猪野泽记录的季风边缘区全新世中期 气候环境演化历史*

通过季风边缘区石羊河古终端湖猪野泽QTL剖面年代学及沉积物粒度、碳酸盐、有机碳、碳氮比和有机质稳定同位素等多项气候代用指标的综合分析,建立了季风边缘区9~3cal.kaB.P. 的古气候演化序列。结果表明,9cal.kaB.P. 到7.8cal.kaB.P. 期间,流域的水分条件和温度逐渐上升,植被状况好转,此时气候逐渐转暖湿;而在7.8~7.5cal.kaB.P. 出现了显著的百年尺度的干旱事件,沉积物主要以砂质沉积为主,此时湖泊生产力显著下降;全新世期间最为暖湿的气候适宜期出现在7.5~5.0cal.kaB.P.;约5.0cal.kaB.P. 以来,该区域出现了较为明显的干旱化趋势。另外,对猪野泽地区的白碱湖的湖泊地貌学和年代学研究表明该区域在7.5~5.0cal.kaB.P. 出现了3次高湖面,并且湖岸堤时序变化指示了全新世后半期湖泊逐渐退缩的过程进而指示该区域出现了显著的干旱化趋势。

MID-HOLOCENE CLIMATE VARIATIONS FROM LAKE RECORDS OF THE EAST ASIAN MONSOON MARGIN: A MULTI-PROXY AND GEOMORPHOLOGICAL STUDY

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 proxies fluctuated in pace. Three shorelines occurred in the time of 7.6~6.6cal.kaB.P. , 6.3~5.9cal.kaB.P. , and 5.17cal.kaB.P. , when the climate proxies especially the C/N values culminated. (3) The environmental indicators reveal a general trend towards aridity since 5.0cal.kaB.P. , which is implied by the shrinking of the Zhuyeze Lake characterized by the development of four shorelines in the time of 5.2cal.kaB.P. , 4.0cal.kaB.P. , 2.4~1.8cal.kaB.P. , and 1.5cal.kaB.P. , respectively.More and more high-resolution, precisely dated, and continuous records of Asian monsoon during the Holocene had been reported in China. We compared our study on the Zhuyeze Lake with other Holocene climate records, including those from the Daihai Lake, the Qinghai Lake, the Hongyuan peat from the Qinghai-Tibet Plateau, the Shanbao cave stalagmite from Shennongjia, Dongge cave stalagmite from Guizhou, and the Zigetang Lake. The comparison of these records suggests a Holocene climate optimum during the first half of Holocene, though the duration and amplitude of the optimum period, as well as its starting and end time, differed a little.A step-wise trend toward aridity during the second half of Holocene is also implied by the comparison. Moreover, the comparison of the climate changes referred in this paper with the summer insolation at 30°N implied that the climate changes in Asian monsoon areas during the Holocene broadly followed summer insolation. The general trend toward aridity since mid-Holocene was found to correspond with orbitally induced lowering of the Northern Hemisphere summer solar insolation during this interval, which indicates that the broad trend is caused by insolation change. However, the insolation change does not account for the short-term dry period recorded in QTL section, occurring in the interval of 7.8~7.5cal.kaB.P. The dry event is also recorded in the Haxi loess-palaeosoil section of Shiyang upriver region, Jiuduntan section of Shiyang midriver, indicating a regional dry event of centennial-scale.