Late Holocene vegetation responses to climate change and human impact on the central Tibetan Plateau

Late Holocene vegetation responses to climate change and human impact on the central Tibetan Plateau

Journal Pre-proofs Late Holocene vegetation responses to climate change and human impact on the central Tibetan Plateau Qingfeng Ma, Liping Zhu, Junbo...

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Journal Pre-proofs Late Holocene vegetation responses to climate change and human impact on the central Tibetan Plateau Qingfeng Ma, Liping Zhu, Junbo Wang, Jianting Ju, Yong Wang, Xinmiao Lü, Thomas Kasper, Torsten Haberzettl PII: DOI: Reference:

S0048-9697(19)35362-8 https://doi.org/10.1016/j.scitotenv.2019.135370 STOTEN 135370

To appear in:

Science of the Total Environment

Received Date: Revised Date: Accepted Date:

6 August 2019 1 November 2019 1 November 2019

Please cite this article as: Q. Ma, L. Zhu, J. Wang, J. Ju, Y. Wang, X. Lü, T. Kasper, T. Haberzettl, Late Holocene vegetation responses to climate change and human impact on the central Tibetan Plateau, Science of the Total Environment (2019), doi: https://doi.org/10.1016/j.scitotenv.2019.135370

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Late Holocene vegetation responses to climate change and human impact on the

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central Tibetan Plateau

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Qingfeng Ma,1 Liping Zhu,1,2,3* Junbo Wang,1,2, Jianting Ju,1 Yong Wang,4 Xinmiao L ü ,1,2

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Thomas Kasper5 and Torsten Haberzettl6

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1Key

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Tibetan Plateau Research (ITP), Chinese Academy of Sciences, Beijing 100101, China

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2CAS

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3 University

Laboratory of Tibetan Environment Changes and Land Surface Processes (TEL), Institute of

Center for Excellence in Tibetan Plateau Earth System, Beijing 100101, China of Chinese Academy of Sciences, Beijing 100049, China

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4 Key

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Basin, School of Geography and Tourism, Anhui Normal University, Wuhu 241002, China

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5Physical

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Germany

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6Physical

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Greifswald, Germany

Laboratory of Earth Surface Processes and Regional Response in the Yangtze-Huaihe River

Geography, Institute of Geography, Friedrich-Schiller-University Jena, 07743 Jena,

Geography, Institute of Geography and Geology, University of Greifswald, 17489

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Abstract

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Understanding long-term environmental changes under natural and anthropic forces is helpful for

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facilitating sustainable development.

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Tibetan Plateau to investigate the impacts of climate and human activities on alpine vegetation

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during the late Holocene, based on a 162-cm-long lacustrine sediment core collected from Tangra

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Yumco. Palynology, charcoal and minerogenic input reveal variations of climate and human

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activity during the past 3400 cal yr BP. Our results show that alpine steppe dominated by

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Artemisia, Cyperaceae and Poaceae was present in the Tangra Yumco area during the entire

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covered period. Only minor human activities are visible between 3400 and 2300 as well as from

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1700 to 400 cal yr BP, when vegetation was mainly influenced by climate. Although human

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activities (presence/grazing) became more intensive between 2300 and 1700 cal yr BP

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corresponding to the Zhang Zhung Kingdom, vegetation change is still mainly affected by a more

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arid climate. Strongest human influence on vegetation was found after 400 cal yr BP, when

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vegetation composition was altered by farming and grazing activities. Our records indicate human

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activities did not have significant impacts on alpine environment until the past few centuries at

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Tangra Yumco on the central Tibetan Plateau.

Here we present a sedimentary record from the central

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Key words

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Pollen analysis, climate reconstruction, human-indicator taxa, late Holocene, Tangra Yumco

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Introduction

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Knowledge of long-term environmental changes is helpful for facilitating sustainable development

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(Mercuri and Florenzano, 2019). The understanding of human impacts on shaping the landscapes

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in the present and past is crucial (Mercuri and Florenzano, 2019; Mercuri and Sadori, 2014). For 2

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this aim, studies focusing on long-term human impacts based on environmental data by using

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various approaches have been carried out in different parts of the world, such as Central and South

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America (Flagg, 2018; Peri et al., 2018), Europe (Luelmo-Lautenschlaeger et al., 2018;

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Florenzano, 2019) and China (Liu et al., 2018; Xu et al., 2018). Human production and life are

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largely based on plants (Mercuri et al., 2018) and in turn cause the variations in the plant species

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composition and vegetation cover. Changes in vegetation cover, either due to climate variability or

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human impact, can influence on climate through feedback mechanisms by altering land surfaces

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themselves and the entire energy budget of an area (Shen et al., 2015; Chen et al., 2013; Zhang et

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al., 2013). Therefore, reconstructing environmental change sequence formed under natural and

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anthropic forces is essential for future predictions and sustainable development.

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Generally, it is a challenge to distinguish human impacts from the effects of climate change

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(Kouli et al., 2018; Kramer et al., 2010; Zhang et al., 2010). Palynology provides an appropriate

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method to analyze vegetation dynamics and changes in the assemblages as response to climatic or

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anthropogenic effects (Edwards et al., 2017; Zhang et al., 2010; Li et al., 2008; Liu et al., 2008).

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Hitherto, numerous studies have detected human impacts on vegetation since the mid to late

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Holocene based on palynological data. In the Mediterranean region, for example, human impact is

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difficult to detect in pollen spectra during the early Holocene, probably because human activity

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did not affect regional pattern of vegetation (Mercuri and Sadori, 2014). During the mid and late

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Holocene, however, both climate and human activities have been the key factors on determining

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vegetation changes (Mercuri and Sadori, 2014; Hoelzmann et al., 2001; Oldfield et al., 2003;

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Zolitschka et al., 2003). In the South America, Flantua et al. (2016) recently discussed evidence

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for human land use and provided an overview seen as important in separating climatically from 3

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anthropogenically driven vegetation change, based on 60 vegetation (pollen) records from across

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the region. They found that human impacts were present in most records during the last 2000

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years by using a wide range of indicators (e.g. appearance of introduced taxa, deforestation, crops

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and indicator of overgrazing).

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An “indicator species approach” is often used to trace human activity in palynological

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records. Indicators for human activity include deforestation (loss of tree taxa), appearance of

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introduced taxa (e.g. Olea, Juglans, Castanea, Eucalyptus, Pinus, Rumex), crop taxa (e.g. cereal

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pollen), presence/elevated abundance of species known as possible disturbance taxa (e.g. Cecropia,

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Chenopodiaceae, Humulus, Plantago)(Kouli et al., 2018; Mercuri et al., 2013; Flantua et al., 2016;

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Wischnewski et al., 2011). Furthermore, several studies form Europe summed some plant taxa to

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develop the indexes for human indicators (Kouli, 2015; Mercuri et al., 2013; Mazier et al., 2009).

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However, there are different human indicator taxa in different region. For example, cereal pollen

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grains are often found in fossil records from farming areas, while they are not relevant in the

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pastoral areas, such as most parts of the Tibetan Plateau (TP) (Ma et al., 2019; Li et al., 2019).

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Another challenge is the ambiguous interpretation of certain pollen types, e.g., high abundance of

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Chenopodiaceae may either reflect a decrease in moisture (Ma et al., 2017a, b; Zhang et al., 2012)

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or stronger human disturbance (Miehe et al., 2014; Zhang et al., 2010) in these semiarid to arid

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regions. Thus, such data have to be interpreted with great care, especially when studying records

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from areas where these taxa are used to trace human activity.

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A further valuable proxy to determine human impact on a landscape, via the interpretation of

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human induced fire, is charcoal, which is produced by an incomplete burning of plants (Miao et al.,

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2019; Xiao et al., 2017; Zhang et al., 2010; Bowman et al., 2009; Patterson et al., 1987). Sharp 4

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increases in microcharcoal concentration in palaeoenvironmental records have been used to

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indicate regional fire occurrence caused naturally by aridification or intentionally by strong

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human-related burning activities. Microcharcoal records from fossil records can provide the

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opportunity to explore climate change and human activity in the past (Miao et al., 2019; Jaffé et al.,

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2013).

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Human activities have profound impacts on the environment, not only in the low altitudes but

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also in the high altitudes. In the Alps, for example, Gilck and Poschlod (2019) found that alpine

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farming began in the Bronze Age (2200-800 BC) in different parts and high-altitude pasture use

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began from 4500 BC, by reviewing the archaeological, linguistic and archaeobotanical studies. On

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the TP, palynological studies also detected human influence on vegetation since the mid-late

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Holocene (Herzschuh et al., 2014; Miehe et al., 2014; Wischnewski et al., 2014, 2011; Kramer et

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al., 2010; Miehe et al., 2009; Schlütz and Lehmkuhl, 2009). However, some studies did not detect

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human impacts, even on the eastern TP (Li et al., 2019; Shen et al., 2006). Furthermore, most of

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these studies targeted sites on the eastern TP, while records from the central and western parts are

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quite rare. This scarcity might be explained by either inappropriate palynological indices for

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human activities, or by only a minor signal of human activity in this high altitude region on the

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central-western TP. Therefore, more studies from the TP are needed to understand human

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influence on the environment.

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For the central TP, one of the most and best investigated archives for environmental change

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is Tangra Yumco. This lake is one of the cultural centers of the ancient Zhang Zhung kingdom

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(Bellezza, 2008; Zhang and Shi, 2016), which spans approximately the period of 2300-1300 cal yr

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BP (Zhang and Shi, 2016). There has frequent human activities, which was proved by founds of 5

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stone tools, a megalith assemblage, abandoned buildings and fields as well as irrigation channels

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(Miehe et al., 2014). Further, studies on peat and lake sediments, as well as paleo-shorelines and

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beach ridges revealed that Tangra Yumco has experienced distinctive Holocene climate-related

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lake level changes (Wang et al., 2017a; Ahlborn et al., 2016; Rades et al., 2015, 2013) with a

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decreasing trend of moisture availability since the Mid-Holocene (Ma et al., 2019; Ahlborn et al.,

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2017). Based on a sediment core from a recessional lake terrace at 4700 m a.s.l., 160 m above the

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present lake level of Tangra Yumco, Miehe et al. (2014) found out that Artemisia steppe

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dominated and human influence increased during the Holocene.

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In this study, we present a late Holocene palynological and charcoal record from the deepest

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part of Tangra Yumco. These data are combined with other paleohydrological records and the

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minerogenic input from Tangra Yumco, which was previously presented as a small part of a

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composite record spanning the past 17500 cal yr BP from Tangra Yumco (Ahlborn et al., 2017).

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By applying this multi-proxy approach, the main aim is to disentangle the climate and human

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influence on the alpine vegetation during the late Holocene.

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Study area

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Tangra Yumco (30°45’-31º22’ N, 86°23’-86°49’ E; Fig. 1) is located at the northern flank of the

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Gangdise Mountain. Its basin belongs to a north-south trending graben (Cao et al., 2009). The

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western mountain range has peaks up to 6132 m a.s.l. (Institute of geography, 1990). Tangra

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Yumco has no outflow but is fed by two major rivers, which drain into the lake from the west and

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southeast. Quaternary deposits are widely distributed on the lake shore (Wang et al., 2017b). Well

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preserved Holocene palaeo-shorelines occur up to ~185 m above the recent lake level of 4545 m

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a.s.l. (Rades et al., 2013), while poorly preserved palaeo-shorelines going back to the Pleistocene 6

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are exposed up to >260 m (Kong et al., 2011). It is one of the largest and deepest lakes on the TP

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with an area of 835.3 km2 (Wang et al., 2017b) and a maximum depth of 230 m (Wang et al.,

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2017b; Haberzettl et al., 2015). The climate of this lake basin is mainly influenced by the Indian

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Summer Monsoon (ISM) (Miehe et al., 2014). The mean annual temperature is 0-2 ℃ and mean

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annual precipitation is 200-250 mm, dominated by the summer rainfall (Institute of Geography,

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1990).

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Alpine steppe and meadow are the two major vegetation types in the basin. Alpine steppe

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mainly occupies the catchment area at altitudes below 4900 m a.s.l., dominated by Stipa purpurea

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along with Orinus thoroldii, Artemisia wellbyi, A. stracheyi, S. basiplumosa. Alpine meadow

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mainly consisting of Kobresia pygmaea and Festuca ovina occurs at altitudes of 4900-5300 m a.s.l.

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Sparse alpine vegetation mainly exists above 5300 m a.s.l., consisting of Saussurea spp., Ajania

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purpurea, Rhodiola, Saxifraga (Miehe et al., 2014; Tibetan Investigation Group, 1988). Crops

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(barley, rape and turnip) can be found at small areas of farmland near the villages to the north and

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northeast of Tangra Yumco (Fig. 1b).

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Figure 1. Study site. (a) Location of Tangra Yumco (red circle) and comparison sites referenced in the text

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(dark circles): 1. Nam Co (Kasper et al., 2012), 2. Basomtso (Li et al., 2017), 3. Lake Naleng (Kramer et al.,

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2010), 4. Lake Dongerwuka (Wischnewski et al., 2014), 5. Lake Ximen (Herzschuh et al., 2014). (b)

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Sampling site of Core TAN 10/4.

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Materials and methods

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Based on a seismic survey (Akita et al., 2015; Wang et al., 2010), the 162-cm-long gravity core 7

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TAN 10/4 (31°15.15’ N, 86°43.37’ E; Fig. 1b) was taken at 223 m water depth in the northern part

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of the lake. The core location was chosen because it was expected to be well suited to capture

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anthropogenic impact, as the short distance to villages and farmlands and the steep morphology of

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the lake basin make it easy for crop pollen and charcoal from human burning activities to be

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transported there (Fig. 1b).

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An age-depth model for core TAN10/4 was developed by Henkel et al. (2016). The original

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length of core TAN10/4 was 162 cm. Event-related deposits were excluded from the cores, which

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were regarded as reworked material of single events. Event layers were identified by their

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lithology, magnetic susceptibility, grain size, water content and Ti content (Akita et al., 2015;

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Henkel et al., 2016). Finally, a sediment profile with an event corrected total length of 130 cm was

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created. Event corrected composite depth (ECCD) was used hereafter. Six radiocarbon ages

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(including one age of wood) for core TAN 10/4 were obtained (Henkel et al., 2016). The age of a

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modern water plant (2070±40 BP) was used for reservoir correction (Henkel et al., 2016), which is

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in the same range as the ages of the sediment-water interface and surface lake sediments (Wang et

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al., 2017b; Haberzettl et al., 2015). Some outliers of ages were excluded from the chronology,

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which were thought too old in comparison to nearby ages in stratigraphic order and alter the

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sedimentation rate without any corresponding changes in the lithology (Haberzettl et al., 2015;

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Henkel et al., 2016; Ahlborn et al., 2017). Therefore, the chronology was established by a linear

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interpolation between youngest median ages in stratigraphic order (Fig. 2). Optically stimulated

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luminescence (OSL) ages of core TAN 10/4 yielded an age offset of approximately 2000 years

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compared to the uncorrected radiocarbon ages (Long et al., 2015). Furthermore, the paleomagnetic

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secular variation (PSV) record of the past 3400 cal yr BP in this core based on the age-depth 8

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model above is in good agreement with the Lake Baikal record, PSV stack for East Asia, as well

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as with the predictions of geomagnetic field models (Henkel et al., 2016; Haberzettl et al., 2015).

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These results support the chronology and the assumption of a constant reservoir effect. For this

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study, 29 samples were subsampled for palynological analysis, each of which had a volume of 4-6

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ml.

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Figure 2.

Chronology of core TAN 10/4 as published in Henkel et al. (2016).

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Table 1. Radiocarbon ages for core TAN 10/4 as published in Henkel et al. (2016) and Haberzettl et al.

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(2015).

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Pollen samples were treated with 10% HCl, 10% NaOH, 40% HF and acetolysis treatments

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and sieved over a 7 μm mesh to remove clay-sized particles. Pollen samples were counted under a

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Zeiss light microscope at 400x magnification. Pollen identifications followed regional guidelines

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from the appropriate references (Wang et al., 1995; Xi and Ning, 1994). More than 300 terrestrial

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pollen grains per sample were counted. Pollen taxa with percentages > 0.5% in at least two sample

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were used in the pollen diagram and ordination analysis. Spores of Sporormiella, Glomus and

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charred particles >10 μm were counted in each sample. Sporormiella and Glomus are presented as

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percentages based on a pollen sum. Charcoal values are presented as concentration (particles/ml).

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Pollen zonation of core TAN 10/4 is based on the constrained incremental sum of squares

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(CONISS) cluster analysis in the Tilia program (Grimm, 2004). A principal component analysis

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(PCA) was used to study the variation in biological assemblages (Birks, 1995). To stabilize

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variances and to optimize the ‘‘signal to noise’’ ratio in the data set, a square root transformation 9

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was applied to the pollen percentage data prior to the PCA. Previous studies revealed that there

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were no trees in the late Holocene on the central TP (Ma et al., 2014; Miehe et al., 2014).

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Therefore, arboreal pollens in the fossil should be transported long distances by air masses from

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forested region. We thereby deducted the arboreal pollen types from pollen assemblages and

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analyzed the principal of pollen types of shrubs and herbs. These analyses were made using the

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Canoco software program (ter Braak and Smilauer, 2002; ter Braak, 1988).

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Semi-quantitative K-values from Core TAN10/4 were obtained using an ITRAX XRF core

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scanner at 0.2 mm resolution. Details on the scanner settings and statistic data treatment are given

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elsewhere (Ahlborn et al., 2017; Henkel et al., 2016). For a better visualization of trends in the record,

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the data were smoothed using the LOWESS at the span of 0.1.

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Results

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Palynological record of core TAN 10/4

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Pollen percentages and concentrations of 26 taxa (abuandance > 0.5% in at least two sample) are

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shown in Fig. 3 and Fig. S1. The pollen record is dominated by herb types, especially Artemisia,

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Cyperaceae and Poaceae. Other herb types mainly include Chenopodiaceae, Brassicaceae,

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Lamiaceae, Ranunculaceae, Fabaceae, Saxifraga, Caryophyllaceae, Polygonum, Crassulaceae, and

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Urtica. Tree and shrub pollen types are dominated by Pinus, Betula, Quercus-evergreen, Picea,

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Tsuga, Ephedra, and Rosaceae. The pollen percentage diagram of core TAN 10/4 can be divided

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into three zones, based on the CONISS cluster analysis (Fig. 3).

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Figure 3.

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plotted on the age scale. Pollen taxa with relatively low percentages have been magnified five times.

Pollen percentage diagram of the selected taxa,spores and charcoal concentration from TAN 10/4

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In Zone 1 (127-65 cm, 3440-1700 cal yr BP), herb assemblages are characterized by high

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Artemisia (mean 50.9%) and Cyperaceae (23.2%). Chenopodiaceae (1.2%) pollen percentages

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were lowest for the entire core. Also the major tree pollen taxa, Pinus (3.4%) and

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Quercus-evergreen (1.1%) are highest of the entire record (Fig. 3). Charcoal concentration is 1631

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grains/ml on average. Glomus (1.0%) and Sporormiella (1.8%) are low. This zone could be

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divided into two subzones: Subzone 1a (3440-2300 cal yr BP) and Subzone 1b (2300-1700 cal yr

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BP). Compared with the Subzone 1a, pollen assemblages of Subzone 1b are characterized by low

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Pinus percentage and highest Cyperaceae percentage. Charcoal is higher than in Subzone 1a.

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In Zone 2 (65-23 cm, 1700-400 cal yr BP), Artemisia percentage (60.1%) shows the highest

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values in the whole record. Cyperaceae (20.9%), Pinus (2.8%) and Quercus-evergreen (0.1%) are

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lower than in Zone 1. Charcoal concentration decreases to 1247 grains/ml. Glomus (1.3%) and

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Sporormiella (2.1%) have no significant changes.

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Zone 3 (23-0 cm, 400 cal yr BP-present) is characterized by high percentages of Brassicaceae

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(1.6-12.8%, mean 5.5%) and a rapidly decreasing trend of Artemisia. Cyperaceae decreases to

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13.7% at 150 cal yr BP, and increases thereafter to the initial value. Urtica and Chenopodiaceae

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show distinctly higher values than Zone 2. Other taxa, such as Polygonum, Poaceae and Pinus,

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increases only slightly. Charcoal concentrations, Glomus and Sporormiella rapidly increase in this

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period. The transition from Zone 2 to 3 is the most intensive change in the entire record.

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Ordination of pollen assemblages

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PCA results show that the first two principal components capture 52.9% (PC 1: 38%, PC 2: 14.9%)

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of the total variance in the pollen data (Fig. 4). PC 1 mainly exhibits high negative values for

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Brassicaceae, Chenopodiaceae and Urtica, and high positive values for Artemisia, Ephedra and 11

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Cyperaceae. PC 2 mainly separates steppe and desert taxa (Artemisia, Chenopodiaceae,

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Caryophyllaceae, Ephedra) from meadow taxa (Cyperaceae, Lamiaceae, Saxifraga, Rosaceae).

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Figure 4.

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K record

PCA result for pollen percentage data from Core TAN 10/4.

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The K record of core TAN 10/4 was previously presented as a small part of a composite

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record spanning the past 17500 cal yr BP from Tangra Yumco (Ahlborn et al., 2017). As no

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details are visible in this composite record, K values of TAN 10/4 are presented in detail in the

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follows. From 3300 to 2300 cal yr BP, the record shows high K values (Fig. 5). During 2300 and

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2000 cal yr BP, the K content decreases and is stable thereafter until 1800 cal yr BP. Between

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1800 and 1300 cal yr BP, values are slightly elevated whereas until 700 cal yr BP the signal is

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rather stable. Subsequently, the lowest values of the entire sequence are reached at around 400 cal

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yr BP. After that, K shows a rapid increasing trend towards recent times.

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Discussion

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Proxies for changes in climate and human activity

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For Tangra Yumco, K has been shown to be highly positive correlated to Fe, Ti and Rb since

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17500 cal yr BP (Ahlborn et al., 2017). As reported in other paleoenvironmental studies based on

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lake sediments from the TP, these elements usually reflect the allochthonous, minerogenic input

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into lakes (Xu et al., 2019; Gyawali et al., 2019; Kasper et al., 2015, 2012). This input is

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considered to be related to surface runoff resulting from precipitation in the catchment brought by

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the ISM on the central TP (Kasper et al., 2015, 2012). As the representative for minerogenic input,

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K is thus used to indicate the intensity of the ISM in this study. 12

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According to our field investigation, the Tangra Yumco basin hosts the world’s highest

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altitude agriculture of barley, rape and turnip. Although these farmland areas are small in the

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northern and northeastern part of the basin (Fig. 1b), our sediment record contains high

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percentages of the related pollen taxa. The significantly high percentages of Brassicaceae in the

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upper part of the record are very likely caused by farming of crops (rape/turnip). Urtica is found in

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wasterlands around settlements and grazing areas (Miehe et al., 2014), which can be an indicator

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for human activity.

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Chenopodiaceae is one common herbaceous type found in samples from steppe and desert

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zones (Ma et al., 2017a; Zhang et al., 2012; Zhang et al., 2010). On regional scale,

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Chenopodiaceae percentages rise along with increasing aridity (Ma et al., 2017a, b; Zhang et al.,

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2012; Zhang et al., 2010). However, human disturbance can also lead to an increase in

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Chenopodiaceae percentage in steppe regions (Miehe et al., 2014; Zhang et al., 2010; Liu et al.,

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2006). Thus, changes in Chenopodiaceae percentage can be related to both human activities and

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climate change, which is needed to be separated through further analysis combined with other

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proxies. As K indicates more precipitation in the area since 400 cal yr BP, high Chenopodiaceae

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percentages in this period are interpreted as vegetation degradation under the impact of human

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activities.

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Fossil charcoal records from lake sediments can provide information about the fire history

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and its relationship to changing climate and vegetation (Xiao et al., 2017; Miao et al., 2016;

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Rimmer et al., 2015; Gavin et al., 2007). Microcharcoal concentrations are mainly controlled by

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climate on long time scales (Miao et al., 2019, 2016; Xiao et al., 2017), and by a combination

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action of climate and human disturbances during the late Holocene (Xiao et al., 2017). As this 13

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study focuses on the late Holocene, high microcharcoal concentrations are used to indicate more

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intense human activities or aridification. However, the first two axes of PCA can be used to

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distinguish between human or climate-induced fires. PC 1 (Fig. 4) distinguishes human-indicator

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taxa (e.g., Brassicaceae and Urtica) from natural vegetation types (Artemisia and Cyperaceae).

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Therefore, PC 1 is interpreted as an indicator for human activity. PC 1 shows low values when it is

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related to intensive human activity and vice versa. PC 2 separates regional steppe and desert taxa

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from meadow taxa. Thus, PC 2 can be served as a moisture indicator. PC 2 shows positive values

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related to humid conditions and negative values related to arid conditions.

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Figure 5. Charcoal, Brassicaceae, spores (Sporormiella and Glomus), PC 1, PC 2 and K data for Tangra

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Yumco compared with lake level record of the lake (Wang et al., 2017a) and paleoclimate records in the

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ISM-dominated region: Basomtso (Li et al., 2017), Nam Co (Kasper et al., 2012) and Arabian Sea (Anderson

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et al., 2002).

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Late Holocene variations in climate and human activity

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The pollen record is dominated by Artemisia, Cyperaceae and Poaceae in the entire sequence (Fig.

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3), which indicates that Artemisia steppe dominated the Tangra Yumco basin during the past 3400

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cal yr BP.

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Between 3400 and 2300 cal yr BP, the lowest charcoal concentration and high PC 2 values

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reflect a relatively humid environment. This is supported by the minerogenic input, which

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indicates more precipitation caused by a strong ISM (Fig. 5). Contemporaneous high values in PC

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1 and low Sporormiella values reveal only minor human activities and low grazing intensity (Fig. 14

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5).

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by a sedimentary record from Nam Co (Kasper et al., 2012).

For the same period, humid conditions with an intense ISM on the central TP is also proved

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From 2300 to 1700 cal yr BP, climate shows a significant drying trend indicated by

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decreasing values in PC 2. This is also proved by decreasing K content, which coincides with the

316

paleohydrological record from Nam Co on the central TP (Fig. 5, Kasper et al., 2012). Human

317

activities became a little stronger in this period, reflected by the decreases in PC 1. This phase

318

corresponds to the Zhang Zhung kingdom period (Huo, 1997), when the Tangra Yumco basin has

319

evolved to one of the centers of the ancient kingdom (Zhang and Shi, 2016). However, our pollen

320

record does not show intense agricultural activities at that time. Slight increases in Sporormiella

321

revealed the occurrence of grazing influence. Therefore, relatively high charcoal concentrations

322

and Chenopodiaceae percentages is suggested to result partly from human disturbance but mostly

323

from the region due to a weakening in the ISM.

324

Between 1700 and 400 cal yr BP, climate gets more arid reflected by decreasing values in PC

325

2, which is supported by low minerogenic input into the lake. A reduced minerogenic input into

326

Nam Co and a lower lake level support the aridity on the central TP (Kasper et al., 2012). In this

327

period, human activities became weaker reflected by high PC 1 values and low charcoal

328

concentrations.

329

Since 400 cal yr BP, increasing PC 2 values indicate moister conditions in the lake basin.

330

Increases in lake level of Tangra Yumco in recent centuries inferred from a record of n-alkanes

331

also indicate a more humid environment (Wang et al., 2017a). Similar tendencies towards moister

332

conditions can be observed in other sediment records from TP (Fig. 5), such as at Nam Co (Kasper

333

et al., 2012) and Lake Basomtso (Li et al., 2017). This is also reported in the fossil record from 15

334

Arabian Sea (Anderson et al., 2002). Increasing Globigerina bulloides abundance during the past

335

four centuries from the Arabian Sea (Fig. 5) revealed that monsoon wind strength increased

336

(Anderson et al., 2002). Enhanced ISM can bring more moisture into the Tangra Yumco basin and

337

thus make its environment to be more humid. During this period, strongest human activities are

338

reflected by sharply decreasing PC 1 values. Higher minerogenic input (increasing K content) and

339

Glomus peaks reflected the strong erosion probably due to the combined action of increasing

340

moisture condition and strong human impacts. High Sporormiella abundance indicates the

341

strongest grazing influence in the whole record. Extremely high percentages of Brassicaceae are

342

probably due to rape and/or turnip cultivation in the basin. Brassicaceae percentage also showed a

343

peak of > 10% in another study in the basin (Miehe et al., 2014), which is in good agreement with

344

our results. According to these data, it is suggested the agriculture was becoming more important

345

in the Tangra Yumco catchment after 400 cal yr BP. Due to the Feudal Serf System in Tibet and

346

progress of agricultural production technology during the Ming and Qing Dynasties, the extent of

347

cultivated land expanded from the Mid-Brahmaputra River and Three-River area in southern and

348

eastern Tibet to the more central region (Wang and Chen, 2014). This might correspond to the

349

relatively higher Brassicaceae percentages at Tangra Yumco core in this period, indicating that

350

farming became a wide spread phenomenon on the TP during this time. Remarkable high values

351

of charcoal concentration, Urtica, Chenopodiaceae percentage also indicate increasing intensity of

352

human impact on the natural environment, with moister conditions.

353 354

Human influence on vegetation on the TP and its significance for paleoclimate reconstructions

355

Recent studies reveal that human impact expanded on the TP at least since the mid-late Holocene. 16

356

Chen et al. (2015) reported archaeological evidences from the northeastern TP to indicate that the

357

first villages were established by 5200 cal yr BP and a novel agropastoral economy facilitated

358

year-round living at higher altitudes since 3600 cal yr BP. Several palynological studies (Table 2)

359

also indicated that human activities have a significant impact on vegetation of the TP during the

360

mid-late Holocene (Herzschuh et al., 2014; Wischnewski et al., 2011; Kramer et al., 2010; Schlütz

361

and Lehmkuhl, 2009). In the Lake Naleng basin, Kramer et al. (2010) indicated that more rapid

362

forest retreat after 3400 cal yr BP was probably promoted by human activities, and that grazing

363

probably increased from 3400 to 2000 cal yr BP and after 1300 cal yr BP. Similar finding was

364

described in the record of Lake Ximen, which reflected human influence expanded from 3000 cal

365

yr BP indicated by higher Potentilla-type and Quercus values (Herzschuh et al., 2014). At Lake

366

Muge, Ni et al. (2019) also held that the large increases of Pinus percentage in pollen spectra

367

during 3500-2300 cal yr BP was related to human activities. However, peat records from

368

Nianbaoyeze Shan pointed out that human impact can trace back to 6000 cal yr BP. This

369

discrepancy of pollen records from the eastern TP may be due to the use of various geological

370

archives. Compared with peat record, the lake pollen record with much larger pollen source area

371

reflects the vegetation change beyond a local scale (Herzschuh et al., 2014). Records covering the

372

last few centuries indicated signs of human impact increasing visible (Wischnewski et al., 2011,

373

2014). These records are from the eastern TP with a relative appropriate environment for human

374

life and agricultural production due to its high mountain canyon topography. However for the

375

central TP at high altitudes where hostile natural environment prevents human surviving, the

376

previous record from the lake terrace of Tangra Yumco reflected human impacts become stronger

377

since 1800 cal yr BP (Miehe et al., 2014). Our results showed low human influence indicated by a 17

378

slight increase in charcoal and Sporormiella during 2300-1700 cal yr BP. Human influence on

379

plant cover may be variable but not detectable in the lake record with large pollen source area

380

(Mercuri et al., 2019). However, vegetation change in Tangra Yumco basin was considered to be

381

mainly forced by climate based on low values of human-indicator taxa in our record. At Tangra

382

Yumco, there were no significant human impacts on alpine vegetation composition until 400 cal

383

yr BP. Thereafter, human activities (e.g., farming and grazing) changed the local vegetation

384

composition, characterized by the rapid increases in Brassicaceae, Urtica, Chenopodiaceae and

385

Sporormiella percentage in the fossil palynological spectra of Tangra Yumco. Thus, more related

386

studies are needed to detect the scope and intensity of human impact on alpine environment.

387 388

Table 2. Palynological records for human impact on vegetation on the Tibetan Plateau.

389

In the regions with strong human impacts on vegetation, paleoclimate reconstructions based

390

on pollen data will be biased due to the marked increase in human influence (Li et al., 2014;

391

Miehe et al., 2009). However despite the human impacts, alpine vegetation on the TP is generally

392

thought to be mainly controlled by climate (Ma et al., 2017a; Herzschuh et al., 2010; Song et al.,

393

2004; Ni, 2000). For example, Herzschuh et al. (2010) proposed that human impact did not blur

394

the general regional signals revealed by the pollen spectra from lake sediments on the central and

395

eastern TP. Similar results were obtained from a study of the relationships amongst modern pollen

396

assemblages, vegetation, climate and human activity on the central-western TP (Ma et al., 2017a).

397

Our pollen results revealed that there were no particularly large vegetation changes since 3400 cal

398

yr BP, with a persistent alpine steppe dominated by Artemisia, Cyperaceae and Poaceae. However,

399

the record occasionally captures the information of human disturbance to local vegetation 18

400

(increases in Brassicaceae, Urtica and Chenopodiaceae) to some extent on the central TP and also

401

suggests the need for palaeoenvironmental renconstructions to take into account human impacts.

402 403

Conclusion

404

The sedimentary record of Tangra Yumco on the central TP reflects impacts of climate and human

405

disturbance on alpine vegetation since 3400 cal yr BP. Minor human activities and a relatively

406

humid environment were detected between 3400 and 2300 cal yr BP. Vegetation change was

407

mainly affected by climate during this interval. From 2300 to 1700 cal yr BP, human activity

408

became only imperceptibly stronger and climate showed a significant drying trend. Vegetation

409

change was partly influenced by human disturbance but mostly by aridification. Between 1700

410

and 400 cal yr BP, human activities became weaker and climate got a bit drier than the previous

411

period. During this time, human activity had no marked impact on vegetation. Since 400 cal yr BP,

412

strongest human activities in the basin are observed and climate tended to be moister. Human

413

activities (e.g., rape farming and grazing) caused the increases in Brassicaceae, Chenopodiaceae

414

Urtica and Sporormiella. Our results recommend that human influence needs to be taken into

415

account in pollen-based climate reconstructions and sustainable development, even in remote

416

regions such as the Tibetan Plateau.

417 418

Acknowledgements

419

We are grateful for the funding from the National Natural Science Foundation of China (grant

420

number 41831177, 41501223), the CAS Strategic Priority Research Program (grant number

421

XDA20020100), MOST Project (grant number 2018YFB05050000), the 13th Five-year 19

422

Information Plan of Chinese Academy of Sciences (grant number XXH13505-06), CAS Field

423

Work Monitoring Stations Project (grant number KFJ-SW-YW038) and DFG priority program

424

1372 (grant number MA 1308/23-1, MA 1308/23-2, MA 1308/23-3). We thank Ping Peng,

425

Ruimin Yang, Xing Hu, Jifeng Zhang, Xiao Lin for their participation in the fieldwork. We are

426

grateful to Gerhard Daut, Heike Schneider, Karoline Henkel and Marieke Ahlborn for their

427

support and help during sample treatments.

428 429

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682 683 684 685 686 687

Highlights 

Impacts of climate and human activities on alpine vegetation are investigated.

31

688 689 690 691 692



Charcoal and human-indicator pollen taxa are used to reflect human activity.



Strongest human influence on vegetation is found after 400 cal yr BP.

ECCD Lab

Section

No.

depth (cm)

(cm) event

conventional

corrected

radiocarbon

composite

age (BP)

Reservoir Error

corrected

(yr)

radiocarbon age (BP)

depth 289070

modern water plant

2070

40

Reservoir

Reservoir

Reservoir

corrected

corrected

corrected

calibrated

calibrated

calibrated

median age

min age

max age

(cal BP)

(cal BP)

(cal BP)

291393

TAN10/1

0

0

2200

30

-60

-60

-60

295002

TAN10/4

0

0

2140

30

-60

-60

-60

295003

TAN10/4

24

16

3450

40

1380

1300

1260

1370

295004

TAN10/4

41

33

3410

40

1340

1270

1220

1320

382663

TAN10/4

78

70

4940

30

2870

2990

2920

3080

295005

TAN10/4

115.5

101

2480

30

2480

2580

2430

2720

295006

TAN10/4

152

124

5260

40

3190

3420

3340

3480

693 694 695 696 697 698 699 700 701 702 703 704 705 706 707 708

Table 1. Radiocarbon ages for core TAN 10/4 as published in Henkel et al. (2016) and Haberzettl et al. (2015).

709 710 711

Table 2.

Palynological records for human impact on vegetation on the Tibetan Plateau.

712 Site

Latitude

Longitude

Elevation

Ocurring period

Human indicators

32

Sequence

References

Lake Naleng

(N)

(E)

(m a.s.l.)

31º06’

99º45’

4200

of human impact

span

3400-0 cal yr BP

11700-0 cal

Kramer et al.,

yr BP

2010

21000-0 cal

Herzschuh et

yr BP

al., 2014

372~12733-0

Schlütz and

cal yr BP

Lehmkuhl, 2009

3500-2300 cal yr

12000-0 cal

Ni et al., 2019

BP

yr BP

Increases in grazing-taxa (i.e.

1870s-1940s,

1800-2005

Wischnewski et

Apiaceae, Liliaceae) and taxa likely

1970s

AD

al., 2011

1400-2000 AD

1400-2003

Wischnewski et

More rapid forest decline; Apperance of Sanguisorba, Rumex and Apiaceae

Lake Ximen

Nianbaoyeze

33º23’

33º

101º06’

101º

4000

3300-4500

Shan

Higher Potentilla-type and Quercus

Increases in grazing taxa (i.e.

3000-0 cal yr BP

6000-0 cal yr BP

Senecio-type, Saussurea-type, Matricaria-type, Rheum, Rumex-type)

Lake Muge

Lake LC6

30º08’

29º50’

101º50’

94º27’

3780

4132

Increase in Pinus

introduced through human cultivation (i.e. Humulus, Fabaceae) Lake

33º13’

101º07’

4307

30º46’

86º40’

4700

Increases in Potentilla-type,

AD

al., 2014

Appearance of Plantago, high

1800-0 cal kyr

11100-0 cal

Miehe et al.,

of Tangra

values of Cercophora-type,

BP

yr BP

2014

Yumco

Glomus, Sporormiella

Dongerwuka Lake terrace

Rumex, Chenopodiaceae

713 714 715 716 717

33

718

719

34

720

721

35

722

723

36

724

37