Response Of Holocene Clay Mineral Records To Climatic Changes And Sedimentary Environments In Huhenaoer,Inner Mongolia

The relative content and crystalline structure of clay minerals are more sensitive to climatic and environmental changes and are proxies for reconstructing paleoclimate and paleoenvironment.However,clay minerals are not only affected by climate change,but also may be influenced by sedimentary environment,latitudinal differences,and post-burial diagenesis,resulting in multi-solvability of their climate and environmental reconstructions,and how clay mineral records and their associated indices respond to climate change and sedimentary environment has become a hot issue of interest to researchers.To this end,this paper establishes clay mineral composition and related index change sequences using Huhenaoer Holocene lake sediments and explores the response mechanisms of clay mineral records to climate change and sedimentary environment in each stage of HH profiles using the climate change sequences and sedimentary environment evolution sequences reconstructed with the help of proxies such as particle size,characteristic elements,and pollen.The main results obtained from the study are as follows:1.The qualitative and semi-quantitative analysis of clay minerals in HH section of Huhenaoer Lake shows that illite,montmorillonite and kaolinite are the main clay mineral assemblage in HH section.Since the Holocene,the illite content has generally increased,the montmorillite content has decreased,the kaolinite content has two peaks,the illite crystallinity has two peaks,and the chemical index has a decreasing trend.2.Using the AMS¹⁴C chronology of the HH section as a framework,the climate change and sedimentary environment evolution sequence of Huhenaoer since the Holocene was reconstructed using characteristic elements,particle size,sporulation,and other indicators:10,100–7900 cal.yr BP,the climate is warm and wet,and the lake is at a high water level;7900–6500 cal.yr BP,the climate cools,precipitation decreases,and the lake is at a low water level or saline meadow depositional environment;6500–4800 cal.yr BP,the climate is cold and dry,precipitation increases slightly,and the lake is at a low water level,slightly higher than the previous period;from 4800–2500 cal.yr BP,the climate is cold and dry,and the lake level is low or intermittently dry;from 2500–1300 cal.yr BP,the climate is cold and dry,the lake level increases slightly,and it is a shallow sedimentary environment;from 1300 cal.yr BP to the present,the climate is warm and humid,and the lake level is high.The lake level is high,but not early Holocene.3.The response mechanism of clay mineral records in HH profile to climate change and depositional environment evolution was discussed by reconstructing the sequence of clay mineral composition and derivative index and characteristic elements,particle size,palynology and other indexes.From 10,100–7900 cal.yr BP,the peak value of kaolinite and the low value of illite may be a response to the intensified weathering in the warm and humid climate of the source area.The content of montmorillonite may be a common response to climate and sedimentary environment.On the one hand,the warm and humid climate conditions in arid areas promoted the formation of more montmorillonite by chemical weathering;on the other hand,the location of the sample point（the center of the lake）on the great lake was far away from the position where the river entered the lake mouth,and montmorillonite with particle size only 1/5 of illite and kaolinite was enriched in the center of the lake due to hydrodynamic differential sedimentation.From 7900 to 6500 cal.yr BP,the peak of kaolinite content may be a response to the depositional environment.The shrinking of the lake makes kaolinite with larger particle size more likely to settle into the center of the lake.The content changes of illite and montmorillonite respond to the changes of climate and sedimentary environment.The cold and dry climate is conducive to the preservation of illite,but not conducive to the formation of montmorillonite.In addition,the retreat of the lake will reduce the sediment differentiation of clay minerals and the proportion of montmorillonite with small particle size.From 6500 to 4800 cal.yr BP,kaolinite content decreased,montmorillonite content increased,and illite content was low,all of which responded to the climate and sedimentary environment.On the one hand,a slight increase in precipitation would convert part of illite into montmorillonite,and the cold and dry climate was not conducive to the formation of kaolinite.On the other hand,the increase of the lake area will make it easier for the smaller size of montmorillonite to be transported to the center of the lake settlement,while the larger size of illite and montmorillonite is not easy to be transported to the center of the lake settlement.From4800 to 2500 cal.yr BP,the increase of illite content and the decrease of montmorillite content are responses to both climate change and sedimentary environment.The cold and dry climate is conducive to the preservation of illite,but not conducive to the formation of montmorillite.In addition,the retreat of the lake will lead to the weakening of the sedimentary differentiation of clay minerals in the process of transport and deposition.The increase of kaolinite is mainly in response to the depositional environment,and the decrease of the distance between the lake center and the inlet makes it easier for the large-size kaolinite to settle in the lake center.From2500 to 1300 cal.yr BP,the decrease of illite content,the increase of montmorillonite content and the decrease of kaolinite content are simultaneous responses to the change of climate and sedimentary environment,and the response mechanism is the same as that during 6500 to 4800 cal.yr BP.Since 1300 cal.yr BP,the increase of kaolinite content and the decrease of illite content are responses to the strengthening of chemical weathering conditions of warm and humid climate in the source area,and the increase of montmorillonite content is a common response to warm and humid climate and lake expansion,and the mechanism is similar to the early Holocene lake invasion stage.4.The trends of illite crystallinity and illite chemical index in the HH profile are consistent with the trends of regional temperature and precipitation and do not show the fluctuations of clay mineral composition changes reflected by changes in the depositional environment,indicating that illite crystallinity and chemical index are not affected by burial science factors related to changes in the lake depositional environment and can faithfully reflect climate change.