Sedimentary Sequency Response To Orbital Forcing The Paleoclimatic Changes In The Lacustrine Records Of The Northeastern Tibetan Plateau Since The Miocene

East Asian landscape during the Cenozoic period has undergone major changes,from the former low altitude in west regions to the high altitude,which was attributed to the uplift of the Tibetan Plateau?TP?.The uplift of the TP has had a profound impact on East Asian paleoenvironment:on the one hand,the high altitude of the TP blocked the warm and humid air currents of the Indian Ocean from transporting to the inland Asia,leading to extremely humid in the southern side of the Himalayas,while the northern side of the inland Asia was perennially dry;on the other hand,the high Tibetan-Himalaya orogen would lead to greater rates of silicate weathering and erosion contributing to lowering of atmospheric CO₂ concentrations to force global cooling.The geological records indicate that the modern geomorphological patterns of the TP may be shaped in three important uplifting and outward growth stages of the 55-35 Ma,30-20Ma and 15-8 Ma periods.These three uplifts of the TP have profoundly affected the Asian paleoclimate,particularly at 30-20 Ma,which may have contributed directly to the origin of the East Asian monsoon system.The East Asian monsoon system is an important component of the global climate system,and its formation and evolution history has profoundly affected the environmental and geomorphological changes in East Asia.It has attracted the attentions of many geologists that carried out research on lake,loess,stalagmite and marine deposits and achieved a series of fruitful results.However,different scholars have different results on the strength evolution of the East Asian monsoon and its changes in key time points.In addition,the studies on the orbital scale of the East Asian monsoon are mainly focused on the Quaternary loess and stalagmites,and it is lack of studies in the pre-Quaternary period and lake sediments,which lead to a significant deficiency for us to understand the variational characteristics of the East Asian monsoon on the orbital scale.We analyzed the lithostratigraphy,magnetostratigraphy and cyclostratigraphy for the Guide and Tianshui basins in the Northeastern TP,and the YZ01 core and Yangqu loess section in the Shanxi Province to reconstruct the evolution characteristics of the East Asian monsoon on the tectonic and orbital scales since 15 Ma.The main conclusions are as follows:1)The astronomical chronology of the Ashigong Formation in the Guide Basin was established,and found that evolution of the Guide paleolake during the middle-late Miocene?¹4-10 Ma?was dominated by¹00 kyr cycles,revealing the variations of the East Asian summer monsoon during¹4-10 Ma.We analyzed the 510 m of the lithology and magnetostratigraphy,and its 370 m?140-510 m?of the redness?a*?,magnetic susceptibility and elemental content of the Guide Basin in the northeastern TP.The lithological characteristics show that our sampling profile belongs to the Ashigong Formation.The paleomagnetic results show that the age of the sampling profile is about¹4.4-10 Ma.A large amount of gypsum deposits in our sampling profile indicate that the Guide Basin was a closed salt lake system during¹4.4 to 10 Ma,confirming previous studies that proposed an early Miocene partitioning of this basin.The results of spectial analysis show that the Guide paleolake evolution was mainly controlled by 100 kyr eccentricity.The maximum lake expansion between 11.4 and 10.5 Ma was coeval to a known peak of monsoonal intensity,indicating that summer monsoon rainfall had played a role in lake evolution.The origin of the¹00 kyr forcing on monsooal moisture supply-different from Quaternary forcing-questions the climatic mechanism of these middle Miocene monsoons and suggests that westerly derived moisture might have also significantly contributed to the regional hydrological budget.2)The astronomical age of the lacustrine deposits in Yaodian site from the Tianshui Basin was established,and was proposed that evolution of the lake basin in the northern TP during 14-7 Ma is mainly dominated by¹00 kyr cycles,and revealed that the origin of¹00 kyr eccentricity was remotely related to the Antarctic ice sheet.We further reported the magnetic susceptibility?MS?and Rb/Sr data?92.5 m?from the lacustrine deposits in the Tianshui Basin?¹0.25-8 Ma?,northern TP.The results of the spectral results,combined with published cyclostratigraphic records of the Guide?¹4-10 Ma?and Qaidam basins?⁸.5-7 Ma?,show that periodic expansion of the lake is mainly driven by the 100 kyr eccentricity during the 14-7 Ma period,which is significantly different from the dominated obliquity in the Quaternary period.These 100kyr cycles corroborate with a significant forcing of the East Asian hydrological cycle by Antarctic ice sheet variations at that time.It is,however,unclear if this forcing affected EASM intensity or westerly-derived moisture supply to the Far East.Regardless of the nature of the main source of precipitation in North Tibet during the Miocene,these results emphasize the existence of a strong teleconnection between Antarctic ice-sheet modulations and the continental climate of Asia.3)Magnetostratigraphy and astronomical age scale of the borehole YZ01 in Xinzhou Basin were established,and was proposed that cut-through Fenwei Graben by the pro-Yellow River drainage was at².5 Ma.We applied a combination of lithostratigraphy,magnetostratigraphy and cyclostratigraphy to date the terrestrial succession within a 365 m borehole in the northeastern Fenhe sub-basin of Fenwei Graben.The change from a shallow lake and/or peatland facies to fluvial gravel deposits occurred at 2.5 Ma,which we interpret as the timing of the steam capture of the Yellow River drainage through the proto-Sanmen Gorge.This river piracy and cutting of the proto-Sanmen Gorge coincided with the onset of Quaternary climate change and base-level drops from eustatic lowstands,and with increased tectonic uplift and eastward extrusion of the Tibetan Plateau.At¹.84Ma,the Fenhe sub-basin is splited by the uplift of the Xizhou Mountains into two depocenters of the northeastern Xin-Ding Graben and the southwestern Taiyuan Basin.This disruption of the Fenhe sub-basin may have been a response to far-field effects of a pulse of outward growth and eastward extrusion of the Tibetan Plateau during1.8-1.7 Ma.Breach of this paleolake took place about 0.43 Ma inferred from occurrence of yellow medium-fine sandstones at about 32 m in the YZ01 drill.4)The spectral analysis of MS and GR from the YZ01 core in Pliocene period shows that¹00 kyr eccentricity intensity changes significantly during⁴.0-3.5 Ma,revealing that the expansions of the Antarctic ice sheet around 3.5 Ma may cause the transformation of sedimentary cycles in the northern hemisphere.Based on spectral analysis of the magnetic susceptibility?MS?and gamma data?GR?from the YZ01 core in Xinzhou city during the Pliocene period,we found that a significant change of¹00 kyr periodic signal occurs during⁴.0-3.5 Ma,which MS becomes weaker and the GR significantly enhanced.Spectral results of the high-resolution?³-4 kyr resolution?iceberg-rafted debris from the IODP U1361 core in the Wilkes Edge of Antarctica show significantly enhanced of 100 kyr signal at³.5Ma.We consider that the intensity transitions of the YZ01 core in Xinzhou are related to the rainfall pattern of the East Asian summer monsoon driven by the expansion of the Antarctic ice sheet.5)Two significant stages in the aridification of the eastern Chinese Loess Plateau since 1.2 MaThe magnetic susceptibility and element concentrations of the Yangqu section in the eastern Loess Plateau yield a history of aridification of the eastern Loess Plateau during the Pleistocene.Two significant stages in drying are recorded in the eastern Chinese Loess Plateau since 1.2 Ma.The first stage at about 0.94 Ma is revealed by an apparent abrupt decrease of Si,Fe and Rb/Sr proportions and a significant increase of Ca content.This event may be related to the increase of global ice volume,especially Northern Hemisphere ice sheets.The second stage of aridification at about 0.31 Ma may be a regional event,which may be attributed to decrease in penetration of the summer monsoon rains to this region.