Late Eocene to early Miocene climate and vegetation of Bulgaria

Review of Palaeobotany and Palynology 153 (2009) 360–374

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Review of Palaeobotany and Palynology j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / r ev p a l b o

Late Eocene to early Miocene climate and vegetation of Bulgaria Vladimir Bozukov a,⁎, Torsten Utescher b, Dimiter Ivanov a a b

Institute of Botany, Bulgarian Academy of Sciences, 1113 Sofia, Bulgaria Steinmann Institute, Bonn University, 53115 Bonn, Germany

a r t i c l e

i n f o

Article history: Received 9 January 2008 Received in revised form 23 October 2008 Accepted 24 October 2008 Available online 5 November 2008 Keywords: Bulgaria Cenozoic macrofloras palaeoclimate vegetation

a b s t r a c t The Bulgarian Palaeogene flora reveals important information concerning floristic transformation and climatic change in southeastern Europe. After the Eocene/Oligocene transition, an invasion of arctotertiary floristic elements took place in the European vegetation. This climatically forced, gradual change from a palaeotropical to an arctotertiary type of vegetation is well reflected in the Bulgarian floras. In the present paper, we analyze 12 palaeofloras covering the time span from the late Eocene to the early Miocene from a palaeoecological and palaeoeclimatic viewpoint. The vegetation change in the Palaeogene was triggered by both global climatic evolution and regional patterns generated by a changing palaeogeography. The signals from both processes are obviously overlapping and in some cases make it impossible to separate their imprints. Hygromesophytic forests without arctotertiary floristic elements still played the major role in the zonal vegetation in the late Eocene. No significant change in vegetation cover at the Eocene/Oligocene transition is apparent, and hygrophytic to hygromesophytic palaeocoenoses and oak-laurel forests dominated the palaeovegetation. Mesophytic to mesoxerophytic communities became important in the early Oligocene, along with the decrease of hydrophytic to hygromesophytic formations. A similar picture is obtained for the late Oligocene, but deciduous arctotertiary elements then reached a higher proportion for the first time. The climatic evolution is more or less consistent with the observed vegetation changes. Warm-temperate conditions persisted throughout this time span, but they show a cooling trend in the late Oligocene, most probably an imprint of global climatic cooling at that time. With respect to changing palaeogeographical patterns, the regressive trend during the early Oligocene is contemporaneous with a slight decrease in annual precipitation. Xerophytic phytocoenoses are reported from most of the sites, but climatic data supporting the existence of such associations are reconstructed only for the Bourgas and Borovets floras. In all the other floras, the majority of taxa indicate that no really dry season existed. © 2008 Elsevier B.V. All rights reserved.

1. Introduction Changes in the vegetation cover of Europe during the Palaeogene stimulate the scientific interest in the fossil floras from this time-span. In the period between the Palaeocene/Eocene Thermal Maximum and the glaciation of Antarctica, culminating in the late Oligocene (Zachos et al., 2001), a worldwide transformation of the floras took place. Early to middle Eocene forests in the mid-latitudes of Europe are strongly thermophilic and dominated by evergreen Fagaceae and Lauraceae. Also, taxa presently occurring in tropical forests are diverse (Mai, 1995; Collinson and Hooker, 2003). Mangrove associations are described from various Eocene sites in Western and Southern Europe underlining the paratropical character of the mid-latitude vegetation belt (e.g., Mai, 1995; Collinson and Hooker, 2003; Utescher and Mosbrugger, 2007). Especially in the study area, mangroves were

⁎ Corresponding author. Tel.: +359 29793768; fax: +359 2719032. E-mail address: [email protected] (V. Bozukov). 0034-6667/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.revpalbo.2008.10.005

common along the coastlines of the Tethyan archipelago (Palamarev et al., 2000). Associated with global cooling from the Mid-Eocene on, mixed deciduous and evergreen forests succeedingly replaced the “paratropical”, evergreen vegetation in various regions of Europe. As indicated by the presence of typical temperate elements these forests appear less thermophilic when compared to the previous type (e.g., Mai and Walther, 1978, 1985; Mai, 1995; Collinson and Hooker, 2003). Sclerophyllous elements, commonly referred to as indicators for seasonally dry climates, sporadically occur in Mid-Eocene floras of Europe but are more common in the late Eocene to Oligocene floral record, especially in the South (Mai, 1995; Hably and Marron, 1998; Mihajlovic, 1990, 1992). Although the environmental interpretation of some indicative taxa, such as notophyllous forms, is still a matter of debate (e.g., Collinson and Hooker, 2003; Utescher et al., 2007b), it is assumed that such associations existed under humid climate conditions with a slightly drier season (e.g., Collinson and Hooker, 2003). The Bulgarian Palaeogene flora reveals important information concerning this climatically forced floristic transformation, which is

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determined also by a specific palaeogeographical configuration that affects floristic exchange pathways. The end of the Eocene marks the beginning of a transformation of the European flora from a palaeotropical to an arctotertiary type, together with the first appearance of species belonging to the genera Alnus, Betula, Cornus, Fagus, etc. During the Oligocene, the distribution area of arctotertiary taxa expanded, and their diversity increased. This transformation can be explained by environmental changes such as climatic shifts, tectonic reorganization of the Tethyan realm, triggering changes in the palaeogeographic settings (Meulencamp and Sissingh, 2003), as well as by intensive volcanic activity. Palaeobotanical studies carried out so far on Bulgarian Palaeogene macrofloras have generally focused on the floristic analysis and on vegetational and ecological reconstructions (Palamarev, 1961; Kitanov and Palamarev, 1962; Palamarev, 1967, 1973; Černjavska et al., 1988; Palamarev and Staneva, 1995; Palamarev et al., 1998, 1999a,b, 2000, 2001). In addition, some taxonomically oriented studies have been published (e.g. Palamarev and Petkova, 1966, 1975). Thus, an integrative study of the vegetation dynamics and climatic evolution in the context of a changing palaeogeography (e.g., geographic isolation or connection) can provide new insights in plant-speciation processes and floristic evolution in the Balkan area. In detail, we focus on the following questions: (1) How is the transformation of the European flora during the Palaeogene reflected in changing palaeocoenoses in the archipelago of the Eastern Paratethys?; (2) Are vegetation changes connected to significant temperature or precipitation changes on a global or on a regional scale?; (3) Can climatic signals and vegetation changes be identified that are referred to fundamental changes in the palaeogeographical settings? To address these questions, we study diverse macrofloras from 12 localities in SW Bulgaria covering the time span from the late Eocene to the late Oligocene/Miocene transition in order to summarize and discuss the vegetational evolution on the basis of the publications cited above. To re-evaluate the floristic record with respect to palaeoclimate a quantitative approach is used. 2. Notes on the geology of the study area In the Palaeogene, the area of the Balkan Peninsula was part of the Northern Peri-Tethys Platform. The evolution of this area is compli-

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cated because of palaeogeographic reorganisations and basin rearrangements (Meulencamp and Sissingh, 2003). Regional uplift caused the closure of marine straits and the disappearance of the connection between Tethys, Arctic, and Atlantic oceans in the late Eocene–early Oligocene. At that time (Eocene–Oligocene transition), the Paratethys appeared as a new discrete palaeogeographic and palaeobiogeographic realm (Fig. 1). The intensified uplift of the Alpine chains forced the isolation of the basin in a north-south direction. The Rhodope massif traditionally is considered as a microplate that has remained in a stable connection to Eurasia since the Triassic (Golonka, 2004). In the Cenozoic, there is evidence for a clockwise rotation combined with extensional tectonics linked to the southwest motion of the Anatolian plate (Dilek, 2006). Since the late Eocene, a northward movement of about 2° latitude can be assumed for the Rhodopes being part of the stable W Eurasian plate. The palaeogeographic evolution of southern Bulgaria is a result of gradual and irregular subsidence of the whole Macedonian–Rhodope (Morava–Rhodope) area (Zagorchev, 1998a). In the Palaeocene to middle Eocene, a West Thracian marine basin covered the southern part of the region, and rapidly subsiding grabens in the central and northern parts were filled with thick olisthostrome formations (Zagorchev, 1998b). The grabens newly-formed in Bartonian/Priabonian time were filled with terrigenous continental sediments resulting from erosion and lateritic weathering under tropical conditions. Later marine transgressions in the Priabonian and early Oligocene (Rupelian) flooded vast territories in the eastern Rhodopes and the Piyanets and Padesh area in SW Bulgaria, while in parts of the Central Rhodopes, sedimentation continued in freshwater basins. In the late Oligocene and earliest Miocene, after the regression, coal-bearing basins (e.g. Bobovdol, Pernik, and Brezhani Basin) formed along important tectonic fractures, and in the earliest Miocene, the sedimentation ended with transpression-related folding and thrusting (Zagorchev, 1992, 1998b). The Palaeogene floras described here originate from sediments in several isolated outcrops: from the so-called Bourgas–Kazanlak Strip, located south of the Balkan Mountains (coal mine Cherno More, Bourgas area), the large tectonic depression in the eastern and central Rhodopes (e.g. Hvoyna, Momchilovtsi), graben structures in the West Rhodopes (like Eleshnitsa, Boukovo etc.), and some coal-bearing basins in SW Bulgaria. They originate from marine, brackish, and continental sediments of various facies type deposited in several

Fig. 1. Palaeogeographic scheme of the Tethys and Paratethys area in the early Oligocene with ocean/land distribution and seaways (from Rögl, 1999). The position of studied area is marked by a square.

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provinces under the conditions of specific volcano–tectonic regimes. The isolated occurrence of the macrofloras, as well as the rapid facies changes in both, vertical and horizontal directions, lead to severe difficulties concerning the correlation of the strata (Černjavska, 1975, 1977). In addition, the scarce faunistic record in terrigenous formations complicates a successful correlation. Radiogeochronological data are available for some areas, but they have a typical minimum analytical error of ±1–1.5 Ma (see Zagorchev, 1998a). Some palynological studies tried to overcome this problem (Černjavska, 1975, 1977). 3. The floras analysed The selected palaeofloras (Table 1) are mainly leaf floras, just one carpoflora is included (Bourgas flora: Palamarev, 1973). For two of the floras palynological and ichnophytological data exist (Hvoyna, Central Rhodopes: Černjavska et al., 1988; Bobovdol, SW Bulgaria: Palamarev et al., 1998). In addition, palynological data are available for some of the floras allowing to complete their taxonomic inventory and to refine the age determination (Boukovo: Palamarev et al., 1999a; Bourgas: Palamarev, 1973; Eleshnitsa: Palamarev et al., 1999b, 2000). To reconstruct vegetation evolution and palaeoclimatic conditions for the Bulgarian Palaeogene only macrofloras (leaves and carpological remains) are considered for palaeoclimatic reconstructions (Table 1). Most of the floras analysed originate from continental deposits preserved in the Palaeogene graben systems of the western and central parts of the Rhodope region. All the other floras originate from coal-bearing sediments in SW and SE Bulgaria (Fig. 2). The fossil flora from the Cherno More coal mine, located near the town of Bourgas (SE Bulgaria) includes 38 seed/fruit taxa (Palamarev, 1973). Based on mammal fauna, the flora is dated as Priabonian, (Nikolov, 1967). This is confirmed by palynological studies (Černjavska, 1970). The Eleshnitsa and Boukovo sites are located in the Mesta Graben. In this area, the leaf bearing strata rest on volcanic rocks radiogeochronologically dated as Rupelian (K–Ar method, 33–28 Ma: Harkovska, 1983; Harkovska et al., 1998; Pecskay et al., 2000). For Eleshnitsa, leaves from two different stratigraphic levels are analysed, one of them (Eleshnitsa-II) above the volcanic formation, the other (Eleshnitsa-I) right below. The lower level is assumed to be of latest Eocene/earliest Oligocene age (according to radiogeochronology; cf. Palamarev et al., 2000), while the upper level belongs entirely to the lower Oligocene. From both horizons, a total of 43 taxa are described. The fossil flora from Boukovo comprises 45 taxa, and, based on the available radiogeochronological data, is of early Oligocene age. The Palaeofloristic analysis and floristic correlations support an early Oligocene age (for details see Palamarev et al., 1999a).

The floristic record (micro- and macrofossils) of the Hvoyna Basin originates from small outcrops situated in the vicinities of the villages of Hvoyna, Malevo, Orehovo, and Pavelsko (central Rhodopes). The age of the sediments is defined by palaeobotanical means as latest Eocene to early Oligocene (Černjavska et al., 1988). The Borino–Teshel flora consists of 27 different morphospecies. It originates from different small outcrops exposing continental sediments of the Borino–Teshel Graben in the western Rhodopes. The Borino–Teshel area belongs to Bratsigovo–Dospat volcanic area. The volcanic activity began in shallow, lacustrine to marshy environments and continued in subaerial conditions (Harkovska, 1984; Harkovska et al., 1998). K/Ar data indicate Rupelian to earliest Chatian age (30– 27 Ma) for the volcanic activities. The flora is dated by palaeobotanical means as early Oligocene (Palamarev et al., 2001). The floristic record from the sandstones near the village of Momchilovtsi (central Rhodopes) comprises 12 taxa. Kitanov and Palamarev (1962) suggest an early Oligocene age for the fossiliferous sediments, also based on floristic correlations. Radiogeochronological data (Harkovska et al., 1998) for Levochevo caldera and Momchilovtsi– Davidkovo dyke swarms provide K/Ar ages of 33.4 ± 1.4–30.2 ± 1.3 Ma, which is consistent with palaeobotanical interpretations. The Polkovnik Serafimovo flora comprising 51 different morphospecies originates from different smaller outcrops exposing continental sediments of the Polkovnik Serafimovo Graben in the central Rhodopes. The flora is dated as early Oligocene, also by palaeobotanical means (Palamarev and Staneva, 1995). Radiogeochronology of batites, andesites and rhyolites near Smolyan (ca. 10–12 km to NW of Polkovnik Serafimovo) gives Rupelian age (32.1 ± 1.3–30.2 ± 1.3 Ma) (Harkovska et al., 1998, Table 1). A total of 127 species and 6 genera fossil make up the palaeoflora from coal-bearing sediments in the Pirin coal mine, close to the village of Brezhani (SW Bulgaria). Based on fossil fauna (fish remains), Gaudant and Vatsev (2003) suggest a Rupelian to Chatian age for the plant-bearing layers. Based on detailed lithostratigraphic correlations with other Oligocene basins along the Pirin massif (the Struma and Mesta graben systems) Zagorchev (1998a, 2007) dates the strata as late Oligocene (according to the chronology of Gradstein et al., 2004). On the basis of palaeofloristic analysis Palamarev (1973) assumed a middle Oligocene (Rupelian) age. The taxonomic composition of the macroflora from Borovets (located near the Borovets winter resort, SW Bulgaria) includes a total of 37 species and 3 genera with open nomenclature. For this floristic complex Palamarev (1961) first cited an Oligocene age, later specified as late Oligocene (Palamarev et al., 2005). The Bobovdol flora was collected from four sites in a sedimentary basin of the same name comprising 54 taxa. For three of them, a late Oligocene age can be assumed, and for the fourth site, Babino, a

Table 1 Floras analysed with data about geographic position, age and method of age determination, and reference literature Locality name

Longitude (degree)

Latitude (degree)

Altitude (m)

Age/method

Reference

Bourgas Hvoyna Eleshnitsa-I Eleshnitsa-II

27° 23′ 24° 41′ 23° 35′ 23° 35′

42° 37′ 41° 52′ 41° 52′ 41° 52′

50 674 861 861

Late Eocene/mammals Late Eocene–early Oligocene/palynology, palaeobotany Late Eocene–early Oligocene/Radiogeochronology (K/Ar method) Early Oligocene/palynology, radiogeochronology (K/Ar method), palaeobotany

Borino-Teshel Momchilovtsi Boukovo Polkovnik Serafimovo Brezhani

24° 20′ 24° 47' 23° 43′ 24° 47' 23° 11'

41° 41° 41° 41° 41°

41′ 40' 43′ 32' 52'

1075 1352 1021 965 625

Early Oligocene/radiogeochronology (K/Ar method) Early Oligocene/radiogeochronology (K/Ar method) Early Oligocene/radiogeochronology (K/Ar method) Early Oligocene/radiogeochronology (K/Ar method) Late Oligocene/fauna, lithological correlations

Borovets Bobovdol Bobovdol–Babino

23° 33′ 23° 01′ 22° 59′

42° 20′ 42° 22′ 42° 21′

1350 694 694

Late Oligocene/palaeobotany Late Oligocene/fauna, palynology, lithological correlations Late Oligocene–early Miocene/fauna, palynology, lithological correlations

Nikolov (1967) Černjavska et al. (1988) Harkovska (1983) Ivanov and Černjavska (1972); Harkovska (1983); Palamarev et al. (2000) Harkovska et al. (1998) Harkovska et al. (1998) Harkovska et al. (1998) Harkovska et al. (1998) Gaudant and Vatsev (2003); Zagorchev (1998a, 2007) Palamarev (1961) Černjavska (1977); Vatsev et al. (2003) Černjavska (1977); Vatsev et al. (2003)

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Fig. 2. Schematic map showing the distribution of different palaeogeographic environments in late Eocene–early Oligocene time in the eastern parts of the Balkan Peninsula (redrawn from Zagorchev, 1992, 1998a). Legend: A – volcanic formations, B – continental deposits, C – marine deposits, D – faults; Floras analysed: 1. Bourgas; 2 . Hvoyna; 3. Eleshnitsa-I; 4. Eleshnitsa-II; 5. Boukovo; 6. Borino–Teshel; 7. Momchilovtsi; 8. Polkovnik Serafimovo; 9. Brezhani; 10. Borovets; 11. Bobovdol; 12. Bobovdol–Babino.

position at the late Oligocene–early Miocene boundary is likely. These datings are based on faunistic data (fish and mammal remains) (see Černjavska, 1977 and discussion therein). Oogons of Chara algae, as well as macro- and micropalaeobotanical analyses (Palamarev et al., 1998) confirm age determination. 4. Methods 4.1. Vegetation reconstruction The method of vegetation reconstruction applied here is widely based on autecology and present ecological requirements of nearest living relatives of the fossil taxa. As for many Palaeogene European floras, the floras presently analysed include taxa with Nearest Living Relatives (NLR's) today growing in disjunct areas, e.g., in eastern Asia and Northern America. The fossil plant associations represent unique floristic combinations having no fully recent analogues. Some extant species with Palaeogene ancestors have a relict distribution in refugia with extremely limited spatial distribution. On the basis of autecological analysis, we reconstruct the most probable palaeocommunities. Here we consider the individual ecological requirements of different fossil taxa and make and attempt to bring together plants with similar ecological and edaphic requirements in order to identify the most probable palaeocoenotic combination. Consequently, the reconstructed palaeocoenoses can be compared to the recent communities most closely related to the fossil ones. The modern analogue communities provide information about the ecologic and climatic conditions of their habitats. The information obtained from modern

analogue plant communities can be transferred to the fossil communities in order to understand past environments. 4.2. Coexistence approach (CA) To obtain quantitative palaeoclimate data that can be compared to the results obtained from vegetation analysis the Coexistence Approach (CA) is used (Mosbrugger and Utescher, 1997). The CA also follows the NLR concept. It is based on climatic requirements of modern plants that are identified as NLRs of the fossil taxa recorded in a flora. This is accomplished by listing climatic conditions of the areas in which these extant representatives exist today. By using a database of extant taxa and their climatic requirements, “coexistence intervals” can be calculated for various climate parameters that allow the majority of plant taxa considered to co-existed. By overlapping all the climatic ranges of the taxa, the coexistence interval is obtained, representing the most probable palaeoclimatic range of the fossil flora studied. For more details on the CA approach, the reader is referred to the above literature. In the present study, all the floras are analyzed with respect to 6 climatic variables. These are mean annual temperature (MAT), temperature of the coldest month (TCM), temperature of the warmest month (TWM), mean annual precipitation (MAP), precipitation in the driest month (MPdry), and precipitation in the warmest month (MPwarm). The results obtained for the single climatic variables are given in Table 3. The number of taxa contributing climate data ranges between 10 and 71 (mean: 32; σ = 16.4) (cf. Table 3.), so the diversity of the Nearest Living Relatives known for each flora is sufficient to

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produce reliable results when the CA is applied (Mosbrugger and Utescher, 1997). 5. Results 5.1. Vegetation structure and dynamics 5.1.1. Late Eocene to early Oligocene floras The flora from the Bourgas area (Cherno More coal mine, SE Bulgaria) is the only flora of Eocene age in the present record. From the taphonomic point of view this fossil flora is a polytopic complex. The following phytocoenotic combinations are present: (1) hydrophytic to hygrophytic palaeocoenoses with the presence of the genera Brasenia, Nuphar, Palaeonymphaea, Aldrovanda, Stratiotes, Cariocoidea, Potamogeton, Limnocarpus; (2) hygrophytic to hygromesophytic swampy palaeocoenoses (genera: Myrica, Osmunda, Nyssa, and Cyrilla, distribution limited); (3) hygromesophytic forest palaeocoenoses. Dominant genera: Quercus, Castanopsis, Engelhardia, Laurus, Gironniera, Omalanthus, Mastixia; accessory: Michelia, Daphnogene, Ficus, Stephania, Pilea, Meliosma, Diospyros, Sambucus, Zanthoxylum, Symplocos, Ilex. Araliaceae were part of the undergrowth. The ground floor vegetation was predominantly composed of ferns (Schizaeaceae); (4) xerophytic shrubby palaeocoenoses (with very limited distribution), included the genera Burtonella (cf. Cleome), and Ephedripites (cf. Ephedra) (Palamarev, 1973). The existence of this community is questionable, because the conclusion about its presence refers to a leaf imprint of Burtonella and pollen grains of Ephedripites. These can only be regarded as potential xerophytic elements growing on specific (e.g. rocky or eroded) places, but do not necessarily indicate dry climatic conditions. The fossil floras from Hvoyna Basin (central Rhodopes) and Eleshnitsa-I (Mesta Graben, SW Bulgaria) are considered to be of transitional late Eocene–early Oligocene age. Based on the flora composition the following associations are identified (Černjavska et al., 1988; Palamarev et al., 2000): (1) hydrophytic to hygrophytic palaeocoenoses. Swamp community (Myrica, Nyssa, and Acer tricuspidatum), distributed in the central Rhodopes (Hvoyna) and herbaceous/shrub palaeocoenoses (Acrostichum, Myrica, Nymphaea, and Bumelia) were distributed in Eleshnitsa-I (Mesta Graben). Acrostichum lanzaeanum (Visiani) E. Reid and Chandler was the most typical component of the latter community and is indicative for a brackish environment. Although the genus Acrostichum is a mangrove element, it also invades the brackish zones in estuaries of tropical rivers; (2) hygrophytic to hygromesophytic palaeocoenosis. Riparian communities with the genera Platanus, Populus, Juglans, Pterocarya, Cyclocarya, Dodonea, Sabal and Palmophyllum were distributed in the central Rhodopes (Hvoyna). For the Eleshnitsa-I site (Mesta Graben), a coastal palm palaeocoenoses composed of Phoenicites spectabilis, P. salicifolius, Phoenicites sp., and Sabal longirahis with hygromesophytic character is reported. These communities were possibly related to the remains of the Eocene sea forming isolated coves with brackish waters, or spread along estuaries of palaeo-rivers where, besides the palms, the mangrove fern Acrostichum lanzaeanum grew; (3) hygromesophytic to mesophytic palaeocoenoses. Woody community dominated by representatives of Magnoliaceae, Lauraceae, Ulmaceae, Sabiaceae, Aquifoliaceae, Ebenaceae, Araliaceae, Celastraceae, Apocynaceae, Arecaceae, and diverse ferns. Probably some of the ferns represent specific stages in these forest communities. Pinus, Doliostrobus, Libocedrites, Chamaecyparis and Sciadopitys were present in some of the habitats in the Hvoyna area. Lauraceae, Dryophyllum and Eotrigonobalanus dominated the hygromesophitic forest in EleshnitsaI with Celastrophyllum in the undergrowth, together with the ferns Rumohra and Cyclosorus. Evergreen palaeotropical elements prevailed in this type of forest palaeocoenoses, while deciduous arctotertiary representatives played a minor role; (4) xerophytic woody and shrubby palaeocoenoses with species of the genera Acacia, Caesalpi-

nites, Cassiophyllum, Myrtus, Ziziphus, Rosa, Rubus, Pyrus, Boehmeria, Punica, and Olea. This type of community was distributed only in the central Rhodopes and was probably associated with drier inland places. There are five floras of early-Oligocene age (see Table 1), namely Eleshnitsa-II and Boukovo (Mesta Graben system), Borino–Teshel (western Rhodopes), Momchilovtsi and Polkovnik Serafimovo (central Rhodopes). Taphonomically, the fossil floras represent a polytopic complex (Kitanov and Palamarev, 1962; Palamarev and Staneva, 1995; Palamarev et al., 1999a, 2000, 2001). The following phytocoenotic combinations can be distinguished on the basis of autecological analysis: (1) hydrophytic to hygrophytic herbaceous and shrub palaeocoenoses with Myrica, Nymphaea, Bumelia, Cyperites, Aponogeton, and Typha. These were the more common plants of these communities, but their proportion varies, probably depending on the local aquatic environment represented in each case. E.g., Equisetum, Cyclosorus, and Bambusa are most common in Momchilovtsi, while Platanus, Myrica and Bumelia prevailed in Polkovnik Serafimovo; (2) hygromesophytic to mesophytic forest palaeocoenoses were the most important communities in the early Oligocene, as they represent the zonal vegetation. A characteristic feature of these coenoses is their high taxonomic diversity. Commonly the dominant plants belong to the families Lauraceae, Fagaceae, and Apocynaceae (Daphnogene, Laurophyllum, Litsea, Persea, Ocotea, Neolitsea, Eotrigonobalanus, Dryophyllum, and Apocynophyllum). The representatives of ferns (Cyclosorus, Cyathea and Blechnum), as well as species of the families Myricaceae, Platanaceae, Betulaceae. Rutaceae, Sapindacee, Oleaceae, Juglandaceae, Theaceae, Celastraceae and Arecaceae should be also allocated to this group. Some of the latter plants were part of the undergrowth and ground-floor vegetation. Evergreen palaeotropical elements prevailed in this type of plant community while deciduous, arctotertiary components (Platanus, Ulmus, and Ailanthus) were of minor importance. The appearance of arctotertiary elements in Europe is related to the Eocene/Oligocene transition (Mai, 1995), followed by their wide extent in the Oligocene. Some representatives of the Fabaceae (Desmodium, Dalbergia, Sophora, Gleditsia and Cassiophyllum) played the role of accompanying species in this plant association and could have formed riverside palaeocoenoses of a riparian or gallery-forest type growing under favourable microclimate conditions (e.g. the Boukovo palaeoflora); (3) Semi-xerophytic or xerophytic and sclerophytic palaeocoenoses were a diverse and heterogenious group. This community comprises species of the genera Myrtus, Colutea, Acacia, Caesalpinites, Casssiophyllum, Sophora, Ceratonia, Cotinus, Carpinus (ex gr. orientalis), Pistacia, Celastrophyllum, and Ziziphus. In most cases, these taxa formed sclerophyllous shrub vegetation dominated by evergreens. The development of this group was connected to open woodless terrains, under the strong impact of volcanic activity and sub-arid climate in times of drought. The most diverse in the subxerophytic community is recognized in the Boukovo flora, while in Borino–Teshel and Momchilovrsi this vegetation type is not represented (Table 2). 5.1.2. Late-Oligocene to late-Oligocene/early-Miocene floras Four of the floras analysed are related to this time span (see Tables 1 and 2): Brezhani (western Pirin Mts, SW Bulgaria), Borovets (Rila Mts, SW Bulgaria), Bobovdol and Bobovdol–Babino (Bobovdol basin, SW Bulgaria). The floristic composition and the ecologic characteristics of the different species are considered as indicators for the presence of the following main palaeocoenoses (Palamarev, 1961, 1967; Palamarev et al., 1998): (1) hydrophytic herbaceous palaeocoenoses composed of Startiotes, Nymphaea, Nelumbo, Nuphar, Myriophyllum, Sparganium, and Salvinia; (2) hydrophytic to hygrophytic (swampy) forest with Glyptostrobus, Taxodium, Comptonia, Myrica, Cyrilla, Andromeda, and Nyssa. In addition herbaceous species of the genera Equisetum, Typha, Phragmites, and Spirematospermum were recognized in the Bobovdol flora but not at Bobovdol–Babino. Hydrophytic and hydrophytic to

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Table 2 Palaeocommunities recorded in the floras analysed

hygrophytic communities absent from the Borovets flora (Table 2); (3) hygromesophytic to mesophytic forest and shrubs palaeocoenoses composed of representatives of genera Schizaeaceae, Cunninghamia, Persea, Litsea, Daphnogene, Neolitsea, Fagus, Trigonobalanopsis, Quercus, Eotrigonobalanus, Castanopsis, Juglans, Engelhardia, Juglans, Platycarya, Pterocarya, Ficus, Alangium, Mastixia, Carpinus, Alnus, Populus, Salix, Platanus, Trema, Zelkova, Gleditsia, Acer, Ilex, Chaneya, Dodonea, Diospyros, and Apocynophyllum. Their representatives were widespread in plains and hilly areas surrounding the basins. The most diverse is the flora from Bobovdol. In the Borovets flora the genus Eotrigonobalanus was most important in the forest vegetation, while in the Bobovdol–Babino flora some of the arctotertiary taxa (Carpinus, Alnus, Populus, Acer) disappeared, but Lauraceae, Sideroxylon, and Engelhardia became dominants; (4) mesoxerophytic

forest and shrub palaeocoenoses composed of thermophyllous elements (Libocedrites, Ziziphus, Quercus, Plumeria, and Chamaerops) existed in the Brezhani and Bobovdol flora. These palaeocoenoses possibly occupied biotopes strongly warmed by insolation and special edaphic conditions. In the Brezhani macroflora, some subxerophytic shrubby palaeocoenoses with Rhus and Myrtaceae) were recognized in addition (Palamarev, 1967). Xerophytic palaeocoenoses of Grevillea, Cassiophyllum, Eugenia, and Pinus hepios Unger were recorded in the Borovets flora, but the rareness of these plants in the fossil record points to a limited distribution of this vegetation type existing under specific edaphic and microclimatic conditions. The presence of various palaeocoenoses in the different regions over the time-span regarded is shown in Table 2. Summarizing up the

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Table 3 Temperature and precipitation data calculated by the CA Locality name

Nt

MAT L (°C)

MAT R (°C)

TCM L (°C)

TCM R (°C)

TWM L (°C)

TWM R (°C)

MAP L (mm)

MAP R (mm)

MPdry L (mm)

MPdry R (mm)

MPwarm L (mm)

MPwarm R (mm)

Bourgas Hvoina Eleshnitsa-I Eleshnitsa-II Boukovo Borino–Teshel Momchilovtsi Polkovnik Serafimovo Brezhani Borovets Bobovdol Bobovdol–Babino

19 51 30 31 38 24 10 40 71 20 27 23

17.5 17 17 16.4 17 17 15.9 17 16.4 15.9 15.7 15.6

19.2 18.5 21.1 21.1 18.8 21.1 21.3 19.5 16.5 16.5 20.5 21.1

11.7 12.6 12.6 12.2 12.2 12.6 12.2 12.6 5.5 1.8 7.7 5

11.7 12.6 12.6 12.6 12.6 12.6 13.3 12.6 10.9 4.8 13.3 12.6

25.4 26 27.8 25.6 27.2 26 25.6 25.6 24.7 25.6 25.6 25.7

28.1 27.9 27.9 28.1 27.9 27.9 28.1 26.1 25.2 27.7 27.5 28.1

1215 1183 1360 1360 1122 1122 1360 1122 1122 897 1217 1122

1864 1356 1377 1384 1355 1384 1613 1356 1187 1237 1237 1355

10 32 32 32 32 32 32 32 32 10 32 32

55 38 42 43 38 38 40 38 37 43 55 38

107 105 90 105 108 90 108 105 105 85 90 108

107 131 131 131 131 187 196 116 116 131 169 131

Nt: number of taxa contributing climate data; for each parameter the minimum (L) and maximum (R) of the coexistence interval are given.

facts it can be stated that hygromesophyte to mesophyte plant associations occurred in all the floras analysed except the oldest Bourgas flora. The latter plant associations most probably represent the zonal vegetation. They are characterized by a high taxonomic diversity, and, considering their proportion in the fossil flora, we can assume that they had a wide spatial distribution when compared to the rest of the palaeocoenoses. In the late-Eocene flora from Bourgas, hygromesophyte forests played a major role in the zonal vegetation, which is a typical feature of ancient Eocene floras. Another characteristic feature of this flora is the absence of arctotertiary elements, confirming its ancient character. The xerophytic component occurs in all the floras studied. These communities are part of the extrazonal vegetation. In the fossil record they are sparse and have low species diversity. The maximum extension of the hygrophyte to hygromesophyte palaeocoenoses is concentrated around the Eocene/ Oligocene boundary, when they probably were part of the zonal

vegetation. Their importance in the vegetation decreased in the early Oligocene. The hydrophyte plant communities are recorded all over the time span studied. In general they have a low taxonomic diversity, and apparently their distribution was closely connected to the presence of aquatic environments and lake/swamp systems. 5.2. Palaeoclimate reconstruction Palaeoclimate data are calculated for 6 different climate variables (MAT, TCM, TWM, MAP, MPdry, MPwarm) using the Coexistence Approach (CA) (cf. chap. 4.2; Mosbrugger and Utescher, 1997). The results obtained are given in Table 3, and climatic ranges for the single sites are shown on Figs. 3–8. As stated above, the results are based on 10 to 71 (mean: 32; σ = 16.4). Nearest Living Relatives of the fossil taxa recorded at the localities studied contribute with climate data. The number of taxa coexisting within the resulting variable

Fig. 3. Coexistence intervals for mean annual temperature (MAT).

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Fig. 4. Coexistence intervals for the mean temperature of the coldest month (TCM).

ranges varies from 89.5% to 100%, all NLRs coexist in 35 out of 60 cases. According to Mosbrugger and Utescher (1997) the results obtained at such a coexistence level can be regarded as highly significant. In 5 out of 60 cases, 2 or even 3 CA intervals have an equal number of coexisting

taxa. This may point to the fact that taxa from various habitats differing in regional climate and moisture supply are combined in the palaeoflora (e.g., Ivanov et al., 2002). Other reasons can be cited, such as uncertainties in modern climatic ranges of taxa, changes of climatic

Fig. 5. Coexistence intervals for the mean temperature of the warmest month (TWM).

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Fig. 6. Coexistence intervals for mean annual precipitation (MAP).

requirements through earth history, and taxonomical uncertainties in the fossil record can (Mosbrugger and Utescher, 1997). The application of the CA allows for an identification of climatic outliers. Concerning temperatures, this is the case for Cyperacites

chavannesii (Heer) Schimper, referred to the extant species Cladium mariscus (Palamarev et al., 1998, 1999b: Bobovdol, Eleshnitsa-I), presently occurring in significantly cooler climates. The genus Dalbergia, cited as NLR for legume-type leaves recorded in the Eleshnitsa-I

Fig. 7. Coexistence intervals for mean precipitation in the driest month (MPdry).

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Fig. 8. Coexistence intervals for mean precipitation in the warmest month (MPwarm).

and the Boukovo floras (D. bella Heer: D. rectinervis Ettingshausen) today comprises ca. 100 mainly tropical species requiring warmer temperature conditions than the resulting values. The same is true for Acrostichum aureum L. (Eleshnitsa-I: A. lanzaeanum) presently restricted to tropical salt marshes and mangroves. Calculation of precipitation variables indicate two outliers Tetraclinis (T. brachyodon (Brongniart) Mai and Walther; Boukovo), today a monotypic relict growing in a Mediterranean type of climate. However, throughout the Cenozoic the genus was widely distributed on the Northern Hemisphere. Thus, the extant taxon apparently does not express the full climatic range of its fossil ancestor. Very high precipitation rates indicated by Acrostichum aureum are not compatible with the vast majority of taxa. 5.2.1. Climate evolution from the late Eocene to the early Oligocene The climatic data resulting for the 9 floras allocated in this time span are listed in Table 3 and CA intervals are shown in Figs. 3–8. MATs calculated for the late Eocene to early Oligocene floras range from 16 °C and 22 °C, and 17 °C to 19 °C when regarding the narrower interval obtained for Bourgas, Hvoyna, and Boukovo. The TCM data range between 12 °C and 13 °C. Taxa delimiting the cooler interval are floristic elements presently growing in the temperate part of East Asia (Ailanthus, Cercidiphyllum, Zanthoxyllum), and the lower limit of the warmer interval is determined by the Mastixiaceae recorded in the flora. From this result, microclimatic differentiations or different altitudinal belts at the site could be inferred. TWM shows some variability in the results with lower values calculated for Polkovnik Serafimovo (at 26 °C), while high summer temperatures between 27 °C and 28 °C are obtained for the Eocene level of Eleshnitsa and the Boukovo flora. The other floras have rather wide, unspecific ranges. Concerning MAP, rates of more than 1100 mm result throughout. For the Eocene Bourgas flora, both Eleshnitsa sites and for Momchilovtsi there is evidence for very wet conditions (above 1400 mm), precipitation rates in the driest month ranged between 30 and 40 mm for most of the sites, in the late-Eocene Bourgas flora taxa with NLRs today restricted to summer-dry climates cause inconsistencies,

as is expressed by the presence of 3 equally significant CA intervals. Here Laurus and Meliosma indicate drier conditions (MPdry below ca. 20 mm), while Iodes and Homalanthus require higher MPdrys at present. 5.2.2. Climate evolution from the late Oligocene to the Oligocene/Miocene transition The time span from the late Oligocene to the Oligocene/Miocene transition is covered by four floras. For the late Oligocene Brezhani and Borovets floras, significantly cooler conditions result, with MAT around 16 °C and TCM from 3 °C to 5 °C at Borovets. For Brezhani two TCM intervals are obtained (5.5 °C–6.2 °C; 9.6 °C–10.9 °C). With TWM around 25 °C lowest summer temperatures of the present analysis result for Brezhani. Also from the late-Oligocene level at Bobovdol there is evidence for cooling. There, two CA intervals are obtained for MAT, the cooler (around 16 °C) close to the one calculated for Borovets. The delimiting factor for the cooler interval is Populus balsamifera L. as NLR of P. balsamoides Göppert occurring in the Bulgarian record for the first time. Engelhardia and evergreen Fagaceae give evidence for higher temperature. Again in this case, the existence of different altitudinal vegetation belts would explain this discrepancy. The second level at Bobovdol (Babino) positioned at the Oligocene/Miocene transition reveals MAT and TWM values similar to those obtained for the early Oligocene floras and point to increasing temperatures at that level. However, the shift of the lower limit of the TCM range to cooler conditions (5 °C) makes it probable that the high early-Oligocene temperature level of the cold season was not attained then. MAP rates tend to be lower at Brezhani and Borovets (b1200 mm), for the latter also drier intervals are obtained with MPdry. The interval 10–13 mm (Fig. 7) is related to the presence of Alnus gaudinii, with Alnus nitida as NLR presently occurring in monsoonal climates. However, the Oligocene ancestor possibly also tolerated more humid conditions in the driest month. For both Bobovdol sites, inferred precipitation rates are at a higher level. Summarizing the results obtained, the palaeoclimate can be described as warm-temperate. With MATs between 16.5 °C and

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22 °C, TCM ranging from 3 °C to 13 °C, TWM from 24.5 °C to 28 °C, MAP rates between 900 mm and 1900 mm and with MPdry between 30 mm and 40 mm for most of the floras, a Cfa type climate (Köppen, 1931) existed. As we see on Fig. 8, the warmest month is obviously not the driest for studied floras. For all the floras summer rainfall was above 80 mm which suggest that the driest month was not in the summer. A TCM was well below 20 °C, and a seasonality of temperature of at least 13 °C clearly indicates an extra-tropical climate throughout the time-span regarded. For the majority of the floras, the proportions of MAP and MPdry point to a certain seasonality of rainfall. However, only for one of the sites, namely Bourgas (late Eocene), there is some evidence for seasonally dry conditions at least for some of the ecosystems or habitats represented by the palaeoflora. 6. Discussion 6.1. Vegetation evolution and palaeoclimate change 6.1.1. Late Eocene to early Oligocene According to Palamarev (1973) and the present data (Section 5.1.), the zonal vegetation of the Bourgas area consisted of mixed hygrophytic to mesohygrophytic evergreen forests. Nearest recent analogues are the subtropical wet forests in southern China, Indochina, and tropical mountain forests of the Malayan Archipelago. The СА data obtained in the present study coincide with vegetation growing in a humid, warm-temperate (“humid subtropical”) climate, with MAT ranging between 17.5 and 19.2 °C and MAP between 1215 and 1864 mm. With an annual range of temperature of at least 13.5 °C a comparatively low seasonality is evident (Figs. 3–7). Černjavska et al. (1988) and Palamarev et al. (2000) proposed that, the dominantly hygrophytic to hygro-mesophytic palaeocoenoses recognized in the transitional late-Eocene/early-Oligocene floras from the Hvoyna Basin and Eleshnitsa-I, had their nearest recent communities as the subtropical mountain rainforests of Taiwan. The hygromesophytic to mesophytic palaeocoenoses correspond to the subtropical oak-laurel forest formations of Burma, Laos, and Vietnam, and the zerophytic palaeocenoses is compared to the subtropical evergreen sclerophyllous broadleaved forest formations in the province of Yuhnnan, China. The quantitative climatic data obtained in this study for the fossil flora with MAT 16.5–18.5 °C, TCM 4.8–7.7 °C, and MAP 1183–1356 mm (cf. Figs. 3, 4, 6) are very close to the recent climate under which the modern reference plant communities cited above grow. In the early-Oligocene floras from Borino–Teshel (Palamarev et al., 2001) and Momchilovtsi (Kitanov and Palamarev, 1962), xerophytic communities are absent. Palaeocoenotically, the vegetation was composed of hygromesophytic and hygromesophytic to mesophytic communities of a subtropical type and small swamp biotopes. According to the СА data, the climatic conditions of both floras are close to the previous ones developed at the Eocene/Oligocene transition (Figs. 3–7). Some minor differences in the values calculated can be noted for the Momchilovtsi flora – higher MAP rates and slightly lower TCMs (Figs. 4, 6). The higher rainfall rates obtained for the Momchilovtsi flora coincide well with the higher proportion of hygromesophytic palaeocoenoses as suggested by Kitanov and Palamarev (1962) (Table 2). According to Palamarev et al. (2001), the vegetation type in the Borino–Teshel Graben in general represents an evergreen Eotrigonobalanus-laurel forest of a hygromesophytic and mesophytic character, which also coincides with lower MAP as compared to the Momchilovtsi flora. The palaeovegetation of Polkovnik Serafimovo and Boukovo differs from the other early Oligocene floras by the absence of coenoses with hydrophytic, hygrophytic and hygromesophytic character. On the basis of the taxonomic composition of the mesophytic and mesophytic to mesoxerophytic forests, Palamarev and Staneva (1995) come to the

conclusion that the vegetation at Polkovnik Serafimovo developed under subtropical, moderately humid and warm conditions with periodic short-term dry climate intervals. The climate estimated for the Boukovo flora is described as subtropical, moderately humid, with a distinct dry phase evidenced by the presence of sclerophyllous shrubby vegetation of “macchia” type, dominated by evergreens (Palamarev et al., 1999a). СА data indicate generally warm conditions for the Boukovo flora together with MAP and MMPdry almost identical to of the other early-Oligocene floras analysed (Figs. 6 and 7). The development of sclerophyllous vegetation can be explained by the specific combination of high temperatures and local edaphic and/or orographic conditions, e.g., the development of plant communities on terrains and soils of low fertility. In summary, the climatic data based on vegetation and climate analysis for the early Oligocene indicate almost equal climatic conditions for most of the sites. Major differences occur in the temperature of the coldest month. The lowest TCMs are calculated for the areas of Momchilovtsi and Polkovnik Serafimovo, with the lower boundary of the CA intervals at 3.7 °C and 5 °C, respectively (Fig. 4). For the Momchilovtsi flora, higher annual precipitation rates result (Fig. 6). 6.1.2. Late Oligocene to Oligocene–Miocene transition For the late Oligocene flora from Brezhani, significant differences in temperature are obtained when compared to the early Oligocene floras (see previous chapter; Figs. 3 and 4). For both, MAT and TCM significantly lower values result. The major vegetation type of the Brezhani palaeoflora is closely related to the recent subtropical monsoon forests of Indochina (Palamarev, 1967). The second important vegetation type in this flora, the subxerophyte community, requires climatic conditions close to that of the present savannah forests of northern and southwest Australia and the Mediterranean (Palamarev, 1967). On the basis of these comparisons, Palamarev (1967) suggests annual temperatures of about 20–23 °C and annual precipitation rates up to 1800 mm for the flora. As is shown on Figs. 3 and 6, the CA data for MAT and MAP are significantly lower. Based on vegetation analysis (Palamarev, 1961), the late Oligocene climate in the Borovets area is supposed to be of subtropical character with dry periods. Thus it resembles in general the early-Oligocene climate as derived from the Boukovo and Polkovnik Serafimovo floras. It is worth mentioning that at all these three locations the vegetation consisted of two types of communities – hygromesophytic, mesophytic, and xerophytic. The СА data support this conclusion (Figs. 3–7). The closest similarities are observed especially for the climatic data from Borovets and Polkovnik Serafimovo. The main difference is in CMM – for the Borovets area the lower boundary of the interval of coexistence equals only 1.8 °C, while the MAP interval ranges from 897 to 1237 mm, these values being the lowest among all the floras analysed. Also the MAT interval (13.8–16.5 °C) tends to be lower, which corresponds to the transition from humid subtropical climatic conditions to a warm temperate climate in the Late Oligocene. Major components in the evergreen vegetation from Borovets were Eotrigonobalanus furcinervis (Rossmässler) H. Walther & Kvaček and species of the genus Myrica (Palamarev, 1961). The palaeoecological analysis of floristic data from the Bobovdol Basin permits a distinction between two phases in the palaeosuccesson (Palamarev et al., 1998). The first one is characterized by the appearance of the arctotertiary species (Acer, Alnus, Carpinus, Populus, Salix, Ulmus, and Zelkova). This phase corresponds to the abovementioned transition from a “humid subtropical” to a warm temperate climate. Quantitative climatic data confirm this conclusion, with MAT intervals between 13.8 and 16.5 °C and MAP ranging between 897 and 1237 mm (Figs. 3, 6). These values are slightly lower than those calculated for the early Oligocene and obviously reflect a global climatic trend. However, this climatic change was only minor and had no strong impact on the vegetation, e.g., in the Bobovdol basin, hydrohygrophytic (swampy) coenoses and hygromesophytic to

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mesophytic forests continued to exist. They are characterized by dominantly palaeotropical elements (Daphnogene, Litsea, Persea, Eotrigonobalanus, Trigonobalanopsis, Engelhardia, Juglans, Trema, Alstonia, Sideroxylon, Reevesia, Alangium, Itea, Symplocos, Araliaceae, Arecaceae, Schizaeaceae), responsible for the appearance of a second “warm” CA interval with MAT from 16.5 to 20.5 °C. The second phase in the palaeosuccession in the Bobovdol Basin (Bobovdol–Babino flora) is correlated with the late-Oligocene/earlyMiocene transition. According to Palamarev et al. (1998) it is characterised by climatic warming with MAT increasing. This warming is evidenced by an increased abundance of thermophyllous taxa belonging to the Lauraceae, dominating the spectrum, and of Sideroxylon salicites (C.O. Weber) Weyland and Engelhardia orsbergensis (P. Wessel and C.O.Weber) Jähnichen, Mai and H. Walther, as well as a reduced importance of arctotertiary elements in the Bobovdol–Babino flora. In general, the composition of the palaoecoenoses did not dramatically change (Table 2), but the representatives of the genera Acer, Alnus, Carpinus, and Populus disappeared. Climate data display higher СА intervals for TCM (5–7 °C) and TWM (Figs. 4 and 5). Higher TCMs allow many thermophyllous species to survive in the winter. A possible trend to higher rainfall is indicated by the increase of the upper boundary of the CA interval from 1237 mm (Borovets) to 1355 mm (Bobovdol) (Fig. 6). The general trend in temperature evolution from the late Eocene to the Oligocene/Miocene transition shows decrease in temperatures and rainfall rates (Figs. 3, 6). At the same time, the vegetation transformed from entirely palaeotropical to palaeotropical/arctotertiary in character. This process was connected with changes in the palaeocoenoses, their taxonomic composition, spatial distribution, and significance in vegetation structure. As shown in Table 2, the importance of hygrophytic and hygromesophytic coenoses in the landscape decreased, and they were gradually replaced by hygromesophytic to mesophytic coenoses, partly also by xerophytic types. Decreasing proportions of hydrophytic and hygrophytic coenoses, being elements of the azonal vegetation, in most cases depends on changes of local conditions. For example, the presence of water bodies and swampy places favours their distribution, even under drier climatic conditions. In the framework of the general trend in climatic change, some isolated deviations in the climate data may be mentioned (Figs. 3–8). As a reason for such deviations specific palaeogeographic settings at the localities studied can be cited. 6.2. Vegetation and climatic change in the context of palaeogeography Besides climate, there are other factors controlling vegetation distribution, e.g., the palaeogeographic position of a site such as palaeo-latitude, palaeo-altitude, and land-sea distribution. From the aspect of plate tectonics, the Rhodope massif was in stable connection with Eurasia during the time span concerned (Golonka, 2004). Since the late Eocene, a northward movement of about 2° latitude can be assumed. Consequently, no major effects can be expected. Changes of the land-sea distribution and the opening/closing of migration routes directly forcing or hampering the migration of plants and animals potentially are of greater importance in our study area. For example, at the Eocene/Oligocene boundary, when the size of the European continent increased, the Turgay Strait became dry, and the Bering Landbridge opened and enabled a mammal and angiosperm exchange from North America to Asia and further to Europe (Rögl, 1999). Later, in the early Oligocene, a closure of seaways caused the first isolation of the Paratethys while the appearance of a land bridge between the Balkans and Asia Minor enabled a floristic exchange and the spreading of arctotertiary species. The Rhodope High represented the main route in this exchange. Its importance in the evolution of the Balkan Palaeoflora is described in details by Palamarev (2003). From the palaeogeographic point of view, the floras studied were located comparatively close to each other (Fig. 2). So we cannot expect

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effects of a latitudinal gradient. More important for potential climatic differences between the floras is their exposition, e.g. their proximity to the seashore, estuaries of rivers and firths, or an inland position. A location close to a large water body provides advantages for many plants by its moderating effect on the climate. Relief and palaeoaltitude were factors controlling the vegetation distribution in the same way as today. The presence of high mountains leads to an altitudinal differentiation of the vegetation. 6.2.1. Eocene to early Oligocene In the late Eocene southern Bulgaria was part of an archipelago surrounded by a comparatively shallow marine basin. At that time a seaway existed to the Indian Ocean and the West Pacific, facilitating oceanic heat transport (Rögl, 1999). This well explains the humid and warm climatic aspect of the Eocene palaeoflora and the climatic data calculated (Bourgas). This palaeogeographic configuration again raises doubts about the existence of xerophytic shrubby palaeocoenoses as described by Palamarev (1973). Apparently the presence of these single fossils with xerophytic nature was not climatically determined. There are, however, indications for the existence of a drier season. At the beginning of the early Oligocene the size of the continent increased and a new ocean-circulation pattern supplied water from the North Sea to the Paratethys. At that time large-scale uplifts, such as Himalaya–Tibet and western North America, changed Earths' global climate (Hay et al., 2002). The uplifted areas strongly influenced the pattern of atmospheric circulation in the Northern Hemisphere, the radiation-distribution balance, and the intensification or reduction of rainfall. Global climatic change influenced the local vegetation in southern Bulgaria through the entire Oligocene (see above). The reduced heat transport connected to changing seaways at the end of the Eocene caused an increase of the proportion of temperate taxa and changes in plant communities. The tendency toward slightly lower MAP rates observed for the early-Oligocene floras when compared to the Eocene might well be connected to the size of the water body decreasing at that time. The difference in palaeocoenotic characteristics of local floras at the Eocene/Oligocene transition from the western Rhodopes (Eleshnitsa-I site: Palamarev et al., 2000) and the middle Rhodopes (Hvoyna Basin: Černjavska et al., 1988) can be explained by the existence of a marine basin close to the Eleshnitsa-I site. The climatic data obtained for both floras do not show any significant difference, but a tendency to somewhat wetter and warmer conditions is apparent at Eleshnitsa-I. The record of palms belonging to Phoenicites and Sabal and a mangrove element of the genus Acrostichum supports this assumption. These communities possibly were related to the remains of the Eocene sea in the form of isolated coves with brackish waters, or they were spread along the estuaries of palaeo-rivers. On the basis of palaeoecological studies on the Hvoyna flora, Černjavska et al. (1988) suggest dry season. They recognise xerophytic plant communities of a probably allochthonous character. The elements of these communities possibly were transported from the inland of the Rhodope High where they grew under specific local edaphic and climatic conditions. This coincides with the direction of palaeotransportation from westsouthwest to the east-northeast, i.e., from land to the lake (Černjavska et al., 1988). The absence of such a palaeo-transport in Eleshnitsa-I explains absence of xerophyte elements in this flora. According to the MPdry data presented herein, however, 51 out of 52 taxa contributing with data indicate that at Hvoyna, precipitation rates above 30 mm can be assumed for the dry season. The regressive trend persisted until the end of the early Oligocene, when the Paratethys became isolated from the Mediterranean. The change of floral composition and coenotic diversity as well as the disappearance of mangrove vegetation can be linked to the retreat of the sea. From that time on, sclerophyllous shrub palaeocoenoses dominated by evergreens occurred (Palamarev et al., 2000; Table 2).

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The regressive phase obviously favoured the migration and spread of temperate deciduous taxa, such as Castanea, Carpinus, Alnus, Populus, Acer, Cornus, Nyssa, and Phellodendron (Palamarev, 1967). As is obvious from the climatic data (Figs. 3–8) the retreat of the sea had almost no impact on the temperatures, but for most of the early Oligocene floras slightly lower annual precipitation resulted. 6.2.2. Late Oligocene to the Oligocene/Miocene transition During the late Oligocene the Paratethys returned to open marine conditions. However palaeoclimatic data point to cooler/drier conditions in the late Oligocene. Obviously the local palaeogeographic signal was overprinted by a cooling trend globally observed at that time. The late-Oligocene floras from Brezhani, Bobovdol, and Borovets grew in different local environments, with freshwater lakes and marshes in the plain of Brezhani and Bobovdol and the influence of the Rhodope High close to Borovets (Fig. 2). That explains the presence of hydrophytic and hygrophytic coenoses in the Brezhani–Bobovdol area and their absence in the Borovets flora. The moderate temperature increase observed at the Oligocene/ Miocene transition (Bobovdol–Babino site) was possibly forced by incursions of warm ocean currents from the Indian Ocean (Rögl, 1999) but might also be connected to global warming at that time (e.g., Zachos et al., 2001). 6.3. Comparison of the results obtained with data from neighbouring areas Aiming to obtain more precise information about the climate change for the time-span from the Late Eocene to the Oligocene/ Miocene boundary we compare our data with paleofloristic records from neighbouring countries. Under the taxonomical point of view, the leaf flora from (F.Y.R. of Macedonia), dated as Late Eocene by marine fauna (Mihajlovič and Ljubotenski, 1994), is very close to the Hvoyna macroflora (Late Eocene–Early Oligocene). Mihajlovič and Ljubotenski (1994) described the vegetation at Ovče Polje which is characterized by both, humid Oak-Laurel and xerophytic Zizyphus communities. The climate was subtropical seasonal dry. This vegetation structure and climate conditions are almost identical to data about Hvoyna Basin. Utesher et al. (2007a) reported a specific and unusual flora from Bogovina (Serbia), characterized as a subtropical needleaved forest. Climate calculations reveals following data: MAT from 17.9 to 21.3 °C, TCM from 7 to 13.3 °C, TWM from 26.4 to 28.1 °C, and MAP from 867 to 1384 mm. These data are in agreement with received by us climate data for early Oligocene floras (see Table 3). The transitional Oligocene–Miocene flora from Ravna Reka (Serbia: Utesher et al., 2007a) is of evergreen vegetation type. Legume-like taxa and other element typical for xerothermic conditions are abundant, possibly pointing to a more pronounced seasonality of precipitation, as compared to the Bobovdol–Babino flora. The climate data also suggest a little bit lower precipitation (see Table 3, and Utesher et al., 2007a: Tables 3–4). Based on pollen data, Pantič and Mihajlovič (1985) described tropical mangrove vegetation from Serbia, in the area of the upper stream of the Pčinja river (Late Eocene–Early Oligocene). The macroflora from the same site shows tropical vegetation with xerophyte palaeocoenoses. The authors suggest an existence of xerophytic vegetation inside the island and mangrove communities at the sea coast. Similar vegetation structure existed in the Eleshnitsa sites – mangrove vegetation is recognised in the first phase of the flora development (Late Eocene– Early Oligocene, Eleshnitsa-I), and sclerophyllous shrub paleocoenoses established during the second phase (Early Oligocene, Eleshnitsa-II). This reveals the conclusion about similar climate conditions in both localities from Serbia and the west Rhodopes. Based on palaeoecological analysis of Late Eocene floras, Petrescu and Givulescu (1987) consider that oak and laurel communities were most characteristic. They developed under climate conditions with

MAT at about 20 °C, and MAP at about 1200 mm. Also these conclusions do not relay on quantitative analysis, they are close to our calculations using the CA on the palaeoflora from the Bourgas Basin (see Figs. 3, 6). There are three references for the Early Oligocene in Romania. In the first one about the paleoflora of the Curtuiuş Beds (Petrescu et al., 1989b) MAT ranges of 19 to 20 °C and a MAP of 1200 mm is reported. In the second one, Petrescu et al. (1989a) consider MAT at about 18 °C and MAP 1800 mm for the paleoflora of the Bizuşa Beds. For the flora from lower fossiliferrous layer in Corneşti-Aghireş, dated as Late Rupelian by Petrescu et al. (1997), the authors suggest a MAT of about 18 °C and a MAP between 1200 and 1600 mm. These results are close to our calculations for the Early Oligocene annual temperatures and precipitations (Figs. 3, 6). The existing difference is easy to explain with the fact that above cited climate reconstructions are non-quantitative, and relay on systematics and leaf morphology interpretations. Petrescu and Givulescu (1986) estimated climate data for Late Oligocene (based on the palaeoflora from the Petroşani Basin) with MAT ranging between 17.3 and 18.2 °C and MAP between 1331 and 1353 mm. For the flora originating from the uppermost layers, exposed at the Corneşti-Aghireş locality (dated as Early Chattian), Petrescu et al. (1997) suggested MAT 16 °C, and MAP 1200–1600 mm. These data are very close to our calculations for Late Oligocene Bulgarian floras. Erdei and Bruch (2004) quantitatively analysed five Late Oligocene floras from Hungary using the CA. The floras originate from the Bükk Mountans (Eger-Wind, Andornaktálya) and from the Transdanubian Range (Pomáz, Vértesszőlős and Kesztölc). The age of deposits containing plant assemblages is defined as Egerian (see Erdei and Bruch, 2004). The analysis of the flora evidences the existence of mesophilous, riparian and swamp plant communities. These floras include both, decidous and evergreen plants, with a significant presence of representatives of Lauraceae, Engelhardia, Platanus neptuni (Ett.) Bůžek, Holý & Kvaček, Ulmus, Zingiberaceae, and some palms. According to Erdei and Bruch (2004) previous investigations on these floras described them as a warm temperate, subtropical seashore vegetation, with nearest recent relatives among warm temperate to subtropical mesophilous forests of East China. When comparing these floras to Bulgarian ones, it is worth to stress some differences in taxonomic composition and the absence of mesoxerophytic and xerophytic communities in the Central Parathetys area. This reveals conclusions about some regional climatic or edaphic differences that determined their distribution. In terms of palaeoclimate, quantitative data from Hungary indicate warm temperate, Cfa type climate (according to the Köppen classification), with MAT mainly between 15.6–18.8 °C, TCM 5.0– 10.2 °C, and TWM 25.6–27.5 °C. The data for annual precipitation ranged from (823) 897 to 1250 (1294) mm. All these data are in accordance with our calculations for the Late Oligocene floras from Bobovdol and Babino. Differing results are only obtained in the CMM calculation for the Borovets area, where a lower limit of the CA interval of 1.8 °C is obtained, but the MAP estimates (CA interval from 897 to 1237 mm) do not significantly differ from the Hungarian floras. The rest of the Bulgarian late Oligocene floras indicate higher rainfall rates, which confirms that the presence of more xerophitic floral elements was probably triggered by local edaphyc/microclimatic conditions. 7. Conclusions Vegetation change in the Palaeogene of the western Rhodopes was triggered by both, global climate evolution and regional patterns generated by a changing palaeogeography. In the records, signals from both processes are obvious, but overlapping and the weakness of the signals in some cases make it impossible to separate or resolve their imprints. Regarding the vegetation evolution in the western

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Rhodopes, the late-Eocene hygromesophytic forests played the major role in the zonal vegetation. Arctotertiary floristic elements were still absent. At the Eocene/Oligocene transition, no significant change occurred in vegetation cover, and hygrophytic to hygromesophytic palaeocoenoses and subtropical oak-laurel forests continued to dominate the zonal vegetation. In the early Oligocene mesophytic to mesoxerophitic communities gained importance while hydrophytic to hygromesophytic formations partially are not recorded in the floras. A similar picture is obtained for the late Oligocene; however, arctotertiary deciduous elements then reached a higher proportion for the first time, decreasing again towards the Oligocene/Miocene boundary while thermophillous taxa became more important. The temperature evolution is overall consistent with the vegetation changes observed. Very warm and wet conditions result for the time span from the late Eocene to the early Oligocene, while in the late Oligocene the climate was significantly cooler and partially also drier. This most distinct signal observed in the climate records most probably is connected to global climatic change. However, minor variations in the floristic composition, and the inferred climate data coincide with changes in the palaeogeographic setting and with specific edaphic conditions. The regressive trend during the early Oligocene is correlated with a slight decrease in annual precipitation rates at most of the sites, whereas for the subsequent transgression no signal is visible in the climate record, possibly overprinted by the global cooling trend. Xerophytic phytocoenoses are reported from most of the sites considered, especially from the later Oligocene on. For only two of the floras (Bourgas, Borovets), CA data support this assumption. In all the other floras, the majority of taxa indicate that no really dry season existed. Consequently it is still unclear whether the mesoxerophytic to xerophytic components grew as extrazonal elements under specific edaphic conditions. In this context it has to be taken into account that in calculations of rainfall rates using the CA erroneous results may be obtained, when the water supply for the plants was not primarily due to precipitation. Here, future research should include additional aspects, such as geochemistry and data from palaeo-soils. However, the coexistence of the xerophytic phytocoenoses with hygro- to hygromesophytic associations comprising typical zonal elements stand against the presence of an overall dry climate. Acknowledgements The present study was carried out in the frame of the DFG project As 103/3-2 436 and the co-operation project Bul. 113/139/0-1, respectively. The financial support of the DFG, Bonn, is gratefully acknowledged. The study is also a contribution to the program “Neogene Climate Evolution in Eurasia—NECLIME”. The authors like to thank I. Zagorchev, Sofia for critical reading and valuable suggestions on the manuscript, and to H. Wright, Minnesota who kindly reviewed the manuscript linguistically. We are particularly indebted to two anonymous reviewers for valuable proposals for the improvement of the manuscript. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.revpalbo.2008.10.005. References Černjavska, S., 1970. Sporopolenovi zoni v nyakoi starotertsierni vuglenosni sedimenti v Bulgaria. Izv. Geol. Inst (Sofia), Ser. Stratigr. i Litol. 19, 79–100 (in Bulgarian, with English Abstr.). Černjavska, S., 1975. Palynological data on the palaeogene flora in Bulgaria. Problems of Balkan flora and Vegetation, pp. 39–42. Černjavska, S., 1977. Palynological studies on the Palaeogene deposits in South Bulgaria. Geol. Balc. 7 (4), 3–26.

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