全新世期间亚洲和非洲干旱区环境的短尺度变化

过去对北非和中国干旱区140 ka BP来环境演化的初步对比表明，两地的干旱度在万年时间尺度上基本同步。这种关系是否也适用于全新世百年尺度的干旱事件，仍是有待研究的问题。本文对撒哈拉地区569个地表淡水指示物和我国北方干旱区158个古土壤和湖泊沉积的14C年代数据进行了时空频数统计分析，揭示出两地沙漠在全新世期间至少经历了三次同时性的、百年-千年尺度的干旱事件, 叠置于季风气候对地球轨道变化响应的总体趋势上；沙漠在纬向上的进退主要受控于季风前沿的伸缩幅度, 说明两个季风系统的短尺度变化可能受统一因素控制。最为显著的干旱事件发生于约4 ka BP。两地干旱区在过去认为的6 ka BP 前后的全新世气候适宜期并没有表现为稳定的湿润环境。我国过去发现的该时期不少的干旱证据被解释为高温所导致的强烈蒸发作用所致。但全新世中期的干旱期在热带的存在及其与冰心记录中低甲烷事件的吻合难以用蒸发来解释，而应与夏季风环流的减弱有关。

HOLOCENE ABRUPT ENVIRONMENTAL CHANGES IN THE ARID REGIONS IN AFRICA AND ASIA

A preliminary comparison between the climatic evolution of the arid regions in North Africa and North China showed that the variations in the continental aridity on time scales of 104 years were approximately synchronous over the last 140 ka BP. Whether this relationship can be established for the Holocene drought events at century-scale reported from tropical and equatorial Africa is still poorly known. Based on 569 reliable radiocarbon dates from surface fresh water indicators from the Sahara and 158 dates on palaeosol and lake sediments from the arid regions in North China, we demonstrate here that the Holocene environments in both regions have been affected by several approximately in phase arid events, inlaid in the slow trends attributable to the orbital forcing. The variations of the southern margins of the deserts, probably associated with the oscillations of the meteorological equator (or ITCZ) in Africa and with the northern monsoon front in China, are documented by the latitudinal distribution of these in-dicators through time. The most startling aspects are the severe interval at 4 000 years BP and a prolonged drier interval around 6 000 years BP. The last is given as the Holocene Optimum. However, this episode is in agreement with an important number of pollen, ice-core, lacustrine and pacific eolian dust records, and also with the lower atmospheric methane concentration recorded in the GRIP ice-core. The existence of this arid interval at low latitudes cannot be explained by higher evapotranspiration in the mid - Holo-cene, but is attributable to the weakening of the summer monsoon circulation.