Climatic Records Of Cenozoic Sediments From Qaidam Basin And Their Implications On Drying Of Asian Inland

A series of significant geological events occurred in the Cenozoic which have led to enormous changes to the earth climatic and environmental system: the rapid stepwise cooling of the climate, the global aridification, episodic tectonic uplifts of Tibetan Plateau and the formation and evolution of Asian monsoon. Among them, the remarkable expanding of arid areas has attracted worldwide attentions due to its profound effects on human beings and scientifically linking with Tibet uplift, Paratethys Sea retreat and global cooling. To understand when and how the Asian inland aridification evolves is of great significance. However, up to now, the scarcity of high-resolution long continuous terrestrial climate records has hampered our insight into the above questions and land-sea comparison. Here we present firstly continuous Cenozoic climate record from the Qaidam Basin, Qinghai Province, NW China in the interior of Asian inland.The Qaidam Basin is a huge intermountain basin in the northern Tibetan Plateau and receives ultra-thick (up to over 10,000 m) and almost continuous Cenozoic sediments in response to the global climate change and uplift of the Tibetan Plateau. A～5000-meter-long continuous sediment outcrop at Lulehe from the north margin of the Qaidam Basin and a 700-meter-long supplementary outcrop at Yahu from the near center of the basin comprise almost a complete Cenozoic stratigraphic sequence (paleomagnetically dated between 53.4 Ma and 1.83 Ma), thus providing a unique opportunity to study the above questions. We carried out detailed analyses of color, carbonate, chloric ion and TOC contents, biomarker and trace elements from these sediments. Through these multiple climatic proxy records and combining a framework research into the sedimentary facies and spectrum analysis, we obtain the following conclusions:1) The redness of color records of the Cenozoic Qaidam stratigraphy can be correlatedwell to the global temperature record, implying the redness can be served as asensitive alternative temperature proxy in Asian inland. 2) The characteristics of the Qaidam multiple climatic proxy records and thus the evolution of the Qaidam Cenozoic climate can roughly be divide as three large stages by two boundary ages at ca. 44.6Ma and 9.7Ma:During 53.4⁴4.6Ma: the records show a low TOC content, high contents of CaCO₃ and Cl^- and maximum redness value with large fluctuations.During 44.6⁹.7Ma: all the proxy records show an overall pattern of substantially lower values of redness and contents of CaCO₃ and Cl^- and higher TOC content, punctuated by some long changes of higher or lower values clearly subdivided as four sub-stages with boundary ages at 33.5Ma, 26Ma and 14¹5Ma.During 9.7～1.81 Ma: The records display a distinct fast persistent stepwise increasing of CaCO₃ and Cl^- contents and Sr/Ba ,and B/Ga ratios and decreasing of redness. The TOC content keeps low and reaches its minimum value.We interpret the above variations of our records as:i) a dry and hot climate dominated the Qaidam Basin during 54-44.6Ma, with a relative homodromous match of temperature and precipitation, i.e. warm vs. humid and cold vs. dry; ii) a relatively humid climate with large long fluctuations prevailed the period of 44.6-9.7Ma. The match of temperature and precipitation is just reversed, i.e., relative warm vs. dry and cold vs. humid; iii) a remarkably persistent and stepwise dry trends or drying of the Qaidam basin began ca. 9.7Ma, with a relative homodromous match of temperature and precipitation again. Further drying events occurred at ca. 8Ma,3.6Ma and 2.6Ma.3) Spectrum analysis of the records shows that Cenozoic climate in Qaidam basin was clearly forced by orbital parameters and modulated by long-term cycles such as 2.3Ma, 1.2Ma and 0.4Ma. Moreover, the amplitude of these periods achieved their maxima during the early Cenozoic (53.4～44.6Ma) and warm Miocene (26～14Ma) and decreased during the cold Oligocene (35～26Ma) and cold middle to late Miocene and thereafter (14～1.8Ma). Detailed comparison between redness record and filtering results also indicates that nearly every cold event coincides with minimum amplitude of 1.2Ma long-term obliquity cycles since 33.5Ma.4) Based on detailed discussion on processes of Tibetan Plateau uplift and Paratethys retreat as well as comparison of our records with those worldwide, we proposed a simple conceptual model to illustrate our observations and interpretations above:A. The Qaidan Basin might be completely controlled by Subtropic High during 54～44Ma, leading to a dry and hot climate. The Westerlies carried limited vapor from Atlantic and Paratethys Sea to the Qaidam Basin and hence gave rise to a positive correlation between temperature and precipitation, i.e., hotter - more vapor - more precipitation in Qaidam Basin.B. The obvious northward move of Qaidam Basin since 44Ma driven by the collision of India with Asia and long global cooling since ca. 50Ma that will cause a southward move of westerlies due to enhanced equator - polar temperature gradient might jointly let the Qaidam Basin at the transitional area of the westerlies and Subtropic High. Consequently, cooling from periodic variations of Earth's orbital parameters will increase relative humidity or reduce evaporation in one side and will force a southward move of the westerlies that carry more vapors from Paratethys Sea to the Qaidam Basin in other side, both will give rise to a humidification of the Qaidam Basin. In a reverse, periodic warming will drive northward move of the Subtropic High and retreat of the westerlies and increase evaporation, thus drying of Qaidam. This mechanism may be responsible for our observation of overall relative humid climate with a relative warm-dry and cold-wet match during this period.C. Since 9.7Ma, the dramatic uplift of Tibetan Plateau and rapid retreat of the Paratethys Sea has not only contributed to the tremendous decrease of vapors in westerlies over areas, but also forced the prevailing west-to-east flow of the westerlies to flow around the Plateau and formed Siberian High, leading to a rapid long aridification of Northwest China. This drying trend may be enhanced by a later rapid global cooling. At the same time, the enhanced Asian summer monsoon by the rapid uplift of Tibetan Plateau might also account for the increase of precipitation during the warm period. Hence, high temperature may also correspond to high precipitation during this period. In one word, both our preliminary records and supposed model indicate that the Tibetan Plateau uplift, Paratethys Sea retreat and global climate changes all have exerted strong impacts on Qaidam Cenozoic climate evolution with their different contributions at different stages, and a positive superimposing of all these three factors may have finally caused the aridification of Qaidam and Asian inland which resembles its present configuration since ca. 9.7 Ma.