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Contents lists available at ScienceDirect
Journal of Arid Environments
Journal of Arid Environments ] (]]]]) ]]]–]]]
0140-19
doi:10.1
� CorE-m
PleasTeng
journal homepage: www.elsevier.com/locate/jaridenv
Holocene vegetation and climate change from a lake sediment
recordin the Tengger Sandy Desert, northwest China
Yan Zhao a,�, Zicheng Yu a,b, Fahu Chen a, Jiajia Li a
a MOE Key Laboratory of Western China’s Environmental Systems,
College of Earth and Environmental Sciences, Lanzhou University,
Lanzhou 730000, Chinab Department of Earth and Environmental
Sciences, Lehigh University, 31 Williams Drive, Bethlehem, PA
18015, USA
a r t i c l e i n f o
Article history:
Received 18 December 2007
Received in revised form
21 April 2008
Accepted 23 June 2008
Keywords:
Arid China
Fossil pollen
Holocene climate change
Lithology
Qingtu paleolake
63/$ - see front matter & 2008 Published by
016/j.jaridenv.2008.06.016
responding author. Tel.: +86 9318912337; fax
ail address: [email protected] (Y. Zhao).
e cite this article as: Zhao, Y., et al.,ger Sandy Desert,
northwest China.
a b s t r a c t
We present lithology and fossil pollen data from a 384 cm
sediment section from Qingtu
paleolake in arid northwest China and discuss their
environmental interpretations. The
chronology was controlled by four accelerator mass spectrometry
(AMS) radiocarbon
dates on peat and bulk lake sediments. Lithology changes suggest
a general sequence of
local environment shifts from a non-lake environment before 7200
cal yr BP, through a
shallow lake during 7200–3500 cal yr BP and a marsh during
3500–3000 cal yr BP, to a
sandy desert after 3000 cal yr BP. Fossil pollen assemblages
suggest a steppe desert
during 7200–5200 cal yr BP, a period of rapid switches between
upland and lowland
pollen types from 5200 to 3000 cal yr BP, and a desert since
3000 cal yr BP. Both lithology
and pollen data indicate that in a generally arid context,
climate was extremely dry in
the early Holocene, relatively wet at 7200–5200 cal yr BP,
highly variable during
5200–3000 cal yr BP, and dry again after 3000 cal yr BP. The
climate change around
Qingtu Lake was likely controlled by the interplay of the East
Asian summer monsoon,
the mid-latitude westerlies and local topography around the
Tibetan Plateau.
& 2008 Published by Elsevier Ltd.
1. Introduction
Northwest China, in the east margin of arid Central Asia, is
located at the boundary between the East Asian summermonsoon and
the Northern Hemisphere’s westerly winds (Lehmkuhl and Haselein,
2000). As a result, the region is sensitiveto changes in the
large-scale westerly and monsoonal circulation systems. Some
studies have indicated that Holoceneclimatic changes in this region
were mostly influenced by expansion and contraction of the summer
monsoonal circulation(e.g. An et al., 2000; Jiang et al., 2006;
Zhou et al., 2001). However, due to the interplay between the
subtropical monsoonsystem and the mid-latitude westerlies, the
region might have experienced complex pattern of climate change
during theHolocene, neither a direct response to the westerlies nor
to summer monsoon.
The Tengger Sandy Desert immediately to the northeast of the
Tibetan Plateau has an average elevation of 1300 m abovesea level,
while the Tibetan Plateau and bordering Qilian Mountains have
elevations ranging from 3500 to 5000 m. Thisdrastic difference in
topography within a few hundred kilometers could have a large
impact on regional climate change byaffecting uplifting/subsiding
air masses (Broccoli and Manabe, 1992). At the present, the region
lies near the northernmargin of the influence of the East Asian
summer monsoon. These physiographic settings of the study region
suggest thatsedimentary records should provide sensitive records of
regional climate, in responding to the interactions of these
factors.
Elsevier Ltd.
: +86 9318912330.
Holocene vegetation and climate change from a lake sediment
record in theJournal of Arid Environments (2008),
doi:10.1016/j.jaridenv.2008.06.016
www.sciencedirect.com/science/journal/yjarewww.elsevier.com/locate/jnlabr/yjaredx.doi.org/10.1016/j.jaridenv.2008.06.016mailto:[email protected]/10.1016/j.jaridenv.2008.06.016Original
Text:Sesert
Original Text:-cm
Original Text:palaeolake
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1000km
TibetanPlateau
90° 100° 110° 120°80°E
30°
20°
40°
50°N
BLWL
BYC
MDWDDW
DGC Qilian MoutainsQinghai
Lake
Shiya
ng R
iver
TenggerDesert
BadanJaranDesert
QTL
Hei R
iver
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EJL
100km100°E
39N°
37N°
41N°
HS
SJC
BDK
5▲
▲▲▲
▲1
2
102°E 104°E
Fig. 1. Location of the study region and site. (A) Map showing
the location of the study region (open square) in China. Six other
paleoclimate sitesmentioned in the text: WL, Wulun Lake; BL, Bosten
Lake; DDW, Dadiwan; MDW, Midiwan; BYC, Bayanchagan; and DGC, Dongge
Cave. (B) Satellite image
showing the location of the study site (solid square) at Qingtu
Lake (QTL) in Gansu, China and other paleoclimate sites (solid
circles) in the region
discussed in the text (SJC, Sanjiaocheng section; HS, Hongshui
River section; BDK, Biandukou; EJL, Eastern Juyan Lake) and five
nearby climate stations
(white triangles; 1: Wushaoling at 3045 m a.s.l.; 2: Qilian at
2787 m; 3: Gulang at 2072 m; 4: Wuwei at 1531 m; and 5: Minqin at
1367 m).
Y. Zhao et al. / Journal of Arid Environments ] (]]]])
]]]–]]]2
2. Study region and study site
The Shiyang River originates from the Qilian Mountains to the
south, runs through the northwest end of the TenggerSandy Desert,
and has a catchment area of 41,163 km2 (lat. 371020–391170N, long.
1001570–1041570E). The Tengger SandyDesert is one of the major
deserts with active sand dunes in northwest China (Fig. 1). Qingtu
Lake (lat. 3910401500N, long.10313604300E, elevation 1302 m a.s.l.)
was a terminal lake in the Shiyang River Basin, which is totally
dried up at the present.
Mean annual precipitation at nearby Minqin meteorological
station (at 1367 m a.s.l., 110 km southwest of the study site)is
about 115 mm and is highly variable seasonally, most of which falls
as rain during the summer months (Fig. 2A); themean annual
temperature is 7.8 1C; and the potential evaporation is about 2640
mm. In the Shiyang River Basin,precipitation increases with
elevation (Fig. 2B). The distribution of modern vegetation in this
region (Hou, 2001; Huang,1997) is strongly related to precipitation
and to elevation: forest at 42500 m, alpine meadow at 2350–2500 m,
desertsteppe at 2000–2350 m and desert at o2000 m. The vegetation
surrounding Qingtu Lake is dominated by desert plants,
e.g.Chenopodiaceae (including Salsola abrotanoides, Kalidium
gracile, Haloxylon ammodendion, and Ceratoides compacta),Ephedra
przewalskii, Nitraria spp., Tamarix chinensis, Ajania fruticulosa
and Kerelinia caspica.
Several studies on Late Quaternary paleoenvironments in the
Tengger Desert have been published (e.g. Pachur et al.,1995; Zhang
et al., 2004a). Evidence from sediment structure, geochemical
composition, and ostracods suggest that theremight be a
mega-paleolake at 39,000–23,000 14C year BP in the Tengger Desert
(Pachur et al., 1995). The stratigraphicevidence shows that no
lakes existed in the Tengger Desert during the last glacial maximum
(LGM) centered around18,000 14C yr BP (Zhang et al., 2004a). The
paleolakes started to develop again around 12,000 14C yr BP. The
extent of theHolocene paleolakes, primarily migratory lakes, was
smaller than that of the Late Pleistocene paleolakes represented
bystratified lake deposits, alternating with fluvial and eolian
deposits, indicating a long-term oscillation trend toward
aridconditions (Pachur et al., 1995; Zhang et al., 2004a). However,
the detailed climate history in this region is still
poorlydocumented and understood, as these records are mostly based
on geomorphic evidence from terrace deposits that mayhave dating
controls and discontinuous deposition problems.
Three papers focusing on climate change in the Shiyang River
drainage along the northwestern edge of the TenggerDesert have been
published in recent years (Chen et al., 2006; Ma et al., 2004; Shi
et al., 2002), but these authors presenteddifferent interpretations
of climate changes. Shi et al. (2002) reconstructed the lake
evolution of the terminal area of theShiyang River drainage based
on the investigations of geomorphology and sedimentology, and
radiocarbon dates, grain sizeand carbonate of the sediments from
nine sections. The chronologies were based on conventional
radiocarbon dates frombulk carbonate-rich organic and carbonate.
They interpreted high carbonate as low lake level and dry climate.
The resultssuggested a coalescent open lake in the terminal area,
indicating moist early Holocene; and closed shallow carbonate
lakesand swamps during the middle and Late Holocene, showing an
aridification trend. Previously published pollen recordsfrom two
sections in this region were from near river and tributary channel
(Chen et al., 2006; Ma et al., 2004; Zhang et al.,2000; Zhu et al.,
2003). A pollen record from Hongshui section (see location in Fig.
1) suggested that the climate was thewarmest and wettest during
7400–5650 cal yr BP, fluctuated during 5650–4450 cal yr BP, and
humid during 4450–3500 cal
Please cite this article as: Zhao, Y., et al., Holocene
vegetation and climate change from a lake sediment record in
theTengger Sandy Desert, northwest China. Journal of Arid
Environments (2008), doi:10.1016/j.jaridenv.2008.06.016
dx.doi.org/10.1016/j.jaridenv.2008.06.016Original Text:°02’-
Original Text:°17’
Original Text:°57’-
Original Text:°57’
Original Text:° 04’15”
Original Text:°36’43”
Original Text:) was
Original Text:º
Original Text:-2500
Original Text:-2350
Original Text:.
Original Text:, Tamarix
Original Text:late
Original Text:palaeoenvironments
Original Text:-23
Original Text:year
Original Text:palaeolakes
Original Text:year
Original Text:palaeolakes
Original Text:palaeolakes
Original Text:Shi et al., 2002; Ma et al., 2004
Original Text:late
Original Text:5650
Original Text:4450
Original Text:3500
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Fig. 2. Regional climate data. (A) Mean monthly precipitation
and temperature at Minqin climate station (from 1953 to 2001),
about 110 km southwest ofQingtu Lake. Mean annual precipitation is
115 mm, and mean annual temperature is 7.8 1C. (B) Relationship
between elevation and annual precipitationfrom five climate
stations in the study region. Elevation of Qingtu Lake is 1302
m.
Y. Zhao et al. / Journal of Arid Environments ] (]]]]) ]]]–]]]
3
yr BP. However, the pollen results from Sanjiaocheng section
(inferred mainly by montane tree pollen from the QilianMountains)
suggested a dry Mid-Holocene during 7100–3800 cal yr BP, and a
wetter early and Late Holocene (Chen et al.,2006). We here present
pollen and lithology results for a dried-up lake (Qingtu Lake)
sediment sequence to discuss lakedevelopment, major vegetation and
climate changes.
3. Materials and methods
3.1. Sampling and dating methods
A 384-cm-long section (QTL-03) was excavated in October 2003 in
the dried-up Qingtu Lake. The profile was describedin the field and
subsampled at 2 cm intervals. Due to the absence of plant
macrofossils, five samples of bulk organic matterwere radiocarbon
dated using accelerator mass spectrometry (AMS) at AMS Dating
Laboratory at Beijing University(PKUAMS, Beijing, China) (Table 1).
Samples at 40 and 374 cm are from organic-rich peat layers; others
are from organicmatter in carbonate layers. Graphite sample
preparation at PKUAMS is based on direct CO2 catalytic reduction on
ironpowder, as described by Vogel et al. (1987). The d13C values
were measured by a VG Sira-24 mass spectrometer forfractionation
correction. All dates were calibrated to calendar years before
present (present ¼ 1950 AD) based on IntCal04dataset (Reimer et
al., 2004), using the program Calib Rev. 5.01. The age-depth model
was based on linear interpolations ofcalibrated ages (Fig. 3).
Calibrated ages were used throughout the paper.
3.2. Magnetic susceptibility, grain size and carbonate content
analysis
Magnetic susceptibility (MS) was measured on Bartington
Instruments’ MS2B susceptibility meter. Analysis ofcarbonate
content was done using a Bascomb calcimeter. Grain-size analysis
was determined using a Mastersizer 2000
Please cite this article as: Zhao, Y., et al., Holocene
vegetation and climate change from a lake sediment record in
theTengger Sandy Desert, northwest China. Journal of Arid
Environments (2008), doi:10.1016/j.jaridenv.2008.06.016
dx.doi.org/10.1016/j.jaridenv.2008.06.016Original Text:mid
Original Text:3800
Original Text:late
Original Text:cm
Original Text:rich
Original Text:data set (
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Table 1AMS radiocarbon dates and calibrated ages from Qingtu
Lake (section QTL-03), northwest China
Laba number Sample depth
(cm)
Material dated d13C (%, VPDB) 14C date7S.E.(yr BP)
2s range (cal BP)c Median age(cal yr BP)c
LAM06-065 40 Peat �28.8 2835735 2857–3042 2950LAM06-033 118
Organic matter �36.2 2675740b 2744–2853 2799LAM06-066 206 Organic
matter �30.1 3860735 4222–4412 4317LAM06-067 374 Peat �24.1 6285735
7159–7278 7219LAM06-034 380 Organic matter �33.4 11,445750
13,209–13,401 13,305
a AMS Dating Lab at Beijing University (PKUAMS), Beijing.b Date
is too young and rejected.c Calibrated based on CALIB rev. 5.01
using IntCal04 calibration dataset (Reimer et al., 2004).
2835±35
2675±40
3860±35
6285±3511445±150
14C date(Cal yr BP)
Peat Silty marl Fine sandMarl
Depth(cm)
Median grainsize (µm)
Hiatus
Age-depth model
Qingtu Lake, Gansu( )2950
( )2799
( )4317
( )7219
( )13305
Coarse sand
Age (cal yr BP)Carbonate (%)MS (10-8m3/g)
Fig. 3. Sediment lithology and chronology at Qingtu Lake,
northwest China. (A) Lithology and AMS dating horizons; (B)
magnetic susceptibility; (C)median grain size; (D) carbonate (%);
and (E) age-depth model of section QTL-03.
Y. Zhao et al. / Journal of Arid Environments ] (]]]])
]]]–]]]4
(Malvern Instruments) after organic matter and carbonate were
removed by H2O2 and HCl, respectively. All these analyseswere done
at 2-cm intervals.
3.3. Pollen analysis
Pollen subsamples of ca. 20 g were taken at mostly 4 cm
intervals from the section. The subsamples were treated with
amodified acetolysis procedure (Fægri and Iversen, 1989), including
HCl, NaOH, HF and acetolysis treatments, and finesieving to remove
clay-sized particles. The concentrate was mounted in glycerol gel.
A known number of Lycopodiumclavatum spores (batch no. 938934) were
initially added to each sample for calculation of pollen
concentration (Maher,1981). Each pollen sample was counted under a
light microscope at 400� magnification in regularly spaced
traverses;1000� magnification was used for critical identification.
Pollen sums are usually 4300 terrestrial pollen
grains.Identifications followed Wang et al. (1995) aided by the
modern reference collections. Pollen percentages were
calculatedbased on the total pollen sum. Pollen diagrams were
plotted using TGView 2.0 (E. Grimm of Illinois State
Museum,Springfield, IL, USA).
Please cite this article as: Zhao, Y., et al., Holocene
vegetation and climate change from a lake sediment record in
theTengger Sandy Desert, northwest China. Journal of Arid
Environments (2008), doi:10.1016/j.jaridenv.2008.06.016
dx.doi.org/10.1016/j.jaridenv.2008.06.016Original Text:Magnetic
Susceptibility
Original Text:Median
Original Text:Carbonate (%);
Original Text:Age
Original Text:(‰-VPDB
Original Text:E
Original Text:445
Original Text:209
Original Text:401
Original Text:305
Original Text:data set (
Original Text:-cm
Original Text:#
Original Text:×
Original Text:magnification
Original Text:-spaced
Original Text:×
Original Text:magnification
Original Text:Illinois
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Y. Zhao et al. / Journal of Arid Environments ] (]]]]) ]]]–]]]
5
4. Results
4.1. Lithology
The lithology of QTL-03 changes from basal sand to marl and to
surface sand, with multiple layers of peat and silty marlwithin
marl section (Fig. 3A). The section below 380 cm consists of coarse
fluvial sand. Between 380 and 100 cm, thesediment is mainly
composed of carbonate (mostly 70–80%) containing numerous mollusk
shells, with two silty marllayers of low carbonate at 215–206 cm
and 170–160 cm. The upper section above 100 m contains three peat
layersalternated with silty marl and marl layers, capped by fine
eolian sand of high magnetic susceptibility.
4.2. Chronology
Although the dates were on bulk organic matters, carbon isotope
values of �28.8% and �24.1% for peat samples at 40and 374 cm
indicate terrestrial origin of organic matter, so we do not expect
a significant old carbon effect, if any, at thesetwo critical
horizons. There is a dating reversal at 40 cm (2835735 14C BP) and
118 cm (2675740 14C BP). Accepting bothdates would result in an age
model that has about 500 years difference in the last 4000 years.
We rejected the date at118 cm from marl layer that is rich in
carbonate as it was more likely problematic due to its low organic
matter content.There is obviously a sediment hiatus around 380 cm
as bracketed by two dates of 13,300 and 7200 cal yr BP (Fig. 3E).
Theremight be another hiatus around 100 cm (3000–3500 cal yr BP)
based on the sharp lithology change from marl to peat andeventually
to sand. Despite the low number of dates, the two reliable dates at
40 and 374 cm on terrestrial peat shouldprovide adequate chronology
for our discussion of major environmental change during the Mid-
and Late Holocene atmulti-millennial timescale.
4.3. Fossil pollen assemblages
We identified a total of 50 pollen types in 88 samples from
section QTL-03. A summary percentage pollen diagram isshown in Fig.
4. Artemisia (up to 87%), Chenopodiaceae (up to 70%), and
Asteraceae (excluding Artemisia) (up to 84%)switched dominance
throughout the sequence. Poaceae (up to 15%), Ephedra (up to 15%)
and Nitraria (up to 30%) aresecondary dominant regional and local
pollen types. Tree pollen are generally o2% through the pollen
sequence, but reachup to 27% at 5200–4300 cal yr BP (252–204 cm).
Aquatic pollen types include Potamogeton, Alisma and Sparganium,
all beingo1%, while Typha is up to 8%.
The percentage pollen diagram is divided into four pollen
assemblage zones, with subzones when necessary, based
onstratigraphically constrained cluster analysis (CONISS) (Grimm,
1987; Fig. 4).
0
1000
2000
3000
4000
5000
6000
7000
Age
(cal
yr B
P)
Picea
20 20 20 20 20 20 2 6 10 14100 200400 80040 60
Pinus
Sabin
a
Betul
aQu
ercus
UlmusRa
nunc
ulus
Poac
eae
Artem
isia
Aster
acea
e
Chen
opod
iacea
e
Nitrar
ia
Ephe
dra
Polle
nsum
Typh
a
Zone
Hiatus
QTL-1
QTL-2
QTL-3
QTL-4a
QTL-4b
(x1000 grains/g) Total sum of squares
CONISS
80 20 20 20
A/C
ratio
Conc
entra
tion
Mountain plants
Percentage pollen diagram from Qingtu Lake, northwestern
ChinaRegional and local plants
Polyg
onum
Cype
racea
emurtcilahTCarbonate (%)
400 80 40 60 80 4060 80
Fig. 4. Percentage pollen diagram of section QTL-03 at Qingtu
Lake, northwest China. Selected taxa shown.
Please cite this article as: Zhao, Y., et al., Holocene
vegetation and climate change from a lake sediment record in
theTengger Sandy Desert, northwest China. Journal of Arid
Environments (2008), doi:10.1016/j.jaridenv.2008.06.016
dx.doi.org/10.1016/j.jaridenv.2008.06.016Original Text:-80
Original Text:-206
Original Text:-160
Original Text:-28
Original Text:‰
Original Text:-24
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Original Text:300
Original Text:-3500
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Y. Zhao et al. / Journal of Arid Environments ] (]]]])
]]]–]]]6
4.3.1. Zone QTL-1 (374–252 cm; 7200–5200 cal yr BP)
The pollen assemblages were dominated by Artemisia (60–70%),
Chenopodiaceae (�25%) and Poaceae (�15%). A/C ratiowas about 3.
This zone had a total pollen concentration of about 30,000–40,000
grains/g. The basal sand sample (380 cm;13,300 cal yr BP) has low
Artemisia (only 15%) and high Chenopodiaceae (34%), with very low
pollen concentration of�1000 grains/g.
4.3.2. Zone QTL-2 (252–204 cm; 5200–4300 cal yr BP)
Pollen assemblages were characterized by Chenopodiaceae (�50%),
Artemisia (20–40%) and Asteraceae (up to �80%). A/C ratio was
mostly o1. Ranunculaceae and Ephedra had high peaks in this zone.
Tree pollen was at its highest value in thiszone, mainly from Pinus
(up to 26%) and Picea. Pollen concentration was around 1000
grains/g.
4.3.3. Zone QTL-3 (204–100 cm; 4300–3500 cal yr BP)
Artemisia increased and reached the maximum value of 87% while
Chenopodiaceae decreases to �15%. A/C ratio reaches410. Poaceae was
up to 10% in this zone. Pollen concentration was 1000–5000
grains/g.
4.3.4. Zone QTL-4 (100–0 cm; 3500–0 cal yr BP)
Pollen assemblages were characterized by consistently low
Artemisia (mostly 10–20%). Nitraria reached the highestvalue
(�32%). A/C ratio was o1. Two subzones were divided at 3000 cal yr
BP (44 cm) based mostly on change inChenopodiaceae and Asteraceae.
At subzone QTL-4a (100–40 cm; 3500–3000 cal yr BP), Asteraceace
dominated pollenassemblages (�95%), with pollen concentration of
500–3000 grains/g. At subzone QTL-4b (40–0 cm; 3000–0 cal yr
BP),pollen assemblages were characterized by Chenopodiaceae (�70%),
with a total pollen concentration at its lowest value ato600
grains/g.
5. Discussion
5.1. Lake development and local environmental change
At Qingtu Lake, the coarse sand deposit before 7200 cal yr BP
suggests a non-lake environment at the site likely under adry
climate during the early Holocene. The date of 13,300 cal yr BP
from the sand deposit indicates a sediment hiatus of6000 year
duration, going back to the Late glacial. The peat layer of high
organic matter and low carbonate during7200–7100 cal yr BP
represent a wetland environment, when the terminal depression began
to become wet. During7100–3500 cal yr BP, marl sediments with high
carbonate content and uniform grain size indicate a stable and
shallow lakeenvironment, except two silty marl layers at 4400 and
4000 cal yr BP suggesting variable and lower lake levels.
Between3500 and 3000 cal yr BP, a highly variable local environment
was suggested by rapidly changing lithology of peat, silty marland
eolian sand, with large variations of magnetic susceptibility and
grain size. Since 3000 cal yr BP, the lake was dried up atthe
coring site as inferred by eolian sand deposit.
Our environmental interpretations of lithology data above are
different from Shi et al. (2002), as they interpreted highcarbonate
representing dry environment. However, our pollen results as
discussed below from the same section at QingtuLake generally
supported our lithologic interpretation that high carbonate
represent wetter and ‘‘steppe’’-dominatedenvironment.
5.2. Holocene regional vegetation history
We focused on the abundance of Artemisia, Chenopodiaceae,
Ephedra, Nitraria, Asteraceae and Poaceae as reflectingvegetation
change in this study. Artemisia is representative of steppe, while
Chenopodiaceae, Ephedra and Nitraria are thetaxa that are
traditionally used to represent desert (El-Moslimany, 1990;
Herzschuh et al., 2004; Liu et al., 1999). Asteraceaepollens at
QTL-03 likely come from A. fruticulosa based on the surface pollen
results in the Shiyang River Basin (Zhu et al.,2003) and on our
vegetation investigation in the field. A. fruticulosa is widespread
in gravel valleys in the regions of desertand steppe desert (Wang,
1988). We speculate that the abrupt increase of A. fruticulosa
pollen around 4300 cal yr BP andduring 3500–3000 cal yr BP at
QTL-03 might suggest the expanding of A. fruticulosa in the
floodplain after flooding. Anincrease of Poaceae can indicate an
expansion of steppe over desert, suggesting relatively moist
environments based onsurface pollen spectra from Inner Mongolia,
Xinjiang and the Tibetan Plateau (Cour et al., 1999; Herzschuh et
al., 2003; Li,1998; Li et al., 2005; Shen et al., 2006; Yu et al.,
2001; Zhao et al., 2007).
The summary pollen diagram shows clear vegetation changes during
the Holocene (Fig. 4). Vegetation was desertcharacterized by
Chenopodiaceae, Artemisia and Ephedra before 7200 cal yr BP. A
steppe desert dominated mainly byArtemisia and Chenopodiaceae, with
high vegetation cover as inferred from pollen concentration,
developed during7200–5200 cal yr BP. Vegetation appeared to be
highly variable during 5200–3000 cal yr BP showing rapid
switchesbetween highland and lowland pollen types: desert dominated
by Chenopodiaceae, Asteraceae (likely Ajania) and Ephedraat
5200–4300 cal yr BP, steppe desert dominated by Artemisia during
4300–3500 cal yr BP, and desert/steppe desert
Please cite this article as: Zhao, Y., et al., Holocene
vegetation and climate change from a lake sediment record in
theTengger Sandy Desert, northwest China. Journal of Arid
Environments (2008), doi:10.1016/j.jaridenv.2008.06.016
dx.doi.org/10.1016/j.jaridenv.2008.06.016Original Text:-70
Original Text:(~
Original Text:(~
Original Text:000 –
Original Text:000
Original Text:300
Original Text:~
Original Text:(~
Original Text:- 40
Original Text:~
Original Text:~
Original Text:-5000
Original Text:20
Original Text:(~
Original Text:40
Original Text:3000
Original Text:(~
Original Text:3000
Original Text:0
Original Text:0
Original Text:(~
Original Text:300
Original Text:late
Original Text:7100
Original Text:– 3500
Original Text:Liu et al., 1999
Original Text:jania
Original Text:jania
Original Text:jania
Original Text:3000
Original Text:jania
Original Text:Li, 1998
Original Text:Yu et al., 2001
Original Text:5200
Original Text:3000
Original Text:4300
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dominated by Asteraceae type (likely Ajania), Artemisia and
Nitraria 3500–3000 cal yr BP. A desert vegetation dominated
byChenopodiaceae returned after 3000 cal yr BP after the lake dried
up at the coring site.
Tree pollen (mostly from Pinus and Picea) at section QTL-03
appears to have been transported from the mountains and isnot
representative of local vegetation around the study site. Pollen
transport by wind and rivers has been confirmed bymodern surface
pollen studies in this region (Zhu et al., 2003). The high
percentages of Pinus and Picea during5200–4300 cal yr BP in the
section QTL-03 probably resulted from overrepresentation of
transported tree pollen from themountains, due to very low local
pollen production as indicated by the extremely low pollen
concentration during thisperiod. Or alternatively, high input of
transported tree pollen reflected an increase of river runoff from
the mountain.Ranunculus pollen at section QTL-03 could also have
been transported from meadow in the high elevation mountain, sohigh
Ranunculus percentage during 5200–3500 cal yr BP was not indicative
of local vegetation. The abrupt increase of A.fruticulosa pollen at
around 4300 cal yr BP and during 3500–3000 cal yr BP aforementioned
followed the high abundances oftree or Ranunculus, supporting the
speculation about the expanding of A. fruticulosa in the floodplain
after increased riverrunoff from the mountains.
The lacustrine section QTL-03 showed different pollen spectra
from Hongshui section (Ma et al., 2004) and Sanjiaochengsection
(Chen et al., 2006) in the Shiyang River Basin, which contain
averagely ca. 40–55% tree pollen (mainly from Picea,Pinus and
Sabina) throughout the Holocene. Modern surface pollen sample
analysis indicated that both sections near riverchannel contain
abundant tree pollen transported by rivers from the Qilian
Mountains (Ma et al., 2004; Zhu et al., 2003).The Qilian Mountains
have elevations averaging 44000 m, while the Tengger Sandy Desert
has an elevation of 1300 m (Fig.1). If both high elevation and low
elevation vegetation respond to the same precipitation change, then
the general moisturepatterns should be the same at both locations.
However, if stream flow was dependent on glacial melting rates in
thesummer, then summer temperature might have played a major role
in river flow, and consequently the river’s ability totransport
tree pollen from highlands. So we speculate that the difference
between our record and the other two previouslypublished records
(Chen et al., 2006; Ma et al., 2004) is due to different changes in
temperature and precipitation betweenhigh and low elevations. In
any case, because their records contain more tree pollen, abundant
river-transported pollenlikely obscure signals from regional and
local vegetation, especially at Sanjiaocheng site.
5.3. Inferred climate change and possible mechanism
The lake development history and vegetation change suggest the
following sequence of Holocene climate change in ageneral arid
context (Table 2). The climate was extremely dry before 7200 cal yr
BP as inferred from dried-up lake andsediment hiatus. The moistest
climate occurred at 7200–5200 cal yr BP, suggested by stable lake
level, steppe desert anddenser vegetation. A highly variable
climate from 5200 to 3000 cal yr BP was indicated by rapid switches
betweenmountain and lowland pollen types. The dry climate persisted
since 3000 cal yr BP as indicated by eolian sand depositionand
dried-up lake, though coarse resolution analysis does not permit
detailed reconstructions.
The Holocene temporal pattern of climate changes at Qingtu Lake
has also been documented at Eastern Juyan Lake(Herzschuh et al.,
2004) in nearby region (see Fig. 1 for location), which showed the
general dry–wet–dry pattern duringthe Holocene (Fig. 5B). However,
the climate pattern at Qingtu was obviously out of phase with the
records from QinghaiLake on the Tibetan Plateau and Biandukou at
the edge of Qilian Mountains (Yu et al., 2006; Fig. 5C), which are
likelyinfluenced by the East Asian summer monsoon. At Qinghai Lake,
ca. 450 km southwest of Qingtu Lake, but at very
differentelevations (3200 m vs. 1367 m), both oxygen isotopes of
lacustrine carbonate (Lister et al., 1991) and pollen record (Shen
etal., 2005) suggested a wet climate in the early and middle
Holocene induced by stronger summer monsoon. At
Biandukou,multi-proxy record of magnetic, optical and geochemical
properties showed that a generally humid early Holocene andstrong
aridification after 4700 cal yr BP (Yu et al., 2006). Many other
records from semi-arid regions and the Loess Plateau(e.g. Dadiwan,
An et al., 2003; Bayanchagan, Jiang et al., 2006; Midiwan, Li et
al., 2003) also appear to correlate well withthose well-dated
records from Qinghai Lake (Lister et al., 1991; Shen et al., 2005)
and Dongge Cave in southwest China asreported by Wang et al. (2005)
(Fig. 6C), which show strong early and Mid-Holocene summer monsoon
(see Fig. 1A forlocations of these records). However, at sites
further north and west of Qingtu Lake mostly influenced by the
westerlies, adifferent pattern appears to emerge. Chen et al.
(2008) reviewed the Holocene moisture change in arid Central Asia
basedon pollen and other independent climate proxies. Most sites
experienced maximum effective moisture between 8000 and4000 cal BP
in the Mid-Holocene and a slightly wet climate around 2000 cal BP
(Fig. 6B). For example, at Hulun Lake inXinjiang, a pollen record
showed a drying climate in the early Holocene, a relatively wet
climate during 7000–5000 cal
Table 2Summary of evidences for moisture change at Qingtu Lake
(section QTL-03), northwest China
Time intervals (ka) Moisture change Evidences for moisture
change
3–0 Very dry Eolian sand, high percentage of Chenopodiaceae
(desert indicator) pollen
5.2–3 Highly variable High variations in carbonate content and
pollen percentages
7.2–5.2 Stable and wet High carbonate percentage, high
percentage of Artemisia (steppe indicator) pollen
47.2 Very dry Sand deposit, sedimentary hiatus
Please cite this article as: Zhao, Y., et al., Holocene
vegetation and climate change from a lake sediment record in
theTengger Sandy Desert, northwest China. Journal of Arid
Environments (2008), doi:10.1016/j.jaridenv.2008.06.016
dx.doi.org/10.1016/j.jaridenv.2008.06.016Original Text:-0 ka
Original Text:very
Original Text:eolian
Original Text:-3 ka
Original Text:highly
Original Text:high
Original Text:-5
Original Text:ka
Original Text:stable
Original Text:high
Original Text:ka
Original Text:very
Original Text:sand
Original Text:4300
Original Text:3500
Original Text:jania
Original Text:3000
Original Text:jania
Original Text:-55
Original Text:Ma et al., 2004
Original Text:5200
Original Text:doesn’t
Original Text:-wet-
Original Text:the
Original Text:, Dadiwan
Original Text:mid
Original Text:mid
Original Text:5000
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Variable
Dry
Wet
Dry
QingtuLake
0
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Age
(ka)
WulunLake
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Wet
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Biandukou QinghaiLake
Dry
Wet
Dry
Eastern JuyanLake
Out-of-phase monsoon pattern Monsoon pattern Westerly
pattern
2
4
3
Nodata
Fig. 5. Correlations of wet-dry climate intervals from
paleoclimate sites in northwest China. (A) Qingtu Lake (this
study); (B) Eastern Juyan Lake(Herzschuh et al., 2004); (C)
Biandukou (Yu et al., 2006); (D) Qinghai Lake (Shen et al., 2005);
(E) Wulun Lake (Yang and Wang, 1996); and (F) Bosten Lake
(Huang, 2006).
Fig. 6. Correlation of Qingtu Lake record with other Holocene
moisture records and possible mechanism interpretations. (A)
Holocene moisture pattern atQingtu Lake (this study); (B) moisture
pattern in arid Central Asia (Chen et al., 2008); (C) oxygen
isotope from Dongge Cave (Wang et al., 2005) with lower
values indicating stronger monsoon precipitation; (D) summer
insolation at 401N (Berger and Loutre, 1991); and (E) proposed
interpretations for QingtuLake moisture record, considering
possible interactions between the Tibetan Plateau-induced air
subsidence and direct monsoon precipitation.
Y. Zhao et al. / Journal of Arid Environments ] (]]]])
]]]–]]]8
yr BP and variable climate after 5000 cal yr BP (Yang and Wang,
1996; Fig. 5E). At Bosten Lake, lithology and pollen datashowed
that a dry climate in the early Holocene, a wet middle Holocene and
a moderately wet and variable climate after3500 cal yr BP (Huang,
2006; Fig. 5F). Comparison with these records suggests that the
Qingtu climate pattern is out of
Please cite this article as: Zhao, Y., et al., Holocene
vegetation and climate change from a lake sediment record in
theTengger Sandy Desert, northwest China. Journal of Arid
Environments (2008), doi:10.1016/j.jaridenv.2008.06.016
dx.doi.org/10.1016/j.jaridenv.2008.06.016Original
Text:Moisture
Original Text:Oxygen
Original Text:Summer
Original Text:Proposed
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9
phase at certain times with the sites influenced by summer
monsoon (Fig. 6C). Also, the pattern is not simply a reflection
ofthe influence of the prevailing westerlies, considering the
different Late Holocene climate pattern (Fig. 6B).
The contrasting climate patterns might have arisen from the
vertical air dynamics modulated by the difference inelevations of
these sites. During the early Holocene, the maximum summer
insolation (Berger and Loutre, 1991) enhancedthe Asian summer
monsoon (Kutzbach, 1981), leading to high precipitation in the
region affected by monsoon. Many recentpaleoclimate studies have
shown an intensification of the monsoon in the early Holocene and
subsequent weakeningaround 5000–4000 cal yr BP (e.g. An et al.,
2000; Morrill et al., 2003; Shao et al., 2006; Wang et al., 2005).
For example, atDongge Cave in South China where the climate is
influenced by both the SE Asian and Indian monsoons, a decrease
inoxygen isotope values in stalagmites at the beginning of the
Holocene indicated an increase in summer monsoonal rains, asa
result of the onset of an intensified monsoon (Wang et al., 2005).
Qingtu Lake is situated in an arid low-lying basin(1302 m a.s.l.)
to the northeast of the Tibetan Plateau with an average elevation
of ca. 5000 m. This difference in elevationmight have been
important in inducing uplifting and subsiding air motions,
modifying regional climate patterns. Broccoliand Manabe (1992)
found that the heating and upward motion of air over the Tibetan
Plateau causes strong air subsidenceto the northwest and north of
the Plateau, inducing dry climate. Observational data during the
1979 monsoon season (He etal., 1987) also supports the hypothesis
that the intense heating and upward motion over the plateau is
accompanied bysubsidence as compensating flow over the surrounding
areas. This uplifting and subsiding dynamic mechanism has
beenconsidered as an explanation for the spatial pattern of
Holocene wet–dry climate periods in Central Asia (Herzschuh,
2006)and in the Qaidam Basin on the Tibetan Plateau (Zhao et al.,
2007, 2008). As a result, the enhanced air subsidence duringthe
early Holocene at the maximum summer insolation might have caused
dry climate at Qingtu Lake.
During the Mid-Holocene, summer monsoon became weaker due to the
decreasing summer insolation. At Dongge Cave,an increase in oxygen
isotope values in stalagmites during the Mid-Holocene indicated a
decrease in summer monsoonalrains (Wang et al., 2005). A similar
drying trend occurred at Qinghai Lake (Lister et al., 1991; Shen et
al., 2005). A weakeneduplifting on the Tibetan Plateau would cause
less subsiding air flow in the surrounding lowland regions, causing
a dryPlateau and a wet lowland relationship. This uplifting and
subsiding mechanism between highlands and lowlands might
beresponsible for a relatively wet climate during 7200–5200 cal yr
BP around Qingtu Lake. The Late part of the middleHolocene
experienced large climate fluctuations during 5200–3000 cal yr BP
around Qingtu Lake, which also correspondedwith the greatest
transported tree pollen and Ranunculus pollen from Qilian Mountain.
This highly variable transitionperiod might have been caused by the
great contrast of climate changes in highland and lowland regions,
induced by thecompeting factors of subsiding dry airflow versus
direct monsoon precipitation.
A general drying trend in the Late Holocene around Qingtu Lake
inferred from our multi-proxy data has beendocumented at many
records from northwest China. For example, climate became drier
after 3000 cal yr BP at Daihai Lake(Xiao et al., 2004), after 3900
cal yr BP at Eastern Juyan Lake (Herzschuh et al., 2004), after
4700 at Biandukou (Yu et al.,2006), and after 4200 cal yr BP at
Qinghai Lake (Shen et al., 2005). This drying pattern has also been
documented by higheroxygen isotope values in stalagmites from
eastern China (e.g. Shao et al., 2006; Wang et al., 2005; Zhang et
al., 2004b). Thedry Late Holocene in the Qingtu Lake region was
possibly in response to weakened monsoon induced by decreased
summerinsolation and subsequent less precipitation.
In summary, the possible controlling factors on the major
environmental change in the Qingtu Lake region are shown inFig. 6.
Before 7200 cal yr BP, strong summer insolation caused enhanced air
subsidence and dry climate in the lowlandregions surrounding the
Tibetan Plateau, including Qingtu Lake. During 7200–5200 cal yr BP,
less subsiding air flow causedby weaker summer insolation and
relative strong monsoon caused a relatively wet climate at Qingtu
Lake. The interactionsbetween subsiding air flow and direct monsoon
precipitation at the lowlands during 5200–3000 cal yr BP might
beresponsible for the variable climate. After 3000 cal yr BP,
subsiding influence continued to decrease due to
decreasedinsolation and at the same time direct monsoon
precipitation became weak as well, causing a dry climate.
Uncited reference
Liu et al. (1998).
Acknowledgments
We thank Dr. D.S. Xia, Mr. J.X. Chao, Dr. M.R. Qiang and Dr. H.
Zhao for field and laboratory assistance. This project wassupported
by the National Natural Science Foundation of China (Grants nos.
40771212 and 40528001) and NSFC InnovationTeam Project (No.
40721061). The final version of the manuscript was prepared when
the senior author was a VisitingResearch Scientist in the
Department of Earth and Environmental Sciences at Lehigh University
(Bethlehem, PA, USA),partially supported by a US National Science
Foundation grant (to Yu).
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theTengger Sandy Desert, northwest China. Journal of Arid
Environments (2008), doi:10.1016/j.jaridenv.2008.06.016
dx.doi.org/10.1016/j.jaridenv.2008.06.016Original Text:late
Original Text:).
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Original Text:Acknowledgements
Original Text:#
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Please cite this article as: Zhao, Y., et al., Holocene
vegetation and climate change from a lake sediment record in
theTengger Sandy Desert, northwest China. Journal of Arid
Environments (2008), doi:10.1016/j.jaridenv.2008.06.016
dx.doi.org/10.1016/j.gloplacha.2007.12.003dx.doi.org/10.1016/j.jaridenv.2008.06.016Original
Text:.
Holocene vegetation and climate change from a lake sediment
record in the Tengger Sandy Desert, northwest
ChinaIntroductionStudy region and study siteMaterials and
methodsSampling and dating methodsMagnetic susceptibility, grain
size and carbonate content analysisPollen analysis
ResultsLithologyChronologyFossil pollen assemblagesZone QTL-1
(374-252cm; 7200-5200calyrBP)Zone QTL-2 (252-204cm;
5200-4300calyrBP)Zone QTL-3 (204-100cm; 4300-3500calyrBP)Zone QTL-4
(100-0cm; 3500-0calyrBP)
DiscussionLake development and local environmental
changeHolocene regional vegetation historyInferred climate change
and possible mechanism
Uncited referenceAcknowledgmentsReferences