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Pollen and charcoal records of a 150 cm long lake sediment core from Taibai Lake in the middle reach of the Yangtze River reveal seven major changes in regional vegetation over the last 1500 years. During the period c. ad 480–710, evergreen broadleaved forest dominated by Castanopsis/Lithocarpus and evergreen oaks occurred in the Taibai Lake catchment. From c. ad 710 to 1050, the vegetation shifted to a mixed conifer and broadleaved forest, with Pinus expanding at the expense of Castanopsis/Lithocarpus. Between c. ad 1050 and 1320, evergreen broadleaved forest reoccupied the studied area. From c. ad 1320 to 1650, the area of primary forest decreased markedly with synchronous reduction in broadleaved trees and Pinus. Between c. ad 1650 and 1740, the biomass declined rapidly, while secondary Pinus forest began to expand. During c. ad 1740–1950, forest extent increased slightly compared with the previous stage, but the landscape was still secondary forest with the minimum proportion of broadleaved trees. After c. ad 1950, the biomass in the surrounding area was very low, with vegetation types similar to that at present (secondary Pinus forest and mixed conifer and broadleaved forest). A detrended correspondence analysis (DCA) with 125 samples and 34 main pollen types is used to separate human and climatic impacts on vegetation. Then, the pollen assemblage, DCA, charcoal record, and magnetic susceptibility are combined to discuss the key factors inducing these vegetation changes. The vegetation changes were mainly controlled by the climatic changes, with the weak impacts of human activities before c. ad 1320. Since then, the intensity of human influences on vegetation increased gradually, entering a transitional period of main controlling factors of vegetation changes from nature to human activities. After c. ad 1740, the vegetation changes were chiefly controlled by human activities, and the climatic signal was weak. [ABSTRACT FROM AUTHOR]

Dating lake sediment using sedimentary event is the supplement and calibration to traditional dating by radionuclide such as 210Pb and 137Cs. Based on the change of spheroidal carbonaceous particle (SCP) concentration, the age sequence of lake sediments can be deduced. It is one of the dating methods using sedimentary event. SCP is formed from combustion of fossil fuel at high temperature up to 1750°C and at a rate of heating of approaching 104 °C/s. It can be dispersed to several hundred kilometers away from its source and deposited with precipitation or dryly deposited, and kept in sediments. Compared with Cs or Pb, there is no evidence for SCP that it decays in lake sediments and is un-removable once stored except by physical disturbance because it is mainly composed of element carbon. Handy method to extract, identify and calculate has been developed. Although fossil fuel has been used early in China, combustion at high temperature started later since emergence of electricity generation. The productivity of SCP is positively related with the generated thermal power, which is reflected as the SCP concentration in lake sediments increases with the increase of generated thermal power. Therefore, reliable sediment markers from the start of the SCP record and the remarkable variation can be used for dating purpose. In China, electricity industry started from the 1950s, and rapid increase of generated power took place since 1978. Based on these time markers, SCP time sequences of lake sediment cores LH and LL-4 from Longgan Lake, the middle reach of the Yangtze River, have been established, which is comparable with the results from 137Cs and 210Pb, and has eliminated the errors of dating using 137Cs and 210Pb. [ABSTRACT FROM AUTHOR]

Abstract: Element content, grain size, pollen and 210Pb dating analysis were performed on 80 cm sediment core from Taibai Lake, a shallow lake in the middle reach of Yangtze River in China, to reveal the response of catchment environment to climate changes and human activities during the past four centuries. The 210Pb dating suggests that 0–39.5 cm of the core represents the past 163 years and bottom of the core is at ca. 1590 AD. Major changes showed in the core are compared with the records from historical documents, including population, cultivation and climate etc. 1700–1810 AD was the period with intensive human activities in Taibai Lake catchment, behaving as deforestation and cereal cultivation expansion, signaled in the sediments by low percentage of pinus pollen and high percentage of Gramineae pollen, high content of <16 μm fraction. Another period with intensive human activities was since 1928 AD, behaving as deforestation, reservoir construction, land reclamation, soil loss. The high sedimentation flux epochs, such as 1900–1920 AD, 1931 AD, 1938–1939 AD and 1954 AD, were correlated with high precipitation; nevertheless, the high sedimentation flux epochs in 1958–1970 AD and 1983–1993 AD were correlated with the land reclamation around Taibai Lake and soil loss due to intensive cultivation development respectively. The two coldest stages during the “Little Ice Age” were in 1650–1700 AD and 1810–1900 AD, consistent with the historical records generally, characterized by weak weathering, high forest coverage and weak human activities in Taibai Lake catchment. [Copyright &y& Elsevier]