Environmental Changes Since The Mid-holocene Recorded By Magnetic Properties Of Aeolian Deposits Over The Ordos Plateau

In the present study,systematic environmental magnetic parameters and sediment grain-size were measured for three Holocene aeolian sections?Jinjie,Wangjiagou and Xini?and 100 surface soils collected from the Ordos Plateau,in order to explore the magnetic property change mechanism of desert aeolian sediments and select the proper proxy for East Asian summer monsoon?EASM?,East Asian winter monsoon?EAWM?and mineral dust concentration.Optically stimulated luminescence?OSL?and radiocarbon(¹⁴C)dating methods were used to establish the chronology framework of the aeolian sections.Based on the chronology and environmental proxies,we reconstructed the moisture,wind intensity and dust variations over the Ordos Plateau since the mid-Holocene,and further explore their interrelation,forcing factors and relationship with human activities.The main results are as follows:Magnetic minerals of aeolian sediments on the Ordos Plateau are dominated by ferromagnetic minerals?magnetite and maghemite?with a few antiferromagnetic minerals?mainly hematite?mixed.Magnetic mineral concentration is relatively low compared with loess-palaeosol sequences on the Chinese Loess Plateau?CLP?.Magnetic grain-size of the Jinjie section is quite fine,and the domain states are mainly coarse stable single domain?SSD?and superparamagnetic?SP?.While,magnetic grain-size of the Wangjiagou and Xini sections is coarse,and the domain states are mainly multi-domain?MD?and pseudo single domain?PSD?.Magnetic susceptibility enhancement of mid-Holocene palaeosol of the Jinjie section is caused by pedogenic fine and ultrafine particles,which is in accord with the “peodgenic mode” of CLP loess.While,magnetic susceptibility enhancements of aeolian sands of the Jinjie section,Wangjiagou section and Xini section are caused by aeolian coarse particles,which is similar with the “dust input mode” of Xinjiang loess.The lithogenic and peodgenic magnetic susceptibility components,symbolized as c₀ and X_pedo respectively,were separated using the mean value concept in a mono-linear regression of multi-variables.The results show that X_ARM/SIRM is linearly positive correlated with the logarithm of X_pedo/c₀,which confirms that it can be used as a proxy for pedogenesis and moisture condition;while,HIRM is linearly positive correlated with c₀,which suggests that it can be used as an indicator for detrital hematite and mineral dust concentrations.The X_ARM/SIRM records of three aeolian sections show that 7.4 4.6 ka was humid over the Ordos Plateau,which corresponded to the Holocene climatic optimum,and climate was gradually drying since 4.6 ka.Our results are in agreement with widespread palaeosol development during the mid-Holocene and aeolian sand or loess accumulation during the late Holocene recorded by stratigraphic chronology from sandy lands of northern China and the CLP,and do not support the view of an early Holocene moisture maximum and a dry mid-Holocene derived from some lacustrine and palustrine records in the Ordos Plateau and surrounding areas.The Jinjie and Wangjiagou sections record five millennial-scale dry events at 6.6 5.6 ka,4.6 4.1 ka,3.5 2.5 ka,1.5 1.0 ka and around 0.5 ka respectively,which are corresponding with the Bond events 4 0?around 5.9 ka,4.2 ka,2.8 ka,1.4 ka and 0.4 ka respectively?recorded by ice-rafted debris?IRD?from the North Atlantic.Besides,the Xini section in the northern Ordos Plateau,where the present-day climate is drier,even records centennial-scale moisture fluctuations during the Holocene climatic optimum,and six moisture peaks were detected at 6.9 ka,6.4 k,6.1 ka,6.0 5.8 ka,5.5 5.3 ka and 4.8 4.6 ka respectively.These moisture peaks all occurred during the decreasing of ENSO frequency,without exception,and lagged the ENSO frequency maximum for about 100 300 a.It suggests that the EASM is influenced by high-latitude?North Atlantic?and low-latitude?ENSO?climatic conditions on millennial-scale and centennial-scale,respectively.Sediment grain-size reveals a long-term weakening of the EAWM since the mid-Holocene,which is in phase with the EASM on orbital timescale.In view of the stable glacial boundary conditions and the significantly decreased rates of atmospheric CO₂ rise since the mid-Holocene,we suggest that orbitally-induced insolation was the major cause of the weakening of both the EASM and EAWM.Decreasing summer insolation and increasing winter insolation reduced the thermal contrast between the ocean and the Asian continent in both summer and winter time,resulting in the weakening of both monsoon systems since the mid-Holocene.There are also centennial-scale fluctuations in the EAWM record during the mid-Holocene?7.1 4.6 ka?,for examples,the EAWM was strong during 7.1 7.0 ka,5.5 5.3 ka and 4.8 4.6 ka;and the EAWM was generally strong with sharply fluctuations during 6.7 6.0 ka;while the EAWM was generally weak during the other times.These centennial-scale EAWM fluctuations are synchronous with North Hemisphere high-latitude temperature recorded by oxygen isotope of GISP2 ice core.It suggests that the EAWM is controlled by North Hemisphere high-latitude temperature on centennial-scale.North Hemisphere high-latitude temperature may influence the EAWM through the Mongolian-Siberian High.The “hard” isothermal remanent magnetizations?HIRMs?of three aeolian sections reveal that mineral dust concentration was low and stable during 7.4 4 ka,and gradually increased since 4 ka.Besides,mineral dust concentration of the Xini section in the northern Ordos Plateau is 2 3 times higher than that of Jinjie and Wangjiagou sections in the eastern Ordos Plateau.In order to verify our results,we synthesized the mass accumulation rate?MAR?on the CLP based on high-resolution stratigraphic chronology results of 12 loess sections.The results show that the MAR was lower than the average during 10 4 ka;was moderate and slightly higher than the average during 4 2 ka;and was sharply increased since 2 ka.The synthesized MAR of the CLP is in agreement with mineral dust concentration of the Ordos Plateau,which confirms the reliability of our HIRM-based mineral dust concentration records on the one hand,and on the other hand,it suggests that the Ordos Plateau might be one of the main source regions of loess deposits on the CLP,at least during the midto late Holocene.The increase of mineral dust concentration since the mid-Holocene is mainly caused by moisture decreasing and human activity intensifying,and the influences from the EAWM is relatively minor.An inverse proportional relationship was detected between mineral dust concentration and moisture.Mineral dust concentration is more sensitive to dry climate.When moisture is low,mineral dust concentration deceases sharply with moisture increasing;when moisture reaches a threshold(X_ARM/SIRM = 0.2 × 10^-3 m/A),mineral dust concentration decreasing rate gradually slows down;and when moisture reaches another threshold(X_ARM/SIRM = 0.4 × 10^-3 m/A),mineral dust concentration tends to stabilize at a low level.The influences from human activity superimpose on the moisture-induced mineral dust concentration changes.The anthropogenic influences on mineral dust concentration were secondary before 2 ka,and significantly increased since 2 ka.