Records of the Mid-Brunhes Event in Chinese loess-paleosol sequences

Journal Pre-proof Records of the Mid-Brunhes Event in Chinese loess-paleosol sequences

Xinwen Xu, Xiaoke Qiang, Sheng Hu, Hui Zhao, Chaofeng Fu, Qing Zhao PII:

S0031-0182(19)30903-4

DOI:

https://doi.org/10.1016/j.palaeo.2020.109596

Reference:

PALAEO 109596

To appear in:

Palaeogeography, Palaeoclimatology, Palaeoecology

Received date:

11 October 2019

Revised date:

6 January 2020

Accepted date:

7 January 2020

Please cite this article as: X. Xu, X. Qiang, S. Hu, et al., Records of the Mid-Brunhes Event in Chinese loess-paleosol sequences, Palaeogeography, Palaeoclimatology, Palaeoecology (2020), https://doi.org/10.1016/j.palaeo.2020.109596

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© 2020 Published by Elsevier.

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Records of the Mid-Brunhes Event in Chinese loess-paleosol sequences Xinwen Xu 1,2, Xiaoke Qiang 2*, Sheng Hu1, Hui Zhao 2, Chaofeng Fu 3, Qing Zhao 4 1. Shaanxi Key Laboratory of Earth Surface System and Environmental Carrying Capacity, Northwest University, Xi′an 710127, China 2. State Key Laboratory of Loess and Quaternary Geology, Institute of Earth

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Environment, Chinese Academy of Sciences, Xi′an 710061, China 3. Key Laboratory of Western Mineral Resources and Geological Engineering,

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Ministry of Education of China, Chang′an University, Xi′an 710054, China

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4. School of Cyberspace Security, Xi'an University of Posts & Telecommunications,

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Xiaoke Qiang

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Corresponding author:

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Xi′an 710121, China

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E-mail: [email protected]

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Abstract: The Mid-Brunhes Event (MBE) was a climatic transition characterized by warmer temperatures, smaller ice volume and higher sea levels during interglacials since marine oxygen isotope stage (MIS) 11. Data from numerous long-term sedimentary sequences indicate that the MBE was expressed throughout the whole

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earth surface; however, conflicting evidence in terrestrial sequences from middle/high latitudes of the Northern Hemisphere mean that the event’s existence and global

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synchronicity are a matter of debate. To address this problem, we investigated the

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MBE signal in paleoclimate records from the Chinese Loess Plateau (CLP). Higher

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magnetic susceptibility (χ) since S5 in the Luochuan, Yimaguan, and Lingtai sections,

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and since S4 in the Jingyuan and Xijin sections indicate that East Asian Summer

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Monsoon (EASM) precipitation strongly affected the central CLP since MIS 13, but exerts a relatively weak influence on the northwestern CLP until MIS 11 during the

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past 800 kyr. From MIS 13 to MIS 11, shrinkage of the Northern Hemisphere ice sheet resulted in northward movement of the Intertropical Convergence Zone and elevated sea-level. An ~25 m sea-level rise could have caused the coastline of the Asian continent to shift northwestward by ~50 - 100 km. These changes induced extension of EASM-domain areas into the northwestern CLP from the central CLP. The response to the MBE in loess-paleosol sequence of the CLP was expressed by variations in the geographic extent of EASM-domain areas, rather than changes in the EASM intensity. The MBE emerged at MIS 11 in the CLP, and was globally synchronous.

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Keywords: Chinese Loess Plateau; East Asian Summer Monsoon; sea level change;

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Monsoon domain areas.

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1. Introduction The Mid-Brunhes Event (MBE) was the most pronounced climatic shift in the amplitude and frequency of glacial/inter-glacial cycles during the past 800 kyr. The event occurred between marine oxygen isotope stage (MIS) 13 and 11 (Jansen et al., 1986; Candy et al., 2010; Kemp et al., 2010; Holden et al., 2011; Yin, 2013; Cronin et al., 2017; Barth et al., 2018). Since ~430 ka (MIS 11), the δ18O signal of marine

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sequences has exhibited extreme glacial ‘troughs’ and interglacial ‘peaks’ (Lisiecki

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and Raymo, 2005), and inter-glaciation intervals have experienced warmer

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temperatures (Jouzel et al., 2007), higher concentrations of atmospheric greenhouse

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sea levels (Spratt and Lisiecki, 2016).

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gases (Luthi et al., 2008), smaller ice volumes (Lisiecki and Raymo, 2005) and higher

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This event was originally described by Jansen et al. (1986), and was thought to be restricted to the Equatorial region and the Southern Hemisphere. Over the past 30

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years, data from numerous of long-term marine sequences (Candy and Mcclymont, 2013; Maiorano et al., 2016; Cronin et al., 2017) and terrestrial successions (Sun et al., 2006a; Gupta et al., 2010; Blain et al., 2012; Cronin et al., 2014; Habicht, 2015; Lu et al., 2019) have indicated that the MBE was expressed throughout the Northern Hemisphere. Nevertheless, conflicting evidence for the expression of the MBE has been reported from middle and high latitudes of the Northern Hemisphere, especially in terrestrial records (Candy et al., 2010). For example, the fossil assemblages in British terrestrial sequence indicated that interglacials before the MBE were equally as warm as those after the MBE in east Britain (Candy et al., 2010). Sea surface and

Journal Pre-proof air temperature records from the North Atlantic (40° to 56° N) show no statistical difference between the magnitudes of interglacial temperatures during MIS 19 to 13 and MIS 11 to 1 (Candy and Mcclymont, 2013). The percentage of arboreal pollen in a terrestrial sequence from Tenaghi Philippon, NE Greece, indicates no change in the magnitude of interglacial tree population expansions after the MBE (Tzedakis et al.,

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2006). Biogenic silica content in Lake Baikal imply that terrestrial productivity levels in central Asia during MIS 17 and 15 were as high as those during MIS 11 and 9

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(Prokopenko et al., 2006). Paleoclimate records from the Chinese Loess Plateau (CLP)

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indicate that the East Asian Summer Monsoon (EASM) was stronger during MIS 13

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than during interglacials since MIS 11 (Sun et al., 2006a; Yin and Guo, 2008; Guo et

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al., 2009; Hao et al., 2012; Meng et al., 2018). Despite the clear evidence for the MBE

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was throughout the whole earth surface, it is still unclear whether it was a global or regional climatic transition (Candy et al., 2010; Past Interglacials Working Group of

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PAGES, 2016; Barth et al., 2018). To improve understanding of this issue, constraining the geographic extent of the MBE and investigating the diversity of MBE expressions in different climate systems is critical (Candy et al., 2010; Yin, 2013; Habicht, 2015; Barth et al., 2018). The East Asian Monsoon (EAM) is a central component of the global climate system (An et al., 2015; Wang et al., 2017), and our knowledge of its history has been improved by a series of paleoclimate studies on long-term loess-paleosol sequences in the CLP (An et al., 1990; Ding et al., 1995; Guo et al., 2002; Sun et al., 2006b, 2018; Qiang et al., 2010; Hao et al., 2012; Zhang et al., 2016; Meng et al., 2018; Lu et al.,

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2019; Lu et al., 2020). In this paper, we investigate the records of the MBE in loess-paleosol sequences in different parts of the CLP. The results improve our understanding of the diversity of the expression of the MBE in different climate systems and its global synchronicity.

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2. Data and methods As illustrated in Figure 1, the monsoon direction is southeast-northwest.

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Following the direction of summer monsoon winds and modern mean annual

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precipitation (MAP) isohyets, Luochuan (Hao et al., 2012), Yimaguan (Hao et al.,

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2012), Lingtai section (Sun et al., 2010) in the center CLP, and Jingyuan (Sun et al.,

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2006a) and Xijin section (Zhang et al., 2016) at the northwestern margin of the CLP

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were collected for analysis. All of these loess-paleosol sequences are well dated, and yield proxies for both the summer monsoon and the winter monsoon. In studies of

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loess-paleosol sequences in the CLP, magnetic susceptibility (χ) was an accepted summer monsoon proxy (An et al., 1991; Sun et al., 2006b; Hao et al., 2012; Song et al., 2014; Zan et al., 2018) while grain size was an indicator for winter monsoon (Sun et al., 2006b, 2010; Hao et al., 2012; Wang et al., 2018). As grain size indices, mean grain size was used for Lingtai and Jingyuan section (Sun et al., 2006a, 2010), the >32 μm particle content for Yimaguan and Luochuan section (Hao et al., 2012), and median grain size for Xijin section (Zhang et al., 2016). All these data were normalized based on original data from the references listed above (Fig. S1). The χ − χmin

normalized χ is deduced by the formula normalized 𝑥 = χmax − χmin, where χmax

Journal Pre-proof and χmin is the maximum and minimum χ value in the loess-paleosol sequence respectively. The normalized grain size was deduced by the same formula. Ancient coastline can be predicted roughly using recent digital elevation models (DEM). DEM used in this paper is a subset from the ETOPO 2v2 database (NGDC, 2006), with a bathymetric and topographic resolution of 2 min. The approximate

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position of the coastline at southeastern Asian continent in the geological time can be represented by isohypse/isobaths. For examples, when global sea level was 10 m

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lower than present, coastline at southeastern Asian continent was represented by -10

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m isobaths. In contrast, when global sea level was 10 m higher than present, coastline

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at southeastern Asian continent was represented by 10 m isohypse. Both the -10 m

3. Results

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isobaths and 10 m isohypse can be determined by interpolation from original data.

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The normalized χ values of the Luochuan, Yimaguan and Lingtai loess sections in the central CLP display similar behavior in terms of both the amplitude and timing of variations during the past 800 kyr. The χ values of paleosol units were much enhanced since S5-1, corresponding to MIS 13 (Fig. 2a). In contrast, the normalized χ values of paleosol units in the Jingyuan and Xijin loess sections at the northwestern margin of the CLP were much enhanced since S4, corresponding to MIS 11 (Fig. 2b). The EASM has been enhanced since MIS 13 in the central CLP (Sun et al., 2010; Hao et al., 2012), and since MIS 11 at the northwestern margin of the CLP (Sun et al., 2006a). The chronology of Luochuan and Yimaguan section was obtained through

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grain-size age model (Hao et al., 2012), while Lingtai (Sun et al., 2010) and Jingyuan section (Sun et al., 2006a) employed orbital tuning method to reconstruct the chronology. The time scale of Xijin section (Zhang et al., 2016) was determined by linear interpolation based on magnetostratigraphy ties and the loess/paleosol boundary ages. Although the chronology of these loess-paleosol sequences were obtained

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through different age model/method, but the typical loess-paleosol couplets (Fig. 2) can be compared with each other as discussed in the related publications (Hao et al.,

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2012; Zhang et al., 2016). The chronological uncertainty was only few thousand years

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(Sun et al., 2006a; Hao et al., 2012). Its effect was negligible in discussing the timing

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of MBE, a climate transition at glacial/interglacial time scales.

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The normalized grain size of all the five loess sections displayed similar behavior

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in terms of both the amplitude and timing of variations during the past 800 kyr. The grain size of loess units was much enhanced since L5, corresponding to MIS 12 (Fig.

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2c). The East Asian Winter Monsoon (EAWM) has been enhanced since MIS 12 at both the center and the northwestern margin of the CLP (Sun et al., 2006a, 2010; Hao et al., 2012).

As illustrated in Figure 3, the wavelet power spectra of χ plotted against time-series show that the precession periodicity (23-kyr and 19-kyr) of χ in the loess-paleosol sequences from the CLP was insignificant compared to the “100-kyr cycle” in the past 800 kyr (Sun et al., 2006b, 2018). The precession periodicity initiated at S5 in the Luochuan, Lingtai and Yimaguan sections (central CLP), but at S4 in the Jingyuan and Xijin sections (northwest margin of CLP) (Fig. 3). These

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results indicate that the EASM precipitation strong influenced areas extended into the northwestern CLP from MIS 13 to 11 (Sun et al., 2006a). During MIS 13, when global sea level was 15 m lower than present (Fig. 4, Spratt and Lisiecki, 2016), areas with DEM > -15 m emerged (Fig. 4). During MIS 11, when global sea level was 10 m higher than present (Fig. 4, Spratt and Lisiecki, 2016),

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areas with DEM < 10 m have been inundated (Fig. 4). From MIS 13 to 11, an ~25 m sea-level rise (Spratt and Lisiecki, 2016) could have caused the coastline of the Asian

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continent to shift northwestward by ~50 - 100 km (Fig. 4). Although we have not

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found any directly geological evidence about the coastline migration induced by this

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sea level fluctuation between MIS 13 and 11. Lithology of ZQ2 sedimentary core

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(21.59° N, 115.08° E) drilling from Pearl River Mouth Basin indicated a continental

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sedimentary environment characterized by clay-silt mixed with sand and gravel (Chen et al., 2005). This set of continental deposits was widespread in structural basins in

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the continental shelf of the northwestern South China Sea like Pearl River Mouth Basin (Chen et al., 2005) and Beibu Gulf basin (Yang et al., 1996), and was considered to be deposited during the last glacial times. The continental deposits in ZQ2 sedimentary core terminated at about 8 m where the ESR age was 11.4 ka (Chen et al., 2005). It is consistent with the age of rapid sea level rise in the northwestern South China Sea since early Holocene (Zong et al., 2012; Xiong et al., 2018). The relative sea level curve indicated that the sea level was ~30 m lower than present at early Holocene (Zong et al., 2012; Spratt and Lisiecki, 2016). Both the continental deposits in ZQ2 sedimentary core (Chen et al., 2005) and reconstructed paleocoastline

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using recent digital elevation model, relative sea-level curves, and data of sediment thickness (Yao et al., 2009), indicated that the paleocoastline was ~50 - 100 km southeastern off its present location at about 10 ka. In this context, we considered that a ~50 - 100 km coastline shift between MIS 13 and 11 was reliable, although this distance might be overestimated to some extent without considering the effect of

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isostatic compensation.

4. Discussion

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The main controls on the climate in the CLP are two seasonally alternating

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monsoon circulations: the warm/humid southerly EASM and the dry/cold northerly

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EAWM (Sun et al., 2006b; Hao et al., 2012; An et al., 2014; Wang et al., 2017). In the

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boreal summer, the EASM system transports heat and moisture from warm tropical oceans to the continental interior of East Asia (An et al., 2014; Wang et al., 2017; Shi

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et al., 2018). The increasing pedogenesis enhances soil magnetism, including χ (Zhou et al., 1990; Liu et al., 2004; Song et al., 2018; Ye et al., 2020). The positive correlation between χ of modern soils and mean annual precipitation in the CLP (Lü et al., 1994; Maher et al., 1995, 2016) indicate that χ was closely correlated with the monsoon precipitation in the past (Zhou et al., 1990; An et al., 1991; Sun et al., 2006a; Maher et al., 2016). Thus, the χ values of loess-paleosol sequences in the CLP are an accepted proxy for the intensity of the EASM (An et al., 1991; Sun et al., 2006b; Hao et al., 2012). As the lower δ18O values in the southeast China speleothem record are

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considered to represent a dominant signal of summer monsoon rainfall (Wang et al., 2001; Cheng et al., 2016), the southeast China speleothem δ18O record has often been interpreted as a pure signal of variation in the amount of EASM precipitation during the Pleistocene (Cheng et al., 2009). The “pure” precession periodicity of southeast China speleothem δ18O indicates that the EASM precipitation is characterized by

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precession cycles (Wang et al., 2001; Cheng et al., 2009). Wavelet power spectra of χ plotted as time-series show that meaningful precession periodicity initiated at S5-1 in

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the Luochuan, Lingtai and Yimaguan sections (the central CLP), but at S4 in the

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Jingyuan and Xijin sections (northwestern margin of the CLP). The EASM

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precipitation is considered to have strongly affected the central CLP since MIS 13

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(S5-1) (Balsam et al., 2004; Hao et al., 2012), but exerts a relatively weak influence

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on the magnetic properties of the primary dust materials at the northwestern CLP until MIS 11 (S4, Sun et al., 2006a; Jin et al., 2019). The EASM precipitation strong

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influenced areas increased from MIS 13 to MIS 11. However, there is confusing evidence that the EASM during MIS 13 might have been the strongest in interglacials since 800 ka (Sun et al., 2006b; Yin and Guo, 2008; Meng et al., 2018; Lu et al., 2020). The dissolution of carbonate minerals (calcite and dolomite, Meng et al., 2018) and paleopedological and geochemical data analysis (Guo et al., 2009) in paleosol units in the CLP indicate that the highest EASM precipitation during interglacials since 800 ka occurred in MIS 13. It is not clear why monsoon-domain areas increased while the EASM weakened from MIS 13 to MIS 11. Understanding the cause of this spatial difference is crucial for verification of whether the Chinese loess-paleosol

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sequences record the MBE, and can provide deep insights into the regional inconsistencies of the timing of the MBE (Sun et al., 2006a). Regional monsoons are affected by the specific features of the underlying surface including the land-sea distribution, ocean circulation and land topography (Wang et al., 2017). Sea-level change varies on geological timescales, and is a major process that

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modifies the underlying surface and hence the monsoon system (Wang et al., 2017). For example, global sea level (GSL) fluctuations markedly affect the land-sea

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distribution between the Asian continent and the West Pacific Ocean (Wang et al.,

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2017). In turn, the land-sea distribution influences monsoon climate patterns by

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determining the Earth's albedo, or the atmospheric circulation (Wang et al., 2017).

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During the Last Glacial Maximum, when the GSL was ~130 m lower than at the

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present day (Hanebuth et al., 2009), the Sunda Shelf and the Sahul Shelf emerged (Wang, 1999; Hanebuth et al., 2009), and the coastline in eastern China shifted

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eastward by ~1000 km (An et al., 1991). The results of numerical simulation show that both the monsoon precipitation and the monsoon-domain extent were generally weakened (Yan et al., 2016). The global monsoon precipitation was reduced by about 10%, and the southeastward shift of the Asian monsoon reduced its domain area by 8.7% (Yan et al., 2016). During MIS 5, when the GSL was ~6 m higher than at present (Dutton and Lambeck, 2012), the high lake level in the Baijian Lake Basin (eastern Alxa Plateau, NW China, Li et al., 2018) and relatively high aggradation rate of the Yabulai alluvial fan (southern Alxa Plateau, NW China, Yu et al., 2019) indicate that EASM exerts a strong influence on the amount and pattern of local precipitation

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at the eastern and southern Alxa Plateau, NW China. In contrast, the EASM has retreated from this area in recent times as a semi-arid climate emerged (An et al., 2012; Li et al., 2018; Chen et al., 2019). It is considered that the GSL fluctuations could affect the amount and pattern of the EASM-transported precipitation penetrates inland in the Asian continent.

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The Xijin and Jingyuan sections are on the northwestern margin of the CLP, between 300 and 400 mm MAP isohyets (Fig. 1). The Luochuan, Lingtai and

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Yimaguan sections are on the central CLP, between 500 and 600 mm MAP isohyets

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(Fig. 1). The spatial difference in χ values (Fig. 2) indicate that the boundary of areas

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that EASM precipitation exerts a strong influence on the pedogenesis of the primary

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dust was located between the 400 (Xijin) and 500 mm (Yimaguan) MAP isohyets

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during MIS 13. During MIS 11, the EASM precipitation strong influenced areas shift northwestward extended over the Jingyuan section (~300 mm MAP isohyets). The

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distance of this migration was ~100 km crossing the MAP isohyets (Fig. 1). From MIS 13 to MIS 11, shrinkage of the Northern Hemisphere ice sheet (Lisiecki and Raymo, 2005) caused a northward shift of the thermal equator and the Intertropical Convergence Zone (ITCZ, Chiang and Bitz, 2005). The results of numerical simulation indicate that northward shift of the position of ITCZ could induce the EASM-domain areas shift northward (Yan et al., 2016). Synchronously, the smaller Northern Hemisphere ice sheet resulted GSL rise. As illustrated in Figure 4, an ~25 m sea-level rise (Spratt and Lisiecki, 2016) could have caused the coastline of the Asian continent to shift northwestward by ~50 - 100 km. The shrinkage of the Northern

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Hemisphere ice sheet caused a northward shift of ITCZ (Chiang and Bitz, 2005) and an ~25 m sea-level rise (Spratt and Lisiecki, 2016), and resulted in the EASM precipitation strong influenced areas expanding into the northwestern CLP from the central CLP from MIS 13 to 11 (Sun et al., 2006a). In this context, we suggested that the MBE recorded in the CLP emerged at MIS 11 (Sun et al., 2006a), rather than MIS

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13 (Barth et al., 2018). The grain size of loess units in both the central part and the northwestern margin of the CLP were much enhanced since L5, indicating stronger

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EAWM winds and colder glacials since MIS 12 (Sun et al., 2006b; Hao et al., 2012;

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Li et al., 2020). The land surface temperature range between glacial and interglacial

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intervals, as inferred from the distributions of soil fossil bacterial glycerol dialkyl

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glycerol tetraethers preserved in loess-paleosol sequences in the central CLP,

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increased after 450 ka (Lu et al., 2019). This finding is consistent with the timing of the MBE expressed by emergence of increased-amplitude glacial cycles (with cooler

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glacial and warmer interglacial). Therefore, we suggest that the response to the MBE in loess-paleosol sequences in the CLP was expressed by variations in the geographic extent of EASM-domain areas, rather than changes in its intensity. This finding shows that the MBE could have been expressed in mid-latitude regions in the Northern Hemisphere such as the CLP (Sun et al., 2006a; Lu et al., 2019). The MBE was a globally synchronous climate transition (Kemp et al., 2010; Holden et al., 2011; Yin, 2013). The misinterpretation of the strong developed paleosol since S5-1 (MIS 13) in loess-paleosol sequences in the CLP as the appearance of the MBE can be attributed

Journal Pre-proof to the fact that the χ values of soils in the CLP were affected by multiple factors (Zhou et al., 1990; Liu et al., 2004; Guo et al., 2009; Hu et al., 2013). As illustrated in Figure 3, the χ spectrum shows both 100-kyr “glacial to interglacial” cycles and 23-kyr precession periodicity. This finding indicates that the EASM was influenced by both the Northern Hemisphere ice sheet (Sun et al., 2006b; Hao et al., 2012; Clemens et al.,

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2018) and regional solar insolation (Cheng et al., 2012; Clemens et al., 2018). Numerous studies have proved that the EASM has been enhanced since MIS 13 (S5-1,

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Sun et al., 2006b; Yin and Guo, 2008; Wang et al., 2014; Meng et al., 2018). During

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MIS 13, when global climate was a cooler interglacial climate, paleo-rainfall

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reconstruction based on dissolution of carbonate minerals in the CLP (Meng et al.,

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2018) and numerical simulation (Yin et al., 2014) indicate that the strong EASM was

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a response to the enhanced zonal sea surface temperature (SST) gradients in the Western Pacific Warm Pool region. The enhanced zonal SST gradients in the Western

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Pacific Warm Pool strengthened Walker circulation, increased rainfall on the CLP (Yin et al., 2014; Meng et al., 2018), and overprinted the global signal (strong EASM during a cooler interglacial) in loess-paleosol sequences. However, the low χ values in S5-1 at the northwestern margin of the CLP (Sun et al., 2006a; Zhang et al., 2016) indicate that regional processes (SST gradients in the Western Pacific Warm Pool) could not control the extent of EASM-domain areas that were dominated by the global climate system (Yan et al., 2016), such as the position of the ITCZ (Chiang and Bitz, 2005). In contrast to MIS 11, during MIS 13 the southward extension of the Northern Hemisphere ice sheet (Lisiecki and Raymo, 2005) caused a southward shift

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of the thermal equator and the ITCZ (Chiang and Bitz, 2005), and a larger polar ice volume resulted in a lower GSL (Spratt and Lisiecki, 2016), which in turn induced a southeastward shift of the coastline of the Asian continent (Fig. 4). The grain-size of aeolian deposits in sedimentary drilling core from CLP and Desert in north China indicated that the Siberian High and westerlies strikingly enhanced at 500 ka (Wang et

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al., 2018). This would block EASM-transported moisture shift northward and weakened the monsoon precipitation to enhance pedogenesis in the northwestern CLP.

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central CLP during MIS 13 (Sun et al., 2006a).

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In this context, the EASM precipitation strong influenced areas were restricted to the

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The missing record of the MBE has mainly been reported from terrestrial

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sequences in middle and high latitudes of the Northern Hemisphere, such as Britain

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(Candy et al., 2010), Tenaghi Philippon (Mediterranean, Tzedakis et al., 2006), Lake Baikal (Prokopenko et al., 2006), and the CLP (Barth et al., 2018). It is reliably

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known that fossil assemblages in British terrestrial sequence are controlled by local summer temperature (Candy et al., 2010), vegetation assemblages in the Mediterranean appear to be driven by insolation (Tzedakis et al., 2006), biogenic silica content in Lake Baikal is driven by local (53°N) insolation (Prokopenko et al., 2006), and χ and geochemical parameters in the CLP are dominated by EASM intensity (Sun et al., 2006b; Guo et al., 2009; Hao et al., 2012; Meng et al., 2018). These proxies are reflections of variations in regional climate such as local insolation (Prokopenko et al., 2006) and variations in the Western Pacific Warm Pool (Yin et al., 2014; Meng et al., 2018). As illustrated in Figure 5, The MBE appears to be

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significant only in terms of greenhouse gases and the shift of the Northern Hemisphere ice sheet, and the climatic variables controlled by those gases (Yin and Berger, 2012). There was no systemic difference in solar insolation during interglacials before and after the MBE (Cheng et al., 2016). The reconstructed EASM rainfall intensity was inconsistent with global temperature since 2.6 Ma (Meng et al.,

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2018). Thus, investigation of the expression of the MBE and its global synchronicity by means of records based on proxies that respond dominantly to regional variations

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is problematic.

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5. Conclusions

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Variations in the χ values of five loess-paleosol sequences from the CLP

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demonstrate that the East Asian Summer Monsoon (EASM) during interglacials strengthened since MIS 13 in the central CLP, and since MIS 11 on the northwestern

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margin of the CLP. This spatial difference in the extent of EASM-dominated areas is attributed to the shift of the Northern Hemisphere ice sheet and its induced sea-level fluctuations. During a cooler interglacial MIS 13, although the EASM was strong, the expanded Northern Hemisphere ice sheet and its induced lower GSL caused a southeastward shift of the position of ITCZ and the coastline of the Asian continent respectively, and restricted the EASM-domain areas to the central CLP. In contrast, during MIS 11, shrinkage of the Northern Hemisphere ice sheet resulting in a northward shift of the ITCZ, and higher GSL caused a northwestward shift of the coastline of the Asian continent. These changes induced EASM precipitation strong

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influenced areas expanded into the northwestern margin of the CLP. The response to the MBE signal in loess-paleosol sequences in the CLP was expressed by changes in the geographic extent of EASM-domain areas which mainly affected by the Northern Hemisphere ice sheet extent and GSL change, rather than changes in the EASM intensity. The increased-amplitude of glacial cycles (with cooler glacial and warmer

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interglacial) since MIS 11/12 in the CLP supports the argument that the MBE was a globally synchronous climate transition. The missing record of the MBE in terrestrial

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sequences is considered to have resulted from the complexity of the responses of

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proxies to forcing factors. It is problematically to detect the MBE (a global climate

Acknowledgments

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variations such as insolation forcing.

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transition) signal by means of proxies that are dominantly controlled by regional

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We thank the Editors and two anonymous reviewers for their inspired advice for the manuscript. This study was supported by National Science Fund of China [41402151, 41572164, 41272208], a major research program of the National Natural Science Foundation of China [91855211], Open Fund of State Key Laboratory of Loess and Quaternary Geology [SKLLQG1218]. We thank Lucy Muir, PhD, from Liwen Bianji, Edanz Editing China (www.liwenbianji.cn/ac), for editing the English text of a draft of this manuscript. Data Availability: The data that are presented in this article are available and can be downloaded

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from the journal website in the data repository.

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Figure captions Fig. 1. Map showing the locations of the Luochuan (LC), Yimaguan (YMG), Lingtai (LT), Jingyuan (JY) and Xijin (XJ) loess sections in the Chinese Loess Plateau. Blue dashed lines represent the modern mean annual precipitation isohyets. Fig. 2. Normalized magnetic susceptibility (χ) and grain size plotted against time

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during the past 800 kyr. a, Normalized χ at Luochuan (LC) (Hao et al., 2012), Yimaguan (YMG) (Hao et al., 2012) and Lingtai (LT) (Sun et al., 2010); b,

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Normalized χ at Jingyuan (JY) (Sun et al., 2006a) and Xijin (XJ) (Zhang et al., 2016);

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c, Normalized grain siz; d, LR04 δ18O (Lisiecki and Raymo, 2005).

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Fig. 3. Wavelet power spectra of χ against time during the past 800 kyr. a, LR04 δ18O

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(Lisiecki and Raymo, 2005); b, χ values in loess-paleosol sequence at Luochuan (Hao

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et al, 2012); wavelet power spectra of χ at c, Lingtai (LT); d, Yimaguan (YMG); e, Luochuan (LC); f, Jingyuan (JY) and g, Xijin (XJ).

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Fig. 4. Monsoon domain shifted northwestward when the Northern Hemisphere ice sheet retreated and global relative sea level rose from MIS 13 to 11. a, LR04 δ18O (Lisiecki and Raymo, 2005); b, global relative sea level (Spratt and Lisiecki, 2016); c, areas emerged/inundated predicted roughly using recent digital elevation models (DEM) when global relative sea level fall/rise, ZQ2, a drilling core exhibited continental deposits before early Holocene when sea level was ~30 m lower than at present; d, summer monsoon precipitation strong influenced areas expanded northwestward from central CLP into the northwestern CLP, blue dashed lines represent the modern mean annual precipitation isohyets.

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Fig. 5. Mid-Brunhes Event (MBE) expressed in different climate compositions. a, LR04 δ18O (Lisiecki and Raymo, 2005); b, global relative sea level (Spratt and Lisiecki, 2016); c, Dome C CO2 (Luthi et al., 2008); d, 21 July insolation at 65° N (Berger, 1978); e, Chinese speleothem δ18O (Cheng et al., 2016). Fig. S1. Magnetic susceptibility (χ) and grain size against time during the past 800 kyr. a, χ at Luochuan (LC), Yimaguan (YMG) and Lingtai (LT); b, χ at Jingyuan (JY) and

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Xijin (XJ); c, the >32 μm particle content at YMG and LC (Hao et al, 2012); d, the

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mean grain size at LT and JY (Sun et al, 2006a, 2010), and median grain size at XJ

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(Zhang et al, 2016).

Journal Pre-proof Declaration of interests √ ☐ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

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☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:

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Highlights: 1. Asian Summer Monsoon rainfall did not penetrate into the northwestern CLP until MIS 11; 2. The response to the MBE in the CLP was expressed by the extent of monsoon-domain areas;

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3. The MBE signal do not expressed in proxies respond dominantly to regional variations.

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