兰州九州台黄土磁性特征及其古气候意义研究

MAGNETIC PROPERTIES OF JIUZHOUTAI LOESS IN LANZHOU AND ITS SIGNIFICANCE OF PALAEOCLIMATE

本文选取黄土高原西部地区具有代表性的九州台黄土进行系统的磁性和粒度特征研究,结果表明其磁性特征主要由磁铁矿控制,同时含有磁赤铁矿和赤铁矿;磁性矿物粒度为假单畴,黄土的粒度组成以粉砂为主。<2μm的细颗粒组分不能很好的反映黄土高原西部地区黄土末次间冰期的成壤强度。与磁化率相比,x_(ARM)/x_(lf)和x_(ARM)/SIRM能更好的反映成壤强度和古气候变化。选取>40μm的粒度百分含量和x_(ARM)/SIRM作为代用指标分别反映冬、夏季风变化,研究结果揭示冬、夏季风趋势并非同步异向变化,在倒数第二次冰期向末次间冰期过渡时,相对于夏季风,冬季风的变化具有一定的滞后性,暗示了冬夏季风的主控因素可能不同。

Lanzhou is located at the intersection of the Qinghai-Tibet Plateau,the Loess Plateau and the northwest desert region, which is also the junction of the three natural belts including the eastern monsoon region, western arid and semi-arid regions and the Qinghai-Tibet alpine region. In this study, we investigated the Jiuzhoutai loess section （36.05°N, 103.88°E ;2056m a. s. 1. ）which is situated at the sixth-level terrace of north shore of the Yellow River in Lanzhou City and is the most representative loess sequences in the western Chinese Loess Plateau. S1 is about 29-36m below the surface, which can be clearly identified with two layers of loess（ S1L1 and S1 L2 ）and three layers of paleosols（ S1S1, S1S2 and S1S3 ）. The pedogenisis intensity is the strongest in S1S3. Two hundred and twenty samples were obtained in S1 paleosol and its upper and lower loess（about 27 -38m）by 5cm intervals. We carried out systemic rock-magnetic investigations including low-frequency magnetic susceptibility （Xlf）, high-frequency magnetic susceptibility （Xlf）, anhysteretic remanent magnetization （ ARM ）, isothermal remanent magnetization （IRM）, saturation isothermal remanent magnetization （SIRM）, hysteresis loop and magnetization curves on all samples. Particle size was also measured by a Mastersize 2000 Laser diffraction particle size analyzer. Results show that the main magnetic minerals of the loess deposit are magnetite, maghaemite, haematite, and the main magnetic grain size of magnetic minerals are PSD. The grain sizes of loess is composed mainly of silt and fine particles cannot reflect pedogenesis of loess very well in West Loess Plateau in the Last Interglacial. There exists a threshold of 30-10-8 m3/kg of the magnetic susceptibility in the western Chinese Loess Plateau. Above the threshold,there is a good positive correlation between magnetic susceptibility and pedogenisis strength and magnetic susceptibility reflect paleoclimate after the sediments deposition. Below the threshold, the magnetic susceptibility may reflect paleoclimate of loess source area. Compared with susceptibility,ZARM/SIRM and XARM/Xlf respond to the strength of pedogenesis and paleoclimate more accurately. The trends of parameters of magnetic and grain-size properties indicate that the changes of winter and summer monsoon were not completely reverse. Relative to the summer monsoon,winter monsoon has a certain lag in the period of transition between penultimate glaciation and the Last Interglacial. The drives of summer and winter monsoon may be different.