环境地球化学国家重点实验室知识库

Karst depressions represent important sinks for sediment, and such sediment can provide a valuable record of the impact of environmental change on soil erosion rates. However, the sediment dynamics of karst depressions are not well understood. This contribution reports a study of the small catchment of a karst depression in Southwest China, with a drainage area of 0.054 km2 , aimed at using the sediment deposits in the depression to reconstruct the erosional response of the catchment to land-use change and, more particularly, the deforestation which took place in 1979. 137Cs and 210Pbex are used as both chronometers and as tracers. Five cores, collected from the bottom of a depression, with an area of 2652 m2 , showed similar 137Cs depth distributions, with a single 137Cs peak, which was attributed to the 1979 deforestation. The 137Cs activity associated with the peaks varied between 5.68
0.64 and 9.19
0.99 Bq kg–1 . The average depth of sediment deposition between 1979 and 2008 deduced from the depths of the 1979 137Cs peak was 74.1 cm. The existence of relatively high 210Pbex activity of 66.33
8.44 Bq kg–1 in the upper section (0–16 cm) of the core analyzed for 210Pbex suggests that recent sedimentation has been very limited. Net erosion rates on the hillslopes contributing runoff and sediment t to the depression were estimated to be 5258 t km–2 year–1 from 1979 to 1990 and 256 t km–2 year–1 from 1991 to 2008, respectively. The high sediment yield in the first period was associated with the severe soil erosion triggered by the 1979 deforestation, which resulted from the changes in land ownership immediately after the Cultural Revolution. Soil erosion has been very limited since 1990 because the thin soils had been totally removed from many parts of the karst slopes and the soils remaining on the other parts of the slopes have been protected by terracing or vegetation rehabilitation.