Investigations / Manych-Kerch spillway

PALEOHYDROLOGICAL RECONSTRUCTION of MANYCH-KERCH SPILLWAY

Scope of this study is reconstruction of water exchange in region between Black Sea and ancient Caspian Sea: along the Manych valley during Late Pleistocene. Manych Valley — the most eastward link of Black Sea — Mediterranean Sea Corridor.

Manych-Kerch spillway

Method of study is analysis of lithology, mineralogy, and geochemistry including isotopic composition of sediments and fossils, for reconstruction of sedimentary environments and paleohydrological calculations, as well as paleomagnetic method (magnetic sensibility) and geomorphologies analyses including 3-D technique.

The Manych-Kerch Spillway is a large trough deeply eroded into solid rocks which connected the Caspian and Black seas. It was inherited from an older strait between the two seas, which existed (with interruptions) since the late Pliocene (the Akchagylian basin). It follows a tectonic depression skirting southern periphery of the Karpinsky Swell (an elevated Mesozoic structure confined between the Donbass and Mangyshlak).

The total length of the spillway amounted to 950–1000 km (depending on the sea level position), with maximum width of 50-55 km and minimum width of 10 km. Its depth reached 30–50 m. The spillway bottom gradient was 0.0001 m, and drop in water level from the Caspian Sea (+50 m) to the Black Sea (-80 to -100 m ) was up to 150 m at the beginning of the excess water flow to 100 m at its termination.

The spillway began from the Khvalynian coastline at the head of Chograi Bay between the Ergeni and Stavropol uplands. The bay is 60 km long and 30 km wide at its entrance, and its depth is between 40 and 50 m. The narrowest section of the spillway (about 10 km) was near the Zunda-Tolga village, where the water flowed over a sill at 20 m above sea level.

Three different age of accumulative bars of spillway were recovered in transversal section of Manych Valley. This bars are 15 – 25 km long, up to 200-300 m width and up to 30 – 35 m relative high. Last one is the oldest generation related with the highest sea level of Khvalynean basin, younger – with lower level and the youngest generation is 5-7 m high. This accumulative bars consists of sill and loam (“chocolate clays”) and contain Caspian Sea mollusk shells of Limnocardiidae family, such as the genus Didacna Eichwald. This genus is not presently found anywhere outside the Caspian Sea.

chocolate clays

The genus is represented by Didacna praetrigonoides (dominant), D. parallela, D. delenda, D. subcatillus, D. ebersini, as well as relatively deep-water (>25 m) D. protracta. Other endemic limnocardiides characteristic of the region are Monodacna caspia, M. laeviscula, Adacna vitrea, and Hypanis plicata.

Of the Early Khvalynian elements, molluscs of the Pontodreissena subgenus are the most common outside the Caspian Sea (Pontodreissena rostriformis and Dreissena polymorpha in semi-freshwater basins). Gastropods are usually represented by the endemic Caspian genera Caspia and Micromelania. These sediments of Khvalynean basin, layers occur above the Late Khazarian (dated to the last interglacial) and below the New Caspian (Holocene) deposits. Grain size composition of this Khvalynean sediments, CaCO3 contain and magnetic sensibility show unstable environment of sedimentation with clear oscillations 6-7 cycles.

Paleohydrological calculations in the narrowest part of Manych Valley near Zunda-Tolga was provided. When the Khvalynian transgression was at its maximum (50 m asl), the spillway depth was up to 30 m (its average depth was 20-25 m). The spillway bed is covered with silt and clay 5 to 10 m thick. The cited data enable us to estimate the flow velocity at ~0.2 m/s and maximum discharge through the Manych Spillway at 40 to 50 thousand m3/s. The total runoff would have amounted to more than 1000 km3/year. This output is six times greater than the Volga River runoff and three times that of the Mississippi River. In the above calculations, sill depth was assumed to be constant at 20 m asl, but with a stream depth of 30 m, the flow velocity would have been much greater. This scenario does not agree with the fine composition of the sediments, however. Such a contradiction may be explained by assuming a higher initial sill level, about 40 m asl or even higher, in which case, the flow discharge would be reduced by several orders of magnitude to near the modern Volga discharge (8 to 10 thousand m3/s). This water along the Manych-Kerch spillway flowed to the Black Sea basin.

So big amount of Caspian water effected great influence on the New Euxinean basin: quick rising of Sea-level from –100 to –50 m asl and occupation of the Caspian fauna. It is funny salinity is not erase and remained near 6-8‰. The possible reason is influence of rivers flow Danube, Dnepr and others with refreshened of the Caspian water.

The Caspian water flow resulted in the formation of a system of interrelated basins in inner Eurasia. They have been studied using various tracers, including lithology (reddish-brown interlayers of chocolate clay type), paleontology (Caspian endemic molluscs, foraminifera, and ostracoda) and isotopes (of oxygen and other elements). These markers have linked together the entire drainage system, providing evidence for a Cascade of Eurasian Basins, beginning in the Aral-Sarykamysh basin, then draining through the Uzboi Spillway into the Khvalynian Sea (Caspian basin), from there through the Manych-Kerch Spillway into the Neweuxinian Sea (Pontic basin), and finally through the Bosphorus, the ancient Sea of Marmara, and the Dardanelles into the Mediterranean Sea.

Cascade of Eurasian Basins