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Abstract: A 76cm sediment core was collected from the Huangdong Reservoir (southeastern China). The radionuclides 137Cs and 210Pb and sediment grain size analysis were applied to establish the chronology and modern sedimentation rates. The chronology established by the 137Cs time markers was consistent with the Constant Flux Constant Sedimentation (CFCS) model and the known time of dam construction (1960AD) recognized from the curve of sediment grain size at the depth of 54cm. The results show that the sedimentation rate of the Huangdong Reservoir is relatively high, implying that soil erosion and water loss were strong. The sedimentation rate before the construction of the reservoir was approximately 1.5cm/a, which belongs to the fluvial flooding deposit. The rate decreased to approximately 1.0cm/a during the reservoir deposition (since 1960s). Pollen and micro-charcoal analysis were performed to test the reliability of pollen-based environment reconstruction, including vegetation cover, deforestation, planting and fire disasters. The results of pollen and micro-charcoal indicated that the herbs (primarily Gramineae), ferns (such as Dicranopteris) and several trees (mainly Pinus) varied following chorography or historical records. We discovered that the most important indicators for local forest-cover changes are pollen concentration and the AP/NAP value (ratio of arboreal pollen to non-arboreal pollen). Human impact on forests is manifested by a sharp decrease in pollen percentage of trees and absolute pollen concentration. Forest fire events were recorded by the peaks of charcoal concentration and the C/P value (ratio of charcoal concentration to pollen concentration). For example, the massive deforestation and intensive fires during the Great Leap Forward movement and the Cultural Revolution are reflected by pollen and micro-charcoal spectra. This work testifies that radionuclide use as a dating method is applicable in southern China, and the 137Cs method has demonstrated its reliability in the small reservoir setting in spite of low absolute values. The environmental proxies, such as grain size, pollen, and charcoal, are sufficiently sensitive to reveal local ecosystem changes caused by natural disasters or human activity. [Copyright &y& Elsevier]

Abstract: Eastern subtropical China is a key region for understanding the variability of the East Asian Summer Monsoon (EASM). Multidisciplinary studies in southeastern China have shown that the summer monsoon intensity declined in the mid-late Holocene. We present a high-resolution pollen record of the last 4000calyr BP in Jinggang Mts of Jiangxi Province, southeastern China. The identified pollen taxa from the core can be statistically divided into three groups corresponding to evergreen, deciduous and wetland communities. The transitions between evergreen and deciduous-coniferous pollen associations is likely caused by temperature fluctuations, indicating that climate was relatively cool at 3800–3200calyr BP and 2200–1300calyr BP and warmer at 3200–2800calyr BP and 1300–800calyr BP. The vegetation study suggests that an Alnus-dominant association represents a secondary forest that usually takes place and expand after repeated forest fires. The charcoal concentration from the core depicts at least six major forest fire events since 4000calyr BP, most of which were followed by the development of an Alnus forest community. This result suggests that the EASM weakened toward the late Holocene and that its related decrease in moisture led to large forest wildfires. Furthermore, the rapid formation of a swamp and the subsequent development of the Alnus wetland community at ~550calyr BP suggest a gradual drying up of the lake, which was likely related to “the Little Ice Age”. As a result of a substantial burning related to an intensification of the human cultivation practices, Alnus reached its highest values in the last 200years along with abundant wetland herbs, pioneer ferns (mainly Dicranopteris) and high charcoal concentrations. The present evidence of several sharp floristic and climate changes coincides with either the collapse or the beginning of some Chinese dynasties, which will need further research on the relationship between natural and human cultural changes. [Copyright &y& Elsevier]