Holocene Environmental Changes in the Ugii Nuur Basin, Mongolia

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Holocene environmental changes in the Ugii Nuur basin, Mongolia

W. Schwanghart a,, M. Frechen b, N.J. Kuhn a, B. Schtt c

a Department of Environmental Sciences, Institute of Geography, University of Basel, Klingelbergstrasse 27, CH-4056 Basel, Switzerlandb Leibniz Institute for Applied Geosciences, Stilleweg 2, Section S3, Geochronology and Isotope Hydrology, D-30655 Hannover, Germanyc Department of Earth Sciences, Geographic Institute, Freie Universitt Berlin, Malteserstrasse 74-100, D-12249 Berlin, Germany

a b s t r a c ta r t i c l e i n f o

Article history:

Received 9 February 2009

Received in revised form 30 April 2009Accepted 10 May 2009

Keywords:

Central Asia

Loess

Lake sediments

Geoarchaeology

Principal component analysis

In this study we investigate the Holocene environmental evolution of the Ugii Nuur basin in the Orkhon

Valley, central Mongolia, in order to gain a better understanding of the climate and landscape dynamics of

the steppe region in Central Asia. We assess terrestrial and lake sediments using mineralogical, geochemical

and statistical techniques and provide absolute chronologies by radiocarbon datings and infra-red stimulated

luminescence datings including fading correction. Our ndings and their discussion in a broader regional

context show that there is a Mid Holocene climate optimum. The shift towards warmer and wetter conditions

involved changes from Aeolian dynamics of sand mobilization to loess-like sediment accumulation owing to

the establishment of dust-trapping vegetation. Between 4 and 2.8 ka BP we track a decline in moisture supply

which is then followed by relatively humid conditions until today. We infer that the region is sensitive to

changes of the westerlies dynamics. In addition, we conclude that the Late Holocene was characterized by

favorable climatic conditions that probably were an important prerequisite for the repetitive colonization of

the Orkhon Valley throughout the last 3000 years.

2009 Elsevier B.V. All rights reserved.

1. Introduction

Information on Holocene environmental evolution is scarce in

Mongolia. At its southern border, numerous investigations on lake

sediments (Wnnemann and Hartmann, 2002; Xiao et al., 2004;

Wnnemann et al., 2006; Hartmann and Wnnemann, 2009),

speleothems (Dykoski et al., 2005; Wang et al., 2008) and loess

proles (Porter, 2001; Roberts et al., 2001; Rost, 2001; Xiao et al.,

2002; An et al., 2005; Maher, 2008) provide spatially distributed

information on landscape and climate evolution in the northern part

of the People's Republic of China. Along the northern border various

lake basins have been studied, most prominently Lake Baikal and Lake

Hovsgol (e.g.Demske et al., 2002; Fedotov et al., 2004; Poberezhnaya

et al., 2006; Hvsgl Drilling Project Group, 2007; Prokopenko et al.,

2007),UvsNuur(Grunert et al., 2000) and various smaller basins(e.g.

Gunin et al., 1999; Tarasov et al., 2000; Peck et al., 2002; Fowell et al.,

2003) providing insight into the evolution of the continental, boreal

region. Thevast steppearea in between these two regions,however, is

only poorly explored (see overview in Harrison et al., 1996;

Herzschuh, 2006).

Knowledge on the environmental history of this extremely

continental area is required to understand its sensitivity to current

global climate change. Northern Mongolia has been warming twice as

fast as the global average during the last 40 years and experiences a

decline in permafrost (Nandintsetseg et al., 2007; Bohannon, 2008),

and projections indicate a more arid climate in the future (Sato et al.,

2007). The manner of how ecosystems react to these changes affectsvital functions of the Mongolian society. Hence, investigating the

Holocene climatic and environmental evolution contributes to an

understanding of ecosystem sensitivity not only because the past few

millennia are the natural baseline from which to differentiate current

climate change, but also because Holocene evolution is linked to

human cultural evolution (Cronin, 1999).

In this study we investigate the Holocene evolution of the Ugii

Nuur basin located in the steppe region of Mongolia using lacustrine

and terrestrial sedimentary archives. The study site gains relevance

not only due to its situation in a poorly investigated region of

Mongolia. It is located in the sphere of inuence of the westerlies

whose role in the climatic evolution of Central Asia is highly disputed

(Chen et al., 2008). Moreover, it is part of the Orkhon Valley that hosts

various archaeological sites that document an human inuence for the

last 3000 years with a climax in the 1314th century (c.) AD when

Dschingis Khan chose the Orkhon Valley as location for his capital

Karakorum (Qara Qorum) (Bemmann et al., in press).

A pollen prole of one of the sediment cores presented in this

study has been investigated byRsch et al. (2005). Yet, climate and

environmental reconstructions based on pollen samples only are less

reliable for arid and semi-arid regions owing to the inuence of long-

distant transport and the often unknown degree of human inuence

on vegetation densities and assemblages (Tarasov et al., 2007c,b).

Hence we complemented this work by an extensive geochemical and

mineralogical approach of lake and terrestrial sediments and provide

new records and dates obtained from the infra-red stimulated

Palaeogeography, Palaeoclimatology, Palaeoecology 279 (2009) 160171

Corresponding author. Tel.: +41 61 26 70737; fax: +41 61 26 73645.

E-mail address:[email protected](W. Schwanghart).

0031-0182/$ see front matter 2009 Elsevier B.V. All rights reserved.

doi:10.1016/j.palaeo.2009.05.007

Contents lists available at ScienceDirect

Palaeogeography, Palaeoclimatology, Palaeoecology

j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / p a l a e o
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luminescence (IRSL) technique including fading correction. We place

emphasis on an interpretation and synthesis of the results and discuss

their signicance in a spatially broader context.

2. Study site

The Ugii Nuur basin is located in central Mongolia (4744N and

10246

E,Fig.1) and comprises elevations between 1328 and 1600 m.Ugii Nuur is a lake with a surface area of approx. 26 km 2 and a

maximum depth of 16 m. It is fed by the Old Orkhon River (Chgschin

Orkhon Gol) and is ephemerally drained by a ca. 8 km long water

course connected to the Orkhon River (Orkhon Gol) (Fig. 2). During

twoeld trips in 2005 and 2006 surface runoff along this outlet was

absent.

Climate is characterized by extreme continentality. Ugii Nuur Sum,

located about 10 km southwest to Lake Ugii Nuur, shows an annual

mean temperature around zero and an annual precipitation of around

250 mm. The continentality causes a strong seasonality. Mean January

air temperatures of21.5 C contrast July air temperatures of 17.0 C.

Precipitation in winter (November to March) mostly occurs in solid

form and rarely exceeds 10 mm/month, while rainfall in summer is

predominantly linked to convective cells associated with warming of

the continental interior (Harrison et al., 1996), cyclonic disturbances

of the westerlies (Weischet and Endlicher, 2000) and intense diurnal

mountain wind systems (Bhner, 2006). In general, rainfall events in

summer are short, intense and accompanied by high wind velocities.

Bedrock in the study area consists of strongly tilted Carboniferous

shists and shales that outcrop on ridges and steep slopes (Fig. 3).

Along the lake shore of Ugii Nuur reddish conglomerates are exposed

that can be attributed to the Late Cretacious and Tertiary and testify

uvial dynamics with high bedload transport generating pebble beds

and channel bar sandstones (Traynor and Sladen, 1995). The origin of

the topographic depression is unclear but may be linked to Miocenevolcanismnearby Ugii Nuur (Kovalenko et al.,1997) orto aneastwest

left-lateral strikeslip fault (Walker et al., 2008).

The shallow subsurface is dominated by vast fanglomerates

accumulated during the Pleistocene (Richter et al., 1963; Grunert

et al., 2000; Schwanghart and Schtt, 2008). The debris layers are

poorly sorted and comprise grain sizes ranging from silt to blocky

debris. Where alluvial sedimentation prevailed, bands with pebble

clusters occur, separated by soil sediments or nes. In at or slightly

inclined areas layering may also be attributed to frost heaving

processes. Soils developed on periglacial debris are dominated by

Kastanozems. On steep slopes these steppe soils are poorly developed,

frequently only a few decimeters thick and classied as Regosols or

Leptosols. The study site is situated within the area of dry, grass

steppes (Hilbig, 1995; Opp and Hilbig, 2003) and is dominated by

grasses (Cleistogenes, Stipa), forbs (Allium), and shrubs (Artemisia,Caragana) while trees are generally lacking (Fig. 3). Present day

Fig.1. Mapof central Asia andlocation of thestudysite.Vegetation data is derived from theGlobal Land Cover Characteristicsdatabase (Running et al.,1995). Elevation data(isolines

with equidistance of 500 m) is based on GTOPO30 data.

161W. Schwanghart et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 279 (2009) 160171


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human activities are largely restricted to herding. The large quantities

of lifestock that have been established since the fall of the sovietregime result in large-scale land degradation.

3. Methods

Our study aims at deriving local and regional paleoclimatic and

paleoenvironmental information from lacustrine and terrestrial

sedimentary archives of the Ugii Nuur basin. Since the acquired data

reects in situ conditions and/or source area characteristics of

material uxes, a regional synthesis of our results is gained by acomparison with existing paleoenvironmental datasets of the region.

3.1. Sediment analysis

We report the ndings from two lake sediment cores: UGI-4 and

UGI-G3. UGI-4 was excavated at the deepest location of Ugii Nuur in

2003 (Fig. 2) witha Usinger piston corer (Walther, 2005).The corehas

Fig. 2.Map of the study site indicating locations of proles described in the text.

Fig. 3.Picture of the Ugii Nuur basin taken in southeast direction. Photo: W. Schwanghart, Aug. 2005.

162 W. Schwanghart et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 279 (2009) 160171


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a length of 630 cm and was sampled with an interval of 10 cm and a

resolution of 2 cm.A gravitational corer was used to retrieve UGI-G3, a

2 m long core with a sample interval of 5 cm and a 2 cm resolution. In

addition, we document two terrestrial proles (UGITP-01, UGITP-08)

located in relativelyat terrain to exclude a strong inuence of lateral

water and material uxes (Fig. 2).

Prior to chemical and mineralogical analysis the samples were

dried, screened on components larger 2 mm and homogenized in an

agate disk swing mill (Lieb-Technik). We used a conductrometric

carbon analyzer (Westhoff, detection limit= 0.02 mass% C) to

determine the total (TC) and inorganic carbon (TIC) content. Total

organic carbon (TOC) content was calculated as the difference of bothvalues. Mineralogical compounds were analyzed by X-ray powder

diffraction (XRD) using a copper k-tube (PW1729/40, Phillips) from

2= 252 and a step width of2=0.01 with each step measured

for 1 min. Contents of mineral components were analyzed semi-

quantitatively (vol%) using the software package Phillips X'Pert

Highscore v. 1.0b (PW3209). After preparing samples using aqua

regia and microwave digestion, we analyzed the element composition

(Ca, Cr, Cu, Fe, K, Mg, Mn, Na, Ni, Pb, PO4, S, Sr) by inductively coupled

plasma atomic emission spectroscopy (ICP-AES, Optima 3000, Perkin

Elmer). TC, H and N were analyzed using a LECO CHN 1000 (detection

limit =0.01 mass-%). Grain size distributions analyzed for one

terrestrial prole were derived from pipette and sieve analysis.

3.2. Statistical analysis

The sequence of element compositions of UGI-4 was statistically

analyzed with a principal component (PC) analysis (PCA) based on the

correlation matrix. Prior to the analysis the data was transformed using

centered log-ratios to account for the constant sum constraint

(Aitchison, 1986; Kucera and Malmgren, 1998). Subsequently all

variables were z-transformed. Since standardization tends to inate

the inuence variables whose variance is small, we excluded H, Na and

PO4 from thePCA owing to their lowconcentrationsand thus lowsignal-

to-noise ratio (Davis, 1986). A VARIMAX rotation of the principal

components (PC)was chosen to enhance the interpretability of the PCs.

3.3. Radiocarbon and luminescence datings

Absolute chronologies were obtained from radiocarbon (14C) and

fading corrected IRSL datings (Tables 1, 2).14C ages were determined

from bulk organic matter in lacustrine sediments and conjointly on

humins and humic acids for terrestrial samples in the Poznan

Radiocarbon Laboratory. Despite the relatively few dates on the

lacustrine core we assume that the chronology is valid due to absence

of e.g. event layers that mayindicate strong ruptures of sedimentation

rates. 14C ages were calibrated with CalPal Online (Danzeglocke et al.,

2008) using the calibration curve CalPal2007 HULU (Weninger and

Jris, 2008). Ages between dated samples were obtained by linear

interpolation. In the following text, radiocarbon ages are provided as

calibrated years unless otherwise noted.Aeolian sediments like loess and dune sands are particularly

suitable for the application of luminescence dating techniques to

determine the deposition age of the sediments (Frechen and

Dodonov, 1998; Frechen, 1999). Luminescence is the light emitted

from crystals such as quartz, feldspar or zircon when they are

stimulated with heat or light after receiving a natural or articial

radiation dose. The equivalent dose is a measure of the past radiation

energy absorbed and, in combination with the dose rate, yields the

time elapsed since the last exposure to sunlight. An important

assumption of luminescence dating techniques is that the mineral

grains were exposed to sunlight sufciently long enough prior to

deposition to reset their luminescence signal. Eight loess samples

were taken in light-tight tubes about 250 g each in the eld.

Furthermore, about 1 kg of sediment was sampled for gammaspec-

trometry to determine the amount of radioactivity in the sediment.

Natural ionising radiation includes alpha, beta and gamma irradiation

released by the radioactive decay of the unstable isotopes of the 238U,235U and 232Th decay chains, 40K and to a minor content 87Rb, 14C and

cosmic radiation. The dose rate depends on a number of external

factors, some of them are even variable during burial time, like

moisture, sediment thickness, grain-size, geochemical weathering of

minerals or salts, mobilisation of clay minerals and radioactive

equilibria/disequilibria of the decay chains and internal factors like

the amount of 40K in potassium-rich feldspars.

Polymineral ne-grained material (411 m) was prepared for the

measurements, as described by Frechen et al. (1996). The Single

Aliquot regenerative Protocol (SAR) was applied to determine

equivalent dose values (Wallinga et al., 2000). The IRSL ages were

fading-corrected following Huntley and Lamothe (2001). A SchottBG39/Corning 759 lter combination was placed between photo-

multiplier and aliquots for the measurements. Alpha efciency was

estimated to 0.080.02 for all samples. Dose rates for all samples

were calculated from potassium, uranium and thorium contents, as

measured by gamma spectrometry in the laboratory, assuming

radioactive equilibrium for the decay chains. Cosmic dose rate was

corrected for the altitude and sediment thickness, as described by

Aitken (1998) and Prescott and Hutton (1994). The natural water

content of the sediment was estimated to 7.52.5%.

4. Results and interpretation

4.1. Lacustrine archives

4.1.1. Geochemical and mineralogical analysis14C dates on UGI-4 age the onset of lake sedimentation to the

Pleistocene/Holocene boundary (seeTable 1). Above a sandy base the

Table 1

List of radiocarbon 14C dates reported in this study.

Sample

number

Milieu Core/

prole

name

Depth Uncal.14C age1 Cal. age1

(cm) (y) (y BP)

POZ-16767 Lacustrine UGI-4 210 3535 30 3809 57



POZ-15083 Terrestrial UGITP-01 50 4010 35 4482 37

POZ-15084 Terrestrial UGITP-01 148 8500 50 9506 24POZ-15085 Terrestrial UGITP-01 198 9670 50 11031 136

Table 2

List of fading corrected IRSL ages of the section UGITP-08.

Sample

number

Depth Doserate DE Uncorrected age g2 days-value Fading corrected age

(cm) (Gy/ka) (Gy) (ka) (% /decade) (ka)

1575 1015 3.74 1.19 0.55 0.12 0.15 0.03 2.90 0.29 0.17 0.07

1576 3640 3.60 0.31 4.67 0.07 1.30 0.11 2.90 0.29 1.59 0.21

1577 6064 3.77 0.30 7.66 0.22 2.03 0.15 2.90 0.29 2.51 0.33

1578 130135 3.37 0.42 31.85 2.37 9.44 0.95 2.90 0.29 11.95 2.11

1579 145150 3.53 0.32 31.10 1.34 8.81 0.72 2.90 0.29 11.12 1.59

1580 160165 3.59 0.52 48.80 4.36 13.60 1.54 2.90 0.29 17.27 3.40



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While former elements reect the allogenic mineral fraction, latter

form the authigenic mineral assemblage in the sediment. It is

important to note, that the concurrent mapping of these two material

fractions on one PC reects an archive entailed concurrence and not a

concurrenceof processes. It maypartly be explained by a rather coarse

sampling resolution that is unable to resolve rapid changes in the

hydrological regime responses of the relatively small lake.

The environmental meaning of the PCs is discussed in detail in

Schwanghart et al. (2008). They conclude that PC1 can serve as a

proxy for lake level variability since carbonate precipitation is heavily

inuenced by activation or deactivation of the lake outlet. During

closed basin conditions evaporation from the water surface is the

main driver to maintain the water balance of the lake, leading to an

enrichment of dissolved carbonate concentrations in the lake water. In

combination with a decline of thermal stratication and generally

higher water temperatures during summer, carbonate precipitation

rates are increased during these periods. Open basin conditions

involve a constant renewal and exchange of the lake water. During

such periods intense carbonate precipitation can be excluded anddeposition of the allogenic mineral fraction prevails. Despite the

metric scale of PC1 the derived lake level curve (Fig. 6) cannot be

translated to precise water surface elevations but remain a qualitative

estimate.

We are aware, that this simplied interpretation of the complex

dynamics of lake sediment formation disregards sedimentuxes that

may be triggered by the exposition of the riparian lake zone or the

sheer fact that sediment characteristics change as the litoral zone

approaches the borehole location during low lake levels. Moreover,

the meandering nature of the Old Orkhon and the Orkhon River and

associated sediment dynamics in the delta area may have caused abypass of the inlet to the lake. At the current state of our knowledge

we cannot exclude uviodynamic effects of the inlet on the lake level

and, hence, paleoclimatic interpretations are limited. Since the lake

level proxy agrees well with other ndings in the area, it is assumed

that these dynamics either did not affect the lake level, or had a short

duration not resolved in our record.

The behavior of Mn and its relation to Fe is reected by PC2.

Schwanghart et al. (2008) conclude that Mn is both related to the

lithogenic sediment fraction and soil derived, dissolved Mn by

groundwater supply. During low lake level phases surface runoff

contribution and delivery of eroded material was largely replaced by

groundwater discharge enriched in dissolved Mn due to its higher

mobility in soils (Schlichting and Schweikle, 1980; Scheffer, 2002).

Due to the large uncertainty associated with the reication of PC2 we

omit its use in the paleoenvironmental interpretation.

TOC and N are associated with PC3. TOC as proxy for paleoclimatic

interpretations can be misleading owing to the variety of sources

and processes generating organic matter in lakes (Hartmann and

Wnnemann, 2009). Since we can infer from C:N ratios that

variability of organic matter is predominantly linked to aquatic

organisms we interpret PC3 to reect biologic lake productivity.

4.2. Terrestrial archives

Landforms in steep terrain in the study area are ridges and convex

to straight slopes characterized by outcrops and relatively thin covers

of blocky debris. An often sharp, concave knick in the lower slope

section passes into a moderately inclined accumulation section of

interngering fans and colluvial material. Texture is spatially highlyvariable, characterized by partly layered debris that is embedded in a

texture of sands and silts (Schwanghart and Schtt, 2008). These

Table 3

Factor loadings and communalities of centered log ratio transformed variables after

Varimax rotation. Loadings N 0.7 and b 0.7 are highlighted bold.

Element Factor loading Communality

PC 1 PC 2 PC 2

Mg 0.980 0.019 0.075 0.97

Cr 0.979 0.179 0.005 0.99

Fe 0.972 0.190 0.019 0.98

Pb 0.964 0.214 0.036 0.98

Ni 0.945 0.258 0.063 0.96K 0.943 0.177 0.135 0.94

Cu 0.833 0.474 0.120 0.93

TIC 0.885 0.246 0.229 0.90

Ca 0.868 0.420 0.151 0.95

Sr 0.791 0.520 0.155 0.92

S 0.767 0.469 0.045 0.81

Mn 0.291 0.841 0.054 0.79

TOC 0.060 0.146 0.842 0.73

N 0.124 0.334 0.711 0.63

Fig. 5.Biplots of the PC loadings (vectors) and PC scores (dots). Note that the biplots follow the convention, forcing the element with largest magnitude in each column of the

loadings matrix to be positive.



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archives are generated by a combination of geomorphological

processes of rock fall, concentrated runoff or sheet wash that produce

cut-ll sedimentary patterns associated with single rainfall events.

Indications of cryoturbation show that denudational processes such as

soil creep and upheaval play a major role in the genesis of these

proles.Terrestrial proles in the at to gently undulating terrain of the

southern part of the study area are spatially less variable. In general,

these terrestrial archives show a predominance of the silt fraction in

the upper part of the proles overlying ne to coarse sands. A horizon

with secondary carbonate formation in variable depth is common,

visible by concretion and carbonate coatings on lower sides of gravel

and pebbles (Klinge, 2001; Lal, 2004), characterized by elevated TIC

concentrations (Figs. 7 and 8) and detected by mineralogical analysis

(Schwanghart and Schtt, 2008).

The terrestrial proles in UGITP-01 and UGITP-08 (Figs. 2, 7 and

8) are located on prole straight and planform straight to convex

slopes with inclinations of 2. UGITP-01 is situated in the lower,

southwest exposed slope section of a broad, saucer-shaped valley.

UGITP-08 is a section partly exposed on a cliff-like scarp surroundingthe backwater of Ugii Nuur (Fig. 2). A thicker silty section in UGITP-

01 than in UGITP-08 is probably owing to enhanced material

contribution by sheetwash that is more intense due to the prole's

topographic situation and a larger upslope contributing area (see

Fig. 2). Nondeclining, but generally low TOC concentrations in the

upper part of UGITP-01 support the interpretation that soil

formation was accompanied by more efcient burial by lateral

material uxes than in UGITP-08. The presence of partly rounded

gravel in UGITP-08 (layer 6) suggests the accumulation of coarse

material subject to uvial erosion or ground by relocation in the

litoral zone of lake Ugii Nuur. Insufcient slope and upslope area for

efcient uvial transport, low lateral variation and a lack of evidence

for paleodrainage at this location suggest that litoral processes

probably formed the gravel layer.

Both fading corrected IRSL ages and calibrated 14C ages provide

Holocene chronologies of the terrestrial sediment sequences under

study. The sand dominated sections can be attributed to the Late

Pleistocene and Early Holocene while the silt dominated sections can

be placed in the Mid to Early Holocene. The pebble-rich section in

UGITP-08 suggests a high lake level stand between these two phases,leading to theaccumulation andgrindingof coarser clastics. Theorigin

of the clastics is Tertiary fanglomerates outcropping along cliffs at the

lake shore. The location of UGITP-08 along a cliff-like scarp at the

backwater of Ugii Nuur suggests a backward retreat due to litoral

erosion simultaneously to or following the silt accumulation phase.

5. Discussion and paleoclimatic and

paleoenvironmental interpretation

5.1. Pre-Holocene evolution (N10 ka)

Lake reservoir lling by the end of the Pleistocene epoch is in good

agreement with other lake systems in Central Asia (Herzschuh, 2006).

Most lakes in NW China and Mongolia had low lake levels or weredried out during the last glacial maximum (LGM) (Wnnemann et al.,

1998; Tarasov et al.,1999; Walther, 1999; Wnnemann and Hartmann,

2002; Waltheret al., 2003; Grunert and Dasch, 2004; Karabanov et al.,

2004; Yanget al., 2004; Hartmann and Wnnemann, 2009) owing toa

largely weakened summer monsoon and moisture depleted wester-

lies (Kutzbach and Guetter, 1986; Foley et al., 1994; Bush, 2004;

Tarasov et al., 2007c).

Despite the rather general notion of a dry LGM, the re-establish-

ment of moisture supply and associated lake phase beginnings were

asynchronous in Central Asia. This may be due to the gradual

establishment of the EA monsoon associated with the summer

insolation maximum in the Late Glacial and Early Holocene (Sirocko

et al., 1993; Harrison et al., 1996; Wanget al., 1999; Blyakharchuk et al.,

2004; Herzschuh, 2006) and discharge connectivity to glaciated

Fig. 6.Factor scores and mineral occurrences of UGI-4.



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Fig. 7.Lithology, total inorganic (TIC) and organic carbon (TOC) of UGITP-01.

Fig. 8.Lithology, total inorganic (TIC) and organic carbon (TOC) of UGITP-08.



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regions (Walther,1999). Different geological settings result in different

rates of aquifer lling and provide an explanation for time lags

between climate amelioration and lake evolution (Hartmann and

Wnnemann, 2009).

Since the Ugii Nuur catchment comprises parts of the Khangay

mountains, glacier retreat in this area (Lehmkuhl and Lang, 2001;

Lehmkuhl et al., 2004) probably contributed to discharge to the Ugii

Nuur basin. Thick periglacial debris covers (Richter et al., 1963;

Lehmkuhl and Lang, 2001; Schwanghart and Schtt, 2008) of theOrkhon Valley suggest that initial discharge served as recharge of

riparian groundwater aquifers and may have delayed lake evolution.

According to Lehmkuhl and Haselein (2000)and Lehmkuhl and

Lang (2001) a phase of sand accumulation is followed by a loess phase

at the end of the Pleistocene (ca. 15 ka). The terrestrial proles

investigated here suggest that sand sheets and/or dunes were active

until at least the Early Holocene, but also Mid Holocene. The sand

originated from oodplains and was transported only over short

distances while silt-sized particles were transported over long

distances due to the lack of a vegetation cover acting as dust trap

(Grunert and Lehmkuhl, 2004).

5.2. Early Holocene (ca. 108 ka)

While the EA Monsoon dominated region of North Eastern China

faced a moisture optimum during theEarly Holocene (An et al., 2000),

various records suggest a dryer climate in the westerlies-dominated

region of Central Asia (Wnnemann et al., 2003; Xiao et al., 2004;

Herzschuh, 2006; Chen et al., 2008; Hartmann and Wnnemann,

2009). A strong seasonal contrast induced by the positive summer

insolation anomaly during this time resulted in convective heating on

the Tibetan Plateau and lead to subsidence of air masses and a dryer

climate northwest of the plateau (Broccoli and Manabe, 1992; Duan

and Wu, 2005). In contrast, moisture availability in the Baikal

watershed was higher during the Early Holocene and declined by

7.58 ka (Prokopenko et al., 2007). The warmer and more humid

climate during this time resulted in afforestation of steppe landscapes

(Naumann, 1999; Westover et al., 2006) and permafrost degradation

(Tarasov et al., 2007a). A wetter Early Holocene in the boreal region ofCentral Asia is explained by the weakening of the Siberian winter

anticyclone that gave way to Atlantic cyclone activity ( Blyakharchuk

et al., 2004). Moreover, subsidence of air masses in the Gobi region

may also have contributed to a northward displacement and

narrowing of the westerlies zone during summer.

Since Ugii Nuur's lake phase started at 10 ka we can infer that

moisture supply to the lake at least exceeded precedent supply.

Allothigenic material input to the lake as indicated by PC1 reects

mobility of surface material during the initial lake phase and ongoing

sand mobility inferred from the terrestrial archives in the Ugii Nuur

basin points to arid conditions. Similar conditions are found in other

regions of Mongolia during the Early Holocene.Grunert et al. (2000)

report dune dynamics from the Uvs Nuur basin and Kowalkowski

(2001) identies a dry-warm phase with an increase of meadowsteppe and steppe vegetation from 11 to 8.5 ka in the Altai Mountains.

According to Gunin et al. (1999)steppe vegetation dominated at most

sites in Mongolia and deserts occupied large depressions in western

Mongolia from 11.5 to 8.9 ka.

5.3. Mid Holocene (ca. 84 ka)

The Mid Holocene atmospheric conditions in Central Asia are

characterized by a southeast retreat of the maximum EA Monsoon

limit due to the weakening of the summer insolation. Particularly in

east China, but also in north China there is evidence for a decline in

moisture supply (An et al., 2000; Hartmann and Wnnemann, 2009).

A trend towards dryer conditions is also found in the Hovsgol and

Baikal region around 7 ka (Prokopenko et al., 2007). Peck et al. (2002)

andFowell et al. (2003)infer a trend towards dryer conditions during

the Mid Holocene for Lake Telmen. Westover et al. (2006)detect a

decline of temperature and humidity by 6 ka in the Altai Mountains,

and signicant afforestation cannot be recognized in most pollen

proles (Gunin et al., 1999).

Many lake systems in westerlies dominated north China and

Mongolia show favorable moisture conditions during the Mid

Holocene (Shi et al., 1993; Dorofeyuk and Tarasov, 1998; Tarasov

et al., 2000; Zhang et al., 2000; Grunert and Dasch, 2004; Herzschuhet al., 2004; Yang et al., 2004; Feng et al., 2005; Xiao et al., 2006; Chen

et al., 2008). Contradictory to the interpretations ofAn et al. (2000)

some authors relate the wetter conditions to further Monsoon

penetration.

Our lake level proxy suggests that Ugii Nuur experienced higher

lake levelsduring theMid Holocene.This is supported by the presence

of litoral material in UGITP-08 whose accumulation likely dates to the

same period and Rsch et al. (2005) infer a trend towards more

woodland. The proxy for biological lake productivity (PC3) also

features the highest values during this time. In addition, we detect a

change from sand dynamics towards loess deposition which has been

shown to be an indicator for the transition from arid to semi-arid

conditions (Yair and Bryan, 2000; Kster et al., 2006). Hence, we infer

that the climate governing the evolution of the Ugii Nuur basin during

the Mid Holocene was characterized by increased precipitation and

temperature providing better conditions for the establishment of

dust-trapping vegetation and supporting algal growth in the lake

during the Mid Holocene.

Due to the inconsistent or asynchronous climatic signals in the

region the atmospheric circulation patterns during the Mid Holocene

cannot be derived in a straight forward manner (Herzschuh, 2006). A

gradual southward spreading of the westerlies zone can be assumed

to be a reaction to the cooling of the Tibetan Plateau (Herzschuh et al.,

2006). According to Chen et al. (2008) higher North Atlantic sea

surface temperatures and high-altitude air temperatures intensied

cyclonic activity and synoptic disturbances along the westerlies that

result in higher convective precipitation rates in westerlies dominated

Central Asia. If so,topographic barriers may have causeda tremendous

effect on the spatial variability of moisture supply (Broccoli andManabe, 1992) and be an explanation for the inconsistent climate

changes in the region.

5.4. Late Holocene (b4 ka)

Since 3 ka EA monsoon dominated areas show lower effective

moisture owing to gradual monsoon weakening since the mid-

Holocene (Overpeck et al., 1996; An et al., 2000; Herzschuh, 2006).

Westerlies-dominated areas, however, lack a uniform decline in

effective moisture (Herzschuh, 2006; Chen et al., 2008) and records

from boreal Central Asia show moisture conditions similar to present

since 4 ka(Gunin et al., 1999; Prokopenko et al., 2007). Various lake

level records in Mongolia provide evidence for only small uctuations(Tarasov et al., 1996; Peck et al., 2002; Fowell et al., 2003; Walther et

al., 2003; Yang et al., 2004).

Some records point at signicant climatic oscillations from 4 to

2.5 ka that are not well dated and understood (Mayewski et al., 2004;

Demske et al., 2005; Prokopenko et al., 2007). Vipper et al. (1989)

report distinct gravely lamina in various Mongolian lakes with an

average calendar age of 3.6 ka that they interpret to reect a shift

towards a regional precipitation regime with more intense storms.

Moreover, modelling studies suggest increased aridity during the Late

Holocene (Bush, 2005) and Prokopenko et al. (2007) report

signicant regional warming between ca. 4 and 2.5 ka in the Baikal

watershed (Prokopenko et al., 2007).

Lower lake levels during 42.8 ka are also shown for Ugii Nuur and

may be associated with increased aridity. Very high concentrations of



10/12

MHC fall into this interval and a signicant increase in TOC and N

associated with this peak suggest a strong biological activity in the lake.

From 2.8 ka to present, the Ugii Nuur record indicates stable high

lake levels that we interpret to reect enhanced moisture supply

during the Late Holocene. Allogenic material delivery to the lake is

high and ages of loess-like sediments show ongoing mineral dust

deposition(Fig. 9). On theone hand, this maybe explained by changes

in rainfall patterns intensifying runoff and bed load transport to the

lake. Moreover, changing surface properties generated by accumula-tion of loess-like deposits and associated reduced inltration

capacities have shown to alter runoff characteristics (Yair and Bryan,

2000).

On the other hand, it is expected that humans have been

signicantly inuencing the ecosystem in the region and associated

lateral material uxes since 2.8 ka. Rsch et al. (2005) infer from

surface pollen records and a pollen record of UGI-4 an increased

human inuence on woodland clearance by the Late Holocene. Tombs

(Kurgans and Khirigsuurs), petroglyphs and wall remnants testify an

anthropogenic inuence since the Palaeolithic in the study site and

the Orkhon valley (Bemmann et al., in press). A cemetery nearby the

Tamir and Orkhon conuence with more than 250 burials is likely of

Xiongnu origin (3 rd2nd c. BC) (Batsaikhan et al., 2006), the memorial

Khosh Tsaidam documents the inuence of Turks (68th c. AD) and

the Uighurs (89th c. AD) established their capital Kharabalghasun

and a rampart in the west to LakeUgiiNuur (Minorsky, 1948; Drompp,

1999, 2005; Bemmann et al., in press).

In medieval times (1314th c. AD) Chengis Khan decided to

establish Karakorum as the capital of the Mongolian empire in the

Orkhon Valley 70 km south of Ugii Nuur. Ugii Nuur was part of the

peri-urban area of Karakorum, a domain where nomadic traditions

mixed with various forms of agricultural activities to supply the

political, economic and cultural center (Shiraishi, 2004; Erdenebat

and Pohl, 2005). Travelogues report grain and vegetable cultivations

on the bank of the Orkhon River, west of the lake ( Shiraishi, 2004).

Together with reports that the spring palace of the Mongolian

emperor located on an elevation in the Orkhon oodplain was

surrounded by numerous lakes (Shiraishi, 2004) that are presently

only lled with water during wet spells (Bemmann et al., in press),these evidences support our inferred favorable moisture conditions

during that time. The operation of furnaces with wood in the capital

(Franken, 2005) may have strongly altered regional tree populations.

Rearing and keeping animals within the surroundings of Karakorum

was common in particular with regard to sheep (von den Driesch et

al., in press) and can be regarded as an additional stress to the

ecosystem by intensifying soil erosion processes especially in the

vicinity of densely populated areas.

The construction of the monastery Erdenet Joo in Karakorum in

1585/86 and an abandoned settlement of seven stone buildings of

Tungian Mandchurs origin (1617th c. AD) in the northwest of Lake

Ugii Nuur (Weiers, 2005; Bemmann et al., in press) provide evidence

for ongoing human activities in this region.

6. Conclusion

In this study we present terrestrial and lake sediment records fromUgii Nuur in central Mongolia. The records allow foran insight into the

environmental and climatic evolution of the steppe region in

Mongolia, a region which is only loosely covered by paleoenviron-

mental studies so far. We conclude that the paleoenvironmental

interpretations of our records provide similar results to those obtained

elsewhere in the westerlies dominated region of Central Asia. A dry

and cold pre-Holocene period is followed by slightly more humid

conditions in the Early Holocene that were not sufcient to generate a

dense vegetation cover to cease Aeolian dynamics of sand mobiliza-

tion. During the Mid Holocene our records point at favorable moisture

conditions and warming associated with change from sand mobiliza-

tion to loess accumulation. Between 4 and 2.8 ka we track a decline in

moisture supply which is then followed by relatively humid condi-

tions until today.

Our ndings and their discussion in a regional paleoenvironmental

context reveal that the areas north of the predominantly EA monsoon

dominated region most likely reacted to changes in the extent of the

westerlies. Yet, one has to keep in mind that especially for heavy rains

in this area interactions between mid latitude frontal systems and

lower latitude cyclone/anticyclone systems are essential (Bao, 1987).

Hence, past changes in the magnitude and frequency distribution of

such singular events may greatly inuence the paleoenvironmental

evolution in the westerlies dominated region. Assessing and inter-

preting sedimentary archives of such events remain a major challenge

due to the complexity of the underlying climatic and surface processes.

In addition, our study provides insight into the Late Holocene

evolution of the Orkhon Valley. At the current state of our knowledge,

we exclude neither natural nor anthropogenic forcings on the

environmental evolution during this time. The effects of humansand livestock on the steppe ecotone in this area are still highly

disputed (Hilbig, 2000; Dulamsuren et al., 2005; Miehe et al., 2007;

Schltz et al., 2008) and population and cattle densities of former

tribes and peoples inhabiting the Orkhon Valley are largely unknown.

Drivers of environmental change beyond natural forcing should be

considered to understand the environmental evolution in this region.

Various results support an interpretation that the environmental

history of the Mongolian plateau is driven by natural forcings rather

than by human activities (Tarasov et al., 2007a; Schltz et al., 2008).

Since most of these results are limited to specic areas, great

difference can be expected among different regions. We suggest that

therepeated colonizationof the Orkhon Valleywas a decisive driverof

environmental evolution in this area throughout the last 3000 years.

The way, humans affected the environmental evolution of this areaby husbandry and pasture farming, rewood use and other econom-

ical practices remains largely unknown. A more detailed survey of the

humanenvironmental interrelations in this area will not only

contribute to a better understanding of the historical evolution, but

will provide us the means to assess the future environmental change

in steppe region of Mongolia.

Acknowledgments

We thank Altangerel Bat-Erdenet, Manuela Burmeister, Dr. Philipp

Hlzmann, Riccardo Klinger, Emmi Krings, Judith Mahnkopf, Dr.

Rper, Heidi Strohm, Dr. Sumiko Tsukamoto and Prof. Dr. Dr. h.c.

Michael Walther for constructive comments, help during eldwork

and technical support.Fig. 9.Grain size distribution in various depths of UGITP-08.



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Holocene Environmental Changes in the Ugii Nuur Basin, Mongolia

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