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Chemical weathering over the last 1200 years recorded in the sediments
of Gonghai Lake, Lvliang Mountains, North China: a high-resolution
proxy of past climate
JIANBAO LIU, JIANHUI CHEN, KANDASAMY SELVARAJ, QINGHAI XU, ZONGLI WANG AND FAHU CHEN
Liu, J., Chen, J., Selvaraj, K., Xu, Q., Wang, Z. & Chen, F.: Chemical weathering over the last 1200 years
recorded in the sediments of Gonghai Lake, Lvliang Mountains, North China: a high-resolution proxy of past
climate. Boreas. 10.1111/bor.12072. ISSN 0300-9483.
Increasing interest in global climate change has led to attempts to understand and quantify the relationship between
chemical weathering processes and environmental conditions, especially climate. This interest necessitates the
identification of new climate proxies for the reconstruction of two important Earth surface processes: physical
erosion and chemical weathering. In this study, an AMS 14C-dated 2.8-m-long sediment core, GH09B1, from Lake
Gonghai in north-central China was subjected to detailed geochemical analyses to evaluate the intensity of chemical
weathering conditions in the catchment. Multivariate statistical analysis of major and trace elemental data of 139
subsamples revealed that the first principal component axis PCA1 explained ∼53% of the variance in the assemblage
of elements/oxides with significant positive correlations between PCA1 scores and the separation of mobile and
soluble elements/oxides from the immobile and resistant elements/oxides, which is thus able to indicate the chemical
weathering in the catchment. These results are supported by the down-core trends of other major and trace
elemental ratios of chemical weathering intensity as well as by pollen data from the same core. Variations in PCA1,
chemical index of alteration (CIA), Rb/Sr ratio and other oxides ratios indicate stronger chemical weathering due
to a wet climate during the Medieval Warm Period (MWP). However, the MWP was interrupted by an interval of
relatively weaker chemical weathering conditions from AD 940–1070. Weak chemical weathering under a dry
climate occurred during the Little Ice Age (LIA), and increased chemical weathering intensity during the Current
Warm Period (CWP). Our proxy records of chemical weathering over the last millennium correlate well with the
available proxy records of precipitation from Gonghai Lake as well as with the speleothem oxygen isotope record
from Wanxiang Cave, but do not show a significant correlation with the temperature record in N China, suggesting
that the chemical weathering intensity in the study area was mainly controlled by the amount of rainfall rather than
by temperature. We conclude that high resolution lacustrine sediment geochemical parameters can be used as
reliable proxies for climate variations at centennial-decadal time scales.
Jianbao Liu, Jianhui Chen (corresponding author: [email protected]), Zongli Wang and Fahu Chen, Key
Laboratory of Western China’s Environmental System (Ministry of Education), Research School of Arid
Environment and Climate Change, Lanzhou University, Lanzhou 730000, China; Kandasamy Selvaraj, State Key
Laboratory of Marine Environmental Science, Xiamen University, Xiamen 361005, China; Qinghai Xu, College of
Resources and Environment, Hebei Normal University, Shijiazhuang 050016, China; received 6th October 2013,
accepted 28th January 2014.
At geological time scales, chemical weathering processes can buffer atmospheric carbon dioxide (CO2)
and thus moderate large fluctuations in global temperature and precipitation through the greenhouse effect
(e.g. White & Blum 1995). On a regional scale, weathering processes are mainly controlled by environmental
conditions such as precipitation and temperature, and
thus knowledge of weathering processes inferred from
geochemical data from lake and marine sediment cores
can provide information on past climatic and environmental changes of the region under investigation.
Previous studies have focused mainly on longer-term
(millennial-scale and greater) chemical weathering
responses to global climate change and atmospheric
CO2 variability (e.g. Brady & Carroll 1994; Lasaga
et al. 1994; McFarlane 1994; Taylor & Lasaga 1999;
Beukema et al. 2011). However, few studies have
addressed the question of whether or not variations in
the chemical constituents of weathered products from a
single watershed could be used as proxies for climate
change, especially at shorter (centennial-decadal) time
scales (Selvaraj et al. 2007; An et al. 2011; Beukema
et al. 2011). Several studies have attempted to reconstruct the chemical weathering history since the Little
Ice Age (LIA) using low-resolution records from
Daihai Lake in N China; however, they used an age
model based on 14C dates of bulk sedimentary organic
carbon, which may potentially be affected by the radiocarbon reservoir effect (Jin et al. 2001a, b; Zhou et al.
2009). In addition, these studies paid less attention to
the mechanism of the chemical weathering-climate link
at shorter time scales and thus little is known about
catchment weathering processes over the last millennium in the mid-latitude region of N China, where the
environment is characterized by semi-arid to semihumid conditions.
In recent years, considerable attention has been paid
to the variation of moisture conditions over the last
millennium in eastern China; this region is heavily
influenced by the East Asian monsoon (EAM) circulation, which is significantly interlinked with economic
activity and cultural development (Zhang et al. 2008;
DOI 10.1111/bor.12072
© 2014 Collegium Boreas. Published by John Wiley & Sons Ltd
2
Jianbao Liu et al.
Cook et al. 2010). However, there is no consensus
regarding the monsoon moisture conditions over the
last millennium in eastern China, because variations
in monsoon strength reflected by different climatic
records spanning this interval show apparent discrepancies (Zheng et al. 2006; Zhang et al. 2008; Man
2009). In addition to possible chronological uncertainties, this could also be related to the fact that different
proxies may have different palaeoclimatic implications,
or that the strength of the Asian monsoon may have
different effects on precipitation in different regions of
eastern China. According to conventional concepts, the
abnormal northward extension of the southerlies into
N China and the associated increased precipitation
could be indicative of a strengthened Asian summer
monsoon (Wang et al. 2008; Liu et al. 2010). It is therefore important to provide additional reconstructions
with a reliable chronology and one or more highresolution climate proxies for the last millennium in N
China.
Here we chose the site of Gonghai Lake, a closedbasin alpine lake located at the northern boundary of
the modern Asian summer monsoon in N China, to
investigate the relationship between chemical weathering conditions and climate change during the last millennium. As the lake is hydrologically closed, the
history of chemical weathering in the watershed is
expected to be recorded in the lake sediments. Based on
AMS 14C dating of terrestrial plant macrofossils, which
provides excellent chronological control, the specific
aims of this study were to reconstruct the highresolution history of weathering intensity in the catchment and to assess its usefulness as a potential proxy
for climatic change over the last 1200 years.
Study site
Gonghai Lake (latitude 38°54′N, longitude 112°14′E,
altitude 1860 m a.s.l.) is located in Ningwu County,
Shanxi Province, on the northern margin of the
Chinese Loess Plateau (CLP) (Fig. 1A). It has a surface
area of ∼0.36 km2 with a maximum water depth of
around 10 m and a flat bottom (Fig. 1B). Gonghai
Lake is a freshwater alpine lake formed on a plateau of
the watershed between the Sanggan and Fenhe rivers.
As the lake is a hydrologically closed basin, the water
source is mainly precipitation (Fig. 1C). The Ningwu
County area is located in a transitional zone between
semi-arid and semi-humid conditions, falling into the
typical fringe of the modern Asian summer monsoon
(Fig. 1A). The mean annual precipitation in Ningwu
(1500 m a.s.l.) is about 468 mm, with about 65% of the
annual precipitation occurring in summer (from June
to August). The regional vegetation in the nearby
Lvliang Mountains is dominated by Larix principisrupprechtii, Pinus tabulaeformis and Populus davidiana
forest, whereas on the plateau (including the drainage
BOREAS
basin of Gonghai Lake), Hippophae rhamnoides scrub,
Bothriochloa ischaemum grassland and Carex spp. are
widely distributed. The exposed bedrock of the zonal
ground surfaces was mainly formed during the
Archaean to Cenozoic, and is comprised mainly of
sandstone, such as Huoshan sandstone (Meng et al.
2007; Liu et al. 2011). The origin of Gonghai Lake and
the distinctive provenance of its lake sediments have
been discussed elsewhere (Zhu et al. 2006; Meng et al.
2007; Chen et al. 2013; Wang et al. 2014). In the study
area, the last tectonic activity occurred ∼16 ka (Wang
et al. 2014), which played an important role in formation of the lake basin. No known crustal uplift or tectonic event has been documented during the Lateglacial
and Holocene periods and thus the effect of erosion or
denudation in the source area on the weathering
rates is unlikely to be very significant during the
Lateglacial−Holocene interval.
Material and methods
In January 2009, a 9.42-m-long sediment core, designated GH09B, was retrieved from the centre of
Gonghai Lake at a water depth of 8.96 m (Fig. 1B)
using a Uwitec Piston Corer. The 2.8-m-long first
section of core GH09B, designated GH09B1, was used
for the present study. In the laboratory, the core was
sliced at 1-cm interval for detailed geochemical analysis. After drying at 105°C, 3 g powdered samples were
compressed (at 30 T pressure) into a 32-mm-diameter
pellet and then stored in desiccators. Concentrations of
32 major, trace and rare earth elements/oxides (Cl, S, P,
As, Ba, Ce, Co, Cr, Cu, Ga, Hf, La, Mn, Nb, Nd, Ni,
Pb, Rb, Sr, Th, Ti, Tl, V, Y, Zn, Zr, Fe2O3, SiO2, Al2O3,
CaO, Na2O, K2O) were determined at the Key Laboratory of Western China’s Environmental Systems, Ministry of Education (MOE), Lanzhou University. We
used a PANalytical PW2403/00 X-ray fluorescence
(XRF) spectrometer equipped with a Super Sharp Tube
for the Rh-anode, with the following settings: 4.0 kW,
60 kV, 160 mA, and a 75 μ UHT Be end window. We
used version 5 of the company’s SuperQ software for
the XRF analysis. The accuracy of the analytical
method was established using six internationally recognized standard reference materials (SRMs): MAG-1,
BCSS-1, PACS-1, MESS-1, NIES-2 and GBW 07314.
Following the manufacturer’s instructions, we calibrated the equipment and found that the analytical
uncertainties (relative standard deviations) were less
than ±5% for most elements and oxides. The reproducibility and accuracy of XRF using blank samples was
better than ±10%. Details of the XRF method have
been described by Selvaraj & Chen (2006) and Selvaraj
et al. (2010). In addition to the sediment core, six
bedrock samples (sandstones) from the catchment of
Gonghai Lake were analysed to calculate the Sr
Chemical weathering over the last 1200 years, N China
BOREAS
100°E
110°E
120°E
112°14'0"E
a
A
Depth
Gonghai
Daihai
920
km
38°54'40"N
38°54'40"N
38°54'30"N
38°54'30"N
110°E
120°E
B
100
112°14'0"E
m
38°54'20"N
460
Mountain
Loess
Dersertand
andsandy
Sandyland
Land
Desert
38°54'20"N
230
Beijing
ains
Gonghai
Wanxiang Cave
3-5 m
1-3 m
Mount
Loess Plateau
5-7 m
Taihan
g
Badain Jaran Desert rt
ese
rD
e
g
ng
Mu Us Desert
Te
L
7-9 m
40°N
SM
Hunshandake
Sandy Land
Lvliang
Mounta
ins
40°N
MA
100°E
112°14'10"E
9-10 m
Keerxing
Sandy Land
0
3
112°14'10"E
C
Fig. 1. A. Location of Gonghai Lake in the Chinese Loess Plateau (modified from Liu 1985). Also shown are Wanxiang Cave, sandy desert,
mountain systems and the modern Asian summer monsoon limit (MASML, dashed line, from Chen et al. 2008). B. Bathymetry of Gonghai
Lake. The dot indicates the location of core GH09B investigated in the present study. C. Panoramic view of Gonghai Lake and its catchment
(modified from Chen et al. 2013). This figure is available in colour at http://www.boreas.dk.
fraction that is associated exclusively with the silicate
fraction in the sediment core. To calculate the chemical
index of alteration (CIA), a suitable proxy to infer
chemical weathering of source rocks in the catchment,
we used the carbonate content obtained from Loss
on Ignition (LOI) measurements to derive the CaO
content associated with the silicate fraction only
(CaO*). However, we obtained a wide range of CaO*
values (1.56–6.80%) and most CaO* values are higher
than the Na2O values in the respective subsamples. The
CIA was therefore calculated using the formula excluding CaO: molar Al2O3/(Al2O3+Na2O+K2O). Similarly,
based on the carbonate content (range: 3.82–8.90%) in
the sediment core, we apportioned the total Sr content
obtained from the XRF method to Sr associated with
silicate and carbonate, and the former Sr values were
used to calculate Rb/Sr ratios of all sediment
subsamples. We also selected 29 subsamples for pollen
analysis, which were processed using the procedure
described in Xu et al. (2007); the analyses were
conducted in the laboratory of Hebei Normal
University.
Sediment lithology and chronology
The lithology of core GH09B1 is uniform and consists
mainly of silty clay with occasional plant fragments.
Terrestrial plant macrofossils were selected from five
depths for accelerator mass spectrometry radiocarbon
(AMS 14C) dating (Table 1) to avoid the reservoir
carbon effect that commonly occurs in the lacustrine
sediments of arid China. The bottom four dates from
core GH09B1 have been published in Liu et al. (2011).
All AMS 14C samples were prepared using the standard
pretreatment (alkali–acid–alkali) and then measured at
the AMS Dating Laboratory of the Institute of Earth
Environment, Chinese Academy of Sciences, Xi’an,
China, and at Beta Analytic, USA. All dates were calibrated to calendar years using OXCAL4.1 software
(Bronk 2009) with the IntCal04 calibration data set
4
Jianbao Liu et al.
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Table 1. AMS 14C dating results of core GH09B1 from Gonghai Lake, N China.
Laboratory
number
Sample
number
Depth
(m)
Material
δ13C
(‰)
Corrected age
(a BP)
Calibrated age
(cal. a BP)
Reference
Beta306751
XA4667
XA4669
XA4670
XA4668
GHB1-35
GHB1-48
GHB1-89
GHB1-153
GHB1-191
0.46
0.63
1.16
1.99
2.49
Stem
Leaf
Stem
Stem
Stem
−25.9
−24.92
−30.66
−30.87
−30.41
−2–280
332–498
565–668
804–963
968–1053
138±140
415±83
617±52
884±80
1011±43
This paper
Liu et al. (2011)
Liu et al. (2011)
Liu et al. (2011)
Liu et al. (2011)
(Reimer et al. 2004). The calendar ages obtained are
precise enough to resolve the intervals of important
climatic events in the past millennium, such as the Medieval Warm Period (MWP), Little Ice Age (LIA) and the
Current Warm Period (CWP). The ages of other samples
were estimated by linear interpolation and the age at the
bottom of core GH09B1 is AD 880 (Fig. 2). There is no
significant change in the silty-clay lithology of core
GH09B1; however, the sediments are somewhat coarser
grained within two limited depth intervals: ∼1.0–2.0 m
and 2.5–2.8 m. Therefore, it is unlikely that there are any
significant changes in the density of mineral grains
affected by changes in either water depth or the energy of
sediment deposition. The sediment accumulation rate is
relatively high (∼239 cm ka−1) (Fig. 2) and the time resolution of the sampling interval ranges from ∼8–18 years,
enabling us to assess changes in weathering intensity on
decadal to centennial time scales.
Results
Principal component analysis (PCA) of the Gonghai
Lake geochemical data
In this study, ordination analysis was used to extract
combinations of variables that are correlated with the
distribution of elements/oxides. Prior to the analysis, an
0.0
AMS 14-C data
0.5
235 cm ka
-1
78 cm ka
-1
Age model
Depth (m)
1.0
262 cm ka
-1
1.5
311 cm ka
2.0
2.5
2000
394 cm ka
1800
1600
1400
1200
1000
-1
indirect ordination approach, i.e. detrended correspondence analysis (DCA), was performed to determine
whether a linear or unimodal model was more appropriate. The results of the DCA with a gradient length of
0.24 (<3.0 standard deviations) suggest that a linear
model is appropriate for the assemblage of elements/
oxides. PCA was then conducted to identify the factors
controlling the composition of elements/oxides. The
first PCA axis (hereafter named PCA1) explains 53.3%
of the variance within the geochemical data set (Fig. 3),
and those elements/oxides usually considered as more
soluble and mobile and that are mainly derived from
chemical weathering due to abundant precipitation (e.g.
CaO, Na2O and Sr) (Mackereth 1966; Engstrom &
Wright 1984) all showed strong positive loadings on
PCA1. In contrast, elements/oxides that are considered
as more insoluble and resistant (e.g. Al2O3, SiO2 and
Fe2O3) produced negative loadings on PCA1 (Fig. 3).
Here, K2O was excluded from the mobile and soluble
group due to the abundance of potassium feldspar in the
catchment that is relatively resistant to chemical weathering (Meng et al. 2007). Thus, the geochemically inert
elements or oxides in the sediments of Gonghai Lake in
general had negative loadings on PCA1. These relationships imply that PCA1 defines a synthetic geochemical
gradient that separates mobile and soluble elements (or
oxides) delivered to Gonghai Lake by catchment weathering processes from immobile and resistant elements
(or oxides). Thus, when arranged stratigraphically
(Fig. 6A), increasing PCA1 sample scores record the
relative importance of soluble and mobile elements (or
oxides) derived from chemical weathering to the overall
geochemical record, suggesting a positive correlation
between PCA1 and chemical weathering intensity. This
also confirms that different degrees of stability of
elements/oxides in the sediments of Gonghai Lake have
been separated under different environmental conditions, suggesting that variations in the PCA1 score could
be used as a proxy for chemical weathering intensity.
Rb/Sr ratio related to chemical weathering intensity in
the sediments of Gonghai Lake
-1
800
Age (cal. a BP)
Fig. 2. Age–depth profile of sediment core GH09B1 with the estimated sedimentation rates.
Strontium (Sr) is a trace element that behaves similarly
to Ca in many geochemical processes. Rubidium (Rb) is
incorporated mainly in potassium (K)-bearing silicates
and displays a more inert geochemical behaviour than Sr
Chemical weathering over the last 1200 years, N China
1.0
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5
P
PCA Axis 2 (18.0%)
Mn
Fe2O3
Co Ce
Ti
Ga
Al2O3
K2O
Rb
As
V
Sr
La Ni
S
Nd
Y Th
-0.4
CO3
Cr
Nb
CaO
Ba
Tl
Pb
Zn Cu
SiO2
Na2O
Hf
Cl
Zr
-1.0
1.5
PCA Axis 1 (53.3%)
Fig. 3. Biplot of PCA results for the first two principal components, showing loadings of analysed elements and oxides. This figure is available
in colour at http://www.boreas.dk.
Although K2O is not excluded from the mobile and
soluble group because of the abundance of potassium
feldspar in the Gonghai Lake basin that is resistant
to chemical weathering, the CIA and Rb/Sr records
(including records of the Rb/Sr ratio calculated using the
total Sr content and Sr that are only associated with
silicates) are still highly correlated (Fig. 5B, C), further
supporting the view that the Rb/Sr ratio can be an
effective weathering proxy for the sediments of Gonghai
Lake.
1.4
Rb/Sr ratio (Sr-Silicate only)
during weathering processes. The strong contrast in
weathering rates between the Ca- and K-containing
minerals, especially Ca- and K-feldspars, leads to the
fractionation of Rb and Sr during weathering (Dasch
1969; Chen et al. 1996), and this results in a significant
increase in the Rb/Sr ratio in the residual weathered
product (Brass 1975; Jin et al. 2001a). Lakes in general
receive large amounts of weathering products from
rocks and/or soils in the catchment. With the enhancement of chemical weathering, a greater amount of Sr is
leached as dissolved Sr2+ from rocks and soils and is
transported into lakes, whereas Rb tends to remain in
rocks and soils, resulting in a decrease of the Rb/Sr ratio
in lake sediments. Analyses of the Gonghai Lake sediments show a negative relationship (r=0.78, p<0.001,
n=139) between Sr concentration and Rb/Sr ratio
(Fig. 4), which we attribute to an affinity of Rb (with K)
for clay minerals and Sr loss (with Ca) to solution during
weathering (Dasch 1969; Chen et al. 1998). Rb/Sr ratios
of six rock samples from the catchment are uniform
(Fig. 5A) and lower than that of the lake sediments
(Fig. 5C). Thus, the variation in the Rb/Sr ratio in the
sediments of Gonghai Lake can be used as an effective
geochemical proxy of chemical weathering intensity (cf.
Selvaraj & Chen 2006; Selvaraj et al. 2010). In addition,
the chemical index of alteration (CIA) is also a measure
of the proportion of aluminium oxide (Al2O3) vs. the
labile oxides (Na2O+K2O+CaO*) (Nesbitt & Young
1982; Selvaraj & Chen 2006; An et al. 2011). Originally,
the CIA index was used to assess the degree of in situ
weathering. For its application to lake sediments, it is
reversed because these labile oxides (Na2O, K2O and
CaO) are easily transported into the lake to reduce the
CIA value by increasing weathering (An et al. 2011).
y=118.40x-0.018
R=0.777 (n=139)
1.2
P<0.001
1.0
0.8
0.6
0.006
0.008
0.010
0.012
1/Sr (Sr-Silicate only)
Fig. 4. Relationship between Rb/Sr ratio and the reciprocal of Sr
concentration for sediments of Gonghai Lake over the last 1200
years. This figure is available in colour at http://www.boreas.dk.
6
Jianbao Liu et al.
BOREAS
Fig. 5. A. Relationship between Rb and
Sr concentrations of six rock samples from
the catchment of Gonghai Lake. B.
Scatterplot of Rb/Sr ratio and CIA in core
GH09B1 from Gonghai Lake. C. Comparison of CIA and Rb/Sr records from
the sediments of Gonghai Lake over the
past 1200 years. This figure is available in
colour at http://www.boreas.dk.
Significance of oxide ratios in the sediments of
Gonghai Lake
The major oxides, such as CaO and Na2O, are usually
considered more soluble and mobile and derived from
chemical weathering and abundant precipitation,
whereas Al2O3, SiO2 and Fe2O3 are considered as more
insoluble and resistant (Mackereth 1966; Engstrom &
Wright 1984). Thus, molar ratios of (CaO+Na2O)/
Al2O3, (CaO+Na2O)/SiO2 and (CaO+Na2O)/Fe2O3 in
Gonghai Lake could also reflect the chemical weathering intensity in the catchment. Higher ratios of major
oxides indicate the predominant input of soluble and
mobile elements into Gonghai Lake during intervals of
elevated hydroclimatic regime in the lake catchment
and thus intense chemical weathering.
Discussion
Origin of weathering products and comparison of
different sedimentary chemical weathering records from
Gonghai Lake
As Gonghai Lake is located in a transitional zone
between the East Asian monsoon system and the loessdesert area of N China (Fig. 1), both soil erosion via
catchment runoff and wind-blown dust could be the
major sources of detrital sediment to the lake basin. On
the Chinese Loess Plateau (CLP), the average loess
accumulation rate was 7 cm ka−1 during the Brunhes
epoch, varying with climate and being roughly four
times higher during glacial periods than during
interglacials (Heller & Evans 1995). In marginal areas
Evolution of chemical weathering over the
last millennium
The base of core GH09B1 is dated at AD 880, thus
enabling us to reconstruct ∼1200 years of weathering
history in arid China. Based on the PCA1 and other
oxide ratios (CIA and Rb/Sr ratio), the record can be
divided into two types of geochemical zone: (i) with
high values of PCA1 and the other oxides (and low
values of CIA and Rb/Sr ratio) and (ii) with low values
of PCA1 and the other oxides (and high values of CIA
and Rb/Sr ratio) (Fig. 6A–F). Intervals of type (i)
occurred from AD 880 to 1230 and from AD 1880 to
the present and roughly correspond to the time inter-
1800
1600
1400
1200
1000
800
0.6
0
B
(CaO+Na2O)/Al2O3
-2
2.40
0.8
1.0
C
0.9
1.2
1.4
Rb/Sr (Sr-Silicate only)
A
2
1.2
7
0.3
0.3
D
0.2
E
1.92
1.44
64
F
0.96
68
65.0
32.5
0.1
(CaO+Na2O)/SiO2
0.6
CWP
0.0
2000
LIA
1800
1600
MWP
1400
1200
1000
G
CIA
PCA 1
2000
4
(CaO+Na2O)/Fe2O3
of the CLP, the loess accumulation rate may be as high
as ∼20 cm ka−1 (Chen et al. 1991). However, the average
accumulation rate in core GH09B1 is ∼250 cm ka−1
(Fig. 2), which is significantly higher than the
maximum deposition rate even during glacial periods
on the CLP. This implies that almost all of the detrital
sediments in core GH09B1 are derived from intense soil
erosion from the catchment, whereas the proportion of
wind-transported particulates is likely to be negligible
compared to the proportion of sediment derived from
runoff. Moreover, previous study of the mineral magnetic properties of core GH09B1 has revealed that the
detrital sediment component is mainly composed of
eroded soil material rather than eolian dust (Chen et al.
2013). Thus, in the closed-basin of Lake Gonghai,
chemically weathered materials derived from rocks and
soils in the catchment are delivered to the lake predominantly via runoff and thus must be related to monsoon
intensity. Therefore, the PCA of geochemical data,
CIA values, Rb/Sr ratios and other major oxide ratios
in sediments, which are positively correlated with each
other over the last 1200 years (Fig. 6A–F), can potentially be used as effective geochemical proxies for the
intensity of chemical weathering of rocks and soils in
the catchment of Gonghai Lake.
To further verify our proposed geochemical result,
subsamples from core GH09B1 were also selected for
pollen analysis and the results are shown in Fig. 6G.
The modern vegetation in the catchment area of
Gonghai Lake consists mainly of grassland and shrubs.
A mixed coniferous and broad-leaved forest is located
below the plateau on which Gonghai Lake is located.
Thus the proportion of tree pollen in lake sediments
from the area might be expected to reflect the amount
of precipitation and hence the summer monsoon intensity. It is therefore of interest that the percentage of tree
pollen in core GH09B1 and our geochemical weathering indices strongly covary over the last 1200 years,
which supports our contention that the intensity of
chemical weathering of rocks and soils in the catchment
of Gonghai Lake was influenced by environmental conditions related to climatic variations.
Chemical weathering over the last 1200 years, N China
Tree pollen (%)
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72
800
Age (AD)
Fig. 6. Variation of proxies for chemical weathering intensity and the
percentage of tree pollen over the past 1200 years in core GH09B1
from Gonghai Lake. A. PCA 1 sample scores. B. Rb/Sr ratio. C.
(CaO+Na2O)/Al2O3. D. (CaO+Na2O)/SiO2. E. (CaO+Na2O)/Fe2O3.
F. CIA. G. Tree pollen percentage. Shaded areas represent the time
intervals of the MWP and the CWP. This figure is available in colour
at http://www.boreas.dk.
vals of the MWP and the Current Warm Period (CWP;
i.e. 20th-century warm period), respectively (Moberg
et al. 2005) (Fig. 7A). An interval of type (ii) lasted
from AD 1230 to 1880, corresponding to the LIA
(Moberg et al. 2005) (Fig. 7A). Therefore, these intervals correlate well with the most distinctive climatic
fluctuations during the last millennium in the Northern
Hemisphere. The PCA1 and other oxide ratios (CIA
and Rb/Sr) exhibit high (low) values in sediments
deposited from AD 880 to 1230 with larger fluctuations, suggesting stronger chemical weathering of
source rocks during the MWP in the study area. In
contrast, in sediments accumulated from AD 1230 to
1880, low values of CIA and Rb/Sr ratio with an
increasing trend suggest that the intensity of chemical
weathering was probably weaker during the LIA, but
increased towards the later part of the LIA. Finally,
high values of these proxies since AD 1880 indicate an
increased chemical weathering intensity during the
CWP. Although the chemical weathering intensity was
generally stronger during the MWP, a pattern of
Jianbao Liu et al.
2000
0.4
1800
BOREAS
1600
1400
1200
1000
800
A
-0.4
-1.2
B
0.75
0.00
64
C
CIA
66
-0.75
-1.50
68
(North China)
1.50
-0.8
Temperature Anomalies
0.0
4
70
D
72
2
0
PCA 1
(Northern Hemisphere)
Temperature Anomalies
8
0.6
Rb/Sr
1.0
-2
92
1.2
F
1.4
88
84
18
δ OVPDB
(Wanxiang Cave)
-8.8
MWP
LIA
CWP
G
-8.4
80
S-300 (GH Summer rainfall)
E
0.8
-8.0
-7.6
2000
1800
1600
1400
1200
1000
800
Age (AD)
Fig. 7. Comparisons of representative chemical weathering records
with other climatic records. A. Northern Hemisphere temperature
records (Moberg et al. 2005). B. Temperature record in north-central
China (Ge et al. 2003). C. CIA index. D. PCA1 scores. E. Rb/Sr ratio.
F. Magnetic proxy records from Gonghai Lake, Shanxi Province, N
China (Liu et al. 2011). G. Speleothem δ18O record from Wanxiang
Cave, Gansu Province, N China (Zhang et al. 2008). Shaded areas
represent the time intervals of the MWP and the CWP. This figure is
available in colour at http://www.boreas.dk.
shorter term oscillations is superimposed on it, of
which the most pronounced lasted from AD 940 to
1070 (Fig. 6A–F). During this period, the values of
PCA1 and other oxide ratios were relatively low but the
CIA and Rb/Sr ratio were relatively high, indicating
that the chemical weathering intensity was reduced at
this time.
Comparison with other climatic records
Climate plays a major role in chemical weathering processes. The major climatic variables, temperature and
precipitation, either individually or in combination, act
as the first order control on the initiation of major
chemical reactions and therefore determine the chemical weathering intensity of rocks (White & Blum 1995;
Blands & Rolls 1998). The inferences drawn above,
based on our proxy records of chemical weathering
from Gonghai Lake, are consistent with several other
published monsoon reconstructions in N China. For
example, Fig. 7 compares proxy records of chemical
weathering from Gonghai Lake (Fig. 7C–E) with the
Asian summer monsoon reconstructed using S-300, a
mineral magnetic rainfall proxy from Gonghai Lake
(Liu et al. 2011) (Fig. 7F), and the oxygen isotope
record of speleothems from Wangxiang Cave (Zhang
et al. 2008) (Fig. 7G), which is also located on the
northwestern margin of the modern Asian summer
monsoon. The correlation coefficient between PCA1
and S-300 is 0.65 (p<0.001, n=139). It is apparent that, on
a multicentennial time scale, the proxy data for the
monsoon (rainfall) variability from different locations
show a consistent pattern with changes in chemical
weathering intensity inferred from geochemical records
from Gonghai Lake over the last millennium. In particular, the stronger Asian monsoon during the MWP
indicated by high values of S-300 in the Gonghai Lake
sediments and the lighter oxygen isotope values
recorded in speleothems from Wanxiang Cave, as well
as the strong chemical weathering intensity indicated
by the high values of PCA1 and oxide ratios and low
values of CIA and Rb/Sr ratio in the sediments of
Gonghai Lake, all indicate a stronger chemical weathering process under a wet climate during the MWP. The
situation was roughly the opposite during the LIA and
consistent since AD 1880. On decadal to centennial
time scales, the secondary weakening of chemical
weathering intensity during AD 940–1070, against the
background of an overall strong chemical weathering
period during the MWP, is also evident in the monsoon
rainfall proxy reconstruction from Gonghai Lake. In
addition, this short-term episode of weaker chemical
weathering is also reflected in the high-resolution
oxygen isotope record from Wangxiang Cave. Therefore, changes in chemical weathering intensity evident
in the sediments of Gonghai Lake seem to respond
sensitively to rainfall variability on submillennial and
even shorter time scales. However, there is no correlation between the geochemical proxy records of
Gonghai Lake and the temperature record reconstructed from historical documentation (Ge et al. 2003)
in north-central China, given that the correlation coefficient between PCA1 and temperature in north-central
China is 0.05 (n=139) (Fig. 7B). This suggests that
chemical weathering in the Gonghai Lake region over
the last 1200 years was mainly controlled by variations
in the amount of precipitation delivered by the Asian
summer monsoon, rather than by temperature. This
possibility is consistent with the fact that over the past
1200 years, temperatures in north-central China only
fluctuated by ±1°C. Clearly, further work on additional
sites in N China is needed to confirm the suggested
dominant role of precipitation in controlling the rate
of chemical weathering during the last millennium.
Finally, we suggest that sediments from lakes in N
China are more suitable for chemical weathering
BOREAS
studies than those from lakes in arid China, where the
geochemistry of sediments is largely influenced by carbonate precipitation.
Conclusions
•
•
•
•
In the closed-basin of Gonghai Lake, the weathering products in the lake sediments are mainly
derived from rocks and soils in the catchment and
are largely supplied to the lake basin via runoff with
minimal aeolian inputs. Such conditions offer the
opportunity of using the sediment geochemical
record to investigate the relationship between
weathering rate in the catchment and climate.
The first component of a PCA of the geochemical
data set of core GH09B1 from Gonghai Lake
defines a synthetic geochemical gradient separating
mobile and soluble elements/oxides delivered to the
lake by catchment processes from that of immobile
and resistant elements/oxides. This suggests that
variations in the PCA1 score can be used as a proxy
for chemical weathering intensity. In addition, the
negative relationship between Sr concentration and
Rb/Sr ratio, due to an affinity of Rb (with K) for
clay minerals and Sr loss (with Ca) to solution
during weathering, suggests that the variation of
Rb/Sr ratios in sediments of Gonghai Lake can also
be used as a proxy of chemical weathering intensity.
These records of chemical weathering intensity,
together with the CIA and other oxide ratios, are all
positively correlated over the last 1200 years.
High resolution geochemical data combined with a
reliable chronology of sediment accumulation in
core GH09B1 enabled us to study the evolution of
chemical weathering in the montane region of N
China for the first time. The results indicate more
intense chemical weathering under a warm and wet
climate during the MWP (AD 880–1230); weaker
chemical weathering with an increasing trend under
a cold and dry climate during the LIA (AD 1230–
1880); and finally increased chemical weathering
intensity during the CWP (AD 1880 to present). In
addition, an interval (AD 940–1070) of relatively
weak chemical weathering occurred within the
context of generally strong chemical weathering
during the MWP.
The major characteristics of chemical weathering
over the last millennium correlate well with other
proxy records of monsoon rainfall derived from
Gonghai Lake as well as with other records from the
monsoon region. All of these reconstructions,
including the reconstruction of chemical weathering
in the sediments of Gonghai Lake and summer
monsoon rainfall, are comparable even on
decadal−centennial time scales. However, we found
no correlation between chemical weathering in the
Chemical weathering over the last 1200 years, N China
9
catchment of Gonghai Lake and a coeval temperature record from north-central China, suggesting
that chemical weathering in the Gonghai Lake
region over the last 1200 years was not controlled by
air temperature, but rather was largely controlled by
the intensity of East Asian summer monsoon rainfall. Our study demonstrates the value of geochemical records from Gonghai Lake as additional,
reliable proxies for exploring the past variability of
Asian monsoon.
Acknowledgements. − We sincerely thank the following: Dr Cao
Xianyong for the help with fieldwork; Dr Wei Haitao and Dr Zhao
Jiaju for making constructive comments and suggestions on the original version of the manuscript; Dr Jan Bloemendal for substantially
improving the English; and two anonymous reviewers and the editor
for their constructive suggestions and comments. The work was supported by the National Basic Research Program of China
(2012CB955301, 2010CB950202), the National Natural Science
Foundation of China (41130102, 41001114), Specialized Research
Fund for the Doctoral Program of Higher Education
(20120211130001), and the Fundamental Research Funds for the
Central Universities (lzujbky-2013-244).
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