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Quaternary International 212 (2010) 2–13

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Quaternary International

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Stable isotope and ostracode species assemblage evidence for lake levelchanges of Nam Co, southern Tibet, during the past 600 years

Claudia Wrozyna a,*, Peter Frenzel a,b, Philip Steeb a,b, Liping Zhu c, Robert van Geldern d,Andreas Mackensen e, Antje Schwalb a

a Institut fur Umweltgeologie, Technische Universitat Braunschweig, Pockelsstr. 3, 38106 Braunschweig, Germanyb Institut fur Geowissenschaften, Friedrich-Schiller-Universitat, Burgweg 11, 07749 Jena, Germanyc Institute of Tibetan Plateau, Chinese Academy of Science, Beijing 100085, PR Chinad Institut fur Geowissenschaftliche Gemeinschaftsaufgaben, Stilleweg 230655, Hannover, Germanye Alfred-Wegener-Institut fur Polar- und Meeresforschung, Columbusstrasse, 27515 Bremerhaven, Germany

a r t i c l e i n f o

Article history:Available online 13 January 2009

* Corresponding author. Tel.: þ49 (0)531 391 7259;E-mail address: c.wrozyna@tu-bs.de (C. Wrozyna)

1040-6182/$ – see front matter � 2009 Elsevier Ltd adoi:10.1016/j.quaint.2008.12.010

a b s t r a c t

Ostracode species assemblages and stable oxygen and carbon isotopes of shells from two 210Pb-datedshort cores of 78 cm and 47 cm length taken from Lake Nam Co, southern Tibet, provide informationabout lake level history of the past approximately 600 years at decadal resolution. A transfer function isapplied to reconstruct paleo-water depths from modern ostracodes. Ostracode assemblages and isotopedata from five species allow a subdivision into phases of lake level evolution. Before w1600 AD relativelylow oxygen isotope values and high abundances of deep water (>30 m) ostracodes (i.e. ?Leucocytheredorsotuberosa, ?Leucocythere dorsotuberosa f. postilirata) suggest a lake level w10 m higher than today.Between w1600 AD and 1800 AD, more positive oxygen isotope values and higher abundances ofshallow water ostracodes (i.e. Ilyocypris cf. mongolica, Candona xizangensis) suggest a lake levelapproximately 40 m below modern level. Between 1800 AD and w1920 AD oxygen isotopes are char-acterized by a trend to more negative values followed by a trend to more positive ones. These trends areinterrupted by several short-term fluctuations but overall, oxygen isotope values suggest a drop in lakelevel. A clearer lake level picture is provided by species assemblages that suggest a continuouslyincreasing lake level since 1900 AD. Precipitation data from meteorological stations nearby show anincrease in precipitation in the last 30 years. The low lake level between 1600 AD and 1800 AD iscontemporary to the Little Ice Age. During that cold period, meltwater input into the lake decreased. Thisunderlines the sensitivity of the level of Lake Nam Co to temperature changes on the Tibetan Plateau.

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1. Introduction

The Tibetan Plateau responds very sensitively to climatic changebecause of its high altitude, strong solar radiation, and the complexinteraction of moisture transport and heating. The Tibetan Plateauis also referred to as the ‘third pole’ and has significant effects on theSouth and East Asian monsoons, and on global climate. Today, theTibetan Plateau is experiencing a temperature increase approxi-mately 40% above the global mean (Wang et al., 2001; IPCC, 2007).Various proxy records from different archives, such as ice cores (e.g.Li and kang, 2006), tree rings (e. g. Brauning, 2006), speleothems(e.g. Denniston et al., 2000), loess deposits (e.g. Chen et al., 2006;Wu et al., 2006), lake and marine sediments, reveal the mode of

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interaction between the components of monsoonal circulation (e.g.Morrill et al., 2006; Xu et al., 2007) and have improved the under-standing of Pleistocene–Holocene evolution and feedback betweenthe large scale atmospheric circulation systems and the regionalresponse of the environmental systems of interior Asia. Theresulting picture, however, is rather incoherent, and a plateau-widecompilation of the development of the monsoon system is stillmissing. This study presents a high-resolution record from a lakesystem in southern Tibet that reveals the timing and response toglobal climate change on a regional scale. Lake Nam Co, the worldlargest high-altitude saline lake system on the plateau, was chosenbecause of its position within the interference zone of the East Asianwinter monsoon, Indian summer monsoon, the Westerlies and thesubtropical jet stream. Lake level changes may provide informationon changes in monsoon impact and associated shifts in precipita-tion amounts, precipitation–evaporation ratios, and meltwaterinput as a consequence of temperature changes.

Fig. 1. Location of Nam Co and the bathymetric map of the eastern part of the lake with the position of the two studied short cores. Bathymetric data from Zhou et al. (2006) withadditions. The N–E transect refers to Fig. 9.

Table 1General geographical, climatological and limnological characterization of LakeNam Co.

Altitude 4718 m a.s.l.N–S 40 kmE–W 70 kmCatchment area 1.06 � 104 km2

Annual precipitation 479 mmAnnual average temperature 1.6 �CMaximum water depth 105 mIce cover 3–5 months (2 m ice thickness)pH 9.5Salinity 2.9–1.3 g l�1

The water depth value follows measurements in 2005 (G. Daut, personalcommunication).

C. Wrozyna et al. / Quaternary International 212 (2010) 2–13 3

Holocene climate records from the region show a cooling trendthrough the Holocene interrupted by several warm (Fontes et al.,1996; Wang et al., 2002; Bao et al., 2003; Herzschuh et al., 2006; Yiet al., 2006; Yu et al., 2006) and cold (Fontes et al., 1996; Bao et al.,2003; Herzschuh et al., 2006; Morrill et al., 2006; Yu et al., 2006)events. Highest lake levels, indicating warm and wet conditions,occurred during the early Holocene (Lister et al., 1991; Feng et al.,2006; Mischke and Wunnemann, 2006; Fan et al., 2007), but sitesdiffer in the duration and onset of high lake levels. During the midHolocene, after w4000 years BP, pollen records and lake sedimentssuggest a short-term fluctuation to drier conditions caused by reducedprecipitation (Herzschuh et al., 2006; Morrill et al., 2006; Yi et al.,2006), which is not displayed in all regions of the plateau. The timingand the onset of climatic episodes, such as the Little Ice Age (LIA), hintat temporal and spatial heterogeneities for the Tibetan Plateau. Herewe provide information from southern Tibet using ostracode speciesassemblages and the stable oxygen and carbon signatures of theirshells. Ostracodes are the most abundant microfossils in Lake Nam Coand have already successfully been used as indicators of lake levelevolution (Lister et al., 1991; Mischke et al., 2005), changes inprecipitation (Holmes et al., 2007) and salinity (Mischke and Wun-nemann, 2006; Mischke et al., 2007) as response to climatic change onthe Tibetan Plateau. Ostracode species assemblages from two shortcores are used to reconstruct the recent lake level dynamics of LakeNam Co with decadal resolution for the past approximately 600 years.In addition, we apply a transfer function based on the water depthdependence of ostracode species distribution from surface sedimentsamples, and integrate results from species assemblages with theisotopic signature of their calcareous valves.

2. Regional setting

Lake Nam Co is located in the southern part (30� 330 N, 90� 540 E)of the Tibetan Plateau about 120 km north of the city of Lhasa (Fig.1).The climate of this region is semi-arid to semi-humid continental(Feng et al., 2006). The catchment area of the surficially closeddrainage basin comprises an area of about 10,000 km2. The lakesurface covers an area of almost 2000 km2, and the maximum depthis at least 105 m (G. Daut, personal communication). Because NamCo has no outflow, the water balance is mainly controlled by inflow,precipitation and evaporation. Precipitation is mainly limited to thesummer months with a mean rainfall of 300 –400 mm per year(T. Yao, personal communication). Several melt-water streams fromglaciers of the Nyainqentanghla Shan (Shan ¼mountains) at the

Fig. 2. Age-depth model for cores Nam Co 4/ and Nam Co 6/1 based on 210Pb activities.

C. Wrozyna et al. / Quaternary International 212 (2010) 2–134

southern margin of the catchment area provide the main watersource. Rivers that enter the lake, mainly in the southern andsouthwestern parts, drain wetlands. The general geographical,climatological and limnological setting is summarized in Table 1.

3. Materials and methods

Two short sediment cores, Nam Co 4/2 and Nam Co 6/1, each79 cm and 47 cm long, were taken in the northwestern part of thelake from 49.2 m and 64 m water depth, respectively, using a UWI-TEC gravity corer during September 2005 (Fig. 2). The chronology ofthe sediments was established using 210Pb and 137Cs datingmethods. Measurements of 210Pb and 137Cs were carried out at theGGA, Hannover. Ages and sedimentation rates were calculated usinga CRS model assuming a constant rate of supply of 210Pb to thesediment according to Appleby and Oldfield (1978). Nineteensamples of 0.5 cm thickness were taken every 0.5 cm for the top19 cm and every 1 cm between 19 and 79 cm for microfossil analysis.Fifteen 0.5 cm thick slides of sediment were taken between 28 and7.5 cm sediment depth from core 6/1 for microfossil analysis. Thesample size for ostracode assemblage analysis depended on theamount needed for 210Pb dating and ranged between 3 and 5 g.

Based on first results from core 4/2 and the observed lithologicalchanges in core 6/1, this depth interval was investigated forcorroborating evidence of lake level change. The water content wasdetermined gravimetrically by drying at 105 �C for 24 h.

The samples from short core 4/2 were processed prior to sepa-ration of ostracode shells with 5% H2O2 for 24 h, washed through1 mm, 200 mm,125 mm and 63 mm sieves and dried at 50 �C. For core6/1 H2O2 treatment was omitted in order to avoid destruction oforganic remains>63 mm. The sediment components such as clastics,charcoal, pyrite and gypsum (inclusive microfossil groups) of thiscore were documented semi-quantitatively by checking the wetresidues and using frequency classes (compare Fig. 8). All size frac-tions >63 mm served for this semi-quantitative frequency estima-tion, whereas we counted the ostracodes of the dried>200 mm sizefraction in detail. For quantitative ostracode analysis, the sampleswere split into subsamples using a micro-splitter and grid bearing

picking trays until they yielded more than 300 and normally lessthan 500 valves. The raw counting data are given in Tables 2 and 3.Counting of these splits served as a base for species abundances andtotal ostracode number calculation. Additionally, the number ofcarapaces (both valves articulated) and the percentage of opaque,coated and abraded valves as an indicator for post-mortem transportand sedimentation conditions were counted (De Deckker, 1988). Todetermine similar sample sites through the core, the Renkonen(1938) percent similarity index was used.

We use a water-depth transfer function based on ostracodespecies dominances for reconstructing paleo-water depth. Thetransfer function was developed using 14 modern samples from NENam Co in a water depth range between 3.7 and 64 m water depth.The paleo-water depth was calculated from dominance values ofLeucocythere dorsotuberosa f. postilirata, Ilyocypris cf. mongolica,Fabaeformiscandona danielopoli and Candona spp. in a weightedaverage method. The standard error for estimated water depth is7.6 m. For details see Wrozyna et al. (accepted). The estimatedwater depth curve of core Nam Co 4/2 is corrected by �9.2 m,because of its distal position to locations of shallow water sedimentredeposition as discussed in Section 5. This difference of �9.2 m isthe present day difference between measured and estimated waterdepth for this core position. For stable isotope analysis we choseshort core Nam Co 4/2, because of its central and isolated positionin lake Nam Co, where an input of allochthonous ostracode valvesshould be at a minimum. Ten to fifteen adult valves of five specieswere selected from the residues of the short core Nam Co 4/2sediments and cleaned with a fine brush and deionized waterunder a stereo-microscope. Valves of Leucocytherella sinensis and?Leucocythere dorsotuberosa were abundant and could be separatedfrom each sample level. In addition, we used valves of ?Leucocy-there dorsotuberosa f. postilirata, Candona xizangensis and Ilyocypriscf. mongolica if there were enough specimens available. Stableisotope analyses were performed at the Alfred-Wegener-Institut furPolar- und Meeresforschung, Bremerhaven, with a Finnigan Mat251 isotope ratio gas mass spectrometer coupled to an automatedcarbonate preparation device (Kiel II). Stable oxygen and carbonisotope ratios are reported in delta per mil notation relative to the

Table 2Encountered ostracode species of core Nam Co 4/2.

Depth(cm)

Valves per g Countedvalves

Carapaces Poorlypreservedvalves

L.sinensis L. dorso-tuberosa

L. dorso-tuberosa f.postilirata

I. cf.mongolica

C. candida C.xizangensis

E. gyirongensis F. danielopoli L. inopinata Juvenilecandonids

Ad. All Ad. All Ad. All Ad. Juv. Ad. Ad. Ad. Ad. Ad.

0.75 81 450 0 3 72 344 13 69 3 23 0 0 0 2 0 0 0 121.75 132 568 1 12 68 399 15 118 1 36 1 1 0 0 0 0 0 142.75 199 778 0 10 115 520 28 174 10 54 0 0 0 1 0 0 0 253.75 189 695 1 0 57 463 33 174 7 42 0 1 0 0 0 0 0 154.75 140 713 0 6 91 542 17 132 2 27 0 2 0 0 0 0 0 85.75 139 698 3 14 155 527 23 116 1 29 2 2 0 0 1 0 0 236.75 127 588 0 9 107 433 13 105 3 23 0 2 0 1 0 0 0 227.75 56 302 1 4 53 231 4 39 3 16 0 0 0 0 0 0 0 178.75 71 326 0 11 61 236 3 53 0 17 0 0 0 0 0 0 1 189.75 67 297 1 20 74 213 7 58 5 15 0 1 0 0 0 0 0 910.75 28 178 1 0 37 145 12 20 1 3 0 0 0 2 0 0 0 811.75 41 290 0 0 63 231 11 33 3 11 2 6 0 1 0 0 0 812.75 112 659 0 10 169 511 27 98 13 33 1 3 0 3 0 0 0 1413.75 85 570 1 5 155 444 40 75 21 37 3 3 0 2 0 0 0 1014.75 133 645 1 6 129 466 40 129 18 42 1 1 0 2 1 0 0 415.75 106 532 4 15 77 382 22 105 10 43 0 0 0 1 0 0 0 116.75 98 458 4 3 109 321 10 97 8 39 0 0 0 0 0 0 0 117.75 204 993 4 10 187 701 24 198 10 83 2 5 0 0 0 0 0 618.75 157 752 4 11 180 524 34 154 11 64 3 3 0 0 0 0 0 319.75 83 436 3 2 98 310 20 80 1 30 4 5 0 2 0 0 0 321.5 410 1566 11 6 227 1040 49 374 8 103 7 12 0 3 0 1 0 3622.5 159 946 13 35 200 712 22 123 9 63 5 9 0 3 0 0 0 3623.5 214 1208 38 72 290 880 32 200 8 72 10 28 0 4 0 0 0 1424.5 208 1372 24 78 294 934 40 180 20 60 7 17 0 8 0 0 2 28

Ad., adult specimens.

C.Wrozyna

etal./

Quaternary

International212

(2010)2–13

5

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C. Wrozyna et al. / Quaternary International 212 (2010) 2–136

international VPDB (Vienna Pee Dee Belemnite) standard (Table 4).Reproducibility is based on repeated analyses of an internal labo-ratory standard (Solnhofen limestone) and was better than 0.08and 0.06 & (1s) for oxygen and carbon isotopes, respectively.

4. Results

4.1. Chronology and lithology

The chronology is based on 210Pb activities and additionalmeasured 137 Cs. The constant decline of 210Pb activities corre-sponds to the natural radioactive decay and to a constant sedi-mentation rate for the last 200 years of sedimentation. Thesediment record reaches back to approximately 1350 AD (Nam Co4/2) and approximately 1400 AD (Nam Co 6/1) (Fig. 2). A chro-nology was established based on the 210Pb dating and the extrap-olation of the measured 210Pb activities assuming long termequilibrium rates of sediment accumulation extended to sedi-mentary levels below detectable unsupported 210Pb (Binford, 1990;Woods et al., 2000). The constant decline of 210Pb and thehomogenous grain size indicate continuous sedimentation andthus support the chronology, however, any extrapolation isa weaker approximation of true dates and any changes in accu-mulation rates may result in an invalid extrapolation (Binford,1990). Artificial radionuclides such as 137Cs as essential chro-nostratigraphic markers (Arnaud et al., 2006) were measured toconfirm the 210Pb chronology but were only detectable in twosamples. They show similar rates (1.05 mm a�1) to the 210Pb derivedsedimentation rates (1.15 mm a�1). We did not use 14C dating forour short core chronologies because 14C dates of a long corerevealed a reservoir effect of 949 14C years (Mugler et al., 2008) thatis larger than the absolute ages of our short cores. A correlation of14C ages corrected for reservoir effect of the long core and 210Pbages of the short core Nam Co 4/2, show lower ages in the long corewith a deviation ranging from 13 to 31 years (mean 20 years) anda reversal in the last sample level of the core where the short coreage is younger. Short core Nam Co 6/1 shows higher age differencesto the corrected 14C ages ranging from 6 to 216 years (mean81 years), increasing with depth. This correlation shows that 210Pbages are useful but contain some uncertainties.

Short core Nam Co 4/2 consists of homogenous grey brown siltswithout any visible layers (Fig. 4). Short core Nam Co 6/1 consists ofsilt and also shows two distinct layers composed of silty fine sand(Fig. 6).

4.2. Ostracoda

In total, ten ostracode species were recorded from sedimentcores Nam Co 4/2, Nam Co 6/1. Only six of these species were foundin the surface sediments from Nam Co (Wrozyna et al., accepted).Besides the cosmopolitan species Limnocythere inopinata andCandona candida all other ostracode species are described from theTibetan Plateau and seem to be endemic.

Both sediment cores contain frequent valves of Leucocytherellasinensis, which represent more than 90% of total valves. In addition,?Leucocythere dorsotuberosa Huang et al., 1985, Candona xizangensisHuang et al., 1985, Ilyocypris cf. mongolica Martens, 1991 and Leuco-cythere dorsotuberosa f. postilirata sensu Pang, 1985 appearfrequently and continuously in all levels. Rare species are Eucyprisgyirongensis (juveniles only), Candona candida, Limnocythere inopi-nata and Fabaeformiscandona danielopoli Yin & Martens,1997 (Fig. 3).

4.2.1. Associations from core Nam Co 4/2Ostracode abundances vary considerably from <100 valves g�1

near the top of the core to more than 3000 valves g�1 before

Table 4Oxygen and carbon isotope values of selected ostracode species of core Nam Co 4/2.

Depth Age (years) L. sinensis L. dorsotuberosa L. dorsotuberosaf. postilirata

C. xizangensis I. cf. mongolica

13C 18O 13C 18O 13C 18O 13C 18O 13C 18O

0.75 2000 1.85 �3.99 �0.791.75 1990 1.37 �4.11 �0.84 �4.26 �1.39 0.003.75 1970 �0.68 �3.46 �0.75 �4.074.75 1960 1.88 �4.17 �0.35 �4.15 �0.91 �4.275.75 1952 1.87 �4.14 �0.27 �4.23 �0.38 0.00 �4.04 �3.386.75 1944 �0.48 �3.80 �5.30 �3.067.75 1936 �0.67 �4.26 �0.89 �4.108.75 1928 2.10 �4.27 0.87 �4.289.75 1920 �0.41 �4.6110.75 1912 2.23 �4.69 0.23 �4.3211.75 1904 2.42 �4.30 �0.24 �4.0212.75 1896 2.16 �4.37 �0.22 �4.74 �0.39 �4.7813.75 1888 2.56 �4.15 �0.55 �4.37 0.32 �4.2514.75 1880 0.36 �4.51 0.57 �4.0915.75 1873 2.24 �4.48 �0.25 �4.49 0.16 �3.6316.75 1866 2.13 �4.42 �0.54 �5.06 0.28 �4.8617.75 1859 2.16 �4.63 �0.34 �4.60 1.15 1.7718.75 1852 2.18 �4.54 �0.63 �4.38 �0.18 �4.8719.75 1845 2.24 �4.19 �0.01 �4.3521.50 1831 3.84 �0.02 �4.57 0.08 �4.72 �2.19 �3.48 1.74 �5.1222.50 1824 2.24 �4.49 �0.12 �4.23 �0.6523.50 1817 2.38 �4.20 0.21 �4.51 0.34 �4.32 1.53 �4.7224.50 1810 2.29 �4.06 1.10 �4.58 0.39 �3.67 1.50 �4.6626.50 1794 2.26 0.15 �3.93 0.10 �3.87 1.77 �4.2127.50 1786 2.42 �3.86 �0.07 �4.19 0.05 �4.2228.50 1778 2.35 �3.52 �3.92 0.42 �4.11 0.90 �4.2929.50 1770 2.28 �4.08 0.72 �4.2231.50 1754 2.16 �3.88 �0.01 �4.07 0.33 �4.13 1.02 �4.3732.50 1746 2.26 �4.09 0.48 �4.33 0.16 �4.71 1.19 �4.3233.50 1738 2.31 �3.92 0.52 �4.26 0.16 �4.39 1.35 �4.3434.50 1730 2.22 �4.04 0.03 �4.63 1.36 �4.4836.50 1716 2.17 �4.05 1.23 �4.4439.50 1695 1.83 �4.13 0.17 �4.1941.50 1681 2.01 �4.50 0.08 �4.89 0.03 �5.05 �0.32 �3.68 1.05 �4.7642.50 1674 1.79 �4.45 0.01 �4.75 �0.28 �5.25 �0.64 �3.61 0.93 �4.9043.50 1667 1.72 �4.38 0.09 �4.78 �0.33 �5.10 �2.11 �3.62 1.09 �4.8344.50 1660 1.81 �4.30 �0.15 �4.75 �0.62 �5.00 �0.01 �3.65 0.81 �4.8446.50 1644 1.89 �4.45 0.16 �4.69 �0.74 �2.93 0.68 �4.6147.50 1636 1.99 �4.51 0.40 �4.70 �0.12 �3.6248.50 1628 1.55 �4.48 �0.21 �4.90 0.32 1.35 �3.80 1.0249.50 1620 1.81 �4.65 0.55 �5.07 0.16 �0.85 �3.70 1.4852.50 1599 1.96 �4.66 0.34 �5.06 0.05 �5.10 1.26 �4.5453.50 1592 2.30 �4.32 0.45 �5.11 0.46 �5.07 1.14 �4.5454.50 1585 1.84 �4.84 0.26 �5.27 �0.08 0.00 �1.67 �3.49 1.29 �5.0368.50 1477 2.06 �4.68 2.08 �4.44 �0.49 �4.86 0.64 �3.20 1.3569.50 1470 2.08 �4.49 0.36 �3.2970.50 1463 �4.77 1.25 �4.5771.50 1456 2.03 �4.65 �0.01 �5.01 �0.08 �5.15 2.11 �3.49 1.26 �4.60

C. Wrozyna et al. / Quaternary International 212 (2010) 2–13 7

1650 AD (50 cm) (Fig. 4). The increasing number of valves is mostlycaused by increasing numbers of juvenile valves so that the pop-ulation age structure is shifted towards juvenile instars.

Adult valves of Leucocytherella sinensis are frequent, appear invarying ratios, and show a decrease from a maximum of 38% around1720 AD (41 cm) to 8% around 1970 AD (3 cm) of the core. ?Leuco-cythere dorsotuberosa shows an opposite trend and slightly increasesfrom around 3% before 1650 AD (50 cm) to more than 6% since1920 AD (10 cm) to Recent. ?L. dorsotuberosa f. postilirata, inter-preted here as the deep-water form of L. dorsotuberosa (compareWrozyna et al., accepted), shows a similar distribution as L. dorso-tuberosa with a distinct maximum after 1920 AD but with generallylower abundances. Candona xizangensis, referred to as a deep waterspecies that prefers fresh water (Yu et al., 2001), shows a generaldecreasing trend from amounts up to 10% with three peaks: between1470 and 1520 AD,, between 1770 and 1520 AD, a last minormaximum between 1920 and 1940 AD (<5%). The distribution ofjuvenile candonids correlates to the distribution of adult valves of

C. xizangensis. Ilyocypris cf. mongolica, a species that seems to preferfine grained sediments (Wrozyna et al., accepted), shows twomaxima in abundance with up to 6% between 1570 and 1630 AD.After 1630 AD these species are very rare with values below 1%. Thedistribution of rare species seems to be irregular, except Eucyprisgyirongensis, shows single peaks similar to those of C. xizangensis(Fig. 4). Articulated valves (almost exclusively L. sinensis) are rare andshow a maximum of 3% between 1780 and 1870 AD. They containonly rarely preserved soft parts. Concomitantly, there is a maximumof abraded and encrusted valves. The ostracode diversity accordingto the Shannon–Wiener diversity index shows a relative low diver-sity (mean 0.83), and generally decreases upcore. Lowest diversityvalues appear between 1520 and 1570 AD. The ostracode basedtransfer function (Wrozyna et al., accepted) estimates water depthsfalling from about 50 m in the lowermost part of the core to valuesoscillating around 20 –30 m in the middle part (Fig. 4). Above1770 AD (35 cm), the transfer function displays estimated waterdepths increasing from about 40 to finally 55 m.

Fig. 3. Ostracode taxa found in both studied short cores from Nam Co. 1–4, ?Leucocythere dorsotuberosa Huang et al., 1985 ¼ Leucocythere dorsotuberosa Huang et al., 1985. Weinclude Leucocythere postilirata Pang, 1985 as a morphotype of ?L. dorsotuberosa; 5, Leucocytherella sinensis Huang et al., 1985; 6, Limnocythere inopinata (Baird, 1843) ¼ Cythereinopinata Baird; 7, Ilyocypris cf. mongolica Martens, 1991; 8–10, Candona xizangensis Huang et al., 1985; 11, Fabaeformiscandona danielopoli Yin and Martens, 1997.

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4.2.2. Associations of core Nam Co 6/1Ostracode data are restricted to the core section from 8 to 28 cm,

because the other core sections were depleted for 210Pb dating andother geochemical analyses. The abundances vary betweenapproximately 100 and 2000 valves per gram with increasing valveratios in the middle part of the core section between 1625 and1820 AD (Fig. 6). The ostracode species appear in similar dominancesas in core Nam Co 4/2 with Leucocytherella sinensis (generally z70%)as the dominant species with a slight decreasing trend within theupper 8 cm. ?Leucocythere dorsotuberosa and its morphotype ?L.dorsotuberosa f. postilirata are represented with similar dominancesas in core Nam Co 4/2 and show an increasing trend toward theupper samples, although ?L. dorsotuberosa f. postilirata hasa distinctive dominance minimum around w1770 AD where Ilyo-cypris cf. mongolica displays a dominance maximum. In contrast to

Fig. 4. Ostracode abundance, dominances of taxa, diversity, ostracode based transf

core Nam Co 4/2, Fabaeformiscandona danielopoli is more frequent(up to 10%) and shows a significant decreasing trend as Candonaxizangensis, too. Poorly preserved valves show a maximum betweenw1780 and w1825 AD. The species diversity is as low as in core NamCo 4/2 (mean 0.88). The ostracode based transfer function suggestsincreasing water depths starting with 22 m and reaching about 50 mwithin the lower part of the studied interval (Fig. 5). Despitea distinctive minimum of approximately 10 m water depth in about15 cm sediment depth, the curve shows values around 50 m witha slight increase in the uppermost part of the section.

4.2.3. IsotopesWe excluded stable isotope analysis of core 6/1, because of its

high ratio of allochthonous material. Ostracode valves fromallochthonous sediment samples are difficult to distinguish from

er function for water depth and simplified lithology of short core Nam Co 4/2.

Fig. 5. Ostracode abundance, dominances of taxa, diversity, ostracode based transfer function for water depth and simplified lithology of short core Nam Co 6/1 with shaded areasdisplaying silty fine sand and light grey areas displaying silt.

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autochthonous specimens and may give a shallow water signal.Core Nam 4/2 is located in the central part of the lake in an isolatedposition, partly surrounded by deeper areas (Fig. 1), and probablyreceives only little allochthonous material if any. Stable isotopevalues of ostracode valves from core Nam Co 4/2 are shown in Fig. 7.For paleoecological and paleoclimatological interpretation, theisotopic composition of the rare species Ilyocypris cf. mongolica andCandona xizangensis were excluded, because of discontinuousisotope data. Oxygen isotope values of shells of Leucocytherellasinensis, ?Leucocythere dorsotuberosa and ?L. dorsotuberosaf. postilirata range from �5.3& to �3.52&. Generally the oxygenisotope trend of these three ostracode species is similar. The valuesshow an increasing trend with lowest values around �5& atw1600 AD (no data available between 70 and 55 cm depth) up to�3.5& at w1800 AD. Between w1800 AD and Recent time, there isa decreasing trend up to w1950 AD followed by an increase.However, these trends are punctuated by short-term fluctuations.

The isotopic range of C. xizangensis shows approximately one permil more positive values for oxygen isotopes and an increasing trendto the top of the core, while I. cf. mongolica is characterized by morenegative oxygen isotope values that are generally below �4&.Carbon isotope values range from �5.3& to 3.8& throughout thecore, displaying individual trends for ostracode species. The carbonisotope values of L. sinensis are the most positive, ranging from�1 to�3&. These values show a slight increase from w1725 to w1840 ADand a decreasing trend to more negative values to the top of the core.?L. dorsotuberosa and ?L. dorsotuberosa f. postilirata show a similarcarbon isotope trend through the core with generally lower valuesthan those of L. sinensis between �1 and þ1& than L. sinensis butshow a decrease towards the top of the core, too.

4.2.4. Other sediment componentsIn addition to the abundant ostracode valves, short core Nam Co

6/1 yields plants, a few remains of fishes, cuticules of cladocerans,and some large diatoms which were counted semi-quantitatively.Additionally, we found pieces of charcoal, pyrite and gypsum. Rockdebris and quartz were frequent in the section between w1680 andw1860 AD (Fig. 8). Most fish remains occur between w1600 andw1790 AD, whereas charcoal, gypsum and pyrite showa decreasing trend from bottom to top. The presence of cladoceransshows the same pattern. Abundances decrease between w1680and w1825 AD and then increase up to the top of the considered

core section. Diatoms are abundant at the base of the core sectionand reappear again after w1860 AD.

5. Interpretation and discussion

Stable oxygen and carbon isotopes values and ostracode speciesassemblages are widely used in paleolimnology and paleoclima-tology (Heaton et al.,1995; Holmes,1996; Schwalb et al.,1999). Thereare only a few examples that have applied these proxies in regions ofthe Tibetan Plateau Tibet (Mischke et al., 2002; Holmes et al., 2007).d18O values of ostracode shells reflect the temperature and theisotopic composition in which the ostracode formed their valves(Holmes, 1996; Schwalb, 2003; Holmes et al., 2007). The 18O valuesof the lake water depend on the mean oxygen isotopic compositionof precipitation in the catchment, and are affected by a series ofregional features affecting the lake system. A changing balance ofinflow vs. evaporation and effective humidity will lead to variationsof d18O. Evaporative enrichment of 18O is regarded as a primarycontrol on the isotopic composition. Additionally, precipitationamount and lake water temperature may play a minor role (Holmeset al., 2007). More positive values of d18O may thus derive froma drier phase. Accordingly, a trend to negative values may result fromwetter conditions in the catchment area or a higher input of melt-water. Although we are lacking isotope data from several levelsbetween 55 and 70 cm, relatively low oxygen isotope values beforeapproximately1600 AD suggest wetter conditions and higher lakelevel than today. This is consistent with cladocerans and ostracodedata from the sediment record of Chen Co, southern Tibet, indicatinga level decline from approximately 1400 AD until 1533 AD (Zhuet al., 2005). The ostracode based transfer function for water depthestimates initially a lake level similar to the present one andsubsequently a level about 10 m lower than today (Figs. 4, 6, 8).

The declining water level is confirmed by a contemporaneousincrease of shallow water species like I. cf. mongolica and higherproportions of L. sinensis, the increase of articulated valves anda reduction of the Renkonen index (e.g. similarity of successiveostracode associations) (Fig. 4) that suggests shallow water condi-tions where reworking of sediment is prominent. A decrease in lakelevel during this period could have been caused by a weakening ofsummer monsoon influence that is associated with a reduction inthe northward extent of the summer monsoon moisture transport(Kaspari et al., 2007). Higher temperatures and decreased rainfall,

Fig. 6. Oxygen (A) and carbon (B) isotope values of selected ostracode taxa from core Nam Co 4/2.

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indicating a warm dry climate (Chen et al., 2005) accompanied bya wetting trend are suggested for the period around 1500 AD(Holmes et al., 2007). Between w1600 and w1800 AD more posi-tive isotopes values and species assemblages indicate evaporativeenrichment and a lower lake level due to drier conditions.A continuously decreasing lake level after w1600 AD is suggestedby an increase in d18O, and the water depth transfer function pointsto a fast drop in lake level of more than 40 m below recent levelafter 1600 AD. In addition, species associations also indicatea relatively short period of lower lake level by a maximum inabundance of L. sinensis and reduced numbers of L. dorsotuberosaand L. dorsotuberosa f. postilirata (Figs. 4, 6). During the lake levelrise, older shallow water lake sediments were reworked andredeposited in deeper water (Fig. 9). Thus, shallow water ostracodeassociations can be found in core Nam Co 6/1, which is located closeto a steep slope, whereas the core Nam Co 4/2 is located in thecentral basin at a slightly elevated position. A probable E–Wrunning bottom current prevented a higher sedimentation rate byholding fine grained sediment in suspension. This produced therelatively low sedimentation rate in the area of core Nam Co 6/1,

despite the redeposition of shallow water material, especiallyostracode valves and carapaces. Dilution by allochthonous sedi-ments causes a shallow water signal in core Nam Co 6-1 duringsuch episodes, whereas the associations of Nam Co 4/2 are mainlyautochthonous and reflect the real water depth. With the slowingdown of the lake level rise and the establishment of deeper water,the estimated water depth for both cores agree (Figs. 4, 6). Higherabundances of Leucocytherella sinensis in comparison with thesecond most abundant species ?Leucocythere dorsotuberosa areconsistent with higher lake water salinities as a consequence ofincreased evaporation rates. Yu et al. (2001) give an upper tolerancelimit of 2 & for ?L. dorsotuberosa, and Huang et al. (1985) suggestsalinities of 13 & for Leucocytherella sinensis. Also, a lower abun-dance of ?L. dorsotuberosa is associated with decreases in theabundance of its morphotype postilirata. Because ?L. dorsotuberosaf. postilirata is an important deep-water indicator in Nam Co(Wrozyna et al., accepted) the water depth would be under-estimated. Therefore the figure of a 40 m lower lake level thantoday in the 17th century may be an overestimation. The lake leveldecrease between w1600 and w1800 AD, probably caused by drier

Fig. 7. Semi-quantitatively determined biogenic remains and sediment components in short core Nam Co 6/1 according to the classes: 1, 1 –2 specimens ; 2, 3 –10 specimens ; 3, 11–100 specimens ; 4, 101 –1000 specimens ; 5, >1000 specimens ; 6 , >1/4 of the sediment ; 7 , >1/2 of the sediment.

Fig. 8. Estimated relative lake level of Nam Co based on the ostracode transfer functionfor paleo-water depths and isotope data.

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conditions, coincides with the ‘Little Ice Age’ (LIA) that occurred inTibet between 1430 AD and the late 19th century (Brauning, 2006).In south and southwest China the LIA occurred between 1550 ADand 1890 AD and is expressed as a cold and wet period (Chu et al.,2002; Chen et al., 2005). While northern Tibet shows wet condi-tions in the early LIA with episodes of aridity in the latter part of theLIA continuing to the present (Holmes et al., 2007), cold and dryconditions are described for southern Tibet between 1533 and1831 AD (Zhu et al., 2005). While oxygen isotopes suggest a periodof drier conditions and a continuous decrease in lake level, thetransfer function suggests a slight lake level increase after 1670 ADto a level ca. 10 m below the modern level (Fig. 9). This agrees withevidence for higher rainfall amounts and decreased temperaturefrom other regions on the Tibetan Plateau (Chen et al., 2005;Holmes et al., 2007) but appears later than in southern China andon the northwestern Tibetan Plateau.

The third period in Lake Nam Co between approx-imately1800 AD and today is characterized initially by a trend towetter conditions until w1920 AD as indicated by a decrease ind18O values and then a shift to generally drier conditions suggestedby a trend to more positive d18O values. The climate of this timeperiod is described from the northwestern Tibetan Plateau and ischaracterized by a trend to drier but more variable climate condi-tions (Holmes et al., 2007). This variability is expressed in variousshort term fluctuations in d18O values and could be attributed tostrengthened monsoonal precipitation. The species assemblagesindicate a delayed lake level increase in comparison to the oxygenisotope signal with a distinct lake level increase between 1800 andw1920 AD, followed by a slight lake level decrease. After approxi-mately 1970 AD d18O values become more negative suggestinga turn to wetter conditions. This is, however, deduced from at leasttwo species (L. sinensis and L. dorsotuberosa) from a single samplelevel and thus remains speculative. However, Zhou et al. (2006)report a recent rising of the lake level with a documented differenceof 70 cm between July 2005 and October 2006.

The carbon isotopic composition of lacustrine carbonatesdepends on the isotopic composition of the dissolved inorganic

carbon (DIC) of the host water where the carbonate precipitatesfrom (e.g. Schwalb, 2003). The isotopic composition of DIC of thelake is affected by complex interactions of inflow composition,exchange between carbon dioxide and the atmosphere, the resi-dence time of the lake water, the rate of photosynthesis and thegroundwater discharge (Craig, 1953; McKenzie, 1985; Fedotov et al.,2006; Wachniew, 2006). Lower d13C values of ostracode calcite are,for example, related to oxidation of bottom organic matter (Fonteset al., 1996). Extensive isotope exchange of the DIC with atmo-spheric CO2 caused by a long water residence time is probably the

Fig. 9. Historical lake level changes of Nam Co based on the ostracode transfer func-tion for paleo-water depths and isotope data.

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main driving force to increase the 13C content (Fontes et al., 1996).Positive d13C values of L. sinensis >1&, in comparison to morenegative values of ?L. dorsotuberosa and ?L. dorsotuberosa f. pos-tilirata up to �1& may suggest different habitat preferences asobserved in benthic foraminifera (Mackensen et al., 2000). Themorphological characteristics of both Leucocythere forms with theircentral curvatures and ornamentations as described in Wrozynaet al. (accepted) support the interpretation that these species areliving on the substrate while L. sinensis may prefer micro-habitatswithin the sediment. The range of w1& is probably indicative for‘‘vital offset’’ (von Grafenstein et al., 1999). These ideas, however,

remain speculative due to lack of information about the ecology of?L. dorsotuberosa and ?L. dorsotuberosa f. postilirata. The d 13C valuesof Leucocytherella sinensis, ?Leucocythere dorsotuberosa and Leu-cocythere dorsotuberosa f. postilirata show a long-term decreasingtrend from the bottom to the top of the sedimentary record thatmay imply an increase in lake productivity over time. Photosyn-thetically active organisms prefer to take up 12C which leads to an13C enrichment of surface water DIC (McKenzie, 1985). Oxicconditions at the lake bottom result in the decomposition oforganic matter and a consequent release of 12C to the lake bottomwaters DIC. Therefore, an increase in productivity in the surfacewaters will lead to relatively negative d13C values of ostracodecalcite formed in oxic bottom waters (Schwalb, 2003). Atmosphericexchange causes the lake DIC source to equilibrate with atmo-spheric CO2. As a result, relatively positive and constant 13C valuesare indicative of lakes subject to no or slow lake level changes orproductivity (Ricketts et al., 2001).

6. Conclusions

A high resolution sedimentary record of two short cores of lakeNam Co covering the last w600 years reveal lake level evolution inresponse to climatic change on the southern Tibetan Plateau. Anostracode-based transfer function was used to reconstruct lakelevel changes. A comparison with stable isotope analysis of ostra-code valves and ostracode species assemblages confirms a lakelevel lowstand of �10 m compared to modern lake level beforew1600 AD. A distinct increase in d18O values and estimated lakelevel drop of up 40 m below modern level for the period betweenw1600 and w1800 is correlated with the LIA. Cold and dryconditions during this episode were apparently persisting inSouthern Tibet, while other regions on the Tibetan Plateau expe-rienced wetter conditions during LIA. After w1800 the climatebecame more variable and was characterized by various short-termfluctuations. There is a trend to wetter conditions and a increasinglake level until w1920 followed by a period with a trend to drierconditions. The last 30 years may reflect a lake level increase due toincreasing precipitation. Generally, the two short cores show thesame amplitude of lake level change but the core obtained from64 m water depth reacts later in time as shown by the appearanceof shallow water sediments and the distribution of ostracodespecies and other biogenic sediment remains such as fishes,cladocerans and plant remains and the estimated water depth. Thisstudy confirms the value of using ostracode-based transfer functioncombined with the isotopic signature of their calcareous valves asindependent tools to reconstruct lake level changes caused bychanges in precipitation vs. evaporation. This shows the need forintegrated studies combining information from species and stableisotope data.

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