Tectonic Uplifts And Climatic Changes In Eastern Qilian Mountains Over The Past 0.8Ma

After the mid-Pleistocene transition (MPT) that occurred about 1.0Ma ago, global climate began the 100kyr glacial cycles. The classical Milankovitch theory is inadequate to explain the genesis of MPT and the strong 100kyr-cycle in climate records satisfactorily. The uplift of the Qinghai-Tibetan (Q-T) Plateau is the most significant geological event in the Cenozoic. High and huge plateau in middle-low latitude not only changed environment of itself and adjacent regions but also had a global significance. The Q-T Plateau was called as "the Driver and Amplifier". Correlation between uplift and climate is one of the kernel topics for Quaternary research. Qilian Mountains, as the boundary between Q-T Plateau and northwest arid region of China, has experienced strong uplift process and left clear evidences. This provides a convenience to discuss tectonic uplifts, climatic changes, and their relations. Terrace series of Shagou river, which flow northward into Tengger desert, is the direct proof for the uplift and northward expansion of Qilian Mountains. From characteristic analysis, it can be concluded that the five major terraces of Shagou river are mainly tectonic genesis, though the climate still played a role. Based on paleomagnetic dating, Thermoluminescence dating, radio carbon dating and loess-paleosol sequence matching, the ages of five major terraces are primarily determined at about 0.83Ma, 0.42Ma, 0.25Ma, 0.14Ma and 0.01Ma, respectively. The tectonic uplift events that reflected by these terraces occurred at the same time and have well parallels with the Q-T Plateau. Though the coincidence is significant, regional difference of tectonic also exists. Based on paleomagnetic dating, loess deposited in eastern Gansu (Hexi) Corridor might begin at 1.4Ma. This was late than that in the Loess Plateau. The possible cause is the stable substrate formed late than those in the Loess Plateau. Shagou loess section is situated in the marginal region of summer monsoon and Tengger desert. High dust deposit rate favors the resolution of climatic changes. Shagou section also lies in the desert-loess transition belt, thus it record could also reflect desert evolution that reflected by sand (>63μm) content. Chronology of this section was established based on some age control points and grain size age model. The result coincides well with those standard deep-sea cores. Shagou section recorded the climatic changes over the past 0.83Ma and the bottom of it is loess layer L9. Grain size and color were chosen for main climatic proxies. Chinese northern desert periodically expanded obviously since 0.6?0.7Ma, and the saw-tooth shaped characteristic became more typical since 0.42Ma. Grain size became coarser during every glacial, and reached the coarsest at glacial maximums. Then, an abrupt and rapid termination (deglaciation) ended the glacial and turned climate into interglacial. During MIS16, the combination effect of maximum global ice volume and maximum glaciation on the Q-T Plateau caused the desert to expand extremely. Spectral analysis indicates that the 100kyr periodicity, which is the same as the global ice volume cycle and the eccentricity, dominates the whole record, both in grain size and color a*. Tilt (or obliquity, 41kyr) cycle is also obvious, while the procession periodicity is weak. The results suggest that in glacial cycle time scale, loess deposit of Shagou section were mainly controlled by high latitude in North Hemisphere, such as ice sheet. Millennial scale variations, which were proved by spectral analysis, also punctuated the whole record over the past 0.8Ma. Climate changes over the last glacial cycle were well recorded in Shagou section. A pause occurred during the penultimate termination (deglaciation), like those in other region over the world. But the TII was abrupt and rapid. The record suggested that the Eemian was rather stable because of being lack of variations in MIS 5e substage. Interstadials (IS) and Cold event (C) in North Atlantic could be found in Shagou section, both in grain size record and in stratigraphy. Thus, the changes in North Atlantic could influence East Asian Monsoon system via westerlies and Siberian High even in interglacial. The last glacial could be divided into three stage according to marine isotope curve, and the millennial variation was superimposed on the global ice volume changes. Differences exist between westerlier region and monsoon region, event in inner monsoon region. From northwest to southeast, the Dansgaard-Oeschger (D-O) cycles reflected by grain size record in loess deposit became weak, while the effect of summer monsoon became strong. Thus, the intense variation in tropicalPacific Ocean could also impact loess record by counteracting the climatic signal transported from high latitude. From Shagou record, it may suggest that influence of North Atlantic was distinct during interglacial, while that of tropical oceans was obvious during glacial. Being close to glaciers and high altitude, Menyuan loess section may be the optimal candidate for reflection of glacial evolution. But the special characteristics of analysis results are beyond our understanding at now. In a certain time scale, tectonic uplift and climatic change are tightly in correlated. The coupling evolutions of glacier, loess and desert are not only controlled by the same global background but also by the amplifying effect of Q-T Plateau. The Plateau affected the westerlier and East Asian monsoon via its dynamic and thermal effects when it reached to threshold altitudes. The tectonic uplifts, which reflected by Shagou river terraces, had the parallels in climatic changes recorded in Shagou loess section. These coupling relation are named as tectonic-climatic events. It is clearly that the uplifts occurred in 0.83Ma is corresponded to desert significant expansion and periodically variation from 0.64Ma to present, the mid-Pleistocene transition and the setup of Chinese climate pattern. The 0.42Ma uplift is corresponded to the beginning of more typical saw-tooth characteristics in the climate record, and 0.14Ma uplift is corresponded to the extreme drought in northern China since the last glacial, respectively. Whether the uplift at 0.25Ma is related to the Mid-Brunhes climatic event needs more works. Though the Q-T Plateau has important role in climatic change, other factors, such as solar insolation and sea-land configuration, could affect climate obviously.