Stratigraphy of the Pliocene deposits of the Black Sea (Ukraine) according to evidence from ostracods (Arthropoda, Crustacea)

This article presents a detailed analysis of the taxonomic composition of the Pliocene (Kimmerian, Kujalnikian) and Eopleistocene (Gurian) ostracods in the northern part of the Black Sea. It presents the patterns of the stratigraphic position of the fossil ostracods in the Miocene Quaternary and their geographic distribution in Western and Eastern Europe (the Pannonian Basin, the Dacian Basin, the Euxinian basin of the Paratethys) and the Mediterranean region.We determined the characteristic species for the Kimmerian, Kujalnikian and Gurian in the northern part of the Black Sea. We established a change in the taxonomic composition of ostracods at the Pliocene (Kujalnikian)/Eopleistocene (Gurian) boundary, namely the disappearance of a large number of Pliocene species and the appearance of new species. Ten species disappeared in the Kujalnikian: Cyprideis pontica, Euxinocythere (M.) crebra, Amnicythere mironovi, Camptocypria lobata, Loxoconcha subcrassula, Loxoconcha verticalitercostata, Xestoleberis (X.) cellulocus, Xestoleberis (P.) communis, Candona (C.) expressa, Ilyocypris caspiensis; one species Amnicythere postbissinuata appeared in the Gurian. The brackish water species Cyprideis pontica is the Kujalnikian index species. The stratigraphic position of Cyprideis pontica in the Mediterranean Basin, Pannonian Basin, Dacian Basin, Euxinian Basin (Black Sea) in the Miocene-Quaternary is analyzed. The time of the disappearance of Cyprideis pontica in the Mediterranean, Pannonian and Dacian basins (Messinian, Pontian/Zanclean, Dacian, Kimmerian boundary) and in the Black Sea (Kujalnikian/Gurian boundary) is established. The diagnostic morphological features of the shell Cyprideis pontica (morphology of the surface pore canals) are established and described, which allows us to place this species in the Neogene deposits. Surface pore canals are different shape, sievetyped, deepened in relation to the surface of the valve. Sieve-shaped lamella contains 110-270 internal pores. The internal pores have a staggered shape, the diameter of the osculum of the internal pore is 302-994 nm; diameter of the central pore is 977 nm-1.8 μm). The evolution of Cyprideis pontica, which was separated from the parent species Cyprideis torosa in the Late Miocene, was reconstructed. In the occupation of a new ecological niche with a reduced oxygen content in deeper water biotopes, in the process of adapting to the conditions of hypoxia and necessity of increasing the volume of water filtration, there was a restructuring of the morphology of the surface pore canals of the shell Cyprideis torosa. This involved an increase in the size of the sieve-shaped lamella, the number of internal pores in the sieve-shaped lamella and the size of the osculum of the inner pore. A new morphotype Cyprideis pontica was thus formed within the existing Parathetys-Mediterranean basins. It had a mosaic, ecologically isolated range that coincided geographically or overlapped with the range of the species Cyprideis torosa (sympatric evolutionary speciation). The range of Cyprideis pontica and the dynamics of its populations in the Euxinian Basin during the Sarmatian-Kujalnikian have been reconstructed.

Ключові слова : cтратиграфічна границя, пліоцен, квартер, остракоди Introduction. In the Geologic Time Scale (2012) the biostratigraphic divisions of the Pliocene deposits and the Quaternary base (2.588 Ma, between the Gelasian and Piacenzian (Riccardi, 2009) was substantiated by chronostratigraphy, event stratigraphy, magnetostratigraphy, radiometric dating ( 40 Ar/ 39 Ar dating, 14 C and 230 Th/ 234 U dating, U/Pb dating, 87 Sr/ 86 Sr), climate change and Milankovich cycles (sedimentation cycles), and oxygen and carbon isotopes δ 18 O, δ 13 C curves. Palaeontological characteristic of the Gelasian and Piacenzian derive from different faunal groups (mammals, planktonic foraminifera, calcareous nannofossils, diatoms, radiolarians, dinoflagellates). However, formally, the Neogene-Quaternary boundary is uniquely substantiated on the appearance at the beginning of the Gelasian of dinoflagellate cysts of Spiniferites pachyderma and Invertocysta tabulata (middle part of D21, northwestern Europe) and the radiolarian Pterocanium prismatium (RN12b, northwestern Europe), as well as the disappearance at the end of the Piacenzian of the radiolarian Anthocyrtidium jenghisi (zone RN12a; northwestern Europe) and the planktonic foraminiferаn Dentoglobigerina altispira (zone PL 5, Pacific Ocean). The Pliocene-Quaternary boundary (2,588 Ma) in the Paratethys region is compared with the boundary of the megacycle Ge1 without paleontological substantiation. It is located in the upper part of the Pannonian and within the Kimmerian (Fig. 29. Neogene-Quaternary Regional Subdivisions: Hilgen, Lourens, Van Dam, 2012).
In the Quaternary stratigraphic scheme for Ukraine, the Pliocene/Quaternary boundary is placed at the level between the Kujalnikian (Gelasian) and the Gurian (Calabrian) at 1.81 Ma (Stratigraphic Code of Ukraine, 2012). This is based on the appearance in the Gurian of the molluscs Dreissena distanta, D.  Geological Journal, 2018). Accordingly, the Pliocene/ Quaternary boundary is now placed at a lower level in the Kujalnikian horizon, which needs additional palaeontological fauna-floristic substantiation.
The purpose of this article is to detail the microfaunistic characteristics (using ostracods) of the Pliocene (Kimmerian, Kujalnikian) and Eopleistocene (Gurian) marine sediments, and to substantiate the Pliocene/Quaternary boundary in the northern part of the Black Sea.
Neogene ostracods of the Paratethys and Mediterranean regions and the Quaternary ostracods of western and eastern Europe, the Black Sea, Caspian Sea and Mediterranean Sea have been well studied in their systematic, ecological, zoogeographical and palaeogeographical aspects. They are used for detailed relative-age determination and correlation of marine deposits, but fossil ostracods as a biostratigraphic tool have not been used in the international stratigraphic scales of the Neogene-Quaternary (Geologic Time Scale, 2004Scale, , 2012 and Ukrainian stratigraphic schemes of the Neogene-Quaternary (Stratigraphic Code of Ukraine, 2012). This biostratigraphic analysis using ostracods was the first of its kind.
Fossil ostracods are a uniquely informative group of fossil microorganisms. In sediments, ostracods are often the only (and usually numerous) representatives of the fossil fauna. They had a predominantly autochthonous type of burial, often beautifully preserved or at least with a sufficient degree of conservation of the shell to allow species identification. Local and regional biostratigraphic divisions in the Neogene-Quaternary deposits of the Black Sea are distinguished using ostracods (Dykan, 2011;Dykan, 2012;Dykan 2016 a, b, c, d, f). Therefore, the application of fossil ostracods to stratigraphic correlation schemes, as a biostratigraphically important group of fossil microorganisms, is scientifically sound and expedient. Material and methods of research. The Neogene-Quaternary and recent ostracods of the Black Sea were collected over a period of forty years  from outcrops on the northern coast of the Black Sea (from the Danube to the Taman Peninsula), well cores (Odessa region, Kerch Peninsula, Taman Peninsula, the estuaries of the northwest coast of the Black Sea) and stations (the shelf and continental slope of the northern part of the Black Sea, SRV NASU) (Fig. 1).
The biostratigraphic conclusions were based on the systematic study of fossil ostracods (the identification of species, the principles and criteria for the determination of taxonomic features, the estimation of the taxonomic weight of the morphological features, the determination of diagnostic features of the different taxonomic ranks, taxonomic diagnoses, unified method of the description of shell morphology) (Dykan, 2006). Electronic-microscopy, taphonomic, statistical, population, geochemical and facial methods, and zoogeographical analyses have been used in the study of fossil ostracods. The determination of the geological ages of the marine deposits was based on biostratigraphic and ecological criteria derived from ostracods. The biostratigraphy is based on the presence of index species; groups of fossil ostracods, which have the upper and lower boundaries, well established first and last appearances of characteristic species; in the presence of periods of optimum of ostracod species (genera); on the ratio of zoogeographical species (Mediterranean, Caspian) and species of different ecological specialization (marine, brackish water, freshwater). Literary sources on the stratigraphic position and geographical distribution of fossil ostracods in the Holarctic belt were also taken into account. Results and their analysis. In the northern part of the Black Sea, Pliocene deposits are represented by Kimmerian and Kujalnikian horizons. The Kimmerian deposits are distributed on the western shelf from the mouth of the Tiligul estuary, on the Crimean continental slope from isobaths 287 m to 1750 m, to the southern slope of the Kerch-Taman shelf to the isobaths 150-200 m. They are presented by a layer of sandy clayey silts on the north-western shelf; oolitic iron ores with clay layers in the Crimean continental slope; iron-bearing sandstone on the eastern part of the northwestern shelf and the Kerch shelf. The thickness of the Kimmerian deposits decreases from east to west and is 40-50 m (Semenenko, 1987;Shuraev, 2015). The lectostratotype of the Kimmerian is the section near the village Arshintsevo (Kamish-Burun)  The Kujalnikian deposits occur in wells on the shelf and in the deep-water zone of the continental slope; their areal distribution coincides with the Kimmerian deposits (Semenenko, 1987). They are presented by sand, silts, sandstones and clay with a total thickness of 40-50 m. The lectostratotype of the (Xestoleberis) chanakovi Liv.&Agal. (Fig. 2).  (Dykan, 2006. Kujalnikian ostracod associations had a brackish water-freshwater-marine composition: brackish water species accounted for 57 %, freshwater -31 %, marine -12 % of the total number of species (Fig. 3).
Gurian deposits are embedded low down in the marine Quaternary deposits and they are mosaically distributed between isobaths 10-90 m in the northwestern and northeastern shelf of the Black Sea.  Table 1). Gurian ostracod associations had a brackish water-freshwater composition: brackish water species dominated (accounting for 69 % of the total number of species ), with freshwater ostracods comprising 31 %; Fig. 3).
Cyprideis pontica is a phylogenetic branch of the species Сyprideis torosa -separating from the parent species C. torosa in the Late Miocene. As a result of the occupation of a new ecological niche in deeper water biotopes of the shelf and continental slope with a reduced oxygen content, part of the population of Cyprideis torosa went through a narrow specialization process. In the process of adaptation to hypoxia and the necessity to increase the volume of water filtration, there was a reorganization of the morphology of the surface pore channels in the species Cyprideis torosa. This involved an increase in the size of the sieve-shaped lamellae, the number of internal pores in these lamellae and the size of the osculum of the inner pore, and a change in the shape of the inner pore on the stack with a rim along the perimeter. As a result of these evolutionary processes, a new morphotype Cyprideis pontica was formed within the Paratethys-Mediterranean basins. It occupied a mosaic, ecologically isolated area that coincided geographically or overlapped with the area occupied by the species Cyprideis torosa (sympatric evolutionary process) (Fig. 6).
The morphology of the shell of Cyprideis pontica Krstić has a diagnostic feature (the morphology of the surface pore canals), which allows this species to be recognised in the Neogene deposits (Dykan, 2016 a, b). The surface pore canals have a different shape (rounded, oval, flower-shaped, irregularly elongated, irregularly oval), different sizes (8-42 μm), sieve-typed, deepened in relation to the surface of the valve. The sieve-shaped lamella contains from 110 to 270 internal pores. The internal pores have a staggered shape and а round osculum (302-994 nm in diameter) with a rim along the perimeter. The central pore is located in the centre of the sieve-shaped lamella, deepened in relation to the surface of the valve, with a round osculum (977 nm-1.8 μm in diameter) (Fig. 7). Conclusions. A monographic study of the Neogene-Quaternary ostracods of the Black Sea, their stratigraphic position and geographical distribution in the Mediterranean-Black Sea-Caspian region allow one to conclude that a change in the taxonomic composition and ecological specialization of ostracods occurred at the Kujalnikian/Gurian boundary in the northern part of the Black Sea. 24% of Pliocene species, including the index species Cyprideis pontica, disappeared in the Kujalnikian. Ostracod associations had a brackish water-freshwater-marine composition. In the Gurian, the new species Amnicythere postbissinuata appeared and brackish water-freshwater associations formed, with the domination of brackish water species.  Krstić, 1968: 1 -right valve, female, adult, lateral external view (x 130), Upper Sarmatian horizon, north-western shelf of the Black Sea, drill hole 67; 2 -left valve, male, adult, lateral external view (х 95), Lower Maeotian horizon, north-western shelf of the Black Sea, drill hole 55; 3fragment of the outer surface of the shell (х 250); 4 -surface porous canals (х 1000);); 5 -sieve-shaped lamella of the surface porous canal (х 5000); 6 -internal pores of the surface porous canal (х 13000)