Loess On West Kunlun Moutains And Aridification Of Asian Inland

The aridification of Asian inland is generally regarded as one of the most important mark of global climatic deterioration since the Late Cenozoic. Many previous researches showed dust due to drying and desertification of Asian inland was carried away by the monsoon and westerlies to desert adjacent areas (such as the Loess Plateau, northern Pacific Ocean and Arctic region), and had an important impact on global substance circulation and climate change by its sun umbrella, ice core and iron fertilization effects, becoming a key link of land-sea-air coupling change. The inland of Northwest China, especially the Taklimakan Desert, is one of the most important source areas of Asia dust. So far, our understanding about the drying process of this region and its environmental impact on East Asia is still quite rough, and the driving mechanism of Asian interior aridity has yet to be thoroughly understood.Loess on the west Kunlun Mountains is so far the thickest loess found in the extreme arid region of China inland, which is a roughly synchronous accompanied product of desert process. During 2006-2007, a 671-meter-long loess deposit core had been retrieved from the northern slope of the West Kunlun Mountains, which provides a good opportunity to study the formation and evolution of drying climate of Central Asia and its possible links with the Tibetan Plateau uplift and global cooling. Based on the detailed analysis about magnetostratigraphy and several climatic proxies for the upper 207m loess deposit core, the following conclusions and new findings are revealed:(1) Detailed plaeomagnetic dating of the upper 207m loess deposit core has been performed and the B/M boundary is found to be located at 170 m of the core, assigning an age of 3-4 Ma for the whole 671m loess deposit, which is the oldest aeolian sediments in the central Asia.(2) The paleoclimatic implication of susceptibility and carbonates content is significantly different between the loess on west Kunlun Mountains and Loess Plateau. Our results suggest that magnetic properties of the Kunlun Mountains loess are dominated by low coercivity ferrimagnetic minerals, i.e. magnetite and maghemite. Hard magnetic minerals, including goethite and hematite, are also found but make a minor contribution.Rock magnetic and grain size studies suggest that magnetic susceptibility of loess from the northern slope of West Kunlun Mountains are not controlled by changes in the concentration of ultrafine pedogenic magnetite/maghemite, but instead controlled by changes in the concentration of aeolian multidomian magnetite/maghemite, which can be further confirmed by the perfect correlation between grain size and magnetic susceptibility of the surface sediments. In addition, the carbonates content increasing with the precipitation for the west Kunlun Mountains loess was also different from loess on Loess Plateau, which was possibly induced by the special surface processes in the extreme arid region of Asian inland.(3) According to the characteristics of multiple climatic proxy records, the drying process of Central Asia since-1Ma can be roughly divided into 5 stages: 0.95-0.87 Ma(207-191m), relative wetting period; 0.87-0.52Ma(191-lllm), development of aridity period; 0.52-0.33Ma(111-67m), significant arid intensification period; 0.33-0.13Ma(67-30m), modulating and transition period; 0.13-0 Ma(30-0m), modern dry climate period. Further research shows that the development of aridity in Asian inland since-1Ma were mainly caused by the tectonic uplift and/or global cooling:the drying event at 0.52 Ma and 0.13 Ma were controlled by the Tibetan Plateau Uplift; the drying event at 0.87 Ma were caused by combined action between the Tibetan Plateau uplift and the increase of the global ice volume.