晚新生代黄土高原风尘序列的粒度和沉积速率与中国北方大气环流演变

文章以风尘沉积的粗粒和细粒组分与季风和西风环流的联系为基础,利用黄土高原中部的洛川剖面、西峰剖面和灵台剖面的粒度和沉积速率记录,讨论了晚新生代中国北方季风环流和西风环流的演变历史,总结了这一时期大气环流演变的基本规律和大气环流演变的动力机制.研究表明,自8～7Ma风尘沉积发育至5Ma左右,西风环流和季风环流都有减弱的趋势;自5Ma以来,西风环流和季风环流的强度都在逐步加强;与此同步,季风环流对风尘沉积的贡献增加,而西风环流对风尘的贡献逐渐减小,这一逐渐发展的大气环流趋势与北半球高纬冰盖的逐步发展有关;大气环流的这种趋势变化在8～7Ma,3.4Ma和1.2～0.9Ma这几个时期存在着突变,可能反映了青藏高原的阶段性隆升对中国北方季风环流演化的决定性作用以及对西风环流结构和强度的重要影响.中国北方大气环流在轨道尺度变化的基本特征是,低空季风环流在冰期加强,在间冰期减弱.西风环流和季风环流在冰期和间冰期的强度和格局可能主要与全球冰量的基本状况和青藏高原原面的性质有关.黄土高原的风尘记录在万年尺度和千年尺度的气候事件上都表现出相当明显的区域差异,可能主要与局部地形的大气环流效应有关.

GRAIN-SIZE AND DUST ACCUMULATION RATE OF LATE CENOZOIC AEOLIAN DEPOSITS AND THE INFERRED ATMOSPHERIC CIRCULATION EVOLUTIONS

Based on the relationship between the grain size components in loess and the atmospheric circulation system this paper aims to reconstructthe evolution history of westerly and monsoonal circulations by using aeolian records of Luochuan, Xifeng, Lingtai and Zhaojiachuan. The result indicates that strength of both westerly and monsoon circulations decreased from 7Ma to 5Ma after initiation of aeolian accumulation over main part of Loess Plateau. Their strength enhanced progressively after 5 Ma, meanwhile the contribution of monsoonal circulation to aeolian sediments increased progressively with respect to the contribution of westerly circulation, and the change of atmospheric circulation should be associated with the progressive development of northern hemisphere ice sheet. This general trend of circulation evolution is marked by several abrupt changes at 8- 7Ma, 3.4Ma and 1.2 - 0.9Ma, which is interpreted as the effects of the uplifted Tibet Plateau on the monsoonal circulation evolution and on the westerly circulation patterns over Late Cenozoic. Grain-size records of the last glacial cycle deposits over Chinese Loess Plateau suggest that the strength of winter monsoon and westerly circulation intensified synchronously in glacial stages and weakened in interglacial stages. However, grain-size records from different sections show differences in millennial and orbital time scales variation, which is explained as the effect of local topography on the loess accumulation at different sites.