137Cs、HCHs测定岩溶洼地沉积物年代及流域产沙强度

以青木关岩溶槽谷区内代表性耕地型与林地型洼地小流域为研究对象,从洼地沉积物入手,探明各洼地沉积物剖面~(137)Cs比活度、六六六(HCHs)、有机质、黏粒含量及容重的深度分布特征;运用~(137)Cs示踪法,辅以HCHs进行沉积物断代,追溯流域近60年来的土壤侵蚀量演变特征及探讨其驱动因素。结果表明:(1)炮台院子耕地型洼地沉积物剖面~(137)Cs、HCHs仅有个别层位检出,无法利用~(137)Cs、HCHs深度分布进行沉积物断代,反映了发育的落水洞对其沉积物剖面影响大。(2)龙洞槽耕地型和劳动村林地型洼地小流域1963—1983,1984—2019年产沙模数分别为231.78,82.04 t/(km~2·a)和68.79,39.46 t/(km~2·a),表明流域生态环境得到明显改善。与1963—1983年时段相比,该地区1984—2019年时段年平均降水量无明显变化,2个小流域产沙模数均显著减小,表明近60年流域产沙强度主要受土地利用方式、水土保持措施等人类活动控制。(3)龙洞槽洼地剖面~(137)Cs、HCHs峰值,表层泥沙~(137)Cs、HCHs数值和不同时期流域产沙模数均明显大于劳动村洼地,主要是由2个洼地分别控制的耕地型与林地型小流域在人类活动影响下产沙强度的差异所致。另外,~(137)Cs和HCHs相结合的示踪方法可较好地用于评估西南岩溶流域产沙量的时间变化。

Determination of Sediment Age and Sediment Yield Intensity Using 137Cs and HCHs Dating in Karst Depressions

We take the representative farmland-type and woodland-type depression small watersheds in the Qingmuguan karst valley region as the research object, and start from the depression sediments, the depth distribution characteristics of ¹³⁷Cs specific activity, HCHs, organic matter, clay content, and bulk density of the sediments in each depression profiles were studied. ¹³⁷Cs tracer method and HCHs were applied to sediment dating, and the evolution characteristics of soil erosion in karst depression small watershed in the past 60 years was traced, and its driving factors were discussed. The results show that:(1) Only a few sediment profiles had detected ¹³⁷Cs and HCHs in the farmland depressions of Fort Yard. The depth distribution of ¹³⁷Cs and HCHs could not be used for sediment dating, which reflected that the developed sinkhole had a great impact on the sediment profile. (2) The sand production modulus of the Longdongcao farmland depression in 1963-1983 and 1984-2019 were 231.78 and 82.04 t/(km²·a), respectively, while those values in Laodong village woodland depression small watershed were 68.79 and 39.46 t/(km²·a), respectively, indicating that the ecological environment of the basin has been significantly improved. Compared with 1963-1983, there was no significant change in the average annual precipitation in the region from 1984 to 2019. The sand yield modulus of the two small watersheds both decreased significantly, indicating that the sand yield intensity of the basin in the past 60 years was mainly controlled by land use, water and soil conservation measures and other human activities. (3) The peak value of ¹³⁷Cs and HCHs of profile, surface sediment ¹³⁷Cs, HCHs values and watershed sand yield modulus in different periods of Longdongcao depression were significantly greater than those of the Laodong village depression. It was mainly caused by the difference in sand production intensity under the influence of human activities, which conducted in the cultivated land and woodland type small watershed of the two depressions. In addition, the combined tracer method of ¹³⁷Cs and HCHs could be used to assess the temporal changes of sediment yield in the southwestern karst basin.