Pedogenesis Of A Thick Loess Profile In Shenyang And Its Application In Geoarchaeology

Thick loess is the combination of the long-term deposition of wind-deposited dust and soil development.Loess is also an important material for archaeological research,and its genetic characteristics are widely used in geoarchaeological studies.However,paleoclimatic records and the genetic characteristics of thick loess in northeast China are still unclear.In this study,the well-preserved,continuously deposited,450 cm thick loess profile（named PAS）excavated from the Paleolithic archaeological site in Shenyang Agricultural University was taken as the main object of study.Genetic characteristics of the PAS profile were synthesized to determine if the parental material was uniform.Using soil development indices,determined the weakest weathering horizon of the profile as the reference parent material.Based on the above,the ratio of water-dispersed clay to clay（WDC/C）was used to characterize the proportion of clay derived from leaching deposition.The Marshall,Barshad,Soil Reconstruction（SRE）and Mass Balance（MB）methods were used to quantify the movement of clay.Meanwhile,the patterns of elemental composition and different forms of silicon in the soil structures and their surface were determined for another profile（NPAS）at a distance of 443 m from the PAS.Subsequently,the basic genetic characteristics of five loess sites in Mulhouse and Lyon of France were analyzed and the published studies of Anhui Xuancheng and Shaanxi Luochuan loesses were cited to compare the genetic characteristics of Chinese（Shenyang,Xuancheng and Luochuan）and eastern France.Then,the particle size composition characteristics,major elements and rare earth elements distribution patterns of the soils within and around the suspected relics were analyzed to determine the uniformity of the soil parent material.Soil organic carbon,available phosphorus content and magnetization rate were analyzed at the level where the suspected relics were located to determine whether human activities might have been present.Finally,the soil development index-climate factor conversion functions were established using a nationwide loess dataset,and the relevant properties of PAS profiles were applied to reconstruct the paleoclimate changes in the Shenyang and compare them with modern climate corresponding to the Marine isotope stages（MIS）.The main results were as follows:（1）The PAS profile parent material is uniform,and the horizon Bt2 is selected as the reference parent material.PAS was a general acidic loess profile with a bottom age of about110 ka;the carbon content was low,silt-based and the change of clay-free particle size distribution and the magnetization was not obvious;except for some elemental changes due to leaching,the overall profile elemental changes were also not obvious;the profile was moderately developed,among which the horizon Bt2 has the weakest degree of weathering.It can be concluded that the profile PAS was a thick loess profile formed by continuous deposition and development under natural conditions with uniform loess-like parent material since 110 ka BP.The horizon Bt2 was selected as the reference parent material.The analysis of the characteristics of this profile can enrich the study of the formation process of thick loess in northeast China and make up for the lack of paleoclimatic records in the region.（2）Determine the increase and decrease of clay at different horizons and the change pattern in different periods.The WDC/C of each horizon in the PAS profile ranged from12.35%to 39.24%,and the increase rate of clay was 3.02%to 16.86%,indicating different degrees of development in each horizon.The lowest clay grain rate in the whole profile occurred in 160–210 cm（3.38%–5.01%）and the highest rate in 270–310 cm（8.96%–16.86%）.Corresponding to the soil age and global climate change,the climate was warm and humid from110–71 ka BP,and the values of WDC/C and clay increase rate were greater in soils formed during this period（210–450 cm）than in soils formed in the relatively cold and dry 71 ka BP to the present（0–210 cm）.The calculated volume and elemental change patterns were similar to those of the clay movement.It was determined that the grayish-white material was soil new-growth silica powder,and the leaching effect was greater on the structural surfaces than in the structures.It was found that the concentration of active silica increased in the upper part of the argic horizon and gradually increased with depth,while the amorphous and water-soluble forms of silica increased abruptly at the beginning depth of the argic horizon and then stabilized.In summary,the increase of clay since 110 ka BP in the thick loess profile of northeast China is divided by 71 ka BP,and a considerable amount of clay came from leaching.The migration of clay was directly related to climate change.The formation of silica powder in soils may be influenced by a combination of factors.（3）The loess of China and eastern France have similar transport and deposition dynamics but differ in their specific composition and degree of development.The loess parent material in Mulhouse was uniform but changed in Lyon.From top to bottom,soil genesis was weakened and sedimentation was enhanced in Mulhouse,while Lyon was the opposite.Comparing the loess in eastern France（Mulhouse）with that in China（Shenyang,Luochuan and Xuancheng）,it can get that that Shenyang,Luochuan and Mulhouse have the highest content of 50–10μm grain size,and Xuancheng has the highest content of<5μm.The sand（>50μm）content of loess in eastern France was higher than in China.The particle size composition of loess in both China and eastern France was mainly composed of<50μm material,reflecting the similarity of transport and deposition dynamics.However,there were still some differences in the specific composition.The order of soil development was Xuancheng>Shenyang>Mulhouse>Luochuan.（4）The suspected relics were the product of natural soil formation processes rather than the relics of human activities.The genetic characteristics of the suspected relics and the surrounding soils showed that they have a uniform parent material,and none of the soil characteristics at the level where the suspected relics were located indicate the presence of human activities,suggesting that they were the products of natural soil formation processes rather than human activities.Further study concluded that these soil features were redox features and the initial stage of the formation of fragipan.（5）A new soil development index-climate factor conversion function was established to quantitatively reconstruct the paleoclimate changes in Shenyang.The newly established transfer functions for soil weathering index and climate were ba1[（K₂O+Na₂O）/Al₂O₃]and mean annual temperature（MAT）:MAT=–29.887ba1+22.361（R2=0.786）,and CPA[100×Al₂O₃/（Al₂O₃+Na₂O）]and mean annual precipitation（MAP）:MAP=0.0843CPA³–20.143CPA²+1630.7CPA–44244（R2=0.892）.By applying these two functions,the changes of MAT and MAP from 110 ka BP to 15 ka BP in Shenyang were quantitatively reconstructed.Comparison of the results with the current MAT of 8.5°C and MAP of 698.5 mm in Shenyang showed that:during MIS 5（110–71 ka BP）,MAT was about 4-5°C higher than the current,and MAP showed several fluctuations but was generally higher,peaking at about 100 mm higher than current.;MIS 5/4 transition（～71 ka BP）showed a strong decrease in both MAT and MAP to a minimum at 60–50 ka BP（9.9°C for MAT and 528 mm for MAP）;during the middle of MIS 3（～47 ka BP）,both MAT and MAP increased;MIS 2（29–14 ka BP）the MAT and MAP gradually decreased again to～9.5°C and～500 mm.The above comparison showed that the reconstructed paleoclimate of Shenyang area was consistent with the global paleoclimate change.In summary,the quantitative reconstructed paleoclimate of Shenyang in this study enriched the record of paleoclimate in northeastern China,and the newly established soil development index-climate factor conversion function can provide a scientific basis for the quantitative reconstruction of paleoclimate on a national scale.