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Impacts of recent human activities on sedimentation rates at Longxi catchment
Alternative Title人为活动对龙溪河流域产沙速率的影响
Thesis Advisor贺秀斌
Degree Grantor中国科学院大学
Place of Conferral北京
Degree Discipline土壤学
Keyword为活动 降雨侵蚀力 产沙速率 核素 龙溪河
Abstract流域侵蚀产沙是自然因素与人为活动综合作用的结果,识别与评价自然因素与人为活动对流域侵蚀产沙的作用强度与贡献,是规划流域水土流失防治与流域管理措施的前提。库湖(塘)泥沙沉积层序特征不仅可追溯其发生年代,沉积剖面中的泥沙理化特性也记录着自然与人类活动印迹。本研究通过长寿湖泥沙剖面的年龄断代和其上游龙溪河流域的降雨与人为活动的分析, 重建了自上世纪50年代以来,龙溪河流域产沙速率的动态变化历史;分析了人为活动对龙溪河流域产沙速率的影响,为流域侵蚀产沙评价提供重要的方法并为流域水土保持与流域管理规划提供科学依据。主要结论如下:(一)在泥沙剖面放射性核素(Cs-137和Pb-210)断代技术的基础上,结合流域降雨侵蚀力与泥沙颗粒对比、泥沙养分(TOC、TN)与流域农事活动记录对比, 建立了泥沙剖面高分辨率(年际)断代技术。雨季的侵蚀性降雨尽管发生低频率但强度高,结合流域气象降雨记录,通过流域降雨侵蚀力与泥沙颗粒对比,精准识别了1982年、1989年和1998年极端降雨事件在泥沙剖面的位置。2005年禁止鱼箱养鱼在泥沙剖面中TOC、TN浓度变化也有较好的反映。(二)基于高分辨率泥沙剖面断代结果计算,龙溪湖流域产沙率即长寿湖多年年均沉积速率为1.64 cm/yr。泥沙沉积速率变化分三个阶段。(1)1956 - 1981年, 泥沙沉积速率为1.25 cm/yr。TOC、TN、微量金属元素含量和平均沉积速率都较低; (2)1982 - 1989年,泥沙沉积速率为2.29 cm/yr。随着社会经济的发展,进入长寿湖的城镇、工业污水排放量逐渐增,进而导致大量TOC、TN和微量金属元素(Cd, Co, Cu, Cr and Ni等)的输入和沉积,TOC、TN的增加主要归因于1989年以后网箱养鱼的发展; (3)1990 - 2013年, 泥沙沉积速率为1.63 cm/yr。表层沉积物中TOC、TN 的增加和微量金属元素的降低表明长寿湖流域生态环境逐步得到改善。渔业的发展促进了长寿湖藻类的生长和初级生产力的增加,并因此爆发2004 - 2005年的水体富营养化事件。(三)研究发现210Pbex 比活度与TOC显著相关。长寿湖沉积物中TOC、TN因1989年以后网箱养鱼的发展而增加,210Pbex 比活度与TOC (R=0.61, p=0.04)、TN (0.51, p=0.04) 显著的统计相关。肥料使用和藻类生长影响长寿湖沉积物中210Pbex 比活度,导致1989年以后210Pbex沉降富集。因此,在使用210Pbex作为湖泊沉积剖面断代工具时,必须考虑沉积物TOC含量。本研210Pb CRS和CIC两种模式分别对长寿湖沉积物样芯进行了定年。
Other AbstractThe Changshou Lake is an in-land reservoir in Longxi catchment of the Three Gorges Reservoir Area. Sedimentary archives from in-land reservoirs such as Changshou Lake provide records of historical sediment dynamics and its related natural and anthropogenic controls. The present research tries to achieve multifold objectives. Firstly, reconstruct the history of natural events and anthropogenic activities in the Longxi catchment and Changshou Lake. Secondly, presents and discuss multiple proxies for an integrated sediment dating index including particulate size distribution, 137Cs activities, mean sedimentation rate, geochemical records, rainfall erosivity estimates and sequential records on human activities within the Longxi catchment. Thirdly, investigate the relationship between total organic carbon and lead activity at Changshou Lake, the possible anthropogenic causes of variations in total organic carbon, its influence on lead distribution and its effects on lead dating at Changshou Lake. Finally, assess the sediment dating efficiency of lead dating models Constant Rate of Supply (CRS), Constant Initial Concentration (CIC) and alternate dating indexes at Changshou Lake.Strategic methodology was adapted to review the literature, collect sediment samples, retrieve valuable data from the catchment annuals, conduct experimental analysis and propose innovative dating index. Sediment core retrieved from Changshou Lake in the Longxi catchment during May 2014 was investigated for Lead-210 excess (210Pbex), Cesium-137 (137Cs) activities, TOC, TN, particle size contents and trace metal distribution. The monthly rainfall, rainy days with magnitude larger than 10 mm, larger than 20 mm and simple rainfall erosivity estimates were calculated for the Longxi catchment during 1957-2009, using data collected at the Changshou, Dianjiang and Liangping meteorological stations provided by the National Meteorological Center of China. Erosive rainfall events with low frequency and large magnitude in the rainy season contribute to a substantial proportion of annual total rainfall, which thus can be used to infer erosion and sediment yield events. The proposed innovative rainfall erosivity dating index was based on the integration of 137Cs chronology, absolute particle size distribution, mean sedimentation rate and erosive rainfall events at corresponding sediment mass depths. Major chronological events were identified using the rainfall erosivity dating index as; the base of the lake or native soil at a mass depth 43 (1956), the Great Leap Forward at mass depth 38.6 (1962), maximum 137Cs fallout at mass depth 37.4 (1963), the national strategic policies at mass depth 35-28 (1966 – 1976), the first major flood event and Industrial Revolution at mass depth 24.1 (1982), a second major flood event and introduction of in-lake fish farming at mass depth 15.4 (1989), a third flood event at mass depth 7.6 (1998) and the ban on in-lake fish farming at mass depth 4.0 during 2004 - 2005.(1) The sedimentary chronology was determined by comparing rainfall erosivity index with depth distribution of 137Cs and absolute particle size, which finally revealed a mean sedimentation rate of 1.64 cm a-1 during 1956 – 2013 and an annual sedimentation rate ranging from 1.1 to 2.3 cm a-1. During the initial period 1956 – 1981, a mean sedimentation rate of 1.25 cm a-1 was recorded. The highest annual sediment accumulation rate of 2.29 cm a-1 during 1982 – 1989 may be ascribed to the 1982 flood event. The historic human activity of natural vegetation disturbance and cultivation on hill slopes became the major sources of sediment accumulation as climate becomes more humid due to catastrophic events. The reduced mean sedimentation rate of 1.63 cm a-1 during 1990 – 2013 can be attributed to soil and water conservation and reforestation policies implemented by the central and local government in the Longxi catchment and Yangtze River basin to control the massive sedimentation problem in this region. The multi-proxy dating index and variation of sedimentation rate divided the sediment profile into three major periods namely reference period, stressed period and present period. The reference period (First period: 1956 - 1982) displays low variability of TOC, TN and trace metal concentrations. The major human activities in Longxi catchment during this period included, economic recovery after the foundation of the People’s Republic of China in 1949 and the Great Leap Forward (1958 - 1962) characterized by widespread iron smelting, agricultural policy encouraged cultivation on steep slopes through terrace formation during the national strategic policies (1966 - 1976) to protect soil fertility. In the stressed period (Second period: 1982 - 1998) industrial and sewerage discharge led to input and deposition of TOC, TN and trace metal concentrations (e.g; Cd, Co, Cu, Cr and Ni). Conversion of agricultural land to rural settlements in the TGRA, household responsibility system during late 1970s, manure and fertilizer usage for in-lake commercial cage fish farming, household and industrial sewerage and agricultural field runoff from the catchment was responsible human activities for the stressed condition at Changshou Lake. In the present period (Third period: 1998 - 2014) increased TOC, TN and decreased trace metals concentrations in the top layers of the sediment core indicated changes in lake ecology. Fish farming promoted algal growth and primary productivity which consequently caused eutrophication until 2004 - 2005. The introduction of in-lake commercial cage fish farming during the late 1980s was associated with excessive use of manure and chemical fertilizer until the late 1990s in Changshou Lake that increased the fish yield multiple times.(2) Increasing trends in TOC and TN were specifically caused by major human activity of in-lake commercial cage fish farming after 1989. The statistically significant correlation between 210Pbex activity, TOC (0.61, p=0.04) and TN (0.51, p=0.04), respectively explained post - 1989 210Pb scavenging. The 210Pbex activity was closely related with coupled peaks of TOC and TN from mass depth 5 - 10 g cm-2. Higher TOC/TN ratio (8.33) indicated submerged macrophytes and native aquatic algal growth as main source of carbon from enhanced primary productivity because of massive fertilizer usage for in-lake fish farming and coherent climate warming. The study supported key hypothesis on vital role of fertilizer usage for in-lake commercial cage fish farming as polluting human activity and algal derived TOC in controlling sedimentary 210Pbex activity at Changshou Lake sediment. 137Cs profile and erosive events as time markers provided reliable and consistent sedimentation rate of (1.64 cm a-1). 210Pbex activity decayed exponentially after peak at mass depth 5.68 g cm-2. 210Pb dating model constant rate of supply provided relatively older age of 1926 for the native river bed at Changshou reservoir, and estimated an average sedimentation rate of 0.09 cm a-1. The 210Pb CRS dates and the estimated sedimentation rate were not in accordance with average sedimentation rate of 1.64 cm a-1 using reliable 137Cs profile. Therefore, violation of 210Pb dating primary assumptions made it inappropriate for sediment dating at Changshou Lake. TOC content must be considered while using 210Pb as dating tool for lake sediment profiles.(3) The age and sedimentation rate estimates using Lead-210 (210Pb) Constant Rate of Supply (CRS) and Constant Initial Concentration (CIC) dating models for Changshou lake sediment core were compared. Whereas, historic records such as Cesium-137 (137Cs), Carbon, Nitrogen signals from fertilizer usage and rainfall erosivity are used as multiple proxies of reliable sediment profile dates. The 210Pb chronologies using CIC model at Changshou Lake validated the average sedimentation rate of (1.65 cm a-1) and reasonable age estimates of 1957 for the lake bed and is validated by 137Cs estimates. Whereas, CRS model estimated relatively older age of 1926 for the Changshou lake native river bed and misleading average sedimentation rate of (0.09 cm a-1) due to possible turbidity flows or preferential association of 210Pb with organic matter.Overall, the dated sedimentary profile from Changshou Lake displays high consistency with archived historical events and reflects the impacts of both natural and anthropogenic controls on sediment production. Human activities such as, deforestation, national strategic policies, industrial revolution and lake eutrophication associated with in-lake commercial cage fish farming since 1989 led to severe limnological variations. 210Pb CIC dating model provided reliable sedimentation rate and age estimates as compared with 137Cs and proposed dating index, whereas 210Pb CRS model was not suitable for sediment dating at Changshou Lake. Available literature identified research gap on impacts of human activities on sedimentation dynamics, chemical and nutrient compositions in the limnological sedimentary record. In order to fill the research gap this research contributes to the available literature on formulating an innovative dating index that logically reconstructed the historic natural events and anthropogenic activities in the Longxi catchment and identified their impact on sediment composition at Changshou Lake through analyzing the variations in multiple proxies of the sediment profile. 
Document Type学位论文
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RAHEEL ANJUM. Impacts of recent human activities on sedimentation rates at Longxi catchment[D]. 北京. 中国科学院大学,2018.
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