Moderate Chemical Weathering of Subtropical Taiwan: Constraints

Moderate Chemical Weathering of Subtropical Taiwan:
Constraints from Solid-Phase Geochemistry
of Sediments and Sedimentary Rocks
Kandasamy Selvaraj and Chen-Tung Arthur Chen
Institute of Marine Geology and Chemistry, National Sun Yat-Sen University, Kaohsiung 804, Taiwan
(e-mail: [email protected])
ABSTRACT
The well-known earthquake-and-storm-triggered extremely high physical weathering rate in Taiwan is consistent
with present geochemical studies of sediments from different subenvironments (offshore, coastal, river, and lake) and
sedimentary rocks of different geological ages, indicating a moderate chemical weathering condition. Major and trace
element concentrations normalized to the average upper crust of Yangtze Craton show that the sediments and the
average composition of sedimentary rocks of Taiwan are depleted in Ca, Mg, Na, and Sr, enriched in Rb and Zr, and
unchanged with respect to K, indicating their moderately altered nature. The mean chemical index of alteration (CIA;
71–75) and plagioclase index of alteration (PIA; 81–86) values of coastal and offshore sediments reveal the sediments’
derivation from sedimentary rocks by moderate silicate chemical weathering processes. The mean CIA value (62) of
sedimentary rocks of Taiwan is similar to that for Chinese sediment (61), further confirming the above inference.
A-CN-K, (A-K)-C-N, and A-CNK-FM plots (A p Al 2 O 3 ; C p CaO∗ ; N p Na 2 O; K p K 2 O; F p FeOT; M p MgO) also
confirm that the sediments and sedimentary rocks in Taiwan have undergone moderate silicate weathering, an
interpretation consistent with CIA and PIA values. The plots also indicate the presence of illite, chlorite, and a
subordinate amount of unaltered feldspars in sediments and sedimentary rocks, which are indicative of the physically
weathered and/or moderately chemically altered nature of sediments. The dominance of illite, chlorite, and unaltered
feldspars as inferred from geochemical data suggests that the immature nature of sediments and sedimentary rocks
is probably a result of low residence times in the source region or river basin and quick removal of materials from
the soil profile by steep, mountainous rivers (physical weathering dominates). Elemental ratios such as Rb/Sr, K/Rb,
molar K/Na, and Al/Na are close to crustal values. Average shale and river particulates such as those from the Yellow
River also indicate moderate chemical weathering conditions for sediments and sedimentary rocks, except for high
alpine lake sediments, where the prevailing extreme chemical weathering condition over erosion is clearly differentiated by higher CIA (80–84) and PIA (92–96) values and by their positions on triangular plots. These inferences
have also been illustratively corroborated by scatter plots of data such as Rb/Sr versus molar K/Na, and Al/Na versus
CIA. Additional evidence from published sources noted here also favors moderate chemical weathering conditions
for Taiwan. Geochemical variation of offshore, coastal, and river sediments is mainly controlled by non–steady state
weathering dominated by erosion. Steady state weathering, however, seems to produce highly weathered sediments
in the alpine region of Taiwan.
Online enhancement: appendix table.
Introduction
the entire island is 5 mm/yr (WRPC 1973). The
average physical denudation rate for Taiwan (1365
mg/cm2/yr) is probably the highest in the world, as
is the chemical denudation rate (50 mg/cm2/yr),
which is ∼27% of the physical denudation rate (Li
1976). Recent estimates of maximum erosion rates
(3–6 mm/yr) and the 30-yr average maximum sediment erosion rate (3.9 mm/yr; Dadson et al. 2003)
Disproportionately high physical and chemical denudation rates occur in the oceanic island of Taiwan under the influence of copious orographic rainfall associated with the Asian monsoon, frequent
typhoon activities, and related storm-triggered
landslides. The average erosion rate estimated for
Manuscript received October 4, 2004; accepted July 12, 2005.
[The Journal of Geology, 2006, volume 114, p. 101–116] 䉷 2006 by The University of Chicago. All rights reserved. 0022-1376/2006/11401-0006$15.00
101
102
K . S E LVA R A J A N D C . - T. A . C H E N
have demonstrated that across Taiwan, cumulative
seismic moment release (earthquakes of magnitude
greater than M w p 5.0) correlated linearly with decadal erosion rates over a sixfold range during a
period between 1900 and 1998. Dadson et al. (2003)
concluded that storm runoff is a first-order control
on erosion rates in Taiwan, and the modern erosion
rates are not controlled by relief and precipitation.
Other recent erosion rates estimated from river water and sediment discharges of the Eastern Central
Range of Taiwan show a wide range of values (2.2–
8.3 mm/yr) resulting from the influence of variable
intensity of storms rather than from lithology, tectonic environment, or climate (Fuller et al. 2003).
Li (1976), however, inferred that the great change
in the physical denudation rate is related to relief
and/or bedrock geology. Indeed, Milliman and Syvitski (1992) projected that mountainous rivers
draining in South Asia and Oceania have much
greater sediment yields (two- to threefold) than
other mountainous rivers of the world, owing to
the influence of human activity, climate, and geology. Chen et al. (2004) listed 13 rivers in Taiwan
among the top 20 worldwide in terms of sediment
yield. Previous studies have documented highly
variable physical and chemical denudation rates for
this island, but no clear consistency between them
has been established because relationships among
climate, physical erosion, and chemical weathering
remain poorly quantified since long-term weathering rates are difficult to measure (Riebe et al.
2001). The purpose of this article is to determine
if the very high, widely variable physical and chemical denudation rates reported from this oceanic
island show any covariance with the intensity
of chemical weathering, especially silicate
weathering.
Geochemical studies have contributed appreciably to the understanding of the growth of the continents through time (Taylor and McLennan 1985).
Numerous factors including source area composition, source area weathering conditions, hydraulic
sorting, adsorption, diagenesis, and metamorphism
affect the composition of siliclastic sediments and
sedimentary rocks over a wide scale (Fedo et al.
1996). Silicate weathering strongly affects the
major-element geochemistry and mineralogy of siliclastic sediments (e.g., Nesbitt and Young 1982;
Johnsson et al. 1988; McLennan 1993), where larger
cations (Al2O3, Ba, Rb) remain fixed in the weathering profile preferentially over smaller cations (Ca,
Na, Sr), which are selectively leached (Nesbitt et
al. 1980). These chemical signatures are ultimately
transferred to the sedimentary record (e.g., Nesbitt
and Young 1982; Wronkiewicz and Condie 1987),
thus providing a useful tool for monitoring source
area weathering conditions. Silicate weathering indexes such as chemical index of alteration (CIA),
plagioclase index of alteration (PIA), and chemical
index of weathering (CIW) are therefore widely used
to interpret the weathering history of modern and
ancient sediments (Harnois 1988; Fedo et al. 1996;
Colin et al. 1999; Tripathi and Rajamani 1999). For
example, high CIA values reflect removal of labile
cations (Ca2⫹, Na⫹, K⫹) relative to stable residual
constituents (Al3⫹, Ti4⫹) during weathering (Nesbitt
and Young 1982). Conversely, low CIA values indicate the near absence of chemical alteration and
might reflect cool and arid conditions (Fedo et al.
1995).
Chemical weathering rate (based on dissolved
loads) includes dissolution of elements from both
carbonate and aluminosilicate source rocks as well
as atmospheric and modern anthropogenic inputs
(Gaillardet et al. 1999; Sarin 2001; Han and Liu
2004). Though silicate weathering is Earth’s longterm sink for atmospheric CO2 (Berner et al. 1983),
in most natural environments, ionic contribution
from carbonate rocks generally dominates the dissolved phase (Han and Liu 2004), with silicate contributing smaller amounts of dissolved solids
(!15%; Sarin 2001). This is especially true where
Cenozoic rocks dominate, such as in Taiwan.
Hence, the chemical weathering rate, especially silicate weathering, of this island may be lower than
previously projected (e.g., 97.09% Ⳳ 2.41% of total
chemical weathering; Lai 2003). Sediments produced by landslides and typhoon activities from
rough, mountainous terrain and rapidly transported
to the continental margin by fluvial systems of Taiwan would be expected to display minimal chemical weathering. That is, silt and mud would be
expected to contain a comparatively low proportion
of aluminous clay minerals and a commensurately
higher proportion of primary minerals (feldspars).
In order to evaluate the chemical weathering intensity and provide accurate conditions of silicate
weathering of Taiwan rocks, a wide representative
range of geochemical data of offshore, coastal, river,
and lake sediments of Taiwan (see table A1, available in the online edition or from the Journal of
Geology office), along with published geochemical
results of sediments from Taiwan Strait (Chao and
Chen 2003) and sedimentary rocks (Lan et al. 2002)
of different geological ages of Taiwan, has been subjected to calculation of weathering indexes (CIA,
PIA, and CIW). A variety of triangular and scatter
plots have also been constructed from the geochemical data to accomplish our aim. To facilitate
the interpretation of sediments and sedimentary
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S U B T R O P I C A L TA I WA N W E AT H E R I N G
103
rocks, elemental values of representative river particulates such as Kaoping (Lai 2003), Yellow and
Yangtze (both from Li et al. 1984), and Amazon
(Martin and Meybeck 1979), as well as loess (Li et
al. 1984) and other important reference compositions such as upper continental crust (UCC), postArchean Australian shale (both from Taylor and
McLennan 1985), average shale (Turekian and
Wedepohl 1961), and North American shale composite (Gromet et al. 1984), have also been included
for comparison.
Methodology
A total of 106 sediment samples were collected
from different subenvironments (12 offshore surface sediments, eight coastal surface sediments,
and core sediment consisting of 16 subsamples, all
from off southwestern Taiwan; one bed sediment
sample from Kaoping River; and core sediment consisting of 69 subsamples from high alpine, anoxic
Great Ghost Lake) in southern Taiwan (fig. 1), with
the help of appropriate sampling devices and techniques. Before chemical analysis, the samples were
freeze-dried and homogenized, and the bulk sediment of each sample was finely ground (!200 mesh)
in an agate mortar. Major (Si, Ti, Al, Fe, Mn, Mg,
Ca, Na, K, and P) and trace (Ba, Rb, Sr, and Zr)
elements were determined using an x-ray fluorescence (XRF) spectrometer (Rigaku RIX 2000)
equipped with an Rh tube at the Institute of Marine
Geology and Chemistry, National Sun Yat-Sen University, Taiwan. Details of the XRF method are described by Chen et al. (2001). The accuracy of the
analytical method was established using six internationally recognized standard reference materials:
MAG-1, BCSS-1, PACS-1, MESS-1, NIES-2, and
GBW 07314. Based on these standards, the accuracy
and precision of the analysis were within Ⳳ1% for
major elements such as Al, Ca, Fe, K, Mg, Na, Si,
and Ti and within Ⳳ5% for Mn, P, Rb, Sr, and Zr.
The precision and accuracy for Ba was within
Ⳳ10%.
A LECO CHN-932 elemental analyzer was employed to determine carbon content at 950⬚C. After
the samples were repeatedly rinsed with 1 N HCl
to remove inorganic carbon (IC), total organic carbon was determined. The amount of IC was estimated by the difference between measured total
carbon and organic carbon. The detection limit of
IC is 0.01%. In the samples that were not measured
for inorganic carbon, the mean value of each sample’s particular subenvironment was used for the
calculation of CaO in only the silicate fraction
(CaO∗). The CIA and PIA values of sedimentary and
Figure 1.
Map of Taiwan showing the study area
(dashed line). Representative surface and core sediment
samples collected from different subenvironments in
southern Taiwan—offshore (open circle), coastal (filled
circle), river (star), and lake (open triangle)—have been
used for this study. The open square represents Kaohsiung City.
metasedimentary rocks as well as reference compositions such as UCC, average shale, post-Archean
Australian shale, and North American shale composite were calculated without correcting CaO for
carbonates and phosphates. In most of the sedimentary rocks of Taiwan, the CaO value is less
than 2% (see table A1), with the exception of argillite (2.53%); the calculated values are, therefore,
slightly lower than actual CIA and PIA values. Loss
on ignition (LOI) was calculated as a percentage of
dry weight after the samples were ignited at 550⬚C
for 1 h (Dean 1974).
Table 1. Summary Statistics of Geochemical Compositions of Sediments and Sedimentary Rocks of Taiwan
Trace elements (ppm)
Major oxides (wt%)
SiO2
TiO2 Al2O3 Fe2O
Offshore sediments (np12):
Minimum
54.33 .69 14.60
Maximum
64.38 .88 17.61
Mean
60.43 .75 15.65
SD
2.84 .06
.94
Coastal sediments (np8):
Minimum
51.55 .52 13.70
Maximum
72.73 .81 16.57
Mean
63.69 .62 14.48
SD
7.32 .09
.88
Offshore core sediment (np16):
Minimum
66.32 .81 14.56
Maximum
68.63 .84 15.34
Mean
67.30 .82 15.01
SD
.65 .01
.21
Core sediment, Great Ghost Lake (np69):
Minimum
50.12 .64 15.38
Maximum
58.74 .82 23.08
Mean
53.78 .76 17.04
SD
1.71 .03
1.44
Bed sediment, Kaoping River (np1):
72.36 .54 12.39
Core tops, Taiwan Strait (np3):c
Minimum
73.12 .29
5.55
Maximum
86.60 .41 12.29
Mean
78.97 .36
8.72
SD
6.91 .06
3.39
Sedimentary rocks of Taiwan (np14):d
Minimum
58.75 .10
6.65
Maximum
86.50 .90 17.97
Mean
70.65 .59 13.37
SD
9.81 .23
4.12
a
Total Fe expressed as Fe2O3.
LOI p loss on ignition.
Analysis from Chao and Chen 2003.
d
Analysis from Lan et al. 2002.
b
c
a
3
MnO MgO CaO Na2O
K2O
P2O5
b
LOI
Total
4.72
8.58
5.79
1.07
.05
.36
.12
.11
1.74
2.75
2.05
.33
.84
4.62
2.47
1.18
.97
1.43
1.16
.14
2.41
3.25
2.76
.26
.11
.21
.14
.03
4.40
9.52
6.91
1.43
96.47
100.01
98.23
1.15
4.26
6.09
5.10
.68
.05
.05
.05
.00
1.36
3.47
2.07
.84
1.52
3.58
2.25
.73
.90
2.45
1.39
.64
2.17
3.52
2.73
.49
.06
.14
.10
.03
4.23
7.19
5.14
1.12
98.29
99.08
98.72
.30
5.76
6.19
5.93
.10
.05
.07
.06
.00
1.64
1.77
1.73
.03
1.12
1.35
1.26
.06
.99
1.27
1.17
.07
2.72
2.86
2.77
.04
.09
.10
.09
.00
2.86
3.89
3.37
.31
4.56
5.55
5.05
.19
.02
.03
.03
.00
.74
1.24
.87
.09
.06
.16
.12
.02
.28
.59
.39
.07
2.41
3.31
2.64
.19
.21
.43
.33
.04
4.75
.04
1.29
1.66
1.31
2.07
2.97
5.51
4.41
1.31
.02
.06
.04
.02
.78
1.62
1.26
.43
1.45
3.86
2.28
1.37
.65
1.37
.96
.37
.71
7.25
4.49
2.27
.01
.17
.09
.05
.05
3.00
1.61
.94
.22
2.53
1.27
.62
.00
3.08
1.70
.85
Ba
Rb
Sr
Zr
381
84
99
178
490
150
187
1432
425
117
137
342
37.8
19.0
22.6
349.2
356
85
113
465
111
143
396
97
124
34.5
7.9
9.6
164
287
207
46.7
98.80
100.80
99.53
.49
436
472
459
10.4
178
198
189
4.7
9.60
23.80
19.38
3.05
100.22
100.54
100.38
.07
465
101
686
228
537
167
45.3
26.9
.09
2.40
98.91
1.33
2.37
1.82
.52
.06
.11
.09
.03
1.39
2.48
2.10
.62
100.27
100.79
100.56
.27
.32
3.84
2.74
1.13
.02
.13
.08
.03
1.25
7.50
3.67
1.90
99.59
101.11
100.26
.41
41
121
48
132
45
126
1.9
3.5
52
70
63
3.6
492
93
335
413
371
39.3
53
88
97
188
70
139
23.6
50.1
110
1034
507
229
7
169
109
51
129
22
291
134
78
161
254
212
20.5
209
47
57
52
5.0
48
593
212
136.3
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S U B T R O P I C A L TA I WA N W E AT H E R I N G
105
Sediment Composition
The average composition of sedimentary rocks of
Taiwan (table 1) has been calculated based on the
data of Lan et al. (2002). The calculation involves
14 samples consisting of four widely varying sedimentary and metasedimentary rocks, such as phyllite (one sample), sandstones (three), argillites (two),
and metapelites (eight), with the assumption that
these four rock types will represent approximately
average values for sedimentary rocks in this small
area (0.024%) of the earth’s surface. Based on SrNd-O isotopic geochemistry of Taiwan granitoids
and metapelites, Lan et al. (1995) suggested that
cover sediments of Taiwan received recycled continental crustal material from South China and the
basement rocks of Taiwan. It has recently been
shown, based on the Nd isotopic composition of
sedimentary and metasedimentary rocks of Taiwan, that these rocks are recycled materials of the
UCC of the South China region (Lan et al. 2002).
South China consists of the Yangtze Craton in the
west and the Cathaysian fold in the east (Jahn et
al. 1990; Lan et al. 1995). Because we are not aware
of published results for the UCC of South China,
in this study, sediment and sedimentary rock compositions of Taiwan were compared with the upper
crust of Yangtze Craton (Gao et al. 1998). The average major (Si, Al, Ti, Fe, Ca, Mg, Na, and K) and
trace (Ba, Rb, Sr, and Zr) element compositions of
sediments from different subenvironments (offshore, coastal, river, and lake) and the average composition of sedimentary rocks (table 1) normalized
with upper crustal values of Yangtze Craton (Gao
et al. 1998) are shown in figure 2. The sediments
and sedimentary rocks have remarkably similar
patterns. The similarity suggests that most of the
sediments are dominantly physically eroded and/
or moderately chemically modified. All the sediments and average composition of sedimentary
rocks are invariably depleted in Ca, Mg, Na, Ba,
and Sr, enriched in Rb and Zr, and unchanged with
respect to K. The degree of elemental variation is
more prominent in lake sediments because of their
highly altered nature. The slight depletion of Ti,
Fe, and Mg shown by the average composition of
sedimentary rocks of Taiwan is similar to UCC normalized values for these elements in the Huanghe
sediments (Yang et al. 2004). Like sedimentary
rocks, coastal and river sediments are depleted in
Al, Ti, and Fe, whereas slight richness of these elements is evident in offshore and lake sediments.
Figure 2. Spider plot showing comparison between average compositions of sediments from different subenvironments (offshore, coastal, river, and lake) and the average composition of sedimentary rocks of Taiwan, both
normalized to the average upper crustal (UC) values of
Yangtze Craton (YC; Gao et al. 1998). Note the similarity
in the behavior of most of the elements between sediments and the average composition of sedimentary rocks.
The similarity suggests that most of the recent sediments
are physically eroded and/or moderately chemically altered products of sedimentary rocks. The depletion of Ca,
Mg, Na, Ba, and Sr relative to the upper crust of Yangtze
Craton could be attributed to their mobility during
weathering.
Silicate Weathering: Geochemical Indicators
and Triangular Plots
Geochemical processes such as weathering and soil
formation are dominated by alteration of feldspars
(and volcanic glass), which accounts for 70% of the
upper crust if the relatively inert quartz is discounted (Nesbitt and Young 1982, 1984). Feldspars
are by far the most abundant labile minerals, and
consequently, the key process during silicate
weathering of the earth’s upper crust is the degradation of feldspars by aggressive soil solutions so
that the proportion of alumina to alkalis typically
increases in the weathered product. A good measure of the degree of weathering can be obtained
by calculation of the CIA using molecular proportions: CIA p 100(Al 2 O 3 /Al 2 O 3 ⫹ CaO∗ ⫹ Na 2 O ⫹
K 2 O), where CaO∗ is the amount of CaO incorporated in the silicate fraction of the rock. The re-
106
K . S E LVA R A J A N D C . - T. A . C H E N
sultant CIA gives a measure of the proportion of
secondary aluminous clay minerals to primary silicate minerals such as feldspars (Nesbitt and Young
1982; Young et al. 1998).
The calculated CIA values for sediments and sedimentary rocks including river particulates, loess,
and important reference compositions presented in
table 2 demonstrate that the surface sediments off
southwestern Taiwan (water depth 105–537 m)
have moderate CIA values of 66–77, with an average of 74. Values for the near coastal sediments
(water depth !100 m) from the same region are
slightly lower but comparable, with CIA ranging
from 59 to 78 and a mean of 73. Sixteen subsamples
of core sediment off southwestern Taiwan (water
depth 294 m) exhibit a narrow range of CIA values
(73–76) with a mean of 75. Three surface sediments
(top 0–15 cm) of three cores from Taiwan Strait, off
central Taiwan (data from Chao and Chen 2003),
also show comparable CIA values (69–72). The CIA
values of surface and core sediments have means
ranging from 71 to 75 and fall within the range of
values for average shale (70–75), indicating that the
sediments are possibly the product of sedimentary
and metasedimentary rocks that have undergone intermediate chemical weathering. To deduce the silicate weathering trends, Nesbitt and Young (1982)
and Nesbitt et al. (1996) used A-CN-K (Al2O3CaO∗ ⫹ Na 2 O-K2O) and A-CNK-FM (Al2O3CaO∗ ⫹ Na 2 O ⫹ K 2 O-FeOT ⫹ MgO) ternary plots.
The data of sediments and sedimentary rocks of Taiwan plotted in Al2O3-CaO∗ ⫹ Na 2 O-K2O compositional space (fig. 3) fall on a trend parallel to the AC ⫹ N join, and the trend approaches the Al2O3-K2O
join at about CIA 80. The CIA values of many weathering profiles and sediments are linear, subparallel
to the A-CN join in the A-CN-K plot (e.g., Nesbitt
and Young 1984; Fedo et al. 1995; Selvaraj et al.
2004). All the samples that display moderate silicate weathering are thus plotted between CIA 60
and 80 (scale shown on the right side of the diagram
for comparison), except for the sediments from high
alpine Great Ghost Lake of southern Taiwan. The
lake sediments that show a narrow range of higher
CIA values between 80 and 84 (mean p 82; table
2), therefore, fall very near to apex A (fig. 3) as advanced weathering leads to an aluminum-rich composition (e.g., Nesbitt and Young 1984; Nath et al.
2000). This indicates extreme chemical weathering
in the high altitude region of this island. The lake
is one of the wettest places in Taiwan (annual
rainfall p 4200 mm; Lou et al. 1997); the presence
of thick vegetation on the gently sloping shore (24⬚–
27⬚ inclination) surrounding the lake acts as an
ideal site for soil development, and the large
amount of humus material (mean total organic
carbon p 11.92%; n p 69) available in this region
is probably responsible for the relatively low pH in
soil solutions of lake catchments caused by production of organic acids. It has been shown by Colin
et al. (1999) that increasing vegetation cover in the
flood plains of the Irrawaddy River during the summer monsoon reinforcement favors soil development. This inference indicates that acidic byproducts of vegetation promote silicate weathering. The
lowest CIA value of loess (46; table 2) indicates lowinput acids to soils as a result of a cool, dry climate.
Most of our sedimentary rocks plot between CIA
60 and 70 in figure 3, close to values for the reference compositions such as the upper crusts of
Yangtze Craton (56) and Central East China (54).
The weathering trend (arrow 2 in fig. 3) connecting
recent sediments and sedimentary rocks falls in a
single line, suggesting that the sediments are the
products of moderately weathered sedimentary
rocks. The weathering trend of sediments and sedimentary rocks shows slight enrichment of K2O
compared with particulate matters of the major rivers Yellow, Yangtze, Brahmaputra, and Amazon and
with upper crustal compositions, indicating the
presence of considerable unaltered K-feldspar in the
samples. In figure 3, arrow 2 intersects the feldspar
join (Pl-Ks) at point x and may represent the original
parent rock composition, which is also slightly rich
in K-feldspar when compared with upper crustal
compositions. Indeed, values for the bed sediment
of the Kaoping River and its suspended particulate
matter fall very close to those for river particulates
of the Yellow River and average world values,
which is evidence for the moderate silicate weathering in the river basin.
The CIA values of sediments from different subenvironments are higher than the calculated CIA
values of sedimentary rocks (argillite, metapelite,
phyllite, and sandstone) in Taiwan (Lan et al. 2002),
which all have mean CIA in the narrow range between 61 and 64 (table 2). These values are strikingly
similar to the mean CIA (61) of Chinese sediment
(Yang et al. 2004), suggesting that sedimentary rocks
of Taiwan have experienced silicate weathering of
intensity similar to that of Chinese sediments. This
further substantiates that the sediments and sedimentary rocks are the product of rocks that experienced moderate chemical weathering in the source
regions. The calculated CIA values of sediments and
sedimentary rocks have been classified according to
the scheme suggested by Fedo et al. (1995) and
shown in table 2. Nearly all the sediments and sedimentary rocks, including metapelites, invariably
show intermediate intensity of chemical weather-
Table 2. Chemical Index of Alteration (CIA), Plagioclase Index of Alteration (PIA), and the Mean Rb/Sr and K/Rb Ratios of Sediments, Sedimentary Rocks,
Riverine Particulates of Taiwan and China, and Important Reference Compositions
CIA
Offshore sediments, southwestern Taiwan
Coastal sediments, southwestern Taiwana
Core sediments, southwestern Taiwana
Lake core sediments, Taiwana
Bed sediment, Kaoping Riverb
Core tops, Taiwan Straita
Earliest Cenozoic to Oligocene metasedimentary to sedimentary rocks of
Taiwan:
Phyllite, Early Cenozoic (Chulai)b
Sandstone, Eocene (Meichi)b
Sandstone, Oligocene (Tsuku)b
Argillite, Oligocene (Kankou)b
Argillite, Oligocene (Tatungshan)b
Metapelites, Jurassic (Tienhsiang)b
River particulates:
Kaoping, Taiwanb
Yangtze, Chinaa
Yellow, Chinaa
Amazonb
Brahmaputraa
Gangesa
River particulate matterb
Shale compositions:
Average shaleb
Post-Archean Australian shaleb
North American shale compositeb
Loess and crustal compositions:
Loessa
Upper continental crust (UCC)b
Mean crustb
Yangtze Craton, China (upper crust)a
Central East China (upper crust)a
East China (total crust)a
PIA
Range Mean Range Mean
Location
a
Mean Mean
Rb/Sr K/Rb
Weathering
intensity
based on
mean CIA
Data source
66–77
59–78
73–76
80–84
…
69–72
74
73
75
82
63
71
71–90
64–90
83–88
92–96
…
79–82
84
83
86
94
66
81
.89
.78
.36
2.67
.72
.55
197
235
508
134
185
220
Intermediate
Intermediate
Intermediate
Extreme
Intermediate
Intermediate
This study
This study
This study
This study
This study
Chao and Chen 2003
…
…
59–67
…
…
45–78
61
63
63
63
64
62
…
…
62–75
…
…
44–86
65
71
69
69
69
67
1.48
.88
1.33
1.71
1.20
.80
237
215
186
191
201
256
Intermediate
Intermediate
Intermediate
Intermediate
Intermediate
Intermediate
Lan
Lan
Lan
Lan
Lan
Lan
…
…
…
…
…
…
…
59
74
61
73
67
75
65
…
…
…
…
…
…
…
62
80
64
77
73
87
68
…
.77
.39
…
…
…
…
…
182
233
…
…
…
…
Incipient
Intermediate
Intermediate
Intermediate
Intermediate
Intermediate
Intermediate
Lai 2003
Li et al. 1984
Li et al. 1984
Martin and Meybeck 1979
Galy and France-Lanord 2001
Galy and France-Lanord 2001
Martin and Meybeck 1979
…
…
…
65
69
57
…
…
…
71
77
60
.82
.80
.88
190
192
264
Intermediate
Intermediate
Incipient
Turekian and Wedepohl 1961
Taylor and McLennan 1985
Gromet et al. 1984
…
…
…
…
…
…
46
46
46
56
54
50
…
…
…
…
…
…
46
45
45
58
56
50
.43
.32
…
.32
.31
.24
196
252
…
248
262
272
Equal to UCC
…
…
…
…
…
Li et al. 1984
Taylor and McLennan 1985
Bowen 1979
Gao et al. 1998
Gao et al. 1998
Gao et al. 1998
et
et
et
et
et
et
al.
al.
al.
al.
al.
al.
2002
2002
2002
2002
2002
2002
Note. Weathering intensity has been given based on the threefold classification of Fedo et al. (1995); weathering intensity for reference compositions such as UCC and mean
crust are not given.
a
Indicates the CaO values of only the silicate fraction.
b
Represents the total CaO used for CIA and PIA calculations.
108
K . S E LVA R A J A N D C . - T. A . C H E N
ing. CIW (100Al 2 O 3 / [Al 2 O3 ⫹ CaO ⫹ Na 2 O]; Harnois 1988) ranges from 70 to 91, with a mean of 81,
for coastal and offshore sediments. These values are
close to those for post-Archean shales, with moderate losses of Ca, Na, and Sr from source area weathering (Condie 1993), and they correspond well with
CIA values. The above interpretation is consistent
with the mineralogy of coastal and offshore sediments, which all show the dominance of quartz,
feldspars, illite, and chlorite composition, with minor amounts of calcite and kaolinite (Chen 1997).
The CIA values are also consistent with sediment
compositions obtained from ODP site 1144 of the
northern South China Sea (Boulay et al. 2003). The
site is believed to be partly sourced by Taiwan’s terrestrial materials.
Monitoring plagioclase weathering alone also
yields additional information for silicate weathering of sediments and sedimentary rocks, and the
PIA can be calculated as 100(Al2O3-K2O)/(Al 2 O 3 ⫹
CaO∗ ⫹ Na 2 O-K2O) in molar proportions. It can
also be represented on a triangle showing molecular
proportions of Al2O3 (minus Al associated with K),
CaO, and Na2O (see the PIA diagram of Fedo et al.
1995). The PIA values for coastal and offshore sediments, in general, fall between 80 and 90 (fig. 4),
with a mean of 85, except for a few samples that
show low PIA values (as low as 64; table 2) and
indicate the presence of small amounts of plagioclase feldspars in sediments. Sediments display low
CaO values and seem to be totally depleted in the
Figure 3. Major element composition of sediments and
sedimentary rocks, river particulates, loess, and other reference compositions plotted as molar proportions on an
Al2O3-(CaO∗ ⫹ Na2O)-K2O(A-CN-K) diagram. Arrow 1
represents the weathering trend of river particulates,
arrow 2 indicates the weathering trend of sediments
and sedimentary rocks of Taiwan, and arrow 3 marks
the limit of weathering. The diagram also represents
the fields of idealized minerals: Pl p plagioclase;
Ks p K-feldspars; Bi p biotite; Sm p smectite; Mu p
muscovite; Il p illite; Ka p kaolinite; Gi p gibbsite;
Ch p chlorite; Gt p garnet. Arrow 2 through the sediments and sedimentary rocks intersects the feldspar join
(Pl-Ks) at point x, which may give an indication of the
original composition of the source material. The relation
between chemical index of alteration (CIA) scale (Nesbitt
and Young 1982) and the triangle is shown at right. Most
of the sediments and sedimentary rocks fall between CIA
60 and 80, indicating intermediate intensity of silicate
weathering. Note that the lake sediments fall above CIA
80, suggesting their extremely altered nature. The reason
for the parallelism between the weathering trends of major river particulates (arrow 1) and sediments and sedimentary rocks of Taiwan (arrow 2) could be explained as
follows: most of the world’s major rivers are perhaps
draining in the terrains consisting of significant amounts
of granite, granodiorite, gneiss, and related geologically
older rocks. In Taiwan island, however, the percentage
of these rocks is very low, and most of the rocks are
geologically younger sedimentary and metasedimentary
clans. This probably causes the chemical variability observed between the two weathering trends, which in turn
suggests the slight but significant differences between
the composition of the original source rocks of sediments
and sedimentary rocks of Taiwan and the composition
of the UCC. The values of A coordinates are given in
parentheses for comparison as well as to avoid confusion
from overlapping points.
Journal of Geology
S U B T R O P I C A L TA I WA N W E AT H E R I N G
109
Figure 4. Triangular plot (after Fedo et al. 1995) showing molar proportions of Al2O3 (minus that associated
with K), CaO, and Na2O. The diagram shows that the
plagioclase index of alteration (PIA) for most of the sediments falls between 80 and 90 (scale shown at right)
because of the presence of small amounts of plagioclase
feldspar resulting from moderate silicate weathering.
Lake sediments fall very close to the Al2O3-K2O apex,
which substantiates that they are the products of highly
weathered sedimentary rocks. An p anorthite; Al p
albite. Symbols and other abbreviations are as in figure
3.
cess operating on a moderate scale over a long period (from Jurassic to recent, even though our study
has some gaps in sampling representation for a few
periods such as Miocene, Pliocene, and Pleistocene). The Pleistocene sediments (ODP site 1144)
of the northern South China Sea have CIA values
of 73–79 (Boulay et al. 2003). Among the sedimentary rocks, metapelites have a wide range of CIA
values (45–78), the lowest value being equal to
those of UCC and loess. The mean value (62;
n p 8), however, is consistent with those of the
other sedimentary rocks (61–64) but lower than
those of the sediments discussed above. The CIA
and PIA values of average composition of sedimentary rocks are about 20% higher than upper crust
values of Yangtze Craton (fig. 2).
To illustrate the approximate mineralogical composition of sediments and sedimentary rocks, major
element data of all the samples have been plotted
in the mafics triangle (fig. 5; Nesbitt and Young
1984; Nesbitt and Wilson 1992). It portrays molecular proportions of Al2O3, CaO∗ ⫹ Na 2 O ⫹ K 2 O,
and FeOT ⫹ MgO (A-CNK-FM). Most of the sediments, sedimentary rocks, and river particulates
(Kaoping, Yellow, and Yangtze) plot within the
compositional triangle of feldspars, garnet, and
chlorite. This implies the approximate mafic minerals composition of samples; however, x-ray diffractogram (XRD) patterns of recent sediments
anorthite component (fig. 4). The sedimentary
rocks, on the other hand, seem to contain a higher
proportion of anorthite than sediments. Lake sediments show very high PIA values (92–96; table 2)
and plot near the Al2O3 apex on the triangle, indicating their highly aluminous character. They are
also virtually depleted in Ca, but all have small
amounts of Na2O (sodic feldspar), which is more
prominent in offshore and coastal sediments. These
differences are well matched with a high degree of
weathering for the lake sediments and moderate
weathering for other sediments. Weathering has
proceeded to a stage at which significant amounts
of Ca and Na have been removed from the sediments because of copious rainfall (source for organic and inorganic acids) in this island. Weathering studies show that Ca, Na, and Sr are rapidly
lost during chemical weathering, and the amount
of these elements lost is proportional to the degree
of weathering (e.g., Wronkiewicz and Condie 1987).
The consistency of CIA and PIA values among
the sediments, sedimentary rocks, and metasedimentary rocks clearly suggests that, in general, the
degree of silicate weathering in this island is a pro-
Figure 5.
Mafics ternary plot of Nesbitt and Young
(1984). A is molar proportions of Al2O3, CNK represents
CaO∗ ⫹ Na2O ⫹ K2O, and FM identifies FeOT ⫹ MgO.
Symbols are as in figure 3. Also shown are the fields of
idealized minerals (see fig. 3 for details; Fs p feldspars).
110
K . S E LVA R A J A N D C . - T. A . C H E N
have not shown any presence of garnet. Therefore,
the coastal and offshore sediments plot away from
the feldspar-garnet join but close to the feldsparchlorite join, suggesting that sediments are essentially composed of feldspars and chlorite, dominated by the latter. This inference is compatible
with the XRD patterns of our coastal and offshore
sediments, all showing prominent feldspar (d p
3.18) and chlorite (d p 7.16 and 3.51) peaks (Chen
1997). The presence of illite and chlorite inferred
from geochemical data is consistent with the earlier clay mineral investigation by Chen (1973), who
found the illite-chlorite dominant suite over the
continental shelf from the East China Sea to the
southern part of Taiwan Strait. The close association of sediments with sedimentary rocks (fig. 5)
indicates that the former is mostly the product of
the latter. The lake sediments again plot toward
higher Al than do other sediments and sedimentary
rocks and well above the line of the feldsparchlorite join, further confirming their higher silicate weathering. Bed sediment of the Kaoping River
plots close to river particulates of the Yellow River
and average world values, as in the A-CN-K plot;
this denotes moderate silicate weathering in the
river basin of the Kaoping and the similarity in the
mineralogical composition of source rocks of the
Kaoping and Yellow rivers. This finding, however,
contradicts some previous reports (e.g., Yang 2001;
Lai 2003) that suggested high chemical/silicate
weathering rates based on dissolved river flux, probably because the dissolved flux also includes the
carbonate dissolved elements. The river particulates of the Amazon and Yangtze rivers, however,
plot close to the sediments, with lower values resulting from the incorporation of total CaO instead
of CaO in only the silicate fraction.
Large-Ion Lithophile Elements: Rb, Sr, K, and Na
The increase in chemical weathering intensity rapidly leaches Sr compared to Rb (Nesbitt and Young
1982); therefore, the Rb/Sr ratio increases with increasing CIA (Ma et al. 2000). Likewise, with the
increase in chemical weathering intensity, K will
normally show depletion against Rb (Wronkiewicz
and Condie 1989), thus leading to a lower K/Rb
ratio. Rb has been considered to be primarily fixed
in weathering residues and less reactive than Ca,
Na, and Sr (Nesbitt et al. 1980). The Rb/Sr ratios
of sediments and sedimentary rocks can thus be
used to monitor the degree of source rock weathering (McLennan et al. 1993). The mean Rb/Sr ratios (0.89 and 0.78) of offshore and coastal surface
sediments of southwestern Taiwan are consistent
with the Rb/Sr ratios of different shale compositions (0.80–0.88; table 2), corroborating that the degree of source rock weathering was moderate. The
lower mean Rb/Sr ratios of core sediments off
southwestern Taiwan (0.36; n p 16) and Taiwan
Strait (0.55; n p 3) are closer to the Rb/Sr ratios of
different upper crustal and loess compositions
(0.31–0.43). The mean Rb/Sr ratios of sandstones
and metapelites are 0.88 and 0.80, respectively, indicating that the recent sediments from coastal and
offshore regions are mainly derived from these sedimentary rocks. This inference is supported by the
similar Rb/Sr ratio (0.71) for bed sediment of the
Kaoping River. High Rb/Sr ratios (1.74–3.77) of lake
sediments and their higher mean value of 2.67
(n p 69) further support their derivation from intensively weathered rocks such as phyllite and argillite (mean Rb/Sr ratios 1.48 and 1.71), which are
dominant in the entire drainage basin (90.3 km2) of
the lake. Rb/Sr ratios in the illite minerals are usually 11, with a maximum value of 6.46 (Chaudhuri
and Brookings 1979), suggesting that the higher illite content in lake sediments might be responsible
for higher Rb/Sr ratios. The moderate chemical
weathering of sediments is also indicated by K/Rb
ratios (table 2) of the investigated sediments that
are not depleted in K (fig. 2), and mean K/Rb ratios
of coastal (235), offshore (197), and river (185) sediments are consistent with K/Rb ratios of Taiwan’s
sedimentary rocks (220), upper crusts of Yangtze
Craton (248), and loess (196).
Molar ratios of K/Na of sediments and sedimentary rocks are correlated well with Rb/Sr ratios as
well as CIA and PIA values (fig. 6). Both ratios are
moderate and consistent with different shale compositions and bed sediment of the Kaoping River,
demonstrating moderate silicate weathering. The
diagram shows the presence of minor plagioclase
(Na and Sr) and K-feldspar (K and Rb) in recent sediments. The close association of sediments with
loess and Yellow River particulates suggests compositional similarity and also that the Taiwan Strait
might have been sourced considerably from the
loess plateau either by the Yellow River or from
dust storm input. High K/Na and Rb/Sr ratios of
lake sediments indicate stronger chemical weathering at higher altitudes as well as preferential dissolution of plagioclase (Na and Sr) relative to Kfeldspar during the silicate weathering process
(Yang et al. 2004). The scatter plot of Al/Na ratio
versus CIA (fig. 7) illustrates the interrelation between both indexes, which reflects the silicate
weathering intensity. The diagram shows that the
degree of chemical weathering of sediments and
sedimentary rocks of Taiwan is moderate, as in-
Journal of Geology
S U B T R O P I C A L TA I WA N W E AT H E R I N G
Figure 6. Diagram showing variations in Rb/Sr versus
molar K/Na in sediments and sedimentary rocks of Taiwan and other reference compositions. Note the two separate fields; low and intermediate values of chemical and
plagioclase indexes of alteration (CIA and PIA) for sediments and sedimentary rocks in field 1, having low Rb/
Sr and K/Na ratios compared to field 2, encircle the high
CIA and PIA samples of Great Ghost Lake (GGL), which
have higher Rb/Sr and K/Na ratios. See figure 3 for explanation of symbols and abbreviations.
dicated by their low Al/Na ratios (0–13.09), which
reveal the presence of small amounts of Na2O (plagioclase) in the samples. The degree of chemical
weathering of lake sediments is high, as shown by
their very high Al/Na ratios (20.75–41.42) resulting
from extreme dissolution of plagioclase. These inferences are consistent with their positions on the
other triangular and scatter plots.
Several lines of evidence have been drawn as additional support for the conclusions arrived at in
this study. The XRD patterns of our offshore and
coastal surface sediments show quartz-feldsparchlorite-illite-calcite-kaolinite association (Chen
1997). Consequently, illite and chlorite are the
most abundant and ubiquitous clay minerals in the
rock formations on the Hengchun Peninsula in
southern Taiwan (Lin and Wang 2001). Both illite
and chlorite may be derived either from the degradation of muscovite and biotite from metamorphic formations or from the erosion of sedimentary
rocks (Chamley 1989). Micas are essential minerals
of rocks such as schist and phyllite. Chlorite is considered a primary mineral of low-grade metamorphic rocks. The basement complex of Taiwan, Tananao schist and the associated choritoid rocks, is
considered the probable source of chlorite in these
sediments. Accordingly, illite, chlorite, and quartz
111
are treated as products of physical erosion or moderate chemical weathering (Chamely 1989). Colin
et al. (1999) identified from the core sediments of
the Bay of Bengal and Andaman Sea that the sediments deposited during the last glacial maximum
are characterized by a decrease in the smectite/
(illite ⫹ chlorite) ratio resulting from increased
physical weathering. If the source rocks experienced intense chemical weathering, feldspars present in the source rock would be altered totally as
aluminous clay (Fedo et al. 1996). Repeated cycles
of weathering and abrasion during transport eventually result in destruction of feldspars and formation of clay minerals (Nesbitt and Young 1996).
As long as feldspars persist, the sediments will remain compositionally immature (Nesbitt and
Young 1996). It has also been mentioned that the
absence of feldspar in sediments and sedimentary
rocks is a characteristic feature of supermature sedimentary rocks (Medaris et al. 2003). Maturity
would have been further enhanced by additional
weathering during fluvial transport in a warm, humid climate (Johnsson et al. 1988) and by preferential destruction of labile minerals and lithic fragments in the high-energy fluvial and shallow
marine environments (Odom et al. 1976). The presence of feldspars in sediments of this study, inferred
from figures 4 and 6 and the illite-chlorite association shown in figure 5, suggests their derivation
by physical weathering (a key process in Taiwan)
and/or moderate chemical weathering.
Direct evidence of chemical weathering was
Figure 7. Scatter plot of Al/Na ratio versus chemical
index of alteration (CIA). Note the interrelation between
both indexes, which reflects the silicate weathering intensity. Symbols and abbreviations are as in figure 3.
112
K . S E LVA R A J A N D C . - T. A . C H E N
found in the record of clay minerals in sedimentary
basins by France-Lanord and Derry (1997). To estimate CO2 consumption from silicate weathering,
they selected the Himalayan-derived sediments of
the late Pleistocene–mid-Miocene age recovered
from the distal Bengal Fan on ODP Leg 116. Before
7 Ma and after 1 Ma, the clay mineral assemblage
in the Bengal Fan was dominantly illite and chlorite
(I-C assemblage), reflecting moderate weathering in
the Ganges-Brahmaputra system. Between 7 and 1
Ma, clays in the Fan were dominantly pedogenic
smectite and kaolinite (S-K assemblage), reflecting
more intense weathering. The I-C sediments have
lost little or no K2O or MgO relative to the source
rocks but have lost about half of their Na2O and
CaO, while the S-K sediments have lost some MgO,
about half of their K2O and CaO, and much of their
Na2O. Therefore, the secondary mineral assemblage of I-C in the sediments of southwestern Taiwan inferred from figures 5 and 6 and their unchanged K and slightly depleted Mg contents (fig.
2) corroborate that these sediments are the products
of physically weathered rocks and/or intermediate
chemical weathering. The clay mineral composition of present-day Ganges River sediment is essentially illite and chlorite, however, indicating
rapid mechanical weathering in the source area
(Singh et al. 2003).
Mean grain sizes (determined with a Coulter LS
particle size analyzer) of offshore surface (6.47–15.08
mm; n p 12) and core (5.13–8.55 mm; n p 13) sediments fall within fine and very fine silt classes of
siliclastic sediments. Coarser coastal sediments
have a wide range of mean sizes, from 65 to 452 mm,
and fall in very fine to medium sand classes. Fine
and medium silt sizes (7.30–27.4 mm; n p 37) are
the characteristics of highly weathered lake sediments. Silt and sand dominance suggests the presence of a relatively higher amount of feldspars than
secondary clay minerals. The insignificant correlation (r p 0.465; P p 0.0001; n p 69) between mean
grain size and CIA of studied sediments (fig. 8) supports the above inference and indicates that the
chemical variability seems to be less likely to be
grain-size controlled because of non–steady state
weathering.
Consequences of Erosion and
Chemical Weathering
Tectonism and climate generally determine the relative rates of erosion and chemical weathering
(McLennan and Taylor 1991). Likewise, the relative
rates of chemical weathering and erosion chiefly
control the composition of siliclastic sediments
Figure 8. Bivariate plot of CIA and mean grain size (mm)
for sediments of Taiwan. Symbols are as in figure 3.
(Nesbitt et al. 1997). Balanced rates of chemical
weathering and erosion result in steady state
weathering, which produces compositionally similar sediments over a long period. Non–steady state
weathering, however, occurs where climate and
tectonism vary greatly, altering the rates of chemical weathering and erosion and resulting in production of chemically diverse sediments. The extreme height of Taiwan’s mountains (4 km) and the
monsoon climate (average precipitation is 2500
mm/yr with an average of four typhoons per year)
result in very rapid physical denudation and fast
transport of sediments to the ocean. In such a
weathering-limited regime, soils are thin because
of a high rate of mechanical denudation (physical
weathering 1 soil formation). The residence time of
secondary products of weathering is much lower
because storage within the high-standing island’s
riverine systems is thought to be minimal (Lyons
et al. 2002); rates of chemical weathering are low,
similar to those of the Himalayas (France-Lanord
and Derry 1997), but high physical weathering rates
continuously create new mineral surfaces that are
responsible for enhanced river chemical fluxes
(Gaillardet et al. 1999). Rivers of Taiwan, therefore,
show high elemental fluxes that are highly related
to huge storm-induced, physically eroded sediments transported in short mountainous rivers
(erosion effect) rather than true alteration of feldspars to clay minerals, i.e., silicate weathering.
Rivers draining in low-altitude areas may have a
better chance of segregation of elements in weathering relative to high-altitude systems, as concluded by Zhang and Liu (2002). They observed that
Journal of Geology
S U B T R O P I C A L TA I WA N W E AT H E R I N G
the segregation factor ([Ca ⫹ K ⫹ Na ⫹ Mg]/[Fe ⫹
Al]) in suspended matter of Chinese rivers increases
with sediment yields in regions dominated by physical weathering. In contrast, the segregation factor
decreases with sediment yield in regions dominated
by chemical and biological weathering because the
reduced water column turbidity allows chemical
and biological reactions to process into depth over
the drainage basin. In addition, an increase in the
height/length (H/L) ratio of the river course corresponds to a reduced segregation of elements in
weathering. Therefore, the very high H/L ratios of
Taiwan rivers and their huge suspended load (∼400
t/yr; Dadson et al. 2003) are also probably not conducive to chemical weathering. Further, extensive
erosion results in reduction of soil particle retention in the weathering crust, coupled with incomplete chemical reaction caused by reduction of
water-particle contact and segregation of elements
(e.g., Al and Si; Zhang et al. 2003).
The ratio between the rates of physical and
chemical denudation in Taiwan ranges from 10 to
40 mg/cm2/yr, approximately. In general, the areas
of lower physical denudation rate show higher
chemical denudation rates and vice versa (see fig.
1 of Li 1976). Similarly, the ratios of physical and
chemical denudation rates of Chinese rivers lie
mostly between 2 and 10 mg/cm2/yr, excluding
loess areas, where the ratio is high, at 90 mg/cm2/
yr. The average physical erosion rate in the Yellow
Basin was about 1.4 # 10 6 kg/km2/yr, while the average chemical erosion rate was 25 # 10 3 kg/km2/
yr. In comparison, the average physical and chemical erosion rates in the Yangtze Basin were
0.29 # 10 6 and 104 # 10 3 kg/km2/yr, respectively
(Li et al. 1984). This implies that intense physical
weathering cannot naturally result in a high silicate
weathering rate, mainly because the strong erosion
prevents the newly formed sediment from accumulating in the catchments area or in the soil profile. As a consequence, the source rocks and sediments have less time to react with the weathering
agents, leading to moderately weathered material.
Hence, because of the combination of active tectonic and climatic regimes (high relief, high rainfall, and storm-induced landslides), the continental
rocks of Taiwan are eroding rapidly at 3–7 mm/yr
(Dadson et al. 2004) through processes of fluvial
bedrock incision (Hartshorn et al. 2002), landsliding (Hovius et al. 2000), and debris flows (Lin et al.
2004). Large volumes of such physically eroded detritus are responsible for high dissolved elemental
fluxes (mass/volume ratio) in the fluvial system.
Chemical weathering has a smaller effect, and
physical denudation and subsequent erosion are
113
more important in controlling sediment compositions of the coastal and offshore sediments studied.
These sediments are produced through non–steady
state weathering dominated by hill-slope mechanical erosion. The possibilities of highly weathered
zones of soil profile are expected to be slight on the
slopes of steep mountains, since the profile materials are eroded before chemical weathering can
produce the appropriate mineralogy or soil zonation (Nesbitt et al. 1997). This non–steady state
weathering is likely to be a characteristic feature
of tectonically active high-standing islands of Asia
and Oceania, such as Taiwan, the Philippines, Indonesia, New Zealand, and Papua New Guinea,
where the entire spectrum of weathering zones developed on bedrock is susceptible to rapid erosion.
Tectonism, moderate relief of lake catchments, and
very high rainfall in the alpine region of Taiwan
result in more vegetation, which stabilizes soils
and aids in the production of organic acids that
decompose most of the feldspars. Deep weathering
profiles are therefore possible in high-altitude areas
of Taiwan where the dominance of chemical weathering rates resulting from high annual acid input
to mineral zones of soils over erosion produces
highly weathered sediments.
Conclusions
In spite of huge orographic rainfall in this mountainous island, chemical weathering is not an intense process, except in the sediments of alpine
Great Ghost Lake, where the CIA and PIA values
show extreme weathering conditions. The CIA
and PIA values between the upper crust of Yangtze
Craton and the sedimentary rocks of Taiwan and
sedimentary rocks and recent sediments increase
by just 20% in each stage. Moreover, sedimentary
rocks are recycled continental crustal materials,
their derivatives are recent sediments, and the
chemical weathering intensity of sediments is
more or less equal to the weathering conditions
of rivers such as Changjiang and Brahmaputra.
Our conclusions in this study emphasize the need
for additional data in this area and the need to
recheck the chemical weathering rates of this orogen with more precise and appropriate techniques.
The dissolved flux data of almost all the rivers in
Taiwan show a very high concentration of nitrate,
clearly indicating the influence of anthropogenic
input in the dissolved loads rather than true
weathering fluxes from dissolution of rockforming minerals. Anthropogenic input appears to
be the major source of riverine nutrients in the
Kaoping River (Yang 2001). For example, anthro-
114
K . S E LVA R A J A N D C . - T. A . C H E N
pogenic inputs of total dissolved nitrogen and
phosphate are about 5–6 # 10 4 and 4–8 # 10 3 kg/
d, respectively. This condition may hold for most
of the west-flowing rivers of Taiwan, where the
total population of the country is residing. Moderate silicate weathering is further supported by
the fact that the concentrations of dissolved silica
in all but six rivers of Taiwan (C.-T. A. Chen, unpub. data) are lower than the reported values of
this parameter in 60 large rivers in the world (Gaillardet et al. 1999). Clearly there is still a need for
additional information and long-term geochemical evidence about chemical weathering of this
orogen. Therefore, our future investigation will
focus on long core sediments of the northwest
Pacific eastern side of the island, where huge
amounts of weathered materials are directly trans-
ported by 11 fast-flowing rivers from the steep
slopes of the Eastern Range of Taiwan (Fuller et
al. 2003), with an aim to reconstruct the long-term
weathering history of Taiwan.
ACKNOWLEDGMENTS
Financial support from the Republic of China National Science Council (NSC 93-2611-M-110-009,
93-2811-M-110-006, and 93-2621-Z-110-004) is
greatly appreciated. We thank B. N. Nath for helpful comments and N. R. Rao for fine-tuning the
revised manuscript. We gratefully acknowledge W.
Nesbitt and an anonymous reviewer for their helpful comments and critiques of the original manuscript.
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