Magnetic Properties Of Shandong Zhangqiu Loess And Its Paleoclimate Significanc

Climate change is one of the most important environmental issues facing the world,and studying the history climate change can provide a scientific basis for understanding the present and predicting future climate change.Loess is the main terrestrial sedimentary Carriers for Quaternary paleoclimate research,and environmental magnetism is the most commonly used proxy for loess paleoclimate research.Shandong loess,as a typical representative of loess outside the Chinese Loess Plateau（CLP）,has the potential to record climate change and sea level change,but there is a lack of systematic environmental magnetism studies on Shandong loess.In this study,the Heiyucun（HYC）loess section at the northern foot of the southern Shandong Mountains was selected as the research object.Through systematic environmental magnetism studies,the types,contents and grain sizes of magnetic minerals in the loess were clarified,and the mechanisms of magnetic property changes in Shandong loess were explored in combination with sediment particle size analysis.Combined with luminescence dating,paleoclimate proxies were selected to reconstruct the climate evolution history of the Last Glacial Maximum（LGM）in Shandong.The main research findings are as follows:（1）The particle size frequency distribution curve of the HYC loess profile shows a bimodal feature,which is very similar to the typical loess on the CLP.Among them,the modal particle size of the coarse-grained fraction is between 10 and 50μm,which is a typical short-distance suspension fraction;the modal particle size of the fine-grained fraction is around 2μm,which is more consistent with the particle size distribution of the long-distance suspension fraction and soil-forming fraction.The texture of HYC loess is homogeneous,dominated by the silt fraction,accounting for68.49%～89.78%of sediment,with a mean value of 82.47%;followed by the sand fraction,accounting for 3.51%～27.24%of sediment,with a mean value of 9.76%;the clay fraction has the least content,ranging from 4.01%to 14.44%,with a mean value of 7.77%.The median particle size of the HYC profile varies from 14.6 to 40.3μm,with a mean value of 23.71μm;the mean particle size varies from 21.5 to 49.4μm,with a mean value of 29.70μm;the modal particle size varies from 14.4 to 49.1μm,with a mean value of 29.82μm.Overall,the particle size of the HYC loess is comparable to that of the Loess Plateau loess.（2）The magnetic minerals in HYC loess include both low coercivity magnetite and maghemite,and also contain high coercivity minerals（hematite and goethite）,with S-ratio values ranging from 0.86 to 0.94 and remanent magnetic coercivity ranging from 42 to 48 m T.The overall magnetic properties are relatively“soft”.The content of magnetic minerals in HYC loess is high,and the variation ofχ_lf ranges from 50.75～76.52×10^-8 m³/kg,with an average value of 60.30×10^-8 m³/kg,which is much higher than the primary magnietic susceptibility of loess in CLP(15～20×10^-8 m³/kg).The magnetic mineral particles in the HYC loess are relatively fine,the magnetic domains are dominated by SSD,and the contribution of SP particles to magnetism varies widely between 10～75%;the equivalent magnetic grain size ranges from 0.1～1μm,with the majority of samples between 0.1～0.2μm.（3）Low coercivity magnetite and maghemite are the main contributors to the magnetic susceptibility and remanent magnetization of the HYC loess,although a certain amount of high coercivity minerals（hematite and goethite）are contained.The source of the loess material did not change significantly during the accumulation of the HYC profile,and the effect of material source changes on the magnetism of the loess was not significant.The wind sorts the grain size of the sediment and of magnetic minerals at the same time,i.e.,the stronger the wind,the coarser the grain size of the sediment and of magnetic minerals.The contribution of podogenic magnetic minerals toχ_lf is only 10%～40%,and the average contribution is about24%;the contribution to theχ_ARM value is about 9%～58%,and the average contribution is about 42%.So,χ_lf,χ_ARM,χ_ARM/SIRM andχ_ARM/χ_lf cannot be used as proxies for summer momsoon and paleoprecipitation.（4）The OSL dating results show that the HYC section is accumulated in the LGM and there are some chronological inversions in the dating results,which may be related to the unstable depositional environment caused by the stronger EAWM during the LGM.Overall,the paleoclimate evolution history recorded in the HYC section can be well compared with the Greenland ice core records,indicating that the Shandong loess has a good response to global climate change and has the potential to record large-scale climate change.