近7.0-0.73MaBP期间甘肃临夏盆地古气候演变的沉积物地球化学记录

THE GEOCHEMICAL RECORD OF PALEOCLIMATE DURING ABOUT 7.0Ma-0.73Ma IN LINXIA BASIN,GANSU PROVINCE

位于青藏高原东北边缘临夏盆地新生代沉积蕴含了丰富的有关青藏高原隆升和环境演变信息。近７．０－０．７３ＭａＢＰ期间沉积物地化元素含量波动特征表明，具相同或相近表生地化性质的元素含量波动呈现大致相同的特征。由常量元素氧化物所揭示的此期间临夏盆地温湿变化明显地可以分成几个阶段，即７．０－５．３５ＭａＢＰ，虽有冷干与暖湿波动，但总体上却呈现出一种较为和缓、稳定的气候特征；至５．３５ＭａＢＰ前后，出现一具全球意义的冷峰，随后气候进入冷干与暖湿频繁、大幅剧烈波动的时期，可能同青藏高原此时开始的整体隆升并导致亚洲季风开始建立但极不稳定的状况有关。而３．４Ｍａ和２．９０Ｍａ两次强烈的构造运动的发生很可能使得青藏高原隆升至一个关键高度，此时亚洲季风已稳定建立，地化元素波动平缓，气候环境以冷湿为主要特征，湖相沉积发育。之后，１．６０Ｍａ前后高原再度隆升，导致黄河朔源侵蚀加剧，最终切穿临夏盆地，排干湖水，且此时高原巨大的山体已成为南来湿润气流的障碍，使内陆地区趋于干旱，温带荒漠开始发育，并在其四周广泛地开始了风成黄土的堆积，而黄土堆积中黄土与古土壤的交替出现仍是冷干与暖湿气候特征的反映。因此，近７．０－０．７３ＭａＢＰ期间临夏盆?

The cenozoic sediment in Linxia basin,which locates at the northeastern fringe of Tibet plateau bears abundant informations on the plateau's uplifts and paleoclimate.The fluctuations of geochemical elements show that those with the same or similar geochemical nature fluctuated identically.The hydro-thermal sequence revealed by oxides during about 7.0Ma-0.73Ma can be divided into several stages,i.e,about 7.0Ma-5.35Ma although it alternated with cold-dry and warm-wet periodes,it generally appeared a mild and stable climate regiem;till to 5.35Ma, a worldwide cold peak occurred.Afterwards,climate stepped into a period characterized by frequent and drastical alternation of cold-dry and warm-wet regiem,which may be related to the beginning of the uplift of the whole Tibet plateau,and resulted in the unstable establishment of east Asia monsoon at this time.Two severe tectonic movement occurred at 3.4Ma and 2.90Ma perhaps made the plateau upheave to an important critical altitude(1500-2000m),and led up to the stable establishment of east Asia monsoon.During 2.4Ma-1.67Ma,geochemical contents changed mildly,climate is characterized by cold-wet state,lacustrine sediment developed continually.At about 1.60Ma,the Tibet plateau uplifted again,and resulted in the Huanghe river headward erosin aggravated,at last cut through Linxia basin,dried out lake water.At this time,the huge plateau had obstacled the moisture current from the south,and made the inland continent tended to drought,the temperate desert began to develop wildly and eolian loess accumulated arround it.The alternation of loess and paleosoil represented cold-dry and warm-wet regiem respectively.So the fluctuation of geochemical contents and the paleoclimatic features revealed by them are of profound tectonic and environment background. \ \