Climate Change And Human Activities Indicated By The Sediments Of Yilong Lake

Yilong Lake is situated in the low latitude plateau region influenced by the southeast and southwest monsoon.Moreover, Yilong Lake is is fundamental for the survival of Shiping County and play a very important role to the local people. Due to its sensitivity to environmental change and human activities, Yilong Lake is an ideal place for the study of sediment records which reflect the past environmental evolution and human activities. Based on above, this study select Yilong Lake as the research object. The sediment core YLH-1 from Yilong Lake was testing indicators of grain-size, carbonate, organic, the stable isotopes of carbon, nitrogen and C/N ratios. Accurate sediment chronology was established by 210 Pb. By comparing the proxies to document and meteorological data, we discussed the environmental evolution and influence of human activities on depositional environment since 1880. And the study in this article has the main conclusions are as follows:(1) The change of proxy climate indicator recorded of Yilong Lake sediment can be used to indicate the environmental evolution and influence of human activities on depositional environment since 1880. According to analysis of proxy climate indicator, we know that the carbonate content and average of median grain-size is higher during the period of 1880~1936a, it is showed the warm climate period. The curve of TOC content is low and flat, the δ13C value is partial positive, the value of δ15N is low, these changes of proxy all conform that the impact on the environment caused by human activity is less. The influence factors of lake environmental change is given priority to natural factors, and this conclusion also be supported by historical record. Therefore, it can be thought that the influence of human activities on the lake basin is limited in this period, and the proxy indicator of lake sediments can effectively indicate climate changes about temperature and precipitation in Yilong lake basin. The lake has been beginning to affect by human activities after the 1950 s, especially in the 1970 s, the beginning of large-scale human activity for turning lakes into fields caused that lake’s nutrition was changed, carbonate content was decreasing and organic matter content was increasing. The climate change showed by the proxy of carbonate and organics can not consistent with the meteorological records any more.(2) From 1950 s to 1990 s, the lake sediment’s CaCO3 content decreased significantly, but the TOC content increased, it may be due to the effect of human activities. Because of the lake organic content increasing, the content of CO2 which released by organic decay also will increase. This process reduces the water’s pH value, and makes part of carbonate mineral dissolution again. This change may be a response for two times large-scale activity of turning lakes into fields at Yilong lake in 1950 s and 1970 s. With the environment of lake basin becoming worse gradually, it was draw attention to concerned department. Some organizations have begun to adopt a series of comprehensive measures to improve the local ecology. From the records of sediment, we can see that the value of median size was low and the content of clay particle increased in this period. C/N value also has a tendency to decrease during this period, showed that the exogenous input of organic matter reduced, but endogenous organize were advantage. These changes response to take measure to close the land for reforestation and to turn lakes into fields.(3) From the change of lake sediment’s index, which can be roughly divided into three stages: The first stage(1880-1936a) showed that is given priority to climate characteristics with warm and wet. Water power of lake basin is larger, rainfall and temperatures increased, the lake surface was extend. The human activities affected on the lake sediment was less in this stage; The second phase(1936-1936 AD), more reflect severe human activity at Yilong lake basin. This period of sediment’s CaCO3 content decreased significantly, but the TOC content increased, it apparent negative correlation. The reason is attributable to human activity intensifies, and changed the normal lacustrine sediments, this also shows that sediment records of this period have not reflect basin climate change; The third stage(1996-2013 AD) reflected that the extent of human activity was reduced, and the ecological environment of the Yilong lake has been improved. It is showed that the CaCO3 content and TOC content have no obviously negative correlation, and the sand content in sediment is reduced.