Establishment Of Absolute Astronomical Time Scale Of Late Miocene Strata Of Jianzha Basin In The Northeastern Margin Of Qinghai-Tibet Plateau,China

The tectonic uplift of the Qinghai-Tibet Plateau play an important role in promoting the evolution of the climatic and geographical patterns of its surrounding areas,especially the northeastern margin of the Qinghai-Tibet Plateau,which is located in a special geographical position,the East Asian monsoon,the westerly circulation and the Indian monsoon jointly control the regional climate system,so it formed a complex climate and geographical patterns.Since the Cenozoic Era,many fault basins began to form with the intense tectonic uplift of the Qinghai-Tibet Plateau,these basins have formed continuous thick Cenozoic sediments,most of them have been studied in Cenozoic stratigraphy and some of them have established chronological scales.These basins have sedimentary records of different geological periods since Cenozoic era,this provides us with a unique opportunity to study the uplift of the Qinghai-Tibet Plateau and the evolution the Asian inland climate.The Jianzha Basin is located in the northeastern margin of Qinghai-Tibet Plateau and contains a thick sequence of late Miocene sediments composed of thickness of the sequence of eolian Red Clay with a bit part fluviolacustrine deposits,which Jiarang section in Jianzha Basin is an ideal section for carrying out cyclostratigraphy study,because of its high resolution studies of magnetostratigraphy and environmental magnetism.Based on the systematic cyclostratigraphy study of?361m sedimentary strata of late Miocene in Jiarang section,the aim is to establish the absolute astronomical time scale of the Jiarang section sedimentary strata and extract the dominant period of earth orbital parameters in the Jianzha Basin that affect the paleoclimate evolution,which provides basic data for the further comparison between land and sea and study of paleoclimatic and paleoenvironmental evolution.The main conclusions of this paper are as follows:(1)We choose the frequency-dependent magnetic susceptibility(?fd)to carry out the spectrum analysis,the result of spectrum analysis in depth domain shows that there is a stable?26m sedimentary cycle representing long eccentricity period in the 3-361m interval,the?6.7m sedimentary cycle representing short eccentricity period,the?3m and?2.5m sedimentary cycle representing obliquity period and the?1.4m and?1.2m sedimentary cycles representing precession period are also reflected,but they are relatively weak and unstable.Based on the identified sedimentary cycles,the long eccentricity period are used to tune ?fd data series in the depth domain of 3-361m.Combined with magnetostratigraphic chronological framework of Jiarang section,the absolute astronomical time scale of about 11.758-5.890 Ma is established.The time domain of astronomically tuned ?fd data series is consistent with the paleomagnetic age data,which indicates that the astronomical time scale is reliable.(2)On the basis of establishing astronomical time scale,spectrum analysis in time domain of the benthic ?18O record and ?fd shows that there are obvious periodic shifts in about 7.2 Ma.Before?7.2 Ma,the 41 ka obliquity period was the dominant period,and after?7.2 Ma,it changed to the 100 ka short eccentricity period.This record reveals that the East Asian monsoon was dominated by 41 ka obliquity period before 7.2 Ma and controlled by 100 ka short eccentricity period after 7.2 Ma.(3)Based on the comparative analysis of magnetic susceptibility of the Jianzha Basin with the ?18O of the benthic foraminifera,Mg/Ga and reconstructed sea surface temperatures of the planktic foraminifera in the southwest Pacific Ocean and ?18O of the benthic foraminifera in the southeast Atlantic Ocean and global marine oxygen isotope record,we suggest that before?7.2 Ma,the 41 ka obliquity period influences the evolution of the Asian inland climate and the East Asian monsoon by regulating the periodic changes of the Antarctic ice sheet;after?7.2 Ma,the 100 ka short eccentricity period influences the evolution of the Asian inland climate and the East Asian monsoon by regulating the periodic changes of the Arctic ice sheet.In addition,the magnetic susceptibility and carbonate content of Jianzha Basin are compared with the magnetic susceptibility of Qinan section,indicating that the uplift of the Qinghai-Tibet Plateau during?8.5?7.2Ma has a significant driving effect on the palaeoclimate evolution of the Jianzha Basin.