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Environmental change and human activities during the 20th century reconstructed from the sediment of Xingyun Lake, Yunnan Province, China
Quaternary International 212 (2010) 14–20
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Environmental change and human activities during the 20th century reconstructed from the sediment of Xingyun Lake, Yunnan Province, China Hongliang Zhang a, b, Shijie Li a, *, Qinglai Feng c, Shitao Zhang d a
State Key Laboratory of Lake Science and Environment, Nanjing Institute of Geography and Limnology, Chinese Academy of Sciences, East Beijing Road, Nanjing 210008, China Graduate School of Chinese Academy of Sciences, Beijing 100049, China c Faculty of Earth Science, China University of Geosciences, Wuhan 430074, China d Kunming University of Science and Technology, Kunming 650093, China b
a r t i c l e i n f o
a b s t r a c t
Article history: Available online 24 July 2009
The history of environmental changes and human activities in the catchment of Xingyun Lake, Yunnan Province, Southwest China, was investigated using an 85 cm sediment core drilled in the northwest part of the lake. The chronology of the sediment, established with the 210Pb method, represented the most recent 160 years of depositional history. A series of lipids, including n-alkanes and n-alkanols, were identified by using GC/MS. The distinct n-alkanes and n-alkanols distributions at different depths are indicators of environmental changes. The carbon preference index (CPI) of the n-alkanes ranges from 0.55 to 3.35. The average chain length (ACL) ranges from 19.2 to 25.7. The ratios of the C27/C31 and C27/ (C15 þ C17 þ C19) are between 0.43–1.63 and 0.05–0.85, respectively. The ratio of the higher carbon components to the lower of n-alkanes (H/L) and n-alkanols changes from 0.16 to 3.18 and 0.24–7.36, respectively. These ratios, compared with the temperature record of the meteorological station in the catchment, reveal a remarkable relationship: high H/L values correspond to warm climate conditions and vice versa. Four stages were recognized according to the sediment records for the past 160 years: a cold period before the 1920s, a warm period from the 1920s to the mid-1950s, a cool period between the mid1950s and 1970s, and a warm period after the 1980s. The proxies also reveal the organic origin of the lake sediment. Human activity led to great environment change, such as the Gehe channel dredging in 1923, denudation and excessive reclamation at the end of the 1950s, and especially the eutrophication of the lake since about the early 1990s. The lipids are a sensitive recorder of the environment changes, although the strong increase of human activities may affect the record of natural changes. Ó 2009 Elsevier Ltd and INQUA. All rights reserved.
1. Introduction Using fossil molecules to reconstruct palaeoclimate, palaeoenvironment and ancient human activities has become an important part of molecular stratigraphy (Brassell et al., 1986; Farrimond and Flannagan, 1995; Fu and Sheng, 1996; Street-Perrott et al., 1997; Evershed et al., 1999, 2004; Sheng et al., 1999; Zhang et al., 1999; Xie et al., 2000, 2002, 2003a,b). Among these, lipids are more frequently used for palaeoclimate and palaeoenvironment reconstruction (Nott et al., 2000; Xie et al., 2003a,b,c; Wang et al., 2004; Zheng et al., 2007). Lake sediments are another important archive for Quaternary research (Cranwell, 1973, 1984; Cranwell et al., 1987; Rieley et al., 1991; Meyers and Ishiwatari, 1993; Huang et al., 1999; Zhang et al.,
* Corresponding author. Tel.: þ86 25 86882002; fax: þ86 25 57714759. E-mail address: [email protected] (S. Li). 1040-6182/$ – see front matter Ó 2009 Elsevier Ltd and INQUA. All rights reserved. doi:10.1016/j.quaint.2009.07.007
1999; Ficken et al., 2000; Schwark et al., 2002; Meyers, 2003; Yao and Li, 2004). Many lipid-based reconstructions focus on environmental changes on longer timescales of centuries to millennia, whereas studies that reconstruct recent environmental changes of the last century or two are relatively rare. Xingyun Lake, located in central Yunnan Province, is a subtropical zone in southwest China, a region strongly influenced by the southwest monsoon system. It is a semi-closed lake with a small catchment that has experienced increased human activities during the past decades. Many studies have been done in recent years on this lake, including sedimentary mineral composition, geochemistry, eutrophication, resource usage and environmental protection issues (Zhao, 1993; Song et al., 1994; Liu and Chen, 2000; Sakamoto et al., 2002; Zhang et al., 2003, 2006). This paper uses lipids in the lake sediment to reconstruct the regional environmental change history in high resolution, and attempts to discriminate environmental signals caused by natural and human induced factors.
H. Zhang et al. / Quaternary International 212 (2010) 14–20
2. Study area Xingyun Lake (24 21027000 N, 102 460 12800 E) is situated at 1722 m a.s.l. It is about 80 km south of Kunming and 20 km east of Yuxi City (Fig. 1). It is a semi-closed shallow lake surrounded by mountains, with a water surface area of 34.7 km2 and the catchment area of 386 km2. The mean water depth is about 7 m and the volume is about 183.3 106 m3. The lake joins Fuxian Lake to the north through a narrow channel of the Gehe River. Annual mean temperatures at regional weather stations range between 13.4 and 16.5 C, and the annual mean precipitation is approximately 880 mm. Water input is primarily through rainfall. The prevailing wind is dominated by the southwest monsoon. The native vegetation is broadleaved deciduous forests. 3. Materials and methods 3.1. Sampling and material A sediment core 85 cm long (XY-1) was drilled using a piston sediment sampler in July 2004 (Fig.1) in the northwest part of Xingyun Lake in 8.2 m water depth. Water transparency was 85 cm. Samples were taken every 4 cm (n ¼ 21) for analysis of phytoliths and every 2 cm (n ¼ 42) for lipid analysis. Samples were subsequently freezedried for storage and analysis. The chronology of the core profile was established by 210Pb dating in the Nanjing Institute of Geography and Limnology, Chinese Academy of Science (Zhang, 2001). 3.2. Experimental The extraction of lipids was carried out at the State Key Laboratory of Geological Processes and Mineral Resources, China University of Geosciences. Freeze-dried samples were ground to <100 mesh and ultrasonicated with chloroform for 20 min; native copper was added to remove sulfur. After weighing and eliminating asphaltenes by precipitation in petroleum ether, the lipids were fractionated by column chromatography into saturated hydrocarbon and aromatic fractions, using hexane and benzene as eluting solvents. The saturated hydrocarbons and the aromatic fractions were analyzed by gas chromatography–mass spectrometer. A Hewlett–Packard 5973A MS, interfaced directly to a 6890 GC equipped with a HP-5MS fused silica capillary column
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(60 m 0.25 mm 0.25 mm), was used for molecular analyses. The operating conditions were as follows: oven temperature ramped from 70 to 280 C at 3 C/min, further to 300 C at 2 C/min, and finally kept at 300 C for 20 min; Helium was the carrier gas. The ionization energy of the mass spectrometer was set at 70 eV; scan range from 50 to 550 amu. The identification of the compounds was based on the published spectra, and the quantification was conducted by using the peak area of the corresponding ion chromatograms. The relative standard deviation (RSD) of parallel analyses for individual samples was less than 10%. 4. Results 4.1. Sediment description and chronology According to the sedimentary properties, the material of the core XY-1 is mainly composed of silty clay and clayey silt, and can be divided into three parts as follows: 0–10 cm depth: Dark gray to gray humic silt with flocculent algal and plant fragments. 10–28 cm: Dark gray to gray silt layer with abundant organic matter and occasional fine sand layers. 28–85 cm: Yellow or grey clay silt with less organic matter and sparse benthic organisms (gastropod spiral shells) at depths of 30– 34 cm and 54–56 cm. Dating is shown in Table 1 and corresponds well to a core taken in 1989 from the same lake (Song et al., 1994). The results show that the XY-1 core profile covers approximately the past 160 years. Furthermore, using known events to check the dating, the grain size increases at a depth of 58–60 cm, which corresponds to the known date of the dredging of the Gehe channel in 1923. This event caused the lake level to drop about 2.5 m. This correspondence suggests that the dating of Core XY-1 is justified. 4.2. Distribution characteristics of n-alkanes The n-alkanes in core XY-1 range from n-C15 to n-C35. Fig 2 shows the n-alkane pattern as distributed in different layers. The chromatographic peaks indicate a bimodal pattern with peaks around n-C17 and n-C29. Moreover, the odd-even carbon predominance is obvious in both the lower (C15–C21) and higher (C22–C35) carbon number of n-alkanes. The carbon preference index (CPI) of the
Fig. 1. Location map showing study area and drilling site.
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H. Zhang et al. / Quaternary International 212 (2010) 14–20
Table 1 210 Pb dating of the core from Xingyun Lake (Zhang, 2001). depth (cm)
210
0–2 8–10 14–16 16–18 26–28 36–38 50–53
158.3 152.2 107.4 106.3 69.8 78.0 67.7
Pb (Bq/kg)
226
Ra (Bq/kg)
31.4 37.9 43.8 41.4 35.1 40.1 42.2
210
Pb (unsupp) (Bq/kg)
126.9 114.3 63.7 64.9 34.7 38.0 25.5
porosity (%)
dry density
sedimentation rate P (cm/a)
sediment flux S (g/cm2 yr)
age (AD)
12.96 12.96 7.262 7.262 7.262 3.447 4.095
0.17 0.17 0.31 0.31 0.31 0.58 0.51
1 1 0.566 0.84 0.67 0.67 0.67
0.1480 0.1480 0.1627 0.2415 0.1926 0.3752 0.3277
1997 1989 1978 1976 1963 1948 1926
n-alkanes ranges from 0.55 to 3.35. The average chain length (ACL) of the n-alkanes ranges from 19.2 to 25.7, which is somewhat lower than that of most lake sediments. In addition, the ratios of the C27/C31 and C27/(C15 þ C17 þ C19) are between 0.43–1.63 and 0.05–0.85, respectively. The ratio of the higher carbon components to the lower (H/L-alk) ranges from 0.16 to 3.18. For the entire core, remarkable changes from the top to the bottom are apparent, such that the lower carbon number component is present in greater abundance at depths of 0–10 cm and 60–85 cm, and the higher carbon number component dominates at the depth of 32–60 cm. There is an absence of hydrocarbon dominance at 10–32 cm (Fig. 2).
4.3. Distribution characteristics of n-alkanols The n-alkanols range from C12 to C30 chain length compounds. The samples indicate the dominance of C16 and C26. The even-odd carbon predominance is characterized in the high carbon number n-alkanols. Furthermore, the ratio of the high carbon number to the lower carbon number of n-alkanols (H/L-achl) in the whole core ranges between 0.24 and 7.36, divided into five specific zones: 1) the value of H/L-achl is lower than 2 at 60–76 cm depth; 2) the value is larger than 2 at 60–38 cm, with the highest ratio around 55 cm depth; 3) the ratio ranges between 1.39 and 2.51 at 20–38 cm
Fig. 2. Distribution of n-alkanes in the sediment core XY-1.
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depth; 4) the value ranges from 2.36 to 4.76 at 12–20 cm with a peak around 20 cm depth; and 5) the value decreases to 0.24 near the top of the core (Fig. 3). 5. Discussion 5.1. Relationship between the lipid records and climate change Changes of n-alkane chain length distribution primarily reflect the seasonal temperature changes of the sediment source region in the study of organic geochemistry, such as atmospheric dust and oceanic sediment. In order to maintain the viscosity of waxy covering and the water balance, higher plants mainly biosynthesize longer waxy lipids in the warmer tropic areas. In the cooler temperate regions, short chain lipid compounds dominate. Therefore, the average carbon chain distribution of n-alkane is a significant potential indicator in the study of palaeoenvironments (Gagosian et al., 1986). The ratio of the lower carbon number to higher carbon number lipids (Rl/h), which has been used widely to discriminate the organic origin, can be an indicator of the different kinds of dominant vegetation, closely related to climate conditions. A higher value of Rl/h corresponds to a relatively cool period, and lower values reflect warm periods. For example, the Younger Dryas (YD) and associated Heinrich Event has been related to the Rl/h proxy in China (Xie et al., 2003a,b,c,d). Terrestrial plant growth is correlated with warm conditions. The lipid record of Xingyun Lake sediment (XY-1 core) also suggests that the ratio of the higher to lower carbon components (H/L-alk) and the ratio of long chain lipids to the short ones (HL-achl) are closely related to trends in air temperature. The vegetation in the catchment is likely the bridge
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that links the lipids record in the lake sediment with climate conditions (Fig. 4). The high values of H/L-alk and H/L-achl suggest that terrestrial carbon from forest and herbaceous plants dominated in the catchment, which would imply that the climate was relatively warm. In contrast, dominance by aquatic algae indicates cool or cold climates. From this point of view, comparing the proxies of the lipids in the lake sediment with the air temperature record in the region reveals a strong relationship that can be divided into five main stages (Fig. 4), as follows: Stage A (85–60 cm depth, 1840 to the early 1920s) H/L-alk ranges from 0.16 to 0.66 (mean 0.30): the ratio has the lowest value in the entire profile. H/L-achl is also low during this period with a mean value of 1.7. This probably demonstrates the cold climate conditions, corresponding to the 3rd cold period of the Little Ice Age (Wang et al., 1998b). The temperature was relatively low, which limited the inputs of terrestrial plant material, and the organic carbon input was mainly dominated by aquatic algae. Stage B (60–32 cm depth, early 1920s to mid-1960s) H/L-alk has a relatively high value, between 0.59 and 3.18, with a mean value of 1.71. The H/L-achl has the highest value with a mean of 3.63 in the entire sediment core. Warm and humid conditions occurred in this stage, which is comparable with the warm period of the early last century in the instrument record (Wang et al., 1998a). The higher plants were more productive, increasing the input of higher plant organic matter. The value of H/L-alk in this stage is variable, which suggests more frequent climatic variability. Stage C (32–20 cm depth, 1960s–1970s) The value of H/L-alk is between 0.86 and 1.54 (mean 1.24), which is lower than in Stage B. H/L-achl has a mean value of 1.99.
Fig. 3. Distribution of carbon number of n-alkanols in the sediment core XY-1.
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Fig. 4. Comparison of the lipid-based proxies in the lake sediments with the anomaly mean temperature for the last 100 years in Kunming (a) (You et al., 1997); anomalous mean P P þ temperature changes from 1880 to 1996 AD in southwest China (b) (Wang et al., 1998a) ðACL alk ¼ nCn = Cn ; H=L alk ¼ Cþ 23 =C22 ; H=L achl ¼ C20 =C19 Þ.
The temperature in this period is lower than that of the Stage B, but warmer than that of the Stage A, in accordance with the instrumental record (Wang et al., 1998a). This period is comparable to the cold period during the 1960s–1970s. Stage D (20–10 cm depth, 1980s to the early 1990s) The ratios represent a response to the warming period from the 1980s to the early 1990s. The value of H/L-alk increased from 1.02 to 2.20 (mean 1.57) and the value of H/L-achl increased to a mean of 3.05, which suggests the increase of the terrestrial organic matter input that may result from the warm climate. Stage E (10–0 cm) The value of H/L-alk ranges from 1.17 to 0.21 (mean 0.81) and shows a decreasing trend towards the top. The value of H/L-achl has a mean of 0.64 in the lowest part. This may indicate that the organic material from aquatic algae plant had remarkably increased after the early 1990s. Comparing the H/L-alk and H/L-achl curve with the anomalous mean temperature curve of the last 100 years in southwest China (You et al., 1997; Wang et al., 1998a), shows a clear correspondence between the two before the early 1990s. After the early 1990s, the values of H/L-alk and H/L-achl show a sharp decreasing trend despite an observed climate warming. This decrease may have resulted from algae blooms associated with lake eutrophication caused by human activities in the catchment. Thus, human impacts disrupted the natural signals produced by climate change. 5.2. Relationship between the lipid record and human activities N-alkanes are less easily degraded than other organic matter, and can be used as an indicator of the organic matter origin (Meyers, 2003). Human activities may affect vegetation in the lake drainage basins, which, in turn, would change the composition of organic matter. The n-C27 alkane usually represents tree organic matter, and the n-C31 alkane reflects organic matter from grasses. Therefore, the ratio of C27/C31 can be a proxy to reflect shifts in vegetation from trees and forest landscape to an open grass-dominated system
(Cranwell, 1973; Rieley et al., 1991; Meyers and Ishiwatari, 1993; Huang et al., 1999). ACL is also as a proxy which is related to the vegetation type. In addition, C27/(C15 þ C17 þ C19) ratios can be an indicator to discriminate the terrestrial or aquatic origins of organic matter. These proxies can be used to demonstrate environmental changes associated with human activities in the catchment. From Fig. 4, it is clear that both the ACL and H/L show a sharp increase between the depths of 60–56 cm, corresponding to the early 1920s. The ratio of C27/(C15 þ C17 þ C19) also has an increasing trend. This may suggest increased organic matter from terrestrial sources. According to the Jiangchuan County Annals, the Gehe channel was dredged in 1923 to allow the water of Xingyun Lake to flow into Fuxian Lake, which resulted in a 2.5 m drop of the Xingyun Lake level and exposure of the lake floor. Therefore, the input of organic matter of terrestrial origin was increased by this human activity during a background of climate warming. The sedimentology also shows that the grain size of the sediment increased sharply at this depth (Zhang et al., 2006). At about 36 cm, the values for ACL, C27/C31, C27/ (C15 þ C17 þ C19) and H/L-alk ratio show a salient change, which would suggest an increasing input of organic matter from grasses. This trend probably corresponds to massive deforestation and land reclamation around 1958, with a subsequent increase in the area used for growing rice. The value of ACL, C27/(C15 þ C17 þ C19) and H/L-alk ratio decreased sharply in the top w8 cm sediments, whereas the ratio of C27/C31 experienced an increasing trend. At this time the H/L-alk curve does not follow the curve of the anomaly in mean temperature. The likely reason is that the algal blooms in Xingyun Lake were becoming increasingly more severe since the early 1990s, due to lake eutrophication associated with the input of pollutant water from industrial processes and urbanization in the catchment (Liu and Chen, 2000; Zhang, 2001). Environmental change at Xingyun Lake revealed that it was hardly disturbed by human activities before 1962 (Zhang, 2001). Prior to the 1960s, the lake water was clean, primary productivity
H. Zhang et al. / Quaternary International 212 (2010) 14–20
was low, and the organic content of the sediment was relatively low, as well. The lipid-based proxies in sediments recorded the natural environmental changes. The main sources of lower carbon number lipids were primarily algae and other microbes that are influenced by temperature but also the nutrient content of the water (Yao and Li, 2004). In this area, the forest cover increased beginning in the 1990s due to a water and soil conservation policy, which caused the terrestrial organic matter input to be decreased. On the other hand, TN and TP inputs to Xingyun Lake from sewage increased approximately 55% and 50% respectively; and inputs from surface runoff increased about 27% and 26% (Zhang et al., 2002). Therefore, the degree of eutrophication increased from 0.23 to 0.58 during the period 1990 to 1999, changing the lake from a mesotropic to a eutrophic status (Zhang et al., 2002). This process produced the lower H/L value and C27/(C15 þ C17 þ C19) ratios in the top w8 cm sediment. Meanwhile, sediment geochemical analyses show that CaO and calcite increased at this time, whereas the d18O of authigenic carbonate and the isotopic d13Corg showed a negative trend (Zhang et al., 2003, 2006). Thus, the lake eutrophication process caused by strong human activities in the catchment had a stronger influence on the geochemical signature in the lake sediments than did the natural environmental processes driven by climate change. 6. Conclusion 1) The core XY-1 from Xingyun Lake in southwestern China has covered a time period of the last 160 years according to the 210Pb dating. A series of lipids, including n-alkanes and n-alkanols were used as sensitive indicator of environmental change. The proxies of the carbon preference index (CPI) of the n-alkanes, the average chain length (ACL), the ratios of the C27/C31, and C27/ (C15 þ C17 þ C19), the ratio of higher carbon components to the lower of n-alkanes and n-alkanols (H/L) in the Xingyun lake sediment have been achieved to reconstruct the environmental change history for the last 160 years. Four stages of these proxies in the period before 1990 were recognized that corresponded with climate change, especially to temperature trends, i.e. a cool period before the 1920s, a warm period between the 1920s and mid-1950s, a cool period between the mid-1950s and 1970s, and a warm period since the 1980s. 2) The ratio of the high carbon number to the low carbon number of the n-alkanes and n-alkanols, as well as the ratios of C27/C31, could be used to identify vegetation change in the catchment, which was influenced by natural climate change and strong human activities. Four stages of climate change before the early of 1990s could be recognized, as well as human activities that include the Gehe river channel dredging in 1923, strong deforestation in 1958, and lake eutrophication after the early 1990s. 3) The lipids in the lake sediment are a mixed proxy of the organic material from the catchment, influenced by both natural processes and human activities. In most lakes, multi-proxy analyses are the best tool for evaluating and understanding the relative influences of natural processes and human impact in a lake’s catchment. Acknowledgments The research was supported by the National Natural Science Foundation of China (grant 40232025) and the research project of Chinese Academy of Sciences (grant 2004CCA02900). The authors thank Prof. Song Xueliang, Zhang Zixiong (Institute of Geoscience, Yunnan province) for the helping in sediment core drilling, also to Prof. Xie Shucheng and Huang Xianyu (China University of Geosciences) for some helpful suggestions and laboratory assistance.
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Environmental change and human activities during the 20th century reconstructed from the sediment of Xingyun Lake, Yunnan Province, China Hongliang Zhang a, b, Shijie Li a, *, Qinglai Feng c, Shitao Zhang d a
State Key Laboratory of Lake Science and Environment, Nanjing Institute of Geography and Limnology, Chinese Academy of Sciences, East Beijing Road, Nanjing 210008, China Graduate School of Chinese Academy of Sciences, Beijing 100049, China c Faculty of Earth Science, China University of Geosciences, Wuhan 430074, China d Kunming University of Science and Technology, Kunming 650093, China b
a r t i c l e i n f o
a b s t r a c t
Article history: Available online 24 July 2009
The history of environmental changes and human activities in the catchment of Xingyun Lake, Yunnan Province, Southwest China, was investigated using an 85 cm sediment core drilled in the northwest part of the lake. The chronology of the sediment, established with the 210Pb method, represented the most recent 160 years of depositional history. A series of lipids, including n-alkanes and n-alkanols, were identified by using GC/MS. The distinct n-alkanes and n-alkanols distributions at different depths are indicators of environmental changes. The carbon preference index (CPI) of the n-alkanes ranges from 0.55 to 3.35. The average chain length (ACL) ranges from 19.2 to 25.7. The ratios of the C27/C31 and C27/ (C15 þ C17 þ C19) are between 0.43–1.63 and 0.05–0.85, respectively. The ratio of the higher carbon components to the lower of n-alkanes (H/L) and n-alkanols changes from 0.16 to 3.18 and 0.24–7.36, respectively. These ratios, compared with the temperature record of the meteorological station in the catchment, reveal a remarkable relationship: high H/L values correspond to warm climate conditions and vice versa. Four stages were recognized according to the sediment records for the past 160 years: a cold period before the 1920s, a warm period from the 1920s to the mid-1950s, a cool period between the mid1950s and 1970s, and a warm period after the 1980s. The proxies also reveal the organic origin of the lake sediment. Human activity led to great environment change, such as the Gehe channel dredging in 1923, denudation and excessive reclamation at the end of the 1950s, and especially the eutrophication of the lake since about the early 1990s. The lipids are a sensitive recorder of the environment changes, although the strong increase of human activities may affect the record of natural changes. Ó 2009 Elsevier Ltd and INQUA. All rights reserved.
1. Introduction Using fossil molecules to reconstruct palaeoclimate, palaeoenvironment and ancient human activities has become an important part of molecular stratigraphy (Brassell et al., 1986; Farrimond and Flannagan, 1995; Fu and Sheng, 1996; Street-Perrott et al., 1997; Evershed et al., 1999, 2004; Sheng et al., 1999; Zhang et al., 1999; Xie et al., 2000, 2002, 2003a,b). Among these, lipids are more frequently used for palaeoclimate and palaeoenvironment reconstruction (Nott et al., 2000; Xie et al., 2003a,b,c; Wang et al., 2004; Zheng et al., 2007). Lake sediments are another important archive for Quaternary research (Cranwell, 1973, 1984; Cranwell et al., 1987; Rieley et al., 1991; Meyers and Ishiwatari, 1993; Huang et al., 1999; Zhang et al.,
* Corresponding author. Tel.: þ86 25 86882002; fax: þ86 25 57714759. E-mail address: [email protected] (S. Li). 1040-6182/$ – see front matter Ó 2009 Elsevier Ltd and INQUA. All rights reserved. doi:10.1016/j.quaint.2009.07.007
1999; Ficken et al., 2000; Schwark et al., 2002; Meyers, 2003; Yao and Li, 2004). Many lipid-based reconstructions focus on environmental changes on longer timescales of centuries to millennia, whereas studies that reconstruct recent environmental changes of the last century or two are relatively rare. Xingyun Lake, located in central Yunnan Province, is a subtropical zone in southwest China, a region strongly influenced by the southwest monsoon system. It is a semi-closed lake with a small catchment that has experienced increased human activities during the past decades. Many studies have been done in recent years on this lake, including sedimentary mineral composition, geochemistry, eutrophication, resource usage and environmental protection issues (Zhao, 1993; Song et al., 1994; Liu and Chen, 2000; Sakamoto et al., 2002; Zhang et al., 2003, 2006). This paper uses lipids in the lake sediment to reconstruct the regional environmental change history in high resolution, and attempts to discriminate environmental signals caused by natural and human induced factors.
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2. Study area Xingyun Lake (24 21027000 N, 102 460 12800 E) is situated at 1722 m a.s.l. It is about 80 km south of Kunming and 20 km east of Yuxi City (Fig. 1). It is a semi-closed shallow lake surrounded by mountains, with a water surface area of 34.7 km2 and the catchment area of 386 km2. The mean water depth is about 7 m and the volume is about 183.3 106 m3. The lake joins Fuxian Lake to the north through a narrow channel of the Gehe River. Annual mean temperatures at regional weather stations range between 13.4 and 16.5 C, and the annual mean precipitation is approximately 880 mm. Water input is primarily through rainfall. The prevailing wind is dominated by the southwest monsoon. The native vegetation is broadleaved deciduous forests. 3. Materials and methods 3.1. Sampling and material A sediment core 85 cm long (XY-1) was drilled using a piston sediment sampler in July 2004 (Fig.1) in the northwest part of Xingyun Lake in 8.2 m water depth. Water transparency was 85 cm. Samples were taken every 4 cm (n ¼ 21) for analysis of phytoliths and every 2 cm (n ¼ 42) for lipid analysis. Samples were subsequently freezedried for storage and analysis. The chronology of the core profile was established by 210Pb dating in the Nanjing Institute of Geography and Limnology, Chinese Academy of Science (Zhang, 2001). 3.2. Experimental The extraction of lipids was carried out at the State Key Laboratory of Geological Processes and Mineral Resources, China University of Geosciences. Freeze-dried samples were ground to <100 mesh and ultrasonicated with chloroform for 20 min; native copper was added to remove sulfur. After weighing and eliminating asphaltenes by precipitation in petroleum ether, the lipids were fractionated by column chromatography into saturated hydrocarbon and aromatic fractions, using hexane and benzene as eluting solvents. The saturated hydrocarbons and the aromatic fractions were analyzed by gas chromatography–mass spectrometer. A Hewlett–Packard 5973A MS, interfaced directly to a 6890 GC equipped with a HP-5MS fused silica capillary column
15
(60 m 0.25 mm 0.25 mm), was used for molecular analyses. The operating conditions were as follows: oven temperature ramped from 70 to 280 C at 3 C/min, further to 300 C at 2 C/min, and finally kept at 300 C for 20 min; Helium was the carrier gas. The ionization energy of the mass spectrometer was set at 70 eV; scan range from 50 to 550 amu. The identification of the compounds was based on the published spectra, and the quantification was conducted by using the peak area of the corresponding ion chromatograms. The relative standard deviation (RSD) of parallel analyses for individual samples was less than 10%. 4. Results 4.1. Sediment description and chronology According to the sedimentary properties, the material of the core XY-1 is mainly composed of silty clay and clayey silt, and can be divided into three parts as follows: 0–10 cm depth: Dark gray to gray humic silt with flocculent algal and plant fragments. 10–28 cm: Dark gray to gray silt layer with abundant organic matter and occasional fine sand layers. 28–85 cm: Yellow or grey clay silt with less organic matter and sparse benthic organisms (gastropod spiral shells) at depths of 30– 34 cm and 54–56 cm. Dating is shown in Table 1 and corresponds well to a core taken in 1989 from the same lake (Song et al., 1994). The results show that the XY-1 core profile covers approximately the past 160 years. Furthermore, using known events to check the dating, the grain size increases at a depth of 58–60 cm, which corresponds to the known date of the dredging of the Gehe channel in 1923. This event caused the lake level to drop about 2.5 m. This correspondence suggests that the dating of Core XY-1 is justified. 4.2. Distribution characteristics of n-alkanes The n-alkanes in core XY-1 range from n-C15 to n-C35. Fig 2 shows the n-alkane pattern as distributed in different layers. The chromatographic peaks indicate a bimodal pattern with peaks around n-C17 and n-C29. Moreover, the odd-even carbon predominance is obvious in both the lower (C15–C21) and higher (C22–C35) carbon number of n-alkanes. The carbon preference index (CPI) of the
Fig. 1. Location map showing study area and drilling site.
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H. Zhang et al. / Quaternary International 212 (2010) 14–20
Table 1 210 Pb dating of the core from Xingyun Lake (Zhang, 2001). depth (cm)
210
0–2 8–10 14–16 16–18 26–28 36–38 50–53
158.3 152.2 107.4 106.3 69.8 78.0 67.7
Pb (Bq/kg)
226
Ra (Bq/kg)
31.4 37.9 43.8 41.4 35.1 40.1 42.2
210
Pb (unsupp) (Bq/kg)
126.9 114.3 63.7 64.9 34.7 38.0 25.5
porosity (%)
dry density
sedimentation rate P (cm/a)
sediment flux S (g/cm2 yr)
age (AD)
12.96 12.96 7.262 7.262 7.262 3.447 4.095
0.17 0.17 0.31 0.31 0.31 0.58 0.51
1 1 0.566 0.84 0.67 0.67 0.67
0.1480 0.1480 0.1627 0.2415 0.1926 0.3752 0.3277
1997 1989 1978 1976 1963 1948 1926
n-alkanes ranges from 0.55 to 3.35. The average chain length (ACL) of the n-alkanes ranges from 19.2 to 25.7, which is somewhat lower than that of most lake sediments. In addition, the ratios of the C27/C31 and C27/(C15 þ C17 þ C19) are between 0.43–1.63 and 0.05–0.85, respectively. The ratio of the higher carbon components to the lower (H/L-alk) ranges from 0.16 to 3.18. For the entire core, remarkable changes from the top to the bottom are apparent, such that the lower carbon number component is present in greater abundance at depths of 0–10 cm and 60–85 cm, and the higher carbon number component dominates at the depth of 32–60 cm. There is an absence of hydrocarbon dominance at 10–32 cm (Fig. 2).
4.3. Distribution characteristics of n-alkanols The n-alkanols range from C12 to C30 chain length compounds. The samples indicate the dominance of C16 and C26. The even-odd carbon predominance is characterized in the high carbon number n-alkanols. Furthermore, the ratio of the high carbon number to the lower carbon number of n-alkanols (H/L-achl) in the whole core ranges between 0.24 and 7.36, divided into five specific zones: 1) the value of H/L-achl is lower than 2 at 60–76 cm depth; 2) the value is larger than 2 at 60–38 cm, with the highest ratio around 55 cm depth; 3) the ratio ranges between 1.39 and 2.51 at 20–38 cm
Fig. 2. Distribution of n-alkanes in the sediment core XY-1.
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depth; 4) the value ranges from 2.36 to 4.76 at 12–20 cm with a peak around 20 cm depth; and 5) the value decreases to 0.24 near the top of the core (Fig. 3). 5. Discussion 5.1. Relationship between the lipid records and climate change Changes of n-alkane chain length distribution primarily reflect the seasonal temperature changes of the sediment source region in the study of organic geochemistry, such as atmospheric dust and oceanic sediment. In order to maintain the viscosity of waxy covering and the water balance, higher plants mainly biosynthesize longer waxy lipids in the warmer tropic areas. In the cooler temperate regions, short chain lipid compounds dominate. Therefore, the average carbon chain distribution of n-alkane is a significant potential indicator in the study of palaeoenvironments (Gagosian et al., 1986). The ratio of the lower carbon number to higher carbon number lipids (Rl/h), which has been used widely to discriminate the organic origin, can be an indicator of the different kinds of dominant vegetation, closely related to climate conditions. A higher value of Rl/h corresponds to a relatively cool period, and lower values reflect warm periods. For example, the Younger Dryas (YD) and associated Heinrich Event has been related to the Rl/h proxy in China (Xie et al., 2003a,b,c,d). Terrestrial plant growth is correlated with warm conditions. The lipid record of Xingyun Lake sediment (XY-1 core) also suggests that the ratio of the higher to lower carbon components (H/L-alk) and the ratio of long chain lipids to the short ones (HL-achl) are closely related to trends in air temperature. The vegetation in the catchment is likely the bridge
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that links the lipids record in the lake sediment with climate conditions (Fig. 4). The high values of H/L-alk and H/L-achl suggest that terrestrial carbon from forest and herbaceous plants dominated in the catchment, which would imply that the climate was relatively warm. In contrast, dominance by aquatic algae indicates cool or cold climates. From this point of view, comparing the proxies of the lipids in the lake sediment with the air temperature record in the region reveals a strong relationship that can be divided into five main stages (Fig. 4), as follows: Stage A (85–60 cm depth, 1840 to the early 1920s) H/L-alk ranges from 0.16 to 0.66 (mean 0.30): the ratio has the lowest value in the entire profile. H/L-achl is also low during this period with a mean value of 1.7. This probably demonstrates the cold climate conditions, corresponding to the 3rd cold period of the Little Ice Age (Wang et al., 1998b). The temperature was relatively low, which limited the inputs of terrestrial plant material, and the organic carbon input was mainly dominated by aquatic algae. Stage B (60–32 cm depth, early 1920s to mid-1960s) H/L-alk has a relatively high value, between 0.59 and 3.18, with a mean value of 1.71. The H/L-achl has the highest value with a mean of 3.63 in the entire sediment core. Warm and humid conditions occurred in this stage, which is comparable with the warm period of the early last century in the instrument record (Wang et al., 1998a). The higher plants were more productive, increasing the input of higher plant organic matter. The value of H/L-alk in this stage is variable, which suggests more frequent climatic variability. Stage C (32–20 cm depth, 1960s–1970s) The value of H/L-alk is between 0.86 and 1.54 (mean 1.24), which is lower than in Stage B. H/L-achl has a mean value of 1.99.
Fig. 3. Distribution of carbon number of n-alkanols in the sediment core XY-1.
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Fig. 4. Comparison of the lipid-based proxies in the lake sediments with the anomaly mean temperature for the last 100 years in Kunming (a) (You et al., 1997); anomalous mean P P þ temperature changes from 1880 to 1996 AD in southwest China (b) (Wang et al., 1998a) ðACL alk ¼ nCn = Cn ; H=L alk ¼ Cþ 23 =C22 ; H=L achl ¼ C20 =C19 Þ.
The temperature in this period is lower than that of the Stage B, but warmer than that of the Stage A, in accordance with the instrumental record (Wang et al., 1998a). This period is comparable to the cold period during the 1960s–1970s. Stage D (20–10 cm depth, 1980s to the early 1990s) The ratios represent a response to the warming period from the 1980s to the early 1990s. The value of H/L-alk increased from 1.02 to 2.20 (mean 1.57) and the value of H/L-achl increased to a mean of 3.05, which suggests the increase of the terrestrial organic matter input that may result from the warm climate. Stage E (10–0 cm) The value of H/L-alk ranges from 1.17 to 0.21 (mean 0.81) and shows a decreasing trend towards the top. The value of H/L-achl has a mean of 0.64 in the lowest part. This may indicate that the organic material from aquatic algae plant had remarkably increased after the early 1990s. Comparing the H/L-alk and H/L-achl curve with the anomalous mean temperature curve of the last 100 years in southwest China (You et al., 1997; Wang et al., 1998a), shows a clear correspondence between the two before the early 1990s. After the early 1990s, the values of H/L-alk and H/L-achl show a sharp decreasing trend despite an observed climate warming. This decrease may have resulted from algae blooms associated with lake eutrophication caused by human activities in the catchment. Thus, human impacts disrupted the natural signals produced by climate change. 5.2. Relationship between the lipid record and human activities N-alkanes are less easily degraded than other organic matter, and can be used as an indicator of the organic matter origin (Meyers, 2003). Human activities may affect vegetation in the lake drainage basins, which, in turn, would change the composition of organic matter. The n-C27 alkane usually represents tree organic matter, and the n-C31 alkane reflects organic matter from grasses. Therefore, the ratio of C27/C31 can be a proxy to reflect shifts in vegetation from trees and forest landscape to an open grass-dominated system
(Cranwell, 1973; Rieley et al., 1991; Meyers and Ishiwatari, 1993; Huang et al., 1999). ACL is also as a proxy which is related to the vegetation type. In addition, C27/(C15 þ C17 þ C19) ratios can be an indicator to discriminate the terrestrial or aquatic origins of organic matter. These proxies can be used to demonstrate environmental changes associated with human activities in the catchment. From Fig. 4, it is clear that both the ACL and H/L show a sharp increase between the depths of 60–56 cm, corresponding to the early 1920s. The ratio of C27/(C15 þ C17 þ C19) also has an increasing trend. This may suggest increased organic matter from terrestrial sources. According to the Jiangchuan County Annals, the Gehe channel was dredged in 1923 to allow the water of Xingyun Lake to flow into Fuxian Lake, which resulted in a 2.5 m drop of the Xingyun Lake level and exposure of the lake floor. Therefore, the input of organic matter of terrestrial origin was increased by this human activity during a background of climate warming. The sedimentology also shows that the grain size of the sediment increased sharply at this depth (Zhang et al., 2006). At about 36 cm, the values for ACL, C27/C31, C27/ (C15 þ C17 þ C19) and H/L-alk ratio show a salient change, which would suggest an increasing input of organic matter from grasses. This trend probably corresponds to massive deforestation and land reclamation around 1958, with a subsequent increase in the area used for growing rice. The value of ACL, C27/(C15 þ C17 þ C19) and H/L-alk ratio decreased sharply in the top w8 cm sediments, whereas the ratio of C27/C31 experienced an increasing trend. At this time the H/L-alk curve does not follow the curve of the anomaly in mean temperature. The likely reason is that the algal blooms in Xingyun Lake were becoming increasingly more severe since the early 1990s, due to lake eutrophication associated with the input of pollutant water from industrial processes and urbanization in the catchment (Liu and Chen, 2000; Zhang, 2001). Environmental change at Xingyun Lake revealed that it was hardly disturbed by human activities before 1962 (Zhang, 2001). Prior to the 1960s, the lake water was clean, primary productivity
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was low, and the organic content of the sediment was relatively low, as well. The lipid-based proxies in sediments recorded the natural environmental changes. The main sources of lower carbon number lipids were primarily algae and other microbes that are influenced by temperature but also the nutrient content of the water (Yao and Li, 2004). In this area, the forest cover increased beginning in the 1990s due to a water and soil conservation policy, which caused the terrestrial organic matter input to be decreased. On the other hand, TN and TP inputs to Xingyun Lake from sewage increased approximately 55% and 50% respectively; and inputs from surface runoff increased about 27% and 26% (Zhang et al., 2002). Therefore, the degree of eutrophication increased from 0.23 to 0.58 during the period 1990 to 1999, changing the lake from a mesotropic to a eutrophic status (Zhang et al., 2002). This process produced the lower H/L value and C27/(C15 þ C17 þ C19) ratios in the top w8 cm sediment. Meanwhile, sediment geochemical analyses show that CaO and calcite increased at this time, whereas the d18O of authigenic carbonate and the isotopic d13Corg showed a negative trend (Zhang et al., 2003, 2006). Thus, the lake eutrophication process caused by strong human activities in the catchment had a stronger influence on the geochemical signature in the lake sediments than did the natural environmental processes driven by climate change. 6. Conclusion 1) The core XY-1 from Xingyun Lake in southwestern China has covered a time period of the last 160 years according to the 210Pb dating. A series of lipids, including n-alkanes and n-alkanols were used as sensitive indicator of environmental change. The proxies of the carbon preference index (CPI) of the n-alkanes, the average chain length (ACL), the ratios of the C27/C31, and C27/ (C15 þ C17 þ C19), the ratio of higher carbon components to the lower of n-alkanes and n-alkanols (H/L) in the Xingyun lake sediment have been achieved to reconstruct the environmental change history for the last 160 years. Four stages of these proxies in the period before 1990 were recognized that corresponded with climate change, especially to temperature trends, i.e. a cool period before the 1920s, a warm period between the 1920s and mid-1950s, a cool period between the mid-1950s and 1970s, and a warm period since the 1980s. 2) The ratio of the high carbon number to the low carbon number of the n-alkanes and n-alkanols, as well as the ratios of C27/C31, could be used to identify vegetation change in the catchment, which was influenced by natural climate change and strong human activities. Four stages of climate change before the early of 1990s could be recognized, as well as human activities that include the Gehe river channel dredging in 1923, strong deforestation in 1958, and lake eutrophication after the early 1990s. 3) The lipids in the lake sediment are a mixed proxy of the organic material from the catchment, influenced by both natural processes and human activities. In most lakes, multi-proxy analyses are the best tool for evaluating and understanding the relative influences of natural processes and human impact in a lake’s catchment. Acknowledgments The research was supported by the National Natural Science Foundation of China (grant 40232025) and the research project of Chinese Academy of Sciences (grant 2004CCA02900). The authors thank Prof. Song Xueliang, Zhang Zixiong (Institute of Geoscience, Yunnan province) for the helping in sediment core drilling, also to Prof. Xie Shucheng and Huang Xianyu (China University of Geosciences) for some helpful suggestions and laboratory assistance.
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