Paleogeographical History of the Aral Sea

Abstract

The Aral Sea basin formed as a result of joint action of tectonic subsidence and processes of arid denudation. The basin itself features rather irregular bottom topography, with depressions divided by an elongated elevation trending from north to south. Fragments of marine terraces occur locally at 54–72 m a.s.l. on the sea coasts. In the marine series infilling the basin there have been distinguished sediments attributable to the Akchagylian and Apsheronian transgressions of the Caspian Sea and those of the Khorezmian suite (Holocene) deposited during the last marine stage of the Aral Sea. The lower portion of the sequence is represented by diversified lacustrine formations containing occasional interlayers of gypsum and shells of brackish-water and freshwater mollusks. The upper part of the sedimentary sequence consists of alternating layers corresponding to transgressive and regressive phases of the Aral Sea; shells of Cerastoderma glaucum (Cardium edule) are typically present.

The history of the Aral Sea may be divided into two unequal parts – a prolonged prehistory and a short epoch of the modern (pre-1961) sea basin. The first stage of the Aral dates back to the Late Pliocene when its basin was filled with water of the Akchagylian and Apsheronian seas; this was followed by long periods in the Pleistocene when subaerial environments persisted in the basin.

The recent marine stage is rather short spanning only the Holocene. It began with a lacustrine-brackish water phase. At that time, lakes of varying salinity existed within the basin; occasionally they dried up and were replaced by solonchak desert. In the mid-Holocene water from the Amudarya turned from the Sarykamysh depression and began to flow into the Aral basin via the Akchadarya channel, thus starting the recent (last) stage of the Aral Sea's history. At that time the Aral was a vast freshened brackish water body of marine type subjected to drastic fluctuations of sea level (within 20 m) and noticeable changes of salinity (up to 10‰) and was inhabited by the mollusk C. glaucum.

The onset and further evolution of the Aral Sea basin were controlled by a number of factors, including climate, hydrology, tectonics, and human impact. It seems, however, that climate was of primary significance, as it controls the hydrologic cycle within the Aral drainage basin, evaporation from the sea surface, and runoff from the Syrdarya and Amudarya rivers; the latter was of prime importance in turning the poorly inundated Aral depression into a large lacustrine–marine basin.