Science of the Total Environment 385 (2007) 196 – 207 www.elsevier.com/locate/scitotenv Environmental response to climate and human impact during the last 400 years in Taibai Lake catchment, middle reach of Yangtze River, China Enfeng Liu, Xiangdong Yang ⁎, Ji Shen, Xuhui Dong, Enlou Zhang, Sumin Wang Nanjing Institute of Geography and Limnology, Chinese Academy of Sciences, 73, East Beijing Road, Nanjing 210008, PR China Received 30 January 2007; received in revised form 21 June 2007; accepted 22 June 2007 Available online 20 July 2007 Abstract Element content, grain size, pollen and 210Pb dating analysis were performed on 80 cm sediment core from Taibai Lake, a shallow lake in the middle reach of Yangtze River in China, to reveal the response of catchment environment to climate changes and human activities during the past four centuries. The 210Pb dating suggests that 0–39.5 cm of the core represents the past 163 years and bottom of the core is at ca. 1590 AD. Major changes showed in the core are compared with the records from historical documents, including population, cultivation and climate etc. 1700–1810 AD was the period with intensive human activities in Taibai Lake catchment, behaving as deforestation and cereal cultivation expansion, signaled in the sediments by low percentage of pinus pollen and high percentage of Gramineae pollen, high content of b 16 μm fraction. Another period with intensive human activities was since 1928 AD, behaving as deforestation, reservoir construction, land reclamation, soil loss. The high sedimentation flux epochs, such as 1900–1920 AD, 1931 AD, 1938–1939 AD and 1954 AD, were correlated with high precipitation; nevertheless, the high sedimentation flux epochs in 1958–1970 AD and 1983–1993 AD were correlated with the land reclamation around Taibai Lake and soil loss due to intensive cultivation development respectively. The two coldest stages during the “Little Ice Age” were in 1650–1700 AD and 1810–1900 AD, consistent with the historical records generally, characterized by weak weathering, high forest coverage and weak human activities in Taibai Lake catchment. © 2007 Elsevier B.V. All rights reserved. Keywords: Human activities; Climate changes; Sedimentary environment; Taibai Lake 1. Introduction The reconstruction of climate and environment changes is essential to understand the impact of natural processes and human activities on the ecosystems. It is especially contributive in the middle and lower reaches of ⁎ Corresponding author. 73, East Beijing Road, Nanjing 210008, P.R. China. Tel.: +86 25 86882149; fax: +86 25 57713063. E-mail address: [email protected] (X. Yang). 0048-9697/$ - see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.scitotenv.2007.06.041 Yangtze River in China. Yangtze River catchment, a sensitive area to climate changes and human activities (Chen et al., 2001; Du et al., 2001; Gao et al., 2004; Liu, 2000; Liu et al., 2000; Xiang et al., 2002; Yasuda et al., 2004; Yin and Li, 2001), was the economic developed, population denseness area in China for thousands of years (Zhang, 1990; Zhang et al., 2005). The development in economy and increase in population in recent decades had exerted great pressure on the ecological environment, especially water pollution, soil loss, siltation on lake and E. Liu et al. / Science of the Total Environment 385 (2007) 196–207 river bed, and lake ecological degeneration in the middle and lower reaches of Yangtze River (Du et al., 2001; Xiang et al., 2002; Yin and Li, 2001; Yang et al., 2002a). In comparison to the environment evolution at millennium scale, there had rapid population increase and frequent climate fluctuation during the last hundreds years (Zhang, 1980, 1990; Yang et al., 2002b), linking closely with current ecological environment. But seldom study was performed on this time scale, especially linking sedimentary and historical records to interpret the ecological environment evolution. It is challenging due to the difficulties obtaining sedimentary records of sufficient temporal resolution and chronological accuracy, and corresponding documentary data. Lake sediments, as the excellent archives of regional environment changes, have the characteristics of continuity, high resolution and abundant information, are playing a more important role in the research on regional human activities and environment change reconstruction (Liu et al., 2000; Huang and Connell, 2000; Tibby, 2003). In this study, the environment response to climate changes and human activities was reconstructed for Taibai Lake catchment, by the systemic analyses of the physical, element geochemistry and pollen proxies in sediment core of Taibai Lake, and the sequential information of population and plantation records in Susong County since 197 17th century, and indirect climate records in the middle and lower reaches of Yangtze River. 2. Modern setting of Taibai Lake catchment and historical records 2.1. Modern setting of Taibai Lake catchment Taibai Lake lies to the north of Yangtze River and the south foot of Dabie Mountain (Fig. 1), where was the ancient Penglize Lake in early Holocene (Qu et al., 1998). The catchment area of Taibai Lake was 607 km2, and the surface area was 28.98 km2 in 2002. The average water depth of Taibai Lake is 3.2 m in recent years, with Vallisneria denseserrulata as the main submerged macrophyte (Jian et al., 2001). The main inflow rivers of Taibai Lake are in the north of its catchment, and the water drains into Longgan Lake from the outlet southeast of Taibai Lake. The pH of Taibai Lake water is 7.4–8.0 and the degree of mineralization is 65–112 mg/L. Its hydrochemical type is of carbonate calcium, with main cations Ca2+, Mg2+, Na+ + K+and main anions HCO3−, SO42− and Cl−, the average concentrations of which are 20.0 mg/L, 3.65 mg/L, 2.97 mg/L and 45.0 mg/L, 3.65 mg/L and 2.67 mg/L respectively (Wang and Dou, 1998). Fig. 1. Location map of Taibai Lake and core site. 198 E. Liu et al. / Science of the Total Environment 385 (2007) 196–207 It is characterized by a subtropical monsoon climate in Taibai Lake catchment as well as in the middle and lower reaches of Yangtze River, with mean annual precipitation 1273 mm concentrating in summer (June– September), mean annual evaporation 1041 mm. The zonal vegetation in the catchment should be evergreen broad-leaved forest; nevertheless, it had been destroyed and substituted by secondary vegetations (mainly pines). 2.2. Cultivation scale and population variations in history Susong County, lies to the northeast of Taibai Lake, was one of the established district since 598 AD, with relatively changeless administrative boundaries since 1645 AD (Annals of Susong County, 1935). The population, plantation scale, and historical events, though not continuous annually, were systematically collected from the Annals of Susong County (1935) by the authors for the first time, and the statistics in recent 50 years were also used in this paper (Fig. 2). The population of Susong County was about eighty thousand in 1590–1670 AD, it was one hundred and forty thousand in 1712 AD and up to three hundred and ninety thousand in 1772 AD (Fig. 2). Thus, the rapid population increase in Susong County was from late 17th century to mid 18th century. Nevertheless, the population decreased rapidly since 1810s and reduced to one hundred and eighty thousand in 1868 AD mainly due to the war dead of peasant uprising, such as the Taiping Heavenly Kingdom (Annals of Susong County, 1935). The population increased slightly in late 19th century, but was still less than that in late 18th century. The cultivation scale in Susong County, mainly rice, wheat and sweet potato (Annals of Susong County, 1935), increased by three folds from late 17th century to late 18th century and then decreased to the level in late 17th century, coincidently with population changes (Fig. 2). Fig. 2. Climate fluctuation in the middle reaches of Yangtze River (Figure A from Zhang, 1980; Figure B from Ye et al., 1994) and the variation of population and cultivation scale in Susong County. E. Liu et al. / Science of the Total Environment 385 (2007) 196–207 In order to understand the variations of population and cultivation in Susong County, the dynasty subrogation for China should be introduced. Ming Dynasty was in 1368– 1644 AD, and Qing Dynasty was in 1644–1911. The population increased slowly from North Song Dynasty (960–1127) to mid Ming Dynasty, which was between 20 million and 100 million in China (Ding, 1993). The rapid population increase was in Qing Dynasty, it was up to 400 million in 1820s from 100 million in mid 17th century in China (Ding, 1993). It can be seen that, population variation in Susong County in Qing Dynasty was not a especial case, it was consistent with that of China. 199 in Fig. 2, which was deduced mainly from chorography by Ye and Zhao (1994) and Zhang (1980). There were two cold stages during the “Little Ice Age” in the studied period, one was 1650s–1700s and the other was 1790s– 1900s (Fig. 2). The annual average temperatures for the two cold periods were 0.55 °C and 0.58 °C lower than that in 1900–1979 AD. Since the end of the “Little Ice Age”, the temperature increased rapidly and plentiful precipitation occurred in 1900–1920s (Fig. 9) (Gong et al., 2001). 3. Materials and methods 3.1. Coring and sampling 2.3. Human activities in recent half-century Human activities influencing Taibai Lake and catchment environment were the construction of water conservancy facilities and land reclamation around the lake. In order to facilitate the agricultural irrigation and flood control, three reservoirs were built in the upper reach of Taibai Lake in 1958–1962 AD, such as Jingzhu, Kaotian and Xianrenba reservoirs (Fig. 1). Correspondingly, land reclamation around Taibai Lake flourished since late 1950s. The area of Taibai Lake was 69.2 km2 in 1930s (Wang and Dou, 1998). It was 63.70 km2, with the water area of 60.10 km2 in mid1950s. Nevertheless, the water area shrunk to 40.01 km2 by mid-1960s due to land reclamation, decreased 33.4% compared with that in mid-1950s, reducing at a rate of 2.0 km2 annually. The water area was only 27.02 km2 in 1978 AD, decreased 32.5% compared with that in mid1960s, reducing at a rate of 1.08 km2 annually. The area of Taibai Lake changed little since 1978 AD (Fig. 3). 2.4. Climate changes inferred from the historical documents Climate change is one of the main factors influencing catchment environment and human activities (Stebich et al., 2005). Here, the temperature change in the middle reach of Yangtze River during the last 450 years is shown The sediment cores (TB-03) about 80 cm long were taken at the deeper area of Taibai Lake with a piston corer in 2003 (Fig. 1). The core sediments were subsampled at a 0.5 cm resolution for 0–50 cm and 1 cm resolution for 50–80 cm. The sub-samples were sealed in plastic bags to be transported to laboratory. 3.2. Methods The frozen dried samples were used for 210 Pb dating, TOC, TN, and metal element analysis. The wet samples were used for grain size and pollen analysis. The main methods used were as following. Pb dating: The concentration of 210Pb for each samples was measured with low level germanium detector gamma-ray spectrometer (EG & G ORTEC, HPGe GWL-120-15). The standard counting errors of which are less than 10%. Supported 210Pb was estimated following the method of Appleby et al. (Appleby and Oldfield, 1978; Appleby et al., 1986; Binford et al., 1993), and was subtracted from the total 210Pb to obtain unsupported 210Pb (210Pbex). (2) Grain size: Particle size spectra of the samples were determined using a Malvern automated laseroptical particle-size analyzer (Mastersizer-2000) (1) 210 Fig. 3. Surface area and shape of Taibai Lake (calculated from relief map for the two former, from remote sensing image for the two latter). 200 E. Liu et al. / Science of the Total Environment 385 (2007) 196–207 after removal of organic matter by 10% H2O2 treatment. The particle size compositions are given in percentage. (3) TOC and TN: The CE-440 elemental analyzer (EAI Company) was used for the determination of TOC and TN content of the samples. (4) Metal elements analysis: The standard EPA method 3052 was used in this paper for metal elements (K, Mg, Ca, Al, Fe, Na) analysis (USEPA, 1996). An accurately weighed sediment sample (∼ 125 mg) was placed in a Teflon nitrification tank, 6.0 ml HNO3, 0.5 ml HCl, and 3.0 ml HF were added. The sealed tank was then placed in a microwave oven (Berghof MWS-3 Digester) and nitrified at 180 ± 5 °C for 15 min. The residue from the tank was then transferred into a Teflon breaker and dissolved with 0.5 ml HClO4 by braising in a heating block at about 200 °C and diluted to 25 ml with double-distilled de-ionized water. The solution was then analyzed for metal elements by inductively coupled plasma-atomic emission spectrometry (Leeman Labs, Profile DV). The accuracy of the analytical determination was established using the reference material GSD9, supplied by the Chinese Academy of Geological Sciences. The analytical results for all elements were found to be in agreement with the certified values, with an accuracy better than 93%. (5) Pollen analysis: Pollen analysis was performed on 115 samples to provide information on terrestrial vegetation in the catchment. Sub-samples with 3– 5 g wet weight were treated with 10% HCl and 15% NaOH, and then separated with a liquid at density of 1.88, which included zinc chloride, sodium polyunsaturated, and mixtures of potassium iodide and cadmium iodide (Nagakawa et al., 1998), and then mounted in glycerol for counting. A calibrated suspension of Lycopodium spores was added at the beginning of processing to enable pollen concentration to be calculated. Pollen identification and counting were performed under the Leitz optics microscope with standard descriptions. For each pollen sample, total count was above 400 grains. Major results were expressed by percentage diagrams. 12 cm of the profile and then a more regular exponential decay in 12–39.5 cm. The large fluctuations for 210Pbex may be caused by the unsteady sedimentation rate (Fig. 5) (Appleby and Oldfield, 1978; Appleby et al., 1986), which could be further proved by the detailed discussion in Section 4.3. According to the characteristics of CRS (constant rate of supply) and CIC (constant initial concentration) models, the CRS model was used to determine the chronologies of Core TB-03, which assumes that there is a constant fallout of 210 Pb from the atmosphere to the lake water and a constant supply rate to the sediments irrespective of any variations which may have occurred in the sediment accumulation processes (Appleby and Oldfield, 1978; Appleby et al., 1986). The CRS model gave 60 dates between 1839 ± 16 (39.5 cm) and 2002 ± 1 AD (Fig. 5). In order to extrapolated the age of the sediment core bellow 39.5 cm, the compaction effect must be evaluated. Water content of sediment was generally 40–60%, up to 70% at the top of the sediment core. The dry bulk density increased with depth at 0–30 cm and then kept generally constant values at 30–80 cm, having an opposite variation with water content and TOC especially in 30–80 cm (Fig. 6). It means that the variations of dry bulk density in 30–80 cm are influenced mainly by the organic matter content, hardly by compaction effect comparing with that in 0–30 cm of the sediment core. There had no correlations for the sedimentation flux and grain size composition at 0–39.5 cm, which reflects the limited influence of the grain size on the 210Pbex concentration and sedimentation flux (sedimentation rate) (Mil-Homens et al., 2006). Thus, the dates bellow 39.5 cm of the sediment core can be extrapolated by the average sedimentation rate at 30–39.5 cm (1839–1895 AD) (Smittenberg et al., 2004; Yamamuro and Kanai, 2005), where the sedimentation flux had no large fluctuations. 4. Results and discussions 4.1. Radiometric dating The distributions of 210Pbex in the sediment profile are shown in Fig. 4. It shows large fluctuations in 0– Fig. 4. Concentration of 210Pbex in the sediment Core TB-03. E. Liu et al. / Science of the Total Environment 385 (2007) 196–207 201 Fig. 5. Sedimentation flux and time-depth curve of Core TB-03. The age at the bottom of the sediment core is ca. 1590 AD and the ages at other depth are shown in Fig. 6–8. Nevertheless, the accuracies of the chronology bellow 39.5 cm of the core are relatively lower comparing with that in 0–39.5 cm. 4.2. Major sediment properties The sediments were composed of grey silty-clay. Fine silt (4–16 μm) and clay (b4 μm) accounted for 60– 90% of the grain size composition, representing the fine grain fraction and having an opposite variation with the coarse silt fraction (16–64 μm); the percentage of sand (N 64 μm) in the sediments was less than 2% (Fig. 6). TOC content in the sediments was generally low, which was 0.8–2.0% (Fig. 6), similar to that in other lakes in the middle and lower reaches of Yangtze River (Wang et al., 2005). The low TOC content and relatively higher primary productivity were probably due to the higher mineralization rate of organic matters in lake water, which was nearly 80% in Taihu Lake (Hu and Pu, 2000). Mineralization of organic matters can also change the original C/N ratio. The research in Taihu Fig. 6. Summary diagram grain sizes, water content, dry density, TOC and C/N ratio of Core TB-03. 202 E. Liu et al. / Science of the Total Environment 385 (2007) 196–207 Fig. 7. Concentration of the metal elements and Na/K ratio of Core TB-03. Lake also revealed that, the C/N ratio was not response to nutrition level and origin changes of organic matters from terrestrial vegetations and primary productivity of algae or submerged macrophyte comparing with the δ13C (Lin et al., 2006). Thus, the low C/N ratio in Taibai Lake sediments and TOC content may not really indicate the origins of organic matters and primary productivity of the lake (Meyers and Ishiwatari, 1993), it was not used in the discussion of environmental evolution in Taibai Lake catchment. The metal elements content variations in the core sediments are presented in Fig. 7, and their correlation coefficients are shown in Table 1. K, Mg, Ca and Fe had similar variations and had significant positive correlations with the fine grain fraction (b 16 μm), but they were all opposite with Na, which had a significant Fig. 8. Percentage pollen diagrams of Core TB-03. E. Liu et al. / Science of the Total Environment 385 (2007) 196–207 203 Table 1 The correlation coefficients matrix for metal elements concentrations and different grain fractions K Mg Ca Al Fe Na b16 μm 16–64 μm Na/K a b Mg Ca Al Fe Na b16 μm 16–64 μm 0.739 a 0.275 a 0.317 a 0.282 a 0.399 a − 0.396 a − 0.451 a 0.054 0.123 0.218 b 0.357 a − 0.351 a − 0.373 a 0.648 a 0.066 0.146 −0.152 −0.203 b − 0.125 0.239 a − 0.244 a − 0.433 a -0.562 a 0.577 a 0.670 a −0.998 a −0.837 a 0.844 a a 0.911 0.724 a 0.347 a 0.405 a 0.285 a 0.426 a − 0.419 a − 0.513 a Correlation is significant at 0.01 level. Correlation is significant at 0.05 level. opposite correlation with the fine grain fraction. The Na/ K molar ratio had a similar variation to the coarse silt fraction (16–64 μm). Na in sediment is primarily enriched in feldspar, especially in plagioclase, and K resides mostly in K feldspar and partly in illite and mica, which imply different weathering degree. So, the Na/K ratio has no correlations with the sedimentation flux, and is one of the proxies indicating the weathering degree of sediments before they are transported into the lake. High Na/K ratio shows low weathering degree of the sediment (Nesbitt and Young, 1982; Sawyer, 1986; Yang et al., 2004). Temperature and moisture are two important factors influencing the weathering of debris in the catchment. The evaporation was lower than precipitation in Taibai Lake catchment, so the weathering was influenced mainly by the temperature (Li and Zhang, 2003). A total of 167 taxa pollen were identified (Fig. 8). The arboreal pollen (AP) was mainly composed of pines, which accounted for 93.8% of the average concentration. Among the herbs (non-arboreal pollen, NAP), Gramineae dominated 57.6% on average. Maximum Gramineae pollen percentage and minimum pines pollen percentage occurred at 62-36 cm and 170 cm of the sediment core. 4.3. Environmental response to climate and human impact in Taibai Lake catchment The accuracy of the sediment records from Taibai Lake is expected to be intrinsically limited by mixing processes which may induced from resuspension of material for the shallow depth. However, the 210Pb activity and the large magnitude changes presented by the sedimentary proxies indicate that mixing processes did not smooth the signals. By the synthetical analysis on sedimentary proxies referenced to statistical results using CONISS software, six environment evolution stages in Taibai Lake catchment during the last 400 years were deduced (Fig. 6–8), and the comparisons between environmental changes and climate or human activity events from the instrumental and historical documents were also performed. 4.3.1. Stage VI (80–70 cm, 1590–1650 AD) It was characterized by little fluctuation for the sedimentary proxies. The high percentage of pinus pollen and low percentage of Gramineae pollen indicated a substantially wooded landscape in the upriver area of Taibai Lake catchment and low human disturbance, which is consistent with the small population in this period. The high content of fine fraction (b 16 μm) and low Na/K ratio showed the well weathering degree of the sediment, which was corresponding with the mild climate during the “Little Ice Age”, with relatively higher temperature (Zhang, 1980; Ye and Zhao, 1994). 4.3.2. Stage V (70–62 cm, 1650–1700 AD) The high Na/K ratio and high percentage of coarse fraction showed the weak weathering, which may be due to the lower temperature during the second cold period during the “Little Ice Age” (Ye and Zhao, 1994; Zhang, 1980). The similar pollen percentage as in stage VI indicated relatively weak human influence on the vegetation composition in Taibai Lake catchment, which is also consistent with the low population and farmland scale. 4.3.3. Stage IV (62–36 cm, 1700–1855 AD) Features of the pollen assemblage changed greatly in this zone. The lower percentage of pinus pollen and higher percentage of Gramineae pollen were unlikely resulted from the climate factors, because this period was corresponding with the warm climate during the “Little Ice Age”, with low frequency of droughts and floods especially in 1700–1810 AD (Ye and Zhao, 204 E. Liu et al. / Science of the Total Environment 385 (2007) 196–207 1994; Zhang, 1980). In this period, population increased largely in Susong County (Fig. 2), which had resulted in deforestation and cultivation expansion largely (Huang and Connell, 2000; Yasuda et al., 2000; Stebich et al., 2005). The pollen component implied that the main deforestation stage was during 1700–1750 AD, and the secondary tree (Pinus) restored rapidly during 1750– 1810 AD after the earlier deforestation. Relative balance was kept between woodland and farmland since 1810 AD due to the decreasing population. The other notable feature in this stage was high content of fine fraction and metals, and low Na/K ratio, which suggested that the large scale deforestation and cultivation had aggravated erosion of the well weathered soils (Huang and Connell, 2000). There was higher content of Ca at 54–57 cm of the sediments, similar features were also presented for Mg, but less distinct, which indicated the high carbonate content in the sediments. Coarse grain size composition, low water content and high dry bulk density were also presented at the same depth. The variation characteristics of the proxies at 54–57 cm may be related to the lower lakelevel during that period, thus more coarse fractions could be transported to the center of the lake (Dearing, 1997; Punning et al., 2006); meanwhile, the carbonate precipitation by the concentration of the lake water, with Ca2+, Mg2+and HCO3− as the main cations and anion (Wang and Dou, 1998), would result in the high content of Ca and Mg in the sediments. The sub-fluctuations for the proxies in this zone indicated climate or human activities fluctuations in Taibai Lake catchment. Grain size composition was coarser, the content of K, Mg and Ca decreased and Na/ K ratio increased since 1810 AD, which implied the weak weathering of the sediment comparing with that in 1700–1810 AD. Historical documents showed that the last cold period during the “Little Ice Age” begun at 1790–1810 AD in the middle and lower reaches of Yangtze River (Ye and Zhao, 1994; Zhang, 1980). So the weak weathering of the sediment since 1810 AD was related to the lower temperature, similar as that in 1650– 1700 AD. The population decreased since 1810 AD, but it was still a large scale comparing with that in 1590– 1700 AD. Due to the hysteresis effect of human influencing on the lake sediment, the sediment from soil erosion still had a big proportion. 4.3.4. Stage III (36–29 cm, 1855–1900 AD) This was a remarkable stage different with the previous. The percentages of pinus and Gramineae pollen resumed to the level before 1700 AD. It reflected a general less human activity pressure and influence on the catchment environment, which was also consistent with the historical documents that the cultivation scale decreased and the population decreased to the lowest level during the last 200 years. Grain size composition became coarser, with high content for the coarse silt and sand, and high Na/K ratio, which indicated the weak weathering degree for the sediments due to the lower temperature in the last cold epoch during the “Little Ice Age”. The sedimentation flux was higher between 1870 and 1880 AD, which may be related to the higher flood frequency (Ye and Zhao, 1994), it was not further analyzed due to the lower age resolution of the subsamples. 4.3.5. Stage II (29–17 cm, 1900–1928 AD) The observation data and historical records showed that the temperature rose rapidly since early 20th century (Wang, 2001; Ye and Zhao, 1994). The grains of the deposits became finer and the Na/K ratio decreased gradually, which was related to the well weathering degree of the sediment under the warmer climate. The population was increasing gradually, but there was no obvious increase in cultivation, and the inconspicuous variation of pollen also indicated the generally weak human activities during 1900–1928 AD comparing with that in 1700–1810 AD. The average sedimentation flux in this stage was 294.6 mg/cm2/a, which was about two folds of that in stage III. The high sedimentation flux was related to the strong erosion in the catchment due to high precipitation (Fig. 9) (Gong et al., 2001). But high sedimentation flux and strong erosion didn’t induce coarse grain size composition of Fig. 9. Sedimentation flux of Taibai Lake and annual anomalies of summer precipitation in the middle and lower reaches of the Yangtze River (lower figure from Gong et al., 2001). E. Liu et al. / Science of the Total Environment 385 (2007) 196–207 the sediment, which showed that grain size composition had no correlations with precipitation or erosion, and it mainly reflected the weathering degree of the sediment as discussed above. 4.3.6. Stage I (17–0 cm, 1928–2002 AD) There was a transition for the elements from zone II to zone I, but no evidence of deposition break. The similar features can also be found in other lakes in 1920s in the middle and lower reaches of Yangtze River, such as Taihu Lake, Yangcheng Lake, Longgan Lake etc. (Ji et al., 1997; Yang et al., 2002b; Liu et al., 2006). There was a rapid economy development in 1920s– 1930s in the middle and lower reaches of Yangtze River (Zhang, 1990). For example, the production of cotton in Xiaogan County, one precinct of Hubei Province, increased 5.7 folds in 1928–1937 AD, and the forest coverage rate decreased rapidly from the 16.65% in late Qing Dynasty due to deforestation (Annals of Xiaogan County, 1990). The rapid development in cultivation and deforestation had induced the change of sediment composition and sedimentary proxies. The decreasing percentage of pinus pollen and the increasing percentage of the Gramineae pollen in 1928–2002 AD were correlated with the continued strengthening deforestation and cereal cultivation. The sedimentation flux fluctuated obviously in this zone. Three peak values (numbers b–d), in 1928 AD, 1937–1942 AD and 1954 AD were corresponding with the high precipitation stages in summer of 1931 AD, 1938-1939 AD and 1954 AD in view of the errors of the 210 Pb dating, and the lower sedimentation flux stages at 1928–1954 AD were all corresponding with the low precipitation periods (Fig. 9). So the sedimentation flux variations in 1928–1954 AD were mainly controlled by the precipitation of Taibai Lake catchment in summer. Though the precipitation chenged largely since 1960s, there were no high values for the sedimentation flux corresponding to the precipitation, which may be due to the storage capacity of the reservoirs in the upper reaches of Taibai Lake catchment built in 1958–1962 AD. In this case, the high sedimentation flux stages in 1958–1970 AD and 1983–1993 AD were attributed to other factors, one of which was the land reclamation around the lake, which could induce aggravated erosion of the catchment and shrinking lake area (Yin and Li, 2001). There was land reclamation in large scale in the middle and lower reaches of Yangtze River. The surface area of Taibai Lake also shrunk 55% from mid-1950s to 1978 AD, so the high sedimentation flux for Taibai Lake in 1958–1970 AD was mainly resulted from the reclamation. But there almost had no land reclamation 205 around Taibai Lakes since 1978 AD, the high sedimentation flux in 1983–1993 AD was not attributed to land reclamation. It was related to the soil loss with the rapidest agriculture development since 1980s, which can also be proved by the relatively high percentage of Gramineae pollen (Fig. 8). Though the sedimentation flux fluctuated obviously and the material source changed in a certain extent, the elements content changed little. This was due to the similar grain size composition and mineral composition for the materials (Mil-Homens et al., 2006). The different cultivation pattern in Qing Dynasty and recent half-century induced the different sediment features for Zone IV and I, though there had similar higher population in the two periods. Cultivation scale extension was the main measure to increase the output of various foodstuffs for the demand of increasing population in the 18th century because of the extensive cultivation pattern in that period, large scale badlands and woodland were reclaimed as farmland. Contrasting with that was the intensive agriculture since 1950s, and the food supply was mainly by the step of increasing grain yield per unit area and the farmland scale increased lesser. Thus, there was similar population increase in Qing Dynasty and the last 50 years, but the sediment features was not identical in the two periods due to the different agricultural production patterns. The population increase in the early agriculture society had greater influence on the catchment environment (Huang and Connell, 2000). 5. Conclusions The synthetical analysis on sedimentary proxies and historical documents revealed the environmental response to climate and human impact in Taibai Lake catchment during the last 400 years. The main conclusions are as follows: (1) The last two cold stages during the “Little Ice Age” were in 1650–1700 AD and 1810–1900 AD reflected by the sedimentary proxies, with the coldest epoch in 1855–1900 AD. The two cold stages were represented by weak weathering, high forest coverage and weak human activities intensity in Taibai Lake catchment. (2) Increasing population was with deforestation and cultivation expansion in 1700–1750 AD, and the stabilized population and cultivation in 1750–1855 AD was with the reforestation of the secondary pines. Deforestation and cultivation destabilized well weathered topsoil, thereby aggregated the erosion of 206 E. Liu et al. / Science of the Total Environment 385 (2007) 196–207 soil, inducing finer grain size composition and higher metal elements contents in the sediment. Though human activities influenced the catchment environment intensively, the chilling climate since 1810 AD during the “Little Ice Age” can also be reflected by the sedimentary proxies. 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