Physicochemical Characteristics Of Loess In Southern Henan And Its Paleoenvironmental Significance

In the context of global warming,Quaternary paleoclimatic changes have attracted the attention of many scholars.The loess-paleosol sedimentary sequence has become one of the three key pillars for interpreting paleoclimate.The loess-paleosol sequence buried in the southern part of Henan retains the relevant information of the original material source and sedimentary environment,and contains rich and superimposed paleoenvironmental information.Studying it will help to invert the fourth transition zone in China.The paleoenvironmental conditions of the period and their regional response to the context of global change.It also provides an important basis for geological exploration and groundwater protection in the region.Previously,some scholars have done a lot of research on the age,formation reasons,and material sources of the loess layer in the surrounding areas?such as the Mangshan and Huaihe River basins?,but most of their profiles are exposed to the surface.The research on buried loess is relatively lacking,and the information mining on its paleoenvironment is not deep enough.Therefore,our research team and Henan National Land Agency drilled in five areas including Zhumadian and Zhengyang in southern Henan,and collected underground burial loess.This study selected Zhumadian to use sedimentary indicators such as particle size,element geochemistry and colorimetry.The index,combined with the optical luminescence and electron spin resonance chronology test,analyzes the physical and chemical characteristics,and explores the causes,ages and material sources of the southern Henan loess.The main conclusions are as follows:?1?The silt samples from the south of Henan Province are mainly composed of silt components?5⁵0?m?,belonging to clay silty sand,and the content of other components is small.The overall particle size parameters have the characteristics of coarse particle size,poor sorting,near symmetry and medium kurtosis.They are very similar to typical loess,and the grain size characteristics indicate the obvious characteristics of wind accumulation.The particle size distribution curve shows a bimodal distribution.The main peak corresponds to a particle size of 20⁴0um.There is also a secondary peak at 1²um.This feature is consistent with many scholars'studies on loess grain size.However,the sub-peak height of the southern Henan loess is lower,which is closer to the right side than the main peak of the lower loess and Xi'an Malan loess,and there is a coarse particle end?>100um?substance.It may indicate that the source of loess in Henan is not the same as the two.The particle size accumulation curve of the southern Henan loess is similar to that of the lower loess,showing a three-stage distribution,suggesting that the motive power of the two is similar and the reasons for formation are similar.From the curve shape:the south Henan loess curve is wider,indicating that its particle size distribution range is larger.From the point of view of the curve turning point:in the second turning point,the southern part of the loess is more right,which proves that it contains more coarse particulate matter,and the handling power is stronger than that of the lower loess.The C-M map analysis shows that the average moving movement of the loess in the south of Henan is close to that of the lower loess,and the maximum moving energy is similar or slightly higher than the above.2)The main chemical components of Yunan loess are SiO₂,Al₂O₃ and Fe₂O₃,the sum of the three is 87.2%,the content of MnO,TiO₂ and P₂O₅ is the least,the average is less than 1%,and the chemical composition appears in different degrees in the vertical direction.Fluctuation.The weathering experienced by all samples in the southern Henan loess is moderate chemical weathering intensity.The chemical weathering intensity of various types of sediments is from weak to strong:UCC<Luochuan loess<Luochuan paleosol<Yunan loess<Henan south pale soil<Zhenjiang Xiayu loess.The values of all rare earth elements are gradually increasing from the Loess Plateau Malan Loess-Yunnan Loess-Zhenjiang Lower Loess.The distribution pattern of rare earth elements in the loess of southern Henan:a"V"type with a negative slope,all of which are relatively rich in light rare earth elements,and Eu is a relatively negative anomaly,while the light and heavy rare earth elements in the paleosol layer are more fractional than the loess layer.More obvious.From the spatial analysis,the fractionation of light and heavy rare earth elements,the southern Henan loess is higher than the Zhenjiang loess and the Loess Plateau Malan loess.?3?The correspondence between chromaticity parameters and stratum is good,especially L*and a*have obvious correspondence with loess-paleosol layer.Loess layer has higher L*and lower a*value,paleosol.In contrast,the above characteristics indicate that chromaticity can be used as a reference index for dividing the strata in the southern Henan loess section.The chromaticity parameters are sensitive to climate change.In most of the horizons,L*is inversely related to Sr,and L*is positively correlated with Rb/Sr,which is consistent with the paleoclimatic significance of the three.Therefore,the chromaticity index in the southern Henan loess has the potential to become an alternative indicator for paleoclimate change.?4?The particle size composition and particle size parameter scatter plot of the southern Henan loess all point to the formation of the wind,but the particle size parameter discriminant shows that most of the samples are aeolian,and a few samples are water,which is judged as a sample of water.Containing>100?m components,probably because the Ruhe River,a tributary of the Huaihe River,flows nearby,and the loess deposits may accept some river sediments during the rainy season.It is also possible that this component is from the secondary transport of winds from the floodplain of the nearby river,and retains some of the grain size characteristics of the floodplain deposits after deposition.The analysis of the content of macroelements in the loess of southern Henan Province suggests that the reason for its formation should be similar to that of typical wind-dust loess,which is basically formed by wind accumulation.?5?The relatively stable macro-trace elements in the sediments of the southern Henan Loess are obviously different from the Loess Plateau profile,indicating that the southern Henan Loess and the Loess Plateau deposits in the inland deserts of Asia are likely to have large differences in material sources..In southern Henan,the winter monsoon has overestimated the transmission of dust from the inland deserts of Asia,which may not be the main source of material for the southern Henan loess.Dust handled at close range may become its main source.?6?The bottom boundary age of the southern Henan loess is 789±79ka.According to the research results of some scholars,the Yellow River has already cut through the Sanmenxia before it,and the large amount of sediment brought by it has begun to shape the North China Plain.According to the wind field simulation published by the Global Paleoclimate Simulation and Comparative Research Program?PMIP?over the past 1000 years,the northeastern wind is prevailing in the south of Henan Province,and a large amount of dust is transported by the wind,which is deposited in the downwind direction to form loess deposits,from Kaifeng.The surface soil particle size characteristics of the Zhumadian in Henan Province also support this result.In the south of Henan Province,the northwest wind is prevailing in the high altitude in the south,and the fine particulate matter from the northwest desert and the Loess Plateau will also enter the yellow loess sediments in the south.Therefore,the material source of the South China loess should be a mixture of multi-source materials,mainly in close-range handling.?7?Electron spin resonance?ESR?dating results show that the bottom boundary age of the southern Henan loess is 789±79 ka,and its deposition begins at the end of the early Pleistocene or the early Pleistocene.According to the data of 4 dating years,the deposition rate of loess in southern Henan Province has increased rapidly since the mid-Pleistocene.This phenomenon may be caused by the increase of the East Asian winter monsoon caused by the uplift of the Qinghai-Tibet Plateau and the increase of the sediment transport of the Yellow River from 150ka to the present.Since the end of early Pleistocene,southern Henan loess has obvious corresponding relationship with deep-sea oxygen isotopes.There are 8-9 cold-warm cycles,which indicate that the Loess in southern Henan responds positively to global climate change.Some peak cycles are different from deep-sea oxygen isotopes,and the response degree is weaker than deep-sea oxygen isotopes.From the bottom to the surface of the whole section,Rb/Sr and CIA show a downward trend of fluctuation,weathering is gradually weakened,and precipitation is gradually reduced.It also reveals the trend of climate change from wet to dry since the end of the Early Pleistocene.