云南楚雄九龙甸水库沉积剖面^137Cs、^210Pbex和细粒泥沙含量的变化及其解译

Interpreting of variations of ^(137)Cs,^(210)Pb_(ex) and fine particle contents in a deposit profile of the Jiulongdian Reservoir,Chuxiong,Yunnan,China

除深度231~137cm的1963年137Cs蓄积峰外(4.26±0.35 Bq.kg-1),九龙甸水库长393cm的沉积剖面还有二个高137Cs或210Pbex含量的异常峰。上异常峰的137Cs和210Pbex含量分别为10.90±0.49 Bq.kg-1和59.20±3.4 Bq.kg-1;下异常峰仅210Pbex含量高43.40±6.4 Bq.kg-1。上、下异常峰分别和1998年春的森林火灾和1960年春的烧荒种地有关。下异常峰未有137Cs检出,可能和当时137Cs累积沉降量还较低有关。1958-1960年大跃进期间的森林砍伐和其后的植被恢复,对流域侵蚀产沙和水库淤积有重要的影响。由于1958-1959年森林砍伐引起了强烈的土壤侵蚀,水库淤积严重,同期的淤沙速率和相应的淤沙模数分别高达83.86×104t.a-1和3255.4 t.km2.a-1。随着森林砍伐的停止和植被的恢复,流域内土壤侵蚀逐渐减缓。1960-1962年期间,水库淤沙速率和相应的淤沙模数分别降至40.88×104t.a-1和1587.0 t.km2.a-1;1963年后,更分别降至6.19~7.73×104t.a-1和240.2~291.5 t.km2.a-1。沉积泥沙粒度的变化对流域植被的破坏和恢复有较好的响应。大跃进期间森林砍伐引起的强烈侵蚀产沙,导致剖面下部泥沙粒度较粗;随着植被逐渐恢复,侵蚀减弱,剖面上部泥沙粒度变细。

Except for the 1963＇ s ^137 Cs peak of 4.26 ± 0.35Bq·kg^-1 at the depth of 231 - 237cm in depth, a deposit profile with a length of 393cm in the Jiulongdian Reservoir has other two unusual ^137Cs and ^210Pbex peaks： the upper peak at the depth of 15 -21cm, which has a ^137Cs concentration of 10.90± 0.49Bq·kg^-1 and a ^210Pbex concentration of 59.20± 3.4Bq·kg^-1, respectively; and the lower peak at the depth of 231 - 237cm, which has only a high ^210Pbex concentration of 43.40 ± 6.4Bq·kg^-1 The upper peak was related to the 1998＇ s forest and shrub fire while the lower oeak to the 1960＇ s fire of reclaiming land for cultivation. No ^137 Cs is detected in the lower peak layer, probably because little ^137 Cs fallout had deposited on the earth surface yet. Timber harvesting during the Great Leap Forward period of 1958 - 1959 and following vegetation rehabilitation has great effects on soil erosion in the catchment and 1958- 1959, the sediment deposition in the reservoir. Due to severe soil erosion caused by timber harvesting in sediment deposit rate in the reservoir and the corresponding specific sediment yield for the catchment were very high： 83.86 × 10^4 t·a^-1 and 255.4 t·km^2 · a^-1, respectively. Since then, soil erosion has become lightening because of stopping of timber harvesting and natural vegetation recovery. The sediment deposit rate in the reservoir and the corresponding specific sediment yield for the catchment decreased to 40.88 × 10^4 t·a^-1 and 1587.0 t·km^2 · a^-1 in 1960 - 1962 respectively, and to 6.19 - 7.73 × 10^4 t · a^-1 and 240.2 - 291.5 t· km^2 · a^-1 since 1963 respectively. Particle size variations of deposited sediments in the reservoir were response to the changes of soil erosion behaviors and severities well. Coarse sediments in the lower part of the profile is related to the severe soil erosion in the period of 1958 - 1959 and following a few years. However, sediments in the upper part of the profile have become finer because of soil erosion reduction due to vegetation rehabilitation by stopping timber harvesting since 1960.