2.6Ma前后大气环流重构的黄土-红粘土沉积证据

RE－ORGANIZATION OF ATMOSPHERIC CIRCULATION AT ABOUT 2.6MA OVER NORTHERN CHINA

为重建第三纪红粘土堆积晚期搬运粉尘的主导性风向，我们在黄土高原采集4个南北向分布的红粘土剖面，分析了它们的沉积学特征；此外，还对黄土高原一条南北向断面上的过去0．13Ma以来的黄土地层进行了沉积特征分析。结果表明，红粘土的颗粒粗细并没有南北向分异现象，从而与粒度从北到南快速变细的黄土沉积形成鲜明对比。黄土颗粒的南北向分异指示了冬季风对大气粉尘的主导性搬运，而红粘土的沉积特征则似乎表明，2．6Ma以前中国北方冬季风很弱或尚未建立，西风环流可能是当时搬运粉尘的主要营力。

Recent studies have shown that the red clay sequence underlying the Quaternary loess of the Chinese LOess Plateau is wind－blown in origin. To address the wind system that tyansported the Tertiary red clay, two north－south transects were studied in the Chinese Loess Plateau.One of the transects was designed to study spatial changes in grain size of the last glacial－nterglasial loess records,and the other to observe particle changes of the Tertiary red clay.The loess transect consists of nine sections, and the red clay transect of four sections. Analyses of closely－－spacedsamples show that there is a strong southward decrease in grain size of both loess and palaeosol horizons of the late Pleistocene, which is compatible with the conclusion that the aeolian materials of the Quaternary in the Loess Plateau are transported by the northerly winter monsoonal winds. Grain size distributions of the red clay sequences, however, do not show such a change. From north to south along the red clay transect the particle size distribution is almost identical in the four sections, suggesting that the winter monsoonal winds might have played a less important role in transporting the red clay matarial. It is suggested that the red clay may have been transported by the westerlies from the dust－source－regions of northwestern China onto the Loess Plateau. A remarkable rearrangement of Anospheric patterns at about 2.6Ma, therefore, has been recorded by the red clay－loess shifL It is speculated that this rearrangement of atmospheric patterns may have been caused by the onset of glaciation in the NOrthern Hemisphere.