Late Pleistocene–Holocene Environmental Changes in Ultra-Continental Subarid Permafrost-Affected Landscapes of the Terekhol' Basin, South Siberia

Catena 112 (2014) 99–111

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Late Pleistocene–Holocene environmental changes in ultra-continental subarid permafrost-affected landscapes of the Terekhol' Basin, South Siberia

Maria Bronnikova a,⁎, Andrey Panin b, Ol'ga Uspenskaya c, Yuliya Fuzeina b, Irina Turova a a Institute of Geography, Russian Academy of Sciences, 119017, Staromonetnyj, 29, Moscow, Russia b Faculty of Geography, Moscow State University, 119992, Vorobjevy Gory, Moscow, Russia c Institute of Horticulture, Russian Academy of Agriculture, 140153, Moscow region, p/o Vereya, Building 500, Russia article info abstract

Keywords: This study is an attempt to use closely related inter-complementary paleo-archives of a local landscape to access Paleo-archives understanding of Late Pleistocene–Holocene environmental changes in the region. The study site is a small Paleosols intermountain basin in the Sayan-Tuva Upland, 51°N., 97°E., 1300 m a.s.l. Paleo-archives covering about Paleoenvironment 13,000 yrs were studied: paleosol-sedimentary sequences on a delta-alluvial fan of a small river, lacustrine sedi- Pleistocene/Holocene transition ments in bottom cores and on palsa-islands and soils of palsa-islands. The following sequence of environmental Holocene changes was established. The fluvial activity in the basin reached its maximum at the end of the Late Pleistocene. South Siberia The sharp decrease of the fluvial activity is terminated by two successive paleosols of Pleistocene–Holocene transition. The older paleosol indicates meadow-steppe (or tundra-steppe) conditions with a shallow permafrost table and impeded drainage. The younger paleosol testifies on sharp aridization, biological activity suppression, contrasting water regime, and warming. Dammed lake appeared in the midst of the trough about 11,000 cal yr BP. Sedimentation on delta-alluvial fan was fairly inconsiderable in Holocene. The first part of Holocene (before 4000 cal yr BP) was most balanced in annual distribution of precipitation. Runoff, even being prominently enhanced in a fluvial and relatively warm sub-phase 8000–6250 cal yr BP, was canalized, without giving seasonal floods. Sub-phase 6250–3800 is characterized by increased continentality and relative aridization caused reduc- tion of runoff, lowering of the lake level, and enhancement of cryogenic processes. The next phase 3800–2000 cal yr BP was more humid. It caused slight revival of fluvial processes and rise of lake level. The last 2000 years climate was more continental and the most arid within the studied period. The lake level and runoff values dropped again, and fluvial activity totally decreased. In contemporary soils aridization is reflected in widely spread Natric features and progressive salinization. © 2013 Elsevier B.V. All rights reserved.

1. Introduction to dynamism of ﬂuvial, lacustrine, slope sedimentation (interchanging intensive sedimentation and relatively continuous hiatuses) during Environmental changes and related evolution of soils in the late Late Pleistocene and Holocene. Pleistocene and the Holocene in South Siberia have not been studied There are only few studies on Holocene soil and environmental satisfactory yet as far as the region is complicated and diverse in respect evolution based on the analysis of soil (or soil-sedimentary) records to landscapes and soils, and hardly reachable in some parts. Soils and of the South Siberia (Dergacheva et al., 2006; Desjatkin, 1984, 2008; pedosedimentary sequences of small intermountain basins of the South Vorob'eva, 2010). Some of these studies analyze local sources not Siberia are one of still insufﬁciently developed and highly promising correlating them with earlier produced data on environmental changes archives of the Late Pleistocene and Holocene environmental changes. in a wider region or the data obtained from other environmental Advantages of soil-sedimentary sequences for paleoenvironmental archives. reconstructions in small inter-mountain basins were closely discussed Studies of different sedimentary Late Pleistocene and Holocene by Vorob'eva (2010).Surfacesoilsofsuchbasins,asarule,havegot archives in the South Siberia and, taking a broader region, in arid sector obvious evidences of polygenesis (recording more than one set of soil of Central Asia are more in number (Blyakharchuk et al., 2004, 2007, forming environmental conditions). Buried soils related to different pe- 2008; Huang et al., 2009; Rudaya et al., 2009; Schwanghart et al., riods and environments are widespread in intrermountain basins due 2009; Vipper et al., 1976 and others). However the data on Late Glacial and Holocene obtained for different localities and applying different

⁎ Corresponding author. Tel.: +7 495 959 00 28; fax: +7 095 959 00 33. methods are often contradictory. Generalizations of data accumulated E-mail address: [email protected] (M. Bronnikova). at the moment are still rare (Blyakharchuk et al., 2004; Zhao et al.,

2009). Despite the growth in understanding of the Late Glacial and Continuous relic permafrost is found in the bottom of Terekhol' Basin. Holocene climate in the South Siberia and generally in arid/subarid It is up to 170 m thick according to geophysical data (Koshurnikov et al., parts of Asia a further work is needed. In editorial to the special issue 2008). Active layer is 120–300 cm deep in July–August in well drained of Quaternary International “Holocene climate variability in arid Asia: conditions. In most of sections it varies around 200 cm. Nature and mechanisms” necessity to extend a net of study sites across Steppe landscapes prevail in the basin's bottom. They occupy all the region was emphasized, in order to escape difficulties in the identi- areas with the exception of the alluvial–lacustrine–thermokarst fication of regional patterns. Studies focused on climatically sensitive area surrounding the Terekhol' Lake and narrow strips of larch areas (such as ultracontinental arid permafrost affected areas) and taiga along transit streams. The never plowed areas are now inter-archive comparisons are among first-priority tasks (Chen et al., occupied by dry steppe communities (Stipa sp., Psathyrostachys 2009). hyalantha (Rupr.) Tzvel., Poa argunensis Roshev., Bupleurum Usually there are a number of paleoenvironmental archives available scorzonerifolium Willd., Veronica incana L., and Orostachys spinosa in local landscapes to access information on environmental changes. (L.) C.A. Mey) and communities of meadow steppes (Stipa sp., Bromopsis There could be independent sedimentary records of various origins (fluvi- inermis (Leyss.) Holub., Potentilla bifurca, P. argunensis Roshev., al, limnic sediments, loess, peat etc.) and soil record in polygenetic surface Thermopsis mongolica Czefr., Heteropappus altaicus (Willd.) Novopokr., soils and buried paleosols (soil memory). Different kinds of paleo- and B. scorzonerifolium Willd). Previously arable lands have lost Stipa archives in most cases are studied independently because of narrow sp. and got some ruderal species such as P. hyalantha (Rupr.) Tzvel., specialization of researchers and common difficulties of interdisciplinary Elytrigia repens (L.) Nevski, Potentilla tanacetifolia Willd. ex Schlecht., studies. Meanwhile each kind of paleo-archive each method used to Artemisia glauca Pall. ex Willd., Chenopodium album L., Descurainia extract paleoecological information has its own approaches, limitations, sophia L. Larch taiga communities cover mountain slopes of the basin. advantages and disadvantages. Therefore a comparative study of different Lake terraces and low islands are occupied mostly by meadows and sedimentary and paleosol archives in a single local geosystem seems to be meadow-steppes, the highest positions of the terraces and high islands the most fruitful and reliable for environmental reconstructions. Data are covered with normal steppe vegetation with Stipa sp. obtained from independent environmental archives of the same age are Cryosols are the major and the most widely spread soils of the complementary and verifying each other. Meanwhile such comparative Terekhol' Basin. Soil cover of steppe landscapes, particularly the sur- studies of different local paleoenvironmental archives are not a common face of the delta-alluvial fan, mostly consists of Turbi-Natri-(Sali)- practice in paleopedology. Only some rare studies can be cited in this field Molliglossi-Calcic Cryosols and Turbi-Protonatri-Molliglossi-Calcic (Sedov et al., 2003, 2009, 2010; Solleiro-Rebolledo et al., 2006). Cryosols. There are two general options of surface soil formation This study is an attempt to use closely related complementary paleo- which proceeds at the background of numerous evidences of archives of a local landscape – asmallintermountainbasin– for most cryogenesis: (1) all the variety of solonization–solodization and comprehensive understanding of Late Pleistocene–Holocene environ- alkalization–dealkalization processes in its different stages (from mental changes in the region. The following archives were studied its initial phase through the mature, sodic stage and later secondary as sources of paleoenvironmental information: salinization up to residual morphological features of Natric horizon), and (2) chernozemic process including formation of the Mollic hori- - soil-sedimentary sequences on a delta-alluvial fan of the Ajyl River — zon (with initial signs of solonization) and accumulation of second- a small river entering the Terekhol' Basin from its south-western ary carbonates in Bk horizons. The specific contemporary soils of the side; alluvial fan were earlier described in details (Bronnikova et al., - sediments of the Terekhol' Lake in bottom cores and on palsa- 2011). A variety of Cambic Cryosols is formed under taiga ecosys- islands; tems, and Histic and Calci-Mollic Cryosols prevail on the lake terraces - soils of palsa-islands. and islands. Catchment areas of the Terekhol' Lake and the Ajyl River lie in The data on the alluvial soil-sedimentary sequences are a major the mountain surroundings of the basin. They are composed of Pre- focus of this paper. Data on lake and island sedimentary sequences Cambrian marbles cut by intrusions of early Paleozoic granitoids. Such (Panin et al., 2012) and island soils (Bronnikova et al., 2010)were geological settings condition high carbonate content of fluvial and published earlier in details. In the present paper these results are gener- especially lake sediments. The Terekhol' Lake is a fresh water body fed alized and discussed as a complimentary source of paleoenvironmental by groundwater, and by a number of small mountain rivers. The lake information. is drained by the Saldam River, left tributary of the Balyktyg-Khem River (one of the sources of the Yenisey River). Terekhol' Lake has a con- 2. Materials and methods siderable area of 33.19 km2, but it is extremely shallow: the average depth is about 0.6 m; less than 1% of the lake area is deeper than 1 m. 2.1. Study site: contemporary environment and study objects Islands make up about 2.85 km2 of the lake area. There is no permafrost below the lake (talik), but all islands have newly formed permafrost The Terekhol' Basin is a small intermountain depression located (Koshurnikov et al., 2008). at the north-east slope of the Sangilen Upland, the south-east part The basin bottom is the basis level for the Ajyl River which dissects of Tyva Republic, South Siberia (50°N., 97°E., about 1300 m above sea its southern slope (Fig. 1B). Ajyl is a small mostly rain-fed mountainous level) (Fig. 1A). river 14 km long draining a catchment area of 40 km2. Seasonal floods, The climate of the study area is ultra-continental subarid, with ex- up to 7 in number, normally take place at the end of May — at the begin- tremely severe winter and relatively hot short summer (the frost-free ning of June. During low water phases between floods the river doesn't season is only 32 days long). The mean annual temperature is −6.1 °С, reach the lake because of seepage into sediments in the eastern part the amplitude of annual temperature variation reaches 55.5 °С, and the of its vast Late Quaternary alluvial fan. sum of temperatures above +10 °С is 1000–2000 °C. The mean annual precipitation is 230–323 mm, and only 11% of annual sum falls in 2.2. Applied approaches and methods aformofsnow(Agroclimatic…,1961). There is no published data available on the seasonal freezing–thawing processes. But it is clear Topographic profiling was accomplished for the delta-alluvial from distribution of annual precipitation sum that soil gets frozen fan, across the bottom of the lake and islands. Topographic surveying while being dry and doesn't get much water due to thawing because was done applying a high-precision Leica Smart Station. The last one of autumn–spring water deficit. includes a GPS base-station (which was set on a local reference point), M. Bronnikova et al. / Catena 112 (2014) 99–111 101

Fig. 1. Location (A), general scheme (B) and geomorphological layout (C) of the Ajyl’s basin. Table of symbols: 1 — top-surfaces, 2 — erosional slopes, 3 — surface of the upper terrace, 4 — ﬂuvial surface considerably transformed by cryogenic processes, 5 — bottom of the valley, 6 — residual Sartan (Vistulian) fan; 7 — major generation of the fan formed between LGM and the beginning of Holocene; 8 — contemporary (late Holocene) fan, 9 — bottom of the erosion gully, 10 — low lake terraces, 11 — borders of the basin, 12 — edges of tributaries’ valleys, 13 — dry former channels, 14 — level of the lower terrace, 15 — ancient channels, 16 — tectonic rockslides, 17 — border of ﬂuvial sediments in the lake bottom; 18 — the border between eastern and western parts of the fan.

a GPS-rover and a digital tacheometer. Key soil-sedimentary and sedi- part, which was dispersed with sodium pyrophosphate (Konert and mentary sections were studied along the produced topographic profiles. Vanderberghe, 1997). Most of the pits were made within the active layer down to the top Mineralogical composition of clay fraction (b1 μm) was ana- of permafrost. Litho-stratigraphic and morphological pedogenetic lyzed by X-ray diffractometry (CuKα emission, Ni-filter) in scan descriptions within the active layer were provided for the soil- mode (scanning pitch is 0.1°, exposure time is 10 s). Samples sedimentary sequences of the alluvial fan and island soils. were pretreated with 10% hydrogen peroxide with water-bath Micromorphological studies of surface and buried soils were heating to remove organic matter, then Mg-saturated with 1 N conducted for the key sections. Undisturbed soil monoliths were MgCl2 for24handwashedoffthesaltexcessbycentrifuging.Mg- studied in thin sections under a polarizing microscope Nikon E200 Pol saturated air-dry, ethylene glycol-saturated, 350 and 550°С heated in plain polarized (PPL) and cross polarized (XPL) transmitted light at oriented samples have been studied. Quantitative estimation of magnifications of 40×, 100×, and 400×. Terminology and approaches clay mineral groups based on areas of their reflections was done of G. Stoops were applied in micromorphological studies (Stoops, according to Biscaye (1965). 2003). Radiocarbon analysis was used as a dating tool in soil-sedimentary Morphological results were supported by some analytical data on sections and sedimentary columns. Radiocarbon dating was done in profile distribution of organic C, carbonates etc. Analytical studies were the Institute of Geography, Russian Academy of Sciences (lab index performed using the standard methods (Vorob'eva, 2006): pH was mea- IGAN) by liquid scintillation counting and in Lund University (lab sured in water suspension by potentiometry; total organic carbon — by index LuS) by Accelerator Mass Spectrometry. Calibration of 14C dates wet oxidation with dichromate potassium and concentrated sulfuric was done in an on-line version of OxCal 4.1 (https://c14.arch.ox.ac.uk/ acid; carbonates were determined by alkalimetric titration; total soluble oxcal/OxCal.html, Bronk Ramsey, 2009), using calibration scale IntCal09. salts were estimated from electroconductivity measurements in water The intervals of calibrated age are given with 95.4% probability. Age extract (soil:water = 1:5). After preparatory successive removal of car- scale of the lake development and age of single events in its history bonates by 10% HCl, and organic matter by 30% H2O2, data on particle were determined basing on age–depth diagrams as interpolation size distribution of silicate residue of the sediment were obtained with of sedimentation rates. Calculating error is estimated in ±200– vibroshaker Analysette 3Pro Frisch screen sizing of fractions N0.1 mm, 250 years taking into account data scattering and the width of con- followed with Analysette 22 Frisch laser granulometry of b0.1 mm fidence interval. 102 M. Bronnikova et al. / Catena 112 (2014) 99–111

Fig. 2. Section across the southeastern part of Terekhol' Lake and the adjacent part of the Terekhol Basin bottom (along the line A–Binthefigure 1C). The water level of Terekhol' Lake is accepted as 0 for the height scale. 1 — unsorted stony-gravely Sartan alluvium (MIS 2); 2 — unsorted sandy-gravely Late Glacial alluvium (N12200 cal. yrs BP); 3 — soils buried in stratified silt loams and silts of the Late Glacial–Early Holocene overbank (flooding) alluvium (10500–12200 cal. yrs BP); 4 — base of Holocene lacustrine silts (around 9500 cal. yrs BP); 5 — washed out surface of the Sartan alluvial fan of the Aiyl River; 6 — permafrost top; 7 — permafrost base; 8 — cores and their numbers. Geomorphological elements: (I) Late Glacial erosional hollow, (II) remnant of the Sartan alluvial fan, (III) Late Glacial alluvial fan of the Aiyl River, (IV) Early Holocene thermokarst basin of Terekhol' Lake, (V) Mid-Holocene palsa — frost heaving hill (Por Bazhin Island).

3. Results Pleistocene–Holocene border. The younger soil was formed at the very beginning of the Preboreal period of the Holocene. Though these two 3.1. The delta-alluvial fan: stratigraphy and chronology soils are very close in age, they are highly different in their genesis as it will be demonstrated below. The main generation of the alluvial fan occupies an area of Soil-sedimentary sequences located to the east from the central axis 2.5 × 4 km (Fig. 1C). Its surface is slightly (up to 2–3°) inclined towards of the fan have the same general stratigraphy: coarse Pre-Holocene the lake. General stratigraphy of the fan presented in Fig. 2, Fig. 3A, E basement (Fig. 3E, Unit 6) overlaid with stratified silt loams and silts of demonstrates the major stratigraphic units in the key sections 6 and the overbank facie usually containing one buried soil (Fig. 3E, Unit 5). 233. Coarse-grained unsorted sandy–gravelly alluvium at the basement The C14 dates obtained from the 2 cm roof layer of its Ab horizon (Fig. 2, Fig. 3A, E, Unit 6) is covered with 150–200 cm of clearly stratified get in the interval from 3702 to 4869 cal years ago (IGAN-3967, IGAN- silt loams and silts, sometimes intercalated with sands (the data on 3819 in Table 1). We consider this time span as an approximate date particle size distribution will be presented below). This upper facie of the soil's burial due to shortly re-established overbank sedimentation. resulted from overbank inundation during seasonal floods. It includes Surface soil was formed on this last phase sediment (Fig. 3E, Unit 4). one or two buried soils, which are distinctively different in their abso- The paleosol buried in the eastern part of the fan between 3700 lute age and a degree of the development in the eastern and western and 4870 cal years ago has much more developed, mature profile in parts of the fan (see details below). An approximate border between comparison to the soils buried around the Pleistocene–Holocene border eastern and western parts of the fan is shown in Fig. 1C. in the western part of the fan. In addition to the dates from the roof of Ab The coarse-grained basement of the Ajyl's fan enters more than horizon, the 2 cm layer at the bottom of organo-accumulative horizons 1 km into the lake. It was found under the lake sediments in a number was dated in some eastern sections with mature buried profiles. Over- of bottom cores and on islands (Fig. 2). The roof of this layer under lapping interval for these dates is 8322–8518 cal yrs BP. lake sediments is related to the Younger Dryas time, and it was dated We also obtained two dates from the bottom 2 cm of the surface A at 11,395–12,837 (Table 1). horizons (IGAN-3489, IGAN-3964). They are young and considerably As it was mentioned above, there are two successive buried soils in different (Table 1). Generally these two dates testify on pretty high subaerial soil-sedimentary sequences of the alluvial fan, within its strat- rates of humus rejuvenation in studied soils. This process is especially ified silt loams and silts of the delta-floodplain overbank facie. These soils fast in soils with strongly developed contemporary Natric features in the eastern part of the fan are related to the Pleistocene–Holocene bor- (the surface soil in section 1), which supposes high mobility of humus der. The older (deeper) of them is wider spread, and better developed in a profile. High rates of humus rejuvenation allow supposing that (Fig. 3A, Unit 3). The younger (upper) one was found in 7 of the 10 stud- soil formation started at the Pleistocene–Holocene border both in the ied sections (Fig. 3A, Unit 2). In the sections where both soils occur, they western and eastern parts of the fan. Pedogenesis at the Pleistocene– are delimited by 20–50 cm of bedded fluvial loams. Clear layering of this Holocene transition was twice interrupted by floodplain sedimentation material testifies on high rates of sedimentation that means very short in the western part of the fan, so that two environmentally different period of time between the two successive phases of pedogenesis. The phases of pedogenesis are well documented in the western sections. dates obtained from these soils have given in some cases overlapping in- At the same time in the eastern part of the fan pedogenesis was not tervals of calibrated C14 age (Table 1). The overlapping interval of cali- interrupted by sedimentation at the Pleistocene–Holocene transition. brated dates obtained from the older soil (IGAN-3407 and IGAN-3441) Thus there are no separate buried soils related to that period in eastern is 11,248–11,802. The narrowest, overlapping interval of three dates sections. The same general stratigraphical pattern in eastern and obtained from a younger soil is 10,565–11,220 (IGAN-3428, IGAN- western parts of the alluvial fan together with different age and maturity 3885, IGAN-3976) (Table 1). Thus the older soil falls exactly on the of buried soils in different parts of the fan could result from west to east M. Bronnikova et al. / Catena 112 (2014) 99–111 103

Fig. 3. Macromorphology of the sections 6 and 233: A. — section 6, B — small deformations and disruptions of 2Ab@ horizon in the older buried soil (Unit 3), C — partial distruction of the 2Ab@ horizon in the Unit 3, D. —syn- and epigenic wedges in the 2Ab@ horizon of the Unite 3, F — section 233. Chrono-stratigraphic units for 3A and 3F — Units 1–5 — overbank silt loams and silts intercalated with sands: Unit 1 — Whole-Holocene surface soil; Unit 2 — Younger soil of Pleistocene-Holocene transition; Unit 3 — Older soil of the Pleistocene-Holocene transition; Unit 4 — Surface soil of approximately last 5000 years; Unit 5 — soil of the first two thirds of the Holocene. Unit 6 — Unsorted sand-gravel basement of the delta-alluvial fan. shift of the river channel and the sedimentation zone of seasonal floods 4Cf horizon). Both soils are developed within the delta-floodplain silts related to it. Thus the river channel was shifting to the west in the first and silt loams (Fig. 4). Soil forming material for the lower paleosol is part of Holocene, and over-bank inundations buried soils in the eastern relatively homogenous weakly stratified silt loam. The upper paleosol part of the fan around 3700–4870 cal. of the first two thirds of the is formed within fine and sharply stratified silts and silt loams (width Holocene. of a strata is less than 2 cm). A stratification of the material above the younger paleosol is not that sharp and relatively coarse (width of lami- 3.2. Buried soils of the Pleistocene–Holocene transition nae is 1–10 cm). The over-permafrost soil sedimentary sequence 6 has the following Soil-sedimentary sequences on the delta alluvial fan in its western horizontal arrangement (Fig. 3A) Anz@-2ABknz-2Bknz-2Anzb-2Bknb1- part will be examined with an example of section 6: N 50.60355° E 2Bknb2-2Cn-2C-2A@b-2Bkgb-2Bgb-3BCgb-4Cf. Surface soil (Fig. 3A, 97.39338° (Fig. 1C). Based on field description and grain size analysis, Unit 1) is classified as Calcic, Molliglossic, Salic, Natric, Turbic Cryosols a stratigraphy of section 6 is as follows. There is a coarse-grained layer (Calcaric, Magnesic). in the basement of the section, right above the permafrost table The upper (younger) buried profile (Fig. 3A, Unit 2) is classified (Fig. 3A, Unit 6). It is represented by unsorted gravelly sands (Fig. 4, as Protovertic Protocalcic Fluvisol (Calcaric). The lower (older) soil 104 M. Bronnikova et al. / Catena 112 (2014) 99–111

Table 1 Results of radiocarbon dating.a

Lab index Section Depth, Areal and morpho-stratigraphic Median Radiocarbon Calibrated date, cal ВР number cm allocation age, BP (probability 95.4%)

IGAN-3489 6 17–20 E.b p., surface soil, Anz hor. (bottom) 832 910 ± 50 734–926 IGAN-3964 233 21–24 W. p., surface soil, A hor. (bottom) 2233 2240 ± 70 2046–2359 IGAN-3967 233 40–43 W. p., Ab@ hor. (roof) 3910 3600 ± 70 3702–4090 IGAN -3968 233 85–88 W. p., ABb@ hor. (bottom) 8116 7300 ± 90 7963–8322 IGAN -3975 233 93–96 W. p., ABkb@ hor. (bottom in a wedge) 8316 7510 ± 90 8072–8518 IGAN -3819 214 90–92 W. p., Ab hor. (roof) 4701 4180 ± 80 4446–4869 IGAN -3441 1 110–120 E. p., the older buried Ab hor. 11,338 9870 ± 110 10,878–11,802 IGAN -3428 6 70–72 E. p., 2Anzb 11,951 10,270 ± 880 9541–14,471 IGAN -3407 6 118–128 E. p., 2A@b 11,731 10,120 ± 130 11,248–12,371 IGAN-3885 192 86–90 E.p., The younger buried A hor. 11,112 9730 ± 190 10,565–11,810 LuS-8398 M-6 705 Por-Bazhyn island, the roof of coarse-grained 12,241 10,460 ± 250 11,395–12,837 layer under lake sediments

a All dates except LuS-8398 were obtained for specimens of humic acids, the date LuS-8398 was got for a stem of Comarum sp. extracted from the sediments. b E.p. — east part of the alluvial fan, W.p. — west part of the alluvial fan.

(Fig. 3A, Unit 3) is classified as Protocalcic Protomolliglossic Turbic creep and solifluction are common for these soils (Fig. 3B, C). Involutions Cryosol (Calcaric, Reductaquic, Thixotropic, Endosceletic). occur in 2A@b horizon at microlevel (Fig. 5B). The surface soil has strong manifestations of solonization (alkalization, 2A@b horizon has a thick platy to angular blocky secondary cryogenic accumulation of exchangeable sodium and magnesium, consolidation macrostructure obtained during diagenesis (Fig. 5C). At the same time and formation of massive prismatic or columnar structure in Bn horizons, there are zoogenic chambers containing welded excrements (Fig. 5D) redistribution of colloids), and salinization (accumulation of readily solu- referred to former biogenic activity. Zoogenic pores and excrements ble salts) processes based both on soil morphology and analytical proper- are sometimes deformed by later cryo-diagenesis. Microstructure of the ties. Diagnostic features of Calcic, Natric and Salic horizons are overlapped first level is well developed, moderately separated granular (Fig. 5E). here within 2ABknz and 2Bknz horizons. Distinctive morphological B horizons have features related to seasonal over-permafrost water features of 2ABknz are dark grayish brown color, highly consolidated logging: diffuse gray or olive mottling related with Fe oxide redistribu- (when dry), massive prismatic structure. There are dark colored, nearly tion, and small Fe–Mn nodules. Besides that B horizons reveal accumu- isotropic clay-humus coatings found at microlevel. lation of secondary carbonates: irregular micritic impregnation of the pH values are strongly alkaline varying between 9 and 10 (Table 2). calcitic crystallitic b-fabric, and rare micritic nodules and coatings. Content of organic carbon in the top horizon is 2.52%. That is really Only relatively stable analytical characteristics are under consider- low compared with neighboring soils nearly untouched by solonization, ation here for paleosols: content of Corg, particle size distribution, where contemporary driving process is Mollic humus formation (6.62% content of CaCO3, total content of iron, contents of dithionite and oxa- in A horizon of section 233). Strong dark staining of water extracts late extractable iron (where it was determined). At the same time pH of Anz@, 2ABknz horizons testifies on high solubility and mobility of values, exchangeable bases, and conductivity of water extract are not humus in strongly alkaline environment. discussed for paleosols, as these parameters characterize contemporary

Maximal content of CaCO3 (22.40%) is located in 2ABknz horizon coin- soil conditions. ciding with maximums of exchangeable sodium (55.7% of exchangeable Content of organic carbon (Table 2) in 2A@b (the top horizon of bases) and conductivity (means readily soluble salts) — 2.08 mS/cm. the older soil) is relatively high for 11–12 thousand year old buried Exchangeable complex of this soil contains up to 58.2% of Mg2+.Soda soil (1.21%), and it decreases gradually with depth. and NaCl prevail among readily soluble salts. Content of carbonates is relatively high. It demonstrates clear Top horizon of the surface soil – Anz@ – (upper 10 cm of it) clearly profile differentiation with the minimum content in 2A@b horizon differs in particle size distribution from subsurface horizons. This litho- and the maximum — in 2Bkgb horizon, where maximum of second- logical discontinuity makes impossible to use the data on particle size ary carbonates is observed morphologically. It is clear that part of distribution in diagnostic of pedogenic textural differentiation. But the secondary carbonates is diagenic. But the distribution pattern allows abovementioned occurrence of clay-humus coatings testifies on contri- one to suppose conditions favorable for eluviation of carbonates bution of pedogenesis in redistribution of clay fraction in surface soil. fromthetophorizonandtheiraccumulationin2Bkgbhorizon The data on the composition of clay minerals reveal strong impover- (means enough atmospheric precipitation and satisfactory intrasoil ishment of Anz@ horizon with expandable minerals at high content of drainage at a part of a year). illites, and gradual rise of labile silicates below. Such distribution pattern Particle size distribution within the older buried soil is mainly of clay minerals is typical for soils subjected to solonization process explained with variations of sedimentation processes. Sand fractions (Sokolova et al., 2005). (2000–63 μm) have a tendency to increase gradually with depth at As it was mentioned above, two buried soils related to Pleistocene– slow decrease of the clay fraction (b2 μm). Distribution of the clay frac- Holocene border are sharply different in their morphology and genesis tion demonstrates a small rise in 2Bkgb and the upper part of 2Bgb in spite of very close intervals of calibrated age. The older soil (Table 1: horizons. This rise is considered to be also lithologically conditioned, 11,248–11,802 cal yrs, IGAN-3428) has very dark brownish-black, taking into account absence of any illuvial clay coatings in these sticky, clearly thixotropic Mollic horizon with an irregular boundary of horizons. relatively shallow (about 40 cm) pocket-like wedges. There are numer- There is no clear profile differentiation of clay minerals in the older ous and variable cryogenic features both at macroscopic and at micro- of buried soils (starting with 2A@b horizon) (Table 3). Existence of scopic levels. Together with syngenetic wedges, epigenetic wedges physical splitting up at very low rates of chemical weathering is charac- and cracks also occur in the soils (Fig. 3D). The last one often infilled teristic of permafrost-affected soils (Alekseev et al., 2003). There is less with sandy and gravelly material derived from frost sorting. Frost than 1% of dithionite extractable iron in B and BC horizons at 7–8% sorting is also exhibited microscopically in micro-zones with linear, of bulk Fe2O3 (Table 2) that also corresponds to the idea of very low clustered and circular distribution pattern of coarse grains (Fig. 5A). rates of chemical weathering supported by possible spring lateral Deformations, disruption and partial or total destroying due to frost removal of reduced iron along the permafrost table. M. Bronnikova et al. / Catena 112 (2014) 99–111 105

as syngenetic wedges, signs of active cryoturbation or soliﬂuction, which are so typical of all other buried and surface soils of the basin. So that the traces of ice lenses actually could be epigenetic (formed in later phases of pedogenesis). Proﬁle is clearly differentiated on the content of carbonates with a tendency to their eluviation from the top horizon (Table 2). Impregnating accumulation of secondary carbonates in 2Bknb1–2Bknb2 horizons is moderate. It is comparable to the content of calcium carbonate in 2Bkgb horizon of the older buried soil and 2.5–2 times less comparable to 2Bknz horizon of the surface soil. There is a spectacular accumulation of expandable minerals in the younger buried soil and especially in its 2Anzb horizon (Table 3). Expandable phase is strongly dominated among clay minerals (up to 50%).

3.3. Buried soils related to the ﬁrst two thirds of the Holocene

Soils on the eastern part of the fan are represented by section 233, N 50.60443 E 97.40260 (Fig. 1C). It has the following over-permafrost profile A-AB@-Bk-Ab@-ABkb@-Bkgb@-2BkCg-2Cf. Surface soil is Calcic Molliglossic Turbic Cryosol (Calcaric, Magnesic). The driving soil forming processes here nowadays are accumulation of Mollic humus and redistribution of carbonates. As opposed to strong evidences of the actual solonization process in the surface soil of section 6, subangular blocky structure and enhanced density are residual in this profile. These features owing to a high share of sodium among exchangeable cations are not supported by contemporary environment. There are low contents of exchangeable sodium (0.3–1.2% of exchangeable bases) and slightly acid to slightly alkaline pH values in the surface profile (Table 2). Buried soil starts from the depth about 40–55 cm. It is classified as Calcic Molliglossic Turbic Cryosol (Calcaric, Reductaquic, Endosceletic). Coarse grained basic layer of the fan is found here at about 1 m from the surface (Fig. 3E, Unit 6). These sandy gravels were covered with very thin layer of early Holocene sandy and gravely silt loams served as a parent material for the buried soil. The last one was overlaid by silt loams resulted from renewed seasonal overbank inundation over the alluvial fan in the middle of the Subboreal. Buried soil here has well developed mature profile as opposed to the Fig. 4. Particle size distribution, section 6. soils of the Pleistocene–Holocene transition. Its Ab@ horizon is very dark brown to black. Highly developed fine granular structure (Fig. 6A) is arranged here in moderately developed subangular blocks (Fig. 6B). At the same time in the surface Mollic horizon granular structure is Generally, in this profile, as well as in the younger paleosol and in not that well shaped and there are no secondary aggregates (Fig. 6C). the surface soil, distribution of total iron is more related with changes There is more than 5% of organic carbon (Table 2) in Ab@ horizon that in the clay content, than to distribution of faint redoximorphic fea- is comparable to its content in the surface A horizon. Numerous features tures. Distribution of oxalate and dithionite extractable forms also related to biological activities are registered in the humus accumulative lacks clear iron impoverishment of B horizons in the older paleosol horizons both in buried and surface soils (Fig. 6D). where redoximorphic features were registered morphologically. Both upper and lower borders of the Ab@ horizon are wedged. There These data testify on short-term redox processes redistributing are involutions at the lower limit of the horizon (Fig. 3E, Unit 5). There iron mostly within a single horizon without a considerable lost. are also strong turbations at the border of the coarse-grained basement The younger (second) buried soil in section 6 is actually embryonic and overlaying silt loams. one. That corresponds to a very short period of its formation. It has a Bk horizon of the buried soil reveals a considerable accumulation of thin, brownish top horizon 2Anzb. There are no obvious signs of former calcium carbonate: 16.5% comparatively to 4.8% in Bk of the surface soil. biological activities in it. Only a slight increase of organic carbon content Morphological forms of calcite are numerous and variable here. These (Table 2) indicates the in situ humus accumulation here comparatively are micritic coatings (Fig. 6E) and infillings (Fig. 6D), very thin micritic to overlying and underlying horizons. coatings surrounding moderately developed and weakly separated 2Anzb horizon of this soil is characterized by a specific morphology granular aggregates (Fig. 6F), calcite-encrusted biogenic structures, of combined vertic and cryogenic features. It has strongly developed and calcified plant tissues. Whereas in Bk horizon of the surface soil very fine highly separated inclined angular blocky or lenticular pedality. secondary carbonates are only represented by small (about 0.2 mm in Peds are extremely hard: it is impossible to crash aggregates by fingers. diameter) micritic nodules (Fig. 6G) (carbonates of calcitic crystallitic Pedal material has a compacted striated or cross-striated b-fabric b-fabric are at least partly lithogenic). (Fig. 5F). In some cases there are traces of ice lens disturbing inclined There are relatively abundant redoximorphic pedofeatures blocks or lenticles and transforming them into granules (Fig. 5G). (small impregnative nodules) found in all horizons of these buried At the same time there are no other obvious cryogenic features such soils (Fig. 6H). 106 M. Bronnikova et al. / Catena 112 (2014) 99–111

Table 2 Analytical characteristics of pedo-sedimentary sequences.

Horizon Depth, cm pH Corg., % CaCO3, % EC of water extract, Exchangeable bases, % Total exchang. Fe2O3 d, % Fe2O3 ox, % Fe2O3 total, % mS/cm bases mEq/100 g Ca2+ Mg2+ K+ Na+

Section 6 Anz@ 0–17 9.2 2.52 3.09 1.106 22.7 29.1 3.3 44.9 12.35 –– 4.96 2ABknz 17–35 9.9 1.05 26.40 2.080 4.1 38.8 1.4 55.7 19.33 –– 5.73 2Bknz 35–70 10.1 0.53 12.26 0.997 5.9 53.4 1.1 39.6 13.48 –– 5.97 2Anzb 70–72 9.7 0.81 4.02 0.865 7.7 50.9 1.1 40.3 24.56 –– 8.32 2Bknb1 72–80 10.1 0.42 10.60 0.490 16.7 52.5 1.1 29.7 13.15 –– 6.00 2Bknb2 80–95 10.1 0.39 10.78 0.320 22.4 58.2 0.9 18.5 11.17 –– 5.57 2Cn 95–102 10.0 0.57 8.94 0.330 29.2 46.6 1.0 23.1 12.66 –– 6.32 2С 102–118 9.6 0.79 8.65 0.245 35.7 50.3 0.8 13.2 15.70 –– 6.88 2A@b 118–128 9.4 1.21 10.60 0.260 33.6 54.9 1.1 10.4 17.85 0.36 0.88 6.65 2Bkgb 128–145 9.4 0.50 16.12 0.194 54.4 35.9 1.3 8.4 10.30 0.27 0.98 6.31 2Bgb 145–184 9.2 0.32 11.42 0.159 45.5 50.00 1.5 3.0 11.00 0.29 0.82 6.34 3BCgb 184–200 9.4 0.32 12.69 0.181 36.0 58.1 1.5 4.4 12.21 0.30 0.78 5.32

Section 233 A0–15 6.7 6.62 0.07 0.288 64.5 34.8 0.5 0.3 30.43 –– – AB@ 15–24 7.7 4.66 0.14 0.326 74.9 23.9 0.5 0.7 27.44 –– – Bk 24–40 8.5 4.02 4.84 0.436 77.6 22.0 0.3 0.1 25.78 –– – Ab@ 40–66 8.3 5.28 3.05 0.401 86.9 12.0 0.5 0.6 26.79 –– – ABkb@ 66–88 8.7 2.65 1.39 0.331 62.1 37.0 0.7 0.2 24.36 –– – Bkgb@ 88–112 9.0 1.19 16.46 0.587 46.5 51.4 0.9 1.2 16.49 –– –

– not determined.

4. Discussion: Pleistocene–Holocene environmental changes in the important to mention that the dates obtained from the 2 cm roof Terekhol' Basin layer of Ab@ horizon are considered to be approximate dates of re- established sedimentation. This impulse buried a soil being formed 4.1. Fluvial activity during the ﬁrst 6–8 kyr of the Holocene.

Retrospective analysis of the river Ajyl basin brought following 4.2. Pedogenesis results (Panin et al., 2012). The Terekhol' Basin was characterized by high fluctuations of river discharge at the end of the Late Pleistocene: As it was already mentioned the Pleistocene–Holocene border was during the Late (Sartan) chryochrone (MIS-2). During MIS-2 the Ajyl marked in the Terekhol' trough by the general climatically conditioned River accumulated a fan overlying the back side of the basin bottom decline of fluvial processes. The transition period is characterized (Fig. 1C). It is supposed to be related to the Last Glacial Maximum by confrontation between fluvial sedimentation and pedogenesis at (LGM, 20–23 thousand years ago cal.) which is regarded as a period of general decrease of fluvial processes. Two successive short phases of strong aridization in South Siberia (Vorob'eva, 2010; Zykin et al., pedogenesis left different paleosols recording a contrast environmental 2002), with characteristic drop of surface and river runoff and enhanced change. accumulation in river valleys (Spasskaya, 2009). This fan was eroded A set of morphological and analytical characteristics in the older after LGM as a result of an enhanced river runoff in the Late Glacial paleosol of the Pleistocene–Holocene transition could be formed in (the event hasn't been dated exactly). The only remnant of the Sartan permafrost-affected environment with relatively shallow permafrost fan has been preserved in the form of a narrow ridge parallel to the table restricting vertical drainage. It is obvious that dry material is not basin side. frost-susceptible. If the water supply is small, no “wet” cryogenic pro- The maximal intensity of fluvial processes in the basin was reached cesses such as segregation of ice, heaving, cryoturbation and solifluction between LGM and the beginning of Holocene. Gravelly sands at the take place (Van Vliet-Lanoё,1998;VanVliet-Lanoё et al., 2004). Thus basement of the alluvial fan (Fig. 3A,E, Unit 6) were accumulated numerous features related to turbation and solifluction in the older at that time. Termination of accumulation of coarse-grained material buried soil prove a sufficient water supply of the soil: namely conditions was imprinted in buried soils of the Pleistocene–Holocene transition. at least seasonally close to field water capacity. Features related to redis- This change is interpreted as a climatically conditioned decrease of the tribution of iron oxides also support seasonal over-moistening and re- river runoff. ducing conditions. At the same time seasonal over-moistening hardly In the Holocene, fluvial sedimentation has never reached its Late was continuous taking into account faint redoximorphic features and Glacial intensity and was characterized by only few short and relatively absence of noticeable impoverishment of subsurface horizons by iron weak impulses of seasonal flood activity. A very short sedimentation compounds. Profile distribution pattern of carbonates supposes their spike terminated the earlier phase of soil formation, and isolated the possible seasonal lateral influx and impregnative accumulation followed older soil in the western segment of the fan (Fig. 3A, Unit 3). Then soil by eluvo-illuvial redistribution in summer when the soil was well formation was re-established for a period of few centuries or even de- drained. There are also features evidencing high biological activity such cades, and again it was interrupted by a short period of sedimentation as dark color of the 2A@ horizon, relatively high for 12,000 year old around 11,000 cal years ago, during which the younger soil (Fig. 3A, soil content of organic carbon, biogenic pores, and excrements. Unit 2) was buried in the western segment of the fan. Granular structure was described in the 2A@b horizon of the In subaerial soil-sedimentary sequences of the alluvial fan the older soil of transitional period. Granular structure is very common for decrease of high fluvial activity is indicated, as it was mentioned Cryosols and supposed to be originating from frost heave and/or from above, by stratified silt loams and silts of the delta-floodplain overbank lateral displacement by solifluction (Van Vliet-Lanoё, 1998, 2010; Van facie with the two successive buried soils of the Pleistocene–Holocene Vliet-Lanoё et al., 2004). Some authors attribute the formation of well border. rounded aggregates to gelifluction in water-saturated environments One more registered impulse of fluvial sedimentation took place (Smith et al., 1991). The granular structure in studied soil most probably between 3700 and 4870 cal years ago (IGAN-3967, IGAN-3819). It is has dual genesis. It was created and supported both by zoogenic activities M. Bronnikova et al. / Catena 112 (2014) 99–111 107

Fig. 5. Buried soils of the Pleistocene–Holocene transition (section 6): micromorphological features. A. 2Bgb horizon: linear distribution of coarse grains, X. B. 2A@b horizon: involution of enriched in humus (darker) and enriched in clay material, ||. C. 2A@b horizon: 2nd order angular blocky structure, ||. D. 2A@b horizon: chamber with welded excrements, ||. E. 2A@b horizon: 1st order granular structure, ||. F. 2Anzb horizon: inclined subangular blocky structure, cross-striated b-fabric, ||. G. 2Anzb horizon: inclined lenticular aggregates (a) and granular aggregated (b), ½ X. H. 2C horizon: internal fabric of a lamina with shattered charcoal particles, ||.

(clearly detected by zoogenic pores with excrements) and by well are variable herbs that occur in the spectrum: Rosaceae, Caryophyllaceae, documented turbation and solifluction. Ranunculaceae, Thalictrum sp., Rumex sp., Fabacea, Lamiaceae, Primulaceae, Strongly stratified overbank sediments, which cover the older Geranium sp., Viola, Compositae,andCirsium. Such a spectrum testifies on soil, contain continuous lamina rich in frost shattered charcoal climatically warm and mild conditions in the basin during the layer's ac- particles (Fig. 5H). This lamina occurs in a number of sections. Possibly cumulation which is in good correspondence with regional palynological it records a prominent fire event which occurred at the Holocene data. Time border 11,000 cal yrs BP considered as a starting point of early transition. Holocene afforestation: expansion of dark-coniferous forests in the South From the layer of overbank sediments at the contact with the older Siberian Mountains (Blyakharchuk et al, 2004, 2007). Thus afforestation buried soil statistically valid pollen data were obtained (layers above of mountain surrounding of the basin took place, induced by general and below this contact were very poor in pollen grains). Arboreal pollen climatic amelioration, but basing on obtained soil data we could say prevails here (67.1%). Picea obovata makes 32% of arboreal sum. There that the landscapes within the basin stayed open. 108 M. Bronnikova et al. / Catena 112 (2014) 99–111

Table 3 Mineralogical composition of clay fraction (b1 μm) in section 6.

Horizons Depth, cm Composition of minerals: Mca — mica, Chl — chlorite, Kln — kaolinite, Sme — smectite, Groups of clay minerals, % Vrm — vermiculite, Ild — interlayered, Qtz — quartz, Fsp — feldspars, Cal — calcite, Dol — dolomite Sme + Ild Mca Kt + Chl

Anz@ 0–17 Mca, Chl (Mg) or Chl-Vrm, Kln, Ild Chl-Sme, Qtz (tra), Fsp (tr), Dol (tr) 8 60 32 2ABknz 17–35 Ild Chl-Sme, Chl (Mg), Mca, Kln, Qtz (tr), Fsp (tr), Cal (tr), Dol (tr) 37 21 42 2Bknz 35–70 Sme, Chl (Mg) or Chl-Vrm, Mca, Kln, Qtz (tr), Fsp (tr), Cal (tr), Dol (tr) 37 31 32 2Anzb 70–72 Sme, Chl (Mg) or Chl-Vrm, Mca, Kln, Qtz (tr), Fsp (tr) 50 26 24 2Bknb1 72–80 Sme, Chl (Mg), Mca, Kln, Qtz (tr), Dol (tr) 42 32 26 2Bknb2 80–95 Sme, Chl (Mg), Mca, Kln, Qtz (tr), Fsp (tr), Dol (tr) 42 29 29 2Cn 95–102 Sme, Chl (Mg) or Chl-Vrm, Mca, Kln, Qtz (tr), Fsp (tr), Dol (tr) 48 27 25 2C 102–118 Sme, Chl (Mg) or Chl-Vrm, Mca, Kln, Qtz (tr), Fsp (tr) 45 32 23 2A@b 118–128 Sme, Chl (Mg), Mca, Kln, Qtz (tr), Fsp (tr), Dol (tr) 35 41 24 2Bkgb 128–145 Sme, Chl (Mg) or Chl-Vrm, Mca, Kln, Qtz (tr), Fsp (tr), Cal 27 39 34 2Bgb 145–184 Sme, Chl (Mg), Mca, Kln, Qtz (tr), Fsp (tr), Dol (tr) 33 40 27 3BCgb 184–200 Sme, Chl (Mg), Mca, Kln, Qtz (tr), Fsp (tr), Dol (tr) 43 32 25

a tr — traces.

The younger paleosol has nearly the same radiocarbon age but Thus the short second phase of pedogenesis at the very beginning of strongly different set of features. This embryonal soil is characterized the Holocene characterized by above-described set of features could be by distinctive accumulation of smectites in it and especially in its reasoned by warming which reduced intensity of cryogenesis and gave 2Anzb horizon. Neoformation of smectites is one of possible reasons way to formation of vertic features, and/or by aridization restricted of their high content in 2Anzb horizon. It is not an evidenced fact but annual water supply and made soil less frost-susceptible. This phase of we cannot exclude it basing on following general considerations. A soil formation was preceded by a short ecological arid stress with exten- principal possibility of smectite neoformation in soils with vertic fea- sive fires and an episode of intensive post-fire erosion–sedimentation. tures and generally in soils with high pH values and abundant magne- Most probably both warming and aridization took place. But the most sium cations was reported earlier (Claridge and Campbell, 2004; important change seems to be in a distribution of annual precipitation Coulombe et al., 1996; Nguetnkam et al., 2007). As far as all parent ma- sum and changes in hydrology of the area. At the background of general terial of studied alluvial soil-sedimentary sequences originates from the warming, seasonality of water supply was enhanced due to a rise of common catchment area, mineralogical composition within the section winter precipitations, a drop of summer precipitations and increased was suggested to be originally similar. We find logical the assumption summer temperatures. These climatically induced changes stimulated on possible partly pedogenic nature of smectites in the Stratigraphic the sharp changes in the character of pedogenesis from cold, cryogenic, unit 2 (Fig. 3A), and especially in the horizon 2Anzb. Processes of chem- permafrost-affected, relatively balanced in water supply with seasonal ical weathering are known to be accelerated in some arid frost-affected over-permafrost water logging to warmer, arid with clear summer– soils in the presence of abundant magnesium ion and high pH (Claridge autumn water deficit and possible spring water saturation. and Campbell, 2004). Such conditions are for an instance realized in Paleosols of the first two thirds of the Holocene have well soils surrounding salt lakes (Furquim et al., 2010). Moreover there are developed profile which is generally similar on its genesis both some evidences of smectite neoformation in permanently frozen to the older paleosols of the Pleistocene–Holocene transition and to ground (Vogt and Larqué, 1998, 2002). Contents of total magnesium the surface Calcic Molliglossic Turbic Cryosol. They differ from the in section 6 are relatively high varying between 2.98 and 7.78%. older soils of Pleistocene–Holocene border mostly by much more de- Hydrocarbonate-magnesium water of the Terekhol' Lake nowadays veloped profile where “Proto”-features had developed to its mature has major mineralization varying between 113 and 169 mg/l, but the state. These paleosols differ from the surface non-solonetzic soils lake could be more saline in more arid phases. Accumulation of smec- by strongly developed granular pedality not only in its top horizon, tites stimulated development of Protovertic features in 2Anzb horizon but also in subsurface horizons and by numerous variable calcitic of the younger buried soil such as high degree of inclined angular- pedofeatures. We regard both of these features as evidences of better blocky pedality, extreme hardness of peds, and cross-striated b-fabric. water supply of this paleosol comparatively to the surface non- Vertic features being normally attributed to subsurface horizons are solonetzic soils. not usual in topsoil, though these features were reported both in A ho- Actually the relations between morphology of secondary calcite rizons of surface and buried soils in coastal landscapes and floodplains and conditions of its formation are not studied enough and to some (floodplains; Imbellone and Mormeneo, 2011; Solleiro-Rebolledo extent contradictory (Durand et al., 2010; Kovda, 2008). Applying the et al., 2011). It should also be stressed that generally vertic features actualistic approach we conclude that such a variety and abundance are regarded to be quite exotic in permafrost affected soils. Recently of secondary calcite in surface soils of the basin are found only in con- soils with vertic properties were discovered in a neighboring Trans- ditions of contemporary or recent additional (lateral) water supply: Baikal region of the South Siberia (Kovda and Lebedeva, 2012). Vertic under a mountain slope and on soils of palsa islands. That means that Cryosols are supposed to be related there with warming of the last soils had been formed during the first two thirds of the Holocene, and decades. had better water supply than contemporary soils in the same positions. At the same time some features normally regarded to be vertic, such Thus, comparatively to contemporary surface soils, the soils formed as stress coatings, striated b-fabric, and high contents of smectites, in in the first two thirds of the Holocene and buried about 4000–5000 cal certain environments are referred to cryogenic horizons (Claridge and yrs BP were formed in conditions of better, well seasonally-balanced Campbell, 2004; Van Vliet-Lanoё,2010). Actually there is no real genetic water supply. There are evident features related to a high biological contradiction between cryogenic and vertic features. Arid or semi-arid activity, intensive “wet” cryogenic processes, additional (lateral) water cold climate with strong seasonality and available source of alkaline supply and related seasonal short-term water logging. These soils salts and magnesium ions favors to accumulation of pedogenic smec- buried by the last considerable sedimentation event over the fan tites. Shrinking–swelling ability of soils rises. As far as freezing means never have any manifestation of solonization process, as opposed to first of all desiccation, labile phase looses water and shrinks, and for- contemporary soils, where Natric features are widespread together mation of ice lenses at the same time provokes stress deformations. with signs of progressive salinization. There is no blocky or columnar In spring time ice lenses melt and expandable clays swell. structure, no high density, and no pedogenic textural contrast in buried M. Bronnikova et al. / Catena 112 (2014) 99–111 109

Fig. 6. Buried soils of the first two thirds of the Holocene (section 233): micromorphological features. A. Ab@ horizon: 1st order granular structure, ||. B. Ab@ horizon: 2nd order blocky structure, ||. C. A horizon: granular structure, ||. D. ABkb@ horizon: chamber with excrements and micritic infilling, ||. E. ABkb@ horizon: micritic coatings, X. F. Bkgb@ horizon: moderately developed weakly separated granular structure and crystallitic b-fabric and strong micritic impregnation, X. G. Bk horizon: crystallitic b-fabric and micritic nodule, X. H. Bkgb@ horizon: Fe nodules, ||. soils. So that most probably alkalization, solonization and salinization exchangeable sodium and magnesium, alkalinization, risen mobility of are the processes related to Subboreal and Subatlantic time of increased clay and humus, consolidation, formation of massive columnar struc- continentality and aridization. ture) followed by salinization (accumulation of readily soluble salts) of Surface soils of the delta-alluvial fan in its western part have nearly former Calcic horizon developed resulting from progressive aridization the whole Holocene age. These soils demonstrate obvious evidences of and a rise of continentality. Actually equilibrium within this triad of polygenesis. That means that a set of their characteristic morphological processes in the ultracontinental arid–subarid landscapes with close and analytical features is formed within more than one evolutionary permafrost table seems to be very labile. In such landscapes these pro- stages coursed by successive environmental changes. As it was cesses are especially sensitive to general fluctuations of water supply discussed earlier, diagnostic features of Calcic, Natric and Salic horizons (as a cumulative result of atmospheric precipitation and its seasonal dis- are overlapped here within a single B horizon. Normally these features tribution, changes of base level of erosion, factors of atmospheric water may occur within a single profile, but they are distributed in a depth redistribution: depth of permafrost table and its roof topography etc.). from top to bottom of a profile as Natric, Calcic, then Salic horizons Typical for Solonetz and diagenetically resistant features such as specific (features). Occurrence of these contradictory features within a single massive blocky or columnar structure, high density, sharp decrease of horizon of the surface soils could be considered as an evidence of their labile phase at risen illite in the composition of clay minerals, have not polygenesis. We suppose that secondary solonization (accumulation of been found either in soils of Pleistocene–Holocene border or in soils 110 M. Bronnikova et al. / Catena 112 (2014) 99–111 buried about 4000 years ago. This fact allows supposing that the time regime. Progressive aridization started about 6200 cal yrs BP. Most before 4000 BP was relatively more humid and more balanced in arid phases occurred 6.2–3.8 and 2.0–0 cal yrs BP. In these phases water supply (with the exception of the short period of sharp newly formed permafrost elevated numerous palsa-islands due to aridization imprinted in the younger paleosols of the Pleistocene– reach-through freezing of the lake and lacustrine sediments. The phase Holocene transition). Only the last 4000 years gave way to processes 3800–2000 cal yrs BP was some more abundant in water. The last of solonization and salinization. 2000 years are characterized by the most severe and steady aridization. The lake level dropped and its flowage decreased considerably. In soil 4.3. History of the Terekhol' Lake and its islands mantle this last aridization is imprinted in a wide distribution of Natric features and in progressive salinization. Together with soil-sedimentary sequences of the delta-alluvial fan, We can conclude that general trends of climatic changes recorded lacustrine sediments were studied in cores from the bottom of the in fluvial soil-sedimentary sequences and in lacustrine sediments Terekhol' Lake and on palsa-islands, as well as soils developed on the of the basin are the same: unstable humidification of the first half of islands of different height-age groups. A story of lacustrine sedimenta- the Holocene, the warmest and most humid interval in the middle of tion, appearance and following evolution of islands and their soils Holocene, increasing aridization in its second half and especially in the were reconstructed basing on litho-stratigraphic studies of dated sedi- last 2000 years. Similar general trends were described earlier basing mentary columns, supported by the data on bulk chemical composition. on studies of different sedimentary and paleosol records in Tyva, Complex group analysis of biological composition supplied with ecolog- Altaj, Trans-Baikal region, Mongolia (Vipper et al., 1976; Rudaya ical analysis was applied for diagnostics of environmental changes in et al., 2009; Dergacheva et al., 2006; Blyakharchuk et al., 2004, the lake and on islands. The method was developed by N.V. Korde 2007, 2008; Schwanghart et al., 2009; Vorob'eva, 2010). (1960) for lacustrine sediments, and later supplemented and success- In spite of revealed similar general regularities, lacustrine record dif- fully approved for other water-related environments (Skrypnikova fers from paleosol one in some important details. The most contradicto- et al., 2011). These data were published in Bronnikova et al. (2010) ry is the Pleistocene–Holocene transition and particularly Pre-Boreal and Panin et al. (2012). Here we only cite general conclusions and period. There are a lot of discrepancies among researches in the under- examine their correlation with above-stated paleopedological results. standing of this epoch. This is explained most probably by drastic, fast, The Terekhol' Lake has thermokarst–alluvial-imponded origination. and oscillatory climatic changes at that time. Lacustrine record of the It appeared 11,000 cal yrs BP as a result of permafrost melting provoked Terekhol' Basin testifies on a short but intensive rise of water influx. by abundant river inundations. Five periods of enhanced river discharge Actually that was the time of the establishing and first expansion of and high lake water level were revealed: 11,000–10,500, 9500–9300, the lake. This event was related to melting of the permafrost due to 7300–6200, 3800–3500 and 2700–2300 cal yrs BP. It was proved that both early Holocene climatic warming and enhanced seasonal inunda- the described fluctuations of water discharge and lake level in the Holo- tions. At the same period soils recorded warming and increased season- cene have no relations to tectonic processes. Thus the fluctuations indi- ality of the water supply. The established lake most probably was due in cate climatic changes. The first half of the Holocene (before 6200 cal yrs no small part to the regulation of local hydrology and changes in soil BP) is characterized by unstable humidification and by sharp short-term water regimes. We suppose that the appearance of vertic features in changes of water influx into the lake. The highest water intervals were soils is related to generally improved drainage of the area after the periods of risen river discharge. Steady aridization is characteristic of lake establishment more than to climatic aridization. the second half of the Holocene. Reach-through winter freezing of the As it was mentioned earlier (Sedov et al., 2009), the lacustrine record lake bottom resulted in formation of palsa islands during cryoarid has higher resolution on the time scale. This conclusion is valid even in epochs 6200–3800 and 2000 cal yrs BP — until now. These two genera- the case of Terekhol' Lake which is characterized by extremely low rates tions of numerous palsas appeared to be stable enough until now. of sedimentation. Particularly there is no detailed paleosol record for a The 3800–2000 cal yrs BP period was somewhat more water abundant, long-term and environmentally labile epoch of the Boreal, the Atlantic mostly due to enhanced underground influx. The most severe and stable and the beginning of the Subboreal. The only one buried soil corre- aridization is detected for the last 2000 years with relatively less arid XX sponds to this long period. It contains generalized palimpsest record. century. It is hardly possible to reconstruct and date it step-by-step. Atthesametimepaleosoldataprovidemorespace-specificand 5. Conclusions definite picture of landscape conditions. That is especially important for very complicated and variable permafrost-affected landscapes of Thus the conducted studies of alluvial soil-sedimentary sequences, ultracontinental mountain regions. Thus, Late Pleistocene–Holocene lacustrine sedimentary sequences, and surface soils of the Terekhol' environmental evolution in the Terekhol' Basin appeared to be gen- Basin allowed one to establish following evolutionary stages for differ- erally low-contrast as based on paleosol data. In spite of fluctuations ent components of the geosystem and environmental conditions corre- of river discharge and lake water level which seem to be consider- sponding to these stages. Maximal intensity of fluvial processes in able,studiedareastaysallthetimeintheframeofmoreorlesscon- the basin took place in the very Late Pleistocene; this stage was termi- tinental permafrost sub-arid to arid conditions with landscapes nated at the Pleistocene–Holocene border by two successive paleosols varying from meadow-steppe (probably tundra-steppe in the very (12,200–11,000 cal yrs BP). First of them was formed in a meadow- beginning) to dry steppe. steppe (or tundra-steppe?) permafrost affected landscape with a considerable water supply and short seasonal over-permafrost water stag- Acknowledgments nation. The second one was formed during a short phase of relative warming and/or sharp aridization, in conditions of suppressed biological This work became possible due to long-term collaboration with activity and seasonally contrast water regime. archaeologists and first of all with Dr. I.A. Arzhantseva. We'd like to The thermokarst–alluvial-imponded Terekhol' Lake appeared in the express our special thanks to the Non-commercial Organization basin about 11,000 cal yrs BP, and then it extended its water area due Cultural Foundation “Por-Bazhyn” for financial support and great to permafrost melting, particularly because of seasonal river floods. help in organization of fieldworks. Funding for field and laboratory Phase between 11,000 and 4000 cal yrs BP was generally unstable research in 2009–2011 was provided by the Russian Foundation concerning moistening, but at the same time well balanced towards pre- for Basic Research (projects 09-04-01742-a, 09-05-00351a, 09-04- cipitation/evaporation ratio and annual distribution of atmospheric pre- 10151-k). We are very grateful to our friendly and highly professional cipitation. There were no seasonal floods due to well balanced water field team: E.D. Sheremetskaja, Dr. A.E. Ivanova, I.G. Shorkunov, S.S. M. Bronnikova et al. / Catena 112 (2014) 99–111 111

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