Soils as indicators of the Pleistocene and Holocene landscape evolution in the Alay Range (Kyrgystan)

Quaternary International 65/66 (2000) 161}169

Soils as indicators of the Pleistocene and Holocene landscape evolution in the Alay Range (Kyrgystan) Wolfgang Zech *, Bruno Glaser , Anatoli Ni
, Maxim Petrov
, Ivan Lemzin Institute of Soil Science and Soil Geography, University of Bayreuth, D-95440 Bayreuth, Germany
Institute of Geology and Geophysics, Academy of Sciences, Tashkent, Republic of Uzbekistan Institute of Seismology, Academy of Sciences, Bishkek, Kyrgyzstan

Abstract Soils derived from Quaternary glacial, glacio#uvial and aeolian deposits in the Alay Range (Kyrgystan) were studied to reconstruct the Pleistocene and Holocene landscape evolution. Geomorphologic studies, radiocarbon dating of A horizons in palaeosols, and iron fractionation were undertaken. During the Last Glacial Maximum (LGM) the Abramov glacier in the Alay Range advanced down the Koksu valley to an altitude of 2500 m asl, about 50 km from the modern snout (ELA lowering about 600 m). In addition, several frontal moraine complexes could be identi"ed at the valley bottom of the Koksu river between 3100 m asl and the modern glacier snout at 3650 m asl. The complexes can be assigned to the following glacier advances: Late Glacial (ELA lowering 200}300 m), early Holocene to end of Late Glacial (Younger Dryas?, ELA lowering about 110 m), Neoglacial (ELA lowering about 50 m), and Little Ice Age (ELA lowering about 50 m). Similar glacier advances could be reconstructed according to the soil development on the lateral moraines near the Abramov glacier at 3600}4000 m asl. Determination of pedogenic iron oxides corroborated the morphologic and radiocarbon results.  2000 Elsevier Science Ltd and INQUA. All rights reserved.

1. Introduction Since glaciers are very sensitive to changes in precipitation and temperature regimes, the reconstruction of their #uctuations, especially during the Holocene and middle or late Pleistocene, provides important palaeoclimatic data. Recent studies reveal that glacial and climatic #uctuations during the Late Pleistocene were not synchronous in Europe, Siberia, North and South America (Dawson, 1992; Gillespie and Molnar, 1995; Benn and Owen, 1998). In Central Europe the Last Glacial Maximum (LGM) occurred about 22}18 Ka BP, whereas in NE Siberia, Alaska and in the central part of the Andes, glaciers had already advanced to a maximum stage during earlier phases of the Late Pleistocene. Kind (1975) for instance reported from NE-Siberia that glaciation was more extensive between 60 and 50 Ka BP (Zyryanka glaciation) and 30 Ka BP (Zigansk stage) than 20 Ka BP (Sartan glaciation). Also in Kamchatka indicators of a pronounced early Late Pleistocene ice advance could be identi"ed (Zech et al., 1996b). Similar results have been

* Corresponding author. E-mail address: [email protected] (W. Zech).

published by Clapperton et al. (1997) with concern to the Andes. The majority of the relevant studies are usually based on geomorphology, lichenometry, tephrochronology, dendrochronology and pollen analysis. Investigations on soil development as an indicator of the Pleistocene and Holocene landscape evolution are less frequent (Mahaney et al., 1996; BaK umler and Zech, 1994). The objective of this paper is to highlight the contribution of soils in determining the reconstruction of Pleistocene and Holocene changes in glaciation and climate in the high mountains of Asia, focussing on the Alay Range in Kyrgystan.

2. Methods During "eld work, the geomorphologic situation was examined by mapping the major features of the landscape, and when possible aided by aerial photograph interpretation. Next, soils of geomorphologic units were examined using soil augering and digging soil pits on the upper part of the moraine ridges, except pro"les 2, 5, and 10 which were dug in concave positions (Figs. 2}4). From each soil horizon three samples (from the frontal and lateral sides of the soil pits) were collected and mixed for

1040-6182/00/$20.00  2000 Elsevier Science Ltd and INQUA. All rights reserved. PII: S 1 0 4 0 - 6 1 8 2 ( 9 9 ) 0 0 0 4 2 - 7

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physical and chemical analyses. Soils were classi"ed according to Soil Survey Sta! (1998). The focus of this paper is on soil morphological descriptions combined with radiocarbon dating. Radiocarbon analyses were carried out on humic acids extracted from buried A or O horizons. All radiocarbon ages were determined using conventional standards. The age-dependent formation of pedogenic iron oxides and hydroxides was used to assess the intensity of soil development, especially along chronosequences (BaK umler et al., 1996). Free pedogenic iron oxides/hydroxides (Fe ) were extrac ted with dithionite}citrate}bicarbonate solution (Mehra and Jackson, 1960), and X-ray-amorphous iron oxides, hydroxides, and associated gels (Fe ) with acid ammonium  oxalate solution (Schwertmann, 1964).

3. Results and discussion 3.1. Alay Range The investigation was carried out in the surroundings of the Abramov Glacier Research Station (3933855N,

7133422E; 3840 m asl), which is situated in the Alay Range, NW of the Pamir, Kyrgystan (Fig. 1, site 5 and Fig. 2, A). Meltwaters from the seven km long Abramov glacier feed the Koksu river which #ows near Darautkurgan (39330N, 72305E; 2470 m asl) into the Kizilsu and via Amu Darya to Lake Aral. Mountains around the Abramov glacier rise to '5000 m asl averaging 4800 m asl. At the Abramov Glacier Research Station (Fig. 2, A), mean annual precipitation is 743 mm and mean annual temperature is !4.13C; the corresponding values in Darautkurgan are 270 mm and #2.93C, respectively. Most of the rain falls in winter (580 mm), whereas in summer the climate is characterised by hot and dry continental conditions (164 mm; Pertziger, 1996). Based on precipitation, temperature, and topography, the actual snowline (equilibrium line altitude, ELA) is located between 4200 and 4280 m asl (Pertziger, 1996). The modern snout of the Abramov glacier descends to 3650 m asl. Both the actual snowline and the lowest part of the glacier are higher in the Alay range than in NW- and N-Tian Shan (Grosswald et al., 1994; Zech et al., 1996a). Bedrock if comprised of Palaeozoic limestone, dolomite, porphyrite, syenite, granite, diorite, and metamorphic rocks.

Fig. 1. Alay range and Abramov glacier (5) N and NW of the Pamir, High Asia, Kyrgystan. (1)}(4) indicate research localities where supplementary studies were carried out.

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163

Fig. 2. Geomorphologic map of the area around the Abramov glacier research station (A).

3.2. Frontal moraines in the Koksu valley The Koksu valley contains typical glacial features. These include U-shaped cross-sections, cirques, moraine terminii and glacio#uvial sediments. The lowest frontal moraines are present at 2500 m asl (Suslov, 1972) which suggest a glacier advance of about 50 km probably during the LGM. Using the methods of HoK fer (1879) and Louis (1954/55), an ELA lowering of 600 m was calculated. This value is lower than ELA depressions for NW- and N-Tian Shan (Grosswald et al., 1994; Zech et al., 2000) and the central part of Nepal Himalayas (Annapurna, Dhalagiri), where Kuhle (1982, 1994) assumes a snowline depression of 1300 m during the LGM. This discrepancy may be explained by the semi-arid

conditions of the Alay Range. Our LGM values for the Abramov glacier correlate with those of Sharma and Owen (1996), who calculated an ELA depression of about 640 m in NW Gharwal in the Central Himalayas. Soils developed from the lowest frontal moraines have A (10 cm), B (40}60 cm) and C horizons and were classi"ed according to the Soil Survey Sta! (1998) as Typic Eutrocryepts (Fig. 3 and Table 1, pro"le 17). The texture of these soils is always determined by aeolian silt similar to those described in other mountain ranges of Asia (Owen et al., 1992). Upvalley, moraines are present at 3100}3200, 3300 and 3350 m asl (Fig. 3), approximately 10}35 km from the modern snout of the Abramov glacier. They are characterized by diorite boulders originating from the Abramov

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Fig. 3. Altitude of moraines in the Koksu valley, with soil pro"les, and their tentative chronology. (A) surface horizon; (Ab) buried A horizon; (Bw) cambic horizon; (C) parent material; (O) peat; (S) sand; (IS) loamy sand; (U) silt; (sU) sandy silt.

Table 1 Basic information about soils developed on terminal moraines at the valley bottom of the Koksu river, Alay Range, Kyrgystan (for details, see Fig. 3) Pro"le no.

9 10 11

15

16 17

Location

Altitude (asl)

Classi"cation (Soil Survey Sta!, 1998)

Crest of Neoglacial moraine close to Schultz glacier, right side 7133528E, 3934011N Swampy depression, right side 7133528E, 3934011N Crest of lowest Little Ice Age moraine close to the mouth of the Schultz glacier right side 7133528E, 3934011N Upper slope of Early Holocene to Late Glacial terminal moraine with large diorite boulders mouth of Alaudin river 7133623E, 3934053N Crest of Late Glacial moraine, mouth of Kimisdikti river, left side 7133835E, 3934149N Crest of LGM moraine, NW of Darautkurgan 7230400E, 3933300N

3550 m

Lithic Eutrorthent

3540 m

Hydric Cryo"brist

3560 m

Lithic Cryorthent

3440 m

Lithic Eutrocryept

3300 m

Typic Eutrocryept

2500 m

Typic Eutrocryept

glacier. Between 3100 and 3200 m asl, besides the diorite boulders coarse dolomite rockfall debris also occurs. The ELA depression varied between 200 and 300 m. These values characterise Late Glacial ice advances as shown by Heuberger and Sgibnev (1998), Zech et al. (2000), and BaK umler and Zech (2000) for surrounding mountain ranges (Fig. 1, research localities 1}4). In the Koksu valley, the soils of Late Glacial age are classi"ed as Typic

Eutrocryepts with ABC pro"les (Fig. 3, pro"le 16) and a silty cambic horizon of about 30}40 cm thickness. Some of these soils have buried A horizons (2Ab in pro"le 16, Fig. 3) which give evidence for a warmer and wetter period than at present with high plant biomass production. According to Solomina and Kamnianski (1998), these favourable climatic conditions occurred during the middle Holocene and terminated around

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3500}4200 years BP. Similar "ndings were reported from Tian Shan (Kovaleva and Evdokimova, 1997) and Tibet (Gasse et al., 1991; Lehmkuhl, 1995). Further upvalley, at 3440 m asl a small but clearly identi"able frontal moraine with large diorite boulders provides evidence for a glacial advance of about 5.6 km based on an ELA depression of about 110 m. Unfortunately, no organic material was found for radiocarbon dating. In the Kichick Alay, NE of Abramov glacier (Fig. 1, site 3), a similar till with big boulders is present at 3490 m asl. It covers a buried A horizon having a radiocarbon age of 7290$80 years BP (Zech et al., 2000). This data suggests that the frontal moraine in the Koksu valley at 3440 m asl also formed during the Early Holocene or probably the Late Glacial (Younger Dryas?). This interpretation is supported by the fact that the cambic B horizon of pro"le 15 (Fig. 3) has comparable morphologic properties to the B horizon of the Late Glacial pro"le 16. Upvalley of pro"le 15 terminal moraines at the valley bottom are never covered by soils with strongly developed brown cambic B horizons. On the basis of soil morphology these moraines can be separated into two groups: (a) grass covered ones with development of AC and ABC pro"les, the B horizon being only weakly developed (Fig. 3 and Table 1, pro"le 9), and (b) moraines with traces of pioneer vegetation and initial A horizons only (Fig. 3 and Table 1, pro"le 11). Group (a) was severely destroyed by glacio#uvial streams that descended from the Abramov and Schultz glaciers. Some relics 1.5 km downvalley the Abramov glacier snout at 3550 m asl are due to a Neoglacial glacier advance when the ELA was 50 m lower. This till buried an A horizon 2030$60 a BP (Fig. 3 and Table 1, pro"le 9). Close by, a swampy depression is located where Hydric Cryo"brists developed (Fig. 3 and Table 1, pro"le 10). The deepest organic layer of these soils, which accumulated directly upon the bedrock, has a radiocarbon age of 3805$145 a BP. This result proves that no glacier advanced deeper downvalley after about 3805 a BP. The lowest terminal moraine of group (b) has a similar altitude (3560 m) as the Neoglacial till with pro"le 9 (Fig. 3). It was also heavily destroyed by glacio#uvial streams. The weak soil development indicates that the moraine is very young and may have been formed only several centuries ago. It represents the maximum glacier advance during little ice age (LIA). The close neighbourhood of the Neoglacial to LIA moraines shows that during the last few centuries glaciers advanced nearly to the same extent than during the Neoglacial. Also, ELA depressions during the LIA were similar (about 50 m) to those during the Neoglacial. Several signi"cant moraine ridges are present at the valley bottom between the lowest LIA terminal moraine at 3560 m asl and the recent Abramov glacier snout at

165

3660 m asl (Fig. 2, I}III). They all belong to group b) moraines with weakly developed soils classi"ed as Lithic Cryorthents (Fig. 3 and Table 1, pro"le 11); they also represent glacier #uctuations during the last centuries. Using the growth of Aspicilia lichens, Solomina and Kamnianski (1998) established a chronology of the Abramov glacier #uctuations during this period. Major ice advances were identi"ed in the 15th century and at the end of the 16th, 18th and 19th centuries. During the 20th century, the Abramov glacier expanded until 1912, before a pronounced retreat began in 1928. A last glacier advance has been recorded in 1972/73. Since then, a further and continuous retreat is observed until the present, in#uencing not only the horizontal but also the vertical dimension of the glacier snout. In comparison to the maximum glacier extent during the LIA (Fig. 2, I), the Abramov glacier retreated about 6 km and thinned about 200 m at the modern snout. 3.3. Lateral moraines near the Abramov glacier In addition to studying terminal moraines at the valley bottom, glacier #uctuations were reconstructed from the remnants of lateral moraines. Fig. 4 shows three small terraces with the soil pro"les no. 12, 13 and 14 on the SE exposed side of the Abramov glacier at 3980, 3950 and 3850 m asl. Similar features were observed on the SW exposed side of the Schultz glacier at 4110, 4080 and 3810 m asl (Fig. 2). Both slopes are relatively steep in contrast to the NW exposed side of the Abramov glacier to the NE of the research station A (Fig. 2). Here, the shallow slope inclination supports the preservation of "ve lateral moraines at 3940, 3910, 3870, 3810 and 3770 m asl (Fig. 4). All these terraces are characterised by the accumulation of erratic boulders and they descend to the Koksu valley. Between these terraces, interpreted as former ice margins, the number of boulders is low. The well preserved terrace at 3910 m asl is about 300 m above the modern valley bottom. Its boulders are densely covered with lichens, but no deep weathering rinds have developed. The soils of this terrace were classi"ed as Typic Haplocryoll (Table 2, pro"le 1) with a 20 cm thick, mollic A horizon, burying a fossil mollic epipedon (Fig. 4, pro"le 1, 4Ab horizon). With a radiocarbon age of 4470$180 a BP, this palaeosol corresponds to the above mentioned mid Holocene Mollisols. Consequently, the lateral moraine at 3910 m asl is older. Diorite boulders are present at the NW exposed slope up to 3940 m asl (Fig. 4) indicating that the Abramov glacier once occupied this altitude. Here a palaeosol developed on a poorly preserved lateral moraine with a 2Bw horizon which gave a radiocarbon age of 24,300$1160 a BP (Fig. 4 and Table 2, pro"le 7). We assume that this palaeosol developed during a warmer interstadial which is postulated for oxygen isotope stage 3, between the Sartan (Late Valdai, isotope stage 2) and

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Fig. 4. Lateral moraines identi"ed along a NW}SE transect between the Abramov glacier research station and hydrometeorological station (see A and B, Fig. 2), their soil pro"les and some radiocarbon data of buried organic materials. A, surface horizon; Bw, cambic horizon; Bg, B horizon with hydromorphic features; (Bc) B horizon with precipitation of CaCO ; (C) parent material; (O) peat; (S) sand; (sL) sandy loam; (IS) loamy sand; (U) silt;  (sU) sandy silt; (suL) sandy silty loam; (tL) clayey loam.

Zyryanka (Early Valdai, isotope stage 4) ice advance (Arkhipov, 1984; Bespaly, 1984; Faustova, 1984; Serebryanny, 1984; Velichko, 1984). Beginning with the Sartan glaciation about 24,000 a BP soli#uction debris and Abramov till began covering the palaeosol. The upper surface of the LGM glacier was even higher than 3940 m asl as indicated by trough shoulders and polished rocks at about 4050 m asl. Consequently, the diorite boulders at 3940 m (soil pro"le 7) and the lateral moraine at 3910 m (soil pro"le 1; Fig. 4) are representing younger, probably Late Glacial glacier advances. The soils of the lower moraine terraces at 3870 and 3810 m asl have a weathering depth of about 30}60 cm and frequently reveal features of Mollisols covered by silty aeolian deposits (Fig. 4, pro"le 4, 2AC horizon). Since these fossil Mollisols developed during the mid Holocene, the terraces may have been formed during the early Holocene or Late Glacial. Further downhill, mainly soils with initial A horizons occur (Fig. 4 and Table 2, pro"le 6), documenting the extension of the LIA glacier snout. Lichens do not or only sparsely cover the diorite boulders. On the upper limit of the LIA till at about 3770}3800 m asl a distinct

discontinuous lateral moraine is present. Soils developed on this moraine are classi"ed as Lithic Eutrocryepts (Fig. 4 and Table 2, pro"le 3). Since, a well-developed buried middle Holocene Mollisol is absent, it is suggested that a late Holocene glacier advance formed these moraines. This view is con"rmed by a radiocarbon age of 985$115 years BP of the shallow 2Ab horizon, detected below the fresh till. These results con"rm the hypothesis that the LIA advances were approximately the same size as the late Holocene to Neoglacial ones (see also Fig. 3, pro"le 9, 2030$60 a BP). Some glaciers in the Tian Shan behaved similarly (Meiners, 1996). On the SE exposed side of the Abramov glacier the highest, very eroded terrace at 3980 m asl is covered by soils with two buried A horizons (2A and 3AB in pro"le 12, Fig. 4). The organic material of the 3AB horizon gave a radiocarbon age of 15,950$350 a BP. Probably, this horizon formed during a warmer period after the LGM. The highest diorite boulders at 3980 m asl on the SE exposed slope, and at 3940 m asl on the NW exposed slope (Fig. 4, soil pro"les 12 and 7) might not have been deposited during the LGM, because the ELA depression during this period was about 600 m, down to 3600 m asl.

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Table 2 Basic information about soils developed on lateral moraines of the Abramov glacier, Alay Range, Kyrgystan (for details, see Fig. 4) Pro"le no.

Location

Altitude (asl) Inclination aspect

Classi"cation (Soil Survey Sta!, 1998)

Crest of highest right side lateral moraine with diorite boulders 713352E, 3931517N

3910 m

Typic Haplocryoll

2

Peat between pro"le no. 1 and 7 713352E, 3933917N

3910 m 03, E

Terric Cryo"brist

3

Crest of right side lateral moraine with many diorite boulders 7133430E, 393393N

3810 m

Lithic Eutrocryept

Crest of right side lateral moraine with diorite boulders 7133459E, 3933923N

3870 m

5

Peat between 1 and 7, right side 713352E, 3933917N

3910 m 03, E

Terric Cryo"brist

6

Till recently covered by glacier, right side 7133441E, 3933928N

3750 m 323, NW

Lithic Cryorthent

7

Highest diorite boulders, weak #attening, right side 7133513E, 3933921N

3940 m

Typic Eutrocryept

12

Crest of lateral moraine, left side diorite boulders 7133528E, 3934011N

3980 m 103, SSO

Typic Eutrocryept

13

Crest of lateral moraine, left side, diorite boulders 713346E, 393405N

3950 m 53, SSO

Typic Eutrocryept

14

Crest of lateral moraine, left side, 7133420E, 393402N

3850 m 103, SSE

Lithic Eutrocryept

1

4

Fig. 5. Fe /Fe ratio of topsoil (䢇) and subsoil horizons (X) of selected   soil pro"les, developed from Late Glacial (LG 1-3), early Holocene to end of Late Glacial (Ho-LG), Neoglacial (NG), and Little Ice Age (LIA) deposits in the Alay Range. Pro"les are indicated by numbers (see Tables 1 and 2).

33, WNW

153, W Typic Haplocryoll

53, SO

53, NW

Their existence at approximately 3900}4000 m asl "ts much better with an ELA depression of 200}300 m as calculated for Late Glacial advances. Fe /Fe ratios of the A and B horizons of soil pro"les,   developed from Late Glacial, Neoglacial and LIA moraines, are plotted in Fig. 5. It is obvious that the Fe /Fe   ratios of the surface horizons and those of the subsurface horizons descend with increasing age of the moraines. This is because Fe increases with increasing soil age.  The di!erentiation between Neoglacial and Late Glacial moraines was clearer using B instead of A horizons. This could be due to the inhibitory e!ect of soil organic matter in the formation of crystalline iron oxides (Schwertmann, 1966). For further investigations, we recommend therefore using B horizons for soil chemical investigations, although A horizons are more weathered. It is unclear, however, to which extent these horizons are in#uenced by soil erosion or human impact.

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4. Conclusions Probably, during the LGM the Abramov glacier in the Alay Range descended to 2500 m asl (ELA lowering about 600 m). Between this till at 2500 m asl and the modern glacier snout at 3650 m asl, several moraine complexes could be identi"ed at the valley bottom of the Koksu river. Those between 3100 and 3300 m asl (ELA depression approximately 200}300 m) may represent Late Glacial glacier advances. A younger, smaller terminal moraine, rich in boulders, is present at 3440 m asl (ELA depression about 110 m), and probably formed during the early Holocene to Late Glacial. Further studies should clarify whether this glacier advance might correlate with Younger Dryas phenomena in other parts of the world. Soils derived from these drifts are classi"ed as Eutrocryepts, frequently containing buried A horizons, and developed during the middle Holocene climatic optimum. Upvalley, two types of frontal moraines are deposited: the older ones are younger than 3000}4000 a BP, but older than about 1000 a BP. They reveal mainly AC or poorly developed ABC soil pro"les. The younger ones are characterised by Cryorthents with AiC soil horizons. These formed during LIA advances. The geomorphologic features and the soil morphological properties of the lateral slopes near the Abramov glacier between 3600 and 4000 m also provided evidence of glacier advances during the LGM or earlier, the Late Glacial with several stages, the Neoglacial, and the LIA. Indicators of a middle Pleistocene glaciation, as identi"ed for the ancient Barkrak and Severtsov glaciers (Fig. 1, sites 1 and 4), could not be observed in the Koksu valley. The question whether the maximal Late Pleistocene glacier advances occured during oxygen isotope stages 2, 3 or 4 is still open.

Acknowledgements The study was "nanced by the German Research Foundation (DFG Ze 154/33). We are grateful to Mr. Houli Mingh for carrying out the soil analyses and to Mr. Th. Engelbrecht for preparing the "gures. We are indebted to Prof. Dr. Geyh, NiedersaK chsiches Landesamt for running the radiocarbon analyses. L.A. Owen and P. Barnard improved signi"cantly the manuscript by valuable critical comments. Dr. Usmanov, director of the Academy of Sciences in Tashkent, provided logistic support.

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