Late Holocene vegetation and climate changes in the Great Hinggan Mountains, northeast China

Journal Pre-proof Late Holocene vegetation and climate changes in the Great Hinggan Mountains, Northeast China Dongxue Han, Chuanyu Gao, Zicheng Yu, Xiaofei Yu, Yunhui Li, Jinxin Cong, Guoping Wang PII:

S1040-6182(19)30848-1

DOI:

https://doi.org/10.1016/j.quaint.2019.11.017

Reference:

JQI 8050

To appear in:

Quaternary International

Received Date: 5 September 2018 Revised Date:

15 September 2019

Accepted Date: 5 November 2019

Please cite this article as: Han, D., Gao, C., Yu, Z., Yu, X., Li, Y., Cong, J., Wang, G., Late Holocene vegetation and climate changes in the Great Hinggan Mountains, Northeast China, Quaternary International, https://doi.org/10.1016/j.quaint.2019.11.017. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Elsevier Ltd and INQUA. All rights reserved.

1

Late Holocene vegetation and climate changes in the Great Hinggan

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Mountains, Northeast China

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Dongxue Hana, b, Chuanyu Gaoa, Zicheng Yuc, d, Xiaofei Yu a, ∗, Yunhui Lia, Jinxin Conga, Guoping

4

Wanga, ∗

5

a

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Agroecology, Chinese Academy of Sciences, Changchun 130102, China

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b

University of Chinese Academy of Sciences, Beijing 100049, China

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c

Department of Earth and Environmental Sciences, Lehigh University, 1 West Packer Avenue,

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Bethlehem, PA 18015, USA

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d

∗

Key Laboratory of Wetland Ecology and Environment, Northeast Institute of Geography and

School of Geographical Sciences, Northeast Normal University, Changchun 130024, China

Corresponding author E-mail addresses: [email protected] (X. Yu), [email protected] (G. Wang).

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ABSTRACT

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High-latitude region is sensitive to global climate change, and pollen record in sediments

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could reflect vegetation variation which responded to climate change. The Great Hinggan

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Mountains are main distribution areas of peatlands in high-latitude region in China and located at

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East Asian summer monsoonal margin, however, the variation of vegetation and climate in this

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region is still unclear. Based on the AMS

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reconstruct vegetation history in Tuqiang (TQ) peatland, and we also used the principal

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component analysis to reconstruct the temperature and effective moisture and compared with other

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palaeoclimate records. Pollen assemblage denoted a coniferous and broad-leaved mixed forest on

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the surrounding mountains, and peatland vegetation gradually evolved from herb community

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(Cyperaceae) to bush community (Ericaceae). The climate of north part in the Great Hinggan

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Mountains was mainly controlled by the East Asian Summer Monsoon which related to

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ocean-atmosphere interactions in the tropical Pacific. During the period from 3300 to 1150 cal yr

24

BP, the dominant plant was Cyperaceae, with coniferous and broad-leaved mixed forests bordered

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the peatland, which pointed to a relatively warm and wet climate. Between 1150 and 600 cal yr BP,

26

the expansion of pine forests indicated a cool climate. The interval of 600-300 cal yr BP, the

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climate became cold and dry which related to the Little Ice Age. Since the 300 cal yr BP, Betula

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and Ericaceae dominated, the climate became warmer and drier. The obvious increase of

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secondary birch forests at the expense of pine forests may be the results of human disturbance,

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anthropogenic activities strengthened gradually and the variation of peatland vegetation was

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influenced mainly by anthropogenic activities rather than by climate since approximately 600 cal

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yr BP.

14

C dating, we analyzed the pollen assemblages to

33

Keywords

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Peatland; Pollen; East Asian monsoon; Anthropogenic disturbance; Little Ice Age; Reconstruction

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1. Introduction

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Climate change mainly depended on natural processes in geological periods, then

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co-determined by natural and human-induced during the Holocene when human civilization

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developed rapidly. The Holocene has become the focus for investigating the global climate change

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and for understanding the forcing mechanisms of climate system (Yu et al., 2002; Yao et al., 2017).

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The driving force of climate change is complex in China because it is located at the margin of

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monsoon which co-influenced by East Asian monsoon and westerlies (Gao et al., 2018), it is

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interesting to estimate the climate change and the possible forcing mechanisms in the climatically

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sensitive region, especially in the margin area of monsoon in northeastern China.

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Great Hinggan Mountains located in a key geographical position at the northern margin of

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the East Asian Summer Monsoon (EASM) and the southern periphery of the permafrost zone,

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which highly sensitive to global climate changes (Zhao et al., 2015; Stebich et al., 2015; Xing et

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al., 2019), are one of the crucial areas of peatlands in China. The previous studies have showed

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that the climate changes were different between the east side and west side in Great Hinggan

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Mountains, and the forcing mechanisms of climate change was more complex (Gao et al., 2018).

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The eastern side of the Great Hinggan Mountains was mainly influenced by the East Asian

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monsoon, while the climate was affected by the westerlies on the western side of the mountains

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(Gao et al., 2018).

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Marsh sediments especially the peats were the crucial records of late Quaternary climate

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change, which deposited in-situ, recorded continuously, and with a high resolution, reliably

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registered regional environment change and wetland development process, as well as regional

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climate change and anthropogenic activities, and had great potential for palaeovegetation,

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palaeoclimate and palaeoenvironment reconstruction. Vegetation as a sensitive indicator which

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responded to climate change, could provide many valuable information of climate change (Zhao et

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al., 2015; Xu et al., 2016). Pollen assemblages were "miniature" of plant which reliably recorded

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the features of ground flora at that time. Different pollen assemblages reflected different plant

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communities, and different plant communities corresponded to different climate environments.

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Pollen records were crucial proxy indicator for reconstructing past plant (Yu et al., 2004).

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Palynological analysis could be used to reconstruct regional vegetation and reveal vegetation

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variability which responded to climatic oscillation such as effective moisture and temperature

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(Zhao et al., 2009, 2011; Wen et al., 2010b; Yu et al., 2017a). Researches on changes of climate

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and environment based on pollen that preserved in strata had become a current international

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important hotspot (Stebich et al., 2015; Zhao et al., 2015; Xu et al., 2016; Zhang et al., 2016; Wu

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et al., 2016; Wen et al., 2017; Yao et al., 2017; Yu et al., 2017a).

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To date, extensive attention had paid to palaeoclimate and palaeoenvironment reconstruction

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in northeast China during the Holocene, mostly concentrated on the Changbai Mountains (Li et al.,

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2011; Chu et al., 2014; Stebich et al., 2015; Xu et al., 2019), Sanjiang Plain (Zhang et al., 2015,

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2016), Hulun Lake (Xiao et al., 2009; Wen et al., 2010a, b), Moon Lake (Wu et al., 2016), Dali

73

Lake (Wen et al., 2017), and so on. However, palaeoclimatic proxy records of the Great Hinggan

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Mountains were relatively poor and insufficient, a detailed vegetation history is urgently needed to

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verify the vegetation succession and how it responded to climate change.

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In the current study, we presented a high-resolution pollen record derived from Tuqiang

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peatland in the Great Hinggan Mountains, northeast China. Based on palynological analysis, AMS

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14

C dating and principal component analysis, we attempted to reconstruct regional vegetation and

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climate changes during the last 3300 cal yr BP, compared with other palaeoclimatic records and

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discussed the possible forcing mechanisms.

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2. Materials and methods

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2.1. Study area and sampling

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The studied peatland (Tuqiang, TQ. 52°56′34.62"N, 122°51′17.46"E) is situated in the

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northern part of Great Hinggan Mountains, Northeast China (Fig. 1), 33 km apart from Mohe

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county, at an elevation of 481 m, belongs to a valley permafrost peatland. A branch of Emuer

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River runs along the southern border of the peatland. It has a cold temperate continental monsoon

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climate, with a long, cold and dry winter from November to April and a short, hot and wet summer

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from July to August, and an ice-free period generally from May to October. The mean annual

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temperature (MAT) is -3.9 °C, with average maximum of 18 °C in July and average minimum

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−29 °C in January. The mean annual precipitation (MAP) is 452 mm and 80% of rainfall occurs

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between May and September (Fick and Hijmans, 2017; Yu et al., 2017b) (Fig. 2). Dominant

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species are consisted of shrubs (Betula fruticosa, Ledum palustre, Vaccinium uliginosum,

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Chamaedaphne calyculata, Salix myrtilloide), sedges (Eriophorum vaginatum) and moss

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(Sphagnum magellanicum, Sphagnum capillifolium), surrounded by Larix gmelinii, Betula

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platyphylla forests on all sides, and a few trees of the same species grow in the site (Yu et al.,

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2017b).

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A 77-cm-long profile (TQ) was collected in November, 2016. From the top down to 40 cm,

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we digged the profile using an iron shovel. On the straight face of the pit, we first isolated a 20 cm

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× 20 cm column of peat by removing material on the either sides. Afterwards, a stainless steel

100

knife was used to slice the sediment samples continuously at 1-cm intervals in the field. Then, we

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extracted samples by drilling the frozen peat layers below the depth of 40 cm. Due to the samples

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are mixture of peat and ice, to get enough samples, 5-cm intervals were used between 40 and 77

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cm of the sediments. A total of 47 samples were collected and packed into tagged sealed

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polyethylene plastic bags, subsequently taken back to the laboratory and storage at -20 °C prior to

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analysis (Xing et al., 2015; Bao et al., 2015; He et al., 2017).

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2.2. Chronology and physicochemistry analyses

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Four samples of TQ profile were selected and submitted to the State Key Laboratory of Loess

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and Quaternary Geology, Institute of Earth Environment, Chinese Academy of Sciences, in Xi’an

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for Accelerator Mass Spectrometry (AMS) 14C dating. Radiocarbon dating results were calibrated

110

into calendar ages before present (0 yr BP＝1950 AD) using the Calib 7.04 software and IntCal 13

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calibration curve (Reimer et al., 2013). The age-depth model was constructed using the ‘Bacon’

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piecewise linear accumulation model (Blaauw and Christen, 2011) in R (R Core team, 2015).

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Subsamples of each peat slice were taken using an aluminum specimen box (volume=17 cm3)

114

and were dried at 105 °C overnight in an oven, weighed, the ratio of dry weight to volume is dry

115

bulk density. A portion of dried subsamples was combusted at 550 °C for 4 h in a muffle furnace

116

and reweighed to determine loss on ignition (LOI) (Heiri et al., 2001; Xing et al., 2015; Liu et al.,

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2019; Xia et al., 2019). The data were both presented as percentages. Dry bulk density and LOI

118

analyses were performed at the Analysis and Test Center, Northeast Institute of Geography and

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Agroecology (IGA), Chinese Academy of Sciences (CAS).

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2.3. Pollen analysis

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Altogether 47 fossil sediment samples were palynologically analyzed following the

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conventional methods (Fægri and Iversen, 1989), preparation of pollen samples in the laboratory

123

involved treatments with hydrochloric acid (HCl), sodium hydroxide (NaOH), hydrofluoric acid

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(HF), a 9:1 mixture of acetic anhydride and concentrated sulfuric acid, sieving with a 10-µm mesh

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screen in ultrasonic bath, and mounting in glycerine (Zhang et al., 2015). A piece of Lycopodium

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spores (27,560 grains/tablet) were added to each sample as tracer at the beginning of the

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pretreatment in order to estimate absolute pollen concentrations (grains/g). Pollen identification

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and counting were carried out under an Olympus BX-53 light microscope with 400 times

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magnification, with the aid of published pollen atlases by Wang et al. (1995) and Tang et al. (2016).

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More than 400 terrestrial pollen grains (440 pollen grains in average) were identified and counted

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per sample. The total sum of terrestrial pollen taxa identified in each sample was used as

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denominator when calculating pollen percentages, while the percentage of Sphagnum was

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calculated based on the sum of terrestrial pollen plus Sphagnum. Pollen diagrams were drawn

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using the Tilia version 1.7.16 and a 10-fold exaggeration was applied to pollen percentages, and

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pollen-assemblage zones were divided based on stratigraphically constrained cluster analysis

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(CONISS).

137

2.4. Numerical analyses

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Numerical analyses were performed using pollen taxa which occurred in at least three

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samples with a percentage of >1%. Total 12 pollen taxa were selected and the analyses were

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carried out on the basis of the square-root transformed pollen percentage data using Canoco 4.5

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(TerBraak and Smilauer, 2003). Detrended correspondence analysis (DCA) was used to determine

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whether linear or unimodal based techniques should be employed in the subsequent ordination

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analysis. The gradient length of the first axis was 0.715 standard deviation (SD) units, which was

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less than 2, showing that the data set has a mainly linear structure and suggesting use of the

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linear-based principal component analysis (PCA). Therefore, PCA was performed to analyse the

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pollen assemblages using inter-species correlations and pollen percentages (Birks and Gordon,

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1985; Chen et al., 2014; Yao et al., 2017).

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3. Results

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3.1. Dating results and physicochemical characteristics

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The chronology of the TQ profile was based on four radiocarbon dates, details information of

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laboratory numbers, test materials and the calibrated ages were showed in Table 1. The lowermost

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part of the profile was dated to 3300 cal yr BP (Fig. 3). The profile can be subdivided into five

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sections from top to bottom: peat with roots (0-10 cm), light brown peat (11-25 cm), dark brown

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peat (26-40 cm), black peat (permafrost) (41-65 cm), silt and clay (66-77 cm). There was an

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increasing trend of loss on ignition from the bottom to the top of the section, ranged between

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10.67% and 96.30%, indicated a higher organic matter content on the top of the profile. Dry bulk

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density declined from 40 cm to the top, fluctuated from 0.06 to 0.61 g/cm3 (Fig. 3).

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3.2. Pollen assemblages in TQ peatland

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A total of 34 families and genera of pollen and spore were identified in the 47 samples from

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the TQ profile, including 9 trees, 3 shrubs, 21 herbs and Sphagnum. Arboreal pollen mainly consist

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of Betula (17.14-50.99%), Pinus (8.33-40.70%), Larix (4.19-18.16%) and Alnus (2.33-18.07%),

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shrub pollen are dominated by Ericaceae (0.71-16.18%) and Salix (0-7.43%), herb pollen was

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mainly from Cyperaceae (0.25-20.79%), Artemisia (1.77-7.93%), Poaceae (0.21-5.45%), and

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Chenopodiaceae (0.22-3.32%), the percentage of Sphagnum was 1.63-48.83%. Pollen

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concentrations range from 1.08×104 to 1.33×106 grains/g. Main pollen taxa (abundance >0.2%)

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were plotted in the diagram (Fig. 4). The pollen diagram of the TQ profile was divided into four

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zones based on the results of pollen assemblages and CONISS analysis.

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3.2.1. Zone 1 (77-28 cm): 3300-1150 cal yr BP

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Pinus, Betula, Larix, Alnus and other arborous pollen predominate pollen assemblages,

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coniferous tree pollen percentages fluctuated from 32.40% to 53.24%, broad-leaved trees pollen

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proportion ranged from 27.80% to 40.60%. Shrub pollen content (1.34-16.63%) was minor, herb

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pollen content (13.26-28.73%) was relatively high, dominated by Cyperaceae pollen

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(3.15-20.79%), with some Artemisia (2.48-6.07%) and Poaceae pollen (0.22-3.77%). The

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proportion of Sphagnum spores was 8.12-35.85%, total pollen concentrations ranging from

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9.77×104 to 1.33×106 grains/g.

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3.2.2. Zone 2 (28-15 cm): 1150-600 cal yr BP

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Coniferous tree pollen percentages (32.88-54.05%) slightly increased, Pinus pollen content

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was 24.27-40.70%, reached the largest values in the whole profile, while broad-leaved trees pollen

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proportion (21.43-37.61%) gradually decreased. Shrub pollen content was 2.17-9.67%, herb pollen

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content was 14.41-26.01%, the percentages of Artemisia and Chenopodiaceae were 3.02-7.21%

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and 0.45-1.63%, relatively. Sphagnum spores content (10.18-42.41%) increased, total pollen

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concentrations (2.92×104 -5.32×105 grains/g) dropped noticeably.

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3.2.3. Zone 3 (15-9 cm): 600-300 cal yr BP

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The percentages of coniferous tree pollen (35.37-46.22%) largely declined, the proportion of

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broad-leaved trees pollen (28.00-48.29%) sharply increased. Shrub pollen content remarkly

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increased (the pollen percentages of Ericaceae and Salix were 1.21-9.62% and 0.24-5.34%,

187

respectively). Herb pollen content reduced rapidly, especially Cyperaceae pollen (3.41-8.89%) and

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Poaceae pollen (0.46-2.22%), the content of Artemisia and Chenopodiaceae was increased,

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4.61-7.93% and 0.23-2.43%, relatively. Sphagnum spores proportions reached a maximum value

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(19.92% -48.83%), total pollen concentrations (3.62×104 -1.94×105 grains/g) decreased.

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3.2.4. Zone 4 (above 9 cm): since 300 cal yr BP

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Arborous pollen proportions significantly rose, the ratio of arborous pollen to non-arborous

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pollen (AP/NAP) reached a maximum peak values. Compared with the previous period,

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broad-leaved trees pollen percentages (50.95-64.78%) continuously increased, with Betula

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(39.57-50.99%) and Alnus pollen contents (9.11-18.07%) were the highest in the whole profile.

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Coniferous tree pollen percentages (18.38-32.22%) distinctly dropped, Ericaceae pollen content

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(2.22-14.69%) increased clearly. Herb pollen content (5.67-11.75%) substantially declined, mainly

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Cyperaceae (0.25-2.89%) and Poaceae pollen (0.21-1.78%), the proportion of Artemisia

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(1.77-6.71%) dropped, while Chenopodiaceae (0.44-3.22%) and Aster (0-0.71%) increased. The

200

proportions of Sphagnum spores (1.63-8.72%) clearly decreased, total pollen concentrations

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(1.08×104 -2.40×104 grains/g) dramatically declined.

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3.3. Results of PCA analysis

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The PCA results based on 12 selected pollen taxa and the total number of samples are shown

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in Figure 5. The first and second principal components explained 52.8% and 15.1% of the 12

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selected pollen assemblages’ variations, respectively, altogether accounted for 67.9% of the total

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variance within fossil pollen assemblages. The 12 main pollen taxa were divided into four groups:

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(1) Ericaceae, Artemisia and Salix; (2) Larix and Sphagnum; (3) Pinus, Cyperaceae, Poaceae and

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Sanguisorba; (4) Betula, Chenopodiaceae and Alnus. Four clusters of samples corresponding to

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pollen assemblage zones are clearly separated from each other on the biplot of PCA scores along

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the first two axes: zone 1 (3300-1150 cal yr BP) characterized by Cyperaceae, zone 2 (1150-600

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cal yr BP) characterized by Larix and Pinus, zone 3 (600-300 cal yr BP) characterized by

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Artemisia and Salix, zone 4 (since 300 cal yr BP) characterized by Betula, Alnus and Ericaceae.

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4. Discussion

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4.1. Vegetation and climate change of TQ peatland

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The modern natural vegetation in TQ peatland is characterized by shrubs and herbs, fifty

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percent of the peat surface is occupied by hummocks covered with continuous moss and shrubs,

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sedge tussocks with hollows cover the rest of the area, forests are distributed on the surrounding

218

mountains (Yu et al., 2017b). Based on the distribution pattern of modern vegetation of TQ

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peatland, we inferred that variations in shrubs and herbs pollen mainly represented the

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development of peatland plants, whereas changes of arborous pollen reflected the forests

221

development on the surrounding mountains (Wen et al., 2010b). The pollen assemblages of TQ

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sedimentary profile reflected a detailed history of vegetation and climate change in TQ peatland

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during the Late Holocene (Fig. 4). PCA analysis could clearly indicate the changes of temperature

224

and humidity (Wen et al., 2010b; Zhao et al. 2015; Xu et al., 2016; Yao et al., 2017). As shown in

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Figure 5, the PCA axis 1 separated the hygrophilic Cyperaceae, Poaceae and Sanguisorba on the

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left from the drought-tolerant Artemisia and Chenopodiaceae on the right. On the other hand, the

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PCA axis 2 separated the cold-tolerant Larix above from the thermophilic Betula and Alnus below.

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This implies that the PCA axis 1 mainly represents effective moisture changes: positive values

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indicate a dry climate, while negative values indicate a wet climate. PCA axis 2 reflects

230

temperature changes: positive and negative values indicate cold and warm conditions,

231

respectively.

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Between 3300 and 1150 cal yr BP, arboreal pollen mainly derived from Pinus and Betula, and

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the proportion of needle trees was a little higher than broad-leaved trees, suggesting a needle and

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broad-leaved mixed forest (needle trees prevailed) on the surrounding mountains. The highest

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percentage of Cyperaceae in herb plants indicated that Cyperaceae was the dominant species in

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peatland. Yu et al. (2017a) suggested high Cyperaceae reflected high moisture levels. The

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percentage of hygrophilous Cyperaceae was the highest in the total grass pollen which implied a

238

wet condition (Fig. 4), PCA axis 1 score curve also reflected the wet climate, PCA axis 2 score

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curve displayed the temperature was moderate (Fig. 6). Overall, we deduced that the climate was

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relatively warm and wet during this period, though fluctuations exist.

241

The interval from 1150 to 600 cal yr BP, the proportion of Pinus increased, coniferous

242

arboreal pollen content appeared its peak value at about 730 cal yr BP, suggested the expansion of

243

pine forests on the surrounding mountains and the cooling of climatic conditions. Wen et al.

244

(2010b) also demonstrated pine forests expanded and birch forests declined marking the

245

temperature fall in the Hulun Lake region. Yao et al. (2017) proposed that Pinus were indicative of

246

low temperature climate. PCA axis 2 score curve further confirmed that temperature declined

247

compared with the previous stage (Fig. 6).

248

During the period of 600-300 cal yr BP, the percentage of Cyperaceae pollen rapidly dropped

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while bushes (Ericaceae and Salix) and Sphagnum increased, marking a transitional period from

250

herbaceous communities to shrub communities. PCA axis 1 and 2 score curves denoted the climate

251

tended to be cold and dry, which corresponded to Little Ice Age (Fig. 6). The broad-leaved trees

252

increased strongly with a drastic reduction of conifers. The currently common Betula platyphylla

253

are drought tolerant and shade intolerant tree species, which are mainly found as pioneer trees on

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sunny slopes (Chen and Li, 2005). Yu et al. (2017a) proposed that frequent burning of forest and

255

grass could facilitate the development of secondary forest. The expanding of birch forest at the

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expense of pine may thus denote drier conditions and/or substantial ecosystem disturbances

257

(Stebich et al., 2015). At this coldest period, a selective lumbering occurred in TQ peatland, people

258

used woods to build houses and make fires during the LIA, meanwhile, logging led to the

259

expansion of secondary birch forests. We presumed that vegetation changes in TQ peatland were

260

mainly caused by anthropogenic activities since about 600 cal yr BP.

261

From 300 cal yr BP to present, broad-leaved trees (Betula and Alnus) proportion was the

262

highest in the entire profile, the advantageous position of Pinus and Larix was replaced by Betula

263

and Alnus (Fig. 4). The expansion of birch forests suggested the climate became warm and dry

264

(Compilatory Commission of Vegetation of China, 1980; Stebich et al., 2015). Bushes kept on

265

increasing, the percentage of Cyperaceae and Sphagnum declined (Fig. 4), denoting a dry

266

condition. PCA axis 1 and 2 score curves exhibited a warm and dry climate (Fig. 6). So there were

267

coniferous and broad-leaved mixed forests which Betula and Alnus flourishing on the mountains

268

and peatland vegetation was predominated by Ericaceae, the climate tended to be warm and dry

269

since about 300 cal yr BP. The continuous reduction of pines and the expansion of birches

270

indicating a clearly intensification of human influence. Increasing historical population levels of

271

Heilongjiang Province supported the proposition of rapidly and obviously intensified

272

anthropogenic activities (Cong et al., 2016).

273

From the above, we speculated that during the last 3300 cal yr BP, Betula, Pinus, Larix and

274

Alnus predominated on the surrounding mountains, denoting a coniferous and broad-leaved mixed

275

forest. Peatland vegetation gradually evolved from Cyperaceae community to Ericaceae

276

community. The climate changed from warm-wet to cool-wet, then cold-dry, final warm-dry.

277

Human beings began to impacting the changes of vegetation and climate since about 600 cal yr

278

BP.

279

4.2. Regional climate change at the EASM margin region

280

TQ peatland is located at the East Asian monsoonal margin region where the regional climate

281

changes are complex during the Holocene (Zhao and Yu, 2012). The late Holocene climate records

282

from TQ peatland are highly comparable with the following proxy records which are also located

283

in the EASM monsoonal domain, such as geochemical records of Hani Peatland, palynological

284

records of sediments in the Hulun Lake, Daihai Lake and Sihailongwan Maar Lake (Fig. 6).

285

During the period between 3300 and 1150 cal yr BP, the smaller scores of PCA axis 2 and 1

286

pointed to a relatively warm and wet climate, showing a close match with pollen-based

287

reconstruction of the mean annual precipitation in the Daihai Lake and Sihailongwan Maar Lake,

288

which revealed high rainfall with a declined trend (Xu et al., 2010; Stebich et al., 2015). The mean

289

annual temperature reconstruction of Hulun Lake and the peat cellulose δ18O records of Hani

290

peatland both marked a warm environment at this period (Wen et al., 2010a; Hong et al., 2009).

291

At about 1000 cal yr BP, PCA axis 2 score reached a peak value, reflecting a higher

292

temperature. Pollen-based mean annual temperature reconstruction in the Hulun Lake also

293

exhibited a higher temperature during the same period (Wen et al., 2010a), which could temporally

294

correspond to the Medieval Warm Period (930-1240 AD). In addition, the peat cellulose δ18O

295

records of Hani peatland recorded a peak value which represented a high temperature at around

296

1100 cal yr BP (Hong et al., 2009), and the δ18O peak value were more remarkable in Jinchuan

297

peatland which was 15 km northwest of Hani peatland (Hong et al., 2001). After the Medieval

298

Warm Period, there was a declining trend of temperature which showed good correspondence with

299

the palaeoclimatic records of Hulun Lake and Hani peatland (Hong et al., 2001; Wen et al.,

300

2010a).

301

During the episode of 600-300 cal yr BP, the highest value of PCA axis 2 scores confirmed

302

the coldest climate, the sedimentary records in the Hulun Lake and Hani peatland also registered a

303

cold period at this time (Hong et al., 2009; Wen et al., 2010a). PCA axis 1 curve in our study and

304

the precipitation curve of Daihai Lake and Sihailongwan Maar Lake all implied the low rainfall at

305

this period (Xu et al., 2010; Stebich et al., 2015). Therefore, the climate was much cold as well as

306

dry at this time, could well relate to the Little Ice Age (1550-1850 AD) (Yu et al., 2017a; Xia et al.,

307

2019).

308

Since 300 cal yr BP, PCA axis 1 and 2 displayed a warm and dry climate. After Little Ice Age,

309

the temperature increased, peat cellulose δ18O records of Hani and Jinchuan peatland also marked

310

an increase trend of temperature after Little Ice Age (Hong et al., 2001, 2009). The pollen-based

311

mean annual temperature reconstruction in the Hulun Lake also provided a modern warming

312

tendency (Wen et al., 2010a.) The mean annual precipitation reconstruction from Sihailongwan

313

Maar Lake and peat cellulose δ13C curve of Hani peatland showed lower rainfall (Hong et al.,

314

2005, 2009; Stebich et al., 2015), the various proxies all implied that the climate tended to be

315

warmer and drier.

316

4.3. Possible forcing mechanisms of climate change at the EASM margin region

317

The EASM plays an important role in the global climate system (Wen et al., 2010b; Stebich

318

et al., 2015; Dong et al., 2015; Xu et al., 2016). TQ peatland is at the north boundary of EASM

319

region, the enhancement and recession of EASM has important impact on the rainfall of study area.

320

The EASM intensity can be reflected directly by rainfall, stronger/weaker EASM circulation

321

transports more/less water vapour from the tropical and subtropical Pacific Ocean, and results in

322

higher/lower rainfall (Chen et al., 2015; Xu et al., 2016). Wen et al. (2010b) demonstrated that

323

temperature change during the Holocene was determined by orbitally induced variations in

324

summer solar radiation in the mid-high-latitude area in Northern Hemisphere. Hong et al. (2001)

325

suggested the thermal state of oceans which showed great correspondence to solar activity could

326

result in drought and humidity conditions by causing perturbations in atmospheric circulation and

327

moisture transport. Ocean-atmosphere interactions play a major role in driving the monsoon

328

dynamic and controlling the rainfall (Wen et al., 2010b; Stebich et al., 2015).

329

From 3300 to 600 cal yr BP, the climate of TQ peatland was humid, the higher sea surface

330

temperature from the western tropical Pacific can lead to a strengthening of monsoon intensity

331

which brought more rainfall to the research region (Hong et al., 2001; Wen et al., 2010b; Stebich

332

et al., 2015). Meanwhile, the record of Sanbao Cave stalagmite oxygen isotopes revealed a strong

333

EASM intensity (Dong et al., 2010). The climate was mainly controlled by East Asian summer

334

monsoon which was related to the ocean-atmosphere interactions.

335

Between 600 and 300 cal yr BP, increasing in the percentages of hematite-stained grains in

336

the North Atlantic provides support for our pollen-based inference of the cold and dry climate

337

which related to Little Ice Age (Bond et al., 1997, 2001). Besides, the stalagmite oxygen isotopes

338

records of Sanbao Cave exhibited a weak EASM strength (Dong et al., 2010). The climate of TQ

339

peatland was mainly controlled by the dry westerly during the LIA.

340

During the latest 300 cal yr BP or so, the climate tended to be warm, and the decrease of the

341

percentages of hematite-stained grains in the North Atlantic supported it (Bond et al., 1997, 2001).

342

The declined EASM intensity led to lower rainfall and the climate became drier (Fig. 7). We

343

presumed the modern warming and drying climate may be influenced more by anthropogenic

344

disturbance with the drastic increase of the Heilongjiang Province population at modern time.

345

5. Conclusions

346

The palaeoclimatic records based on palynological analysis of TQ peatland concluded the

347

history of vegetation and climate changes during the late Holocene in the Great Hinggan

348

Mountains, NE China, showed strong similarities with other palaeoclimatic proxy records in the

349

East Asian summer monsoon region. The climate of north part in the Great Hinggan Mountains

350

was mainly controlled by the East Asian Monsoon which related to ocean-atmosphere interactions

351

in the tropical Pacific. From 3300 to 1150 cal yr BP, there were conifers and broad-leaved mixed

352

forests on the surrounding mountains, Cyperaceae prevailed in the peatland which reflected a

353

relatively warm and wet climate. Between 1150 and 600 cal yr BP, pine forests expanded and the

354

climate became cool. During the episode of 600-300 cal yr BP, Ericaceae increased with the

355

decrease of Cyperaceae, it was a transitional period from humid to dry condition, the climate

356

became coldest at approximately 500 cal yr BP, which could correspond to Little Ice Age. From

357

300 cal yr BP to present, birch forests expanded at the expense of pine forests, the climate became

358

warmer and drier. The sharply reduction of Pinus and the expansion of Betula marked intensified

359

anthropogenic activities (selective deforestation). The anthropogenic disturbance might have a

360

crucial impact on the variation of vegetation and climate since about 600 cal yr BP. Though there

361

still have some shortages, in future, we will associate the fossil pollen records with modern surface

362

pollen assemblages to reconstruct the palaeoclimate quantitatively, also address anthropogenic

363

influences

364

inference/reconstructions.

on

modern

pollen

assemblages

and

how

they

would

affect

climate

365

Acknowledgements

366

The authors gratefully acknowledge the assistance of the Analysis and Test Center of the

367

Northeast Institute of Geography and Agroecology of the Chinese Academy of Sciences. This

368

work was supported by the National Key Research and Development Project (No.

369

2016YFA0602301), and the National Natural Science Foundation of China (No. 41571191,

370

41701217).

371

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530

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531

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532

Figure captions

533

Fig. 1. Locations of Tuqiang peatland in the Great Hinggan Mountains and other sites discussed in this study. 1,

534

Tuqiang Peatland (TQ); 2, Hulun Lake (Wen et al., 2010a); 3, Hani and Jinchuan Peatlands and Sihailongwan

535

Maar Lake in the Changbai Mountains (Hong et al., 2009, 2001; Stebich et al., 2015); 4, Daihai Lake (Xu et al.,

536

2010); 5, Sanbao Cave, Hubei Province, China (Dong et al., 2010).The dashed line is the summer monsoon limit

537

(Chen et al., 2010).

538

Fig. 2. Climate diagrams showing monthly changes in annual precipitation and temperature in the TQ peatland.

539

Climate data was derived from the WorldClim dataset with 1-km spatial resolution (Fick and Hijmans, 2017).

540

Fig. 3. Lithology, loss on ignition (LOI), dry bulk density and age-depth model of TQ sedimentary profile. Zones

541

derived from fossil pollen data.

542

Fig. 4. Simplified pollen percentage diagram of TQ profile. The gray shading is 10× exaggeration of the original

543

data.

544

Fig. 5. PCA ordination of 12 pollen taxa from TQ profile.

545

Fig. 6. Percentages of conifers and broad-leaved trees, historical population of Heilongjiang Province (Cong et al.,

546

2016), PCA axis 1 and 2 curves of TQ profile, compared with Tann of Hulun Lake (Wen et al., 2010a), Pann of

547

Daihai Lake (Xu et al., 2010) and Sihailongwan Maar Lake sediments (Stebich et al., 2015), and peat cellulose

548

δ18O and δ13C of Hani Peatland (Hong et al., 2005; Hong et al., 2009). Horizontal dotted lines bracket the stages

549

characterizing the pattern of changes in the vegetation and climate over the TQ peatland region during the late

550

Holocene.

551

Fig. 7. Comparison of PCA axis 1 and 2 scores curves from TQ peatland with other selected proxy records.

552

Hematite-stained grains (%) from the North Atlantic (Bond et al., 1997, 2001), sea surface temperature (SST, ℃)

553

from the western tropical Pacific (WTP) (Stott et al., 2004), δ18O of Sanbao Cave stalagmite (Dong et al., 2010),

554

July insolation at 45°N (Berger and Loutre, 1991), the residual atmospheric △14C record (~2000-year moving

555

average) (Stuiver et al., 1998).

556

Table 1

557

Calibrated Accelerator Mass Spectrometry (AMS) radiocarbon dates

Depth Lab No.

Calibrated age

(14C yr BP)

(cal yr BP) (2σ range)

Dated material

(cm)

558

AMS 14C age

22

XA-7561

Bulk peat

1065 ± 30

928-1008

36

XA-7562

Bulk peat

1645 ± 30

1516-1618

57

XA-7563

Bulk peat

2020 ± 30

1893-2053

77

XA-7564

Bulk peat

3045 ± 35

3163-3357