Quaternary Ostracods from the Tibetan Plateau and Their Significance for Environmental and Climate-Change Studies

Chapter 15

Quaternary Ostracods from the Tibetan Plateau and Their Significance for Environmental and Climate-Change Studies Steffen Mischke* University of Potsdam, Institute for Earth and Environmental Sciences, Karl-Liebknecht-Str. 24, 14476 Potsdam, Germany, and Freie Universita¨t Berlin, Institute of Geological Sciences, Malteserstr. 74-100, 12249 Berlin, Germany *Corresponding author: e-mail: [email protected]

ABSTRACT Tibetan Plateau lakes are diverse in basin morphology, water chemistry and geological history. Limnological parameters including salinity, ion composition and water depth are closely linked to climatic conditions in this continental, alpine and relatively dry part of Central Asia. Analyses of Quaternary ostracods focused initially on the biostratigraphical assessment of lacustrine sediments of the Qaidam Basin for oil and gas exploration, but more recent palaeoenvironmental and palaeoclimatic studies have focused on lakes Qinghai and Nam Co. Stratigraphical and palaeoenvironmental inferences have been made from analyses of ostracod assemblages, stable isotopes and trace element ratios of ostracod shells and shell length variations. Consideration of species’ ecological characteristics facilitates the use of ostracods as indicator taxa and the application of transfer functions for quantitative palaeoenvironmental reconstructions. Ostracod shell chemistry has been used successfully in environmental and climate-change studies, but assemblage composition data have provided the clearest evidence for sometimes dramatic hydrological changes. Keywords: Ostracoda, Tibetan Plateau, Shell chemistry, Palaeoecology, Palaeoenvironment, Climate change

15.1 INTRODUCTION Studies of Quaternary ostracods from the Tibetan Plateau date back only a few decades, beginning in the 1950s as a result of the petroleum exploration in the Qaidam Basin at its northern margin (Fig. 15.1). Many of the early reports from this period were published in the Chinese language in various Chinese journals and publication series from individual research institutes, poorly known and more or less inaccessible for non-Chinese readers (Sun, 1988; Hou

and Gou, 2002, 2007). The early reports focused mainly on the taxonomic description of recorded species and on the discussion of their stratigraphical context. As a result of the economic significance of Quaternary ostracods from the Qaidam Basin for the oil and gas exploration, papers often summarized unpublished reports without a presentation of the detailed original data of ostracod records from individual sites (Huang, 1964, 1979, 1984, 1987; Chen and Bowler, 1986; Wang and Zhu, 1991). Some of the earlier studies included the investigation of modern ostracods and the relevant habitats of the Tibetan Plateau as a basis for palaeoecological inferences although the majority focused on the stratigraphical use of Quaternary ostracods from the Qaidam Basin (Huang et al., 1985; Yang, 1988; Yang et al., 1995, 1997; Sun et al., 1995, 1997, 2003). Shell chemistry analyses were not applied earlier than in the late 1980s and early 1990s, using ostracod records from Qinghai Lake (Zhang et al., 1989; Huang and Meng, 1991; Lister et al., 1991; Zhang et al., 1994; Fig. 15.1). These studies initiated the recent concentration of research on late glacial and Holocene ostracods as proxies for environmental and climatic change on the Tibetan Plateau (Li et al., 1997a; Peng, 1997; Zhu et al., 2002; Mischke et al., 2005a, 2008a; Wrozyna et al., 2010). Stable isotope analysis of ostracod calcite is now a standard procedure of palaeoenvironmental research on the Tibetan Plateau (Henderson et al., 2003; Holmes et al., 2007; Li et al., 2007; Liu et al., 2007a; Wrozyna et al., 2010). Along with species composition data, ostracod-based stable isotope records provide important data for the discussion of environmental change and inferences of potential moisture sources (the Indian

Developments in Quaternary Science. Vol. 17, http://dx.doi.org/10.1016/B978-0-444-53636-5.00015-9 ISSN: 1571-0866, # 2012 Elsevier B.V. All rights reserved.

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and East Asian summer monsoons, westerlies-derived precipitation) during the late glacial and Holocene (Lister et al., 1991; Holmes et al., 2007; Mischke et al., 2008a; Wrozyna et al., 2010). The most recent development in the application of Quaternary ostracods from the Tibetan Plateau is represented by transfer-function-derived inferences of past environmental conditions (Frenzel et al., 2010; Mischke et al., 2007, 2010a, 2010b; Wrozyna et al., 2009a). This chapter summarizes the previous and ongoing work on Quaternary ostracods from the Tibetan Plateau and discusses difficulties and recent trends in ostracod research in the region.

15.2 CHARACTERISTICS OF THE TIBETAN PLATEAU The Indo-Asian tectonic collision in the Eocene c. 55 Ma caused the formation of the Tibetan Plateau, which has an area of 2.5 million km2 and an average elevation of over 4500 m above sea level (asl). The creation and uplift of a large plateau region triggered the onset and intensification of the Asian summer monsoon through its effect as a heating plate in summer time and the drawing of moist air from the Indian Ocean and South and East China Seas (Molnar et al., 1993). The tectonic mechanisms and uplift history are still intensively discussed, but several periods of enhanced uplift c. 14, 8 and 4 Ma occurred as a result of northward shortening and crustal thickening (Me´tivier et al., 1998; Blisniuk et al., 2001; Tapponnier et al., 2001; Fang et al., 2007). The major SW–NE crustal shortening was accompanied by intensive left-lateral W–E strike-slip faulting, and the clockwise rotation and southward extrusion outside the plateau and towards South China and Indochina at the southeastern edge of the Tibetan Plateau (Tapponnier et al., 2001; Royden et al., 2008). The W–E orientation of mountain ranges on the Tibetan Plateau and major strike-slip faults result in a belt-like pattern of exposed basement rocks. Mesozoic, mainly Triassic and Jurassic, metamorphic and sedimentary rocks are exposed in the southernmost part of the Tibetan Plateau south of the Indus-Zangbo suture; granitic and volcanic rocks farther to the north are mainly replaced by Jurassic rocks in the central part of the Tibetan Plateau and by Triassic clastic rocks in a band south of the Kunlun Fault (Ma, 1999; Tapponnier et al., 2001). A massive sequence of Quaternary fluvio-lacustrine sediments is found in the Qaidam Basin in the north and Permian–Triassic metamorphic and sedimentary rocks in the northeastern part of the Tibetan Plateau, while igneous, mainly plutonic rocks occur more frequently in the southern part of the Tibetan Plateau. The valleys, basins and plains are filled by Pleistocene and Holocene alluvial to lacustrine sediments and partly covered by a relatively thin (c. 1 m) layer of loess or dune

Ostracoda as Proxies for Quaternary Climate Change

sand in the arid to semi-arid western and northern part of the Tibetan Plateau. The Tibetan Plateau receives c. 80% of the precipitation during the summer half year through the uplift of warm air above its surface and the penetration of moisture-charged air from the southern and eastern seas and lowlands. Consequently, mean annual precipitation is c. 800 mm at the southern and eastern margins of the Tibetan Plateau and c. 2000 mm in the upper reaches of the Brahmaputra catchment at maximum (Zhu, 1999). In contrast, the most continental regions in the western and northern part of the Tibetan Plateau receive less than 100 mm per year. Cyclones triggered by the westerly jet stream may provide sporadic precipitation with Atlantic Ocean, Mediterranean Sea or continental water sources in the west. A SW–NE aligned band of c. 300 mm annual precipitation runs through the central Tibetan Plateau with less precipitation on its northwestern and more precipitation on its southeastern side. In the winter half year, a stable high pressure cell over Siberia and northern Mongolia brings dry and cold air from northern directions and causes the annual change in wind direction typical for a monsoon climate. Mean annual temperature reflects the precipitation pattern and continentality with 10–12
C at the southern and southeastern margin of the Tibetan Plateau and  4
C in the northwestern part (Wang, 1999). This regional contrast corresponds to a difference between 4
C and  16
C for the mean January temperature and 18
C and 6
C for the mean July temperature. However, the large topographical differences between mainly W–E aligned mountain ranges in the central part of the Tibetan Plateau and the intermediate plains, between high mountains at the plateau margins and deeply incised valleys and between differently facing slopes cause large climatic gradients within small areas. The strong NW–SE moisture and temperature gradient is clearly reflected by the vegetation with subtropical, subalpine Picea and Abies forest in the deeply incised valleys of the southeastern plateau margin, Rhododendron-dominated scrub vegetation at slightly higher altitude or more continental position and alpine Kobresia meadows in a large area of the southern and eastern Tibetan Plateau limited roughly by the 400 mm precipitation isohyet. Stipa- or Carex-dominated steppe occupies the drier part of the Tibetan Plateau farther in the north and west, with the most arid regions characterized by dwarf shrub semi-deserts and deserts (Sun et al., 1999). In spite of the arid conditions in many regions of the Tibetan Plateau, several thousand lakes, ponds, streams and rivers exist and provide potential habitats for living ostracods. There are more than 1600 lakes with an area > 1 km2 (Zheng, 1997). Many of the large lakes on the plains in the northern and western part of the Tibetan Plateau are saline closed-basin lakes with tectonic origin while meltwater-fed freshwater lakes fill glacially eroded

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Quaternary Ostracods from the Tibetan Plateau and Their Significance

265

valleys in the mountain regions. In addition, numerous groundwater-supported shallow lakes and ponds are spread over the slightly undulating plateau surface. The large climatic and geological differences on the Tibetan Plateau cause large limnological ranges. The pH values of the lakes are mainly neutral to alkaline between 7 and 10, salinities are mainly between < 0.5 and 50 g L 1 and can reach c. 550 g L 1 at maximum (Zheng and Liu, 2009). Most lakes have a water depth of only a few metres but lakes such as Donggi Cona, Mapam Yumco, Nam Co (Co ¼ lake) and Tangra Yumco have a maximum depth of c. 100 m or even more (Yao, 2008; Wrozyna et al., 2009a; Mischke et al., 2010c).

and climatic conditions. Recent studies of ostracod assemblages from modern water bodies of the Tibetan Plateau and palaeoenvironmental records provide the basis to assess the environmental and societal impact of Recent global change on the Tibetan Plateau, especially with respect to moisture changes and runoff of large rivers in its densely populated eastern and southern foreland and to compare the amplitude and timing of environmental change with those of the pre-industrial history (Mischke et al., 2003, 2007, 2010c; Zhang et al., 2006; Wrozyna et al., 2009a; Li et al., 2010).

15.3 SIGNIFICANCE OF OSTRACOD RESEARCH ON THE TIBETAN PLATEAU

15.4.1 Study Regions and Main Stratigraphic Focus

In contrast to the large number of geophysical, mineralogical and palaeontological studies to decipher the uplift history of the Tibetan Plateau, investigations that include ostracod studies are very rare (Wu et al., 2001; Kempf et al., 2009). Research on Quaternary ostracods from the Tibetan Plateau mainly contributed to our understanding of Quaternary environments in two ways: (1) the Quaternary stratigraphy of the Qaidam Basin and (2) the palaeoenvironmental history of the Tibetan Plateau, especially in the late Pleistocene and Holocene. Quaternary ostracods have been used intensively to differentiate stratigraphic zones in the Qaidam Basin but most of the existing data are unpublished and only accessible to local and national oil and gas exploration institutions such as the Headquarter of Petroleum Exploration and Development of Qinghai Province in Dunhuang (Gansu, China) and the University of Petroleum in Beijing (China). Relatively few summarizing papers about Quaternary ostracods from the Qaidam Basin have been published to date (Sun et al., 1995, 1997; Yang et al., 1997; Liu et al., 1998). Scientific drillings beyond economic interests have been conducted in the Qaidam Basin since 2006 by the Qinghai Institute of Salt Lakes (Chinese Academy of Sciences), the Research and Development Center of Saline Lake and Epithermal Deposits (Chinese Academy of Geological Sciences) and the Institute of Tibetan Plateau Research (Chinese Academy of Sciences), and new exciting insights into the basin evolution, the uplift of the Tibetan Plateau and the environmental and climatic history in the Qaidam Basin are expected. The large potential of ostracod analysis for palaeoenvironmental reconstructions is also utilized in other regions of the Tibetan Plateau (Zhang et al., 1989; Li et al., 1991, 1997a; Peng, 1997). Ostracod assemblage data, results of shell chemistry analysis and, more recently, the application of ostracod-based transfer functions show that ostracods serve as reliable indicators of Quaternary aquatic environments

Research work on Quaternary ostracods from the Tibetan Plateau has been mainly conducted in the easily accessible northern and eastern part of the plateau, including the Qaidam Basin and the Qinghai Lake region (Fig. 15.1). Apart from the exploration activities in the Qaidam Basin, assemblage or shell geochemistry data were presented for Pleistocene or Holocene records from its eastern part and its northernmost tip (Holmes et al., 2007; Zhang et al., 2008, 2009; Mischke et al., 2006, 2010a; Zhao et al., 2009). Intensive investigations on Pleistocene, Holocene and Recent ostracods were performed at China’s largest Lake Qinghai and its vicinity (Zhang et al., 1989, 1994, 2006; Lister et al., 1991; Henderson et al., 2003; Henderson, 2004; Mischke et al., 2005a; Liu et al., 2007a; Li et al., 2010). Additional late glacial and Holocene records from the Yellow and Yangtze River source areas in the southwest of Lake Qinghai have been published in recent years (Mischke et al., 2008a, 2010b, 2010d; Fig. 15.1). The south-central region of the Tibetan Plateau around Lhasa represents an additional ‘hot spot’ of past and ongoing research on Quaternary ostracods. Late Holocene or Pleistocene long-term environmental change was addressed by the studies of Zhu et al. (2002) and Jin et al. (2009). The investigations of Holocene ostracods and stable isotopes, and modern assemblages of Nam Co and nearby water bodies represent the most detailed application of ostracod analysis in this region (Qu et al., 2006; Frenzel et al., 2010; Wrozyna et al., 2009a, 2009b, 2010, 2012; Xie et al., 2009; Zhu et al., 2010). Apart from the northeastern and south-central part of the Tibetan Plateau, research work which includes analysis of Quaternary ostracods is scattered over the western region of the Tibetan Plateau (Fig. 15.1). The existing studies mainly address environmental change on late glacial and Holocene time scales as a result of the growing interest in environmental change in the most recent history of the Tibetan Plateau and the easier

15.4 STUDIES OF QUATERNARY OSTRACODS FROM THE TIBETAN PLATEAU

266

Ostracoda as Proxies for Quaternary Climate Change

FIGURE 15.1 Map of the Tibetan Plateau (grey shading > 3000 m asl) with positions of Quaternary ostracod records. Circles indicate records with quantitative ostracod assemblage data and squares mark records without detailed assemblage data. Stable isotope data obtained from ostracod shells are indicated by black symbols. Dotted and broken lines mark the better studied northeastern part of the Tibetan Plateau with the Qaidam Basin and Lake Qinghai and the south-central region around Lhasa. See Table 15.1 for references.

accessibility of late glacial and Holocene sediments in comparison to the older Pleistocene sequences (Fig. 15.2; Table 15.1). Fewer studies deal with Early, Middle and Late Pleistocene records prior to the global Last Glacial Maximum (LGM), reflecting the difficulties in accessing and especially in dating older sediments (Fig. 15.2).

15.4.2

Ostracod Assemblage Studies

Reconstructions of Quaternary environments on the Tibetan Plateau derived from ostracod assemblages are mainly based on comparisons with ecological data of modern ostracods. Literature data from Europe, northern America

FIGURE 15.2 Number of Quaternary ostracod records from the Tibetan Plateau with respect to stratigraphy. LGM, global Last Glacial Maximum; P., Pleistocene; see Table 15.1 for references.

and especially the former Soviet Union were used during the initial period of ostracod research on the Tibetan Plateau (Stepanaitys, 1959; Mandelstam and Schneider, 1963; Hartmann, 1964; Neale, 1969; Sywula, 1974). An increasing amount of ostracod species distribution and ecological data has been compiled from the Tibetan Plateau since the 1980s and in the most recent years (Huang et al., 1985; Mischke et al., 2003, 2007, 2010c; Wrozyna et al., 2009b). The earlier studies provided relatively basic information on the species distribution while the more recent research typically includes observations of a relatively large number of ecologically relevant parameters. The modern ostracod assemblages of Lakes Nam Co and Donggi Cona were investigated to examine their distribution pattern with respect to water depth and macrophyte abundance (Wrozyna et al., 2009a; Mischke et al., 2010c). Regional data sets of modern ostracod assemblages for the eastern Tibetan Plateau or the Nam Co region were established or are currently being developed (Mischke et al., 2003, 2007; Li et al., 2010; Wrozyna, Frenzel, Schwalb, and Mischke, unpublished data). The large and deep lakes in the southwestern and central part of the Tibetan Plateau apparently host a characteristic ostracod assemblage dominated by Leucocytherella sinensis Huang, 1982, Leucocythere dorsotuberosa Huang, 1982 and the probably ecophenotypic variants of the latter (Fig. 15.3). The author recorded this assemblage in Bangong Co, La’nga Co, Mapam Yumco and Peiko Co. The assemblage was also recorded in Nam Co, Pumayum Co and Tangra Yumco (Wrozyna et al., 2009b; unpubl. data

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Quaternary Ostracods from the Tibetan Plateau and Their Significance

TABLE 15.1 Records of Quaternary Ostracods from the Tibetan Plateau Latitude (
N)

Longitude (
E)

Bangong

33.500

Chen Co

Average spacing (a)

Altitude (m)

Lake type

Data

Period (ka)

79.667

4300

open

S

40–25

28.933

90.600

4438

closed

A

1.4–0

6

Zhu et al., 2002

Co Ngoin (Cuoe)

31.467

91.508

4532

closed

I,L,S

2,010– 840; 0.233–0

9669; 3

Yin et al., 2001; Jin et al., 2009

Donggi Cona

35.345

98.436

4090

open

A,I,C

18.9–0

222

Mischke et al., 2010b, 2010c

Gahai

*37.000

*100.560

*3197

closed

S,I

1.6–0

37

Henderson, 2004; Mischke et al., 2008b

Gonghe

36.200

100.200

2900

open

S

Pliocene780

Heqing

26.500

100.167

2200

open

A,E

143–11.9

190

Peng, 2000, 2002; Peng and Wang, 2003; Hu et al., 2008

Hurleg

37.295

96.879

2813

open

S

1.7–0

20

Zhao et al., 2009

Koucha

34.012

97.240

4530

open

A,I, E,C

16.3–0

131

Mischke et al., 2008a

Kuhai

35.306

99.183

4150

closed

A,I

14.9–0

75

Mischke et al., 2010d

Kunlun Pass

35.667

94.033

4700

open

S

3,580–780

Luanhaizi

37.593

3200

open

A,I

~50–0

323

Mischke et al., 2005a

Nam Co

30.750

90.500

4718

closed

A,I, W

8.4–0

210–8

Qu et al., 2006; Frenzel et al., 2010; Wrozyna et al., 2009a, 2009b, 2010, 2012; Xie et al., 2009; Zhu et al., 2010

Peiko Co

28.900

85.700

4580

closed

A

16–5.1

95

Peng, 1997

Qinghai

36.900

100.200

3193

closed

I,L,S, E

17.5–0

200–8

Zhang et al., 1989, 1994, 2004; Lister et al., 1991; Henderson et al., 2003; Liu et al., 2007a

Quan Ji

36.974

96.259

2730

open

A,C

400–120

1107

Mischke et al., 2006; Mischke et al., 2010a

Shell Bar

36.515

96.200

2700

-

A

43.5–22.4

237

Chen and Bowler, 1986, Zhang et al., 2008

SilingBangkog

31.800

89.400

4570

closed

S

PlioceneQuaternary

*34.830

*80.050

5060

closed

A

0.15–0

1

Zhu et al., 2007

Sugan

38.850

93.900

2792

closed

I,S,E

0.85–0

4

Holmes et al., 2007; Zhang et al., 2009

Tianshuihai

35.350

79.417

4840

closed

A

240–22

705

Li et al., 1997a, 1997b

Zabuye

31.350

84.067

4421

closed

A

128–1.4

1305

Liu et al., 2007b; Li et al., 2008

Zada

31.500

80.000

36504500

-

S

9,000– 1,000

Lake basin

South Hongshan

101.35

Author(s) Li et al., 1991

Huang, 1984

Pang, 1985; Wu et al., 2001; Wang et al., 2008

Pang et al., 1985

Kempf et al., 2009

Abbreviations for data types: A, quantitative assemblage data; C, conductivity transfer function applied; E, trace element ratio data; I, stable O and C isotope data; L, shell length data; S, presence–absence data for selected taxa; W, water depth transfer function applied. * Position from Google Earth.

268

of Peter Frenzel, Antje Schwalb and Claudia Wrozyna). Apart from the mesohaline Tangra Yumco, these lakes are fresh to slightly oligohaline and all are deeper than 40 m. The mean altitude of these lakes is 4650 m asl (range: 4250–5030 m asl). In contrast, the lakes in the northern and eastern part of the Tibetan Plateau have more speciesrich assemblages with widespread taxa such as Limnocythere, Ilyocypris, Fabaeformiscandona and Candona (Fig. 15.3). It may be speculated that the L. sinensis– L. dorsotuberosa assemblage in central and southwestern Tibetan Plateau lakes represents the Quaternary local, cold-water adapted ostracod assemblage which survived the Quaternary cold and dry glacials in the deep lakes, but which was possibly not able to compete with the widespread ostracod taxa in the lower and more humid northern and eastern part of the plateau. Based on the ostracod records from more than 200 waterbodies published by Mischke et al. (2003, 2007) and Li et al. (2010) and the more detailed analyses on Lake Donggi Cona (Mischke et al., 2010c), a number of species were determined to occur under specific ecological conditions, and therefore can be regarded as indicator species for palaeoenvironmental reconstruction. However, it has to be pointed out that these indicator species were only identified for the northern and eastern part of the Tibetan Plateau due to the low diversity and dominance of ostracod assemblages by L. sinensis and L. dorsotuberosa in its southwestern and central parts. Typical inhabitants of freshwater bodies on the northern and eastern Tibetan Plateau are Cypris pubera O.F. Mu¨ller, 1776, Candona candida (O.F. Mu¨ller, 1776), Fabaeformiscandona danielopoli Yin and Martens, 1997, Ilyocypris echinata Huang, 1979, Paralimnocythere psammophila (Flo¨ssner, 1965), Trajancypris clavata (Baird, 1838) and species of the Pseudocandona compressa-group (Fig. 15.3). Somewhat higher salinities are often indicated by Candona neglecta Sars, 1887, Limnocythere inopinata (Baird, 1843), Heterocypris salina (Brady, 1868) and Sarscypridopsis aculeata (Costa, 1847). Saline lakes typically host monospecific populations of Eucypris mareotica (Fischer, 1855), better known by its junior synonym Eucypris inflata (Sars, 1903; Fig. 15.3). Saline lakes are sometimes also inhabited by Leucocythere sp. (Mischke et al., 2010d). Shallower water bodies are indicated by I. echinata, species of the Pseudocandona compressa-group, C. candida and Eucypris afghanistanensis Hartmann, 1964. Ilyocypris sebeiensis Yang and Sun, 2004 and Leucocythere sp. often occur in shallow waters but occupy deeper sites if shallower waters are inhabited by other species due to apparently lower competitive abilities. Cytherissa lacustris (Sars, 1863) is found exclusively in deep freshwater lakes. Temporary water bodies are preferentially occupied by Heterocypris incongruens (Ramdohr, 1808) and H. salina (Fig. 15.3). Stagnant, permanent waters are characterized

Ostracoda as Proxies for Quaternary Climate Change

by I. echinata, Cyclocypris ovum (Jurine, 1820) and F. danielopoli, in contrast to flowing waters, which are indicated by Fabaeformiscandona rawsoni (Tressler, 1957), L. inopinata and smooth-shelled species of Ilyocypris such as Ilyocypris bradyi Sars, 1890 and Ilyocypris lacustris (Sars, 1863). The knowledge of the ecological preferences of the modern ostracods from the Tibetan Plateau provides a reliable basis for the interpretation of Quaternary ostracod assemblages from drilled or exposed sections. In addition to the indicator species approach, inferences may result from the application of ostracod-based transfer functions. A first conductivity transfer function for ostracods from the Tibetan Plateau was established by Mischke et al. (2007) for a data set of 145 lakes from the eastern part of the plateau, of which 100 lakes provided sufficient ostracod shells for statistical analysis. This transfer function had already been applied to Middle Pleistocene and late glacial to Holocene ostracod assemblages for an estimation of palaeosalinities (Mischke et al., 2008a, 2010a, 2010b). Furthermore, two lake-specific transfer functions for water depth demonstrated that ostracods have a relatively strict water-depth relationship in response to macrophyte presence and abundance, wave action and substrate type, light and nutrient conditions and other factors (Wrozyna et al., 2009a, Mischke et al., 2010c). However, the application of ostracod-based transfer functions for water depth is hampered by the indirect response of ostracods to water depth through other parameters, which have the potential to determine the water-depth relationship of ostracods without any change in lake level. Detailed multi-proxy studies of lake sediment cores are often not sufficient to provide reliable information on changes of water turbidity, ice cover duration and macrophyte density and types through time. Thus, water-depth reconstructions derived from ostracod-based transfer functions will likely provide uncertain results which should be viewed with caution (Wrozyna et al., 2009a; Frenzel et al., 2010; Mischke et al., 2010c).

15.4.3 Stable Isotope Geochemistry of Ostracod Shells Stable isotope analysis of ostracod calcite has been used to generate records from nine sites on the Tibetan Plateau to date (Fig. 15.1; Table 15.1). These records represent different time scales, covering the past c. 1000 years, the late glacial and Holocene, or a large part of the early Pleistocene in one instance. The latter is a relatively low-resolution but longterm, continuous record, which perceived a number of dry– wet oscillations in the early Pleistocene and provided a good basis for more detailed analysis of the timing and amplitude of specific early Pleistocene periods and a comparative assessment of early Pleistocene inferences from the

FIGURE 15.3 Typical Quaternary ostracods from the Tibetan Plateau. 1, Candona candida, RV ev; 2, Candona neglecta, RV ev; 3, Pseudocandona compressa-group, RV ev; 4,5, Fabaeformiscandona rawsoni: 4, ♀ RV iv, 5, ♂ RV ev; 6, Fabaeformiscandona danielopoli, LV iv; 7, Sarscypridopsis aculeata, RV ev; 8, Limnocythere inopinata, RV ev; 9,10, Eucypris mareotica: 9, RV ev, 10, LV iv; 11, Eucypris afghanistanensis, RV iv; 12,13, Ilyocypris sebeiensis: 12, RV ev, 13, LV iv; 14,15, Ilyocypris echinata: 14, LV ev, 15 LV, iv; 16,17, Leucocythere dorsotuberosa: 16, ♀ RV ev, 17, ♂ RV ev; 18,19, Leucocytherella sinensis: 18, ♀ RV ev, 19, ♂ RV ev; 20, Heterocypris salina, RV iv; 21, Heterocypris incongruens, RV iv. Leucocythere dorsotuberosa and Leucocytherella sinensis dominate in the southwestern and central part of the Tibetan Plateau while the other taxa are abundant in the northern and eastern part. All specimens housed in the Institute for Geological Sciences of Freie Universita¨t Berlin. RV, right valve; LV, left valve; ev, external view; iv, internal view.

270

Co Ngoin record and late Pleistocene and Holocene inferences from nearby sites such as Nam Co (Jin et al., 2009). Late glacial and Holocene stable isotope records derived from ostracod shell analysis have been presented for five sites on the northeastern Tibetan Plateau so far (Fig. 15.4). Of the sites investigated, Lakes Kuhai and Qinghai are both closed-basin lakes, while the other three lakes are currently open-basin freshwater lakes, but apparently lacked outflow sometime during the late glacial and Holocene. Thus, d18O trends for the different lakes cannot be compared in a straightforward way, although some similarities do exist (Fig. 15.4). Generally, low d18O values in the late glacial to Holocene transition period indicate enhanced meltwater discharge to the lakes as a result of late and post-glacial warming. Increasing d18O values in the midand late Holocene apparently reflect insolation-driven decreasing monsoon precipitation and stronger evaporation effects (Fig. 15.4). The present freshwater Lakes Koucha and Donggi Cona experienced periods of significantly increased salinity, clearly marked by the almost monospecific occurrence of E. mareotica. Lake Koucha was saline in the warm

Ostracoda as Proxies for Quaternary Climate Change

early Holocene while Lake Donggi Cona became a saline lake in the preceding cold Greenland Stadial 1 (GS 1 ¼ Younger Dryas in the North Atlantic region; Mischke et al., 2008a, 2010b). In addition to this contrast in timing of the saline lake periods, the d18O trends of both records display an opposite pattern in the saline lake phases, with significantly elevated values in Lake Donggi Cona and lower values in Lake Koucha (Fig. 15.4). This twofold contradiction probably reflects low water temperatures, a shallow lake level and permanency of the water during the GS 1 in the Donggi Cona basin, and significantly higher water temperatures and possibly the intermittent desiccation of Lake Koucha during the early Holocene. The permanency of the saline water body in the Donggi Cona basin and apparently occurring periods of desiccation during the saline lake period of Lake Kuhai are the main reasons behind the difference, which caused the opposite d18O trends in the saline lake periods of both lakes. In general, dissimilarities in the d18O trends for the five lake records result not only from the individual open- and closed-basin histories but also from catchment-specific differences in the relative contributions

FIGURE 15.4 Comparison of late glacial and Holocene ostracod shell d18O records (% VPDB) from five lakes on the northeastern Tibetan Plateau (Liu et al., 2007a; Mischke et al., 2005a, 2008a, 2010b, 2010d). Dotted line is the insolation trend at 30
N in June from Berger and Loutre (1991). Blue shading indicates freshwater (blue) to saline (white) conditions inferred from quantitative ostracod assemblage data. Gaps in the Lake Kuhai isotope record due to the lack of ostracod shells during periods with anoxic bottom-water conditions.

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Quaternary Ostracods from the Tibetan Plateau and Their Significance

of snow, ice and frozen ground to the lakes’ hydrological budgets, as well as local precipitation–evaporation conditions (Mischke et al., 2008a, 2010b, 2010d). Ostracod shell-derived stable isotope records for the past c. 1000 years were presented for Lake Qinghai and its small brother, Lake Gahai, and for the Lakes Sugan and Nam Co (Fig. 15.1). All four lakes are currently closed-basin lakes but the d18O records show poor agreement (Henderson, 2004; Holmes et al., 2007; Wrozyna et al., 2010). The sites are very different with respect to lake size (range: 48–4300 km2), catchment area (range: c. 300–30,000 km2), maximum water depth (range: 6–105 m) and local precipitation (range: 16–479 mm). These differences and related consequences such as mixing of the water column or thermal stratification, and uncertainties in the core chronologies probably cause the lack of correspondence between the d18O records. It is once more clear that the processes that govern the stable isotope signatures of ostracod shells are catchment- and lake-specific, requiring the detailed assessment of individual sites.

15.4.4 Trace Element Geochemistry of Ostracod Shells Sr/Ca ratios and Mg/Ca ratios of ostracod shells were studied for the records from Lakes Sugan, Qinghai, Koucha and Heqing representing different time scales of late Pleistocene or longer, down to the past 850 years (Zhang et al., 1994, 2009; Hu et al., 2008; Mischke et al., 2008a). Different methodological approaches were used in these studies. Zhang et al. (1994) performed Sr/Ca ratio measurements on a single shell of E. mareotica at each sampled level for a Holocene lake sediment core from Qinghai Lake. The use of a single shell per sampled level surely causes a large uncertainty with respect to the trace element variability within any given stratigraphic level. Thus, the significance of the Sr/Ca variations along the core cannot be sufficiently assessed. However, the authors presented an almost perfect linear relationship (r2 ¼ 0.96) between the Sr/Ca ratios of E. mareotica shells and the salinity of Lake Qinghai and nearby water bodies in a range from 12% to 33% and applied this relationship to their Holocene Sr/ Ca record. Palaeosalinities were reconstructed and even more, past lake levels calculated. In addition to the poor data basis of the single-shell analysis per sampled level of the Holocene core, the corresponding calibration data set to establish the present Sr/Ca ratio–salinity relationship of Lake Qinghai was based on only 12 single-shell samples. A verification and data set enlargement are surely required for a more reliable reconstruction of the Holocene salinity history of Lake Qinghai. Furthermore, aragonite and calcite have been precipitating from Lake Qinghai waters since the early phase of the late glacial (Shen et al., 2005). Sr is preferentially co-precipitated into aragonite. Thus, Sr/Ca ratios

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of ostracod shells will reflect the Sr/Ca ratios of the host water in response to the relative precipitation of the different carbonate mineral phases rather than salinity (Henderson and Holmes, 2009). Mg/Ca ratios of ostracod shells do not represent an alternative appropriate proxy for salinity in Lake Qinghai due to the high Mg/Ca ratio in present-day lake waters and the suppression of Mg uptake by ostracods to maintain the precipitation of low-Mg calcite shells (Henderson and Holmes, 2009). Hu et al. (2008) prepared two separate Sr/Ca and Mg/Ca data sets for shells of two species of a mainly late Pleistocene lake sediment record from the southeastern margin of the Tibetan Plateau. Twenty shells of either Ilyocypris microspinata Huang, 1982 or Lineocypris jiangsuensis Hou and Ho, 1982 were used per stratigraphic level, resulting in a species-specific average Sr/Ca and Mg/Ca ratio for each sampled horizon. They detected low Sr/Ca ratios of ostracod shells during periods of aragonite precipitation and a corresponding decoupling of Sr/Ca ratios of ostracod shells and Sr concentrations of sediments. The trends for Sr/Ca ratios of ostracod shells for both species followed the Sr concentration of the sediments in the units without aragonite formation. Sr/Ca ratios were regarded as representing the palaeosalinity in the aragonite-free core units. The obtained Mg/Ca ratios seemed to be unaffected from periods of aragonite precipitation and were used to infer palaeosalinities in a qualitative way. Unfortunately, the authors do not present any other salinity proxy such as ostracod assemblage data, diatom, macrophyte or d18O data that could be used to assess the assumed trace element–salinity relationship. In contrast to Zhang et al. (1994) or Hu et al. (2008), Mischke et al. (2008a) and Zhang et al. (2009) performed trace element analysis on single shells using mainly four single-shell samples per stratigraphic horizon of their lake sediment records. The study of Mischke et al. (2008a) of a late glacial and Holocene sequence was not accompanied by an examination of Sr/Ca and Mg/Ca ratios and salinities of water samples from the present lake and surrounding water bodies. Instead, the authors inferred palaeosalinities from distinct changes in the ostracod species composition, which included the almost monospecific occurrence of E. mareotica in one interval and a species-rich freshwater assemblage in another. In addition, palaeoconductivities were calculated for the late glacial and Holocene record based on an ostracod transfer function for the Tibetan Plateau (Mischke et al., 2007). The measured Sr/Ca and Mg/Ca ratios did not trace the assemblageinferred gradual increase in salinity but the trace element ratios were higher and significantly more variable at a sampled stratigraphic level in the generally more saline period of the Lake Koucha record. In contrast, trace element ratios were significantly lower and less variable in the freshwater period of the lake. The larger variability

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of the trace element ratios in the saline lake period was interpreted to result from larger hydrochemical fluctuations in a saline, shallow lake in comparison to a freshwater lake with larger depth and volume. Trace element ratios apparently helped to detect periods of higher salinity in the lake but could not be used for quantitative estimations of palaeosalinities. The record from Lake Sugan presented by Zhang et al. (2009) is marked by the occurrence of authigenic aragonite and possibly dolomite in its lower part where Sr/Ca ratios of single ostracod shells are generally low and stable for the four single-shell samples from a sampled stratigraphic level. Sr/Ca ratios in the upper part lacking aragonite are higher and the ranges for several single-shell results from a sampled stratigraphic level are broader. Sr/Ca ratios reflect salinity variations in the upper sediment section that does not contain aragonite. Independent evidence for this inference comes from stable isotope data of ostracod shells and treering-derived reconstructions of regional precipitation. Apart from a few exceptions, Mg/Ca ratios are low and apparently decoupled from salinity variations. The Mg/Ca ratio of Lake Sugan water is very high (> 40) and the authors concluded that ostracods physiologically excluded excess Mg and are therefore not useful indicators of the host water Mg/Ca history in this setting. The exceptions of high Mg/Ca ratios in ostracod shells and broad ranges for Mg/Ca data from individual stratigraphic levels correspond to lower Sr/Ca ratios and d18O values as a result of short-term freshwater inflows. Lake water Mg/Ca ratios were significantly reduced during these periods and the Mg partitioning of ostracods increased in comparison to the periods of high lake water Mg/Ca ratios. The study of Zhang et al. (2009) benefited from a survey of present-day water samples from the lake, from rivers, springs and groundwater, and made it clear that a good understanding of the lake sediment mineralogy and the hydrochemical conditions in the catchment is required for a sound application of trace element analysis on ostracod shells.

15.5

CHALLENGES AND FUTURE TRENDS

15.5.1 Taxonomy of Quaternary Ostracods from the Tibetan Plateau Two different taxonomic schools are currently used with respect to Quaternary ostracods from the Tibetan Plateau. The followers of the ‘Chinese School’ tend to differentiate different species based on slightly different variations in shell morphology including the presence and number of nodes or tubercles, while those of the ‘Western School’ treat these slightly differing forms as ecophenotypic variations of a single species. For example, Limnocythere dubiosa Daday, 1903, L. binoda Huang, 1964 and L. inopinata are regarded as different species by many Chinese ostracodologists while these forms are viewed as ecophenotypic forms of

L. inopinata by most non-Chinese and some Chinese researchers (Mischke, 2001; Yang et al., 2008). Similarly, Ilyocypris gibba (Ramdohr, 1808), I. bradyi and I. biplicata (Koch, 1838) are differentiated on the basis of the presence or absence of nodes by the members of the former school while those of the latter do not regard the presence of nodes as a good criterion (Yang et al., 2002), even if the species themselves are valid. Another example of this taxonomic controversy is the relatively large number of species of Leucocythere (Leucocytherella parasculpta Huang, 1985, L. pseudosculpta Yang, 1985, L. subsculpta Huang, 1982, L. tropis Huang, 1984, L. postilirata Pang, 1985, L. postistrumifera Pang, 1991) or the ecophenotypic variability of one or two species of Leucocythere. Similarly, the different species of Leucocytherella or Limnocytherellina (L. sinensis Huang, 1982, L. trinoda Huang, 1985, L. glabra You and Huang, 1982, L. biechinata Huang, 1985, L. triechinata Huang, 1985, L. subtriechinata Huang, 1985, L. quadriechinata Yang, 1985, L. quinquechinata Huang, 1985; Limnocytherellina bispinosa Pang, 1985, L. trispinosa Pang, 1985 and L. kunlunensis Pang, 1985) might represent ecophenotypic forms of only one species (Wrozyna et al., 2009b). In addition to this controversial assessment of morphological differences in ostracod shells, the relative isolation of Chinese researchers until the death of Chairman Mao in 1976 probably resulted in a number of species being described as new due to the lack of information on earlier descriptions in the ‘western’ literature. A detailed check using the originally described material could help to assess whether, for example, Candona houae Huang, 1964 and Fabaeformiscandona hyalina (Brady and Robertson, 1870) are both valid species or synonymous. In addition, it is not clear whether specimens identified as Candona arcina Liepin, 1963 and C. nyensis Gutentag and Benson, 1962 by the followers of the ‘Chinese School’ actually represent different species or junior synonyms of F. rawsoni. Similarly, it is open whether F. danielopoli and E. inflata are younger synonyms of F. gyirongensis (Huang, 1982) and E. mareotica, respectively. However, the tremendous work required to sort out synonymies and check the taxonomic status of many ostracod species described by members of the ‘Chinese School’ can only be done as international joint work which involves Chinese scientists with access to described specimens hosted in local research institutes in China. This work and future field surveys must include collections of living specimens for soft-part analysis to support the taxonomic assessment of closely related shell morphotypes. Collected living material or dry mud and rearing of ostracods from eggs could allow culture experiments under controlled conditions and attempts to examine the role of certain ecological parameters with respect to possible ecophenotypic shell characteristics. Yin et al. (1999) had already tested the significance of temperature and salinity on nodation in L. inopinata but were not able to ascertain direct influence of these parameters.

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Quaternary Ostracods from the Tibetan Plateau and Their Significance

15.5.2 Ecology of Quaternary Ostracods from the Tibetan Plateau The ecological preferences of Quaternary and modern ostracods from the Tibetan Plateau are still poorly known for the species restricted to the plateau and adjoining mountain ranges. Taxa such as L. dorsotuberosa, L. sinensis and I. echinata occur frequently in water bodies of the Tibetan Plateau and similarly in late glacial and Holocene lake records and a better understanding of their ecological optima and tolerances would result in more detailed palaeolimnological inferences and environmental reconstructions. Other likely endemic taxa are not widely occurring in modern waterbodies of the Tibetan Plateau and their palaeoecological significance is not known so far. For example, the middle and late Pleistocene lake record from Heqing Basin at the southeastern margin of the Tibetan Plateau was studied in detail by Peng (2000, 2002; Peng and Wang, 2003) and provided a species-rich continuous ostracod record. However, palaeoenvironmental interpretations remained relatively speculative and unclear due to the abundance of taxa with poorly known ecological preferences such as species of the genera Planocypria, Lineocypris, Callistoilyocypris, Yunnanicandona, Neochinocythere, Parachinocythere, Fuxianhucythere and Heterometacypris (Peng and Wang, 2003). More detailed sedimentological and geochemical studies and research on other organism remains are required to allow a better ecological assessment of the most abundant taxa here. Although information with respect to conductivity and water depth is available for the habitats of some more abundant living ostracods from the Tibetan Plateau, knowledge of life cycles of ostracod species is sparse. Such knowledge is required for ostracod-based reconstructions of the permanency and seasonality of Quaternary lakes, and for the interpretation of stable isotope and trace element records derived from shell geochemistry analysis. All year round operating research stations on the Tibetan Plateau such as the monitoring station of the Institute of Tibetan Plateau Research at Lake Nam Co or the station of the Research and Development Center of Saline Lake and Epithermal Deposits at Lake Zabuye could provide suitable conditions for detailed ostracod life cycle analysis. Further emphasis should be placed on the ecological significance of shell morphological variations such as noding or shell size. Noding in L. inopinata might turn out to be an important tool for palaeoenvironmental reconstructions and other species such as L. sinensis could similarly serve as important indicator species through shell morphological patterns. The shell size of L. inopinata has been examined in a study of c. 60 surface samples from lakes and smaller water bodies of the Tibetan Plateau by Yin et al. (2001). A positive correlation between the length of adult shells and conductivity was observed for conductivities < 5.62 mS cm 1 and a negative correlation for higher conductivities. The negative

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correlation between shell length and conductivity at values > 5.62 mS cm 1 is based on only 14 samples and is not similarly true for the relationship between shell length and the measured total dissolved solids (TDS; Yin et al., 2001). However, the authors established three different equations for three conductivity ranges (< 2.0, 2.0–5.62, > 5.62 mS cm 1) to calculate palaeoconductivities and applied their method to shell length measurements from L. inopinata specimens of a short core in Co Ngoin (Cuoe) Lake. The reconstructed conductivities show a step-like decline from c. 20 mS cm 1 in the lower core half to c. 3 mS cm 1 in the upper part, although measured shell lengths are mostly similar in both parts. The conductivity equation used apparently determined the reconstructed step-like conductivity decline and it remains unclear how the use of one equation in the lower core part and another equation in the upper part was justified. Zhang et al. (2004) used the negative relationship between shell length of L. inopinata and conductivity in more saline waters to reconstruct palaeosalinities for the past 900 years in Lake Qinghai. They observed a clear positive correlation between shell length data and carbonate contents of lake sediments, which could suggest that shell length data actually have the potential to trace the salinity history in saline water bodies depleted in Ca2 þ or HCO3  ions by evaporative concentration and the loss of carbonate through precipitation. Ca2 þ availability limits the carbonate precipitation in Lake Qinghai and also other large saline lakes in Central Asia, and a larger freshwater input may result in enhanced carbonate precipitation (Henderson and Holmes, 2009; Mischke et al., 2010e). However, Roberts et al. (2002), Henderson and Holmes (2009) and Holmes et al. (2009) discussed that various factors such as food availability, temperature and developmental constraints may influence the ostracod shell length in addition to conductivity alone and it is evident that a better calibration data set, especially one including more samples from higher conductivity sites, is clearly required before conductivity calculations based on shell length data are possible.

15.5.3 Analysis of Ostracod Shell Chemistry–d18O and d13C Values Stable isotope studies of ostracod shells from Tibetan Plateau lake sediments face the problem that microhabitat characteristics and life cycle features of many species are not known. A prevailing epibenthonic occurrence of the ostracods on sediment substrate or macrophytes is usually assumed, although this has not been convincingly demonstrated for species from the Tibetan Plateau so far. Consequently, stable isotope records derived from analysis of ostracod shells could reflect pore-water instead of bottom-water conditions if shells of endobenthonic ostracods have been analyzed. More detailed investigations of living ostracods from the Tibetan Plateau and their

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microhabitats could make sure that stable isotope records actually represent bottom-water conditions in a lake in response to catchment hydrology and regional climate. Furthermore, palaeolimnological studies of sensitively changing lake ecosystems such as small and shallow basins would certainly benefit from a better knowledge of the timing and place of ostracod shell formation. Another unresolved problem associated with stable isotope studies of ostracod shells from the Tibetan Plateau is the unclear order of vital offsets for the most abundant species. Li and Liu (2010) made a first valuable attempt in this respect by examining the d18O fractionation of E. mareotica in cultivation experiments. They demonstrated that d18O values of E. mareotica shells do not differ significantly from inorganic calcite precipitated in isotopic equilibrium with the host water. Similar studies of other relatively abundant ostracod taxa from the Tibetan Plateau should follow to provide a sound basis for the comparison of ostracod shellderived d18O and d13C records. This comparability is required, not necessarily for intersite comparisons of different records from the Tibetan Plateau, but for the use of stacked isotope records from lake sediments that contained different ostracod species in different parts of the section. Up to now, this stacking of species-specific stable isotope profiles from individual lake sediment records was done based on the comparative analysis of subsets of shells of different species from few sampled stratigraphic levels of a record (Mischke et al., 2008a, 2010b). However, this procedure provided only a very rough estimation of the probable order of magnitude of vital effects and additional stable isotope differences as a result of different microhabitat preferences. Stable isotope analysis on several single ostracod shell samples from a given stratigraphic level has not been performed on material from the Tibetan Plateau so far although

Ostracoda as Proxies for Quaternary Climate Change

similar studies on trace element ratios showed that important inferences concerning parameters such as water depth and temperature fluctuations can be derived (Mischke et al., 2005b, 2008a).

15.5.4 Analysis of Ostracod Shell Chemistry–Trace Element Ratios Trace element studies of ostracod shells from the Tibetan Plateau provide a powerful tool for qualitative salinity inferences if shell geochemistry analysis is accompanied by hydrochemical investigations of water bodies (springs, streams, ponds, lakes) in the catchment (Zhang et al., 2009). The water chemistry of lakes and ponds in the Tibetan Plateau is highly diverse due to the large geological and climatical differences of individual catchments (Zheng and Liu, 2009). Waters may be dominated by either Ca2 þ and HCO3  or Naþ and Cl; alternatively, they may be transitional (Fig. 15.5). In general, Mg/Ca ratios of Tibetan Plateau waters tend to increase with increasing conductivity (Fig. 15.6). Sr/Ca ratios do not show a clear correlation with specific conductivity due to their dependency on other factors such as aragonite precipitation and the Ca2 þ decrease or increase during evaporative concentration and carbonate precipitation in response to the initial Ca2 þ and HCO3  concentrations (Eugster and Jones, 1979; Fig. 15.6). Thus, the Sr/Ca and Mg/Ca ratios of water bodies in a specific catchment need to be investigated to understand the local trace element ratio behaviour in response to conductivity (or salinity) changes. Mg/Ca ratios may be especially high in the brackish to saline lakes of the Tibetan Plateau. In such cases, high Mg/Ca ratios of host waters are not recorded by correspondingly high ratios in ostracod shells due to the physiological discrimination of Mg during the

FIGURE 15.5 Major anion and cation composition of 220 lakes and ponds on the eastern Tibetan Plateau. Positions of sampled water bodies marked on the map of China.

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Quaternary Ostracods from the Tibetan Plateau and Their Significance

275

FIGURE 15.6 Molar Sr/Ca and Mg/Ca ratios of water samples from 220 lakes and ponds on the eastern Tibetan Plateau versus specific conductivity.

FIGURE 15.7 Molar Sr/Ca and Mg/Ca ratios of water samples from Lake Donggi Cona and nearby water bodies versus specific conductivity. Sample sites are marked on the map. Arrows indicate flow direction of streams.

formation of calcitic ostracod shells (Henderson and Holmes, 2009; Zhang et al., 2009). However, the study of local hydrochemical conditions, supported by mineralogical analyses of the sediments, represents a valuable opportunity to assess whether trace element ratios of ostracod shells reflect trace element ratios of host waters and whether these can be used as palaeosalinity proxies. For example, water samples from Lake Donggi Cona and streams, ponds and a spring in its vicinity show that Sr/Ca ratios increase only within the lower part of the salinity range but not in the upper part, as a result of the low initial Ca2 þ concentration and Ca2 þ removal with carbonate precipitation (Mischke et al., 2010c; Fig. 15.7). The Mg/Ca ratios of stream water samples do not

correlate with conductivity probably as a result of local differences in catchment geology. The lake and pond samples suggest that Mg/Ca ratios possibly correlate with conductivity, in waters that show greater evaporative concentration, but a more detailed study of the local hydrochemical characteristics is required for a sound assessment of the trace element ratio trends in waters of increasing conductivity in Lake Donggi Cona and nearby sites (Fig. 15.7). Information derived from ostracod species composition changes, d18O values of authigenic fine-grained carbonate or ostracod shells or macrophyte remains is required as independent salinity proxy for a trace element ratio study of Pleistocene or Holocene ostracod shells from Lake Donggi Cona.

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15.5.5 Other Aspects of Quaternary Ostracod Studies from the Tibetan Plateau The majority of studies on the Tibetan Plateau that involve Quaternary ostracods deal with late glacial and Holocene lake records for palaeoenvironmental reconstructions and the assessment of its regional climatic implications. Although absolute dating techniques have been significantly improved and diversified in recent years, chronologies of late glacial and Holocene records are still the crucial points of many records. Not only the number of radiocarbon dates per record should be increased, but also other independent age data should be included, either from the same record itself by the application of varve counting, OSL dating or uranium series measurements in appropriate geochemical settings, or from other geological archives such as nearby peat records, tree-ring chronologies, exposed shorelines or glacial remains in the catchment (Zhou et al., 2007; Liu et al., 2010). In particular, OSL dating was successfully applied to finegrained lake sediments from the Tibetan Plateau in recent years (Fan et al., 2010; Liu et al., 2010; Long et al., 2010). At present, the required sample size for AMS radiocarbon dating has made dating of pollen concentrates from lake sediments possible and this has helped to reduce the age bias through the lake reservoir effect (‘dead’ or ‘old carbon effect’; Mischke et al., 2008a). Salinity inferences from ostracod geochemistry or species composition data would benefit from a comparative assessment with other salinity proxies such as d18O records from authigenic or biogenic carbonate, mineralogical analysis of lake sediments or other organism remains. A diatom-based transfer function and two independent chironomid transfer functions for conductivity or salinity of lakes on the Tibetan Plateau have been established but were not applied to any late glacial or Holocene material so far (Yang et al., 2003; Zhang et al., 2007; Plank, 2010). The comparative application of the diatom- or chironomidbased conductivity transfer function together with those established for ostracods could provide reliable palaeosalinity estimations even for the more saline sections of lake sediment records, which often contain ostracod shells of just a single species (Mischke et al., 2007).

15.6

CONCLUSION

Studies of Quaternary ostracods from the Tibetan Plateau provide valuable information with respect to the tectonic and climatic history of this region, the related environmental consequences and the present discussion of global warming and resulting consequences for moisture availability. Examples of Quaternary ostracod records from the Tibetan Plateau show that dramatic changes took place in the Pleistocene and Holocene, including the occupation of large basins by freshwater lakes and their subsequent

salinization and desiccation. Although ecological information is still poorly known for many species from the Tibetan Plateau, Quaternary ostracod assemblages serve as a reliable convincing proxy to trace the history of lakes in this region. Based on this successful demonstration of the potential of ostracods analysis, research on Quaternary ostracods from the Tibetan Plateau should be intensified to improve our understanding of the Quaternary environmental conditions in this vast region and to serve as a reference of the natural environmental variability and baseline of rapid environmental change in times of global warming. The large number of water bodies in the Tibetan Plateau, its diversity in terms of the geological conditions, the water chemistry and climatic setting and the still relatively low human impact on most of these water bodies represent a unique natural laboratory of manifold aquatic conditions, which offers fantastic research opportunities along many lines of investigation and which clearly should be used more intensively in the future.

ACKNOWLEDGEMENTS I wish to thank Chengjun Zhang for his constant help during sampling surveys and Alexandra Oppelt for providing additional sample material from the southwestern Tibetan Plateau. The author is currently a Heisenberg Fellowship recipient of the German Research Foundation (DFG) which also funded the author’s work in the Tibetan Plateau. The invitation of the editors to write this paper on studies of Quaternary ostracods from the Tibetan Plateau is highly appreciated. Peter Frenzel and Jonathan A. Holmes provided very helpful constructive reviews of the original manuscript.

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Chapter 15

Quaternary Ostracods from the Tibetan Plateau and Their Significance

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