Pleistocene stratigraphy of the Vychegda River basin, European North-East

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Pleistocene stratigraphy of the Vychegda River basin, European North-East Nataliya Zaretskayaa,b,∗, Andrei Panina,c, Anatoliy Molod'kovd, Svetlana Trofimovae, Aleksandra Simakovab, Dmitriy Baranova,c a

Institute of Geography, Russian Academy of Sciences, Staromonetny lane 29, Moscow, 119107, Russia Geological Institute, Russian Academy of Sciences, Pyzhevsky lane 7/1, Moscow, 119107, Russia Faculty of Geography, Moscow State University, Vorobiovy Gory 1, Moscow, 119991, Russia d Research Laboratory for Quaternary Geochronology, Department of Geology at Tallinn University of Technology, Ehitajate Rd. 5, 19086, Tallinn, Estonia e Institute of Plant and Animal Ecology, Ural Division of Russian Academy of Sciences, 8 Marta str., 202, Ekaterinburg, 620144, Russia b c

A R T I C LE I N FO

A B S T R A C T

Keywords: Middle and Upper Pleistocene European North-East Stratigraphy Chronology Palynological and carpological record

The paper is devoted to the Middle and Upper Pleistocene stratigraphy of the Vychegda River basin, located in the far northeast of Europe, in the Timan–Pechora–Vychegda subzone of the Middle–Late Pleistocene glaciations. The available litho-/chrono-/biostratigraphy data were have been obtained from more than 20 natural outcrops along the valley of Vychegda and its tributaries, which were studied lithostratigraphically. Also radiocarbon, OSL, IR-OSL and 230Th/U dating was performed, together with pollen and palaeocarpological analyses. The oldest glacial deposits are related to the Pechora horizon (MIS 8), and the oldest numerically dated relate to the Rodionovo horizon (Schöningen, MIS 7), embracing the chronostratigraphic archive of the last ~250 ka. The Vychegda horizon (Drenthe + Warthe, MIS 6) includes surficial tills, glaciofluvial and glaciolacustrine sediments dated back to 186–111 ka. This ice sheet produced the ice-dammed lake which overflowed into the KamaVolga and Caspian basin. The Last Interglacial–Glacial cycle is associated with the Sula horizon (Eemian + Lower Weichselian, MIS 5), and the Nenetsky superhorizon with the Laya (Middle Weichselian, MIS 4), Byzovaya (Middle Weichselian, MIS 3) and Polar (Upper Weichselian, MIS 2) horizons, reflecting alluvial and aeolian sedimentary environments. The chronostratigraphic record of the Byzovaya (67–27 ka) and Polar (27–1 1.7 ka) horizons is well supported with 14C, OSL, IR-OSL and 230Th/U-dates and biostratigraphic data. Unlike the predecessors' descriptions no glaciolacustrine deposits had been identified for these horizons. The Holocene floodplain, aeolian and peat bog deposits dated from 11.7 ka are widespread over the Vychegda catchment area.

1. Introduction The river Vychegda, starting in the moraine highlands of Zhezhim–Parma (southern Timan Ridge), flows almost westwards towards the southeastern limits of the Last Scandinavian Ice Sheet, through thousands of years forming a valuable archive for understanding the history of Timan-Pechora-Vychegda subzone of the Middle-Late Pleistocene glaciations (Fig. 1A). Vychegda is the largest tributary of the Severnaya Dvina River, its length is 1130 km and the catchment area is 121000 km2. Within the administrative-territorial system of Russia, the river is mostly located in the Komi Republic, and its lower reaches are in the Arkhangelsk region; the city of Kotlas was built at its confluence with Severnaya Dvina. Climate is temperate continental, with rather high amount of precipitation (500–600 mm/ yr), which led to the development of a dense erosion network (Chalov,

2012). The Vychegda catchment is a slightly undulating plain which elevates at 140–220 m a.s.l., lying on Triassic sandstones, clays and siltstones. Tectonically it is located in the Mezen’ Depression. This geological setting caused the long-lasting accumulation of the Quaternary sediments of the maximum thickness ca 200 m (Semenova et al., 2016). For stratigraphic subdivision of the Quaternary deposits we use the preliminary stratigraphic chart of Timan-Pechora-Vychegda region (Guslitser et al., 1986) correlated with North European stages. The main goal of this work is to present and discuss the chronostratigraphic data obtained over decades of geological surveys and research investigations, including recent studies performed by the authors at the new key-sections supported by radiometric and luminescence ages, which are intended for the Database of Quaternary Terrestrial European Stratigraphy (DATESTRA) (http://datestra-seqs.strikingly.com/).

∗

Corresponding author. Institute of Geography, Russian Academy of Sciences, Staromonetny lane 29, Moscow, 119107, Russia. E-mail addresses: [email protected] (N. Zaretskaya), [email protected] (A. Panin), [email protected] (A. Molod'kov), svetlana.trofi[email protected] (S. Trofimova), [email protected] (A. Simakova), [email protected] (D. Baranov). https://doi.org/10.1016/j.quaint.2019.09.020 Received 18 April 2019; Received in revised form 16 September 2019; Accepted 17 September 2019 1040-6182/ © 2019 Elsevier Ltd and INQUA. All rights reserved.

Please cite this article as: Nataliya Zaretskaya, et al., Quaternary International, https://doi.org/10.1016/j.quaint.2019.09.020

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Fig. 1. General overview of Northeastern Europe (A) (Vychegda catchment area is ensquared) and Vychegda River catchment area with sections and their coordinates (B) Legend for 1B: 1 – Sheryag (61.587439°N, 54.355665°E); 2 – Kuryador (61.685767°N, 54.888648°E); 3 – Nydyb (60.318536°N, 51.582042°E); 4 – Myjoldino (61.717246°N, 54.895601°E); 5 – Kam-21 (62.740570°N; 48.731630°E); 6 – Don I (61.597790°N, 53.884430°E); 7 – Charsovo (62.005635°N, 50.642741°E); 8 – Gam (62.106053°N, 49.621885°E); 9 – Ozyag (61.812470°N, 53.376137°E); 10 – Kyltovka (62.379011°N, 50.934209°E); 11 – Baika (61.268270°N, 46.795020°E); 12 – Kechoyag (61.966190°N, 50.676270°E); 13 – Nem (61.604387°N, 54.902934°E); 14 – Ozyag III (61.798659°N, 53.332026°E); 15 – Storozhevsk (61.913581°N, 52.425867°E); 16 – Niobdino 61.923166°N, 52.160106°E); 17 – Don (61.548584°N, 53.811870°E); 18 – Biostation (61.802491°N, 51.839041°E); 19 – Gusiha (61.333911°N, 46.971350°E); 20 – Ust’-Timsher (61.839293°N, 54.892729°E); 21 – Myjoldino-site (61.809955°N, 54.885802°E); 22 – Liokvozhjol’ (61.860772°N, 52.132241°E). Number of the section corresponds to the number in the text and in the Table 2.

2. Methods and materials

et al., 2015). Quartz-based optically stimulated luminescence (OSL) dating was carried out at Nordic Centre for Luminescence Dating, Aarhus University, Denmark (the technique described by Murray and Wintle, 2000, 2003), and feldspar-based infrared optically stimulated luminescence (IR–OSL) dating at the Research Laboratory for Quaternary Geochronology (RLQG), Department of geology, Tallinn University of Technology according to Molod'kov and Bitinas (2006). Pollen analysis was performed following the probe maceration procedure adopted in GIN. Pollen diagrams were constructed in Tilia 1.5.12 program (Grimm, 1991), which allows to calculate the general spectrum (arboreal pollen + nonarboreal pollen + spores = 100%) and individual components as a portion of the total amount of grains. The palynological analysis was carried out on an optical microscope Motic BA 400 with a camera Moticam 2300, at working magnification ×400. Samples for carpological analysis were processed according to the

Field studies of intra-valley sections within the Vychegda catchment area comprised documenting the lithostratigraphic units, geochronological sampling (14C, 230Th/U, IR–OSL, OSL) and ground penetrating radar (GPR) scanning of terrace surfaces, both in summer and in winter. 230 Th/U sampling from buried organic-bearing strata was performed in parallel with radiocarbon one. Radiocarbon dating of different materials (wood, peat, silty peat, buried soil) was performed mostly in the laboratory of Geological Institute of the Russian Academy of Sciences (GIN), with sample pretreatment and measurement technique described in (Zaretskaya et al., 2012). Calibration of radiocarbon dates was performed using Calib v. 704 software and IntCal 2013 calibration curve (Reimer et al., 2013). 230 Th/U dating (buried soils) was done in the laboratory of geochronology of the St-Petersburg University (the technique by Maksimov 2

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Table 1 Principal correlation of stratigraphic units discussed in the text: North European – Russian General – NE regions of the East European Plain. North European Stratigraphic Chart (Head, Gibbard, 2015)

Russian General Stratigraphic Scheme (Zastrozhnov et al., 2018)

System

Series

Series (division)

Links

Steps

QUATERNARY

HOLOCENE PLEISTOCENE

HOLOCENE NEOPLEISTOCENE

UPPER

III4 III3 III2 III1

Subseries

LATE

MIDDLE

MIDDLE

LOWER

MIS

II6 II5 II4 II3 II2 II1 I8

3.1. Mid-Middle Pleistocene (Russian lower Neopleistocene) deposits

3.2. Late Middle Pleistocene (Russian Middle Neopleistocene) deposits include

Chirva

Superhorizon

HOLOCENE Weichselian

Eemian Saalian

Holsteinian Elsterian

Horizon

HOLOCENE Polar Byzovaya Laya

Upper Middle

Nenetsky

Lower

Sula

Drenthe + Warthe Schoningen Cooling stadial Reinsdorf Fuhne

TimanUralian

Vychegda Rodionovo Pechora Chirva

KomiPermiak

Pomus

3.2.2. Pechora horizon Pechora horizon (MIS 8) is represented by till and glaciofluvial deposits. Till was studied in numerous drilling cores in topographic depressions. It is presented by dark-grey and grey clayey diamicton with angular fragments of sedimentary rocks (limestone, sandstone etc.) and rare pebbles and boulders of Timan provenance. Its maximal thickness within the Vychegda catchment area is around 95 m (Semenova et al., 2016). In the lenses of the intra-till sands, the Dycrostonyx simplicior Feifar lemming molars with an evolutionary stability index of minus 4.5 were found, which was considered by B.I. Guslitser as the confirmation of the Pechora age (Kalberg, 1967). The subdued moraine landscape of the Pechora glaciation may be observed in the “knee” of Vychegda valley, where its course turns from submeridional (upper reach) to sublatitudinal (middle reach). The Pechora till (grey diamicton with cobbles, ca ~5 m thick) overlain by glaciofluvial deposits (~3.5 m, yellow unsorted sand) could be observed in the right bank of Vychegda near Sheryag (Fig. 1B: 1; Table 2).

The oldest Quaternary deposits in the Vychegda catchment area are dated to the middle of the Middle Pleistocene (or Early Neopleistocene according to the Russian Stratigraphic Chart) and related to Pomus horizon (Elsterian stage, MIS 12) (Guslitser et al., 1986; Semenova et al., 2016; Zastrozhnov et al., 2018). These are glacial deposits (matrix-supported diamicton with pebbles and cobbles of sedimentary and igneous rocks) found only in drilling cores in the middle reaches of river Vychegda (Semenova et al., 2016). Its stratigraphic position was estimated only by geological situation; pollen data are unreliable due to the very poor preservation of pollen grains. The maximum thickness of Pomus horizon is ~50 m (Semenova et al., 2016).

formations

Stage

3.2.1. Chirva horizon The horizon is presented by alluvial deposits, found by drilling cores mostly in buried palaeovalleys (Semenova et al., 2016). It is underlain by the Pomus (MIS 12) till, and is overlain by the Pechora (MIS 8) till (Fig. 4). It is composed of sands with lenses of gravel and pebble and sometimes with peat and clay beds containing freshwater diatom species Pinnularia borealis Ehrenberg, P. aff. Fascolata (Lagerat) Hust., P. braimii var. Amphioephala (A. Mayer) Hust. (Loseva, 2000). In addition, the stratigraphic position of the formation is supported by its occurrence under the Pechora tills dated to the Middle Pleistocene by Dycrostonyx lemmings (Guslitser et al., 1986). Pollen data allowed reconstruction of forest type vegetation consisting of coniferous (Picea sec. Omoriса, Pinus strobus) and mixed forests with broad-leaved (Corylus, Tilia cordata, Ulmus etc.) species, characteristic for the Chirva interglacial (Traat et al., 1960). The average thickness of the Chirva strata within the Vychegda catchment area is 25 m (Semenova et al., 2016). Timan-Uralian superhorizon includes Pechora, Rodionovo and Vychegda horizons (Semenova et al., 2016 Zastrozhnov et al., 2018). Pechora and Vychegda horizons are represented by glacial complexes, and Rodionovo horizon – by interglacial strata (Table 1).

3. Results and key-sections

Neopleistocene

NE regions of the East European Plain (Astakhov, 2013; Andreicheva et al., 2015; Zastrozhnov et al., 2018)

(Holsteinian stage, MIS 11) and Timan-Uralian superhorizon (Saalian stage), including Pechora (MIS 8), Rodionovo (MIS 7) and Vychegda (MIS 6) horizons (Guslitser et al., 1986; Semenova et al., 2016; Zastrozhnov et al., 2018) (Table 1).

standard procedure (Nikitin, 1969), and analyzed using the stereomicroscope Carl Zeiss Stemi 2000-C. Identification of plant fossils is based on the collection and herbarium of the museum of Institute of Ecology of Plants and Animals, Ural Branch of RAS, and using the Atlas of the Pleistocene vascular plant macrofossils of Central and Eastern Europe (Velichkevich, Zastawniak, 2006). The ecological interpretations of the results are based on (Tolmachev, 1976; Schmidt, 2005). To better understand the Pleistocene chronostratigraphy of the Vychegda catchment area, we used the published data by the Norwegian Geological Survey (Lyså et al., 2011, 2014) and explanatory notes for maps of the Russian Geological Survey (Semenova et al., 2016). The regional stratigraphic terms and names of superhorizons and horizons have been obtained from the regional stratigraphic chart of the Timan-Pechora-Vychegda region (Guslitser et al., 1986), correlated with the International Stratigraphic chart (Cohen and Gibbard, 2011; Head and Gibbard, 2015) and the NW European stratigraphic subdivisions of the Quaternary in the recent works of Zhamoida et al., 2019, Astakhov (2013) and by Zastrozhnov et al. (2018) (Table 1). The peculiarities of the correlation between the International Stratigraphic Chart and General Quaternary Stratigraphic Chart of Russia are well explained in the recent Russian works (Astakhov, 2013; Korsakova, 2019).

Middle

1 2 3 4 5a-d 5e 6 7 8 9 10 11 12

North European chronostratigraphic subdivisions (Ehlers, 1996; Turner, 1998)

horizon 3

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Fig. 2. Key-sections of the Vychegda valley: Kuryador (2; 61.685767°N, 54.888648°E), Kechoyag (12; 61.966190°N, 50.676270°E) and Baika (11; 61.268270°N, 46.795020°E). Legend: 1 – paleosol; 2 – clayey silt; 3 – sand; 4 – silt; 5 - pant detritus; 6 – ice wedge casts; 7 – ripple marks; 8 – cross-bedding; 9 – radiocarbon and calibrated dates (this work; after Maksimov et al., 2015; Zaretskaya et al., 2019); 10 – OSL dates (after Lyså et al., 2011); 11–230Th/U date (after Maksimov et al., 2015; Zaretskaya et al., 2019).

et al., 2016). Pollen spectra of the Rodionovo deposits indicate the interglacial conditions characterized by the development of birch forests (up to 90%) with an admixture of alder (up to 10%) with broad-leaf (oak, elm, maple, linden, hazel) and coniferous (Pinus sylvestris L., Piсea

3.2.3. Rodionovo horizon Rodionovo horizon (Schöningen stage, MIS 7) is represented by alluvial lenses of sand and gravel (Fig. 4) up to 20 m thick in cores separating diamictons of the Pechora and Vychegda tills (Semenova

Fig. 3. Key-sections of the Vychegda valley: Biostation-1 and 2 (18; 61.802491°N, 51.839041°E) and Gusikha (19. 61.333911°N, 46.971350°E). Legend: 1 – sand; 2 – silt; 3 – peat; 4 – silty peat; 5 – ripple marks; 6 – cross-bedding; 7 – OSL date (after Karmanov et al., 2013); 8 – radiocarbon and calibrated dates (after Zaretskaya et al., 2014, 2018; 2019); 9 – signs of aeolian sedimentation. 4

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Fig. 4. Chronostratigraphic chart of the Vychegda River basin: a,l II ch – alluvial and lacustine deposits, Chirva horizon; g II pč – glacial deposits, Pechora horizon; a,l II rd – alluvial and lacustine deposits, Rodionovo horizon; g II vč – glacial deposits, Vychegda horizon; fg, lg II vč – glacifluvial and glaciolacustrine deposits, Vychegda horizon; fg II vč – glaciolacustrine deposits of proglacial lake, Vychegda horizon; a III sl – alluvial deposits, Sula horizon; a III la – alluvial deposits, Laya horizon; a III bz – alluvial deposits, Byzovaya horizon; eol III pl – aeolian deposits, Polar horizon; a III pl – alluvial deposits, Polar horizon; a IV – alluvial deposits, Holocene; 19–11.7 ka – OSL- and IR-OSL ages; 11.7 cal. ka – radiocarbon calibrated ages.

(Semenova et al., 2016). In the upper reaches of Vychegda in the right bank, outside the Vychegda marginal moraine glaciofluvial sands were OSL dated to 135 ± 10 (RISØ– 063810) and 111 ± 8 (RISØ– 063811) ka in the Myjoldino section (Fig. 1B: 4) (Lyså et al., 2011). Glaciolacustrine deposits of the Vychegda horizon form the vast plains in the northern part of the Vychegda catchment area atop of the Vychegda till, where they are represented mainly by sands with interlayers of silt and clays. They are usually capped by Holocene mires. These sands were IR-OSL-dated to 143.0 ± 25.0 ka (RLQG– 2127– 123) in the Kam– 21 section (Fig. 1B: 5; Table 2) (Semenova et al., 2016). Glaciolacustrine deposits of proglacial lakes also form plains and terraces within the major river valleys of the eastern and southeastern part of the Vychegda catchment area, along the Vychegda ice marginal zone. The typical altitude of the terrace surface is 120–125 m a.s.l., and up to 150 m a.s.l. in the upper reaches of Vychegda. Most of the glaciolacustrine deposits reside on the Pechora till (Semenova et al., 2016). During our field works in the Kadam– Donty palaeo-valley in the upper reaches of Vychegda, a small quarry on the sandy terrace (Don I section, Fig. 1B: 6) had been sampled and studied (Table 2). The 3.5 m section represents an interlayering of gravels, sands with oblique lamination (layers fall to the south) and varve-like beds of 0.5 m thickness indicative of a lacustrine origin of the deposits. The lake was dammed by one of the MIS 6 marginal moraines that cross the Vychegda valley. The bottom sand of the section (depth 3.25 m) indicating proglacial lake with unstable hydrodynamics was dated back to the MIS 6/5 boundary – 130.8 ± 12.8 ka (RLQG– 2359– 085). The lake seems to have survived in the following interglacial as the overlying laminated sand (depth 2.65 m) provided the IR-OSL date of 124.6 ± 14.3 ka (RLQG– 2360– 085).

sect. Omorica, up to 2–3%). The presence of Piсea sect. Omorica supports a Middle Pleistocene age of the described strata (Semenova et al., 2016). In the natural exposure within the study area the Rodionovo deposits are reliably identified only at Kuryador section in the upper reach of Vychegda (Fig. 1B: 2; Table 2). Here coarse alluvial (fluvial channel) sands and gravels apparently 3 m thick with the OSL dates of 239 ± 21 (RISØ– 063801), 249 ± 22 (RISØ– 063802) and 203 ± 16 (RISØ– 063804) ka (Lyså et al., 2011) are exposed at the base of the section (Fig. 3). The top of the formation is eroded with ca 200 ka years of the sedimentary record missing.

3.2.4. Vychegda horizon Vychegda horizon (Drenthe + Warthe stage, MIS 6) is the major landforming stratigraphic unit throughout the Vychegda basin. The horizon is represented mainly by an assemblage of till, glaciofluvial and glaciolacustrine sediments (Fig. 4). The formation of these deposits is ascribed to the Scandinavian and Novaya Zemlya ice sheets (Andreicheva et al., 2015). The two different ice sheets are clearly identified in the map pattern from the orientation and configuration of the marginal landforms (Semenova et al., 2016). The tills are represented by dark grey, brownish grey and brown diamictons with poor and well-rounded clasts and intra-till sand lenses of total thickness up to 50 m. The marginal moraine of the Vychegda ice sheet forms the Zhezhim– Parma upland and is traced in the eastern and south-eastern parts of the Vychegda catchment (Semenova et al., 2016). Glaciofluvial deposits of the Vychegda horizon are widespread in river valleys and only rarely on interfluvial plateaus (Fig. 4; Table 2). They are sands, gravelly sands, gravels-pebble sediments, and sandy silts up to 20 m thick on top of the till. The terraces of this time are inclined at 8–10° to the thalwegs suggesting a rapid fall of water levels in the streams (Semenova et al., 2016). In the southern part of the Vychegda glaciated terrain these deposits were dated back to 186.0 ± 14.0 ka (RLQG– 2375– 095) in the Nydyb section (Fig. 1B: 3)

3.3. Late Pleistocene (Russian upper Neopleistocene) deposits and horizons The formations of Sula horizon (Eemian + Lower Weichselian, MIS 5

6

Komi-Permiak

Timan-Uralian

Sula

Nenetsky

Super-horizon

3

Byzovaya

8

9 10 11

Rodionovo

Pechora

Chirva

12

7

Vychegda

Pomusovka

4 5a-d 5e 6

Laya

2

Polar

Cores

Cores

Cores, Sher'jag (1)

Kam-21 (5) Myjoldino (4) Nydyb (3) Many (high river banks through the valley) Kur'jador (2)

Gam (8) Charsovo (7) No Don I (6)

Kuryador (2) Baika (11) Kyltovka (10)

Kechoyag (12)+ others (13–16)

Gusikha (19) Biostation (18) + others (20–22) Don (17) Oz'jag (9) Kur'jador (2) Oz'jag (9) Baika (11)

Many

Sedimentary environment

46-35 cal ka BP 47-39 ka 59-47 ka 67-60 ka 130-111 ka 101-92 ka 124 ka 130 ka 143 ka 135-111 ka 186 ka -

Eolian

Alluvial

Alluvial, subaerial (paleosol)

Silt, sand

Glacial

boulder silt with poor and wellrounded clasts Sand, gravel

Silt with pebbles and boulders

Sand, gravel, peat, clay

Sand Sandy silt and clay with sedimentary rock clasts

Glaciolacustrine glacifluvial

Sand, gravel, pebbles

Glacial

Alluvial

Glacifluvial Glacial

Alluvial

Proglacial lake

Sand, laminated sand

Sand, gravelly sand

Alluvial, eolian

32-27 cal ka BP

Periglacial alluvial

Sand, silt

Sand, silt, plant detritus, peat, silty peat Sand, clayey silt, peat, silty peat, buried soil

23 ka 19 ka 17–14 ka 14 ka

Alluvial, oxbow lake

Sand, silt, peal, plant detritus

-

-

-

249-203 ka

17–12 cal ka BP

Alluvial (flood-plain), eolian, peat-bog

Sand, clay silt, peat

11.7– cal ka BP to present

14 C, OSL, IROSL and 230Th/ U ages

Biostratigraphy

-

mixed forests with broad-leaved species (freshwater)

Birch forests with alder and minor presence of broad-leaf and coniferous specie -

-

-

(Dyrostonyx simplicior Femfar, e.s.i. - 4.5)

-

-

-

-

-

-

-

Forest-tundra with shrub birch; rare spruce and larch in the valleys Shrubs and meadow grasses; moss-grass bogs (Dicrostonyx torquatus)

-

-

Oxbow lake plants

Macro-fossil (fauna)

-

-

Shrub tundra with areas of foresttundra; northern taiga

Spruce and pine-birch forests with wetlands

Spore plants, sparse vegetation, low pollen productivity

-

Pine forests with spruce and birch ⇒ spruce forests ⇒ coniferous forests with birch ⇒ south taiga with fir-trees Shrubs and herb communities; sparse spruce woodlands

1

Lithology

Holocene

Sections (cores)

Pollen and diatom

MIS

Horizon

NE regions of the East European Plain (Astakhov, 2013; Andreicheva et al., 2015; Zastrozhnov et al., 2018)

Semenova et al., 2016; Loseva (2000); Yakobson, 1999; Traat et al. (1960) Semenova et al., 2016

This work; Semenova et al., 2016

Semenova et al., 2016; Lyså et al. (2011)

Semenova et al., 2016; Lyså et al. (2011); Semenova et al., 2016 Semenova et al., 2016

This work; Zaretskaya et al. (2019); Maksimov et al. (2015); Lyså et al. (2011); Guslitser, Duriagina (1983) Semenova et al., 2016; Lyså et al. (2014) This work

This work; Semenova et al., 2016 This work; Semenova et al., 2016; Maksimov et al. (2015); Lyså et al. (2011); Sidorchuk et al. (2001) This work; Zaretskaya et al. (2019)

This work; Zaretskaya et al. (2018); Zaretskaya et al. (2014)

Chernov et al. (2015); Andreicheva et al. (2015); Karmanov et al. (2013)

References

Table 2 Neopleistocene (Middle and Late Pleistocene) stratigraphy, referred key sections and characteristics of their stratigraphic subdivisions in the Vychegda River basin (northeastern Europe).

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River, in its left bank, at an altitude of 60 m a.s.l. (Fig. 2). The whole outcrop sequence is about 14 m thick. Byzovaya horizon is traced in the lower 6.5 m of the section and is composed of alluvial light grey and light brown sands, detrital lenses at the very bottom of the section and an organic (gyttja, peat) layer with large ice wedge casts in the upper part of the layer. Pollen and palaeocarpological data from detrital lenses are as follows. In the lower layers, north-taiga landscapes were reconstructed (coniferous-birch forests, meadow communities and wetlands) but upwards spruce pollen increased and Tilia pollen appeared. The climate was warm and humid. Local conditions indicate a moss-grass bog in a temperate climate, although macrospores of a relatively cryophilic species, Selaginella selaginoides, have been found. Upwards, in the layer of horizontally-bedded sands, a double lens of highly cryoturbated peaty soil and gyttja is traced, dated by 14C to 42280 ± 460 (45.9–45.2 cal kyr BP, GIN– 15708) and 43550 ± 500 (47.3–46.1 cal kyr BP, GIN– 15709) respectively. The results of pollen analysis revealed a cold-warm-cold rhythm. At the bottom part, shrub tundra with patches of forest-tundra (with Selaginella selaginoides) was reconstructed; the climate was relatively cold. In the middle part, birch, spruce and pine pollen dominated, and the amount of spores reduced; there was a warming and aridization of climate. In the upper complex, shrub tundra with birch dominated, and spore species as Selaginella selaginoides, Sphagnum, Botrychium expanded again; the climate became cold and humid. Ice wedges indicate the following strong cooling episode. Another section with Byzovaya horizon is Kyltovka (Fig. 1B: 10), located in the northern part of Vychegda basin (Lavrov, Potapenko, 2005). This is a 13-m outcrop in the right bank of River Kyltovka, exposing horizontally or indistinctly bedded sands and silts (sometimes with peat lenses) with gravel and pebble beds. Three radiocarbon dates have been obtained from the depth of 11.2 m (spruce log, 47520 ± 1000 (LU–566), out of calibration curve limits), 6.7 m (spruce log, 42100 ± 1700 (GIN–606), 47-44 cal kyr BP) and 5 m (peat, 39170 ± 470 (LU–588), 43.8–43 cal kyr BP); pollen data reconstructed the taiga (coniferous forest) conditions (Lavrov, Potapenko, 2005). The upper layers of Byzovaya horizon also are traced in the bottom parts of the first Vychegda terrace (Fig. 4; Table 2). They are composed of alluvial sands and silts with lenses of plant detritus, peat and loamy peat. A series of 14C dates from 5 sections (Nem (13), Ozyag III (14), Storozhevsk (15), Niobdino (16) and Kechoyag (Fig. 1B: 12; Fig. 2)) range from 28500 ± 260 (33.3–32.4 cal kyr BP, GIN–15080) to 22800 ± 150 (27.4–27.0 cal kyr BP, GIN–15692) (Zaretskaya et al., 2019; this work). Results of pollen analysis from the Kechoyag section correspond to spruce and pine-birch forests with wetlands. Results of carpological analysis are characteristic for interstadial flora and showed the local conditions of marshy biotopes of the forest-tundra zone with shrub birches in the vicinity of low-running water; floodplain was inhabited by sparse larch and spruce forests.

5) and Nenetsky superhorizon, including Laya (Middle Weichselian, MIS 4), Byzovaya (Middle Weichselian, MIS 3) and Polar (Upper Weichselian, MIS 2) horizons, are attributed to the Late Neopleistocene/Late Pleistocene (Semenova et al., 2016; Zastrozhnov et al., 2018) (Table 1). 3.3.1. Sula horizon The lower part of Sula horizon (Eemian stage, MIS 5e) is actually not found in the Vychegda river basin. The closest section with the same stratigraphic position of the deposits is the Tolokonka one located 100 km downstream the Vychegda – Severnaya Dvina confluence. These are oxbow-lake deposits (peat and silty peat) 2 m thick that lay at the very bottom of the section. The layer contains large Picea logs and megafauna remains, such as mammoth, horse and elk bones. In the upper part, this layer was dated by 230Th/U to 120.4 ± 11.1/9.2 (L/L model) and 104.0 ± 9.4/8.0 (TSD-model) ka (Zaretskaya et al., 2013). 3.3.2. Sula and Laya undivided horizon The undivided horizon including the upper Sula (Lower Weichselian stage, MIS 5d-a) and Laya (Middle Weichselian stage, MIS 4) layers composed the 3rd terrace of Vychegda (Fig. 4). This terrace is traced in the middle and lower reaches of the river; its height is 18–20 m above the low-water stage of the river and elevation ranges between 90 (lower reaches) and 100 (middle reaches) m a.s.l. There are two sections of this terrace (Table 2): Charsovo (Fig. 1B: 7) and Gam (Fig. 1B: 8) in the lower reach (Lyså et al., 2014). At Gam (exposure 18 m high) and Charsovo (10 m high), the lower boundary of this horizon is located almost at the river shoreline, directly overlying the bedrock. Sediments are represented by horizontally and cross-bedded sands and gravelly sands interbedding with clayey silts with OSL dates ranging between 130 and 111 ka (Charsovo) and 101–92 ka (Gam) (Lyså et al., 2014). 3.3.3. Byzovaya horizon Byzovaya horizon (MIS 3) is traced along the Vychegda valley and some of its tributaries and makes the middle and bottom parts of the second and first river terraces (Fig. 4). The almost full lithostratigraphic sequences of Byzovaya horizon are presented in two key sections: Kuryador (Fig. 1B: 2; Fig. 2) in the upper and Baika (Fig. 1B: 11; Fig. 2) in the lower reaches of the river Vychegda (Table 2). The Kuryador 15 m high exposure is located in the upper reaches of Vychegda River, in its right bank, at an altitude of 122 m a.s.l. (Fig. 2). The Byzovaya horizon here is traced in the middle part of the section and embraces two units. The lower unit (3.7 m thick, 111.6–114.3 m a.s.l.) is presented by interbedding of greyish sands and clay silts (overbank alluvial facies) OSL-dated to 67–60 ka (Lyså et al., 2011). They are covered by thin peat layer dated by 14C back to 41700 ± 600 (45.6–44.7 cal kyr BP, GIN– 14324 (Zaretskaya et al., 2019)) overlain by fine-grained light grey sands with ripple marks (back-water facies) and ice-wedge casts, OSL-dated to 59–47 ka (Lyså et al., 2011). The upper unit (0.7–1 m, 114.3–115 m a.s.l.) is composed of buried paleosol, covered by meadow deposits with high organic content. Traces of rodent holes and plant remains of shrub vegetation indicate the subaerial sedimentary environment (Zaretskaya et al., 2019). A series of 10 consecutive radiocarbon dates - from 39610 ± 360 (44–43.4 cal kyr BP, GIN–15076-2) to 26200 ± 400 (31.1–30.6 cal kyr BP, GIN–14320) were obtained and lay in the time interval of ~44 to 30 cal kyr BP; the lowermost layer of buried soil was 14C-dated (44–41.6 cal kyr BP) in parallel with 230Th/U (46.8–38.8 ka BP), the results are in good agreement (Zaretskaya et al., 2019; Maksimov et al., 2015). Pollen record allowed reconstructing four cold phases with birch sparse woodlands and Artemisia grasslands alternating with three warmer phases of pine and spruce forests with alder and fern presence (Andreicheva et al., 2015). Remains of Dicrostonyx torquatus teeth (Guslitser, Duriagina, 1983) also indicate the interstadial climatic conditions. The Baika section is located in the very lower reaches of Vychegda

3.3.4. Polar horizon The sediments of Polar (MIS 2) horizon within the Vychegda catchment area form many massifs of the 1st terrace in the Vychegda River and tributary valleys as well as aeolian dunes on these terraces and aeolian blankets on the interfluves (Fig. 4). Well sorted monotone sands or silts form the upper parts of the Kuryador (2) and Baika (11) sections (Fig. 2; Table 2). At Kuryador, these are fine sands and silts with unclear broken bedding. The upper part of the layer was OSL-dated to 16.9–14.0 ka (Lyså et al., 2011). Such silt/sand covers are widespread in the interfluves in the upper reaches of the Vychegda basin. At Baika, these are monotone sands forming the upper 7 m of the section. Pollen analysis from this layer revealed the strong domination of spore plants (Sphagnum, Polipodiaceae, Lycopodium pungens, Selaginella selaginoides), and sparse vegetation with low pollen productivity, characteristic for cryo-arid conditions of the LGM (Sidorchuk et al., 2001). Also, large drop-shaped 7

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dunes probably of the LGM age occur on the first terrace along the Vychegda River in its lower reach. Sediments of Polar horizon forming the 1st Vychegda terrace are represented by alluvial sands (Fig. 4; Table 2). The terrace height is usually 5–6 m above the water level, but could be higher in case of aeolian cap on its surface. According to our recent studies, being of equal height above the river bed level, this terrace ages differ at various locations and may be separated into two groups: pre-LGM and of the Late Glacial. In the alluvia of the older terrace massifs, ice wedge casts and cryoturbations are common. One of the examples of this terrace is the Ozyag section (Fig. 1B: 9; 15.5 m high) where well-sorted alluvial sands 4 m thick with horizontal and oblique bedding are exposed, IROSL dated back to 19.6 ± 1.5 (RLQG– 2377– 095). They are overlain by thick (11.5 m) layer of monotonous medium-size aeolian sands dated by IR-OSL to 14.4 ± 1.1 ka (RLQG– 2260– 084). Another example of this terrace is the Don section (Fig. 1B: 17; middle reach of Vychegda River) where the IR-OSL date 23.1 ± 2.0 (RLQG–2362–085) was obtained from under the cryoturbation layer in the channel alluvial sand. The younger terrace is of post-LGM age (Table 2), being of the same height and containing the organic-bearing deposits of the Late Glacial at the bottom parts (17–11 cal kyr BP) (Zaretskaya et al., 2014). One of the key-sections of this terrace is Biostation 1 and 2 (Fig. 1B: 18; 12 m outcrop, 89 m a.s.l.) in the right bank of Vychegda in its middle reach, recently studied by authors (Fig. 3). The lower unit in this section is related to the Late Pleistocene – Late Glacial transition and is composed of three sub-units. The lowermost subunit – coarse to middle-grained brownish rusty sands with horizontal and cross-bedding (river channel facies) was probably formed not long after the final phase of the last glaciation. The second sub-unit is composed of interbedded organicbearing deposits (silty peat, plant detritus, even logs), sands and silts. Radiocarbon dates obtained through this layer range from 13890 ± 50 (GIN–14192) to 10300 ± 50 (GIN–14195) (17–12 cal kyr BP) (Zaretskaya et al., 2014). Plant macrofossil analysis showed the oxbow lake sedimentary environment. Pollen analysis enabled to distinguish short-termed climatic oscillations of the Late Glacial. Vegetation was represented predominantly by shrubs (birch, willow, and alder) and herb communities (grasses, sedges, sages, and goosefoot). The presence of spruce pollen may indicate that very sparse spruce woodlands occurred in the Vychegda River valley. The uppermost sub-unit is represented by thick (several meters) layer of light-grey or yellow sands, horizontally and cross-bedded, and demonstrate a very high accumulation rate at the very end of the Late Glacial and during the Early Holocene. The upper part of the section is composed of thin-layered light-grey aeolian sands OSL-dated to 5850 ± 670 (GdTL– 1167) (Karmanov et al., 2013), which shows the Middle-Holocene age of aeolian activity in this region. Other sections of the younger 1st Vychegda terrace with similar ages and structure were studied in the upper and middle reaches of the river; these are Ust-Timsher (Fig. 1B: 2), Myjoldino-site (Fig. 1B: 21) and Liokvozhjol (Fig. 1B: 22) (Zaretskaya et al., 2014). The second characteristic section of this terrace is Gusikha (Fig. 1B: 19; surroundings of the Solvychegodsk town) recently studied in the lower reach of Vychegda. This is a 5.7 m terrace at the right bank of the river, which is composed of alluvial horizontally, cross- and obliquebedded sands overlain by peat interbedded with aeolian sands (Fig. 3). The bottom part of the peat filling the ice-wedge cracks was radiocarbon dated at 10050 ± 40 BP (11.4–11.7 cal kyr BP, GIN– 15704) (Zaretskaya et al., 2018). Apparently, the ice wedge casts may be related to the sharp cooling of the Young Dryas. The results of palynological analysis indicate the dominance of forest-tundra vegetation. A twig from the underlying bed of alluvium was dated at 11290 ± 70 (13.1–13.2 cal kyr BP, GIN– 15706), confirming the Late Glacial age of this first terrace.

3.4. Holocene horizon Holocene horizon is represented by floodplain and peat bog deposits dated from 11700 cal BP till now (Chernov et al., 2015). 4. Discussion 4.1. Mid-Middle Pleistocene (Russian lower Neopleistocene) deposits The oldest Pleistocene sediments of Pomus glaciation in the Vychegda catchment area had been found only in cores. As no related chronological and almost no biostratigraphic data exist on these deposits, their occurrence in the Vychegda basin remains debatable. 4.2. Late Middle Pleistocene (Russian Middle Neopleistocene) deposits The reliable stratigraphic succession from the Timan-Uralian superhorizon starts from the end of the Middle Pleistocene (MIS 8-6). Glacial deposits (tills) of Pechora horizon (MIS 8) are well defined in the eastern part of the Vychegda basin outside the marginal moraines of the Vychegda glaciation, which are well expressed geomorphologically. Reliable Pleistocene chronological record in the Vychegda catchment area starts from the Rodionovo horizon (MIS 7) (Fig. 4). OSL dates ranging from 249 to 203 ka were obtained on the alluvia forming this horizon in the Kuryador section (Lyså et al., 2011), and are confirmed by the stratigraphic position and location beyond the eastern limits of the maximal middle Pleistocene glaciation. In the neighboring Pechora region the 230Th/U dates of inter-till peat layers from the stratotipic sections Rodionovo and Seida 1 showed the time span 240–186 and 236–181 ka BP respectively, and the pollen diagrams from the peat indicated the southern boreal forest with broad-leaved trees and presence of Piсea sect. Omorica (Astakhov, 2013). Data from the Kuryador section indicates the alluvial sedimentation pattern during the early Rodionovo interglacial and runoff direction the same as modern. Vychegda horizon (till and glaciofluvial sediments), deposited by the maximal Middle Pleistocene glaciation (MIS 6), is traced through the whole Vychegda basin (except its very eastern part), modeling the interfluves' surface and high river banks (up to 50 m above the water level) (Fig. 4). OSL and IR-OSL dates of glaciofluvial and glaciolacustrine sands associated with the moraines range from 186 to 111 ka (Lyså et al., 2011; Semenova et al., 2016), suggesting a long period of glaciation and oscillations of ice lobes in the Vychegda catchment area. Proglacial lake facies of Vychegda horizon form vast sand fields beyond the marginal moraines of the MIS 6 glaciation that cross the middle and upper course of the Vychegda valley. The easternmost area of such sands is located in the valley widening upstream the maximum ice limit at Ust-Kulom. The IR-OSL dates 130–124 ka from the top of the lacustrine sands at elevation 140 m a.s.l. probably relate to the very end of MIS 6, but the moraine-dammed lake could have survived also in the beginning of MIS 5. The high elevation of lake deposits evidence probable lake expansion far upstream the Vychegda valley and the possibility of the overflow into the Kama basin through the Kel'tma palaeo-valley filled with sand/silts deposits that may be associated with the above proglacial lake laid (Yakovlev, 1956). 4.3. Late Pleistocene (Russian upper Neopleistocene) deposits and horizons The lower Sula horizon, related to the MIS 5e (Eemian), is not well recognized neither dated within the Vychegda catchment area, though it is reliably identified and well dated in the neighboring areas of the Severnaya Dvina valley (Tolokonka section (Zaretskaya et al., 2013)) and Pechora valley (Sula section (Murray et al., 2007; Astakhov, 2013)). Probably, during this epoch, drainage network was cutting through the moraine landscapes left by the Vychegda glaciation and sedimentation environment was rather erosional than depositional. Further sedimentary and chronostratigraphic record still remains 8

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31–24 ka (Astakhov, Svendsen, 2011; Astakhov, 2013). The stratotype site of the Bysovaya Palaeolithic site embraces the debris flow sediments with artefacts and mammoth bones radiocarbon dated to 35.5–29.9 cal kyr BP, overlain by the aeolian coversand which discontinuously accumulated 33–15 ka, according to OSL-dates (Heggen et al., 2012). Thus we conclude the sedimentary environment during the formation of Bysovaya horizon within the Vychegda catchment area as ice-free with mainly alluvial and then aeolian accumulation similar to the neighbor Pechora valley, but with warmer climatic conditions causing the formation of palaeosoils and other plant organic-bearing layers and no mammoth fauna remains. Polar horizon is determined in the upper part of the 2nd and the most of 1st terrace of Vychegda and its tributaries, tracing through the whole valley from upper to lower reaches (Fig. 4). The upper parts of the 2nd terrace are interpreted as LGM proglacial lake deposits by several research groups (Lavrov, Potapenko, 2005; Lyså et al., 2011, 2014; Larsen et al., 2013). This interpretation is disproved by the absence of ice blockage of Vychegda valley during the LGM (Semenova et al., 2016; Zaretskaya et al., 2018) and our arguments for the aeolian origin of the sands that cover the top of the 2nd terrace, namely the massive structure of sand massifs and the absence of hydraulic gradational bedding. Large dunes were found along the Vychegda river bed in its lower reach. Recent pollen analysis of the upper part of Baika section revealed the traces of soil fungi and acarian remains. One of the most debatable is the interpretation of the Kuryador section in the upper Vychegda (Fig. 2). Here, Lyså et al. (2011) propose the occurrence of the LGM lacustrine deposits at ca 115–121 m a.s.l. (6–12 m above the river; their unit K3a). However the sandy silts with fractured bedding in the upper part of this layer, interpreted by Lyså et al. (2011) as water- and sand-escape structures, look rather as the result of niveo-aeolian sedimentation – aeolian deposition in winter with subsequent deformation of bedding due to the melting of snow (Koster, 1988; Astakhov, Svendsen, 2011). Loess-like origin of this deposit is supported also by high carbonate content (up to 4.5%), massive structure and presence of grass root casts revealed in microscopic studies by Andreicheva (2009). Silts in lower half of unit K3a by Lisa et al. (2011) shows only unclear, dotted bedding. Thin section study by Andreicheva (2009) revealed the absence of microlamination and abundance of thin plant fabric. The sum of data allows the interpretation of these deposits as being accumulated in a shallow, relatively warm and highly vegetated lake of local origin. It cannot be associated with any huge dammed lake that overflowed the Kel'tma paleovalley at 130 m a.s.l. and therefore must have been at least 15 m deep at Kuryador. The transitional sedimentary pattern within the Vychegda catchment basin was similar to the neighboring Pechora lowland: the Middle Weichselian alluvium continuously changed to the Late Weichselian aeolian cover, with the OSL-dates of 22–19 ka at the sections Bolshaya Rogovaya, Mamontovaya Kuria etc. (Astakhov, 2012). At the Bysovaya section and site, the OSL-dates for the Late Weichselian aeolian cover sand range between 24.4 and 15 ka, with cold and dry sedimentation pattern (Heggen et al., 2012). Sediments of the Polar horizon form also the older fragments of the 1st terrace. Though a number of researchers proposed lacustrine or glaciolacustrine origin of this terrace (Lavrov, Potapenko, 2005; Lyså et al., 2011, 2014; Larsen et al., 2013), its morphology bears traces of migration of river channel much larger than the present-day Vychegda River and having mostly braided pattern (Karmanov et al., 2013; Zaretskaya et al., 2014). We consider the sands that composed the base of this terrace representing channel alluvium. Their fluvial origin is emphasized by the occurrence of ice wedge casts. Terrace sands are mostly buried under the silts – overbank alluvial facies, and 2–3 m thick peat cover deposited through the Holocene. Highest sand massifs not buried by silts are either channel levees or aeolian dunes. Some of the dunes border banks of former river channels and thus visualize the palaeochannels that are invisible now under the silt and peat cover.

controversial; similar in lithological definitions, it is different in interpretations, with alternatives between proglacial-lacustrine and alluvial, or proglacial-lacustrine and subaerial, and even glacial and alluvial (Guslitser, Duriagina, 1983; Lavrov, Potapenko, 2005; Lyså et al., 2011, 2014; Andreicheva et al., 2015; Zaretskaya et al., 2018, 2019). The following upper Sula + Laya horizon coincide to MIS 5d – MIS 4 and is represented in the 3rd terrace of Vychegda (Fig. 4), which is traced mostly in the lower reaches of the river. River incision took place before the start of terrace accumulation, after the renewal of westward runoff. The origin of the terrace was interpreted as of proglacial lake (Lavrov, Potapenko, 2005) or alluvial (Lyså et al., 2014). As according to the latest data the runoff of Vychegda–Severnaya Dvina fluvial system had not been blocked by the MIS 4 glaciation (Helmens et al., 2000; Evzerov, 2010; Semenova et al., 2012), the proglacial origin is ambiguous and the alluvial origin of the Sula + Laya terrace is preferential. The sediments in the Gam and Charsovo sections with OSL dates 130–92 ka were interpreted as fluvial channel sands, and palaeocurrent indicators evidence westerly flow direction, same as the present-day Vychegda current (Lyså et al., 2014). In the neighboring Pechora valley the OSL dates from the 3rd fluvial terrace which is incised into the floor of the Lake Komi, have yielded the ages of 90–60 ka BP (Svendsen et al., 2004). The 2nd terrace of Vychegda relates to Byzovaya horizon (Fig. 4). Two outcrops – Kuryador and Baika – could be appointed as key-sections of this horizon (Fig. 2). At Kuryador, Byzovaya horizon was studied by several research groups and interpretations of lithostratigraphic data are very controversial: 1) the White Sea proglacial lake deposits which had been accumulate due to the blockage of Severnaya Dvina by the Barents Sea ice sheet (Lyså et al., 2011); this hypothesis is disproved by geological data from the White Sea area (Korsakova, Kolka, 2009; Evzerov, 2010; Semenova et al., 2012; Zaretskaya et al., 2019), where non-glacial and marine sedimentation environment is reconstructed for the MIS 3; 2) The lake deposits (Andreicheva et al., 2015); this hypothesis is disproved by sedimentological and plant macrofossil data (Maksimov et al., 2015; Zaretskaya et al., 2019); 3) We consider the deposits of the 2nd Vychegda terrace as the river alluvium (floodplain, floodplain lake and oxbow lake facies) interbedded with paleosols; this interpretation is confirmed by sedimentological and plant macrofossil remains of dry-meadow vegetation (Maksimov et al., 2015; Zaretskaya et al., 2019) and traces of burrowing rodent holes under the paleosol layers. The chronostratigraphic record of Byzovaya horizon is well supported with 14C, OSL- and 230Th/U-dates and biostratigraphic information, thus its partitioning is quite detailed: 12 episodes of warming and cooling were defined based upon the data from Vychegda basin (Zaretskaya et al., 2019). Alluvial sedimentary environment predominated within the river valleys; paleosols or peat layers formed during the warmer episodes. The upper (terminal) layers of Byzovaya horizon form the bottom parts of the 1st Vychegda (MIS 3/2) terrace, comprising the time interval 32–27 cal kyr BP; alluvial sedimentation prevailed together with thin organic layers accumulation, and eolian accumulation started at Kuryador. We suppose another Vychegda incision right before the 32 cal kyr BP, probably caused by climate changes. Palynological record from Kechoyag key-section (Fig. 2) demonstrates the cooling of MIS 3/ MIS 2 transition. Alluvial and aeolian sedimentation had not been ceased after the transition to early phase of the Polar horizon formation. In the neighbor Pechora catchment area Bysovaya horizon is also represented in a series of sections of the 2nd terrace of the rivers Pechora (Bysovaya section and site), Shapkina, Bolshaya Rogovaya and Usa (Mamontovaya Kuria section), though it is composed mainly by channel alluvium with ice wedge casts, lack of floodplain and oxbow lake sediments and pollen spectra reflecting periglacial sedimentary environment (Astakhov, Svendsen, 2011). Channel alluvium at the Mamontovaya Kuria section had been OSL-dated to 50–38 ka, enclosed mammoth bones– 37–32 cal kyr BP, and overlying floodplain sands – to 9

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within the Severnaya Dvina valley, the sedimentary pattern was mostly alluvial, and prevailed during the Holocene.

Incision of Vychegda into this terrace occurred in the Late Glacial due to both climatic reasons and the drainage of the proglacial lake in the Severnaya Dvina valley with corresponding formation of the younger fragments of the 1st terrace (Zaretskaya et al., 2014). Thus the younger part of the Polar horizon forms the bottom parts of the younger generation of this terrace. The chronostratigraphic record of Polar horizon is well supported with 14C, OSL and IR-OSL-dates and biostratigraphic data and we can separate two stages of its formation: 27–17 ka (around LGM) and 17–11.7 ka (deglaciation).

Data availability Geological maps cited in the text are almost available from the website of VSEGEI (A.P. Karpinsky Russian Geological Research Institute): https://vsegei.ru/en/ The Calib v. 704 calibration software and the IntCal 2013 calibration curve are available from the RADIOCARBON home page: http:// radiocarbon.webhost.uits.arizona.edu/ The collection of reference plants is available in the herbarium of the museum of Institute of Ecology of Plants and Animals, Ural Branch of RAS, Ekaterinburg, https://ipae.uran.ru/

4.4. Holocene horizon Holocene horizon (floodplain, aeolian and peat bog deposits) is widespread over the Vychegda catchment area and is still accumulating from 11.7 cal kyr BP.

Acknowledgements

5. Conclusions

This work was supported by the Russian Foundation for Basic researches [grant number 17-05-00706], studies of lower reach of Vychegda were sponsored by Russian Science Foundation [grant number 17-17-01289], following the plan of the scientific research of the Institute of Geography, Russian Academy of Sciences [0127-20190008] and the Geological Institute of Russian Academy of Sciences [0135-2019-0059].

We collected the data for the Middle and Upper Pleistocene stratigraphy and chronology of the Vychegda River basin which are currently available from geological reports, maps, papers and our own studies. More than 20 sedimentary sections had been studied within the valley of Vychegda and its tributaries which provided the litho-, chrono- and biostratigraphic information concerning the Timan-Pechora-Vychegda subzone of the Middle-Late Pleistocene glaciations (W-Ib) for the last 250 ka. However, the sections which might be suggested as key-sections are only five: Kuryador, Baika, Kechoyag, Biostation and Gusikha, comprising collateral archives, presenting the variety of results by different methods which complement and validate each other. The rest of the mentioned sections provide useful supplementary data. The main stratigraphic and eventual “benchmarks” are as follows:

References Andreicheva, L.N., 2009. Paleogeographicheskiye obstanovki formiriovaniya otlozheniy v opornom razreze verkhnego pleistocena ’Kuryador‘ na verkhney Vychegde [Paleogeographic settings of the sedimentation at Kuryador, the key section for the Upper Pleistocene in the Upper Vychegda River]. Vestnik Instituta Geologii Komi Nauchnogo Zentra UrO RAN 10/11, 4–6 (In Russian). Andreicheva, L.N., Marchenko-Vagapova, T.I., Buravskaya, M.N., Golubeva, YuV., 2015. Prirodnaya Sreda Neopleistocena-Golozena Evropeiskovo Severo-Vostoka [Palaeoenvironment of the Neopleistocene-Holocene in the European North-East]. GEOS Press, Moscow, pp. 223 (In Russian). Astakhov, V.I., 2013. Pleistocene glaciations of the Northern Russia – a modern view. Boreas 42, 1–24. https://doi.org/10.1111/j.1502-3885.2012.00269.x. Astakhov, V.I., Svendsen, J.I., 2011. Pokrovnaya formatsia finalnogo pleistocena na krainem severo-vostoke Yevropeiskoy Rossii [Cover formation of the final Pleistocene in the far North-East of the European Russia]. Reg. Geol. Metall. 47, 12–27 (In Russian). Chalov, R.S. (Ed.), 2012. Ruslovye Process I Vodnie Puti Na Rekah Basseina Severnoi Dviny [Channel Processes and Waterways on the Rivers of the Severnaya Dvina Catchment Area]. Moscow State University & “Sevvodput”, Moscow 492 pp. (In Russian). Chernov, A.V., Zaretskaya, N.E., Panin, A.V., 2015. Evolyuciya i dinamika verhnej i srednej Vychegdy v Pozdnelednikov'e i golocene [Evolution and dynamic of the upper and middle Vychegda in the Late Glacial and Holocene]. Rev. Russ. Geogr. Soc. 147 (5), 27–49 (In Russian). Ehlers, J., 1996. Quaternary and Glacial Geology. John Wiley and Sons Ltd., Chichester 578 pp. Evzerov, V.Ya, 2010. Oledenenia Belogo moria [glaciations in the White Sea area]. White Sea system I. Nauchni Mir 76–94 (In Russian). Grimm, E., 1991. TILIA and TILIAGRAPH. Illinois State Museum, Springfield 56 pp. Guslitser, B.I., Duriagina, D.A., 1983. Prirodnye obstanovki v bassejne verhnej Vychegdy v sredne – pozdnevaldajskoe vremya [Palaeoenvironment within the Upper Vychegda during the Middle-Late Valday time]. In: Geology and Resources of the European North-East of the USSR. Proceedings of the Geological Institute, Syktyvkar, pp. 26–27 (In Russian). Guslitser, B.I., Loseva, E.I., Lavrov, A.S., Stepanov, A.N., 1986. Timan-Pechora-Vychegda Region (Chart II) (Timano-Pechoro-Vychegodskij Region (Skhema II)). Decision of the 2nd Inter-departamental Meeting on the Quaternary System of the EasternEuropean Platform. Leningrad. pp. 25–38 (In Russian). Head, M.J., Gibbard, P.L., 2015. Formal subdivision of the quaternary system/period: past, present, and future. Quat. Int. 383, 4–35. Heggen, H.P., Svendsen, J.I., Mangerud, J., Lohne, O.S., 2012. A new palaeoenvironmental model for the evolution of the Byzovaya Palaeolithic site, northern Russia. Boreas 41, 527–545. https://doi.org/10.1111/j.1502-3885.2012.00259.x. Helmens, K.F., Räsänen, M.E., Johansson, P., Jungner, H., Korjonen, K., 2000. The last interglacial-glacial cycle in NE fennoscandia: a nearly continuous record from Sokli (Finnish Lapland). Quat. Sci. Rev. 19, 1605–1623. Kalberg, E.A. (Ed.), 1967. Gosudarstvennaya Geologicheskaya Karta SSSR Mashtaba 1;200000. List P-39-XXI Mezen’ [State Geological Map of the Russia at Scale1:200000. Sheet P-39- XXI Mezen’]. All-Russian geological institute (VSEGEI) Press, Leningrad (in Russian). Karmanov, V.N., Chernov, A.V., Zaretskaya, N.E., Panin, A.V., Volokitin, A.V., 2013.

• The “numerical” history (eventual stratigraphy) of the Vychegda •

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catchment area started from Rodionovo time and horizon (249 - 203 ka); the lack of geological data do not permit reconstructions of the sedimentation pattern of the Rodionovo interglacial in this area. The Vychegda glaciation (186–111 ka) covered almost the whole study area. The rather huge associated proglacial lake formed in the valleys of Vychegda and its tributaries in the eastern part of catchment area, overflowing the water dividing line between Vychegda and Kama rivers (135 m a.s.l.) through the Kel'tma spillway or even covering the Vychegda-Kama-Pechora watershed area and could have survived also in the beginning of MIS 5 (Sula interglacial). After the proglacial lake degradation (Sula interglacial, MIS 5e) and during the Laya cold stage (MIS 5d – MIS 4) the sedimentary environment was mainly erosional then depositional. The northwestward runoff of Vychegda–Severnaya Dvina fluvial system had not been blocked by the MIS 4 glaciation, and the 3rd Vychegda terrace has been formed. During Bysovaya interstadial (MIS 3) the alluvial sedimentary environment prevailed within the Vychegda catchment area and the 2nd (67–32 ka BP) and the 1st (32–27 ka BP) terraces were forming. Abrupt and short-period climatic oscillations during the MIS 3 comprise 12 episodes of cooling and warming; buried soils or peat layers formed during the warmer episodes. Polar horizon was formed during the Late Weichselian (27–17 ka (around LGM) and 17–11.7 ka (Late Glacial)) and is determined in the valley of Vychegda and its tributaries. Its origin is alluvial (the most of 1st terrace) and aeolian (the upper part of the 2nd terrace). No glacial neither glaciolacustrine deposits of the Late Weichselian (LGM) age had been identified within the Vychegda catchment area. The transition from the alluvial to aeolian accumulation in the 2nd terrace is sedimentologically continuous, indicating the aridization of climate during the LGM, and low water level of Vychegda River. During the Late Glacial, after the drainage of the proglacial lake 10

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