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   During the early Sarmatian (about 14-15 m.y. ago) the Paratethys Basins were isolated from the ocean. The inflow of fresh water exceeded the amount lost through evaporation and led to a rise of sea level in the Forecar-pathian and Euxinian Basins, and a decrease in salinity. After the elevation of the Carpathians during the Bes-sarabian (about 12.5 m. y. ago) these basins were iso-lated from the Pannonian Basin which turned into an al-most freshwater lake, while in the other parts of the Paratethys the basins decreased in area, the Pannonian remained almost the same size until nearly the end of the Chersonian. The equilibrium between the inflow and evaporation in the Euxinian Basin was maintained mainly at the expense of the rivers flowing from the north (Volga, Don and Dnepr). The Danube was not yet in existence. Part of the waters which comprise the con-temporary Danube flowed into a strongly desalinized Pannonian Basin and the other part drained into the Forecarpathian Basin. In the present study an attempt to establish the palaeoclimatic changes which occured dur-ing the late Bessarabian and Chersonian is made on the basis of both palynological and sedimentological data.
    The studied material originated from two boreholes (C-12, near the village of Deleina, and C-136A, near the town of Balchik) in northern Bulgaria (Fig. 1). It consists of aragonite sediments (aragonitites) and clays of Bes-sarabian and Chersonian age. The total thickness of the aragonite sediments from borehole C-136A (the Topola Formation) is about 67 m. Massive aragonite sediments of Bessarabian age occur in the lower part of the section. They alternate with micritic limestones, dolomites and clays. The middle and upper parts of the section are rep-resented by materials of Chersonian age, composed of aragonite and clay laminae. In borehole C-12 the Cher-sonian clays are about 83 m thick (the Florentin Forma-tion). They are interbedded with laminated aragonite-clay materials and rare limestone beds.

Palynological data
   During the Bessarabian changes in the composition of the plant palaeocommunities are apparent in the territory of northern Bulgaria (Figs 2, 3). They are characterized mainly by a decrease (in comparison with the Volhynian) in the role of thermophilous plants like Engelhardia, Araliaceae, Arecaceae, Theaceae, Pandanus and Sapota-ceae, which, in the upper part of the Bessarabian sedi-ments and in the Chersonian ones, are presented by sin-gle pollen grains or not at all. Meanwhile an increase in the proportions of arctotertiary elements (Abies, Cedrus, Castanea, Quercus, Ulmus, Zelkova, Pterocarya, Carya, Fagus, Betula and Carpimts) can be seen. Dominant in the composition of mixed mesophilous forest were spe-cies of the genera Quercus and Ulmus, accompanied by Castanea, Eucommia, Fagus, Betula, Carya, Juglans and Acer (PI. 1, figs 3-18). Wet habitats were occupied by swamp and riparian forest composed of species of the Taxodiaceae, Myrica, Nyssa, Alnus, Planera, Liquidam-bar and Salix.
   A significant change in the fossil vegetation is the in-creasing role of subxerophytic and xerophytic shrub pa-laeocommunities. They were composed of species of Celtis, Pistacia, Rhus, Oleaceae, Ephedra, Spiraea, and also Quercus (ex. sect. Ilex), Robinia, Arbutus, Berberis, Paliurus, Celastrus and Caesalpinites, remains of which occur as macrofossils (Palamarev 1989, 1991, Palamarev & Ivanov 1998). At that time herbaceous palaeocommu-nities appeared (Chenopodiaceae, Artemisia, Caryophyl-laceae, Asteraceae, Apiaceae etc.). Palynological data re-lating to the Chersonian palaeoflora (Figs 2, 3) show increasing shares of Chenopodiaceae (16-23%), Celtis, Asteraceae, Artemisia, Caryophyllaceae etc. (Ivanov 1995). This is evidence of the appearance of open land-scapes and the development of xerophytic herbaceous palaeocommunities. Simultaneously the distribution of swamp and riparian forest contracted
   These changes in the fossil vegetation reflect the in-creasingly arid climate during the late Bessarabian -Chersonian period. The appearance and development of this semi-arid phase were associated with the retreat of the Miocene sea from the territory of northern Bulgaria. The climate at that time could be described as warm-temperate with long dry periods and minimal rainfall.