Introduction
The connection of the Black Sea with the Mediterranean has been episodic during the Quaternary, whereas fewer periodic intrusions of water from the Caspian Sea have occurred via the Manych Corridor during the Pleistocene (Chepalyga, Reference Chepalyga2002, Reference Chepalyga2007; Bahr et al., Reference Bahr, Lamy, Arz, Major, Kwiecien and Wefer2008; Badertscher et al., Reference Badertscher, Fleitmann, Cheng, Edwards, Göktürk, Zumbühl, Leuenberger and Tüysüz2011; Yanina, Reference Yanina2014). These intrusions have greatly influenced the environmental conditions of the Black Sea, eventually creating the biota that now inhabits the basin (e.g., Mudie et al., Reference Mudie, Rochon, Aksu and Gillespie2002; Yanko-Hombach et al., Reference Yanko-Hombach, Gilbert and Dolukhanov2007; Marret et al., Reference Marret, Mudie, Aksu and Hiscott2009; Boomer et al., Reference Boomer, Guichard and Lericolais2010). The present two-way connection between the Black Sea and Marmara Sea was established in the early Holocene (Grigor’ev et al., Reference Grigor’ev, Isagulova and Fedorov1984; Yanko and Troitskaya, Reference Yanko and Troitskaya1987; Hiscott et al., Reference Hiscott, Aksu, Mudie, Marret, Abrajano, Kaminski, Evans, Çakiroğlu and Yaşar2007; Soulet, Ménot, Lericolais et al., Reference Soulet, Ménot, Lericolais and Bard2011), creating a substantial salinity increase in the Black Sea. However, quantitative estimation of such conditions and their timing have been heavily debated (Ryan et al., Reference Ryan, Pitman, Major, Shimkus, Moskalenko, Jones, Dimitrov, Gorür, Sakinç and Yüce1997, Reference Ryan, Major, Lericolais and Goldstein2003; Aksu et al., Reference Aksu, Hiscott, Mudie, Rochon, Kaminski, Abrajano and Yaşar2002; Hiscott et al., Reference Hiscott, Aksu, Mudie, Marret, Abrajano, Kaminski, Evans, Çakiroğlu and Yaşar2007; Ivanova et al., Reference Ivanova, Murdmaa, Chepalyga, Cronin, Pasechnik, Levchenko, Howe, Manushkina and Platonova2007, Reference Ivanova, Marret, Zenina, Murdmaa, Chepalyga, Bradley, Schornikov, Levchenko and Zyryanova2015; Yanko-Hombach et al., Reference Yanko-Hombach, Gilbert and Dolukhanov2007, Reference Yanko-Hombach, Mudie, Kadurin and Larchenkov2014; Marret et al., Reference Marret, Mudie, Aksu and Hiscott2009; Nicholas et al., Reference Nicholas, Chivas, Murray-Wallace and Fink2011). Most of these studies have yielded well-preserved macro- and microfossil proxy records, providing insights of past environmental conditions during the Holocene (Atanassova, Reference Atanassova2005; Hiscott et al., Reference Hiscott, Aksu, Mudie, Marret, Abrajano, Kaminski, Evans, Çakiroğlu and Yaşar2007; Ivanova et al., Reference Ivanova, Murdmaa, Chepalyga, Cronin, Pasechnik, Levchenko, Howe, Manushkina and Platonova2007, Reference Ivanova, Murdmaa, Karpuk, Schornikov, Marret, Cronin, Buynevich and Platonova2012, Reference Ivanova, Marret, Zenina, Murdmaa, Chepalyga, Bradley, Schornikov, Levchenko and Zyryanova2015; Mudie et al., Reference Mudie, Marret, Aksu, Hiscott and Gillespie2007; Yanko-Hombach et al., Reference Yanko-Hombach, Gilbert and Dolukhanov2007, Reference Yanko-Hombach, Mudie, Kadurin and Larchenkov2014; Verleye et al., Reference Verleye, Mertens, Louwye and Arz2009; Boomer et al., Reference Boomer, Guichard and Lericolais2010; Bradley et al., Reference Bradley, Marret, Mudie, Aksu and Hiscott2012; Mertens et al., Reference Mertens, Bradley, Takano, Mudie, Marret, Aksu and Hiscott2012). However, understanding the temporal and spatial changes in assemblages from a variety of locations, taxonomic refinements (Boomer et al., Reference Boomer, Guichard and Lericolais2010), and more information about modern habitat preferences are a prerequisite for improving Holocene paleoenvironmental reconstructions, which is the case for ostracods. This microfossil group is particularly suitable for paleoenvironmental reconstruction because its members are abundant, inhabit a wide range of habitats, and are sensitive to environmental change, and their specific assemblage composition reflects definite bottom-water conditions (Athersuch et al., Reference Athersuch, Horne and Whittaker1989; Schornikov and Zenina, Reference Schornikov and Zenina2014).
Two main migration pathways have been postulated to explain the composition of Holocene Black Sea ostracod fauna. The first pathway (Fig. 1) enabled the migration of Caspian species during periodic intrusions of water during the Pleistocene, via the Manych Corridor, with the last connection with the Caspian Sea occurring during the Neoeuxinian transgression between 14 and 15 cal ka BP (Yanina, Reference Yanina2014). These species are tolerant of low-salinity environments such as the present conditions in the Caspian Sea (Boomer et al., Reference Boomer, Grafenstein, von, Guichard and Bieda2005). During periods of relative higher salinity, for example in the Holocene or Eemian (Shumilovskikh et al., Reference Shumilovskikh, Marret, Fleitmann, Arz, Nowaczyk and Behling2013), the Caspian species did not inhabit the main body of the Azov–Black Sea basin but survived in low-salinity refuges (Schornikov, Reference Schornikov2011). The second pathway (Fig. 1a) allowed the migration of marine species during periods of connection with the Mediterranean (Hiscott et al., Reference Hiscott, Aksu, Mudie, Marret, Abrajano, Kaminski, Evans, Çakiroğlu and Yaşar2007; Boomer et al., Reference Boomer, Guichard and Lericolais2010). Improving our knowledge about the ecological affinities of these two groups will better constrain environmental reconstructions from a period in which the Black Sea was isolated from other water bodies up to its reconnection with the Mediterranean.
Fig. 1 The study region. (a) Overview of study area. (b) Ponto-Caspian basins during the late Pleistocene in the time of the connection of the Neoeuxinian (Black Sea) Basin and the Khvalynian (Caspian) Basin (adapted from Yanina, Reference Yanina2014). (c) Location of cores Ak-2575 and Ak-521. (d) Sample locations in the Caspian Sea. (e) Modern distribution of fauna of the Caspian type in the Azov–Black Sea basin (Schornikov, Reference Schornikov1969, Reference Schornikov2011; Opreanu, Reference Opreanu2008; and new data from the Kuban Delta).
This article aims to provide new insights into the Holocene environmental changes on the northeastern (Caucasian) Black Sea shelf based on ostracod data from two gravity cores (Ak-521 and Ak-2575). We interpret the fossil ostracod data using modern species and habitat information from living fauna collected in the Caspian Sea and Azov–Black Sea basin. The article provides a more detailed examination of the fossil data presented by Ivanova et al. (Reference Ivanova, Marret, Zenina, Murdmaa, Chepalyga, Bradley, Schornikov, Levchenko and Zyryanova2015) using the newly collected modern data. In addition, new fossil data with taxonomic revisions from core Ak-521, first published at a lower resolution by Ivanova et al. (Reference Ivanova, Murdmaa, Chepalyga, Cronin, Pasechnik, Levchenko, Howe, Manushkina and Platonova2007), are presented. Six species of the Caspian type are reported for the first time in the Black Sea fossil assemblages. Five of these species, hypothesized to belong to genera of the Caspian type, are not found in the Caspian Sea. They are new undescribed species and shown in open nomenclature.
Regional setting
The Black Sea has a unique set of characteristics, with surface water salinity about half of oceanic salinity ranging between 18 and 22 practical salinity units (psu) (Murray, Reference Murray1991; Sorokin, Reference Sorokin2002). Strong stratification of the water column results in the appearance of a hydrogen sulfide zone in the deep water. In the southwest, the Black Sea exchanges water with the Marmara Sea, via the Bosporus Strait, with outflowing brackish surface water replaced by inflowing saline deep water. In the northeast, the Black Sea exchanges water with the Sea of Azov, via the Kerch Strait (Fig. 1). Compared with the Black Sea, the Sea of Azov is geologically young and very shallow (<10 m); it has a small surface area and much smaller volume. Therefore, it can be considered as a gulf, or part of the Black Sea (Chepalyga, Reference Chepalyga2002). In this article, we use the term Azov–Black Sea basin. The Caspian Sea is the largest enclosed body of water on Earth, with water depth ranging from quite shallow in the north (<20 m) to relatively deep in the south basin (1025 m; Kosarev, Reference Kosarev2005). The Volga River is the largest water inflow into the Caspian Sea and discharges into the north Caspian basin. The Caspian Sea is closed with no outlet. In the twentieth century, it has experienced large changes in sea level with drops and increases of ~1–3 m observed (Arpe and Leroy, Reference Arpe and Leroy2007). The salinity is brackish with ranges between 1 and 13 psu. The salinity increases in a southward direction.
The two sediment cores were taken from the Caucasian shelf (Fig. 1a and c) between Arkhipo-Osipovka and Dzhubga. In this area, the shelf narrows to a width of 4 to 12 km and generally has a flat surface that slopes slightly (0.2°–0.6°) towards the shelf break at depths of 100–120 m. The shelf can be divided into three portions: a coastal area that extends from the shoreline to depths of 25–30 m; a central shelf platform between 30 and 70 m depth; and the outer shelf that ranges from a depth of 80 m to a sharp shelf break at 95–105 m (Torgunakov et al., Reference Torgunakov, Merklin, Shimkus, Moscalenko and Lobkovsky2002).
The surface circulation on the northeastern shelf is dominated by the counterclockwise rotating peripheral rim current (Bogatko et al., Reference Bogatko, Boguslavskii, Belyakov and Ivanov1979; Öǧuz et al., Reference Öğuz, Latun, Latif, Vladimirov, Sur, Markov, Özsoy, Kotovshchikov, Eremeev and Ünlüata1993). On the outer shelf, two water masses are identified in the water column. A well-ventilated surface water mass is present in the upper 50–90 m, above a strong seasonal pycnocline. Salinity measurements at ~44.53° N and 37.93° E during February, May, July, and October 2015 show that annual salinity values vary between 17.5 and 19.4 psu (Fig. 2). Seasonal variability depends on depth, with less than 25 m measurements having a range of ~0.5 psu, 25–75 m depths spanning less than 0.5 m, and greater than 75 m ranging around 1 psu. Annual temperature values vary between 7.5°C and 25°C with greatest variability at depths 0–50 m (Fig. 2). At depths below 50 m, variability decreases to ~2.5°C.
Fig. 2 Variation in temperature and salinity with depth at four time intervals from 44.53° N and 37.93° E on the northeastern Black Sea shelf.
Materials and methods
Two gravity cores, Ak-521 (44.26° N, 38.54° E; water depth, −101 m; 200 cm long) and Ak-2575 (44.22° N, 38.63° E; water depth, −99 m; 186 cm long), were retrieved during cruises of the R/V Akvanavt in 2001 and 2007, respectively.
Fossil ostracod analysis
Samples were sieved through a 100 µm (Ak-521) or 63 µm (Ak-2575) mesh using distilled water. For core Ak-2575, dry fractions (>2 mm, 0.1–2 mm, and 0.063–0.1 mm) were weighed to calculate the percent of each fraction in the samples and the total number of ostracods per gram of sediment. The dry fractions 0.1–2 mm (both cores) and 0.063–0.1 mm (Ak-2575 only) were analyzed using a binocular microscope. Ostracod valves were described to the species level where possible. Ivanova et al. (Reference Ivanova, Murdmaa, Chepalyga, Cronin, Pasechnik, Levchenko, Howe, Manushkina and Platonova2007) briefly described a subset of 18 ostracod samples from Ak-521. In this article, we present the full ostracod record (37 samples), taxonomic revision of previously published work (Table 1), integration with ecological data (Fig. 1e, Tables 2 and 3), and species images (Supplementary Figs 1–3). Ostracod assemblages from core Ak-2575 were analyzed in 2 cm thick slices from 93 contiguous samples (Supplementary Table 1). Previously, ostracods from this core were briefly described in Zenina et al. (Reference Zenina, Schornikov, Ivanova, Bradley and Marret2013) and Ivanova et al. (Reference Ivanova, Marret, Zenina, Murdmaa, Chepalyga, Bradley, Schornikov, Levchenko and Zyryanova2015). SEM images were taken using a Zeiss EVO 40.
Table 1 Revised ostracod taxonomy for Ak-521 (A) from Ivanova et al. (Reference Ivanova, Murdmaa, Chepalyga, Cronin, Pasechnik, Levchenko, Howe, Manushkina and Platonova2007), Ak-2575 (B) from Ivanova et al. (Reference Ivanova, Marret, Zenina, Murdmaa, Chepalyga, Bradley, Schornikov, Levchenko and Zyryanova2015), and fauna of Caspian type (C) from Opreanu (Reference Opreanu2008)
Table 2 Modern distribution of ostracod species of Caspian type in Azov–Black Sea basin based on literature data (Schornikov, Reference Schornikov1969, Reference Schornikov2011; Opreanu, Reference Opreanu2008)
Table 3 Modern distribution of some collected ostracod species in the Caspian Sea. For samples I–IX (see Fig. 1c), asterisk indicates species is found living, and no asterisk indicates only valves and shells; salinity, temperature, O2, and pH are noted only for sites with living ostracods
The ostracod accumulation rate (OAR) of valves in AK-2575 was estimated using the following equation:
$${\rm Accumulation}\,{\rm rate \ of }\,{\rm valves }\left\ ( {{\rm cm}^{{\rm 2}} \,{\rm per}\,{\rm ka}} \right)\,{\rm {\equals}}\,V{\rm {\times}}D{\rm {\times}}S,$$
where V is the valve count per gram of dry sediment from grain-size fractions between 63 and 2000 μm; D is the sediment wet density (1.4 g/cm3); and S is the sedimentation rate (cm/ka), which is based on the age model by Ivanova et al. (Reference Ivanova, Marret, Zenina, Murdmaa, Chepalyga, Bradley, Schornikov, Levchenko and Zyryanova2015).
Modern ostracod samples
Nine samples collected from the Caspian Sea with a small dredge were analyzed (Fig. 1c). Samples I–VI were collected by M.A. Zenina between July 28 and August 1, 2014. Samples VII–IX were collected by E.I. Schornikov on August 5, 1956 (VII, VIII) and July 16, 1952 (IX). In this study, we also provide information about ostracod fauna living in the Azov–Black Sea basin and do not consider species living only in the Caspian Sea.
Lithostratigraphy and chronostratigraphy
The age model for the two gravity cores Ak-521 and Ak-2575 published by Ivanova et al. (Reference Ivanova, Marret, Zenina, Murdmaa, Chepalyga, Bradley, Schornikov, Levchenko and Zyryanova2015) is based on 19 calibrated radiocarbon dates (9 for Ak-521, 10 for Ak-2575). Regional stratigraphic units with calibrated dates of their boundaries (Balabanov, Reference Balabanov2009; Ivanova et al., Reference Ivanova, Marret, Zenina, Murdmaa, Chepalyga, Bradley, Schornikov, Levchenko and Zyryanova2015) are also taken into account. The proposed transgression phases include Neoeuxinian (11–10 cal ka BP), Bugazian (10–8.8 cal ka BP), Vityazevian (8.8–7.8 cal ka BP), Kalamitian (7.8–6.9 cal ka BP), Dzhemetinian (6.9–2.6 cal ka BP), and Nymphean (2.6–0 cal ka BP) (Balabanov, Reference Balabanov2009).
Both cores were retrieved close to the shelf break and recovered similar sections of Holocene deposits. This included a ~0.5 m thick coquina at their base composed of semifreshwater Caspian-type mollusk shells (dominated by Dreissena rostriformis) in its lower part and a mixed (Caspian + Mediterranean) mollusk fauna in the upper part (Ivanova et al., Reference Ivanova, Murdmaa, Chepalyga, Cronin, Pasechnik, Levchenko, Howe, Manushkina and Platonova2007, Reference Ivanova, Marret, Zenina, Murdmaa, Chepalyga, Bradley, Schornikov, Levchenko and Zyryanova2015). The coquina marks a high-energy bottom environment at the shelf edge that commenced at the end of the Neoeuxinian stage as shown by the oldest date from the shelly mud layer underlying the coquina in core Ak-521 (11.87 cal ka BP) and lasted up to ~7.4 cal ka BP according to the age model (Ivanova et al., Reference Ivanova, Marret, Zenina, Murdmaa, Chepalyga, Bradley, Schornikov, Levchenko and Zyryanova2015). The sharp, possibly erosional, basal contact of the coquina suggests a hiatus at its base. Another hiatus likely separates the Dreissena-dominated coquina from the mixed-fauna one, both belonging to a slowly accumulating high-energy shelf edge facies.
The gradual, although distinct, upper contact of the coquina reflects slowing down of bottom hydrodynamics, thus allowing deposition of fine-grained terrigenous mud that was inhabited by Mytilus galloprovincialis (the Mytilus mud facies). Sedimentation rates increased up to 95.2 cm/ka during the time interval 7.4–6.8 cal ka BP, and to 111.1 cm/ka in the interval 6.8–6.5 cal ka BP (Ivanova et al., Reference Ivanova, Marret, Zenina, Murdmaa, Chepalyga, Bradley, Schornikov, Levchenko and Zyryanova2015). Intercalation of the Mytilus mud by thin muddy Mytilus coquina beds suggests pulsating sedimentation possibly related to alternation of extreme floods delivering very large amounts of terrigenous suspended material to the shelf edge with periods of depleted mud supply.
Sedimentation rates decreased during the interval 6.5–5.8 cal ka BP to values less than 4.4 cm/ka. A hiatus likely occurs between the 6.27 and 3.59 cal ka BP dates that separates the Mytilus mud facies from the overlying Modiolus phaseolinus mud facies characterized by low sedimentation rates of 5.0–19.3 cm/ka. The M. phaseolinus–dominated shelly mud comprises the uppermost parts of both core sections up to the sediment surface. At present, it covers a wide area of the outer shelf landward to depths of ~60 m.
Results
Ostracod division into ecological groups based on their modern distribution
The ostracods of Caspian type in the Azov–Black Sea region dwell in shallow water (down to 5 m; Fig. 1e, Table 2), but in the Caspian Sea, they can inhabit considerably greater depths (Table 3). For example, living specimens of Graviacypris elongata were found in depths of 50–68 m in the Caspian Sea. Most of the species found in both cores (Table 4) are abundant in silty-sandy mud. Tyrrhenocythere amnicola donetziensis and Euxinocythere virgata were also quite numerous on shelly ground in the Caspian Sea. T. amnicola donetziensis, E. virgata, G. elongata, Xestoleberis chanakovi, and Sarmatina? cf. azeri can inhabit depths of 30 m. These species are found living in the Caspian Sea (Table 3). Owing to the lack of adequate data on the ecology of Caspian-type species, it is not yet possible to separate groups solely based on habitat.
Table 4 Species composition of ostracod fauna from the northeastern outer shelf. Occurrence of species: +++, abundant; ++, common; +, rare. Bg, Bugazian (10–8.8 cal ka BP); Vt, Vityazevian (8.8–7.8 cal ka BP); Kl, Kalamitian (7.8–6.9 cal ka BP); Dz, Dzhemetinian (6.9–2.6 cal ka BP) according to Balabanov (Reference Balabanov2007). C, species of Caspian type; F/O, freshwater-oligohaline species; M, species of Mediterranean type. I, IIA, IIB, IIIA, and IIIB indicate assemblages (see Figs. 3 and 4)
It is possible to differentiate ostracod species of the Mediterranean type (Table 4) based on habit preferences (Ivanova et al., Reference Ivanova, Schornikov, Marret, Murdmaa, Zenina, Aliev and Bradley2014; Schornikov et al., Reference Schornikov, Zenina and Ivanova2014). Mud dwellers include Palmoconcha agilis, Cytheroma variabilis, Cytheroma marinovi, Bythocythere sp., Carinocythereis carinata, Paradoxostoma simile, and Xestoleberis cornelii. Sandy mud dwellers include Hiltermannicythere rubra, Leptocythere multipunctata, Callistocythere diffusa, and Pontocythere tchernjawskii. Sagmatocythere rennata is a sand dweller.
Fossil distribution of ostracod species
In total, 37 ostracod species were recorded in the samples taken from the two cores (Table 4). These species can be split into three groups based on their habitat preference type: 2 are freshwater/oligohaline taxa, 23 taxa are of the Caspian type, and 12 taxa are of the Mediterranean type. Both cores are subdivided into three sections based on assemblage groupings. These sections appear to depend on the variation in salinity and habitat type during the Holocene.
Assemblage I
Assemblage I spans the interval from 11.9 to 7.4 cal ka BP, which corresponds to core depths 187–144 cm in Ak-2575 and 192–148 cm in Ak-521. The ostracod abundance for core AK-2575 varies from greater than 1 to 31 valves/g, and OARs vary from 6.8 to 870.8 valves/ka. Twenty-nine species are identified for this period including 23 species of the Caspian type. Two are of the freshwater/oligohaline type, and 3 species are the earliest Mediterranean ostracods to appear in the Black Sea (Table 4, Figs. 3 and 4). The most abundant species are Loxocaspia lepida, Loxocaspia sublepida, Amnicythere martha, Euxinocythere relicta, Amnicythere stepanaitysae, and G. elongata. The two freshwater/oligohaline taxa are Cypria lubeziensis, which is only found in the deepest samples of core Ak-521 (195–190 cm), and Fabaeformiscandona sp., which is recorded in both cores between ~9.6 and 7.4 cal ka BP. Strong polymorphism is exhibited in a number of Caspian species, especially A. stepanaitysae, L. lepida, E. relicta, and A. martha. This is highlighted for A. stepanaitysae and E. relicta in Supplementary Figures 1 and 2. The total number of species found per sample decreases towards the upper parts of the interval of assemblage I. Only three taxa, L. lepida, L. sublepida, and G. elongata, are found in the uppermost part of the assemblage interval, around 7.4 cal ka BP (150–152 cm in Ak-2575 and 153–158 cm in Ak-521). In the youngest samples, L. lepida increases in relative abundance. There are also Mediterranean-type species including H. rubra and P. agilis in both cores, and sparse specimens of L. multipunctata in Ak-2575 are represented both by adult and juvenile valves.
Fig. 3 Occurrence of ostracods in core Ak-2575. Lithology and calendar ages are based on Ivanova et al. (Reference Ivanova, Marret, Zenina, Murdmaa, Chepalyga, Bradley, Schornikov, Levchenko and Zyryanova2015). The 2σ ranges for the calibrated radiocarbon dates (all cal yr BP) are as follows: 2610 (2360–2800), 3590 (3390–3830), 6270 (6000–6450), 6510 (6300–6690), 6820 (6600–7030), 7150 (6910–7330), 7415 (7250–7580), 7520 (7410–7680), 9440 (9270–9540), and 9550 (9470–9690).
Fig. 4 Occurrence of ostracods in core Ak-521. Lithology and calendar ages are based on Ivanova et al. (Reference Ivanova, Marret, Zenina, Murdmaa, Chepalyga, Bradley, Schornikov, Levchenko and Zyryanova2015). The 2σ ranges for the calibrated radiocarbon dates (all cal yr BP) are as follows: 4540 (4280–4830), 6120 (5920–6302), 6410 (6270–6640), 6870 (6650–7160), 7420 (7170–7620), 8740 (8380–9130), 8710 (8350–9130), 9150 (8700–9500), and 11,870 (11,250–12,430). See sediment and ostracod occurrence legend in Figure 3.
Assemblage II
Assemblage II occurs between 7.4 and 6.7 cal ka BP, which corresponds to core depths of 144–90 cm in Ak-2575 and 148–88 cm in Ak-521. This assemblage contains a mixture of Caspian and Mediterranean species. The low number of ostracods recorded in Ak-2575 (<100 valves per sample) prevented determination of the relative abundance of species. The ostracod abundance for core Ak-2575 is low (0–3 valves/g). Among the ostracods of Caspian type, L. lepida and G. elongata are common. In contrast, L. sublepida is represented by only two valves in the sample 136–138 cm from Ak-2575. In both cores, the regular appearance of L. multipunctata is noted from ~7.4 cal ka BP. Furthermore, the species composition in the two cores is slightly different. For example, P. agilis was not recorded in core Ak-521.
Assemblage II can be further subdivided into two subassemblages occurring at 7.4–7.1 cal ka BP (IIA) and 7.0–6.8 cal ka BP (IIB). The fauna of the Caspian type prevails in subassemblage (IIA), whereas taxa of the Mediterranean type, such as H. rubra, L. multipunctata (in both cores), and P. agilis (in core Ak-2575), are still only minor components of the assemblage. OARs in the lower and middle parts (122 to 96 cm) of assemblage II are low (0 to 65.1 valves/ka), but in the upper part (up to 90 cm, ~6.8 cal ka BP), the value increases to 362 valves/ka.
In the upper part of IIA, G. elongata disappears from the record. In the interval 7.1–7.0 cal ka BP (122–116 cm in core Ak-2575), the record is barren. There are no data from Ak-521 in this period because of the lower sampling resolution. In subassemblage IIB, species diversity of Mediterranean ostracods becomes higher. Along with the taxa recorded in IIA, the fauna of this period also includes S. rennata, X. cornelii, C. marinovi, and C. variabilis. The only species of the Caspian type found during this period is L. lepida, which disappears from the record around ~6.8 cal ka BP. The most typical species in subassemblage IIA are L. multipunctata and S. rennata. H. rubra increases in relative abundance upwards in the assemblage.
Assemblage III
Assemblage III spans the interval from 6.8 cal ka BP (90 cm in core Ak-2575 and 88 cm in core Ak-521) to present (Figs. 3 and 4). It is characterized by a depleted fauna of Mediterranean-type species, typical of water depths of greater than 50 m. Accumulation rates are relatively high at the oldest boundary before falling in younger samples. Two species of the Caspian type found in core Ak-2575 include a reworked valve of X. chanakovi (found in the sample at 22–20 cm) and two reworked valves of L. lepida (found in the sample at 84–82 cm). The ostracod abundance for core Ak-2575 in this period is between greater than 1 and 41 valves/g. Fauna of the Mediterranean type in core Ak-2575 is more diverse and is represented by 12 species, whereas in core AK-521 it only consists of 9 species (Figs. 3 and 4). However, this is most likely because of the more detailed study of core Ak-2575. Indeed, species not recorded in Ak-521 samples such as C. carinata, C. diffusa, and P. tchernjawskii are noted as minor components in samples from core Ak-2575.
The upwards disappearance of the relatively shallow-water S. rennata and decrease in abundances of L. multipunctata and X. cornelii coincide with the increased occurrence of P. agilis and appearance of Bythocythere sp. in the uppermost part of the cores. This section is split into two subassemblages in Ak-2575 depending on the abundance and occurrence of these species. X. cornelii and H. rubra are the most abundant species in subassemblage IIIA during the interval 6.8–6.6 cal ka BP (90–64 cm in Ak-2575). However, the two subassemblages cannot be identified in Ak-521 because of the lower sampling resolution. Hence, the data indicate a similar species diversity and relative abundance within this assemblage in both cores. The abundance of the sandy mud dweller L. multipunctata decreases in the lower part of the interval, whereas the mud dweller P. agilis gradually becomes more common in the younger samples. In Ak-2575, two subassemblages are divided by a short interval from 6.5 to 6.3 cal ka BP in which only rare valves of P. agilis and H. rubra are found. The subassemblage IIIB from the interval 6.3–0 cal ka BP is of a relatively colder type. This is indicated by considerable numbers of P. agilis generally dominating the ostracod fauna and by the recording of Bythocythere sp., which prefers lower temperatures and inhabits modern assemblages of the Black Sea only at depths of greater than 70 m (Schornikov, Reference Schornikov1969). H. rubra decreases in relative abundance during this period. A short-term spike of maximal total ostracod abundance (up to 40 valves/g of dry sediment) occurs just above the hiatus, at ~4.2 cal ka BP.
Discussion
Present-day ostracod distribution and implication for paleoenvironmental reconstruction
This study investigated the present-day environmental ranges of Caspian-type species. These species currently inhabit the main body of the Caspian Sea (Boomer et al., Reference Boomer, Grafenstein, von, Guichard and Bieda2005) but are limited in the Black Sea region to estuaries, rivers, lagoons, and lakes (this study, Fig. 1d and Table 2; Schnornikov, 1969, 2011; Opreanu, Reference Opreanu2008). They currently account for ~20% of all known ostracods in the Black Sea (Schornikov, Reference Schornikov2012). These Caspian-type species have the highest relative abundance in early Holocene sediment records in the Black Sea (Hiscott et al., Reference Hiscott, Aksu, Mudie, Marret, Abrajano, Kaminski, Evans, Çakiroğlu and Yaşar2007; Boomer et al., Reference Boomer, Guichard and Lericolais2010; Bradley et al., Reference Bradley, Horne, Williams, Marret, Aksu and Hiscott2011; Williams et al., Reference Williams, Hiscott and Aksu2011; Ivanova et al., Reference Ivanova, Marret, Zenina, Murdmaa, Chepalyga, Bradley, Schornikov, Levchenko and Zyryanova2015; this study, Figs. 3 and 4). Currently, environmental reconstructions are limited to suggesting that the early Holocene Black Sea was similar to the modern Caspian Sea. The modern data presented in this study allow some refinement of early Holocene benthic reconstructions in the Black Sea, a period in which salinity and rates of change are contested (see Yanko-Hombach et al., Reference Yanko-Hombach, Gilbert and Dolukhanov2007; Bradley et al., Reference Bradley, Marret, Mudie, Aksu and Hiscott2012).
In-situ data show that ostracods of the Caspian type previously found in shallow waters (0–5 m) in the Black Sea region (Schornikov Reference Schornikov1969, Reference Schornikov2011; Opreanu, Reference Opreanu2008; this study, Table 2 and Fig. 1e) are able to inhabit considerably greater depths in the modern Caspian Sea (Table 3). These two basins are known to be characterized by different salt compositions, with chlorine type in the Black Sea basin and sulfate-hydrocarbonate type in the Caspian Sea (Nevesskaya, Reference Nevesskaya1965; Gidrometizdat, Reference Gidrometizdat1975). Our data suggest that the different salt compositions may affect the ostracod species distribution. Notably, species of the Caspian type recently inhabiting mesohaline depths in the Caspian Sea (Fig. 1d, Table 3) can tolerate less saline conditions in the Black Sea basin (Fig. 1c, Table 2). This hypothesis is supported by previous findings with other groups of animals (Morduhai-Boltovskoy, Reference Morduhai-Boltovkoy1960).
In contrast to the Caspian-type species located in the fringe areas of the Black Sea, Mediterranean-type species inhabit the oxygenated, shallow-water areas that are free of hydrogen sulfide pollution (Caraion, Reference Caraion1962; Schornikov, Reference Schornikov1969, Reference Schornikov2012) and represent ~80% of reported ostracods in the modern Black Sea (Schornikov, Reference Schornikov2012). This fauna consists of species that can tolerate marine conditions in the Mediterranean Sea, as well as brackish salinities and lower temperatures of the Black Sea. For many Mediterranean species, the low salinity (half that of the Mediterranean Sea) and low temperatures are the main barriers to migration into the Black Sea, and thus the Black Sea fauna is a depleted representative of modern Mediterranean fauna. The fauna of the Sea of Azov is further depleted, relative to the Black Sea, because of the lower salinity in the shallow sea, being 1.3 times lower than that of the Black Sea (Caraion, Reference Caraion1962; Schornikov, Reference Schornikov1969).
Studies of Black Sea ostracods have referred to endemic marine species (Briceag and Ion, Reference Briceag and Ion2014)—for example, Pontocythere bacescoi (Caraion, 1960). The finding of P. bacescoi valves in the northern part of the Aegean Sea (Schornikov, Reference Schornikov1969) suggests that this species is not endemic to the Black Sea. Indeed, it is unlikely that any Mediterranean-type species are endemic to the Azov–Black Sea basin. The Caspian-type ostracods discussed in this study are likely endemic to the Ponto-Caspian region. Studies of late Quaternary Black Sea and Caspian Sea ostracod assemblages illustrate that they have strong affinities to the Neogene assemblages of the eastern Paratethyan basin (Boomer, Reference Boomer2012). Partial or total isolation from the global ocean and generally intense freshwater input from rivers favored endemism of the ostracod fauna.
Periodic connections between the Black Sea and the Caspian Sea in the Quaternary allowed species migration between the two basins (Chepalyga et al., Reference Chepalyga2007; Badertscher et al., Reference Badertscher, Fleitmann, Cheng, Edwards, Göktürk, Zumbühl, Leuenberger and Tüysüz2011). They were last connected ~15–14 cal ka BP (Yanina, Reference Yanina2014); therefore, any major migration of Caspian-type ostracods occurred prior to the start of the Holocene, although some members of this group were present in the Black Sea prior to this connection (Boomer et al., Reference Boomer, Guichard and Lericolais2010). Therefore, this means that assemblage change in Holocene ostracod records (this study; Hiscott et al., Reference Hiscott, Aksu, Mudie, Marret, Abrajano, Kaminski, Evans, Çakiroğlu and Yaşar2007; Boomer et al., Reference Boomer, Guichard and Lericolais2010; Bradley et al., Reference Bradley, Horne, Williams, Marret, Aksu and Hiscott2011; Williams et al., Reference Williams, Hiscott and Aksu2011; Ivanova et al., Reference Ivanova, Marret, Zenina, Murdmaa, Chepalyga, Bradley, Schornikov, Levchenko and Zyryanova2015) is a reflection of the migration and/or expansion of Mediterranean-type ostracod populations. This process was driven by the input of water from the Marmara Sea, commencing in the early Holocene (discussed subsequently) and highlighted by various authors as the dominant control of environmental changes in the Black Sea (Yanko-Hombach et al., Reference Yanko-Hombach, Gilbert and Dolukhanov2007; Marret et al., Reference Marret, Mudie, Aksu and Hiscott2009; Nicholas et al., Reference Nicholas, Chivas, Murray-Wallace and Fink2011; Soulet, Ménot, Lericolais et al., Reference Soulet, Ménot, Lericolais and Bard2011).
Paleoenvironmental records from the northeastern shelf
The data collected from cores Ak-521 and Ak-2575 provide detailed information about the benthic paleoenvironments at the northeastern Black Sea shelf edge throughout the Holocene. Estimates of water depth in the early Holocene are difficult to quantify using ostracods because of the lack of modern species information. However, the depth was shallower than present but increased until ~4 cal ka BP, which corresponds to sea-level curve estimates for the Black Sea (Balabanov, Reference Balabanov2007; Brückner et al., Reference Brückner, Kelterbaum, Marunchak, Porotov and Vött2010). The ostracod records show that prior to ~7.4 cal ka BP, the shelf was covered in low-salinity water, but salinity values increased until ~5.7–4 cal ka BP. The increases in water depth and salinity are linked to the establishment of the two-way flow through the Bosporus in the early Holocene (Grigor’ev et al., Reference Grigor’ev, Isagulova and Fedorov1984; Yanko and Troitskaya, Reference Yanko and Troitskaya1987; Mudie et al., 2004, Reference Mudie, Marret, Aksu, Hiscott and Gillespie2007; Hiscott et al., Reference Hiscott, Aksu, Mudie, Marret, Abrajano, Kaminski, Evans, Çakiroğlu and Yaşar2007; Yanko-Hombach et al., Reference Yanko-Hombach, Gilbert and Dolukhanov2007).
Following establishment of the two-way flow in the Bosporus, the data support other studies from around the basin (e.g., Mertens et al., Reference Mertens, Bradley, Takano, Mudie, Marret, Aksu and Hiscott2012) that suggest changes in global sea level were the main factor in controlling water level and salinity in the Black Sea. These changes would have been moderated by changes in the precipitation-evaporation budget and discharge from major rivers (Giosan et al., Reference Giosan, Coolen, Kaplan, Constantinescu, Filip, Filipova-Marinova, Kettner and Thom2012). River inflow of the Danube, Dnieper, and Dniester is responsible for ~85% of runoff into the modern Black Sea (Likhodedova and Konikov, Reference Likhodedova and Konikov2007) and therefore disproportionally affects the northwestern Black Sea. However, our records are not sensitive enough to detect these secondary controls on water depth and salinity on the northeastern shelf. The boundaries between the assemblages and subassemblages are indistinct. The data in this study do not provide evidence of any of the rapid changes in environmental conditions that were hypothesized by Ryan et al. (Reference Ryan, Major, Lericolais and Goldstein2003).
Transformation of the Neoeuxinian environment towards the Holocene marine environment (assemblage I)
The Caspian assemblage present between 9.6 and 7.4 ka cal BP suggests that salinity values ranged between 6 and 11 psu, sea level was greater than 50 m, and salt composition was similar to Caspian Sea water (Nevessakaya, Reference Nevesskaya1965; Chepalyga, Reference Chepalyga2007; Ivanova et al., Reference Ivanova, Murdmaa, Chepalyga, Cronin, Pasechnik, Levchenko, Howe, Manushkina and Platonova2007, Reference Ivanova, Murdmaa, Karpuk, Schornikov, Marret, Cronin, Buynevich and Platonova2012, Reference Ivanova, Marret, Zenina, Murdmaa, Chepalyga, Bradley, Schornikov, Levchenko and Zyryanova2015; Yanko-Hombach et al., Reference Yanko-Hombach, Gilbert and Dolukhanov2007, Reference Yanko-Hombach, Mudie, Kadurin and Larchenkov2014). A significant portion of the sediment sequence is represented by a coquina. This coquina is rich in ostracod valves belonging to different ecological groups, possibly because of a variety of biotopes alternating during the very slow and discontinuous coquina deposition in a high-energy bottom-water environment. Reworking of ostracod valves might also contribute to the observed high species diversity in the coquina. In Ak-521, assemblage I contains species of the Caspian type such as Amnicythere sp. 1, Loxocaspia cf. immodulata, Euxinocythere? sp., Amnicythere? sp. 1, and Amnicythere sp. 2, which were previously not recorded in either the Black Sea or the Caspian Sea (Agalarova et al., Reference Agalarova, Kadyrova and Kulieva1961; Caraion, Reference Caraion1962, Reference Caraion1967; Mandelstam et al., Reference Mandelstam, Markova, Rosyeva and Stepanaitys1962; Schornikov, Reference Schornikov1969; Stancheva Reference Stancheva1989a, Reference Stancheva1989b; Boomer et al., Reference Boomer, Grafenstein, von, Guichard and Bieda2005, Reference Boomer, Guichard and Lericolais2010; Leroy et al., Reference Leroy, López-Merino, Tudryn, Chalié and Gasse2014; Yanko-Hombach et al., Reference Yanko-Hombach, Mudie, Kadurin and Larchenkov2014). In the younger sections of this assemblage, there is a gradual disappearance of certain species including T. amnicola donetziensis, Euxinocythere baquana, and E. relicta. This suggests that salinity was increasing throughout the early to mid-Holocene.
The reason for this increase in salinity is linked to the greater input of higher salinity water into the Black Sea from the Mediterranean Sea, via the Marmara Sea (see Hiscott et al., Reference Hiscott, Aksu, Mudie, Marret, Abrajano, Kaminski, Evans, Çakiroğlu and Yaşar2007; Mertens et al., Reference Mertens, Bradley, Takano, Mudie, Marret, Aksu and Hiscott2012) and a rise in sea level (Balabanov, Reference Balabanov2007). This increase was potentially moderated by the higher levels of precipitation discussed by Göktürk et al. (Reference Göktürk, Fleitmann, Badertscher, Cheng, Edwards, Leuenberger, Fankhauser, Tüysüz and Kramers2011). However, detailed information about past climates of the northeastern Black Sea region is lacking. In comparison with western Europe, there are very few studies concerning the Holocene climate conditions for the Black Sea region. The European paleoclimate reconstructions from Mauri et al. (Reference Mauri, Davies, Collins and Kaplan2015) provide some insights on winter and summer conditions for different time slices from the onset of the Holocene, but they are based on a low number of palynological records around the Black Sea. Benthic salinity of less than 2 psu, suggested by Soulet, Ménot, Garreta et al. (Reference Soulet, Ménot, Garreta, Rostek, Zaragosi, Lericolais and Bard2011) is unlikely because the freshwater/oligohaline component is poorly represented, with only two species of ostracods present.
Although Caspian fauna dominate in the early Holocene, the first Mediterranean ostracods appeared on the northeastern Black Sea shelf at least by ~9.6 cal ka BP (Figs. 3 and 4). They were represented by P. agilis, H. rubra, and L. multipunctata, although the total number of valves is low. In shallower areas, the species composition of the first Mediterranean ostracods is more diverse because of the greater variety of habitats. C. diffusa and Callistocythere mediterranea (Müller, 1894) are recorded in sediment; C. variabilis, C. marinovi, and Cytherois spp. are found together with species of the Caspian type on the northeastern shelf (Schornikov collections). This suggests that saline water input from the Mediterranean reached the northeastern shelf relatively quickly after the initial establishment of the two-way flow. This suggestion is supported by the persistent occurrence of euryhaline dinoflagellate cysts and rare specimens of foraminifer Ammonia tepida from 9.6 cal ka BP in Ak-2575 (Ivanova et al., Reference Ivanova, Marret, Zenina, Murdmaa, Chepalyga, Bradley, Schornikov, Levchenko and Zyryanova2015), as well as by the foraminiferal data from the northwestern and southeastern shelves (Yanko-Hombach et al., Reference Yanko-Hombach, Mudie, Kadurin and Larchenkov2014).
Coexistence of Mediterranean and Caspian fauna (assemblage II)
At about 7.4 cal ka BP, salinity reaches a critical limit of ~11–12 psu, and the salt composition of water changes from the Caspian type to the normal oceanic type (Nevesskaya, Reference Nevesskaya1965; Chepalyga, Reference Chepalyga2002; Yanko-Hombach et al., Reference Yanko-Hombach, Gilbert and Dolukhanov2007). These conditions are unsuitable for the majority of the Caspian-type fauna. However, they are also not optimal for Mediterranean ostracods, because the salinity is at the lower limit of their tolerance (Schornikov, Reference Schornikov1969). This explains the low ostracod abundance just after the transition between assemblage I and assemblage II. Soft shelly mud started to accumulate over the coquina at the northeastern shelf edge owing to a decrease in bottom-water dynamics and an increase in fine-grained terrigenous material supply. These environmental changes coincided with the beginning of the large-scale migration of the Mediterranean ostracod fauna to the Black Sea shelf that characterized assemblage IIA development.
The data presented in this study assume ecological conditions that allowed the coexistence of Mediterranean and Caspian species (Ivanova et al., Reference Ivanova, Murdmaa, Chepalyga, Cronin, Pasechnik, Levchenko, Howe, Manushkina and Platonova2007, Reference Ivanova, Murdmaa, Karpuk, Schornikov, Marret, Cronin, Buynevich and Platonova2012, Reference Ivanova, Marret, Zenina, Murdmaa, Chepalyga, Bradley, Schornikov, Levchenko and Zyryanova2015). According to other studies, after the onset of two-way circulation in the Bosporus Strait, ostracod valves of both fauna types occur simultaneously over an extended period of time (Stancheva, Reference Stancheva1989a; Ivanova et al., Reference Ivanova, Murdmaa, Chepalyga, Cronin, Pasechnik, Levchenko, Howe, Manushkina and Platonova2007, Reference Ivanova, Marret, Zenina, Murdmaa, Chepalyga, Bradley, Schornikov, Levchenko and Zyryanova2015; Yanko-Hombach et al., Reference Yanko-Hombach, Mudie, Kadurin and Larchenkov2014). Living specimens of the Caspian and Mediterranean types are not reported together in the same samples from the Black Sea and the Sea of Azov. Along with the fauna of the Mediterranean type, only C. torosa was found in the Sea of Azov. This species is able to coexist with representatives of the Caspian type.
However, in different areas of the Black Sea, the appearance of Mediterranean species and thus the lower boundary of the mixed assemblage seem to be slightly diachronous. According to the data from the Bulgarian shelf, this level corresponds to the Bugazian/Vityazevian boundary (Stancheva, Reference Stancheva1989a). Hiscott et al. (Reference Hiscott, Aksu, Mudie, Marret, Abrajano, Kaminski, Evans, Çakiroğlu and Yaşar2007) discuss the transition from the Caspian to the Mediterranean type on the southwestern shelf from 7.3 to 6.0 cal ka BP. Using calibration procedures in Ivanova et al. (Reference Ivanova, Marret, Zenina, Murdmaa, Chepalyga, Bradley, Schornikov, Levchenko and Zyryanova2015), this corresponds to ~7.9 to 6.2 cal ka BP. Note that the datum level of the large-scale migration of Mediterranean species cannot be ascertained using the quantitative data on their occurrence (Yanko-Hombach et al., Reference Yanko-Hombach, Mudie, Kadurin and Larchenkov2014). The less abundant occurrence of Mediterranean ostracods during the large-scale migration event can possibly be explained by their slower migration rates into the Black Sea compared with mollusks, as the majority of Podocopa are benthic animals without any pelagic ontogenetic stages.
The Caspian-type ostracod fauna prevails over the Mediterranean fauna throughout the assemblage IIA interval. This occurs at a very low level of total ostracod abundance (Fig. 4), likely because of dilution by rapidly accumulating shelly mud of the Mytilus mud facies (Ivanova et al., Reference Ivanova, Marret, Zenina, Murdmaa, Chepalyga, Bradley, Schornikov, Levchenko and Zyryanova2015). Moderate OAR values support this assumption for core Ak-2575. However, a persistent presence of authigenic gypsum crystals suggests that episodic upwelling of the anoxic deep water onto the shelf edge might suppress populations of the ostracod fauna. In Ak-521, gypsum crystals are rare, and the ostracod fauna is more abundant.
The gradual transition from assemblage IIA to IIB at ~7.1–7 cal ka BP is expressed by a disappearance of most Caspian-type ostracod species except for L. lepida. Relatively high diversity and abundance of species of the Mediterranean type in the IIB interval (from Ak-2575) indicate that an increase in salinity should exceed the limits of tolerance for species of the Caspian type. In this context, the late disappearance of only one ostracod species, L. lepida, and its late occurrence in significant amounts (10 valves per sample) needs further investigation, as a salinity of ~18 psu was reported at that period (Mertens et al., Reference Mertens, Bradley, Takano, Mudie, Marret, Aksu and Hiscott2012). It is unlikely that L. lepida could in situ coexist with a relatively diverse Mediterranean fauna for such a long time because of the significantly different tolerances to salinity conditions (absolute value and salt composition balance). Thus, we assume that the specimens of L. lepida found in younger samples are reworked from shallow-water areas where it could inhabit low-salinity conditions. In the early Holocene and late Pleistocene, this species was one of the most abundant (Stancheva, Reference Stancheva1989a, Reference Stancheva1989b; Boomer et al., Reference Boomer, Guichard and Lericolais2010). Moreover, reworked valves of the Caspian-type ostracods occur in grab samples together with recent ostracods mainly near the southern extremity of the Crimean Peninsula at depths of 60–100 m, in the near-Bosporus area at a depth of 105–415 m (Schornikov, Reference Schornikov2011), and in our grab and drag samples from the northeastern shelf edge at depths of 80–100 m.
Assemblage IIB differs from IIA because of the higher diversity of Mediterranean-type ostracod fauna. It contains more stenohaline species along with euryhaline species, which previously dominated. Thus, it demonstrates a migration event via the Bosporus Strait related to increases in salinity. Significant changes in bottom-water temperature are documented by variations in the species composition of assemblage II. S. rennata occurred in significant numbers at the point when Mediterranean-type species increased. At present, this species inhabits depths of 15 to 30 m (Schornikov, Reference Schornikov1969) where the bottom-water temperature is significantly higher over the year than on the outer shelf (Fig. 1). Abundant L. multipunctata occurs within the same time interval in both cores, although at present it is usually more abundant in shallower depths.
Hence, assemblage II suggests that the bottom-water temperature was considerably warmer than at present on the outer shelf during the colonization by Mediterranean fauna (7.4–6.7 cal ka BP). However, it is not clear when the warm climatic conditions responsible for the bottom-water warming commenced. The warm period during the mid-Holocene (7.4–6.7 cal ka BP) as highlighted by our ostracod records is identified for winter conditions (1°C to 2°C above modern preindustrial conditions) from reconstructed climate conditions by Mauri et al. (Reference Mauri, Davies, Collins and Kaplan2015). In addition, warm and humid climates were estimated from the pollen record from Yenicağa Lake in northern Anatolia (Bottema et al., Reference Bottema, Woldring and Aytuğ1995), Lake Van in central Anatolia (Wick et al., Reference Wick, Lemcke and Sturm2003; Litt et al., Reference Litt, Krastel, Sturm, Kipfer, Örcen, Heumann, Franz, Ülgen and Niessen2009), northern Anatolia (Shumilovskikh et al., Reference Shumilovskikh, Tarasov, Arz, Fleitmann, Marret, Nowaczyk, Plessen, Schlütz and Behling2012), and Georgia (Kvavadze and Connor, Reference Kvavadze and Connor2005), as well as from the Bulgarian Black Sea shelf (Filipova-Marinova, Reference Filipova-Marinova2006). According to Shumilovskikh et al. (Reference Shumilovskikh, Tarasov, Arz, Fleitmann, Marret, Nowaczyk, Plessen, Schlütz and Behling2012), a warm and humid phase occurred in northern Anatolia at ~8.5–5 cal ka BP.
Onset of the recent bottom environment on the northeastern Black Sea shelf (assemblage III)
Assemblage III consists of only Mediterranean-type species. The composition of Mediterranean-type fauna in both transitional (II) and marine (III) assemblages is very impoverished because both studied cores were collected from a relatively deep area (99–101 m) covered with rather uniform soft shelly silty mud. On shallower areas of the eastern shelf, upper Holocene ostracod assemblages are much more diverse (Schornikov et al., Reference Schornikov, Zenina and Ivanova2014).
Salinity almost reaches present-day values during the gradual end of the assemblage II phase. Further development of the ostracod fauna mainly depended on changes in the bottom-water temperature and sedimentation processes.
Disappearance of relatively warm-water S. rennata and the decrease in abundance of L. multipunctata at ~6.8 cal ka BP indicate a gradual reduction in bottom-water temperature. Other explanations seem unlikely, as there is no evidence of large salinity changes and the biotope (soft silty-pelitic mud) is similar both below and above this level (Fig. 5A). Therefore, the above-mentioned changes in composition of the ostracod fauna likely reflect a climate (and bottom water) cooling trend. However, the accumulation rate of ostracod valves sharply accelerated at the transition from assemblage IIIA (Fig. 4). This suggests high productivity of the ostracod fauna, although concentration of valves in sediments is rather low owing to their dilution by rapidly accumulating terrigenous mud.
Figure 5 Lithology, stratigraphy, benthic ostracod diversity, and assemblages from Ak-2575 (1) and Ak-521 (2). (A) Lithology (see Fig. 3 for sediment type). (B) Calibrated ages of accelerator mass spectrometry (AMS) 14C–dated levels (see Fig. 3 for 2σ range, cal yr BP). (C) Grain size. (D) Ostracod taxa per sample and ecological affinities overview. (E) Ostracod accumulation rates (OARs) of valves (valves/ka yr). (F) Ostracod abundance. (G) Ostracod assemblages. (H) Stratigraphy. (I) Lithology (see Fig. 2 for sediment type). (J) Calibrated ages of AMS 14C–dated levels (see Fig. 4 for 2σ range, cal yr BP). (K) Ostracod taxa per sample and ecological affinities overview. (L) Ostracod assemblages. (M) Stratigraphy. (A–D, F, H–J, M) from Ivanova et al. (Reference Ivanova, Marret, Zenina, Murdmaa, Chepalyga, Bradley, Schornikov, Levchenko and Zyryanova2015). Ne, Neoeuxinian (11.8–10 cal ka BP); Bg, Bugazian (10–8.8 cal ka BP); Vt, Vityazevian (8.8–7.8 cal ka BP); Kl, Kalamitian (7.8–6.9 cal ka BP); and Dz, Dzhemetinian (6.9–2.6 cal ka BP).
Figure 6 Holocene ostracod assemblages and major events affecting the faunal changes.
Sedimentation rates slowed down after the very rapid terrigenous mud accumulation related to the Kalamitian high sea-level stand (Ivanova et al., Reference Ivanova, Murdmaa, Chepalyga, Cronin, Pasechnik, Levchenko, Howe, Manushkina and Platonova2007, Reference Ivanova, Marret, Zenina, Murdmaa, Chepalyga, Bradley, Schornikov, Levchenko and Zyryanova2015). The hiatus (>1000 years) that likely corresponds to erosion during the Kundukian regressive phase (Chepalyga, Reference Chepalyga2002) separates assemblages IIIA and IIIB. This correlates with Mytilus mud and M. phaseolinus mud facies, respectively. Ostracod abundance and OAR increased during the transition from IIB to IIIA, reflecting favorable conditions for ostracods, and fell to almost zero values after ~6.5 cal ka BP, according to the data from core Ak-2575 (Fig. 5E). The short period of suppressed ostracod fauna before the hiatus might be related to an anoxic water upwelling event.
The species composition of assemblage III suggests the cooling trend continued up to the time of the modern assemblage onset at ~5.7–4 cal ka BP, although water conditions were still somewhat warmer than present. Increasing percentages of P. agilis during the time interval from 6.7 to 4 cal ka BP and appearance of Bythocythere sp. at ~4 cal ka BP confirms this suggestion. Bythocythere sp. is found in modern assemblages of the Black Sea only at depths of greater than 70 m (Schornikov, Reference Schornikov1969). Therefore, its presence serves as evidence of bottom-water temperature reduction to present values. The changes in species composition were simultaneous with the gradual transition from Mytilus mud to M. phaseolinus mud facies (Ivanova et al., Reference Ivanova, Marret, Zenina, Murdmaa, Chepalyga, Bradley, Schornikov, Levchenko and Zyryanova2015). P. agilis is the most abundant ostracod species on the modern M. phaseolinus mud (Schornikov, Reference Schornikov1969). Changes in assemblage III may be caused either by facies replacement that was accompanied by the slowing down of mud sedimentation rates (and thus, also OAR; Fig. 4) or by some bottom-water cooling. Formation of modern assemblages at ~5.7–4 cal ka BP indicates the onset of environmental conditions close to present.
Conclusions
Ostracod records of cores Ak-521 and Ak-2575 are generally synchronous and represent the response of ostracod assemblages at the northeastern Black Sea shelf edge to the major Holocene paleoenvironmental events. During the period ~11.8 to 7.4 cal ka BP, the ostracod fauna of the Caspian type was represented by a relatively diverse range of species. However, the first Mediterranean migrants had already appeared at ~9.6 cal ka BP. A significant portion of the sediment sequence in this period is represented by a coquina that is rich in ostracod valves belonging to different ecological groups. This is potentially explained by a variety of biotopes alternating during the very slow coquina deposition interrupted by hiatuses, in a high-energy bottom-water environment that resulted in washing away of mud, reworking of mollusk shells in the coquina, as well as ostracod valves. Many species in the Caspian assemblage are the same as those in the modern Caspian Sea.
The composition of Mediterranean-type fauna in the Mytilus mud facies overlying the coquina is very impoverished compared with that from shallower areas, likely because of a uniform biotope represented by shelly silty mud. A transitional assemblage occurs between 7.4 and 6.8 cal ka BP that contains a mixture of both Caspian- and Mediterranean-type species. This occurs at a very low level of total ostracod abundance, likely because of high accumulation rates of the mud. The Caspian-type ostracod fauna dominates over the Mediterranean type until ~7.1 cal ka BP. After this period, Mediterranean fauna become more abundant. Remains of the Caspian-type fauna found in sediments younger than ~7.1 cal ka BP were likely reworked from shallower freshened areas. They were unlikely to reside together with a diverse range of Mediterranean species, which occupy bottom conditions with a salinity much higher than the tolerance level for the Caspian species. After 6.8 cal ka BP, ostracod fauna represented only by Mediterranean-type species became more abundant.
Changes in the ostracod fauna of the northeastern Black Sea shelf edge during the Holocene reflect the increase in bottom-water salinity after the opening of the two-way circulation through the Bosporus Strait and a bottom-water cooling trend starting from the mid-Holocene optimum (at ~6.8 cal ka BP) to recent conditions. The bottom-water temperature was considerably warmer than at present on the outer shelf during the active colonization by Mediterranean fauna (~7.4 to 6.8 cal ka BP). After 6.8 cal ka BP, warmer-water assemblages were gradually replaced by colder-water ones. The cooling trend continued up to the onset of the modern assemblage at ~5.7–4 cal ka BP when water conditions were still somewhat warmer than present. The modern ostracod assemblage is typical of the M. phaseolina mud facies on the outer shelf.
Acknowledgments
Our paper is a contribution to CLIMSEAS, the European project Marie Curie, CLIMSEAS-PIRSES-GA-2009-247512: “Climate Change and Inland Seas: Phenomena, Feedback and Uncertainties. The Physical Science Basis.” We appreciate the discussions with Andrey Chepalyga. We are thankful to Andrey Zatsepin, the chief of the complex Black Sea expedition by P.P. Shirshov Institute of Oceanology, Russian Academy of Science (RAS), for providing the conductivity-temperature-depth data, and to the participants of several cruises and field trips in the Black Sea and Caspian Sea for the assistance with ostracod sampling. This study was partly funded by the Russian Science Foundation grant 14-50-00095 (EVI and IOM), project 0149-2014-0029 (MAZ) by P.P. Shirshov Institute of Oceanology RAS, and the Leverhulme Trust (FM and LRB; project “The Black Sea environmental conditions during the Meso- and Neolithic period”). This work was supported by the NERC Radiocarbon Facility NRCF010001 (allocation number 1729.1013; FM).
Supplementary Material
To view supplementary material for this article, please visit https://doi.org/10.1017/qua.2016.2
