Past climate and vegetation in Southeast Bulgaria — a study based on the late Miocene pollen record from the Tundzha Basin
The results of palynological studies on the late Miocene freshwater deposits of the Tundzha Basin (Southeast Bulgaria, SE Europe) are presented. The basin is relatively well known in terms of geology and palaeogeography. The age of sediments in the Tundzha Basin ranges between the late Miocene to the Pliocene, based on mammal and diatom fossils. We carried out a palynological analysis of clayey sediments interlayered with coal beds from four cores and from one outcrop, aiming to obtain information about the composition and the structure of fossil vegetation. The ratios between the main floristic elements and the composition of the fossil flora are analysed and discussed from a palaeoecological point of view. Several main vegetation palaeocommunities were recorded: swamp forests, mixed mesophytic, communities of aquatic plants, and herbaceous palaeocoenoses. The changes in vegetation and in plant diversity are identified. The palaeoclimate analysis indicates a warm temperature climate with high rainfall and mild winter temperatures, without seasonal drier conditions. The early Pontian climate was about 3–4 °C warmer than today, with rainfalls per year at least 300 mm higher than today. The results of palaeoecological analysis of the flora and of the quantitative palaeoclimate data show that the climate in the Southeast Bulgaria indicates a climate change towards slight cooling and some drying. This event is consistent with the period of accumulation of the upper, undivided part of the Elhovo Formation.
KeywordsPalynology Palaeobotany Coexistence approach Neogene Tundzha Basin Bulgaria
Arboreal Pollen grains
Climatic Amplitude Method
Climate Leaf Analysis Multivariate Programme
European Leaf Physiognomic Approach.
Leaf Margin Analysis
Non-Arboreal Pollen grains
Changes in climate and vegetation during the Miocene are the subject of scientific interest which has encouraged studies of fossil floras and palaeoenvironments. After the middle Miocene climatic optimum (MMCO), the Earth climate recorded a progressive cooling trend (Zachos et al. 2001). This reveals a global transformation in biodiversity and ecosystems. For the eastern Paratethys, the emergence of open habitats and the distribution of herbaceous vegetation during the late Miocene characterized the flora and the vegetation turnover (Ivanov et al. 2002, 2007a). The territory of the Balkan Peninsula with its numerous Miocene lakes and swamps served as a key region for the study of the Neogene evolution of flora and vegetation, for the migration routes and for the exchange corridor of many plant species between Central-Eastern Europe and Asia Minor (Meulenkamp et al. 1996; Rögl 1998, 1999; Meulenkamp and Sissingh 2003; Popov et al. 2006; Akgün et al. 2007; Akkiraz et al. 2008; Ivanov et al. 2011; Alçiçek and Jiménez-Moreno 2013; Biltekin et al. 2015; Durak and Akkiraz 2016; Ivanov and Worobiec 2017; Kayseri-Özer 2017; Kayseri-Özer et al. 2017; Yavuz et al. 2017). The territory of Bulgaria apparently provides substantial information for many of these processes, e.g., the survival of a number of palaeotropical species in various refuges and the processes of plant speciation (Palamarev 1989; Palamarev and Ivanov 1998, 2001, 2004; Palamarev et al. 1999; Ivanov 2015).
The spatial distribution of plants and vegetation strongly depends on climatic conditions. Thus, through reconstruction of the vegetation from the past, conclusions can be drawn about past climates. Based on this assumption, several quantitative methods have been developed during the last few decades aiming to reconstruct the climate of the past, e.g., the Climate Leaf Analysis Multivariate Programme (CLAMP) (Wolfe 1993), the Coexistence Approach (CA) (Mosbrugger and Utescher 1997; Utescher et al. 2014), the Leaf Margin Analysis (Wilf 1997), the Climatic Amplitude Method (Fauquette et al. 1998), and the European Leaf Physiognomic Approach (ELPA) (Traiser et al. 2005). In this way, many climate reconstructions and a number of local and regional climatic reconstructions have been proposed for the Neogene period (Bertini 2002, 2006; Bruch and Gabrielyan 2002; Ivanov et al. 2002, 2007a, 2007b, 2007c, 2011; Bruch and Kovar-Eder 2003; Fauquette and Bertini 2003; Uhl et al. 2003, 2006, 2007b, 2007a; Bruch et al. 2004, 2006, 2007, 2011; Mosbrugger et al. 2005; Traiser et al. 2005, 2007; Fauquette et al. 2006, 2007; Jiménez-Moreno 2006; Jiménez-Moreno and Suc 2007; Jiménez-Moreno et al. 2007c, 2007a, 2007b, 2008a, 2008b, 2009; Utescher et al. 2007, 2009b, 2009a, 2011a, 2011b, 2013, 2015; Alçiçek and Jiménez-Moreno 2013; Ivanov 2015; Ivanov and Worobiec 2017; Yavuz et al. 2017).
Intensive investigations on the Miocene vegetation and on climate dynamics were performed in the Neogene basins in Bulgaria over the last years, using pollen analysis (e.g., Utescher et al. 2009b; Ivanov et al. 2010, 2011; Hristova and Ivanov 2014; Ivanov 2015; Ivanov and Worobiec 2017). This area plays a key role in the network of palaeoecological studies conducted in different parts of the Balkan Peninsula in relation to Southeast-European Neogene vegetation and flora history, aiming to reveal the chronological succession of the main vegetation phases, the climate changes behind them, species migration and distribution (Akgün et al. 2007; Jiménez-Moreno et al. 2007c, 2007a; Akkiraz et al. 2008; Bozukov et al. 2009; Alçiçek and Jiménez-Moreno 2013; Biltekin et al. 2015; Ivanov 2015; Durak and Akkiraz 2016; Kayseri-Özer 2017; Kayseri-Özer et al. 2017; Yavuz et al. 2017). Nevertheless, there are only few studies in the Southeast Bulgaria on past vegetation and climate (Palamarev and Bozukov 2004; Ivanov and Lazarova 2005; Ivanov et al. 2007b; Ivanov 2004, 2010). The aim of this paper is to present new results on pollen analysis from sediments of the Tundzha Basin and to summarize the available data about the vegetation ecology and climate in this area during the late Miocene.
2 Geology and palaeogeography
The Neogene sediments of the Tundzha Basin are assigned to the Elhovo Formation (Kojumdgieva et al. 1984) with two members (Fig. 1): the Izgrev Member and the Duganovo Member, and one undivided part (Prustnik Limestone Formation; access to Angelova et al. 1991). It is represented by an irregular alternation of claystone, sandstone and rare conglomerates. The thickness of the Elhovo Formation is ca. 150–200 m, but locally it reaches up to 300 m. Within these deposits, large lenses of gray and black clays, diatomite clays and lignites are grouped within the Izgrev Member, which locally occurs in the middle part of the basin (Fig. 1). The total thickness of the Izgrev Member reaches up to 40 m, with three main coal seams, each of them with a thickness varying from 3 m up to 8 m.
The lignite seams accumulated in a rheotrophic, low-lying mire. A vegetation rich in decay resistant conifers dominated in the Elhovo Formation together with mesophytic angiosperm species. The peat accumulation occurred in an environment subject to a low subsidence rate, in which the woods were sustained severe mechanical destruction prior to the burial. Peat accumulation was terminated by a major flooding event, expressed by a short-lived lake (Zdravkov et al. 2007). The Elhovo Formation is unconformably overlain by a few meters of the Pleistocene-Holocene sediments.
The vertebrate fauna recorded to the upper part of the Elhovo Formation (Kojumdgieva et al. 1984; Nikolov 1985) reveals a Pontian (=late Messinian)-Pliocene age (MN 13–14). The results of the diatom analysis (Temniskova-Topalova et al. 1996; Temniskova-Topalova and Ognjanova-umenova 1997) confirmed the late Miocene age (Pontian age for Elhovo Formation). The lithological and facies characters and the specific cyclicity of the sediments of the Tundzha Basin gave grounds to some authors to define these sediments as analogous to the Neogene sediments of the Upper Thracian Basin (Dragomanov et al. 1984). However, similar correlations were confirmed neither by biostratigraphic data, nor by detailed sedimentological studies. Even more, significant differences occur in the nature of sedimentation processes in the two basins, with specific periods of sedimentation interruption and denudation surfaces.
The sediments of the Elhovo Formation are deposited in alluvial, fluvial and locally lacustrine-marshy environments (Nakov et al. 2001). As a result of extensive tectonic movements at the beginning of the late Miocene, a number of freshwater pools appeared in the Balkans, including the Tundzha Basin. During the Maeotian, two low areas were formed: Yambol and Elhovo (Savov 1983). The initial alluvial sedimentation had been predominantly replaced by lake and swamp environments (Izgrev Member). Gradually, the basin was filled, and at the end of the Pontian and the early Pliocene, the alluvial sedimentation was restored.
3 Material and methods
3.1 Studied sections
Twenty-eight samples from the upper part of the Elhovo Formation from three outcrops were collected for pollen and spores analyses: 1) the outcrop in the abandoned quarry in Prastnitsata, 200 m west of the Izgrev village, Elhovo district (Kojumdgieva et al. 1984), including about 1.5 m greenish clayey alleurites with limestone and green muds, 6–7 m white and yellowish fine-grained sands with layers of medium to coarse grain sands (six samples); 2) the outcrop along the road from Elhovo to Golyam Manastir village (SR-1), close to the bridge over the Sinapovska River (18 samples); and, 3) the outcrop Hanovo on the right bank of the Tundzha River between the Hanovo and Tenevo villages, including cross-bedding sands with thin layers of sandy clays (four samples). Samples of these outcrops proved to be barren, except for some of the samples of the outcrop SR-1 (Sinapovska River outcrop).
The profile near Sinapovska River (SR-1) includes about 5 m of sandstones with three layers of about 0.5 m of green to purple aleuritic clays, followed by 5–10 m of cross-bedded sands (for details see Ivanov et al. 2007c). Leaf imprints and pollen have been found in the clay layers. The sedimentological analysis of the flora-bearing sediments (Ivanov et al. 2007c) explains the conditions for the accummulation of sediments and for the preservation of the fossil material. Good preservation of plant debris is related to the relatively rapid sedimentation (accumulation) rate of the alluvial clay material in which they were deposited. This material underwent significant compaction due to the pressure of the overlying sediments. But the high sedimentation rate is inappropriate for the accumulation of sufficient pollen, which is why the established pollen complexes are comparatively pure.
The total number of studied samples from the Tundzha Basin is 64: 27 were barren, but 35 from four cores and two from the outcrop SR-1 contained enough pollen for study.
Tracing the changes in the percentage values of the different pollen type curves permitted the identification of pollen zones in the investigated cores. Differentiation of the pollen zones is based on sediments with a specified fossil content, or specific palaeontological characters (characteristic pollen complexes, type and frequency of palynomorphs), which distinguish them from the neighbouring sediments (Gordon and Birks 1972). The presented pollen zones for each core were regarded as Local Pollen Zones (LPZ) indexed by letters and digits. The palynological subdivision was applied only for the core C-432, which contains enough samples for correlation.
3.2 Methods for vegetation and climate reconstructions
The principles of autecology were used for the reconstruction of vegetation, as well as the data on ecological requirements of the nearest analogues of the fossil taxa. As many Neogene European floras, the flora of the Tundhza Basin includes taxa whose nearest living relatives (NLR) now grow in distant areas, e.g., East Asia and North America. The palaeocoenoses were reconstructed with the help of autecological analysis, assuming that the ecological requirements of fossil taxa are similar to those of their recent analogues; taxa with similar ecological and edaphic requirements were grouped and the main palaeocommunities were identified.
Charts showing the results of the pollen analyses are illustrated by two types of pollen diagrams: detailed and synthetic. The first diagrams include all identified plants and show their individual presence. In the second type of diagrams, the plants were ordered into ecological groups following Suc (1984) and Jiménez-Moreno et al. (2005) and they provide information for the general trends in vegetation change. The synthetic pollen diagram was plotted with pollen taxa arranged in different groups on the basis of ecological criteria to clearly show the temporal changes in vegetation.
Mega-mesothermic (subtropical) elements: “taxodioid” Cupressaceae pollen, Taxodium-type, Symplocos, Engelhardia, Platycarya, Myrica, Sapotaceae, Distylium, Hamamelis, Corylopsis, Castanea-Castanopsis type, Cyrillaceae-Clethraceae, Reevesia, Theaceae, Alangium, Chloranthaceae, Parthenocissus, Araliaceae, Arecaceae and others;
Cathaya: pollen of Cathaya sp.;
Mesothermic elements: (Quercus, Carya, Pterocarya, Carpinus betulus, Carpinus orientalis, Ostrya, Parrotia, Eucommia, Juglans, Zelkova, Ulmus, Tilia, Acer, Liquid ambar, Alnus, Salix, Populus, Rhus, Celtis, Platanus, Nyssa, Ilex, Lonicera, Caprifoliaceae, Vitaceae, Fraxinus, Betula, Sequoia-type, Fagus, Hedera, Ilex, Tilia, etc.;
Pinus + Pinaceae: Pinus diploxylon type and undetermined Pinaceae pollen;
Mid-altitude trees (Meso-microthermic elements): Tsuga, Cedrus, Sciadopitys;
High-altitude trees (microthermic elements): Abies, Picea, Keteleeria;
Cupressaceae: Cupressus-Juniperus-type and/or pollen irrespective of environmental interpretations, including unspecified pollen grains;
Xerophytes: xerophyte taxa e.g. Quercus ilex-coccifera-type, Olea-type (Oleaceae), Caesalpiniaceae, Pistacia, Rhus and others;
Herbs: Poaceae, Amaranthaceae, Asteraceae-Asteroideae, Asteraceae-Cichorioideae, Centaurea, Plantago, Brassicaceae, Lamiaceae, Valerianaceae, Polygonaceae, Knautia (Dipsacoideae), Rosaceae, Malvaceae, Geraniaceae, Erodium, Caryophyllaceae, etc.;
Steppe elements: Artemisia, Ephedra.
The palaeoclimate reconstructions in this work are based on the Coexistence Approach (CA) (Mosbrugger and Utescher 1997; Utescher et al. 2014), and based on the assumption that climatic requirements of the fossil plants for environmental conditions are similar to those of their recent analogues. It should be noted that the Coexistence Approach uses only the presence or absence of taxa, without analyzing their relative frequency. Tests have shown that the approach yields good results when applied to fossil floras with more than ten taxa with a known contemporary analogue. The approach is valid for various types of fossils: leaves, fruits and seeds, spores and pollen grains. This method permits to analyze also carpological data and to compare the two types of fossil associations. This method provides a robust palaeoclimatic proxy although its reliability has been questioned by some authors (Grimm and Denk 2012; Grimm et al. 2016). A lot of studies were undertaken for testing different climate reconstruction methods (CAMethod, LMA, CLAMP, ELPA, etc.), which emphasized some differences in the results when comparing the CA and other proxies. But in most cases, similar results were obtained (Bruch et al. 2002; Uhl et al. 2003; Yang et al. 2007; Jacques et al. 2011, 2014; Xing et al. 2012; Bondarenko et al. 2013). The results are consistent with respect to global climate reconstructions, and in general they are consistent with the data obtained from a large variety of other proxies, for example isotope geochemistry, small mammals or other independent palaeoclimatic approaches.
The Palaeoflora Database (Utescher and Mosbrugger, 1990–2018) has been used for palaeoclimatic reconstructions. The graphic presentations of palaeoclimate results are illustrated by the respective figures, where the coexistence intervals (CA-intervals) for each pollen spectrum (=local pollen flora) are represented by four parameters. Besides the respective CA-intervals, the graphics also show a curve of the CA mean values. This curve does not mean that these are the most probable values (the values of the respective climate parameter could remain within the boundaries of the range), but they illustrate approximately the changeability of climate and the dynamics of climate values over time (Pross et al. 2000; Ivanov et al. 2002).
4.1 Palynological subdivision of the Elhovo formation
Local pollen zone Tu-1
Ulmus - Betula - Carya
Age: late Miocene.
Distribution: 79.0–61.0 m.
The core is marked by high values of the Ulmus pollen, which is represented by values ranging mainly in the range of 13%–20% and with a maximum of 29.8% at 65.0 m. The quantity of Carya pollen is 4%–9%, which are the highest values in the core. The Betula pollen is also represented with higher values (3%–5%) in this part of the profile. Fagus is represented with higher values in the lower part of the zone (3.9%–7.4%), and is below 1% and marked by a sharp drop in the upper part (interval 64.5–61.0 m). Similar dynamics of the quantitative values are characteristic for inaperturate pollen referred to Glyptostrobus — 3%-6% at the base and a drop to about 1% in the upper part. Carpinus orientalis/Ostrya type, Ericaceae, Nyssa, Poaceae, Typha, Sparganium and Tricolporopollenites sibiricum are also registered with higher values. Pinus diploxylon type is represented by constant values ranging in narrow range between 17% and 20%, with single deviations from them, e.g., 11.3% at 73.0 m or 26.1% at 74.0 m. Cathaya has low values not exceeding 1.5%. The pollen of herbs is low (less than 1%), with the exception of Poaceae, Asteraceae, partially Amaranthaceae: Chenopodioideae. Higher values for these three pollen types trigger higher NAP (Non-Arboreal Pollen grains) values, reaching a percentage of 13.1%, which is the maximum for the entire profile. Local elements also have a broader involvement in the pollen spectrum of this zone, reaching maximum value of 15.6% at 73.0 m.
Local pollen zone Tu-2
Engelhardia - Quercus - Fraxinus
Age: late Miocene.
Distribution: 60.0–46.0 m.
Quercus records higher values in this part of the profile. While in the previous zone it is discovered in quantities of about 2%, in this zone its values vary between 6% and 11%. The change in the Engelhardia is similar, after a relatively poor presence in the Tu-1 zone (2%–5%), the participation rate increased to 8%–10% and even 11.8% (maximum value for the whole profile registered at 48 m). The most significant is the increase in the participation of Fraxinus: it reaches up to 12%–16% from 2%–5%. Parallel to this, Tsuga values increase up to 4%, and also Corylopsis, but less pronounced. Oleaceae (up to 4.7%), Buxus (up to 2.6%) and Pistacia (up to 1%) are shown at the top of the higher-value zone. Platanus pollen is below 1% across in the profile, but at 48.0 m it has a peak of 9.6%. At the same depth (48.0 m), Alnus, whose pollen in the rest of the profile has a constant participation of 1%–2%, also shows its maximum percentage. In the range of 48.0–46.0 m, the Myrica (up to 3.6%) and Salix (up to 6.3%) were recorded. Lower values in this zone are registered for Betula, Fagus, Ulmus and Carya, which were predominant in the previous zone. Pollen of herbaceous plants (NAP) is also presented with lower values. The local elements with reduced pollen spectra in this pollen zone are Typha and Sparganium. Pinus pollen reaches a peak at 54.0 m (45%), followed by a decreasing trend. Cathaya, as well as in the Tu-1 zone, is low at 1.0%–1.5%.
The pollen diagrams of the cores C-96, C-127 and C-146 are not divided into pollen zones due to the small number of studied samples (four to six in each core). The analysis of pollen content and the quantification of fossil palynomorphs show a similarity to the local pollen zone (LPZ) Tu-2 on core C-432. The major pollen types found in the cores C-96, C-127 and C-146 are of similar values in all pollen spectra. Quercus pollen records high values ranging from 6% to 11%. In this respect, the proximity to the quantitative coverage of this type of pollen is almost identical to its participation in the LPZ Tu-2. Ulmus has variable values, with about 2% in most samples up to a maximum of 12.5%. With similar values, this type of pollen is recorded in the upper parts of the LPZ Tu-2. Similar values are represented by Tsuga and Picea, for which values of 1%–2% were established in four profiles. Similar quantities are observed in the pollen of Betula, Fagus, Oleaceae, Pterocarya, Carya, Engelhardia, Alnus, Salix, and Myrica ranging from 1%–2% to 3%–4%.
The main differences in both profiles refer to Pinus pollen. Pinus diploxylon-type in LPZ Tu-2 has a quantitative value close to the core C-96 (except the maximum at 54.0 m), followed by a decreasing trend observed in both profiles. The more significant is the presence of Cathaya, which in the LPZ Tu-2 was presented with lower values (1.0%–1.5%), and only in the pollen spectrum of 46.0 m was registered with higher values (3.6%). In the core C-96, this type of pollen is registered with higher values of 11%–17%, which in the upper part of the section reduced to 5%. Higher values may be explained partly by local features in the structure of vegetation, suggesting a greater involvement of Cathaya in the pollen rain. Another possibility is related to a discrepancy in stratigraphic levels, e.g., the cut-out interval from the core is a later stage of the LPZ Tu-2, at the end of which higher values of this pollen type were recorded. The lack of other fossils, lithological and stratigraphic data makes the correlation of the two cores less reliable.
The pollen flora from the “Sinapovska River” outcrop (SR-1) differs significantly from the flora found in the sediments of the Izgrev Member of the Elhovo Formation. The profile includes layers of greenish to violet aleuretic clays, which refer to the uppermost levels of Elhovo Formation and correspond to a later stage in the development of the flora in the area. A characteristic feature of the pollen flora is the significant involvement of pollen from herbaceous plants and the lack of representatives of spore plants. Herbaceous plants are subject to significant taxonomic diversity and to a high percentage participation, e.g., Amaranthaceae: Chenopodioideae (11.6%), Asteroideae (8.5%), Poaceae (7.1%), Dipsacoideae (Caprifoliaceae) (5.4%), and Artemisia (2.7%). The composition of the spore-pollen complex differs significantly from the pollen complexes of the studied samples from cores C-432, C-96, C-127 and C-146. At the same time, the low content of pollen in the studied samples makes the separation of an independent pollen zone in the outcrop Sinapovska River (SR-1) uncertain.
4.2 Fossil flora and vegetation of the Tundzha Basin
Taxonomic composition of the fossil pollen flora from the Tundzha Basin
Carpinus betulus type
Carpinus orientalis/Ostrya type
Carya sp. 1 and sp. 2
Pterocarya sp. 1 and sp. 2
Quercus sp. 1 and sp. 2
Engelhardia sp. 1 and sp. 2
Juglans sp. 1 and sp. 2
The thermophillous elements are relatively limited in the composition of the flora in terms of their floral diversity. Grass plants are poorly represented in quantitative terms, although they are covered with 27 taxa, which is about a fifth of the palaeoflora diversity. The pollen of wood and shrub components (AP) is predominant. This implies the dominance of the forest-type vegetation in the areas around the basin. This does not apply to the pollen flora from the outcrop of Sinapovska River, where the grass component is much better represented.
An interesting feature of the palaeoflora is the morphological variability of the pollen of the Juglandaceae family, observed in all recorded genera. In the case of genus Engelhardia (Pl. 3: 1–9) the variability can be considered within the range of the natural variation of the morphological features as it shows smooth transitions without distinct differences in grain outline and in morphometric characteristics. This pollen type can be assumed to be within the range of the variability of Engelhardia wallichiana-type (Ivanov 2004). The pollen illustrated on Pl. 3–10 and Pl. 3–11 is morphologically close to Engelhardia spicata-type, and more specifically to the pollen of modern species of E. rigida Blume and E. spicata Blume.
Two morphotypes were found in the Carya pollen (Pl. 2: 29–33), which differ in size of pollen grains and thickness of the exine. The pollen of Pterocarya is also represented by two pollen types (Pl. 3: 14 and 15), with a major difference between them in the shape of apertures and in the exine thickness, the first closer to the modern species Pterocarya pterocarpa (Michx.) Kunth. (Pl. 3–14) and the second closer to Pterocarya serrata Schneider (Pl. 3–15).
Exine thickness, pollen grain outlines and aperture shape are the diagnostic characters allowing the separation of two morphotypes in the fossil pollen of Juglans (Pl. 3: 12 and 13), corresponding to the artificial species Juglandipollis juglandoides Kohlman-Adamska (Pl. 3–12) and Juglandipollis maculosus (Pot.) Kohlman-Adamska (Pl. 3–13).
The palaeoflora from the outcrop SR-1 has a more limited floristic composition, as the palynomorphs are poorly preserved due to taphonomic reasons (see above Chapter 4.1.). The high sedimentation rate at which fossil deposition is formed explains the poor pollen content of the recorded fossil complexes (Ivanov et al. 2007c). The palaeobotanical studies on the composition of the macroflora include mainly the results of the leaves from the outcrop SR-1 (Palamarev and Bozukov 2004). The macroflora is represented by 33 species belonging to 16 families. Scarce palaeofloristic data are also reported for carpoids from the Elhovo Formation (including the Izgrev Member) — Potamogeton, Phelodendron, Polycnemum, Portulaca, Arenaria and Chenopodium (Palamarev 1990; Mai and Palamarev 1997). A total of 35 genera were found in the macroflora composition, and 11 of them were confirmed by palynomorphs. 64 species are reported in the present study as new fossil taxa for the studied area.
The composition of the mixed mesophytic forest communities varied, and besides the families already mentioned, the representatives of Magnolia, Betula, Corylus, Carpinus, Fagus, Acer, Tilia, Castanea, Corylopsis, Parrotia, Eucommia, Pterocarya, Juglans, Ilex, Buxus and others participate in their structure. Thermophilous plant species of the genera and families Platycarya, Engelhardia, Symplocos, Sapotaceae, and Araliaceae are also present in pollen spectra with varying frequencies in sediments of different age and position. Of these, only the representatives of the Engelhardia probably had a dominant role at certain stages of vegetation development. The reasons for such an assumption are provided by the data dealing with quantitative values of this genus illustrated in Figs. 3, 4, 5 and 6.
The variegated palaeofloristic composition of mixed mesophytic forest communities suggests the presence of vertical differentiation of palaeoflora and of palaeocenoses and the existence of a belt of mountain forest palaeocenoses. The components involved in the construction of mountain palaeocenoses include representatives of the genera Tsuga, Abies, Keteleeria, Picea, Cedrus and Cathaya, generating mixed communities with the participation of Betula, Fagus, Acer and Ericaceae.
Herbaceous palaeocenoses have a relatively limited distribution, demonstrated by low percentages of their pollen. This indicates their limited importance for the formation of the vegetation cover compared to the forest palaeocenoses. Their main components were Amaranthaceae: Chenopodioideae, Asteraceae, Caryophyllaceae, Apiaceae, Brassicaceae, Poaceae, Ranunculaceae, Achillea, Artemisia, Aster, Centaurea, Mentha/Salvia, Polygonum, Plantaginaceae, Thalictrum and others. Probably some of them have been involved in the structure of herbs cover in the mesophytic forest communities, while others have occupied open (or erosional) terrains near the basin and the river valleys.
Swamp forests were comparatively limited, as evidenced by the percentage contribution of their components to pollen spectra. Representatives of ‘Taxodioid’ Cupressaceae and Alnus were predominant in these forests, which are supposed by the slightly higher pollen values found in pollen spectra (1%–2% to 5%–7%). They were accompanied by species belonging to the genera Glyptostrobus, Sciadopitys, Nyssa, Myrica, Osmunda, Salix, and Cyrillacaeae/Clethraceae, typically represented at around 1%, rarely higher. The presence of pollen from some pollen types characteristic of coastal forests (e.g., Salix, Pterocarya, Platanus, Liquidambar, etc.) can be interpreted as an indication of the distribution of this type of palaeocenoses in the valleys of the inflowing rivers and in the coastal areas. Fraxinus, which in some of the analyzed samples, was recorded with high values compared to other taxa, probably also participated in the composition of riparian forests, swamps or transitional areas with mixed mesophytic forest palaeocenoses. Components of these palaeocenoses were probably the liana species of Vitaceae, Humulus and Hedera.
The spread of swamp forests is likely to be directly related to the water level in the basin. During high water stands of the lake, the diatomite clays wеre deposited, while at low water stands, the marsh-swampy vegetation was widespread, as precursors of coal seams. The Tundzha Basin was an extensive graben structure formed in the final stage of the continental collision at the southern edge of the Alpine Orogen. Typically, this type of basins has a similar development, starting with lake-river sedimentation and deposition of conglomerates and sands, gradually passing into clay sedimentation and subsequently swamping and forming thick coal beds covered by lake sediments (Zdravkov et al. 2007). This sequence reflects the drowning of the palaeomire due to high subsidence rates. When subsidence rates decreased, the lake was filled with river-delta sediments. The high number of lignite layers in the Tundzha Basin is the evidence of a relatively low subsidence speed, which allowed the frequent change between lacustrine (diatomaceous and black clays) and swampy environments (lignite).. The high peat content of lignite indicates that swamps were often flooded, and the marsh complex was of the so-called rheolytic marshes (Zdravkov et al. 2007).
The geochemical analysis of the coals showed that lignites originated from coniferous wood, which is significantly more resistant to oxidation processes than that of herbaceous plants and it is better stored (Zdravkov et al. 2007). Probably the main coal precursors were the representatives of Taxodioideae (Taxodium, Glyptostrobus), as in most Miocene lignite basins in Bulgaria.
During the periods of peat accumulation, the (ground)water table was probably not above the peat surface. The basis for such asumption is the complete absence of algal remains and of sapropelic coal (Zdravkov et al. 2007). According to Zdravkov et al. (2007), the vegetation rich in decay-resistant conifers, accompanied by mesophytic broadleaf species, prevailed during these intervals. Due to the lack of samples for pollen analysis from coal beds, this assumption cannot be confirmed or rejected. The studied samples were collected from diatomitic and black clays formed in lake environments. The results of the diatom analysis (Temniskova-Topalova et al. 1996) show that during the period of accumulation of diatomaceous clays, the lake had a depth of approximatively 15.0 m. This means that during high water stands in the Tundzha Basin, vast territories flooded and the marshland had been completely submerged. This explains the low participation of swamp palaeocoenoses components, which have been preserved on the outskirts of the lake complex, in conditions suitable for their ecology.
The representatives of aquatic vegetation (euhydrophyte and hygrohydrophyte grasslands) found in the studied pollen spectra are in low quantities and they have a relatively poor species composition. In the open water, typical hydatophytes evolved, such as, in Potamogeton, in Nuphar, and in Nymphaeaceae. In the peripheral areas of the basin, plant communities of helophyite species of Alisma, Persicaria, Typha and Sparganium were developed. The low occurrence of pollen from aquatic plants in the pollen spectra supports the features of the lake basin: rather deep (predominant planktonic species of diatom algae), poorly developed shallow water (suitable for the development of hygrohydrophyte grasslands) and low eutrophicity (Temniskova-Topalova et al. 1996).
The xerophytes (Oleaceae, Celtis, Rhus, Buxus, Pistacia and some grasses) also have a limited distribution occupying possibly eroded or rocky terrains near the lake. The development of this vegetation type was directly related to edaphic and microclimatic factors. The quantitative contribution of sub-xerophytes and xerophytes in pollen spectra does not give reason to assume that they have the character of zonal vegetation.
5 Climatic analysis of the fossil flora of Tundzha Basin
The climate reconstruction, based on the palaeoecological data from the Izgrev Member of the Elhovo Formation, shows relatively constant values for observed climate parameters. The lower limit of the coexistence intervals for the mean annual temperature is limited in all the analyzed pollen floras at 15.6 °C. The upper limit is in most cases set at 16.5 °C, only in few cases higher values (18.4 °C and 19.4 °C) are observed thus forming wider ranges. The average temperature was typically about 16 °C with few exceptions. These annual temperatures show the relatively high precision of results obtained with the Coexistence Approach. The stability of the data for these parameters and the absence of significant deviations indicate no significant climate change.
Winter temperatures also show relatively constant values without significant changes. The most common coexistence intervals are 5.0–7.0 °C, and the most common mean values are 6.0 °C. In some cases, the upper limit of calculated cold temperature values indicates higher values and wider ranges, for example, 5.0–8.1 °C and 5.0–9.6 °C. In the coldest month temperature, the lower limit of intervals is important because low winter temperatures are often a limiting factor for the spread of many plants. The persistence of values above 5.0 °C indicates a mild and wet winter without extreme minimum temperatures.
Perhaps the wider ranges for the two temperature ratios are related to the incomplete fossil record rather than to a possible climate change. As far as they occur almost in synchronicity in the analyzed pollen flora, a slightly warmer climate with a higher average annual temperature due to higher winter temperatures is not excluded, with less seasonal climate change. The latter assumption is supported by the results obtained for the average temperature of the warmest month. The obtained WMTs are 24.7–27.8 °C and 24.7–27.3 °C (with one exception at 61 m; Fig. 11) and the average summer temperatures are in the range of 26.0–26.3 °C. There is no synchronization between wider WMT intervals and the other two indicators — CMT and MAT. This suggests a less pronounced seasonality, related only to a change in winter temperatures.
The mean annual precipitation also does not show drastic deviations. The intervals for annual rainfall are relatively broad ranging from 823 mm/m2 to 1308 mm/m2, and the average curve is slightly above 1000 mm.
The dynamics of the pollen quantitative values of the various fossil taxa showed two stages in vegetation development, the boundary between them being established in the pollen spectrum of 60.00 m in core C-432 (Fig. 3). The representatives of Ulmus and Carya dominate the mesophytic forest communities of the lower part of the profile (LPZ Tu-1). The representatives of Betula, Fagus and Carpinus orientalis/Ostrya also played an important role in the construction of this type of palaeocoenosis. The mega-mesothermic elements (Figs. 7, 8, 9 and 10) are represented with lower values and the grasses have a wider distribution reaching a maximum in the range of 64.5–62.5 m in core C-432. Among the latter, a major role is played by species belonging to the families Asteraceae, Poaceae, and partially Amaranthaceae: Chenopodioideae. Hydrophytic forest palaeocenoses also had a wider spread, and Glyptostrobus was dominant. The representatives of the Nyssa were also important components of the swamp forests. The hydrophytic herbaceous vegetation represented by Typha and Sparganium has also been established. These data testify to the development of the flora in a warm temperate and humid climate.
In the range of 60.0–46.0 m (LPZ Tu-2) in core C-432 (Fig. 3), a significant change in the composition and structure of the vegetation was recorded. The change is associated with an increase in Quercus, Fraxinus, Engelhardia, Oleaceae, Buxus, Cyperaceae, Typha, Sparganium and NAP. An interesting fact is that in the diatom flora of core C-432 changes also occur in this interval (Temniskova-Topalova et al. 1996). It is likely that a climate change took place. The beginning of this change is recorded to the top of the profile C-432, and a later result of this change is reflected in the flora from the outcrop SR-1 (see below). The most significant change in the composition of the mesophytic forest communities was a change of dominant taxa — a reduced distribution of Ulmus and Carya is observed, and at the same time, a rapid increase in the values of Quercus and Engelhardia. The participation of Fraxinus pollen, which plays an important role in the construction of riparian forest palaeocenoses, is stronger. The participation of Oleaceae, Buxus and Pistacia in the composition of the vegetation increases in the upper part of the profile. Mega-mesothermic elements (Fig. 7) are presented with higher values, which may indicate warming climate.
The profiles C-96, C-127 and C-146 show a trend towards the reduction of coniferous pollen (Cathaya and Pinus), but the group of mesothermal and subtropical species does not change significantly (Figs. 4, 5 and 6). This change can be correlated with that of the top of core C-432, or it may even be a sequel. The increase in herbaceous pollen at the top of the cores C-96, C-127 and C-146 (Figs. 4 and 7) coincided with some increases in NAP in C-432 and the increased participation of some (sub-) xerophytes in the same range. These data could indicate a certain climate change associated with increased seasonality and the occurrence of drier habitats.
The pollen record from the outcrop SR-1 differs from the four cores. It includes 47 taxa from 30 families (Ivanov et al. 2007c). The fossil macroflora (Palamarev and Bozukov 2004) includes a total of 33 taxa of 16 families of leaf imprints. Based on the composition of the established macro- and micro-flora, the following main plant communities are distinguished: mixed mesophytic forests composed of representatives of Magnolia, Lindera, Daphnogene, Ocotea, Quercus, Carya, Acer, Ulmus, Zelkova, Corylus, Fagus, Castanea, Tilia, Wisteria, etc.; riparian forests involving species of the genera Salix, Populus, Pterocarya, Liquidambar, Alnus, Rhamnus, Nyssa, Myrica, and Bambusoideae; xero-mesophyte tree and shrub communities of Robinia, Arbutus, Paliurus, Pistacia, Parrotia, Oleaceae; herbaceous palaeocenoses composited by the following families and genera: Amaranthaceae: Chenopodioideae, Asteraceae, Artemisia, Centaurea, Plantaginaceae, Caryophyllaceae, Brassicaceae, Apiaceae, Poaceae, Dipsacoideae (Caprifoliaceae); hydrophytic vegetation of Typha, Sparganium, Cyperaceae, Nymphaeaceae.
The representatives of the riparian forests are represented with the greatest number of leaf imprints. This is related, on one hand, to the better storage possibilities (spread around the water basin) and, on another hand, to the relatively limited distribution of mesophytic forest palaeocenoses. In spatial terms, mesophytic forests have been in close contact with riparian and coastal forests, occupying damp habitats in lowered areas of relief without forming a fully developed mesophytic forest belt (Ivanov et al. 2007c). Palynological data also suggest that the mesophytic forests were fragmented, as their representatives have low values and low affinity in pollen spectra. They differ significantly from the data on the mixed mesophytic forests that existed during the accumulation of the sediments of the Izgrev Member, when they were building the zonal vegetation. During this period of vegetation development, Quercus and Ulmus were the dominants in forest vegetation, with Betula, Carya, Carpinus, Corylus, Acer, Juglans, Engelhardia, Tilia and others. The floristic elements, whose distribution today is bound to temperate climates, but also with some thermophillous taxa, are regular in the pollen record of Engelhardia, Platycarya, Castanea-Castanopsis, Corylopsis (see above). The presence of pollen from Pinus, Tsuga, Cedrus, Keteleeria and Picea implies the presence of mountain forest communities. Their low percentages suggest that the recorded pollen is likely to be a result of a long distant transport, as there were not enough high-mountain systems near the sedimentation site. Xerophytes also played an important role in shaping the palaeolandscape. Their representatives were spread over drier and more eroded terrains, along rocky slopes.
Pollen data suggest a wide spread of grass palaecoenosis (NAP = 45.8%). It is quite possible the existence of at least two types of herbaceous communities: mesophytic grasslands inhabiting humid habitats near ponds (wetland prairies, wet prairies; see Hofmann and Zetter 2005), and more xerophytic herbs spread over drier terrains. As some recent studies on the prevalence of modern pollen and the AP/NAP ratio to their vegetation produce (Favre et al. 2008), small changes in herbs pollen values usually do not take into account real changes in herbaceous vegetation. While the sharp change in the ratio of woody and grassy pollen in favor of the latter is usually a sure indication of open habitats. Palamarev et al. (1999) also testify to the prevalence of xerophytic grasslands, made up of representatives of Polycnemum, Chenopodium, Arenaria and Portulaca, who formed semi-grade species communities on open and eroded terrains.
Popescu (2006) provided palaeoecological data from the Southwest Black Sea Region (DSDP Site 380A) and noted an increase in the grass component and the steppe/forest index (SFI) in the late Miocene-Pliocene. These data correspond to the high NAP values found in this study. A sharp increase and high values of the grass component were also recorded for the upper sequences of the late Miocene sediments of the Karlovo and Staniantsi Basins (Utescher et al. 2009b; Ivanov et al. 2010).
All palaeoclimatic data provided in the current study indicate a warm to subtropical climate with values for all temperatures of about 4 °C higher than today and with precipitations that were at least 300 mm higher than today. The climate was steady and stable over the period of sedimentary deposition. This assumption of slight climate change to the top of the profile (LPZ Tu-2), based on the analysis of vegetation changes, finds no confirmation in the climate reconstructions. Perhaps such a change was less than the resolution of the Coexistence Approach, which explains why it was not registered in other palaeoclimate reconstructions.
The climate reconstruction, based on the palaeo-floristic data from outcrop SR-1,, shows different values for the monitored parameters. The calculations made for the ranges of the individual palaeoclimate values based on the data from fossil macroflora (Ivanov et al. 2007a) show that the annual temperatures were in the range of 14.4–15.8 °C, the winter temperatures were 3.7–5.8 °C, the summer temperatures were 25.6–26.4 °C, and the annual rainfall was in the range of 961–1179 mm. These values are several degrees higher than the current values for the temperatures in the Elhovo-Yambol area, and significantly higher in terms of the amount of precipitation. Calculated values for the same climate parameters, based on palynological data, show wider CA intervals: MAT as 13.6–18.4 °C, CMT as 2.4–9.4 °C, WMT as 22.8–26.1 °C, and MAP as 740–1206 mm. The wider coexistence intervals derived from the palynological data are explained by the lower taxonomic resolution of the pollen analysis. The wider annual precipitation interval (740–1206 mm) may reflect also the diversity in the climatic conditions of a larger area, and the presence of habitats with a drier character.
The results of the macro- and micro-flora analysis from outcrop SR-1 show a high degree of similarity, which increases the reliability of the resulting palaeoclimate quantification. They are also in line with the palaeoecological analysis of the flora, which implies the development of the vegetation in a temperate climate with a possible dry period in the year. Compared to the results on palaeoclimate, during the deposition of the Izgrev Member, a climate-cooling trend, which is reflected in all temperature parameters, and a lower amount of annual rainfall, is now reported.
The results from the pollen analysis of the Neogene sediments from the Tundzha Basin include spore and pollen flora permitting to outline the main vegetation palaeocommunities: namely mixed mesophytic forests, swamp forests, communities of aquatic plants, and herbaceous palaeocoenoses. The dominant species in the zonal vegetation were floristic elements growing in warmtemperate to subtropical climate conditions, while thermophillous floristic elements were not well represented. The studied palaeoflora shows a stage in the long-term evolution of the Late Neogene floras in the Balkan Peninsula, connected with the reduction of palaeotropical elements, the dominance of arcto-tertiary taxa in the vegetation structure, and the increased distribution of herbaceous vegetation. Palaeoclimate results obtained with the Coexistance Approach show that the climate in the Tundzha Basin was warm temperate and permanently humid.
The results from the palaeoecological analysis of the flora and the quantitative data on the palaeoclimate recorded from the top of the sediment succession (the outcrop SR-1) show a trend in climate change towards the decline of temperature and of humiditity and a wider distribution of herbaceous vegetation.
The authors are grateful to V. Mosbrugger and T. Utescher (Germany) for the kindly provided access to the Palaeoflora database and Climstat software used by us for climate reconstructions. This work is a contribution to the International Network Programe NECLIME (www.neclime.de) and Project B-1525/2005 NSF of Bulgaria. The authors are thankfull for the critical reading and the valuable comments of three anonymous peer-reviewers.
DI carried out pollen analysis of core C-432, ML carried out pollen analysis of cores C-96, C-127 and C-146. Interpretetaions, analysis, discussion and conclusions have been done by DI. The design and draft of the manuscript was prepared by DI. All authors read and approved the final manuscript.
The authors declare that they have no competing interests.
Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.
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