SCIENCE CHINA Earth Sciences

Transcription

1 SCIENCE CHINA Earth Sciences RESEARCH PAPER August 2010 Vol.53 No.8: doi: /s Geochronology of recent lake sediments from Longgan Lake, middle reach of the Yangtze River, influenced by disturbance of human activities WU YanHong 1,2*, LIU EnFeng 1, BING HaiJian 1,3, YANG XiangDong 1, XUE Bin 1 & XIA WeiLan 1 1 State Key Laboratory of Lake Science and Environment, Nanjing Institute of Geography and Limnology, Chinese Academy of Sciences, Nanjing , China; 2 Institute of Mountain Hazards and Environment, Chinese Academy of Sciences, Chengdu , China; 3 Graduate University of Chinese Academy of Sciences, Beijing , China Received April 3, 2009; accepted December 5, 2009; published online May 27, 2010 The impacts of human activities in Longgan Lake region on the distribution of 210 Pb in lake sediment made it difficult to establish geochronological sequence. However, environmental proxies recorded these human activities and could be used as time marker to check 210 Pb dating results. Based on the analysis of 210 Pb exc distribution in cores LS-1 and LGL-1, and in combination with the human activities records in grain-size and element concentration, the sedimentation rates in the upper layers of these two cores were determined to be 0.19 and 0.23 cm/a respectively. The MAR (mass accumulation rates) of these two cores demonstrated that natural condition, such as precipitation and runoffs, led to the variation of MAR before human disturbance, and human activities controlled the sedimentary processes afterwards, which obviously reduced the particles influx into the lakes. During 1940s when human activity had not changed the water system within the catchment, 210 Pb activity was abnormally strong in lakes along middle to lower reaches of the Yangtze River, which might be attributed to less precipitation and can be used as a time marker for geochronological research. 210 Pb, geochronology, environmental index, human activity, Longgan Lake Citation: Wu Y H, Liu E F, Bing H J, et al. Geochronology of recent lake sediments from Longgan Lake, middle reach of the Yangtze River, influenced by disturbance of human activities. Sci China Earth Sci, 2010, 53: , doi: /s Lake sediment plays a key role in lake environmental research. The process of lake environmental change, flux and history of the pollutant, lake environmental background and present facies, and mechanism of lake environmental change can be detected by using some environmental proxies of lake sediment. In recent decades, the anthropogenic impact on the lake environmental change can also be quantitatively deduced from lake sediment proxies. Precise geochronology is of crucial importance in above studies. *Corresponding author ( yhwu@niglas.ac.cn) 210 Pb and 137 Cs dating methods are widely used in the recent geochronology study of lake sediment [1 5]. 210 Pb (half life is 22.3 years) is a kind of natural radionuclides. The total activity of 210 Pb in lake sediment is composed of supported 210 Pb ( 210 Pb sup ) and unsupported or excessive 210 Pb ( 210 Pb exc ). The former is sourced from 226 Ra decay in the natural uranium series, whereas the latter is from atmospheric deposition of 222 Rn decay. 137 Cs is a kind of artificial radionuclides from the atmospheric testing of nuclear weapons. 210 Pb and 137 Cs are removed from the atmosphere by precipitation or dry deposition, falling onto the land sur- Science China Press and Springer-Verlag Berlin Heidelberg 2010 earth.scichina.com

2 WU YanHong, et al. Sci China Earth Sci August (2010) Vol.53 No face or into lakes and oceans, being absorbed by particles or organic matter and being kept in sediment. Using 137 Cs for dating is according to its time markers (emerging at the beginning of the 1950s and peaking at 1963 in north hemisphere). 210 Pb dating can provide absolute age for lake sediment according to its natural decay after deposition. There are two simple models, commonly referred to as the CIC (Constant Initial Concentration) and CRS (Constant Rate of 210 Pb Supply) models. CIC model is based on the assumption that the initial concentration of 210 Pb is constant, whereas CRS on the assumption is that the atmospheric fallout of 210 Pb is constant, but the flux of particles changes the 210 Pb concentration in lake sediment. One of the fundamental assumptions of 137 Cs and 210 Pb sediment dating is that there is no transition of them besides decay. However, the mixing of surface sediment particulates [6, 7], bio-disturbance and wind and wave forced disturbance [8], chemical transition of elements [9], sudden change of primary productivity and sedimentation flux of particles [10] and anthropogenic disturbance [7], often cause the abnormally vertical distribution of 210 Pb exc and 137 Cs, resulting in negative influence on dating result. A large number of lakes are distributed along the middle and lower reaches of the Yangtze River where human activity is relatively intensive. Since the 1950s, lakes in this region have met with two serious issues, i.e., lake volume decrease and eutrophication. The sedimentation rate (SR) is necessary for understanding the process of lake volume decrease, while geochronology for eutrophication history. Therefore, studies on geochronology and the SR of lake sediment, including 137 Cs and 210 Pb dating, have been widely developed [8, 11 16]. Since the 1950s, within the middle reach of the Yangtze Rive catchments, the prominent human activity was building a large number of reservoirs in the upper reaches of lakes, isolating lakes from the Yangtze River by building gates in lower reaches of lakes and reclaiming lakeshore into agrarian land. The intensive human activities must have been recorded by lake sediment and have caused errors of 137 Cs and 210 Pb dating. Here we present a study on element geochemistry, grain size proxies, and 137 Cs and 210 Pb data of two sediment cores from Longgan Lake in the middle reach of the Yangtze River, to discuss human impact on the SR and hopefully provide a reasonable example for establishing geochronogical sequence for modern lake sediment in this region. present due to the reclaiming. When the lake water level is 21.1 m a.s.l. the average water depth is 3.78 m. Longgan Lake is a pass water lake, which collects the water from the Taibai Lake and the Erlang River to the north of it, and discharges into the Yangtze River passing through Huangda Lake, Bo Lake, and Xiaochi [13, 17, 18]. 2 Materials and methods Two sediment cores, LS-1 and LGL-1, were collected using gravity corer from the open area of Longgan Lake in October 2007 and March 2008 respectively (Figure 1). The lithology of these two cores is of grey and black silty clay. Sediment was pushed out and sliced at intervals of 1 cm for 210 Pb and other proxy analyses. 137 Cs, 210 Pb, and 226 Ra activities were detected using an Ortec HPGe GWL series well-type coaxial low background intrinsic germanium detector after samples were dried in low temperature (<40 C) and weighted. The excessive 210 Pb ( 210 Pb exc ) was obtained by subtracting the activity of 226 Ra from the total activity of 210 Pb ( 210 Pb tot ). Pretreatment of samples for grain size analysis was done according to the following procedures. About 5 g sample was taken, 5% HCl was added to remove carbonate, and 5% NaOH to remove organic matter, and the sample was then washed to neutral. The residues were dispersed with an ultrasonic oscillator for 15 minutes after mixing with dispersant solution ((Na 2 PO 3 ) 6 ). Grain-size was then determined using a Malvern Laser Grainsize Meter. Aliquots of samples were digested by mixed acid (nitric acid, hydrofluoric acid, and perchloric acid). After being digested, the element concentrations were determined by inductively coupled plasma-atomic emission spectrometry (ICP-AES) with standard solution SPEX TM from the US as the standard. The uncertainty of standard solution SPEX TM is 2%. Quality control was assured by the analysis of duplicate samples, blanks, and reference materials (GSD-9 and 1 General description of the research area Longgan Lake ( N, E) is located in Susong County, Anhui Province and Huangmei County, Hubei Province, which was km 2 of water surface area in the early 1950s, and shrank to km 2 at Figure 1 Sketch map of sampling site.

3 1190 WU YanHong, et al. Sci China Earth Sci August (2010) Vol.53 No.8 GSD-11, Chinese geological reference materials). 3 Results In both LS-1 and LGL-1, 226 Ra activity varied slightly from the bottom to top, indicating relatively steady sediment source in two core sites (Figure 2). 210 Pb came to equilibrium with 226 Ra below 35.5 cm in LS-1 core, while 37.5 cm in LGL-1 core. There is a parallel variation for 210 Pb tot and 210 Pb exc in two cores. They increased upwards and came to a high level from 12.5 to 9.5 cm in LS-1 core and from 18.5 to 13.5 cm in LGL-1 core. The abrupt decline of 210 Pb tot and 210 Pb exc occurred following this high level stage, and they increased prominently afterwards and came to their peak values at the surface layer. The logarithmic value of 210 Pb exc (ln 210 Pb exc ) increased in a linear style above the high level stage. The parallel variations of 210 Pb tot and 210 Pb exc were more visible when using mass depth instead of depth, and the high level stage of 210 Pb tot and 210 Pb exc emerged at the same mass depth (Figure 3). The activity of 137 Cs was very low in both LS-1 core and LGL-1 core, although it can be detected from 9.5 cm and 15.5 cm above in two cores respectively (Figure 2). Sediments in both LS-1 and LGL-1 cores were composed mainly of fine particles; the portion of clay and silty exceeded 80% (Figure 4). In LS-1 core, the portion of clay Figure 3 Variation of nuclide elements in LS-1 (a) and LGL-1 (b) cores with mass depth. Figure 2 Distribution of nuclide elements in LS-1 (a) and LGL-1 (b) cores. Figure 4 Proxies of grain size varied in LS-1 core (above) and LGL-1 core (below).

4 WU YanHong, et al. Sci China Earth Sci August (2010) Vol.53 No particles (<16 μm) decreased from 9.5 cm upwards, while the portion of sand (32 64 μm) increased. In LGL-1 core, the portion of silty particles (16 32 μm) increased obviously from 13.5 cm upwards but the portion of sand decreased, and the coarser particles (>64 μm) increased in the surface layer. The median diameter (D(0.5)) varied slightly around 10 μm in LGL-1 core, but showed an obvious increase trends in LS-1 core from 9.5 cm upwards. The sorting coefficient showed a decline trends in two cores, especially above 9.5 cm in LS-1 core and 13.5 cm in LGL-1 core, where the sorting coefficients were at their valley values. A slight increase of sorting coefficient occurred in the top 4 cm in LGL-1 core. From the bottom to top, concentrations of Al and some heavy metals (Pb, Co, Ni, Cu, and Zn) declined continuously in LS-1 core until 9.5 cm where a sudden increase occurred. In LGL-1 core the concentrations of Al and heavy metals increased by three steps, which are below 31.5 cm, cm, and above 13.5 cm (Figure 5). 4 Discussions 4.1 Human activities recorded by lake sediment As mentioned above, since the 1950s, within the middle reach of the Yangtze River catchments, the most prominent human activity was building reservoirs, gates, dams, and dikes and reclaiming. Within Longgan Lake catchments, Diaoyutai Reservoir was built in 1955 at the upper reach of the Erlang River 1), and Jingzhu Reservoir and Yongan Reservoir were built in 1959 at the upper reach of Taibai Lake, which is another major inflow of Longgan Lake [15]. The major pathway of outflow of Longgan Lake is passing by Daguan Lake, Huang Lake, and Bo Lake, and discharging into the Yangtze River through Huayang Gate and Yangwan Gate, which were built in ). The large scale reclaiming in Longgan Lake catchments started from the beginning of the 1950s when Huayang Farm and Longganhu Farm were established [17, 18]. Building water conservancy facility at the upper reach resulted in the decrease of inflow flux, the changing of the hydrologic condition, and the decrease of particles transported by runoffs. Variation of the SR reflected these changes. Assuming that the high value stages of 210 Pb from 18.5 to 13.5 cm in LGL-1 core and from 12.5 to 9.5 cm in LS-1 core occurred synchronously, the SRs of the upper layer were quite discrepant. LGL-1 core was at the juncture of Longhu bay and open area of Longgan Lake where the sediment source was from the Erlang River. LS-1 core was at the west part of Longgan Lake where the sediment source was from Taibai Lake. After regulated by Taibai Lake and reservoirs in its upper reach, the load of particles was relatively lower, and the SR in the west part of Longgan Lake was hence slower than that in the east part. With the building of gates at the lower reach of the lake, Longgan Lake was isolated from the Yangtze River, and its hydrologic condition was changed resulting in the weakening of sorting reflected by the decrease of sorting coefficient of lake sediment. As shown in Figure 4, obvious decrease of the sorting coefficient occurred at 9.6 cm in LS-1 core and 13.5 cm in LGL-1 core respectively. Large scale reclaiming within lake catchments caused the increase of silty portion in the upper layer of LGL-1 core and the increase of sandy portion and D(0.5) in the upper layer of LS-1 core. The concentrations of Pb, Co, Ni, Cu, and Zn in LGL-1 core and LS-1 core were positively correlated with silty particles content. In LGL-1 core, the concentration of Cu is correlated well with the content of 4 32 μm portion (R = 0.80, n = 40). Therefore, the variation of heavy metal content indicated the intensity of human activities. In the Longgan Lake catchment, human activities caused the change of grain size composition as well as the increase of heavy metal concentrations. 4.2 Dating lake sediment of Longgan Lake under the impacts of human activity Figure 5 Variation of Al (mg/g) and some heavy metals (mg/kg) in LS-1 core (above) and LGL-1 core (below). The activity of 137 Cs in LS-1 core was too low, so the peak value cannot be used as time marker credibly, although it 1) Compilation Committee of Susong County Annals. Susong County Annals. Unpublished document.

5 1192 WU YanHong, et al. Sci China Earth Sci August (2010) Vol.53 No.8 was detected in the upper 9.5 cm. In LGL-1 core, there was no obvious peak of 137 Cs activity, indicating that the forces from wind, wave, and bio-disturbance modified the distribution of 137 Cs. Therefore, the time marker of the emerging of 137 Cs at the beginning of the 1950s was available in Longgan Lake, but other time markers, such as 1963 s time marker, were not suitable for dating sediment of Longgan Lake. The record of human activity at the sedimentation proxies provided the possibility for dating sediment by Event Dating approach. In LS-1 and LGL-1 cores, the obvious changes of grain size composition and heavy metal concentrations at 9.5 cm and 13.5 cm respectively, was corresponding to the end of the 1950s, when reservoirs and gates were built. Since the 210 Pb exc was exponentially correlated with the depth in the upper 9.5 cm in LS-1 core and LGL-1 core, with R of 0.98 (n = 10) and 0.93 (n = 14) respectively, CIC model was used to calculate the SRs for the upper layer of these two cores. According to CIC model, the average SRs were 0.19 and 0.26 cm/a for the upper layer of LS-1 and LGL-1 cores respectively. The age at 9.5 cm in LS-1 core was 1960 AD, and it was 1957 AD at 13.5 cm in LGL-1 core. These ages were corresponding to the time when reservoirs and gates were built in Longgan Lake catchments. The result of 210 Pb dating and Event dating validated each other. Below 9.5 cm of LS-1 core and 13.5 cm of LGL-1 core, 210 Pb exc is distributed without obvious exponential relation with depth (Figures 2, 3), so that CRS model was used to establish time sequence and calculate the MAR for the lower part of two cores (Table 1). Table 1 MAR and geochronology of LS-1 core and LGL-1 core LS-1 LGL-1 Depth (cm) Mass depth (g/cm 2 ) MAR (g/(cm 2 a)) Age (AD) Depth (cm) Mass depth (g/cm 2 ) MAR (g/(cm 2 a)) Age (AD)

6 WU YanHong, et al. Sci China Earth Sci August (2010) Vol.53 No Figure 6 Variations of MAR in LS-1 and LGL-1 cores. Indicated by time sequence, the high value stages of 210 Pb at depth of cm in LGL-1 core and cm in LS-1 core were corresponding to the period from the end of the 1940s to the beginning of the 1950s, when 137 Cs emerged in both cores (Figure 2). Therefore, the dating result of 210 Pb is reasonable. The high value stage of 210 Pb was also found in the northwest bay of Longgan Lake [13, 18] and other lakes in middle to lower reaches of the Yangtze River, such as in Chaohu Lake [16], Gucheng Lake [19], Taibai Lake [15], Dongting Lake [8, 14], and Poyang Lake [8]. If more data can be provided from other regions, this high value stage of 210 Pb should be an effective time marker. Wan et al. [10] ascribed the sudden increase of 210 Pb exc flux to the enhancement of lake productivity change. Which caused the high value stage of 210 Pb in lakes of the middle to lower reaches of the Yangtze River? Former studies demonstrated that the productivity in lakes of the middle to lower reaches of the Yangtze River was not so high in the 1940s and 1950s [20]. The variation of MAR may be the possible interpretation for the increase of 210 Pb activity. The MAR varied synchronously in LS-1 core and LGL-1 core. The higher MAR occurred around the 1940s (Figure 6). The summer precipitation in the 1940s was relatively high in the middle to lower reaches of the Yangtze River [15, 21], which brought more particles into the lake. Since the beginning of the 1950s, many reservoirs have been built in the upper reach of the lake catchments. More particles were deposited in reservoirs causing high MAR [22], nevertheless fewer particles were discharged into the lake. On the other hand, the precipitation was low around 1950 AD, and fewer particles were loaded by runoffs. Therefore, the 210 Pb concentration in sediment increased. Since the later 1950s, the SRs in LS-1 core and LGL-1 core were both lower but varied asynchronously. The decrease of MAR started since the early 1950s in LGL-1 core, while since the later 1950s in LS-1 core. The discrepancy should be ascribed to the time difference of building of Diaoyutai Reservoir and Jingzhu Reservoir. The decrease trends of MAR in LS-1 core were consistent with the results of Tabai Lake [15] but different from the result in Dayuan Lake [13, 18]. The reason should be that the Dayuan Lake is a relatively closed lake bay collecting the materials from surrounding small rivers, and the nearby reclamation brought abundant particles causing the enhancement of MAR. 5 Conclusions (1) Building reservoirs in the upper reach of lake catchments decreased the particles discharges and resulted in the decrease of SR. Building gates at the outflow isolated the lake from the Yangtze River, and lake hydrologic conditions hence were changed. The relatively lenitic condition resulted in the decrease of sorting coefficient of grain size. The changes of particles discharge and hydrologic condition influenced the distribution of 210 Pb in the lake sediment, confusing the dating result when using conventional methods. Meanwhile, the records of human activity in sedimentation proxies are significant as time markers, which can verify the conventional radionuclides dating methods and improve the precision of dating. (2) 210 Pb exc is exponentially distributed in the upper layer in LS-1 and LGL-1 cores, and the average SR of the upper layers of these two cores were calculated as 0.19 and 0.23 cm/a respectively using CIC model. The time of human activity deduced by the average SR coincided with the document records and 137 Cs dating result. The MAR of the whole core indicated that natural factors, such as precipitation and runoffs, controlled the variation of MAR before the construction of water conservancy facility, and human ac-

7 1194 WU YanHong, et al. Sci China Earth Sci August (2010) Vol.53 No.8 tivity changed the hydrologic condition of Longgan Lake and reduced the particles flux into the lake. (3) The high value stage of 210 Pb emerged before human activity changed the structure of water system, which was ascribed to less precipitation at the period from the end of the 1940s to the beginning of the 1950s. This abnormality of 210 Pb accumulation can be used as a time marker to verify conventional radionuclides dating result. This work was supported by National Natural Science Foundation of China (Grant No ), Chinese Academy of Sciences (Grant No. KZCX2-YW-319). 1 Wan G J, Santschi P H, Sturm M, et al. Natural ( 210 Pb, 7 Be) and fallout ( 137 Cs, 239,240 Pu, 90 Sr) radionuclides as geochemical tracers of sedimentation in Greifensee, Switzerland. Geochem Geol, 1987, 63: Wan G J. Progrosses on 137 Cs and 210 Pb ex dating of lake sediments (in Chinese). Adv Earth Sci, 1995, 10: Wan G J. 210 Pb dating for recent sedimentation (in Chinese). Quat Sci, 1997, 3: Appleby P G, Oldfield F. The caculation of lead-210 dates assuming a constant rate of supply of unsupported 210 Pb to the sediment. Catena, 1978, 5: Krishnaswamy S, Lal D, Martin J M, et al. Geochronology of lake sediments. Earth Planet Sci Lett, 1971, 11: Dominik J, Mangini A, Muller G. Determination of recent deposition rates in Lake Constance with radioscopic methods. Sedimentology, 1981, 28: Miguel M, Bolivar J P, Carcia-Tenorio R. Mixing, sediment accumulation and focusing using 210 Pb and 137 Cs. J Paleolimnol, 2003, 29: Xiang L, Lu X X, Higgitt D L, et al. Recent lake sedimentation in the middle and lower Yangtze Basin infered from 137 Cs and 210 Pb measurements. J Asian Earth Sci, 2002, 21: Benoit G, Rozan T F. 210 Pb and 137 Cs dating methods in lakes: A retrospective study. J Paleolimnol, 2001, 25: Wan G J, Chen J A, Xu S Q, et al. Sudden enhancement of sedimentation flux of 210 Pb ex as an indicator of lake productivity as exemplified by Lake Chenghai. Sci China Ser D-Earth Sci, 2005, 48: Rose N L, Boyle J F, Du Y, et al. Sedimentary evidence for changes in the pollution status of Taihu in the Jiangsu region of eastern China. J Paleolimnol, 2004, 32: Yi C, Appleby P G, Boyle J F, et al. The sedimentary record of a significant flooding event in Lake Taihu on the Yangtze delta, China. J Coastal Res, 2004, 43: Wu Y H, Wang S M, Xia W L, et al. Dating recent lake sediments using spheroidal carbonaceous partical (SCP). Chin Sci Bull, 2005, 50: Du Y, Cai S, Zhang X, et al. Interpretation of the environmental change of Dongting Lake, middle reach of Yangtze River, China, by 210 Pb measurement and satellite image analysis. Geomorphology, 2001, 41: Liu E F, Yang X D, Shen J, et al. Sedimentary flux of Lake Taibai, Hubei Province and correlations with precipitation and human activities in the catchment during the last century (in Chinese). J Lake Sci, 2007, 19: Yao S C, Xue B, Li S J, et al. Sedimentation rates in Honghu, Chaohu and Taihu Lakes in the middle and lower reaches of the Yangtze River (in Chinese). Resourc Environ Yangtze Basin, 2006, 15: Yang X D, Wang S M, Shen J, et al. Lacustrine environment responses to human activities in the past 300 years in Longganhu Lake catchment, southeast China. Sci China Ser D-Earth Sci, 2002, 45: Wu Y, Luecke A, Wang S. Assessment of nutrient sources and paleoproductivity during the past century in Longgan Lake, middle reaches of the Yangtze River, China. J Paleolimnol, 2008, 39: Yao S C, Xue B, Zhu Y X, et al. Sediment lead pollution records from lakes at middle and lower reaches of Changjiang River Basin: Case study in Honghu, Guchenghu and Taihu Lakes (in Chinese). Quat Sci, 2008, 28: Yang X D, Shen J, Dong X H, et al. Historical trophic evolutions and their ecological responses from shallow lakes in the middle and lower reaches of the Yangtze River: Case studies on Longgan Lake and Taibai Lake. Sci China Ser D-Earth Sci, 2006, 49(Suppl): Gong D Y, Zhu J H, Wang S W. Flooding 1990s along the Yangtze River, has it concern of global warming? J Geograph Sci, 2001, 11: Dai X, Dearing J A, Yu L, et al. The recent history of hydro-geomophological processes in the upper Hangbu river system, Anhui Province, China. Geomorphology, 2009, 106: