末次间冰期甘肃沙尘暴演化历史的黄土记录分析

The Evolution of Dust Storms Since Last Interglacial in Gansu as Reconstructed from Loess Record

本文通过对兰州黄土记录的粒度分析， 探讨了末次间冰期 （即黄土磁化率第五阶段 Ｌ Ｏ Ｍ Ｓ Ｓ－ ５） 甘肃沙尘暴形成演化的历史。结果表明， 此阶段沙尘暴经历了由弱到强的演化； 这与岁差周期影响下的冬季风演化有关， 表现为 ５ａ、５ｃ 和 ５ｅ 亚阶段 （持续约 １３６ ｋａ） 相对较弱，而 ５ｂ 和 ５ｄ 亚阶段 （持续约 ６４ ｋａ） 相对加强； 各亚阶段沙尘暴发生频率高低和强度大小的比较结果分别为 ５ｃ＞ ５ｂ＞ ５ａ＞ ５ｄ ５ｅ 和 ５ｄ＞ ５ｂ＞ ５ａ＞ ５ｅ＞ ５ｃ； 从极细砂含量的异常增多来看， 至少经历了 １０ 次强沙尘暴事件， 反映冬季风的突发性加强； 从＜ ４５０φ的粗颗粒组分含量来看，各亚阶段沙尘暴之平均强度均较 “９３０５０５”沙尘暴大得多； 沙尘暴形成演化除受轨道因素外也有某些非轨道和突发性的因素。

The present dust storms in Gansu are severe. The records show that about six extraordinary dust storms have happened in the corridor area since 1950 A.D.. The evolution history of dust storm during the last interglacial are reconstructed through analyzing the particle size of loess, and by comparing it with the present extraordinary heavy dust storm numbered “930505”. The main information on dust storm from loess record includes: The deposition rate of loess may indirectly reflect the frequency of dust storm. The general grain size has the good relationship with the intensity of dust storm. While the coarse grains (fine sand) in loess implies an extraordinary dust storm event (EDS). The grain size curves of some proxies gave a general process of the dust storm. Comparing the grain size curves with the magnetic susceptibility curve of loess, we may analyse factors influencing the formation and evolution of the dust storm. Also we can discuss the possible mechanism of dust storm formation. The loess samples were taken from a newly dug loess well (45 m deep) at Gaolanshan in Lanzhou, Gansu province. This well penetrates S 1 paleosol and spans over the past 150 000 years. Since the resolution of loess record in this area is high and the paleosols are poorly developed, a new loess division method called the loess magnetic susceptibility stage (LOMSS), which is basically corresponding to the marine isotope stage MIS), is adopted in this paper. 180 samples of loess of LOMSS-5 and partly LOMSS-4 and LOMSS-6 at 5 cm interval were analyzed by using the SKC 2000 Particle size Analytical System. According to the magnetic susceptibility proxy and some grain size proxies of loess (<4 25φand 4 25～4 50φ), the high resolution loess formed during the last interglacial (LOMSS-5) can be divided into five sub stages, LOMSS-5a to LOMSS-5e, that are corresponding to MIS-5a to MIS-5e respectively. Studies show that the LOMSS-5 dust storms might be significantly weaker than those in LOMSS-4 and LOMSS-6. During 5a, 5c and 5e sub stages, the dust storms were comparatively weaker, and it must have been impacted by the orbital factors (Precession Cycle). 5b and 5d sub stages are a little stronger. From beginning to end, the Last Interglacial dust storms have experienced a growing process in intensity. According to the deposition rate of loess and the content of coarse materials in loess, the occurring frequency of dust storm can be lined up as 5c>5b>5a>5d5e , with 5d>5b>5a>5e>5c in intensity.