Isotopic studies of environmental changes in the basins of the Caspian Sea and the Aral Sea
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based on the fact that the G2H value in pore water was +6‰. That value suggests that the process took place in a drying up water body. And also concentration of Na+ and Cl-there was 2.5 to 3 g/L suggests that the gulf was a terminal lake without connection to the middle Caspian Sea. Around 8.5 ka BP the G2H value in water decreased up to -6‰, which indicates the beginning of dilution of the water in the former terminal salt lake by river runoff. During the period from 8.5 to 2.2 ka the value of G2H in the pore water decreased from -6 to -17‰, which points to an increase of the river runoff. The gulf was rapidly filled up and after ~3 ka BP periodically discharged water to the middle basin of the Caspian Sea. After the events of the New Caspian transgression at about 2.2 ka, the Karyn-Zharyk River system dried up and since that time the gulf is recharged by water from the Caspian Sea. All the measured values prove this conclusion [6].
(7) Four transgressive and three regressive periods during the Holocene history of the Aral Sea were found by study of the shore terraces and bottom sediments. The sea level rose from 53 m to 57 m asl and dropped from 44 m to 35 m asl. The sea level of 57 m asl indicates that at this time water from the Aral Sea discharged in the southern basin of the Caspian Sea through the Usboy river. Two peat layers of 0.2 and 0.4 m thickness with an age of 4,5 and 1.3 ka BP, respectively, were found in a sediment core. From 1.3 ka BP up to present time sedimentation continues at variable hydrological regimes with periodic rises and drops of the sea level. These findings are confirmed by alternate clay and sand lamination and salt crystal inclusions identified in the bottom sediment section [6].
(8) The results of isotopic, mineral and chemical analysis of bottom sediments from the Lake Issyk- Kul show that within 7.6 to 6 ka BP sedimentation was connected with steady recharge of cold water and discharge into the Chu River. Low concentrations of organic carbon, monohydrocalcite, G18O and Sr/Ca, and slightly increased concentrations of CaCO3and MgCO3 at the upper boundary of the core interval prove the above conclusion. During the time interval from 6 to 4.3 ka BP, sedimentation continued at a reduced river water inflow, lower water level in the lake, increased salinity, and decreased sedimentation rate. This conclusion is proved by an increase in G18O in mollusk shells, a decrease in Mg2+concentration in the deposited calcite, and an increase in concentration of organic matter and Sr/Ca ratio in shells. Within the upper interval from 4.3 to the present time periodic change in the lake regime was subject to periodic changes with deceases and increases of the lake level up to the Chu River bed through which the excess water was discharged [6].
(9) It was found by means of G2H, G18O and 14C data, that the Yaskhan lens of fresh groundwater, which is located in the central part of the Karakumy desert in West Turkmenia (~40oN, 63oE) and has mineralization of less than 0.6 g/L and a 14C contents of 17-22 pmc, has formed about 12 ka BP [1, 5].
(10) Impact of the pluvial epoch on the groundwater in the Central Asian and Near East region was demonstrated by isotope studies carried out in the Syr Darya, Amu Darya, Chu and Ily River basins and the Syrian desert [1, 5].
4. Environmental changes and natural events
On the basis of the above findings, the relationship between natural events and the most severe environmental changes in the Caspian and Aral basins can be described as follows:
(1) The opposite change in sedimentation rate between the southern and middle basin of the Caspian Sea accompanied by changes in isotopic composition of carbonates, salinity of water, chemistry and mineral contents of sediments, provides evidence of a change in the hydrological regime over the Central Asian and the European part of the catchment basin in the Late Pleistocene and Holocene. This change in the hydrological regime was caused by a drastic change of climate conditions, which started around 23-25 ka BP and finished about 10-12 ka BP. It was the latest long-periodic cycle of the Earth's climate evolution. As a result, the temperate humid climate in
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the Central Asian region changed to hot semi-arid and arid climate, and the cold semi-arid climate in the Volga and Ural basins alternated to a temperate humid regime. The onset of aridity in the Central Asian region was accompanied by melting of the vast glaciers in the Pamir and Tien Shan and by runoff of enormous volumes of melt water to the Aral Sea and through the Usboy River to the Caspian Sea (Fig. 1). Judging by isotope data of carbonates and by salinity of pore water of the sediments, the process continued within 12 to 8 ka BP and periodically appeared later on.
(2) Extremely different sedimentation rate, salinity of pore water, changes of other measuring parameters in the southern and middle basin of the Caspian Sea and also the shape of bathymetric section of submarine part of the Apsheron Sill evidence that up to recent times the Caspian Sea was separated by the 30-50 km wide natural dam, the Apsheron Sill (Fig. 2). The geological history of the Apsheron Sill proves this conclusion [7]. Obviously, the merging of the lakes happened after the dam was washed out by water overflowing from one lake to another. The structure of the bathymetric section shows that the direction of the eroding flow was mainly from south to north. The measured data suggest that the unification of the two lakes was completed between 12 to 8 ka BP.
(3) The Aral Sea started to discharge water to the Caspian Sea after the Turonean Lowlands were filled in up to +56 m asl which is the mark of the Usboy River’s outflow in the Kugunek Hills.
From 23 to 8 ka BP the Turonean Lowlands were filled up to that mark forming an expansive low water Aral-Sarikamish Sea. During that period excess water discharged to the Caspian Sea by the Usboy River. At present a dry and highly eroded bed of the Usboy River at the foot of the Kopet- Dag Mountains evidences this (Fig. 2).
(4) The registered periodic change in contents of clay minerals in the Caspian bottom sediments mark at least eight transgression and regression cycles of changes in the hydrologic regime in each basin of the sea with a period of ~1.5-2 ka. More detailed records of oxygen and carbon isotopes in carbonates demonstrates periodicity in the runoff with shorter periods in the order of hundreds of years.
As follows from the available data on runoff, precipitation, temperature and sea level variation during the last 150 years, there are inter-annual changes in the hydrologic regime of the Caspian and Aral Sea basins, each 30-50 years the sea level amplitude reaches 1-2 m or more. That period of hydroclimatic variation seems to be important for human activities.
The above effects and also the same facts observed in other regions of the Earth encouraged the authors to search for the physical cause of short-periodic hydroclimatic change on the Earth by following Milancowich’s astronomical theory of the long-term climate variation. In order to derive a possible short-periodic latitudinal effect in the variation of the solar energy flux we decided to treat the problem of the Earth's rotation and oscillation in its own force field. The results proved to be promising. A novel approach to studying climatic and water level changes in the Caspian and Aral Seas will be published elsewhere [10].
5. Conclusions
Isotope studies of environmental changes in the Caspian and Aral Sea basins give qualitative confirmation that short-periodic variation of the climate change is a natural process which has physical justification. The theoretical results summarized in this paper suggest that all other observed and proposed causes of global climate change are secondary effects in comparison to the primary effects discussed here. A further study of the connection between these physical causes and the observed climatic changes appears to be useful and could be carried out within the framework of an international project with the involvement of international organizations such as the IAEA and UNESCO.
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FIG. 1. Effect of latitudinal shift of the humid climate zone of the Caspian-Aral region. (a) Catchment basin change from the Volga-Ural to the Amu Darya -Syr Darya; (b) Regression of the Caspian and transgression of the Aral Seas during the transitional period of the climate zone shift.
FIG. 2. (a) Bathymetric map of the Apsheron Sill with a 10 m isobathic interval and cross section of the Sill along latitudes (b) 40o10’, (c) 40o20’, (d) 40o30’.
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[5] FERRONSKY, V.I., POLYAKOV, V.A.,.LOBOV, A.L., Isotope study of impact of climatic changes on hydrological cycle in Central Asian and Caspian arid region, IAEA TECDOC-1207, Vienna (2001) 59-76.
[6] FERRONSKY, V.I., BREZGUNOV, V.S., VLASOVA, L.S., KARPYCHEV, Yu.A., POLYAKOV, V.A., BOBKOV, A.F., ROMANOVSKY, V.V., JOHNSON, T., RICKETTS, D., RASMUSSEN, K., The Kara Bogaz Gol Bay, Lake Issyk Kul and Aral Sea sediments as archive of climate change in the Aral-Caspian basin, Proc. Intern. Conference, Vienna, IAEA (2002) 144-153.
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[8] KUPRIN, P.N., FERRONSKY, V.I., POPOVCHAK, V.P., SHLIKOV, V.G., ZOLOTOVA, L.A., KALISHEVA, M.V., Bottom sediments of the Caspian Sea as an indicator of changes in its water regime, Water Res. 30 (2003) 136-153
[9] FERRONSKY, V.I., POLYAKOV, V.A., BREZGUNOV, V.S., VLASOVA, L.S. POLYAKOV, V.A., FROEHLICH, K., ROZANSKY, K., Investigation of water-exchange processes in the Caspian Sea on the basis of isotopic and oceanographic data, Water Res. 30 (2003) 10-22.
[10] FERRONSKY, V.I., POLYAKOV, V.A., FROEHLICH, K., KUPRIN, P.N., BREZGUNOV, V.S., VLASOVA, L.S., KARPYCHEV, Yu.A., A novel approach to explain climatic and water level changes in the Caspian and Aral Seas (in preparation).
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