Vegetation succession and climate change during the early pleistocene (2.2-1.8 Ma) in the Nihewan Basin, northern China

Journal Pre-proof Vegetation succession and climate change during the early Pleistocene (2.2-1.8 Ma) in the Nihewan Basin, northern China Guoqiang Ding, Yuecong Li, Zhen Zhang, Wensheng Zhang, Yong Wang, Zhenqing Chi, Gaihui Shen, Baoshuo Fan PII:

S0031-0182(19)30025-2

DOI:

https://doi.org/10.1016/j.palaeo.2019.109375

Reference:

PALAEO 109375

To appear in:

Palaeogeography, Palaeoclimatology, Palaeoecology

Received Date: 11 January 2019 Revised Date:

28 August 2019

Accepted Date: 13 September 2019

Please cite this article as: Ding, G., Li, Y., Zhang, Z., Zhang, W., Wang, Y., Chi, Z., Shen, G., Fan, B., Vegetation succession and climate change during the early Pleistocene (2.2-1.8 Ma) in the Nihewan Basin, northern China, Palaeogeography, Palaeoclimatology, Palaeoecology, https://doi.org/10.1016/ j.palaeo.2019.109375. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier B.V.

Fig. 1. Location of the study area in China (left) and topography of the study area (right).

Fig. 2 Lithology and palaeomagnetic age diagram of core NHA from the Nihewan Basin (GPTS: the geomagnetic polarity timescale) (Hilgen et al., 2012; Singer, 2014; Singer et al., 2014)

Fig. 3. Pollen percentage and concentration diagram of core NHA showing the selected taxa in the Nihewan Basin

Fig. 4 Grain-size parameter curves of core NHA in the Nihewan Basin

Fig. 5 Principal component analysis of pollen taxa from core NHA in the Nihewan Basin

Fig. 6. Pollen records and sediment grain size from core NHA in the Reunion event period a. Pinus pollen percentage; b. Dark coniferous (Abies and Picea) pollen percentage; c. Broad-leaved tree (Anacardiaceae, Betula, Quercus and Ulmus etc) pollen percentage; d. Median grain-size

Fig. 7. Comparison of PCA axis 1 and axis 2 scores of core NHA with environmental proxy indicators a. PCA axis1 scores; b. PCA axis2 scores; c. ＞ 30 μm grain size percentages of core NHA; d. Sediment grain size on the Loess Plateau (Sun et al., 2010); e. SST record from ODP Site 882 in the Subarctic Pacific (Martínez-Garcia et al., 2010); f. SST record from ODP Site 1090 in the Subantarctic Atlantic (Martínez-Garcia et al., 2010); g. Eurasian ice volume relative to present (Bintanja, R. and R.S.W. van de Wal, 2008) and h. LR04 benthic δ18O stack (Lisiecki and Raymo, 2005).

Highlights 1. Pinus pollen percentage was higher than 60 % in the study period, indicating vegetation was dominated by pine forest. 2. From 1.92 to 1.78 Ma (Olduvai event), it was the coldest and driest interval indicated by higher pollen percentages of Picea, Artemisia and Chenopodiaceae. 3. During Reunion (2.15-2.14 Ma) event, with the increase of Quercus and decrease of Picea in the pollen assemblages, it suggested a transition from cold to warm climate.

1

Vegetation succession and climate change during the early Pleistocene (2.2-1.8

2

Ma) in the Nihewan Basin, northern China

3

Guoqiang Dinga,b,c, Yuecong Lia,b*, Zhen Zhanga,b, Wensheng Zhanga,b, Yong Wangd, Zhenqing Chid*, Gaihui

4

Shena,b, Baoshuo Fana,b

5

a. College of Resources and Environment Science, Hebei Normal University, Shijiazhuang 050024, PR China

6

b. Key Laboratory of Environmental Evolution and Ecological Construction of Hebei

7

c. Key Laboratory of West China's Environmental System (Ministry of Education), Lanzhou University, Lanzhou

8

730000, PR China

9

d. Institute of Geology, Chinese Academy of Geological Sciences, Beijing 100037, PR China

10

* Corresponding author:

11

Yuecong Li, College of Resources and Environment Science, Hebei Normal University, 20 Road East, 2nd Ring

12

South, Shijiazhuang 050024, PR China

13

E-mail: [email protected]

14

Tel: 138 3119 0396

15

Zhenqing Chi, Institute of Geology, Chinese Academy of Geological Sciences, No. 26 Baiwanzhuang Street, Beijing

16

100037, PR China

17

E-mail: [email protected]

18

Tel: 139 0118 4032

19

Abstract: The Nihewan Formation, northern China is ideal for studying environmental changes

20

during the early Pleistocene. In conjunction with palaeomagnetic measurements, pollen and grain-

21

size analyses were conducted on 120 samples from a ~24 m long section (2.2-1.8 Ma) of core NHA

22

from the Nihewan Basin, in order to reconstruct past vegetation and climatic changes. The pollen

23

assemblages were dominated by Pinus, indicating that the vegetation was primarily pine forest and

24

that the climate was relatively warm and wet. From 2.15-1.92 Ma, deciduous broad-leaved tree

25

pollen significantly increased to > 10 % of the total, showing that more broad-leaved trees grew in

26

the study area during the warmest and wettest period in the study section. From 1.92-1.78 Ma

27

(coeval with the Olduvai event), Pinus and broad-leaved tree pollen types decreased. The

28

percentages of Picea (>20 %), Artemisia and Chenopodiaceae increased, indicating that spruce

29

forests expanded, the openness of the forested areas increased and the climate became cold and dry.

30

The vegetation changes reconstructed during the Olduvai period indicates that the climate in the

31

Nihewan Basin was cold and dry, relating to global cooling facilitated by the uplift of the Tibetan

32

Plateau and the strengthening of the winter monsoon.

33

Keywords: pollen assemblages; palaeovegetation; palaeoclimate; Reunion event; Olduvai event.

34

1. Introduction

35

The early Pleistocene was an important sub-epoch during which global climate underwent

36

fundamental changes (Leinen and Heath, 1981; Bailey et al., 2012). In general, the East Asian

37

climate was warm and humid, and the East Asian winter and summer monsoons exhibited

38

synchronous changes during the Late Pliocene and Early Pleistocene (Clemens et al., 2008; Jiang et

39

al., 2010; Andreev, 2012; Andreev et al., 2016). Since the early Pleistocene, the atmospheric

40

circulation strengthened and glaciations begun. In particular, the East Asian climate exhibited

41

glacial-interglacial cyclicity with the winter and summer monsoons operating independently from

42

one another (Ding et al., 1990; Sun et al., 1998; Fang et al., 2003; Zhang et al., 2016). Two

43

palaeomagnetic reversal events occurred in the period 2.2-1.8 Ma (Deng et al. 2006; 2008; 2019;

44

Liu et al., 2010; 2012). Since early Pleistocene environmental changes also coincided with the

45

expansion of early humans, it is an important period to study (Potts, 1998; Demenocal, 2004;

46

Bonnefille et al., 2010; Colcord et al., 2018).

47

The Nihewan strata constitute a continuous record of early Pleistocene climate changes in

48

northern China. For example, Xia and Liu (1984) compared the profiles of the Hongya, Haojiatai

49

and Hutouliang Stations and determined that the Nihewan Formation is a large stratigraphic unit

50

which contains deposits spanning the Pleistocene epoch. Early Pleistocene climate changes can be

51

divided into three stages: (i) warm and humid in the early stage; (ii) mild and semi-humid in the

52

middle stage with frequent climate fluctuations; and (iii) mild and semi-humid in the late stage.

53

However, palynological evidence from the whole Nihewan section revealed up to 15-16 climatic

54

changes (Chen, 1988). Zhou et al. (1991) inferred that the climate transitioned from warm-humid to

55

cool-dry climate in the early Pleistocene in the Nihewan area. The Nihewan layer has been divided

56

into three sections based on exposures at the Dadaopo and Donggou sites and can be compared with

57

the Loess Plateau red clay, the Wucheng loess and the Lishi loess (Yuan et al. 1996). The Nihewan

58

Formation has been dated using magnetostratigraphy, which established an age framework. (Zhu et

59

al. 2001; 2004; Deng et al. 2006; 2008; 2019; Wang et al. 2004; 2008). However, the overall

60

temporal resolution remains low and thus existing frameworks of vegetation and climate change

61

remain rather general for the earliest Pleistocene (prior to 1.8 Ma).

62

This paper presents a detailed pollen record for the early Pleistocene (2.2-1.8 Ma) from core

63

NHA, Nihewan Basin. These data will be used to investigate the relationships between the

64

vegetation succession and climate change in the Nihewan Basin, northern China during the early

65

Pleistocene.

66

2. Study areas

67

Located in Yangyuan County of Hebei Province, Nihewan Basin (40°05′-40°20′N, 114°25′-

68

114°44′E) is a late Cenozoic fault basin developed in the transitional zone between the North China

69

Plain and the Inner Mongolian Plateau. The basin is surrounded by mountains and dissected along

70

a southwest-northeast trend by the Sanggan River. There are extensive sedimentary exposures

71

consisting of well-developed late Cenozoic lacustrine-fluvial deposits rich in mammalian fossils

72

known as the Nihewan Fauna (Barbour, 1924, 1925; Teilhard de Chardin and Leroy, 1942). The

73

total area of the basin is approximately 2,000 km2, and the average elevation is approximately 1000

74

m a.s.l. (Fig. 1). The region has a distinct continental monsoon climate with an annual temperature

75

range of 7 to 8°C, whilst the annual precipitation ranges from 360 to 420 mm. The vegetation

76

succession is transitional from warm temperate deciduous broad-leaved forest to temperate semi-

77

arid and arid grassland. The interior of the basin is dominated by semi-arid and arid shrubs and the

78

surrounding mountains are dominated by forests (Xia and Liu, 1984; Zhou et al., 1991).

79

3. Materials and methods

80

3.1. Sampling collection and lithology

81

Lithostratigraphic division of strata is critical for investigating regional geology. To

82

determine the lithostratigraphy of the Nihewan Formation we collected a 365.82 m long core

83

NHA (40°13′0.4″N, 114°38′32.3″E; 938 m a.s.l.) at Haojiatai which is located in the eastern part

84

of the Nihewan Basin, Yangyuan County, Hebei Province (Fig. 1). The borehole was systematically

85

described and sampled at high resolution. The results indicate that the Nihewan Basin stratigraphy

86

is coeval with sediments on the Loess Plateau, specifically the Haojiatai, Salawusu, and Xiaodukou

87

Formations. In addition the Nihewan Formation is correlated with the Lishi and Wucheng Loess.

88

The sediments exposed at this profile represent a stratotype for the fluvio-lacustrine strata in the

89

North China region. A total of 120 samples were collected from 129.8-106 m deep in the middle

90

section of the core. The sediment samples in this study section are mainly composed of horizontally

91

bedded blue-grey, black-grey, brown-black clay and silt, containing abundant charcoal and mollusc

92

fossils. The lithology and fossil content indicate that the depositional environment was a lacustrine

93

swamp and that there is a high degree of stratigraphic continuity (Min et al., 2015).

94

3.2. Palaeomagnetic measurements

95

Bulk magnetic susceptibility was determined for all samples on a Bartington MS2B at a

96

frequency of 0.47 kHz. Natural remanent magnetization (NRM) and demagnetization measurements

97

of all samples were made on a 2G-755 three-axis cryogenic magnetometer shielded by a Helmholtz

98

configuration at the palaeomagnetic laboratory of the Institute of Geomechanics, Chinese Academy

99

of Geological Sciences (CAGS) (Wang et al., 2004). The samples were subjected to progressive

100

thermal demagnetization (40-60 ºC intervals to a maximum of 680 ºC) using a TD-48 thermal

101

demagnetizer or stepwise alternating field (AF) demagnetization (more than 10 steps to a maximum

102

of 100 mT) using a Schonstedt GSD-5 demagnetizer. The directions of the NRM components were

103

determined with principal component analysis (PCA) using at least four temperature steps or AF

104

steps for each component. Remanent coercivity analysis and thermomagnetic analysis were carried

105

out at the palaeomagnetic laboratory, Department of Earth Science, University of Bergen, Norway.

106

3.3. Pollen analysis

107

Laboratory pollen was extracted using a modified HCL-NaOH-HF procedure (Fægri and

108

Iversen, 1989). For each sample, 50 g was weighed before chemical treatment. Then one tablet of

109

Lycopodium spores (27560 grains) was added as an indicator to each sample to calculate the pollen

110

concentration. After this treatment, pollen and spores were concentrated using heavy liquid (Zinc

111

bromide) flotation. The procedures were carried out at the College of Resources and Environmental

112

Sciences of Hebei Normal University. The pollen was identified and counted using a Zeiss Imager

113

A2 optical microscope at ×400 magnification. For most samples, more than 300 identified pollen

114

and spores were counted.

115

3.4. Grain-size analysis

116

Measurements were carried out at the Laboratory of Environmental Evolution, College of

117

Resources and Environmental Sciences, Hebei Normal University. In preparation for grain-size

118

analysis, samples were treated to remove organic matter and calcium carbonate by conventional

119

methods. Grain-size analyses (particle sizes ranging from 0-3500 µm) were undertaken using a

120

Malvern Mastersizer 3000 laser particle size analyser. At least three runs were performed on each

121

sample, and the average was taken as the final result. The repeated measurement error was less than

122

2 %.

123

3.5. Principal component analysis (PCA)

124

PCA is a quantitative method widely used in ecology and related fields. The main aim of this

125

method is to transform high-dimensional data into low-dimensional data, thereby simplifying

126

complex problems with multiple variables (Weng et al., 1993; Davies and Fall, 2001). In this paper,

127

we used Canoco 5 software to analyse the principal components. To clarify the environmental

128

significance of pollen more clearly and to reduce the error, the pollen type in the pollen assemblage

129

with percentages > 1 % for at least 10 samples was selected as a representative type on which to

130

carry out the PCA (Braak and Smilauer, 2002; 2012).

131

4. Results

132

4.1. Palaeomagnetic data

133

The core chronostratigraphic sequence was determined based on palaeomagnetic stratigraphy.

134

The sedimentation rate and magnetic stratigraphy of the core show that the Matuyama/Gauss

135

boundary begins at 156.6 m depth. Two transient positive polarity drift events are present at 125.4-

136

121.2 m and from 116.4-105.8 m, which correspond to the Reunion and Olduvai intervals,

137

respectively, according to the existing palaeomagnetic chronology and stratigraphic framework

138

(Zhu et al., 2007; Ogg, 2012; Deng et al., 2019). The ages of 120 samples in the 129.8-106 m section

139

were calculated by linear interpolation according to the existing palaeomagnetic chronological

140

nodes and average deposition rate (Fig. 2).

141

4.2. Pollen assemblages

142

Ninety pollen types were identified within the 120 pollen samples in the Haojiatai NHA drill

143

core. These include 24 tree pollen types, 17 shrub pollen types, 40 herb pollen types and 9 fern spore

144

types. A total of 46126 grains of pollen were counted (excluding algae). An average of 384 pollen

145

and spores were counted for each sample, with an average concentration of 423 grains/g. Among

146

the identified pollen samples, Pinus, Picea, Abies, Quercus, Ulmus, Betula and Anacardiaceae are

147

the most common tree pollen types; Ostryopsis, Elaeagnus, Rosaceae and Corylus are the most

148

common shrub pollen types; Artemisia, Chenopodiaceae, Poaceae, Asteraceae, Brassicaceae,

149

Labiatae, Cyperaceae, Urtica and Humulus are the most common herb pollen types; and

150

Polypodiaceae and Triletes dominate the fern spores. According to the results of CONISS statistical

151

analysis, together with changes in pollen concentrations, the record can be divided into five pollen

152

assemblage zones (Fig. 3), which are herein described from bottom to top.

153

Zone 1 (129.8-124.6 m; 2.21-2.15 Ma; 27 samples) has an average number of 678 pollen grains

154

and an average concentration of 1078 grains/g, which are the highest values of identified grains and

155

pollen concentrations in the study section. In pollen assemblages, the tree pollen content is dominant,

156

with an average of 93 % (range: 73.1-99.8 %), and is the highest in the study section. Pinus pollen

157

accounts for over 70% of the assemblage whilst Picea pollen content ranges from 10-20 %. The

158

percentages of broad-leaved tree pollen are the lowest in the study section, accounting for

159

approximately 2 %. The average pollen content of shrubs is less than 1 %. The average pollen

160

content of herbs is 3 % (range: 0-14.4 %) and is the lowest in the study section, with Poaceae

161

(average 1.2 %) being the most common. The fern spore content is the highest in the study section,

162

with an average of 3.3 % (range: 0-12 %), and Polypodiaceae (average 2.9 %) is the most common.

163

Zone 2 (124.6-119.2 m; 2.15-2.06 Ma; 27 samples) has an average number of 355 pollen grains

164

and an average pollen concentration of 204 grains/g, both of which are significantly lower than those

165

in zone 1. The tree pollen abundance is significantly lower than in zone 1, with an average content

166

of 80.2 % (range: 52.1-99.7 %), but Pinus pollen is still dominant (>50 %) whilst Picea pollen

167

contributes less than 5 % to the assemblage. The pollen content of broad-leaved trees is the highest

168

in the study section, with an average of 11.5 %, and Quercus (average 5.7 %), Betula (average

169

3.3 %) and Anacardiaceae (average 1.9 %) are dominant. The average pollen content of shrubs is

170

9.4 %, and Rosaceae (average 3.2 %), Elaeagnus (average 3.1 %) and Corylus (average 2.3 %) are

171

the common types. The herb pollen content is higher than that in zone 1, with an average of 9.6 %

172

(range: 0.1-18.9 %), among which Poaceae (average 3.8 %) and Chenopodiaceae (average 2.6 %)

173

account for the highest percentages. The fern spore content is less than 1 %.

174

Zone 3 (119.2-114.4 m; 2.06-1.92 Ma; 24 samples) has an average count of 396 pollen grains

175

and shows a slight increase in pollen concentration to 505 grains/g. In the pollen assemblage, the

176

tree pollen content is the lowest in the study section, with an average of 77.1 % (range: 47.5-97.8 %).

177

Pinus pollen content accounts for >50 %, and the Picea pollen content is slightly higher than in zone

178

2 but <10 % in total. The pollen content of broad-leaved trees is lower than in zone 2. Ulmus

179

(average 6.7 %) accounts for a larger proportion and Betula (average 1.5 %) is more common. The

180

shrub pollen abundance is lower than in zone 2, having decreased to 3.7 %. The herb pollen content

181

is higher than in zone 2, (average 16.7 %, range 0-44.3 %); the abundances of Chenopodiaceae

182

(average 5.2 %), Artemisia (average 2.9 %) and Poaceae (average 2.1 %) are high, and Urtica and

183

Humulus are also common. The fern spore content is also slightly higher than in zone 2, having

184

increased to 2.5 %.

185

Zone 4 (114.4-109.6 m; 1.92-1.84 Ma; 24 samples) has an average number of 146 pollen grains,

186

and a pollen concentration of 28 grains/g which are the lowest in the study section. In the pollen

187

assemblages, the tree pollen content is slightly higher than in zone 3, averaging 79.2 % (range: 48.4-

188

97.9 %). The Pinus pollen content is often higher than 40 %, and whilst Picea pollen content is

189

higher than that in zone 3 it remains less than 10 % Notably however two obvious peaks (mostly

190

higher than 50 %) occur during this period. The pollen content of broad-leaved trees decreases

191

significantly; the dominant pollen type is Ulmus (average 3.6 %). The pollen content of shrubs

192

decreases continuously, with an average of 1.7 %. The herb pollen increases to the highest level in

193

the study section, with an average content of 18.16 % (range: 1.9-48.4 %); Chenopodiaceae

194

(average 7.6 %), Artemisia (average 5 %) and Poaceae (average 2.2 %) are the most common. The

195

fern spore content is approximately 1 %.

196

Zone 5 (109.6-106 m; 1.84-1.78 Ma; 18 samples) contains 292 pollen grains on average and a

197

higher pollen concentration than in zone 4 (185 grains/g). In the pollen assemblages, the average

198

content of tree pollen is 80 % (range: 42.5-97.2 %). The Pinus pollen content is significantly lower

199

than in zone 4, ranging from 30-50 %. The Picea pollen content is significantly higher than in zone

200

4 (>20 %) which is the highest in the study section. The content of broad-leaved trees decreases to

201

4.2 %, and Ulmus (average 2.5 %) is predominant. The pollen content of herbs decreases slightly

202

to 15.7 % (range: 1.7-51.7 %), whilst Artemisia (average 7.9 %) and Chenopodiaceae (average

203

5.9 %) are the dominant species. The fern spore content increases to 3.3 %, with Polypodiaceae

204

(average 1.7 %) and Triletes (average 1.3 %) being the most common.

205

4.3. Grain-size analysis

206

4.3.1 Indicative significance of grain-size

207

In the late Cenozoic, the creation of the lake in the Nihewan Basin was facilitated by continuous

208

fault-driven subsidence resulting in water accumulation in the Yangyuan and Yuxian Basins (Zhou

209

et al., 1991). Thereafter, the lake system was influenced by global climate changes. The interval

210

studied in this paper represents a stable stage of lake sedimentation which primarily reflects climate

211

rather than tectonic processes (Xia, 1992; Ding et al., 2018). Thus grain-size coarsening reflects

212

aridification and vice versa controlled by lake expansion and varied depth (Pei et al., 2009; Pan and

213

Chen, 2010).

214

4.3.2 Grain-size distributions

215

The sediment samples in the study section are mainly horizontally bedded blue-grey clay and

216

silt layers that contain charcoal and mollusc fossils. The grain-size data show that the sediments are

217

mainly silt (4-63 μm; accounting for an average of 72.5 %), followed by clay (<4 μm; accounting

218

for an average of 19.2 %), and sand (>63 μm; accounting for an average of 8.3 %). The grain-size

219

of the drill core is generally fine, with an average median diameter of 15 μm. According to the

220

characteristics of grain-size variation of sediments, the study section is divided into 5 zones

221

consistent with pollen assemblages from bottom to top as follows (Fig. 4).

222

Zone 1 (129.8-124.6 m; 2.21-2.15 Ma) mainly contains blue-grey clay. The sediments in this

223

zone are fine-grained (median 13.3 μm, range: 6.3-42.2 μm). The average percentage of silt is 74.3 %

224

(range: 61-79.6 %), which is the highest value in the study section; the average clay concentration

225

is 19.7 % (range: 7.7-31.1 %), and the average sand concentration is approximately 6 % (range: 1.2-

226

30.7 %), which is the lowest in the study section.

227

Zone 2 (124.6-119.2 m; 2.15-2.06 Ma) mainly contains dark brown silty clay, with silt and

228

lenticular fine sands which are in contact with the underlying zone. The median particle size is

229

greater than in zone 1, with an average of 17.9 μm (range: 5.7-57.5 μm), and shows a significant

230

peak. Compared with zone 1, the average content of silt is lower, reaching 68.9 % (range: 50.9-

231

79.4 %), which is the lowest silt content in the study section. The sand content is higher in zone 2

232

than in zone 1, with an average of 11.3 % (range: 0-43.5 %), and the clay content (mean 19.8 %,

233

range: 5.1-37.5 %) exhibits no significant difference.

234

Zone 3 (119.2-114.4 m; 2.06-1.92 Ma) is mainly composed of a blue-grey and bronze-coloured

235

clay layer and a blue-grey silt lens. The median particle size is significantly lower in this zone than

236

that of zone 2, (average 9.9 μm), which is the lowest in the study section. Compared with zone 1,

237

zone 3 has a higher average clay content (23 %, range: 14.6-35.9 %), which is the maximum value

238

in the study section. The silt content is higher in this zone than in zone 2 (average 73.8 %, range:

239

63.5-83.9 %), and sand content is significantly lower in this zone than in zone 2 (average 3.1 %,

240

range: 0-9 %), which is the lowest in the study section.

241

Zone 4 (114.4-109.6 m; 1.92-1.84 Ma) consists of reddish brown and cyan clays, silt and a fine

242

sand layer. The median particle size in this zone is higher than that in zone 3, with an average of

243

13.3 μm (range: 6.3-31.5 μm). Compared to zone 3, zone 4 has a lower clay content of 18.7 % (range:

244

5.9-33.6 %), which is the second lowest value in the study section; a higher sand content of 7.5 %

245

(range: 0-22.7 %), and a similar silt content (average 73.9 %, range: 65.7 %-81.6 %).

246

Zone 5 (109.6-106 m; 1.84-1.78 Ma) is mainly composed of blue-grey silt and clay, with a fine

247

sand layer and calcium carbonate. The particle size is obviously higher in this zone than that in zone

248

4, (median: 22.4 μm), which is the highest in the study section. Compared to zone 4, zone 5 has a

249

higher sand content (average 15.1 %, range: 2.9-37.7 %), but a slightly lower enema silt content

250

(average 71.9 %, range: 55-79.6 %); and a lower clay content (average 13 %, range: 5.5-22.1 %).

251

4.4. Ecological significance of the pollen types

252

The palaeoclimatic interpretation of the established pollen zones is further described by

253

applying PCA to the pollen data. Twenty principal pollen taxa from core NHA were chosen for

254

the PCA. The PCA biplot of pollen percentages is shown in Fig. 5. The first principal component

255

(axis 1) has an eigenvalue of 0.3614, and the second principal component (axis 2) has an

256

eigenvalue of 0.2148. Axis 1 reflects regional temperature because the highest value represents

257

coniferous forests, including Picea and Abies, which always appear on shaded slopes under

258

cold climate conditions. The lowest value represents broad-leaved trees (Quercus and Betula),

259

warm shrubs and herbs, which generally prefer to grow under warm climate conditions. Axis 2

260

reflects regional moisture variations because the highest value represents Artemisia and

261

Chenopodiaceae and the lowest value represents Pinus. Pinus generally appears in humid

262

environments, while Artemisia and Chenopodiaceae usually live in dry environments.

263

5. Discussion

264

5.1. Vegetation succession and climate change from 2.2 to 1.8 Ma

265

Currently, most of the vegetation types in the study area belong to the warm temperate

266

deciduous broad-leaved forest area, whereas the north western area belongs to the temperate

267

grassland zone. The temperate continental monsoon climate zone has an average annual temperature

268

of 7-8 ºC and a mean annual precipitation of 360-420 mm (Ding et al., 2018). The vegetation

269

succession and climate change history since the early Pleistocene in the Nihewan Basin were

270

revealed through palynological and sediment grain-size analyses.

271

The pollen assemblages in the study area were dominated by arboreal pollen, among which

272

Pinus was dominant. Xu et al. (2007) analyzed 205 surface pollen samples from different

273

communities in Northern China to understand the quantitative relationship between pollen and its

274

original vegetation. Pinus is the overrepresented pollen type. The pollen percentages themselves

275

exhibit some relations with plant cover. Pine forest might be present when Pinus pollen percentages

276

exceed 30 %. Results from the core demonstrate that the pollen content of Pinus was more than

277

60 %, indicating that the vegetation in the study area was primarily pine forest and thus that the

278

climate was warm and wet, but there were also variations. For example, from 2.21-2.15 Ma in

279

addition to the thermophilic Pinus pollen, the cryophilic Picea pollen content was >10 % in most

280

samples (up to 80 %). This finding indicates that the primary vegetation type was pine forest whilst

281

spruce forest was present in the mountains. These forests are shown to have expanded during

282

separate times (approximately 2.19-2.18 Ma) when a short-term cold period occurred. The pollen

283

content of broad-leaved trees increased significantly from 2.15-1.92 Ma and included Quercus

284

(average 3.3 %), Betula (average 2.5 %) and Ulmus (average 3.4 %). The pollen derived from broad-

285

leaved forest species has also been investigated in surface soils. Quercus has a higher correlation

286

coefficient, where pollen percentage is obviously lower than the plant cover. There is a weak

287

relationship between Ulmus pollen percentages and vegetation cover particularly where other arbors

288

are present. Nevertheless, where their pollen percentages are >1%, Ulmus trees should exist (Li et

289

al., 2005; Xu et al., 2007). This indicated that broad-leaved trees of a certain area appear in the

290

vegetation, which was the warmest and wettest period. Picea was dominant on several occasions

291

from 1.92-1.78 Ma, whilst the pollen content of Artemisia and Chenopodiaceae increased. The

292

spruce forest area expanded, the openness of forestland increased, and the abundance of xerophytes

293

increased, thus indicating that this interval was the coldest and driest in the study section.

294

5.2. Evidence for palaeovegetation and palaeoclimate during the Reunion event period

295

The Reunion event (2.15-2.14 Ma) was a very short geomagnetic reversal which occurred

296

during the Matuyama negative polarity epoch (Deng et al., 2019). So far most studies have focused

297

on the timing of this event rather than the palaeoenvironmental conditions (Yan et al., 2001; Yang

298

et al., 2007; Liu et al., 2012; Deng et al., 2019). Our results indicate that the depositional rate was

299

relatively fast and the sediment grain-size was relatively coarse. According to the variations in

300

pollen assemblages, two phases of environmental change can be inferred: 1) from 2.15-2.145 Ma,

301

although the pollen assemblages were dominated by Pinus, the cryophilic Picea increased

302

significantly (>10 %) ; and 2) from 2.145-2.14 Ma, the concentration of Picea pollen decreased

303

significantly and the broad-leaved tree increased significantly, in particular the thermophilic

304

Quercus (>5 %) and Anacardiaceae ( >3 %). The results indicate that in the early stage, besides pine

305

forest, there is obvious spruce forest in the mountain highlands, while in the late stage, the area of

306

spruce forest reduced, and the area of thermophilic broad-leaved trees expanded (represented by

307

Quercus and Anacardiaceae) (Fig. 6a; 6b; 6c). In addition, the score of PCA axis 1 changed to a

308

positive value (Fig. 7a) and sediment particle changed from coarse to fine (Fig. 6d), indicating that

309

the climate during the Reunion period was relatively cold from 2.15-2.145 Ma but then gradually

310

warmed. The global LR04 δ18O record (Fig. 7h) (Lisiecki and Raymo, 2005), Pacific and Atlantic

311

Ocean sea surface temperature (Fig. 7e; 7f) (Martínez-Garcia et al., 2010) and the decrease in

312

Eurasian ice volume (Fig. 7g) (Bintanja, R. and R.S.W. van de Wal, 2008) all imply the environment

313

of the Reunion event gradually changed from cold-dry to warm-humid.

314

5.3. Evidence for palaeovegetation and palaeoclimate during the Olduvai event period

315

The Olduvai positive event (1.945-1.778 Ma) occurred during the Matuyama negative polarity

316

epoch and lasted approximately 0.17 Ma (Lepre and Kent, 2010; Deng et al., 2019). This signifies

317

an important climate transition period during the Quaternary as it intersected with the emergency of

318

early humans. Whilst previous studies highlighted that the Olduvai event was cold, the processes

319

and characteristics of temperature and precipitation change still remain controversial (Wei, 2004;

320

Wu et al., 2010; Nutz, 2017; Tian et al., 2018). The available evidence indicates there were large

321

spatiotemporal differences in global cooling during the Olduvai event (Wu et al., 2010; Zhang, 2014;

322

Tian et al., 2018).

323

In this study, the sediment grain size during the Olduvai period was higher than before. The

324

pollen assemblages are characterized by a drop in the thermophilic Pinus but an increase in

325

cryophilic Picea as well as the drought-tolerant Artemisia and Chenopodiaceae. Collectively this

326

indicates that the climate was generally cold-dry during the Olduvai event, but there are significant

327

differences at different stages. At the beginning of the Olduvai event (1.95-1.92 Ma), the pollen

328

assemblages were still dominated by Pinus (>70 %) indicative of warm-humid conditions

329

dominated by pine forest. During the middle of the Olduvai event (1.92-1.84 Ma), the pollen

330

assemblage was characterized by alternating Pinus and Picea. Pollen types indicative of xeric herbs

331

such as Artemisia and Chenopodiaceae exhibited a large increase thereby indicating that the

332

surrounding vegetation in the lake area was comprised of mainly coniferous forest comprising pine

333

and spruce. The score of PCA axis 1 (Fig. 7a) fluctuated significantly whilst that of PCA axis 2 (Fig.

334

7b) was significantly positive, indicating that temperature and humidity began to decrease albeit

335

with major fluctuations. From 1.84-1.78 Ma (the late of the Olduvai event), the concentration of

336

Picea pollen exceeded 30 % (up to 60 %), indicating further expansion of spruce forest. Furthermore,

337

the score of PCA axis 1 was significantly positive. The sediment grain-size increased sharply

338

indicative of cold, dry conditions. Such cold, dry conditions in the Olduvai period have been

339

identified from records elsewhere in the world. Based on reconstructed bathymetric changes and

340

coastline migration of Lake Turkana, Nutz et al. (2017) also demonstrated that cold and dry

341

conditions persisted in Kenya from 1.87-1.76 Ma. In Italy, glaciation as represented by the

342

expansion of herbaceous vegetation was dated to around 1.84 Ma (Nebout and Grazzini, 1991). In

343

addition widespread glaciation with accelerated glacial erosion was also recorded in the coastal

344

mountains of British Columbia, Canada (Shuster et al., 2005). Sudden changes in the relative

345

abundances of diatom species in Lake Baikal, Siberia at 1.8 Ma also suggest global cooling and

346

Eurasian glacial expansion (Grachev et al., 1998).

347

The cold and dry climate of the Olduvai palaeomagnetic reversal period in the Nihewan Basin,

348

as in other areas, was mainly affected by global cooling, the strengthening of the winter monsoon

349

and the uplift of the Tibetan Plateau (Zhang, 2014; Li, 2015; Tian et al., 2018). During this period,

350

the SSTs in the Subarctic Pacific and Subantarctic Atlantic (Fig. 7e; 7f) were significantly lower

351

than in the previous period (Martínez-Garcia et al., 2010). In addition, there was a positive excursion

352

in the marine oxygen isotope record (Fig. 7h) (Lisiecki and Raymo, 2005), and the Eurasian ice

353

volume (Fig. 7g) increased significantly (Bintanja, R. and R.S.W. van de Wal, 2008), indicating that

354

the global climate became colder (Shackleton et al., 1984; Kennett, 1995). The expansion of sea ice

355

and Arctic land ice caused by rapid cooling in the high latitudes of the Northern Hemisphere may

356

have led to the intensification of the Siberian high-pressure system (Ruddiman and Kutzbach, 1989;

357

Guo et al., 2004) and further strengthened the winter monsoon (An et al., 2001; Tian et al., 2005).

358

The >30 μm coarse particle percentage was one of the most sensitive indicators for reconstructing

359

past changes in the East Asian winter monsoon (Lu and An., 1997; 1998). The percentage of coarse

360

particles that were >30 μm in the sediments of the study area increased significantly during the

361

Olduvai event (Fig. 7c; 7d) indicating that the winter monsoon had intensified (Wan et al., 2007).

362

Other studies have shown that the uplift of the Tibetan Plateau likely had a profound impact on the

363

global atmospheric circulation system (William, 1997; Shi et al., 1999; Dong et al., 2006; Li et al.,

364

2015) achieved via blocking the northward migration of the southwest monsoon (Li and Fang, 1998;

365

Dong et al., 2011; Qin et al., 2011). Instead the climate in the study area was more affected by the

366

high-latitude East Asian winter monsoon. Therefore, the strengthened winter monsoon and the

367

weakened summer monsoon together may have caused the cold and dry conditions in the Nihewan

368

region during the Olduvai period (An et al., 2001; Ding et al., 2005; Dong et al., 2006,2011; Yan et

369

al., 2014).

370

6. Conclusions

371

The early Pleistocene (2.2-1.8 Ma) pollen record from core NHA was used to investigate

372

the vegetation succession and climate changes in the Nihewan Basin. The results showed that the

373

pollen assemblages were dominated by Pinus, indicating that the vegetation in the study area was

374

primarily pine forest and that the climate was relatively warm and wet. From 2.15 Ma to 1.92 Ma,

375

the broad-leaved tree pollen abundance significantly increased to >10 %, indicating that this period

376

was the warmest and wettest in the entire record. During the Reunion (2.15-2.14 Ma) event,

377

increased Quercus but reduced Picea pollen concentrations suggested a transition from a cold to

378

warm climate. During the Olduvai event (1.92-1.78 Ma), the pollen contents of Picea, Artemisia

379

and Chenopodiaceae increased, indicative of the expansion of spruce forests and thus evidence for

380

very cold, dry conditions. The cold and dry climate in the Nihewan Basin during the Olduvai

381

period was strongly related to global cooling and resultant winter monsoon enhancement.

382

Additionally, the rapid uplift of the Tibetan Plateau also played an important role in the process

383

of the climate becoming cooler and drier.

384

Acknowledgements

385

This study is supported by the National Natural Science Foundation of China (Grant Nos.

386

41877433, 41701230, 41472157); China Geological Survey Project (Grant Nos. DD20160345). We

387

express our gratitude to Dr. Chris Oldknow for improving the English.

388

References

389

An Z.S., Kutzbach J.E., Prell W.L., Porter S.C., 2001. Evolution of Asian monsoons and phased

390

391 392

uplift of the Himalaya-Tibetan plateau since Late Miocene times. Nature. 411, 62-66.

Andreev, A., 2012. Late Pliocene/early Pleistocene environments of the siberian arctic inferred from the lake El'gygytgyn pollen record. Quat. Int. 279-280, 19.

393

Andreev, A.A., Tarasov, P.E., Wennrich, V., Melles, M., 2016. Millennial-scale vegetation changes

394

in the north-eastern Russian Arctic during the Pliocene/Pleistocene transition (2.7-2.5Ma)

395

inferred from the pollen record of Lake El'gygytgyn. Quat. Sci. Rev. 1-14.

396 397

Bintanja, R., van de Wal, R.S.W., 2008. North American ice-sheet dynamics and the onset of 100,000-year glacial cycles. Nature. 454, 869-872.

398

Bailey, I., Foster, G.L., Wilson, P.A., Jovane, L., Storey, C.D., Trueman, C.N., Becker, J., 2012. Flux

399

and provenance of ice-rafted debris in the earliest Pleistocene sub-polar North Atlantic Ocean

400

comparable to the last glacial maximum. Earth Planet. Sci. Lett. 341-344, 222-233.

401 402

403 404

Barbour, G.B., 1924. Preliminary observation in Kalgan area. Bulletin of the Geological Society of China. 3, 153-167.

Barbour, G.B., 1925. The deposits of the Sang kan ho Valley. Bulletin of the Geological Society of China. 4, 53-55.

405 406

407 408

409 410

Bonnefille, R., Probst, J. L., Ortlieb, L., Fauredenard, L., 2010. Cenozoic vegetation, climate changes and hominid evolution in tropical africa. Global Planet. Change. 72, 390-411.

Braak, T., Smilauer, P.N., 2002. Canoco reference manual and CanoDraw for Windows users guide: software for canonical community ord. Ithaca Ny Usa Www.

Braak, T., Smilauer, P.N., 2012. Canoco reference manual and user's guide: software for ordination, version 5.0. Ithaca USA: Microcomputer Power. 496.

411

Chen, M.N., 1988. Study on the Nihewan formation. Ocean Press, China. pp. 1-145.

412

Clemens, S.C., Prell, W.L., Sun, Y.B., Liu, Z.Y., Chen, G.S., 2008. Southern Hemisphere forcing of

413

Pliocene δ18O and the evolution of Indo-Asian monsoons. Paleoceanography. 23, PA4210.

414

Colcord, D.E., Shilling, A.M., Sauer, P.E., Freeman, K.H., Njau, J.K., Stanistreet, I.G., Stollhofen,

415

H., Schick, K.D., Toth, N., Brassell, S.C., 2018. Sub-Milankovitch paleoclimatic and

416

paleoenvironmental variability in East Africa recorded by Pleistocene lacustrine sediments

417

from Olduvai Gorge, Tanzania. Palaeogeogr. Palaeoclimatol. Palaeoecol. 495, 284-291.

418 419

420 421

Davies, C.P., Fall, P.L., 2001. Modern pollen precipitation from an elevational transect in central Jordan and its relationship to vegetation. J. Biogeogr. 28, 1195-1210.

Demenocal, P. B., 2004. African climate change and faunal evolution during the PliocenePleistocene. Earth Planet. Sci. Lett. 220, 3-24.

422

Deng, C.L., Wei, Q., Zhu, R.X., Wang, H.Q., Zhang, R., Ao, H., Chang, L., Pan, Y.X., 2006.

423

Magneto stratigraphic age of Xiantai Paleolithic site in the Nihewan Basin and implications for

424

early human colonization of Northeast Asia. Earth Planet. Sci. Lett. 44, 336-348.

425 426

427 428

Deng, C.L., Zhu, R.X., Zhang, R., Ao, H., Pan, Y.X., 2008. Timing of the Nihewan formation and faunas. Quat. Res. 69, 77-90.

Deng C.L., Hao Q.Z., Guo Z.T., Zhu R.X., 2019. Quaternary integrative stratigraphy and timescale of China. Sci. Chin. Earth Sci. 62, 324-348.

429

Ding, G.Q., Shen, G.H., Li, Y.C., Wang, Y., Chi, Z.Q., Yang, X.L., Li, B., 2018. Late Pliocene

430

palynological records of vegetation and climate changes in the Nihewan Basin. Quat. Sci. 38,

431

336-347.

432 433

Ding, Z.L., Han, J.T., Liu, C., 1990. Preliminary Determination of Climate Transition Events in Northern China around 2.5 Ma. Chin. Sci. Bull. 35, 1090-1092.

434

Ding, Z. L., Derbyshire, E., Yang, S.L., Sun, J.M., Liu, T.S., 2005. Stepwise expansion of desert

435

environment across northern China in the past 3.5 Ma and implications for monsoon evolution.

436

Earth Planet. Sci. Lett. 237, 45-55.

437

Dong, M., Fang, X.M., Wu, F.L., Shi, Z.T., 2006. 1.95 Ma B. P. Climate Transition Event and Critical

438

Height of the Effect of Qinghai - Tibet Plateau Uplift on the Atmosphere. Journal of Yunnan

439

Normal University. 61-65.

440 441

Dong, M., Fang, X.M., Ming, Q.Z., Shi, Z.T., Su, H., 2011. Evolution of early pleistocene environment in Linxia basin, Gansu province. Journal of Lanzhou University. 47, 1-5.

442

Fægri, K., Iversen, J., 1989. A Textbook of Pollen Analysis. J. Biogeogr. 12, 328.

443

Fang, X.M., Lü, L.Q., Mason, J.A., Yang, S.L., An, Z.S., Li, J.J., Guo Z.L., 2003. Pedogenic

444

response to millennial summer monsoon enhancements on the Tibetan plateau. Quat. Int. 106,

445

79-88.

446

Grachev, M.A., Vorobyova, S.S., Likhoshway, Y.V., Goldberg, E.L., Ziborova, G.A., Levina, O.V.,

447

Khlystov O.M., 1998. A high-resolution diatom record of the palaeoclimates of east siberia for

448

the last 2.5 Ma from lake baikal. Quat. Sci. Rev. 17, 1101-1106.

449

Guo, Z.T., Peng, S.Z., Hao, Q.Z., Biscaye, Pierre, E., An, Z.S., Liu, T.S., 2004. Late Miocene-

450

Pliocene development of Asian aridification as recorded in the Red-Earth Formation in

451

northern China. Global Planet. Change. 41, 135-145.

452

Hilgen, F.J., Lourens, L.J., Van Dam J.A., 2012. The Neogene Period. In: Gradstein F M, Ogg J G,

453

Schmitz M D, Ogg G M, eds. The Geologic Time Scale 2012, Vol. 2. Amsterdam: Elsevier.

454

923-978.

455

Jiang, H.C., Mao, X., Xu, H.Y., Thompson, J., Ma, X.L., 2010. ~4Ma coarsening of sediments from

456

Baikal, Chinese Loess Plateau and South China Sea and implications for the onset of NH

457

glaciation. Palaeogeogr. Palaeoclimatol. Palaeoecol. 298, 201-209.

458

Kennett, J.P., 1995. A review of polar climatic evolution during the Neogene, based on the marine

459

sediment record. In: Vrba, E.S., Denton, G.H., Partridge, T.C., Burckle, L.H. (Eds.),

460

Paleoclimate and Evolution, with Emphasis on Human Or igins. Yale University Press, New

461

Haven. pp. 49-64.

462 463

464

Leinen, M., Heath, G. R., 1981. Sedimentary indicators of atmospheric activity in the northern hemisphere during the Cenozoic. Palaeogeogr. Palaeoclimatol. Palaeoecol. 36, 1-21.

Lepre, C. J., Kent, D. V., 2010. New magnetostratigraphy for the olduvai subchron in the koobi fora

465

formation, Northwest kenya, with implications for early homo. Earth Planet. Sci. Lett. 290,

466

362-374.

467 468

469 470

471 472

473 474

475 476

Li, J.J., Fang, X.M., 1998. Study on the uplift of Qinghai-Tibet Plateau and environmental changes. Chin. Sci. Bull. 43, 1568-1574.

Li, J.J., Zhou, S.Z., Zhao, Z.J., Zhang, J., 2015. The Qingzang Movement: The major uplift of the Qinghai-Tibetan Plateau. Sci. Chin. Ser. D. 58, 2113-2122.

Li, X.J., 2015. The early Pleistocene climate change recorded in the northern South China Sea sediments. Chin Acad Sci. Univ. 19-38.

Li, Y.C., Xu, Q.H., Xiao, J.L., Yang, X.L., 2005. Indication of some major pollen taxa in surface samples to their parent plants of forest in northern China. Quat. Sci. 25, 598-608.

Lisiecki, L.E., Raymo, M.E., 2005. A Plio-Pleistocene stack of 57 globally distributed benthic delta 18O

records. Paleoceanography. 20, 522-533.

477

Liu, P., Deng, C., Li, S., Zhu, R.., 2010. Magneto stratigraphic dating of the Huojiadi Paleolithic

478

site in the Nihewan Basin, North China. Palaeogeogr. Palaeoclimatol. Palaeoecol. 298, 399-

479

408.

480

Liu, P., Deng C.L., Li S.H., Cai S.H., Cheng H.J., Yuan B.Y., Wei Q., Zhu R.X., 2012. Magneto

481

stratigraphic dating of the Xiashagou Fauna and implication for sequencing the mammalian

482

faunas in the Nihewan Basin, North China. Palaeogeogr. Palaeoclimatol. Palaeoecol. 315-316,

483

75-85.

484

Lu, H.Y., An, Z.S., 1997. Paleoclimate significance of grain-size composition in Luochuan Loess.

485

486 487

Chin. Sci. Bull. 42, 67-69.

Lu, H.Y., An, Z.S., 1998. Paleoclimate significance of grain-size composition of loess on the Loess Plateau. Sci. Chin. Ser. D. 28, 278-283.

488

Martínez-Garcia, A., Rosell-Melé, A., McClymont, E.L., Gersonde, R., Haug, G.H., 2010. Subpolar

489

link to the emergence of the modern equatorial Pacific cold tongue. Science. 328, 1550-1553.

490

Min, L.R., Chi, Z.Q., Wang, Y., Dong, J., Wang, Y.L., Zhu, G.X., 2015. Lithostratigraphic division

491

and correlation of Haojiatai NHA borehole from Nihewan basin in Yangyuan, Hebei. Geol.

492

Chin. 42, 1068-1078.

493 494

Nebout, N.C., Grazzini, C.V., 1991. Late Pliocene Northern Hemisphere glaciations: The continental and marine responses in the central Mediterranean. Quat. Sci. Rev. 10, 319-334.

495

Nutz, A., Schuster, M., Xavier, B., Rubino, J.L., 2017. Orbitally-driven evolution of Lake Turkana

496

(Turkana Depression, Kenya, EARS) between 1.95 and 1.72 Ma: A sequence stratigraphy

497

perspective. J. Afr. Earth. Sci. 125, 230-243.

498

Ogg, J.G., 2012. Geomagnetic polarity time scale. Geologic Time Scale. pp. 85-113.

499

Pan, A.D., Chen, B.S., 2010. Late Quaternary paleoenvironment of Gahai Lake in Qaidam Basin.

500

501 502

503

Meteorological Press, Beijing. pp. 29-38.

Pei, S.W., Li, X.L., Liu, D.C., Ma, N., Peng, F., 2009. Preliminary study on the living environment of hominids at the Donggutuo site, Nihewan Basin. Chin. Sci. Bull. 54, 3896-3904.

Potts, R., 1998. Variability selection in human evolution. Evol. Anthropol. 7, 81-96.

504

Qin, F., Ferguson, D. K., Zetter, R., Wang, Y.F., Syabryaj, S., Li, J.F., Yang, J.A., Li, C.S., 2011.

505

Late Pliocene vegetation and climate of Zhangcun region, Shanxi, North China. Global Change

506

Biol. 17, 1850-1870.

507

Ruddiman W F., Kutzbach, J.E., 1989. Forcing of Late Cenozoic Northern Hemisphere Climate by

508

Plateau Uplift in Southern Asia and the American West. J. Geophys. Res: Atmos. 94, 18409-

509

18427.

510

Shackleton, N.J., Backman, J., Zimmerman, H., Kent, D.V., Hall, M.A., Roberts, D.G., Schnitker,

511

D., Baldauf, J.G., Desprairies. A., Homrighausen, R., Huddlestun, P., Keene, J.B., Kaltenback,

512

A.J., Krumsiek, K.A.O., Morton, A.C. J., Murray, W., Westberg-Smith, J., 1984. Oxygen

513

isotope calibration of the onset of ice-rafting and history of glaciation in the North Atlantic

514

region. Nature. 307, 620-623.

515

Shi, Y.F., Li, J.J., Li, B.Y.,

Yao, T.D., Wang, S.M.,

Li, S.J., Cui, Z.J., Wang, F.B., Pan,

516

B.T., Fang, X.M., Zhang, Q.S., 1999. Uplift of the Qinghai-Xizang (Tibetan) Plateau and

517

East Asia environmental change during Late cenozoic. Acta. Geogr. Sci. 54, 10-20.

518

Singer, B.S., 2014. A Quaternary geomagnetic instability time scale. Quat Geochron. 21, 29-52.

519

Singer, B.S., Guillou, H., Jicha, B.R., Zanella, E., Camps, P., 2014. Refining the Quaternary

520

Geomagnetic Instability Time Scale (GITS): Lava flow recordings of the Blake and Post-Blake

521

excursions. Quat Geochron. 21, 16-28.

522 523

Shuster, D.L., Ehlers, T. A., Rusmoren, M.E., Farley, K.A., 2005. Rapid glacial erosion at 1.8 Ma revealed by 4He/3He thermochronometry. Science. 310, 1668-1670.

524

Sun, D.H., Chen, M.Y., JohnShaw, Lu, H.Y., Sun, Y.B., Yue, L.P., 1998. Magnetic stratigraphic age

525

and paleoclimatic records of the dust accumulation sequence of the Late Cenozoic Loess

526

Plateau. Sci. Chin. Ser. D. 28, 79-84.

527

Sun, Y.B., An, Z.S., Clemens, S.C., Bloemendal, J., Vandenberghe, J., 2010. Seven million years of

528

wind and precipitation variability on the Chinese Loess Plateau. Earth Planet. Sci. Lett. 297,

529

525-535.

530 531

532 533

Teilhard de Chardin, P., Leroy, P., 1942. Chinese fossil mammals, a complete bibliography and analyzed, tabulated, annotated and indexed. Inst. Geo-Biologie. Pekin 8, 1-142.

Tian, J., Wang, P. X., Cheng, X. R., 2005. Forcing mechanism of the Pleistocene east Asian monsoon variations in a phase perspective. Sci. China. Ser. D. 48, 1708-1717.

534

Tian, YY., Wei, M.J., Cai, M.T., Wang, J.P., Li, X.L., 2018. Late Pliocene and early Pleistocene

535

environmental evolution from the sporopollen record of core PL02 from the Yinchuan Basin,

536

northwest China. Quat. Int. 476, 26-33.

537

Wan, S.M., Li, A.C., Berend, J., Stuut, W., Xu, F.J., 2007. East Asian monsoon evolution since

538

nearly 20 Ma revealed by the grain-size of ODP1146 station in the northern South China Sea.

539

Sci. Chin. Ser. D. 37, 761.

540

Wang, X.S., Yang, Z.Y., Lovlie, R., Min, L.R., 2004. High-resolution magnetic stratigraphy of

541

fluvio-lacustrine succession in the Nihewan Basin, China. Quat. Sci. Rev. 23, 1187-1198.

542

Wang, X.S., Lovlie, R., Su, P., Fan, X.Z., 2008. Magnetic signature of environmental change

543

reflected by Pleistocene lacustrine sediments from the Nihewan Basin, North China.

544

Palaeogeogr. Palaeoclimatol. Palaeoecol. 260, 452-462.

545

Wei, Z.X., 2004. Quaternary Environmental Evolution in Eastern Yangtze Delta: Coupling of

546

Neotectonic Movement, Paleoclimate and Sea-level Fluctuation. East Chin Norm. Univ. 74-78.

547

Weng, C.Y., Sun, X.G., Chen, Y.S., 1993. Numerical characteristics of pollen assemblages of surface

548

samples from the West Kunlun Mountains. Acta. Bot. Sin. 69-79.

549

William, F.R., 1997. Tectonic uplift and climate change. New York: Plenum Press. pp. 124-147.

550

Wu, F.L., Fang, X.M., Miao, Y.F., Dong, M., 2010. Environmental indicators from comparison of

551

sporopollen in early Pleistocene lacustrine sediments from different climatic zones. Chin. Sci.

552

Bull. 55, 2981-2988.

553 554

Xia, Z.K., Liu, X.Q., 1984. On paleogeography of the Nihewan basin during the accumulation of Nihewan. Ser Geol. & Quat Geol. 4, 101-110.

555

Xia, Z.K., 1992. Underwater loess and paleoclimate. Acta Geog. Sin. 47, 58-65.

556

Xu, Q.H., Li, Y.C., Yang, X.L., Zheng, Z.H., 2007. Quantitative relationship between pollen and

557

vegetation in northern China. Sci. Chin. Ser. D. 50, 582-599.

558

Yang T., Hyodo, M., Yang, Z., Ping, L., Fu J.L., Mishima, T., 2007. Early and middle Matuyama

559

geomagnetic excursions recorded in the Chinese loess-paleosol sediments. Earth Planets and

560

Space. 59, 825-840.

561

Yan, X.L., Miao, Y.F., Zan, J.B., Zhang, W.L., Wu, S., 2014. Late cenozoic fluvial-lacustrine

562

susceptibility increases in the linxia basin and their implications for Tibetan plateau uplift. Quat.

563

Int. 334-335, 132-140.

564

Yan, W., Tang, X.Z., Cheng, Z., Cheng, M.H., Gu, S.C., 2001. Origin and environmental

565

significance of red and black sedimentary interlayers in coral reef of Nanyong 2. Chin. Sci.

566

Bull. 46, 1476-1480.

567 568

Yuan, B.Y., Cui, J.X., Zhu, R.X., Tian, W.L., Li, R.Q., Wang, Q., Yan, F.H., 1996. Age stratum division and contrast of Nihewan formation. Sci. Chin. Ser. D. 26, 67-73.

569

Zhang, J., Li, J.J., Guo, B.H., Ma, Z.H., Li, X.M., Ye, X.Y., Yu, H., Liu, J., Yang, C., Zhang, S.D.,

570

2016. Magnetostratigraphic age and monsoonal evolution recorded by the thickest Quaternary

571

loess deposit of the Lanzhou region, western Chinese Loess Plateau. Quat. Sci. Rev, 139, 17-

572

29.

573 574

575 576

Zhang, S.D., 2014. Paleoclimatic Significance indicated by the Magnetic Susceptibility of Loess over the 2.2 Ma at Xijin, Lanzhou. Lanzhou. Univ. 55-56.

Zhou, Y.R., Li, H.Z., Liu, Q.S., Li, R.Q., Sun, X.P., 1991. Cenozoic Paleography research of Nihewan basin. Science Press, Beijing. pp. 1-46.

577

Zhu R.X., Hoffman, K.A., Potts R., Deng, C.L., Pan, Y.X., Guo, B., Shi, C.D., Guo, Z.T., Yuan, B.Y.,

578

Hou, Y.M., Huang, W.W., 2001. Earliest presence of humans in Northeast Asia. Nature. 413,

579

413-417.

580

Zhu, R.X., Hoffman, K.A., Potts, R., Deng, C.L., Pan, Y.X., Guo, B., Shi, C.D., Guo, Z.T., Yuan,

581

B.Y., Hou, Y.M., Huang, W.W., 2004. New evidence on the earliest human presence a high

582

northern latitude in Northeast Asia. Nature. 431, 559-562.

583

Zhu, R.X., Deng, C.L., Pan, Y.X., 2007. Magneto chronology of the fluvio-lacustrine sequences in

584

the Nihewan Basin and its implications for early human colonization of Northeast Asia. Quat.

585

Sci. 27, 922-944.

586

Figure captions

587

Fig. 1. Location of the study area in China (left) and topography of the study area (right).

588

Fig. 2 Lithology and palaeomagnetic age diagram of core NHA from the Nihewan Basin (GPTS:

589

the geomagnetic polarity timescale) (Hilgen et al., 2012; Singer, 2014; Singer et al., 2014)

590

Fig. 3. Pollen percentage and concentration diagram of core NHA showing the selected taxa in the

591

Nihewan Basin

592

Fig. 4 Grain-size parameter curves of core NHA in the Nihewan Basin

593

Fig. 5 Principal component analysis of pollen taxa from core NHA in the Nihewan Basin

594

Fig. 6. Pollen records and sediment grain size from core NHA in the Reunion event period

595

a. Pinus pollen percentage; b. Dark coniferous (Abies and Picea) pollen percentage; c. Broad-

596

leaved tree (Anacardiaceae, Betula, Quercus and Ulmus etc) pollen percentage; d. Median grain-

597

size

598

Fig. 7. Comparison of PCA axis 1 and axis 2 scores of core NHA with environmental proxy

599

indicators

600

a. PCA axis1 scores; b. PCA axis2 scores; c.＞30 μm grain size percentages of core NHA; d.

601

Sediment grain size on the Loess Plateau (Sun et al., 2010); e. SST record from ODP Site 882 in the

602

Subarctic Pacific (Martínez-Garcia et al., 2010); f. SST record from ODP Site 1090 in the

603

Subantarctic Atlantic (Martínez-Garcia et al., 2010); g. Eurasian ice volume relative to present

604

(Bintanja, R. and R.S.W. van de Wal, 2008) and h. LR04 benthic δ18O stack (Lisiecki and Raymo,

605

2005).

606