Loess microstructure indication indexes for the study of palaeoclimatic conditions in northwest China

The Loess Plateau of China, located to the west of the Liupan Mountains and north of the Qinling Mountains stretching across the Yellow River, is the main loess deposit area in Northwest China. The loess in the northwest of China has deposits with large thickness and extensive distribution. Scanning electron microscopy (SEM) and energy spectrum analysis of loess have been performed to study the relationship between the loess microstructures and the forming climates Era. The relevant indexes were evaluated including the sand‐dropping speed (Vn), certain sedimentary depths (hn sand particle volume (Vd) and element ratios of Ca/Fe, K/Al, Si/Al and Ca/Mg. The microstructure indexes of loess accumulation and evolution reflect paleoclimate conditions and time scales to a certain extent. The important discovery is the microscopic sand‐dropping speed (Vn) and the sedimentary depth hn) calculation method of different sedimentary ages. These indices are compared with the record of major aeolian‐forming climates from the Guliya ice core, and provide a reliable benchmark for studying climate change It also can be used as important indicators of monsoonal change and environmental evolution reconstruction. The index of sand sedimentation speed (Vn) got from loess microstructure could reflect sand‐dropping speed and loess deposition course. According the article can serve as new indicators of climatic changes of different forming loess layers. It can also be concluded that the climatic indexes obtained from loess microstructure can reflect climate conditions of loess forming. The loess forming climatic parameters are synchronous correspond to Tengger Desert and Guliya ice core for studying climate change, then microscopic parameters can also be used for preliminary analysis of loess climate formation and has be found corresponding evidence, and the loess climatic parameters correspond to the other two indexes. The analysis of loess microstructure indexes is very useful in researching climate change. Loess microstructure indexes can find new indicators and information about the monsoon climate evolution and paleoclimate changes.


| INTRODUCTION
Environment changes in "Global Climate Changes Project" are a major focus of research because of their impacts on human population and well-being.Notably, the loess deposits preserved significant messages of monsoon climate evolution, including the information of loess formation, palaeo-climatic records and time scales (Liu, 1985;Busacca, 1989;Whalley et al., 1982;Hong et al., 2003;Guo et al., 2002;Liu et al., 2000).In addition, there are lots of climate indicators derived from loess sediments (An et al., 2006;Lu et al., 2004), for example, loess particle sizes can be used to reflect wind intensity (Ding et al., 1999;Lu & An, 1998;Baotian & Jianmin, 1999).The sand particles can serve as proxy indicator changes of desert boundaries (Dong & Hui, 1997;Sun et al., 2002;Chang et al., 2019).Relevant climatic indexes can also be found.Liu (1985) introduced loess ratio of SiO2/TiO2 first to indicate winter monsoon intensity, while some scholars also proposed other climatic indexes to serve as the same purpose (Diao & Wen, 1997;When, 1989).
Soils are usually composed of different types of substances or soil-forming microstructures, and these characteristics can be used to explain climatic or environmental conditions at the time the soil was formed.The ratio of major elements in palaeosol can be used as an important index to evaluate the degree of chemical weathering and soil-formation (Chen et al., 2018;Sheldon & Tabor, 2009).For example, Na 2 O/K 2 O indicates salinization, (CaO + MgO)/Al 2 O 3 indicates calcification, FeO/ Fe 2 O 3 indicates oxidation, Al 2 O3/SiO2 indicates claying and Al2O3/(CaO + MgO + Na2O + K2O) represents alkali saturation (Retallack, 2007;Nesbitt & Young, 1982).According to the soil formation characteristics and element ratio of the palaeosol, the palaeosol profile formed in the similar palaeo-environment and palaeo-climate conditions is classified as a pedotype.
The loess deposits are by large thickness and extensive in Northwestern China.Studying the loess microstructure is not only helpful to understand the engineering geological properties of loess but also to understand the local climate characteristics at that time (Deng et al., 2009(Deng et al., , 2013)).The loess also hinders the application of such indicators in climatologist (Huang & Pang, 2002;Jia et al., 2005).
Previously, loess microinstruction was mainly used for less mechanical properties and its engineering applications.For example, the changes of microscopic images before and after dynamic trial tests, explains their influence characteristics on dynamic behaviours of less from the perspective of the micro structure.(Li et al., 2018).Some papers are devoted to climate research (Wei, 2012;Xiao et al.,1995;Xue & Zhao, 2003;Yves & Jennifer, 2010).The main studied micro-structure and characteristics of less in China under different climatic environments and ages.The characteristics of less structure in less region under different climate environment are comprehensively analysed, and the regional differences of less and the characteristics of less micro structures in different formation ages are studied (Deng et al., 2009;Gao, 2015;Zhao et al., 2017).The differences of physical and mechanical parameters of less microstructure in different soil forming years were tested and analysed, and the microstructure characteristics of soil forming environment in different regions and the changes of microstructure characteristics caused by climate environment were studied (Deng et al., 2009).
Because northwest loess is cold and dry and has less cementing clay, its microstructure particals is more clear.Therefore, its sand particle size can be easily obtained.It can be concluded that the loess microstructure and their corresponding parameters can reflect the dry and cold or warm and wet climate conditions, and even the slight changes in temperature, which can be used as new parameters for the analysis of climate change (Markus & Aldo, 2004).It can even be attempted to evaluate the analysis of the climate change by the application of the microstructure climate index.With an in-depth study, chemical index information and sand grain information can be then microscopic parameters can also be used for preliminary analysis of loess climate formation and has be found corresponding evidence, and the loess climatic parameters correspond to the other two indexes.The analysis of loess microstructure indexes is very useful in researching climate change.Loess microstructure indexes can find new indicators and information about the monsoon climate evolution and paleoclimate changes.

K E Y W O R D S
climate change, climate index, loess microstructure, northwest of China loess, sand-dropping indexe extracted from the microscopic information of deep soil layer, which can help with to analyse the climatic and environmental conditions at that time, which could then be used to reconstruct the palaeoclimate (Chen et al., 1996;Gao et al., 2018).This paper attempts to obtain climatic indicators through an in-depth research on the loess microstructure and information on climate change during the deposition of loess.
Micromorphology is of outstanding importance because it enables the reconstruction of original properties and polygenetic soil development, especially in pedocomplexes (Guo, 1997;Guo et al., 2002).The scanning electron microscope (SEM) and energy spectrum analysis was used to study the relationship between the indexes of loess microstructures and the forming climates in paper (Deng et al., 2009).In the paper, the mentioned indexes are the diameter of sand grains, Ca/Fe value and the characteristic shapes of the loess from SEM microstructures pictures.The characteristics of loess microstructure in loess region under different climate environment are comprehensively analysed.

| Samples collection
The Loess Plateau of China, located to the west of the Liupan Mountains and north of the Qinling Mountains stretching across the Yellow River, is the main loess deposit area in Northwest China.The formation of loess is related to deserts and cold/dry climates.Most loess samples were collected strictly from typical aeolian loess Q 3 2 more than 2-20 m below surface.The sampling method is adopted by manual exploration well sampling, in order to keeping the original loess' microstructure: each sampling site depth is 2-20 m and sampling interval 2-5 m.The geomorphological unit system includes the Yellow River first tributary terrace (Yongdeng Dongshan), intermountain alluvial and diluvial basins (Gl-lanzhou and Xc-Lanzhou), Yellow River IV-terrace (Gxt-Lanzhou) and mountain ridge mound platform (Yuzhong), which is covered with Quaternary loess accumulation.The selected samples are 70-300 km away from the Tengger desert, such as Gaolan, Lintao, Yuzhong, Jingtai, Yongdeng and Yuzhong (sampling depth, 2, 4, 6, 8 and 10 m); Jingtai (2 and 3 m); Lintao (4, 8, 12, 16, 20 m); Yongdeng (4 and 10 m); Gxt-Lanzhou (4, 8, 10, and 12 m); Bl-Lanzhou (5 and 8 m); Xicha (6, 8, and 10 m); Tsp-Lanzhou (6, 8, and 10 m); and Gaolan (5-6 m).The sampling method is adopted by manual exploration well sampling; in order to keeping the original loess' microstructure, each sampling site depth is 2-20 m, sampling interval 2-5 m.Sampling sites are shown in Figure 1.From Figure 1, it may seen that the sampling sites are 30-400 km close to the Tengger Desert and are Q 3 2 typical loess samples deposited by aeolian dust.The geomorphic systems include the Yellow River's first tributary terrace (Yongdeng Dongshan).
Gl-Lanzhou and Xc-Lanzhou belong to intermountain alluvial and diluvial basins.Gxt-Lanzhou is part of the Yellow River IV-terrace.Yuzhong is the mountain ridge mound platform, which is covered with the Quaternary loess accumulation.These loess samples are selected for electron microscopic analysis.The test instrument was obtained from "German high power electron microscopy instrument"(Gemini, ZEISS).It was prepared in a dustfree environment, in accordance with the size of the sample Table 1.Conductive carbon powder was sprayed on the top of the sample.The conductive adhesive was applied around and at the bottom of the sample table, which was connected with the sample table.The sample as kept in vacuum, the number and magnification were selected, and images of magnification of 500 times were taken.Then, the energy spectrum was used for the analysis of chemical elements in the loess microscopic images.

| Chemical element ratios
Loess minerals include quartz, feldspar, illite and monitmorillonite.It continuous accumulates in the form of clay particles in loess microstructure.Here, the energy spectrum is used to analyse the chemical elements of loess microscopic image to obtain the ratios of chemical values, considering that the comparison needs to adopt the parameter proportion relationship, that is, the relative content comparison.There are two types of loess mineral chemical values ratio, large power sand grains and fine clay particle.On energy dispersive spectrum, the energy spectrum was used to analyse the chemical elements of loess microscopic images to obtain the ratios of chemical values.That the comparison needs to adopt the parameter proportion relationship, that is, the relative content comparison, such as Si/Al, Ca/Fe, Ca/Mg and K/Al, must also be considered.There are two types of loess mineral chemical value ratios: large power sand grains and fine clay particles.The changes of the climate periods in humidity and temperature are shown by comparing the chemical indexes, such as the ratios of Si/Al, Ca/Mg, Ca/Fe and K/Al in big particles and fine clay particles in present in Table 3.

| The ratio of Si/Al
During the period loess deposition and continue weathering process, Al 2 O 3 become enrichment in smaller clay from loess particles.Since Al is difficult to disslove, its oxides and hydroxides are easy to preserve.It is also more difficult for SiO 2 to undergo degradation and change its size.If Si/Al value in large particles are higher, it indicates rich supply deposition at that period of time.Likewise, if Si/Al ratio is higher, it indicates the climate was likely drier and colder.

| The ratio of Ca/Mg
The secondary carbonate material is formed by rainfall, ground water or biological activities.Fine clay particles were often present in solution and lead to the formation of secondary carbonate HCO 3 − in process of loess formation.
The content of Mg 2+ is controlled by two factors, temperature and rainfall.If the of Ca/Mg ratio in big particles is higher, the temperature would be lower, thus leading to a dry and cold climate.If the Ca/mg ratio of small clays is higher, there would be more rainfall and a warmer climate.
2.2.3 | The ratio of K/Al Besides, the K/Al ratio is considered as an evolution index, since K + is dissolvable and easily transferable whereas AlO 2− is stable.The K/Al ratio can be an indicator of evolution.The lower K/Al ratio in large particles indicates an intense evolution and raining climate.from microstructure diagrams of cemented fine particles.The former (12 m) is rich in Ti, Fe, Ca, Si, Al and K, while the latter (8 m) is rich in Si, Ca, Fe and Ti.The analysis of particle composition by energy spectrum reflects the indicators of different climate periods.Figures 2 and 3 are the energy spectra of small particles of soil samples from the observation platform, which formed during the maximum glacial period (12 m) and the deglaciation period (8 m).It is clear that the former is rich in Ti, Fe and Ca, followed by Si, Al and K. Compared ratios of 8 m and 12 m of Guanxiangtai in Lanzhou.The Ca/Mg ratios of the large particles were 10.1 and 2.5, respectively, while the Ca/Fe ratios of the small particles were 0.51 and 9.215, and the K/Al ratios of the former were 2.50 and 0.43.A higher ratio of Ca/Mg, a lower ratio of Ca/Fe, and higher Si/Al and K/Al values can be seen at a depth of 12 m.The former is cold and dry at 12 m (The glacial minimum period), which indicate more cold, humid and windy climate, while the latter is warm and wet at 8 m (about deglaciation period).In the 8 m sample, a lower ratio of Ca/Mg and a higher ratio of Ca/Fe, and a lower of Si/Al and K/Al values were estimated, which indicates a more warm and rainy period with weak wind in the glacial minimum.The energy spectrum analysis can give compared with indexes in different climatic periods.

F I G U R E 2
The energy spectrum of Guanxiangtai, Lanzhou, 12 m.

F I G U R E 3
The energy spectrum of Guanxiangtai, Lanzhou, 8 m.

| Sediment speed and depth calculate formulas
The grain size composition in loess is closely related to both wind transport speed and distance from the Gobi Desert.It is an important indicator of monsoon changes and changes in environmental conditions.Usually, only the average loess sedimentary thickness per 0.1 Ma can be estimated (Liu, 1985).Wind intensity can affect sand sizes composition (Sun et al., 2002).However, it is difficult to estimate sand sedimentation speed in one period because the sedimentary thickness is a mixture of sand and clay, and the sand-dropping rate is not known.Here, we estimated the sand-dropping speed through analysis of loess microstructures.
The basic principle is to use microscopic image of certain period to calculate the content of sand particles in unit loess, so as to obtain the speed of sand falling in different climate periods.The average volume of sand particles can be converted from the content of sand particles in the microscopic image to compare the changes of sand fall in different climate periods.
First, the size and number of sand particles from the microscopic imajes of a certain sedimentary layer is extracted.Second, we calculated the average loess particle size and average volume.Third, based on the microscopic image, the sand dropping the sand dropping rate is calculated.In order to know a particular climate period's sand dropping speed, the following calculation method was used: In Formula (1.3, 1.2) M loess and M sand are the mass of loess and sand in the image, respectively, and V loess is the average loess settling velocity.The microscopic photograph the thin layer of loess quality are M loess , the mass of sand are M SAND , by formula (1.2) and (1.3) where ρ d is the sand density; V d is the average volume of sand particles; m is the number of sand particles (500 times images).Z is the thickness layers of the image.h t is the width of microstructure images, ρ g is the dry density of the loess layer, and S t is the area measured in the images.
Malan loess of the Gansu plateau, west of Liupan Mountain, is ~30 to 40 m thick and has a sedimentation speed of ~4.6 to 6.2 m/0.1 Ma (Chen et al., 1996).We assume that the sedimentation speed of loess in the entire west Gansu was about 5,000 mm/0.1 Ma.Using formulas (1) and ( 2), the loess accumulation speed V-loess in Lanzhou was about 5-6 m/0.1 Ma (Chen et al., 1996).Here, the assumed sedimentation speed of loess is 5,000 mm/0.1 Ma: so V-loess = B = 5,000 mm/0.1 Ma in Formula (1.4).
The sedimentation speed (v n ) is calculated by Equation (2).Climate era sedimentary depths is (h n ) and mean is thickness of sand sedimentary thickness (h n ) in certain climatic period can be obtained by Equation ( 3), where t n is the duration of the certain climatic period.The sedimentation speed V n of per 0.1 Ma in unit is mm/0.1 Ma.
In (1), M loess and M SAND is the mass of loess and sand in the image; − loess is the average loess' settling velocity, For the Formula ( 2) and (3), B is 5,000 mm/10 KaBp; ρ d is the sand density; V d is the average volume of sand particles; m is the number of sand particles (500 times microstructure images); Z is the thickness of layers in the image; h t is the width of microstructure images; ρ g is the dry density of the loess layer; S t is the area measured in the images; and t n is the duration of a certain climatic period.
Since the 1990s, there has various studies of glacial aging Glacial tills in Rongbuk Valley are labelled as the last glacial (0.18 Ma), the early last glacial (0.72 Ma), and last glacial (2.02 Ma), respectively.
Calculated according to Formula (2), the V n (sand-dropping speed) value of interglacial is about 2,011 mm/0.1 Ma.The V n of glacial maximum is 3,119 mm/0.1 Ma and glacial minimum is 2,644 mm/0.1 Ma and post-glacial is 2,819 mm/0.1 Ma.
By adding the sedimentary thickness (Formula 3), we can calculate the deposit depth.Shown in Table 2 is the sample from Lijiawan/Lintao.The glacial maximum hn is 4.68 m.The V n value of the interglacial is 2,011 mm/0.1 Ma.The h n of glacial maximum is 3,119 mm/0.1 Ma, the glacial minimum is 2,644 mm/0.1 Ma, and the post-glacial is 2,819 mm/0.1 Ma.The sediment depth and deposition time are combined for a sediment reduction rate for comparison. (1 The sediment depth and deposition time are unified as the sediment reduction rate for comparison.According to Equations ( 2) and ( 3), the ratio of the mass of sand grains to the mass of soil in the microscopic image of the loess deposited at certain time.The deposition thickness is obtained by multiplying the time of this period by the sand fall rate.The deposition thickness of each period is gradually accumulated to obtain the deposition depth.
For example, in Table 2, the surface accumulation of 2-3 m, the Holocene period in Lintao loess sedimentary thickness h n = 0.36 m; the sedimentary depth is about 2.4-3.4 m.The Holocene early of h n = 1.2 m and depth is 3.5-5 m.Post glacial of h n = 0.56 m and depth is 4-6 m.Glacial minimum h n = 0.79 m and depth is about 6.5-8.0 m.Glacial maximum accumulative total depth hn is 4.68 m and the deposited depth is about 9-13 m.Interglacial of h n = 2.01 m and the depth is about 13-16 m.

| RESULTS
The analysis of the loess microstructure can provide indicators of palaeoclimate changes.Through the study of the loess microstructure in Northwest China, we obtained information about monsoon frequency and other palaeoclimate changes.The microstructure indexes were evaluated including the sand-dropping speed (V s ) and sand particle volume (V d ), as well as elemental ratios of Ca/Fe, K/Al, Si/Al, and Ca/Mg.These indexes were compared with the Guliya ice core record, providing a reliable dataset for the study of climate changes.The analysis of the difference in the formation of microstructures can provide a basis for the study of palaeoclimates.The analysis of the microstructure indexes can provide a theoretical basis for the study of the difference climate index in the loess certain depth.The analysis of loess microstructure indexes can provide indicators of palaeoclimate changes.Through the study of the loess microstructure in Northwest China, we obtained information about monsoon frequency and paleoclimate changes.
To further determine the influence of the climate environment, the climate inferences from the corresponding microstructure indexes, as categorized by climate stage.
The sampling and its depth.According to sedimentary loess depth calculated by formula (1), ( 2) and ( 3), the division result of loess depth in the deposition period was calculated and shown Table 2. Sampling sites are shown in Figure 1.In order to further determine the influence of the climate environment on the formation of loess, the climate evidence of the corresponding microstructure indexes divided by climate stage and its sampling sites and depth are show in Table 3. Specific application way is follow.

| Early and middle Holocene
The end of the Younger Dryas event marked the beginning of the Holocene, and the temperature increased drastically.According to the records of the Guliya ice core, the early Holocene had rapidly increasing temperatures, increased rainfall, desalination of lake water, and lush vegetation.
Figure 4 shows the microstructure images of Jingtai at 3.5 m, which were fully cemented clay particles.The evidence from the structural parameters is shown in Table 3.For Jingtai (3-3.5 m), the Ca/Mg ratio was 1.5-1.9, the Ca/ Fe ratio 0.48, and sand-dropping speed 1,200-2,158 mm/ Ma with an average sand volume of 5,040-6,803 μm 3 .The loess Holocene samples indicated that it was formed in a warm and humid climate.

| The late Holocene
During 3-4 kaBP, the temperature dropped sharply and glaciers expanded.The Azha Glacier can be dated back to 2,980 ± 150 aBP and the Kunlun Glacier to 3,083 ± 120 aBP.Correspondingly, the climate of the loess plateau became drier and lakes shrank (Peterson et al., 2000;Porter, 2000;Lu et al., 2004;Liu et al., 2000).
As shown in Table 1, in the Holocene, for example, in Xiquan, Jingtai (2-2.5 m), as shown in Figure 5, the ratio of Si/Al was 20.95 and that of Ca/Mg 6.37.The microstructure of photographs in Figure 5 shows that the clay is weaker, and sand particles were ~40 μm, with an average sand volume of 7,694-8,765 μm 3 .Figure 6 shows the chemical indexes of samples from two different depths in Jingtai.The samples from 2.5 m had high ratios of Si/Al (20.95) and Ca/Mg (4.33).As for the 3.5 m sample, it showed a higher ratio of Si/Al (6.63) and lower ratios of Ca/Fe (2.71) and K/Al (0.53).The weak cementation of clay particles indicates aridity and coldness, with intense winter wind and scarce precipitation in summer.Compared with the early Holocene, the climate in the late Holocene is colder and dryer.
The above shows that the climate in these two periods is completely different.The climate was warm and humid in the early and middle Holocene, and that was colder and dryer in the late Holocene.However, in the late Holocene, it was drier and colder, and the winter monsoon had a stronger impact on sand-dropping.Compared with the early Holocene, these two high ratios indicate that the climate in the late Holocene is colder and dryer than the early and middle Holocene.

| The post-glacial
The Guliya ice core indicates that there was a temperature decrease of 11°C around 12.2-10.9kaBP during the Younger Dryas event (Lu et al., 2004;Liu et al., 2000; Peterson  et al., 2000;Porter, 2000).As Table 2, the post-glacial loess sedimentary layer is about 4-6 m underground.Figure 7a,b shows the samples at 4 and 6 m in Yuzhong.At 4 m, the sand-dropping rate was 3,417 mm/years, the Ca/Mg index 47.2, and Ca/Fe 0.2.Loess samples at 6 m were 4.46 and 2.23, and the sand-dropping speed was 1,047 mm/years (4 m) and 2,219 mm/years (6 m).Loess samples of 4 m suggest the climate was drier and colder than loess samples at 6 m.
So the formation of climate and environment is more dry and cold than loess samples of 6 m.It shows that this period microstructure is mainly characterized by bigger sand particles and loosely accumulated weak cementation.This indicates the majority of sedimentary particles are larger than the glacial maximum (10 m).

| The last deglaciation
During the last deglaciation period (12-15 kaBP), the climate turned warmer and humid, and plateau lakes expanded.The calculated loess sedimentary layer is 7-8 m underground.In Figure 7c,e, the loess samples from Lintao (8 m) and Yuzhong (8 m) are shown, respectively.The particle size is 12-35 μm.The sand-dropping speed in the Lijiawan loess samples was only 2,639-2,644 mm/Ma.During this period, the microstructure is bracketed as closely-packed cement between particles.The particles decreased and the cement strength increased.The microstructure datum showed that the sand effect weakened, and the climate was relatively warm and humid.In addition, the particle size of loess sample from Yuzhong decreased to 4,011 μm 3 at 8 m.This indicates weak wind-blown sand action and increased clay (<25 μm) falling number, proving that it was warmer and humid.

| The Last Glacial Maximum
The Guliya ice core records showed the Last Glacial Maximum was around 23 kaBP and the temperature decreased by 9°C.Zoige lake sediments showed the temperature decreased 6°C around 18 kaBP, colder than present temperatures.The glacial maximum was 15-30 kaBP at the end of the Late Glacial Maximum.According to the records of the Guliya ice core, there was a rapid increase in temperatures, increased rainfall, desalination of lake water, and lush vegetation (Peterson et al., 2000;Porter, 2000;Lu et al., 2004;Liu et al., 2000).
The glacial maximum was around 30-15 kaBP, Figure 7d,f shows the images of samples from Yuzhong (10 m) and Lintao (12 m).The largest particle size is more than 100 μm, indicating increased wind-blown sand movement.Loess samples of Yuzhong of 10 m contrast with weaker cemented-in particles (Figure 7d).The particles size increased as did the mean sand dropping rate.The Ca/ Mg ratio was significantly higher: the Ca/Fe ratio increased to 90.8 (in the Lintao 12 m loess samples).The loess sample at 10 m had a sand-dropping speed of 2,147 mm/Ma, lower than the deglaciation period of 3,417 mm/Ma.This was also the main period of loess accumulation in Western China.The microstructure of this period shows characteristic weak cementation with more large sand particles.Larger sand particles had accumulated than in the post-glacial because of a combination of cold temperatures, high precipitation and intense wind-blown sand action.Aridity and frequent wind-blown sand movement are caused by high sand sedimentation speed and the thickest loess sedimentary layer.

| Interglacial
The Guliya ice core records show that the temperature in the warmest stage of the interglacial was 5°C higher than the present.Figure 7g,h are sample images from Lintao 16 m and 20 m.The low ratios of Ca/Fe (0.19) of both samples and the lower ratio of Ca/Mg (2.19) are at 20 m.This reflects an intensely warm and humid climate and is also consistent with a warm and humid climate.Clay particles and crystal minerals accumulated around particles (at an average size of 30 μm) in base cementation.
This should have been the result.According to the records of Guliya ice core, there are more changes in each climate period.These analyses are in good agreement with corresponding glacial records (Peterson et al., 2000;Porter, 2000;Lu et al., 2004;Liu et al., 2000).

| Application analysis on the climate evolution
For Example 1, the following part is the microstructure index analysis of several typical loess sections in Lijiawan of Lintao.Figure 8 shows loess sampled from Lintao.The sand dropping index and average sand volume reached a maximum at 12 m and were lower at 4, 8, 16 and 20 m; Si/ Al at 12 m was much higher than that at 8 and 16 m.Its sand sedimentation speed reached 3,231 mm/10 ka (V s ).Ca/Mg and Ca/Fe in large particles were higher in 12 m.At 16 m, sand sedimentation speed decreased greatly.Ca/Mg and Ca/Fe in fine clay particles were lower that indicates warmer and humid climate.
At 20 m, lower ratios of Si/Al and K/Al than those of other layers and a high ratio of Ca/Fe in fine clay particles indicate cold temperatures and low humidity.The loess sample in the 12 m layer is also a calcified film and calcareous enrichment layer.The 12 m layer is little cold and dry.The 16 m layers were solid calcareous crystalloids.This indicates the condition of severe aridity around the glacial maximum and then warm and wet in the next period.
For Example 2, shown in Figure 9, the climatic index in Xiaguanying (Xgy-Lanzhou) samples is presented.In the 2 m layer of the early Holocene, the high ratio of Ca/Mg and the highest ratio of Ca/Fe among all layers indicated a colder and drier climate.In the 4 m layer, the F I G U R E 8 Climatic Index of Microstructure of Loess Q32 in Lintao.{sand sedimentation speed (V s ), sand particle volume (V), element ratio of grain groups (Si/al, K/al, Ca/Mg and Ca/fe}.ratios of and Ca/Fe were higher than those of the 6 m layer.It indicate colder climate than the latter and little humidity.The 8 m deep, the sand sedimentation speed decreased greatly by lower ratios of Ca/Fe and K/Al, than those of 6 and 10 m layers indicate warmer and humid climate.The ratio of K/Al (fine) and Ca/Fe (fine) index of the 10 m layer were greater than those of the 6 and 8 m layers, and the sand sedimentation speed decreased.At 10 m, the Si/Al ratio and Si/Al ratio of large particles were higher than fine clay particles.Si/Al ratio in fine clay particles is higher.The ratio of K/Al and Ca/Fe index in fine clay particles of 10 m layer are greater than those of 6 m layer.

| Loess climate indicators in different climatic periods
Table 3 and Figures 10 and 11 show results of analysing the mean value of climatic period parameters at each sedimentary depth.In Figure 10, the Ca/Mg, Ca/Fe, K/ Al, sand reduction volume (V) and sand-droping dropping (V s ) had obvious changes.The average sand reduction rate and the average particle volume also varied significantly (Figure 11).The sand drop volume at 9-12 m is the highest, followed by 5-6, 2-3, 7-8, 3-4, and 16-20 m.So the climate represented by 2-3 m, 5-6 m, and 9-12 m were colder and dry, and the sand drop rate were the highest at 9-12 m.Therefore, the glacial maximum experienced a great decline in temperature, while the post-glacial experienced a severe aridity.
In Table 4.1, by analysing, the mean value of climatic period parameters from the loess microstructure are shown: the early and middle Holocene (dry, colder raining, fast dropping sand);the late Holocene (rainy, warm and sandy), post-glacial (10-12 kaBP) (less sand-drops and then little dry and cold); the last deglaciation (15-12 kaBP) (Warmer, wetter with moderate sanding); last Glacial Maximum (25-15 ka) (most dry and cold, windy and sandy); and interglacial (warm, damp).
In addition to the Tengger sand source in the north, the west of the loess sampling site is blocked by the Tibetan Plateau.The Qinghai-Tibet Plateau climate change seriously affects the loess climate index, and the three are correlated.In Table 4.2, the loess micro climatic index and Guliya ice core records are provided.
Table 5 shows the climate characteristics and the loess microstructure index range: ( 1

| The sand dropping speed and climate era sedimentary depth (h n )
The sand dropping speed velocity is obviously changed in different climate ages.The index of sand sedimentation speed calculated by Formulae (2) and (3).The calculated sand-dropping speed is that, the interglacial is about 2,011 mm/0.1 Ma, the glacial maximum is 3,119 mm/0.1 Ma, post-glacial is 2,819 mm/0.1 Ma.Chlimate era sedimentary depth (h n ) calculation result, and glacial maximum accumulative depth hn is 4.68 m, at a depth of 9-13 m.Interglacial of h n = 2.01 m and is 13-16 m.The Holocene period sedimentary thickness hn (0.36 m) is about 2.4-3.4 m deep.The Holocene early of h n (1.2 m) depth is 3.5-5 m.Post-glacial of h n (0.56 m) depth is 4-6 m.For glacial minimum h n = 0.79 m, the depth is about 6.5-8.0.Glacial maximum accumulative depth hn is 4.68 m and the deposited depth is about 9-13 m.Interglacial of h n = 2.01 m and depth is about 13-16 m.The microscopic sand-dropping speed (V s ) and Climate era sedimentary depth (h n ) calculation method can be used to analysis of loess different depth's sedimentary ages.Dongshan).Gl-Lanzhou and Xc-Lanzhou belong to intermountain alluvial and diluvial basins.Gxt-Lanzhou is part of the Yellow River IV-terrace.Yuzhong is the mountain ridge mound platform, which is covered with the Quaternary loess accumulation.
In addition to the north sand source, the west of the loess sampling site is blocked by the Tibetan Plateau.The Qinghai-Tibet Plateau climate change seriously affects the loess climate index, and the three are correlated.The three regions are involved in the study.One is the Qinghai-Tibet Plateau where Guliya ice core was secured, the second is the Loess Plateau where loess samples were secured, and the third is Tenger desert where sand came from.The geographical map of the three regions be provided in Figure 1.The relativity of the three regions, in palaeoclimate should be expounded, so that the record of Guliya ice core may be used to compare with the indexes of loess microstructure.

| Tengger Desert
The study of the paleoclimate evolution in monsoon region by sediment samples were obtained by shallow drilling in the ancient lacustrine area in the northern margin of Tengger Desert, and climate evolution was analysed with a series of profiles.Climate proxies such as particle size, magnetic susceptibility and palynology were selected, finally find the fluctuating aridity stage from the end of late Pleistocene to Holocene and after (14.2-0 kaBP): the climate changed abruptly to dry and cold, and then gradually returned to warm and wet.With the increase of temperature and humidity, the "optimum period of Holocene" occurred, followed by sudden drought events.In the later period, the climate gradually became arid, the temperature fluctuated frequently, and the lake dried up gradually.The above stages of climate evolution in the study area are basically consistent with the global climate background and previous research conclusions.
In Table 4.2, the loess micro climatic index and Guliya ice core records are given and the compared. 4.4.2| The late Holocene (4-3 kaBP, 1.8-2 m) The period of significant cooling event and glacier advance.Its important feature is that the plateau interior climate is drying.The plateau lake continued to retreat for a dry and cold event (1.8-2 m) loess sample had a high Ca/Mg index (14.56)and Ca/Fe value (8.75).Sand volume dropping speed was large, and Si/Al and K/Al was small.The climate is dry, colder, raining and fast dropping sand.
4.4.3| The early and middle Holocene (1-0.2 kaBP, 3-4 m) After the dry-cold Younger Dryas event ended, the early and middle Holocene happened, and the temperature increased sharply.The temperature increased rapidly and precipitation increased.The lake was fading and vegetation was flourishing.Forest grassland environment cold wet climate, 3-4 m, Ca/Mg and Ca/Fe index is lower; K/ Al ratio and Si/Al ratio are little change than 2 m soil layer but the sand volume and sand dropping speed is lower.Therefore, the climate is cold and wet and warm breeze. 4.4.4| Post-glacial (10-12 kaBP, loess 4-6 m)   Temperature drop reached 11°C, the Younger Dryas event.It is also reflected in lake sediment records.It did experience a dry and cold period of intense dust fall about 10,000-12,000 years ago.The post-glacial soil layer is about 6-4 m.Si/Al ratio is range about 8-34.6,Ca/Mg 76.3-12.5.The sand dropping speed is 3,636 mm/million, the particle size is 9,953 μm 3 , the Ca/Fe ratio is 31.4,Ca/ Mg value is 76.3, K/Al value is 5.79, the leaching rate is low, the precipitation is less, the pedoforming temperature is low, and the climate is dry and cold. 4.4.5 | The last deglaciation (15-12 kaBP, loess 7-8 m) As the climate became warmer and wetter, the plateau lake once expanded.A period of warmer and wetter weather.The Ca/Fe ratio, K/Al ratio and Si/Al ratio are all low, while the Ca/Mg ratio is high, indicating a cold and wet climate.The particle size decreased, the number of small particles increased and the consolidation strength increased, indicating that the climate of this period was weakened by aeolian sand. 4.4.6 | Last Glacial Maximum (30-15 kaBP,   loess 9-12 m)   The temperature drop reaches 9°C; the temperature was 6°C lower than that of modern times.The glacier area was 7.5 times than that of modern times due to significant In the interior of the plateau, there is a tendency of drying, and lakes generally shrink.The climate became drier, with more wind and sand, and vegetation reduced to a savanna environment.Dust fall was more concentrated.
The period is the Longxi loess extensive accumulation of the main period.The sedimentation speed of gravel was large, and the sedimentary loess layer is thickest.The loess particle accumulation density was high, which may the simultaneous action of cold, heavy rainfall and strong wind and sand.When the Ca/Mg 65-90.8,K/Al ratio is up to 5.35, and the large grit volume was up to 15,461 μm 3 , the sediment reduction rate is up to 3,556 mm/million.The maximum particle size is above 70 μm.Therefore, it was Dry and cold, windy and sandy. 4.4.7 | Interglacial (30-40 kaBP, 12-16 m)   The temperature of the most warm period of the last interglacial period recorded by the Guliya ice core was 5°C higher than that of modern times.The palynological records of the Ruoige Lake core revealed that it was wet and rainy at that time, and the last interglacial section was particularly warm and wet, 12-16 m: the ratios of Ca/Fe and Ca/Mg of the 16-20 m soil samples were small, which also showed that it were warm and wet during this period.
The clay particles were tightly packed around the larger particles (average diameter 30 μm) with clay particles and crystalline minerals.The microstructure was the base of cementation, which should be formed in a strong warm and humid climate.
According to the records of Guliya ice core, there are more changes in each climate period.(Peterson et al., 2000).The end of the Younger Dryas event marked the beginning of the Holocene, the early Holocene, showed a soared rapidly in temperature (Porter, 2000).The temperature increased drastically, rainfall increased, desalination of lake water and lush vegetation, characterized by warm and humid.However, during 4-3 kaBP, the late Holocene, the temperature dropped sharply and the glacier expanded.The Azha Glacier can be dated back to 2,980 ± 150 aBP and the Kunlun Glacier to 3,083 ± 120 aBP.Correspondingly, climate of loess plateau became drier and lakes shrank continuously.In the post-glacial, the Guliya ice core indicates there was temperature decrease of 11°Caround 12.2-10.9kaBP (Lu et al., 2004).The last deglaciation period about 15-12 kaBP, the climate turned warmer and humid, and plateau lakes expanded.The glacial maximum, it was around 30-15 kaBP at the end of the late Glacial Maximum, according to the records of Guliya ice core, showed a soared rapidly in temperature, increased rainfall, desalination of lake water and lush vegetation.The Guliya ice core records show that the temperature in the warmest stage of the Interglacial was The temperature of the most warm period of the last interglacial period recorded by the Guliya ice core was 5°C higher than that of modern times.The palynological records of the Ruoige Lake core revealed that it was wet and rainy at that time, and the last interglacial section was particularly warm and wet.| 43 JIN 5°C higher than the present Guliya ice core provide a reliable benchmark for climate change and can be used as important indicators of monsoonal change and environmental evolution reconstruction (Liu et al., 2000).
The microscopic parameters can also be used for preliminary analysis of loess climate formation and can also be found corresponding evidence.The three indexes changes are synchronous, and the loess climatic parameters correspond to the other two indexes.The microscopic parameters can also be used for preliminary analysis of loess climate formation and be found corresponding evidence.The three indexes changes are synchronous, and the loess climatic parameters correspond to the other two indexes.4. The microstructure's pictures of loess can serve as a direct proof of climatic changes.For instance, the glacial maximum was characterized by closely packed large and coarse particles; the glacial minimum by fine clay particles and more clay particles; the post-glacial by large and loose particles, indicating a colder and drier climate than the glacial maximum; and the interglacial by fine particles and more crystalloids, indicating a warm and humid climate.It can provides a theoretical basis for analysing the difference of soil depth and its mechanics engineering properties.

| Applicability and limitations
For most of the loess formation age is similar, so it has universal significance.Here, assumed sedimentation speed of loess in entire west Gansu is based on the average settlement rate of loess; therefore, it is also applicable to the aeolian loess in Shanxi, Shaanxi, Xinjiang and other places.Because of the aeolian loess of Northwest China accumulates sand and powder particles rapidly and few cemented clay materials, so there's loess particles clearly visible under the electron microscope (500), and can easy get important information (such as chemical element, particle volume) indicators.The microstructure's pictures of loess can also serve as direct proof of climatic changes.For instance, the glacial maximum was characterized by closely packed large and coarse particles; the glacial minimum by fine clay particles and more clay particles; the post-glacial by large and loose particles, indicating a colder and drier climate than the glacial maximum; and the interglacial by fine particles and more crystalloids, indicating a warm and humid climate.It can provide a theoretical basis for analysing the difference of soil depth and its mechanics engineering properties.However, it is difficult to extract the size of sand grains in some places, because sufficient rainfall, its loess is more likely covered by many clay particles.It may be obtained by increasing magnification by microelectron microscopy to 1,000-2,000.There is a great difference between Q 2 loess and Q 3 loess: the Q 2 loess' particles in the microstructure are not clear, and it is generally difficult to use the analysis method presented in this paper.

ACKNO WLE DGE MENTS
This work was supported by The Gansu Earthquake Administration innovation team special fund (2019TD-01-02), and the National Natural Science Foundation of China (No. 51578518).The author claims that all data used in this article are true and reliable, obtained from the author's experiment and have not been submitted to other publications.

F
The chemical index of samples from two different depths (2 m, 3 m) in Jingtai, Gansu.F I G U R E 7The microstructure of Samples from different depths in Yuzhongi, Lintao.

4. 4 |
The relativity of the three regions paleoclimate As shown in Table 4.2, and Figure 1, the loess sampling sites are 30-00 km close to the Tengger Desert and are Q 3 2 typical loess deposited by aeolian dust.The geomorphic systems include the Yellow River's first tributary terrace F I G U R E 9 Climatic index of microstructure of Loess Q 3 2 in Xiaguanying.

F
Sand velocity and grain volume in different climatic periods.
The ratios of Ca/Fe and Ca/Mg of the 16-20 m soil samples were small, which also showed that it were warm and wet during this period.The clay particles were tightly packed around the larger particles (average diameter 30um) with clay particles and crystalline minerals.The microstructure was the base of cementation, which should be formed in a strong warm and humid climate.

1.
According to the article, it can serve some new indicators of climatic changes and the climates of different forming layers.The changes of the climate periods in humidity and temperature are shown by compared the chemical indexes, such as the ratios of Si/Al, Ca/ Mg, Ca/Fe and K/Al in big particles and the ratios of Si/Al, Ca/Mg, Ca/Fe and K/Al in fine clay particles.The climatic indexes obtained from microstructure of loess in China, can reflect conditions of humidity, sand sedimentation and even subtle changes in temperature of the ancient loess forming ages.2. The index of sand sedimentation speed got from loess microstructure could be a new method of reflect sanddropping speed and loess deposition course.Through analysis by loess microstructure indexes, we can knowledge of desert boundary expanding and sanddropping centre.It is very useful for research climate change.Through loess microstructure indexes, it can be concluded that the glacial maximum experienced a great decline in temperature, while the post-glacial experienced a severe aridity.According to the Tenger palaeoclimate, the differences between loess and loess climate indexes are compared, the average climate indexes of loess formation depth in different climatic periods are listed.The climatic characteristics of Guliya ice core are listed.The results show that the two results are similar.3. The Qinghai-Tibet Plateau climate change seriously affected the loess climate index.Guliya ice core provide a reliable benchmark for studying climate change and can be used as important indicators of monsoonal change and environmental evolution reconstruction.
T A B L E 2T A B L E 3 The chemical ratios indexes and sand sediment velocity in different stratum layers.

T A B L E 4 . 1
The loess micro-element ratios and sand reduction rate in different climatic periods.

microstructure indexes The element ratio Sand dropping speed vs/ mm/10 Ka Sand Volume (V d )/μm 3 Forming climate Si/Al Ca/Fe K/Al Ca/Mg
The loess micro climatic index and ice core records.

time Guliya ice core. Records Loess micro climatic index (average) The late Holocene (4-3 kaBP) (1.8-2 m) The period of significant cooling event and glacier advance. Its important feature is that the plateau interior climate is drying. The plateau lake continued to retreat for a dry and cold event. 2 m: Ca/Fe (8.75), Ca/Mg (14.56), Si/Al (7.2), K/Al (0.45), Sand volume (5,921.5 μm 3 ), Sand dropping speed (1,970.5 mm/10 Ka)
(1.8-2 m) loess sample had a high ca/Mg index (14.56)and Ca/Fe value (8.75).Sand Volume and dropping Speed is large, Si/Al and K/Al.Is small.The climate is dry, colder, raining, fast dropping sand.