Quaternary sediment characteristics and paleoclimate implications of deposits in the Three Gorges and Yichang areas of the Yangtze River

Journal Pre-proof Quaternary sediment characteristics and paleoclimate implications of deposits in the Three Gorges and Yichang areas of the Yangtze River

Fang Xiang, Hengxu Huang, James Ogg, Hongbo Zhu, Dongya Kang PII:

S0169-555X(19)30472-6

DOI:

https://doi.org/10.1016/j.geomorph.2019.106981

Reference:

GEOMOR 106981

To appear in:

Geomorphology

Received date:

3 July 2019

Revised date:

26 November 2019

Accepted date:

26 November 2019

Please cite this article as: F. Xiang, H. Huang, J. Ogg, et al., Quaternary sediment characteristics and paleoclimate implications of deposits in the Three Gorges and Yichang areas of the Yangtze River, Geomorphology(2019), https://doi.org/10.1016/ j.geomorph.2019.106981

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© 2019 Published by Elsevier.

Journal Pre-proof Quaternary sediment characteristics and paleoclimate implications of deposits in the Three Gorges and Yichang areas of the Yangtze River

Fang Xiang 

, Hengxu Huang , James Ogg

①②③

③

③④

③

③

, Hongbo Zhu , Dongya Kang

①State Key Laboratory of Oil and Gas reservoir Geology and exploitation, Chengdu University of Technology，Chengdu，China ②Shangdong Provincial Key Laboratory of Depositional Mineralization & Sedimentary Mineral, Shangdong

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University of Science and Technology, Qingdao, China ③Institute of sedimentary geology, Chengdu University of Technology, Chengdu, China

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④Earth, Atmosphere, Planet Science college, Purdue University, West Lafayette, Indiana, USA

Abstract: The Three Gorges and Yichang areas of the Yangtze River are close to the eastern

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edge of the Qinghai-Tibet Plateau. Sediment proxies of paleoclimate trends in these areas are of

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great significance for the interpretation of the regional climatic impact of the episodic uplift of

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the Qinghai-Tibet Plateau. We examined the macro-sedimentary characteristics, clay mineralogy and geochemistry of sediments from the fan delta and lake deposits and the two river terraces of

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early and middle Quaternary age in the Yichang area and from the planation surfaces and the five main river terraces of early to late Quaternary age in the Three Gorges areas. The early and middle Quaternary had a generally warm-wet paleoclimate from 1260 to 300 ka. The regional climate changed to generally dry-cold conditions during late Quaternary (110-10 ka), that can be



Corresponding author: Fang Xiang, professor of geology at the Institute of Sedimentary Geology, Chengdu University of

Technology. She is engaged in teaching and research of sedimentary petrology, lithofacies palaeogeography, Quaternary Geology and geomorphology, paleoclimate and palaeoenvironment. Email: [email protected]; phone: 0086-13308200692 

Corresponding author: Hengxu Huang, doctoral candidate of sedimentology at the Institute of Sedimentary Geology, Chengdu

University of Technology. He is engaged in research of sedimentary petrology, lithofacies palaeogeography, Quaternary Geology and geomorphology. Email: [email protected]; phone: 0086-15694003769 1

Journal Pre-proof correlated to glacial cycles in the Tibetan Plateau. During the late Quaternary, especially after its Gonghe Movement (150 ka) uplift, the Qinghai-Tibet Plateau had a significant impact on regional climate. Comparing the formation age and source direction of the Wushan loess in the Three Gorge with loess in the western Sichuan (the Ganzi loess), in the Loess Plateau and distributing in the middle and lower reaches of the Yangtze River (the Xiashu loess), it can be found that coupling of the Kun-Huang Movement of the Qinghai-Tibet Plateau occurred at 1100-600 ka and Mid-Pleistocene Transition and global cooling resulted in the sharp increase of

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East Asian winter monsoon and plateau monsoon, forming the western Sichuan loess and Xiashu loess. The Gonghe movement (150 ka) of the Plateau once again enhanced the plateau monsoon

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and blocked the Indian summer monsoon moving north-eastward. As a result, the cold-dry

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loess from the Zoige Basin to Wushan area.

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climate appeared in the Sichuan Basin and the Three Gorges area, resulting in the deposition of

1. Introduction

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monsoon

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Keywords: Yangtze Three Gorges; Quaternary paleoclimate; uplift of the Qinghai-Tibet Plateau;

The Yangtze Three Gorges and Yichang areas are located close to the eastern edge of the Qinghai-Tibet Plateau in the transitional region between the second and third topography steps of China. During the Quaternary, this region's topography, geomorphology and climate evolution have been closely associated with the uplift history of the Qinghai-Tibet Plateau (Sun, 1998; Shi et al., 1999). The climatic characteristics in these areas were also affected by trends in global climate, in the Asian monsoon (Cheng et al., 2005) and in the westerly circulation (Tan, 2011). Sediment proxies of paleoclimate in these areas are of great significance for the reconstruction of the paleoclimate trends of southern China and understanding the impacts of the phases of uplift of the Qinghai-Tibet Plateau on that climate history. 2

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investigations

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geomorphology,

paleontological

types

and

sediment

characteristics obtained different conclusions about Quaternary paleoclimate characteristics in these areas: 1) From studies of geomorphology and δ18O in the calcareous sediments of terraces, Li (1940), Ching (1965), Kang (1987), Tang and Tao (1997) and Wei et al. (2009) believed that there were 3 to 4 glacial-interglacial cycles in the Three Gorges and adjacent areas. 2) Wang (2006a), in contrast, suggested that there were no significant Quaternary glaciers in

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the mountain areas of the Western Hubei, and that the Quaternary paleoclimate was always warm and humid.

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3) Xu et al. (1974), Huang and Fang (1991) and Cheng et al. (2006) interpreted the

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earlier-middle Pleistocene paleoclimate in Chongqing and the Three Gorges as warm and wet

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subtropical climate.

4) By analyzing the grain size characteristics and chemical characteristics of the Wushan

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loess and the carbon and oxygen isotope characteristics of the Shennongjia cave stalagmites,

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Zhang et al. (2013b) and Zhang (2014) considered there had been a tendency for the paleoclimate to gradually become drier since the early Pleistocene.

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To help constrain these varied interpretations, we applied mineralogical and geochemical methods to derive paleoclimate proxies from sediments of this region. However, mainly middle and upper Quaternary sediments in the Yangtze Three Gorges are preserved in terraces. Although investigations of the Qinghai-Tibet Plateau glaciers (Zhao et al., 2011; Zhang et al., 2013a), of loess in northwest China (Liu, 1985), and red earth paleosols of South China (Xiong et al., 1999; Xie et al., 2003) produced useful paleoclimatic interpretations, there are few such studies using terrace deposits in this area, even though sediments characteristics were potentially controlled by climate. Although these terraces are widely distributed, discontinuous sedimentation and subsequent weathering and erosion significantly affected the terrace sediments. 3

Journal Pre-proof Aeolian loess is considered to be an ideal geological material for studying the evolution process of the atmosphere. The loess on the Loess Plateau records the strengthening of the East Asian monsoon in the glacial climate (Li, 2018), also the formation of the western Sichuan loess on the eastern edge of the Qinghai-Tibet Plateau may reflect when the Indian monsoon has been blocked after the Qinghai-Tibet plateau rose to a new critical height (Pan et al., 1999; Chen et al., 2002).The Wushan Loess, located in the Three Gorges area of the Yangtze River and near Wushan county, is another important loess deposit. Because the loess is located in the

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“extension‖ area of the dust particles in the East Asian winter monsoon and the Qinghai-Tibet Plateau monsoon, the loess has been receiving attention since its discovery (Zhao et al., 2012).

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Based on previous studies of grain size characteristics, REE compositions and magnetic fabric

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characteristics of Wushan loess, researchers confirmed that the loess is a wind-induced

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sedimentary product (Zhang et al., 2010; Li et al., 2010; Zhang et al., 2013a; Zhang et al. 2013b; Wu et al., 2014). According to the analysis of elemental geochemical characteristics and the

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optically stimulated luminescence dating (OSL) of the loess, the Wushan loess is thought to be

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an aeolian deposit of the late Pleistocene dry and cold environment, with its source material

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coming from the western Sichuan region (Liu, 1983; Zhao et al., 2012; Zhang et al., 2014). However, the formation mechanism of the Wushan loess and its relationship with the Asian monsoon and the Qinghai-Tibet Plateau need further study. Discussion of its formation mechanism can provide evidence for learning about the climate change process in the Three Gorges area and the entire Sichuan Basin during the Quaternary, as well as providing a reference for understanding the climate effects caused by the uplift process of Qinghai-Tibet Plateau and the evolution of the Asian monsoon. This paper applies sediment chronology methods to obtain improved ages for these Quaternary deposits of the Yichang and Three Gorges areas, and then discusses the macro-sedimentary characteristics, clay minerals and element characteristics of the sediments to 4

Journal Pre-proof interpret larger scale paleoclimate trends in the study areas. By comparing these new data to previous research on the glacial history of the Qinghai-Tibet Plateau and loess in China, this paper also explores the relationship of paleoclimate in the Three Gorges and adjacent areas to Quaternary glacial-interglacial stages and to the uplift episodes of the Qinghai-Tibet Plateau and effect of Asia monsoon evolution.

2. Geological and geographical background of the study area

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The study includes the area near the Fengjie and Wushan counties in the Three Gorges and near Yichang city (Fig. 1). The Three Gorges cuts through a low mountain region with an

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elevation of less than 2000 m. From the Wushan area, the highest point in the Three Gorges, the

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elevation falls below 500 m to the west of Wanzhou city and then only about 40 m to the east of

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Yichang city.

The current climate of the Three Gorges and Yichang areas is subtropical monsoonal humid

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in the transition between the mid-subtropical and the northern subtropical zones. It is warm in

Fig. 1. Location of the study area and sampling sites. (a) Sampling sites in the Three Gorges area. Fj-02 down to WS-06 are samples name, and the detail of these samples is described in text and Table 1. (b) Sampling sites in Yichang area. 5

Journal Pre-proof winter and hot in summer, with an uneven spatial and temporal distribution of rainfall. The average temperature is 16-18°C and the annual precipitation is about 1000-1300 mm. Rainfall is influenced by monsoon and topographical factors, and vertical climate changes are significant. The sediments of the early and middle Quaternary are well preserved in the Yichang area to the east of the Three Gorges. The Yunchi Formation (Q1y) was mainly alluvial fan to fan delta sediments of early Pleistocene, and Shanxiyao Formation (Q2s) was a fan delta and lacustrine deposit of middle Pleistocene (Xiang et al., 2007). The middle to upper Quaternary is preserved

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as the gravel, sand and clay deposits on the 5 main terraces of the Yangtze River. In the Three Gorges area, there are three widespread planation surfaces. The highest is the

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E’xi planation surface with an elevation between 1700-2000 m; the second is the Shanyuan

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planation surface with an elevation of about 800-1200 m. Both the E'xi and Shanyuan planation

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surfaces are generally considered to have formed before the Quaternary (Shen, 1965; Liu, 1983; Tian et al., 1996). The third surface is the Yunmeng planation surface with an elevation of about

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600 m (Tian et al., 1996) and is thought to have formed by fluvial processes mainly during the

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early Pleistocene (Li et al. 2001, Xie et al., 2006, Wang et al., 2016). Our results, discussed below, indicate that the termination of the Yunmeng planation corresponds to the topmost part of

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the Shanxiyao Formation to the east of the Three Gorges. Below the Yunmeng planation surface are five main terraces along the slopes of the Yangtze Three Gorges (Tian et al., 1996; Xiang et al., 2005; Zhao et al., 2012). The terraces are well preserved in Fengjie and Wushan regions. The higher 5th and 4th generally display pedestal terrace forms on basement rock, whereas the 3rd, 2nd and the lowest 1st terrace are mostly accumulation terraces. The height difference between 2nd and 1st terraces is relatively small, generally about 10-25 m, while the height differences among the other terraces are 35-60 m with a maximum of 100 m. The upper parts of the outcrops of the 5th and 4th terraces in the Three Gorges display an appearance of reticulated red clay (Liu, 1983). These reticulate mottling soil patterns typically 6

Journal Pre-proof form during extended periods of wet, warm climates (Hu et al., 2015). In the Sichuan Basin, in the upper part of the 3rd and 2nd terrace sediments, the reticulation networking can be found locally with kaolinite bands and iron-manganese mottling and nodules. The lower portions of the 3rd and 2nd terrace sediments have calcium nodules, often indicative of relatively dry climates (He et al., 2015). The Wushan Loess (35-100 ka BP) can be seen in the top of the 2nd and 3rd terrace sediments in the Wushan and Zigui counties. In the Jialing River and Fu River basins and in the northern part of the Chengdu plain, the ―Chengdu clay‖ was distributed on the 2nd terrace

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and other higher landscape. Both the Chengdu clay and the Wushan Loess were considered to be periglacial eolian loess formed during a glacier age (Liu, 1983, Zhao et al., 2012).

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In the 1st terrace, the reticulate red clay is not found, and the upper part of the outcrop is

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yellow-brown sandy clay. Along the Yangtze River, the lower gravel layer of the 1st terrace

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commonly has calcareous cementation and forms the characteristic ―Jiangbei Conglomerate‖

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(Xia et al., 2010; Zhang, 2012).

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3. Outcrop sedimentary characteristics and sample collection Descriptions of outcrops observed in the Fengjie, Wushan and Yichang areas follow. The

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locations for the outcrops are shown in Figure 1, and composite sections for the assembled outcrops are in Figure 2-4.

3.1. Lower part of the Yunchi Formation (early Pleistocene age) in Yichang area An outcrop of the lower Yunchi Formation is located on the west side of Luyingchong Reservoir (N30°32′21.9″, E111°26″8.2″, and elevation about 84 ±9 m). Approximately 4-5 m thick sediment package is mainly layered brownish-red gravels with a brownish-red sand and silty clay filling among the gravels. Sample 028-01 was collected from the interstitial material (Fig. 2). Another outcrop (N30°32'25.2″, E111°26′14.2″, elevation about 110 ±5 m) on the northwest side of the same Luyingchong Reservoir is entirely composed of a brick-red clayey-sandy gravel layer and localized sandy layer with cross bedding. The dip of the largest 7

Journal Pre-proof flat surface of gravels is consistent with the dip of the cross bedding in sand layer. Horizontal gravel layer with iron-rich cements can be found locally. In the upper part of the outcrop, the size of gravels becomes larger, and a red weathered soil layer about 1 m thick can be seen on the top. Sample 021-01 was collected from brick-red clayey interstitial material in the lower part of the outcrop. The dating sample 021-ESR was collected from sand at the bottom of the outcrop.

3.2. Upper part of the Yunchi Formation and lower part of the Shanxiyao Formation (early

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Pleistocene age) in Yichang area

The outcrop is located at Lijiayuanzi, Yunchi county, Yichang city (N30°28'22″,

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E111°27'11″, and elevation of the mid-low part about 109 ±10 m). From the top to the bottom,

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Formation (layers C and D) (Fig. 2).

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four layers were distinguished in the Shanxiyao Formation (layers A and B) and the Yunchi

Shanxiyao Formation layer A is a coarse and poorly sorted gravel layer that is 2-6 m thick.

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Layer B is 6-m-thick gravel layer with brownish-red clay-sandy interstitial materials. The upper

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layer A is horizontal and the lower layer B has cross bedding. Dating sample 017-ESR was collected in sandy interstitial materials at the same layer in another profile.

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Yunchi Formation layer C is a gray-white to yellow semi-consolidated sand layer with some 1-2-cm-thick gravel layers It is 8-m thick and of homogenous texture. There is an erosional surface with a ferric banding on the top of this layer. Parallel bedding can be found in the upper sand layer and cross bedding appears in the lower part. The underlying layer D is a brown-yellow gravel layer about 12-m thick. Disseminated red-brown ferric precipitates are distributed in this layer. The whole layer is made up of 3-4 gravel-to-sand fining-upwards sequences that have thicknesses of about 1 to 3 m. Cross bedding, about 1-m thick, appears in the upper part, which has the same dip as the largest flat surface of gravels and dips to the southwest. Dating sample 015-ESR is collected from sandy interstitial materials. This layer D has an unconformable basal contact onto the brownish-red argillaceous siltstones of the 8

Journal Pre-proof Fangjiahe Formation of Paleogene age. Our previous studies (Xiang et al., 2007) have shown that the Yunchi Formation records alluvial fan and fan delta deposition, and the Shanxiya

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Formation, alluvial fan and lacustrine deposition.

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Formations in Yichang area.

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Fig.2. Composite section of sedimentary characteristics and sample location of Yunchi and Shanxiyao

3.3. Middle and upper parts of the Shanxiyao Formation (early Pleistocene age) in Yichang area

The outcrop in the vicinity of the Shanxiyao Brickyard in the Zhijiang city (N30°28′48″, E111°27′53″, elevation about 160 ±6 m) represents a 2-3-m-thick lacustrine deposit. The lower and middle parts are composed of several sequences of yellow fine-grained sand to silt deposits with red-brown iron-rich cements. Climbing-ripple bedding becoming ripple bedding upwards is in the upper part, and the uppermost portion is brick-red clay with no reticulate structure. On the top is maroon clay with irregular reticulate structure and iron-manganese nodules. Clay sample 018-01 was collected from brick-red clay in the uppermost portion (Fig. 2). 9

Journal Pre-proof 3.4. Upper part and top of the Shanxiyao Formation (early Pleistocene age) in Yichang area The outcrop near the Three Gorges Airport (N30°33'41.2″, E111°27′52.7″, elevation about 192 ±5 m) has an exposed thickness of about 3-4 m of reddish-brown sediment of lacustrine deposition. The lower part is a gravel layer of about 1-m thick. The size of gravels increases upward from smaller (5 cm) to larger (10-20 cm) pebbles. The interstitial material is sandy clay with a small amount of reticulate structure, from which the sample YCJC-01 was collected (Fig. 2). Dating sample 024-ESR was collected from the sandy interstitial material. Sample SXJC-01

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was collected from the red clay of the upper part which has a coarse reticulate structure. This red

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clay layer is widely distributed and constitutes a flat landform about 190 m above sea level.

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3.5. Yunmeng planation (early Pleistocene age) surface outcrop at Fengjie

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The Yunmeng planation surface on the northern bank of the Yangtze River in Fengjie county (N31°03′24.5″, E109°32′51.8″, elevation about 570 ±6 m) is identified by its platform-like

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topography. At other sites of the same height, the Permian limestone formed dome-shaped

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topography by erosion, and solution cracks, clints and red clay filling material were observed. Below an overlying slide-related gravel layer, there are three layers of fluvial deposits: (1) An

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uppermost gravel layer about 30-80-cm thick, intercalated with brownish-red sandy clay, has sub-circular pebbles about 1.5-4-cm long composed of quartzite, flint, siliceous limestone, sandstone and other components in a preferred orientation. Dating sample PT-ESR was collected from the sandy interstitial material (Fig. 3). (2) A gray clay layer is 1-3-cm thick from which sample PT-01 was collected. (3) A layer of lower brownish and yellow-brown clayey sand is about 8-15-cm thick.

3.6. The 5th and 4th terrace (middle Pleistocene age) outcrops in Yichang area (Fig. 3) The 5th terrace (N30°42′23.7″, E111°17′57.4″, elevation about 143 ±10 m) is on basement rock of the Cretaceous Wulong Formation of purple mudstone and gray sandstone covered by 10

Journal Pre-proof 15-23 m of sediments. The upper part has been largely destroyed by cultivated land, but some brown-red sand and clay layers are preserved. The lower part is gravel layer with imbricated structure. The interstitial material is brown-red clay, from which sample YC-01 was collected. The 4th terrace (N30°41'50‖, E111°18′, elevation about 114 ±6 m) is also on basement rock, with thickness of about 12-10 m. It is composed of a gravel layer filled with reddish-brown sandy clay. The composition of gravels is complex. Sandy interlayers appear locally. Clay sample 00B-01 was collected from interstitial materials, and dating sample YC-ESR was

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collected from the sandy interlayer.

Yichang area.

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Fig.3. Composite section of sedimentary characteristics and sample location of the 5th and 4th terraces in

3.7. The 5th through 1st terrace (middle to later Pleistocene age) outcrops at Wushan (Fig. 4) Terraces are developed in the left bank of the Yangtze River in the Wushan county. These are on a basement rock of the Triassic Badong Formation composed mainly of mudstone, limestone and sandstone. The 5th terrace (N31°05′18.4″, E109°52′50.8″, elevation about 320 ±10 m) is composed of brown-yellow clay 10-35 m thick on basement rock. No gravel layer is found. Clay sample WS-06 was collected. At another outcrop at a similar elevation, there is 10-40cm long earthy yellow angular limestone debris deposited with brown-yellow clay on the terrace surface, and 11

Journal Pre-proof calcium crusts 7-8cm thick are visible covering partings in the clay. Some parts of calcium crust consist of 3-4cm high stalactites. The formation of these stalactites is obviously later than the 5th

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terrace, so dating sample WS-ESR01 was collected to constrain the age of the 5th terrace.

and terraces in the Three Gorges area.

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Fig.4. Composite section of sedimentary characteristics and sample location of the Yunmeng planation surface

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The 4th terrace (N31°05′17.9″, E109°52′55.5″, elevation about 290 ±9 m) is 6-24 m thick

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brown clay on basement rock. Clay sample WS-04 was collected at 5m depth. No gravel was found in the outcrop, and peat or organic-rich clay was occasionally seen in its lower part. The 3rd terrace (N31°05′02.9″, E109°52′59.2″, elevation about 220 ±11 m) is a pedestal terrace composed mainly of yellow loess-like clay. The calcium carbonate nodules form 3 horizontal layers lying at the middle and lower part of the profile. Sample WS-03 was collected from the clay, and sample WS-ESR02 was collected from caliche nodules. The 2nd terrace (N31°04′35″, E109°52′57.7″, elevation about 145 ±9 m) is an accumulation terrace composed of loess-like clay deposits 20-30 m thick with calcium carbonate nodules that form a horizontal layers lying at the lower part of the profile. Sample WS-02 was collected from the clay. 12

Journal Pre-proof The 1st terrace (N31°04′16.6″, E109°52′44.4″, elevation about 120 ±8 m) is an accumulation terrace with loess-like clay deposits 40-50 m thick, from which clay sample WS-01 was collected at the middle of the outcrop. 3.8. The 3rd and 2nd terrace (later Pleistocene age) outcrops at Fengjie (Fig. 4) The 3rd terrace of the Yangtze River is an accumulation terrace on the left bank of the Meixi River, Fengjie county (N31°03′17.0", E109°31′13.9″, elevation about 158 m ±6 m). It has a thickness of 7-8 m and mainly brown-yellow silty clay with intermittently distributed calcareous

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nodules. Six cyclothems of calcareous nodules and clay can be found. Clay sample FJ-02 was collected at 6 m depth.

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The 2nd terrace at N31°02′25.4″, E109°30′38.8″ (elevation about 134 ±7 m) is also an

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accumulation terrace with 10-20 m of sediment. The sediment is mainly khaki-colored silty clay

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with a lenticular pebble layer having a maximum length of 3-4 m and a thickness of 8-10 cm. The gravel composition is mainly sub-circular or sub-angular pebbles of limestone, flint,

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collected at 13 m depth.

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siltstone and fine-grained sandstone that range from 0.5 to 4 cm in size. Clay sample FJ-01 was

4. Research methods

In the Three Gorges area, there are three major planation surfaces, and the lowest surface is the Yunmeng planation with an elevation of about 600 m (Shen, 1965; Liu, 1983; Tian et al., 1996; Li et al. 2001, Xie et al., 2006, Wang et al., 2016). It can be distinguished from terraces by its altitude, platform-like mountain top and a relatively thin package of fluvial sediments as described above. Due to the discontinuous distribution and poor preservation of the planation surface and terraces, sampling at a high resolution could not be obtained. Therefore this study focused on the larger scale changes in paleoclimate, rather than trends within each deposit. Analyses of the topographic settings and sedimentary characteristics of the different 13

Journal Pre-proof outcrops were supplemented by the samples' clay mineralogy and elemental composition data to develop evidence of paleoclimatic changes. The chronologic ages of the outcrops were determined by Electron Spin Resonance (ESR) analyses combined with research data from prior studies.

4.1. Electron Spin Resonance (ESR) In our study, because of its wider dating range, the ESR dating method was used for samples

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older than 10 thousand years, while the formation ages of the 3rd to 1st terrace sediments mainly came from the results of previous achievements. For the ESR dating of detrital quartz, Liang et

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al. (1993) found that the quartz E'- center has an average life of 109-1010 years. The quartz E'-

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center is mostly distributed in an outer shell with a thickness of less than 1mm. So, the E'- center

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accumulated in the parent rock is basically lost during the process of geological transportation away from the parent rock, and as a result, the age obtained by testing signals of the E'-center of

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quartz newly generated by α and β rays in sediments basically represents the deposited age of

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sediments . Hennig et al. (1983), Ye et al. (1998), Zhou et al. (2002), Kowalewski and Rimstidt (2003), Zhou et al. (2005) and Liu et al. (2010) used ESR methods to acquire the ages of river

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terraces and Pleistocene marine and lacustrine sediments. These ages can be consistent with dating data from OSL, U-Th and paleomagnetic data, which confirms that the ESR data is credible.

ESR ages were analyzed and tested at the Sichuan Provincial Key Open Laboratory of Earth Science and Technology, using JES-FE2XG electron spin resonance instrument, RJF2 microcomputer data acquisition system with a Cobalt-60 dual-plate source. The dating object was the alpha-quartz E'-centers. The g-factor value of dating signal was 2.0005 ±0.0003. The quartz particles selected from the 1kg dating sample were divided into 4 portions, three of which were irradiated by artificially β-rays with doses of 1351 Gy, 1813 Gy and 3652 Gy. The ESR signals were then tested after 4 samples were subjected to a heat activation treatment. 14

Journal Pre-proof 4.2. Clay mineral analyses Clay mineral analysis was performed at the Key Laboratory of Marine Geology, Ministry of Education, Tongji University, Liu et al.( 2003), Petschick(2010) and Sun et al.(2017) described the method used for this technique. The relative proportions of smectite (including illite and smectite mixed-layer mineral), illite, kaolinite and chlorite were determined mainly by the area ratio of the (001) peak of smectite (17 Å), the (001) peak of illite (10 Å), the (002) peak of kaolinite (3.57 Å) and the (004) peak of chlorite (3.54 Å) (Sun et al., 2017; Liu et al., 2003) (Fig.

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5). Replicate analyses of a few selected samples provided a precision of ±2%. The illite chemistry index (ICI) was calculated as the ratio of the 5 Å and 10 Å illite peak

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areas (Esquevin, 1969) in the ethylene glycol-saturated curves (Fig. 5). Values of the illite

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chemistry index lower than 0.15 reflect the appearance of Fe-Mg-rich illite (the result of

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physical weathering), while values higher than 0.4 reflect the appearance of Al-rich illite (the

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result of hydrolysis) (Petschick et al., 1996).

Fig. 5 Typical X-ray diffraction patterns of clay minerals of sample FJ-01. The different preparation conditions: EG-ethylene-glycol solvation; N-air drying; H-heating

15

Journal Pre-proof 4.3. Major and trace element analyses and Chemical index of alteration Major elements of Al2O3, TFe2O3, Na2O, MgO and CaO, the ratios of elements such as Rb/Sr, TFe2O3/MgO, Na2O+CaO/Al2O3, K2O/Na2 O, and the chemical index of alteration (CIA) (Al2O3*100/ (Al2O3+CaO*+Na2O+K2O)) (McLennan, 1993), were used for paleoclimatic proxies. Elemental analyses were carried out at the Sichuan Metallurgical Geology and Minerals Testing Center. Samples were conventional pre-treated and tested using the Optima 5300V Inductively Coupled Plasma Spectrometer. The ambient temperature is 23°C. Accuracy is ±0.1

oo

f

nm, the wavelength range is from 165 to 782 nm, and the detection limit is 0.1-1 ppm. The CIA formula is calculated using the mole fraction of oxides, where CaO* is the total

pr

CaO mole fraction in a sample after adjusting the total CaO to obtain the portion from chemical

e-

deposition, that is CaO*=CaO-(10/3×P2O5). When CaO*
Pr

directly, but when CaO*>Na2O, then the CaO* is replaced with the Na2O mole fraction (McLennan, 1993). Nesbitt and Young (1989) reported that a CIA ratio of 50-65% indicated a

al

cold and dry climate; at 65-85%, it was a warm and moist condition; and at 85-100%, it was a

rn

hot and humid tropical-subtropical condition.

When climate is hot and humid, the elements of Sr, Na2O, MgO and CaO are easily leached,

Jo u

therefore leaving enrichment in Al2O3 and Fe2O3. Therefore, in hot and humid climates, Rb/Sr, Al2O3, Fe2O3, K2O/Na2O and CIA are relatively high, whereas Na2O, MgO, CaO and NaO+CaO/Al2O3 are relatively low; and these chemical indexes contain opposite trends in cold and dry conditions (Zhao et al., 2012; Li et al., 2013). Considering our clay samples all in the oxidized state, the total Fe2O3 (TFe2O3) in a sample is used directly, even though one sample may be affected by the provenance composition and there is a certain deviation expected (such as sample PT-01, from gray clay layer, including almost only illite).

16

Journal Pre-proof 5. Results 5.1 Results of ESR dating and age of sediments The analytical ESR results are given in Table 1. The precision of dating results was 10.0%. Table 1. ESR dating results of samples. Sampling

Sample Sedimentary site number

Fengjie

PT-ESR

Yunmeng planation

sandy interstitial

surface

material

terrace (corresponding to

pr sandy interstitial

the Yunchi Formation

material

al

sandy interstitial

Shanxiyao Formation

material

lower part of the Yunchi

sandy interstitial

rn

Lower part of the

Jo u

021-ESR

(Ma)

Quartz

0.689±0.06

authigenous 0.395±0.039 calcite

authigenous

calcareous nodule

Upper and middle part of 015-ESR

017-ESR

e-

3rd terrace

Pr

WS- ESR02

mineral

stalactites

T4)

Wushan

Yichang

oo

calcium crust on the T5 WS- ESR01

Age

f

site

Dating Sample type

Formation

material

upper part of the

sandy interstitial

Shanxiyao Formation

material

024-ESR

0.101±0.01 calcite

Quartz

1.08±0.108

Quartz

0.87±0.087

Quartz

1.15±0.115

Quartz

0.75±0.075

Quartz

0.308±0.03

sandy interstitial YC-ESR

4th terrace material

By merging with the results of previous sedimentary dating research (Liu, 1983; Yang, 1990; Chen, 1996; Tian, 1996; Li et al., 2001; Xiang et al., 2007), age span and uncertainty of the Quaternary deposits in the study areas were obtained (Table 2). The mean ages for the depositional units were calculated by interpolation of all samples from those units. 17

Journal Pre-proof 5.2 Results of clay and geochemical analyses 5.2.1. Clay mineral analyses Data from 12 samples were obtained. Another 3 samples from reticulated red clay (010-01, SXJC-01, YCJC-01) had complex diffraction peaks, which were complex mixed-layer clay minerals forming under strong heat-wet conditions, and could not be used to calculate the relative content of clay minerals and ICI. Observation with the Scanning Electron Microscopy and Energy Spectrum system indicates that all the clay minerals in the 12 samples have poor

oo

f

crystal form and small crystal size. Some of crystals have obvious fractures and rounding edges (Fig.6). These characteristics indicate that the samples contain mainly clast-derived clays.

Jo u

rn

al

Pr

e-

pr

However, there are also authigenic clay minerals in each sample, which are mainly

Fig. 6. Scanning electron microscopy characteristics of clay minerals. (a) Clastic illite and smectite mixed-layer clay mineral and kaolinite. Crystal form is poorly developed, and authigenic smectite can be found. Sample 021-01. (b) Clastic illite. Crystal form is poor. Sample PT-01. (c) Clastic kaolinite. Edges are rounded, and some cracks can be found. Sample 018-1. (d) Clastic illite and smectite mixed-layer clay mineral, poorly developed crystal form. Sample FJ-01. (e) Authigenic leafy chlorite with fine blades. Sample WS-06. (f) Clastic clay changing to authigenic illite. Sample WS-03. (g) Clastic kaolinite transforming to illite. Sample 018-01. (h) Authigenic smectite, partially changing from kaolinite. Sample 021-01. 18

Journal Pre-proof curled-sheeted smectite (or illite and smectite mixed-layer mineral) and a few leafy chlorites (Fig. 6). The XRD analytical results of these samples are given in Table 3. 5.2.2 Major and trace element change trends In the Yichang area, Rb/Sr, TFe2O3 and TFe2O3/MgO gradually increase from the Yunchi Formation to the overlying Shanxiyao Formation and gradually decrease from the 5th terrace to the younger 4th terrace (Tables 4 and 5; Figures 7 and 8). Al2O3 content increases upward from the base of the Yunchi Formation, increases again from the base of the Shanxiyao Formation,

oo

f

and gradually decreases from the 5th terrace to the 4th terrace. Na 2O does not change much from the Yunchi Formation to the Shanxiyao Formation, and gradually increases from the 5th terrace

pr

to the 4th terrace. MgO, CaO and Na2O+CaO/Al2O3 have a decreasing trend from Yunchi

e-

Formation to Shanxiyao Formation, and a gradual increase from the 5th terrace to the 4th terrace.

Pr

K2O/Na2O is similar in the Yunchi Formation and the Shanxiyao Formation, with a significant decrease from the 5th terrace to the 4th terrace. CIA in the Yunchi Formation, in the Shanxiyao

al

Table 2. Ranges of sediment chronology data from ESR dating and

rn

previous date (Liu, 1983; Yang, 1990; Chen, 1996; Tian et al., 1996; Li et al., 2001; Xiang et al., 2007).

Jo u

Sample sampling site/topographic unit

Age (ka)

Yunchi Formation in Yichang area

1260-960 (ESR)

Shanxiyao Formation in Yichang area

950-740 (ESR)

River sandy sediments on the Yunmeng 750 (ESR) planation surface in Fengjie 5th terrace of the Yangtze river

730-700 (ESR)

4th terrace of the Yangtze river

500-300 (ESR)

3rd terrace of the Yangtze river

110-90 (TL)

2nd terrace of the Yangtze river

50-30 (C14)

1st terrace of the Yangtze river

30-10 (C14)

19

Journal Pre-proof Formation, and in the 5th and 4th terraces all exceeds 75%, with the largest value (>82%) in the Shanxiyao Formation, and with a decreasing trend from the 5th terrace to the 4th terrace (Table 5). Table 3. XRD analytical data of clay minerals. Sampling

Sample

Sedimentary

Age

site

number

location

(ka)

FJ-01

2nd terrace

FJ-02

3rd terrace

S

I

K

C

ICI

40

14.57

56.17

11.05

18.21

0.39

100

40.58

44.77

9.07

5.58

0.43

98.40

0.72

0.00

0.43

Yunmeng planation PT-01

750

1st terrace

10

25.25

57.08

5.22

12.45

0.36

WS-02

2nd terrace

40

27.59

39.92

5.36

27.13

0.46

WS-03

3rd terrace

100

16.78

71.85

7.85

3.53

0.38

WS-04

4th terrace

400

29.48

57.10

7.78

5.64

0.43

WS-06

5th terrace

700

64.32

25.20

6.93

3.55

0.56

00B-01

4th terrace

400

15.83

29.89

54.28

0.00

0.38

800

15.57

34.80

49.63

0.00

0.43

1100

63.62

5.62

30.76

0.00

0.47

1260

88.79

4.04

7.17

0.00

0.44

al

Pr

e-

WS-01

rn

Wushan

0.88

pr

surface

oo

f

Fengjie

middle part of the Shanxiyao

Jo u

018-01

Formation

Yichang

middle part of the

028-01

Yunchi Formation lower part of the 021-01 Yunchi Formation  S- smectite; I- illite; K-kaolinite; Ch-chlorite; ICI-illite chemical index; a precision of ±2%

In the Three Gorges area, Rb/Sr and Al2O3 have a progressively decreasing trend from the planation surface to the youngest 1st terrace. Na2O, MgO and CaO have a gradually increasing trend from the planation surface to the 1st terrace. A trend of TFe2O3 is not obvious; TFe2O3/MgO gradually decreases from the 5th terrace to the 1st terrace, but with a significant 20

Journal Pre-proof peak in the 3rd terrace. Na2O+CaO/Al2 O3 has the smallest ratio in the planation surface with a ratio similar to that in the Shanxiyao Formation at Yichang. This Na2O+CaO/Al2O3 ratio has a slight increase from the 5th terrace to the 3rd terrace, and then it increases significantly in the 2nd and 1st terraces. K2O/Na2O has the largest enrichment in the planation surface, is similar in the 5th terrace to the 3rd terrace with a decreasing trend, and then decreases significantly in the 2nd and 1st terraces. CIA slightly decreases from the planation surface and 5th terrace to the 3rd

oo

5.2.3 Paleoclimatic inferences from clay mineral analyses

f

terrace, and then decreases significantly in the 2nd and 1st terraces, where it is <65%.

The clay minerals in the Yunchi and Shanxiyao Formations of the Yichang area are

pr

dominated by smectite and kaolinite without chlorite (Table 3). The clay minerals in the

e-

planation surface and terraces in Three Gorges area are dominated by smectite and illite, with a

Pr

small amount of chlorite, whereas the content of kaolinite is low. However, this contrast in clay composition partly reflects the impact of the provenance source material on the type and content

al

of clay minerals in those sediments: the Huangling Dome to the west of Yichang is composed of

rn

mainly medium-acidic magmatic rocks and metamorphic rocks with abundant feldspar, which can provide a lot of kaolinite to the clay after chemical weathering. For this reason, this study

Jo u

uses the illite chemical index (ICI) to discuss the paleoclimate, because this index is thought to reflect mainly the degree of chemical hydrolysis in the sediments (Esquevin, 1969; Chamley, 1989; Petschick et al., 1996; Liu et al., 2012; Sun et al., 2017).

21

Journal Pre-proof Table 4. Elemental composition of clay samples (only for those elements discussed in the text). ω(B)/10-6

ω(B)/10-2

Sampling

Sample

Sedimentary

Age

site

number

location

(ka)

Rb

Sr

Al2O3

TFe2O3

Na2O

MgO

CaO

K2O

FJ-01

2nd terrace

40

94

84.6

15.51

6.55

1.50

2.37

2.63

FJ-02

3rd terrace

100

118

77.37

14.42

6.22

1.23

1.74

4.62

2.46

750

146

122.6

16.92

2.65

f o

2.53

0.79

0.19

3.86

1.59

2.51

6.41

2.39

o r p

Fengjie Yunmeng PT-01

e

planation surface

Wushan

r P

0.46

WS-01

1st terrace

10

79

70.63

11.87

5.39

WS-02

2nd terrace

40

70

56.97

11.35

4.87

1.64

1.91

6.82

1.90

WS-03

3rd terrace

100

99

69.4

13.08

5.75

1.21

1.35

1.04

2.31

WS-04

4th terrace

400

98

75.95

14.35

6.31

1.11

1.64

1.13

2.40

WS-06

5th terrace

700

95

74.45

14.4

6.56

0.79

1.27

1.02

1.94

00B-01

4th terrace

400

74

70.56

18.06

8.38

1.31

1.13

0.82

1.98

YC-01

5th terrace

700

81

96.25

19.02

9.26

0.57

0.91

0.45

1.88

740

79

81.12

15.23

8.34

0.24

0.56

0.27

1.46

Yichang

rn

u o

J

l a

upper part of the SXJC-01

Shanxiyao Formation

22

Journal Pre-proof middle part of 018-01

Shanxiyao

800

83

75.09

14.81

7.40

0.30

0.65

0.21

2.03

830

72

71.99

12.45

6.61

0.22

0.55

f o

0.23

1.26

1100

63

63

17.05

7.83

0.31

0.79

0.42

1.86

1260

68

57.28

10.32

0.25

0.75

0.49

1.76

Formation lower part of YCJC-01

Shanxiyao Formation

o r p

middle part of 028-01

e

Yunchi Formation lower part of 021-01

l a

Yunchi Formation

r P

6.52

n r u

o J

23

Journal Pre-proof The highest ICI values are in the Yunchi and Shanxiyao Formations in the Yichang area, and in the Yunmeng planation surface and 5th terrace in the Three Gorges area (Table 3 and Figure 8). There is a decrease of ICI into the 4th terrace. At the 3rd terrace, the ICI continues to decrease. After increase in the 2nd terrace, the value decreases from the 2nd terrace to the 1st terrace. The presence of authigenic smectite (or illite and smectite mixed-layer clay) and chlorite as observed by SEM is basically compatible with the decreasing trend of ICI. These trends indicate that the paleoclimate signals preserved in the later terraces within these areas had

oo

f

changed to drier and colder conditions.

From above macroscopic sedimentary characteristics, we can find there are karstification,

pr

reticulated clay and peat in the Yunmeng planation surface, the Yunchi and Shanxiyao

e-

Formations, the 5th and 4th terraces. But in the 3rd to 1st terraces, loess, calcareous cement and

Pr

calcareous nodules appeared. These characteristics also reflect the paleoclimate changing from warm and wet to dry and cold. The long-term change process of paleoclimate reflected by the

al

macroscopic sedimentary characteristics is consistent with the climate change process reflected

rn

by the ICI values.

Sampling

Jo u

Table 5. Elemental ratios and CIA within clay samples. Sample

Age

Rb/Sr

Na2O+CaO/

K2O/

CIA

Al2O3

Na2O

(%)

Sedimentary location

site

number FJ-01 FJ-02

(ka)

2nd terrace

40

0.68

0.26

1.75

60.83

3rd terrace

100

0.79

0.41

2.01

67.86

749

1.43

0.04

8.41

76.14

Fengjie Yunmeng planation PT-01 surface WS-01

1st terrace

10

0.42

0.67

1.50

59.74

WS-02

2nd terrace

40

0.43

0.74

1.16

59.86

WS-03

3rd terrace

100

0.91

0.17

1.91

67.00

WS-04

4th terrace

400

0.98

0.16

2.16

68.73

Wushan

24

WS-06

5th terrace

700

1.13

0.13

2.45

73.14

00B-01

4th terrace

400

0.54

0.12

1.51

75.50

YC-01

5th terrace

700

1.16

0.05

3.29

83.25

SXJC-0

upper part of 740

1.66

0.03

6.00

85.95

1

Shanxiyao Formation

800

1.70

0.03

6.69

82.76

830

1.62

0.04

5.63

85.20

1100

1.20

0.04

6.01

83.78

1260

pr

Journal Pre-proof

0.07

6.94

78.82

middle part of 018-01 Shanxiyao Formation lower part of

1

Shanxiyao Formation

oo

YCJC-0

f

Yichang

middle part of Yunchi 028-01 Formation

Pr

Formation

0.93

e-

lower part of Yunchi 021-01

al

6. Discussion

6.1. Quaternary paleoclimate characteristics in the study areas

rn

The combination of the ESR dating of these deposits, combined with their

Jo u

macro-sedimentary characteristics and clay mineral and elemental characteristics, indicates that the paleoclimate in Yichang and Three Gorges areas underwent a change from warm and wet conditions during the early and middle Quaternary into cold and dry conditions with some fluctuations during the late Quaternary. It relatively warm and wet during the deposition of the Yunchi Formation (1260-960 ka), then it became even warmer and wetter during the Shanxiyao Formation (950-740 ka) and the coeval Yunmeng planation stage. There was a gradual decline in warmth and moisture during the 5th terrace (730-700 ka) and the 4th terrace (500-300 ka) intervals. During the 3rd and the 2nd terraces interval (110-30 ka), the paleoclimate was relatively dry and cold. Then, after 30 ka and during the 1st terrace (10 ka), the average paleoclimate 25

Journal Pre-proof became significantly dry and cold. In general, the Yichang area on the eastern side of the current

oo

f

Three Gorges highlands was warmer and wetter than within the Three Gorges area.

pr

Fig. 8. Ratio of elements and illite chemical index of samples in stratigraphic order. (T-terrace;

Pr

e-

SXY-Shanxiyao Formation; YP- Yunmeng planation surface; YC- Yunchi Formation)

6.2. Paleoclimate records and glacial events in the Eastern and western China

al

Li (1940), Ching (1965) and Wei (2009) believed that there were 3 or 4 Quaternary glacial

rn

cycles in the western Hubei Province, and these corresponded to the named Quaternary glacial periods of the Lushan Mountains (Jing and Liu, 1999) (Table 6). Tian et al. (1996) and Tang and

Dagu Ice Age.

Jo u

Tao (1997) proposed that the formation of Three Gorges was the result of glacial erosion of the

During the Poyang Ice Age (ca. 1700 ka), the altitude of the Qinghai-Tibet Plateau was 2,000-2,500 m (Table 6). The presence of the Great Plateau strengthened the heat source and the summer monsoon, but the altitude of the Plateau was not enough to block the monsoon vapor. At this time, the Tibetan Plateau had not yet entered the cryosphere (Shi et al., 1999). During the Dagu Ice Age at ca. 1000 ka, the Qinghai-Tibet plateau reached 3,000-3,500 m and entered the cryosphere for the first time. However, the Yunchi Formation in the Yichang area recorded a warm and humid climate. Although there are no sedimentary records in the Three Gorges area during the same period, probably that area had a similar climate to the adjacent 26

Journal Pre-proof Yichang area. Table 6. Comparison of the Eastern China Ice Ages with the paleoclimate in the Three Gorges and Yichang areas. Eastern China Ice Ages and dates

Qinghai-Tibet

(Jing and Liu, 1999)

elevation (m)

Sedimentary units and paleoclimate in the Three Gorges Ice Age

(Shi et al., 1999)

date

Lulin Ice Age

3rd terrace-1st terrace (10-110 ka) >4,000 (150 ka) cold and dry 12-200 ka

oo

f

Lushan- Lulin

Lushan Ice Age

pr

interglaciation

200-400 ka

3,500

4th terrace (300-500 ka), degree of

Dagu - Lushan 400-900 ka

al

interglaciation

warmth and moisture was reduced

Pr

e-

(150-600 ka)

5th terrace and Shanxiyao

rn

2,500-3,500

Poyang-Dagu

warm and wet

900-1100 ka

Jo u

Dagu Ice Age

Formation (700-950 ka)

(600-1100 ka)

Yunchi Formation (960-1260 ka) warm and wet

1100-1500 ka

interglaciation

2,000-2,500 Poyang Ice Age

1500-1800 ka (1500-1800 ka)

During the Lushan Ice Age (ca. 300 ka), the Plateau was relatively stable with an elevation of about 3,500 m; therefore, it produced a strengthened cold source. This was the formation period of the 4th terrace in the Three Gorges, and its paleoclimate proxies indicate there was still a warm and humid climate. However, compared with the previous interval, the degree of warmth 27

Journal Pre-proof and moisture was reduced. During this interval of 1260 to 300 ka, there was no significant glacial record in the area from Yichang to Fengjie (Table 6). This conclusion is basically consistent with the view of Shi and Deng (1982), Shi et al. (1989) and Shi (2010) that the middle and lower reaches of the Yangtze River were not affected by glaciation during the early and

Jo u

rn

al

Pr

e-

pr

oo

f

middle Quaternary.

Fig. 9. Comparison of the paleoclimate change trends in the Three Gorges and adjacent areas and the glacial-interglacial cycle in the Northern Tibetan Plateau (glacial-interglacial cycle date from Zhang et al., 2013).

By comparing to the paleoclimatic data of the Qinghai-Tibet Plateau glaciers (Zhao et al., 2011; Zhang et al., 2013a) (Fig. 9), we can find the paleoclimate mainly was warm and wet from the Yunchi to Shanxiyao Formations (1260-740ka) in Yichang area and at Yunmeng planation surface (750 ka) in the Three Gorges. However, at the same period of time in the Northern Tibetan Plateau, significant glacier events can be identified. The dry and cold sedimentary records can only be found in the 3rd to 1st terrace sediments (110-10ka) in the study area, and 28

Journal Pre-proof the paleoclimate change trend seems to coincide with the glacier-interglacial cycles in the Northern Tibetan Plateau. All of these implied that cold and dry paleoclimate in the Tibetan Plateau had no effect on the area to the west of the Three Gorges before 110ka.

6.3. Implications for progressive uplift of the Qinghai-Tibet Plateau and evolution of Monsoon Li et al. (2018) believed the distribution area of loess enlarged from north (Loess Plateau) to

oo

f

southeast (Xiashu loess at Nanjing and Yizheng in Jiangsu Province, and Xuancheng in Anhui Province) (Fig. 10). This change was thought to be an effect of the East Asian monsoon

pr

strengthening from 1160 to 800 ka (Zheng et al., 2002). In the west of Sichuan Province, another

e-

kind of loess also was found, the typical of which was known as Ganzi Loess (or Chuanxi

Pr

Loess). Fang et al. (1996) found that Ganzi Loess was formed by the plateau monsoon (may be influenced by westerlies too), and was sediment blown by wind from exposed materials (such as

al

moraine, cold weathering clastics) at the east edge of the Plateau. A review of Wang et al. (2005),

rn

Wang et al. (2006b), Qiao et al. (2007) and Sheng et al. (2010) indicates that Chuanxi loess was deposited first in the drainge area of the Yalong River basin (at the west of the Daxue Mountain)

Jo u

at 1160 ka, then appeared in the Dadu River basin (at the west of the Qionglai Mountain) at 200 ka, and deposited in Zoige Basin, Litang county at 128-120 ka (Fig.10). Loess was also found in Wushan county (Fig. 4) from 100 ka to 35 ka (Zhao et al., 2012), which was believed to have the same provenance as the Chuanxi Loess. So the distribution of Chuanxi loess and Wushan Loess showed that the plateau monsoon strengthened and region of influence enlarged eastward from 1160 ka to 100 ka. From the sediment records of the Three Gorges and the starting age of the Wushan loess, it can be found that the paleoclimate mainly was warm and wet from 1260 ka to 110 ka in the Sichuan basin. At the same time, previous studies on the Quaternary sporopollen flora in the Yangtze River Basin (Liu, 1991) showed that the vegetation changes in the middle and lower 29

Journal Pre-proof reaches of the Yangtze River (west of Hukou County, Jiangxi Province) have been consistent since the Quaternary, reflecting the area was controlled by the East Asian monsoon circulation since the Quaternary. The vegetations in the high mountains and high areas in the upper reaches of the Yangtze River changed with the uplift height of the Qinghai-Tibet Plateau. However, the Sichuan Basin between these areas had unique characteristics of vegetation change: in the early-middle Pleistocene, the main vegetation was coniferous and broad-leaved mixed forest, reflecting the warm and humid climate, and then transitioned to a coniferous forest and

oo

f

grassland with evergreen broad-leaved and deciduous broad-leaved mixed forest appearing at the middle period in the late Pleistocene, reflecting the overall cold-dry climate.

pr

From the previous studies on the uplift of the Qinghai-Tibet Plateau and the environmental

e-

changes in East Asia (Shi et al., 1999), we can see that the Kunlun-Yellow River movement

Pr

(Kun-Huang Movement) occurring at 1100 ka-600 ka caused the Qinghai-Tibet Plateau to rise up to a height of 3000-3500 meters, and the Plateau entered the cryosphere. The cold source of

al

the Plateau was enhanced, which coupled with the Mid-Pleistocene Transition (MPT)

rn

(Ruddiman et al., 1986; Clark et al, 2006) and the global cooling and resultant increase of Siberian high pressure. These changes led to the sharp increase of East Asian winter monsoon

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and caused the distribution of loess expanding from the Loess Plateau to the southeast, thus forming the Xiashu loess (starting at 900-850 ka) in the lower reaches of the Yangtze River. Also as the plateau monsoon system formed and the temperature of the Arabian Sea surface decreased, the Indian summer monsoon weakened (Shi et al., 1999). Coincident with these changes a cold-drying environment appeared in the west of Sichuan. The loess first deposited in the Yalong River basin in Ganzi (starting at 1160 ka), and then deposited in the Jinchuan area of the Dadu River basin (starting at 200 ka) after the plateau monsoon crossed the Daxue Mountain (present average elevation of 5000m). The latest stage of uplift of the Qinghai-Tibet Plateau, which is called the Gonghe Movement (ca. 150 ka), lifted the Qinghai-Tibet Plateau to more than 4000 m, including the elevating of several mountains on the eastern edge of the plateau above the height 30

Journal Pre-proof of equilibrium line of glacier formation (Zhao et al., 2011). The strengthening cold source of the Plateau may cause the plateau monsoon to cross the Qionglai Mountain (present average elevation of 4000m) and deposit loess in the Zoige Basin, Litang county (starting at 128-120 ka), and starting 100 ka even affected Wushan county to the west of the Wu Mountain (present

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highest elevation of 2400m).

Fig.10. Time contrast of Wushan loess, Chuanxi loess, loess in Loess Plateau and Xiashu loess and its relationship with uplift of the Qinghai-Tibetan Plateau and monsoon.(data from Shi et al., 1999; Wang et al. 2005; Wang et al., 2006b; Qiao et al., 2007; Sheng et al., 2010; Zhao et al., 2012; Li et al., 2018) 31

Journal Pre-proof The warm and humid climate in the Sichuan Basin at the early-middle Pleistocene was related to the Indian summer monsoon. Although the main influence area of the Indian summer monsoon in southwestern China is to the south of Kunming in present, Chen et al. (2002) found that the Indian monsoon had a significant impact on the area of Ganzi in 500-1150 ka, and after 500 ka, the impact of the Indian monsoon on the study area was gradually weakened. This shows that although the Qinghai-Tibet Plateau experienced a significant uplift after the Kun-Huang movement, the elevation of its southeastern margin (such as the Hengduan Mountains) had not

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blocked the input of Indian monsoon water vapor. At the same time, due to the blocking effect of the Qinling and Daba Mountains, the enhanced East Asian winter monsoon did not affect the

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Sichuan Basin and even the Yichang area. After the Gonghe Movement, the uplift of the

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Qinghai-Tibet Plateau and its surrounding mountain systems blocked the north-eastward

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movement of the Indian summer monsoon. The influence area of the Indian summer monsoon retreated to the outside of the Sichuan Basin, resulting in the emergence of a cold and dry

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climate in the Basin, combined with the enhancement of the plateau monsoon effect, resulting in

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loess depositing in the Basin. Since the Wushan loess originated from the eastern margin of the Plateau and the western Sichuan, it was believed that the East Asian winter monsoon could not

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affect the area to the west of Wu Mountain at and prior to 35ka (age of the top of Wushan loess). From the marine oxygen isotope curve, it can be found that MIS3 is a weak warm period, which is the interglacial stage in the last glacial period, but Shi and Zhao (2009) found that climate was more warm and humid in 30-40ka than that is in present. The reason for this phenomenon was explained by the change of the precession cycle, which caused the solar radiation to increase at 50°N-30°S at 30-40ka, and increased the temperature difference between sea and land, especially deepening the effects of thermal low pressure and heat source of the Tibetan Plateau, and then enhancing the southwest and southeast summer monsoon and westerly winds. This effect may explain the appearance of reticulated red clay in the upper part of the T2 terrace (30-50ka) in the Sichuan Basin and the low illite content and the high illite chemical 32

Journal Pre-proof index in the 2nd terrace in the Three Gorges (Table 3).

7 Conclusions The Electron Spin Resonance (ESR) dating of Quaternary deposits in the Fengjie and Wushan localities within the Three Gorges and in the Yichang area to the east of the Three Gorges, combined with macro-sedimentary, clay mineral and element characteristics of these sediments, indicates a progression in regional paleoclimate. The Yichang and the Three Gorges

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areas have similar paleoclimate trends, although the degree of warmth and moisture in Yichang area was generally slightly greater than in the Three Gorges area.

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1). The early and middle Quaternary was warm and wet during the deposition of the Yunchi

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and Shanxiyao Formations, and the 5th and 4th terraces (1260 ka to 300 ka). There were no

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glacier records found in the Three Gorges and adjacent areas at least before 110 Ka. 2). During the late Quaternary, the climate was drier and colder when the 3rd through 1st

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terraces formed (110-10ka), especially after 30ka.

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3). The paleoclimate mainly was warm and wet from 1260 to 740 ka in the Yichang and Three Gorges, but significant glacier events can be identified at the same period of time in the

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Northern Tibetan Plateau. The dry and cold sedimentary records can only be find in the 3rd to 1st terrace sediments (110-30ka) in the study area, and the paleoclimate change trend can be coincide with the glacier-interglaciar cycle in the Northern Tibetan Plateau. Cold and dry paleoclimate in the Tibetan Plateau had no effect on the area to the west of the Three Gorges before 110 ka. 4). A sharp increase in the East Asian winter monsoon from 1160 to 800 ka caused the distribution of loess in eastern China to expand to the southeast and form the Xiashu loess in the lower reaches of the Yangtze River. In the same period, in the Sichuan and adjacent areas, the distribution of Chuanxi loess and Wushan loess showed that the plateau monsoon strengthened and region of influence enlarged eastward from 1160 ka to 100 ka. After the Gonghe Movement 33

Journal Pre-proof (150 ka) lifting the Qinghai-Tibet Plateau to more than 4000 m, the strengthening cold source of the Plateau may have strengthened the plateau monsoon, and weakened and blocked the Indian monsoon. As a result, the warm and humid climate disappeared in the Sichuan Basin and adjacent areas in the early-middle Pleistocene, and a cold and dry climate appeared in the Sichuan Basin and Three Gorges area resulting in the Wushan loess (starting at 100 ka).

Acknowledgements

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This study was supported by the projects of the National Natural Science Foundation of China (Grant No. 41572093, 41972101, 41072083), the project of Innovation team of

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Sedimentary Geology (Chengdu University of Techology) (KYTD201703) and the Open

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Foundation of Shandong Provincial Key Laboratory of Depositional Mineralization &

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Sedimentary Mineral. The Authors thank Doctor Mingshi Feng in State Key Laboratory of Oil and Gas Reservoir Geology and Exploitation at Chengdu University of Technology for his

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instructive discussions and help for sample testing. The Authors also thank two reviewers very

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Highlights

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Paleoclimate changed from warm-wet to dry-cold in Quaternary in Three Gorges. Paleoclimate in Three Gorges can only correspond with glacier-cycle after 150 Ka. Tibetan Plateau’s Gonghe Movement had a significant impact on paleoclimate. Gonghe movement enhanced the Plateau monsoon and blocked the Indian summer monsoon. Cold-dry climate and loess appeared in Sichuan basin after Gonghe movement.

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