Climate and Vegetation changes in the Northern Eurasia during the Younger Dryas

Abstract. The climate development of the Late Glacial/Holocene transition is one of the most riveting problem of environmental change under short-term cooling. This paper is devoted to a special reconstruction of climate and Vegetation in the Northern Eurasia with special reference to chronological level at 10,500 years b. p., i. e. just in the middle of the Younger Dryas . Reconstruction of Vegetation was based on detailed palaeo-climate maps with numeric estimation of temperature and precipitation parameters. Concluding results were affirmed by independent palaeodata.


Introduction
The reconstruction of natural zonality and its dynamics in the past provides the adequate possibilities to estimate the modern and future environmental condition.Studies ofthe time interval during the climate transit from the The simulated values were compared with the results of independent analysis of fossil beetle faunas synchronous with the Younger Dryas time.Nowadays the significance of beetle fauna for the palaeogeographical reconstruc¬ tion.for the determination ofthe landscape type and for the specification of climatic parameters has been well proved (kiselev et al. 1984).Climatic changes are com¬ pletely reflected in quantitative and qualitative character- istics ofthe fossil beetle fauna represented in palaeontological annals.Palaeoclimatic reconstructions were used as a data base for the Vegetation reconstructions for the former USSR during the Younger Dryas.For this purpose we have used data showing the correlation between Vegetation zone position and climatic patterns distribu¬ tion.They were derived from the Statistical processing of 280 sites on modern Vegetation and climate map for the investigated area.

Results
The results of the independent analysis of the beetle fauna of the Northern Eurasia synchronous with the Younger Dryas cooling give evidence of stable tempera¬ ture regime (kiselev 1981,1988.kiselev etal. 1984.kise¬ lev et al. 1987).According to the data from Kolyma and Indigirka river basins the climate similar to the modern one has established 11.000-10.000years b.p. (average July temperatures were 12°-13 °C and mean annual pre¬ cipitation 150-200 mm).In the areas ofthe modern Arc¬ tic coast ofthe West Chukotka average July temperatures stabilized on the 12 °C level and precipitation was equal to 200 mm.For the mountains ofthe lower Kolyma river basin the average July temperatures were 10°C-12°C while mean annual precipitation 200-300 mm.These Parameters (mostly temperature) were lower than the modern ones.According to the fossil beetle fauna ana¬ lysis the climate ofthe Western Siberia coastland (Gydan peninsula) and the European North-East (Yugorsky peninsula) during the Younger Dryas was similar to the modern.

General climatic characteristic during the Younger Dryas
In most parts ofthe former USSR the Younger Dryas was marked by the cooling and decrease of precipitation in comparison with the present data.The analysis of compiled geomap maps (fig. 1) shows that the cooling was not significant within high latitude areas of Eurasia (not more than 1 °C), and on the coast ofthe Arctic Ocean the average July temperatures were even a little bit higher than the modern ones.
Data obtained from the study of the fossil soils in the areas ofthe lower reaches of Ob and Yenisey rivers, on the Yamal peninsula.Ayon island in Chaunskaya guba, on Svalbard and Novaya Zemlya archipelagoes proved the stability ofthe temperature regime on the far North ofthe former USSR.The soil formation started in this region 11.000 years b.p. and finished near 7,000 years b.p. (evseev et al. 1986).This period is characterized by soils with well developed profiles.Their formation took place during climate conditions close to modern ones.

Ô/ n <?
<? Map of temperature and precipitation deviations in the Northern Eurasia for the Younger Dryas 1.
Average July temperature deviations 2.
Mean annual precipitation deviations 3.

Glaciers in the ocean
From geomap simulations it is evident that July tempera¬ tures increased gradually southward to the Polar Circle.
On the 50 N temperature anomalies (the difference be¬ tween palaeo and modern temperatures) reached 3°C.Within these latitudes negative deviations most abruptly were expressed on the Pacific coast.In the Kamchatka peninsula the difference was equal to 2°C but in Chukotka peninsula average July temperatures were equal to the modern ones.The maximum cooling (3°-4 °C) took place in the European sector.simultaneously with greater meridional atmospheric and oceanic flow, severe snowfallsand flux oficebergs in the North Atlantic (MIL¬ LERS kaufman 1991 ).GEOMAPsimulation shows that the most significant dryness and cooling were expressed in the Mediterranean on the 30 N. The distribution of negative temperature anomalies showed that they were subjected to the latitude zonality.while the distribution of negative precipitation anomalies was rather longitudinal.During the Younger Dryas the whole territory ofthe former USSR and Western Europe had significantly lesser precipitation than nowadays.The minimum dif¬ ference was observed on the extreme ofthe North-Eastern part ofthe continent, the maximum in the regions ex¬ posed to the North Atlantic margin.Within the Continen¬ tal regions of East and Middle Siberia this difference didn't exceed 50 mm per year, while in the West Siberia region it was about 100 mm.In Middle Asia and Altai negative anomalies were negligible (about 50 mm per year), but rather discernible as it is evident by analytical information (serebryanny et al. 1980,'murzaeya et al. 1984, butvilovsky et al. 1992).Probably the zone of maximum humidity was shifted to high mountains, and middle and low mountains had drier environments.
Westward from the Urals the differences in precipitation increased to 150 mm and on the Atlantic coast the dif¬ ference has become more noticable and reached 400 mm.Hence the sharp deviation in the climate regime ofthe Younger Dryas according to the simulations took place within the middle latitudes adjacent to the Atlantic coast, whereas in the Continental region essential climate change was not found.
The attachment of maximum cooling to the North Atlan¬ tic coastlands led us to the decision to analyse the tem¬ peratures of the Atlantic surface water during the Younger Dryas.The map of the model is based on 19 values of palaeotemperatures.determined by Prof.
M K barash These values were derived from marine micropalaeontological data.According to the geomap simulating temperature deviations it is clear that the area with maximum cooling lied in the Eastern part ofthe North Atlantic between 35 N and 45 N and the adjacent part of Mediterranean sea (fig. 2).On the Iberian Penin¬ sula the negative temperature deviations were 8 °C.and within the Mediterranean, especially in its Western and Central parts, the negative anomalies reached 9°C.These conclusion were proved in some features by recon¬ struction of ruddiman and MciNTYRE (1981).According to their opinion, the polar front on ocean shifted rapidly from the Islandwhere it had been situated during the proceeding Alleröd warm phasetowards the Iberian Peninsula latitudes during the time interval 10,000-10,500 years b.p whereas its position in the Western Atlantic, near American coast, didn't change.
Major trends in Vegetation history during the Younger Dryas During the Younger Dryas the Vegetation zones in North¬ ern Eurasia had a rather distinctive pattern (Fig. 3).Since the cooling practically didn't touch the high latitudes areas ofthe Northern Hemisphere. the borders and the areas of Arctic and typical tundra zone analogues were approximately similar to the present ones.This fact is supported by the vast palaeobotanical data (muratova 1973, khotinsky 1977).Several data referto typical tun¬ dra existence in the Ust-Port region (70 N, 83 E) (kind  1974).The southward penetration of tundra in North-Eastern Europe is also supported by palynological data (mkiphorova 1982).The areas of mountain tundra didn't increase to great extent while forest tundra signifi¬ cantly expanded southward to the areas which nowadays are occupied by Northern taiga.This phenomena was marked more distinctly within the limits ofthe Russian Piain (khotinsky 1977), kaplina et al. 1982).
Climatic simulations are well proved by fossil beetle fauna.It included typical species for the present Hypo- Vegetation map of the Northern Eurasia for the Younger Dryas, 10.500 years b.p.  b).The progressive decrease of this beetle in modern environment is well correlated with the increase ofthe Vegetation cover.except the sedges above mentioned.Nowadays it is impossible to verify a signifi¬ cant accumulation of beetle Morychus viridisvihich could be compared with the quantities of this beetle during the Pleistocene.As it is shown in the Table 1.almost every¬ where the significant role of xerophytic tundra biotop species (maximum 40%, minimum 20%) was marked.
Habitats ofthe hydro-mesomorphic biotops, especially the ground-beetles.were not so typical.The shares ofthe representatives of the steppe and meadow steppe were equally corresponding to 5-10% and 10-15% in average.The species connected mainly with the willows (weevil Lepurus and Phunchaenusjplayed a rather negligible role.
The most significant elements ofthe palaeolandscapes were those with the considerable drainage and with the patches ofthe Vegetation cover.The plant communities with the dominance of Arcticus Alpimis species, the pillows of ZVmvcombined with the different species of Saxi¬ fraga.Polygonumand grasses Poa. which are spread nowa¬ days.could be considered as their modern analogue.The second place was ocuppied by the different variants of biotops of steppe meadows and steppe spots dominated by Artemisia.The plant community with the dominance of Carex arguneiisis as a habitat of Morychus viridis were also widely spread in this area.The distribution ofthe analogues ofthe moss and Eriophorum vaginaltmilundra and mesophytic shrubs or shrubs associations (with the participation of birch, willow shrubs and even willow ar- Ecological spectra of the Late Glacial beetle faunas of the Northern Eurasia Ecological groups index: Aaquatic; Rriparian, flood piain; H-Mhydro-mesic Tundra; X -xeric Tundra; Dr - Carex-Dryas type; Grgrassland-meadow type; s -Steppe type; Trshrubs-trees Vegetation.
boreal) was limited by the relief depressions including Valleys and by the coasts of the natural reservoirs.The rate ofthese communities increased in the North (includ¬ ing the dry shelf) and in the West ofthe area.This demonstrates the zonal and sector dissimilarity ofthe Vegetation cover.In comparison with the continental lowlands aver¬ age July temperatures decreased to 10 °C and precipita¬ tion increased towards the modern level.The beetle fauna ofthe coastal plane from the lower reaches of river Lena was characterized by approximate equal participa¬ tion ofthe xeromorphic and hydro/mesomorphics forms of tundra communities.The rate of meadow beetles signi- ficantly decreased, and the steppe beetles were com¬ pletely absent.Note that the last group was marked in big variety of the forms to the East of the Verkhoyanskiy ridge.Pill-beetle Morychus viridis was defaced not very often and as a rule solitary, whereas the beetles trophically connected with arboreal and shrubby willow (Rhyn- chaenusarcticus, Doryiomusand Lepyrusjwere abundanl. In general tundra type landscapes prevailed in the vast coastlands ofthe Arctic Ocean from the mouth of river Lena to the Taimyr Peninsula.These landscapes differed from the present tundra by larger participation of the grass/herbs and shrub/herbs communities.Beetle fauna on the Gydan peninsula indicated the stable climate con¬ ditions and relatively uniform landscapes throughout the most part ofthe Late Quaternary.The dominance ofthe tundra beetle communities assembled in hydro-mesomorphic ecological group was typical.The presence of several beetle forms typical for meadow biotops ofthe West Siberian boreal zone (f.g. weevil Otiorrhynchuspolilus Cf//., Ot. arcticus F.) is also worthy of notice.The pecu- liarities ofthe beetle fauna composition, especially their ecological demands suggest a decrease of precipitation combined with a synchronous increase of average July temperature to 11°C-12°C The area of Northern Taiga was twice bigger than nowa¬ days.It spread out on the region of present Middle and Southern Taiga and partly even ofthe mixed deciduous forests.During the Younger Dryas, the Northern Taiga can be identified as an open woodland because of lesser precipitation especially within the European Piain.The vestiges of climatic aridity were of widespread occurrence according to the palaeovegetation records.The pollen of tundra and cool steppe primitive plant aggrega- tion combined with the arboreal pollen was rather typi¬ cal for forest zone spectra.The set of plant communities seem to be different from the modern one.
khotinsky (1977) was the first who demonstrated on the base of palaeobotanic studies the suppressing of forest Vegetation and development of steppe and often tundra communities on the whole investigated area.The de- clined forest communities were replaced by open wood¬ land of pine and birch, unusual for these regions.The Vegetation of these forests with the dominance of the steppe xerophytic and representatives ofthe tundra flora was identified by frenzel (1968) as "tundra-steppe".
The idea ofthe extensive distribution of peculiar "cool" landscapes which didn't have any modern analogues was confirmed by other palaeobotanic studies.karaulova (1991) Supports the idea ofthe presence oflarch-birch open woodland in Siberia (61 N, 70 E) and birch-pine open woodland in the Far East (43 N. 132 E).The dis¬ tribution ofthe wormwood-goosefoot communities with the representatives of forest and tundra zones was marked in the Polar Urals (surova etal. 1975).The peri¬ glacial pine and birch forest steppe existed in Eastern Eu¬ rope.Such landscapes with pine parklands and prevailing wormwood/grass communities were described at the slopes of glint near St. Petersburg at the end of the Younger Dryas (serebryanny 1978).The wormwood xerophytic communities penetrated to Southern Taiga in the West Siberia at the end ofthe Late Glacial (pia\- chenko 1968).The Middle Taiga was spread within limited areas near the Urals mountains, nowadays occu¬ pied by mixed forest.At 10,500 years b.p. the Southern Taiga practically disappeared, only few spots seemed to be present in the Fast East.In general, both areas of Middle and Southern Taiga were by seven times smaller than the modern zones.
The role ofthe forest steppe transition zone increased significantly during the cooling phase because of its Northward expansion.Under drier and colder climate, the forest steppe occupied the area of present broadleaved forests in the West and larch forests in the East of the former USSR (vipper 1975).The significant shift of the steppe border Northward was caused by increasing aridity.During the Younger Dryas there were few broadleaved forests now distributed on the vast areas as im¬ portant phytomass accumulators.Nevertheless, in the last years we have some indications about the preservation of forest Stands in the Southern Middle Russian heights, Eastern Crimea, Lower Volga region and even Central Kazakhstan (ta serebryan- naya.n.s.bolikhovskaya.p.e. tarasov.verbal communi¬ cation).These Stands are treated as the remnants of forest zone which shifted southwards during the maximum of Late Pleistocene glaciation.As this information is quite new, we have not succeded to put it into the model, and the Southern fringes of our maps probably will be considerably refined in the nearest future.

Discussions
The problem of instability ofthe climatic System "oceanatmospherecryosphere" is very intricate.This complicated self-regulated System maintains in the condition dose to the unstable equilibrium.According to regu- larities ofthe nature development in certain time interval when one of the components changes its structure or when the unstable equilibrium reaches its critical limit the whole System disrupts.This phenomena creates short-term climatic oscillations: warm periods alternate the cool ones.The extensive palaeoclimate information including numeric estimations of temperature suggest that the time scale ofthe climate change during the transi¬ tion from glacial to interglacial period is much more complicated than the classical geochronological scales (milankovich 1938,emiliani 1966, emiliani et al. 1975).
Late Glacial and Holocene provide the unique opportunity of rather detailed study of climate change.In this climatic stratigraphic scale one can underline the events of two ranks: major glacial/interglacial, which are well recognized in the Northern Hemisphere and series of short-term climate oscillations of different intensity as well as within every major interval.This oscillations coincided with the final glacial phases and have distinct tendency towards damping.The time sequence which took place during the glacial contraction is well known.
Warm and cool periods with 6 °C average global tempera¬ ture amplitude were marked at the end of deglaciation within 2.000 years.The last of them was the cooling at 10.800-10,300 years b.p. (Younger Dryas).
During cool periods the temperature decrease reached 5°-6 °C within one Century contrary to the present oscil¬ lation, which doesn't exceed 1 °C (budyko 1986).The main reason of their manifestation was the regulär dis- ruptions ofthe System "ocean -land/surfaceatmo- sphere" equilibrium.Evidently these disruptions took place simultaneously with the termination of insolation change.Short-term stages and interstages.which interrupted the long-term warming and cooling during the Pleistocene, were obviously derived by the leading im¬ pact ofthe ocean (dupplessy et al. 1981. barash 1988).
Due to its size.the ocean is the main accumulator ofthe solar radiation received by the Earth.which subse- quently releases to the atmosphere.Having tremendous mass and heating capacity, which significantly exceed the same Parameters ofthe land surface.the ocean has great impact on the atmosphere towards its stability.Its impact on the atmosphere comes mainly through transit of clear heat energy and phase transformation.These processes to some extend determine the main features and the pe- culiarities of the atmosphere circulation and also the possibility of long term prediction of its condition (GROSSVALD& MURATOVA 1985).
The nature of sharp coolings are not certainly determined.Evidently the main reason was the disruption of the glacial shelf in the Polar basin and Northern part of the Norwegian sea, arctic icebergs influx to the middle latitudes within the Atlantic and climate effect ofthe ice melting (ruddiman et al. 1981).The mechanism ofthe glacial surging effect on climate doesn't complete with the absorption of heat due to the ice melting in the ocean; it also involves a marked expansion of relatively fresh water in the surface water layer, an increase ofthe winter ice cover and a sharp increase ofthe ocean surface al¬ bedo.
During the last years several scenarios devoted to the cooling during the Younger Dryas have appeared (bori sova 1990, klimanov 1990).These scenarios are based on the linear interpolation between separate points in which according to the palynological analysis the climate Parameters were determined.The scenario by klimanov (1990) doesn't coincide with geomap simulations.He supposes that 10.500 years b.p. the maximum cooling took place within high latitudes.Whereas the reconstruc¬ tion by borisoya (1990) correlates well with geomap simulations.According to her data negative average July temperature deviations during the Younger Dryas had the trend to increase from the North to the South, it was especially marked in Western Europe.On the map of mean annual precipitation anomalies.borisova has underlined the area of their sharp decreasing within North-Western and Western Europe.On the other terri¬ tories mean annual precipitation was a little bit lower than the present one.

Conclusions
The climate Simulation worked out for the last "nearest" to the modern time sharp short-term cooling in the Northern Hemisphere has distinctly showed that this cold phase was derived by the cooling ofthe North Atlan¬ tic waters.It took place probably due to the increased meridional atmospheric and ocean circulation.The dis¬ tribution of negative temperature anomalies in the ocean and land surface during the Younger Dryas shows that the "epicenter" ofthe cooling was attached to the Eastern part ofthe North Atlantic between 35 N and 45 N.Maxi¬ mum cooling in Europe and North America was accompanied by significant decrease in precipitation and took mainly place in Continental margins.On the land surface its manifestation was weaker than on the ocean.Within high latitudes the temperatures during the Younger Dryas practically didn't differ from the modern ones and were even higher than I °C as it is evident by pollen and fossil beetle fauna analysis.The middle part of the Younger Dryas was marked by distinct sharp cooling and aridity.This time interval was proceeded by the Böl¬ ling-Alleröd epoch of significant warming on the land surface and in the ocean according to the palaeobotanical and micropalaeontological data (mercer 1969).Dur¬ ing this warm epoch, the temperature ofthe upper layer in the Bay Biscay and adjacent parts ofthe Atlantic were similar to the modern ones.It means that low tempera¬ tures ofthe Younger Dryas were not inherited from the Glacial period but were consequences ofthe indepen¬ dent climatic process at the final stage of deglaciation.The decreasing of temperature and significant aridity caused the changes in the distribution of Vegetation cover and floristic composition.Forests transformed to open woodlands.The zone of broad leaved forests almost disappeared, some of the broad leaved species were preserved in the composition ofthe open woodland on the spots which were favorable for their growth.The areas occupied by forest tundra.Northern taiga.forest steppe and steppe increased.To our opinion, the increase of forest tundra and Northern taiga areas was determined mainly by cooling.while increase of forest steppe and steppe areas was caused by aridity.
The analysis ofthe climate and Vegetation maps gives the possibility to determine the areas where landscapes have undergone great changes in comparison with the present ones.It is very important not only for the understanding of environmental dynamics but also for the prediction of its future development under climate change. Fig.1 Fig 2 Map of temperature deviations ofthe North Atlantic for the Younger Dryas.