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Sediment Quality Assessment and Management: Insight and Progress Edited by M. Munawar© 2003 Ecovision World Monograph Series Aquatic Ecosystem Health & Management Society Effects of palaeoclimate on chemical and mechanical erosions in watershed recorded in lake sediments: the case of Daihai Lake (northern China) Z. Jin1*, S. Wang1, J. Shen1, F. Li2, X. Lu2 1Key Laboratory of Lake Sedimentation and Environment, Nanjing Institute of Geography and Limnology, Chinese Academy of Sciences, Nanjing 210008 China. * 2Department of Earth Sciences, Nanjing University, Nanjing 210093 China. Keywords: closed lake, Rb/Sr ratio, chemical weathering, Little Ice Age, Medieval Warm Period Introduction Weathering and erosion are two universal behaviours at the earth’s surface. The term “weathering” implies that chemical weathering is strongly affected by climate, principally by precipitation and temperature that control its intensity and outcome (Lasaga et al., 1994; Blum et al., 1998; Taylor and Lasaga, 1999). A major uncertainty in present climate models is the sensitivity of weathering rates to climatic variables. It is important to understand surface geochemical process if we wish to correctly interpret the sediment records in oceans, rivers and/or lakes, because their inputs are chiefly from detritus and solute by weathering and erosion. In recent years, the intense interest in past and present global climate change has renewed efforts to quantitatively understand relationship between chemical weathering and climate, including PAGES/PANASH (1995). Though many studies have been realized on intensity and control factors of chemical weathering, understanding chemical weathering of a single watershed and/or rock is still limited (Blum et al., 1998; Taylor and Lasaga, 1999). Solute concentration sometimes used for judging chemical weathering is, however, not an effective method for reflecting weathering rates between different watersheds with diverse climate conditions. The concentrations of major cations 308 in streams are evidently controlled by the evapotranspiration, especially in the watershed with lower precipitation as Daihai area (McFarlane, 1994). The Daihai Lake, Inner Mongolia, northern China, is located in a transitional zone of semi-arid and semi-humid, where is a climate sensitive area responds to East Asian Monsoon. It is 133.46 km2 in surface area, steep-sided, flatbottomed , and 16.05 m deep (maximum). It drains 2289 km2 of hummocky terrain. The Daihai Lake has a closed hydrological condition and lake sediments provide a good record of the history of chemical and mechanical erosions in watershed. Here we provide a vertical distribution of the Rb/Sr ratio in the Daihai Lake sediment sequences, and reconstruct the history of the weathering intensity in the last 2,200 years and further discuss how this geochemical parameter responds to palaeoclimate change by comparing with magnetic susceptibility, CaCO3 content, organic carbon concentration (Corg) and clay mineralogy. Sampling and analytical methods Sediment samples for analysis were taken from a depth of 12.50 m in the central part of the Daihai. The sediment core is 400 centimetres long and comprises brown to grey-black silty clay and mud. Dating control was provided by measurements of 210Pb, 137Cs and accelerator mass spectrometer (AMS) 14C date (Table 1). According to 210Pb and 137Cs activity by γ ray of surface 14 cm sediments, modern sedimentation rates average 1.6~1.8 mm/yr. The silty clay was therefore deposited continuously over the last 2,200 years. The core was divided into 240 samples, each of 1 cm above 80 cm and 2 cm below it. The sample was split into two representative sub-samples, one was left uncrushed for clay minerals analysis and the other crushed to a fine power. Firstly, the carbonate, ferruginous cement and organic matter in each subsample were removed by HCl and H2O2. Afterwards, the < 2 μm fraction and then the < 1 μm fraction of each sub-sample were separated by centrifugation. The oriented clay-aggregate samples were prepared by dropping the aqueous suspension on glass slides and drying them in the air (air-dried, AD). To achieve constant thickness during X-ray diffraction (XRD) examination (Kisch, 1991), a thickness of 3 mg cm-2 was made for each specimen. After the AD oriented clay-aggregate Table1. Dating data from the Daihai Lake sediments, northern China. Depth (m) Material Age (a B.P.) Method Laboratory 0.14 Surface sediment 80±5 210 Pb, 137 Cs Lake Sedimentation and Environment, CAS 2.00-2.02 Wood fragment 1560±70 AMS-14 C AMS Dating Lab., Tokyo University 4.00-4.02 Wood fragment 2280±120 309 was examined by XRD, all oriented clay-aggregate samples were saturated with ethylene glycolate (EG) using an atomizer, and then examined by XRD again. Both AD and EG oriented clay-aggregate samples were examined twice by D/Max-Rb type XRD at the Centre of MaterialsAnalysis, Nanjing University. Each specimen was scanned at 1°2θ min-1 with Cu-Kα radiation, 40 kV, 80 mA, 0.01°2θ steps and scanning range of 2-36°2θ. The error is less than 5%. The CaCO3 contents were measured to an accuracy of ±2% by gran titration with HCl (Shen et al., 1994). The magnetic susceptibility was measured with a Bartington MS2 Meter. We used a XRF spectrograph for Rb and Sr measurements. For each XRF measurement, a 5 g sample was sieved to a size-fraction of < 200 mesh, and then compacted into a round disc and measured with the VP-320 XRF spectrometer (Japan). The relative standard deviation is about 1%. After CaCO3 is removed by 10% HCl and than dried in the air, the organic carbon concentration (Corg) of 43 samples are determined by using the CE-440 elemental analyzer (USA). The error is less than 0.2%. Results and discussion Rubidium and strontium are easily fractionated during common crustal processes at the Earth’s surface, in response to weathering and to different geochemical behaviours (Glodstein, 1988; Chen et al., 1999). It is evident from the studies on active and relict weathering profiles and on loess-palaeosol sequences that the Rb/Sr ratios in weathering products would increase significantly with the enhancement of weathering intensity (Dasch, 1969; Chen et al., 1998). Accordingly, the Rb/Sr ratios in lake/sea sediments would decrease significantly with the enhancement of weathering intensity in watersheds. The open behaviour of Sr reflects its response to weathering, Sr, however, would not be changed during mechanical erosion and sedimentation processes, and even long-term contact with lake water or pore water. It takes an extremely long time to reach equilibrium with Sr of various water bodies (Deurer et al., 1978; Ingram and Sloan, 1992). Moreover, because short-term Sr contents change is involved here, we should not consider radiogenic Sr increases over time from 87Rb decay due to long half-time of 87Rb decay (4.88×1010y). Based on the variations of Rb/Sr ratios in the lake sediment sequence, we can rebuild the palaeoclimatic change history based on chemical weathering. The results of Rb/Sr ratio, CaCO3 content, Corg and magnetic susceptibility of the Daihai Lake sediments are showed in Figure 1. The Rb/Sr ratio is in the range of 0.11 to 0.68. Periodicity in the Rb/Sr ratios reflect periodic climatic change during past 2,000 years (Grove, 1988; PAGES/ PANASH, 1995). There are three higher Rb/Sr ratio sections in Figure 1, indicating that there have been three stages 310 of lower intensity of chemical weathering during the last 2,200 years in the Daihai watershed. They are during 1870-1230 a B.P. (years before present), 900-670 a B.P. and 490-90 a B.P. In the short-term, silicate weathering is sensitive to change in air temperature and precipitation. For example, there is less chemical weathering in arid glacial periods than in the more humid interglacial (Deurer et al., 1978; Brady and Carroll, 1994; Blum et al., 1998). On the other hand, bedrock exposure in the lake watershed can be reasonably assumed not to have changed over at least two thousand years that is further supported by similar silty clay in recovered cores by weathering and erosion. Thus, the three sections with higher Rb/Sr ratios in the lake sediment probably represent three relative cold and/or arid stages during the last 2,200 years in the Daihai area. The latest is the Little Ice Age (LIA) with two cool periods, verified by historical documents (Chu, 1973; Grove, 1988; Wang et al., 1990). The cold period since 900 a B.P. appear to be equivalent to the beginning time of the LIA in European continent (Grove and Switsur, 1994). Accordingly, there are relative warm and humid periods between three cold stages during the past 2,200 years, including the Medieval Warm Period (MWP) and modern warming since the 20th century (Figure 1). Both the LIA and MWP are Fig. 1. Reconstructed MWP (Medieval Warm Period) and LIA (Little Ice Age) in the Daihai area, northern China, based on variations of Rb/Sr ratio, CaCO3 content, Corg and magnetic susceptibility record in the lake sediments. 311 two well-known global climate events since the neoglaciation (Chu, 1973; Shi and Wang, 1979; Grove, 1988; Thompson et al., 1993; Jones et al., 1998). An increase Rb/Sr ratio but a decrease both CaCO3 and Corg correspond to a cold period, vice versa. The lithologies exposed in the Daihai Lake watershed are Tertiary basalt and Archean metamorphic intrusive igneous rocks (Wang et al., 1990). Much of the Ca and Sr in basalt or high- calcium igneous rocks are held within plagioclase feldspar. Plagioclase decomposes more readily than other principal minerals in these rocks, resulting commonly in the marked loss of Ca and Sr during more intense weathering. In the Daihai watershed, cold and arid climate resulted from acute reduction of precipitation during the LIA (Wang et al., 1990; Wu and Liu, 1998). The extent of chemical weathering and plant growth certainly would have been limited under these conditions, contributing more plagioclase feldspar, coarser fragment and higher Rb/Sr ratio, but lower CaCO3 and Corg in the sediment. The sediment contains more ostracoda and Betula pollen due to salty lake water during arid periods. The palaeoclimatic change is also reflected by magnetic susceptibility variation of the sediment samples, illustrating a negative correlation between Rb/Sr ratio and magnetic susceptibility for lake sediment, as shown in Figure 1. The high positive correlation between the frequency-dependent magnetic susceptibility, which is used to identify ferromagnetic grain size, and the magnetic susceptibility indicates that the susceptibility depends mainly upon ultra-fine-grained ferromagnetic minerals formed during weathering and pedogenesis (Maher, 1986). Therefore, susceptibility in the loess-palaeosol sequences is used as an indicator of monsoon change (An et al., 1991). Under cold environments, fine-grained magnetic mineral sediments would decrease significantly with the reduction of weathering intensity (Wu, 1993), showing that the peak of the Rb/Sr ratio is contrary to the low values of magnetic susceptibility (Figure 1). A very significant feature is that the negative correlation between Rb/Sr ratio and magnetic susceptibility for the lake sediment shown in Figure 1 responds to a striking similarity between both in the loess-palaeosol sequences (Chen et al., 1999). However, the Rb/Sr ratio may provide more detailed information than magnetic susceptibility. For example, there are two obvious peaks after the Little Ice Age shown in Rb/Sr ratio, but they are not seen in magnetic susceptibility curve in Figure 1. Magnetic susceptibility is abnormal in the near surface layer and may be affected by human activities (Wu, 1993; Anderson and Hallet, 1996). It is suggested that the Rb/Sr ratio responds sensitively to palaeoclimatic change that affects chemical weathering and pedogenetic intensity in the watersheds and may be particularly useful proxy of palaeoclimate and that under some conditions it may be better than magnetic susceptibility in tracing palaeoclimate change recorded in the lake sediment and loess-palaeosol sequence (Chen et al., 1998, 1999). According to a high-resolution lake record characterized by lower Rb/Sr 312 ratios and higher both CaCO3 and Corg in the sediments (Figure 1), there exists the Medieval Warm Period in the northern China during 900-1200 a B.P., which a warm and humid environment demonstrated by significant increase of chemical weathering and by a progressive increase of biologic productivity (high Corg), and by a high lake levels. The Medieval Warm Period has not only an inner climatic fluctuant, but also the strongest chemical weathering during the last 2,200 years. There are tree spore pollen such as Pinus and Betula and ostracoda as Candona in the sediments (Wang et al., 1990). During the warm and humid periods, leaching and chemical weathering are relative strong and some active elements such as K, Na, Ca, Mg and Sr are readily released from the basalt and metamorphic rock in the Daihai Lake watershed. At such periods, these elements were transported into lake by surface runoff and result in the observed lower values of Rb/Sr ratio (Figure 1). This can be confirmed by the positive relationship between Sr contents and CaCO3 percentage in the sediment samples, as illustrated in Figure 2. Significant amounts of calcium are lost during rock weathering, hence CaCO3 is enriched relative to Sr during the period of the Little Ice Age (Figure 2). Furthermore, the MWP in northern China is contemporaneous with the world-wide event identified in the lakes, oceans, land mollusc sequences, polar ice cores, pollens and historical documents (for reviews, see Crowley and Lowery, 2000). It should be noted that analysis of lake sediments have shown little variation of Rb contents, thus variations of the Rb/Sr ratio mainly reflect Sr activity during chemical and mechanical erosions. The negative relationship between Sr content and the Rb/Sr ratio, as illustrated in Figure 3, are thought to confirm this. The high Rb/Sr ratio in fine-grained silicate clastic sediments reflects the affinity of Rb (with K) for clay minerals, and the loss of Sr (with Ca) to solution during weathering. The acid-leaching experiment has proved that Rb is stable while Sr is ready to be removed out from the deposits (Chen et al., 1998). The Rb/Sr ratio in the lake sediments deposited continuously, therefore, could be an indicator of the extent of rock weathering in the watershed during leaching process. Though most detritus constitutes in the core are same, including quartz, feldspar, micas and carbonate, the content of clay minerals differs. For instance, during periods with higher Rb/Sr ratios, clays content are relative high but illite, chlorite and muscovite are dominant. Talc and kaolinite, illite, chlorite and muscovite and lower clay contents occur in the sediments during periods with lower Rb/Sr ratios. Moreover, there are deposits of gypsum (< 2 μm) and gowerite (< 0.5 μm) during the MWP and after the LIA. Summary and conclusions Variations of Rb/Sr ratios in the lake sediment sequence are used as a good proxy of chemical weathering in watershed due to different geochemical behaviour 313 between rubidium and strontium during chemical and mechanical erosions. Rb/Sr ratio, CaCO3 content, organic carbon concentration (Corg), magnetic susceptibility and clay minerals of 4.0 m sediments samples recovered from Daihai Lake, northern China, are presented in the paper. Weathering and palaeoclimatic change history during the last 2,200 years is reconstructed by the variations of Rb/Sr ratios in the Fig. 2. Plot of CaCO3 vs. Sr contents in the Daihai Lake sediments. Note the separation of both the Little Ice Age and the Medieval Warm Period from other periods. Fig. 3. Negative correlation between the Rb/Sr ratios and Sr content for the Daihai Lake sediments. 314 sediment sequence, including the Little Ice Age and Medieval Warm Period. Our results suggest that the evolution processes of weathering and palaeoclimate can be rebuilt by the variations of Rb/Sr ratios in the lake sequence, matching with magnetic susceptibility, Corg, CaCO3 contents and clay mineralogy. The lake sediments deposited in different palaeoclimate conditions display an evident variation of Rb and Sr distribution, reflecting intensities of weathering and erosion by variation of the Rb/Sr ratios. According to the variations of Rb/Sr ratios in the Daihai Lake sediment sequence, a weathering and palaeoclimatic change history since 2,200 a B.P. is reconstructed, including the LIA and MWP. By combining these data with magnetic susceptibility, Corg, CaCO3 contents and clay mineralogy, variations of Rb/Sr ratios can be used as a good proxy of chemical weathering, thus recording change history of precipitation and temperature in watershed. Furthermore, since the enhancement of weathering intensity including the MWP supplies more Sr to lakes and rivers, Sr behaviour on the earth surface in single watershed likely provides geochemical evidence for the gradual increase in marine 87Sr/86Sr ratios over the past 40 Ma (Lasaga et al., 1994; Blum and Erel, 1995; Taylor and Lasaga, 1999, Yang et al., 2000). Acknowledgements This work was supported by the National Natural Sciences Foundation of China through grants 40003001, 49702028 and 49894170-04. We thank Mr. Jeffery Haller from Indiana, USA, in Guangzhou University for his valuable comments on the English manuscript. We also thank two anonymous reviewers for their insight suggestion and critical reviews of the manuscript. 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