Holocene Environmental Changes Inferred From an Aeolian-Palaeosol-Lacustrine Profile in the Mu Us Desert, Northern China

An excavated profile of aeolian-palaeosol-lacustrine sediments (the Wapianliang profile), located at the southeastern part of the Mu Us Desert, Northern China, was studied to reconstruct regional Holocene environmental changes. A chronology was established based on three AMS 14C and two OSL dates, and variations in the lithology and grain size, magnetic susceptibility, soil micromorphology, and chemical elements were used to explore the regional depositional environments during the Holocene. The results showed that since around 14 ka BP, this region had experienced seven alternations of wetting and drying. A shallow lake, which was identified by celadon lacustrine sediments with sporadic freshwater gastropod fossils, occurred in this area from around 13.0 ka BP to 9.9 ka BP. There existed two obvious intervals of soil formation, inferred from the environmental proxies of the palaeosol/sandy palaeosol layers, with relatively fine average grain-size, high magnetic susceptibility value, remarkable pedogenesis features, and strong chemical weathering, in particular, a well-developed palaeosol layer dating from the middle Holocene (8.6 ka BP to 4.2 ka BP). A weakly-developed palaeosol layer (from around 1.2 ka BP) at the upper part of the profile is possibly an indication of the Medieval Warm Period. This implies a forest steppe environment at both of these sedimentary stages. After 0.9 ka BP, a desert environment returned, analogous to before around 13.0 ± 1.4 ka BP, between 9.9 ± 1.1 ka BP to 8.6 ka BP, and between 4.2 ka BP to 1.6 ka BP, indicating the aggravation of aeolian activity and the expansion of mobile sand dunes. The variations in sedimentary environments were mainly triggered by changes in the East Asian Summer Monsoon (EASM).


INTRODUCTION
The Holocene (from around 11.7 ka BP to the present) may appear as a blink of an eye considering the long history of the Earth, yet it is vitally important to understanding current and recent environmental conditions (Qin, 2011). As an interglacial period, the climate patterns during the Holocene are relatively warm-moist, particularly the ubiquitous and long-term Holocene Megathermal period (8.5 ka BP to 3.0 ka BP) (Shi et al., 1994). Previous studies have revealed that this was a prosperous period, with high lake levels (Liu et al., 2018a;Chen et al., 2021), massive glacial thaw (Nesje and Kvamme, 1991;Nesje, 1992), well-developed palaeosol (Lu et al., 2005;Yue et al., 2021), and extensive prehistoric human activities (Nicoll, 2004;Liu et al., 2021). In particular, in ecologically fragile monsoon marginal and agricultural and pastoral interlaced zones, numerous sedimentary profiles or cores have recorded multiple sedimentary facies variations (Zhang et al., 2011;Chen et al., 2015;Shanahan et al., 2015;Liu et al., 2018b;. The frequent variations in lacustrine and aeolian activities recorded in individual profiles indicate the quick response and feedback of the regional climatic and environmental changes (Li et al., 2012;Liu et al., 2018a).
The Mu Us Desert in Northern China is located in a desertloess transitional zone, on the northern margin of the East Asian Summer Monsoon (EASM), within a farming-grazing transitional belt (Figure 1). The area is highly sensitive to environmental variation driven by climate change and/or human impacts and is therefore suitable for studying palaeoclimatic and palaeoenvironmental history (Zhou et al., 2002;Ding et al., 2005). Since the 1920s, efforts to understand Holocene palaeoclimate in the Mu Us Desert have found that under the background of climatic variation during the Holocene, aeolian and fluvio-lacustrine deposits coexisted, and the differences that arose concerned the proportion of these different lithologies over time (Liu et al., 2018a), which has been confirmed by various geological records (Ding et al., 2021;Shu et al., 2021). Based on the chronology, lithology, and environmental indices, during the Holocene Climate Optimum period, one or two (perhaps even four) layers of palaeosol and sandy palaeosol were widely developed, corresponding to that the sand dunes were mostly fixed (Gao et al., 2001;Xu Z. et al., 2015;Jia et al., 2015;Zhao S. et al., 2016).
In general, owing to the limits in the distribution area and ideal exposed profiles, together with the frequent meandering Salawusu palaeo-river (a major river in the southern area of Mu Us Desert) (Zhao H. et al., 2016), to date, little attention has been paid to the reconstruction of these sedimentary conditions, especially since an individual profile has identified the presence of multiple sedimentary facies (Zhao H. et al, 2016;Liu et al., 2018b;Jia et al., 2018). Based on extensive field campaigns, a proper section, Wapianliang (WPL), has been found, with aeolian sand, palaeosol, and lacustrine sediments. Therefore, in this paper, we selected the WPL section and used grain size, magnetic susceptibility, soil micromorphology, chemical elements, and lithology analyses to explore the variations in regional depositional environments during the Holocene in the southeastern marginal areas of the Mu Us Desert. Our main objective is to further understand the changes in the regional sedimentary environment during the Holocene.

Regional Setting
The Mu Us Desert, with a total area of about 39,000 km 2 , is located along the margin of a region influenced by the EASM (Figure 1), which dominates the regional climate. The mean annual rainfall is between 250 and 440 mm and the mean annual temperature is between 6.0 and 8.5°C. Warm and humid air brought by the EASM delivers more than 60-70% of the annual precipitation, which falls mainly between June and August (Liu et al., 2018a). The modern vegetation consists mostly of Artemisia ordosica Krasch., Tamarix chinensis Lour., and Hippophae rhamnoides Linn. Regional landscape types are varied, including stabilized, semistabilized, and active sand dunes, grassland, and interlinked sequences of lakes and marshes (Department of Geography of Peking University et al., 1983;Zhou et al., 1996).

Sampling and Laboratory Measurements
An excavation has exposed a stratum in the southeastern margin of the Mu Us Desert, namely section WPL (N38°34′58″, E109°21′39″, altitude 1226 m), which was chosen for study. The WPL section is an aeolian sand-palaeosol-lacustrine sediments sedimentary sequence about 6.20 m thick. The detailed lithological description is presented in Table 1.
Three Accelerator Mass Spectrometry (AMS) 14 C dates of the organic sediments were obtained from the WPL section. The analyses were made at the Beta Analytic Radiocarbon Dating Laboratory, and details of the pretreatment procedure are given at http://www.radiocarbon.com/pretreatment-carbon-dating. htm#Washes. The AMS 14 C dates were converted to calendar ages using the program Calib 7.02 based on the INTCAL 13 calibration (Reimer et al., 2013). In addition, for the lacustrine layer, we collected two samples for Optical Stimulated Luminescence (OSL) dating. The material from the middle part of the sample tube, which had not been exposed to light, were used for equivalent dose (De) measurements. Pure quartz grains 90-125 μm size were extracted using the procedure employed by the OSL Laboratory of the Qinghai Institute of Salt Lakes, Chinese Academy of Sciences. De measurements were conducted using a Risø DA-20 TL/OSL reader equipped with blue diodes (k 470 ± 20 nm) and a 90Sr/90Y radioactive beta source. The luminescence was detected using a U-340 filter. The material that was possibly exposed to light at each end of the tube was used to measure the concentrations of Uranium (U), Thorium (Th), and Potassium (K) by neutron-activation-analysis (NAA). The water content was measured by weighing the samples before and after drying. Details of the pretreatment procedure of OSL dating samples are presented in Lai and Brückner (2008). Finally, we obtained the boundary ages of the aeolian sand, sandy palaeosol, and the lacustrine deposit units using linear interpolation.
216 samples were collected from the WPL section at 2 or 4 cm intervals for laboratory measurements. The grain size measurements were made using a Mastersizer 2000 with a measurement range of 0.02-2000 μm at the State Key Laboratory of Earth Surface Processes and Resource Ecology, Beijing Normal University (see Lu et al. (2015) for a description of the pretreatment procedure). Measurements of magnetic susceptibility were made using a Bartington Instruments MS2 meter and MS2B sensor at the same laboratory, and after airdrying, the samples were disaggregated and packed into 8 cm 3 plastic boxes. The thin sections of the soil sample from 260 cm depth were produced by the Cambrian Geological Technology Limited Company, Langfang City, Hebei Province, and the identification of the soil micromorphology was conducted using a Leica Microscope. The elemental content analyses of samples were made with an Axios wavelength-dispersive X-ray fluorescence spectrometer using a superlong, sharp-pointed ceramic X-ray light tube with a power of 4 kW and a pipe flow of 160 mA at the Key Laboratory of the Desert and Desertification, Northwest Institute of Eco-Environment and Resources, Chinese Academy of Sciences, Lanzhou, China. The pretreatment procedure is presented in Ding et al. (2019).
The Ti/Sr ratio was used to reflect the amount of precipitation indirectly, particularly in the aeolian deposits. Generally, Ti is a stable element, whereas Sr is more active, therefore, the variation in the Ti/Sr ratio depends on the eluviation of Sr (Chen et al., 1999;Liu et al., 2013). The Ratio of Elements (Re) is the ratio of (K 2 O+Na 2 O+CaO+MgO)/(Fe 2 O 3 +MnO 2 ) as proposed by Liu et al. (2002). In Northern China, K, Na, Ca, and Mg are usually concentrated when the climate is dry, while sediments are rich in Fe and Mn when the climate is wet (Guan, 1992).

Chronology
The sandy palaeosol layers contained organic material, which meets the requirements necessary for AMS 14 C measurements. Three samples for dating were chosen and the results for the WPL profile were listed in Table 2. All of these dates were obtained from bulk organic matter. For the lower part of the section, to obtain the age of the lacustrine sediment layer, two samples were collected using 4 cm-diameter lucifuge steel tubes for the OSL dating, with the results listed in Table 3. Generally, all five ages were in good chronological order. We established a final age model using piecewise linear fitting ( Figure 2) and the age of the boundary sample was determined by linear interpolation and extrapolation. Because of lack of precise dating data directly derived from the lacustrine layer (Stage B), here we selected the OSL ages from upper and lower aeolian sands to control and represent the possible development period of the lacustrine sediments Stage B to a certain degree.

Palaeoclimatic Record of the WPL Profile
To summarize the main phases of the site's sedimentary evolution, the record was divided into seven stages, labeled A to G from bottom to top ( Figure 3). Stage A. We interpreted the pale yellow-brown aeolian sand layer A to represent the sand of the late glacial period (before 13.0 ± 1.4 ka BP), as indicated by it having the highest medium sand content (47%), the lowest clay and silt contents (0 and 2%), and the best sorting (0.63ɸ, the minimum of all the identified stages). The curve of the grain size frequency distribution for this stage showed a near symmetrical single peak. Meanwhile, the low-frequency magnetic susceptibility (χl f ) and the organic matter content (OM) showed low values (Figure 3). Under a same provenance circumstance, the value of χ lf is predominantly affected by climate, particularly precipitation, as characterized by the different degrees of pedogenesis (Tite and Linington, 1975;An et al., 1991;Verosub et al., 1993). The lower values of χ lf and OM mean there was little precipitation, sparse vegetation, and weak pedogenesis.
Considering the geochemical element indices, the value of Ti/ Sr was low and Re was high, representing an arid environment with low precipitation. All the selected environmental proxies indicate an aeolian sand-dominated environment, with an arid climate, poor precipitation, and rare vegetation during the late glacial period (before 13.0 ± 1.4 ka BP) in our study area.
Stage B. Between around 13.0 ± 1.4 ka BP and 9.9 ± 1.1 ka BP, the former aeolian sand was succeeded by light gray lacustrine sand, with sporadic freshwater gastropod fossils, which reflects a shallow lake environment. In detail, the grain size parameters exhibit a higher content of silt and very fine sand (13.84%, highest of the whole profile) and worse sorting. The grain size frequency distribution curve for this stage shows three peaks, indicating a mixture of multiple types of interactions between water and wind. For the other indices, the higher χ lf and less obvious crest value of OM indicate the enhancement of the input magnetic materials during this time. In addition, the lower value of Re and higher value of Ti/Sr indicate a relatively humid environment with more precipitation (Figure 3). As a whole, we speculate that during this interval, the regional environment existed within a vivifying situation, with lake-dominated conditions and a relatively humid climate, abundant precipitation, and surrounded by lush vegetation at the beginning of the Holocene.
Stage C. For the interval from around 9.9 ± 1.1 ka BP to 8.6 ka BP, the lithology is seen to be typical aeolian sand, characterized by the highest mean grain size (Mz) with medium sand, coarse Frontiers in Earth Science | www.frontiersin.org December 2021 | Volume 9 | Article 799935 5 sand, and very coarse sand, and better sorting. The grain size frequency distribution curve showed a single peak, similar to Stage A. In addition, the low χ lf value and the near-zero content of OM all indicate an extremely weak degree of pedogenesis and a sparsely vegetated environment. The higher value of Re and lower value of Ti/Sr indicate that during this period, there existed a relatively arid climate, with less precipitation (Figure 3).
Stage D. This stage represents the period from between 8.6 ka BP and 4.2 ka BP. The lithology is dark gray-brown sandy palaeosol, with sporadic pseudomyceliums. The Mz of this stage was 243.88 μm, the minimum of all the sedimentary layers, with the highest content of clay (the only one, over 1%) and silt, and the worst sorting. The frequency distribution curve for grain size showed a double peak or three-peak patterns (Figure 3, low part, D-WPL066, D-WPL106, D-WPL118, D-WPL124), pointing to the diversity of sources, such as the various processes of pedogenesis. Considering soil micromorphology, sample SM-1 displayed a relatively coarse sand structure, which contained quartz, feldspar, mica, and other lithic minerals, with the total content of silt and clay minerals being less than 10%. In detail, the soil particles were loosely arranged, with few clay minerals. The main quartz was single-crystal quartz (41%), with the development of the secondary enlargement, and the distribution of clay line. Its surface was clean and displayed no cleavage. The feldspar was comprised of alkali feldspar and plagioclase (45%) and developed alteration of micacization and kaolinization. As a whole, all of these characteristics imply a relatively weak degree of pedogenesis during Stage D. Given the expression of the soil character, we called this stage sandy palaeosol. In addition, it shows the highest value of χ lf and OM content, indicating the existence of pedogenesis and a flourishing vegetated environment. Together with the results of the element geochemical index, which saw the lowest value of Re and highest value of Ti/Sr, during this period there existed widely developed palaeosol, with a relatively humid climate, more precipitation, and flourishing vegetation ( Figure 3). Thereinto, at a depth of between 230 and 210 cm, the transitional layer exhibited obvious transitional characteristics, from the stage D to stage E ( Figure 3).
Stage E. For the interval from 4.2 ka BP to 1.6 ka BP (Late Holocene), the lithology is light brown aeolian sand. The grain composition mainly consists of medium, fine, and coarse sand (approximatively 80%). The sorting for Stage E is better than for Stage D and the grain size frequency curve mainly showed a typical single-peak pattern, similar to stages A and C. In addition, the lower values of χ lf and OM content indicate a weak degree of pedogenesis and less vegetation. The value of Re sightly increased, together with a lower value of Ti/Sr. These results indicate that during this Late Holocene period, the regional climate was relatively arid, with less precipitation, and less vegetation coverage (Figure 3).
Stage F. At this stage (from around 1.6 ka BP to 0.9 ka BP, or around 1.2 ka BP), we obtained an AMS 14 C dating age (1177 ± 96 yr cal BP) using organic material. In the weakly developed sandy palaeosol layer, the clay content was relatively high, second to the Stage D sandy palaeosol layer. The grain size frequency distribution curve mainly showed a double peak pattern, and the curve was relatively smooth, implying the weak effect of pedogenesis. The higher values of χ lf and OM content all confirmed the development of soil and the existence of more vegetation. For the geochemistry of elements, the valley value of Ti/Sr and the crest value of Re indicate a relatively humid climate, with more precipitation (Figure 3). We, therefore, speculate that Stage F was a period of weakly developed palaeosol.
Stage G. For the top layer, we interpreted the pale yellowbrown aeolian sand to represent the sand of recent millennial deposits (around 0.9 ka BP to the present). Its grain composition was composed of medium, coarse, and fine sand (over 80%), and the frequency distribution curve of the grain size showed a single peak pattern. The lower value of χ lf and OM content, together with the geochemistry results (lower Ti/Sr value and higher value of Re) (Figure 3) indicated that after 0.9 ka BP, there existed a relatively arid climate with less precipitation.
Based on these proxies', since the late glacial period, the WPL section with aeolian sand, palaeosol, and lacustrine sediments has documented at least seven stratigraphic cycles, varying between desert and non-desert periods. Generally, for the four desert periods, Stages A, C, E, and G, their mean grain sizes were coarser, sorting was better, the frequency curves had mainly single-peaked forms, and the values of χ lf and OM content were lower, with the combination of elements showing obvious arid climate patterns. Conversely, over the three non-desert periods, which include the lacustrine deposits and sandy palaeosol or weakly developed sandy palaeosol, Stages B, D, and F, their mean grain sizes were finer, sorting was worse, the frequency curves showed double-peaked or multimodal patterns, and the values of χ lf and OM content were higher. Furthermore, the combination of elements showed obvious humid climate patterns.

Development of Regional Sedimentary Conditions Inferred From the WPL Profile and Its Climatic Control
At each stage, the sedimentation reflects the different regional environments, hence, the study of sedimentary facies provides perspective on what the environment could have been at that time. In order to better understand the geomorphic processes and variations in the sedimentary conditions, we considered the possible forcing mechanisms.
Based on the results of section 3.1.2, we speculated upon the possible regional sedimentary conditions for each stage. As seen from Figure 4A, we considered Stages A, C, E, and G, that is, from before 13.0 ± 1.4 ka BP, from between 9.9 ± 1.1 ka BP to 8.6 ka BP, from 4.2 ka BP to 1.6 ka BP, and from 0.9 ka BP to the present, respectively, to indicate cold and dry desert environments, characterized by strong aeolian activity with coarser grain sizes, better sorting, lower OM content, lower χ lf , and Ti/Sr values, and higher Re values (see image Figure 4D, which was taken at the hinterland of the modern Mu Us Desert). The emergence or disappearance of aeolian sand was strongly affected by the intensity of the EASM, whether during the Last Glaciation period or the Late Holocene ( Figures 5A-F). This should be detailly introduced and discussed later.
For Stage B, from around 13.0 ± 1.4 ka BP to 9.9 ± 1.1 ka BP as shown in Figure 4B, we speculated that it was a shallow lake environment, which is attributed to a relatively warm and moist Frontiers in Earth Science | www.frontiersin.org December 2021 | Volume 9 | Article 799935 climate during the Last Deglaciation and early Holocene (Zhou et al., 2002), similar to the landscape shown in Figure 4E, showing the area at Manai Lake in the modern Mu Us Desert. As seen from Figure 5H, since 15 ka BP, orbital variations have driven changes in summer insolation, which led to thermal contrasts between land and sea (An et al., 2000). The enhanced intensity of monsoons could bring more precipitation ( Figures 5A,C,E,F), consistent with the lower Re values ( Figure 5B). On the other hand, it is acknowledged that the lakes or marshes that formed during the Last Deglaciation and early Holocene were related to strong aeolian activity, mainly the formation of blowouts. With regards to blowouts and their relationship to the foundation of lakes, Shen et al. (2005) believed Hongjiannao Lake in the eastern margin of the Mu Us Desert was the result of a wind-erosion lake. Because of the uncertainties in the dating, we can only speculate that within the period between around 13.0 ± 1.4 ka BP to around 9.9 ± 1.1 ka BP, there existed an obvious lake environment stage, as evident by sporadic gastropod fossils. In addition, the rise in temperature ( Figure 5D) produced meltwater from the permafrost (Xu H. et al., 2015), providing another possible source of lake water. A similar view was expressed by Liu et al. (2018a) and this profile provides new evidence for this idea. In fact, examples of emerging lakes and marshes since 15 ka BP to the early Holocene are very common in our studied area, such as Midiwan (Zhou et al., 2002), Tanyaogou (Jia et al., 2018), and Dishaogouwan Left (Liu et al., 2018b). Considering Stages D and F, from 8.6 ka BP to 4.2 ka BP and from 1.6 ka BP to 0.9 ka BP, respectively, based on the results of the analysis of the environmental proxies and soil micromorphology, we speculate that these stages indicate a forest steppe environment ( Figure 4C), with the lithology of sandy palaeosol or weakly developed sandy palaeosol. Therefore, we chose the dense herbal and woody plants symbols in Figure 4. The timing of Stages D and F correspond to the Holocene Megathermal and Medieval Warm Period (MWP, 600-1280 AD), with an obvious decrease in the degree of drought ( Figure 4B). Particularly for Stage D, the soil characteristics are obvious, meaning the fixation of sand dunes, the existence of the process of pedogenesis, and the flourishing of vegetation (Shi et al., 1994;Mason et al., 2009;Xu Z. et al., 2015;Jia et al., 2015). Meanwhile, the suitability of climate is also reflected in these stages having the most precipitation ( Figure 5A) and higher temperatures ( Figure 5D) during the middle Holocene. The landscape during this period was close to that shown in Figure 4F, which was taken at Tongwan City in the southern margin of the modern Mu Us Desert. We propose that an increased intensity in the EASM was the main reason for the presence of the Holocene Optimum and MWP. The WPL section is mainly located within the positive landform area, therefore, the period of development of the soil corresponds to the lacustrine and sandy peat-dominated period based on the synthesis of variations in hydrological conditions inferred from peat-containing profiles ( Figure 5G). Therefore, the optimum timing of the climate was mostly similar, i.e., 8.5 to 3.0 ka BP and 8.6 to 4.2 ka BP, respectively. Furthermore, for the return of sandy palaeosol during Stage F, the short-lived enhancement of the summer monsoon might be responsible, with the redness of Qinghai Lake also capturing this returned process ( Figure 5C).
On the whole, these variations in the sedimentary conditions mentioned above are the products of climatic and environmental changes, which were mainly triggered by the EASM (Figure 5). As a result, the alternations between wetting and drying were FIGURE 4 | The sketch map of regional sedimentary conditions. (A) Desert environment dominated by active dunes. (B) Shallow lake environment. (C) Forest steppe environment with luxuriant vegetation. Thereinto, the differences between the periods 8.6 ka BP to 4.2 ka BP and from 1.6 ka BP to 0.9 ka BP were influenced by the amount of vegetation in the vicinity; (D) landscape photo of a desert environment, which was taken at the hinderland of the modern Mu Us Desert; (E) landscape photo of a shallow lake, which was taken at Manai Lake in the modern Mu Us Desert; (F) landscape photo of a forest steppe environment, which was taken at Tongwan City in the southern margin of the modern Mu Us Desert.

Regional Comparisons and Environmental Significances
The WPL profile recorded signals of varying temporal scales, from centennial to millennial, characterized by the transition of sedimentary facies and conditions. As mentioned above, in the Mu Us Desert, as a process of the first paludification, peatland development was typically initiated in topographic depressions and in lakeshore and river valley environments (Liu et al., 2018a), as noted in the early-Holocene wetness records widely seen in valleys (Shu et al., 2021). Our studied section belonged to a topographically depressed area, which saw blowout-based shallow lakes forming during Stage B. As the main body of sediments, the Holocene aeolian deposits in our profile were very thick (over 5 m) and lasted for a relatively long time (over 10 ka). The prolonged and prosperous Holocene Optimum, which was characterized by the widely distributed palaeosol and sandy palaeosol stratigraphies, is widely recognized over the Northern China dune fields. In addition to Mu Us Desert region, more sedimentary sequences in the Hulunbuir Sandy Land, the Horqin sandy land, and the Otindag Sandy Land were constantly established ( Figure 6). Generally, as seen from Figure 6B, the light yellow shaded area indicates the age of the middle Holocene palaeosol of the WPL profile and in this regard, it shows that the climate of the middle Holocene in these sandy areas was similarly humid and moist. In particular, between 7.5 and 3.5 ka BP, all the sandy lands in the eastern portion of the desert belt (sand seas and sandy lands) in northern China were stabilized and the intensity of aeolian activity was significantly weakened (Yang et al., 2019). However, previous studies have also demonstrated that the chronological framework about the Holocene palaeosols displayed obvious spatial heterogeneities (Gao et al., 1993;Yang et al., 2019). The differences over time and space could be strongly affected by the East Asian monsoon (Jiang and Liu, 2007). Furthermore, with respect to the reason for these inconsistencies, the instability, Annual precipitation records from Gonghai Lake, northern China (Chen et al., 2015). (B) Re from this paper. (C) Redness curve of Qinghai Lake which reflects precipitation . (D) Alkenone-based summer temperature record from Lake Qinghai (Hou et al., 2016). (E) Dongge cave stalagmite D4 oxygen isotope record (Dykoski et al., 2005). (F) Dongge cave stalagmite oxygen isotope record . (G) Synthesis of variations in hydrological conditions inferred from peat-containing profiles (Liu et al., 2018a). (H) Northern Hemisphere summer insolation at 30°N (Berger and Loutre, 1991).
Frontiers in Earth Science | www.frontiersin.org December 2021 | Volume 9 | Article 799935 8 and multiperiod aeolian activities could also impact this region .
More in-depth palaeoenvironmental studies are required for a full understanding of the relationships between aeolian activity and climate change in northern China (Yang et al., 2019). Profiles such as Wapianliang with aeolian-palaeosollacustrine sediments could provide us with more useful information about the varied sedimentary conditions, including the geomorphological processes under Holocene climatic patterns. Furthermore, the study of the transportation and replenishment of materials from centennial to millennial scales may be a way forward in enhancing our understanding of these processes.

CONCLUSION
We have presented a new aeolian-palaeosol-lacustrine profile, Wapianliang, from the Mu Us Desert and have used the results to reconstruct Holocene environmental changes, particularly the sedimentary conditions. The results indicate that since 14 ka BP, this region had experienced seven alternating periods of wetter and dryer climate. Correspondingly, the sedimentary conditions changed from a shallow lake (around 13.0 to 9.9 ka BP), forest steppe environment (around 8.6 ka BP to 4.2 ka BP, and around 1.2 ka BP), and desert environments (the remaining periods). In addition, the variations in depositional environments were mainly triggered by the East Asian monsoon. This section, therefore, provides useful information for enhancing our understanding of the paleoenvironmental evolution of the Mu Us Desert.

DATA AVAILABILITY STATEMENT
The original contributions presented in the study are included in the article/Supplementary Material, further inquiries can be directed to the corresponding author/s.

AUTHOR CONTRIBUTIONS
XL and RL contributed to conception, design of the study and writing of draft of the manuscript. ZDi and ZL conducted investigation and methodology. YL performed the statistical analysis. ZDo provided the funding and resources. All authors contributed to manuscript revision, read, and approved the submitted version.

FUNDING
This study was supported by the National Natural Science Foundation of China (No. 41901094, 41930641, 41330748) and Fundamental Research Funds for the Central Universities (GK202103148).

ACKNOWLEDGMENTS
We thank the editor and anonymous reviewers who have significantly improved this paper. We thank Du Jing for assistance in both field and laboratory, and MJEditor (www. mjeditor.com) for English improvement. Frontiers in Earth Science | www.frontiersin.org December 2021 | Volume 9 | Article 799935 9