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Article

Paleontological Evidence for a Northward Shift of the Climate Zone During the Qin and Han Dynasties—A Case of Paleontology from Lake Deposits in the Salawusu River Basin, Mu Us Desert, China

by
Dongfeng Niu
1,
Baosheng Li
2,3,*,
Shuhuan Du
4,*,
Xiaohao Wen
2,
Yansheng Wen
2,
Peixian Shu
3,
Qingyuan Bai
5,
Fengnian Wang
6,
Yuejun Si
1 and
Min Chen
2
1
School of Geography, Lingnan Normal University, Zhanjiang 524048, China
2
School of Geography, South China Normal University, Guangzhou 510631, China
3
State Key Laboratory of Loess and Quaternary Geology, Institute of Earth Environment, Chinese Academy of Sciences, Xi’an 710061, China
4
State Key Laboratory of Tropical Oceanography, South China Sea Institute of Oceanology, Chinese Academy of Sciences, Guangzhou 511458, China
5
Publicity Department of Wushen Banner Municipal Committee of the Communist Party, Inner Mongolia Autonomous Region, Ordos 017300, China
6
School of Geography and Tourism, Huizhou University, Huizhou 516007, China
*
Authors to whom correspondence should be addressed.
Water 2025, 17(17), 2587; https://doi.org/10.3390/w17172587
Submission received: 16 June 2025 / Revised: 23 August 2025 / Accepted: 26 August 2025 / Published: 1 September 2025
(This article belongs to the Special Issue Trend Prediction and Analysis of Climate and Hydrological Changes in the Basin)
Browse Figures
Versions Notes

Abstract

The lacustrine horizon (thickness of 8.75 cm thick) of the Qin and Han dynasties (221 BC–220 AD) was determined based on AMS-14C analysis conducted by the Beta Analytic Radiocarbon Dating Laboratory on the Dishaogouwan section (37°43′ N, 108°31′ E) in the Salawusu River Basin, Mu Us Desert, located in the temperate zone of China. The identification results of the ostracod and charophyta fossils from the four samples at this horizon show the following results: 1. All the samples contain 458 ostracod fossil valves, belonging to six genera and eight species. Their quantity (valves) and percentage, in descending order of abundance, are Candoniella albicans (Brady), 255/55.68%, Ilyocypris bradyi Sars, 73/15.94%, Eucypris inflata Sars, 46/10.04%, Cyclocypris serena Koch, 26/5.68%, Candona kirgizica Mandelstam, 18/3.93%, Ilyocypris biplicata (Koch), 17/3.71%, Candoniella mirabilis Schneider14/3.06% and Leucocytherella sinensis Huang, 6/1.31%. 2. All the samples contain 99 fossil charophyte gyrogonites, belonging to one genera and four species. In terms of quantity/percentage, the Chara sp. is the most abundant, with 41 pieces (41.41%), followed by Chara braunii Gemlin, with 26 pieces (26.26%); Chara leptosperma Braun and Chara canescens Loiseleur account for 19 pieces (19.19%) and 13 pieces (13.13%), respectively. Based on the analysis of the ecological environment of the existing species of these ostracods and charophytes, combined with the fossilized Ilyocypris brady, Ilyocypris biplicata, and Gyraulus convexiusculus Hutton found in all the samples—which indicate very warm, even subtropical climates then—it can be concluded that during the Qin and Han Dynasties, the Salawusu River Basin was primarily characterized by a freshwater lake environment under a warm climate, with the average annual temperature and precipitation in this area approximately 2.1 °C and 100 mm higher than they are currently. The prevailing East Asian summer monsoon pushed the warm temperate climate at least 110 km northwestward from this basin. During this period, there were at least four episodes of brief subtropical climate fluctuations, occurring approximately every 110 years.

1. Introduction

Since Bond et al. discovered in the 1990s that there were multiple millennium–centennial scale climate fluctuations during the Holocene recorded in the North Atlantic [1], the instability of the climate during this period has been successively confirmed in various geological records around the world. In the middle-to-low-latitude regions of China, under the influence of the East Asian monsoon, similar climate fluctuation events have also been widely recorded such as in stalagmites [2], lakes [3,4], peats [5], and deserts [6]. Along with the progress of research, many scholars are paying more attention to the relationship between climate and human activities during the Holocene. For example, Zhang et al. [7] demonstrated through big-data historical analysis that, in agrarian societies, climate change and its natural impacts were almost decisive for human societal development. And, according to pollen-based monsoon climate records and radiocarbon probability density, Xu et al. [8] revealed that synchronous ~500-year cyclic changes occurred between the East Asian monsoon and human activity during the Holocene. The warm-humid/cold-dry phases of the monsoon climate cycles closely corresponded to the intensification/weakening of human activity and the flourishing/decline of prehistoric cultures. These studies not only provide historical context for the man-land relationship but also offer valuable references for today’s consideration of the relationship between highly developed human societies and nature, the environment, and ecological balance.
Recently, Qin et al. [9] used multiparameter analysis of tree-ring records and process-based physiological modeling to demonstrate that the boundary of the Asian summer monsoon in northern China shifted approximately 60 to 100 km northwestward during 270–77 BC. We have found that this view point was supported, to some extent, by the ecological evidence of plant geography recorded in Chinese history. For example, according to Sima Qian’s records in the “Shi Ji: Biographies of Merchants”, the species of bamboo currently growing in the subtropical regions south of 32° N in China were once widely distributed in the Guanzhong region (34–35° N) during the Western Han Dynasty, and at that time, subtropical plants, such as the lacquer tree, could also be grown in the Yellow River Basin at 35° N [10]. In addition, Zhu’s study on Chinese phenology showed that the entire Qin-Han period, which began in 221 BC and ended in 220 AD, was a period of high temperatures [11]. This result raises an interesting question of whether we can find an answer to the phenomenon of the northward drift of the warm climate during the Qin and Han dynasties in the field of geological paleobiology. Could the extreme northern boundary of the subtropical climate extend beyond 35° N into the arid interior of China? When focusing the investigation of this issue on the extensively developed Holocene lake deposits in the Salawusu River Basin of the Mu Us Desert at 37° N, it appears that the geological and paleontological phenomena preserved from the Qin and Han dynasties there provide a compelling explanation for this occurrence. However, when determining the age of the lacustrine horizon during the Qin and Han dynasties, the influence of the carbon reservoir effect on the chronological data is inevitably involved. Many studies in the past have revealed substantial reservoir effects in lacustrine sediments from northern, western, and central China [12]. Fortunately, in recent years, Wen et al. [13] conducted an analysis of the Holocene lacustrine samples from the Salawusu River Basin at the Beta Analytic Radiocarbon Dating Laboratory. They analyzed five AMS 14C assays, and a reservoir correction of 530 years was determined (see Supplementary Material in reference [13] for full details of the methods, samples, and results used to derive this number), which is consistent with reviews of the issue [12]. As a result, the accuracy of dating the horizon in the Qin and Han dynasties has been improved, and it has also made it possible to explore the phenomenon of “a northward shift of the climate zone during the Qin and Han dynasties”. This article attempts to present research results addressing this question by dating the lacustrine strata from the Qin and Han periods in the region and analyzing ostracod and charophyte fossils, as well as gastropod shells.

2. An Overview of the Regional and Holocene Lacustrine Strata

2.1. General Overview of the Region

The Salawusu River originates at the northern foot of Baiyu Mountain on the northern Chinese Loess Plateau in Shaanxi Province, stretching from 37°10′ N to 37°59′ N and from 108°10′ E to 108°58′ E (Figure 1a). Four tributaries of the river converge in Xinqiao, Jingbian County, Shaanxi Province, and then form the main stream, which meanders northward to flow into the southeastern depression of the Ordos Plateau, mostly covered by the Mu Us Desert. Finally, it flows into the Wuding River, a tributary of the Yellow River, at Batu Bay, with the main stream of the river extending over 110 km. The Salawusu River Basin, both up and down reaches of the Xinqiao, belongs to the Loess Plateau-Loess Hill region and the southeastern depression of the Ordos Plateau, respectively (Figure 1b).
In the comprehensive natural regionalization of China, the Salawusu River Basin is located in the transition zone between the arid temperate zone in Northwest China (II10) and the warm temperate zone (I3) in the eastern monsoon region [14] (Figure 1a). According to the meteorological data of Wushen Banner, the mean annual air temperature is approximately 6–7 °C, with the lowest monthly mean temperature in January (−10.7 °C) and the highest in July (21.6 °C). The mean annual precipitation is 350–400 mm, mainly falling from July to September and accounting for 60–70% of the annual precipitation. There are often heavy rainfall events that last for several days. The annual evaporation ranges from 2200 mm to 2800 mm. Since the basin is located at the intersection of the East Asian winter monsoon and the summer monsoon, it has transitional characteristics between a temperate continental arid climate and a warm temperate humid monsoon climate.
Due to the recent tectonic uplift over the past thousand years [15], the Salawusu River has deeply incised its bed and gradually formed a 60–80-m-deep gorge—the “Salawusu Valley”—with well-developed meandering channels (Figure 1c). The Upper Pleistocene-Holocene Series is widely exposed in the valley.

2.2. The Holocene Lacustrine Strata

During the Holocene, lakes were extensively developed in the middle and lower basins of the Salawusu River [13]. Figure 2 shows six identified stratigraphic sections with chronological markers, published by various scholars (Sections ❶–❻ in Figure 2, with their distribution locations shown in Figure 1c). To conduct a comparative study, the figure also lists the section of the Hailiutu River Basin northeast of the Salawusu River (Section ❼, location shown in Figure 1a). From Sections ❶–❺, it can be seen that each profile includes lake deposits from the Qin and Han dynasties, corresponding to a 14C age of 2171–1730 a BP (calculated from the year 1950). Such as the OSL dating of Section ❶, which shows 3200 a and 1750 a, and the AMS-14C dating of Section ❺, which shows 3560 a and 1600 a, indicating the lacustrine existence in the basin during the Qin and Han periods. Additionally, the lacustrine facies dating of Section ❻, 2332 ± 181 a, with the negative error yielding 2151, also falls within this period.
To determine the lacustrine deposits of the Qin and Han periods, an investigation was conducted on the Dishaogouwan section (DGS1) (37°43′26.3″ N, 108°31′2.3″ E; location shown in Figure 1d). The relevant authors of this article have previously discussed the Holocene changes in the Salawusu River Valley based on the analysis of trace elements [21], CaCO3 [22], and grain size [23] in the section. However, with the emergence of the newly determined AMS-14C age results for this section, as described below, and with reference to the OSL ages in [16,17], it was decided to update the age in the original section and no longer use some of the originally referenced ages. This section exhibits continuous lacustrine sedimentation at a depth of approximately 300–440 cm (Figure 3a), characterized by interbedded blue-grey clayey silt, silty fine sand, and silty very fine sand, containing numerous mollusk shells.

3. Methods

3.1. Age Determination

We selected three mollusk shell samples from DGS1 for radiocarbon (14C) dating at the Beta Analytic Radiocarbon Dating Laboratory. The choice of such materials was primarily based on their distinct advantages over total organic carbon (TOC) samples for 14C dating. The carbon source of mollusk shells mainly comes from inorganic carbon (DIC) dissolved in water at the time of growth. It is fixed as calcium carbonate in the growth zone, and the internal structure is dense. By removing the outer organic membrane and treating the samples with weak acid, secondary attachments can be effectively eliminated, ensuring high sample purity. However, freshwater mollusk shells are subject to a “reservoir effect”, which leads to lower 14C activity and, consequently, older apparent ages, necessitating regional correction values for accurate calibration. Beta Analytic employed conventional pretreatment techniques to determine three accelerator mass spectrometry (AMS) 14C dates for bulk organic materials from the DGS1 section [24]. Calibration was performed using the IntCal13 calibration curve [25,26,27] provided by Calib software version 8.1.0. According to the aforementioned research conducted by Wen et al. [13], the average 14C reservoir age in the study area is 530 years. This information was used to determine the corrected ages of these three samples.
In order to understand the sedimentary environment of QHDH, four samples (“QHS1–QHS4”) were analyzed for their grain size and CaCO3 content at this horizon. As shown in Figure 3c, QHDH is characterized by fine particles and high calcium content—silt and clay fine particles make up a significant proportion, ranging from 37.54% to 56.00%, with an average of 48.37%, and an average grain size Mx (φ) ranging from 3.88φ to 4.71φ, with an average of 4.36φ; CaCO3 distribution ranges from 29.60% to 32.00%, with an average of 30.9%.

3.2. The Processing Methods for Ostracod and Charophyte Fossil Samples

Microfossil samples (10 g each) underwent 24-h deionized water immersion, wet-sieving through a 63 µm mesh, and oven-drying at 60 °C. Taxonomic identification was conducted using a stereoscopic microscope Stemi 2000 (Carl Zeiss AG, Jena, Germany), with absolute abundance standardized to 100 g sediment equivalents.
Ostracod identifications were primarily based on the comprehensive taxonomic treatments in Meisch [28], Hou et al. [29], and Yu [30]. For specific genera and species, the original descriptions and subsequent revisions from relevant regional stratigraphic studies were consulted. The key diagnostic features used for identification include the shell shape, hinge structure, edge grooves, and surface decorations (such as nodules, mesh-like textures, and pits).
The identification of fossil charophyte gyrogonites primarily follows the classification system established by Han et al. [31], which is based on the morphology of the apical apparatus (apical pore and spiral cells). Morphometric parameters measured for each morphotype include height, width, isopolarity index (ISI = height × 100/width), and number of coils.
For the QHDH section (see Figure 3c), ostracods and charophytes were identified from four samples (QHS1–QHS4). Samples QHS1 and QHS2 correspond to the time interval of 0–221a BC, while QHS3 and QHS4 represent 0–220 a AD, with an average temporal resolution of approximately 110 years per sample.

3.3. Mollusk Shell Samples

A 10-kg sample was collected from the entire QHDH (located at a depth of 334.51–325.76 cm in the section), with a thickness of 8.75 cm. After immersion in water and sieving, 437 individual mollusk shells were obtained. Professor Chen Deniu from the Institute of Zoology, Chinese Academy of Sciences, identified these specimens.

3.4. Scanning Electron Microscopy

Scanning electron microscopy (SEM) was carried out at the SCSIO, CAS, with the Hitachi SU3500 (Hitachi High-Technologies Corp., Tokyo, Japan) scanning electron microscope. The SEM is used to scan the ostracod and charophyte fossils.

4. Results

4.1. Age

Table 1 lists the AMS-14C results and related parameters of the three mollusk shells from DGS1, as determined by the Beta Analytic Radiocarbon Dating Laboratory. Among them, the age of sample Beta-498920 falls within the time range of the Qin-Han period. The other two samples—Beta-498921 and Beta-498919, although earlier and later than the Qin-Han period, respectively, both provide conditions for obtaining the horizon from this period using the average sedimentation rate. This will be discussed in the following Section 5.

4.2. Ostracod

All the samples contain 458 ostracod fossil valves, belonging to six genera and eight species. The assemblage, in descending abundance order, include Candoniella albicans (Brady); Ilyocypris bradyi Sars; Eucypris inflata Sars; Cyclocypris serena Koch; Candona kirgizica Mandelstam; Ilyocypris biplicata (Koch); Candoniella mirabilis Schneider and Leucocytherella sinensis Huang (Table 2, Figure 4).
The ostracod fossils relative abundance in the QHDH ranges from 1040 to 1320 valves per 100 g of sediment, with ostracod abundance being highest in QSH2, followed by QHS4, QSH1, and QSH3 in descending order. Across all four layers, C. albicans maintains absolute dominance as the primary species, while I. bradyi consistently serves as the secondary dominant species in QHS3, QHS2, and QHS1, however, in QHS4, E. inflata slightly exceeds I. bradyi in abundance, becoming the subdominant species, followed by C. serena and C. kirgizica (tied for fourth place) and I. biplicata and C. mirabilis (fifth place). Notably, L. sinensis is absent in most layers, appearing only in low amounts in QSH2 and QSH3, where it remains the least abundant species. In QSH3, C. kirgizica ranks third, while E. inflata and C. serena are tied for fourth, with C. mirabilis surpassing I. biplicata in abundance and L. sinensis being the rarest. In QSH2, C. kirgizica is the least abundant, followed by C. mirabilis and L. sinensis (jointly second-lowest), while I. biplicata, E. inflata, and C. serena increase progressively in abundance. In QSH1, aside from the dominant C. albicans, subdominant I. bradyi, and third-ranked E. inflata, other species exhibit minimal presence—C. kirgizica and I. biplicata each constitute 3.77%, with only three valves of C. mirabilis and two valves of C. serena detected.

4.3. Charophyte

These samples contain 99 charophyte specimens, belonging to 1 genera and 4 species (Table 3, Figure 5). Based on specimens/percentage, Chara sp. was the most abundant, with 41 specimens/41.41%, followed by Chara braunii Gmelin, with 26 specimens/26.26%, Chara leptosperma Braun and Chara canescens Loiseleur had 19 specimens/19.19% and 13 specimens/13.13%, respectively.
Chara braunii Gmelin, Flor. Badens. Alsat. 4 (Supp.):646.1826; Robinson, Bull. New York Bot. Gard. 4:258–259.1906; H. Et J.Groves, Philipp. Journ. Sci. 7:70.1912; Nordstedt. Proc. Roy. Bot. Soc. Victoria 31:5.1918; Han, Kao et Liu, Journ. Sichuan Univ. 1963:121–123, 1963; Jao et Li, Acta Phytotax, Sinica 12:365.1974.
Distribution in China: Beijing; Hebei: Ping Shan; Shanxi: Taiyuan; Inner Mongolia: Linhe, Wuyuan, Tuoketo; Shaanxi: Yulin, Xi’an, Xianyang; Xinjiang: Burqin; Shihezi; Taiwan: Tainan. It is commonly found in rice fields, fish ponds, lotus fields, ditches, waterlogged pits, and saline waterlogged areas.
Global distribution: Widely distributed worldwide.
This species is a cosmopolitan species, and its morphology often varies with environmental conditions.
Description: The egg capsule is nearly ellipsoidal. It measures 460–620 µm in length and 350–600 µm in width, with the maximum width located at the midpoint. The number of spiral rings when viewed from the side is 9–12. The basal pore is pentagonal.
Chara canescens Loiseleur, Loiseleur-Doslongchamps, Not. Pl. Fl. France 139.1810; Robinson, Bull. New York Bot. Gard. 4 (12–14):262–263. 1906; J. Groes et Bullock-Webster, Brit. Charoph. 2, 14. 1924; Zaneveld, Blumea 4:175–176,1940; Han, Acta Phytotax. Sinica. 16: 90–91.1978.
Distribution in China: Shanxi: Datong; Inner Mongolia: Bayannur League; Gansu: Lanzhou; Zhangye; Linze; Xinjiang: Urumqi; Tibet: Duiqing Lake. It is commonly found in water ditches, ponds, reed marshes, lakes, and swamps.
Global Distribution Asia: China, India, Afghanistan, Mongolia; Europe: former Soviet Union; North America: United States, Canada; Africa: Algeria and Egypt.
Description: The egg case is nearly ellipsoidal. It measures 600–630 µm in length and 360–380 µm in width, with the maximum width located at the midpoint. It has 11–13 lateral spiral rings.
Chara leptosperma Braun, Braun et Nordstedt, Abh. Kon. Akad. Wiss Berlin 1882:184.1882; Ling, Journ. Shanxin Univ. 1984: 92–93.1984.
Distribution in China: Shanxi: Hejin; Inner Mongolia: Bayangol, Linhe, Chengkou, Wulate Qianqi, Maowusu Desert (MG79034). Grows along rivers, water ditches, and small water pools in deserts.
Global distribution: Asia: China, Afghanistan, Iran; Central America: Mexico.
Description: The egg case is elongated and oval in shape. It measures 520–560 µm in length and 380–400 µm in width. It features 10–11 lateral spiral rings.
In these four layers of QHDH, C. sp. consistently appears as the dominant species in QHS4, QHS3, and QHS2. The secondary dominant species C. braunii shows its highest percentage in QHS1, followed by C. sp., C. canescens, and C. leptosperma in descending order. In QHS2, C. leptosperma ranks second in abundance, while C. braunii and C. canescens share the third position. Both QHS3 and QHS4 exhibit similar patterns with C. leptosperma as the secondary dominant species and the remaining two species (C. braunii and C. canescens) sharing equal third-place percentages.
The morphometric data of gyrogonite fossils for gyrogonite width, height, isopolarity index, and number of convolutions for the gyrogonite fossils showed in Figure 6.

4.4. Mollusk Shells

All of these belong to the class Gastropoda, under two families, two genera, and three species: Gyraulus sibiricus Dunker, Galba pervia Martens, and Gyraulus convexiusculus Hutton (Figure 7). Among them, the first species overwhelmingly dominates in numbers, accounting for 422 individuals/96.60%, while the latter two species have 10 individuals/2.30% and eight individuals/1.80% respectively. These are almost all young gastropod shells (97%), with shell heights ranging from 0.35 to 7.30 mm and widths ranging from 2.2 to 5.60 mm. It is worth noting that although individuals of G. convexiusculus are few, they can still be found at each of the depths where ostracod and charophyte fossil samples are distributed—comprising 1–3% of individuals.

5. Analysis and Discussion

5.1. The Determination of the Horizon During the Qin and Han Dynasties

As shown in Table 1 and Figure 3, the age of 1990 ± 80 a at a depth of 332 cm in DGS1 (equivalent to the late Western Han Dynasty around 40 BC) falls precisely within the Qin and Han periods. Regardless of whether the age is considered with its positive or negative margin of error, it still falls within this range. Based on the ages above and below this point, the depth of lake sediment accumulation during the Qin-Han period was obtained by applying linear interpolation according to the sedimentation rates. As a result, the average sedimentation rate between 332 cm/1990 a and 352 cm/3410 a is 0.14 mm/a, while the rate between 320 cm/1440 a and 332 cm/1990 a is 0.24 mm/a. According to these two average sedimentation rates, the lacustrine horizon containing a large number of mollusk shells (as shown in Figure 3b), at a depth of 334.51–325.76 cm (with a thickness of 8.75 cm) in the 10LS and 11LS layers, belongs to the Qin and Han Dynasties Horizon (QHDH), as shown in Figure 3c. It can be seen that the lacustrine average sedimentation rate from 3410 to 1440 a BP, including the Qin and Han dynasties, is not high, with an average of only 0.19 mm/a. This rate is also quite similar to the rate obtained from the OSL dating results. For example, the OSL date for the lacustrine (peat) section at a depth of 270–230 cm in Section ❶ is 3200–1750 a BP [16], with a sedimentation rate of 0.21 mm/a. Additionally, the AMS-14C dating of the lacustrine facies at a depth of 108–62 cm in the nearby Hailiutu River Basin, shown in Section ❼, ranges from 4333 to 2247 a BP [20], which also seems to support this issue, with an average sedimentation rate of 0.22 mm/a. Based on this, it can be deduced that the sedimentation rate of lakes during the Qin and Han periods might have had a broader regional range.

5.2. Ecological Analysis

Below, to ascertain the climatic environment of the Qin and Han dynasties in this basin, we first turn to an understanding of the ecological preferences of the extant species of these fossils.
(1)
Ecological preferences of the extant species
(a) Ostracod
The majority of extant ostracod species identified in this horizon exhibit a broad global distribution across lacustrine and other aquatic environments. At the same time, a minority appears to be associated with specific natural ecological zones. Below is a summary of their inferred salinity tolerance and preferred water temperature ranges:
C. albicans: A strictly freshwater species, tolerating salinity < 1‰; adapted to 10–20 °C, with optimal growth at 15–29 °C [32,33].
I. bradyi: Tolerates salinity 0–3‰; thrives at 10.5–20 °C, with an optimal range of 15–25 °C [34].
I. biplicata: Tolerates salinity 0–8‰ (optimal 1–4‰); adapted to 20–28 °C, with optimal growth at 25–30 °C [32].
E. inflata: Tolerates salinity 0.5–15‰, most abundant in low-salinity environments (<3‰); adapted to 15–30 °C, optimal at 20–25 °C [35].
C. serena: A strictly freshwater species, tolerating salinity < 1‰; thrives at 15–25 °C [36].
C. kirgizica: Tolerates salinity 0.5–8‰; (optimal < 3‰) [32]. At present, no literature is available regarding the water temperature to which this species is adapted. However, reference [32] mentions that the water temperature in the Yarlung Zangbo River, where C. kirgizica is found, ranges from 12.31 to 27.81 °C, with most variations between 16.01 and 19.51 °C.
C. mirabilis: Tolerates salinity < 3‰ (optimal 0.5–2‰); thrives at 10–20 °C [37].
L. sinensis: Tolerates salinity 0.5–3‰ adapted to 15–25 °C [32].
(b) Charophyte
Among the charophyte fossils in this horizon, extant species such as N. hyalina, C. braunii, and C. leptosperma are strictly freshwater taxa (accounting for 86.86% of the assemblage), while C. canescens can tolerate transitional freshwater-brackish conditions. However, their global climatic distributions and preferred water temperatures differ: C. leptosperma and C. braunii are widely distributed in temperate to subtropical static or slow-flowing waters, with growth temperature ranges of 10–28 °C and 15–30 °C, respectively, and optimal growth at 18–24 °C and 20–26 °C. N. hyalina and C. canescens are predominantly found in temperate lacustrine environments [38], with growth temperature ranges of 5–22 °C and 8–25 °C, respectively, and optimal growth occurring at 12–18 °C and 15–20 °C, respectively.
(c) Gastropod shell
Based on references [19,39], the geographical distribution and habitat of the QHDH’s three gastropods are briefly described below:
G. sibiricus: In China, it is primarily found in the northern regions and the Tibetan Plateau; abroad, it is distributed in countries such as Russia, Kazakhstan, and others. It widely inhabits lakes, creeks, ponds, rice fields, small puddles, swamps, and other water bodies. It prefers warm, humid temperatures but can also adapt to cooler environments.
G. pervia: Widely distributed across both northern and southern China, found throughout various provinces and regions of the country. It is also found abroad in Russia, Mongolia, North Korea, Japan, as well as Southeast Asia and South Asia. It typically inhabits calm, still waters, such as lake shores and streams, and prefers warm, humid environments.
G. convexiusculus: It is widely distributed in the Yangtze River basin and its southern regions in China, as well as in Taiwan, extending further south to Southeast Asia and South Asian countries. It inhabits water bodies such as lakes, streams, ditches, ponds, rice fields, small puddles, and marshes, and is a species that thrives in very warm environments.
(2)
Paleoecological analysis
(a) Ostracod
Except for the I. biplicata, C. kirgizica, and E. inflata, the salinity tolerance of the other 5 species of ostracods is ≤3‰, accounting for 82.32% of the total members. Among them, the total number of C. albicans and C. serena (Figure 4 (1, 4)) with extremely low salinity tolerance accounts for 61.36% of the total. Although I. biplicata, C. kirgizica, and E. inflata can reach upper limits of 4‰, 8‰, and 15‰ respectively in terms of salinity, their lower limit ranges are between 0.5‰ and 1‰. Thus, it can be seen that all eight species of ostracods can survive only when the lake salinity is ≤1‰. Conversely, if the salinity were greater than 1‰, more than 60% of C. albicans and C. serena would not be able to survive. The particularly high abundance of C. albicans, both in total amount and across individual samples (see Table 2), further supports this view. As early as the 1920s, Sars G.O. [40] classified this species as a freshwater one of Western Europe. In the 1990s, Chinese scholars [41,42] shared the same view, pointing out that “when its abundance value is high, it can indicate the freshwater dilution of the water body”. In this sense, the Salawusu River Basin during the Qin and Han periods must have been a freshwater lake, called the “Paleo-lake of the Qin and Han Dynasties,” abbreviated as “QHDL.” Of course, there was also a possibility for a slow-flowing part of a large river at the edge of the lake.
In terms of water temperature, among these ostracods, three species have a minimum water temperature requirement of 15 °C. While the lower limit of the most optimal water temperature range for the dominant species C. albicans and the subordinate species I. bradyi is also 15 °C. However, considering that the lower limit of water temperature for the I. biplicata is higher, the average water temperature of QHDL is estimated to be ≥ 15 °C.
(b) Charophyte
The total sum of the C. leptosperma, N. hyalina and C. braunii charophytes (Figure 5), which clearly live in freshwater, account for 84% of the total—a proportion very close to that of the five species of ostracods, all of which have a salinity tolerance of ≤3‰, as mentioned above. Only the C. canescens is suitable for freshwater and brackish water transition zones, but it is not solely a brackish water species, thus also implying that these charophytes represent lake ecosystems where freshwater predominates. As mentioned above, the water temperature where these charophytes coexist should not be lower than 15 °C, the lower limit of the temperature range that C. braunii can tolerate.
Due to the higher lower limit of the optimal growth temperature for both C. leptosperma and C. Braunii, with its relatively high abundance—these two accounting for 60.60%—suggest that the water temperature at that time was consistent with the temperature indicated by the ostracods, ≥15 °C.
(c) Gastropod
The large number of freshwater gastropod shells in QHDH perfectly illustrates the concept of a “freshwater-dominated lake.” Regarding water temperature, although the G. sibiricus can be widely distributed across the Eurasian continent, including the Qinghai-Tibet Plateau, and can adapt to certain cool environments, warm climates are the best choice for its survival and reproduction. In QHDL, up to 96.60% of this species suggests that the food sources it relies on—algae, humus, and aquatic vascular plants—must have been provided by substantial biomass nurtured under favorable hydrothermal conditions. Moreover, the G. pervia that coexists with it, especially the typical tropical and subtropical species G. convexiusculus [19,39], clearly indicates that the average water temperature at that time should not have been <15 °C.

5.3. Discussion

As can be seen from the above, whether it is the ostracods in the QHDL, charophytes, or gastropods, they clearly indicate that this was a freshwater lake, even though the C. kirgizica suggests slight fluctuations in the lake’s salinity. Significantly, the dominant species C. albicans not only indicates freshwater but may also imply the existence of water bodies with optimal temperature conditions in the area at that time. Although Candona has been reported in the Late Cenozoic strata of the northern hemisphere’s high latitudes and the Qinghai-Tibet Plateau [43] and is considered a heat-averse species, extant Candona can be found in the Gulf of Mexico in tropical and subtropical regions [44]. Fossilized Candona has also been found in the Miocene Guantao Formation, which exhibits typical subtropical monsoon climate characteristics [45]. Interestingly, Liu et al. [33] conducted an investigation on the extant species of C. albicans in Dongting Lake, southern China (29°14′ N–29°22′ N, 112°47′–112°59′ E). The results showed that it existed as the dominant species, with an average summer water temperature of 27.8–29 °C, and its relative abundance varied between 30 and 370 valves per 100 g. The relative abundance of the four QHDH samples, with 560–740 valves per 100 g, far exceeded this range of abundance. Based on this, it can be inferred that when C. albicans was the dominant species in the QHDL or had a high abundance, it might suggest that the water temperature in the lake was relatively high at that time.
Furthermore, considering the habitat of the Ilyocypris in QHDL, it seems to indicate that the water temperature of this lake was relatively high at that time. It is generally believed that this species is thermophilic, with water temperatures during the suitable growth period reaching as high as 20–30 °C. Among them, the sub-ordinate dominant species I. bradyi in QHDL often survives in lake-bottom water temperatures of 17 °C under humid and hot climatic conditions. In recent years, studies by Chen Liang et al. [46] on the extant I. bradyi in Dongting Lake have shown that the water temperature when it is the dominant species, is 20.33 °C. Furthermore, I. biplicata provides even more significant evidence of this. I. biplicata is also known as Ilyocypris gibba, and the two are synonyms [28]. Lüttig [47] pointed out that I. gibba prefers high temperatures of 20–30 °C, and its extant species can be found in the Neotropical region [46]. Additionally, this species has been observed in Lake Metztitlán, Mexico (20°40′53″ N, 98°51′55″ W) [48], and in the waters of Qixianling, Hainan, China (18°14′–18°44′ N, 109°35′–109°45′ E) [49]. Geologically, fossils of I. biplicata have been found in the Neogene Changpo Formation of Hainan Island, which represents a subtropical monsoon climate [50,51], as well as in the warm interglacial MIS5a and MIS5c strata of the Salawusu River basin [52]. Interestingly, like QHDH, the MIS5a and MIS5c strata in this basin also contain G. convexiusculus shells, and the former also contains G. filaris Gredler, an indicator of subtropical climate [19].
Based on the understanding of the dominant species C. albicans, the sub-dominant species I. brady and the typical warm species I. biplicata, considering the water temperature (≥15 °C) of the previously determined charophyte and the ecological environment of the above-cited G. convexiusculus and G. pervia of QHDH, it can be considered reliable that the average water temperature of QHPL is ≥15 °C.
On the basis of determining the above-mentioned water temperature, we can now discuss one of the key indicators of the climate zone discussed in this article: air temperature. To achieve this, an attempt was made to deduce the possible temperature of the basin at the time by comparing the water temperature with the current water temperature of the Yan River in Yan’an, which is located about 160 km southeast of Dishaogouwan and lies in the warm temperate zone. Jiang [53] averaged the water temperature data from five monitoring sections of the Yanhe River in 2017 by season, yielding the following temperatures for spring, summer, autumn, and winter: 9.0 °C, 26.6 °C, 13.8 °C, and 2.54 °C, respectively, the annual average temperature was approximately 13 °C. This means that the water temperature of the QHPL has reached or is at least slightly higher than that of the Yanhe River. Closely related to the latter is the temperature: the annual average temperature of 9.9 °C in Yan’an, which is located on the banks of the Yan River. Based on this, it can be approximated that this is equivalent to the temperature of the Salawusu River Basin at that time. This temperature is 2.1 °C higher than the annual average temperature of 7.8 °C at the nearest Jingbian (The city center 37°36′ N, 108°47′ E) meteorological station, which is located about 25 km southeast of Dishaogouwan.
Next, let us discuss another key indicator of the climate zone that this article focuses on: precipitation. The precipitation during the Qin and Han Dynasties is discussed, first with the aid of tree-ring research results from Qin Chun et al. [9]. They pointed out that the precipitation in the Asian summer monsoon region of northern China during 270–77 BC was 18–34% higher than it is today. The boundary of the summer monsoon extends as deep as 200 km into the Ordos Plateau. Dishaogouwan is located on the southern edge of the plateau. According to Qin Chun et al., the precipitation in this area had been more abundant during that time. The annual average precipitation at the meteorological station in Jingbian County is 395 mm, which can also be approximately regarded as the precipitation of Dishaogouwan. If calculated based on the median value of 26% of the increased precipitation range from Qin Chun et al., the precipitation in the basin from 221 to 77 a BC within the above-mentioned time range reached almost 500 mm, which is exactly the same as present-day Yan’an City (the central urban area of Yan’an, 36°58′ N). Here, let us take a look at the ostracods and charophytes in QHS1 and QHS2 of QHDH. As shown in Figure 3, these two samples represent the entire period from 221 BC to 0 AD, including the 221–77 BC period mentioned in reference [9]. From Table 2 and Table 3, it is easy to see the distribution of the high-abundance, low-salinity (≤3‰) ostracod fossils (QHS1 and QHS2 account for 81.13% and 86.37% respectively) and the three types of strictly freshwater charophyte fossils with high content (QHS1 and QHS2 account for 87.18% and 84.21% respectively) in these two samples. Such a distribution not only supports the aforementioned view of Qin Chun et al. regarding the high precipitation in summer monsoon areas during the Qin and Han dynasties, it also, to some extent, explains the view that the precipitation in the QHPL basin had reached the same level as that of present-day Yan’an. Because without sufficient precipitation, it would be impossible for such a combination of freshwater ancient biological species to exist in the QHPL basin of the inland Mu Us Desert. For instance, Wushen Banner, which is about 130 km north of Dishaogouwan, currently has an average annual precipitation of 351.4 mm. However, all the lakes around this banner are saline-alkali lakes. It should be emphasized that the water and heat environments indicated by the ostracods and charophytes in QHS3–QHS4, representing the 220 years of the late Han Dynasty, do not show significant differences from those in QHS1-QHS2. Moreover, G. pervia and G. convexiusculus, which indicate warm species were found in each sample. Therefore, the above view of 500 mm/a of precipitation applies throughout the Qin and Han Dynasties.
Obviously, the paleontology discussed above and the paleoecology it indicates strongly support the large-scale northward shift of the warm climate during the Qin and Han dynasties. At that time, the Salawusu River Basin at least reached a warm temperate monsoon climate similar to that of today’s Yan’an, with average temperatures and precipitation approximately 2.1 °C and 100 mm higher than they are today, respectively. The ruin of the Great Wall (37°39′59.9″ N, 109°02′33.4″ E) in Jingbian, about 40 km southeast of Dishaogouwan, is precisely a “landmark” marking the boundary between the temperate and warm temperate zones. The ruin is located approximately 110 km from Yan’an City. This signifies that approximately 110 km north of Yan’an, at this landmark, the climate transitions from the warm temperate to temperate. Based on this understanding and the established fact that during the Qin and Han periods “the Salawusu River Basin at least reached a warm temperate monsoon climate similar to that of today’s Yan’an,” we speculate that the powerful East Asian summer monsoon at that time had pushed the temperate/warm temperate boundary northward by at least 110 km relative to this basin—roughly one latitude—reaching the urban center of Wushen Banner (38°36′ N, 108°51′ E). So, was the Salawusu River Basin always in a warm-temperate zone during the Qin and Han dynasties? The presence of the subdominant species I. bradyi, as well as I. biplicata, G. pervia, and G. convexiusculus, which indicate a very warm climate, especially the appearance of G. convexiusculus, suggests that there must have been some subtropical climate fluctuations during this period. However, due to the extremely small number of this species, it can be interpreted as a brief extreme situation of the warm climate development in this basin during the Qin and Han Dynasties. Thus, the answer to the question raised at the beginning of this article—“Could the extreme northern boundary of the subtropical climate extend beyond 35° N into the arid interior of China?”—has been provided. As QHDH’s QHS1–QHS4, all containing species indicative of a subtropical climate, it can be concluded that the Salawusu River Basin, during that period, experienced at least four episodes of brief subtropical climate fluctuations, occurring approximately every 110 years. In addition, the view of the northward migration of climate zones can also be indirectly illustrated by the following related examples.
(1)
In the QHDH, nearly 50% of the material is composed of silt and clay, with an average Mx (φ) value of 4.36 φ, equivalent to silt particles. The particle size composition significantly differs from that of the Mu Us Desert. In 333 samples of dune sand from the desert, the sand particle content is above 92%, with the remainder consisting of silt and clay. The average Mx (φ) value of all samples is 2.42φ, corresponding to fine sand particles [19]. This significant difference reveals that the influence of the Mu Us desertification, associated with the East Asian winter monsoon during the Qin and Han dynasties, was greatly weakened. It is worth noting that the average sedimentation rate during the Qin and Han dynasties was only approximately 0.2 mm per year—a rate equivalent to the size of a single grain of sand. This also clearly indicates the decline and weakening of the winter monsoon during that period. On the other hand, the average CaCO3 content of QHDH (30% or more) shows a significant contrast with that of the paleo-sand dunes in the Mu Us Desert (525 samples), which is only 0.47%. This suggests that, due to the influence of the strong East Asian summer monsoon, the Salawusu River Basin during the Qin and Han dynasties had extremely favorable water and heat conditions, resulting in a large amount of soluble Ca being discharged.
(2)
The QHDL in the basin is actually equivalent to the “Chengchuan Paleo-Lake”, first discovered by Hou Renzhi in 1964 [54], later named by Zhu Shiguang [55], and connected to the Salawusu River Basin during the Qin and Han dynasties, which was referred to as “Sheyanze” in the Han Dynasty [56] (Author’s Note: Sheyanze means Sheyan Lake). Recent research indicates that the area of this lake at that time was as large as 130 km2 (Wen et al., 2025, unpublished data [57]). It is significant that during the Qin and Han dynasties, there were nearly 100 towns on the Ordos Plateau and its surrounding areas [9]. At that time, merely the predecessor of the ancient city of Tongwan (location shown in Figure 1b), Sheyan County, had a considerable population during the reign of Emperor Wu of the Western Han Dynasty (141–87 BC). At that time, the county had 4508 households and a population of more than 26,000 [58].
If there had been no large-scale northward shift of the warm-temperate zone driven by the summer monsoon during the Qin and Han dynasties, how could the aforementioned physical and chemical behaviors have occurred? How can the biomass necessary for the prosperity of so many towns and populations in Ordos be sustained? What is significant is that the warm climate during the Qin and Han dynasties may have had a much wider range and might have even been a climatic event linked to global changes. In China, the annual average temperature variation in the mid-eastern Qinghai-Tibet Plateau over the past 2485 years, reconstructed using tree-ring widths, shows that the temperature departure during the Qin and Han dynasties was almost entirely within the range of 2 to 2.40 °C [59]. During the same period, the oxygen isotope fluctuations of the Dongge Cave stalagmite, which indicate the significance of the Asian summer monsoon, showed that, apart from the recent hundred years with isotope values ranging from −7.8 to −8.8, the values during the Qin and Han dynasties were approximately the lowest. The vast majority of the δ18O values were within the range of −7.3 to −7.7 [2]. The latter, when compared with the oxygen isotope variations in the ice cores from high-latitude Greenland [60,61], shows a broad similarity in terms of frequent decadal-scale and centennial-scale fluctuations. The basic same rhythm of this change in oxygen isotopes seems to indicate that the increase in temperature during the Qin and Han dynasties was at least a widespread climatic event across the Northern Hemisphere. The solar radiation curves of Charles A. P. and Kenneth J. H. show that the temperature departure during the Qin and Han dynasties was mainly between 2 and 2.25 °C [62] (which is very similar to that in the eastern part of the Qinghai-Tibet Plateau mentioned above). In this sense, the “northward shift of the climate zone during the Qin and Han dynasties” discussed in this article represents a regional response to global warming during that period. Obviously, such a temperature was decisive in driving global warming during that period. From this perspective, the “northward shift of the climate zone during the Qin and Han dynasties”, discussed in this paper, was a regional response to the global warming of that period.

Author Contributions

Methodology, S.D.; Software, P.S. and M.C.; Formal analysis, D.N. and Y.S.; Investigation, Q.B.; Resources, S.D. and F.W.; Data curation, Y.W.; Writing—original draft, D.N., B.L. and S.D.; Writing—review & editing, X.W.; Supervision, B.L.; Funding acquisition, D.N., S.D. and X.W. All authors have read and agreed to the published version of the manuscript.

Funding

This work was supported by the “Continental Desert Evolution since 150 ka BP-East Asian Monsoonal Environment Changes from Typical Section and Chronological Ages” (No. salawusu20230703); the Hainan Provincial Joint Project of Sanya Yazhou Bay Science and Technology City (2021JJLH0048), the Guangdong Basic and Applied Basic Research Foundation (2023A1515010705) and the National Natural Science Foundation of China (42171005).

Data Availability Statement

The original contributions presented in this study are included in the article. Further inquiries can be directed to the corresponding authors.

Acknowledgments

The authors acknowledge the three anonymous reviewers for their professional comments.

Conflicts of Interest

The authors declare no conflict of interest.

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Figure 1. The Salawusu River Basin in the Mu Us Desert of the Ordos Plateau and its location within the comprehensive natural regionalization of northern China (a), the locations through which the river flows (b), and the distribution of the Holocene sections (c,d). Legends: 1. Eastern Monsoon Region; 2. Northwestern Arid Region; 3. the boundary between I3 and II10; 4. Great Wall; 5. desert; 6. sand dune or sandy land; 7. scrub and woodland; 8. river; 9. Salawusu River meander; 10. geomorphic unit boundary; 11. southeast depression of Ordos Plateau; 12. Loess Plateau-loess hilly region; 13. the distribution location of the Holocene stratigraphic sections in this paper; 14. ancient city ruins; 15. DGS1 position.
Figure 1. The Salawusu River Basin in the Mu Us Desert of the Ordos Plateau and its location within the comprehensive natural regionalization of northern China (a), the locations through which the river flows (b), and the distribution of the Holocene sections (c,d). Legends: 1. Eastern Monsoon Region; 2. Northwestern Arid Region; 3. the boundary between I3 and II10; 4. Great Wall; 5. desert; 6. sand dune or sandy land; 7. scrub and woodland; 8. river; 9. Salawusu River meander; 10. geomorphic unit boundary; 11. southeast depression of Ordos Plateau; 12. Loess Plateau-loess hilly region; 13. the distribution location of the Holocene stratigraphic sections in this paper; 14. ancient city ruins; 15. DGS1 position.
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Figure 2. The Holocene Series indicating the existence of lacustrine and swamp facies in the Salawusu River Basin from the Qin and Han Dynasties. Legends: 1. Modern mobile dune sand; 2. Paleo-mobile dune sand; 3. Lacustrine facies; 4. Congeliturbated fold; 5. Gastropod fossils; 6. Peat; 7. The transition layer between lacustrine and peat; 8. Cultivated meadow soil; 9. Grassy cortex; 10. Black soil or paleosoil; 11. Light yellow secondary silt; 12. Secondary river silt layer; 13. Light yellow fluvial silt or fine sand; 14. Extremely fine fluvial sand; 15. Alluvial or fluvial deposit; 16. The position and dating of the OSL sample/a BP; 17. The position and dating of the AMS-14C/cal a BP; 18. Traditional 14C dating/cal a BP. Note: In this figure, ❶ according to Reference [16]; ❷ and ❺ according to Reference [13]; ❸ according to Reference [17]; ❹according to Reference [18]; ❻ according to Reference [19]; ❼ according to Reference [20].
Figure 2. The Holocene Series indicating the existence of lacustrine and swamp facies in the Salawusu River Basin from the Qin and Han Dynasties. Legends: 1. Modern mobile dune sand; 2. Paleo-mobile dune sand; 3. Lacustrine facies; 4. Congeliturbated fold; 5. Gastropod fossils; 6. Peat; 7. The transition layer between lacustrine and peat; 8. Cultivated meadow soil; 9. Grassy cortex; 10. Black soil or paleosoil; 11. Light yellow secondary silt; 12. Secondary river silt layer; 13. Light yellow fluvial silt or fine sand; 14. Extremely fine fluvial sand; 15. Alluvial or fluvial deposit; 16. The position and dating of the OSL sample/a BP; 17. The position and dating of the AMS-14C/cal a BP; 18. Traditional 14C dating/cal a BP. Note: In this figure, ❶ according to Reference [16]; ❷ and ❺ according to Reference [13]; ❸ according to Reference [17]; ❹according to Reference [18]; ❻ according to Reference [19]; ❼ according to Reference [20].
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Figure 3. DGS1 columnar section and its age (a), the burial status of gastropod fossils in the horizon of the Qin and Han dynasties (b), the grain size components; and the CaCO3 content (c). Legends: 1. Modern-mobile dune sand; 2. Paleo-mobile dune sand; 3. Paleo-fixed dune sand; 4. Black paleo-soil; 5. Loess-like silty fine sand; 6. Lacustrine facies (clayey silt); 7. Lacustrine facies (silty very fine sand); 8. Lacustrine facies (silty fine sand); 9. Peat; 10. Gastropod shell; 11. The AMS-14C age determined by the Beta Laboratory; 12. Traditional 14C dating, based on reference [21]; 13. OSL dating, based on reference [17]; 14. Grain-size, CaCO3, ostracod, and charophyte fossil samples; 15. Grain-size content; 16. CaCO3 content. Note: In this figure, the Arabic numerals followed by M, D, F, LS, and CS represent modern mobile dunes, paleo-mobile dune sand, paleo-fixed dune sand, lacustrine-swamp facies, and loess-like silty fine sand, respectively.
Figure 3. DGS1 columnar section and its age (a), the burial status of gastropod fossils in the horizon of the Qin and Han dynasties (b), the grain size components; and the CaCO3 content (c). Legends: 1. Modern-mobile dune sand; 2. Paleo-mobile dune sand; 3. Paleo-fixed dune sand; 4. Black paleo-soil; 5. Loess-like silty fine sand; 6. Lacustrine facies (clayey silt); 7. Lacustrine facies (silty very fine sand); 8. Lacustrine facies (silty fine sand); 9. Peat; 10. Gastropod shell; 11. The AMS-14C age determined by the Beta Laboratory; 12. Traditional 14C dating, based on reference [21]; 13. OSL dating, based on reference [17]; 14. Grain-size, CaCO3, ostracod, and charophyte fossil samples; 15. Grain-size content; 16. CaCO3 content. Note: In this figure, the Arabic numerals followed by M, D, F, LS, and CS represent modern mobile dunes, paleo-mobile dune sand, paleo-fixed dune sand, lacustrine-swamp facies, and loess-like silty fine sand, respectively.
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Figure 4. Electron microscope image of fossil ostracods in the QHDH. 1. Candoniella albicans (Brady), left valve; 2. Ilyocypris bradyi Sars, right valve; 3. Eucypris inflata Sars, left valve; 4. Cyclocypris serena Koch, dorsal; 5. Candona kirgizica Mandelstam, left valve; 6. Ilyocypris biplicata (Koch), left valve; 7. Candoniella mirabilis Schneider, right valve; 8. Leucocytherella sinensis Huang, left valve.
Figure 4. Electron microscope image of fossil ostracods in the QHDH. 1. Candoniella albicans (Brady), left valve; 2. Ilyocypris bradyi Sars, right valve; 3. Eucypris inflata Sars, left valve; 4. Cyclocypris serena Koch, dorsal; 5. Candona kirgizica Mandelstam, left valve; 6. Ilyocypris biplicata (Koch), left valve; 7. Candoniella mirabilis Schneider, right valve; 8. Leucocytherella sinensis Huang, left valve.
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Figure 5. SEM fossils charophyte in the QHDH. 1. Chara sp.: 1a, apical view; 1b, lateral view; 1c, basal view; 2. Chara braunii Gemlin: 2a, apical view; 2b, lateral view; 2c, basal view; 3. Chara leptosperma Braun; 3a, apical view; 3b, lateral view; 3c, basal view; 4. Chara canescens Loiseleur: 4a, apical view; 4b, lateral view; 4c, basal view.
Figure 5. SEM fossils charophyte in the QHDH. 1. Chara sp.: 1a, apical view; 1b, lateral view; 1c, basal view; 2. Chara braunii Gemlin: 2a, apical view; 2b, lateral view; 2c, basal view; 3. Chara leptosperma Braun; 3a, apical view; 3b, lateral view; 3c, basal view; 4. Chara canescens Loiseleur: 4a, apical view; 4b, lateral view; 4c, basal view.
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Figure 6. Morphometric data of gyrogonite fossils from the QHDH for width, height, isopolarity index, and number of convolutions. (A). Chara sp.; (B). Chara braunii Gmelin; (C). Chara leptosperma Braun; (D). Chara canescens Loiseleur.
Figure 6. Morphometric data of gyrogonite fossils from the QHDH for width, height, isopolarity index, and number of convolutions. (A). Chara sp.; (B). Chara braunii Gmelin; (C). Chara leptosperma Braun; (D). Chara canescens Loiseleur.
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Figure 7. The Gyraulus convexiusculus Hutton in QHDH.
Figure 7. The Gyraulus convexiusculus Hutton in QHDH.
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Table 1. Several AMS-14C results of DGS1 and their related parameters.
Table 1. Several AMS-14C results of DGS1 and their related parameters.
Depth/cmLaboratory NumberMaterialsδ13C/‰Conventional 14C Ages (A BP)Calibrated Ages
(Cal. A BP, ±2σ)		Reservoir-Corrected Ages (Cal. A BP, ±2σ)
320Beta-498919shell−23.802080 ± 301550 ± 301440 ± 80
332Beta-498920shell−23.902570 ± 302040 ± 301990 ± 80
352Beta-498921shell−9.103710 ± 303180 ± 303410 ± 50
Table 2. Identification results of QHDH ostracod fossils.
Table 2. Identification results of QHDH ostracod fossils.
Depth/cmCandoniella albicansIlyocypris bradyiEucypris inflataCyclocypris serenaCandona kirgizicaIlyocypris biplicataCandoniella mirabilisLeucocytherella sinensis
QHS4/325.7–328.3874 (65.49%)11 (9.73%)16 (14.16%)4 (3.54%)4 (3.54%)2 (1.77%)2 (1.77%)/
QHS3/328.39–331.0056 (53.85%)12 (11.54%)8 (7.69%)8 (7.69%)9 (8.65%)4 (3.85%)5 (4.81%)2 (1.92%)
QHS2/331.01–332.7659 (44.70%)35 (26.52%)10 (7.58%)12 (9.09%)1 (0.76%)7 (5.30%)4 (3.03%)4 (3.03%)
QHS1/332.77–334.5166 (62.26%)15 (14.15%)12 (11.32%)2 (1.89%)4 (3.77%)4 (3.77%)3 (2.83%)/
Table 3. Identification results of QHDH charophyte fossils.
Table 3. Identification results of QHDH charophyte fossils.
Depth/cmChara sp.Chara braunii
Gmelin		Chara leptosperma
Braun		Chara canescens
Loiseleur
QHS4/325.76–328.383 (42.86%)1 (14.29%)2 (28.57%)1(14.29%)
QHS3/328.39–331.0016 (47.06%)4 (11.76%)10 (29.41%)4 (11.76%)
QHS2/331.01–332.769 (47.37%)3 (15.79%)4 (21.05%)3 (15.79%)
QHS1/332.77–334.5113 (33.33%)18 (46.15%)3 (7.69%)5 (12.82%)
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Niu, D.; Li, B.; Du, S.; Wen, X.; Wen, Y.; Shu, P.; Bai, Q.; Wang, F.; Si, Y.; Chen, M. Paleontological Evidence for a Northward Shift of the Climate Zone During the Qin and Han Dynasties—A Case of Paleontology from Lake Deposits in the Salawusu River Basin, Mu Us Desert, China. Water 2025, 17, 2587. https://doi.org/10.3390/w17172587

AMA Style

Niu D, Li B, Du S, Wen X, Wen Y, Shu P, Bai Q, Wang F, Si Y, Chen M. Paleontological Evidence for a Northward Shift of the Climate Zone During the Qin and Han Dynasties—A Case of Paleontology from Lake Deposits in the Salawusu River Basin, Mu Us Desert, China. Water. 2025; 17(17):2587. https://doi.org/10.3390/w17172587

Chicago/Turabian Style

Niu, Dongfeng, Baosheng Li, Shuhuan Du, Xiaohao Wen, Yansheng Wen, Peixian Shu, Qingyuan Bai, Fengnian Wang, Yuejun Si, and Min Chen. 2025. "Paleontological Evidence for a Northward Shift of the Climate Zone During the Qin and Han Dynasties—A Case of Paleontology from Lake Deposits in the Salawusu River Basin, Mu Us Desert, China" Water 17, no. 17: 2587. https://doi.org/10.3390/w17172587

APA Style

Niu, D., Li, B., Du, S., Wen, X., Wen, Y., Shu, P., Bai, Q., Wang, F., Si, Y., & Chen, M. (2025). Paleontological Evidence for a Northward Shift of the Climate Zone During the Qin and Han Dynasties—A Case of Paleontology from Lake Deposits in the Salawusu River Basin, Mu Us Desert, China. Water, 17(17), 2587. https://doi.org/10.3390/w17172587

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