我国季风边缘区湖泊沉积记录的全新世亚洲夏季风衰退事件

亚洲夏季风是全球季风系统的重要组成部分,亚洲夏季风的变化对其控制区域自然生态系统的多样性和生态平衡,以及社会经济发展有重要的影响。本文选择位于现代亚洲夏季风边缘区对季风变化响应敏感的湖泊达连海为研究对象,基于陆生植物残体和全有机质的AMS14C定年建立了钻孔顶部24.6 m沉积物的年代框架,利用粒度指标重建了全新世研究区水文变化过程以及亚洲夏季风衰退事件序列。结果显示,沉积物中存在数层砂层,代表了湖泊低水位时期,进而指示了亚洲夏季风衰退事件。这些事件处在11.6~11.3 cal.ka B.P.、10.4~9.5 cal.ka B.P.、6.4~6.0 cal.ka B.P.、4.6~4.4 cal.ka B.P.、3.7~3.4 cal.ka B.P.、3.1~2.9 cal.ka B.P.以及2.0~0.9 cal.ka B.P.,可以发现中晚全新世以来亚洲夏季风衰退事件发生的频率显著增加。进一步与北半球高纬地区与低纬地区的气候突变事件记录对比显示,全新世百年-千年时间尺度上亚洲夏季风强度的变化与低纬ENSO活动存在密切的联系。

Abrupt decreasing of Holocene Asian summer monsoon recorded by lake sediments from monsoon margin

Asian summer monsoon is one of the important components of the global monsoonal system, whose decreasing has strong effects on the environmental changes, ecology safety, economic development, as well as social stability in its domains. The boundary regions of the modern summer monsoon are ideal places to study the monsoon intensity. However, the continuous sequence of decreasing monsoon events during the Holocene is still lacking. Dalianhai Lake is the terminal lake of the Shazhuyu River in the Gonghe basin(35.5°~36.9°N, 98.8°~101.4°E), northeastern Tibetan Plateau, which is located in the margin area of modern Asian summer monsoon system. In 2012, a 200-m-long core DLH12A(36°14'25.2"N, 100°23'55.0"E;2852 m a.s.l.) was obtained from dry lake bed. In the present study, we focus on the upmost 24.6 m sediments which are composed of greyish-brown clay or silty clay between 24.6 m and 17.0 m, and greyish silty clay in the 0-17 m interval. Seven layers of well sorted sand were found at the depth of 6.0 m, 12.9 m, 14.6 m, 16.5 m, 19.0 m, 23.0 m and 24.3 m. Based on AMS14C(accelerator mass spectrometry radiocarbon) dating results from 5 terrestrial plant remains and 8 bulk organic matter samples, and the applying of multi-steps and layers polynomial curve fitting, the age-depth chronology model of the studied core was successfully established for the period from 11.7~0 cal.ka B. P. Hydrological variations in the basin and decreasing events of the Asian summer monsoon were reconstructed based on analyzing of grain size(contents of > 63 μm particles) and SEM(scanning electron microscope) images of the sediments. The well-sorted sand layers in the studied sediments represent centennial-to millennial-scale decrease of the lake level, further indicating the decrease of precipitation and weakening of the Asian summer monsoon. All these events occurred during the period from 11700 cal.a B. P. to 9500 cal.a B. P. and since 6500 cal.a B. P.(11.6~11.3 cal.ka B. P., 10.4~9.5 cal.ka B. P., 6.4~6.0 cal.ka B. P., 4.6~4.4 cal.ka B. P., 3.7~3.4 cal.ka B. P., 3.1~2.9 cal.ka B. P. and 2.0~0.9 cal.ka B. P.). The abrupt weakening events of the Asian summer monsoon during the Holocene can be well compared with the ENSO events in the low altitude, indicating that the abrupt decreasing of Asian summer monsoon may be closely linked to the ENSO events during the Holocene.