Early Holocene Climate Recorded in Geomorphological Features in Western Tibet

Palaeogeography, Palaeoclimatology, Palaeoecology 199 (2003) 141^151 www.elsevier.com/locate/palaeo

Early Holocene climate recorded in geomorphological features in Western Tibet

E.T. Brown a;Ã, R. Bendick b, D.L. Bourle's c;1, V. Gaur d, P. Molnar b, G.M. Raisbeck c, F. Yiou c

a Large Lakes Observatory, University of Minnesota, Duluth, MN 55812, USA b Department of Geological Sciences, University of Colorado, Boulder, CO 80309, USA c Centre de Spectrome¤trie Nucleaire et de Spectrome¤trie de Masse, CNRS-IN2P3, F91405 Campus Orsay, France d Indian Institute of Astrophysics, II Block, Koramangala, Bangalore 560 034, India

Received 31 May 2002; accepted 4 June 2003

Abstract

Cosmic ray exposure ages for formation of perched alluvial terraces and for abandonment of an alluvial/debris- flow fan on opposite sides of the Tangtse Valley (the outflow at the northwest end of Lake Panggong, which is in the Karakorum Range of Western Tibet) provide evidence of a humid period at V11.5 to V7 ka. This is consistent with other regional records and supports a controversial chronology for the sedimentary record from Lake Panggong. Fan abandonment appears to have occurred at V11.5 ka as the climate presumably became more humid in response to initiation of enhanced monsoonal circulation, consistent with previously reported onset of humid conditions in a sedimentary record from the easternmost basin of the lake. In contrast, the terraces did not form until about 7 ka with downcutting of the transverse valley by overflow from Lake Panggong. This lag can be explained in light of the bathymetry of Lake Panggong; the modern lake consists of five basins separated by shallow sills, and outflow through the Tangtse Valley could not occur until the water level was substantially above its present level. The easternmost basin receives the inflow of the major rivers feeding the lake, making its chemistry highly sensitive to changes in precipitation. However, sustained wet conditions are required to fill the basins to the west to the sill depth necessary for overflow through the Tangtse Valley and resultant downcutting and terrace formation. ß 2003 Elsevier B.V. All rights reserved.

Keywords: cosmogenic nuclides; dating; paleoclimate; Karakorum; debris £ow; £uvial features

1. Introduction tral Asia re£ect the interaction of global, regional, and local processes. The relative positions of the Records of changing climate conditions in Cen- Siberian High and the monsoonal circulation cells determine regional moisture transport patterns. The Tibetan Plateau exerts a strong in£uence on 1 Present address: CEREGE, Europo“le Me¤diterrane¤en de the interaction of these weather systems; summer l’Arbois, F13545 Aix-en-Provence Cedex 04, France. * Corresponding author. Tel: +1-218-726-8891; heating of the plateau enhances the continental Fax: +1-218-726-6979. low-pressure system that drives the Indian E-mail address: [email protected] (E.T. Brown). Summer Monsoon. Variability in these systems

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Fig. 1. Map of the Lake Panggong region in Ladakh, India and China, modi¢ed after Fontes et al. (1996). The Tangtse Valley site, presented in more detail in Fig. 2, is outlined and shaded. The approximate location of the eastern Lake Panggong coring site of Fontes et al. (1996), Gasse et al. (1996), Van Campo et al. (1996) and Hui et al. (1996) is also marked. is superimposed on global climate shifts on Mi- trolled by a vertical component of slip on the lankovitch timescales. Local factors, such as Karakorum strike-slip fault (Searle, 1996; Searle changes in meltwater discharge from mountain et al., 1998). The lake presently consists of three glaciers or tectonic and geomorphological events basins up to V40 m deep that are separated by also can a¡ect climate conditions at a given site. shallow (1^2 m at present) sills. These separations, Unraveling records of past climate change in Asia and the diversity of terrain drained by rivers £ow- thus have the potential to provide information on ing into the systems, mean that the present lake a broad range of processes. Quantitative study of has signi¢cant heterogeneity in chemical composi- the formation of geomorphological features is be- tion, with the westernmost basin approximately ginning to constrain the timing of climatic events 10 times more saline than the easternmost (Ou, (e.g. Phillips et al., 2000; Richards et al., 2000; 1981). Taylor and Mitchell, 2000). Here we study the The lake has changed dramatically in the past; timing of formation of a group of alluvial and a series of ancient shorelines ring the basin debris-£ow deposits adjacent to Lake Panggong (Fontes et al., 1996). During high stands the (or Banggong Co) in the Karakorum Range of lake £owed out through the Tangtse Valley to Ladakh (northeastern India) with the goal of the Shyok River and ultimately to the Indus. In understanding the linkages between the processes the Tangtse Valley three alluvial terraces are that formed them and regional climate forcing. perched along the northeast side of the valley at Lake Panggong is a closed basin lake situated elevations up to 40 m above the present valley in a V200-km-long submerged valley (Fig. 1). Its £oor. In addition, there are a number of aban- out£ow channel, the Tangtse Valley, may be con- doned alluvial/debris-£ow fans throughout valley.

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Fig. 2. CORONA satellite photographs of the Tangtse Valley study site (available from the US Geological Survey, http:// edcwww.cr.usgs.gov/glis/hyper/guide/disp). This valley was the out£ow of Lake Panggong during high stands. The £uvial terraces and abandoned debris-£ow fan on the north and south sides of the valley, respectively, are highlighted. The trace of the Karako- rum Fault, with clear evidence of right lateral movement, appears to cross from the southwest to the northeast side of the valley a few km downstream of the terraces and fans we studied.

A record of changing climate conditions is pre- tectonics (e.g. Lasserre et al., 1999); such applica- served in Lake Panggong sediments as well. A tions require a robust chronology. Development series of papers report a continuous paleoclimate of the radiocarbon chronology for this core re- record from sediments of the easternmost basin. quired signi¢cant normalizations for reservoir ef- Several lines of evidence ^ N18O in authigenic in- fects due to the presence of carbonate rocks, and organic carbonates (Fontes et al., 1996), terres- additional sources of dead carbon, in the drainage trial plant pollen (Van Campo et al., 1996), and basin. These ages thus may be subject to system- changing lacustrine biota (Hui et al., 1996) ^ in- atic uncertainties associated with the large (6670 dicate that generally humid conditions prevailed year) reservoir e¡ect determined for this lake in the Panggong basin from 9.5 to 6.2 14C ka. (Fontes et al., 1996). Because of the magnitude This humid period was interrupted by an episode of these corrections, the chronology of Fontes et of drier conditions between 8.6 and 7.7 14Cka al. (1996) remains somewhat controversial (Lehm- and was then followed by a general trend of in- kuhl and Haselein, 2000). creasing aridity after V6.2 14C ka, with a particularly dry period occurring at 3.9^3.2 14C ka, that ultimately led to closed basin conditions after 2. Materials and methods V1.2 14C ka. These conclusions have been utilized in studies 2.1. Fieldwork of Central Asian paleoclimate (e.g. Denniston et al., 2000; Phadtare, 2000; Phillips et al., 2000; To obtain additional information on the history Ricketts et al., 1999; Yan et al., 1999) and neo- of Holocene climate in this region, we collected

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Fig. 3. Photograph of the abandoned fans, showing locations of debris £ows o¡set by 2 and 40 m, as well as a scarp along the trace of the Karakorum Fault. This photo was taken from the highest of the perched terraces, looking southwest across the valley. samples for cosmic ray exposure dating from geo- There are numerous debris-£ow fans, both ac- morphological features likely to have been formed tive and abandoned throughout the Tangtse Val- at the time of the humid episode recorded in Pan- ley. In an earlier study (Brown et al., 2002), we ggong sediments. These dates may provide inde- examined the history of abandonment of alluvial/ pendent evidence for the timing of this humid debris-£ow fans on the southwest side of the period, and provide information on the interplay Tangtse Valley that had been deposited over the of Holocene climate history with landform evolu- Karakorum Fault, and subsequently displaced by tion. movement on the fault. Nearly directly across the In the Tangtse Valley, the out£ow channel at valley from the perched terraces, one of these the northwest end of Lake Panggong, we found a abandoned fans contains material from two de- series of three perched terraces at elevations ap- bris £ows that were deposited nearly perpendicu- proximately 40 m, 14 m and 7 m above the pres- lar to the strike of the fault and subsequently ently active stream (Figs. 2 and 3). The uppermost o¡set by right lateral movement on the fault by of these terraces has a spatial extent of approxi- V2 m and V45 m (Brown et al., 2002). The mately 500U100 m while the lower two are con- debris £ow with the greater o¡set is larger, with siderably smaller: 200U50 m and 100U15 m, re- a distance of V10 m between the boulders form- spectively. Determining the timing of the river ing its levees. It appears to have been the last incision that formed these terraces will provide debris £ow to deposit boulders on the surface, insights into interplay between tectonic and cli- before the stream debouching onto the fan from matic events that a¡ect landform evolution. the mountains to the southwest incised the fan.

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Fig. 4. Pro¢les across the fan and the terraces. Small insets show details of the three £uvial terraces and of the fault with its 5 m of reverse faulting. These correspond to the heavy lines in the larger plot.

The source of the debris £ow lies at the mouth of two debris £ows and incorporated them in the the canyon above the fan. In contrast, the ridge surfaces of the three alluvial terraces. We col- that forms the western boundary of the aban- lected material from boulders embedded in the doned fan supplied material to the smaller debris surfaces and with well-developed desert varnish, £ow (V5 m between levees). This debris £ow was avoiding boulders that showed obvious evidence deposited recently, 1000s of years after abandon- of recent disturbance (rolling) or loss of material ment of the fan (Brown et al., 2002). through spalling. To evaluate exposure prior to Using a topographic map of the area based on deposition in the alluvial terraces, we collected kinematic GPS data (Brown et al., 2002, in press) ‘geological blanks’ (Brown et al., 1998) from the we created a pro¢le across the valley, including active stream in the valley immediately to the west the debris fan and the perched terraces (Fig. 4). of Tangtse Valley debris-£ow fan (Figs. 2 and 3). The data were collected at a 10-s sampling inter- In addition, material from the debris £ow o¡set val while the antenna was moved across the fan, by V2 m might be considered to be a geological providing sampling at a spacing of about 10 m. blank for the debris £ow o¡set by V45 m. GPS data were then processed relative to a ¢xed base station at the £oor of the valley, where the 2.2. Analytical methods position was measured continuously for 3 days. Our mapping quanti¢ed the right lateral o¡set Following methods described elsewhere (Brown of the debris £ows (2 m and 45 þ 10 m) and of et al., 1991, 2002), quartz was puri¢ed from sam- the morphology of the fan in general (40 þ 5) ples of granite and granidiorite and targets were (Brown et al., 2002). The consistency of these lat- prepared for analyses of 10Be. These analyses were ter o¡sets suggests that fan abandonment oc- undertaken at the Tande¤tron Accelerator Mass curred shortly after deposition of the 45-m o¡set Spectrometer at Gif-sur-Yvette, France (Raisbeck debris £ow. et al., 1994). Analytical uncertainties (reported as To determine the timing of formation of these 1c) are based on counting statistics and conserva- features, we sampled quartz-bearing granidiorite tive assumptions of a 5% uncertainty in machine boulders (1^3 m diameter) in the levees of the calibration and a 50% uncertainty in the chemical

PALAEO 3176 9-9-03 146 E.T. Brown et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 199 (2003) 141^151 and analytical blank correction. These blank cor- 1995). Comparison of these ages with absolute rections were 6 1% for samples from surfaces to age requires propagation of an additional 15% be dated and 6 10% for the ‘geological blanks’. uncertainty for the production rates. Ratios of 10Be/9Be were measured relative to the NIST standard (SRM 4325), which has a certi¢ed value of 2.68U10311 atom/atom. 3. Results Production rates of cosmogenic nuclides at the Earth’s surface vary with time, altitude, and loca- The data from the perched terraces suggest rap- tion. Apparent exposure ages were calculated us- id downcutting and abandonment of the three ing production rates calculated for the altitude terraces at approximately 8 ka (Table 1). There and geomagnetic inclination of our site following is considerable scatter in the dates for the oldest the calculations of Dunai (2000) but neglecting terrace, with exposure ages ranging from 8.6 to production by muons, which produce only V1% 47.5 10Be ka. We suppose that the two high values of 10Be at sea level (Braucher et al., 1998; Brown may result from exposure of material during a et al., 1995) and contribute an even smaller long low stand of the lake during the Late Pleis- amount at higher altitudes. At our study area, tocene glacial period when there was minimal riv- production rates calculated using the commonly er out£ow. In contrast, ages from the lower two used polynomials of Lal (1991) are V15% lower terraces are more tightly grouped (7.5^10.5 10Be than those of Dunai (2000) and would yield cor- ka) with values close to the two youngest ages for respondingly older exposure ages. This di¡erence the highest terrace (8.6 and 10.4 10Be ka). This is linked to a lower attenuation length in air im- suggests that disection of the upper terrace and plicit in the work of Dunai (2000) that is consis- formation of the lower two terraces may have tent with recent experimental results (Brown et occurred during a brief period of valley incision. al., 2000). The average location of the geomag- Low levels of 10Be for the boulders in the active netic North Pole over the past 10 000 years is streambed, and for the debris £ow with a 2-m very close to the geographic North Pole (Ohno o¡set, suggest modest levels of prior exposure and Hamano, 1992), so we utilize production for material on the alluvial terraces. Using a rates calculated for magnetic inclinations relative mean age of 2.0 þ 1.2 10Be ka as a geological to the geographic pole. Production rates at the blank for the seven tightly grouped samples sug- individual sampling sites were normalized to elim- gests that downcutting occurred at about 7.2 þ 1.7 inate the e¡ects of shielding by local topography 10Be ka BP. by integrating ¢eld measurements of skylines with In an earlier study, which had the primary goal the sin2:3a dependence of the cosmic ray £ux on of determining slip rates on the Karakorum Fault, angle a above horizontal (Conversi and Rothwell, we estimated the timing of abandonment of the 1954). This correction was always less than 10%. debris-£ow fan on the southwest side of the val- Because these production rates are based on mea- ley, facing the perched terraces (Brown et al., surements calibrated against standards consistent 2002). The distributions of 10Be concentrations with a value of 3.06U10311 for the NIST stan- of boulders incorporated in the two debris £ows dard (see discussions in Brown et al., 2000; Mid- cluster tightly enough to suggest that they can be dleton et al., 1993), they were decreased by a fac- used to date the £ows. In both cases, seven or tor of 0.875 for use with our measurements, which eight of the 10 measured concentrations cluster were calibrated against the NIST certi¢ed value within a fairly narrow range and two or three (2.68U10311). For comparison of exposure ages outlying samples have much higher concentra- within a given landform or region, we report tions. This suggests that there are two distinct ages in terms of ‘10Be years’ propagating experi- populations of boulders within each debris £ow: mental uncertainties for individual samples, but most boulders were exhumed from depth when disregarding systematic uncertainties in produc- the debris £ows occurred, and a few previously tion rates (Brown et al., 1998; Gosse et al., exposed on the fan surface were entrained in the

PALAEO 3176 9-9-03 E.T. Brown et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 199 (2003) 141^151 147 debris £ows as they passed over the fan. Van der o¡sets, respectively. Using the average cosmic ray Woerd et al. (1998) took a similar approach in exposure ages of boulders in the 2-m o¡set debris evaluating 10Be results from alluvial terraces £ow and the active stream (2.0 þ 1.2) as a geolog- along the Kunlun Fault. Disregarding the out- ical blank, we adopt an age of 11.6 þ 1.8 10Be ka liers, the 10Be concentrations (using the produc- for deposition of the large debris £ow and also for tion rates described above) correspond to ages abandonment of the debris-£ow fan. These results ranging from 1.0 to 4.2 10Be ka and 12.4 to 15.6 imply that slip along the Karakorum Fault con- 10Be ka for the debris £ows with 2-m and 40-m sists of right lateral movement at a rate of 4 mm/

Table 1 Analytical results and exposure ages Sample ID 10Be 10Be error ‘Age’ ‘Age’ error ‘Age’ error (103 atoms/g) (103 atoms/g) (10Be ka) (10Be ka) (calendar ka) Debris £ow ^ 2-m o¡seta KK98-1 143 38 1.8 0.5 0.5 KK98-3 207 32 2.5 0.4 0.6 KK98-4b 442 51 5.5 0.6 1.0 KK98-5 111 15 1.4 0.2 0.3 KK98-6 345 44 4.2 0.5 0.8 KK98-7 300 59 3.7 0.7 0.9 KK98-8b 789 59 9.7 0.7 1.6 KK98-14 107 14 1.3 0.2 0.3 KK98-15 80 14 1.0 0.2 0.2 Active streama KK97-41 42 10 0.5 0.1 0.1 KK97-41.5 147 35 1.8 0.4 0.5 mean for ‘geological blanks’ Debris £ow ^ 40-m o¡seta KK97-36b 1968 132 24.4 1.6 4.0 KK97-37b 2307 198 28.6 2.5 4.9 KK97-38 1016 111 12.5 1.4 2.3 KK97-39 1214 90 15.0 1.1 2.5 KK97-40 1264 100 15.6 1.2 2.7 KK98-9 1068 74 13.2 0.9 2.2 KK98-10 1040 162 12.8 2.0 2.8 KK98-11 1008 111 12.4 1.4 2.3 KK98-12 1141 85 14.1 1.0 2.4 KK98-13b 2814 170 34.9 2.1 5.6 mean Upper terrace KK97-24 850 74 10.4 0.9 1.8 KK97-26b 1303 129 16.0 1.6 2.9 KK97-27b 3831 302 47.5 3.7 8.0 KK97-28 702 78 8.6 1.0 1.6 Middle terrace KK97-29 750 101 9.2 1.2 1.9 KK97-30 697 75 8.6 0.9 1.6 Lower terrace KK97-33 850 91 10.5 1.1 1.9 KK97-34 607 51 7.5 0.6 1.3 KK97-35 814 70 10.0 0.9 1.7 mean of terraces a Values previously published in Brown et al. (2002). b Outlying samples not included in means.

PALAEO 3176 9-9-03 148 E.T. Brown et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 199 (2003) 141^151 yr (for timescales of 104 yr) accompanied by a small amount of reverse thrusting, raising the valley £oor at a rate of V0.5 mm/yr (Brown et al., 2002). For comparison with the timing of terrace downcutting we do not propagate systematic uncertainties in the geological blank for the system, yielding ages of 7.2 þ 1.1 10Be ka and 11.6 þ 1.3 10Be ka, for terrace and debris-£ow fan abandonment, respectively.

4. Discussion

Fig. 5. Plot of age versus elevation above valley £oor. Verti- Enhanced precipitation and river £ow can lead cal lines show means and standard deviations (disregarding to abandonment of fans by two mechanisms: in- the high outliers, see text) of the ages for fan abandonment cision of the fan due to increased streampower on and valley incision. The results indicate that abandonment of the fan, or downcutting at the fan toe by en- the fan occurred at 11.6 þ 1.3 10Be ka, whereas valley incision 10 hanced £ow in a transverse stream leading to for- was later, at 7.2 þ 1.1 Be ka. Because this plot compares ages within a single study site, the uncertainties are based on mation of a knick point that propagates upward counting statistics only, and do not include uncertainties in on the fan (Bull, 1991; Harvey, 2002a,b; Leeder the ‘geological blank’ for the system or in cosmogenic nu- and Mack, 2001). Fan abandonment through the clide production rates. ¢rst mechanism could be decoupled from valley incision, whereas abandonment by the latter particularly when interpreted in light of the lake’s mechanism would be coeval with valley downcut- bathymetry. That work indicated initiation of a ting. Our results indicate that the abandonment of humid period at 9.5 14C ka (10.9 ka BP), with a the alluvial fans on the southwest side of the val- quite rapid response in lake chemistry. This may ley preceded, by V4 kyr, the river incision that correspond to the timing of incision of the debris led to formation of the terraces perched on the £ow/alluvial fan in response to enhanced precip- northeast side of the valley (Fig. 5). itation. The core examined in that work was from The di¡erence between the time of abandon- the easternmost of the Panggong basins, which ment of the debris-£ow fan (11.6 þ 1.3 10Be ka) receives the bulk of riverine runo¡ to the lake and the valley incision (7.2 þ 1.110Be ka) may be (Fontes et al., 1996)(Fig. 1). At the initiation of associated with the lag between initiation of hu- a humid period, the chemistry of the easternmost mid conditions and commencement of out£ow basin should respond quickly; once it ¢lled be- from Panggong through the Tangste Valley. The yond the sill depth (presently 1^2 m below lake threshold for rainfall required to initiate fan level) it would become an open basin lake, over- abandonment by incision from the head is likely £owing to the other basins to the west and north. to be lower than the threshold for over£ow of the After an extended humid period, the lake would lake and incision by the trunk stream. The trunk reach the levels recorded by the high strandlines drainage (Tangtse) did not respond strongly to and begin over£ow at its western end, ultimately the enhanced precipitation until Lake Panggong incising the Tangste Valley. began to over£ow its sill. At that time, there was It may be possible to obtain information on the rapid downcutting in the valley, leaving the three frequency of these downcutting events by examin- perched terraces. ing the terrace elevations in light of the rate of the This scenario is consistent with earlier results vertical component of slip rate on the Karakorum on changes in the chemistry and biota of Lake Fault, which intersects the Tangtse River a few Panggong (Fontes et al., 1996; Gasse et al., km downstream of the terraces. At this site 1996; Hui et al., 1996; Van Campo et al., 1996), high-frequency tectonic events (the estimated re-

PALAEO 3176 9-9-03 E.T. Brown et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 199 (2003) 141^151 149 currence interval for movement on the Karako- 2. The alluvial terraces appear to have been rum Fault is V500 years (Brown et al., 2002)) formed approximately 4 kyr after abandonment are punctuated by lower frequency climatic pulses of the alluvial/debris-£ow fans on the opposite in which over£owing lake waters downcut the val- side of the valley. This is consistent with a two- ley. Suppose that the valley £oor northeast of the step process in response to continued humid con- fault is raised above the baselevel southwest of the ditions in the basin. In this scenario, fan aban- fault during dry intervals primarily through tec- donment would have occurred at ca. 11.5 10Be tonic movement rather than accumulation of sedi- ka as the climate became more humid presumably ment, and that downcutting events return the riv- in response to enhanced monsoonal circulation, erbed to baselevel. Then the height of the terraces but the terraces would not have been formed until above the valley £oor should correspond to the about 7 10Be ka with downcutting of the valley at amount of vertical movement on the fault that the base of the fan in response to initiation of occurred since the previous climatic pulse. The over£ow from Lake Panggong. This proposed ratio of the elevation of the highest terrace above lag between the onset of humid conditions (also the valley £oor (40 m) to the vertical component recorded in sediments of the eastern basin) and of slip on this segment of the Karakorum Fault of initiation of out£ow is consistent with a several V0.5 mm/yr would imply that the penultimate thousand year process of ¢lling the lake’s multiple major downcutting event occurred at V80 ka. basins before over£ow could occur. The suggestion that alluvial landforms are produced by low-frequency climate events is consistent with other studies hypothesizing landform Acknowledgements evolution punctuated by Milankovitch-scale climate cycles (Benedetti et al., 2000; Molnar et Funding for this project was provided by the al., 1994; Ritz et al., 1995; Winter et al., 1993). National Science Foundation, Division of Earth At our study site, this could be linked to a region- Sciences (Grants EAR-9705841 and EAR- al increase in precipitation due to greater Indian 9706502 to E.T.B. and P.M., respectively). Dis- Summer Monsoon strength during the warm pe- cussions with Franc°oise Gasse, Doug Ricketts, riods immediately following glacial maxima Hugh Sinclair, and John Swenson and thoughtful (Clemens et al., 1991, 1996; Colin et al., 1998; reviews by Devendra Lal and Adrian Harvey im- Marcantonio et al., 2001; Reichart et al., 1997). proved this study considerably. We thank Jacques Lestringuez and Dominique Debo¥e for their ex- pertise in AMS measurements and Jason Agnich 5. Summary and conclusions for skilled assistance in sample preparation. Tan- de¤tron operation is supported by CNRS, IN2P3, 1. Our results indicate that £uvial incision and and CEA. downcutting events occurred at V11.5 and V7 10Be ka BP, suggesting enhanced precipitation during that time. This is consistent with a growing body of literature suggesting that the period from References 10 to 6 ka BP was humid in Central Asia, most Benedetti, L., Tapponnier, P., King, G.C.P., Meyer, B., Ma- notably the Lake Panggong sedimentary record nighetti, I., 2000. Growth folding and active thrusting in the (Fontes et al., 1996; Gasse et al., 1996) that in- Montello region, Veneto, northern Italy. J. Geophys. Res. dicates that wet conditions prevailed from 9.5 to B105, 739^766. 6.2 14C ka (10.9^7.1 ka BP). Furthermore, this Braucher, R., Colin, F., Brown, E.T., Bourle's, D.L., Bamba, consistency indicates that the V6.7-kyr reservoir O., Raisbeck, G.M., Yiou, F., Koud, J.M., 1998. African laterite dynamics using in situ-produced 10Be. Geochim. e¡ect inferred for radiocarbon dates from Lake Cosmochim. Acta 62, 1501^1507. Panggong sediments by Fontes et al. (1996) is Brown, E.T., Bendick, R., Bourle's, D.L., Gaur, V., Molnar, reasonable. P., Raisbeck, G.M., Yiou, F., 2002. Slip rates of the Ka-

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