INTRODUCTION

Regressive–transgressive cycles became the most important stages in the formation of recent conditions of the Sea of Azov, according to classical views [1, 2]. The specific regional fluctuations of the sea level in the Sea of Azov–Black Sea basin, the eustatic variations, and the analysis of drilling cores and bottom deposits’ columns allow up to 7–10 alternating transgressions and regressions to be identified in the Holocene [1–3]. In particular, during the Phanagorian regression (3100–2200 years BP), the sea level was 6–7 m lower than at present. The formation of the Don delta and the Azov spits occurred during the maximum of the Ancient Azov (4000–6000 years BP) and Nymphean (2400–1500 years BP) transgressions.

The structure of the Taganrog Bay coastal spits and the bottom sediments of the Sea of Azov provide important information on the paleogeography of the region and the nature of sea level changes and the hydrological regime. The main reason for the proliferation of the Azov accumulative spits is the continuous inflow of organogenic shell material from the Azov Sea into the coastal zone. The main source of sediment for the formation of the above-water part of the Dolgaya Spit apparently occurred in the area of the Zhelezinskaya, Akhtarskaya, and Eleninskaya banks [1, 2].

Information on the composition and habitat of recent benthos species is presented in a number of works [4–8]. There are also a few summaries covering the Holocene history of the malacofauna of this sea [9]. Given the frequent occurrence and highly informative value of shell material, this work is based on a detailed malacofaunistic analysis of samples obtained during drilling of coastal accumulative landforms and sampling of offshore bottom sediment columns, combined with the series of radiocarbon dates obtained.

MATERIALS AND METHODS

The core boring and cable-churn drilling, performed from 2017 to 2022, on Beglitskaya, Pavlo-Ochakovskaya, Chumburskaya, and Dolgaya spits, as well as in the delta of the Kagalnik and Don rivers, provided 30 cores with a length from 10 to 26.5 m. About 140 samples were subjected to malacofaunal analysis. Bottom sediments sampling was carried out from the board of the R/V Deneb with a straight-flow coring tube with weights and a petal valve. The malacofauna analysis of three columns with thickness of 260, 298, and 300 cm (Fig. 1) collected in the Sea of Azov in 2019 were used in this work.

Fig. 1.
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Schematic map of sampling sites in the Sea of Azov and on Dolgaya Spit.

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The main object of the lithological and malacofaunistic analysis was the largest spit in the Sea of Azov, Dolgaya Spit (Fig. 1). Samples were taken from the cores containing shell material, at intervals of 5–10 cm. Shell material was also obtained from eight outcrops with a thickness from 95 to 250 cm, located in the root, central, and distal parts of the spit. Of these, the material was collected from both layers containing complete shells and interlayers composed of shell detritus, with spacing not exceeding 50 cm (usually 20–25 cm). Taxonomic determination of the malacofauna was performed using a some of sources [5, 6].

To determine the absolute age, samples 15–40 g in size of predominantly closed shells of Cerastoderma were collected. Fifty-five radiocarbon dates were obtained at the Laboratory of Geomorphological and Paleogeographical Research, St. Petersburg State University. The age values are given on the basis of the calibration program “OxCal 4.4.1.”

MALACOFAUNA SURVEY

The accumulative body of the Dolgaya Spit was formed in the time interval from 1920 ± 110 (LU-9756) to 2500 ± 150 years BP (LU-9757), during the Nymphea transgression. In general, this transgression was associated with the accumulation of deposits of New Azovian age (from 3100 years BP), which are ubiquitous in the Sea of Azov [0, 1, 10].

Carbonate sediments of the Dolgaya Spit are mainly shells of euryhaline species Cerastoderma glaucum, Abra segmentum, and Hydrobia sp., capable of withstanding desalination of up to 5‰ (Table 1). Black Sea species such as Gastrana fragilis, Polititapes aureus, Chamelea gallina, and Lucinella divaricata, being indicators of Ancient Azovian fauna, occur sporadically and are confined to deeply buried layers. The brackish-water mollusk Theodoxus major, living under the optimum salinity of 5–7‰, was observed in large numbers in the estuary-type sediments; isolated specimens were found in the shell cover of the spit.

Table 1. Mollusk shells found in the sediments of Dolgaya Spit
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The key role in the formation of carbonate sediments in the Sea of Azov is played by C. glaucum, which is characterized by high productivity and strong shells, as well as by A. segmentum and Mytilaster lineatus. The mollusks Anadara kagoshimensis and Mya arenaria, introduced in the last century, also make a significant contribution [11]. The total annual production of zoobenthos in the Sea of Azov is estimated to be 19–20 million tons [4], of which C. glaucum production is 13–14 million tons per year, reaching a biomass of 2 kg/m2 [11].

Dolgaya Spit has been developing for more than two thousand years, acquiring the shape of reclined beach ridge of detritus. The formation of the spit has continued due to the ejection of fresh shells and live mollusks during strong winds with a speed above 20 m/s and storm surges of the sea. Accumulations of carbonate material are similar in composition and consist of deposited and redeposited material. This sequence is characterized by layers containing complete shells alternating with those containing shell detritus and sand. A shell stratum up to 2.5 m-thick in the central part of the spit is represented by a complex of species of equal preservation of C. glaucum, A. segmentum, Barnea candida, and Hydrobia sp. with overwhelming domination of shells and detritus of the genus Cerastoderma, with a significant contribution of Abra shells. In addition, there were some Hydrobia sp., Lentidium mediterraneum, and Th. major. Toward the bottom of the section studied, the number of Abra shells increases and their preservation improves; fine elongated cerastoderm specimens appear. The deposits are denser in the distal and root parts of the spit; compacted interlayers of detritus and sand are thicker than layers containing whole shells. The deposits of the cover are looser in the central part. The shells are mainly represented by species presently inhabiting the water area near Dolgaya Spit. We found rare redeposited shells of the Black Sea mollusks Ch. gallina, G. fragilis, and Solen vagina.

Drilling revealed more ancient horizons containing shell material of the New Azovian (recent Azov) and Ancient Azovian (depauperated version of the Black Sea one) malacofaunae. Holocene mollusk shells were observed up to a depth of 7.8–8.0 m. In particular, the modern Hydrobia–Cerastoderma–Abra–Lentidium–Parthenina malacocenosis inhabiting the Dolgaya Spit area was noted in horizons deeper than 420 cm (Borehole 3) and 320 cm (Borehole 6). Shells from these horizons are dated from 910 ± 100 to 2240 ± 130 yr. BP. A transitional version of the fauna, 2980 ± 100 yr. BP, was recorded at a depth of 550–570 cm (Borehole 5). Shells of gastropods Bittium reticulatum, Rissoa sp., and Hydrobia sp. contained in the horizon may indicate the presence of aquatic vegetation, while A. segmentum is indicative of shallow water and a possible oxygen deficit in lagoon or bay conditions. Fossil plants and numerous shells of the eurybionts Hydrobia, Cerastoderma, and Abra were observed in the 430–440 cm horizon (Borehole 9; 2890 ± 90 yr. BP), confirming the presence of a bay or estuary with aquatic vegetation and salinity close to the recent one.

The Ancient Azovian malacofaunal complex in the distal part of the spit, observed in the layer 750–780 cm (Borehole 1; 7860 ± 100 l), contains, in addition to the Sea of Azov fauna, whole shells of juvenile Ch. gallina and Tritia reticulata. The layers opened at 615–780 cm in Borehole 3 and at 650–670 cm in Borehole 4, being similar in taxonomic composition, type of bottom sediments, and content of coarse debris and detritus of shells of the family Mytilidae, contain well-preserved shells of the Ancient Azovian species L. divaricata, G. fragilis, and Parvicardium exiguum, and valves of P. aureus. The soil here is characterized by a high content of small fragments of Mytilidae shells, indicating the high intensity of the processes that took place.

Borehole 13 uncovered layers containing malacofauna with other ecological characteristics in the root part of the spit on the eastern bank. Lagoon-type taxocenosis was observed in the dark gray clayey silt of the 158–165 cm horizon (6560 ± 90 yr. BP), with Hydrobia being the leading species, numerous Abra and Rissoa, and scarce Cerastoderma and Mytilaster. A significant component is brackish-water Th. major, which indicates that the salinity of the water body did not exceed 10–12‰. Probably, there was a firth, weakly connected with the sea or completely isolated from it in the current Lebyazhie Lake basin near Dolgaya Spit. Overlying the layers of this core (100–110 cm), Th. major is replaced by the gastropod Setia valvatoides, which now inhabits aquatic vegetation of the estuaries of the Taman Gulf, indicating a more intense communication of the estuary with the sea and the respective increase of its salinity up to marine one.

Previously published data [10] and analysis of the bottom sediment columns allowed us to identify sources of shell material on Dolgaya Spit. Shell deposits in the area of the Zhelezinskaya sand bank were indicated, in addition to the coastal malacocenoses, as one of them. The surface layers of bottom sediments (to a depth of 90 cm) at the northwestern end of the bank contain a large number of shells of the Cerastoderma genus [10]. Up to the depth of 50 cm, there is a shell layer, 2400 ± 180 yr. old, which contains over 40% of carbonate material (mainly Bivalvia shells). The presence of shells of the Black Sea species G. fragilis in these layers may indicate continuous processes of deposition, removal, and redeposition of carbonate material.

A bottom deposits column was sampled on the Elena Bank in the vicinity of Dolgaya Spit (sampling site 7). The layers of uncovered bottom sediment ranged in age from 550 ± 80 yr at the surface to 2930 ± 130 yr. at the base of the column and showed that there was a mollusk taxocenosis typical of the silty sands of the Sea of Azov, with the dominating species C. glaucum and Hydrobia acuta and scarce A. segmentum, Parthenina interstincta, and Cylichnina variabilis. B. reticulatum, withstanding the lowering of salinity to 10‰, which was massively represented in the sediments of Dolgaya Spit in the layers older than 4000 yr., becomes an important part of the assemblage in the layers underlying the 60–70 cm horizon (1960 ± 250 yr. BP) [11]. The halophilous marine species Retusa truncatula and Ebala pointeli and coastal vegetation inhabitant Rissoa sp. were also found here.

A more diverse mollusk taxocenosis was found in the bottom deposits columns from the southeastern part of the Sea of Azov (sampling sites 41 and 59). The prevailing species in the surface layers up to a depth of 120–130 cm (1260 ± 70 yr. BP) are Cerastoderma, Abra, Hydrobia, and Lentidium. Bittium and Parthenina are codominants on site 59, and the gastropods Ebala, Rissoa, Cylichnina, and Retusa are numerous on both stations. In the underlying layers, starting from a depth of 130 cm, there are significant differences in the composition of the malacofauna. There is a sharp decrease in the number of L. mediterraneum shells at both sites and of Abra shells at site 59. A significant increase in the proportion of the gastropods Bittium and Rissoa is characteristic of site 41 in the layers deeper than 200 cm (over 2190 ± 110 yr. BP), which may indicate the existence of shallow water with aquatic vegetation. At site 59 at a depth of 185–200 cm (3140 ± 180 yr. BP), predominant species include Cerastoderma, Hydrobia, and Bittium, and starting from the depth of 153–165 cm (2280 ± 150 yr. BP), isolated shells of Ch. gallina and P. aureus, markers of the ancient Black Sea fauna, were recorded.

DISCUSSION

An analysis of the mollusk taxocenoses extracted from borehole cores and outcrops on Dolgaya Spit revealed several patterns. The sedimentary cover on the spit is composed of shells of different ages and different faunas. Both modern mollusks of the New Azovian fauna and the Black Sea species that inhabited this area during the Ancient Azovian transgression were found here. The surface layers contain a mixture of shells from both faunas, which may be related to wave activity and, as a consequence, to the redeposition of shell material; up to 90% of the carbonate material here is represented by Cerastoderma shells.

The accumulated bed of carbonate material has a heterogeneous layered structure, characterized by alternation of loose layers with well-preserved shells and interlayers of dense detritus with muddy and sandy cement. The study of outcrops in the distal and central parts of the spit showed that there are differences in the composition and preservation of material. The presence of complete shells of Cerastoderma and Chamelea and near-hinge fragments of more fragile Barnea and Abra in the distal part of the spit may indicate intense wave activity during its formation. The good preservation of most species in the central part may indicate less vigorous water dynamics and thus less intense erosion and redeposition of material.

The deep layers of cores reveal bottom sediments of water bodies that existed before the formation of the spit. The separate deposition of faunas observed here allows changes in habitat conditions and, as a consequence, in the composition of malacocenoses to be identified.

CONCLUSIONS

Sampling of bottom sediment columns up to 4 m in length and drilling of sediment strata with sampling of 25-m cores on Azov spits (in particular, Dolgaya Spit), as well as malacofaunistic studies in combination with absolute dating of shells have permitted a new characterization of the zoobenthos of different Holocene stages and reconstruction of the sea level fluctuation history in the Sea of Azov basin for the last 5000 years. The current level of the seas and oceans was established during the climatic optimum of 6000‒ 4000 years BP [12]. Mass bottom fauna freezing during the Nymphea transgression in the time interval from 1920 ± 110 (LU-1756) to 2500 ± 150 (LU-9757) years resulted in the accumulation of a huge mass of biogenic deposits at the junction of Taganrog Bay of the Sea of Azov.

The upper part of the Dolgaya Spit deposits uncovered by boreholes is composed of shell material with an admixture of clay and loam [2]. Shells of the bivalve mollusk C. glaucum predominate in different horizons of the shell deposits. The proportion of shells of this species in the surface horizons varies from 96 to 57%. The shares of A. segmentum (up to 38%), B. candida (up to 24%), and H. acuta (up to 13%) increase (Fig. 2) toward the base of the sections. The thickness of shell deposits increases from 4 m at the root of the spit to 7 m in its distal part. Upper Holocene shell layers are often underlain by horizons with features of the subaerial conditions of formation. The lagoon deposits often have a black hue with a typical smell of hydrogen sulfide. Apparently, they were formed during the Phanagorian regression 3100–2500 years BP. In general, the malacofauna collected from the shelf is almost identical to that from the spit. The difference in the number of taxa can be explained by the differences in the conditions in the sampling sites of columns (deep water areas) and cores (coast, shallow water) (Table 1).

Fig. 2.
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Proportion of shells of the dominating species in different horizons of Dolgaya Spit.

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Major sources of shell material for the formation of the Azov type coastal spits obviously existed in the southeastern part of the Azov shelf in the area of Zhelezinskaya, Akhtarskaya, and Elena banks, in the western coastal part in the area of Arabat spit, as well as in the bays of the Sea of Azov [1, 2, 11]. It can be assumed that a most mollusk shells constituting the main body of the spits were formed under conditions favorable for the development of benthic organisms during the Nymphaean transgression.

The intra-century climate cyclicity caused noticeable variations in climate in the region of the Sea of Azov. For example, droughts and low water cycles were repeated during the last millennium and in the twentieth and twenty-first centuries with an interval of about 30–40 years [13]. The transfer and deposition of shell material in the area of the spits was carried out by storm waves, which formed a series of beach ridges (e.g., on the Dolgaya spit there are up to fifty of them), clearly distinguished on the surface of coastal accumulative formations.