王晓雪^1,,
聂军胜¹,
JoelSaylor²,
张蕊³
1. 兰州大学资源环境学院, 西部环境教育部重点实验室, 甘肃 兰州 730000
2. 休斯敦大学地球与大气科学学院, 休斯敦 77204
3. 江苏师范大学, 江苏 徐州 221116

基金项目: 中央高校基本科研业务费专项项目（批准号：lzujbky-2018-ot05）和政府间国际科技创新合作重点专项二级子课题（批准号：2016YFE0109500）共同资助


详细信息

作者简介: 王晓雪, 女, 26岁, 硕士研究生, 第四纪地质学专业, E-mail:wangxx16@lzu.edu.cn

中图分类号: P534.61⁺2;P318.4;P532

收稿日期:2018-05-07
修回日期:2018-07-25
刊出日期:2018-09-30

Environmental magnetic characteristics of Late Miocene fluvio-lacustrine sediments in Zhada Basin and Indian monsoon evolution

Wang Xiaoxue^1,,
Nie Junsheng¹,
Joel Saylor²,
Zhang Rui³
1. Key Laboratory of Western China's Environmental System(Ministry of Education), College of Earth and Environmental Sciences, Lanzhou University, Lanzhou 730000, Gansu
2. Department of Earth and Atmospheric Sciences, University of Houston, Houston, 10 Texas 77204, USA
3. Jiangsu Normal University, Xuzhou 221116, Jiangsu


MSC: P534.61⁺2;P318.4;P532

--> Received Date: 07 May 2018
Revised Date: 25 July 2018
Publish Date: 30 September 2018


摘要
摘要:晚新生代印度季风的演化一直以来是地学研究的热点问题，然而目前对印度季风的研究多集中在海洋沉积物上。磁学参数作为指示气候的重要指标，已被广泛应用于黄土高原等地区的古气候重建中，对揭示东亚季风的演化历史和驱动机制做出了重要的贡献，但利用磁学指标对印度季风变化历史的研究还很稀少。文章运用磁学方法对藏南札达盆地的河湖相厚225.5 m、年龄范围在9.4~6.0 Ma的沉积物进行分析，发现磁学参数值在8.6 Ma前后发生了明显变化，对应沉积环境和岩相的改变（由河流环境和粗颗粒沉积转变为潮上环境和较细的沉积）。8.6 Ma前磁学参数值较高，磁化率（χ）高的原因可能是风化生成的细颗粒含量多导致的，而HIRM高是由于风化形成赤铁矿增多引起的。鉴于研究剖面的沉积速率在8.6 Ma发生了快速的增加以及以前的研究揭示这段沉积物风化程度较低，我们把8.6 Ma后所有的磁学参数值的降低归因于季风增强造成沉积剥蚀搬运速率增加从而导致沉积物来不及风化，细颗粒的亚铁磁性矿物和赤铁矿没有大量生成。考虑到札达盆地记录的气候变湿时间与阿拉伯海叶蜡碳同位素记录的南亚干旱化得以缓解的时间一致，我们认为这些记录共同指示了印度夏季风的增强。在综合分析青藏高原隆升、特提斯海退缩、南北半球冰量变化和印度洋表水温度变化的基础上，我们把印度夏季风的增强归因为南极冰盖的增加或者是青藏高原的向东北扩张。这一研究加深了对印度季风变化历史和驱动机制的认识。
关键词: 札达盆地/
环境磁学/
沉积相/
印度季风/
晚中新世

Abstract:Although magnetic parameters have been successfully applied to understanding the evolution of the East Asian summer monsoon and its forcing mechanisms, few studies applied these parameters to sediments of South Asia. Instead, previous investigations of the evolution of the Late Cenozoic Indian summer monsoon have relied largely on marine sediments.
The Zhada basin(30°50'~32°20'N, 79°00'~80°30'E) is situated just north of the high Himalayan ridge crest, bounded by the South Tibetan Detachment System to the southwest. The(paleo) environment of the Zhada basin is very sensitive to strengthening and weakening of the Indian summer monsoon because its climate is primarily controlled by interactions between dry westerly air and moist Indian summer monsoon air. The stratigraphy in the Zhada basin can be divided in three intervals, which were deposited(in stratigraphic order) in a fluvial/supra-littoral, littoral/lacustrine, and mixture of fluvial-supra-littoral and alluvial fan environments. The age model of the section has been established using paleomagnetic dating. We measured multi-magnetic parameters(frequency-dependent magnetic susceptibility, SIRM, and HIRM) in SZ section(31°18'~32°24'N, 79°42'~80°54'E) for 0~225.5 m in thickness and 9.4~6.0 Ma in climate change, which comprise the fluvial/supra-littoral interval in order to better understand evolution and forcing mechanism of the Indian summer monsoon. Before 8.6 Ma, there is a relatively high value of magnetism indicators. The reason may be that the content of ultrafine particles and hematite by weathering is increased. These parameters consistently decreased during the supra-littoral interval after 8.6 Ma. We infer that this interval does not have a reduced condition which can result in reductive dissolution of magnetic minerals, because previous research shows that the sediments deposited within this interval were weakly weathered. Instead, we suggest that the decrease of magnetic parameters records a decrease of magnetic minerals due to rapid erosion and limited chemical weathering. In this situation, few pedogenic magnetic minerals will be generated, resulting in decrease in magnetic parameter values. We notice that faster erosion in Zhada basin is synchronous with temporary wetting in South Asia continent as is recorded by Arabian Sea leaf wax carbon isotopes. We argue that these records demonstrate that the Indian summer monsoon intensified at 8.6 Ma. After considering factors which might affect intensity of Asian monsoon(uplift of the Tibetan Plateau, retreat of Paratethys, high latitude ice volume variations, oceanic heat and salinity variations), we infer that this phase of Indian summer monsoon intensification was likely caused by growth of the Antarctic ice sheets and/or NE growth of the Tibetan Plateau.
Key words:Zhada basin/
environmental magnetism/
Late Miocene/
Asian monsoon/



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