Pliocene deposits in the southeastern Russian Plain are more widespread than on the Don Plain. Sediments of Pontian age (early Pliocene, approximately 5.4 - 4.7 Ma) are found in the southwest of Astrakhan district and in Kalmykiya, where they are represented by limestones and clays with interbedded sands and shells (the Novorossiysk substage). These sediments contain rare shells of Congeria sp. and a variety of ostracodes: Cyprideis torosa, Bakunella guriana and others. These deposits were probably formed in a brackish water shallow inlet of the Pontian marine basin.
Continental deposits of the Pliocene on the southeastern Russian Plain (the Kushumskaya suite) are more widespread and accumulated from late Miocene through the early and middle Pliocene time. These layers overlie eroded deposits of varying ages and origins and underlie Akchagylian marine layers.
Upper Pliocene deposits in this region are mainly of marine origin and are related to a transgressions of the Caspian Sea. The Akchagylian stage (poorly dated, but estimated to range from 3.5 - 2.0 Ma) is subdivided into three substages on the basis of mollusk and ostracode faunas and on palynological data. The Apsheronian stage (about 2.0 - 0.7 Ma) is thought to be a part of Pliocene in this region, although some researchers regard it as a part of Eopleistocene. Apsheronian deposits are very widespread and are represented both by marine and continental facies.
At the beginning of Pontian time, the Zanklian transgression united all the marine basins of southern Europe, including the Caspian Sea. At the end of the early Pontian (~5.0 Ma) there was a major regression, and the Caspian Sea lost its connection to the Black Sea. During the early Pontian in the southwestern region, forest and forest-steppe vegetation dominated under warm and rather dry climatic conditions (estimated mean January temperature was 3° C, and that of July 24° C). In the Don River basin steppe vegetation was predominant at this time, and only riparian forests existed in this region (Populus tremula, Alnus incana, A. glutinosa, etc.). Unfortunately the modern climatic ranges of these species are too broad to estimate paleotemperatures from this area. The southeastern part of the Russian Plain was occupied by forest-steppe and steppe vegetation. The climate was rather warm, as indicated by the presence of thermophilous mollusks and ostracodes. Caspian Sea surface temperatures were estimated from strontium isotopes on Unio and Valvata shells to range between 11° and 22° C, which is characteristic of the subtropical zone (N. Yasamanov, pers. comm.).
During the middle Pliocene (the Kimmerian stage of the Black Sea regional stratigraphy), marine basins occupied only the depressions of the Black and Caspian Seas. In the circum-Black Sea region broad-leaved forests with thermophilous elements were widespread. The reconstructed temperatures for the interval from 4.0 to 4.2 Ma are the warmest of the Pliocene, and at the end of Kimmerian the area of forest cover was reduced due to drier climatic conditions. Faunal assemblages also changed, as animals typical of open landscapes, such as Equus and Paracamelus, appeared.
Middle Pliocene deposits are scattered on the south-central Russian Plain, and palynological data are sparse. The lower Kimmerian fauna in this region includes steppe species (Cricetulus, Spalax, etc.). Later in the Kimmerian faunal assemblage became more heat-loving and included some tropical elements. Its composition also included characteristic forest inhabitants (for example, Pliopetaurista), which may imply more moist conditions. Terrestrial mollusk faunas also point to warm climatic conditions: thermophilous species of the forest-steppe zone were present, along with several subtropical taxa (Striatura, Hawaia, and five species of Gastropoda). Based on these data, the lower Kimmerian can be regarded as the warmest part of the Pliocene.
At the beginning of the middle Pliocene steppe landscapes dominated by Chenopodiaceae were prevalent in the southeastern Russian Plain. During late Kushum time, steppe vegetation was partly replaced by forest-steppe and forest. Along with coniferous forests, broad-leaved vegetation was widespread, with different species of Ulmus, Celtis, Juglans, Pterocarya, Liquidambar and others. The climate remained warm (mean January temperature = ~ 1° C, July temperature = ~ 20° C) and became more humid. Farther to the south on coast of the Caspian Sea, shells of the heat- loving mollusks Unio and Valvata were found in the layers of the same age, confirming the interpretation of a warm middle Pliocene.
The Caspian Sea regressed to near its modern level in the beginning of the late Pliocene (a continuation of the lowering of sea level that began in Kimmerian time), and a river pattern similar to that of today was established. A reduction of forest cover took place during the Kujalnik stage in the northern Black Sea region, and forest-steppe vegetation was replaced by steppe. The flora was very similar to that of today, and only the presence of rare pollen grains of exotic taxa allow us to consider these layers to be of Pliocene age. Clearly the climate became drier and cooler during the late Pliocene.
During the early Akchagylian substage (corresponding to the Kujalnik) in the Don river basin, steppe vegetation existed with patches of riparian forests. The middle Akchagylian is marked with cooling which is reflected in a spread of Betula (arboreal pollen spectra are dominated by Betula in these deposits), and micromammal faunal assemblages confirm the existence of dry open landscapes. A new stage of forest expansion indicates that warming took place later in the middle Akchagylian. More humid conditions allowed broad-leaved trees such as Quercus and Tilia to grow in this region, and thermophilous mollusks, including Striatura, Hawaia, Parmacella and others, were identified in deposits of this age.
Profound cooling occurred in late Akchagylian time, when exotic thermophilic taxa died out. The northern part of Don River basin was occupied by forest-steppe vegetation, while the southern part had steppe vegetation. These interpretations from pollen data are confirmed by fauna assemblage that contain the steppe taxa Equus stenonis, Paracamelus and Struthio. The final part of late Pliocene on the Don Plain is characterized by temperate climate and forest-steppe vegetation.
On the southeastern Russian Plain, late Pliocene vegetation was dominated by steppe and forest-steppe. Coniferous trees were more common than broad- leaved ones in the forests, and the flora included a small number of subtropical elements than in Kushum time. The paleobotanical data suggest that mean January temperature was about 0°C, and that of July was about 20°C. The ostracode assemblages include paleoarctic taxa such as Candona and Iliocypris, indicating cooler water conditions than occurred earlier in the Pliocene.
The middle part of the Akchagylian was the warmest within the late Pliocene; vegetation was dominated by mixed coniferous-broadleaved forests, and heat-loving taxa of mollusks were abundant in the Caspian Sea. Mean January temperature was about 2°C, that of July 21°C.
In the late Akchagylian a new cooling took place, marked by the spread of coniferous forests of Pinus, Picea and Tsuga. The flora became more depauperate than earlier, and increasing aridity is interpreted from a spread of the steppe vegetation at the end of Akchagylian time. During the final part of the Pliocene (the Apsheronian stage) further cooling and drying occurred on the southeastern Russian Plain, and steppe vegetation became dominated by Artemisia and Chenopodiaceae associations.
In general, for any stage within the Pliocene the same pattern can be observed: greater continentality and aridity in the east than in the west. Overall, since the time of maximum warmth during the middle Pliocene Kimmerian stage a similar pattern toward greater continentality and aridity through time can be seen in any given subregion of the southern Russian Plain. The climatic oscillations on the background of this general trend can be seen more clearly by time sequence of landscape change in the southeastern Russian Plain. The middle Pliocene (early Kimmerian) time can be regarded as the climatic optimum of the Pliocene in the southern Russian Plain.
For the warm global climates of the Pliocene, the highest amplitude climatic fluctuations were expressed in the eastern, more continental, part of the southern Russian Plain. In contrast, under the cold global climates of the late Quaternary, climatic fluctuations were greater in western Europe than in the east.