化学蚀变指数指示古气候变化的适用性探讨

摘要

腾格里沙漠南缘土门剖面末次冰消期层段由3层沙丘砂、2层湖相沉积、2层古土壤和5层黄土构成,对各沉积相进行了年代学研究与主量元素含量测试,确定该层段为末次冰消期,计算了各沉积相CIA指数,结果显示CIA指数由大到小依次为沙丘砂>黄土>古土壤>湖相沉积.通过对比,发现CIA指示的各沉积相化学风化作用强度(气候环境)与已有粒度成分指示的该时期气候变化研究结论相矛盾,通过对该矛盾现象的深入研究和探讨,得到以下认识:1)研究层段的古土壤、湖相沉积、黄土的沉积物质可能来源于土门剖面东南一侧的黄土高原,沙丘砂沉积物质受北侧的腾格里沙漠沙丘砂强烈影响,各沉积相的物源化学元素存在差别使得不同沉积相CIA指数的对比存在先天性不足;2)土门剖面所处的低洼地形会产生汇水作用,使易迁移的钙、钠、钾等元素随降水迁移而汇聚于此,对不同沉积相的CIA值产生干扰,导致CIA指数偏小,其不能反映真实化学风化作用强度;3)应用CIA指数作为古气候变化的替代指标需综合考虑沉积相物源差异及沉积环境的影响,CIA指数用于指示古气候变化存在一定的适用性问题.%In the southern part of the Tengger Desert, the last glacial dissolution stratum of its Tumen profile consists of three layers of dune sand,two layers of lacustrine sediments,two layers of paleosol and five layers of sand loess. Through the chronological study of the sedimentary facies and testing of the main elements,the results define the stra-tum attributes to the last glacial dissolution stratum;once all the CIAs calculated,the order of CIA index appears as sand dune>loess>paleosoil>lacustrine sediments from the largest to the smallest.By contrast, it is found that the weathering intensity(climatic environment)of all the sedimentary facies indicated by CIA is in contradiction with the research conclusion that the granularity indicates the climate change in this period.With a deep study and discussion of the contradiction phenomenon,the following conclusions are obtained:1)The sedimentary material of the paleosol, lacustrine sediments and loess may originate from the Loess Plateau,on the southeast side of the soil door section.The provenance of sand dune is strongly influenced by the dune sand from the Tengger Desert in the north.The differences of the chemical elements of sedimentary facies make the comparison between the CIA index of different sedimentary fa-cies congenitally deficient.2)The low-lying topography where the soil door section locates at will produce water-catch-ing effect,so that the migratory elements such as calcium,sodium,potassium and others will migrate with the precipi -tation of water to the surface of the sedimentary facies and the CIA indexes of different sedimentary facies are influ-enced to be too small to reflect the true weathering intensity.3)The application of CIA indexes as a surrogate indica -tor of palaeoclimate change shall comprehensively consider the sedimentary facies provenance differences and the effect of the sedimentary environment; hence, the CIA index has limited application in indicating the paleoclimatic change.