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Cosmogenic 10Be and 26Al dating of paleolake shorelines in Tibet
Journal of Asian Earth Sciences 41 (2011) 263–273
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Cosmogenic
10
Be and
26
Al dating of paleolake shorelines in Tibet
Ping Kong a,⇑, Chunguang Na a, Roderick Brown b, Derek Fabel b, Stewart Freeman c, Wei Xiao a, Yajun Wang a a
Key Laboratory of the Earth’s Deep Interior, Institute of Geology and Geophysics, Chinese Academy of Sciences, P.O. Box 9825, Beijing 100029, China Department of Geographical and Earth Sciences, University of Glasgow, Glasgow G12 8QQ, Scotland, UK c Scottish Universities Environmental Research Centre, Rankine Avenue, Scottish Enterprise Technology Park, East Kilbride G75 0QF, Scotland, UK b
a r t i c l e
i n f o
Article history: Received 11 February 2010 Received in revised form 28 January 2011 Accepted 25 February 2011 Available online 10 March 2011 Keywords: Tibetan Plateau Lake shoreline Exposure age Monsoon Cosmogenic nuclide
a b s t r a c t We have used in situ produced cosmogenic nuclides 10Be and 26Al to date lacustrine shorelines around eight lakes in Tibet. Two lakes located south of the Yarlung Tsangpo River discharged at high lake levels. One of them, Drolung Co (Co is a local term, meaning lake), became closed (its water level fell below its outlet channel) at 3.8 ka and lowered over 120 m vertically since then. This implies a significant change in intensity of precipitation driven by the Indian monsoon over southern Tibet in the middle-late Holocene. Nam Co and Dajia Co, located to the north of the Yarlung Tsangpo River, also drained at high lake levels and became closed 53–36 ka and 48 ka, respectively. Of four other lakes developed closely in the Qiangtang basin, north of the Yarlung Tsangpo River, Tangra Yum Co and Siling Co (located at 31– 32°N) show exposure ages for high lake shorelines of over 220 ka, the oldest lake shorelines reported for Tibet, while towards the north and west, the high lake of Zhari Nam Co appeared after the Last Glacial Maximum (LGM). The different pattern of lake development to the north of Yarlung Tsangpo River may reflect regional climate effects. Ó 2011 Elsevier Ltd. All rights reserved.
1. Introduction The Tibetan Plateau between 74–98°E and 28–40°N is covered by lakes, which have a present total surface area exceeding 30,000 km2. Ancient, preserved lake shores identified from field investigations and mapped using satellite imagery suggest that the paleolakes cover a much more extensive area of approximately 84,000 km2 (Lehmkuhl and Haselein, 2000), or even more (Zhao et al., 2002, 2003; Zheng et al., 2006). Most of these high elevation lakes are surrounded by conspicuous sequences of shorelines. We interpret the shorelines as indicating the most recent high lake levels as older remnants could have been obscured during repeated rises and falls of lake levels. The changes of lake levels are the net result of precipitation, input from melting glaciers, outflow and evaporation, which are related to paleo-environmental changes on the Tibetan Plateau. Studies that can quantify the spatial and temporal pattern of these changes to the lake systems provide constraints on paleo-precipitation distribution and help to clarify the role that the Tibetan Plateau has played on global climate. Currently precipitation on the plateau comes mainly from the Indian monsoon and the mid-latitude westerlies. The influence of the Indian monsoon decreases significantly from east to west, whereas the westerlies primarily affect
⇑ Corresponding author. Tel.: +86 10 82998317; fax: +86 10 62010846. E-mail address: [email protected] (P. Kong). 1367-9120/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.jseaes.2011.02.016
the northern and western parts of the plateau (Benn and Owen, 1998). Quantitative studies of the ages of high lake shorelines are few, leading to various hypotheses about the pattern of development of Tibetan lakes. Fang (1991) gives a general overview on the evolution of lakes in China over the last 30 ka. He showed that lakes throughout China were at high levels during 30–24 ka, 22.5–20 ka and 9.5–3.5 ka. Some recent studies, however, suggest that lakes in Tibet and western China behave synchronously, with their largest sizes during 40–30 ka due to an enhanced Indian monsoon (Shi et al., 2001; Yang et al., 2004). By dating lacustrine sediments using a U–Th disequilibrium method, Zhao et al. (2002, 2003) preferred a combined eastern Qiangtang lake as large as 150,000 km2 in the period of 116–72 ka. Using cosmogenic nuclide dating, Kong et al. (2007a) studied the exposure ages of the highest bedrock terraces around Sumxi Co, a small closed lake located in northwest Tibet. They identified an age for the exposure of the highest preserved shoreline of 11–12.8 ka, different from the previous inference of early-mid Holocene (7–8 ka; Gasse et al., 1991) or 25–30 ka (Zhang et al., 1998). Clearly more precise ages related to high lakes are needed in order to understand paleo-environmental changes on the Tibetan Plateau. The cosmogenic nuclide exposure dating method has proven reliable in dating fluvial terraces (Hetzel et al., 2002; Brown et al., 2003), and is especially useful for bedrock terraces which are common in Tibet. Here we report on the first systematic study of lake shorelines using cosmogenic nuclide exposure dating. The main target of this work is to find out whether high lakes formed
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synchronously on the Tibetan Plateau and how they reflect spatial and temporal changes of paleo-climate. 2. Geological settings and sampling Tibet has the largest area of lakes in China. Most of Tibet lakes occur in tectonic depressions caused by a number of E–W and S–N trending faults. For example, Nam Co and Zhari Nam Co (Fig. 1b) occur in the E–W trending depression between Gangdise and north
Qiangtang. This depression is the geomorphological manifestation of the Gar-Nam rift which starts from Ge’gyai, Pakistan to the west Nyaniqentanglha mountain range. The occurrence of Peiku Co is related to E–W trending Jilong-Tingri-Cuona rift. Tangra Yum Co, Tangqung Co and Xuro Co are located in a graben induced by a S–N normal fault. Siling Co occurs in the intersection of E–W and S–N trending rift zones (Fig. 1b). Most of S–N Tibetan normal faults are thought to have formed in the Miocene (Blisniuk et al., 2001) or less than 5 Ma ago (Mahéo
Fig. 1. Tibet lakes studied in this work. (a) Study area in Tibet; (b) Enlarged gray part in (a) which shows the locations of eight lakes. Data in red indicate exposure ages obtained for high lake shorelines around the eight lakes.
P. Kong et al. / Journal of Asian Earth Sciences 41 (2011) 263–273
et al., 2007). The 1 Ma apatite (U–Th)/He ages for samples from Xuro Co (central Tangra Yum Co) suggest continued rapid exhumation at rates of >1.0 mm/yr (Dewane et al., 2006). The initiation of E–W trending faults in Tibet may have occurred early but is less well constrained (Zhang et al., 1998). We have determined exposure ages of shorelines around eight lakes in this study (Fig. 1). Two lakes are located south of the Yarlung Tsangpo River, and six are located north of the river. In contrast to lakes north of the river, shorelines around lakes south of Yarlung Tsangpo River cannot be clearly identified in the field and are not much higher than the current water levels. The exceptions are at Peiku Co and Drolung Co where shorelines are obviously higher than the current water levels and can be easily recognized from their relics on the surrounding mountains. Lakes located south of Yarlung Tsangpo River are believed to have discharged during high lake stages (Zhang et al., 1998). We have collected two kinds of terrace samples: bedrock and terrace deposits. All samples are from the surface, 1–2 cm thick. From bedrock we sampled mainly quartz veins which protrude 1–2 cm out of bedrock. From terrace deposits, we collected over 100 quartz clasts of 1–2 cm size for each sample. These samples were not covered by soil or organic materials. The elevations of the samples were obtained with a handheld GPS.
2.1. Peiku Co Peiku Co (85°290 –85°400 E, 28°460 –29°020 N) lies in Jilong County, 20–30 km south of the Yarlung Tsangpo River, at an elevation of 4580 m with an area of 300 km2 (Fig. 2). Current water input is mainly from precipitation driven by the Indian monsoon and melting glaciers of Mount Xixapangma. This lake is located in the depression caused by Jilong-Tingri-Cuona fault, although Deng and Liu (1998) believed it is an ice-dammed lake. From the Google-Earth image we could identify high lake terraces at 4680– 4700 m. At 4680 m, the lake drains to Langqiang Co, then to Pengqu River, and finally enters into Nepal at Chentang. The sample TA02 we analyzed is taken from a wide and flat depositional terrace at 4682 m.
2.2. Drolung Co
265
2.4. Zhari Nam Co Nam Co, Siling Co, Zhari Nam Co and Tangra Yum Co are the four largest lakes in Tibet. Zhari Nam Co (85°200 -85°540 E, 30°440 – 31°050 N) is located to the west, in Cuoqin County, at an elevation of 4613 m with an area of 1023 km2 (Fig. 4). Average water depth is 3.6 m with a maximum of 5.6 m. Current water input is mainly from precipitation and glacial meltwater. Lacustrine deposits form a sequence of clear benches on surrounding valley sides. We have analyzed two quartz-pebble samples (TD06 at 4761 m and TD07 at 4718 m) taken from the highest sedimentary benches in the northwest direction of the lake and a beach pebble sample (TD11 at 4615 m) taken from the lake water level. 2.5. Tangra Yum Co Tangra Yum Co (86°230 –86°490 E, 30°450 –31°220 N) is located in Nima County, about 100 km east of Zhari Nam Co, with an elevation of 4535 m and an area of 1000 km2 (Fig. 4). The higher shorelines are bedrock terraces and lacustrine deposits occur only below 4750 m. The high bedrock terraces and the lower sedimentary units appear to be remnants of two high lake events, as the low terraces are well preserved and the high terraces are obscured and preserved only at some locations. This lake shows the largest change in water depth found in Tibet so far. We have analyzed three samples (TH01 at 4900 m, TH02 at 4835 m and TH03 at 4825 m) from the high bedrock terraces. 2.6. Tangqung Co Tangqung Co (86°410 –86°480 E, 31°310 –31°370 N) is located to the north of Tangra Yum Co, in Nima County at an elevation of 4475 m (Fig. 4). Lacustrine deposits are well preserved between Tangqung Co and Tangra Yum Co at elevations below 4740 m, which suggests that the two lakes were connected at high lake levels. We have sampled and analyzed two lacustrine deposits (TI01 at 4739 m and TI02 at 4705 m) preserved between Tangqung Co and Tangra Yum Co and a beach sand sample (TI04 at 4476 m) taken from the lake water level. We have also analyzed a sample (TI05 at 4710 m) taken from a lower bedrock terrace to the north of the lake. 2.7. Siling Co
0
0
0
0
Drolung Co (85°22 –85°26 E, 29°06 –29°09 N) is a small lake located in Jilong County, 20 km south of the Yarlung Tsangpo River, at an elevation of 4600 m with an area of 17 km2 (Fig. 2). Currently no glaciated mountains are located in the catchment, which measures 540 km2. Water input is mainly from precipitation driven by the Indian Monsoon. Shorelines form rings on surrounding mountains and can be easily recognized. Higher shorelines are bedrock terraces, and the highest terrace is at 4723 m. At this elevation, the lake discharges through Rongna pass to Yarlung Tsangpo River. We sampled quartz veins (TB01 and TB02) from the two highest bedrock terraces around this lake.
Siling Co (88°320 –89°220 E, 31°330 –32°020 N) is located in central Tibet, near the juncture of three counties: Bange, Shenzha and Nima. The present lake level is 4530 m with an area of 1820 km2 (Fig. 5) and a maximum water depth of 33 m. Currently the lake is mainly fed by precipitation and melting glaciers of Tanggula mountain. The area of the catchment is the largest in Tibet, 45,530 km2. Shorelines cutting in bedrock terraces surround the lake. We have taken samples from the highest bedrock terrace at 4600 m to the north and to the southeast. Two samples (TJ04 at 4596 m and TJ05 at 4593 m) taken from north were analyzed in this study. 2.8. Nam Co
2.3. Dajia Co Dajia Co (85°400 –85°460 E, 29°440 –29°580 N) is a small lake located in Angren County, 60 km north of the Yarlung Tsangpo River, at an elevation of 5100 m (Fig. 3). Current water input is from precipitation driven by the Indian monsoon and melting glaciers of surrounding mountains. At 5200 m, the lake discharges south into Yarlung Tsangpo River. In the southeast corner of the lake, there is a flat bedrock outcrop showing continuity in elevation with the highest depositional terrace to its north (Fig. 3). We sampled this highest bedrock terrace (TC01) at 5186 m.
Nam Co (90°150 –91°020 E, 30°300 –30°560 N) is the largest lake in Tibet, at an elevation of 4718 m with a surface area of 1980 km2 (Wu and Zhu, 2008). The catchment area measures 10,610 km2. It lies in the hinterland of the Tibetan Plateau, located north of the Gangdise and Nyainqentanglha mountains, 120 km north of Lhasa. Current water input is mainly from precipitation and melting glaciers of the Nyainqentanglha mountain range. Lacustrine deposits occur clearly below 4750 m (Fig. 5). At 4750 m, the lake discharges to Ren Co, Mujiu Co and then to Siling Co. We have sampled and analyzed lacustrine pebbles located below 4740 m.
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Fig. 2. Google-Earth images and photos showing lacustrine shorelines around Peiku Co and Drolung Co, located at the south of the Yarlung Tsangpo River. (a) Image showing lake shorelines and sample sites around the two lakes; (b–e) Photos and images focusing on the sample sites.
3. Experiment and results Chemical preparations were carried out in the cosmogenic nuclide lab at the Institute of Geology and Geophysics, Chinese Academy of Sciences, in Beijing. Bedrock and all pebble clasts were first crushed to 0.1–1.0 mm size. Meteoric 10Be was removed from the
samples by 4-5 iterations of ultrasonic leaching at 80 °C with a mixed solution of dilute HF and HNO3 (Kohl and Nishiizumi, 1992). About 10 g of pure quartz for each sample was completely dissolved together with 0.6 mg 9Be carrier. Beryllium and Al were separated by ion chromatography, hydroxides were precipitated, and then baked to oxides at 850 °C.
P. Kong et al. / Journal of Asian Earth Sciences 41 (2011) 263–273
267
Fig. 3. Google-Earth images and photos showing lacustrine shorelines around Dajia lake, north of the Yarlung Tsangpo River. (a) Image showing lake shorelines and sample site around the lake; (b and c) Photo and image focusing on the sample site.
Total Al concentrations in aliquots of the dissolved quartz samples were quantified by Inductively Coupled Plasma – Optical Emission Spectroscopy at the Institute of Geology and Geophysics, Chinese Academy of Sciences, and the 10Be and 26Al concentrations were measured by AMS at the Scottish Universities Environmental Research Centre in Glasgow, UK (Table 1). The 10Be/9Be ratios of the samples were normalized to the NIST standard SRM-4325. A value of 3.00 1011 for the 10Be/9Be ratio of the standard was used instead of the quoted value (Schnabel et al., 2007). The 10Be/9Be and 26 Al/27Al ratios of the chemical procedural blanks are at levels of 7 1015 and 2 1014, respectively. These values were used to correct the measured ratios for the samples. Production rates of cosmogenic nuclides at the Earth’s surface vary with latitude and elevation. We calculated site specific production rates using the scaling method of Stone (2000). The resultant scaling factors are combined with the high-latitude, sea-level production rate of 5.1 ± 0.3 atoms/g quartz/yr and 31.1 ± 1.9 atoms/g quartz/yr for 10Be and 26Al, respectively, to give the minimum model exposure ages presented in Table 1. Half lives of 1.5 Ma and 0.71 Ma are used for 10Be and 26Al, respectively. Exposure ages listed in the last two columns are corrected for geomagnetic field intensity variations according to the method of Dunai (2001). At latitudes of 30°N where most of our samples are located, the changes of production rates due to variations of geomagnetic field intensity are <10% when exposure ages are <40 ka, and the changes of production rates are 10–15% when exposure ages >40 ka. Except TI04 and TH03, other samples have 26Al/10Be concentration ratios close to 6, suggesting that these samples have undergone simple exposure histories. A two-isotope plot of 10Be and 26 Al data also shows that most samples are consistent with a model of constant exposure (Fig. 6). The deviation of the 26Al/10Be ratios
from 6 for TI04 and TH03 is probably due to Al concentration errors induced during HF volatilization. The chemical procedure has been improved by separation of aliquots for Al concentration measurement before HF volatilization (Kong et al., 2009). Therefore, the following discussions are made mainly based on 10Be data. The ages shown in Table 1 are minimum exposure ages. To better constrain the true exposure ages, we need to correct for erosion of the landforms. From TH02, having the oldest 10Be age, we get a maximum erosion rate of 1.58 mm/ka assuming its 10Be concentration reaching steady-state (For calculation, refer to Kong et al., 2007b). This value is significantly lower than those previously obtained for northwest Tibet (Kong et al., 2007b) and for central Tibet (Lal et al., 2003). Introducing erosion rates of 1.5 mm/ka would increase the exposure ages estimated based on 10Be at an age of 20 ka by 3%, and for those of 200 ka by 50%. However, if we consider erosion rates of 10 mm/ka as obtained for northwest Tibet (Kong et al., 2007b) and for central Tibet (Lal et al., 2003), the exposure ages estimated based on 10Be at an age of 20 ka will increase by 25%. Snow or ice cover on the samples should also be considered in derivation of the true exposure ages. Recent studies suggest that glaciers during the last glacial period limit to mountain fronts and do not form a united ice-sheet on the plateau, especially in the central Tibet (Owen, 2009). In the field we did not find glacial deposits around the lakes, which may indicate that the dated lakes were not affected by glacial activities during the last glacial period. Seasonal snow cover will reduce the cosmic ray intensity reaching to and reacting with the surface rocks. If we assume 10 cm thick snow cover on the samples for 3 months during a year, the true exposure ages of the terraces will increase 1% from the minimum exposure ages. Thus, seasonal snow cover will not affect the exposure ages obviously.
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Fig. 4. Google-Earth images and photos showing lacustrine shorelines around Zhari Nam Co, Tangra Yum Co and Tangqung Co, located in the west Qiangtang. (a) Image showing lake shorelines and sample sites around the three lakes; (b–e) Photos and image focusing on the sample sites.
P. Kong et al. / Journal of Asian Earth Sciences 41 (2011) 263–273
269
Fig. 5. Google-Earth images and photos showing lacustrine shorelines around Siling Co and Nam Co, located in the east Qiangtang. (a) Image showing lake shorelines and sample sites around the two lakes; (b–e) Photos and images focusing on the sample sites.
4. Discussion 4.1. Inherited cosmogenic nuclides We have taken samples from shorelines around eight lakes; shorelines around four lakes are cut into bedrock and the other four are lacustrine deposits. The lacustrine deposits come from erosion of surrounding landforms, containing inherited cosmo-
genic nuclides. Therefore, deposits at high lake levels contain cosmogenic nuclides produced both before deposition and after deposition. To correct for the inherited cosmogenic nuclides we have also sampled deposits at the lake water level. These samples may contain only inherited cosmogenic nuclides if they have only recently been exposed. We have studied three such samples; one sample (TD11) has cosmogenic nuclide concentrations not obviously distinguishable from those for high lake deposits. This
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Table 1 Exposure ages of lacustrine shorelines around eight Tibet lakes. Lake name
Sample name
Sample type
Long. (E)
Lat. (N)
Elev. (m)
10 Be atoms/g (106)
Peiku
TA02
Terrace pebble
85°30
28°48
4682
1.53 ± 0.07
Drolung
TB01 TB02
Bedrock Bedrock
85°22 85°22
29°08 29°08
4723 4710
0.29 ± 0.02 0.31 ± 0.02
26
Al atoms/g (106)
1.83 ± 0.31 1.97 ± 0.15
26
Al/10Be
6.2 ± 1.2 6.4 ± 0.6
10
Prod. Be
Prod. 26 Al
Min. exp. age (10Be, ka)
80.6
492
19.1 ± 1.5
82.8 82.3
505 502
3.6 ± 0.2 3.8 ± 0.3
Min. exp. age (26Al, ka)
Exp. agea (10Be, ka)
Exp. agea (26Al, ka)
18.7 3.6 ± 0.6 3.9 ± 0.3
3.7 3.9
3.7 4.0
Dajia
TC01
Bedrock
85°46
29°47
5186
5.53 ± 0.13
36.1 ± 2.2
6.5 ± 0.4
104
633
53.9 ± 3.8
58.5 ± 4.5
48.0
52.1
Zhari Nam
TD06
Terrace pebble Terrace pebble Beach pebble
85°17
31°04
4761
1.30 ± 0.06
8.11 ± 0.54
6.3 ± 0.6
88.8
542
14.7 ± 1.0
15.1 ± 1.1
14.4
14.8
85°17
31°04
4718
2.50 ± 0.06
13.5 ± 0.7
5.4 ± 0.3
87
531
28.9 ± 2.2
25.8 ± 1.8
27.0
24.1
85°17
31°04
4615
1.62 ± 0.04
82.8
506
19.9 ± 1.4
TD07 TD11
19.5
Tangra Yum
TH01 TH02 TH03
Bedrock Bedrock Bedrock
86°32 86°32 86°32
31°05 31°05 31°05
4900 4835 4825
15.2 ± 0.3 26.7 ± 0.6 12.9 ± 0.3
104 ± 5 168 ± 9 991 ± 5
6.8 ± 0.4 6.3 ± 0.4 7.7 ± 0.4
95.1 91.9 91.5
581 561 558
166 ± 12 312 ± 22 146 ± 10
196 ± 14 356 ± 26 195 ± 14
145 271 127
170 305 169
Tangqung
TI01
86°39
31°25
4739
1.81 ± 0.04
11.3 ± 0.6
6.2 ± 0.4
88.7
541
20.5 ± 1.4
21.1 ± 1.5
20.1
20.7
86°39
31°25
4705
2.00 ± 0.06
12.0 ± 0.7
6.1 ± 0.4
87.3
533
22.9 ± 1.7
22.8 ± 1.6
22.3
22.3
86°39
31°25
4476
0.21 ± 0.02
0.99 ± 0.15
4.7 ± 1
78.2
477
2.7 ± 0.2
2.1 ± 0.2
2.8
2.2
TI05
Terrace pebble Terrace pebble Beach pebble Bedrock
86°47
31°37
4710
0.64 ± 0.02
4.59 ± 0.28
7.2 ± 0.5
87.5
534
7.3 ± 0.5
8.6 ± 0.6
7.4
8.7
Siling
TJ04 TJ05
Bedrock Bedrock
89°0 89°0
32°07 32°07
4596 4593
19.8 ± 0.7 13.1 ± 0.3
113 ± 6 88.9 ± 4.9
5.7 ± 0.4 6.8 ± 0.4
84.5 84.4
515 515
248 ± 19 161 ± 12
248 ± 18 189 ± 14
216 141
214 164
Nam
TL02
Terrace pebble Beach pebble
90°36
30°34
4738
4.85 ± 0.09
26.5 ± 1.4
5.5 ± 0.5
86.6
528
56.8 ± 4.0
51.4 ± 3.7
53.0
47.9
90°36
30°34
4719
1.49 ± 0.04
8.78 ± 0.49
5.9 ± 0.3
85.7
523
17.5 ± 1.2
16.9 ± 1.2
17.2
16.6
TI02 TI04
TL03
Note: The 10Be and 26Al exposure ages are calculated using scaling factors from Stone (2000). The errors with exposure ages also include 6% from production rate, 3% from Be carrier and 3% from ICP-OES for Al. a These exposure ages are corrected for geomagnetic field intensity variations (Dunai, 2001).
sample is from Zhari Nam Co and the lake is fed both by precipitation and meltwater from glaciers. Thus, the elevation of the water level may be significantly affected by temperature changes. Deposits found at the current water level may not be newly exposed; they may have been exposed, then buried by water and re-exposed recently. The highest bedrock samples may also contain inherited cosmogenic nuclides if they were not eroded enough to remove all cosmogenic nuclides. Except for Dajia Co, we have analyzed at least two bedrock samples per lake. The lower bedrock bench is at least 3 m lower than the highest terrace, ensuring removal of all inherited cosmogenic nuclides. The similar and consistent exposure ages for the highest and lower bedrock terraces of Drolung Co suggest that samples from bedrock terraces do not contain inheritance. Kong et al. (2007a) studied bedrock terraces around Sumxi Co and found no inheritance of cosmogenic 10Be in these terraces. Thus, bedrock terraces are good targets for cosmogenic exposure dating. Exposure ages of the bedrock terraces around Tangra Yum Co and Siling Co are systematically old. For such old ages, the effects of erosion are significant. The differences between the highest and lower terraces may reflect differential erosion.
Fig. 6. Plot of 26Al/10Be vs. 10Be concentrations for shorelines around Tibet lakes. The deviation of the 26Al/10Be ratios from 6 is probably due to Al concentration errors induced during the process of HF volatilization.
4.2. Spatial and temporal pattern of high lakes From Google-Earth images, we find that the high lake shorelines stand at similar elevations for a specific lake studied in this work. We cannot identify the influence of dip-slip faults on these lakes, especially on the high lake shorelines. Therefore, we interpret these shorelines and water level variations as reflecting climatic changes, rather than responding to tectonic activities.
4.2.1. Lakes in southern Tibet Peiku Co and Drolung Co are located in southern Tibet. From Google-Earth images we found that the sample TA02 we collected for Peiku Co is from an alluvial fan (Fig. 2d). As the lower part of the fan was obscured by the high lake terraces (Fig. 2d), the high lake of Peiku Co must have occurred after the formation of the fan. Considering the deposit sample to contain cosmogenic nuclides
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271
Fig. 7. High lake deposits of Nam Co suggested by Zhao et al. (2002, 2003). Grasses have not been converted to charcoal and appear as layers embedding in the fine deposits. Wood and grass layers embedded in the deposits were dated with 14C, which show ages between 1.7 and 2.3 ka.
produced before deposition, the highest lake level should have occurred within 18.7 ka. Studies of 14C deposited in a peat layer 20 m below the highest terrace give an age of 13.8 ka (Deng and Liu, 1998), which is consistent with our results. Analysis of the two bedrock samples from Drolung Co gives quite consistent exposure ages, 3.7 ka and 3.9 ka. At 4720 m, the lake discharges through Rongna pass to Yarlung Tsangpo River. Our data suggest that the lake became closed at 3.8 ka. Zhang et al. (1998) obtained an age of 3 ka BP, by 14C dating of the high lake terrace, for the close of Yamdrok lake. Yamdrok lake is also located in southern Tibet, 480 km east of Drolung Co, and once discharged to the Yarlung Tsangpo River. The two sampled shorelines around Drolung Co are 123 m and 110 m higher than the current lake water level, respectively. Thus the lake level has lowered by over 120 m vertically during the past 3.8 ka. As no glaciated mountains are located in the catchment, water in Drolung Co is mainly derived from precipitation driven by the Indian monsoon. There is evidence that the intensity of precipitation driven by the Indian Monsoon remained at consistently high levels between 8 and 10 ka and gradually decreased thereafter (Fleitmann et al., 2003; Wu et al., 2006). Our results suggest that the precipitation–evaporation balance in the catchment where Drolung Co is located changed from positive to negative at a time around 3.7–3.9 ka. Based on Kutzbach’s hydrologic and energy balance model, Wang et al. (2008) calculated that the annual precipitation in Drolung Co catchment at 3.8 ka is 530 mm/a, which is 200 mm/a higher than at present. This suggests precipitation increasing over 30% along with 1 °C temperature increase within the domain of the Indian monsoon, which is much higher than the average of 6% worldwide (Wentz et al., 2007). Thus global warming has much stronger impacts on the population living in the domain of the Indian monsoon.
4.2.2. Dajia Co and Zhari Nam Co Dajia Co is the highest lake we have studied in this work. The 10 Be exposure age for the highest bedrock terrace is 48 ka. This suggests that the high water level of Dajia Co occurred during the early part of Marine Isotope Stage 3 (MIS-3: 58–32 ka). We have analyzed three pebble samples for Zhari Nam Co, two from the highest terraces (TD6 and TD7) and one from the current lake water level (TD11). From Table 1 we see that the exposure age for the water level sample is not distinguishable from the two high terrace samples. This may suggest short exposure with high inherited cosmogenic nuclide concentrations or re-cycling of terrace pebbles, that is, the pebbles were removed during lake level changes. In both cases, the exposure ages of TD6 and TD7 provide the maximum age limit of high lake occurrence. That is, the high lake level should have occurred within 15 ka.
4.2.3. Tangra Yum Co and Tangqung Co The three samples from high lake bedrock terraces around Tangra Yum Co have exposure ages of 145 ka, 271 ka and 127 ka. The differences between the three exposure ages may reflect the effects of erosion, since the exposure ages do not show relationship with elevation. Xu et al. (2006) found lacustrine deposits around Xuro Co at elevations below 4900 m. Tangra Yum Co, Xuro Co and Tangqung Co formed a united lake at high lake levels (Fig. 4). They determined, using the method of electron spin resonance (ESR), the deposition ages for lacustrine sediments at 4900 m and 4820 m as 369 ka and 230 ka, respectively. Their results reflect deposition time at high lake levels, whereas cosmogenic dates reflect the time that the high lake started to retreat. Both results of this study and of Xu et al. (2006) suggest preservation of very old lake shorelines around Tangra Yum Co.
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The exposure age of the pebble sample taken from the beach of Tangqung Co is 2.8 ka, which suggests that inheritance is not significant for deposits around this lake. This result differs from that for Zhari Nam Co and may suggest that Tangqung Co shrank continuously below 4750 m. However, the significant difference in exposure age between the deposit terrace (TI02: 22.3 ka) and the bedrock terrace (TI05: 7.4 ka) which is at the same elevation is hard to explain. One possibility is the deposits around Tangqung Co at high elevations (P4710 m) are not totally derived from surrounding mountains but are partly mixed with re-cycled material related to the united lake. Tangra Yum Co and Tangqung Co were connected at their high lake levels. The highest lake shorelines, however, are recorded at different elevations for the two lakes. Such a phenomenon occurs often on the Tibetan Plateau. Zheng et al. (2006) and Xu et al. (2006) attribute this to differential neotectonic activity. However, our analyses of the highest terraces at Tangra Yum Co and Tangqung Co indicate significantly different exposure ages: <20 ka for Tangqung Co and >271 ka for Tangra Yum Co. Obviously the highest shorelines around the two lakes do not reflect the same lake expansion event. As the highest lake level in the catchment was reached very early, >271 ka, most of the high lake terraces were erased by later erosion. Thus, the mid-Pleistocene high lake was only recorded in a few very stable landforms. 4.2.4. Nam Co and Siling Co From 1: 100,000 topographic maps (contour interval: 40 m) we determined that water in Nam Co overflows at 4750 m to Ren Co, Mujiu Co, then flow into Siling Co. Shorelines below 4750 m are clearly seen around Nam Co. We have analyzed two pebble samples for Nam Co, one from the 4740 m shoreline and the other from the current water level. The corresponding exposures ages (after geomagnetic correction) are 53 ka and 17 ka, respectively. Assuming that the concentration of 10Be for the water level pebbles are totally due to inheritance, the exposure ages for the high lake sample, after correction of inheritance, are 36 ka. As Nam Co is also fed by meltwater from glaciers, the water level may have risen and fallen repeatedly in the past. Thus the concentration of 10Be for the water level pebbles may not be completely attributed to inheritance. In such a case the inheritance will be lower and the corrected ages larger. In other word the high lake of Nam Co should have occurred at the time 53–36 ka, during MIS-3. Thus, Nam Co was drained and became closed during MIS-3. The results agree well with those reported in Lehmkuhl and Haselein (2000). Siling Co is another lake with very old shoreline exposure ages (>216 ka). Considering erosion, we believe that the high lake level occurred even earlier. Zhao et al. (2002, 2003) claimed that they have found lacustrine deposits around Nam Co at 4850 m and suggested that Nam Co and Siling Co were connected through Ren Co, forming a united lake 139 m higher than the Nam Co water level at 120 ka. The highest terrace of Siling Co is at about 4600 m. If Nam Co and Siling Co were to form a combined lake, the exposure ages of the Siling terraces should be younger than 120 ka. The exposure age of the high Siling terrace is, however, at least as old as 216 ka. In addition, we have also determined the ages of grass layers that are embedded in the sediments claimed by Zhao et al. (2002, 2003) as lacustrine deposits at 4750–4850 m using 14C dating (Fig. 7). The results show ages between 1.7 and 2.3 ka, suggesting that these deposits are not related to the high lake of Nam Co. All these results argue against the assumed combined lake within east Qiangtang at 120 ka. 4.3. Occurrence of high lakes in Tibet The exposure ages for high lake shorelines around the eight lakes are marked in Fig. 1b. Four out of the eight lakes discharged
at high lake levels. In fact the exposure ages of high lake terraces around these four lakes do not tell exactly when the highest lakes occurred. What we can infer is Nam Co and Dajia Co discharged and became closed during MIS-3 and these two lakes did not overflow and reach the levels achieved during MIS-3 after LGM. Peiku Co and Drolung Co discharged and became closed after the LGM. High lake levels at Tangra Yum Co and Siling Co occurred 220 ka ago and the high lake of Zhari Nam Co occurred after the LGM. These limited data show that there were no synchronous high lake levels in the analyzed area of Tibet.
4.4. Indian monsoon and the westerlies Changes in water level of lakes located to the south of the Yarlung Tsangpo River are strongly affected by the intensity of precipitation driven by the Indian monsoon. From our results and previous works (Zhang et al., 1998), we see that lakes located to the south of the Yarlung Tsangpo River discharged during the early Holocene when the Indian monsoon was strong. Along with the decrease in the strength of the Indian monsoon, the precipitation– evaporation balance of lakes changed from positive to negative and the lake Drolung Co became closed at 3.8 ka. Our results show that high lakes of Dajia Co and Nam Co occurred during MIS-3, when the Indian monsoon was more intense than at Holocene. As temperatures during MIS-3 are also high and input of meltwater from glaciers is significant, it is not straight forward to attribute the occurrence of high water levels of these two lakes to the enhanced Indian monsoon. Dajia Co is located in a catchment with few glaciers. Thus the occurrence of high water levels at Dajia Co is not likely due to melting of glaciers. We suggest that the Indian monsoon provides most of water input to Dajia Co and Nam Co, and the high precipitation during MIS-3 was driven by the greater intensity of the Indian monsoon, leading to the substantial expansion of these two lakes. The preservation of very old lacustrine shorelines around Tangra Yum Co and Siling Co suggests low erosion rates and perhaps low precipitation at their latitude (31–32°N). Toward the north and west, high water levels of Zhari Nam Co and Sumxi Co (Kong et al., 2007a) occurred after the LGM. The different pattern of lake development between these regions may likely suggest that the Zhari Nam Co and Sumxi Co area is outside the influence of the Indian monsoon.
5. Conclusions The patterns of lake level changes are different for lakes located to the south and to the north of the Yarlung Tsangpo River. Lakes located to the south of the Yarlung Tsangpo River discharged at high lake levels, whereas lakes located to the north of the Yarlung Tsangpo River developed during their recent history after the LGM as closed lakes. Our data suggest that the intensity of precipitation driven by the Indian monsoon may have significantly changed in southern Tibet during the middle-late Holocene. This study analyzed six lakes located to the north of the Yarlung Tsangpo River. High lake levels occurred before 271 ka at Tangra Yum Co and 216 ka at Siling Co, located at 31°N and 32°N, respectively. Toward the south and east, the high lake levels of Dajia Co and Nam Co occurred during MIS-3, whereas toward the north and west, high lake levels of Zhari Nam Co and Sumxi Co (Kong et al., 2007a) occurred after the LGM. Our data do not show synchronous high lake levels in Tibet. The different pattern of lake development in the north of the Yarlung Tsangpo River may result from regional climatic control.
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Acknowledgment Funding for this project was provided by the National Science foundation of China (Grants 40573041 and 40125010). References Benn, D.I., Owen, L.A., 1998. The role of the Indian summer monsoon and the midlatitude westerlies in Himalayan glaciation: review and speculative discussion. Journal of Geological Society of London 155, 353–363. Blisniuk, P.M., Hacker, B.R., Glodny, J., Ratschbacher, L., Bi, S., Wu, Z., McWilliams, M.O., Calvert, A., 2001. Normal faulting in central Tibet since at least 13.5 Myr ago. Nature 412, 628–632. Brown, E.T., Bendick, R., Bourlès, D.L., Gaur, V., Molnar, P., Raisbeck, G.M., Yiou, F., 2003. Early Holocene climate recorded in geomorphological features in Western Tibet. Palaeogeogrphy Palaeoclimatology Palaeoecology 199, 141–151. Deng, X., Liu, Y., 1998. An explanation of the dam of the Peikucuo lake in the north slope of Mt. Xixiabangma. Journal of Glaciology and Geocryology 20, 85–87 (in Chinese). Dewane, T.J., Stockli, D.F., Hager, C., Taylor, M., Ding, L., Lee, J., Wallis, S., 2006. Timing of Cenozoic E–W Extension in the Tangra Yum Co-Kung Co Rift, Southcentral Tibet. AGU Fall Meeting, Abstract #T34C-04. Dunai, T.J., 2001. Influence of secular variation of the geomagnetic field on production rates of in situ produced cosmogenic nuclides. Earth and Planetary Science Letters 193, 197–212. Fang, J.Q., 1991. Lake evolution during the past 30,000 years in China, and its implications for environmental change. Quaternary Research 36, 37–60. Fleitmann, D., Burns, S.J., Mudelsee, M., Neff, U., Kramers, J., Mangini, A., Matter, A., 2003. Holocene forcing of the Indian Monsoon recorded in a stalagmite from southern Oman. Science 300, 1737–1739. Gasse, F., Arnold, M., Fontes, J.C., Fort, M., Gibert, E., Huc, A., Li, B., Li, Y., Liu, Q., Mélières, F., Van Campo, E., Wang, F., Zhang, Q., 1991. A 13,000-year climate record from western Tibet. Nature 353, 742–745. Hetzel, R., Niedermann, S., Ivy-Ochs, S., Kubik, P.W., Tao, M., Gao, B., 2002. 21Ne versus 10Be and 26Al exposure ages of fluvial terraces: the influence of crustal Ne in quartz. Earth and Planetary Science Letters 201, 575–591. Kohl, C.P., Nishiizumi, K., 1992. Chemical isolation of quartz for measurement of insitu-produced cosmogenic nuclides. Geochimica et Cosmochimica Acta 56, 3583–3587. Kong, P., Na, C., Fink, D., Huang, F., Ding, L., 2007a. Cosmogenic 10Be inferred lakelevel changes in Sumxi Co Basin, Western Tibet. Journal of Asian Earth Science 29, 698–703. Kong, P., Na, C., Fink, D., Ding, L., Huang, F., 2007b. Erosion in the northwest Tibet from in situ produced cosmogenic nuclides 10Be and 26Al in bedrocks. Earth Surface Processes and Landforms 32, 116–125. Kong, P., Granger, D., Wu, F.Y., Caffee, M.W., Wang, Y.J., Zhao, X.T., Zheng, Y., 2009. Cosmogenic nuclide burial ages and provenance of the Xigeda paleo-lake: implications for evolution of the Middle Yangtze River. Earth and Planetary Science Letters 278, 131–141. Lal, D., Harris, N.B.W., Sharma, K.K., Gu, Z., Ding, L., Liu, T., Dong, W., Caffee, M.W., Jull, A.J.T., 2003. Erosion history of the Tibetan plateau since the last interglacial: Constraints from the first studies of cosmogenic 10Be from Tibetan bedrock. Earth and Planetary Science Letters 217, 33–42.
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Contents lists available at ScienceDirect
Journal of Asian Earth Sciences journal homepage: www.elsevier.com/locate/jseaes
Cosmogenic
10
Be and
26
Al dating of paleolake shorelines in Tibet
Ping Kong a,⇑, Chunguang Na a, Roderick Brown b, Derek Fabel b, Stewart Freeman c, Wei Xiao a, Yajun Wang a a
Key Laboratory of the Earth’s Deep Interior, Institute of Geology and Geophysics, Chinese Academy of Sciences, P.O. Box 9825, Beijing 100029, China Department of Geographical and Earth Sciences, University of Glasgow, Glasgow G12 8QQ, Scotland, UK c Scottish Universities Environmental Research Centre, Rankine Avenue, Scottish Enterprise Technology Park, East Kilbride G75 0QF, Scotland, UK b
a r t i c l e
i n f o
Article history: Received 11 February 2010 Received in revised form 28 January 2011 Accepted 25 February 2011 Available online 10 March 2011 Keywords: Tibetan Plateau Lake shoreline Exposure age Monsoon Cosmogenic nuclide
a b s t r a c t We have used in situ produced cosmogenic nuclides 10Be and 26Al to date lacustrine shorelines around eight lakes in Tibet. Two lakes located south of the Yarlung Tsangpo River discharged at high lake levels. One of them, Drolung Co (Co is a local term, meaning lake), became closed (its water level fell below its outlet channel) at 3.8 ka and lowered over 120 m vertically since then. This implies a significant change in intensity of precipitation driven by the Indian monsoon over southern Tibet in the middle-late Holocene. Nam Co and Dajia Co, located to the north of the Yarlung Tsangpo River, also drained at high lake levels and became closed 53–36 ka and 48 ka, respectively. Of four other lakes developed closely in the Qiangtang basin, north of the Yarlung Tsangpo River, Tangra Yum Co and Siling Co (located at 31– 32°N) show exposure ages for high lake shorelines of over 220 ka, the oldest lake shorelines reported for Tibet, while towards the north and west, the high lake of Zhari Nam Co appeared after the Last Glacial Maximum (LGM). The different pattern of lake development to the north of Yarlung Tsangpo River may reflect regional climate effects. Ó 2011 Elsevier Ltd. All rights reserved.
1. Introduction The Tibetan Plateau between 74–98°E and 28–40°N is covered by lakes, which have a present total surface area exceeding 30,000 km2. Ancient, preserved lake shores identified from field investigations and mapped using satellite imagery suggest that the paleolakes cover a much more extensive area of approximately 84,000 km2 (Lehmkuhl and Haselein, 2000), or even more (Zhao et al., 2002, 2003; Zheng et al., 2006). Most of these high elevation lakes are surrounded by conspicuous sequences of shorelines. We interpret the shorelines as indicating the most recent high lake levels as older remnants could have been obscured during repeated rises and falls of lake levels. The changes of lake levels are the net result of precipitation, input from melting glaciers, outflow and evaporation, which are related to paleo-environmental changes on the Tibetan Plateau. Studies that can quantify the spatial and temporal pattern of these changes to the lake systems provide constraints on paleo-precipitation distribution and help to clarify the role that the Tibetan Plateau has played on global climate. Currently precipitation on the plateau comes mainly from the Indian monsoon and the mid-latitude westerlies. The influence of the Indian monsoon decreases significantly from east to west, whereas the westerlies primarily affect
⇑ Corresponding author. Tel.: +86 10 82998317; fax: +86 10 62010846. E-mail address: [email protected] (P. Kong). 1367-9120/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.jseaes.2011.02.016
the northern and western parts of the plateau (Benn and Owen, 1998). Quantitative studies of the ages of high lake shorelines are few, leading to various hypotheses about the pattern of development of Tibetan lakes. Fang (1991) gives a general overview on the evolution of lakes in China over the last 30 ka. He showed that lakes throughout China were at high levels during 30–24 ka, 22.5–20 ka and 9.5–3.5 ka. Some recent studies, however, suggest that lakes in Tibet and western China behave synchronously, with their largest sizes during 40–30 ka due to an enhanced Indian monsoon (Shi et al., 2001; Yang et al., 2004). By dating lacustrine sediments using a U–Th disequilibrium method, Zhao et al. (2002, 2003) preferred a combined eastern Qiangtang lake as large as 150,000 km2 in the period of 116–72 ka. Using cosmogenic nuclide dating, Kong et al. (2007a) studied the exposure ages of the highest bedrock terraces around Sumxi Co, a small closed lake located in northwest Tibet. They identified an age for the exposure of the highest preserved shoreline of 11–12.8 ka, different from the previous inference of early-mid Holocene (7–8 ka; Gasse et al., 1991) or 25–30 ka (Zhang et al., 1998). Clearly more precise ages related to high lakes are needed in order to understand paleo-environmental changes on the Tibetan Plateau. The cosmogenic nuclide exposure dating method has proven reliable in dating fluvial terraces (Hetzel et al., 2002; Brown et al., 2003), and is especially useful for bedrock terraces which are common in Tibet. Here we report on the first systematic study of lake shorelines using cosmogenic nuclide exposure dating. The main target of this work is to find out whether high lakes formed
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synchronously on the Tibetan Plateau and how they reflect spatial and temporal changes of paleo-climate. 2. Geological settings and sampling Tibet has the largest area of lakes in China. Most of Tibet lakes occur in tectonic depressions caused by a number of E–W and S–N trending faults. For example, Nam Co and Zhari Nam Co (Fig. 1b) occur in the E–W trending depression between Gangdise and north
Qiangtang. This depression is the geomorphological manifestation of the Gar-Nam rift which starts from Ge’gyai, Pakistan to the west Nyaniqentanglha mountain range. The occurrence of Peiku Co is related to E–W trending Jilong-Tingri-Cuona rift. Tangra Yum Co, Tangqung Co and Xuro Co are located in a graben induced by a S–N normal fault. Siling Co occurs in the intersection of E–W and S–N trending rift zones (Fig. 1b). Most of S–N Tibetan normal faults are thought to have formed in the Miocene (Blisniuk et al., 2001) or less than 5 Ma ago (Mahéo
Fig. 1. Tibet lakes studied in this work. (a) Study area in Tibet; (b) Enlarged gray part in (a) which shows the locations of eight lakes. Data in red indicate exposure ages obtained for high lake shorelines around the eight lakes.
P. Kong et al. / Journal of Asian Earth Sciences 41 (2011) 263–273
et al., 2007). The 1 Ma apatite (U–Th)/He ages for samples from Xuro Co (central Tangra Yum Co) suggest continued rapid exhumation at rates of >1.0 mm/yr (Dewane et al., 2006). The initiation of E–W trending faults in Tibet may have occurred early but is less well constrained (Zhang et al., 1998). We have determined exposure ages of shorelines around eight lakes in this study (Fig. 1). Two lakes are located south of the Yarlung Tsangpo River, and six are located north of the river. In contrast to lakes north of the river, shorelines around lakes south of Yarlung Tsangpo River cannot be clearly identified in the field and are not much higher than the current water levels. The exceptions are at Peiku Co and Drolung Co where shorelines are obviously higher than the current water levels and can be easily recognized from their relics on the surrounding mountains. Lakes located south of Yarlung Tsangpo River are believed to have discharged during high lake stages (Zhang et al., 1998). We have collected two kinds of terrace samples: bedrock and terrace deposits. All samples are from the surface, 1–2 cm thick. From bedrock we sampled mainly quartz veins which protrude 1–2 cm out of bedrock. From terrace deposits, we collected over 100 quartz clasts of 1–2 cm size for each sample. These samples were not covered by soil or organic materials. The elevations of the samples were obtained with a handheld GPS.
2.1. Peiku Co Peiku Co (85°290 –85°400 E, 28°460 –29°020 N) lies in Jilong County, 20–30 km south of the Yarlung Tsangpo River, at an elevation of 4580 m with an area of 300 km2 (Fig. 2). Current water input is mainly from precipitation driven by the Indian monsoon and melting glaciers of Mount Xixapangma. This lake is located in the depression caused by Jilong-Tingri-Cuona fault, although Deng and Liu (1998) believed it is an ice-dammed lake. From the Google-Earth image we could identify high lake terraces at 4680– 4700 m. At 4680 m, the lake drains to Langqiang Co, then to Pengqu River, and finally enters into Nepal at Chentang. The sample TA02 we analyzed is taken from a wide and flat depositional terrace at 4682 m.
2.2. Drolung Co
265
2.4. Zhari Nam Co Nam Co, Siling Co, Zhari Nam Co and Tangra Yum Co are the four largest lakes in Tibet. Zhari Nam Co (85°200 -85°540 E, 30°440 – 31°050 N) is located to the west, in Cuoqin County, at an elevation of 4613 m with an area of 1023 km2 (Fig. 4). Average water depth is 3.6 m with a maximum of 5.6 m. Current water input is mainly from precipitation and glacial meltwater. Lacustrine deposits form a sequence of clear benches on surrounding valley sides. We have analyzed two quartz-pebble samples (TD06 at 4761 m and TD07 at 4718 m) taken from the highest sedimentary benches in the northwest direction of the lake and a beach pebble sample (TD11 at 4615 m) taken from the lake water level. 2.5. Tangra Yum Co Tangra Yum Co (86°230 –86°490 E, 30°450 –31°220 N) is located in Nima County, about 100 km east of Zhari Nam Co, with an elevation of 4535 m and an area of 1000 km2 (Fig. 4). The higher shorelines are bedrock terraces and lacustrine deposits occur only below 4750 m. The high bedrock terraces and the lower sedimentary units appear to be remnants of two high lake events, as the low terraces are well preserved and the high terraces are obscured and preserved only at some locations. This lake shows the largest change in water depth found in Tibet so far. We have analyzed three samples (TH01 at 4900 m, TH02 at 4835 m and TH03 at 4825 m) from the high bedrock terraces. 2.6. Tangqung Co Tangqung Co (86°410 –86°480 E, 31°310 –31°370 N) is located to the north of Tangra Yum Co, in Nima County at an elevation of 4475 m (Fig. 4). Lacustrine deposits are well preserved between Tangqung Co and Tangra Yum Co at elevations below 4740 m, which suggests that the two lakes were connected at high lake levels. We have sampled and analyzed two lacustrine deposits (TI01 at 4739 m and TI02 at 4705 m) preserved between Tangqung Co and Tangra Yum Co and a beach sand sample (TI04 at 4476 m) taken from the lake water level. We have also analyzed a sample (TI05 at 4710 m) taken from a lower bedrock terrace to the north of the lake. 2.7. Siling Co
0
0
0
0
Drolung Co (85°22 –85°26 E, 29°06 –29°09 N) is a small lake located in Jilong County, 20 km south of the Yarlung Tsangpo River, at an elevation of 4600 m with an area of 17 km2 (Fig. 2). Currently no glaciated mountains are located in the catchment, which measures 540 km2. Water input is mainly from precipitation driven by the Indian Monsoon. Shorelines form rings on surrounding mountains and can be easily recognized. Higher shorelines are bedrock terraces, and the highest terrace is at 4723 m. At this elevation, the lake discharges through Rongna pass to Yarlung Tsangpo River. We sampled quartz veins (TB01 and TB02) from the two highest bedrock terraces around this lake.
Siling Co (88°320 –89°220 E, 31°330 –32°020 N) is located in central Tibet, near the juncture of three counties: Bange, Shenzha and Nima. The present lake level is 4530 m with an area of 1820 km2 (Fig. 5) and a maximum water depth of 33 m. Currently the lake is mainly fed by precipitation and melting glaciers of Tanggula mountain. The area of the catchment is the largest in Tibet, 45,530 km2. Shorelines cutting in bedrock terraces surround the lake. We have taken samples from the highest bedrock terrace at 4600 m to the north and to the southeast. Two samples (TJ04 at 4596 m and TJ05 at 4593 m) taken from north were analyzed in this study. 2.8. Nam Co
2.3. Dajia Co Dajia Co (85°400 –85°460 E, 29°440 –29°580 N) is a small lake located in Angren County, 60 km north of the Yarlung Tsangpo River, at an elevation of 5100 m (Fig. 3). Current water input is from precipitation driven by the Indian monsoon and melting glaciers of surrounding mountains. At 5200 m, the lake discharges south into Yarlung Tsangpo River. In the southeast corner of the lake, there is a flat bedrock outcrop showing continuity in elevation with the highest depositional terrace to its north (Fig. 3). We sampled this highest bedrock terrace (TC01) at 5186 m.
Nam Co (90°150 –91°020 E, 30°300 –30°560 N) is the largest lake in Tibet, at an elevation of 4718 m with a surface area of 1980 km2 (Wu and Zhu, 2008). The catchment area measures 10,610 km2. It lies in the hinterland of the Tibetan Plateau, located north of the Gangdise and Nyainqentanglha mountains, 120 km north of Lhasa. Current water input is mainly from precipitation and melting glaciers of the Nyainqentanglha mountain range. Lacustrine deposits occur clearly below 4750 m (Fig. 5). At 4750 m, the lake discharges to Ren Co, Mujiu Co and then to Siling Co. We have sampled and analyzed lacustrine pebbles located below 4740 m.
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Fig. 2. Google-Earth images and photos showing lacustrine shorelines around Peiku Co and Drolung Co, located at the south of the Yarlung Tsangpo River. (a) Image showing lake shorelines and sample sites around the two lakes; (b–e) Photos and images focusing on the sample sites.
3. Experiment and results Chemical preparations were carried out in the cosmogenic nuclide lab at the Institute of Geology and Geophysics, Chinese Academy of Sciences, in Beijing. Bedrock and all pebble clasts were first crushed to 0.1–1.0 mm size. Meteoric 10Be was removed from the
samples by 4-5 iterations of ultrasonic leaching at 80 °C with a mixed solution of dilute HF and HNO3 (Kohl and Nishiizumi, 1992). About 10 g of pure quartz for each sample was completely dissolved together with 0.6 mg 9Be carrier. Beryllium and Al were separated by ion chromatography, hydroxides were precipitated, and then baked to oxides at 850 °C.
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Fig. 3. Google-Earth images and photos showing lacustrine shorelines around Dajia lake, north of the Yarlung Tsangpo River. (a) Image showing lake shorelines and sample site around the lake; (b and c) Photo and image focusing on the sample site.
Total Al concentrations in aliquots of the dissolved quartz samples were quantified by Inductively Coupled Plasma – Optical Emission Spectroscopy at the Institute of Geology and Geophysics, Chinese Academy of Sciences, and the 10Be and 26Al concentrations were measured by AMS at the Scottish Universities Environmental Research Centre in Glasgow, UK (Table 1). The 10Be/9Be ratios of the samples were normalized to the NIST standard SRM-4325. A value of 3.00 1011 for the 10Be/9Be ratio of the standard was used instead of the quoted value (Schnabel et al., 2007). The 10Be/9Be and 26 Al/27Al ratios of the chemical procedural blanks are at levels of 7 1015 and 2 1014, respectively. These values were used to correct the measured ratios for the samples. Production rates of cosmogenic nuclides at the Earth’s surface vary with latitude and elevation. We calculated site specific production rates using the scaling method of Stone (2000). The resultant scaling factors are combined with the high-latitude, sea-level production rate of 5.1 ± 0.3 atoms/g quartz/yr and 31.1 ± 1.9 atoms/g quartz/yr for 10Be and 26Al, respectively, to give the minimum model exposure ages presented in Table 1. Half lives of 1.5 Ma and 0.71 Ma are used for 10Be and 26Al, respectively. Exposure ages listed in the last two columns are corrected for geomagnetic field intensity variations according to the method of Dunai (2001). At latitudes of 30°N where most of our samples are located, the changes of production rates due to variations of geomagnetic field intensity are <10% when exposure ages are <40 ka, and the changes of production rates are 10–15% when exposure ages >40 ka. Except TI04 and TH03, other samples have 26Al/10Be concentration ratios close to 6, suggesting that these samples have undergone simple exposure histories. A two-isotope plot of 10Be and 26 Al data also shows that most samples are consistent with a model of constant exposure (Fig. 6). The deviation of the 26Al/10Be ratios
from 6 for TI04 and TH03 is probably due to Al concentration errors induced during HF volatilization. The chemical procedure has been improved by separation of aliquots for Al concentration measurement before HF volatilization (Kong et al., 2009). Therefore, the following discussions are made mainly based on 10Be data. The ages shown in Table 1 are minimum exposure ages. To better constrain the true exposure ages, we need to correct for erosion of the landforms. From TH02, having the oldest 10Be age, we get a maximum erosion rate of 1.58 mm/ka assuming its 10Be concentration reaching steady-state (For calculation, refer to Kong et al., 2007b). This value is significantly lower than those previously obtained for northwest Tibet (Kong et al., 2007b) and for central Tibet (Lal et al., 2003). Introducing erosion rates of 1.5 mm/ka would increase the exposure ages estimated based on 10Be at an age of 20 ka by 3%, and for those of 200 ka by 50%. However, if we consider erosion rates of 10 mm/ka as obtained for northwest Tibet (Kong et al., 2007b) and for central Tibet (Lal et al., 2003), the exposure ages estimated based on 10Be at an age of 20 ka will increase by 25%. Snow or ice cover on the samples should also be considered in derivation of the true exposure ages. Recent studies suggest that glaciers during the last glacial period limit to mountain fronts and do not form a united ice-sheet on the plateau, especially in the central Tibet (Owen, 2009). In the field we did not find glacial deposits around the lakes, which may indicate that the dated lakes were not affected by glacial activities during the last glacial period. Seasonal snow cover will reduce the cosmic ray intensity reaching to and reacting with the surface rocks. If we assume 10 cm thick snow cover on the samples for 3 months during a year, the true exposure ages of the terraces will increase 1% from the minimum exposure ages. Thus, seasonal snow cover will not affect the exposure ages obviously.
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Fig. 4. Google-Earth images and photos showing lacustrine shorelines around Zhari Nam Co, Tangra Yum Co and Tangqung Co, located in the west Qiangtang. (a) Image showing lake shorelines and sample sites around the three lakes; (b–e) Photos and image focusing on the sample sites.
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Fig. 5. Google-Earth images and photos showing lacustrine shorelines around Siling Co and Nam Co, located in the east Qiangtang. (a) Image showing lake shorelines and sample sites around the two lakes; (b–e) Photos and images focusing on the sample sites.
4. Discussion 4.1. Inherited cosmogenic nuclides We have taken samples from shorelines around eight lakes; shorelines around four lakes are cut into bedrock and the other four are lacustrine deposits. The lacustrine deposits come from erosion of surrounding landforms, containing inherited cosmo-
genic nuclides. Therefore, deposits at high lake levels contain cosmogenic nuclides produced both before deposition and after deposition. To correct for the inherited cosmogenic nuclides we have also sampled deposits at the lake water level. These samples may contain only inherited cosmogenic nuclides if they have only recently been exposed. We have studied three such samples; one sample (TD11) has cosmogenic nuclide concentrations not obviously distinguishable from those for high lake deposits. This
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Table 1 Exposure ages of lacustrine shorelines around eight Tibet lakes. Lake name
Sample name
Sample type
Long. (E)
Lat. (N)
Elev. (m)
10 Be atoms/g (106)
Peiku
TA02
Terrace pebble
85°30
28°48
4682
1.53 ± 0.07
Drolung
TB01 TB02
Bedrock Bedrock
85°22 85°22
29°08 29°08
4723 4710
0.29 ± 0.02 0.31 ± 0.02
26
Al atoms/g (106)
1.83 ± 0.31 1.97 ± 0.15
26
Al/10Be
6.2 ± 1.2 6.4 ± 0.6
10
Prod. Be
Prod. 26 Al
Min. exp. age (10Be, ka)
80.6
492
19.1 ± 1.5
82.8 82.3
505 502
3.6 ± 0.2 3.8 ± 0.3
Min. exp. age (26Al, ka)
Exp. agea (10Be, ka)
Exp. agea (26Al, ka)
18.7 3.6 ± 0.6 3.9 ± 0.3
3.7 3.9
3.7 4.0
Dajia
TC01
Bedrock
85°46
29°47
5186
5.53 ± 0.13
36.1 ± 2.2
6.5 ± 0.4
104
633
53.9 ± 3.8
58.5 ± 4.5
48.0
52.1
Zhari Nam
TD06
Terrace pebble Terrace pebble Beach pebble
85°17
31°04
4761
1.30 ± 0.06
8.11 ± 0.54
6.3 ± 0.6
88.8
542
14.7 ± 1.0
15.1 ± 1.1
14.4
14.8
85°17
31°04
4718
2.50 ± 0.06
13.5 ± 0.7
5.4 ± 0.3
87
531
28.9 ± 2.2
25.8 ± 1.8
27.0
24.1
85°17
31°04
4615
1.62 ± 0.04
82.8
506
19.9 ± 1.4
TD07 TD11
19.5
Tangra Yum
TH01 TH02 TH03
Bedrock Bedrock Bedrock
86°32 86°32 86°32
31°05 31°05 31°05
4900 4835 4825
15.2 ± 0.3 26.7 ± 0.6 12.9 ± 0.3
104 ± 5 168 ± 9 991 ± 5
6.8 ± 0.4 6.3 ± 0.4 7.7 ± 0.4
95.1 91.9 91.5
581 561 558
166 ± 12 312 ± 22 146 ± 10
196 ± 14 356 ± 26 195 ± 14
145 271 127
170 305 169
Tangqung
TI01
86°39
31°25
4739
1.81 ± 0.04
11.3 ± 0.6
6.2 ± 0.4
88.7
541
20.5 ± 1.4
21.1 ± 1.5
20.1
20.7
86°39
31°25
4705
2.00 ± 0.06
12.0 ± 0.7
6.1 ± 0.4
87.3
533
22.9 ± 1.7
22.8 ± 1.6
22.3
22.3
86°39
31°25
4476
0.21 ± 0.02
0.99 ± 0.15
4.7 ± 1
78.2
477
2.7 ± 0.2
2.1 ± 0.2
2.8
2.2
TI05
Terrace pebble Terrace pebble Beach pebble Bedrock
86°47
31°37
4710
0.64 ± 0.02
4.59 ± 0.28
7.2 ± 0.5
87.5
534
7.3 ± 0.5
8.6 ± 0.6
7.4
8.7
Siling
TJ04 TJ05
Bedrock Bedrock
89°0 89°0
32°07 32°07
4596 4593
19.8 ± 0.7 13.1 ± 0.3
113 ± 6 88.9 ± 4.9
5.7 ± 0.4 6.8 ± 0.4
84.5 84.4
515 515
248 ± 19 161 ± 12
248 ± 18 189 ± 14
216 141
214 164
Nam
TL02
Terrace pebble Beach pebble
90°36
30°34
4738
4.85 ± 0.09
26.5 ± 1.4
5.5 ± 0.5
86.6
528
56.8 ± 4.0
51.4 ± 3.7
53.0
47.9
90°36
30°34
4719
1.49 ± 0.04
8.78 ± 0.49
5.9 ± 0.3
85.7
523
17.5 ± 1.2
16.9 ± 1.2
17.2
16.6
TI02 TI04
TL03
Note: The 10Be and 26Al exposure ages are calculated using scaling factors from Stone (2000). The errors with exposure ages also include 6% from production rate, 3% from Be carrier and 3% from ICP-OES for Al. a These exposure ages are corrected for geomagnetic field intensity variations (Dunai, 2001).
sample is from Zhari Nam Co and the lake is fed both by precipitation and meltwater from glaciers. Thus, the elevation of the water level may be significantly affected by temperature changes. Deposits found at the current water level may not be newly exposed; they may have been exposed, then buried by water and re-exposed recently. The highest bedrock samples may also contain inherited cosmogenic nuclides if they were not eroded enough to remove all cosmogenic nuclides. Except for Dajia Co, we have analyzed at least two bedrock samples per lake. The lower bedrock bench is at least 3 m lower than the highest terrace, ensuring removal of all inherited cosmogenic nuclides. The similar and consistent exposure ages for the highest and lower bedrock terraces of Drolung Co suggest that samples from bedrock terraces do not contain inheritance. Kong et al. (2007a) studied bedrock terraces around Sumxi Co and found no inheritance of cosmogenic 10Be in these terraces. Thus, bedrock terraces are good targets for cosmogenic exposure dating. Exposure ages of the bedrock terraces around Tangra Yum Co and Siling Co are systematically old. For such old ages, the effects of erosion are significant. The differences between the highest and lower terraces may reflect differential erosion.
Fig. 6. Plot of 26Al/10Be vs. 10Be concentrations for shorelines around Tibet lakes. The deviation of the 26Al/10Be ratios from 6 is probably due to Al concentration errors induced during the process of HF volatilization.
4.2. Spatial and temporal pattern of high lakes From Google-Earth images, we find that the high lake shorelines stand at similar elevations for a specific lake studied in this work. We cannot identify the influence of dip-slip faults on these lakes, especially on the high lake shorelines. Therefore, we interpret these shorelines and water level variations as reflecting climatic changes, rather than responding to tectonic activities.
4.2.1. Lakes in southern Tibet Peiku Co and Drolung Co are located in southern Tibet. From Google-Earth images we found that the sample TA02 we collected for Peiku Co is from an alluvial fan (Fig. 2d). As the lower part of the fan was obscured by the high lake terraces (Fig. 2d), the high lake of Peiku Co must have occurred after the formation of the fan. Considering the deposit sample to contain cosmogenic nuclides
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Fig. 7. High lake deposits of Nam Co suggested by Zhao et al. (2002, 2003). Grasses have not been converted to charcoal and appear as layers embedding in the fine deposits. Wood and grass layers embedded in the deposits were dated with 14C, which show ages between 1.7 and 2.3 ka.
produced before deposition, the highest lake level should have occurred within 18.7 ka. Studies of 14C deposited in a peat layer 20 m below the highest terrace give an age of 13.8 ka (Deng and Liu, 1998), which is consistent with our results. Analysis of the two bedrock samples from Drolung Co gives quite consistent exposure ages, 3.7 ka and 3.9 ka. At 4720 m, the lake discharges through Rongna pass to Yarlung Tsangpo River. Our data suggest that the lake became closed at 3.8 ka. Zhang et al. (1998) obtained an age of 3 ka BP, by 14C dating of the high lake terrace, for the close of Yamdrok lake. Yamdrok lake is also located in southern Tibet, 480 km east of Drolung Co, and once discharged to the Yarlung Tsangpo River. The two sampled shorelines around Drolung Co are 123 m and 110 m higher than the current lake water level, respectively. Thus the lake level has lowered by over 120 m vertically during the past 3.8 ka. As no glaciated mountains are located in the catchment, water in Drolung Co is mainly derived from precipitation driven by the Indian monsoon. There is evidence that the intensity of precipitation driven by the Indian Monsoon remained at consistently high levels between 8 and 10 ka and gradually decreased thereafter (Fleitmann et al., 2003; Wu et al., 2006). Our results suggest that the precipitation–evaporation balance in the catchment where Drolung Co is located changed from positive to negative at a time around 3.7–3.9 ka. Based on Kutzbach’s hydrologic and energy balance model, Wang et al. (2008) calculated that the annual precipitation in Drolung Co catchment at 3.8 ka is 530 mm/a, which is 200 mm/a higher than at present. This suggests precipitation increasing over 30% along with 1 °C temperature increase within the domain of the Indian monsoon, which is much higher than the average of 6% worldwide (Wentz et al., 2007). Thus global warming has much stronger impacts on the population living in the domain of the Indian monsoon.
4.2.2. Dajia Co and Zhari Nam Co Dajia Co is the highest lake we have studied in this work. The 10 Be exposure age for the highest bedrock terrace is 48 ka. This suggests that the high water level of Dajia Co occurred during the early part of Marine Isotope Stage 3 (MIS-3: 58–32 ka). We have analyzed three pebble samples for Zhari Nam Co, two from the highest terraces (TD6 and TD7) and one from the current lake water level (TD11). From Table 1 we see that the exposure age for the water level sample is not distinguishable from the two high terrace samples. This may suggest short exposure with high inherited cosmogenic nuclide concentrations or re-cycling of terrace pebbles, that is, the pebbles were removed during lake level changes. In both cases, the exposure ages of TD6 and TD7 provide the maximum age limit of high lake occurrence. That is, the high lake level should have occurred within 15 ka.
4.2.3. Tangra Yum Co and Tangqung Co The three samples from high lake bedrock terraces around Tangra Yum Co have exposure ages of 145 ka, 271 ka and 127 ka. The differences between the three exposure ages may reflect the effects of erosion, since the exposure ages do not show relationship with elevation. Xu et al. (2006) found lacustrine deposits around Xuro Co at elevations below 4900 m. Tangra Yum Co, Xuro Co and Tangqung Co formed a united lake at high lake levels (Fig. 4). They determined, using the method of electron spin resonance (ESR), the deposition ages for lacustrine sediments at 4900 m and 4820 m as 369 ka and 230 ka, respectively. Their results reflect deposition time at high lake levels, whereas cosmogenic dates reflect the time that the high lake started to retreat. Both results of this study and of Xu et al. (2006) suggest preservation of very old lake shorelines around Tangra Yum Co.
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The exposure age of the pebble sample taken from the beach of Tangqung Co is 2.8 ka, which suggests that inheritance is not significant for deposits around this lake. This result differs from that for Zhari Nam Co and may suggest that Tangqung Co shrank continuously below 4750 m. However, the significant difference in exposure age between the deposit terrace (TI02: 22.3 ka) and the bedrock terrace (TI05: 7.4 ka) which is at the same elevation is hard to explain. One possibility is the deposits around Tangqung Co at high elevations (P4710 m) are not totally derived from surrounding mountains but are partly mixed with re-cycled material related to the united lake. Tangra Yum Co and Tangqung Co were connected at their high lake levels. The highest lake shorelines, however, are recorded at different elevations for the two lakes. Such a phenomenon occurs often on the Tibetan Plateau. Zheng et al. (2006) and Xu et al. (2006) attribute this to differential neotectonic activity. However, our analyses of the highest terraces at Tangra Yum Co and Tangqung Co indicate significantly different exposure ages: <20 ka for Tangqung Co and >271 ka for Tangra Yum Co. Obviously the highest shorelines around the two lakes do not reflect the same lake expansion event. As the highest lake level in the catchment was reached very early, >271 ka, most of the high lake terraces were erased by later erosion. Thus, the mid-Pleistocene high lake was only recorded in a few very stable landforms. 4.2.4. Nam Co and Siling Co From 1: 100,000 topographic maps (contour interval: 40 m) we determined that water in Nam Co overflows at 4750 m to Ren Co, Mujiu Co, then flow into Siling Co. Shorelines below 4750 m are clearly seen around Nam Co. We have analyzed two pebble samples for Nam Co, one from the 4740 m shoreline and the other from the current water level. The corresponding exposures ages (after geomagnetic correction) are 53 ka and 17 ka, respectively. Assuming that the concentration of 10Be for the water level pebbles are totally due to inheritance, the exposure ages for the high lake sample, after correction of inheritance, are 36 ka. As Nam Co is also fed by meltwater from glaciers, the water level may have risen and fallen repeatedly in the past. Thus the concentration of 10Be for the water level pebbles may not be completely attributed to inheritance. In such a case the inheritance will be lower and the corrected ages larger. In other word the high lake of Nam Co should have occurred at the time 53–36 ka, during MIS-3. Thus, Nam Co was drained and became closed during MIS-3. The results agree well with those reported in Lehmkuhl and Haselein (2000). Siling Co is another lake with very old shoreline exposure ages (>216 ka). Considering erosion, we believe that the high lake level occurred even earlier. Zhao et al. (2002, 2003) claimed that they have found lacustrine deposits around Nam Co at 4850 m and suggested that Nam Co and Siling Co were connected through Ren Co, forming a united lake 139 m higher than the Nam Co water level at 120 ka. The highest terrace of Siling Co is at about 4600 m. If Nam Co and Siling Co were to form a combined lake, the exposure ages of the Siling terraces should be younger than 120 ka. The exposure age of the high Siling terrace is, however, at least as old as 216 ka. In addition, we have also determined the ages of grass layers that are embedded in the sediments claimed by Zhao et al. (2002, 2003) as lacustrine deposits at 4750–4850 m using 14C dating (Fig. 7). The results show ages between 1.7 and 2.3 ka, suggesting that these deposits are not related to the high lake of Nam Co. All these results argue against the assumed combined lake within east Qiangtang at 120 ka. 4.3. Occurrence of high lakes in Tibet The exposure ages for high lake shorelines around the eight lakes are marked in Fig. 1b. Four out of the eight lakes discharged
at high lake levels. In fact the exposure ages of high lake terraces around these four lakes do not tell exactly when the highest lakes occurred. What we can infer is Nam Co and Dajia Co discharged and became closed during MIS-3 and these two lakes did not overflow and reach the levels achieved during MIS-3 after LGM. Peiku Co and Drolung Co discharged and became closed after the LGM. High lake levels at Tangra Yum Co and Siling Co occurred 220 ka ago and the high lake of Zhari Nam Co occurred after the LGM. These limited data show that there were no synchronous high lake levels in the analyzed area of Tibet.
4.4. Indian monsoon and the westerlies Changes in water level of lakes located to the south of the Yarlung Tsangpo River are strongly affected by the intensity of precipitation driven by the Indian monsoon. From our results and previous works (Zhang et al., 1998), we see that lakes located to the south of the Yarlung Tsangpo River discharged during the early Holocene when the Indian monsoon was strong. Along with the decrease in the strength of the Indian monsoon, the precipitation– evaporation balance of lakes changed from positive to negative and the lake Drolung Co became closed at 3.8 ka. Our results show that high lakes of Dajia Co and Nam Co occurred during MIS-3, when the Indian monsoon was more intense than at Holocene. As temperatures during MIS-3 are also high and input of meltwater from glaciers is significant, it is not straight forward to attribute the occurrence of high water levels of these two lakes to the enhanced Indian monsoon. Dajia Co is located in a catchment with few glaciers. Thus the occurrence of high water levels at Dajia Co is not likely due to melting of glaciers. We suggest that the Indian monsoon provides most of water input to Dajia Co and Nam Co, and the high precipitation during MIS-3 was driven by the greater intensity of the Indian monsoon, leading to the substantial expansion of these two lakes. The preservation of very old lacustrine shorelines around Tangra Yum Co and Siling Co suggests low erosion rates and perhaps low precipitation at their latitude (31–32°N). Toward the north and west, high water levels of Zhari Nam Co and Sumxi Co (Kong et al., 2007a) occurred after the LGM. The different pattern of lake development between these regions may likely suggest that the Zhari Nam Co and Sumxi Co area is outside the influence of the Indian monsoon.
5. Conclusions The patterns of lake level changes are different for lakes located to the south and to the north of the Yarlung Tsangpo River. Lakes located to the south of the Yarlung Tsangpo River discharged at high lake levels, whereas lakes located to the north of the Yarlung Tsangpo River developed during their recent history after the LGM as closed lakes. Our data suggest that the intensity of precipitation driven by the Indian monsoon may have significantly changed in southern Tibet during the middle-late Holocene. This study analyzed six lakes located to the north of the Yarlung Tsangpo River. High lake levels occurred before 271 ka at Tangra Yum Co and 216 ka at Siling Co, located at 31°N and 32°N, respectively. Toward the south and east, the high lake levels of Dajia Co and Nam Co occurred during MIS-3, whereas toward the north and west, high lake levels of Zhari Nam Co and Sumxi Co (Kong et al., 2007a) occurred after the LGM. Our data do not show synchronous high lake levels in Tibet. The different pattern of lake development in the north of the Yarlung Tsangpo River may result from regional climatic control.
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