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Geographic and geomorphologic context of Caspian Sea level fluctuations over the Late Pleistocene and Holocene

Contexte géographique et géomorphologique des fluctuations du niveau de la mer Caspienne depuis le Dernier Maximum Glaciaire
Catherine Kuzucuoǧlu et Suzanne Leroy
p. 71-92

Résumés

Cet article a pour objectif de présenter l’état des connaissances sur les changements de niveau de la mer Caspienne dans le cadre de son bassin versant, et de présenter des données-références de la chronologie des hauts et bas niveaux. Les facteurs contrôlant ces changements de niveaux sont exposés, qu’il s’agisse de contrôles par le contexte climatique, ou de tout autre contrôle. Après une introduction présentant le cadre physique de la mer Caspienne (bassins et seuils, structure géologique et reliefs environnants), les composantes climatiques influençant les changements de niveau dans les bassins sont présentées. Les changements de niveau sont discutés séparément pour la fin du Pléistocène et pour l’Holocène, car l’étude de chaque période requiert des méthodes différentes, et aussi parce qu’un large éventail de facteurs influence le niveau de la mer Caspienne. C’est pourquoi, après une synthèse des publications sur le sujet, et après un bref rappel des limitations méthodologiques, l’article propose une discussion sur le rôle du climat versus les autres facteurs de contrôle de changements de niveaux. Ces autres facteurs sont l’insolation, les changements d’extension du bassin versant de la mer Caspienne, spécifiquement vers le nord en direction de la zone Arctique et vers le sud-est jusqu’au Pamir, la grande extension latitudinale de la zone considérée, et aussi des facteurs locaux dépendant du relief et des connections hydrographiques (passées ou présentes), de la tectonique, de la dynamique des masses d’eau, et d’autres particularités géographiques et géomorphologiques dans le bassin versant.

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The authors would like to thank their laboratories for having provided infrastructure for the writing of this synthesis paper. The also thank the anonymous reviewers of the paper. This article was initially written as an oral contribution to a colloquium organized by the Maison de l’Archéologie et de l'Ethnologie (Nanterre, France) and the Musée d’Archéologie Nationale (Saint-Germain-en-Laye, France) in 2015. The title of the colloquium was Caspian Sea Shores: Contacts, Spaces and Territories along the Caspian Sea during the Bronze and Iron Ages, and it was organized by C. Lorre, R. Vallet, M. Mashkour and J. Bendezu-Sarmiento. The first author thanks this organizing committee to have brought us together in this occasion. Considering the archaeological orientation of the initial paper to be published soon in the colloquium publication, the paper presented here concentrates on geological and palaeohydrological data, including up-to-date Late Pleistocene and Holocene CSL changes data.

1 - Introduction

1In regional studies about past societies and climates in the Caspian Sea watershed, references to the specificity of this continental and brackish waterbody, and especially to its level changes (Furlani et al., 2012; Haghani et al., 2016; Haghani & Leroy, 2016; Leroy et al., 2019b; Ollivier et al., 2015, 2016), are usually put forward. This latter subject is of high significance for the understanding of past geographic and environmental changes in the Caspian Sea (CS) basin, especially since the Last Glacial Maximum (LGM) and in relation to archaeological, palaeogeographical and palaeoclimatic topics, thus:

2What is the relationship between Caspian Sea level (CSL) and geomorphological systems of the watershed?

3What is the relationship between CSL and climate?

4Based on such questions, the purpose of this review is to present data on the physical geography of the CS watershed, on the water level changes identified at diverse time scales (Pleistocene, Late Pleistocene, Holocene), and on their impacts on areas connected to the CS water body.

5These questions are important for the population living around the CS. One may still wonder what may have been the impact of CSL changes on the environment exploited and associated with human societies? Remains of Homo georgicus, a possible precursor of Asian Homo habilis (Vekua et al, 2000; Vekua et al.., 2002; Lordkipanidze et al., 2013), have been found at Dmanisi, in Georgia in the upper drainage area of the Kura River, a tributary of the CS. In addition, the mountain rivers of the Caucasus being very rich in archaeological remains since, at least, the Neolithic and possibly before, i.e. during the Palaeolithic (presence of Mousterian sites on the CS coast, see Dolukhanov et al., 2010), did CSL changes interfere with human environments and migrations during the Pleistocene and the Holocene? What kind of control or limitation did the CSL changes exert on human societies movements, developments, settlements and resources? Did CSL changes interfere with human mobility? e.g. are there possible linkages of specific CSL changes with momenta in the evolution of human societies?

2 - Physical geography of the Caspian Sea

6The following references, amongst many others, have been used for the bibliographic review of CSL changes through time: Varushchenko et al. (1987), Kostianoy & Kosarev (2005), Mischke (2020) and Leontiev et al. (2023).

2.1 - Setting

7The Caspian water body is very important at the global level, since it contains 40% of all lacustrine waters on Earth, which makes it the largest enclosed mass of water. Although in the distant past, it was a sea connected to the Black Sea (as part of the Paratethys Sea), it functions now as a lake. Its water volume is c. 78,200 km3, and the sea surface covers c. 371,000 km2 (Leontiev et al. 2023). Occupying a very deep basin, it is positioned mid-way:

8- west-east: between the Black Sea (i.e. the Eastern Mediterranean system) and the Aral Sea (i.e. a central Asian closed basin) and the Karakum desert;

9- north-south: between the lowlands of northern European steppes and cold forests, and a mountainous barrier formed by the northern edge of the Alpine/Zagros/Himalaya system (fig. 1). The N-S extension of the CS reaches nearly 1200 km while its average width is only 300 km (Leroy et al. 2020). Current elevation of water surface is at -28 m bsl (fig. 2).

Fig. 1: Wide geographic context of the Caspian Sea and its neighbours (Aral Sea and Black Sea). Fig. 1 : Contexte géographique général de la mer Caspienne et de ses voisines (mer d’Aral et mer Noire).

Fig. 1: Wide geographic context of the Caspian Sea and its neighbours (Aral Sea and Black Sea). Fig. 1 : Contexte géographique général de la mer Caspienne et de ses voisines (mer d’Aral et mer Noire).

Source: General Bathymetric Chart of the Oceans (GEBCO) (modified) (open source, online).
Source : General Bathymetric Chart of the Oceans (GEBCO)(modifié) (source ouverte, en ligne).

Fig. 2: The topographic setting of submarine and continental relief of the Caspian Sea. Fig. 2 : Topographie du relief continental et sous-marin du bassin de la mer Caspienne.

Fig. 2: The topographic setting of submarine and continental relief of the Caspian Sea. Fig. 2 : Topographie du relief continental et sous-marin du bassin de la mer Caspienne.

(A) Relief and marine currents. (B) Sub-marine topography and main rivers flowing in the Caspian Sea (with watershed evidenced by broken orange line). Source: online General Bathymetric Chart of the Oceans (GEBCO) (open source, modified).
(A) Relief et courants marins. (B Topographie sous-marine et principales rivières affluentes de la mer Caspienne (la limite du bassin drainé vers la mer Caspienne est soulignée par un tireté orange). Source : General Bathymetric Chart of the Oceans (GEBCO), (Source ouverte, modifiée).

2.2 - Submarine basins and sills

10Below the surface, the topography of the CS bottom presents three distinct basins (fig. 2). Along the foot of the Alborz range in Iran, the southern basin reaches 1050 m bsl. The limit between the southern and the middle basins, is formed by the Apsheron Sill. The top surface of this sill is 80 m bsl; it is interrupted from SSW to N by one narrow, elongated depression reaching 240 m depths (Ferronsky et al., 1999). It opens to the middle CS as a broad mouth outlet. Northwards, another topographic step, the Mangyshlak Threshold, leads to a very shallow basin, the North basin, corresponding to a wide depression gently sloping south (fig. 2), the bottom of which is blanketed by a thick mantle of alluvial material deposited by rivers flowing from the north and east, alternating with lacustrine sediment.

11In the northern Caspian basin, the mean water depth is only 5 m. In its southernmost part, the bottom of this shallow depression reaches 40 m bsl. In the northern part, three coalescent submarine river deltas expanding over the depression form a belt very sensitive to minor level oscillations. This northern basin (fig. 2) extends over c. 100,000 km2 and is limited south by the SW-NE oriented boundary with the middle Caspian built by the Mangyshlak Threshold. This latter slope is formed by structural rises which, reaching the water surface, form the Tyuleny and Kulaly islands emerging along the Zhemchuzhny shoals (Leontiev et al., 2023).

12South of the Mangyshlak Threshold, the middle Caspian basin (fig. 2) expands over 138,000 km2 (water surface) and has a maximal depth of 788 m. This irregular depression is limited by an abrupt slope to the west, and a gentler gradient slope to the east. The shallowest part of the basin is a shelf reaching 100 to 140 m bsl. The western slopes of the basin are imprinted by landslides and canyons, while eroded remains of ancient river valleys are recognizable east in direction of the central depression. These landforms record a submerged continental plain deepening eastward (Putans et al., 2010).

13Limiting the middle basin to the south, the Apsheron Sill is a NW-SE oriented, 80 m deep, belt of shoals and islands built by the oldest rocks of the basin (Precambrian, Carboniferous: Ferronsky et al., 1999). Deepening down a 50-60 m cliff toward south, it overlooks the southern Caspian basin (fig. 3) which contains two thirds of the water mass of the lake. On the surface, the southern depression covers c. 150,000 km2 and its bottom reaches > 1023 m bsl. Its bottom is relatively flat below a shelf which, from being narrow to the west widens south-eastward at the foot of the Alborz highlands, becoming much larger eastwards in direction of the Turkmenistan lowlands.

Fig. 3: Main structural features in the southern part of the Caspian Sea watershed. Fig. 3 : Principales lignes structurales dans le bassin méridional de la mer Caspienne.

Fig. 3: Main structural features in the southern part of the Caspian Sea watershed. Fig. 3 : Principales lignes structurales dans le bassin méridional de la mer Caspienne.

Source: modified from Özyavaş et al. (2010).
Source : modifié d’après Özyavaş et al. (2010).

14Adapting to this succession of basins deepening south, water circulation follows a counterclockwise movement (north-to-south along the western shore), developing south a complex pattern presenting several occurrences of subsidiary movements (fig. 3) (Lahijani et al., 2019). When these currents coincide with strong winds, the sea surface is often ruffled by wave action, while the highest storm waves occur near the Apsheron Peninsula (Leroy et al., 2022b).

3 - Geological structures

15Some of the structures controlling submarine reliefs of the CS are very old. The best examples are in the southern basin that rests above a very ancient sub-oceanic-type basaltic floor which belongs to the ‘Tethys Ocean’ that disappeared in the region during the Mesozoic c. 150 Ma ago. Despite huge accumulations of sedimentary layers many km thick (Ghorbani, 2013; Berberian, 2020), this oceanic basaltic bedrock geologically defines the Caspian as a sea. Other structures and rocks occurring in the CS are continental-type crustal structures (i.e. ‘continental crust’ or ‘continent plates’) dating back to the Cambrian Epoch, i.e. at least back to c. 541 Ma ago (Kazanci & Gulbabazadeh, 2013). These structures record tectonic deformations caused by continental plate tectonics. For example, between the north and middle basins, the Mangyshlak threshold is formed by structural remnants of an outlying part of the orogenesis that occurred c. 300 Ma ago (Zanchetta et al., 2009). Westwards, the submarine shelf of the Middle basin belongs to the sagging edge of the Greater Caucasus continental sub-plate, while in the east the submerged Turan sub-plate swells up in the Kara-Bogaz Gol area.

16Starting during the Miocene, 26 Ma ago, and continuing until 10 Ma, the Alpine orogenesis generated folds of various magnitudes that still structure today’s Caucasus range and its extension to the Kopet Dagh. On the western shores of the CS, the Apsheron Peninsula results from this folding activity. About 23 Ma (Mid-Miocene), the collision between the western Asian and Arabian continental plates caused an intra-plate compression generating deformations such as folds, thrusts, uplifts, faults. These tectonic movements affected all regions from today’s Mediterranean area to Western Turkmenistan, generating new structures which still control the regional landforms and submerged reliefs in the Caspian region (Kuandykov et al., 2010) (fig. 3). In addition, and starting c. 15 Ma ago, the regional uplift that occurred in the northern Mediterranean from the Dinarids to the Taurus and Zagros, also affected the Caucasus area, causing the closure of the CS (Kazanci & Gulbabazadeh, 2013). Around 5.5 Ma ago in the Caucasus range, folding and faulting added other deformations to those caused by the uplift, forming the anticline structuring the Greater Caucasus, and the more complex uplifted and thrusted folds and basins of the Lesser Caucasus. Structural compression also generated the two important strike-slip fault-controlled highs which separate the three CS basins (Mangyshlak Threshold and Apsheron Sill) and the linear troughs limiting the parallel Upper and Lower Caucasus ranges (fig. 1). The linearity of the bordering steep slopes of the thresholds is typical of such active strike-slip fault system (Jackson et al., 2002). Today, the folding activity still affects the Caspian area, especially along and around the middle and southern Caspian basins (Jackson et al., 2002). It is the cause of several deadly earthquakes (Leroy et al., 2022b).

17The CS contains several large reservoirs of oil and gas. In the south basin, the hydrocarbon formed in the Oligo-Miocene Maikop formation, and is trapped in younger Pliocene sands (e.g. Smith-Rouch, 2006; Washburn, 2018; Lebedev & Kostianoy, 2019; Zeinalzadeh et al., 2021). It is capped by Pliocene clays. This geologically relatively young system is favoured by the strong subsidence of the south basin and the rapid surrounding orogenesis. This leads to very high sedimentation rates, especially in the south basin (Brunet et al., 2003).

18During the early Pleistocene (starting 2.6 Ma ago), glacial advances and retreats started, crossing the northern Russian Plains and hilly areas, as did also mountain glaciers in the uplifting ranges around the Caspian basins (i.e. Caucasus, Alborz, Pamir) – cf. multiple articles from Mamedov (1997), Rychagov (1997), to Bolikhovskaya & Makshaev (2020) –. During glacial advances, temporary oceanic connections between the Barents Sea (Ostendo, 2023) and the Ponto-Caspian region occurred across the northern Russian plains (Hoyle et al., 2020). Pollen analysis in Azerbaijan has revealed that glacial-interglacial vegetation changes occurred regularly at the scale of the obliquity (41 ka) during the Akchagylian highstand (Hoyle et al., 2020). These deposits are unusually well-dated as they contain volcanic ashes, produced by volcanoes, so far still unidentified.

19On later occasions, this geographical context favoured connections/disconnections of the CS with the Black Sea and Aral Sea, which happened also to be at times connected with ice meltwater flowing from northern regions of Russia (Tudryn et al., 2013, 2016) (fig. 4). Responding to climatic changes, the CS shrank and expanded many times. Similar regular changes to those forced by obliquity (and later by eccentricity) that impacted vegetation, must have occurred together with water level although these could not be highlighted yet. The most recent major change in CS volume is known as the Khvalynian highstand (e.g. Arslanov et al., 2016; Tudryn et al., 2013, 2016, 2022). In addition, similar paths (now dry) connecting the CS and the Aral Sea are still visible north of the Karakum desert and along the Uzboy palaeoriver (Boomer et al., 2000, 2009; Létolle, 2002; Létolle et al., 2007) (fig. 4). These hydrological changes are recorded in terraces marking old shorelines, especially visible in places of orogenesis (such as in Azerbaijan), and in sediment deposited on the water bottom.

Fig. 4: Thresholds and paths connecting the basins occupied by the Aral, Caspian and Black Seas. Fig. 4 : Seuils et corridors de connections entre les bassins occupés par la mer d’Aral, la mer Caspienne et la mer Noire.

Fig. 4: Thresholds and paths connecting the basins occupied by the Aral, Caspian and Black Seas. Fig. 4 : Seuils et corridors de connections entre les bassins occupés par la mer d’Aral, la mer Caspienne et la mer Noire.

(1) Manych Strait (Black Sea connection) is today topping at +40 m aGsl (i.e. +66 m aCsl). (2) Volgograd-Don Strait (Black Sea connection), topping at +78m aGsl (i.e. +104 m aCsl). (3) Aral Sea, and the Uzboy valley eventually capturing either or both Amu Darya and Aral Sea Basin. Source inset 1: modified from Tudryn et al. (2013) (see also Létolle et al. 2007). Source inset 2: GeoMapApp open application, modified with colors spanning every 25 m from -25 m to 400 m, where deep blue is below -400 m, and white is above +400 m aGsl).
(1) Corridor de Manych (avec la mer Noire), altitude max. à +40 m aGsl (i.e. +66 m aCsl). (2)  Corridor entre la rivière Don et la mer Noire, susceptible de capturer le Don via la Volga, altitude max. à +78 m aGsl). (3) Corridor formé par la vallée de l’Uzboy vers la mer d’Aral puis le bassin de la mer d’Aral (ex : Amu Darya). Source encadré 1 : modifié de Tudryn et al. (2013) (voir aussi Létolle et al., 2007). Source encadré 2 : fond construit avec l’application GéoMapApp (ouverte en ligne) coloré avec une palette de couleurs changeant chaque 25 m de -25 m à 400 m. Le fond bleu correspond aux profondeurs sous -400 m, et le fond blanc aux altitudes supérieures à +400 m aGsl.

4 - Climate components and their relation to relative changes in sea level

20The CS basin is the meeting place for several and much contrasted regional climates. The continental climate dominates the northern CS, whereas the central and southeastern and southwestern parts of the sea are in a dry climate belt (fig. 5). The south shore has a subtropical humid climate, favouring the expansion of the warm-humid forests (Hyrcanian vegetation) in the Alborz Range (Akhani et al., 2010). Generally, the northern part of the watershed of the Caspian (i.e. northern Russia) is dry, cold and moderately continental steppe, getting warmer and wetter southwards. Eastwards, aridity increases, causing a steppe vegetation to transform into desert especially in the south-east corner of the CS.

Fig. 5: Physical parameters of the Caspian Sea water: precipitation, air and surface temperatures, salinity. Fig. 5 : Paramètres physiques de la mer Noire : précipitations, températures de l’air et de surface, taux de salinité.

Fig. 5: Physical parameters of the Caspian Sea water: precipitation, air and surface temperatures, salinity. Fig. 5 : Paramètres physiques de la mer Noire : précipitations, températures de l’air et de surface, taux de salinité.

Sources: Redrawn from the Caspian Environment Programme 2009, with the addition of Kavak (2012), modified, for mean surface temperature map.
Sources : Caspian Environment Programme, 2009 (modifié), et Kavak, 2012 (modifié) pour la carte de températures de surface de la mer.

4.1 - Atmospheric circulation in the Caspian watershed

21According to Nandini-Weiss et al. (2019), the mid-latitude westerly winds bring humidity-bearing depressions controlled by the North Atlantic Oscillations (NAO) during most of the year. When arriving, these depressions download rainfall and snowfall, especially over the Russian plains and Caucasus ranges. Conversely, precipitation in the east and most of the southeast and southwest region is low (see fig. 1 in Leroy et al., 2013a). In detail, northerlies and north-westerlies compose nearly two-thirds of air masses inflows over Russia and the Caucasus. The northern slopes of the Alborz Mountains receive very high mounts of precipitation especially from air loaded with water from the CS (Molavi-Arabshahi et al., 2016). Additional atmospheric circulation features influencing the CS catchment area include, during winter: (1) polar and subtropical jet streams at upper tropospheric levels (Molavi-Arabshahi et al., 2016), and (2) cold and clear air of the Asian anticyclone, i.e. the Siberian High.

22Today, a controversy remains about the role of the NAO in triggering the hydrology of the Caspian catchment region and, hence, of the CSL. While Rodionov (1994) and Panin et al. (2015) argue in favour of NAO-driven CSL changes through precipitation influences, other studies could not find this connection with NAO, suggesting additionally an important role of the El Niño-Southern Oscillation (ENSO) in driving CSL variations (e.g. Arpe & Leroy, 2007; Arpe et al., 2000).

4.2 - Water balance

23As for all land-locked water masses, the water balance of the CS results directly and only from water inflow and outflow. Inflow is almost totally controlled by (i) precipitation over the continental basin and the CS itself, and by (ii) river and ground water runoff collected in the watershed by more than 130 rivers running into the lake (Rodionov, 1994; Arpe et al., 2000; Leroy et al., 2020).

24Apart from a small CS water discharged into the Kara-Bogaz Gol (gulf east of the CS), output is mainly due to evaporation from the sea water surface. All these components of the balance are directly linked to atmospheric circulation, if no other controlling factor occurs (e.g. impacts of geological, hydrographical or human origins).

4.2.1 - Inputs: Precipitation and river inflow

25Although much contrasted in topography and climate belts, the drainage area (c. 3.7×106 km2) around the Caspian is c. 10 times larger than the surface of the CS itself. Water input is composed of (i) precipitation on the water surface (as a small component), and (ii) river run-off, which is composed by more than 130 rivers running into the CS (Rodionov, 1994; Arpe et al., 2000).

26In the CS watershed, the seasonal cycle of precipitation varies strongly, with maximal precipitation in summer over the northern part (Volga basin), in spring over the western part (Kura/Terek area), and in autumn-winter over the southern CS (the eastern CS coastal area being a desert) (fig. 5) (Lahijani et al., 2008; Molavi-Arabshahi et al., 2016; Nandini-Weiss et al., 2019). Meanwhile, evaporation over the CS is the highest in autumn (September) (Rodionov, 1994) while the precipitation total amount over the water surface varies between 200 and 1700 mm/year.

27Five rivers deliver most of the water to the sea. Of these, the most important is the Volga that contributes to the budget with c. 80 to 90% of the total water input (fig. 5). In the northern drainage areas of the CS, the Volga River and its tributary reach the low circum-Arctic regions of Russia characterized by very humid soils and abundant springs. Worth pointing, the highest land separating the northernmost limits, i.e. between the Volga River basin and the Barents and the Baltic seas, consists of low elevations which are only c. 150 m asl. Toward the East, the northern watershed of the Volga River gets close to the Ural highlands, from where the Ural River flows in direction of the Volga River for a while (fig. 6). Outflowing in the western part of the northern and middle basins of the CS, the Terek River and other steep-sloped streams drain the northern Caucasus and Khokh highlands.

Fig. 6: Annual water discharge by main rivers into the Caspian Sea. Fig. 6 : Apports des débits annuels des principales rivières de la mer Caspienne.

Fig. 6: Annual water discharge by main rivers into the Caspian Sea. Fig. 6 : Apports des débits annuels des principales rivières de la mer Caspienne.

Source: Redrawn from P. Rekacewicz (le Monde Diplomatique) assisted by L. Margueritte and C. Marin, updated by R. Pravettoni (GRID-Arendal), and V. Novikov (Zoi Environment Network). Open-source file.
Source : Modifié de P. Rekacewicz (le Monde Diplomatique), assisté de L. Margueritte et C. Marin, mis à jour par R. Pravettoni (GRID-Arendal) et V. Novikov (Zoi Environment Network). Source ouverte.

28Combined annual flows from these three northern rivers (Volga, Ural, Terek) account to c. 88% of all river water entering the CS In addition, the Caucasian rivers flowing from the west (Sulak, Samur, Kura, and a high number of smaller rivers), account for only c. 7% of the total flow into the CS. The rivers Kura and Aras drain most of the southern Caucasus (UNECE, 2011). They both join in Azerbaijan before flowing to the CS, south of the Baku Peninsula. Note that the Aras spring is located as far as the vicinity of Erzurum in Turkey. This river drains also parts of Eastern Anatolia and northern Iran (figs. 1 & 2).

29The remaining river input (5%) comes from Iranian rivers entering the southern shores, and a very low contribution from the south-eastern shores (Atrek and Gorgan rivers). The eastern shores of the CS are so arid that they provide almost no permanent water to the sea.

4.2.2 - Outputs (temperatures, evaporation) and resulting salt content of the sea

30Monthly average Caspian Sea Surface Temperatures (CSST), range from -10 C in the north to 12 °C in the south in winter (Sea Temperature Info 2023a, 2023b). Seasonally, summer CSST are rather evenly distributed with average July to August figures ranging between 24 and 27 C. During winter, ice formation affects the northern CS, which usually freezes completely by January. Occasionally, ice floating along the western shore may come as far south as the Apsheron Peninsula. In the southern part of the basin, air temperatures may rise to 41 C in July and aridity is extensive (fig. 5).

31Yearlong, the evaporation from the sea surface is very high (1015 mm/year), with a distribution exhibiting a very high contrast from north to east (Kavak, 2012).

32The resulting salinity of the water varies much from North to South (fig. 5). In the northernmost part of the basin, water inputs by the rivers Volga and Ural abundantly dilute the Caspian water, while the waters become brackish in the middle and south basins (Leroy et al., 2013a). The monthly variability of the surface salinity (SSS) between 1960 and 2003 in the three basins of the Caspian Sea, has been studied by Dyakonov & Ibrayev (2019: Fig. 8). Results show that:

33- in the northern basin, SSS oscillated between 4.5 psu (min. reached in 1995) and 10 psu (max. reached in 1967, 1969 and 1977);

34- in the middle and southern basins, salinity values are clearly higher, with much less monthly variations, with seasonal data varying between 11 and 12.3 psu, and 12.6 and 13 psu, respectively;

35- the SSS overall evolution in the Caspian Sea between 1960 and 2003 shows a decreasing trend in the three basins;

36Such differences in salt concentration between the water bodies composing the water mass of the CS provoke superficial currents mixing parts of these water bodies (fig. 2). The salt content of the water is thus unstable through space and time. This is because of high seasonal and spatial variations due to seasonal characteristics of (i) river run-off discharge into the CS, (ii) intensity of turn-over of water along the vertical mass of the CS water within the deeper middle and southern deep basins as well as the occurrence of some upwelling along the eastern shores, and (iii) heat waves related to the desert air masses in the east and south-east of the Caspian basin and lowlands where they cause a high evaporation to provoke high salt concentration increase on the surface water (fig. 5).

37At the mid-point of its eastern shores, the CS flows into the Kara-Bogaz Gol depression, the surface of which is positioned a few meters below today’s CS water surface level and has a very high evaporation. This depression is the smallest part of the CS, not only because of its small surface (shrinking quickly today, its watered area used to account for 3% of that of the sea), but also by its significantly low water volume. Shallow water stretches extend between sand bars. Water quickly evaporates and salinity reaches extremely high values, i.e. more than twenty times the water salinity of the Middle Caspian (Giralt et al., 2003; Leroy et al., 2006).

5 - Water level changes since 1840

5.1 - Monitoring data

38Monitoring data derive from a combination of gauges and of satellite imagery. It is useful to remind us here that the levelling of the CSL was done in reference to the Baltic Sea. During the past 150 years, large and rapid fluctuations of ~ >3 m occurred between 25.8 and 29 m bsl (Arpe et al., 2000; Arpe & Leroy, 2007; Kostianoy et al., 2014; Chen et al., 2017).

39After a rather stable level weakly oscillating around -26 m between 1840 and 1930, the level suddenly dropped by c. 1.8 m between 1930-41 (also catastrophic arid years in the mid-US and mid-Turkey: Kuzucuoğlu & Gramond, 2006) and 1942. From ~-27.7 m in 1942, the decrease continued to -28.2 m in 1950, and reaching a minimum of ~-29 m in 1977. After 1978, a rise occurred with a +13.09 cm.yr-1 ascent rate, until the CSL reached a peak at ~-26.6 m bsl in 1995. After this date, the CSL decreased rather rapidly to ~-27.3 m in 2002. This decrease was followed by a small rebound up to ~-26.8 m in 2005, after which the direction reversed again, with a declining trend, at a -6.72 cm.yr-1 rate during the following decades (Chen et al., 2017) (fig. 7).

Fig. 7: Level curve of the Caspian Sea from AD January 1993 to March 2022. Fig. 7 : Courbe du niveau de la mer Caspienne, de janvier 1993 à mars 2022.

Fig. 7: Level curve of the Caspian Sea from AD January 1993 to March 2022. Fig. 7 : Courbe du niveau de la mer Caspienne, de janvier 1993 à mars 2022.

Source: monthly mean Caspian Sea level changes observed by tide gauges (1993-1997) and satellite altimetry (1997 to 2022, provided by Legos/CNES. http://hydroweb.theia-land.fr/​). A systematic bias between tide gauge and altimeter series is removed (using a 4-year overlapping period 1993-1996).
Source : les données moyennes mensuelles du niveau de la mer Caspienne, relevées à parti de gauges locales (1993–1997) et les altitudes relevées par satellites (1997 to 2022), proviennent de Legos/CNES (http://hydroweb.theia-land.fr/​). Un biais systématique entre les séries des gauges et des altitudes a été éliminé (en utilisant le recouvrement de 4 années des données, soit 1993 à 1996).

40In March 2022, the level was at c. -28 m, meaning so far that (i) the post-1995 drop has reached -2.3 m in total, and that (ii) the CSL decrease rate has been of 11 cm.yr-1 between 2015 and 2022 (according to USDA 2022a and 2022b) (fig. 7).

5.2 - Possible causes of CS level changes between 1979 and 2022

41According to Arpe et al. (2000), evaporation of the CS water balance can be accentuated by the westerlies action over the CS basin, causing water vapour to leave the Caspian area, and to fall as precipitation further east. These authors also suggest that, during such occurrences, the exported CS water may cause an additional deficit in the balance of the Caspian water mass.

42It has also been proposed that extensive reservoir construction –for irrigation and industry– during the last decades, that led to a cascade of reservoirs over the Volga-Kama and its tributaries (fig. 8), contributed to accentuate the CSL decrease trend observed after the 1940s and after the 1995s (Terskii et al., 2022). However, the water stored in the reservoirs would have had little impact on the water reaching the CS once they are full.

Fig. 8: Dams in the Rivers Volga - Kuma watersheds. Fig. 8 : Barrages dans les bassins de la Volga et de la Kuma.

Fig. 8: Dams in the Rivers Volga - Kuma watersheds. Fig. 8 : Barrages dans les bassins de la Volga et de la Kuma.

Source of background map: Redrawn from K. Musser (under Creative Commons Attribution-Share-Alike 2.5 Generic). Open-source file).
Source de la carte de fond : modifiée à partir K. Musser (under Creative Commons Attribution-Share-Alike 2.5 Generic). Source ouverte.

43Nevertheless, due to the significant role of the Volga discharge in the water balance of the CS, it is important to stress that some part of the water level changes responds to changes in the Volga water discharges, meaning that some climatic events or drainage changes in the vast Volga catchment basin may also have impacted the CS level. The Caspian catchment is dominated (c. 88%) by the input from the Volga, Ural and Terek rivers. These drainage basins are subject to seasonally and spatially contrasted (i) W-E oriented Westerlies and (ii) the NE-SW Siberian Highs, precipitation and river flow in the Volga and Ural watersheds that control most of the changes in the northern Russian water input to the CS (e.g. Arpe et al., 2000; Kislov et al., 2014; Nandini-Weiss et al., 2019). As a result, reconstruction and prediction of water level changes must use modelling of (i) past and future Siberian Highs affecting humidity in the northern parts of the Caspian watershed, and (ii) include NAO and ENSO dynamics. Several research groups are trying to forecast CSL changes with some success by anticipating changes over the Volga drainage basin, as it takes a few months for the water to reach the CS (Arpe et al., 2000). Recent models including the CS area and recognising it as a sea (thus involving horizontal exchanges within the water masses), show that a longer-term forecast remains very difficult, with as many possibilities to have a drop than a rise (Fig. 5 in Koriche et al., 2021).

44In addition, rhythm and time range of water input to the waterbody are not only characterized by the time-space amounts and distribution of rain/snow fall/ice melting amounts in the northernmost part of the water divide, but also by:

45- possible changes in water divides (captures, diversions);

46- damming of river valleys… This latter remark being very important when considering the river drainage of the Volga (fig. 8). This leads then to examining longer time scales in the next sections.

47The impact of such water level changes, close to 3 m in the XXth century, has been felt on shoreline displacement, infrastructures, environment, and life overall (above as well as under water surface) (Leroy et al., 2022b). This will have happened many times in the past (Leroy et al., 2022a).

6 - Reconstructing level changes in the Caspian Sea

6.1 - Selection of field work areas

48The amplitude of CS water level changes submits the sedimentary records in the shallow environments of the basin to frequent emersion (leading to erosion and/or absence of sedimentation). However, shallow areas have been and still are subject to many investigations because of easy access and also because they are likely to be affected by a range of hazards and disasters, with water level changes at the top of all risks (Leroy et al., 2022b). These shallow areas (fig. 9) are coastal areas marine/coastal terraces and deltas. These are especially present in the northern part of the Caspian shelf where the water depth is ca 5 m, especially the whole of the Volga and Ural off-shores and on-shores parts of the river deltas (e.g. Kaplin & Selivanov, 1995; Rychagov, 1997; Hoogendoorn et al., 2005; Kakroodi et al., 2011; Kazancı & Gulbabazadeh, 2013; Leroy et al., 2013a; Richards et al., 2014; Arslanov et al., 2016; Haghani et al., 2016a,b; Haghani & Leroy, 2020; Leroy et al., 2022a); in the eastern lowlands which connected and disconnected through time with the CS (Leroy et al., 2019a), as well as with the Kara-Bogaz Gol (e.g. Leroy et al., 2006), the Aral Sea, Amu Darya and Syr Darya deltas (e.g. Létolle, 2002; Boomer et al., 2000, 2009), in the river deltas on the western shores, i.e. the deltas of the Terek, Kura and Aras rivers (e.g. Hoogendoorn et al., 2005).

Fig. 9: The northern part of the Caspian Sea depression, with curves marked every 10 m between -40 m and +5 m above Global Sea Level. Fig. 9 : Partie nord de la dépression occupée par la mer Caspienne, avec courbes de niveau espacées de 10 m entre -40 m et +5 m au-dessus du niveau de l’océan (aGSL).

Fig. 9: The northern part of the Caspian Sea depression, with curves marked every 10 m between -40 m and +5 m above Global Sea Level. Fig. 9 : Partie nord de la dépression occupée par la mer Caspienne, avec courbes de niveau espacées de 10 m entre -40 m et +5 m au-dessus du niveau de l’océan (aGSL).

Comment: This map shows (i) how sensitive is the northern part of the Caspian Sea northern basin to even small changes in elevation of the water level, and (ii) position of areas in the watershed where overflow (river capture, flow diversion) may happen during water level rises in direction of neighbor river/sea basins. Source: based on GeoMapApp open application on internet.
Commentaire : Cette carte illustre (i) la sensitivité de la partie nord de la mer Caspienne et de son bassin proche à des changements de niveau de la mer, même de faibles amplitudes, et (ii) les points qui, par leur altitude, sont susceptibles d’ouvrir des déversements possibles (captures de rivières, transferts vers des bassins externes) quand le niveau de la mer atteint leurs altitudes. Source du fond de carte : produite à partir des données de GeoMapApp, source ouverte en ligne.

49In these areas, highstand sediment have been studied in outcrops, especially along rivers. In particular, the mapping of the topography and the seismic profiling of the northern depression, today mostly immerged by water and by river sediments prograding in direction of the deeper basins, have been used for establishing the stratigraphic successions of progradation (sedimentation) vs retreat (erosion) of lake sediment units recording progression/regression successive phases of the CSL (Yanina et al., 2018; Bezrodnykh et al., 2020). In addition, cores at various depths have been retrieved, both in the northern part of the sea and in its southeastern part. Results from these cores have completed climatic reconstructions on the basis of physical and biological parameters.

50In addition, alluvial terraces in valleys of tributary rivers of the CS, such as in the Kura (Furlani et al., 2012, Ollivier et al., 2015, 2016,), the Volga (Panin et al., 2011; Tudryn et al. 2013, 2014, 2016; Panin & Matlakhova, 2015) and the Manych Strait (Dolukhanov, et al. 2010).

51With present mean depths c. 5 m and maximal depths hardly surpassing 10 m (fig. 9), the northern part of the Caspian contains today only c. 1% of the volume of the lake (although it represents one third of the total area of the CS). An important consequence of this shallowness is that this area repeatedly dried up in past geological epochs. Studies of cores and superficial landforms over the lake bottom, as well as terrestrial river landforms in the proximity of the waterbody, thus provide information on both wide and short magnitude level oscillations. In times of low magnitude changes, records from the northern basin provide more precise information on the water level changes than the other basins. At any case, this part of the basin dries easily; in such cases it is transformed into a terrestrial lowland which is rather silent during level falls that would reach > c. 100 m magnitude below today’s level of the lake. Another such example is the Kara-Bogaz Gol which is today filled only by a few meters of water. Its outlines repeatedly changed during past geological epochs, becoming either very vast with diluted waters expanding over the closed depressions around, or completely dry and transformed into a sebkha (Giralt et al., 2003).

52In the deeper basins (e.g. the middle and the south basins), the sequences are expected to be continuous. At these depths, changes may be recorded in salinity and sediment-water source proxies, such as biota and isotopic modifications. However, few deep-water cores exist (e.g. Leroy et al., 2019a). Deep lacustrine deposits have been until now poorly explored due to the lack of vessel devoted to research. Only ad hoc solutions have been used for the investigations published so far, at the exception of a few large projects in the deep areas (e.g. Chalié et al., 1997; Jelinowska et al., 1998,1999; Ferronsky et al., 1999; Kuprin et al., 2003; Boomer et al., 2005; Leroy et al., 2013b, 2014, 2019b, 2022a).

53Ideally both shallow and deep sequences should be studied in parallel. As usual in lake sediment records of lake level changes, cores are also used in association with stratigraphy studied in and out today’s lake domain (e.g. inundated valley bottoms and/or terraces, past/present coastal deposits, sediment from lake bottom).

6.2 - Selection of proxies

54Field and laboratory analyses tackle geological data about stratigraphy, facies, geochemistry, as well as data about biologic, chemical and lithologic characteristics of the sediments. In the CS, research has been traditionally based on the study of fossil molluscs that are often used to define loosely some stratigraphical periods. Some analyses address the ecological systems of the water at various depths (which depend on light, temperature, salinity of the water, such as dinocyst and ostracod assemblages) (Leroy et al., 2006, 2007, 2018; Boomer et al., 2005), while the sediments themselves inform about origin and process (river input, evaporation impact, etc.) with analyses such as Sr isotopes of bulk sediment (Pierret et al., 2012), magnetic minerals (Jelinowska et al., 1998,1999, Tudryn et al., 2013), and Nd isotopes (Tudryn et al., 2016).

55However, the CS has some peculiarities that constrain so far accurate constructions. Only some are listed here: (i) Foraminifera do not live below 50 m water depth, hence their limited value for stable isotope analyses commonly used elsewhere; (ii) The problem of the radiocarbon reservoir is very acute and is still poorly understood (see discussion in Leroy et al., 2022a). However, the OSL technique is increasingly applied in the region, especially where organic matter is missing and where the sediment is beyond the range of radiocarbon dating (Kurbanov et al., 2020); (iii) It is important to note here the absence of stratotypes, leading to possible confusions; (iv) Some of the biota are endemic and their ecology remains poorly known. For biota, only few training sets are available (e.g. Leroy et al., 2018); (v) The presence of large hydrocarbon reservoirs restricts scientific research, and data obtained by industry (e.g. seismic profiles) are often under embargo.

7 - Changes in the Caspian Sea level during the Late Pleistocene

56The chronology of the main highstands and main lowstands of the Late Pleistocene is limited by serious problems of dating. This has led to the co-existence of several parallel chronologies, often incompatible with each other (see discussions in Kislov et al., 2014; Yanina, 2014, Arslanov et al., 2016; Yanko-Hombach & Kislov, 2018). An in-depth critical review of all the ages (14C, OSL and U-Th) obtained for on-land and underwater samples has allowed proposing a new chronology (tab. 1) (Tudryn et al., 2022).

Tab. 1: Chronology of the late Pleistocene (Marine Isotopic Stages 3 and 2) water levels. Tab. 1 : Chronologie des niveaux marins du Pléistocène final (stades isotopiques marins 3 et 2).

Tab. 1: Chronology of the late Pleistocene (Marine Isotopic Stages 3 and 2) water levels. Tab. 1 : Chronologie des niveaux marins du Pléistocène final (stades isotopiques marins 3 et 2).

Source: from Tudryn et al. (2022). (1) First phase of the lowstand of Atelian, (2) Second phase of the lowstand of Atelian and of the following highstand of Early Khvalynian.
Source : Tudryn et al. (2022). (1) Première phase de le la régression atélienne, (2) Seconde phase de la régression atélienne et de la transgression du Khvalynien inférieur.

57During the highest highstands of the Last Glacial period (MIS 3 and 2), the CS was connected to the Black Sea through a palaeo-valley (today dry) called the Manych Strait. Of these, two highstands have been dated, which are known as the Hyrcanian and the Khvalynian (e.g. Dolukhanov et al., 2010; Yanko-Hombach & Kislov, 2018) (tabs. 1 & 2; figs. 10 & 11). Tudryn et al. (2022) suggest that the Hyrcanian transgression, fed by waters deriving from the Hindu Kush-Pamir-Tien Shan region, interrupted a lowstand called the Atelian regression (tab. 2) during the upper part of which the water level of the CS would have decreased down to 90 m bsl.

Tab. 2: Detailed chronology of water levels during the last 2200 years. Tab. 2 : Chronologie détaillée des niveaux de la mer Caspienne pendant les 2200 dernières années.

Tab. 2: Detailed chronology of water levels during the last 2200 years. Tab. 2 : Chronologie détaillée des niveaux de la mer Caspienne pendant les 2200 dernières années.

According to Leroy et al. (2022a).
D’après Leroy et al. (2022a).

58After the Atelian lowstand, two transgressions occurred: the Early and the Late Khvalynian transgressions, separated by a regression. This succession probably corresponds to the peak period of the glaciation, when the climate in these northern regions (corresponding to areas larger than today’s northern Russia) was dominantly very cold, i.e., during the LGM (fig. 10). During the intermediate regression, the CSL then dropped possibly to -45 m (Yanina, 2014; Rolanov et al., 2015) while, during the highstands, the surface of the resulting waterbody must have been considerably larger than today, as must have also been the Black Sea body. Another change, during both highstands, was the considerable modification in water salinity, as the water of the lake became mostly fresh because of excess water from precipitation and meltwater origins.

Fig. 10: Possible directions of melting delivered by the Fenno-Scandian inlandsis to northern Russia and Baltic areas during the c. 9000 yrs duration of the inlandsis retreat in direction of Iceland and northern Scandinavia. Fig. 10 : Directions possibles des écoulements issus de la fonte de l’inlandsis fennoscandien en Russie septentrionale et dans le bassin de la mer Baltique, pendant les 9000 ans qu’a duré le retrait de l’inlandsis vers le nord de la Scandinavie et l’Islande.

Fig. 10: Possible directions of melting delivered by the Fenno-Scandian inlandsis to northern Russia and Baltic areas during the c. 9000 yrs duration of the inlandsis retreat in direction of Iceland and northern Scandinavia. Fig. 10 : Directions possibles des écoulements issus de la fonte de l’inlandsis fennoscandien en Russie septentrionale et dans le bassin de la mer Baltique, pendant les 9000 ans qu’a duré le retrait de l’inlandsis vers le nord de la Scandinavie et l’Islande.

(A) Extension of the Fenno-Scandian inlandsis at Late Glacial Maximum, based on Svensen et al. (2004) and http://www. Glacier-climat.com/cg/quaternaire-de-planete/. (B) Circulation directions of melt water during the melting of the LGM inlandsis, based on ground topography, with occurrences of flat areas possibly causing instability of melt water directions, in turn possibly causing river captures over northern Scandinavia and northern Russia: 1/ Deep Sea basins below -1000 m bGsl (“below Global Sea level”); note that slopes descending to the Middle and Southern Caspian deep basins are less abrupt than those in the Black Sea and Mediterranean Sea (the intermediate levels from bottom of CS depressions to 0 Bsl, appear in green and yellow colors), 2/ Areas with surfaces between 0 and -300 m below Global sea level (bBsl), 3/ Areas from 0 to 300 m above Global sea level (aBsl), 4/ Areas between 300 m and 500 m topographic curves (aBsl), 5/ River Volga’s today river valley (with main direction of flow), 6/ River Dniepr valley today, 7 to 9/ Outflows during the first retreat of the inlandsis between 20 and 13 ka ca BP, 10/ Outflows from the 13 ka position of the retreating inlandsis, 11/ Spots of indefinite control on water flows (possible river flow confusions over flat surfaces), 12/ Area suitable for river connections/diversions/captures during the LGM and during the Late Glacial, 13/ Topographic connections of valleys through the Aral Sea basin, from north (Russian plains), north-east (Syr-Darya) and south-east (Amu-Darya). Source of map: interpretation of the topographic map of today’s emerged land north of the Caspian Sea associated with the maximum expansion area of the LGM inlandsis over Scandinavia, the Baltic Sea, the northern Atlantic and North Russia (20 ka cal BP).
(A) Étendue de l’inlandsis Fenno-Scandien au dernier Maximum Glaciaire, modifié de Svendsen et al. (2004) et de http://www. Glacier-climat.com/cg/quaternaire-de-planete/. (10) Routes possibles des eaux issues de la fonte de l’inlandsis fennoscandien à partir de la fin du LGM, basées sur la topographie du relief, et en tenant compte de larges surfaces planes susceptibles de causer des incertitudes dans les directions des flux, pouvant ainsi causer des captures entre bassins versants en Scandinavie et Russie septentrionales : 1/ Bassins marins sous -1000 m bGsl (sous le niveau marin mondial), notez que les talus séparant les fonds des trois bassins de la mer Caspienne sont moins abrupts que ceux relevés en mer Noire et mer Méditerranée, les fonds entre les bassins sud et moyen de la mer Caspienne (les niveaux intermédiaires) à partir du fond de la dépression marine jusqu’au niveau de surface (0m Bsl) sont colorés en vert et jaune, 2/ Zones situées entre 0 et -300 m au-dessous du niveau marin global (aBsl), 3/ Zones de 0 à 300 m au-dessus du niveau mondial de la mer (aBsl), 4/ Zones entre 300 m et 500 m au-dessus du niveau marin global (aBsl), 5/ Vallée actuelle de la Volga (la direction de la rivière est indiquée) ; 6. Vallée actuelle de la rivière Dniepr, 7 à 9/ Apports d’eau en provenance de l’inlandsis pendant la 1ère phase de son retrait (entre 20 et 13 ka cal BP), 10/ Apports d’eau le long de la limite sud de l’inlandsis à partir de 13 ka cal BP, 11/ Secteurs susceptibles d’avoir provoqué des confusions/changements d’orientation des flux d’eau de fonte (grands terrains plats), 12/ Secteurs susceptibles d’avoir provoqué des connections, changements de cours et captures pendant le LGM et le Tardiglaciaire, du nord (plaines septentrionales russes) et du nord-est (bassin du Syr-Darya) vers le sud et du sud-est (bassin de l’Amu-Darya) vers le bassin de la mer d’Aral. Source : interprétation de la carte topographique des terres aujourd’hui émergées au nord de la mer Caspienne, associée avec l’expansion maximum de l’inlandsis du LGM sur les bassins du nord de la Scandinavie, de la mer Baltique, de l’Océan Atlantique et du nord de la Russie (20 ka cal BP).

59Dated end of the LGM at c. 23-22 to 13-12 ka uncal BP (Arslanov et al., 2016; Tudryn et al., 2022), a late part of the Khvalynian transgression was triggered by the melting of the Fennoscandian inlandsis (fig. 10). According to several authors, e.g. Yanina (2014) and Tudryn et al. (2022), the Manych Strait was reached again, remaining active in connecting the CS and the Black Sea. Indeed, in the Black Sea, red layers characterized and dated at 18-15.5 cal ka BP, present a mineralogy hinting at a Caspian source (Bahr et al., 2005). However, Lericolais et al. (2013) and Soulet et al. (2013) showed, on the basis of red layers Nd isotope studies, that this sediment originated in the Baltic shield, and was transported with meltwater from the Scandinavian icesheet directly to the Black Sea by the River Dniepr. Similarly, the same material known as chocolate clays in the CS lowlands and as far south as the middle basin, has been transported to the CS by the Volga River (Tudryn et al., 2016).

60It is possible also that, during some intervals of the Quaternary, the drainage area of the Aral Sea and of the Amu Darya may have been connected to the CS through the Uzboy palaeo-valley, adding to the volume of the water and to its extension. However, according to Dolukhanov et al. (2010), the level of the CS topped at only a few meters below the threshold.

8 - Changes in sea levels during the Holocene

8.1 - From 12 to c. 4 ka cal BP

61At the end of the Late Glacial Late Khvalynian highstand, a regression phase started with high sediment-load floods between 12 and 11 ka cal BP, while meltwater from northern Russia profusely fed by a melting permafrost, maintained high freshwater input into the CS (from 11.5 to 10.5 ka cal BP) (Mayev 2010; Leroy et al. 2014, 2019b; Bezrodnykh & Sorokin, 2016; Bezrodnykh et al., 2020). Called the Mangyshlak lowstand, this regression reached the ~90 m bsl. According to Bezrodnykh et al. (2020) in a study north of the Mangyshlak Threshold, the water level remained quite low until c. 10.4 ka cal BP (tab. 2).

62After this major lowstand, the CSL shows a generally small amplitude of changes. The Mangyshlak lowstand was followed by a Neocaspian transgression. Depending on authors however, it is not clear when this Neocaspian period started during the Holocene (see fig. 3.5 in Leroy et al., 2019a). This Neocaspian transgression is identified as starting soon after the Mangyshlak lowstand by several authors (Leroy et al., 2020), although Leroy et al. (2019a) (see fig. 3.5 in Leroy et al., 2019a) suggest it to be more likely after 4 cal ka BP. Indeed, a transgressive post-Mangyshlak highstand is recorded as occurring in some cores from the two deep basins, with characteristics more like those of the Khvalynian highstand. In these very deep basins, field observations, dating and coring, have demonstrated the occurrence of the NeoCaspian transgression, with a starting date at -4 ka cal BP (Leroy et al., 2019a).

8.2 - Dating post-glacial transgressions and regressions

63In the northern basin, the underwater sediment coring program along and across the Volga River delta in direction of the Mangyshlak Threshold in the south, has been associated with tens of 14C carbon dates on molluscs (Yanina et al., 2018; Bezrodnykh et al., 2020). Cores have been recovered from five zones distributed along a N-S profile in direction of the Mangyshlak Threshold. According to the results from these cores, erosion surfaces truncate several phases of sediment deposited by the freshwater discharges of the Volga River, testifying for several periods of delta-construction by sediment discharges through the basin during the regression that followed the Khvalynian highstand (i.e. from c. 11 ka cal BP to 10.4 ka cal BP) (tab. 2).

8.3 - Between 8400 and 4000 ka cal BP

64Freshwater river flood sediments invaded more or less regularly the area corresponding to the Volga River delta, sometimes far south into the depressions bordering the northern Mangyshlak Threshold. These results record freshwater discharges and lateral movements of the Volga delta system, during successive progression/regression phases of the Volga sediments and water in direction of the Mangyshlak Threshold. Magnitudes of these fluctuations and seaward progression phases have been studied by Yanina et al. (2018) and Bezrodnykh et al. (2020), evidencing sediment layers (associated with the dated shells) alternating with erosion phases pertaining to lateral movements of the delta and its surficial channels, coast lines and associated coastal wetlands (especially in the northern parts of the delta). Results show that, above eroded Holocene onset sediments (12-10.5 ka cal BP), an Early Holocene progression phase occurred in direction of Mangychlak Threshold, followed by a northward regression phase in the delta realm. This progression started c. 8.4 ka cal BP, and reached the Mangyshlak Threshold c. 7.5 ka cal BP. The following regression started c. 7 ka cal BP, ending back near the continent at c. 6.7 ka cal BP (tab. 2).

65Later, between 5.5 and 4.5 ka, the CS level a smaller progression occurred in the northern basin, but with level fluctuations ending with the decline of water input from the continent.

66In the southern and middle basins, the fine details of Holocene CSL variations after 4 ka cal BP are not well recorded so far (Leroy et al., 2019a), because the deep basin sequences were not sensitive enough to moderate fluctuations. However, Leroy et al. (2019a) demonstrated that during the Holocene, the surface water of these basins had also a low salinity water, with probably less impact of evaporation than in the north because of the high depth of the water mass. This persistence of low salinity environments was due (like in the northern basin) to the arrival of continental freshwater from the HinduKush-Pamir-Tien-Shan and other mountain ranges in the south which allowed this freshwater input to last steadily until c. 4000-3900 cal BP, when the water salinity and most likely water levels changed sharply.

67In summary, the Holocene data in the CS realm show that, although the Early to Late Holocene marine events seem distinct in the northern and the rest of the CS, with the Mangyshlak threshold possibly separating the Volga River-influenced northern shallow basin on the one hand, and the deeper and saline middle and southern basins on the other hand, there was a relative abundance of freshwater input in both basins, producing low salinity marine environments until the end of freshwater input in the middle and southern basins near 4 ka.

8.4 - The marine Neocaspian transgression (from c. 1900/1700 BC to 1700 AD)

68Within the Neocaspian transgression, only small-scale changes occurred, perhaps no more than 15 m in amplitude. In the northern basin where coastal areas offer more potential data, a small progression dated c. 3500-2500 BC was followed by a sharp, c. 200 year-long, northward regression which started about 1950 BC, and left the northern basin dry by 1750-1650 BC (Bezrodnykh et al., 2020). In the southern basin, 2nd millennium BC events are well recorded by low levels. There, a rise occurred during the last centuries of the 1st millennium BC (Bezrodnych et al., 2020).

69Decreases in precipitation and water levels followed, with water level rising again c. 200 BC, reaching a peak c. 50 BC-AD 50 (tab 2) (Leroy et al., 2022a). During the Roman period, moisture is recorded in the watershed and in the lake (high levels). At the end of the 1st millennium AD, a dry spell parallels cold aridity recorded in central Asia (Ellenblum, 2012).

70Comparison with insolation-based water level changes during this period, confirms the increasing drying and instability of climate (both at the scale of the watersheds and of the lake itself), as well as the acceleration of the CSL lowering trend. Noteworthy is also the fact that low-stands parallel precipitation depletion in the northern to mid-latitudes (Leroy et al., 2022a), i.e. changes in atmospheric humidity circulations. This succession of level changes is presented in table 2, as they have been reconstructed by Leroy et al. (2022a).

71Recently, a revision of chronology and trends of CSL changes took place in the southern basin, with ages and elevations of water level indicators in the last 2200 years. This revision is based on a recalibration of radiocarbon dates following a ‘freshwater offset reservoir’ correction of 351 years ± 33 years (Leroy et al., 2022a). The results show that the ‘2600 year BP highstand’ evidenced by Kroonenberg et al. (2007) could not be found, for it is possibly younger than thought. With a correction based on the (re)-calibration of the radiocarbon dates as explained above, this highstand should be better referred to as the Parthian highstand (around 50 BC-50 AD). In addition, extreme caution should be applied when referring to the ages of Derbent lowstand (often attributed to the Medieval period), as it encompasses two lowstands: the mid-Sasanian lowstand and the later Medieval moderate lowstand. The Little Ice Age is expressed by a highstand (Leroy et al., 2011; Haghani et al., 2016a).

8.5 - Impact of Caspian Sea level changes on river terracing in the Caucasian valleys

72Geomorphological research in terrestrial terraces in valleys also provides chronologies of accumulation/erosion of sediments deposited by rivers, which may be compared with their base-level changes (altitude changes of the river mouths) along the CS lake shores.

73In the Kura valley, Ollivier et al. (2015, 2016) show, for example, parallelism between alluvial deposition in valleys and high levels of the CS during the Late Pleistocene (fig. 11). During the Late Holocene, accumulation/erosion phases recorded by alluvial deposits, point to:

74- the global decrease of the mean level of the CS since 5500 cal BP (paralleled by similar trends in the coastal area around the Volga River delta);

75- oscillations in the sediment deposition, that parallels water level rises, while erosion of these deposits respond to decreases of the base level of the river in the coastal plain.

Fig. 11: Alluvial terraces in the Kura River valley and relations with Caspian Sea level changes. Figure 11 : Terrasses alluviales dans la vallée de la rivière Kura et leurs relations avec les changements de niveau de la mer Caspienne.

Fig. 11: Alluvial terraces in the Kura River valley and relations with Caspian Sea level changes. Figure 11 : Terrasses alluviales dans la vallée de la rivière Kura et leurs relations avec les changements de niveau de la mer Caspienne.

Source: Modified from Ollivier et al. (2016)
Source : Ollivier et al., 2016 (modifié).

76Accordingly, such results are very important for they contribute to the understanding of the dynamics of terrestrial environments occupied by human societies which, often concentrated in the river valleys, are possibly threatened by physical backpressures (erosion, floods) triggered by changes in the base levels of the river channels at the Caspian shore.

9 - Causes as reconstructed by palaeo-models and proxies

77In the last decades, based on proxy methods and palaeo-models (specific climatic models applied to the past), there have been many reconstructions of water level changes (e.g. Arpe et al., 2000; Arpe & Leroy 2007; Giralt et al. 2003; Shiklomanov et al. 2016; Bezrodnykh et al., 2020; Koriche et al., 2022), which remain often contradictory.

78Regarding earlier changes, the drivers for CSL changes are even less clear-cut and are still much debated, as they seem to be diverse and to follow different timing and mechanisms including solar activity, glaciation/deglaciation timing (insolation), variations in river basins, human impact, etc. (e.g. Allen et al. 2008; Clauer et al., 2000; Kroonenberg et al., 2007; Leroy et al., 2007, 2019a; Panin et al., 2015; Yanko-Hombach & Kislov 2018 ; Koriche et al., 2022). It is important to account, not only for (i) climate and climatic contrasts in the catchment basin, but also for (ii) possible variations in the northern and eastern limits of the catchment basins, (iii) moving connections with other river basins and seas, as well as (iv) seismo-tectonic impacts. As for the recent times, water level reconstructions diverge between authors. See for example figure 3.5 of Leroy et al. (2019a), where eight water level curves spanning since the LGM are compared and turn out often to be inconsistent besides major trends.

9.1 - Climate

79Climatic changes during the Pleistocene caused alternations of (i) cold periods during which the water was retained as ice at high latitudes, and deglacial phases during which ice-broke up, feeding southwards meltwater flooding in direction of the collecting basins (Black Sea, CS and Aral Sea: fig. 4). A striking example of such alternation occurred at the end of the Last Glacial. During the LGM (NGRIP-MIS2, ice core yearly counts: c. 28-18 ka) (26.5 to 20-19 ka in Clark et al., 2009, using 14C, 10Be, and 3He ages), an ice cap developed over today’s Scandinavia and northern Russia, covering the upper part of today’s Volga drainage area (fig. 10).

80At the peak of the glaciation, it may be considered that the ice cap covering Scandinavia and northern Russia deprived the CS basin from an important water volume that would have been otherwise delivered to the CS through the Volga basin. These facts and their ages are confirmed by new data. These show a drastic CSL decrease during the LGM (Leroy et al., 2019b). In addition to the freezing process, the resulting dry air that accompanied the ice-covered areas, must have had a further decreasing impact upon water level.

81Afterwards, MIS 2 (from 22 to 12.5 ka cal BP, e.g. Clark et al., 2009) corresponds to a high CS level up to + 40 m above today’s level (tab. 1) (Tudryn et al., 2022).

82Thus, at the beginning of deglaciation, thawing of ice cap/or glaciers (in the mountains) would have delivered high discharges of freshwater into the tributary rivers to the CS, the level of which rose, while its salinity decreased drastically. After important deviations of the meltwater toward large river valleys in Scandinavia and Russia (see such impact in western Europe: Toucanne et al., 2010, for the Fleuve Manche), the initial water volume delivered by increasing temperatures, decreased stepwise. As a result, when the Scandinavian Icesheet became too small to overlap the CS drainage basin at 13.8 cal ka BP, indicators of river input to the CS (such as reworked palynomorphs) sharply declined (tab. 1) (Tudryn et al., 2016).

9.2 - Rivers and connections to other drainage basins

83In the case of the CS, one may not simply link glacials to lowstands and interglacials to highstands (Leroy et al. 2007). For example, the decline of water input in the CS at 13.8 ka cal BP was linked to northern river derivations to the Fleuve Manche, rather than reniewed cooling of the cold and dry Younger Dryas. The lowstand after that of the LGM, called the Mangyshlak lowstand, is dated at the beginning of the Holocene in several sequences (e.g. Leroy et al., 2019a). Its causes are not well known and may be a combination of dry climate at the beginning of the Holocene and a deviation of some rivers.

84It is likely that the post-Mangyshlak highstand was caused by water inflow from the eastern drainage, i. e. the Amu-Darya, as the inlandsis had by that time entirely withdrawn from the CS drainage basin and another source of water has to be involved, such as inflow from the Pamir-Hindu-Kush (e.g., Leroy et al., 2007, 2020).

85According to Leroy et al. (2019a), this humidity was due to impact of the monsoon system along the northern slopes of the Hindu-Kush and Pamir and possibly indirectly northern Iran. Glaciers reacting to monsoon influences started to melt, providing abundant freshwater discharges into the Caspian hydrological systems, through the Amu-Darya River system for example.

86This configuration of geographic and climatic events may possibly explain why the 4.2 ka BP event is not marked by an “event” in the CS. It corresponded to a major shift in water quality at 4000-3900 cal BP (Leroy et al. 2019a). In comparison, in the Caucasus region, no dry event is likewise recorded at that time during which however shifts to wetter climatic conditions occurred (Messager et al., 2013; Joannin et al., 2014).

87Concerning the last 2200 years, the drivers of water level changes are a combination of climate and human activities. Historical inflows from the Amu-Darya have been recorded for example at the beginning of the Little Ice Age (LIA) and were caused by human-made river diversions (Haghani et al., 2016; Leroy et al., 2022a). Mongol armies destroyed the Gurganj dam in AD 1219  causing the Amu-Darya to flow in the CS for an unknown duration (Haghani et al., 2016; Leroy et al., 2022a).

9.3 - Thresholds controlling the connection of the Caspian Sea with its neighbouring basins

88These thresholds provide critical elevation limits for identifying major highstands and for establishing the timeframe of minor highstands. The lowest threshold of the CS basin, at ~25 m asl in the Manych Strait (figs. 4 & 10), has allowed past connections of the CS with the Black Sea. This altitude may have been slightly higher in the Late Quaternary because of the activity of the faults controlling the dynamics of the strait (Jackson et al., 2002; Berberian, 2020).

89Eastwards, the threshold of the Aral Basin with the CS is today positioned at c. 69 m asl. In today’s configuration of the relief, a level of the Aral Sea reaching 69 m asl would mean that the Aral Sea was flowing into the CS basin through outlets cascading in a stair of lakes (Chepalyga, 2007). Before reaching this level, the Amu Darya and Syr-Darya River discharges would have been boosted by melt water from the Hindu-Kush, Pamir and the western Tian-Chan (e.g. Leroy et al. 2019a). It is also possible that the Amu-Darya may have flown directly in the CS without going through the Aral Sea.

90Finally, a third threshold may have (or not) played a role in CS/Black Sea connections -although possibly earlier in the Pleistocene, depending on possible local tectonics. West of Volgograd, a bench is as low as 76 m asl today between the Don and Volga rivers (fig. 10). This point is positioned on the bottom of an uplifted fossil valley, today occupied by the Don-Volga canal. This palaeo-valley records a past connection between the Don (today flowing at 45 m asl near Kalatch-on-the-Don), and the Volga (today flowing at 9 m asl south of Volgograd). According to the greater incision of the Volga valley and to faulted-like (uplifted) right bank of the Volga River, the Don River may have been once a palaeo-tributary of the Volga, captured and extracted from the Volga drainage area by the Black Sea basin. The cause of such a capture would be either a tectonic movement uplifting the bench (?) or, possibly an incision provoked by an important level drop of the river baseline in the Black Sea basin that would have triggered the capture of the Don River by a small tributary of the Black Sea. A former shoreline is indeed visible on Google Earth imagery near Konstantinov, at 45 m asl. Whatever this possibility is feasible or not, its verification requires field research, especially for testing the possibility of tectonic movements having deformed the terrain (e.g., a possible uplift of the threshold).

9.4 - Seismo-tectonic movements within the lake may also led to water level changes

91According to Kuprin (2002) and Kuprin et al. (2003), the Apsheron Sill limited, perhaps even blocked the flow (southwards or northwards) between the middle basin and the south basin during lowstands, while subsiding very recently owing to tectonic movement.

92Other types of impact on the hydrology of the CS have been pointed out but have a minor importance only. Three examples from the seismo-tectonic follow. Özyavaș et al. (2010) underline the time-correspondence between a significant centimetric water level drop and two strong earthquakes along the Apsheron Sill in 2000 (fig. 3). Another specific geological impact on the dynamics of the CS consists in the formation of submarine mud volcanoes formed over the lake bottom along the Apsheron Sill (Huseyinov & Guliyev, 2004), in direction of the SE Caspian lowland in Iran (we must be aware of a possible confusion between mud volcanoes over the Gorgan Plain surface and archaeological tells: Omrani & Raghimi, 2018, pp. 622). These volcanoes are caused by discharges of deep gases onto the surface, in relation to the tectonic compression still occurring in the south-CS basin region (fig. 3) (Leroy et al., 2022b). Finally, Clauer et al. (2000) suggest that tectonics may have induced and is still inducing a sub-surface inflow of saline water by 2 to 9% of the total water input to the CS and Kara-Bogaz Gol.

10 - Conclusions

10.1 - The Volga discharge as a relation to climatic change

93As the CS water balance depends mostly on precipitation and evaporation over the Volga drainage (fig. 6), any change in these factors drive variations in the Caspian water level. Accordingly, past climatic changes over the drainage basin have had strong control on CSL variations. Notably, the development of ice caps over northern Scandinavia and Western Russia during glacial periods trapped humidity, keeping water from flowing out to the downstream basins. Conversely, ice melting pulses provoked by temperature rise triggered rapid and abundant meltwater discharges.

94Such a melting occurred at the end of the LGM in the Fennoscandian ice cap which was covering N. Europe by a > 2000 m high ice accumulation. During warmer periods, the melting of this ice produced huge deliveries of melt water south of the retreating ice cap (fig. 10). This melting gave first birth to very large wetlands and lakes over the lowlands extending through northern Europe. While climate was warming, these systems connected to each other, captured each other, and overflew downslope, reaching the headwaters of the Volga basin system. The water discharge formed large floods that flew downstream into the closed basins of the CS, Black Sea, and Aral Sea, until they were directed to the English Channel.

95During the Holocene, pulses in the CS water level occurred (tab. 2). In the southern basins, the salinity can be the best marker of climate change, both when precipitation/temperature changes occurred on the surface of the water, and/or when freshwater input by local rivers varied in response to climatic variations in watersheds, possibly different from those in the northern basin.

10.2 - Other factors interfering with water level on the long-term or short-term, according to their time-resolution

96Among such factors are (i) the solar activity which can interfere with lake levels, (ii) the tectonic activity that can modify thresholds and river paths or provoke river captures, and (iii) human activity (dam/canal constructions/destructions; water withdrawals, etc…). Regarding the solar control on the water level, a comparison with mid-to Late-Holocene data collected from the field and the laboratory shows that the insolation model provides a useful “background signal” of the solar control on the climate components intervening on the CSL changes. However, field data indicate (e.g., fluvial terraces) a lack of close parallelism, due to local factors such as hydrography and types of environments associated with the water level change records, through time as well as through space. There are however periods during which climate-triggered humidity is well evidenced in the CS proper basin-, although direct human interferences certainly occurred at times.

97These contrasts suggest that the level of the CS responds to a mix of local vs regional geographic, environmental and climatic components. It seems that, within a global frame of climate being controlled by atmospheric circulations, teleconnections and insolation, the latitude extension of the humidity collectors (esp. the Volga watershed) and the relief control (esp. Caucasus, Alborz and Kopet Dagh highlands: fig. 3) can modify the water balance of the CS as well as its physical characteristics with high diversity.

10.3 - Overflow into the Black Sea basin

98Indeed, if the water balance of the CS reaches up to > 25 m, the CS waters would flow (today) over to the Black Sea, impacting its physiochemical and biological characteristics. Conversely, comparisons between sedimentological records retrieved from sediments in the southern and middle parts of the CS do not show any chronological and environmental parallelism unless the Apsheron Sill let water passing through.

10.4 - Changes in the vegetation

99Pollen analyses in Caspian sediments show changes of regional vegetation, but do not show any direct link between vegetation and water levels. This conclusion is quite understandable as the water levels are under the direct control of water input delivered by a very large –and possibly changing– watershed, i.e., by exogenous factors (i.e. sub-regional changes in hydrographic connections such as river captures and diversions, especially in the land East of the CS). Dinoflagellate cysts are on the contrary a useful proxy for water level reconstruction, via the information they provide on past salinity. In addition, flora and fauna on land and in the CS (e.g., molluscs and ostracods) respond also to other geographical components of the ecosystems such as local precipitation control, local bedrock reliefs and soils, local water resources and more recently human activities.

10.5 - In the tributary river valleys

100The impacts of level changes on the river profiles upstream from river mouths (which are directly controlled by water level) varied in timing and intensity along their profiles (fig. 11). Accordingly, deltas and coastal plains record intensively baseline level changes, with more details than in the river terraces in the valleys. Rivers feeding the CS respond to changes in their baselevel by: (i) erosion in case of base level fall, and (ii) accumulation in case of base level rise. These changes impact the river differently according to distance to the mouth, and –if any- to other control(s) on the profile, such as tectonics. Indeed, tectonic movements are regularly recorded in the Caucasus and along the Alborz ranges, where impacts on human societies and their activities, movements, settlements etc. are mostly felt.

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Table des illustrations

Titre Fig. 1: Wide geographic context of the Caspian Sea and its neighbours (Aral Sea and Black Sea). Fig. 1 : Contexte géographique général de la mer Caspienne et de ses voisines (mer d’Aral et mer Noire).
Légende Source: General Bathymetric Chart of the Oceans (GEBCO) (modified) (open source, online). Source : General Bathymetric Chart of the Oceans (GEBCO)(modifié) (source ouverte, en ligne).
URL http://journals.openedition.org/quaternaire/docannexe/image/17991/img-1.png
Fichier image/png, 1,1M
Titre Fig. 2: The topographic setting of submarine and continental relief of the Caspian Sea. Fig. 2 : Topographie du relief continental et sous-marin du bassin de la mer Caspienne.
Légende (A) Relief and marine currents. (B) Sub-marine topography and main rivers flowing in the Caspian Sea (with watershed evidenced by broken orange line). Source: online General Bathymetric Chart of the Oceans (GEBCO) (open source, modified). (A) Relief et courants marins. (B Topographie sous-marine et principales rivières affluentes de la mer Caspienne (la limite du bassin drainé vers la mer Caspienne est soulignée par un tireté orange). Source : General Bathymetric Chart of the Oceans (GEBCO), (Source ouverte, modifiée).
URL http://journals.openedition.org/quaternaire/docannexe/image/17991/img-2.png
Fichier image/png, 2,1M
Titre Fig. 3: Main structural features in the southern part of the Caspian Sea watershed. Fig. 3 : Principales lignes structurales dans le bassin méridional de la mer Caspienne.
Légende Source: modified from Özyavaş et al. (2010).Source : modifié d’après Özyavaş et al. (2010).
URL http://journals.openedition.org/quaternaire/docannexe/image/17991/img-3.png
Fichier image/png, 835k
Titre Fig. 4: Thresholds and paths connecting the basins occupied by the Aral, Caspian and Black Seas. Fig. 4 : Seuils et corridors de connections entre les bassins occupés par la mer d’Aral, la mer Caspienne et la mer Noire.
Légende (1) Manych Strait (Black Sea connection) is today topping at +40 m aGsl (i.e. +66 m aCsl). (2) Volgograd-Don Strait (Black Sea connection), topping at +78m aGsl (i.e. +104 m aCsl). (3) Aral Sea, and the Uzboy valley eventually capturing either or both Amu Darya and Aral Sea Basin. Source inset 1: modified from Tudryn et al. (2013) (see also Létolle et al. 2007). Source inset 2: GeoMapApp open application, modified with colors spanning every 25 m from -25 m to 400 m, where deep blue is below -400 m, and white is above +400 m aGsl).(1) Corridor de Manych (avec la mer Noire), altitude max. à +40 m aGsl (i.e. +66 m aCsl). (2)  Corridor entre la rivière Don et la mer Noire, susceptible de capturer le Don via la Volga, altitude max. à +78 m aGsl). (3) Corridor formé par la vallée de l’Uzboy vers la mer d’Aral puis le bassin de la mer d’Aral (ex : Amu Darya). Source encadré 1 : modifié de Tudryn et al. (2013) (voir aussi Létolle et al., 2007). Source encadré 2 : fond construit avec l’application GéoMapApp (ouverte en ligne) coloré avec une palette de couleurs changeant chaque 25 m de -25 m à 400 m. Le fond bleu correspond aux profondeurs sous -400 m, et le fond blanc aux altitudes supérieures à +400 m aGsl.
URL http://journals.openedition.org/quaternaire/docannexe/image/17991/img-4.png
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Titre Fig. 5: Physical parameters of the Caspian Sea water: precipitation, air and surface temperatures, salinity. Fig. 5 : Paramètres physiques de la mer Noire : précipitations, températures de l’air et de surface, taux de salinité.
Légende Sources: Redrawn from the Caspian Environment Programme 2009, with the addition of Kavak (2012), modified, for mean surface temperature map.Sources : Caspian Environment Programme, 2009 (modifié), et Kavak, 2012 (modifié) pour la carte de températures de surface de la mer.
URL http://journals.openedition.org/quaternaire/docannexe/image/17991/img-5.png
Fichier image/png, 668k
Titre Fig. 6: Annual water discharge by main rivers into the Caspian Sea. Fig. 6 : Apports des débits annuels des principales rivières de la mer Caspienne.
Légende Source: Redrawn from P. Rekacewicz (le Monde Diplomatique) assisted by L. Margueritte and C. Marin, updated by R. Pravettoni (GRID-Arendal), and V. Novikov (Zoi Environment Network). Open-source file.Source : Modifié de P. Rekacewicz (le Monde Diplomatique), assisté de L. Margueritte et C. Marin, mis à jour par R. Pravettoni (GRID-Arendal) et V. Novikov (Zoi Environment Network). Source ouverte.
URL http://journals.openedition.org/quaternaire/docannexe/image/17991/img-6.png
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Titre Fig. 7: Level curve of the Caspian Sea from AD January 1993 to March 2022. Fig. 7 : Courbe du niveau de la mer Caspienne, de janvier 1993 à mars 2022.
Légende Source: monthly mean Caspian Sea level changes observed by tide gauges (1993-1997) and satellite altimetry (1997 to 2022, provided by Legos/CNES. http://hydroweb.theia-land.fr/​). A systematic bias between tide gauge and altimeter series is removed (using a 4-year overlapping period 1993-1996).Source : les données moyennes mensuelles du niveau de la mer Caspienne, relevées à parti de gauges locales (1993–1997) et les altitudes relevées par satellites (1997 to 2022), proviennent de Legos/CNES (http://hydroweb.theia-land.fr/​). Un biais systématique entre les séries des gauges et des altitudes a été éliminé (en utilisant le recouvrement de 4 années des données, soit 1993 à 1996).
URL http://journals.openedition.org/quaternaire/docannexe/image/17991/img-7.png
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Titre Fig. 8: Dams in the Rivers Volga - Kuma watersheds. Fig. 8 : Barrages dans les bassins de la Volga et de la Kuma.
Légende Source of background map: Redrawn from K. Musser (under Creative Commons Attribution-Share-Alike 2.5 Generic). Open-source file).Source de la carte de fond : modifiée à partir K. Musser (under Creative Commons Attribution-Share-Alike 2.5 Generic). Source ouverte.
URL http://journals.openedition.org/quaternaire/docannexe/image/17991/img-8.png
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Titre Fig. 9: The northern part of the Caspian Sea depression, with curves marked every 10 m between -40 m and +5 m above Global Sea Level. Fig. 9 : Partie nord de la dépression occupée par la mer Caspienne, avec courbes de niveau espacées de 10 m entre -40 m et +5 m au-dessus du niveau de l’océan (aGSL).
Légende Comment: This map shows (i) how sensitive is the northern part of the Caspian Sea northern basin to even small changes in elevation of the water level, and (ii) position of areas in the watershed where overflow (river capture, flow diversion) may happen during water level rises in direction of neighbor river/sea basins. Source: based on GeoMapApp open application on internet.Commentaire : Cette carte illustre (i) la sensitivité de la partie nord de la mer Caspienne et de son bassin proche à des changements de niveau de la mer, même de faibles amplitudes, et (ii) les points qui, par leur altitude, sont susceptibles d’ouvrir des déversements possibles (captures de rivières, transferts vers des bassins externes) quand le niveau de la mer atteint leurs altitudes. Source du fond de carte : produite à partir des données de GeoMapApp, source ouverte en ligne.
URL http://journals.openedition.org/quaternaire/docannexe/image/17991/img-9.png
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Titre Tab. 1: Chronology of the late Pleistocene (Marine Isotopic Stages 3 and 2) water levels. Tab. 1 : Chronologie des niveaux marins du Pléistocène final (stades isotopiques marins 3 et 2).
Légende Source: from Tudryn et al. (2022). (1) First phase of the lowstand of Atelian, (2) Second phase of the lowstand of Atelian and of the following highstand of Early Khvalynian.Source : Tudryn et al. (2022). (1) Première phase de le la régression atélienne, (2) Seconde phase de la régression atélienne et de la transgression du Khvalynien inférieur.
URL http://journals.openedition.org/quaternaire/docannexe/image/17991/img-10.png
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Titre Tab. 2: Detailed chronology of water levels during the last 2200 years. Tab. 2 : Chronologie détaillée des niveaux de la mer Caspienne pendant les 2200 dernières années.
Légende According to Leroy et al. (2022a).D’après Leroy et al. (2022a).
URL http://journals.openedition.org/quaternaire/docannexe/image/17991/img-11.png
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Titre Fig. 10: Possible directions of melting delivered by the Fenno-Scandian inlandsis to northern Russia and Baltic areas during the c. 9000 yrs duration of the inlandsis retreat in direction of Iceland and northern Scandinavia. Fig. 10 : Directions possibles des écoulements issus de la fonte de l’inlandsis fennoscandien en Russie septentrionale et dans le bassin de la mer Baltique, pendant les 9000 ans qu’a duré le retrait de l’inlandsis vers le nord de la Scandinavie et l’Islande.
Légende (A) Extension of the Fenno-Scandian inlandsis at Late Glacial Maximum, based on Svensen et al. (2004) and http://www. Glacier-climat.com/cg/quaternaire-de-planete/. (B) Circulation directions of melt water during the melting of the LGM inlandsis, based on ground topography, with occurrences of flat areas possibly causing instability of melt water directions, in turn possibly causing river captures over northern Scandinavia and northern Russia: 1/ Deep Sea basins below -1000 m bGsl (“below Global Sea level”); note that slopes descending to the Middle and Southern Caspian deep basins are less abrupt than those in the Black Sea and Mediterranean Sea (the intermediate levels from bottom of CS depressions to 0 Bsl, appear in green and yellow colors), 2/ Areas with surfaces between 0 and -300 m below Global sea level (bBsl), 3/ Areas from 0 to 300 m above Global sea level (aBsl), 4/ Areas between 300 m and 500 m topographic curves (aBsl), 5/ River Volga’s today river valley (with main direction of flow), 6/ River Dniepr valley today, 7 to 9/ Outflows during the first retreat of the inlandsis between 20 and 13 ka ca BP, 10/ Outflows from the 13 ka position of the retreating inlandsis, 11/ Spots of indefinite control on water flows (possible river flow confusions over flat surfaces), 12/ Area suitable for river connections/diversions/captures during the LGM and during the Late Glacial, 13/ Topographic connections of valleys through the Aral Sea basin, from north (Russian plains), north-east (Syr-Darya) and south-east (Amu-Darya). Source of map: interpretation of the topographic map of today’s emerged land north of the Caspian Sea associated with the maximum expansion area of the LGM inlandsis over Scandinavia, the Baltic Sea, the northern Atlantic and North Russia (20 ka cal BP).(A) Étendue de l’inlandsis Fenno-Scandien au dernier Maximum Glaciaire, modifié de Svendsen et al. (2004) et de http://www. Glacier-climat.com/cg/quaternaire-de-planete/. (10) Routes possibles des eaux issues de la fonte de l’inlandsis fennoscandien à partir de la fin du LGM, basées sur la topographie du relief, et en tenant compte de larges surfaces planes susceptibles de causer des incertitudes dans les directions des flux, pouvant ainsi causer des captures entre bassins versants en Scandinavie et Russie septentrionales : 1/ Bassins marins sous -1000 m bGsl (sous le niveau marin mondial), notez que les talus séparant les fonds des trois bassins de la mer Caspienne sont moins abrupts que ceux relevés en mer Noire et mer Méditerranée, les fonds entre les bassins sud et moyen de la mer Caspienne (les niveaux intermédiaires) à partir du fond de la dépression marine jusqu’au niveau de surface (0m Bsl) sont colorés en vert et jaune, 2/ Zones situées entre 0 et -300 m au-dessous du niveau marin global (aBsl), 3/ Zones de 0 à 300 m au-dessus du niveau mondial de la mer (aBsl), 4/ Zones entre 300 m et 500 m au-dessus du niveau marin global (aBsl), 5/ Vallée actuelle de la Volga (la direction de la rivière est indiquée) ; 6. Vallée actuelle de la rivière Dniepr, 7 à 9/ Apports d’eau en provenance de l’inlandsis pendant la 1ère phase de son retrait (entre 20 et 13 ka cal BP), 10/ Apports d’eau le long de la limite sud de l’inlandsis à partir de 13 ka cal BP, 11/ Secteurs susceptibles d’avoir provoqué des confusions/changements d’orientation des flux d’eau de fonte (grands terrains plats), 12/ Secteurs susceptibles d’avoir provoqué des connections, changements de cours et captures pendant le LGM et le Tardiglaciaire, du nord (plaines septentrionales russes) et du nord-est (bassin du Syr-Darya) vers le sud et du sud-est (bassin de l’Amu-Darya) vers le bassin de la mer d’Aral. Source : interprétation de la carte topographique des terres aujourd’hui émergées au nord de la mer Caspienne, associée avec l’expansion maximum de l’inlandsis du LGM sur les bassins du nord de la Scandinavie, de la mer Baltique, de l’Océan Atlantique et du nord de la Russie (20 ka cal BP).
URL http://journals.openedition.org/quaternaire/docannexe/image/17991/img-12.png
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Titre Fig. 11: Alluvial terraces in the Kura River valley and relations with Caspian Sea level changes. Figure 11 : Terrasses alluviales dans la vallée de la rivière Kura et leurs relations avec les changements de niveau de la mer Caspienne.
Légende Source: Modified from Ollivier et al. (2016)Source : Ollivier et al., 2016 (modifié).
URL http://journals.openedition.org/quaternaire/docannexe/image/17991/img-13.png
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Catherine Kuzucuoǧlu et Suzanne Leroy, « Geographic and geomorphologic context of Caspian Sea level fluctuations over the Late Pleistocene and Holocene »Quaternaire, vol. 34/2 | 2023, 71-92.

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Catherine Kuzucuoǧlu et Suzanne Leroy, « Geographic and geomorphologic context of Caspian Sea level fluctuations over the Late Pleistocene and Holocene »Quaternaire [En ligne], vol. 34/2 | 2023, mis en ligne le 17 juillet 2023, consulté le 07 février 2025. URL : http://journals.openedition.org/quaternaire/17991 ; DOI : https://doi.org/10.4000/quaternaire.17991

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Catherine Kuzucuoǧlu

Laboratoire de Géographie Physique, LGP, UMR 8591, 2 rue Henri Dunant, FR-94320, THIAIS. Email: catherine.kuzucuoglu[at]lgp.cnrs.fr

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Suzanne Leroy

Aix Marseille Univ, CNRS, Minist Culture, LAMPEA, UMR 7269, 5 rue du Château de l'Horloge, FR-13097, Aix-en-Provence. Email: suzleroy[at]hotmail.com
School of Environmental Sciences, University of Liverpool, UK-L69 7ZT LIVERPOOL.

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