黄土高原北部晚新近纪“吴起古湖”的古地磁年代学与古环境记录  

Palaeomagnetic chronology and paleoenvironmental records of late Neogene Wuqi paleolake in northern Chinese Loess Plateau

作　　者：孙蕗[1] 岳乐平[1,2] 王建其[1] 李建星[1,3] 徐永[1] 张佳音[1] 马冀[1] 

机构地区：[1]西北大学大陆动力学国家重点实验室,西安710069 [2]中国科学院地球环境研究所,黄土与第四纪地质国家重点实验室,西安710075 [3]中国地质调查局西安地质调查中心,西安710054

出　　处：《地球物理学报》2010年第6期1451-1462,共12页Chinese Journal of Geophysics

基　　金：国家自然科学基金项目(40772025);国家重点基础研究发展计划"973"课题(2003CB214607);大陆动力学国家重点实验室与第四纪地质国家重点实验室开放基金(SKLLQG0511)资助

摘　　要：晚新近纪以来中国北方堆积的风成红粘土-黄土序列是古气候信息的良好载体,在黄土高原北部的吴起地区存在一套湖相沉积物,层位上介于第四纪黄土与晚新近纪红粘土之间,但该古湖存在的具体时限尚不清楚.我们利用岩石磁学及古地磁年代学方法并结合多种气候指标对代表"吴起古湖"的土佛寺剖面进行了初步研究,结果表明湖相沉积物中特征剩磁的载体主要为磁铁矿和赤铁矿,在此基础上得到的磁极性序列能很好地与国际地磁极性年表对比,剖面起始于4.28 Ma B.P.左右,湖相沉积层起始于3.0 Ma B.P.左右,结束于1.2 Ma B.P.左右.岩性、磁化率、粒度和沉积速率变化均反映出这一时期该地区环境变化的总体趋势是由暖湿向冷干发展,期间"吴起古湖"的演化大致经历了三个阶段:3.0～2.5 Ma B.P.为古湖形成时期,水体相对较深;2.50～2.05 Ma B.P.期间受古气候变化的影响湖水明显变浅;2.05～1.20 Ma B.P.时古湖中仍有一定量的水体存在,但随后逐步缩小并最终消亡.该地区气候条件在1.2 Ma B.P.左右的恶化是导致吴起古湖消亡的主要因素,区域构造活动引起的北洛河溯源侵蚀并切穿湖盆可能也是原因之一.Aeolian red clay and loess deposited in northern China since late Neogene is an ideal palaeoclimate recorder. A profile consisting of loess, lacustrine deposits and red clay was found at Wuqi County in northern Chinese Loess Plateau. Primary investigations were carried out on the typical Tufosi section in order to provide the age constraints of the lacustrine deposits （namely Wuqi paleolake）. Rock magnetic results demonstrate that the magnetic carriers of the characteristic remanent magnetization of lacustrine deposits are mainly magnetite and hematite. The correlation between magnetic polarity sequence and Geomagnetic Polarity Time Scale is very well. The age of the bottom of the section is 4.28 Ma B.P. approximately. Wuqi paleolake formed at about 3.0 Ma B.P. and died out around 1.2 Ma B.P. The regional environment became colder and drier during this period as implied by the variations of lithology, magnetic susceptibility, grain size and sedimentation rates. The evolution history of Wuqi paleolake can be divided into three stages： the paleolake was formed during the period of 3.0~2.5 Ma B.P. and the water body was relatively deep; the lake became shallow markedly as the result of palaeoclimate change during 2.50~2.05 Ma B.P., during 2.05~1.20 Ma B.P., reduction of the water body led to the extinction of the lake. Climatic deterioration in this region around 1.2 Ma B.P. was the main reason for the end of Wuqi paleolake, and the headward erosion of North Luohe River caused by regional tectonism was probably a factor, too.

关 键 词：吴起古湖 古地磁年代学 古环境 黄土高原 晚新近纪 

分 类 号：P318[天文地球—固体地球物理学]