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Little Ice Age glacier fluctuations reconstructed for the southeastern Tibetan Plateau using tree rings
Quaternary International 283 (2013) 134e138
Contents lists available at SciVerse ScienceDirect
Quaternary International journal homepage: www.elsevier.com/locate/quaint
Little Ice Age glacier fluctuations reconstructed for the southeastern Tibetan Plateau using tree rings Haifeng Zhu a, Peng Xu b, *, Xuemei Shao b, Haijiang Luo c a
Key Laboratory of Tibetan Environment Changes and Land Surface Processes, Institute of Tibetan Plateau Research, Chinese Academy of Sciences, Beijing 100085, China Institute of Geographic Sciences and Natural Resources Research, Chinese Academy of Sciences, Beijing 100101, China c Environment Monitoring Center of China, Beijing 100012, China b
a r t i c l e i n f o
a b s t r a c t
Article history: Available online 16 April 2012
Long-term records are essential for evaluating recent glacier fluctuation and its linkage to climate change. Moraines of two glaciers (Gawalong glacier and Xincuo glacier) in the southeastern Tibetan Plateau were dated. Germination date of the oldest tree yields a minimum age before 1760s for the outermost lateral moraine of Gawalong glacier in southern Bomi. A tilted tree indicated that the glacier readvanced to its maximum position since the early 20th century at around 1987e1992. Xincuo glacier in eastern Gongbujiangda reached its Little Ice Age (LIA) maximum before 1876. The difference implies that detailed dating of moraines is necessary for studying LIA glacier response to climate change in the study area. Trees growing on LIA glacier deposits proved to be very useful for deriving age estimates of historic glacier fluctuations in the southeastern Tibetan Plateau. Ó 2012 Elsevier Ltd and INQUA. All rights reserved.
1. Introduction The Tibetan Plateau is experiencing rapid glacier retreat in recent decades (Yao et al., 2004). To valuate the significance of the retreat and its linkage to climate change, it is important to make an analysis by putting it in a long-term context. However, there are few long-term observations on glacier fluctuations of this area (Shi et al., 2006). Thus, it is essential to reconstruct past glacier activities by dating glacier moraines. Dendrochronological techniques have been employed to date glacier moraines by the age of trees growing on moraine deposits in several mountain regions, such as the European Alps (Holzhauser and Zumbuhl, 1999) and the Canadian Rockies (Smith et al., 1995; Luckman, 2000). In the southeastern TP, many temperate glaciers terminate at elevations below the upper timberline (Li et al., 1986). During earlier advances in the Little Ice Age (LIA) they formed lateral and terminal moraines presently covered by trees. Several studies demonstrated the potential to infer LIA glacier fluctuations in the southeastern TP using dendrochronological methods (Li et al., 1986; Bräuning, 2006; Xu et al., 2012). However, the proxy evidence of glacier fluctuations is still too scarce to understand the
* Corresponding author. E-mail address: [email protected] (P. Xu). 1040-6182/$ e see front matter Ó 2012 Elsevier Ltd and INQUA. All rights reserved. doi:10.1016/j.quaint.2012.04.011
general glacier variability in this area. The purpose of this paper is to present new tree-ring dating evidence of LIA glacier moraines in the southeastern TP. In view of the fact that most dendrochronological studies in China concentrated on climate reconstructions and some ecological issues (e.g., Zhang et al., 2003; Gou et al., 2008; Liu et al., 2009; Cai et al., 2010; Shao et al., 2010; Yang et al., 2010; Liang et al., 2011, 2012; Zhu et al., 2011), this study is an attempt to broaden the status of the art and to provide useful information on glacier response to climate changes (Su and Shi, 2002; Liu et al., 2005). 2. Study area The study area is located in Linzhi, southeastern Tibet, China, which links the eastern Himalayas and the eastern Nyainqentanglha Mountains. Due to the influence of the south Asian monsoon through the Valley of Yarlung Zangbo River, the local climate is warm and humid. According to the record of the Bomi meteorological station (29.52 N, 95.46 E, 2736 m a.s.l.,1961e2008), the total annual precipitation is 835 mm, with 74.9% falling in the summer-half year (AprileSeptember). The mean annual temperature is 8.7 C, with the lowest monthly mean temperature of 0.2 C in January and the highest of 16.6 C in July. Favored by the topographical and the water/heat conditions, modern temperate glaciers are well developed in this area (Yang et al., 1983).
H. Zhu et al. / Quaternary International 283 (2013) 134e138
3. Materials and methods 3.1. Studied glaciers and tree-ring sampling sites The two glaciers we studied were Gawalong glacier (29.78 N, 95.70 E) and Xincuo glacier (30.09 N, 94.27 E), which are located in south Bomi and east Gongbujiangda, respectively (Fig. 1). The moraine of Gawalong glacier is probably composed of an old core on which the large blocks were deposited during younger glacier advances (Bräuning, 2006). Dead trees were found on the lower part of the outer slope of the terminal moraine. Only old living trees on its crest (GWL1) and inner (GWL2) parts were sampled to date the recently formed terminal moraine. Samples were taken from the old trees at the upper lateral moraine (GWL3) (Table 1 and Fig. 2). No dead ones were found, suggesting that the deposits of younger glacier advances may fully cover the old core. Near the glacier tongue, a disk was taken from a tilted living tree whose damage date could indicate the accurate time of glacier impact (Luckman, 2000). The sampled trees were mostly Larix griffithii, with two Picea balfouriana. At the Xincuo glacier, moraines were covered by Hippophae thibetana and Abies georgei. Tree-ring samples were taken from trees growing on both the outer (XCW) and the inner sides (XCN) of the terminal moraine (Table 1 and Fig. 3). When taking increment cores from old trees growing on the lateral/terminal moraines of glaciers, their positions were recorded using GPS with an error of about 6 m. 3.2. Strategies of dating glacier moraines The age of the oldest tree provides an estimate of the minimum age for the relevant moraine. To get more accurate ages of the sampled trees, two to three increment cores per tree were taken at different sampling heights at the Gawalong glacier. The missing rings to the pith (pith offset) were estimated, since increment borers often miss the piths of trees. First, the length of the missing radius was estimated according to the curvature of the innermost
135
Table 1 Information of tree-ring samples from Gawalong and Xincuo glaciers. Glacier
ID
Latitude (N)
Longitude (E)
Sample depth (core/tree) Larix griffithii
Gawalong
GWL1 GWL2 GWL3
29.781 29.780 29.773
95.699 95.699 95.699
Xincuo
XCN XCW
30.088 30.087
94.265 94.264
55/22 20/8 36/15 Hippophae hibetana 5/5 5/5
Picea balfouriana e e 4/2 Abies georgei 1/1 2/2
rings (Duncan, 1989; Rozas, 2003). Then, the length was divided by the mean width of the innermost 5 rings to estimate the pith offset. Finally, the pith year of the increment cores was calculated by subtracting the pith offset from the year of the innermost ring. The increment cores sampled at different heights were able to provide a rough estimate of vertical growth rate on average (Xu et al., 2012). Then, the sampling height was divided by the vertical growth rate to obtain the sampling height age. The tree germination date was finally estimated by subtracting the height age from the pith year. For Xincuo glacier, tree-ring samples were taken at the ground level since it is easy to get fresh increment cores. This sampling method could produce a relatively minor source of error in the determination of tree age in glacier forefields (McCarthy et al., 1991). Hence, No age adjustments were conducted for sampling heights. To acquire a more accurate estimate of the moraine surface age, the ecesis interval (time from surface stabilization to seedling germination) needs to be added to the age of the oldest tree (McCarthy and Luckman, 1993). The lack of dated surfaces, however, prevented sampling to estimate ecesis intervals for these sampled species. Thus, estimates for ecesis interval length were obtained from other studies. In the nearby area, 5 years was used as ecesis intervals for different coniferous species since glacier moraines were very near to slopes covered by seed trees (Bräuning, 2006). For Midui glacier, estimates of a 12-year ecesis for P.
Fig. 1. Map of glaciers where the Little Ice Age moraines were dated in this and earlier studies in the southeastern Tibetan Plateau.
136
H. Zhu et al. / Quaternary International 283 (2013) 134e138
Fig. 2. Overview of Gawalong glacier forefields and tree-ring sampling sites (GWL1, GWL2 and GWL3; middle). The arrow-directed graphs are the position of trees and their age structures. The symbols in each sub-graph indicate the sampled trees of each site, respectively. The top five oldest trees of each site were enhanced with larger symbols. The ages of trees are written as: germination date (pith offset-height age-※) S. ※ represents the oldest tree, and S for Picea balfouriana. Ring-width patterns of the tilted tree are presented in the top-right panel. The maps are from http://maps.google.com/.
balfouriana and 3 years for Populus pseudoglauca were used, based on the trees growing after the outburst of a proglacial lake in AD 1988 (Xu et al., 2012). In the Canadian Rockies, ecesis values of 10e20 years are most commonly applied, although they may have a wide range from 10 to 50 years due to different dispersal manner of seeds and geographic environments, such as soils, climate conditions, and distance to the seed trees (Luckman, 2000 and references therein). The geographic conditions in this study were quite similar to the glaciers in the above studies. Hence, a 10-year ecesis interval was used for the pioneering coniferous species. Seed-providing H. thibetana are also very near to the Xincuo glacier moraine. Their fruits are food of birds and other animals, so that seeds may be easily dispersed to the moraine. In addition, Hippophae is a typical clonal plant which has a high capability to adapt to harsh conditions. Accordingly, 5 years was used as its ecesis time at Xincuo glacier.
4. Results and discussion 4.1. Dating of Gawalong glacier moraines The germination dates of the oldest trees in the three sampling sites were AD 1768 (GWL3), 1840 (GWL1) and 1918 (GWL2), respectively (Fig. 2). All of the oldest trees are L. griffithii, indicating that this tree is the pioneering species in this area. Their age was far below the maximum potential of the species, which can live more than 700 years (Bräuning, 2006). The oldest ages decreased from the outer slope towards the top and then the inner side of the moraine. These age characteristics suggest that the studied trees are individuals of the primary succession after the glacier moraines were formed. According to the maximum tree ages and their positions, the lateral moraine must be older than 1758. The glacier end retreated from its LIA
Fig. 3. Overview of Xincuo glacier, tree-ring sampling site (left) and tree positions and ages. The symbols in each sub-graph indicate the sampled trees of each site, respectively. The top five oldest trees of each site were enhanced with larger symbols. The age is written as: germination date (pith offset-height age-※) F. ※ represents the oldest tree, and F for Abies georgei. The maps are from http://maps.google.com/.
H. Zhu et al. / Quaternary International 283 (2013) 134e138
maximum extent no later than 1830, and left the inner side of the moraine before 1908. The tilted tree on the inner moraine slope was dated from 1970 to 2009 (Fig. 2). Tree-ring widths across the disk present patterns of reaction wood after 1987. The growth rate at this near glacier site decreased abruptly to a lower level after 1991. Afterwards, however, the growth rates are relatively stable and the tree remained alive to date, providing evidence that the glacier readvanced near to the tilted tree during 1987e1991. Combined with the oldest ages (1918) of the nearby trees, it can be concluded that the position of the tilted tree may be the glacier’s maximum extent since the early 20th century.
137
5. Conclusion Trees growing on LIA glacier deposits proved to be very useful for deriving age estimates of historic glacier advances in the southeastern Tibetan Plateau, where LIA glaciers advanced below the regional upper timberline. The outermost moraines of two glaciers since the LIA in the southeastern TP were dated using tree rings in this study. The dated ages are consistent with others from the nearby area. However, there are also obvious differences when glaciers reached their LIA maximum. This implies that detailed dating of moraines is needed when studying the response of glaciers to climate changes since the LIA and predicting possible future glacier changes.
4.2. Dating of Xincuo glacier moraine Acknowledgements On the terminal moraine of Xincuo glacier, the germination dates of the oldest sampled H. thibetana were 1881 and 1901 for the outer and the inner moraines, respectively (Fig. 3). They are more than 20 years older than sampled A. georgei on both sides (1909 and 1925), suggesting that H. thibetana is the first colonizer of the primary succession on the moraines after the glacier retreated. Taking into account the ecesis time, the LIA maximum terminal moraine may have been formed before 1876. The end of the glacier tongue left the inner side of the moraine before 1896. 4.3. Comparisons with other dated moraines nearby on the TP The ages of the moraines in this study are comparable with several other glacier moraines (Fig. 1) in the nearby area of the TP (Bräuning, 2006; Xu et al., 2012). The minimum age (1758) of GWL3 is quite similar to that of the LIA maximum moraine of Midui glacier (1767) (Xu et al., 2012), Xinluhai glacier (1777) and Gyalaperi glacier (1760) (Bräuning, 2006). Moraines formed during the same period as Xincuo glacier (1876) occur at Midui glacier (1875), Xinluhai glacier (1885) and Poge glacier (1857e1888) (Li et al., 1986). In addition, the readvance to the recent maximum extent of Gyalaperi glacier is consistent with that of Gawalong during 1987e1991. These agreements support earlier studies’ suggestions that there may be high spatial coherency of glacier fluctuations on the TP or even the European Alps and the Canadian Rockies (Bräuning, 2006; Xu et al., 2012). The coherency may be due to the good correlation between glacier fluctuations and temperature variations (Bräuning, 2006; Gou et al., 2006; Liang et al., 2008, 2009; Xu et al., 2012). Despite the temporal agreements of moraines from different areas, there are differences when glaciers reached their LIA maximum extent. For example, Xinluhai glacier, Midui glacier, Gyalaperi glacier and Gawalong glacier all reached their LIA maximum extent in the late 18th century. However, this is not the case for Xincuo glacier which attained its maximum before 1876, as well as the Arza glacier during 1813e1852. Lichen-dated age of a terminal moraine at Ruoguo glacier suggested a glacier readvance to the maximum LIA extent at around 1822 (Li et al., 1986). Besides, there are also differences in glacier fluctuations in the 20th century. Gawalong glacier and Gyalaperi glacier readvanced near to the LIA maximum in the late 20th century. Other glaciers, such as Xincuo, Xinluhai, and Midui glaciers showed generally steady retreat from the LIA maximum positions. Liu et al. (2005) investigated variations of 88 glaciers from 1980 to 2001 in the Gangrigabu mountain area and found that 60% of local glaciers retreated and 40% readvanced. Precipitation variation and differences in response time due to glacier size may be some of the reasons for these spatial differences (Liu et al., 2005; Shi et al., 2006). However, to better understand factors causing the differences on long timescale, more dating evidence of glacier moraines and long-term precipitation reconstructions are needed.
We would also like to thank Zhang Yong, Xu Haiqing, Wang Yafeng, Xu Guobao, Wang Wenzhi involved in the field work. This study was jointly funded by grants from the National Natural Science Foundation of China (40801033 and 41130529) and DFG, German Research Foundation (BR 1895/21-1). Appendix A. Supplementary data Supplementary data related to this article can be found online at doi:10.1016/j.quaint.2012.04.011. References Bräuning, A., 2006. Tree-ring evidence of ‘Little Ice Age’ glacier advances in southern Tibet. Holocene 16 (3), 369e380. Cai, Q.F., Liu, Y., Bao, G., Lei, Y., Sun, B., 2010. Tree-ring-based May July mean temperature history for Lüliang Mountains, China, since 1836. Chinese Science Bulletin 55. doi:10.1007/s11434-11010-13235-z. Duncan, R.P., 1989. An evaluation of errors in tree age estimates based on increment cores in Kahikatea (Dacrycarpus Dacrydioides). New Zealand Natural Sciences 16, 31e37. Gou, X., Chen, F., Yang, M., Jacoby, G., Peng, J., Zhang, Y., 2006. A comparison of treering records and glacier variations over the past 700 years, northeastern Tibetan Plateau. Annals of Glaciology 43, 86e90. Gou, X.H., Peng, J.F., Chen, F.H., Yang, M.X., Levia, D.F., Li, J.B., 2008. A dendrochronological analysis of maximum summer half-year temperature variations over the past 700 years on the norheastern Tibetan Plateau. Theoretical and Applied Climatology. doi:10.1007/s00704-00007-00336-y. Holzhauser, H., Zumbuhl, H.J., 1999. Glacier fluctuations in the Western Swiss and French Alps in the 16th century. Climatic Change 43 (1), 223e237. Li, J., Zheng, B., Yan, X., Xie, Y., 1986. Glaciers of Xizang. Science Press, Beijing, 1e328 pp. (in Chinese). Liang, E.Y., Shao, X.M., Qin, N.S., 2008. Tree-ring based summer temperature reconstruction for the source region of the Yangtze River on the Tibetan Plateau. Global and Planetary Change 61 (3e4), 313e320. Liang, E.Y., Shao, X.M., Xu, Y., 2009. Tree-ring evidence of recent abnormal warming on the southeast Tibetan Plateau. Theoretical and Applied Climatology 98 (1e2), 9e18. Liang, E., Wang, Y., Eckstein, D., Luo, T., 2011. Little change in the fir tree-line position on the southeastern Tibetan Plateau after 200 years of warming. New Phytologist. doi:10.1111/j.1469-8137.2010.03623.x. Liang, E., Lu, X.M., Ren, P., Li, X.X., Zhu, L.P., Eckstein, D., 2012. Annual increments of juniper dwarf shrubs above the tree line on the central Tibetan Plateau: a useful climatic proxy. Annals of Botany 109 (4), 721e728. Liu, S.Y., Shang Guan, D.H., Ding, Y.J., Han, H.D., Zhang, Y., Wang, J., Xie, C.W., Ding, L.F., Li, G., 2005. Glacier variations since the early 20th century in the Gangrigabu range, southeast Tibetan Plateau. Journal of Glaciology and Geocryology 27 (1), 55e63 (in Chinese). Liu, Y., An, Z.S., Linderholm, H.W., Chen, D.L., Song, H.M., Cai, Q.F., Sun, J.Y., Tian, H., 2009. Annual temperatures during the last 2485 years in the mid-eastern Tibetan Plateau inferred from tree rings. Science in China Series D-Earth Sciences 52 (3), 348e359. Luckman, B.H., 2000. The Little Ice Age in the Canadian Rockies. Geomorphology 32 (3e4), 357e384. McCarthy, D.P., Luckman, B.H., 1993. Estimating ecesis for tree-ring dating of moraines e a comparative-study from the Canadian cordillera. Arctic and Alpine Research 25 (1), 63e68. McCarthy, D.P., Luckman, B.H., Kelly, P.E., 1991. Sampling height-age error correction for spruce seedlings in glacial forefields, Canadian cordillera. Arctic and Alpine Research 23 (4), 451e455.
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Rozas, V., 2003. Tree age estimates in Fagus sylvatica and Quercus robur: testing previous and improved methods. Plant Ecology 167 (2), 193e212. Shao, X., Xu, Y., Yin, Z.Y., Liang, E., Zhu, H., Wang, S., 2010. Climatic implications of a 3585-year tree-ring width chronology from the northeastern Qinghai-Tibetan Plateau. Quaternary Science Reviews 29 (17e18), 2111e2122. Shi, Y.F., Liu, S.Y., Shang Guan, D.H., Li, D.L., Ye, B.S., Shen, Y.P., 2006. Two peculiar phenomena of climatic and glacial variations in the Tibetan Plateau. Advances in Climate Change Research 2 (4), 154e160 (in Chinese). Smith, C.J., Mccarthy, D.P., Colenutt, M.E., 1995. Little Ice Age glacial activity in Peter Lougheed and Elk Lakes Provincial Parks, Canadian Rocky Mountains. Canadian Journal of Earth Sciences 32 (5), 579e589. Su, Z., Shi, Y.F., 2002. Response of monsoonal temperate glaciers to global warming since the Little Ice Age. Quaternary International 97e8, 123e131. Xu, P., Zhu, H., et al., 2012. Tree ring-dated fluctuation history of Midui glacier since the little ice age in the southeastern Tibetan Plateau. Science China-Earth Sciences 55 (4), 521e529.
Yang, Y.C., Li, B.Y., Yin, Z.S., Zhang, Q.S., Wang, F.B., Jing, K., Chen, Z.M., 1983. Geomorphology of Tibet. Science Press, Beijing (in Chinese). Yang, B., Kang, X.C., Brauning, A., Liu, J., Qin, C., Liu, J.J., 2010. A 622-year regional temperature history of southeast Tibet derived from tree rings. Holocene 20 (2), 181e190. Yao, T.D., Wang, Y.Q., Liu, S.Y., Pu, J.C., Shen, Y.P., Lu, A.X., 2004. Recent glacial retreat in High Asia in China and its impact on water resource in Northwest China. Science in China Series D-Earth Sciences 47 (12), 1065e1075. Zhang, Q.B., Cheng, G.D., Yao, T.D., Kang, X.C., Huang, J.G., 2003. A 2,326-year treering record of climate variability on the northeastern Qinghai-Tibetan Plateau. Geophysical Research Letters 30 (14), 1739e1741. Zhu, H.F., Shao, X.M., Yin, Z.Y., Xu, P., Xu, Y., Tian, H., 2011. August temperature variability in the southeastern Tibetan Plateau since AD 1385 inferred from tree rings. Palaeogeography, Palaeoclimatology, Palaeoecology 305 (1e4), 84e92.
Contents lists available at SciVerse ScienceDirect
Quaternary International journal homepage: www.elsevier.com/locate/quaint
Little Ice Age glacier fluctuations reconstructed for the southeastern Tibetan Plateau using tree rings Haifeng Zhu a, Peng Xu b, *, Xuemei Shao b, Haijiang Luo c a
Key Laboratory of Tibetan Environment Changes and Land Surface Processes, Institute of Tibetan Plateau Research, Chinese Academy of Sciences, Beijing 100085, China Institute of Geographic Sciences and Natural Resources Research, Chinese Academy of Sciences, Beijing 100101, China c Environment Monitoring Center of China, Beijing 100012, China b
a r t i c l e i n f o
a b s t r a c t
Article history: Available online 16 April 2012
Long-term records are essential for evaluating recent glacier fluctuation and its linkage to climate change. Moraines of two glaciers (Gawalong glacier and Xincuo glacier) in the southeastern Tibetan Plateau were dated. Germination date of the oldest tree yields a minimum age before 1760s for the outermost lateral moraine of Gawalong glacier in southern Bomi. A tilted tree indicated that the glacier readvanced to its maximum position since the early 20th century at around 1987e1992. Xincuo glacier in eastern Gongbujiangda reached its Little Ice Age (LIA) maximum before 1876. The difference implies that detailed dating of moraines is necessary for studying LIA glacier response to climate change in the study area. Trees growing on LIA glacier deposits proved to be very useful for deriving age estimates of historic glacier fluctuations in the southeastern Tibetan Plateau. Ó 2012 Elsevier Ltd and INQUA. All rights reserved.
1. Introduction The Tibetan Plateau is experiencing rapid glacier retreat in recent decades (Yao et al., 2004). To valuate the significance of the retreat and its linkage to climate change, it is important to make an analysis by putting it in a long-term context. However, there are few long-term observations on glacier fluctuations of this area (Shi et al., 2006). Thus, it is essential to reconstruct past glacier activities by dating glacier moraines. Dendrochronological techniques have been employed to date glacier moraines by the age of trees growing on moraine deposits in several mountain regions, such as the European Alps (Holzhauser and Zumbuhl, 1999) and the Canadian Rockies (Smith et al., 1995; Luckman, 2000). In the southeastern TP, many temperate glaciers terminate at elevations below the upper timberline (Li et al., 1986). During earlier advances in the Little Ice Age (LIA) they formed lateral and terminal moraines presently covered by trees. Several studies demonstrated the potential to infer LIA glacier fluctuations in the southeastern TP using dendrochronological methods (Li et al., 1986; Bräuning, 2006; Xu et al., 2012). However, the proxy evidence of glacier fluctuations is still too scarce to understand the
* Corresponding author. E-mail address: [email protected] (P. Xu). 1040-6182/$ e see front matter Ó 2012 Elsevier Ltd and INQUA. All rights reserved. doi:10.1016/j.quaint.2012.04.011
general glacier variability in this area. The purpose of this paper is to present new tree-ring dating evidence of LIA glacier moraines in the southeastern TP. In view of the fact that most dendrochronological studies in China concentrated on climate reconstructions and some ecological issues (e.g., Zhang et al., 2003; Gou et al., 2008; Liu et al., 2009; Cai et al., 2010; Shao et al., 2010; Yang et al., 2010; Liang et al., 2011, 2012; Zhu et al., 2011), this study is an attempt to broaden the status of the art and to provide useful information on glacier response to climate changes (Su and Shi, 2002; Liu et al., 2005). 2. Study area The study area is located in Linzhi, southeastern Tibet, China, which links the eastern Himalayas and the eastern Nyainqentanglha Mountains. Due to the influence of the south Asian monsoon through the Valley of Yarlung Zangbo River, the local climate is warm and humid. According to the record of the Bomi meteorological station (29.52 N, 95.46 E, 2736 m a.s.l.,1961e2008), the total annual precipitation is 835 mm, with 74.9% falling in the summer-half year (AprileSeptember). The mean annual temperature is 8.7 C, with the lowest monthly mean temperature of 0.2 C in January and the highest of 16.6 C in July. Favored by the topographical and the water/heat conditions, modern temperate glaciers are well developed in this area (Yang et al., 1983).
H. Zhu et al. / Quaternary International 283 (2013) 134e138
3. Materials and methods 3.1. Studied glaciers and tree-ring sampling sites The two glaciers we studied were Gawalong glacier (29.78 N, 95.70 E) and Xincuo glacier (30.09 N, 94.27 E), which are located in south Bomi and east Gongbujiangda, respectively (Fig. 1). The moraine of Gawalong glacier is probably composed of an old core on which the large blocks were deposited during younger glacier advances (Bräuning, 2006). Dead trees were found on the lower part of the outer slope of the terminal moraine. Only old living trees on its crest (GWL1) and inner (GWL2) parts were sampled to date the recently formed terminal moraine. Samples were taken from the old trees at the upper lateral moraine (GWL3) (Table 1 and Fig. 2). No dead ones were found, suggesting that the deposits of younger glacier advances may fully cover the old core. Near the glacier tongue, a disk was taken from a tilted living tree whose damage date could indicate the accurate time of glacier impact (Luckman, 2000). The sampled trees were mostly Larix griffithii, with two Picea balfouriana. At the Xincuo glacier, moraines were covered by Hippophae thibetana and Abies georgei. Tree-ring samples were taken from trees growing on both the outer (XCW) and the inner sides (XCN) of the terminal moraine (Table 1 and Fig. 3). When taking increment cores from old trees growing on the lateral/terminal moraines of glaciers, their positions were recorded using GPS with an error of about 6 m. 3.2. Strategies of dating glacier moraines The age of the oldest tree provides an estimate of the minimum age for the relevant moraine. To get more accurate ages of the sampled trees, two to three increment cores per tree were taken at different sampling heights at the Gawalong glacier. The missing rings to the pith (pith offset) were estimated, since increment borers often miss the piths of trees. First, the length of the missing radius was estimated according to the curvature of the innermost
135
Table 1 Information of tree-ring samples from Gawalong and Xincuo glaciers. Glacier
ID
Latitude (N)
Longitude (E)
Sample depth (core/tree) Larix griffithii
Gawalong
GWL1 GWL2 GWL3
29.781 29.780 29.773
95.699 95.699 95.699
Xincuo
XCN XCW
30.088 30.087
94.265 94.264
55/22 20/8 36/15 Hippophae hibetana 5/5 5/5
Picea balfouriana e e 4/2 Abies georgei 1/1 2/2
rings (Duncan, 1989; Rozas, 2003). Then, the length was divided by the mean width of the innermost 5 rings to estimate the pith offset. Finally, the pith year of the increment cores was calculated by subtracting the pith offset from the year of the innermost ring. The increment cores sampled at different heights were able to provide a rough estimate of vertical growth rate on average (Xu et al., 2012). Then, the sampling height was divided by the vertical growth rate to obtain the sampling height age. The tree germination date was finally estimated by subtracting the height age from the pith year. For Xincuo glacier, tree-ring samples were taken at the ground level since it is easy to get fresh increment cores. This sampling method could produce a relatively minor source of error in the determination of tree age in glacier forefields (McCarthy et al., 1991). Hence, No age adjustments were conducted for sampling heights. To acquire a more accurate estimate of the moraine surface age, the ecesis interval (time from surface stabilization to seedling germination) needs to be added to the age of the oldest tree (McCarthy and Luckman, 1993). The lack of dated surfaces, however, prevented sampling to estimate ecesis intervals for these sampled species. Thus, estimates for ecesis interval length were obtained from other studies. In the nearby area, 5 years was used as ecesis intervals for different coniferous species since glacier moraines were very near to slopes covered by seed trees (Bräuning, 2006). For Midui glacier, estimates of a 12-year ecesis for P.
Fig. 1. Map of glaciers where the Little Ice Age moraines were dated in this and earlier studies in the southeastern Tibetan Plateau.
136
H. Zhu et al. / Quaternary International 283 (2013) 134e138
Fig. 2. Overview of Gawalong glacier forefields and tree-ring sampling sites (GWL1, GWL2 and GWL3; middle). The arrow-directed graphs are the position of trees and their age structures. The symbols in each sub-graph indicate the sampled trees of each site, respectively. The top five oldest trees of each site were enhanced with larger symbols. The ages of trees are written as: germination date (pith offset-height age-※) S. ※ represents the oldest tree, and S for Picea balfouriana. Ring-width patterns of the tilted tree are presented in the top-right panel. The maps are from http://maps.google.com/.
balfouriana and 3 years for Populus pseudoglauca were used, based on the trees growing after the outburst of a proglacial lake in AD 1988 (Xu et al., 2012). In the Canadian Rockies, ecesis values of 10e20 years are most commonly applied, although they may have a wide range from 10 to 50 years due to different dispersal manner of seeds and geographic environments, such as soils, climate conditions, and distance to the seed trees (Luckman, 2000 and references therein). The geographic conditions in this study were quite similar to the glaciers in the above studies. Hence, a 10-year ecesis interval was used for the pioneering coniferous species. Seed-providing H. thibetana are also very near to the Xincuo glacier moraine. Their fruits are food of birds and other animals, so that seeds may be easily dispersed to the moraine. In addition, Hippophae is a typical clonal plant which has a high capability to adapt to harsh conditions. Accordingly, 5 years was used as its ecesis time at Xincuo glacier.
4. Results and discussion 4.1. Dating of Gawalong glacier moraines The germination dates of the oldest trees in the three sampling sites were AD 1768 (GWL3), 1840 (GWL1) and 1918 (GWL2), respectively (Fig. 2). All of the oldest trees are L. griffithii, indicating that this tree is the pioneering species in this area. Their age was far below the maximum potential of the species, which can live more than 700 years (Bräuning, 2006). The oldest ages decreased from the outer slope towards the top and then the inner side of the moraine. These age characteristics suggest that the studied trees are individuals of the primary succession after the glacier moraines were formed. According to the maximum tree ages and their positions, the lateral moraine must be older than 1758. The glacier end retreated from its LIA
Fig. 3. Overview of Xincuo glacier, tree-ring sampling site (left) and tree positions and ages. The symbols in each sub-graph indicate the sampled trees of each site, respectively. The top five oldest trees of each site were enhanced with larger symbols. The age is written as: germination date (pith offset-height age-※) F. ※ represents the oldest tree, and F for Abies georgei. The maps are from http://maps.google.com/.
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maximum extent no later than 1830, and left the inner side of the moraine before 1908. The tilted tree on the inner moraine slope was dated from 1970 to 2009 (Fig. 2). Tree-ring widths across the disk present patterns of reaction wood after 1987. The growth rate at this near glacier site decreased abruptly to a lower level after 1991. Afterwards, however, the growth rates are relatively stable and the tree remained alive to date, providing evidence that the glacier readvanced near to the tilted tree during 1987e1991. Combined with the oldest ages (1918) of the nearby trees, it can be concluded that the position of the tilted tree may be the glacier’s maximum extent since the early 20th century.
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5. Conclusion Trees growing on LIA glacier deposits proved to be very useful for deriving age estimates of historic glacier advances in the southeastern Tibetan Plateau, where LIA glaciers advanced below the regional upper timberline. The outermost moraines of two glaciers since the LIA in the southeastern TP were dated using tree rings in this study. The dated ages are consistent with others from the nearby area. However, there are also obvious differences when glaciers reached their LIA maximum. This implies that detailed dating of moraines is needed when studying the response of glaciers to climate changes since the LIA and predicting possible future glacier changes.
4.2. Dating of Xincuo glacier moraine Acknowledgements On the terminal moraine of Xincuo glacier, the germination dates of the oldest sampled H. thibetana were 1881 and 1901 for the outer and the inner moraines, respectively (Fig. 3). They are more than 20 years older than sampled A. georgei on both sides (1909 and 1925), suggesting that H. thibetana is the first colonizer of the primary succession on the moraines after the glacier retreated. Taking into account the ecesis time, the LIA maximum terminal moraine may have been formed before 1876. The end of the glacier tongue left the inner side of the moraine before 1896. 4.3. Comparisons with other dated moraines nearby on the TP The ages of the moraines in this study are comparable with several other glacier moraines (Fig. 1) in the nearby area of the TP (Bräuning, 2006; Xu et al., 2012). The minimum age (1758) of GWL3 is quite similar to that of the LIA maximum moraine of Midui glacier (1767) (Xu et al., 2012), Xinluhai glacier (1777) and Gyalaperi glacier (1760) (Bräuning, 2006). Moraines formed during the same period as Xincuo glacier (1876) occur at Midui glacier (1875), Xinluhai glacier (1885) and Poge glacier (1857e1888) (Li et al., 1986). In addition, the readvance to the recent maximum extent of Gyalaperi glacier is consistent with that of Gawalong during 1987e1991. These agreements support earlier studies’ suggestions that there may be high spatial coherency of glacier fluctuations on the TP or even the European Alps and the Canadian Rockies (Bräuning, 2006; Xu et al., 2012). The coherency may be due to the good correlation between glacier fluctuations and temperature variations (Bräuning, 2006; Gou et al., 2006; Liang et al., 2008, 2009; Xu et al., 2012). Despite the temporal agreements of moraines from different areas, there are differences when glaciers reached their LIA maximum extent. For example, Xinluhai glacier, Midui glacier, Gyalaperi glacier and Gawalong glacier all reached their LIA maximum extent in the late 18th century. However, this is not the case for Xincuo glacier which attained its maximum before 1876, as well as the Arza glacier during 1813e1852. Lichen-dated age of a terminal moraine at Ruoguo glacier suggested a glacier readvance to the maximum LIA extent at around 1822 (Li et al., 1986). Besides, there are also differences in glacier fluctuations in the 20th century. Gawalong glacier and Gyalaperi glacier readvanced near to the LIA maximum in the late 20th century. Other glaciers, such as Xincuo, Xinluhai, and Midui glaciers showed generally steady retreat from the LIA maximum positions. Liu et al. (2005) investigated variations of 88 glaciers from 1980 to 2001 in the Gangrigabu mountain area and found that 60% of local glaciers retreated and 40% readvanced. Precipitation variation and differences in response time due to glacier size may be some of the reasons for these spatial differences (Liu et al., 2005; Shi et al., 2006). However, to better understand factors causing the differences on long timescale, more dating evidence of glacier moraines and long-term precipitation reconstructions are needed.
We would also like to thank Zhang Yong, Xu Haiqing, Wang Yafeng, Xu Guobao, Wang Wenzhi involved in the field work. This study was jointly funded by grants from the National Natural Science Foundation of China (40801033 and 41130529) and DFG, German Research Foundation (BR 1895/21-1). Appendix A. Supplementary data Supplementary data related to this article can be found online at doi:10.1016/j.quaint.2012.04.011. References Bräuning, A., 2006. Tree-ring evidence of ‘Little Ice Age’ glacier advances in southern Tibet. Holocene 16 (3), 369e380. Cai, Q.F., Liu, Y., Bao, G., Lei, Y., Sun, B., 2010. Tree-ring-based May July mean temperature history for Lüliang Mountains, China, since 1836. Chinese Science Bulletin 55. doi:10.1007/s11434-11010-13235-z. Duncan, R.P., 1989. An evaluation of errors in tree age estimates based on increment cores in Kahikatea (Dacrycarpus Dacrydioides). New Zealand Natural Sciences 16, 31e37. Gou, X., Chen, F., Yang, M., Jacoby, G., Peng, J., Zhang, Y., 2006. A comparison of treering records and glacier variations over the past 700 years, northeastern Tibetan Plateau. Annals of Glaciology 43, 86e90. Gou, X.H., Peng, J.F., Chen, F.H., Yang, M.X., Levia, D.F., Li, J.B., 2008. A dendrochronological analysis of maximum summer half-year temperature variations over the past 700 years on the norheastern Tibetan Plateau. Theoretical and Applied Climatology. doi:10.1007/s00704-00007-00336-y. Holzhauser, H., Zumbuhl, H.J., 1999. Glacier fluctuations in the Western Swiss and French Alps in the 16th century. Climatic Change 43 (1), 223e237. Li, J., Zheng, B., Yan, X., Xie, Y., 1986. Glaciers of Xizang. Science Press, Beijing, 1e328 pp. (in Chinese). Liang, E.Y., Shao, X.M., Qin, N.S., 2008. Tree-ring based summer temperature reconstruction for the source region of the Yangtze River on the Tibetan Plateau. Global and Planetary Change 61 (3e4), 313e320. Liang, E.Y., Shao, X.M., Xu, Y., 2009. Tree-ring evidence of recent abnormal warming on the southeast Tibetan Plateau. Theoretical and Applied Climatology 98 (1e2), 9e18. Liang, E., Wang, Y., Eckstein, D., Luo, T., 2011. Little change in the fir tree-line position on the southeastern Tibetan Plateau after 200 years of warming. New Phytologist. doi:10.1111/j.1469-8137.2010.03623.x. Liang, E., Lu, X.M., Ren, P., Li, X.X., Zhu, L.P., Eckstein, D., 2012. Annual increments of juniper dwarf shrubs above the tree line on the central Tibetan Plateau: a useful climatic proxy. Annals of Botany 109 (4), 721e728. Liu, S.Y., Shang Guan, D.H., Ding, Y.J., Han, H.D., Zhang, Y., Wang, J., Xie, C.W., Ding, L.F., Li, G., 2005. Glacier variations since the early 20th century in the Gangrigabu range, southeast Tibetan Plateau. Journal of Glaciology and Geocryology 27 (1), 55e63 (in Chinese). Liu, Y., An, Z.S., Linderholm, H.W., Chen, D.L., Song, H.M., Cai, Q.F., Sun, J.Y., Tian, H., 2009. Annual temperatures during the last 2485 years in the mid-eastern Tibetan Plateau inferred from tree rings. Science in China Series D-Earth Sciences 52 (3), 348e359. Luckman, B.H., 2000. The Little Ice Age in the Canadian Rockies. Geomorphology 32 (3e4), 357e384. McCarthy, D.P., Luckman, B.H., 1993. Estimating ecesis for tree-ring dating of moraines e a comparative-study from the Canadian cordillera. Arctic and Alpine Research 25 (1), 63e68. McCarthy, D.P., Luckman, B.H., Kelly, P.E., 1991. Sampling height-age error correction for spruce seedlings in glacial forefields, Canadian cordillera. Arctic and Alpine Research 23 (4), 451e455.
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Rozas, V., 2003. Tree age estimates in Fagus sylvatica and Quercus robur: testing previous and improved methods. Plant Ecology 167 (2), 193e212. Shao, X., Xu, Y., Yin, Z.Y., Liang, E., Zhu, H., Wang, S., 2010. Climatic implications of a 3585-year tree-ring width chronology from the northeastern Qinghai-Tibetan Plateau. Quaternary Science Reviews 29 (17e18), 2111e2122. Shi, Y.F., Liu, S.Y., Shang Guan, D.H., Li, D.L., Ye, B.S., Shen, Y.P., 2006. Two peculiar phenomena of climatic and glacial variations in the Tibetan Plateau. Advances in Climate Change Research 2 (4), 154e160 (in Chinese). Smith, C.J., Mccarthy, D.P., Colenutt, M.E., 1995. Little Ice Age glacial activity in Peter Lougheed and Elk Lakes Provincial Parks, Canadian Rocky Mountains. Canadian Journal of Earth Sciences 32 (5), 579e589. Su, Z., Shi, Y.F., 2002. Response of monsoonal temperate glaciers to global warming since the Little Ice Age. Quaternary International 97e8, 123e131. Xu, P., Zhu, H., et al., 2012. Tree ring-dated fluctuation history of Midui glacier since the little ice age in the southeastern Tibetan Plateau. Science China-Earth Sciences 55 (4), 521e529.
Yang, Y.C., Li, B.Y., Yin, Z.S., Zhang, Q.S., Wang, F.B., Jing, K., Chen, Z.M., 1983. Geomorphology of Tibet. Science Press, Beijing (in Chinese). Yang, B., Kang, X.C., Brauning, A., Liu, J., Qin, C., Liu, J.J., 2010. A 622-year regional temperature history of southeast Tibet derived from tree rings. Holocene 20 (2), 181e190. Yao, T.D., Wang, Y.Q., Liu, S.Y., Pu, J.C., Shen, Y.P., Lu, A.X., 2004. Recent glacial retreat in High Asia in China and its impact on water resource in Northwest China. Science in China Series D-Earth Sciences 47 (12), 1065e1075. Zhang, Q.B., Cheng, G.D., Yao, T.D., Kang, X.C., Huang, J.G., 2003. A 2,326-year treering record of climate variability on the northeastern Qinghai-Tibetan Plateau. Geophysical Research Letters 30 (14), 1739e1741. Zhu, H.F., Shao, X.M., Yin, Z.Y., Xu, P., Xu, Y., Tian, H., 2011. August temperature variability in the southeastern Tibetan Plateau since AD 1385 inferred from tree rings. Palaeogeography, Palaeoclimatology, Palaeoecology 305 (1e4), 84e92.