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СПИСАНИЕ НА БЪЛГАРСКОТО ГЕОЛОГИЧЕСКО ДРУЖЕСТВО, год. 81, кн. 3, 2020, с. 135–137REVIEW OF THE BULGARIAN GEOLOGICAL SOCIETY, vol. 81, part 3, 2020, p. 135–137

Национална конференция с международно участие „ГЕОНАУКИ 2020“National Conference with international participation “GEOSCIENCES 2020”

Climate changes during the latest Bessarabian–еarliest Khersonian (Sarmatian) according to integrated sedimentological and biostratigraphic data, Northeastern BulgariaКлиматични промени през късния Бесараб–ранния Херсон (Сармат) по интегрирани седиментоложки и биостратиграфски данни, Североизточна България Elena Koleva-Rekalova, Marlena Yaneva, Petar Nikolov, Nadja Ognjanova-Rumenova, Nikolai NikolovЕлена Колева-Рекалова, Марлена Янева, Петър Николов, Надя Огнянова-Руменова, Николай Николов

Geological Institute, Bulgarian Academy of Sciences, Acad. G. Bonchev str., bl. 24, 1113 Sofia; E-mail: e_koleva@geology.bas.bg

Keywords: aragonite sediments, clay minerals, mollusks, diatoms, Northeastern Bulgaria.

IntroductionIt is well-known that climate changes on land play a significant role in the type of sedimentation in the basins, as well as in the appearance, flourishing and extinction of marine fossil groups. This short com-munication presents a little known in the special-ized literature period of climate aridization, most pronounced during the latest Bessarabian across the territory of Northeastern Bulgaria. To characterize the main manifestations of this severe climate, both literature data and new studies on the sediments of the Topola Formation and the different fossil groups found in them have been used.

Geological settingThe Miocene sedimentary rocks comprise a signifi-cant part of the Black Sea Region in Northestern Bulgaria. According to the structural and palaeoge-ographical regions distinguished by Kojumdgieva and Popov (1981a), they fill the shallow Varna-Balchik Bay of the Euxinian-Caspian Basin. The object of the present study are the sediments of the Topola Formation. They crop out as a narrow band along the Black Sea Coast, starting from the village of Kranevo, through the town of Balchik and ending west of the Cape Kaliakra. Inland, these sediments were found in some drill-cores during the explora-tion of the Dobrudja Coal Field. The chronostrati-graphic range of the Topola Formation coincides

with the upper part of the Bessarabian and lower part of the Khersonian Substages of the Sarmatian Stage (Popov, Kojumdjieva, 1987). The thickness of the sediments composing the formation varies from 40 to 90 m. The formation is represented mainly by aragonite sediments (Koleva-Rekalova, 1994). The Bessarabian aragonite sediments are massive and rarely laminated. They are irregularly interbedded with clayey layers, micritic limestones and sparse dolomites. The Khersonian aragonite sediments are mostly laminated. The content of aragonite in the aragonite sediments varies from 85 to 95% (Kole-va-Rekalova, 1994).

Material and methodsThe section Zelenka, located between the village of Balgarevo and Cape Kaliakra, was investigated in detail (Fig. 1a–c). The description of the section, aragonite sediments, mollusks and diatomaceous flora were presented in previous publications (Ya-neva et al., 2019; Ognjanova-Rumenova, 2019). New data about the clay mineralogy of the bed 7 (Fig. 1a–b) are presented here. Clay minerals have been widely used to study paleoclimatic and region-al paleoenvironmental reconstructions (Chamly, 1987; Dianto et al., 2018). The clay mineral com-position in the sediments reflects climatic changes, particularly in tropical environments with efficient chemical weathering, whereas in cold temperate environments, clay minerals are useful as source

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indicator (Dianto et al., 2018). Two samples col-lected from a light gray and dark gray layers from bed 7 were decarbonatized with 2% HCl and then analyzed by Differential Thermal Analysis (DTA).

DiscussionAfter the rise of the Carpathians during 11.6–11.3 Ma (Palcu et al., 2019, and references therein) the Sar-matian Sea was divided into two parts. The Pannon-ian Basin was formed in the western part, and the Euxinian-Caspian Basin continued in the eastern part. In the eastern part, climate aridization on the land started. As a result, inflows of fresh waters (± terrigenous components) into the basin decreased significantly, or were sporadic. This led to an in-crease in the salinity and alkalinity of seawater. Un-der these conditions, the formation of the aragonite sediments began (Koleva-Rekalova, 1994).

The changed composition of the seawater (in-creased salinity and particularly high alkalinity) had a negative impact on the fauna. For example, during the middle Bessarabian (zone Flintina tutkowskii) there was a great species diversity and an increased amount of foraminifera (Darakchieva, 1989; Kole-va-Rekalova, Darakchieva, 2020) (Fig. 1d). But dur-ing the latest Bessarabian (zone Protelphidium sub-granosum) the species Protelphidium subgranosum

existed mainly, and the representatives of the genera Meandroloculina, Dogielina and Nodobaculariella disappeared (Darakchieva, 1989). A change is also observed in mollusks. During the latest Bessarabi-an, only two species remained (Kojumdgieva, Po-pov, 1987). Some species are defined in the studied section Zelenka, predominantly in bed 4 (Yaneva et al., 2019). In general, mollusks are found only in limestones and extremely rarely in aragonite sedi-ments. Most likely, during the occasional inflows of fresh waters into the basin, brackish environments were created. These conditions were favorable for the existence of certain mollusks. Conversely, the reduction in water salinity led to the dissolution of aragonite in bottom muds, it precipitated as low-Mg calcite (stable under new conditions) and the subse-quent lithification process leading to the formation of limestone (Koleva-Rekalova, 1994). Micritic limestone (bed 4) containing only single shells of mollusks is represented on Figures 1a–c.

In the Upper Bessarabian, there are aragonite sediments and layers rich in clay minerals. Prob-ably, they were also introduced into the basin with freshwater inflows, and as as a result brackish to fresh water conditions were created. This is evi-denced by the available diatomaceous flora in the clay layers, studied in the Zelenka section (Ogn-janova-Rumenova, 2019).

Fig. 1. a, field photograph of section Zelenka (Topola Formation, latest Bessarabian); b, close view of the clayey bed 7; c, close view of the micrite limestone containing single shells of mollusks (red circle), bed 4; d, Middle Bessarabian limestone, enriched mainly in foraminifera and rare gastropods and molds of mollusks (Odartsi Formation, Tyulenovo section, after Koleva-Rekalova, Darakchieva, 2020)

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The studied clayey layers (bed 7) are composed of clay minerals, calcite, few silt-sized quartz grains and diatom frustules. Some organic matter and rare pyrite crystals are recorded. The carbonate content is very similar in both samples: the lighter one con-tains 51.68% carbnate, and the darker one – 48.57%. The dominant clay mineral in both samples is smec-tite with minor participation of illite, and the bulk amount of clay minerals is the same. The difference between the samples is in the content of organic matter, as it is more abundant in the darker one.

The formation of smectite as a main clay min-eral requires mild to intermediate weathering and slow drainage in the hinterland. High amounts of smectites preferentially form during weathering un-der arid climatic conditions (McKinley at al., 2003). Smectite is reported to be the dominant clay-mineral weathering product in modern desert environments (Chamley, 1989). Arid climates lead to mineralogi-cally immature sediment that increases the chance of preserving smectite minerals in the resulting rock (McKinley at al., 2003). Thus, during the formation of aragonite in the basin under arid climate condi-tions, weathering produced smectite minerals in the hinterland. Later on, during the transportation and deposition of clayey minerals, the climate changed to more humid with more precipitation for freshwa-ter streams to appear, which brought the clay min-erals into the basin and refreshed the environment.

In the earliest Khersonian, the climate in the study area was most likely seasonal, as evidenced by the presence of laminated aragonite-clayey sedi-ments of the varve type (Koleva-Rekalova, 1994, 1997). During the summer season, as a result of higher temperatures, aragonite muds were formed, and during the cold seasons – sediments enriched in clay minerals.

Climate aridity during the studied period was first reported by Belokrys (1967). It has been prov-en by the existence of dolomite sediments in south-ern Ukraine, which according to the author, were formed in lagoon environments.

ConclusionThe present study traces the climate changes through the latest Bessarabian-earliest Khersonian on the territory of Northeastern Bulgaria. During the lat-est Bessarabian the climate was predominantly arid with some humid periods, and during the earliest Khersonian it was seasonal. The climate had a sig-nificant influence on the type of sedimentation in the Balchik part of the Varna-Balchik Depression, as well as on the existence and disappearance of various organism groups. The conclusions are based

on literature data and new sedimentological and bi-ostratigraphic investigations.

Acknowledgements: This work has been carried out in the framework of the National Science Pro-gram “Environmental Protection and Reduction of Risks of Adverse Events and Natural Disasters”, approved by the Resolution of the Council of Min-isters № 577/17.08.2018 and supported by the Min-istry of Education and Science (MES) of Bulgaria (Agreement № Д01-322/18.12.2019).

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