Study Area, Material, and Methods

Abstract

This chapter provides an overview of the study area known as the Ponto-Caspian, which includes the Black Sea, the Sea of Azov, the Caspian Sea, the Aral Sea (currently dried up), and connecting straits. The Black Sea, with a maximum depth of 2212 m, is the easternmost of the seas of the Atlantic Ocean basin and can be considered as the most isolated sea of the modern Global Ocean. The Sea of Azov is connected to the Black Sea via the Kerch Strait. On average, the level of the Black Sea is 7–11 cm lower than that of the Sea of Azov and 30 cm higher than that of the Sea of Marmara. The Black Sea exhibits the standard oceanic provinces of the continental shelf, slope, and abyssal plain. The extensive continental shelf accounts for 25% of the sea area; the 200-m isobath is commonly taken as the shelf boundary of the Global Ocean. The bottom relief is largely smooth due to the distribution of sediment discharge from major lowland European rivers, such as the Danube, Dnieper, Dniester, and the Southern Bug, that together discharge 56.8 million tons of sediments annually.

The average bottom-water salinity of the Black Sea (17 psu) is only half that of the Eastern (39 psu, practical salinity units) and Western (34 psu) Mediterranean. Salinity varies spatially, temporally, and with depth, in association with freshwater discharge into the basin and the influence of subsurface inflow from the Sea of Marmara with salinity of ~35 psu. The maximum salinity of the Sea of Azov is 13 psu.

The Caspian Sea is the world’s largest inland body of water, variously classed as an isolated lake without an outflow (i.e., an endorheic basin). It lies at the junction of Europe and Asia, with the Caucasus Mountains to the west and the steppes of Central Asia to the east. The Caspian Sea, like the Black Sea, is a remnant of the ancient Paratethys Sea. It became landlocked about 5.5 million years ago due to tectonic uplift and falling sea level.

The Caspian Sea is divided into three distinct physical regions: the Northern, Middle, and Southern Caspian. The Northern Caspian only includes the Caspian shelf and is very shallow, with an average depth of only 5–6 m; it accounts for less than 1% of the total water volume. More than 130 rivers flow into the Caspian, with the Volga River being the largest. The Northern Caspian water is almost fresh, becoming more brackish toward the south. The sea is most saline off the Iranian shore, where the watershed contributes little inflow. Currently, the mean salinity of the Caspian is 12.8 psu on average, varying from 1 psu near the Volga outlet to a high of 200 psu in the Kara-Bogaz-Gol, where intense evaporation occurs.

The Aral Sea was also an endorheic lake; lying between Kazakhstan and Uzbekistan, it began shrinking in the 1960s and had largely dried up by the 2010s. Formerly it was the fourth largest lake in the world with an area of 68,000 km². The shrinking of the Aral Sea has been called “one of the planet’s worst environmental disasters.”

In the Black Sea, the Sea of Azov, and the Kerch Strait, sediment samples have been collected since mid-1970s using various research vessels. In limans (marshes) and river deltas, the samples were collected from small vessels by hand corer or diving. As a rule, the sampling campaigns took place in May–June. Samples for foraminiferal analysis were obtained from multiple organizations of the Black Sea countries. In total, approximately 32,000 samples from 1500 grabs, multicorers, box corers, 4300 gravity/piston cores, and 56 boreholes (up to 40 m in length) were investigated. The samples were obtained in limans, lagoons, river deltas, shelf, and continental slope of the Black Sea, the Kerch Strait, the Sea of Azov, and the Caspian Sea. The total length of investigated sediment cores reached 8000 m. Marine research was supplemented by the study of 112 Quaternary outcrops including stratotypes located on the Crimean and Kerch Peninsula, Caucasian, and northwestern coast of the Caspian Sea.

At most marine stations, environmental (e.g., salinity, depth, temperature, dissolved oxygen) and foraminiferal parameters were measured. The total assemblage, including live (stained) and dead (empty) tests of foraminifera, was determined and expressed as the number of tests (abundance) per 50 g of dry sediment. To identify possible interrelations among foraminiferal characteristics and environmental parameters, a range of statistical methods were applied.

The inner structure of foraminiferal tests and their wall ultrastructure were studied in 320 thin sections and 120 artificial models. All species of benthic foraminifera were morphologically examined, taxonomically identified, and SEM imaged. Particular attention was paid to morphological deformities of foraminiferal tests as indicators of environmental stress. The collections of benthic foraminifera from the Black Sea, Caspian Sea, and Sea of Azov are stored in the Paleontological Museum of Odessa National University, Ukraine.

The stratigraphic work follows the Russian subdivision of the Quaternary System (Zhamoida, Stratigr Geol Correl, 12:321–330, 2004) into the Eopleistocene (1.8–0.8 Ma), Neopleistocene (0.8–0.01 Ma), and Holocene (0.01–0.0 Ma). The resulting high-resolution stratigraphy and geochronology of the Late Quaternary (Neopleistocene and Holocene) sediments are based on the combination of bio- and ecostratigraphic criteria supplemented by absolute dating. Biostratigraphic criteria include precise taxonomic analysis of benthic foraminifera to trace species evolution and to discover species indicators for certain time intervals. Due to the shortness of the studied time interval, the application of these criteria is limited and so is supplemented by ecostratigraphic criteria.