THE FUTURE CHEMICAL EVOLUTION OF THE ARAL SEA FROM2000 TO THE YEARS 2050

and N.G.O. BOROFFKALab. Appl. Geol., Univ. P & M. Curie, Paris, France (rene.letolle@wanadoo.fr)

Départment of Zoology, University of St. Petersburg, Russia (aral@zin.ru)Geoforschung Zentrum, Potsdam, Germany

(Received 7 May 2003; accepted in final form 11 December 2003)

Abstract. The future of Southern Aral Sea water in Kazakhstan, and of the sequence of mineralevaporitic sediments precipitated since the beginning of the present regression from gypsum to bloed-ite (astrakhanite) was studied. A comparison is made with previous regressive deposits discoveredearlier in cores by I.V. Rubanov. Absence of mirabilite deposits in sediments of the eastern basin isattributed to deflation processes during the XIVth century AD.

1. Introduction

From 1960 on, distraction of water from the Aral Sea in Kazakhstan for irrigationhas led to the shrinking of one of the biggest lakes in the world, with appallingconsequences on the biomass, and the life of the inhabitants of the shore (Boomeret al. 2000). Many publications deal with attempts for the Aral Sea to recover,or at last to be kept in its present state: Shivaryova, 1996, predicted that the BigSea would become two lakes in 2030, if the water input remained the same. Thisrevelation was realized much sooner. As there is no evidence that the climaticconditions (Mason et al. 1994; Small et al. 1999), and that the present hydrologicalsituation may change appreciably in future in the watershed of the two rivers, Amuand Syr Darya, the only surface water to reach Aral in the future will remain thesmall amount of precipitation (ca. 10 cm per year), and a variable but small inputof drainage waters (a few km3 per year). If so, apart from the Small NorthernSea which receives most of the remaining flux of Syr Darya, the southern basinswill show a constant decrease of their area and depth, so that an almost completedessication of the south basin is foreseeable in the next decades (Tables Ia and Iband Figure 1). What will be the physical and chemical evolution of these southernbasins in the time to come? A companion paper (Aladin et al. 2004) gives theevolution of the biomass, which follows the evolution of the water chemistry.

2. Water Balance of the Aral Sea

Table Ia shows our own evaluations of the water mass of the western and easternbasins, based on bathymetry as shown on the most recent maps of the Aral Sea.

C© Springer 2005

1RENÉ LÉTOLLE , NICHOLAS ALADIN , IGOR FILIPOV

Mitigation and Adaptation Strategies for Global Change (2005) 10: 51–70

1 2

1

2

R. LETOLLE ET AL.

Figure 1. Variation of depth, area, salinity and volume of Aral Sea over 70 year .

Volumes are evaluated as that of a prism of water 5 m thick for the western and twoor one meter for the astern basin, taken between the areas of limiting surface water.

Table Ib shows the total volume for the western basin (87.3 km3) correspondstightly with Nikolaieva estimations (85 km3). With these data may be calculated thequantity of water necessary to keep the lake level at a given altitude. The sum forthe Eastern volume (63.1 km3) is lower than in Nikolaieva’s estimation: 70.1 km3)due to the fact that was not considered here the region north of a transect Kulandy-Barsa Kelmes, especially the Tchebas bay, which will be isolated in 2003 from themain water body, and may receive some underground water, which will slow itsown dessication.

From 1989 on, the Small Aral sea (Maloye More) became an isolated watermass with its own destiny, and subsists today through the derivation of the sub-

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THE FUTURE CHEMICAL EVOLUTION OF THE ARAL SEA

TABLE Ia

Morphometry of the Aral sea in Kazakhstan (Nikolaieva 1969, 1971).

Water level (in m. asl Small Sea Eastern basin Big Sea Western basin Big Sea

general oceanic sea level)

AREA VOLUME AREA VOLUME AREA VOLUME

(km2) (km3) (km2) (km3) (km2) (km3)

53 Year 1960 5992 79.7 46466 681.2 13 628 302.8

51 5361 68.7 40 885 593.8 13 364 275.9

48 4830 53.5 37 556 476.3 12 962 236.3

43 3846 31.9 31 417 304.1 11 385 175.2

33 1363 6.0 15 817 70.1 6 203 85.0

23 2 689 40.8

13 1 597 20.6

3 954 8.60

0 0

TABLE Ib

Total volume of water in western and eastern basins of Aral Sea.

Western basin Eastern basin

Depth Free water Total water Depth Free water Total water

(asl) area in volume (asl) area in volume

(km2) (km3) (km2) (km3)

–16 m (0 m) 0 0 24 m (0 m) 0 0

–12 54.6 0.136 25 15 0.01

–7 342.75 1.126 27 2334 2.36

–2 614 3.516 28 4400 9.11

3 920 7.36 29 6680 14.65

8 1251.7 12.79 31 12880 34.25

13 1607 19.94 33 15930 63.05

18 2596.5 30.45

23 2844 44.05

28 4134 61.49

33 6203 87.32

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R. LETOLLE ET AL.

sisting Syr Darya water to its depression. On the contrary, the Big Sea (BolchoieMore) just receives a small input of drainage waters from the Amu Darya, whichis less than 10 km3 per year. So its regressing level, which is monitored almostevery day by the US National Oceanic Atmospheric Administration (NOAA) infrared weather satellites, and the Topex Poseidon sea level campaign from 1995 to2003, shows a quasi-regular regression of about 0.5 m per year, a shift of 0.3 min spring, and a drop of 0.8 m in summer and autumn, with small fluctuationsdue to tides, wind effects and sporadic inputs from the deltas1. This tendency ismore or less regular since the 1960s. Hence the salinity of the remaining southlake increases regularly from 1987 on, since the Maloye More was cut off from theBolchoie More, with large local variations in chemistry and global salinity due topoor mixing of waters on low depth bottoms, especially in the south-east basin.

Evaporation, which is commonly stated as 1 m per year in the Aral Sea, by com-parison with past meteorological stations of the Pri-Aral, seems by combination ofknown water inputs in the last forty years with the variation of the level of thelake, nearer 1.3 m than 1 m (Figure 2). Evaporation flux is the essential mechanismleading to the shrinkage of the lake (infiltration represents at most 5% of the waterbalance), and will decrease in time as saturation water vapour pressure decreaseswhen salinity increases (Dickson et al. 1965). Diminution of the evaporation fluxfrom a brine is also linked to the ‘activity’ of water at high salt concentrations,which is not precisely known due to lack of experimental data (Jauzein and Hubert1984) in spite of theoretical models. 58 m is the highest possible level of the AralSea, due to the low altitude of the Amu Darya delta sediments at the south of pastlake Aibugir, SW of Kungrad City. This corresponds exactly to the higher possiblelevel of the Aral Sea for a variety of combinations between lake area (Table I),evaporation intensity, and surface input (rivers and precipitations). Before 1960,the lake level oscillated around 53m asl, damped by evaporation.

As also the ratio of evaporating area to volume will be decreasing (Table I) inthe western basin, this ratio is much larger in the eastern basin, and the drop inlevel of the two basins should be more quicker in the latter.

A question is: how much water is presently carried by rivers to the Aral Sea(except the Maloye More)? Taking the total output of Syr Darya as 10 km3 per year,and 5000 km2 as the mean area of the Small sea (which fluctuates widely by about15%), evaporation on Maloye More (taken as 1.3 m per year) uses about 6.5 km3

per year, which lets 3–5 km3 to pour southwards the past dyke. A part of this flowis lost in the lowland south of Kokaral peninsula, so that perhaps no more than 2–3 km3 pour into the southern basins, which were yet linked in 2002. But presently(may 2003) the link of the Western basin with the residual flow from the Syr Daryais a small channel (1.5 m deep plus or minus 0.5 m due to wind, variations ofevaporation and inputs etc.) and this channel will be closed (Figure 3a) in a few

1 Thanks are due to J.F. Cretaux, from CNES, Toulouse (France) for providing Aral Sea level datafrom Topex-Poseidon.

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THE FUTURE CHEMICAL EVOLUTION OF THE ARAL SEA

Figure 2. Variation of evaporation and water inputs for a steady state of the Aral Sea for given waterlevels.

years if the water level goes on dropping. Examination of the precise bathymetry ofthe straits, between Chernychev and Vozrojdenje peninsulas shows it is shallowerby one or two meters than the straits between the Barsakelmes and Vozrojdenjepeninsulas (Figure 3b). Deposition of sediments (2 mm per year) during a fewcenturies have partly choked up this channel. So in the years to come all Syr Daryaremaining flux will go to the eastern basin, much larger and less deep than thewestern basin, but this water will not be enough to equilibrate evaporation evenwhen the eastern basin will have shrunk to an area of a few thousands km2 (2000to 3000 approximately): that is with a level a few meters below the present level.

The same NOAA pictures show that the western basin received in autumn 2002drainage water from the western part of the Amu Darya delta, and in the futurethis may compensate partly for the closing of the Kulandy-Vozroshdenie channel.

55

R. LETOLLE ET AL.

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56

THE FUTURE CHEMICAL EVOLUTION OF THE ARAL SEA

Anyway, the level drop of the western basin will only slow, but as injection of saltedwater (5–7 g per litre will go on, chemical concentration will go on, just slower.

EVAPORATION AND CHEMICAL EVOLUTION OF THE ARAL SEA

Before 1960, the water and chemical composition of the lake were in quasi-steadystate, which means that as much water and chemicals arrived and went away, andthe turn-over time was about 15 years for water. It was calculated that equilibrationof the ‘matter balance’ needed about 5% of dissolved matter was to be eliminatedthrough sprays and infiltration of the sediments. The essential factors of the waterbalance was in fact competition between surface inputs and evaporation.

Salinity grew from 10.2 g l−1 in 1947 to 041.8 in 19982, and 80 g l−1 in August2002 in Chebichev bay. 90 g/l have been found at the bottom of western basin inNovember 2002. As soon as salinity reached 30 g l−1 (Brodskaya 1952), gypsumbegan to precipitate directly from open water. In earlier conditions, Aral Sea waterwas already saturated relative to calcite, which was the only deposit in addition todetrital material brought by rivers and wind. Calculations show that in the ‘normalstate’ (salinity of 10–12 g/l and chemical composition of Aral water as taken from1947 data (Bortnika and Tchistyaevoi 1990), and taking into account the effect ofionic strength (a chemical parameter linked to the chemical activity of dissolvedions, see for example Glynn 1990; Stumm and Morgan 1996), Aral Sea water wasslightly unsaturated relative to gypsum. Spontaneous precipitation of gypsum inopen water (not considering deposits on shores, solontchaks, where conditions ofevaporation are completely different than those in open water) should have begunsome time between 1980 and 1990. At the present time (2003) dissolved calcium(Ca) precipitates as CaCO3 and gypsum, and the increase in salinity is due to therise of Sodium (Na+), Magnesium (Mg++), Sulfate (SO4

−), Chlorine (Cl−), andaccessorily of Potassium (K+) ions. Chemical composition has evolved as given inTable II and Figure 4.

EVAPORITES FORMATION IN THE ARAL SEA

As shown by theoretical data and experiments carried on Aral Sea water by Lepesk-hov and Bodayeva (1952), and Rubanov and Timokhina (1982), the first next salt toprecipitate directly from water will be mirabilite, associated with sodium chloride(NaCl). Loss of water by mirabilite crystals gives powdery thenardite (Na2 SO4)which is blown away by wind and is the essential reason for lung diseases in thePri-Aral area.

The solubility of gypsum, which is 2.41 g.l−1 at O ◦C and 2.59 g. l−1 at 18◦ inpure water, decreases by a factor of 3 for a salinity of 20 g.l−1. Mirabilite shouldprecipitate at 4 ◦C from Aral water for a salinity of about 150 g.l−1 (Figure 5) Atthat time, NaCl would be the main component of dissolved salts, and mirabilite

2 And not 48.1 g/l as indicated by error in Letolle and Chesterikoff (2000).

57

R. LETOLLE ET AL.

Figure 4. Evolution of the chemical composition of Aral Sea water with time (meg l−1).

58

THE FUTURE CHEMICAL EVOLUTION OF THE ARAL SEA

TABLE II

Chemical composition of Aral sea water: in meq.l−1.

Ca++ Mg++ Na++ K+ SO4− Cl− HCO3

− NO3−

Aral sea (Blinov 1947) 27.8 40.61 98.39 2.07 65.21 98.64 3.19 –

Bolchoie More, Cape

Aktiumsik, May 1998

(Letolle and Chesterikoff, 2000) 59.9 201.8 512.5 19.9 281.1 520.6 2.14 < 0.1

Tchebas bay Aug.19, 2002 49.8 314.8 791.1 150.7 350 897 6.4 <0.1

Cherdintsev bay, Aug.25, 2002 54.3 315.8 826 145.6 345.8 855 5.0 + <0.1

CO3−

= 1.0

Figure 5. Stability of salts in the Na-Mg-Cl-SO4diagram with the increase of evaporation, afterRubanov (1982, 1986).

59

R. LETOLLE ET AL.

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60

THE FUTURE CHEMICAL EVOLUTION OF THE ARAL SEA

TABLE IIIa

Mineral content of Aral Sea water.

Mineral Composition Solubility for pure salts

(moles l−1) at 25 ◦C

Calcite CaCO3 0.00014

Gypsum CaSO4·2H2O 0.0154

Mirabilite Na2SO4·10H2O 1.96

Epsomite MgSO4·7H2O 3.07

Thenardite Na2SO4 3.45

Hexahydrite MgSO4·6H2O 4.15

Bischofite MgCl2·6H2O 5.84

Halite NaCl 6.15

Bloedite (Astrakhanite) Na2SO4, MgSO4, 5H20

TABLE IIIb

Theoretical minerals from AralSea water.

Minerals Anhydrous

minerals (g l−1

CaCO3 0.14

CaSO4 1.36

NaCl 4.31

Na2SO4 1.74

MgSO4 3.34

MgCl2 1.30

K2SO4 0.15

KCl 0.03

total 12.37

would remain stable during winter. The sequence of salts for further evaporiticdeposits3 (Table II and Figures 6a and 6b), which does not imply Ca+, but only Na+and Mg+ ions, is not known in old aralian sediments of Aral, as will be discussedlater. For other salts, not found presently in the Aral environment, except thenardite,solubilities are shown in Tables IIIa and IIIb:

3 There is a wealth of key theoretical papers on the evaporitic sequence of brines, based on Pitzer(1973); Pabalan and Spitzer (1987), Voigt (2001), give an important bibliography.

61

R. LETOLLE ET AL.

Of these, no K+ salts are known from Quaternary deposits of Central Asia(Rubanov 1977), except in Kara Bogaz Gol. As concerns bloedite Rubanov foundit only on the recently dessicated eastern shores of the Aral, together with halite(Rubanov 1994).

There is a striking correspondence between the map of solid sulfates in sedimentcores (Figure 7) drawn by Rubanov (1984), and the isobaths of the Western basinof Aral Sea. The greatest thickness of beds, more than 4 m, appear in the southof the present western basin and is deduced from Rubanov’s observations (1994).Rubanov (1977, 1984, 1994) discovered these mirabilite beds in cores a few meterslength taken from the deeper parts of the western Bolchoie More (Figure 8), alongthe Chink cliff, and also from western Maloye More), but not in the eastern basin.These beds in holocene sediments, C14-dated by Rubanov and others (1984), canbe explained only if the Aral Sea was almost dry some centuries ago. This may becorrelated with various historical events (see Létolle and Mainguet 1997; Boomeret al. 2000). Conservation of mirabilite in Aral sediments when water reappearedagain in the Aral Sea can be explained only by a very fast flood of water with aheavy load of argillaceous suspended matter which sealed off most of mirabilitedeposits from redissolution. Mirabilite is surrounded by a ring of thenardite thengypsum, the thickness of which decreases radially from the deepest point of thethrough.

3. The Future of South Aral Sea

The future of Maloye More is completely independent now from that of the Bol-choie More. As it seems very probable that the water input from Amu Darya andSyr Darya will not increase in the future, and separation of Western and Easternbasins will occur in the very next years, their water will evolve towards a high-Mg+brine, where Mg+ salts would precipitate when the water content of the basinswill be of a few tens of km3. However, as there will be some water input to theresidual lakes, would it be only through rain and snow (about 10 cm height peryear on the catchment area), the chemical composition of dissolved matter wouldoscillate seasonally between summer and spring. One has also to consider thatdissolved Mg+ percolating in sediments could react with Ca CO3 minerals to formprotodolomite eventually becoming dolomite, which is found in some other lakesof Uzbekistan (Rubanov 1977, who did not consider formation in situ of dolomite,which is in any case a very long diagenetic process). May we imagine how thedrying process will go on?

There are precise correlations between area, volume, area/volume ratio, andresidual salinity (Table I: data taken from Nikolaeva, 1970–71, and ours), andit is possible to extrapolate the present regression curves for depth, surface andchemical composition to the time when salinity becomes higher than 150 g l−1, ifhydrologic and climatic conditions do not change.

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THE FUTURE CHEMICAL EVOLUTION OF THE ARAL SEA

Figure 7. Map of the deposited salts at the Aral Sea bottom (Rubanov 1984) with 2002 coast, andresidual state in 2030 (in black). 1: Diagenetic (secondary) gypsum in deltaic sediments; 2: isolatedauthigenic ( primary) gypsum crystals in marine sediment; 3: isolated thin beds; 4: beds less than10 cm total thickness; 5: from 10 to 50 cm; 6: more than 50 cm; 7: mirabilite in sea sediments;8: authigenic gypsum and deliquescent salts; 9: deliquescent salts (a) halite, (b) astrakhanite, (c)thenardite and mirabilite.

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Figure 8. Mineralogy of the western basin cores (adapted from Rubanov 1977): 1 – mirabilite; 2 –clay and marls; 3 – aleurite; 4 – quartz sand; 5 – gypsum; 6 – geological substratum; 7 – fauna; 8 –boreholes location.

Gypsum precipitation began between ten and twenty years ago, as we have seenearlier. Calcium used in precipitation of calcite takes between 5 to 7% of all thecalcium stock, which was 600 Mt in 1947, for a water stock of 1090 km3 and anarea of 69 000 km2. In 1998, the respective values were approximately 250 Mt,210 km3 and 22 500 km2. When gypsum precipitation began directly from Aralwater, say in 1985, the SO−

4 stock was almost intact, as was the Ca stock, and atthat time the area and volume of the south basin can be estimated as 35 000 km2

and 300 km3 respectively. From these values can be estimated the quantity of CA+

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which disappeared in deposited gypsum, which amounts to 1500 Mt. Density ofgypsum being 2.32, this corresponds to 675 mm3, distributed on 35 000 km2, thatis a thickness of 19.3 mm for a ten or so years of deposition, evidently unequallydistributed on the sea bottom.

The remaining Ca+ in 1998 represented about 1100 Mt of gypsum, corres-ponding to a mean thickness of 48.1 mm which adds to the previous 19.3 mm.As solubility of gypsum decreases considerably with the concentration of salts –‘the common salt effect’ – so that the speed of its precipitation on the bottomwill accelerate notwithstanding a diminished evaporation, and will have exhaustedmost of the dissolved Ca+ stock before the beginning of mirabilite deposition, thatis, following our estimation, in a few tens years. As the total theoretical thicknessof precipitated gypsum is less than 10 cm, we have to consider now how the muchthicker beds of gypsum observed in Rubanov’s cores could have been obtainedin the past in the sediments at some places in the middle South Aral depressions(Figure 8). First, currents in a high density water are able to displace gypsum crys-tallites without dissolving them, and accumulate them in the deeper parts, due totheir density. Second, rain precipitations on the dry shores of the depressions createtemporary short brooklets which also carry crystals to the remaining solontchaks.Therefore gypsum beds observed between detrital beds in Rubanov’s cores may beconsidered also as essentially of detrital or secondary origin.

The continuation of the present situation in the water balance of the Aral Seawill lead to a similar sequence of salt deposition in the drying process of the Aral(Figures 5 and 6), which is very classical and has been studied theoretically byHarvie et al. (1984). An interesting comparison can be made with the Kara BogazGol, fed by Caspian water – the composition of which is not quite different of theAral water of pre-1960 – which was in steady state for centuries with regards toevaporation-water input equilibrium, and changed drastically in the 1970’s due tothe closing of the narrow channel between the two water masses. The lowering ofwater in Kara Bogaz Gol (Terziev et al. 1986) went with the formation of a circularzonation of evaporites directly on the floor of the gulf where they form beds severalmeters thick (Kolosov et al. 1974) until the quasi-dessication of the bay, before itwas reopened to Caspian waters.

The same NOAA pictures show that at mid-january 2003, the northern partsof Bolchoie More are frozen, due to the fact that low salt content water comingfrom the Syr Darya overfloats salty lake water (50–80 g l−1) which freezes onlyat a lower temperature (freezing temperature lowers by 0.052 ◦C for a shift of1 g l−1 salinity). Therefore, if such a situation exist for long, and as the inputwater contain much less salts than the present Aral, this will just slow the sequenceof events leading to mirabilite and halite precipitation in the two basins, whichwould occur some years later than predicted below: as the input of ‘fresh’ water isseasonal, it may be predicted that a part of existing evaporates will dissolve againand may react with other mineral to give a variety of diagenetic minerals (Eugsterand Hardie 1978).

65

R. LETOLLE ET AL.

A coarse estimation of the total mass of salts to be deposited in the future inthe western basin from its present 87 km3 (2003 AD) of water is about 6 billiontons salts, which, in case of a complete drying, would correspond to 1–0.5 meterthickness. The same value is to be foreseen for the eastern basin. Same calculationsshow that complete precipitation of 1000 km3 of Aral 1960 water would give about540 × 106 Mg of Mg+ which would occupy, considered as mirabilite 4460 MM3

(d = 1.49); considering the deposition of this salt in a conical volume with height4 m (the maximal thickness observed by Rubanov), one finds an area of 3300 km2,which approximates closely the area where mirabilite of earlier regression episodesis found in the western basin of the Aral Sea another conclusion could be that mostof precipitated mirabilite during the last regression episode is yet present there. Itspresent existence implies a very fast reconquest of the flats by water with a bigcharge of suspensions (clays and silt).

Precipitation of mirabilite will begin to occur directly from water in winter inthe residual western basin at some time when the volume of the western basin willreach about 25 km3, which could be attained before years 2030–2040 in presentconditions. Then summer precipitation of mirabilite will occur about years 2050–2070. A little quantity of halite will precipitate also on the shores, where evapora-tion is more efficient. Gypsum may evolve further towards glauberite CaNa2(SO4)2(Létolle and Mainguet 1993, p. 224). At that time, the volume of the ‘sebkhra’will be about 7–8 km3 for a salinity of 220–250 g l−1. Bloedite (astrakhanite) willappear if the volume is reduced to 2–3 km3. This model does not consider saltsprecipitated on shores of the retiring water. Na+ and Cl− will yet be the maindissolved ions, with smaller concentrations of SO4

− and Mg++. HCO3− will havecompletely disappeared.

Evolution of the eastern basin will be more rapid than for the western basin,due to the much greater ratio of area to volume. Winter mirabilite should appear atthe bottom before the years 2020 and if the Syr Darya input is sufficient, the waterdepth would stabilize around 3 m and the evolution of brine will go on as for thewestern basin.

Precipitation in open water of sodium chloride (NaCl) would begin when Cl−content will be approximately 2 500 meq; a coarse extrapolation shows that thiscould happen a little after 2020 in the present water balance condition; the samefor winter mirabilite will occur when total salt content is approximately 140 g l−1.

Due to the fact that the two southern basins cannot dry completely, anotherquestion then arises: what will be the fate of the remaining stock of Mg+, Na+ andCl− subsisting in the brine lakes? As seen earlier, the quasi-steady state chemicalbalance of the lake, before 1960 was explained through infiltration in the sedimentsof the Aral Sea, representing about 5% of the water budget of the lake. Thus saltwater seeps through the sediments towards the deep cretaceous aquifers, fed withvery old precipitation waters from the piedmont of distant mountains (Karatau,etc.), and the salinity of these deep waters is sometimes higher than 150 g l−1.Anyway, a part of the future brine on the eastern shores of the remaining lakes will

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THE FUTURE CHEMICAL EVOLUTION OF THE ARAL SEA

be blown away to the south-east, which may augment the impact of dust on thepopulation’s health.

Present projects consider to carry water from Amu darya and Syr Darya to thetwo relict basins. As the quantity of water which will eventually reach them will bethe order of the present inputs (except during exceptional flood years), the resultwill be just a delay in the further shrinking of the basins. But as the salinity ofthe two rivers goes on rising slowly, more and more salt will add anyway to theremaining stock, and the precipitation of solid salts will go on slower; equilibriumcould then be attained not in a dozen years, but perhaps in one century. . .

Formation of dolomite through interaction of magnesian waters with limestonegeologic strata may be possible, helped by the existence of faults all along theChink cliff, and may explain the disappearance of most remaining ions from thewestern basin, and not only through wind action on dry shores.

4. Lessons of the Past

Rubanov’s discoveries did not show mirabilite in the eastern basin, and no traceanywhere in the cores of further terms, such as bloedite and halite, above mirabilitebeds. Here is a problem, since the end of an evaporitic sequence at the mirabilitestages implies that exists a mechanism to eliminate excess ions (Cl−, SO4

−, Na+,Mg++, K+): either a discontinuous draining of the ‘sebkhras’, or infiltration. Thefirst is not possible, due to the morphology of the Aral depression; the intensityof the second is not important and would have needed a very long time to beeffective. The most plausible hypothesis to explain the absence of most evaporiticbeds of past regressions in the eastern basins (Figure 7) is that they were submit-ted to aeolian erosion and transport, which has been demonstrated to be able todisplace million tons of salts on the recently dessicated shores of the Aral. Theupper part of the evaporitic sediments could have therefore been blown away aftertheir deposition by dominant winds from the North-east, and this abrasion wouldhave been less important in the western basins, due to the various reliefs from theVozrhozdenie ‘ridge’, which, in any case would have deflected the eastern windsup, as this is easily observed south of Beltau ridge, in the south-east corner of thepast Aral Sea, as well as in all hills of south Kyzyl Kum desert.

Here will not be discussed the Holocene reconstructions of the Aral Sea evolu-tion (Létolle and Mainguet 1997; Boomer et al. 2000; Létolle 2000). The Aral Seaexisted in 1146 when Al Idrisi draw his map of Asia and wrote that Aral perimeterwas 300 miles. The last historical drying episode of the Aral sea, which producedaccumulation of sodium sulphates in the troughs of the western part of Aral basin),is commonly associated with the flooding by Timur in 1406 of the city of Urgenchin the Amu Darya delta, the flow of which was diverted to the Caspian through thepast Uzboy channel. Remains of buildings and saxaul stumps have been observedon the dried floor between the Kokaral and Barsa Kelmes past islands. Syr Darya

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then became the main tributary to Aral, and its water flow was not sufficient tokeep the Aral at its previous level. With a mean interannual input of 25 km3 waterper year, the volume of Aral would have been stabilized at about 200–220 km3

after one century regression to what is presently its present state and 80 g/l salinity.This is too much an input to explain mirabilite. In fact, some time after the TimurLeng episode at the beginning of the XVth century A.D when all Amu Darya wasdiverted to the Caspian Sea, Aral had also lost most of the Syr Darya water input,as shown by the testimony of Babur.4 Syr Darya certainly lost itself at that time inthe Kyzyl Kum desert, another part going into the Golodnaia Stepa and Aidarkuldepression (where Rubanov found halite 1977), and into salt marshes SW and NEof Kzyl Orda. Perhaps a small part of the Syr Darya flow went yet to the Easternbasin (where no halite has been found) through its SW arms Kuwan Darya andYany Darya. Taking 30 km3 per year at that time as the mean Syr Darya flow in itslower course, about 22 000 km2 should be necessary to evaporate completely thisflow of water, and precipitate the 0.2 g l−1 equivalent gypsum transported. Thiscorresponds to the formation of a gypsum bed growing about 1cm per year. Thefirst Russian explorers at the end of the XVIth century saw the Syr Darya flowingagain to the Aral, so that the duration of this regression episode lasted at mostthree centuries, more probably 150 years as Jenkinson’s account in 1556 seems toindicate that the Amu Darya course had just been re-established to the Aral. Allthe evaporitic deposits hypothetically formed in the present lower course of SyrDarya were surely washed off when the river returned to its previous north-easternAralian mouth. The sudden arrival of a big mass of water loaded with sediments (atlast 1 g/l) covered in a very short time the evaporitic basins, preventing mirabiliteand gypsum to dissolve entirely, and protecting them; then the sea was filled againwill water in less than one century. Analysis of the present regression gives a rathergood image of past events which happened to the Aral Sea in the Middle Ages.

Acknowledgement

This paper is dedicated to I.V. Rubanov. The help of program CLIMAN INTASAral-30 is acknowledged.

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