Full text of DOI:10.1007/BF02875338

Vol. 44 No. 7
SCIENCE IN CHINA (Series D)
July 2001
Weak chemical weathering during the Little Ice Age
recorded by lake sediments
JIN Zhangdong (金章东)1, WANG Sumin (王苏民)1, SHEN Ji (沈 吉)1,
ZHANG Enlou (张恩楼) 1, JI Junfeng (季峻峰) 2 & LI Fuchun (李福春)2
1. Lake Sediment and Environment Laboratory, Nanjing Institute of Geography & Limnology, Chinese Academy of
Sciences, Nanjing 210008, China;
2. Department of Earth Sciences, Nanjing University, Nanjing 210093, China
Correspondence should be addressed to Jin Zhangdong (email: zhdjin@niglas.ac.cn)
Received July 18, 2000
Abstract Low magnetic susceptibility, low Sr content and hence high Rb/Sr ratio in the lake
sediment sequence indicate a weak chemical weathering process under arid and cold climate of
the Little Ice Age in a single closed lake watershed. According to different geochemical behavior
between rubidium and strontium in earth surface processes, variation of Rb/Sr ratios in the lake
sediment sequence can be used as an effective geochemical proxy with definite climatic signifi-
cance of chemical weathering in watershed. Unlike chemical weathering process in tropic zone
and modern temperate-humid climate, concordant changes in both Sr content and magnetic sus-
ceptibility with δ18O values of Dunde ice core suggest that the weak chemical weathering was con-
trolled by air temperature during the Little Ice Age maximum. After the Little Ice Age, chemical
weathering intensity was controlled also gradually by precipitation with increasing in temperature.
Keywords: closed lake, Rb/Sr ratio, chemical weathering, the Little Ice Age.
Studies of climatic variability, especially of the Little Ice Age, are essential for modern cli-
matic change and for evaluation of its trend in future. Chemical weathering is a universal link of
climatic and environmental reconstruction between continent and ocean, catchment and lake. In
recent years, the intensive interest in past global climatic change has renewed efforts to quantita-
tively understand process, products and contribution to sediments of chemical weathering under
—
different tectonic and climatic conditions[1 3]. The chemical weathering process is restricted by
regionally environmental conditions, thus chemical constituents of weathering products have re-
corded paleoclimatic and paleoenvironmental changes[1, 2]. Many studies have, however, been fo-
cused on millionnial- and/or millennial-scale chemical weathering responding to global paleocli-
matic and atmospheric CO2 concentration changes[2, 4, 5], or on chemical weathering process under
present climatic conditions[1], little on short-time scale chemical weathering behavior in single
watershed, including the Little Ice Age. Solute concentration, which has been commonly used for
judging chemical weathering[4], on the other hand, is not an effective method for comparing the
weathering rates and watersheds with diverse climatic conditions, because the concentration of
major cations in watershed streams are evidently controlled by the evapotranspiration[5], especially
in the watersheds with lower precipitation.

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Due to different geochemical behavior between rubidium (Rb) and strontium (Sr) in chemical
weathering process, variation of Rb/Sr ratios has been used as an effective geochemical proxy of
—
chemical weathering intensity in nature[6 9]. In this paper, we provide a high-resolution vertical
distribution of Rb/Sr ratio in sediments from the Daihai Lake (112°32′31″—112°48′40″E,
40°28′7″—40°39′6″N) by combining magnetic susceptibility with clay mineralogy, to discuss
chemical weathering process recorded in lake sediment of the Little Ice Age and to reconstruct the
history of the weathering intensity and paleoclimatic change in the last 500 years. The Daihai
Lake, Inner Mongolia, is located in a transitional climate zone between semi-arid and semi-humid
area which is a climatically sensitive area to the East Asian Monsoon. The lake gets along with
chronically closed hydrological condition and hence the history of chemical weathering in water-
shed should be recorded in its sediments, so the closed lake is an ideal place for studying paleo-
climatic change.
1
Sampling and analytical methods
A 78 cm-long sediment core for analysis was recovered from a water depth of 12.50 m in the
central part of the Daihai Lake, and 78 samples were taken at 1-cm interval. The core sediments
are composed of brown to grey-black silty clay and mud. According to radionuclide 210Pb dating,
modern sedimentary rate in the Daihai Lake is about 1.6—1.8 mm/a[10], so the core sediments re-
covered were deposited continuously during the last 500 years, namely a resolution of about 6 a
for each sample. Samples were divided into two parts: one part was left uncrushed for clay miner-
als analysis and the other was grounded to a fine power in a mortar for analyzing Rb and Sr con-
tents and magnetic susceptibility.
Magnetic susceptibility was measured with Bartington MS2 Meter at Lake Sediment and En-
vironment Laboratory, Chinese Academy of Sciences. Rb and Sr contents in each sample were
measured with a Japanese VP-320 XRF spectrometer. The relative standard deviation is less than
1×10−6. Clastic mineral constituents were analyzed by D/Max-Rb type X-ray diffraction (XRD).
Meanwhile, to obtain clay fraction of sample, the carbonate and organic matter in sediment were
removed by HCl and H2O2. Afterwards, the ＜2 μm fraction and then the ＜0.5 μm fraction were
separated from each sample by centrifugation respectively. The suspension was dropped on glass
slide to gain an oriented clay specimens with a thickness of 3 mg/cm2 and then they were dried in
the air (air-dried, AD). After the AD oriented specimens were examined by XRD, all oriented
specimens were saturated with glycolate liquid (GL) using an atomizer and then examined imme-
diately by XRD again to determine the clay minerals and contents. Both AD and GL oriented
specimens were studied twice by XRD at the Center of Materials Analysis, Nanjing University.
Each specimen was scanned at 1°2θ/min with Cu-K-Alpha radiation, 40 kV, 80 mA, 0.01°2θ
steps and scanning range of 2—36°2θ. The error of ranges is less than 5%.

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2
Analytical results
As shown in fig. 1, Rb/Sr ratio and magnetic susceptibility are in the ranges of 0.41—0.68
and 50—137 (10−8 m3·kg−1) respectively, with a negative correlation. Due to little variation of Rb
contents ((109—125)×10−6) in lake sediments, variation of Rb/Sr ratios relies chiefly upon Sr ac-
tivity during chemical weathering. The negative relationship between Sr content and Rb/Sr ratio,
as shown in fig. 2, could confirm this. It is caused by the strong affinity of Rb for clay minerals
and Sr loss to solution during weathering leaching process[8, 11]
.
Fig. 1. Rb/Sr ratio and magnetic susceptibility of lake sediments from the Daihai Lake, northern China.
Though the clastic debris constituents in the whole core are the same on the whole including
mainly quartz, feldspar, micas and carbonate, clay minerals are different. For example, at the in-
tervals with higher Rb/Sr ratios, clay contents are 9%—12%, and illite, chlorite and muscovite are
dominant in the sediments. At lower Rb/Sr periods,
there are talc and kaolinite besides illite, chlorite
and muscovite and relatively high clay contents
(15%—18%). Moreover, some gypsum (<2μm)
was found at 50 cm depth (AD1695 to AD1700).
3
Discussion
3.1 Rb/Sr and chemical weathering intensity
Rubidium and strontium are easily fraction-
ated in earth surface geochemical process due to
different geochemical behavior[6, 7]. Sr, however,
would not be changed during erosion and sedi-
Fig. 2. Relationship between the Rb/Sr ratio and Sr
content in the Daihai Lake sediments.

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655
mentation process, and even long-term contact with lake water or interlayer medium water, and
not reached equilibrium with Sr of various water bodies[12]. Moreover, the radiogenic Sr increases
from 87Rb decay should not be considered for centennial-scale Sr contents change here due to a
long half-time of 87Rb decay (4.88×1010 a). It is seen from the studies on active and relict weath-
ering profiles and loess-paleosol sequence that Rb/Sr ratio in weathering products would increase
significantly with the enhancement of weathering intensity[7, 8, 11]. Consequently, Rb/Sr ratio in
lake and ocean sediments would decrease with the enhanced weathering in watershed, resulting in
more Sr into basin. In addition, the bedrocks exposed in the Daihai Lake watershed are chiefly the
Tertiary basalt and the Archean metamorphic igneous rock. Here we can assume reasonably that
the rock types did not change over at least one thousand years. By variation of Rb/Sr ratios in lake
sediments, we can reconstruct the chemical weathering history affected by paleoclimatic change.
Three sections of higher Rb/Sr ratio in the core depths of 62—54 cm (AD 1615—1665), 41
—38 cm (AD 1745—1765) and 29—19 cm (AD 1820—1880)[10] are shown in fig. 1. It can be
seen that there are three stages with lower intensity of chemical weathering in the last 500 years in
the Daihai watershed. Correspondingly, there are three relatively strong chemical weathering pe-
riods with lower Rb/Sr ratios between them. The acid-leaching experiments[11] have proved that
Rb is stable while Sr is easy to remove into solution, resulting in an increasing of Rb/Sr ratio in
weathering relic products. The Rb/Sr ratio variability for lake sediments deposited continuously,
therefore, substantially reflects the leaching degree of source area.
3.2 Weak chemical weathering during the Little Ice Age
The climatic study of the Little Ice Age plays a significant role in understanding present cli-
—
mate and in predicting future trend, and has become a frontier in current PAGES[13 15]. In the
Daihai watershed, cold and arid climate caused an acute reduction of precipitation (about 50 mm/a)
during the Little Ice Age according to historical document[16]. The chemical weathering certainly
would be weakened under this condition. Thus, three weak weathering periods with higher Rb/Sr
ratios in lake sediments probably represent relatively arid and/or cold stages during the last 500
years in the Daihai area. Through comparison with historical documents, these higher Rb/Sr sec-
tions are equivalent in age to three periods of arid and cool climate[14, 16]. The first is the coldest
“Little Ice Age” maximum, and the other two are two cold and dry periods. Especially, during AD
1615—1665, an increase in Rb/Sr ratio significantly suggests that this period should be the Little
Ice Age maximum[14, 15, 17], in agreement with historical records. The coldest period around AD
1625—1630[13, 17] has the highest Rb/Sr ratio (at depth about 59 cm) from the Daihai lake sedi-
ments (fig. 1). The weak chemical weathering is also reflected by magnetic susceptibility variation
from lake sediments. In arid and cold periods, fine-grained magnetic mineral into basin would
decrease significantly with the reduction of weathering and pedogenesis[18], illustrating that peak
of Rb/Sr ratio is correlative to low value of magnetic susceptibility (fig. 1). A very significant fea-
ture is that the negative correlation between Rb/Sr ratio and magnetic susceptibility of lake sedi-

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ment shown in fig. 1 responds to the striking similarity between them in the loess-paleosol se-
quence[7]. This suggests that Rb/Sr ratio respond sensitively to paleoclimatic change that affects
chemical weathering and pedogenetic intensity.
3.3 Chemical weathering and paleoclimate
In the short-term period, silicate weathering is a function of changes in air temperature and
precipitation. For example, there is less chemical weathering in arid glacial periods than in more
humid interglacial[1, 5, 19]. Thus Rb/Sr ratio preserved well in sediments can further be regarded as
a proxy for temperature and precipitation, especially in the watershed with lower precipitation as
the Daihai area[2, 5]
.
Fig. 3 shows approximately 50 a running average of Sr contents in the Daihai Lake sediments,
precipitation in the Daihai area[20] and δ18O values from the Dunde ice core[21] during the last 500
years. Because oxygen isotopic ratios from ice core, δ18Oice, primarily reflect air temperature[21, 22]
,
and because both Dunde and Daihai are located in a transitional zone of the East Asian mon-
soon[21], variation of δ18Oice values from the Dunde ice core can reflect air temperature change
trend in the Daihai area. Much of Sr in either basalt or high-calcium igneous rock is held within
plagioclase feldspar. Plagioclase decomposes more easily than other principal minerals in these
rocks, resulting commonly in the marked loss of Sr during more intense weathering process. Dur-
ing the Little Ice Age, cold and arid condition resulted from acute reduction of precipitation
largely weaken the intensity of chemical weathering[16, 20] (fig. 3), bringing about more coarser
plagioclase fragments and lower Sr content (i.e. higher Rb/Sr ratio) into basin and more brackish
ostracoda and Betula pollen during the periods with low lake water level[10, 16]. The coincident
peaks between Sr contents in lake sediments and δ18O values from the Dunde ice core[21] during
Fig. 3. Approximately 50 a running average of Sr contents in the Daihai Lake sediments, precipitation in the Daihai
area and δ18O values from the Dunde ice core during the last 500 years. The precipitation shown is from tree-ring
reconstructed climate data[20]. The δ18O values are from the Dunde ice core data[21]. Three arrows in the figure show
three stages of weak chemical weathering during the Little Ice Age.
the last 500 years indicate that Sr leaching capacity chiefly depended upon air temperature varia-

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CHEMICAL WEATHERING RECORDED IN LAKE SEDIMENTS
657
tions under arid and cold condition[23]. The weaker the chemical weathering, the lower the air
temperature, in the two cool periods after the Little Ice Age maximum (fig. 3). As both air tem-
perature and precipitation increased after the Little Ice Age, the chemical weathering relied upon
precipitation variations under modern humid and warm climatic conditions[19, 23], showing high Sr
contents in lake sediments with increase in precipitation (fig. 3). Consequently, the chemical
weathering rate could indicate air temperature and precipitation in single watershed under
semi-arid and semi-humid climatic conditions. Weak chemical weathering, therefore, reflects re-
gionally arid and cold paleoclimatic environment during the Little Ice Age.
4
Conclusion
The sediments deposited continuously in a closed lake under different paleoclimatic condi-
tions display an evident variation of Sr distribution, reflecting chemical weathering history in sin-
gle watershed by variation of Rb/Sr ratio. Low magnetic susceptibility, low Sr content and hence
high Rb/Sr ratio in lake sediments deposited during the global Little Ice Age indicates a weak
chemical weathering process in a single watershed which was controlled by arid and cold climate.
Our work reported here on a geochemical investigation of a single watershed, with the aim at
evaluating chemical weathering variation in the Little Ice Age, indirectly supports large-scale re-
gional studies which suggested that both silicate and carbonate chemical erosion rates increased
by about 20% during the Holocene compared with the Last Glacial Maximum (LGM, 14C 18 ka
BP)[23]. Furthermore, we find that air temperature plays a more important role in chemical weath-
ering than precipitation during the dry and cold “Little Ice Age” maximum in single watershed
located in a transitional zone of semi-arid and semi-humid, on the basis of variation trends of Sr
content of lake sediments, precipitation from tree-ring data and δ18O value from ice core during
the last 500 years.
A good negative correlation between Rb/Sr ratio and magnetic susceptibility during the Little
Ice Age as shown in fig. 1 further suggests that the Rb/Sr ratio in lake sediments can also be re-
garded as a good indicator for magnetic susceptibility of East Asian summer monsoon intensity
and front position which are closely related to weathering behavior[7, 11, 18]. The Rb/Sr ratio, how-
ever, could provide more detailed information than magnetic susceptibility. For instance, there are
two obvious peaks after the Little Ice Age maximum shown by Rb/Sr ratio (fig. 1), but the mag-
netic susceptibility curve does not show the changes, which correspond to sub-peaks of two
post-glacial climate fluctuation that were documented by historical records [10, 16]. Instead, mag-
netic susceptibility is abnormally high near the surface layer that might be affected by human ac-
tivities[18]. Those suggest that the Rb/Sr proxy should be much better than magnetic susceptibility
in tracing paleoclimatic change recorded in lake sediments and loess-paleosol sequence[7, 11]
.
Acknowledgements This work was supported by the National Natural Science Foundation of China (Grant No.
40003001), National Key Basic Research Program (Grant No. G1999043400) and Post Doctoral Foundation of China. Thanks
are due to Prof. Zhu Jinchu in Nanjing University for his valuable comments on the English manuscript and Liu Di, Dr Huang
Jianhua for assistance with laboratory work.

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