MID-HOLOCENE CLIMATE VARIATIONS FROM LAKE RECORDS OF THE EAST ASIAN MONSOON MARGIN: A MULTI-PROXY AND GEOMORPHOLOGICAL STUDY
猪野泽记录的季风边缘区全新世中期 气候环境演化历史*

The East Asian monsoon marginal areas of China are very profit to study the past environment changes because of their sensitivities to climate changes. Therefore, these areas received intensive attention from palaeoclimatologists in recent years. There have been a large number of researches on the Holocene climatic variations in these areas as yet, but quite different opinions have been expressed as to the climatic conditions of the mid-Holocene. Traditionally, the mid-Holocene in most parts of China was thought to be warmer with higher precipitation, resulted from a strong Asian summer monsoon. However, some recent researches have proposed a mid-Holocene drought interval of millennial-scale in East Asian monsoon marginal areas. Thus whether mid-Holocene was dry or humid remains an open issue. Here, the Zhuyeze palaeolake, the terminal lake of the Shiyang drainage lying in Asian monsoon marginal area, was selected for reconstructing the details of climate variations during the Holocene, especially mid-Holocene, on the basis of a sedimentological and geomorphological analysis.QTL section of 6.92m depth was taken from the Zhuyeze palaeolake. Eleven samples from QTL section were collected for 14 C dating. The materials used for 14 C dating include organic carbon (bulk samples) from the organic-rich sediments or peat, and inorganic carbon from shells. Based on the dating results and the reservoir effect analysis, Multi-proxy analysis of QTL section, including grain size, carbonate, TOC, C/N, and δ13 C of organic matter, was used to document regional climatic changes during 9.0~3.0cal.kaB.P. The record showed a major environmental change at 9.0~7.8cal.kaB.P. , attributed to a climate trend towards warmth and humidity. This event was followed by a typical regional drought event which occurred during 7.8~7.5cal.kaB.P. A warm and humid climate prevailing from 7.5 to 5.0cal.kaB.P. , attributed to the warm/humid Holocene Optimum in this region is also detected. After that, the climate gradually became drier.Moreover, well-preserved lake geomorphological sequences such as terraces or shorelines are probably the best evidence for reconstructing palaeolake levels. Extensive field investigations on the Zhuyeze Lake identified one lake terrace and nine lake shorelines. These shoreline sediments are found to be rich in shells, which were collected from every shoreline for AMS age dating. The comparisons between sedimentological and geomorphological evidences reflecting climate changes during the Holocene indicated that: (1) The multi-proxy study indicated a warm/humid climate prevailing during 7.5 and 5.0cal.kaB.P. , which was evidenced by high water levels marked by the development of three shorelines in the time of 7.6~6.6cal.kaB.P. , 6.3~5.9cal.kaB.P. , and 5.17cal.kaB.P. , respectively. This period (7.5~5.0cal.kaB.P.) is attributed to the Holocene optimum in the studied region. (2) During the Holocene optimum, the sedimentological and geomorphological proxie