Study On The Spatial Difference And Influence Mechanism Of Paleoclimate During MIS-13 Recorded By Mid-latitude Asian Loess

The earth climate has undergone two climatic transitions since Quaternary glacial period.The first is mid-Pleistocene transition(MPT)or mid-Pleistocene revolution(MPR).It is characterized by an increase in mean global ice volume and a change in the dominant period from 41 to 100 ka.Its timing is often considered to be at about 900 ka BP.The second is mid-Brunches event(MBE),roughly corresponds to the transition between MIS-12 and MIS-11 about 430 ka ago.The MBE is characterized by a further increase of ice-volume variations,large amplitude glacial-interglacial fluctuations,and increased dissolution of carbonates worldwide.The intermediate period between MPR and MBE is characterized by a less clear pattern,with significantly weaker amplitude of ice volume variations than after MBE.It is widely spread Quaternary loess sequences in the mid-latitude Asian inland,these sequences contain a wealth of paleo-climate information,which could be contrasted with deep sea oxygen and the polar ice core.In Chinese Loess Plateau(CLP),S5 soil layer developed three sub-layers: S5-1,S5-2 and S5-3,the extremely high magnetic susceptibility indicates S5-1 paleosol layer,corresponding to MIS-13 occurred about 500 ka years ago which is a special stage of paleoclimate evolution,had undergone the most intense pedogenesis during the last 800 ka.It may indicate the extremely strong East Asian summer monsoon(EASM).This seems to be paradoxical with a relatively cool interglacial recorded by deep sea oxygen and Antarctic ice core.Although researchers have proposed the viewpoint that the extremely strong EASM occurring in the relatively cool interglacial MIS-13,it still needs more evidence to support.In the meantime,regional diversity might exist in the relative intensity of the S5-1 soil formation.Studies have found that different from the central and eastern CLP,the S5-1 paleosol is weakly developed in the western CLP while the S4 paleosol(developed during MIS-11)is the most developed in the Quaternary loess-paleosol sequences.This difference is caused by atmospheric circulation pattern or soil prolonged pedogensis process? It is a still debate.Compared with the CLP,few studies have focused on the loess-paleosol sequences in central Asia,the climate conditions which are controlled by westerlies circulation.Therefore,there are some doubts: are there spatial differences about MIS-13 paleoclimate condition? If there are,what are mechanisms that cause these differences?In order to understand these questions,we took samples at Xifeng,located in central CLP,and Darai Kalon,located in Tajikistan,respectively,and collected the proxy records which are included MIS-13 or S5-1 paleosol from public literatures around global world.The orbital tuning,magnetic parameters,grain size and colorness are used as dating methods and paleoclimatic proxies for two profiles,respectively.LOVELIM simulation is used to explain climate mechanisms.Conclusions are as follows:(1)We investigate the iron mineralogical properties of the S5-1 paleosol at the Xifeng loess section located in the central CLP,the results show that magnetic enhancement of the S5-1 paleosol unit can be ascribed to produce a large number of fine-grained ferrimagnetic minerals.The prolonged period of pedogenesis is not mainly factor that impact the magnetic enhancement on S5-1 paleosol or the soil strong development during MIS-13.Changes in the ratio of pedogenic hematite to goethite,and in magnetic enhancement,indicate that the S5-1 paleosol unit developed under extremely humid conditions.(2)Numerous available loess records which are collected from the whole CLP show that in central CLP,S5-1 is indeed the strongest developed as suggested by previous studies,whereas in the western CLP,it is weakly developed and the S4 paleosol developed during MIS-11 is the strongest.This locally spatial difference which is recorded by loess is mainly controlled by the atmospheric circulation at that time.As compared to MIS-11,the northern front of the EASM during MIS-13 didn?t penetrate into western CLP,but was located more northerly and eastward.Based on sea surface temperature(SST)records in the tropical Pacific,we suggest that the temporal and spatial abnormal variation of the EASM during MIS-13 could be related to a strong La Ni?a or La Ni?a-like climate condition.Under such condition,the Western Pacific Subtropical High(WPSH)become weaker and retreats more northeastward,leading to more precipitation in the mid-east region of northern China but less rainfall in the western inland.(3)In this study,we use magnetism proxies,grain size and Lightness to research the paleosol development condition and to discuss paleoclimatic condition during MIS-13.Magnetism results indicate climate is the dominant factor of magnetic susceptibility enhancement of the paleosol units.The >25 ?m(GT25)fraction can be used to indicate the intensity of atmospheric circulation in low level.Magnetic parameters,Lightness and GT25 indicate that the S5-1 paleosol is relatively weak development,suggesting a relatively warm and dry environment,and weak surface winds during MIS-13,while that S3,corresponding to MIS-9,is the most developed soil,suggesting a relatively warm and humid environment,and strong surface winds during MIS-9.By comparing to Indian summer monsoon(ISM)index,we suggest the low the atmospheric greenhouse gages(GHG)concentrations during MIS-13 may cause the lower temperature over Southern Hemisphere high latitudes,and finally lead to Indian summer monsoon(ISM)strengthened and extension via atmospheric circulation process,in turn,the Subtropical High and westerlies are also extended into northward.Therefore,Tajikistan at central Asia is controlled by Subtropical High for a prolonged period,which may cause the dry environment,and lead to the weaker S5-1 paleosol development.(4)By comparing the MIS-13 environment condition with MIS-9 based on simulation results,we find the moisture conditions between the CLP and Tajikistan are consistent with our proxy records under the combined effects of GHG and insolation.In order to understand mechanism of the paleoclimate spatial difference during MIS-13,we further compare geological records from globally other regions.The results find compared MIS-13 with the subsequent interglacial intervals,in the Northern Hemisphere(NH),in addition to AM region,it is boundary by 40?N,the most of proxy records in the subtropical and warm temperate zones indicate an arid environment between 20?N and 40?N,while almost all geological records indicate a warm and humid environment in the north of 40?N,and the tropic is also a warm and humid environment in south of 20?N.By contrast,in the Southern Hemisphere(SH),almost all geological records indicate a relatively dry environment from mid-latitude subtropical(40?-20?S)to low-latitude tropical zones(20?S-0?).There are also regional differences about the maximum values of SST of different ocean drillings by comparing MIS-13 with MIS-9.In this paper,we combine the simulation result with geological records to explain the possible driving and feedback mechanisms of paleoclimate spatial difference during MIS-13.We suggest that this paleoclimate spatial difference may be result from external forcing factors and internal feedback of climate system.Specifically,on the one hand,high latitudes of NH summer could receive more insolation than that of SH summer with lower GHG concentrations during MIS-13,which are prone to causing the colder SH and warmer NH,that is,asymmetry of hemispheric climates.This asymmetry could cause different climatic zones migration to north,which may lead to the arid environment which has been indicated by different geological records of SH.The northern movement of Intertropical Convergence Zone(ITCZ)leads to the strong summer monsoon in tropical monsoon regions of the NH.The northern movement of ITCZ also may strength Atlantic Meridional Overturning Circulation(AMOC),which will transfer more heat from the SH to NH,will further aggravate the asymmetry of hemispheric climates,and could cause the SST anomaly between the east and west of tropical Pacific.This SST anomaly could generate a strong La Ni?a-like climate condition,which will cause the anomaly of location and intensity of WPSH.On the other hand,the configuration of external driving factors could cause meridional temperature gradient increases between the low and high latitudes of NH,which will lead to the westerlies circulation strengthened and northward,and will generate a positive North Atlantic Oscillation(NAO)-like climate condition.Meanwhile,the positive NAO also could promote the northward transport of surface ocean water flow in the North Atlantic Ocean to further enhance the AMOC,the strengthened AMOC further strengthen the La Ni?a-like climate condition,and in turn,the La Ni?a-like climate condition may promote the positive NAO.Therefore,we suggest that such a feedback mechanism that it causes the spatial differences of paleoclimate during MIS-13 in mid-latitude Asia.