人类活动影响下的固城湖环境变迁

2005年在江苏固城湖湖心采集了连续岩芯,进行了多环境代用指标(有机碳、氮、磷以及金属元素)的分析.210Pb CRS模式计算表明,1980s后沉积速率平均约0.067 cm/a,与137Cs 1986年时标得到的沉积速率吻合.而1920s至1980s,固城湖沉积速率变化较大,在0.056-0.167cm/a之间,其中1960s沉积速率最高,对应于该阶段固城湖强烈的人类围垦活动.元素Cu、Pb、Zn、Cr的含量在表层增长迅速,对比参考元素的变化,表明固城湖受到了一定程度的人为造成的重金属污染.岩芯0.5m到1.78m深度的AMS 14C的树轮校正年龄在8000-6500BC之间,光释光数据表明同等深度其绝对年龄在7-8ka之间,多环境代用指标在岩芯30cm深度左右出现了大的转折,表明近代存在沉积物缺失的可能。这也许与该区春秋末期以来的反复围垦活动有关,人类活动改变了湖泊沉积的模式.有资料表明,固城湖彻底与太湖隔绝是在公元1556年胥溪河河段下坝建成之后.若依此时间对应于岩芯30cm深度,得到的平均沉积速率为0.067cm/a,与1980s以来的沉积速率一致,有可能指示固城湖至下坝建成以来才出现沉积物的持续沉积.

Environment changes influenced by human activities at Lake Gucheng

Sedimentary cores were taken from the center part of Lake Gucheng, Jiangsu in 2005. The total organic carbon, total nitrogen, phosphorus, metals were analyzed. Calculations of sedimentation rates were achieved using 210Pb CRS method. The results demonstrated that average sedimentation was 0.067 cm/a after 1980s corresponding to that from Chernobyl accident marker occurred at 1986. During the period of 1920s and 1980s, the sedimentation rates varied between 0.056 and 0.167cm/a. The high sedimentation rates found in 1960s may be associated with the intense reclamation since 1960s. Cu, Pb, Zn, Cr showed up-core increase in the top 30 cm sediment. Combined with the ratio of these elements to Ti it was concluded that the origin of Cu, Pb, Zn, Cr was not wholly from natural processes. The age was between 6500-8000a BC based on AMS 14C results from depth 0.5 m to 1.78 m, which was comparable with OSL age of 7-8 ka from the same depth. Sharp transition of index including TOC, TN and metals emerged at depth of 0.3 m may indicate that sediments could be eliminated due to erosion or human impact such as cultivation. It was reported that Lake Gucheng was isolated from Lake Taihu due to establishment of Xiaba at 1556 A.D. Taken the depth of 30 cm as the time when Xiaba was established, the average sedimentation rate was calculated to be 0.067cm/a which was in agreement with the results of 210Pb and 137Cs method. Therefore, continual deposition may occur after the isolation of Lake Gucheng from Lake Taihu.