黔中喀斯特丘原区小河水库沉积物的 矿物磁性特征及其土壤侵蚀意义 *

对贵州中部小河水库的沉积物柱芯XS-1进行矿物磁性分析(χ_lf,ARM,IRM_20mT,SIRM 和IRM_-300mT)并计算了HIRM,χ_ARM 和χ_ARM/SIRM。主要根据沉积物的矿物磁性特征, 将这一岩芯的主要部分划分为11个小段。 通过解译这些矿物磁性参数,结合完成的¹³⁷ Cs、粒度、TOC和C/N比值分析结果,定性地推测了过去45年(1960~2005年)小河水库所在汇水流域内的土壤侵蚀变化。在此基础上,通过对各阶段平均磁性参数的主成分分析,得到各阶段土壤侵蚀的相对强度变化的定量信息,并结合人类活动和降水资料初步探讨了影响土壤侵蚀的相对强度变化的主要因素。结果表明,1960~1968年、1973~1974 年和1998~2000 年是小河流域内土壤侵蚀最为严重的3个阶段。1960~1968年人类砍伐树木是造成该阶段土壤侵蚀强烈的主要原因,而1973~1974年和1998~2000年流域内土壤侵蚀强烈则是由于降水的偏多造成的。1968年以来,夏季降水是影响土壤侵蚀变化的主要因素。

MINERAL-MAGNETIC CHARACTERISTICS OF SEDIMENTS FROM XIAOHE RESERVOIR IN KARST HILLY PLAIN, CENTRAL GUIZHOU PROVINCE AND THEIR IMPLICATIONS ON SOIL EROSION

Soil erosion as well as the resultant 'rocky desertification’ is a serious environmental-ecological problem in the southwest part of China dominated with carbonate-rocks and karst landscapes. Analysis of mineral magnetism of sediments from lakes or reservoirs has been proved to be a valuable approach to investigations on soil erosion occurring in catchments where these natural or artificial water-bodies are located and has thus been increasingly applied. The Xiaohe Reservoir (26.57°N,106.40°E, 1237m a.s.l.) was built in the central part of Guizhou Province, Southwest China. The catchment of the reservoir is a relatively developed area in the karst hilly plain. A 32-cm long sediment-core (named 'XS-1’) was sampled from the reservoir. Some mineral magnetic parameters (χ_lf, ARM, IRM_20mT, SIRM, and IRM_-300mT) were measured and HIRM, χ_ARM, and χ_ARM/SIRM are further calculated. The part of this core above the level of 29cm covering the period of 1960~2005 was divided into 11 intervals in the term of mainly variations in mineral-magnetism. Soil-erosion occurring during the past 45 years (1960~2005) in the drainage basin is qualitatively inferred by interpreting the mineral-magnetic characteristics in combination with analyses of ¹³⁷ Cs, grain size, TOC, C/N, as well as depositional rates calculated. On this basis, the quantitative information on relative intensity of soil erosion in each of the 11 corresponding phases was extracted by the principal component analysis from the data of magnetic parameters. The primary causes of the inferred changes in the relative intensity of soil erosion are deduced by human activity records and rainfall data. During 1960~1968, 1973~1974, and 1998~2000, soil erosion was intensive in this catchment. The mass deforestation might have mainly caused the serious erosion during 1960~1968 while the intense erosion occurring during 1973~1974 and 1998~2000 may be attributed largely to abundant precipitation. In particular, variations in summer precipitation may have controlled the fluctuations in the relative intensity of soil erosion since 1968 in this catchment. These results, though still remaining to be refined, may be significant for further investigation of soil erosion and soil and water conservation for the same or similar small drainage basins in karst hilly plain regions.