The Magnetic Properties of Jiaxian Red Clay

详细信息   

• 责任者：Zhao Guoyong (Key Laboratory of Western China’s Environmental Systems ; Ministry of Education ; Research School of Arid Environment and Climate Change ; Lanzhou University ; Lanzhou 730000 ; China) ; Liu Xiuming
• 刊名：Quaternary Sciences
• 出版年：2013
• 卷期：33(3)
• 关键词：红粘土 ; red clay ; 环境磁学 ; environmental magnetism ; 陕西 ; Shaanxi Province
• 页码：p.415-423
• 图表：illus., 1 table, 59 refs.
• 分类号：1400
• issnNo：ISSN1001-7410
• isbnNo：CN11-2708/P

摘要

Both Chinese loess-paleosol sequences and red clay are aeolian deposits. However, red clay has not been studied as intensive as loess. The question why the strongly developed red clay shows low magnetic susceptibility remains unclear. There are some controversies on whether magnetic susceptibility of red clay can be used as a proxy to trace the variation of summer paleomonsoon and what the pathway of aeolian dust transportation for red clay is. In addition, the paleoclimatic significance of red clay needs to be further explored. Therefore, more research on red clay is urgently needed. Jiaxian is located in the northern edge of the Chinese Loess Plateau, close to the Mu Us desert, in the transitional zone of monsoon and non-monsoon areas, where the present annual precipitation is about 400mm. The Jiaxian red clay （38 ~15＇50＂N, 110~06＇14.9＂E ; 1171 _＋8m a. s. 1. ） is located at Fangta Town, which is about 30km eastwards to Yulin downtown and is about 50km northwest to Jiaxian County. In order to obtain high-resolution paleoclimatic record, 1800 powder samples were collected in the 60m thick red clay,with 3 samples collected every 10cm; 90 powder samples were collected every 5cm in the overlying Wucheng loess. In total, 1890 samples were collected in the 64.5m thick section. The samples from Jiaxian section were provided by Ding Zhongli and Yang Shiling. In order to better analyze the data, combining with field observations,we divided the selected samples into 5 types according to pedogenetic intensity, loess （ 48.3m ）, the weakly developed horizons （ Bw, 47.27m ）, the developed horizons （ W-Bt, 17.27m）, the strongly developed horizons （ S-Bt, 26. 37m ） and abnormally strongly developed horizons （ abn. S-Bt, 64.5 m）.

    

  Systematic magnetic analyses show that ferrimagnetic minerals （ magnetite and maghemite） dominate the Jiaxian red clay. Antiferromagnetic minerals （ hematite and goethite, mainly hematite） are also present. The strongly developed paleosols have relatively high X, Xfd, XARM, Ms, Mrs, Rb/Sr, CIA index and ZARM/X, lOW SIRM/X, Bc and B o. This suggests that the strongly developed paleosols contain more soft magnetic minerals （magnetite and maghemite） than that of the weakly developed paleosols,which is in line with observations that the hysteresis loops close at low fields,low remanent magnetization at the same alternating field, and the high ratio of IRM/SIRM in strongly developed paleosols. The weakly developed paleosols have more hard magnetic minerals （ hematite and goethite）. The magnetic grain sizes of red clay sequence nearly are below 0.2μm. When Z〉64 x 10-s m3/kg, the grain size is less than 0. 1 μm. Magnetic grain size decreases with increasing X. The unit RC18,at the bottommost of Jiaxian section,is strongly developed palesols with the lowest value of %, Xfd, ZARM and highest value of Rb/Sr, CIA index, XARM/X,which is in contrast with the overlying strongly developed paleosols. This can be attributed to changes in magnetic mineralogy and grain size：the hematite content increases, SP maghemite sharply deceases and the relative content of SD hematite increases, which corresponds to the previously suggested transformation pathway （ ferrihydrite→ SP maghemite →SD maghemite→ SD hematite） during pedogenesis progress.