-C, Shea K-S, Lo W, Lin W-H, et al. 2000. Geologic map of Taiwan. Central Geological Survey, Ministry of Economic
Affairs: Taiwan.
Clift P, Gaedicke C. 2002. Accelerated mass flux to the Arabian Sea during the middle to late Miocene. Geology 30: 207–210.
Dadson SJ, Hovius N, Chen H, Dade WB, Hsieh ML, et al. 2003. Links between erosion, runoff variablity and seismicity in the Taiwan
orogen. Nature 426: 648.
Dadson SJ, Hovius N, Chen H, Dade WB, Lin JC, et al. 2004. Earthquake-triggered increase in sediment delivery from an active mountain
belt. Geology 32: 733–736.
Dunai TJ. 2000. Scaling factors for production rates of in situ produced cosmogenic nuclides: a critical reevaluation. Earth and Planetary
Science Letters 176: 157–169.
Elmore D, Ma X, Miller T, Mueller K, Perry M, et al. 1997. Status and plans for the PRIME Lab AMS facility. Nuclear Instruments and
Methods in Physics Research Section B 123: 69–72.
Fabryka-Martin JT. 1988. Production of radionuclides in the Earth and their hydrogeologic significance, with emphasison chlorine-36 and
iodine-129. PhD thesis, University of Arizona, Tucson.
Copyright © 2005 John Wiley & Sons, Ltd.
Earth Surf. Process. Landforms 30, 955–971 (2005)
970
M. Schaller et al.
France-Lanord C, Derry LA. 1997. Organic carbon burial forcing of the carbon cycle from Himalayan erosion. Nature 390: 65–67.
Gaillardet J, Dupré B, Louvat P, Allègre CJ. 1999. Global silicate weathering and CO2 consumption rates deduced from the chemistry of
large rivers. Chemical Geology 159: 3–30.
Gosse JC, Phillips FM. 2001. Terrestrial in situ cosmogenic nculides: theory and application. Quaternary Science Review 20: 1475–1560.
Hartshorn K, Hovius N, Dade WB, Slingerland R. 2002. Climate-driven bedrock incision in an acitve mountain belt. Science 297: 2036–
2038.
Heisinger B, Lal D, Jull AJT, Kubik PW, Ivy-Ochs S, et al. 2002a. Production of selected cosmogenic radionuclides by muons: 1. Fast
muons. Earth and Planetary Science Letters 200: 345–355.
Heisinger B, Lal D, Jull AJT, Kubik PW, Ivy-Ochs S, et al. 2002b. Production of selected cosmogenic radionuclides by muons: 2. Capture
of negative muons. Earth and Planetary Science Letters 200: 357–269.
Ho CS. 1988. An Introduction to the Geology of Taiwan: Explanatory Text of the Geological Map of Taiwan (2nd edn). Central Geological
Survey, Ministry of Economic Affairs: Taipei.
Hovius N, Stark CP, Chu H-T, Lin J-C. 2000. Supply and removal of sediment in a landslide-dominated mountain belt; Central Range,
Taiwan. Journal of Geology 108: 73–89.
Howard AD, Kerby G. 1983. Channel changes in badlands. Geological Society of America Bulletin 94: 739–752.
Hsieh ML, Liew PM, Chen Y. 2003. Control of Bedload Sediment Supply Upon Bedrock Incision and River Longitudinal Profile in the
Eastern Central Range, Taiwan. Eos Trans. AGU 84(46), Fall Meet. Suppl., Abstract T21C-0465.
Ivy-Ochs S, Synal H-A, Roth C, Schaller M. 2004. Inital results from isotope dilution for Cl and 36Cl measurements at the PSI/ETH Zurich
AMS facility. Nuclear Instruments and Methods in Physics Research B 223–224: 623–627.
Joint Typhoon Warning System. 1972–2001. Yearly TC Occurrence for all Months. http://www.npmoc.navy.mil/jtwc/climostats/trackclimo/
allaor.html
Koons PO. 1989. The topographic evolution of collisional mountain belts; a numerical look at the Southern Alps, New Zealand. American
Journal of Science 289: 1041–1069.
Kubik PW, Korschinek G, Nolte E, Ratzinger U, Enrst H, et al. 1984. Accelerator mass spectrometry of 36Cl in limestone and some
paleontological samples using completely stripped ions. Nuclear Instruments and Methods in Physics Research B5: 326–330.
Kuo CM, Liew PM. 2000. Vegetational history and climatic fluctuations based on pollen analysis of the Toushe peat bog, central Taiwan
since the Last Glacial Maximum. Journal of the Geological Society of China 43: 379–392.
Lal D. 1991. Cosmic ray labeling of erosion surfaces: in situ nuclide production rates and erosion models. Earth and Planetary Science
Letters 104: 424–439.
Lavé J, Avouac JP. 2000. Active folding of fluvial terraces across the Siwaliks Hills, Himalayas of central Nepal. Journal of Geophysical
Research, B, Solid Earth and Planets 105: 5735–5770.
Liew PM. 1988. Sedimentology and River-terrace Correlation of the Liwu River. National Taiwan University: Taipei (in Chinese).
Liew PM, Hsieh ML, Lai CK. 1990. Tectonic significance of Holocene marine terraces in the Coastal Range, eastern Taiwan. Tectonophysics
183: 121–127.
Liew PM, Huang SY. 1994. A 5000-year pollen record from Chitsai lake, central Taiwan. Terrestrial, Atmosphere and Oceanic Sciences 5:
411–419.
Liu B, Phillips F, Fabryka-Martin J, Fowler M, Stone W. 1994. Cosmogenic (super 36) Cl accumulation in unstable landforms; 1, Effects of
the thermal neutron distribution. Water Resources Research 30: 3115–3125.
Liu T-K, Hsieh S, Chen Y-G, Chen W-S. 2001. Thermo-kinematic evolution of the Taiwan oblique-collision mountain belt as revealed by
zircon fission track dating. Earth and Planetary Science Letters 186: 45–56.
Masarik J, Frank M, Schäfer JM, Wieler R. 2001. Correction of in situ cosmogenic nuclide production rates for geomagnetic field intensity
variations during the past 800,000 years. Geochimica et Cosmochimica Acta 65: 2995–3003.
Merritts DJ, Vincent KR, Wohl EE. 1994. Long river profiles, tectonism, and eustasy; a guide to interpreting fluvial terraces. Journal of
Geophysical Research, B, Solid Earth and Planets 99: 14031–14050.
Mitchell S, Matmon A, Bierman P, Enzel Y, Caffee M, Rizzo D. 2001. Displacement history of a limestone normal fault scarp, northern
Israel, from cosmogenic (super 36) Cl. Journal of Geophysical Research 106: 4247–4264.
Molnar P. 2001. Climate change, flooding in arid environments, and erosion rates. Geology 29: 1071–1074.
Montgomery DR, Buffington JM. 1997. Channel-reach morphology in moutain drainage basins. Geological Society of America Bulletin 109:
596–611.
Nishiizumi K, Winterer EL, Kohl CP, Klein J. 1989. Cosmic Ray Production Rates of 10Be and 26Al in Quartz From Glacially Polished
Rocks. Journal of Geophysical Research 94: 17907–17915.
Phillips F, Leavy B, Jannik N, Elmore D, Kubik P. 1986. The accumulation of cosmogenic chlorine-36 in rocks; a method for surface
exposure dating. Science 231: 41–43.
Pratt B, Burbank DW, Heimsath A, Ojha T. 2002. Impulsive alluviation during early Holocene strenghtened monsoons, central Nepal
Himalaya. Geology 30: 911–914.
Pratt-Sitaula BA, Burbank DW, Heimsath AM, Ojha T. 2004. Landscape disequilibrium on 1000–10 000 year scales, Marsyandi River,
Nepal, central Himalaya. Geomorphology 58: 223–241.
Raymo ME, Ruddiman WF. 1992. Tectonic forcing of Late Cenozoic climate. Nature 359: 117–122.
Schaller M, von Blanckenburg F, Veldkamp A, Tebbens LA, Hovius N, Kubik PW. 2002. A 30 000 yr record of erosion rates from
cosmogenic 10Be in Middle European river terraces. Earth and Planetary Science Letters 204: 307–320.
Seno T, Stein S, Grip AE. 1993. A model for the motion of the Philippine Sea plate consistent with NUVEL-1 and geologic data. Journal of
Geophysical Research 98: 17941–17948.
Copyright © 2005 John Wiley & Sons, Ltd.
Earth Surf. Process. Landforms 30, 955–971 (2005)
Fluvial bedrock incision in Taiwan
971
Snyder NP, Whipple KX, Tucker GE, Merritts DJ. 2000. Landscape response to tectonic forcing; digital elevation model analysis of stream
profiles in the Mendocino triple junction region, Northern California. Geological Society of America Bulletin 112: 1250–1263.
Snyder NP, Whipple KX, Tucker GE, Merritts DJ. 2003. Channel response to tectonic forcing; field analysis of stream morphology and
hydrology in the Mendocino triple junction region, Northern California. Geomorphology 53: 97–127.
Stone JO, Allan GL, Fifield LK, Cresswell RG. 1996. Cosmogenic chlorine-36 from calcium spallation. Geochimica et Cosmochimica Acta
60: 679–692.
Teng LS. 1990. Geotectonic evolution of late Cenozoic arc-continent collision in Taiwan. Tectonophysics 183: 57–76.
Teng LS. 1996. Extensional collapse of the northern Taiwan mountain belt. Geology 24: 949–952.
Tinkler K, Wohl EE. 1998. Rivers over Rock: Fluvial Processes in Bedrock Channels. Geophysical Monograph Series 107: 323pp.
Tucker GE. 2004. Drainage basin sensitivity to tectonic and climatic forcing; implications of a stochastic model for the role of entrainment
and erosion thresholds. Earth Surface Processes and Landforms 29: 185–205.
Tucker GE, Bras RL. 2000. A stochastic approach to modeling the role of rainfall variability in drainage basin evolution. Water Resources
Research 36: 1953–1964.
Water Recources Agency, 1970–2003. Hydrological Yearbook of Taiwan, Republic of China. Ministry of Economic Affairs: Taipei.
Whipple KX, Kirby E, Brockelhurst SH. 1999. Geomorphic limits to climate induced increases in topographic relief. Nature 401: 39–43.
Willett SD. 1999. Orogeny and orography, the effects of erosion on the structure of mountain belts. Journal of Geophysical Research, B,
Solid Earth and Planets 104: 28957–28982.
Willett SD, Fisher D, Fuller CW, Yeh EC, Lu CY. 2003. Erosion rates and orogenic wedge kinematics in Taiwan inferred from apatite fission
track thermochronometry. Geology 31: 945–948.
Wu CC, Kuo YH. 1999. Typhoons affecting Taiwan: Current understanding and future challenges. Bulletin of the American Meteorological
Society 80: 67–80.
Yoshimura K, Nakao S, Noto M, Inokura Y, Urata K, et al. 2001. Geochemical and stable isotope studies on natural water in the Taroko
Gorge karst area, Taiwan-chemical weathering of carbonate rocks by deep source CO2 and sulfuric acid. Chemical Geology 177: 415–
430.
Yu SB, Chen HY, Kuo LC. 1997. Velocity field of GPS stations in the Tiawan area. Tectonophysics 274: 41–59.
Zreda MG, Phillips FM. 2000. Cosmogenic nuclide build-up in surface material. In Quaternary Geochronology: Methods and Applications,
Noller JS, Sowers JM, Lettis WR (eds). American Geophysical Union: Washington DC; 61–76.
Zreda M, Phillips F, Elmore D, Kubik P, Sharma P, Dorn R. 1991. Cosmogenic chlorine-36 production rates in terrestrial rocks. Earth and
Planetary Science Letters 105: 94–109.
Copyright © 2005 John Wiley & Sons, Ltd.
Earth Surf. Process. Landforms 30, 955–971 (2005)