近13万年来黄土高原干湿气候的时空变迁

第四纪时期，黄土高原粉尘物质的搬运、沉积及其后的成土过程受控于干湿气候的变化。对黄土高原这一特定地区而言，风成沉积序列在地域上的差异主要与东亚地区的季风环流在时间和空间上的变化有关。本文的研究揭示出，近１３万年里，最为显著的成壤期有６期，与这６个成壤期对应的时段也应当是夏季风环流加强、气候温湿的时期；在空间上，全新世适宜期及末次间冰期中与深海氧同位素阶段５ａ，５ｃ，５ｅ对应的时期，夏季风足可以深入到毛乌素沙漠腹地，并具有占优势的环境效应。在阶段３的早、晚期及５ｂ时期夏季风虽然也能深入沙漠－黄土边界带，但其环境效应在黄土高原北部及毛乌素沙漠南缘已不再显著。在阶段２、阶段４及阶段３的中期夏季风已不能深入沙漠－黄土边界带。

SPATIAL AND TEMPORAL CHANGES OF DRY AND WETCLIMATE DURING THE LAST 130 000 YEARS INTHE LOESS PLATEAU

he aeolian sequences in the Loess Plateau respond in a strong,coherent fashion to the cyclic variations of the winter and summer monson circulation.The loess deposit in the Loess Plateau is mainly transported by the winter monsoon.During the glacial period,the strength of winter monsoon increased,which resulted in the rapid accumulation of loess material in the Loess Plateau.During the interglacial time,the winter monsoon weakened and then greatly reduced the loess accumulation.Nevertheless,pedogenesis of aeolian sediment,after its deposition,is mainly controlled by the strength of the summer monsoon circulation.Thus,the aeolian sequences of one region should record the winter and summer monsoon circulation of this specific area and,the aeolian sequences of different sites should be good archives for discussing the spatial and temporal changes of dry wet climate during the Quaternary period. In this paper,from the Mu Us Desert to the Loess Plateau (along the N-S transect),the authors chose six typical sections.Based on the stratigraphic correlation and the magnetic susceptibility profiles,changes of monsoonal circulation in space and time have been studied.Our results indicated that during the Holocene and the last interglacial time there were four remarkable soil forming periods,corresponding to the Holocene Optimum and the Stage 5a,5c,and 5e of the marine oxygen isotope records.During the last glacial time,there were two pedogenic periods,coinciding with the Stage 3 of the marine oxygen isotope records.Thus,there were six soil forming episodes over the last 130 000 years,these episodes should coincide with the intensified summer monsoon circulation during the last glacial interglacial cycle.In space,the aeolian sequences of the last glacial time changed from the sand loess alternations in the desert loess transitional zone to the silty loess layer in the northern Loess Plateau and then to the loess soil alternations in the central and southern Loess Plateau.The last interglacial soil in the desert loess transitional zone consists of three soil layers and two interbedded loess units.In the northern Loess Plateau,the last interglacial soil has two subdivided soil layers and one interbedded loess horizon.In the central and southern Loess Plateau,no loess intercalations occur in the last interglacial soil,which could not be easily subdivided in the field.The above stratigraphic correlation indicated that during the Holocene Optimum and the substages of 5a,5c,and 5e,the enhanced summer monsoon could easily reach the fat interior of the Mu Us Desert,as demonstrated by the development of the four soil horizons.During the earlier and later periods of Stage 3 and sub stage 5b,the summer monsoon could still reach the marginal zone of Mu Us Desert,but the geological records showed that the summer monsoon was not the dominant monsoonal circulation.During Stage 2 and Stage 4,as well as the middle episode of Stage 3,summer monsoon could not reach the desert loess transitional zone,this can be proved by the existence of three sand intercalations of Yulin section,these periods should coincide with the stages of the most intensified winter monsoon circulation over the last 130 000 years.