| 其他摘要 | The objective of this study is to combine carbon and nitrogen isotopes, coupled with 210Pb dating, 14C dating, TOC, TN, C/N ratio and TP, to discuss the evolution process of the recent lacustrine environment and the origin of organic matters in the sediments of Taihu Lake, Chaohu Lake and Longgan Lake, and to investigate the paleoenvironmental changes of Taihu Lake. The main conclusions are summarized as follow:
1. The values of δ13Corg and C/N ratios in Taihu Lake sediments indicate that the organic matters are mainly derived from algae. No terrestrial organic matters were inputted to Zhushan Bay (T3 core), but some to Meiliang Bay (T2 core). The sedimentary organic matters in the center location of Taihu Lake (T4 core) are partly derived from hydrophyte that is different species from Zhushan Bay and Meiliang Bay. The sediment profiles show that after 1920s, the environmental changes are related to human activity.
2. The profiles of δ13Corg and δ15N in Chaohu Lake sediments suggest that the sedimentary organic matters are derived mainly from aquatic algae and partly from city and agricultural pollutants. The construction of Chaohu Gate has a serious effect on lake productivity and hydrophyte species, and the development of eutrophication.
3. The values of δ13Corg and δ15N in Longgan Lake sediments show that the sedimentary organic matters are derived mainly from native aquatic organic matters. Less land organic matters and city pollutants are inputted to this lake. In 1960s, the inning around Longgan Lake caused the increasing of nutrients input. Because lake grass can help the accumulation of nutrients, and decelerate the degree of eutrophication.
4. The difference of NO3－ and SO42- content in the surface porewater among Taihu Lake, Chaohu Lake and Longgan Lake sediments is related to the lakes pollution status, abundance of organic matters, hydrodynamic conditions, accumulation and release of nitrogen and even to the difference of productivity between lake algae and grass.
5. The NH4+-N content in the surface sediments reflects a different pollution status and organic abundance between the lakes, Meiliang Bay in Taihu Lake being more impacted than Chaohu Lake and Longgan Lake. The NH4+-N content in the surface sediments of the three lakes suggests a potential transference from sediments to the above water.
6. Regular datum curves of sediment particle size, δ13Corg, δ15N, TOC, TN, C/N ratio, TP content can be found in the sedimentary profiles of Zhushan Bay and Meiliang Bay in the western Taihu Lake, corresponding to carbon dating. The results indicate three major periods in the historical evolvement and the organic matters origin in each period. From 6870 a B.P. to 6670 a B.P., the Taihu area was covered by salt water. The organic matters were derived from an autochthonous source in this area, but Zhushan Bay was characterized by an increasing input of C4 Plants. From 6670 a B.P. to 5140 a B.P., a lagoon probably appeared and a sedimentation hiatus occurred. Since 5140 a B.P., Taihu Lake is totally a freshwater lake. Organic matters in the sediments have mainly an autochthonous origin. Also a hiatus can be estimated in the sedimentation rate.
7. The organic matter in the freshwater lakes of the middle and lower reaches of the Yangtze River has mainly an autochthonous origin. However, the input of terrestrial organic matter from human activity into the lakes could not be neglected, because the increase of lakes productivity is affected by the input of terrestrial organic matter. Due to the distance to the shore, hydraulic status and distinct hydrophytes, a difference exists among the recent sedimentary evolution of the middle and lower reaches of the Yangtze River. But the main reason for the lakes eutrophication is the environmental change related to human activity. |
修改评论