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Timing of multiple late Quaternary glaciations in the Hunza Valley, Karakoram Mountains, northern Pakistan: De®ned by cosmogenic radionuclide dating of moraines Lewis A. Owen* Department of Earth Sciences, University of California, Riverside, California 92521-0423, USA Robert C. Finkel Marc W. Caffee² Center for Accelerator Mass Spectrometry, Lawrence Livermore National Laboratory, Livermore, California 94550, USA Lyn Gualtieri Quaternary Research Center, Box 351360, University of Washington, Seattle, Washington 98195-1360, USA ABSTRACT Mountains, southwest Asian summer monsoon, through its control on the moisture monsoon. ¯ux, is the primary agent regulating glaciation Moraines and associated landforms in on the plateau and in its bordering mountain the upper Hunza Valley, Karakoram INTRODUCTION ranges. Mountains, northern Pakistan, provide an The moraine successions in the glaciated valleys along the entire length of the Trans- excellent record of multiple glaciations. The geologic archive attests to numerous During the late Quaternary, glaciers ad- changes in climate (Bradley, 1999; Lowe and Himalayan mountain belt record changes of vanced at least eight times. By using 10Be Walker, 1997). Global climate can be read in moisture and temperature, and therefore mon- and 26Al surface-exposure dating on mo- marine sediments and polar ice cores (Brad- soon variability, for at least the last two glacial raine boulders and scoured bedrock, we de- ley, 1999; Lowe and Walker, 1997). Regional cycles. Earlier studies suggested that glacia- termined the timing of glaciation for four climate changes are evidenced in many forms. tions may have been asynchronous in different of these glacial advances: ca. 54.7±43.2 ka In mountain areas, these changes are recorded parts of the Himalaya and with global glaci- (Borit Jheel glacial stage), ca. 25.7±21.8 ka in spectacular glacial landforms, such as mo- ations (Benn and Owen, 1998). (Ghulkin I glacial stage), ca. 18.4±15.3 ka raines and valley ®lls. To investigate the climate factors at work (Ghulkin II glacial stage), and ca. 10.8±9.0 Understanding the coupling between re- during the late Quaternary in the mountainous ka (Batura glacial stage). For two of the gional climate events and global forcing is im- region bordering the Tibetan plateau, we have older advances, the Yunz and Shanoz gla- portant because it can yield information on dated several glacial successions in the Kara- cial stages, our data set a limit of .60 ka. how the climate system works. In addition, the koram Mountains, situated in Pakistan at the Although, at present, the uncertainties in effects of climate change on human popula- western end of the Trans-Himalayan mountain dating make it impossible to describe un- tions are related to regional climate events, not belt. The Karakoram range, one of the highest equivocally the climate processes control- to globally averaged forcing. Given the like- on Earth, has been uplifted to between 7000 ling glaciations in the Hunza Valley, the re- lihood of future global climate change, explo- and 8000 masl (meters above sea level) by the sults suggest that precipitation changes ration of the coupling in past climate changes collision of the Indian and Eurasian plates (cf. related to oscillations in the southwest is of great importance. Searle, 1991). The rapid uplift (»0.1 mm´yr±1) Asian monsoonal system combine with A regional climate system of particular in- and intense denudation (Searle, 1991) of this cooling that is broadly associated with terest is the southwest Asian summer mon- range produced thick Quaternary valley ®lls Heinrich events to cause glacial advances in soon. The precipitation brought by the south- and impressive successions of moraines and this region. west Asian summer monsoon waters a region glacially eroded landforms in this region (Der- of central Asia that directly affects the liveli- byshire et al., 1984; Owen, 1989; Owen and hood of 25% of the world's population Derbyshire, 1993; Shroder et al., 1989, 1993). Keywords: cosmogenic radionuclide dat- . (Benn and Owen, 1998). The existence of the Despite the excellent potential to recon- ing, glaciation, Hunza Valley, Karakoram monsoon is intimately related to the uplift of struct the nature of late Quaternary climate the Tibetan plateau and greatly in¯uences the change from the glacial geologic evidence, lit- *E-mail: [email protected]. climate in this elevated region (Ruddiman and tle research has been undertaken in this region ²Present address: Department of Physics/PRIME Lab, Purdue University, 1396 Physics Building, Kutzbach, 1990; Raymo and Ruddiman, (Derbyshire et al., 1984; Owen, 1988, 1989; West Lafayette, Indiana 47907-1396, USA. 1992). However, it is not evident whether the Shroder et al., 1989, 1993; Scott, 1992). This GSA Bulletin; May 2002; v. 114; no. 5; p. 593-604; 7 ®gures; 2 tables. For permission to copy, contact [email protected] q 2002 Geological Society of America 593 OWEN et al. paucity of research is partly due to dif®culties rectly the timing of glaciations in the region to high rates of sediment transport and depo- of access, both physical and political, and between the Ghulkin and Batura glaciers. sition that accelerate the rate of glacial ero- partly due to failure to recognize the potential sion. Collins (1998) estimated that rates of importance of Himalayan glacial systems for GEOMORPHIC CONTEXT FOR abrasion beneath the Batura Glacier are be- the study of global change. These dif®culties COSMOGENIC RADIONUCLIDE tween 3.4 and 4.2 mm´yr ±1. These high ero- have been compounded by the absence of geo- DATING sion rates help ensure that boulders and bed- chronological control. During recent years, rock surfaces are deeply eroded and thus however, cosmogenic radionuclide surface- The glaciers in the Hunza Valley have high- reduce the likelihood of signi®cant cosmogen- exposure dating has come to provide a new altitude source areas (.4500 masl) with an- ic radionuclide buildup during exposure in and productive method to date glacial land- nual precipitation totals of .2000 mm. Their outcrop. (c) The fresh surfaces exposed by forms in glaciated high-mountain and semi- snouts extend into the semiarid valley ¯oors boulder breakup also reduce the likelihood arid environments (Nishiizumi et al., 1993; (;2700 masl) where the annual precipitation that inherited cosmogenic radionuclides will Gosse et al., 1995a, 1995b; Phillips et al., totals ,200 mm´yr ±1 and summer tempera- be present. 1996, 2000; Sloan et al., 1998; Owen et al., tures are frequently .25 8C (Goudie et al., The second condition involves the possibil- 2001). We have therefore mapped and dated 1984). The glaciers are several hundred meters ity that a boulder has an exposure age greater glacial landforms and sediments in the Kara- thick and .5 km long; the Batura Glacier ex- than the age of formation of the moraine from koram Mountains to provide evidence for the ceeds 50 km in length. These glaciers are also which the boulder is sampled. Boulders can episodic advance and retreat of glaciers during among the steepest in the world (Ghulkin Gla- contain inherited cosmogenic radionuclides if the Quaternary and to allow reconstruction of cierÐ8.18; Pasu GlacierÐ6.08; Batura Gla- the boulders have been reworked from older local climate oscillations and rates of glacial cierÐ2.48). The Hunza Valley glaciers are deposits. With each successive glaciation, the erosion and sedimentation. Our reconstruction classi®ed as high-activity glaciers (Andrews, advancing glaciers may override previous gla- of the extent and timing of each glaciation has 1975), as evidenced by considerable snowfall cial deposits. Some of the older glacial debris then been used to test the synchronicity of re- in upper regions and high ablation rates in may then be reworked and deposited within gional and global climate changes. In addition, lower regions. The ¯ow rate for these glaciers the younger moraines. Younger moraines may these results allow a test of whether each gla- is relatively fast: 520±1000 m´yr ±1 for the Ba- also lie stratigraphically lower than older mo- ciation in this region can be correlated with tura Glacier (Batura Glacier Investigation raines if the younger moraines are inset. In the intensity of the southwest Asian summer Group, 1979; Shi and Zhang, 1984) and 160 this case, colluvial processes may transport monsoon. m´yr ±1 for the Pasu Glacier (Pillewizer, 1957). older material onto the younger surface. Both The steep and long valley slopes adjacent cases, therefore, contain the potential that the to the glaciers are important in delivering exposure age of a boulder is greater than that RATIONALE large quantities of snow and rock to glacier of the moraine from which the boulder was surfaces. Large blocks of rock that fall onto collected. The upper Hunza Valley was chosen for this the glacier during avalanches disintegrate eas- To account for all of these possibilities, we study because it contains a well-preserved and ily (cf. Hewitt, 1988), exposing fresh rock sur- have collected multiple samples from each very detailed record of multiple glaciations in faces. This supraglacial debris is then trans- moraine. Concordance between ages for sev- the Karakoram Mountains (Derbyshire and ported rapidly toward the glacier snout where eral boulders from the same moraine increases Owen, 1997) (Figs. 1, 2, and 3). The ®rst com- the debris is deposited to form high (»100 m) the con®dence with which the measured cos- prehensive study of the Hunza Valley was un- latero-frontal moraines (Fig. 4; Owen and mogenic radionuclide content can be inter- dertaken by Derbyshire et al. (1984), who con- Derbyshire, 1989; Owen, 1994) that continue preted to indicate the exposure age of the mo- centrated on the area between the Batura and to build up until the glacier retreats. raine helps samples that may have had Ghulkin glaciers (Fig. 1). Subsequent work on Two conditions must be met if cosmogenic complex exposure histories.