sediments in the Dalian Lake, Qinghai Province, China

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1 137 Cs tracing of lacustrine sediments in the Dalian Lake, Qinghai Province, China YAN Ping 1,2, DONG Guangrong 3 & DONG Zhibao 3 1. China Certer of Desert Research at Beijing Normal University, Beijing , China; 2. Key Laboratory of Environmental Change and Natural Disaster, the Ministry of Education of China, Beijing Normal University, Beijing , China; 3. Cold and Arid Regions Environmental and Engineering Research Institute, Chinese Academy of Sciences, Lanzhou , China Correspondence should be addressed to Yan Ping ( yping@ bnu.edu.cn) Abstract Based upon the analysis of 137 Cs and grain-size parameters, the total amounts of 137 Cs deposition by wind and water in the Dalian Lake of Gonghe Basin, Qinghai Province are defined. The vertical profile of 137 Cs in the lacustrine sediments exhibits three 137 Cs activity maxima and one minimum. The major maximum corresponds to the global 137 Cs fallout in 1963; the two secondary maxima correspond to the leakage of the Chernobyl reactor in 1986 and complete desiccation of the lake in 1994, respectively. The minimum corresponds to aeolian deposition caused by extensive reclamation in the late 1980s and early 1990s. The 137 Cs data set was used to evaluate average sedimentation rate of the Dalian Lake since The deposition was stable in and , and it was rapid during This preliminary study reveals that 137 Cs has the same potential to trace environmental changes introduce by human interference in arid/semiarid regions as in humid regions. Keywords: 137 Cs technique, Dalian Lake, sedimentation rate. Caesium-137 ( 137 Cs, half-life 30.2 a) is a manmade radionuclide produced in the global fallout of debris resulting from nuclear weapon tests during the 1950s and 1960s. 137 Cs is strongly absorbed by clay and organic matter in the topsoil and not susceptible to leaching and plant s assimilation, usually concentrated in the surface horizons. After its deposition, its redistribution is mainly associated with soil physical processes, such as soil erosion and tillage. Since its introduction in the 1960s [1,2], 137 Cs has been widely used as a valuable tracer in soil erosion and sediment delivery, and as an indication of environmental changes and human disturbance for recent a [3]. While the 137 Cs technique is widely applied to water erosion research in humid regions [3, 4], there are few studies for arid and semi-arid environments [5, 6]. Recently, some investigators have estimated average wind erosion rate over a relatively long period, using 137 Cs measurement [5 13]. However, no time series of wind erosion and its relationship with recent environment changes has been obtained. The lacustrine sediments in the arid and semi -arid regions can be regarded as a prefect record of wind erosion and environmental changes [14]. Here we report a preliminary result of lacustrine sediment sampling and its 137 Cs measurement from the Dalian Lake, Qinghai Province, China in The specific objectives were to determine the sediment accumulation rate and its temporal variations, and to discuss the use of 137 Cs tracing in the study of environmental changes in wind erosion regions. 1 Study area The Dalian Lake is located at N, E, with an elevation of 2850 m above sea level. Formerly as the terminal basin of the Shazhuyu River, the lake is divided into two sections (fig. 1), the northern section is Lower Dalian Lake and the southern section is Upper Dalian Lake; they were dried up entirely in 1973 and 1994, respectively. The lake can be regarded as a playa [15] at its present stage of evolution. The Dalian Lake region has a semiarid highland climate, with an average annual air temperature of 4.8. The region receives 246 mm precipitation, but experiences 1717 mm evaporation annually. The arid index (K) is 1.44 on average. The mean annual wind speed is 2.5 m s -1, with gales (>17.0 m s -1 ) occurring in 50.6 days per year and dust storm (vis ibility < 1000 m) 20.7 days per year. The Shazhuyu River is charging into the Dalian Lake, which was once a branch of the Yellow River during the Late Pleistocene. Around 200 thousand years ago, as a result of unsymmetrical tectonic uplift and climatic change in the Qinghai-Tibet Plateau, it turned into an interior river [16]. It has a mainstream of 133 km in length and a total watershed area of 6512 km 2. The mean discharge of the river is 1.78 m 3 s 1 and the total runoff volume amounts to m 3 a 1[17]. Since the 1950s, in the upper and middle reaches of the river, 14 reservoirs, with 15 irrigation ditches, have been constructed. This cause increase in the water storage capacity (i.e. 60% of the total runoff volume). The engineering project has greatly benefited the middle reaches of the river by improving the water utilization, but the natural distribution of water resources was modified such that the water supply of the lower reaches was almost cut off, causing complete desiccation of the Dalian Lake. During the 1980s, 2.97 km 2 of the Upper Dalian was developed as a fish pond, but now abandoned due to water exhaustion. After desiccation in the 1970s, the Lower Dalian was subjected to continuous blown sand accumulation and now turned into a sand basin. Xu [18] has pointed out that the desiccation of the Dalian Lake is continuation of separation and shrinkage of the lakes in the Gonghe Basin since the Late Pleistocene, especially the Holocene, reflecting modern regional environmental changes in the Gonghe Basin or the Qing- Chinese Science Bulletin Vol. 46 Supp. December

2 hai-tibet Plateau. It is surprising that the present-day evolution of the Dalian Lake occurred in such a short period of some 20 years, demonstrating a strong influence of human activities on the regional environment. This series of changes have been recorded by 137 Cs fallout and redistribution in the lacustrine sediments in the playa. Therefore, some relevant information on recent environment changes can be identified by means of 137 Cs measurements for the lacustrine sediments. 2 Sampling and measurement 137 Cs samples were collected from the central Upper Dalian Lake (fig. 1) in July of The sampling depth was 45 cm. For in the upper 0 20 cm, 137 Cs samples were collected by a scraper-plate [19, 20] in 2 cm intervals; in the lower cm, the sampling interval was 5 cm. In addition, grain-size and bulk density were analysed for the samples from the 0 20 cm layer. In total, Cs samples, 10 grain-size samples and 10 bulk density samples were analyzed. 137 Cs samples were air-dried, ground and passed through 1 mm sieve after taking away rejected coarse grains and grass residues by sieving. Each of the samples, weighing about 400 g, was tested at the Nuclear Physics Laboratory of Sichuan University, Chengdu, China. Measurements of 137 Cs activities in soil samples were undertaken by gamma-ray spectrometry, using a high resolution, low background, low energy, hyperpure N-type germanium coaxial γ -ray detector (EG&G ORTEC LOAX HPGe) connected to an ORTEC amplifier and multi-channel analyzer (4069 Channels). The samples were placed on the top of the detector head and counted for over s, providing a precision of ca.±10% at the 90% significance level of confidence for the gamma-ray spectrometry measurements. The 137 Cs activities in the samples were obtained from the peak area in the spectrum associated with 662 kev of 137 Cs. 3 Results and discussion ( ) Total amount of 137 Cs deposition. The total 137 Cs areal activity, i.e. 137 Cs point inventory (Bq m -2 ) was calculated by Sutherland and de Jong [7], Walling and Quine [20] : n 3 = Ci Bdi Di 10 i= 1 CPI, (1) where i is sampling depth and n is maximum number of sample depths with detectable 137 Cs, Ci is 137 Cs activity for sample i (Bq kg 1 ), Bdi is the bulk density (t m 3 ) for depth i, and Di is the depth increment (m) for sample i. On the basis of eq. (1), the total amount of 137 Cs deposition of lacustrine sediments within 0 20 cm was ( ± ) Bq m 2, and within 0 45 cm, ( ± ) Bq m -2. The sampling depth (0 45 cm) did not reach the limit of detectable 137 Cs, but from the 137 Cs depth profile (fig. 2) most 137 Cs was accumulated in the depth lower than 20 cm below the bed, and the 137 Cs activity decreased rapidly below the depth of 30 cm. The total amount of 137 Cs deposition by water and wind erosion during recent three decades in the Dalian Lake can be estimated to be approximately 2.3 times of the 137 Cs reference inventory of the Gonghe Basin, i.e. ( ± ) Bq m 2[16]. ( ) Maximum and minimum 137 Cs deposition (1) A maximum at cm horizon contained the highest 137 Cs activity, i.e. (39.20 ± 2.27) Bq kg -1 for the whole profile, suggesting that it may represent the period of maximum radionuclide fallout associated with the peak of atomic weapon testing in 1963 [3, 22]. (2) Two minimum values in the 6 10 cm section have the lowest 137 Cs activities (2.36±0.18) Bq kg 1 and Fig. 1. Location of Dalianhai Lake in Gonghe Basin (after Duan et al., 1994). 84 Chinese Science Bulletin Vol. 46 Supp. December 2001

3 Fig. 2. Profile of 137 Cs activity and grain-size distribution in the sediments of Dalian Lake. (2.21 ± 0.22) Bq kg 1 ), corresponding to aeolian sand s 137 Cs activity in the Gonghe Basin [21]. It may be relevant to the sand deposition. Grain-size analysis (fig. 2) shows that there are six horizons of sediment with similar grain-size composition in 0 20 cm section. Silt ( mm) is the main comp onent of particle in the six horizons, with a mean grain-size of mm and a sorting coefficient of around 1.0 (i.e. poorly sorted according to Fork and Ward [23] and Wu [23] ). Apparently, these horizons have the same origin of lacustrine sediment. Except for these horizons, the two horizons of 6 8 cm and 8 10 cm sharply varied in grain-size of sediment, consisting mainly of sand (> mm). Mean grain-size of the two horizon are and mm, respectively; 76.56% and 90.86% of fractional percentage are concentrated on the size grades from sand to very fine sand. The sorting coefficients were and 0.620, respectively ( moderately sorted and moderately well sorted ). These characteristics are in agreement with those of the sand in the Gonghe basin [21], indicating their aeolian origin. For the other two horizons at 4 6 cm and cm below the bed, two 137 Cs minimum values (6 10 cm) have the transitive characteristics of the above horizons, consisting mainly of silt and sand, with mean grain-size of mm and mm, respectively, and sorting coefficients of more than 1.5 ( poorly sorted ). These two horizons represent clearly transition from lacustrine sediment to eaolian sand. The interre lationship between these horizons indicate that abnormal sand accumulation with the 137 Cs minimum values was neither formed in the period of high water level of the lake, nor after the lake was dried up thoroughly. The environment for sand accumulation should be a shallow water lake or a swamp during the short periods before the lake was dried up. From the materials of the Dalian Lake desiccation [18,25], it can be accurately dated to be in the period between the late 1980s to the early 1990s. During this period, there were no remarked fluctuations in climate, i.e. abrupt dry events [21] ; thus, it is only possible that excessive human disturbance resulted in the exceptional aeolian sand deposition. In , as an auxiliary project of Liangtang Reservoir [25], the expanded farmland for irrigation were drawn up in Xiakaligang and Laihaita Villages (fig. 1). The reclaimed wasteland of more than 400 ha, 85% of the planned project were not suitable to cultivation because of the coarse sand through the whole tillage layer and were finally abandoned. Without natural vegetation protection after the destruction, the sandy surface became bare and exposed to intense wind erosion, causing a new shifting dune belt over the Terrace of the Shazhuyu River in the northern Dalian Lake. The sand dune has been constantly encroached on the Dalian Lake along the southern Terrance, filling up most part of the Lower Dalian, and a part of the Upper Dalian under the desiccation process. Because of the lag effect of aeolian process and the transition between the horizons, this 137 Cs minimum can be dated to be (3) For the secondary peaks of cm and 2 4 cm horizons, the former represents the Chernobyl accident in 1986 with a medium 137 Cs activity of (12.60 ± 0.81) Bq kg 1, which is very close to the results by Collins et al. [22] in the Severn Basin, Bewdley, UK and by Wan [27] in the Erhai Lake, Yunnan, China. The latter may represent a stagnant phrase before the lake desiccation i.e As the lake was about to dry up, the central lake experienced clay and 137 Cs accumulation by stagnant water, with a high clay content of 2.5 times larger than an average level and a 137 Cs activity of (10.79 ± 0.75) Bq kg 1. ( ) Deposition rate and its different stages. Assuming that there are no distinct variations in porosity and bulk density of the sediments, the deposition rate becomes [27] : P = Zm /(Tc Tm), (2) where P is the deposition rate (cm a 1 ), Zm is the horizon depth of 137 Cs maximum or minimum (cm), Tc is the sampling year (a) and Tm is the dated year (a) of 137 Cs maximum or minimum. Chinese Science Bulletin Vol. 46 Supp. December

4 Table 1 Deposition rates on the basis of 137 Cs dating for Dalian Lake Dated year 137 Cs peak or valley Environment event Depth/cm Deposition rate/cm a 1a) secondary maximum (2) Lake s desiccation minimum aeolian sand deposition secondary maximum (1) Chernobyl accident maximum global 137 Cs fallout peak a) The left refers to progressively average rate from a certain dated year to 1998, and the right refers to average rate of interval between two dated years. The time-averaged deposition rate of the Dalian Lake since 1963 is 0.79 cm a 1, according to the calculations (table 1). The period can be divided into three periods by different deposition rates. They are two slow deposition periods ( and ) and one rapid deposition period ( ). The slow periods have deposition rates of cm a 1, at a normal level of lacustrine sediment deposition rates reviewed by Wise [4], indicating a relatively stable phase of lacustrine sediment deposition. The rapid deposition period has deposition rates of cm a 1, times more than the stable deposition rate; this was caused by the abnormal aeolian sand deposition, as described above. 4 Conclusions The total amount of 137 Cs deposition in lacustrine sediments within the 0 20 cm depth was ( ± ) Bq m 2, and it is within the 0 45 cm depth ( ± ) Bq m Cs was accumulated mostly in the depth lower than 20 cm below the bed, and the 137 Cs activity decreased rapidly below the depth of 30 cm. The total amount of 137 Cs deposition by water and wind erosion during recent several decades in the Dalian Lake is estimated to be 2 or more times of the 137 Cs reference inventory of the Gonghe Basin. Combining 137 Cs measurement with the relevant grain-size analysis, 3 maxima and 1 minimum are present in the 137 Cs profile, which can be used to date the lacustrine sediment deposition. The major maximum corresponds to the global 137 Cs fallout in 1963; the two secondary maxima correspond the leakage of the Chernobyl reactor in 1986 and the desiccation event of the Lake in 1994, respectively. The minimum corresponds to aeolian deposition caused by the extensive land reclamation during the late 1980s and the early 1990s. The 137 Cs data set can be used to evaluate sedimentation rates of the Dalian Lake. In accordance with the different deposition rates, the deposition are divided into slow and rapid deposition periods, with the rates of cm a 1 (representing stable phase of lacustrine sediment deposition) and cm (caused by the abnormal aeolian sand deposition), respectively. Present-day human activities with water diversion and land use can accelerate the desiccation of the playas and activate aeolian sediment transport processes. 137 Cs tracing has the same potential to identify environmental changes introduced by human interference in the arid/ semiarid regions as in humid regions. Acknowledgements We should thank Dr. Wang, X. M. and Dr. Li, X. Z. (Institute of Desert Research), Yang, H. H. (Desert Control Experimental Station of Qinghai Province), Lu, F. G. (Hainan Prefecture Agriculture & Pasture Planing Team, Qinghai Province), for their assistance during field investigation and samples collection. We also thank Prof. Zhang Y. Y. and Bai, L. X. (Physics Department of Sichuan University), Wen, A. B. (Institute of Mountain Hazards and Environment), and Song, W. J. (Institute of Desert Research) for samples analysis and other relevant helps. This work was supported by the National Key Basic Research Program of China (Grant No ) and the National Natural Science Foundation of China (Grant No ). References 1. Ritchie, J. C., Spraberry, J. A., McHenry, J. R., Estimating soil erosion from the redistribution of fallout Cs-137.Soil Science Society of America Proceedings, 1974, 38: Rogowski, A. S., Tamura, T., Movement of cesium-137 by runoff, erosion and infiltration on the alluvial Captina silt loam. Health Physics, 1965, 11: Ritchie, J. C., McHenry, J. R., Application of radioactive fallout caesium-137 for measuring soil erosion and sediment accumulation rates and patterns: a review, Journal of Environment Quality, 1990, 19: Wise, S. M., Caesium -137 and lead-210: a review of the techniques and some applications in geomorphology, in Timescales in geomorphology (eds. Cullingford, R. A., Davidson, D. A., Lewin, J.), Chichester: John Wiley & Sons, 1980, Sutherland, R. A., Kowalchuk, T., de Jong, E., Cesium-137 est i- mates of sediment redistribution by wind. Soil Science, 1991, 151: Chappell, A., The limitations of using 137 Cs for estimating soil redistribution in semi-arid environments, Geomorphology, 1999, 29: Sutherland, R. A., de Jong, E., Estimation of sediment redistribution within agricultural fields using caesium -137, Crystal Springs, Saskatchewan, Canada. Applied Geography, 1990, 10: Chappell, A., The spatial variability of net soil flux in the south-west Niger, West Africa. PhD thesis, Department of Geography, University College London, a 1 86 Chinese Science Bulletin Vol. 46 Supp. December 2001

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