A Case Study of the Dajiuhu Basin

sustainability

Article Changes in Paleovegetation and Paleoclimate in China since the Late Middle Pleistocene: A Case Study of the Dajiuhu Basin

Liwei Wu 1,2,3, Xinling Li 1,* , Qinghai Xu 1,*, Manyue Li 4, Qiufeng Zheng 3, Maoheng Zhang 3, Dandan Wang 1 and Zhiguo He 1

1 Hebei Key Laboratory of Environmental Change and Ecological Construction, Hebei Technology Innovation Center for Remote Sensing Identiﬁcation of Environmental Change, College of Resources and Environment Science, Hebei Normal University, Shijiazhuang 050024, China; [email protected] (L.W.); [email protected] (D.W.); [email protected] (Z.H.) 2 Department of Geography, Xinzhou Teachers University, Xinzhou 034000, China 3 College of Geography Science, Nanjing Normal University, Nanjing 210046, China; [email protected] (Q.Z.); [email protected] (M.Z.) 4 Institute of Nihewan Archaeology, Hebei Normal University, Shijiazhuang 050024, China; [email protected] * Correspondence: [email protected] (X.L.); [email protected] (Q.X.)

Abstract: The East Asian monsoon system is an important part of global atmospheric circulation; however, records of the East Asian monsoon from different regions exhibit different evolutionary rhythms. Here, we show a high-resolution record of grain size and pollen data from a lacustrine sediment core of Dajiuhu Lake in Shennongjia, Hubei Province, China, in order to reconstruct the paleovegetation and paleoeclimate evolution of the Dajiuhu Basin since the late Middle Pleistocene (~237.9 ka to the present). The results show that grain size and pollen record of the core DJH-2 are Citation: Wu, L.; Li, X.; Xu, Q.; Li, M.; consistent with the δ18O record of stalagmites from Sanbao Cave in the same area, which is closely Zheng, Q.; Zhang, M.; Wang, D.; He, related to the changes of insolation at the precessional (~20-kyr) scale in the Northern Hemisphere. Z. Changes in Paleovegetation and This is different from the records of the Asian summer monsoon recorded in the Loess Plateau of Paleoclimate in China since the Late North China, which exhibited dominant 100-kyr change cyclicities. We suggest that the difference Middle Pleistocene: A Case Study of between paleoclimatic records from North and South China is closely related to the east–west- the Dajiuhu Basin. Sustainability 2021, oriented mountain ranges of the Qinling Mountains in central China that blocked weakened East 13, 4848. https://doi.org/10.3390/ Asia summer monsoons across the mountains during glacial periods. su13094848

Keywords: Asian summer monsoon; precessional; Dajiuhu; pollen; late Middle Pleistocene Academic Editor: Lucio Di Matteo

Received: 23 December 2020 Accepted: 21 April 2021 Published: 26 April 2021 1. Introduction The East Asian monsoon system is an important part of global atmospheric circulation Publisher’s Note: MDPI stays neutral and plays a major role in controlling precipitation in China and driving climate change in with regard to jurisdictional claims in East Asia [1–6]. Numerous studies have reconstructed regional climatic change in East Asia published maps and institutional afﬁl- and the spatio-temporal evolution of the Asian monsoon on multiple time scales, using iations. various proxies [7–14]. However, records of the East Asian monsoon from different regions exhibit different evolutionary rhythms [10,15], which cause an important controversy. For example, the Chinese loess deposit sequences [16] were found to be dominated by eccentricity (100 kyr) variations, which were closely linked to the high latitude ice-volume Copyright: © 2021 by the authors. dynamics. In high-resolution records from stalagmites in South China, including those Licensee MDPI, Basel, Switzerland. from Hulu Cave in Jiangsu Province [7,17] and Sanbao Cave in Hubei Province [10,18], the This article is an open access article most striking feature was the changes of ~20 kyr precessional cycles, which indicates that distributed under the terms and the tropical/subtropical monsoons responded dominantly and directly to the changes of conditions of the Creative Commons summer insolation in the Northern Hemisphere. Therefore, it is important to obtain a great Attribution (CC BY) license (https:// number of continuous and high-resolution records of monsoonal climate change from East creativecommons.org/licenses/by/ Asia district in order to resolve these differences. 4.0/).

Sustainability 2021, 13, 4848. https://doi.org/10.3390/su13094848 https://www.mdpi.com/journal/sustainability Sustainability 2021, 13, x FOR PEER REVIEW 2 of 14

indicates that the tropical/subtropical monsoons responded dominantly and directly to the changes of summer insolation in the Northern Hemisphere. Therefore, it is important to obtain a great number of continuous and high-resolution records of monsoonal climate Sustainability 2021, 13, 4848 change from East Asia district in order to resolve these differences. 2 of 13 In this study, we used high-resolution grain size and pollen records from the Dajiuhu Basin in the eastern monsoon region of China to reconstruct the regional vegetation and climate Inchanges this study, since we the used late high-resolution Middle Pleistoc grainene. size The and pollenbasin recordsis formed from by the lacustrine Dajiuhu and peatBasin sediments. in the eastern Our aims monsoon were region to determine of China to the reconstruct periodic thecomponents regional vegetation of the record, and as climate changes since the late Middle Pleistocene. The basin is formed by lacustrine and well as to consider their implications for the East Asian monsoon, both in the past and the peat sediments. Our aims were to determine the periodic components of the record, as future.well as to consider their implications for the East Asian monsoon, both in the past and the future. 2. Study Site 2. Study Site The Dajiuhu Basin (Figure 1) is located in the southwest part of Shennongjia in China (31°24′~31°33The Dajiuhu′ N, 109°56 Basin′~110°11 (Figure1′) E). is located It lies within in the southwest a closed sub-alpine part of Shennongjia basin, which in China consists (31◦240~31◦330 N, 109◦560~110◦110 E). It lies within a closed sub-alpine basin, which consists of the eastern range of the Daba Mountains. The altitude of the basin floor is 1730 m a.s.l. of the eastern range of the Daba Mountains. The altitude of the basin floor is 1730 m a.s.l. 2 (a.s.l.,(a.s.l., above above sea sea level) level) and and it itcovers covers an an area area of of ~16 ~16 km km2.. The The basin basin floor floor is flatflat andand isis sur- roundedsurrounded by steep by steep mountains mountains with with altitudes altitudes of of 2200 2200 to to 2400 2400 mm a.s.l.a.s.l. At At present, present, the the basin basin is occupiedis occupied by relatively by relatively undisturbed undisturbed peats peats de depositedposited within within wetlands wetlands which which occupy occupy 47.4% 47.4% of theof the basin basin area. area. Dajiuhu Dajiuhu is is a a closed alpinealpine lake lake basin basin that that is rechargedis recharged by atmospheric by atmospheric precipitationprecipitation and and a astream stream from from thethe surroundingsurrounding mountains mountains [19 ].[19].

FigureFigure 1. Environmental 1. Environmental setting setting of the of the study study area. area. (A (A) )Map Map of of East AsiaAsia showing showing the the current current northern northern limit limit of the of East the East AsianAsian summer summer monsoon monsoon and and the the locations locations of of the the sites sites mentio mentionedned inin thethe text. text. EASM: EASM: East East Asian Asian Summer Summer Monsoon. Monsoon. ISM: ISM: IndianIndian Summer Summer Monsoon. Monsoon. (B) (SatelliteB) Satellite image image of of the the Dajiuhu Dajiuhu Basin showingshowing the the location location of coreof core DJH-2. DJH-2. The The yellow yellow line is line is the shoreline.the shoreline. (C) Schematic (C) Schematic diagram diagram showing showing vertical vertical vegetation vegetation zoneszones aroundaround Dajiuhu. Dajiuhu. m m a.s.l.: a.s.l.: meter meter above above sea level.sea level. (I) Evergreen(I) Evergreen broad-leaved broad-leaved forest; forest; (II) (II) mixed mixed evergreen evergreen and and deciduous broad-leaved broad-leaved forest; forest; (III) (III) broad-leaved broad-leaved deciduous deciduous forest;forest; (IV) mixed (IV) mixed broad-leaved-coniferous broad-leaved-coniferous forest; forest; (V) (V) sub-alpine sub-alpine coniferousconiferous forest. forest. The region belongs to the warm temperate monsoon climate zone and has four distinct seasons.The region At the belongs closest meteorological to the warm stationtemperate in Jiuhu monsoon town, the climate mean annualzone and temperature has four dis- tinctaverages seasons. 7.4 At◦C, the with closest a mean meteorological warmest month station (July) in temperature Jiuhu town, of the 17.2 mean◦C and annual mean tem- peraturecoldest averages month (January) 7.4 °C, temperature with a mean of − warmest4.3 ◦C. The month mean annual(July) temperature precipitation is of 1560 17.2 mm, °C and most of which falls during April to October [19]. The vegetation of the area exhibits a Sustainability 2021, 13, 4848 3 of 13

distinct altitudinal zonation, as follows (Figure1C): (I) Below 900 m a.s.l. the vegetation is evergreen broadleaved forest dominated by Lauraceae and Fagaceae. (II) Within the ranges of 900–1500 m a.s.l., the vegetation is evergreen and deciduous broadleaved mixed forest dominated by Quercus aliena, Quercus serrata, Carpinus spp., Castanea seguinii, Betula luminifera, and Toxicodendron verniciﬂuum. (III) Within the ranges of 1500–1800 m a.s.l., the vegetation is broadleaved forest dominated by Fagus engleriana and Fagus pashanica. (IV) Within the ranges of 1800–2400 m a.s.l., the vegetation is coniferous and broadleaved mixed forest. Among the broadleaved trees, deciduous broadleaved trees such as trees of Quercus aliena are the most common. Coniferous trees include Pinus, Abies, Picea, and Tsuga chiensis. (V) Above 2400 m a.s.l., the vegetation is subalpine coniferous forest, with Pinus, Abies, Picea, and Tsuga chiensis. The understory includes Fargesia spathacea, Lespedeza bicolor, and other shrubs, as well as numerous ferns [20,21]. Bogs and peat deposits are widely distributed in the lake basin, and many aquatic and wetland plants occur, such as Carex argyi, Sanguisorba, Juncus effuses, Festuca rubra, and Sphagnum palustre [22].

3. Materials and Methods 3.1. Core Acquisition In July 2006, sediment sampling was conducted at the depocenter of Dajiuhu Lake using a drill rig with casing. Sediment cores were extracted to a maximum depth of 43.6 m and designated as DJH-2 (31◦2902800 N, 109◦5904000 E) (Figure1). The sediment recovery rate was greater than 95%. The sediments of the core mainly contained clay and peat, which deposits in a lacustrine sedimentary environment.

3.2. Chronology The determination of the chronostratigraphic sequence of the core was based on accelerator mass spectrometry (AMS) 14C dating [23] and amino acid racemization (AAR) dating [24–26]. The AMS14C dating was conducted at the Keck Carbon Cycle AMS facility at the University of California, Irvine (KCCAMS/UCI). The AMS14C ages were corrected to calendar years using the tree ring curve and OxCal program (version 4.1 online, https://c14.arch.ox.ac.uk/oxcal.html, accessed on 25 April 2021). The AAR dating was conducted in the Chromatography Laboratory, College of Life Sciences, Nanjing Normal University. A total of 10 samples collected from peat or organic-rich horizons of the upper 545 cm were used for AMS14C dating. In addition, 5 samples were collected from organic- rich horizons below the depth of 545 cm for AAR dating. AAR dating is a method of determining the geological age using the racemization of residual amino acids in geological bodies (such as fossils). If the sample is not selected properly, it will directly affect the reliability of the age data. Most areas of the Dajiuhu Basin are in a marsh sedimentary environment, and plants are often buried quickly after death, and then forming peat in a marsh hypoxia environment. In this study, the layers with high content of peat or organic matter and plant residue samples were selected to ensure the reliability of the dating age. The sedimentation rate and the dating of the core indicated that the onset of this section of core DJH-2 occurred at ~237.9 ka [27,28]. A detailed chronological framework was established for regional paleovegetation research. The results for all of the dating samples are listed in Table1. Sustainability 2021, 13, 4848 4 of 13

Table 1. Dating results for samples from the core DJH-2.

Depth (cm) Dating Method Age (cal. Year B.P.) 33 AMS14C 1115 ± 37 51 AMS14C 1635 ± 9 83 AMS14C 2600 ± 20 99 AMS14C 3300 ± 37 117 AMS14C 3865 ± 53 144 AMS14C 7410 ± 35 270 AMS14C 16,650 ± 80 396 AMS14C 23,730 ± 80 517 AMS14C 35,790 ± 300 545 AMS14C 39,140 ± 350 560 AAR 40,935 573.8 AAR 83,209 729.1 AAR 53,441 1030.1 AAR 75,737 3110.6 AAR 191,005

3.3. Sample Processing and Analysis The core sections were spilt, photographed, and described, and then cut into segments at 2–13 cm intervals. The particle size analysis was performed using a Malvern Mastersizer 2000 laser particle size analyzer. A total of 2096 samples were used for grain-size analysis. The sediments of core DJH-2 were divided into 3 grades: clay (<4 µm, 8 phi), fine silt (4–16 µm, 8–6 phi), and silt (16–63 µm, 6–4 phi) according to grain size [29]. Samples were sequentially treated with H2O2 and HCl to remove organic matter and carbonates, respectively, and grain-size frequency distributions were measured with a laser particle analyzer. The measurements were made at the Academy of Geographical Sciences, Nanjing Normal University. Pollen analysis was conducted on 362 samples with intervals of about 12 cm, which were pretreated using a modified HCl–NaOH–HF procedures [30]. Pollen identifications were made in the Institute of Northeast Geography and Agricultural Ecology, Chinese Academy of Sciences. Pollen was mounted in glycerin and counted under the light microscope with ×400 magnification. For most samples, more than 300 pollen grains were counted. The identification of pollen grains was performed using the Pollen Flora of China [31] and An Illustrated Handbook of Quaternary Pollen and Spores in China [32], together with the use of the modern pollen reference collection (Environmental Archaeology Laboratory of Hebei Normal University).

4. Results 4.1. Lithology The core can be divided into the following ﬁve sedimentary units: 43.6–37.0 m: sand and gravel layers; most gravels are angular, with average diameters of ~2–20 mm, and a maximum of ~80 mm, which shows a decreasing trend from bottom to top. It is interpreted as a pluvial fan (a fan-like accumulation terrain that is formed after a temporary or seasonal river rushes out of the mountain pass) environment. 37.0–29.4 m: alternating layers of gray silty clay and peat, interpreted as a lacustrine sedimentary environment. 29.4–14.9 m: gray and yellow-gray clay or silty clay with occasional gravel layers (average diameters of ~1–3 mm with a maximum of 30 mm); interpreted as shallow water faces. 14.9–2.3 m: alternating gray silty clay and peat, with occasional angular gravels with average diameters of ~6–7 mm and a maximum of 20 mm; interpreted as a lacustrine sedimentary environment. 2.3–0 m: peat, deposited within a lacustrine sedimentary environment.

4.2. Environmental Implications of the Grain-Size Record The Dajiuhu Basin is located in a typical East Asian monsoon-dominated region, where precipitation is regarded as one of the best proxies for summer monsoon intensity [33–35], and it has been widely used in studies of both the modern summer monsoon and the paleo- Sustainability 2021, 13, x FOR PEER REVIEW 5 of 14

4.2. Environmental Implications of the Grain-Size Record Sustainability 2021, 13, 4848 The Dajiuhu Basin is located in a typical East Asian monsoon-dominated region,5 of 13 where precipitation is regarded as one of the best proxies for summer monsoon intensity [33–35], and it has been widely used in studies of both the modern summer monsoon and themonsoon paleo-monsoon [36–41]. The[36–41]. grain The size grain of the size clastic of the component clastic component of loess and of aquaticloess and sediments aquatic sedimentsis widely is used widely to gain used insights to gain insights into hydrological into hydrological activity activity within thewithin catchment the catchment [42,43]. [42,43].The Dajiuhu The Dajiuhu has no has inflowing no inflowing or outflowing or outflowing rivers, rivers, and isand fed is byfedprecipitation by precipitation and and the thestream stream from from the surroundingthe surrounding mountains. mountains. In humid In humid regions, regions, the grain the size grain of clasticsize of material clastic materialis mainly is mainly affected affected by variations by variations in slop in flow slop intensity, flow intensity, with coarserwith coarser particles particles primarily pri- marilyreflecting reflecting high rainfallhigh rainfall [44,45 [44,45].]. The greaterThe greater the hydrodynamicthe hydrodynamic force, force, the the stronger stronger the thecarrying carrying capacity capacity of of coarse coarse particles. particles. High High content content ofof clayclay indicates stable stable sedimentary sedimentary environmentenvironment and and weak weak hydrodynamic hydrodynamic conditions. conditions. Depth Depth plots plots of various of various grain-size grain-size frac- tionsfractions of the of core the DJH-2 core DJH-2 are shown are shown in Figure in Figure2, and 2the, and grain-size the grain-size characteristic characteristic of specific of intervalsspecific intervalsof the core of are the described core are described below. below.

FigureFigure 2. 2. DepthDepth plot plot of ofvarious various grain-size grain-size components components of core of core DJH-2. DJH-2. Data Data points points withwith stars starsin front in frontof them of themare the are time the intervaltime interval of MIS of 5 MISboundaries 5 boundaries adjusted adjusted with the with 250 the ka 250 interval ka interval of the ofstalagmites the stalagmites δ18O inδ 18SanbaoO in Sanbao Cave. MZ: Cave. median MZ: median grain size.grain size. 43.6–31.1 m (237.9–191.0 ka). The proportion of the 4–16 µm fraction decreased, and that43.6–31.1 of the fraction m (237.9–191.0 >16 µm increased ka). The sharplyproportion and of fluctuated the 4–16 frequently.μm fraction The decreased, median grainand thatsize of (MZ) the fraction ranged from>16 μ 6.2m increased to 47.6 µm, sharply and the and average fluctuated was frequently. 11.4 µm. The median grain size (MZ)31.1–14.9 ranged m from (191.0–130.0 6.2 to 47.6 ka). μThem, and proportion the average of the was 4–16 11.4µ μmm. fraction increased and maintained31.1–14.9 high m (191.0–130.0 values and theka). proportion The proportion of the of >16 theµm 4–16 fraction μm fraction decreased. increased The median and maintainedgrain size (MZ) high rangedvalues and from the 6.8 proportion to 52.5 µm, of and the the>16 average μm fraction was 11.7decreased.µm. The median grain 14.9–7size (MZ) m (130.0–71.0 ranged from ka). 6.8The to proportion52.5 μm, and of 4–16the averageµm fraction was 11.7 first μ decreasedm. and then increased,14.9–7 whilem (130.0–71.0 that of the ka). fraction The proportion >16 µm first of 4–16 increased μm fraction and then first decreased. decreased The and median then increased,grain size while (MZ) rangedthat of fromthe fraction 7.3 to 35.8 >16 µμm,m andfirst theincreased average and was then 15.3 decreased.µm. The median grain7.0–2.3 size m (71.0–13.8(MZ) ranged ka). fromBoth 7.3 the to 4–16 35.8µ μmm, and and >16 theµ maverage fractions was fluctuated 15.3 μm. frequently but7.0–2.3 exhibited m (71.0–13.8 trends of decreasing,ka). Both theincreasing, 4–16 μm and and >16 then μm decreasingfractions fluctuated values. The frequently median butgrain exhibited size (MZ) trends ranged of decrea from 8.0sing, to 40.6increasing,µm, and and the then average decreasing was 14.2 values.µm. The median grain 2.3–0.3size (MZ) m (13.8–1.0ranged from ka). The8.0 to proportion 40.6 μm, and of the the 4–16 averageµm fraction was 14.2 decreased, μm. and that of the fraction >16 µm increased. The median grain size (MZ) ranged from 10.1 to 53.9 µm, the average was 19.5 µm. Sustainability 2021, 13, 4848 6 of 13

4.3. Pollen Source Area and Vegetation Implications of the Pollen Assemblages from Dajiuhu Lake Within the 362 pollen samples analyzed, 126 taxa were identiﬁed. Tree and shrub pollen types included Abies, Picea, Pinus, Acer, Tsuga, Betula, Carpinus, Corylus, Fagus, deciduous Quercus (Quercus (D)), Juglans, Pterocarya, Ulmus, evergreen Quercus (Quercus (E)), Rhus orientalis, Euphorbia, and Sapium. Herbaceous pollen types included Chenopodiaceae, Artemisia, Asteraceae, Poaceae, and Ranunculaceae. Aquatic hygrophyte pollen types included Cyperaceae, Typhaceae, Myriophyllum, and Potamogeton. Fern spores included Loxogrammae, Polypodiaceae, and Humata. Dajiuhu is a closed alpine lake, and we inferred that pollen is transported to the lake from different vegetation zones on different altitudes surrounding the Dajiuhu Basin. According to the distribution of modern vegetation and its composition in the study area, we combined the major pollen types into three ecological groups: (1) Coniferous forest, consisting of, e.g., Pinus, Picea, and Abies, which are often used as indicators of a cold environment. (2) Broadleaved forest comprising Betula, Carpinus, Corylus, etc., and warm-and-humid broadleaved forest such as Quercus (E), Euphorbia, Fagus, Pterocarya, and Sapium. Warm-and-humid broadleaved forest was used as indicators of a warm and humid paleoenvironment. Quercus (E) belongs to subtropical broadleaved trees, whereas Carpinus belongs to temperate broadleaved trees. If Carpinus appears in the place of Quercus (E), it indicates that the climate is getting colder. (3) Subalpine meadow, dominated by Artemisia, Chenopodiaceae, Asteraceae, Poaceae, Sanguisorba, and Cyperaceae. The pollen diagram Sustainability 2021, 13, x FOR PEER REVIEW 7 of 14 for core DJH-2 is shown in Figure3, and a detailed description of the pollen zones is given in the Supplementary Materials.

Figure 3.FigurePollen 3. Pollen percentage percentage diagram diagram of ofcore core DJH-2 DJH-2 showing showing selected taxa. selected Pollen assemblage taxa. Pollen zones ar assemblagee divided by CONISS zones analysis. are divided by CONISS analysis.

The modern vertical vegetation distribution surrounding the Dajiuhu Basin suggests that broadleaved forest, coniferous forest, subalpine meadow with increasing of elevation, and the basal vegetation zone are occupied by broadleaved forest. As the climate warms up, the content of broadleaved trees in vegetation assemblage will increase. As the climate cools down, the content of coniferous trees will increase, with the tree line moving down. Sustainability 2021, 13, 4848 7 of 13

According to the grain size records and pollen assemblage characteristics of core DJH- 2 (see Supplementary Materials), we were able to divide the evolution of the vegetation and climate of the study area into five periods, which are equivalent to the pollen zones described in the Supplementary Material; these areas are summarized below. Zone I (43.6–31.1 m, 237.9–191.0 ka). The percentage of coniferous trees was highest (average 22.6%) in the record. The average percentage of warm-and-humid broadleaved tree pollen was 15.2%. There were secondary fluctuations within the zone. At 43.6–35.5 m, the coniferous trees such as Picea (average 12.3%) and Abies (average 9.7%) were abundant. Moreover, the percentages of herbaceous types such as A+C (Artemisia+Chenopodiaceae, average 3.5%) and Asteraceae (average 7.1%) were high. At 35.5–31.1 m, Picea (average 6.3%) and Abies (average 4.1%) decreased sharply. Meanwhile, the A+C (average 1.1%) and Asteraceae (average 0.9%) decreased sharply. However, the warm-and-humid broadleaved trees such as Quercus (E) (average 7.7%) and Fagus (average 6.9%) were increased. Zone II (31.1–14.9 m, 191.0–130.0 ka). The percentage of coniferous trees (average 18.5%) increased, and that of the warm-and-humid broadleaved trees (average 14%) decreased substantially. Zone III (14.9–7.0 m, 130.0–71.0 ka). The percentage of coniferous trees (average 9.8%) decreased substantially, and that of warm-and-humid broadleaved trees (average 17.5%) increased sharply. There were secondary fluctuations within the zone. At 14.9–8.4 m, the coniferous trees such as Picea (average 4.1%) and Abies (average 3.8%) were higher. The warm-and-humid broadleaved trees such as Quercus (E) (average 5.5%), Fagus (average 4.5%), and Euphorbia (average 2.1%) were lower. At 8.4–7.0 m, the Picea (average 1.4%) and Abies (average 1.7%) decreased slightly. However, the Quercus (E) (average 11.4%), Fagus (average 7.6%), and Euphorbia (average 4.3%) were increased. Zone IV (7.0–2.3 m, 71.0–13.8 ka). The percentage of coniferous taxa (average 14.1%) increased and that of warm-and-humid broadleaved taxa (average 12.8%) decreased slightly. Zone V (2.3–0.3 m, 13.8–1.0 ka). The tree pollen representation reached its highest level within the core, and that of warm-and-humid broadleaved trees (average 31%) was also the highest; however, the percentage of coniferous trees (average 2.2%) was the lowest in this zone.

5. Discussion 5.1. Climate and Environmental Change within the Dajiuhu Basin since the Late Middle Pleistocene According to the stalagmite δ18O record of Sanbao Cave [10,18], in the same area, together with the dating results for core DJH-2, we found that there was a close correspondence between the variations in the pollen record in core DJH-2 and the glacial and interglacial period for the last 240 ka. The grain-size record and pollen assemblages of core DJH-2 indicated that the environmental evolution of Dajiuhu Basin during the studied intervals underwent the following series of changes. Period I (237.9–191.0 ka, 43.6–31.1 m). On the basis of the AAR dating, we found that this interval corresponded to MIS 7 (MIS, marine isotope stages). The median grain size (MZ) and the content of the fraction >16 µm increased substantially, which indicated increased precipitation and runoff; this indicated that the East Asian summer monsoon was stronger, which in turn led to a rise in lake level and an increase in hydrodynamic forces. The site was a lacustrine sedimentary environment within which alternating layers of dark gray or black organic clay or peat were deposited. The representation of coniferous trees decreased substantially, while that of warm-and-humid broadleaved trees increased. This indicates that the climate was temperate and wet, and the local vegetation was evergreen and deciduous broadleaved mixed forest. Period II (191.0–130.0 ka, 31.1–14.9 m). On the basis of the AAR dating, we found that this interval corresponded to MIS 6. The content of the fraction >16 µm decreased substantially, which indicated relatively weak precipitation and runoff; this indicated that the East Asian summer monsoon weakened. The water level of the lake was dropped down, within which gray and yellow-gray clay or silty clay was deposited, with occasional Sustainability 2021, 13, 4848 8 of 13

gravel layers. The tree pollen representation decreased substantially, and the pollen spectra were dominated by coniferous tree taxa; there were increases in Artemisia, Chenopodiaceae, and Cyperaceae. This suggests that vegetation landscape was dominated by coniferous forest with alpine meadow developed in some area. Period III (130.0–71.0 ka, 14.9–7.0 m). This interval corresponded roughly to MIS 5. The median grain size (MZ) and the content of the fraction >16 µm fluctuated frequently but remained at an overall high level. These changes indicated fluctuations in precipitation and runoff, which indicated that there was an increase in the strength of the East Asian summer monsoon and hence in precipitation. The site was a lacustrine sedimentary environment within which alternating layers of gray silty clay and peat were deposited, as well as occasionally angular gravel. The representation of coniferous trees decreased substantially while that of warm-and-humid broadleaved trees increased, which indicates that the climate was temperate and wet. These vegetation characteristics showed that the climate of the Dajiuhu Basin had entered an interglacial period and that the landscape was occupied by evergreen and deciduous broadleaved forest. Period IV (71.0–13.8 ka, 7.0–2.3 m). This interval corresponded approximately to MIS 4– 2. The median grain size (MZ) and the content of the fraction >16 µm fluctuated frequently, with first an increasing and then a decreasing trend. These changes indicated fluctuations in precipitation and runoff. This indicated that the summer monsoon weakened, strengthened, and then weakened again. The sediments were mainly gray clay, silty clay, and dark organic clay. In the early period, the representation of coniferous tree taxa was high, and that of warm-and-humid broadleaved tree taxa was low. Steppe vegetation (e.g., Chenopodiaceae and Artemisia) predominated in the mountain foothills. These vegetation characteristics showed that the climate of the Dajiuhu Basin had entered a glacial period, and the vegetation was coniferous forest with mountain meadow. In the middle part, the vegetation changed substantially to broadleaved deciduous forest dominated by Fagus and Quercus (E), which indicated a substantial rise in temperature. In the late period, broadleaved forest was replaced by coniferous and broadleaved mixed forest. Period V (13.8–1.0 ka, 2.3–0.3 m). This interval corresponded to MIS 1. The median grain size (MZ) and the content of the fraction >16 µm increased substantially, which indicated increased precipitation and runoff; this indicated that the East Asian summer monsoon strengthened. The site was a lacustrine sedimentary environment within which peat and clay were deposited. The content of coniferous trees was the lowest. There was a rapid increase in evergreen and deciduous broadleaved forest, indicating a substantial rise in temperature.

5.2. Regional Vegetation Contrasted between Different Interglacial Periods In Figure4, selected indexes from core DJH-2 (coniferous trees, warm-and-humid broadleaved trees) and the median grain size (MZ) are compared with various paleoclimatic records from different latitudes. During MIS 7, MIS 5e, and MIS 1, the magnetic susceptibility of Lake El’gygytgyn [46], the magnetic susceptibility of Xifeng section [47], and 10Be rainfall record of Chinese loess [48] increased substantially, indicating that the East Asian Monsoon strengthened and that the climate was warm and wet. The δ18O record of plankton in the South China Sea [49] and the stalagmite δ18O record of Sanbao Cave was low, also showing that the climate warmed during these periods. The grain size record of Dajiuhu Lake indicated that the depositional environment during these periods was wetland with peat accumulation. The median grain size (MZ) and the representation of warm-and-humid broadleaved forest in core DJH-2 also suggested that climate was warm and wet at these times. The proportion of warm-and-humid broadleaved trees increased substantially, conﬁrming that the climate in the Dajiuhu area was warm and wet, which is consistent with comparable records on a global scale. Sustainability 2021, 13, x FOR PEER REVIEW 10 of 14

warm and wet at these times. The proportion of warm-and-humid broadleaved trees in- Sustainability 2021, 13, 4848 creased substantially, confirming that the climate in the Dajiuhu area was warm and9 wet, of 13 which is consistent with comparable records on a global scale.

Figure 4. Comparison of major paleoclimate paleoclimate indexes (warm-and-hu (warm-and-humidmid broadleaved forest, coniferous, and grain size) of ◦ core DJH-2 with with paleocli paleoclimaticmatic records records from from different different latitu latitudesdes of of the the Northern Northern Hemisphere. Hemisphere. (a ()a 21) 21 July July insolation insolation at at65° 65 N [50].N[50 (].b) (Lakeb) Lake El’gygytgyn El’gygytgyn magnetic magnetic susceptibility susceptibility profile profile [46]. [46 (c].) Xifeng (c) Xifeng magnetic magnetic susceptibility susceptibility profile profile (MSUS) (MSUS) [47]. [ 47(d].) The(d) The loess loess 10Be10 rainfallBe rainfall proxy proxy record record [48]. [48 (].e) ( eSpeleothem) Speleothem 18O18 Orecord record from from Sanbao Sanbao Cave Cave [10,18]. [10,18]. ( (ff)) Planktonic Planktonic 18OO record record from the South China Sea [49]. [49]. ( g)) Warm-and-humid broadleaved trees pollen da datata and their ~20-kyr filtering filtering (red (red line) line) from corecore DJH-2DJH-2 (this(this study). study). ( h(h)) Coniferous Coniferous tree tree pollen pollen data data and and 10-point 10-point running running mean mean (purple (purple line) line) from from core core DJH-2 DJH-2 (this (this study). (i) Grain size record of core DJH-2. study). (i) Grain size record of core DJH-2.

TheThe proportion proportion of of coniferous trees and warm-and-humid broadleaved trees from corecore DJH-2 DJH-2 was was also also compared compared with with the the trees trees and shrubs of the pollen record in Lake El’gygytgynEl’gygytgyn [[50–52]50–52] (Figure(Figure5 ).5). In In Lake Lake El’gygytgyn, El’gygytgyn, the the proportion proportion of trees of trees and shrubsand shrubs also Sustainability 2021, 13, x FOR PEER REVIEW alsoincreased increased during during MIS 7,MIS MIS 7, 5,MIS and 5, MISand 1.MIS During 1. During MIS 1,MIS the 1, vegetation the vegetation11 of was 14 characterized was charac- terizedby deciduous by deciduous forest and forest forest-tundra and forest-tundra [50,51]. During [50,51]. MIS During 5, the MIS vegetation 5, the vegetation was dominated was dominatedby deciduous by taxa,deciduous with a taxa, high with percentage a high ofpercentageBetula and ofAlnus Betulapollen and Alnus [50,51 pollen]. During [50,51]. MIS During7, the vegetation MIS 7, the wasvegetation dominated was dominated by tundra, whichby tundra, was which mainly was alder mainly and birch alder [ and52]. birch [52].

Figure 5. ComparisonFigure 5. Comparison of the trees andof the shrubs trees ofand pollen shrubs record of po inllen Lake record El’gygytgyn in Lake [El’gygytgyn50–52] with the [50–52] proportion with of coniferous trees and warm-and-humidthe proportion of broadleaved coniferous trees trees and from warm-a core DJH-2.nd-humid broadleaved trees from core DJH-2.

However, the responseHowever, of the the response regional ofvege thetation regional to climate vegetation change to climate during changethe three during the three interglacials ininterglacials core DJH-2 was in core substantially DJH-2 was different. substantially During different. MIS 7 in During core DJH-2, MIS 7 inthe core DJH-2, the regional vegetationregional consisted vegetation main consistedly of broadleaved mainly of forest broadleaved with Fagus forest, Quercus with Fagus(D), and, Quercus (D), and Quercus (E), and the proportion of coniferous forest was low. During MIS 5e in core DJH- 2, the proportion of subtropical evergreen broadleaved forest, with Rhus orientalis and Eu- phorbiaceae, was greater than during MIS 7, while the amount of Picea and Abies forest was less than during MIS 7. We speculate that MIS 5e was warmer than MIS 7 in core DJH- 2. The magnetic susceptibility of Xifeng section, the 10Be rainfall record of Chinese loess, and the δ18O record of plankton in the South China Sea also supported this interpretation. However, the magnetic susceptibility of Lake El’gygytgyn and the stalagmite δ18O record of Sanbao Cave during MIS 5e were not warmer than MIS 7. From the perspective of vegetation restoration at Dajiuhu, the pollen representation of broadleaved taxa was greatest during MIS 1, and it appeared that MIS 1 was the warmest and wettest interglacial within the studied interval. In terms of the conditions at the site of core DJH-2, a lacustrine environment was present during MIS 5e, while a wetland environment occurred during MIS 1. Since MIS 1, the pollen record of core DJH-2 indicated that a large amount of evergreen and deciduous broadleaved mixed forest had developed within the Dajiuhu Basin, which is the equivalent of the lowermost vegetation zone within the surrounding mountains. Thus, there would have been more broadleaved trees pollens entering Dajiuhu at this period.

5.3. Origin of the Observed Vegetation and Climate Changes within the Dajiuhu Basin The record of the development of coniferous and warm-and-humid broadleaved forest in the Dajiuhu Basin is consistent with the stalagmite δ18O record of Sanbao Cave in the same area, the variations of which are closely related to changes in Northern Hemi- sphere insolation (21 July at 65° N) [53], which in turn are directly related to precession (Figure 4). When insolation was high, coniferous forest was poorly represented in the Da- jiuhu Basin, while warm-and-humid broadleaved forest was well represented, and vice versa (Figure 4). An important feature of the pollen record from the Dajiuhu Basin is the occurrence of ~20-kyr precessional cycles in the vegetation record, which are clearly evident in the record of the percentages of warm-and-humid broadleaved trees in core DJH- 2 (Figure 4g). The east–west-oriented mountain ranges (>1500 m a.s.l.) between South and Sustainability 2021, 13, 4848 10 of 13

Quercus (E), and the proportion of coniferous forest was low. During MIS 5e in core DJH-2, the proportion of subtropical evergreen broadleaved forest, with Rhus orientalis and Euphorbiaceae, was greater than during MIS 7, while the amount of Picea and Abies forest was less than during MIS 7. We speculate that MIS 5e was warmer than MIS 7 in core DJH-2. The magnetic susceptibility of Xifeng section, the 10Be rainfall record of Chinese loess, and the δ18O record of plankton in the South China Sea also supported this interpretation. However, the magnetic susceptibility of Lake El’gygytgyn and the stalagmite δ18O record of Sanbao Cave during MIS 5e were not warmer than MIS 7. From the perspective of vegetation restoration at Dajiuhu, the pollen representation of broadleaved taxa was greatest during MIS 1, and it appeared that MIS 1 was the warmest and wettest interglacial within the studied interval. In terms of the conditions at the site of core DJH-2, a lacustrine environment was present during MIS 5e, while a wetland environment occurred during MIS 1. Since MIS 1, the pollen record of core DJH-2 indicated that a large amount of evergreen and deciduous broadleaved mixed forest had developed within the Dajiuhu Basin, which is the equivalent of the lowermost vegetation zone within the surrounding mountains. Thus, there would have been more broadleaved trees pollens entering Dajiuhu at this period.

5.3. Origin of the Observed Vegetation and Climate Changes within the Dajiuhu Basin The record of the development of coniferous and warm-and-humid broadleaved forest in the Dajiuhu Basin is consistent with the stalagmite δ18O record of Sanbao Cave in the same area, the variations of which are closely related to changes in Northern Hemisphere insolation (21 July at 65◦ N) [53], which in turn are directly related to precession (Figure4). When insolation was high, coniferous forest was poorly represented in the Dajiuhu Basin, while warm-and-humid broadleaved forest was well represented, and vice versa (Figure4). An important feature of the pollen record from the Dajiuhu Basin is the occurrence of ~20-kyr precessional cycles in the vegetation record, which are clearly evident in the record of the percentages of warm- and-humid broadleaved trees in core DJH-2 (Figure4g). The east–west-oriented mountain ranges (>1500 m a.s.l.) between South and North China tend to block the northward movement of near-ground airﬂows, and thus strong summer monsoon could cross the mountains during interglacial periods. During glacial periods, when the sea level was low and Northern Hemisphere ice volume was large, the Northern boundary of ITCZ (intertropical convergence zone) lay more south, and the East Asian Monsoon was too weak to cross the Qinling Mountains at these times. For these reasons, North China was beyond the northern limit of the summer monsoon during glacial periods; in contrast, however, the stronger summer monsoon during interglacial periods was able to reach North China. Thus, the paleoclimatic records from the region south of the Qinling Mountains, where the Dajiuhu Basin is located, exhibit a dominant 20-kyr precessional cyclicity, while in contrast the summer monsoon records from North China primarily show a cyclicity similar to that of ice volume variations.

6. Conclusions (1) A ~240 kyr pollen record from the sediments of Dajiuhu Lake in Hubei Province indicates the occurrence of major fluctuations in the proportions of coniferous and warm-and-humid broadleaved forest. The fluctuations are consistent with the stalagmite δ18O record from Sanbao Cave in the same area, which are closely related to changes in precession-driven Northern Hemisphere insolation. (2) The cyclicity of the East Asian summer monsoon evident in records from south of the Qinling Mountains is mainly controlled by precession, via a direct response to insolation at the precessional (~20-kyr) scale in the Northern Hemisphere. The difference between paleoclimatic records from North and South China is closely related to the east–west-oriented mountain ranges in central China that block the movement of near-ground airflows; thus, strong summer monsoon could cross the mountains during interglacial periods. During glacial periods, when the sea level Sustainability 2021, 13, 4848 11 of 13

was low and Northern Hemisphere ice volume was large, the Northern boundary of ITCZ lay more south, and the East Asian Monsoon was too weak to cross the Qinling Mountains at these times. (3) There were differences in the vegetation composition of the Dajiuhu Basin during the interglacial maxima of MIS 7, MIS 5e, and MIS 1. The extent of warm-and-humid broadleaved forest during the Holocene indicated peak conditions of warm and humid compared to the previous interglacial periods. During the Holocene, the Dajiuhu Basin was occupied by evergreen and deciduous broadleaved mixed forest, similar to the composition of the lowermost vegetation zone within the surrounding mountains.

Supplementary Materials: The following are available online at https://www.mdpi.com/article/10 .3390/su13094848/s1. Author Contributions: L.W., Q.X., and X.L. designed the study; M.L. and D.W. provided useful advice on data processing; L.W. wrote the paper; M.Z., Q.Z., and Z.H. provided laboratory assistance. All authors discussed the results and reviewed the manuscript. All authors have read and agreed to the published version of the manuscript. Funding: This work was supported by the National Natural Science Foundation of China (grant nos. 41931181, 41630753, and 42002197), the Hebei Key Laboratory of Environmental Change and Ecological Construction and Key Discipline Fund Project in Hebei Province, and the Science Foundation of Hebei Normal University (grant no. L2019B35). Institutional Review Board Statement: Not applicable. Informed Consent Statement: Not applicable. Data Availability Statement: The data presented in this study are available on request from the corresponding author. Acknowledgments: We would like to thank Qiufeng Zheng and Maoheng Zhang for their help in laboratory assistance. Thanks to Shengrui Zhang for his assistance in discussing the results. Conﬂicts of Interest: The authors declare no conﬂict of interest.

References 1. An, Z.; Wu, X.; Wang, P.; Wang, S.; Dong, G.; Sun, X.; Zhang, D.; Lu, Y.C.; Zheng, S.H.; Zhao, S.L. Paleomonsoons of China over the last 130,000 years—Paleomonsoon variation. Sci. China B 1991, 34, 1007–1015. 2. An, Z.S. The history and variability of the East Asian paleomonsoon climate. Quat. Sci. Rev. 2000, 19, 171–187. [CrossRef] 3. Chen, F.; Xu, Q.; Chen, J.; Birks, H.J.B.; Liu, J.; Zhang, S.; Jin, L.; An, C.; Telford, R.J.; Cao, X.; et al. East Asian summer monsoon precipitation variability since the last deglaciation. Sci. Rep. 2015, 5, 11186. [CrossRef] 4. Cheng, H.; Edwards, R.L.; Broecker, W.S.; Denton, G.H.; Kong, X.; Wang, Y.; Zhang, R.; Wang, X. Ice age terminations. Science 2009, 326, 248–252. [CrossRef] 5. Ding, Z.L.; Yang, S.L.; Sun, J.M.; Liu, T.S. Iron geochemistry of loess and red clay deposits in the Chinese Loess Plateau and implications for long-term Asian monsoon evolution in the last 7.0 Ma. Earth Planet. Sci. Lett. 2001, 185, 99–109. [CrossRef] 6. Wang, P.X. Global monsoon in a geological perspective. Chin. Sci. Bull. 2009, 54, 1113–1136. [CrossRef] 7. Wang, Y.J.; Cheng, H.; Edwards, R.L.; An, Z.S.; Wu, J.Y.; Shen, C.-C.; Dorale, J.A. A high-resolution absolute-dated Late Pleistocene monsoon record from Hulu Cave, China. Science 2001, 294, 2345–2348. [CrossRef] 8. Wang, P.; Clemens, S.; Beaufort, L.; Braconnot, P.; Ganssen, G.; Jian, Z.; Kershaw, P.; Sarnthein, M. Evolution and variability of the Asian monsoon system: State of the art and outstanding issues. Quat. Sci. Rev. 2005, 24, 595–629. [CrossRef] 9. Wang, Y.; Cheng, H.; Edwards, R.L.; He, Y.; Kong, X.; An, Z.; Wu, J.; Kelly, M.J.; Dykoski, C.A.; Li, X. The Holocene Asian monsoon: Links to solar changes and North Atlantic climate. Science 2005, 308, 854–857. [CrossRef][PubMed] 10. Wang, Y.; Cheng, H.; Edwards, R.L.; Kong, X.; Shao, X.; Chen, S.; Wu, J.; Jiang, X.; Wang, X.; An, Z. Millennial-and orbital-scale changes in the East Asianmonsoon over the past 224,000 years. Nature 2008, 7182, 1090–1093. [CrossRef] 11. Yuan, D.; Cheng, H.; Edwards, R.L.; Dykoski, C.A.; Kelly, M.J.; Zhang, M.; Qing, J.; Lin, Y.; Wang, Y.; Wu, J.; et al. Timing, duration, and transitions of the Last Interglacial Asian Monsoon. Science 2004, 304, 575–578. [CrossRef] 12. Cao, X.Y.; Ni, J.; Herzschuh, U.; Wang, Y.B.; Zhao, Y. A late Quaternary pollen dataset from eastern continental Asia for vegetation and climate reconstructions: Setup and evaluation. Rev. Palaeobot. Palynol. 2013, 194, 21–37. [CrossRef] 13. Ran, M.; Feng, Z. Holocene moisture variations across China and driving mechanisms: A synthesis of climate records. Quat. Int. 2013, 313–314, 179–193. [CrossRef] Sustainability 2021, 13, 4848 12 of 13

14. Yang, X.; Wei, G.; Yang, J.; Jia, G.; Huang, C.; Xie, L.; Huang, W.; Argyrios, K. Paleoenvironmental shifts and precipitation variations recorded in tropical maar lake sediments during the Holocene in Southern China. Holocene 2014, 24, 1216–1225. [CrossRef] 15. Tian, J.; Wang, P.X.; Cheng, X.R.; Wang, R.J.; Sun, X.J. Forcing mechanism of the Pleistocene east Asian monsoon variations in a phase perspective. Sci. China 2005, 48, 1708–1717. [CrossRef] 16. Ding, Z.L.; Derbyshire, E.; Yang, S.L.; Yu, Z.W.; Xiong, S.F.; Liu, T.S. Stacked 2.6 Ma grain size record from the Chinese loess based on ﬁve sections and correlation with the deep-sea δ18O record. Palaeoceanography 2002, 17, 1–21. [CrossRef] 17. Cheng, H.; Edwards, R.L.; Wang, Y.J.; Kong, X.G.; Ming, Y.; Kelly, M.J. A penultimate glacial monsoon record from Hulu Cave and two-phase glacial terminations. Geology 2006, 34, 217–220. [CrossRef] 18. Cheng, H.; Edwards, R.L.; Sinha, A.; Spötl, C.; Yi, L.; Chen, S.; Kelly, H.C.R.L.E.M.; Kathayat, G.; Wang, X.; Li, X.; et al. The Asian monsoon over the past 640,000 years and ice age terminations. Nature 2016, 534, 640–646. [CrossRef] 19. Editorial Committee of Local Chroniclea in Shennongjia Forest. Shennongjia Records, 1st ed.; Hubei Science and Technology Press: Wuhan, China, 1996. 20. Ban, J.D.; Qi, G.S. The Vegetation Study of Western Hubei, 1st ed.; Huazhong University of Science and Technology Press: Wuhan, China, 1995. 21. Zhao, C.M.; Chen, W.L. Vegetation of Shennongjia and the basic characteristics of biodiversity. In Biodiversity Conservation and Regional Sustainable Development. Fourth National Conference on the Conservation and Sustainable Use of Biodeversity Symnosium; China Forestry Publishing House: Wuhan, China, 2002; pp. 269–279. 22. Luo, T.; Lun, Z.J.; Gu, Y.S.; Qin, Y.M.; Zhang, Z.Q.; Zhang, B. Plant community survey and ecological protection of Dajiuhu wetlands in Shennongjia area. Wetl. Sci. 2015, 3, 153–160. 23. Brook, R.C. Bayesian analysis of radiocarbon dates. Radiocarbon 2009, 51, 337–360. 24. Kaufman, D.S.; Manley, W.F. A new procedure for determining DL amino acid ratios in fossils using reverse phase liquid chromatography. Quat. Sci. Rev. 1998, 17, 987–1000. [CrossRef] 25. Goodfriend, G.A. Patterns of racemization and epimerization of amino acids in land snail shells over the course of the holocene. Geochem. Et Cosmochim. Acta 1991, 55, 292–302. [CrossRef] 26. Wang, J.K.; Hong, H.H.; Zhong, Y.M.; Chen, C.H. Geochronology Amino Acids, 1st ed.; China Ocean Press: Beijing, China, 1986. 27. Zheng, Q.F.; Zhang, M.H.; Li, J.J.; Shu, Q.; Chen, Y.; Xiao, J.Y. The climate and environment change of shennongjia area recorded in the core of Dajiuhu Basin since the late Middle Pleistocene. Geogr. Res. 2014, 33, 1167–1177. 28. Zhu, Y.; Zhao, Z.J.; Chen, Y.; Zhang, M.H.; Lei, G.L. Record of environmental and millennial-scale climatic changes during MIS-3 by peat at Shennongjia. Quat. Sci. 2013, 33, 155–166. 29. Wentworth, C.K. A Scale of Grade and Class Terms of Clastic Sediments. J. Geol. 1922, 30, 377–392. [CrossRef] 30. Faegri, K.; Kaland, P.E.; Krzywinski, K. Textbook of Pollen Analysis, 4th ed.; John Wiley and Sons: Chichester, UK, 1989. 31. Wang, F.X.; Chien, N.F.; Zhang, Y.L.; Yang, H.Q. Pollen Flora of China, 2nd ed.; Science Press: Beijing, China, 1995. (In Chinese) 32. Tang, L.Y.; Mao, L.M.; Shu, J.W.; Li, C.H.; Shen, C.M.; Zhou, Z.Z. An Illustrated Handbook of Quaternary Pollen and Spores in China; Science Press: Beijing, China, 2016. (In Chinese) 33. Parthasarathy, B.; Munot, A.A.; Kothawale, D.R. All-India monthly and seasonal rainfall series: 1871–1993. Theor. Appl. Climatol. 1994, 49, 217–224. [CrossRef] 34. Wang, B.; Fan, Z. Choice of South Asian summer monsoon indices. Bull. Am. Meteorol. Soc. 1999, 80, 629–638. [CrossRef] 35. Wang, B.; Wu, Z.; Li, J.; Liu, J.; Chang, C.-P.; Ding, Y.; Wu, G. How to measure the strength of the East Asian summer monsoon. J. Clim. 2008, 21, 4449–4463. [CrossRef] 36. Maher, B.A.; Thompson, R.; Zhou, L.P. Spatial and temporal reconstructions of changes in the Asian palaeomonsoon: A new mineral magnetic approach. Earth Planet. Sci. Lett. 1994, 125, 461–471. [CrossRef] 37. Hoelzmann, P.; Kruse, H.J.; Rottinger, F. Precipitation estimates for the eastern Saharan palaeomonsoon based on a water balance model of the West Nubian Palaeolake Basin. Glob. Planet. Chang. 2000, 26, 105–120. [CrossRef] 38. Zhu, C.; Ma, C.M.; Yu, S.Y.; Tang, L.Y.; Zhang, W.P.; Lu, X.F. A detailed pollen record of vegetation and climate changes in central China during the past 16000 years. Boreas 2009, 39, 69–76. [CrossRef] 39. Xu, H.; Hong, Y.T.; Hong, B. Decreasing Asian summer monsoon intensity after 1860 AD in the global warming epoch. Clim. Dyn. 2012, 39, 2079–2088. [CrossRef] 40. Zhou, X.; Sun, L.G.; Huang, W.; Cheng, W.H.; Jia, N. Precipitation in the Yellow River drainage basin and East Asian monsoon strength on a decadal time scale. Quat. Res. 2012, 78, 486–491. [CrossRef] 41. Wang, Y.B.; Herzschuh, U.; Shumilovskikh, L.S.; Mischke, S.; Birks, H.; Wischnewski, J. Quantitative reconstruction of precipitation changes on the NE Tibetan Plateau since the Last Glacial Maximum—Extending the concept of pollen source area to pollen-based climate reconstructions from large lakes. Clim. Past 2014, 10, 21–39. [CrossRef] 42. Újvári, G.; Kok, J.F.; Varga, G.; Kovács, J. The physics of wind-blown loess: Implications for grain size proxy interpretations in Quaternary paleoclimate studies. Earth Sci. Rev. 2013, 154, 247–278. [CrossRef] 43. Folk, L.R.; Ward, W.C. Brazos River bar: A study in the signiﬁcance of grain size parameters. J. Sediment. Petrol. 1957, 27, 3–26. [CrossRef] 44. Kashiwaya, K.; Yamamoto, A.; Fukuyama, K. Time variations of erosional force and grain size in Pleistocene lake sediments. Quat. Res. 1987, 28, 61–68. [CrossRef] Sustainability 2021, 13, 4848 13 of 13

45. Chen, J.A.; Wang, G.J.; Zhang, D.D. Environmental records of lacustrine sediments in different time scales: Sediment grain size as an example. Sci. China Ser. D Earth Sci. 2004, 47, 954–960. [CrossRef] 46. Melles, M.; Brigham-Grette, J.; Minyuk, P.S.; Nowaczyk, N.R.; Wennrich, V.; DeConto, R.M.; Anderson, P.M.; Andreev, A.A.; Coletti, A.; Cook, T.L.; et al. 2.8 Million Years of Arctic Climate Change from Lake El’gygytgyn, NE Russia. Science 2012, 337, 315–320. [CrossRef] 47. Guo, Z.T.; Berger, A.; Yin, Q.Z.; Qin, L. Strong asymmetry of hemispheric climate during MIS-13 inferred from corrlating China loess and Antarctica ice records. Clim. Past 2009, 5, 21–31. [CrossRef] 48. Beck, J.W.; Zhou, W.; Li, C.; Wu, Z.; White, L.; Xian, F.; Kong, X.; An, Z. A 550,000-year record of East Asian monsoon rainfall from 10Be in loess. Science 2018, 360, 877–881. [CrossRef][PubMed] 49. Caballero-Gill, R.P.; Clemens, S.C.; Prell, W.L. Direct correlation of Chinese speleothem δ18O and South China Sea planktonic δ18O: Transferring a speleothem chronology to the benthic marine chronology. Paleoceanogr. Paleoclimatology 2012, 27, 2203. 50. Lozhkin, A.V.; Anderson, P.M.; Matrosova, T.V.; Minyuk, P.S. The pollen record from El’gygytgyn Lake: Implications for vegetation and climate histories of northern Chukotka since the late middle Pleistocene. J. Paleolimnol. 2007, 37, 135–153. [CrossRef] 51. Lozhkin, A.V.; Anderson, P.M. Vegetation responses to interglacial warming in the Arctic, examples from Lake El’gygytgyn, northeast Siberia. Clim. Past Discuss. 2013, 9, 245–267. 52. Zhao, W.; Andreev, A.A.; Tarasov, P.E.; Wennrich, V.; Melles, M. Vegetation and climate during the penultimate interglacial of the northeastern Russian Arctic: The Lake El’gygytgyn pollen record. Boreas 2019, 48, 507. [CrossRef] 53. Berger, A.; Loutre, M.F. Insolation values for the climate of the last 10 million years. Quat. Sci. Rev. 1991, 10, 297–317. [CrossRef]