Chinese Science Bulletin **** Vol. ** No.** **** water from the Kaidu River drainage, which originates in

1432

the glaciated mountain ranges of Tienshan. The wide-

Holocene lake deposits of spread Keidu River delta at the western side of the lake is

a favourable area for intensive farming, the latter being

Bosten Lake, southern the main source of nutrients entering the lake via the nu-

merous channels. As a result, the lake became strongly

Xinjiang, China eutrophic over the past few decades. An outflow into the

Tarim Basin supports the irrigated area of the Korla oasis.

1 2,3 1

B. W nnemann, CHEN Fahu, F. Riedel, The river waters had once reached the terminal paleolake

3 1 3

ZHANG Chengjun, S. Mischke, CHEN Guangjie, Lop Nor. As changes in the water balance of Bosten Lake

1 3

D. Demske & MING Jin are strongly controlled by the precipitation pattern and

melt water supply in the catchment area of the lake and

1. Interdisciplinary Centre Ecosystem Dynamics in Central Asia, Freie

Universit t Berlin, Berlin 12249, Germany; tectonic influence has not significantly modified the

2. Desert and Desertification Lab., Cold and Arid Regions Environ-

hydrology in Late Quaternary[1], investigations on the lake

mental and Engineering Research Institute, Chinese Academy of Sci-

sediments seem to be favourable for reconstructing the

ences, Lanzhou 730000, China;

Holocene lake history and its relation to regional climate

3. Center for Arid Environment and Paleoclimate Research, National Lab.

of Western China s Environment Systems, Lanzhou University, Lan- and environmental change. Mineralogical and stable iso-

zhou 730000, China

tope studies on lacustrine carbonates, stable isotopes and

Correspondence should be addressed to Bernd W nnemann (e-mail:

palynological data from sediment sequences some 20 km

abpnzw@r.postjobfree.com)

apart from the lake indicate that several climate spells

Abstract A 9.25-m-long sediment core from Bosten Lake, since 12 ka from warm to cold and dry to wet conditions

Xinjiang, provides detailed information about changes in the and vice versa may have occurred[1,2]. The question how

water budget and biological acticity over the last 8400 cal- the lake itself reacted to climatic deteriorations during the

endar years. The chronology is constructed from six AMS

Holocene is unsolved. In this respect a 9.25-m-long sedi-

radiocarbon dates on the terrestrial plant remains. Based on

ment core, core XBWu 46, from the northwestern part of

analyses of TOC, CO3, detrital compounds and biogenic SiO2,

the lake (41 54 45 N, 86 43 61 E) was obtained by us-

lake level fluctuations and periods of remarkably-negative

water budget appeared at 8.4 8.2 cal ka, 7.38 7.25 cal ka,

ing a piston coring system. The water depth at the drilling

5.7 5.5 cal ka, 3.7 3.4 cal ka and 3.3 2.9 cal ka, respec-

site was 6.25 m based on a water level of 1046 m a.s.l. in

tively. As they are in-phase with low lake levels at Sumxi Co

summer 2000 (Fig. 1).

and Bangong Co in western Tibet Plateau and with paleo-

lakes in Inner Mongolia, a climate-induced change to some- 1 Methods

what drier and warmer conditions is inferred. A further drop

Sediment samples were taken every 2 cm, giving a

in lake level after 1320 AD of about 200 yr duration may be

mean temporal resolution of 20 years. In total, 437 sam-

attributed to a negative water balance prior to the main

ples were analyzed for TOC and CO3 content on homoge-

phase of the Little Ice Age. Deep and stable lake phases of

nized samples using two methods: loss on ignition (LOI

1500 yr and 1800 yr duration at 7.2 5.7 cal ka and 5.5 3.7

550 and LOI 880 ) and carbon analyser (Str hlein

cal ka coincide with maximum moisture during the Holocene

instruments). The residue (expressed as silicates) is de-

Megathermal in China. The long term trend towards aridity

since about 4.3 cal ka can clearly be recognised. The reduced rived from the LOI excluding the remaining CaO and the

water budget of Bosten Lake from 640 1200 AD may be biogenic silica. The latter was dissolved on original sam-

attributed to local effects. ples by 1 mol/L NaOH and quantified by a photometer.

For the age control we separated organic matter from

Keywords: Bosten Lake, Holocene, sediments, water balance, lake

level, climate change. the samples at 1, 63, 367, 587 and 848 cm depth respec-

tively. Radiocarbon-AMS dates on terrestrial plants re-

DOI: 10.1360/02wd0270

mains and organic matter provided ages of 102 24 yr BP

( 13C: 23.75, Ki 13113: 1809 1925 cal AD), 1207

Bosten Lake (86 40 87 26 E, 41 56 42 14 N)

23 yr BP ( 13C: 9.39, Ki 13114: 809 cal AD), 3866

is the largest freshwater body in northwestern China and

30 yr BP ( 13C: 26.66, Ki 13115: 4,220 cal BP), 4949

covers an area of about 1000 km2 with a maximum water

33 yr BP ( 13C: 9.61, Ki 13116: 5689 cal BP) and

depth of 14 m in the centre and southeastern part of the

7368 36 yr BP ( 13C: 25.61, Ki 13117: 8175 cal BP).

lake respectively. It is located in the southern part of the

As the calculated sedimentation rates varied between 0.56

tectonically influenced Yanqi Basin in southern Xinjiang.

and 2.03 mm/yr, although these strong differences could

The area is part of the arid region of western China with

68.2 mm mean annual precipitation and a mean annual

temperature of 6.3 [1]. Bosten Lake is mainly supplied by

Chinese Science Bulletin Vol. 48 No. 14 July 2003 1429

REPORTS

Fig. 1. Core XBWu 46 from Boston Lake, southern Xinjiang, China: lithology, geochemical proxies and biogenic SiO2 related to depth and radiocar-

bon ages (14C yr BP and calendar years BP/AD). Calibration refers to calib 4.3 programme[3].

not be inferred from sediment structure and composition, minerals). Neither authigenic carbonate, nor fossil re-

we calculated mean sedimentation rates for the lower part mains (diatoms, ostracods and molluscs) have been found.

The angular shaped grains indicate the influence of frost

(367 890 cm depth) to 1.37 mm/yr and 0.94 mm/yr for

weathering due to a very low lake level and total freezing

the upper part (0 367 cm depth). As a result, age correc-

of the water body down to the bed during wintertime. It is

tions for those with high 13C values yield age deviations

also possible that the lake did not reach the drilling site at

of 450 and 340 yrs. respectively, referred to the calibrated

that time and the Kaidu River delta extended further to

mean age of 2 sigma probabilities. The uppermost date

east. This is supported by the fact that terrestrial fungal

was corrected to 1980 AD.

spores have been identified, even indicating the total lack

2 Results of a waterbody at that position. However, based on the

recent lake level a decline of approx. 14 m at least at

Based on the assumption that the lake level decline

about 8400 calendar years can be assumed.

of Bosten Lake promotes the extent of the Kaidu River

Unit 2 (890 830 cm depth, 8.4 8.0 cal ka). Al-

delta and deposition of coarser material eastward while

ternating layers of peat, lake carbonate rich in fossil re-

rising lake levels result in the retreat of the subaerial part

mains and sand mark the onset of lake formation although

of the delta, and only fine-grained suspension load can

the shallow lake still fluctuated frequently. The input of

reach the coring site, we can use the sediment proxies of

fluvial sand between 8.1 and 8.3 cal ka, centered at about

TOC, CO3, detrital compounds and biogenic silica to dis-

8.2 cal ka, is related to a drop in lake level of 200 yrs du-

tinguish between high and low lake levels as well as

ration, although the lake did not dry up as blooming peri-

variations in bioactivity. Seven different units reflect

ods of diatoms indicate. High percentages of TOC and

changes in sedimentation processes due to variable water

CO3 alternate with extreme low values due to the input of

budget of the lake:

fluvial sand.

Unit 1 (925 890 cm depth, approx. 8.5 8.4 cal

Unit 3 (830 560 cm depth, 8.0 5.8 cal ka). The

ka). The sediment is composed of dark coloured me-

continuous increase of lake carbonate from 25% to 30%

dium to coarse sand of fluvial origin, containing high per-

CO3, high values of TOC (between 1.5% and 3.5 %) and

centages of very angular grains (quartz and opaque

Chinese Science Bulletin

1430 Vol. 48 No. 14 July 2003

the decrease of detrital input of silicates (from 65% to and the increase of lake carbonate. Abundant ostracods

45% dry weight) are indications for a relatively stable and and molluscs also point to well oxygenated freshwater

biologically active lake, lasting for more than 2000 years. conditions. A change to reduced water depth and quality

Numerous aquatic plant remains, molluscs and ostracods can be inferred from the uppermost sediment sequence.

but low amounts of diatoms may reflect an oligotrophic The total lack of oxygen in the upper sediment points to

freshwater lake with active life at its bottom. A minor de- reducing conditions in that sequence. This is the reason

cline in lake level from 7.38 to 7.25 cal ka is assumed by a that living molluscs and ostracods were not found on the

higher input of sand into the lake body. sediment surface.

Unit 4 (560 378 cm depth, 5.8 4.3 cal ka). This 3 Molluscs and ostracods

unit is marked by remarkably low values of TOC (1% Altogether 12 mollusc species have been identified

2%), while CO3 is still increasing continuously to 40%. and attributed to the genera Bithynia (Caenogastropoda,

Lowest TOC and carbonate values as well as high detrital Rissooidea), Valvata (Heterobranchia, Allogastropoda)

input of silicate from 5.73 to 5.5 cal ka are interpreted as a Gyraulus, Radix (both Heterobranchia, Basommatophora)

decline in lake level due to a reduced water supply via the and Pisidium (Autolamellibranchiata, Veneroida). None of

drainages and/or increase in the evaporation/precipitation the species is endemic.

ratio. This deterioration may not have been very severe, Ostracods are abundant in nearly all parts of the

because stable and possibly deep lake conditions occurred sediment. They comprise 15 species: Darwinula steven-

just afterwards, lasting for about 1400 years. Due to the soni, Candona candida, Candona neglecta, Fabaeformis-

low TOC values we assume that the lake was deepest at candona caudata, Fabaeformiscandona cf. levanderi,

that time, and subaquatic vegetation development at the Fabaeformiscandona hyalina, Pseudocandona compressa,

lake bottom was reduced. Pollen spectra, ostracods re- Cyclocypris ovum, Ilyocypris cf. gibba, Ilyocypris cf.

cords and geochemical analyses (not shown here) support bradyi, Herpetocypris chevreuxi, Cypridopsis vidua,

this assumption. These results will be discussed else- Limnocythere inopinata, Cytherissa lacustris and Cypri-

where. deis torosa. Notably F. cf. levanderi, P. compressa, C.

Unit 5 (378 118 cm depth, 4.3 cal ka 640 AD). ovum, I. cf. gibba, F. caudata and C. lacustris were not

recorded in the uppermost centimeters of the core[4]. Es-

This unit of silty and sandy lake carbonate is characterized

by strong fluctuations in carbonate content, detrital input pecially the absence of the latter two species point to

of silicate and TOC. They indicate frequent changes in the shallow water conditions, while the first six taxa as well

water balance of the lake. Remarkable lake level declines as the presence of further 4 taxa, notably Candona welt-

neri, Pseudocandona cf. hartwigi, I. salebrosa, Hetero-

occurred at 3.7 3.4 cal ka, 3.3 2.95 cal ka and 2.23 cal

cypris salina and Sarscypridopsis aculeata are indicative

ka 80 AD, identified by the increase of fluvial input of

of changes in water quality during the past decades, which

silty/sandy material via the Kaidu River system.

may coincide with the industrial pollution of the lake wa-

Unit 6 (118 65 cm depth, 640 1320 AD). Most

ter.

of that sequence is characterized by very low TOC,

4 Discussion and conclusion

carbonate and biogenic silica contents, while detrital input

is dominant. We infer that the lake level decline was as Considerable changes in TOC content, total carbon-

strong as during lower lake level periods of unit 5, also ate, detrital input (silicates) and biogenic silica of

indicated by the increase of Mg/Ca mole ratio and other XBWu46-core from Bosten Lake indicate variations in the

geochemical data, which will be discussed in detail else- water budget and biological activity of the lake since 8400

where. However, the lake did not dry up. A freshwater cal yr BP. The record starts with evidence for a very low

input may have resulted in a short-term lake level rise lake level from 8.5 to 8.3 cal ka. The frost-disturbed

after 1200 AD. structure and angular sand all indicate a quite cold climate

during the short period although our core did not penetrate

Unit 7 (65 0 cm depth, 1320 AD-present). Three

the sand layer at the drilling site. The onset of a positive

different sediment types have been recorded: a brown

water budget with rising, but fluctuating lake levels cen-

peaty lake carbonate in the lower part, greyish-white

ters on 8.2 cal ka, indicating increasing moisture in the

sandy lake carbonate in the middle part and dark brown

catchment area of Bosten Lake. This result does not

silty mud in the upper 12 cm.

clearly fit to the cold event, recorded in Greenland ice

Between 1300 AD and 1520 AD, coeval with the

cores[5,6]. However, the possible correlation of the low

onset of the Little Ice Age, the lake shrank due to the lack

lake level and cold climate event with global 8.2 ka cold

of water supply, leading to the development of peaty mud

event still exists, considering our chronology is estab-

at the drilling site. With regard to the present lake level, a

lished according to the average sedimentation rate, which

decline of 6 m must be assumed. A return to a positive

may not be suitable for the bottom sand layer. After the

water budget and increasing lake level between 1520 and

sandy layer a reverse trend is seen: a rising lake level, peat

1870 AD is documented by the decrease of detrital input

Chinese Science Bulletin Vol. 48 No. 14 July 2003 1431

REPORTS

35

development and precipitation of calcite require increas-

3. Stuiver, M., Reimer, P.J., Bard, E. et al., INTCAL98 radiocarbon

ing moisture and a regional warming. Thus it follows that

age calibration, 24,000 0 cal BP, Radiocarbon, 1998, 40: 1041

high lake levels and stable water budgets from 8.1 to 3.7

1083.

cal ka must be attributed to warm and moist climatic con- 4. Mischke, S., Schudack, M., Sub-recent Ostracoda from Bosten

ditions representing the Holocene Megathermal. Reversed Lake, NW China, J. Micropalaeontology, 2001, 20: 12.

5. Grootes, P. M., Stuiver, M., Oxygen 18/16 variability in

trends with drops in lake level from 7.38 to 7.25 cal ka

Greenland snow and ice with 103 to 105-year time resolution, J.

and 5.73 to 5.5 cal ka agree with records from western

Geophys. Res., 1997, 102: 264**-*****.

China[7,8] and Inner Mongolia[9 11]. From 3.7 cal ka on- 6. Johnsen, S. J., Dahl-Jensen, D., Gundestrup, N. et al., Oxygen

ward Bosten Lake underwent significant lake level fluc- isotope and palaeotemperature records from six Greenland

tuations, resulting in periods of falling lake level from 3.7 ice-core stations: Camp Century, Dye-3, GRIP, GISP2, Renland

and NorthGRIP, J. Quaternary Sci., 2001, 16: 299 307.

to 3.4 cal ka, 3.3 to 2.9 cal ka, 2.3 cal ka to 80 AD, 640 to

7. Gasse, F., Arnold, M., Fontes, J. C. et al., A 13,000-year climate

1210 AD and 1320 to 1520 AD respectively. The latter

record from western Tibet. Nature, 1991, 353: 742 745.

desiccation period may be partly attributed to the onset of 8. Gasse, F., Fontes, J. Ch., van Campo, E. et al., Holocene environ-

climatic deterioration during the Little Ice Age, although mental changes in Bangong Co basin (Western Tibet), Part 4:

during the main period of that cold event (1500 1870 Discussion and conclusions, Palaeogeography, Palaeoclimatology,

AD[12]) Bosten Lake had a positive water budget. The Palaeoecology, 1996, 120: 79 92.

9. W nnemann, B., Pachur, H. -J., Zhang, H., Evidences for climatic

higher frequency of lake level changes supports the argu-

and environmental changes in the deserts of Inner Mongolia,

ments for a long-term trend towards aridity[8] since about China since the Late Pleistocene, Quaternary Deserts and Climate

4.3 cal ka (3900 14C yr BP). However, there is clear evi- Change (eds. Alsharhan, A. S., Glennie K. W.), Rotterdam:

dence that the lake has never dried up completely since Balkema, 1998, 381 394.

10. Chen Fahu, Qi Shi, Jiang-Ming Wang, Environmental changes

8.3 cal ka.

documented by sedimentation of Lake Yiema in arid China since

the Late Glaciation, J. Palaeolimnology, 1999, 22(2): 159 169.

Acknowledgements Fieldwork was partly supported by China Na-

11. Zhang, H. C., Ma, Y. Z., W nnemann, B. et al., A Holocene cli-

tional Cooperation Project (Grant No. 2002CB714004).

matic record from arid northwestern China. Palaeogeography,

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Chinese Science Bulletin

1432 Vol. 48 No. 14 July 2003

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