ISSN 1028-334X, Doklady Earth Sciences, 2006, Vol. 411, No. 8, pp. 1331–1335. © Pleiades Publishing, Inc., 2006. Original Russian Text © E.V. Bezrukova, A.V. Belov, A.A. Abzaeva, P.P. Letunova, L.A. Orlova, L.P. Sokolova, N.V. Kulagina, E.E. Fisher, 2006, published in Doklady Akademii Nauk, 2006, Vol. 411, No. 2, pp. 254–258.
GEOGRAPHY
First High-Resolution Dated Records of Vegetation and Climate Changes on the Lake Baikal Northern Shore in the Middle–Late Holocene E. V. Bezrukovaa, b, A. V. Belovc, A. A. Abzaevaa, b, P. P. Letunovaa, b, L. A. Orlovad, L. P. Sokolovac, N. V. Kulaginae, E. E. Fisherc Presented by Academician A. P. Derevyanko October 17, 2005 Received November 10, 2005
DOI: 10.1134/S1028334X0608037X
The northern shore of Lake Baikal (hereafter, northern Baikal), which is located in the middle part of Central Asia, represents a very interesting, but poorly studied, biogeographic and paleogeographic region [1, 2]. This region incorporates a complicated phytocoenosis represented by plant communities of the Ural–Siberia, Angarida, and Beringia phratries of taiga-type (Boreal) formations. Interactions between these communities are characterized by the arduous dynamic relations developed in the late Holocene. This connection was reflected not only in the ecotone nature of the structure of present-day vegetation, but also in the fact that the eastern and western boundaries of dominant species of these major heterochronous caenogenetic vegetation units pass precisely along the study region [3]. Therefore, the vegetation of this region is highly sensitive to climate changes of different time scales. Naturally, such properties of vegetation are of great interest for reconstructing the environment. The study region is located in the land fragment of the North Baikal Basin, where the vegetation is repre-
a
Institute of Archaeology and Ethnography, Siberian Division, Russian Academy of Sciences, ul. Favorskogo 1a, Irkutsk, 664033 Russia b Institute of Geochemistry, Siberian Division, Russian Academy of Sciences, ul. Favorskogo 1a, Irkutsk, 664033 Russia; e-mail:
[email protected] c Institute of Geography, Siberian Division, Russian Academy of Sciences, ul. Ulan-Batorskaya 1, Irkutsk, 664033 Russia d Institute of Geology, Siberian Division, Russian Academy of Sciences, pr. akademika Koptyuga 3, Novosibirsk, 630090 Russia e Institute of the Earth’s Crust, Siberian Division, Russian Academy of Sciences, ul. Lermontova 128, Irkutsk, 664033 Russia
sented by a unique assemblage of forest–meadow–bog communities of the Kichera and Upper Angara continental deltas. The climate of the basin is temperate continental due to the warming effect of the Baikal watermass. At the same time, the permafrost developed here indicates severe conditions in the region. The Baikal and Barguzin ranges make up the edges of the North Baikal Basin on the west and east, respectively. The macroslopes of ranges facing Lake Baikal are characterized by asymmetric altitude zonation of the vegetation. Light coniferous forests with Siberian larch (Larix sibirica) and Dahurian (Larix gmelini) larch predominate on the slopes of the Baikal Range, whereas dark coniferous forests with spruce (Picea), fir (Abies), and cedar (Pinus) predominate on the Barguzin Range slope in the taiga belt. Overtopped larch forests are located on waterlogged terraces of Baikal. scrub pine (Pinus pumila) predominates in the belt below bald peaks. This work reports new data on integrated biostratigraphic studies of continuous peat bog sections (Fig. 1), which served as the basis for comprehensive paleogeographic analysis of environmental changes in the northern Baikal region in the middle–late Holocene. The Verkhnyaya Zaimka section (Fig. 1, Site 1) is located on the right slope of the Upper Angara River valley within an overgrown lake drained by man-made dams. Sparse pine–cedar–larch forests with birch (Betula) and undergrowth (Duschekia and low birch) are growing on the bog margins. The soil cover is made up of ledum, sedge, and less commonly bog whortleberries and red whortleberries. The section is 110 cm thick. The age of sediments is defined by three 14C data (table, Fig. 2). Radiocarbon dates are given in accordance with the calibration scale [4]. Based on simple interpolation of the growth rate of peat in the Verkhnyaya Zaimka section, the age of the section base is estimated at ~980 yr. In the context of
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BEZRUKOVA et al. 110°
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Fig. 1. Map of the studied region: (1) Verkhnyaya Zaimka, (2) Ukta.
global paleoenvironmental changes, this estimate allows us to assign the beginning of the peat bog formation to the second half of the Medieval Optimum of the late Holocene (~600 to ~1200 yr B.P. or 1400–800 AD) [5, 6]. Arboreal pollen (pine, birch, and cedar) predominates, while pollen from fir and larch is substantial in the spore-and-pollen spectra (SPS) from sediments of this period. The content of grass and sedge pollen is maximal here. According to the SPS composition, cedar (with fir) and pine–larch (with birch) forests predominated in the northern Baikal region 980–840 yr
B.P. (Zone 3). The climate of the medieval warming episode could include warmer winter seasons and a high snow cover. The lake was overgrown with sedge– grass associations with willow under conditions of mainly groundwater alimentation. The next stage of vegetation formation (Zone 2) lasted from 840 to 600 yr B.P., with SPS dominated by pollen of birch, larch, and shrubs. The first maximum of sphagnum spores was established in this period. Its vegetation was characterized by a considerable expansion of the steppe-type birch and larch forests because of the maximum development of wormwood–herbs associations. Poaceae associations, which predominated in the bog at the beginning of the period, gave way to heather (Erica) undershrub, sphagnum, and Middendorff’s birch thicket by the end of the period, indicating the beginning of the formation of a transition peat bog with mainly atmospheric alimentation. A deficit of atmospheric moisture during this period is also recorded in the high values of the steppe-forest index (SFI), which reflects the interaction between forest and steppe taxa (indicators of changes in the atmospheric moisture level [7]): SFI = (Artemisia + Chenopodiaceae)/(the same + arboreal pollen) · 100 (in relative units). Cedar and pine pollen predominate in the Zone 1 SPS (the last 600 yr). However, the amount of spruce and larch pollen increases in Subzone 1c, whereas the amount of Ericaceae pollen and sphagnum spores increases in Subzone 1a. According to the SPS composition, pine and cedar forests predominated in the studied territory over the last 600 yr. The predominance of these tree species was accompanied by changes in the role of fir, spruce, and larch. Successions in the structure of vegetation of the bog massif are also established. Cedar forests predominated during the formation of pollen spectra of Subzone 1d (600–410 yr B.P.) and sphagnum predominated in the bog. However, the next short period (410– 330 yr B.P.) was marked by a considerable increase in the area of larch forests and spruce valley forests along with a growing role of scrub pine. Heather undershrub appeared again in the bog massif. Such vegetation changes coincide in time with the maximum of the Little Ice Age. The spectra of Subzone 1b (310–200 yr B.P.)
Rate of peat growth and time resolution of sampling intervals Section Verkhnyaya Zaimka Ukta
Section interval, Horizon thickcm ness, mm
Accumulation Sampling inter- Calculated peat Time resolution time, yr val, cm growth rate, mm/yr of record, yr
0–27 27–67 67–107
270 400 300
320 260 410
6 6 6
0.8 1.5 0.7
53 43 68
0–11 11–57 57–107 107–150 150–187
110 460 500 430 370
250 1410 2115 320 460
3 6 6 6 6
0.5 0.3 0.2 1.3 0.8
84 230 350 53 75
DOKLADY EARTH SCIENCES Vol. 411 No. 8 2006
1a 1b 1c
320
1d 580 2 3
0 80
20 60
60 45 3 60
3 8 30 60 12 30 12 3 12 50 100200 6 3 3 4 2 2 3 6 100 200 0 %
990
0 160 320 400 480 560 700 840 980
Calendar age
Pa ly no zo ne s 14 C de age pt / h
Tr
1333
Depth, cm
0 10 20 30 40 50 60 70 80 90 100 110
ee s Sh ru G bs ra ss Sp or es Pi nu L a s sy rix lve str Pi is nu s Ab s ie ib Pi s s iric ce ibi a a r Be ob ica tu ov la a Be alb ta tu l aPi a na typa nu n D s p a-ty u u p Sa sch mil a li ek a Cy x ia f ru pe tic r os Er ace a ic ae Po ac ac eae ea e Ar te Ra mi n si Li unc a lia ul Ca ce ace n ae ae Iri nab da is Aq cea ua e Po tic sa s Po cea ly e p Sp odi ha ac gn eae um To ta po l lle n
FIRST HIGH-RESOLUTION DATED RECORDS OF VEGETATION
500 0 5
Fig. 2. Spore-and-pollen diagram of the Verkhnyaya Zaimka section.
indicate the predominance of pine and cedar forests with fir. A high peat bog overgrown with heather undershrub and sphagnum formed during the last 200 yr. The surrounding vegetation included pine–cedar forests with birch and larch. The trend of birch forest expansion evidently reflects human impact. The Ukta section (Fig. 1, Site 2) is located on the old Baikal terrace in the Ukta Creek mouth (near the Kichera River mouth) within a vast waterlogged massif amidst peat hummocks with scrub pine and dwarf Arctic birch undergrowth. Some areas are covered with pine–cedar–birch forests disturbed by fires and tree felling. The surface of the peat massif is covered with ledum, red whortleberry, sedge, and mosses. The section interval (193 cm) was sampled with a spacing of 6 cm. Five 14C ages obtained for this section were recalculated into calendar years (Fig. 3). The time resolution of the record varies from 55 to 350 yr (table). The results of the palynological study are represented in the spore-and-pollen diagram (Fig. 3). The maximum abundance of pollen of spruce, fir, larch, birch, and meadow–bog grass is established in Zone 5 SPS (5300– 5100 yr B.P.). These spectra characterize vegetation of the north taiga affinity dominated by cedar forests with fir. Spruce and larch forests reduced in valleys. Birch forests predominated around the bog massif, while Equisetum and Carex–Poaceae associations predominated in the massif. Vegetation of such a type was characteristic of the latest Atlantic interval of the Holocene in the Baikal Basin [8, 9] under conditions of warming and reduction of precipitation. The Pine sylvestris and Larix pollen became predominant in Zone 4 SPS, indicating a wide distribution of light coniferous larch–pine forests 5100–4450 yr B.P. The horsetail–sedge bog still persisted in the area of the studied section. A noticeable peak of spruce pollen in the middle part of the zone corresponded to a short period of expansion of spruce– larch forests 4800–4550 yr B.P. during a cooling event known in North America [10], North and Central DOKLADY EARTH SCIENCES Vol. 411 No. 8 2006
Europe [11], and West Siberia [12]. A considerable increase in the abundance of the Siberian cedar pollen in Zone 3 SPS indicates the expansion of cedar forests under conditions of higher atmospheric humidity between 4450 and 2300 yr B.P. This interval includes two short episodes of fir forest expansion, probably corresponding to higher humidity in West Siberia approximately 4200 and 2800 yr B.P. [11]. The boundary of the Holocene Subatlantic interval (~2300 yr B.P.) was marked by a significant change in the composition of vegetation (Zone 2), which is reflected in a fast and substantial distribution of birch forests around the massif and Middendorff’s birch thicket within the massif. Such a sharp change in the composition of dominating wood species could be related to fire. However, data on variations in landscape–climatic conditions on the western slope of the Central Baikal Basin demonstrate that a short period of cooling and a decrease in the effective humidity persisted here 2290 yr B.P., which coincided with climate worsening in the Northern Hemisphere at that time [13]. Pine pollen predominated in Zone 1 SPS, but the abundance of birch and undershrub pollen increased in the spectra of Subzones 1a–1c. Such a composition of the spectra confirms the persistence of pine and cedar forests over the past 1700 yr, but the expansion of larch forests is noted at 1700–1200 yr B.P. However, they again gave way to cedar (with fir and birch) forests 1200 to 800 yr B.P. The expansion episode of dark coniferous forests (with fir) corresponds to the Medieval Optimum, which is also recorded in sediments of the Verkhnyaya Zaimka section (Fig. 2). From 900 yr B.P. and nearly up to 450 yr B.P., scrub pine became widespread and vegetation of the bog massif was represented by heather undershrub. The formation of a sphagnum bog after 450 yr B.P. with subsequent expansion of larch and spruce may indicate cooling in the Little Ice Age.
Depth from the section surface, cm
1a
1
250
1b 1c 2
1600
3
3775
4095 4 5 0
100 25 40 500 5 40 50
80 2
90 30 10 10 1 3 5 16 2 16 40 6
60 3 5 8% 0
500
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4555
0 150 300 600 900 1200 1500 2100 2800 3500 4200 4300 4400 4500 4600 4800 5000 5200 5400
Calendar age
0 10 20 30 40 50 60 70 80 90 100 110 120 130 140 150 160 170 180 190
C4 de age pt / h
Tr
Sh ru G bs ra ss Sp or Ab es ie s Pi sib ce iri a c Pi obo a nu va s s ta ib Pi i nu rica ss La ylv es ri tri Be x s tu la Pi alb nu as t Be pu ypa tu mil l a a D u n Sa sch ana lix eki -ty a pa Ar fru te tic m Ra is os a nu ia Po nc sa ula c Aq e ce ua ae ae Er tic ic s Cy ac pe eae H race er b ae Sp s ha gn um D ip Po has ly iu p m Eq od ui iac se ea To tum e ta an l po d ll sp en or es St ep pe Pa /fo ly re no st zo in ne de x s
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Fig. 3. Spore-and-pollen diagram of the Ukta section. Hatched areas correspond to short-term episodes of climate changes quasisynchronous for the Northern Hemisphere, which were distinguished for the first time based on the results of the palynological and radiocarbon analyses of the studied territory.
Thus, the first continuous high-resolution dated palynological records of vegetation and climate changes in the northern Baikal area revealed an unstable environment in the second half of the middle and late Holocene. We have recorded for the first time a signal of the medieval warming at 1200–800 yr B.P. for the North Baikal Basin. This is manifested in the increasing role of fir in the mid-taiga forest mountain belt, indicating an increase in winter temperatures and intensification of winter cyclonic activity (an increase in winter precipitation). We can also distinguish a cooling episode in the Little Ice Age 600–150 yr B.P. expressed by expansion of the area of scrub pine, spruce, and larch under conditions of a decrease in heat and effective humidity. Both episodes of variations in the paleoclimate could be related to changes in the intensity of secular cycles of thermohaline circulation in the North Atlantic [14]. We have also identified for the first time chronological frames of paleogeographic events in the North Baikal Basin for the last 5300 yr and have correlated them with environmental changes in the Temperate Zone of the Northern Hemisphere. It is significant that the medieval warming episode in the studied territory was expressed as the stage of warming and higher humidity, unlike other territories marked by aridization [6]. This fact is also confirmed by data on variations in the content of oxygen isotopes in silica of the Baikalian diatom valves [15]. The SFI values are always low for both records, thus confirming the existence of permanently humid climatic conditions over the last 5300 yr in the studied territory. However, this environment was
interrupted by short-term episodes of decreasing humidity 800, 600, and 150 yr B.P. The suggested succession of vegetation and climate changes in the northern Baikal region provides new insights into the general evolution of the lake ecosystem in the Holocene. Our work is aimed at elucidation of specific features and regularities in the evolution of the paleoecosystem for simulating possible changes in the near future. ACKNOWLEDGMENTS This work was supported by the Russian Foundation for Basic Research (project nos. 04-05-64078 and 06-05-64671) and the Siberian Division of the Russian Academy of Sciences (integration project no. 6.10). REFERENCES 1. L. N. Tyulina, in Geobotanical Studies in Baikal (Nauka, Moscow, 1967), pp. 5–44 [in Russian]. 2. A. V. Belov, Investigation of the Earth from Space, No. 6, 97 (1980). 3. A. V. Belov, V. F. Lyamkin, and L. P. Sokolova, Cartographic Study of Biota (Oblmashinform, Irkutsk, 2002) [in Russian]. 4. P. J. Reimer, M. G. L. Baillie, E. Bard, et al., Radiocarbon 46, 1029 (2004). 5. I. Murdmaa, L. Polyak, E. Ivanova, and N. Khromova, Palaeogeogr., Palaeoclimatol., Palaeoecol. 209, 141 (2004). DOKLADY EARTH SCIENCES Vol. 411 No. 8 2006
FIRST HIGH-RESOLUTION DATED RECORDS OF VEGETATION 6. K. V. Kremenetskii, T. Boettger, G. M. MacDonald, et al., Palaeogeogr., Palaeoclimatol., Palaeoecol. 209, 113 (2004). 7. A. Traverse, Paleopalynology (Unwin Hyman, Boston, 1988). 8. E. V. Bezrukova, A. A. Abzaeva, P. P. Letunova, et al., Quatern. Int. 136, 18 (2005). 9. D. Demske, G. Heumann, W. Granoszewski, et al., Global Planet. Change 46, 255 (2005). 10. R. Booth, S. T. Jackson, S. L. Forman, et al., The Holocene 15, 32 (2005).
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11. S. Tonkov and E. Marinova, The Holocene 15, 663 (2005). 12. V. S. Volkova and I. V. Mikhailova, in Principal Regularities of Global and Regional Changes in the Climate and Environment in the Late Cenozoic of Siberia (IAE SO RAN, Novosibirsk, 2002), pp. 58–71 [in Russian]. 13. E. V. Bezrukova, S. K. Krivonogov, A. A. Abzaeva, et al., Geol. Geofiz. 46 (1), 21 (2005). 14. A. Hiller, T. Boettger, and K. Kremenetski, The Holocene 11, 491 (2001). 15. D. W. Morley, M. J. Leng, A. W. Mackay, and H. J. Sloane, Global Planet. Change 46, 221 (2005).
