内蒙古达里湖全新世有机碳氮同位素记录与环境演变

HOLOCENE ENVIRONMENT VARIATIONS RECORDED BY STABLE CARBON AND NITROGEN ISOTOPES OF SEDIMENTARY ORGANIC MATTER FROM DALI LAKE IN INNER MONGOLIA

内蒙古中东部的达里湖为一水文封闭型湖泊,位于现今东亚夏季风的北部边缘区,对区域环境变化十分敏感.本文对达里湖沉积中心提取的岩芯（DL04沉积岩芯）顶部8.5m沉积物进行了有机地球化学分析.15个全岩样品有机质的放射性碳测年结果表明：岩芯顶部8.5m涵盖了过去大约11500年.按约50年分辨率分析的225个总有机碳（TOC）、总氮（TN）含量、总有机碳/总氮（C/N）原子比值数据以及221个有机碳和有机氮稳定同位素（δ13C和δ13C）数据,详细揭示了全新世东亚夏季风边缘区的水文和生态变化历史.在距今11500 ～ 9800日历年（ca1.aB.P.）期间,所有地球化学指标均呈逐渐增大趋势,指示入湖河流逐渐增强,达里湖开始扩张,水生植物生产率逐渐升高;在9800 ～ 7700cal.aB.P.期间,TOC和TN含量以及C/N比值维持稳定高值,δ13C和δ15N值较小,指示湖泊维持高湖面状态,陆源输入和浮游植物生产率较高;在7700～ 5900cal.aB.P.期间,C/N比值和δ15N维持低值,TOC和TN含量呈现更高值,并且波动变化,δ13C值逐渐增大,表明湖面维持高水平,湖水显著变暖,浮游植物生产率显著升高,流域植被大幅度扩张;在5900 ～ 4850cal.aB.P.期间,TOC和TN含量,C/N比值以及δ13C值显著减小,δ15N值显著增大,暗示地表径流显著减弱,达里湖湖面显著下降,湖泊生产率快速下降;从4850cal.a B.P.开始,TOC和TN含量以及C/N比值呈逐渐减小趋势,δ13C和δ15N值呈逐渐增大趋势,表明湖面逐渐下降,湖水盐度、碱度升高,湖水可能变冷,湖泊生产率逐渐下降,流域植被收缩.全新世东亚夏季风边缘区水文和生态环境的变化可能直接或间接受北半球夏季太阳辐射量和区域季风降水强度的共同控制.

Dali Lake （43°13＇～43°23＇N, 116°29＇- 116°45＇E） Mongolia, is located at the current northern margin of the , a hydrologically closed lake in the central-eastern Inner East Asian summer monsoon that represents a particularly sensitive region to environment changes. The upper 8.5m of a sediment core （DL04 sediment core, 43°15.68＇N, 116°36.26＇E） extracted in the depoeenter of Dali Lake was sampled for organic geochemical measurements in this study. 15 radiocarbon dating data of total organic matter from bulk samples indicate that the upper 8.5m of the sediment core covers the last 11500 years. 225 data of total organic carbon （TOC） and total nitrogen （TN） concentrations and TOC/TN （C/N） atomic ratio, combined with 221 data of stable isotopes （δ13C and δ15N） of sedimentary organic matter analyzed at ca.50a resolution, provide a detailed history of changes in the hydrology and ecology of the East Asian summer monsoon margin during the Holocene. From 11500cal.a B.P. to 9800cal.a B.P., ali geochemical indicators displayed increasing trends in their values, indicating gradual increases in surface runoffs and the lake started to expand with gradual increases in primary productivity. From 9800cal.a B,P. to 7700 cal.a B.P., high, stable TOC and TN concentrations and C/N ratios combined with low δ13C and δ15N values suggested a steady status of high stands and productivities of the lake with a balanced terrestrial input. From 7700 cal.a B.P. to 5900cal.a B.P., C/N ratio and δ15N maintained low values, TOC and TN concentrations showed higher values while fluctuating and δ13C values showed an increasing trend, implying a continued high lake level and warmer lake waters with a further increase in phytoplankton productivity and terrestrial input. During the interval between 5900cal. a B.P. and 4850cal. a B.P., TOC and TN concentrations, C/N ratio and δ13C value decreased markedly while δ15N value increased, denoting weakened surface runoffs and significant drops in lake level and declines in aquatic production. Since 4850cal. a B.P., TOC and TN concentrations and C/N ratio displayed decreasing trends while δ13C and δ15N values displayed increasing trends, indicating gradual declines in lake level and increases in water salinity and alkalinity, together with decreases in aquatic production due to possibly gradual decreases in water temperature and declines in terrestrial ecosystem. The changes in the hydrology and ecology of the East Asian summer monsoon margin during the Holocene were likely to be directly or indirectly controlled by variations in the Northern Hemisphere summer insolation and the regional monsoon precipitation intensity.