Journal of Paleolimnology 18: 189–206, 1997. 189c 1997 Kluwer Academic Publishers. Printed in Belgium.

A preliminary investigation of siliceous microfossil succession in lateQuaternary sediments from Lake Baikal, Siberia

M. L. Julius1, E. F. Stoermer1, S. M. Colman2 & T. C. Moore1

1Center for Great Lakes and Aquatic Sciences, University of Michigan, 2200 Bonisteel Blvd., Ann Arbor, MI48109-2099, USA2U.S. Geological Survey, Woods Hole, MA 02543, USA

Received 19 June 1996; accepted 16 November 1996

Key words: Lake Baikal, Russia, paleolimnology, diatoms, chrysophyte cysts, climate change

Abstract

Siliceous microfossil assemblage succession was analyzed in a 100 m sediment core from Lake Baikal, Siberia.The core was recovered from the lake’s central basin at a water depth of 365 m. Microfossil abundance variedgreatly within the intervals sampled, ranging from samples devoid of siliceous microfossils to samples with upto 3.49� 1011 microfossils g�1 sediment. Fluctuations in abundance appear to reflect trends in the marine �18Orecord, with peak microfossil levels generally representing climate optima.

Microfossil taxa present in sampled intervals changed considerably with core depth. Within each sample a smallnumber of endemic diatom species dominated the assemblage. Changes in dominant endemic taxa between sampledintervals ranged from extirpation of some taxa, to shifts in quantitative abundance. Differences in microfossilcomposition and the association of variations in abundance with climate fluctuations suggest rapid speciation inresponse to major climatic excursions.

Introduction

Lake Baikal (Figure 1) is the world’s deepest (1620 m)and most voluminous lake (23 000 km3). It is alsoperhaps the world’s oldest continuously existing lake.Lake Baikal lies within the central third of the BaikalRift Zone. Extensional forces in the rift zone formed,and continue to alter, the lake’s basins. The lake con-sists of three basins (North, Central, and South). TheSouth Basin has a maximum depth of approximately1400 m and is divided from the deeper (ca. 1600 m)Central Basin by an underwater ridge which expandsnortheasterly across the lake in the Selenga Deltaregion. The Central Basin is separated from the shal-lower (ca. 800 m) North Basin by the Academic Ridge,a structural and bathymetric high, which extends north-easterly from Ol’khon Island.

Sediments in Lake Baikal have recorded much ofthe lake’s history and surrounding environmental con-ditions. Recent multichannel seismic profiling revealedup to 7.5 km of sediment in the lake (Hutchinson

et al., 1992). Three tectono-stratigraphic units havebeen identified in the sediments of the lake’s Southand Central Basins, with a total thickness of over 7 kmin the South and 7.5 km in the Central Basin. The NorthBasin appears to have simpler sedimentary structure,containing only two distinct units with a total thicknessof 4.4 km. Current sedimentation rates in the South andCentral Basins are estimated at 0.1–1.0 mm yr�1 basedupon 137Cs and 210Pb dating (Edgington et al., 1991).Sedimentation rates are estimated between 0.03 mmyr�1 for Academic Ridge and 0.3 mm yr�1 for theCentral Basin, based upon accelerator-mass spectrom-eter (AMS) analyses radiocarbon dating (Colman et al.,1993). These relatively low sedimentation rates and thehigh volume of deposited sediment suggest the poten-tial for reconstructing the entire history of the lake andits surrounding environments.

Investigations of modern, large Pleistocene-agelakes have shown changes due to anthropogenic effects(Stoermer et al., 1990). These changes included majorfluctuations in presence and abundance of specific

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Figure 1. Map of Lake Baikal showing location of surrounding mountain ranges and lake basins. N-North Basin, C-Central Basin, S-SouthBasin, and X-site of core. Bathymetric contour interval 500 m.

diatom taxa. Earlier paleolimnological investigationsof Lake Baikal demonstrated great changes in diatompopulations over time. Chernyeava (1970) recordeddiatom populations in the upper meter of sedimentfrom seven transect cores in Baikal’s North Basin. Shefound changes in diatom assemblages ranging fromvariations in abundance to extirpation of some taxa.The floristic changes revealed by her study could notbe attributed directly to anthropogenic influences.Highresolution sampling techniques were not employed andthe near surface sediment was not sampled. Large,within-system variations occur in large lakes subject-ed to different degrees and histories of perturbations(Stoermer et al., 1993). Because of this, knowledgeof assemblages being deposited under present condi-tions is needed to aid in the interpretation of assem-blages in older lake sediments. This is particularly truefor Lake Baikal because of its many endemic species.Phytoplankton assemblages contain Baikalian specieswhich are not known to exist in other lakes, makingknowledge of the components and influences on thesurficial sediment record crucial for paleolimnologicalinvestigations.

Modern records of Lake Baikal plankton assem-blages are incomplete, making plankton ecology dif-ficult to interpret. Stoermer et al. (1995) investigated

a number of surficial samples from stations in eachof Lake Baikal’s basins. These results can serve as awithin-lake calibration set and show general north tosouth decrease in weight abundance of diatoms. Thisagrees with earlier work by Popovskaya (1991), whoindicated regional differences in phytoplankton abun-dance. Endemic diatoms (Cyclotella baicalensis Skv.,C. ornata (Skv.) Flower, C. minuta (Skv.) Antipova,Aulacoseira baicalensis C. Meyer, and Aulacoseiraskortzowii Edlund, Stoermer, and Taylor) were dom-inant in all basins. The interaction of these taxa issimilar to that of Cyclotella and Aulacoseira speciesfound in other large northern lakes (Stoermer et al.,1981). Cyclotella species dominate the summer flo-ra and certain Aulacoseira species are more abundantin colder portions of the year. Non-endemic diatomtaxa (Synedra acus Kutz., Stephanodiscus binderanus(Kutz.) Krieg., and Cyclostephanos dubius (Fricke)Round) were more abundant in the South Basin than inthe North Basin. The southern end of Lake Baikal hasbeen the site of regional urbanization and industrial-ization (Galazii, 1991; Belt, 1992), and the presence ofthese taxa has been attributed to anthropogenic influ-ences (Stoermer et al., 1995).

Investigations of longer sediment cores utilizedinformation from the surficial sediments to recon-

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struct pre-modern lake environments. Edlund et al.(1995) analyzed diatom succession in two short cores.One core from the South Basin recorded influences ofanthropogenic activity and showed two zones of highanthropogenic influence: one estimated from 1950 to1991 and the other occurring during the late 1700’s.These periods of perturbation are in agreement withthe history of human activity in the area. The 1950’swere a period of rapid industrialization in the area,including the construction of a dam at Irkutsk whichcontrolled and significantly changed the water levelsin the lake (Stewart, 1990). The earlier activity cor-responds to a shift in the lifestyle of humans in thearea. Prior to this period the native Buryat people wereprimarily nomadic. The late 1700’s marked the peakinflux of European peoples in the region, who grad-ually influenced the native people to shift to a moreagrarian economy, including slash and burn agricul-tural techniques (Humphrey, 1983). The second corereported by Edlund et al. (1995) was retrieved fromthe North Basin. Microfossils in these sediments wereless influenced by anthropogenic activity and appearto reflect fluctuations in past climate. Major changeswere present in the abundance of dominant diatomsdeposited during a period known as the Little Ice Age(ca. 1600–1850), which was characterized by coolerclimate conditions in Siberia (Khotinskiy, 1984).

Lake Baikal’s sediment record is well suited forpaleolimnological climate reconstruction on a longertime scale. Glaciations have occurred in the lake’sdrainage basin, but glacial scouring has been limited tothe surrounding mountain ranges rather than the lakebed (Peck et al., 1994), suggesting sediment records ofglacial activity should be well preserved. Two majorglacial events are recorded in land sections in Siberiaduring, approximately, the last 100 000 years. Theseare known locally as the Zyryanka Glaciation (ca. 110,000–50 000 BP) and the Sartan Glaciation (ca. 26 000–11 000 BP) (Arkhipov, 1984). These glacial periodswere interrupted by a warm period known as the Kar-ginsky Interstade (ca. 50 000–26 000 BP). Baikal’shigh latitude also provides good potential to recordglobal variations in climate, because the lake is sensi-tive to long-term changes in insolation patterns reflect-ing changes in the Earth’s orbital parameters (Kuzminet al., 1993; Peck et al., 1994; Colman et al., 1995).

Previous paleolimnological investigations docu-mented climate fluctuations recorded in Baikal’s sed-iment record. Cores taken throughout Baikal’s basinssuggested that changes in taxonomic composition ofdiatom microfossils reflected climatic shifts between

cold-dry and warm-moist periods (Bradbury et al.,1994). Unfortunately, available cores were a maxi-mum of 11 m in length, limiting the time interval thatcould be examined. Peck et al. (1994) found evidenceof glacial cycles in the magnetic properties record ofLake Baikal sediments. This study examined numer-ous cores from Lake Baikal with lengths up to 10 m.Influences of the most recent glacial event, the SartanGlaciation, were evident in the concentration, grain-size, and minerology of the magnetic sediment com-ponents. Glacial intervals were characterized by highconcentrations of magnetite and relatively large pro-portions of high coercivity minerals. Interglacial eventsshowed the reverse, with increased diatom concentra-tion diluting the magnetic minerals.

Recent studies of the cores from Academic Ridgeof Lake Baikal (Colman et al., 1995) not only indi-cate that higher levels of biogenic silica are associatedwith interglacial and interstadial intervals, but also thatthe detailed character of the variations in the biogenicsilica content closely match the marine oxygen iso-tope record of climatic and global ice volume change.The spectral character of these variations is also verysimilar to that of the marine oxygen isotope record.This lends support to the conclusion that variations inthe Earth’s orbit have driven glacial to interglacial cli-matic change in this interior continental area in a waythat is nearly identical to the fluctuations seen in themarine realm (Colman et al., 1995). Based on thesenew findings, we relate the classical Sartan Glaciationto oxygen isotope stage 2 (12 050–24 110 BP; Martin-son et al., 1987), the Karginsky Interstade to oxygenisotope stage 3 (24 110–58 960 BP; Martinson et al.,1987), and the Zyryanka Glaciation to oxygen isotopestage 4 (58 960–73 910 BP; Martinson et al., 1987).

In this paper, siliceous microfossil compositionis examined in a preliminary study of core materi-al deposited during several glacial-interglacial cycles.As part of the cooperative Baikal Drilling Project, a100 m sediment core was recovered near the village ofBuguldeika on the southwestern side of Lake Baikal.Biogenic silica composition, and siliceous microfos-sil enumeration were performed on the first, relativelywidely-spaced set of samples. Some age control wassupplied by AMS dating. These data were compiled tocreate an initial record of past environments, focusingparticularly on the major glacial-interglacial cycles ofthe last 500 000 years.

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Table 1. Taxa occurring at greater than 5% in any sample

1. Aulacoseira baicalensis (C. Meyer) Simonsen2. Aulacoseira skvortzowii Edlund, Taylor, and Stoermer3. Cyclostephanos species #14. Cyclotella gracilis Nikiteeva et Likhoshway5. Cyclotella minuta (Skv.) Antipova6. Cyclotella ornata (Skv.) Flower7. Cyclotella ornata var. #18. Stephanodiscus carconensis Grun.9. Stephanodiscus carconensis var. minor Grun.

10. Stephanodiscus carconensis var. pusilla (Grun.) Gasse

11. Stephanodiscus flabellatus Khursevich et Loginova

12. Stephanodiscus grandis Khursevich et Loginova

13. Stephanodiscus species #6

14. Stephanodiscus species #7

15. Synedra acus Kutzing

16. Cyst #1

17. Cyst #18

Materials and methods

Core recovery

Two cores, each approximately 100 in length, wererecovered on the southwestern side of Lake Baikalnear the village of Buguldeika (Figure 1) (Kuzminet al., 1993). Most of the core was obtained usingan advanced hydraulic piston core system with a 2-mcore barrel; some of the lower sections were obtainedwith a rotary action tool with a lined core receiver.Drilling was performed in a water depth of 365 m uti-lizing the coring rig mounted on a barge frozen in thelake. The coring operation proceeded in the followingmanner. The drill string was lowered with the bottomplate and buoyant collar attached.The bottom plate wasdeployed upon reaching bottom, indicated by a reduc-tion in weight on the drill string. The core receiver wasemployed utilizing a pressure differential between thedrill string and core barrel. The core barrel was raised,and the drill string was lowered with rotation allowingthe flushing of the cavity created by sediment removal.The core was extracted using a wire-line system andthe above process was repeated for a total of 31 individ-ual core drives. Core recovery was approximately 75%in the first drill hole. Two samples were taken fromeach core drive, for a total of 62 samples. Splits weredistributed to various members of the Baikal DrillingProject for analysis.

Table 2. AMS radiocarbon ages for Lake Baikal sediments

Depth Material Date Error Sed. rate

(cm) (�1000) (�1000) (mm/year)

84.5 TOC 5.02 0.09

188.5 TOC 11.39 0.15 0.16

264.5 TOC 16.07 0.24 0.16

392 TOC 21.74 0.58 0.22

450 TOC 22.43 0.63 0.84

515 TOC 29.6 1.6 0.091

1162 TOC 35.2 3.2 1.15

1490 TOC 30.7 1.8 ? � 3.00

1567 TOC 33.1 0 0.32

� Sedimentation rate calculated from 515 cm sample.

Chemical analysis

Biogenic silica measurements were made using induc-tively coupled plasma (ICP) analysis and extractionmethods of weight percent biogenic silica followingthe procedures of Mortloch and Froelich (1989). Allanalyses were corrected for clay dissolution using alu-minum concentration (Carter & Colman,1994). Radio-carbon ages are based on AMS analysis of total organiccarbon, as described by Colman et al. (1993). Most ofthe organic carbon is autochthonous (i.e. algal in origin(Colman et al., 1993)). AMS dates were used to calcu-late bulk sedimentation rates for the sediment intervalsbetween dated samples.

Siliceous microfossil analysis

For siliceous microfossil analysis, a portion of eachsection was freeze-dried to reduce microfossil break-age associated with other drying methods. A dry,weighed subsample of each section was boiled for30 min in 30% H2O2 at 110 �C. Twenty-five ml of con-centrated HNO3 was added to the peroxide-sedimentsuspension, resulting in a rapid exothermic reactionwithin 5 min. The solution was then heated at 120 �Cfor an hour. Samples were then rinsed six times withdistilled water to remove oxidation byproducts. Theentire portion of each sample was settled upon 18 mmcircular cover slips in Battarbee chambers (Battarbee,1973). Replicate slides were prepared from each sam-ple using HyraxTM mounting medium. At least 500microfossils or four 9 mm transects were enumerat-ed, using brightfield oil immersion optics (N.A.>1.32)capable of 1200� magnification. All recognizablealgal remains were counted and recorded according to

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their size relative to a whole specimen of their species.These functional categories were ‘reconstituted’ byaddition and reported in terms of the base morpho-logical unit, one valve of the species for diatoms andone cyst for chrysophyte cyst morphotypes. Countswere converted to absolute abundance and reportedas diatom valves or microfossils g�1 dry sediment.Counts were also converted to percent abundance andreported relative to total microfossil abundance. Dis-cussion of sedimentary distribution of taxa will be lim-ited to the most abundant taxa. These are defined asthose occurring at >5% relative abundance in at leastone sample (Table 1).

Results

Stratigraphy and age model

Biogenic silica analysis identified 13 silica peaks inthe sediment core (Figure 2). Biogenic silica levels(corrected for clay dissolution) ranged from peak val-ues of 4.91–25.41%, to interpeak values of as low as0%. Microfossil (g�1 sediment) data produced peaksalmost identical to biogenic silica data (Figure 3).Differences between the two data sets included theabsence of a microfossil peak corresponding to silicaPeak 13 and the merger of silica Peaks 5 and 6 in themicrofossil data. Peak microfossil values range from2.71� 108–3.49� 1011 (g�1 sediment), and interpeakvalues included intervals lacking microfossils.

AMS dating results (Table 2) are comparable toothers performed on Lake Baikal (Toyoda et al., 1993;Bradbury et al., 1994). Based on the earlier studies ofcores from Lake Baikal (see introduction) and on theAMS radiocarbon ages from this drill core (Table 2),we take these intervals of high biogenic silica to repre-sent interglacial and interstadial intervals. AMS radio-carbon ages were determined on total organic carbonand are thus subject to errors associated with redepo-sition and contamination. Because of these difficul-ties, we do not trust the ages older than approximate-ly 30 000 BP, and believe that errors in the youngerage determinations may be considerably more than theanalytical precision given in Table 2. The youngestsix ages, however, do fall on a linear trend (Figure 4),with an intercept at the sediment surface near a zero age(about 1000 BP) and with a slope equivalent to an accu-mulation rate of about 19 cm ky�1. This would placethe barren sample at 290 cm (Figure 2) within oxygenisotope stage 2 and the sample at 454 cm toward the

Figure 2. Profile of dry weight percent biogenic silica vs depth inLake Baikal sediment. Numbers identify peak biogenic silica levels.

end of the earlier interstadial associated with oxygenisotope stage 3.

Extrapolating sedimentation rates is a dangerousexercise, especially in environments that are likely toundergo substantial changes in sediment accumulationdriven by changes in depostional processes and pat-terns as well as by substantial changes in climate. Ifwe take the biogenic silica peak 2 as representing theacme of oxygen isotope stage 3 (stage 3.3 of Martin-son et al., 1987) and biogenic silica peaks 3 and 4 asbeing associated with oxygen isotope stage 5, then wecan develop an age model for this core that reachesto at least 16 m (Figure 4). This model suggests thataccumulation rates diminished somewhat during thelast interstadal (stage 3).

The widely spaced peaks in biogenic silica in thedeeper part of the core suggest increased accumulationrates; however, the pattern is not sufficiently distinctiveto confidently match the peaks in biogenic silica withspecific interglacial and interstadial intervals of theoxygen isotope record; thus, we cannot extend an agemodel to the base of the recovered core section. We canonly suggest that, based on the age model for the upperpart of the core and on the number of silica peaks in

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Table 3. Dominant microfossil assemblage taxa in peak intervals in Lake Baikal sediments withcorrelation to zones identified by Bradbury et al. (1994)

This study Bradbury et. al., (1994) �18O Period

Peak 1 Stage 1 Aulacoseira baicalensisassemblage 1� Zone V Aulacoseira skvortzowii

Cyclotella minutaSynedra acus

assemblage 2� Zone VI Aulacoseira baicalensisAulacoseira skvortzowiiStephanodiscus flabellatus

Peak 2 Zone VII Stage 2 Aulacoseira baicalensisCyclotella gracilisC. minutaC. ornataStephanodiscus flabellatus

Peak 3 Zone VIII Stage 5 Aulacoseira baicalensisCyclotella minutaStephanodiscus carconensisS. grandis

Peak 4 Stage 5 Aulacoseira baicalensisCyclotella minutaC. minutaStephanodiscus carconensisS. grandis

Peak 5–6 Aulacoseira skvortzowiiassemblage 1� Stephanodiscus carconensis

S. grandisCyst #1Cyst #18

assemblage 2� Cyclotella ornata var. #1Stephanodiscus grandis

Peak 7 Cyclotella ornata var. #1Stephanodiscus grandis

Peak 8 Aulacoseira skvortzowiiStephanodiscus carconensisS. carconensis var. minorS. carconensis var. pusillaS. grandis

Peak 9 Stephanodiscus carconensisassemblage 1� S. carconensis var. minor

S. grandisassemblage 2� Aulacoseira skvortzowii

Stephanodiscus carconensisS. grandisCyst #1Cyst #18

Peak 10 Stephanodiscus carconensisS. carconensis var. minorS. carconensis var.pusillaS. grandis

Peak 11 Aulacoseira skvortzowiiCyclotella ornata var. #1Stephanodiscus carconensisS. carconensis var. minorS. grandis

Peak 12 Aulacoseira skvortzowiiStephanodiscus sp. #6Stephanodiscus sp. #7

� One of multiple assemblages within a single silica peak.

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Figure 3. Abundance (valves and cysts� 1010 g�1 dry wt sediment)in Lake Baikal sediment. Numbers identify peak biogenic silicalevels.

the remainder of the record, the base of the section isnear 400 000 to 500 000 BP. The Brunhes-Matuyamamagnetic reversal boundary was not encounterd in thecore (BDP93 Baikal Drilling Project Members, 1996);therefore, the base of the core is certainly younger thanabout 800 000 BP.

Microfossil assemblages

Three types of microfossils were encountered in thesediment core: diatoms, chrysophyte cysts, and spongespicules. Of these, the diatoms were most abundant.Planktonic diatom taxa comprise 66–93% of micro-fossil assemblages. Benthic diatom taxa occurred lessfrequently, comprising 0.6–5% of microfossil assem-blages. Chrysophyte cysts are also common in LakeBaikal sediments, comprising 2–31% of microfos-sil assemblages. Chrysophyte cyst morphotypes werevery diverse with over 100 different morphotypes iden-tified. Siliceous components of chrysophytes otherthan cysts were not encountered in microfossil enu-merations. Sponge spicules were rare and did not com-prise a significant portion of the microfossil assem-

blages, but because of their size and solid construction,sponge spicules may constitute a significant portion ofbiogenic silica (Conley & Schelske, 1993).

Of the 62 sediment samples analyzed for siliceousmicrofossils 18 contained significant quantities ofthese remains. This is reflected in the peaks producedin the microfossils g�1 sediment profile (Figure 3).Assemblages in each of the 18 samples were dominat-ed by a few planktonic taxa endemic to Lake Baikal(Figures 5–9). Endemic taxa comprised over 45% ofthe microfossil assemblages in each sample. Endemictaxa varied in abundance within each of the samplesand some species were extirpated. Persistent replace-ment of dominant taxa was the most striking feature ofthe sediment record.

A variety of nomenclatural schemes have beenapplied to endemic Baikalian taxa, and there are dif-ferences in opinion concerning what encompasses agiven species and the proper specific epithets. Becauseof this, an explanation of our concept is presented here.For chrysophyte cyst morphotypes an arbitrary numer-ical designator is used, because we cannot, at thispoint, correlate these structures with known species. Inmost cases, the most recent taxonomic interpretation ofdiatom systematics is used in naming diatom taxa. Thisincluded Nikiteeva and Likhoshway’s (1994) descrip-tion of Cyclotella gracilis, which was previously iden-tified as an unnamed species (Bradbury et al., 1994).Cyclotella ornata was separated from C. baicalensisand C. minuta. Observations in this study fully sup-ported Flower’s (1993) conclusion that these are allindividual species. We also separated Cyclotella orna-ta var. #1 from this group. This taxon appeared todiffer morphologically from Cyclotella ornata, butspecimens found are very eroded, making definitivetaxonomic separation tenuous. All three morphotypesof Aulacoseira baicalensis (nominate, squarosa, com-pacta) are reported individually. These morphotypeswere considered formae by Skvortzow (1937), but theopinion of Stoermer et al. (1995) was followed here.Stoermer et al. considered morphological differencesecophenotypic and similar to differences in A. islandi-ca morphology in the Laurentian Great Lakes (Stoer-mer et al., 1981, 1985). The taxon reported here asAulacoseira skvortzowii has been considered a sporan-gial frustule of A. baicalensis (Skvortzow, 1937; p. 303lns 4–6), and a separate morphotype of A. islandica(Kozhova et al., 1982; Genkal & Popovskaya, 1991).This entity has been described as a species (Edlundet al., 1996) and we also treat it as a distinct species.Deep core taxa identified as Stephanodiscus carco-

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Figure 4. Proposed age model for first 16 m of Lake Baikal sedi-ments.

nensis Grunow, S. carconensis var. minor Grun., andS. carconensis var. pusilla Grun. follow the interpreta-tions of Gasse (1980). Although superficially similar tothese taxa, small variations in morphology are noticedin light microscope observations of the Lake Baikalforms and more detailed ultrastructural investigationmay prove them to be separate entities.

Descriptions of microfossil assemblages will begiven from bottom to top to simplify presentation.A summary of these descriptions with correlation toBradbury et al. (1994) is given in Table 3. Diatomswere found first in samples from 6800–7100 cm. Thiscorresponded to Peak 12 on the microfossil abundanceprofile (Figure 3). Sediment samples below this depthwere devoid of siliceous microfossil remains. Twoundescribed Stephanodiscus species (Stephanodiscussp. #6, Stephanodiscus sp. #7) dominated the florain this sediment interval. Each of these taxa share asimilar ‘Bauplan’ and are not represented in any othersediment interval.

Microfossils were not present in sediment samplesfrom 5700–6800 cm. Microfossil remains appearedagain between 5300–5700 cm, corresponding toPeak 11 on the microfossil abundance profile (Fig-ure 3). A variety of Baikal diatoms were present inthis interval. Stephanodiscus grandis Khursevich etLoginova, S. carconensis, S. carconensis var. minor,Aulacoseira skvortzowii, and Cyclotella ornata var. #1were all major components of the diatom flora in thissediment interval. Chrysophyte cysts #1 and #18 werealso major components of the microfossil flora. Eachof these microfossil taxa was represented in other sed-iment intervals and formed significant portions of thepopulation in each of these intervals. Stephanodiscusgrandis, in particular, dominated in most of the sam-

ples in which it is represented, comprising 20–59% ofthe population. Because of the large size of Stephan-odiscus grandis, it probably comprised an even greaterportion of the phytoplankton biomass.

Microfossils occurred infrequently in samplesbetween 4950–5300 cm and reappeared in abundancebetween 4850–4950 cm. This represents Peak 10 onthe microfossil abundance profile (Figure 3). Stephan-odiscus grandis, S. carconensis, S. carconensis var.minor, S. carconensis var. pusilla were the major taxain the microfossil flora. Stephanodiscus carconensisvar. pusilla occurred in other sediment intervals andfrequently comprised a significant portion of the pop-ulation.

Siliceous remnants became rare in samples between4000–4950 cm. Stephanodiscus grandis, S. carconen-sis, S. carconensis var. minor, Aulacoseira skvortzowii,and chrysophyte cysts #1 and #18 appeared in abun-dance between 3550–4000 cm. This is represented asPeak 9 on the microfossil abundance profile (Figure 3).Aulacoseira skvortzowii levels fluctuated greatly in thisinterval, constituting >56% of the population in onesample and <1% in another.

Microfossil numbers decreased sharply in the sed-iment between 3000–3500 cm, and reappeared in sig-nificant numbers between 2700–3000 cm. This corre-sponds to Peak 8 on the microfossil abundance profile(Figure 3). Stephanodiscus grandis, S. carconensis,S. carconensis var. minor, S. carconensis var. pusilla,and Aulacoseira skvortzowii were the dominant mem-bers of the flora in this interval.

Microfossils were very rare and often absent in thesediment samples 2500–2700 cm. Microfossil num-bers increased between 2300–2500 cm, representingPeak 7 on the microfossil abundance profile (Fig-ure 3). An alteration in the population compositionoccurred in this interval. Cyclotella ornata var. #1constituted almost the entire microfossil assemblage,replacing Stephanodiscus spp. as the dominant taxa. Adrop in microfossil abundance occured in the sedimentsamples between 2000–2300 cm and then numbersincreased between 1700–2000 cm. Two distinct micro-fossil assemblages existed within this peak. The lowermicrofossil assemblage was similar to that found inPeak 7, consisting almost entirely of Cyclotella ornatavar. #1. The upper microfossil assemblage (1708 cm)is dominated by Aulacoseira skvortzowii, Stephanodis-cus grandis, S. carconensis, and chrysophyte cysts #1and #18. These two distinct assemblages support theconclusion that both biogenic silica Peaks 5 and 6 (Fig-ure 2) are represented by the single peak identified as

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5–6 on the microfossil abundance profile (Figure 3).Silica Peak 6 corresponds to the assemblage domi-nated by Cyclotella ornata var. #1, and silica Peak 5relates to the more diverse assemblage.

Microfossil numbers diminished again in the sed-iment samples between 1600–1700 cm. Abundanceincreased significantly between 1150–1600 cm. Thiscorresponds to Peaks 3 and 4 on the microfossil abun-dance profile (Figure 3). The two peaks were dividedby a slight drop in abundance of microfossils between1250–1450 cm. It is important to note the minimumpoint of this drop in abundance still represented greatermicrofossil numbers (g�1 sediment) than most of thepeaks previously identified. The major components ofthe flora in this sediment interval consisted of Aula-coseira baicalensis, Cyclotella ornata, C. minuta,Stephanodiscus grandis, and S. carconensis. Aulaco-seira baicalensis, Cyclotella ornata, and C. minutaare major components of modern Lake Baikal plank-ton assemblages. This sediment interval representedthe earliest record of these modern taxa in this sedi-ment core, and the last occurrence of Stephanodiscusgrandis.

A decrease in microfossil abundance occurredbetween 700–1150 cm. A rise in abundance corre-sponding to Peak 2 on the microfossil abundance pro-file (Figure 3) occurred in sediment samples between550-700 cm. Aulacoseira baicalensis, Cyclostephanossp. #1, Cyclotella ornata, C. minuta, C. gracilis,and Stephanodiscus flabellatus Khursevich et Logino-va were major components of microfossil assemblage.Cyclotella gracilis only occurred within this sedimentlevel. Stephanodiscus grandis, S. carconensis, S. car-conensis var. minor, and S. carconensis var. pusillawere absent in this microfossil abundance peak andwere never present in sediment deposited after thisinterval.

Diatom microfossil abundance dropped for the finaltime in the sediment interval between 202–550 cm.Microfossils appeared in abundance in the sedimentfrom 202 cm to the top of the core. This corresponds toPeak 1 on the microfossil abundance profile (Figure 3),and was the largest microfossil abundance peak identi-fied. Two microfossil assemblages were present in thisinterval. Aulacoseira baicalensis, Aulacoseira skvort-zowii, and Stephanodiscus flabellatus dominated theearlier portion of the sediment interval. Aulacoseirabaicalensis, Aulacoseira skvortzowii, Cyclotella min-uta, and Synedra acus were the major components ofthe assemblage in the latter sediment interval. The lat-ter assemblage was very similar to the modern Lake

Baikal plankton community and was the only occur-rence of a close analog to modern assemblages.

Discussion

Lake Baikal’s large size, depth, geographic location,and high degree of isolation from human activities pro-duce unique limnological conditions. These factors,coupled with the lake’s ancient age, have produced alarge number of endemic species. This high degree ofendemism is present in the planktonic species, espe-cially in the diatom community. Because of this, com-parisons of microfossil assemblages with modern cali-bration sets were impossible. Interpretations of down-core change must be based upon our knowledge of theautecology of modern Baikalian algae. An analyticalapproach of this nature has proven successful in pale-olimnological interpretations of other large lake sys-tems (Stoermer et al., 1993). In this method, resolutionof past environmental conditions is dependent on thepresence of extant algal species in the sediment core.Lake Baikal’s microfossil assemblages are dominatedby a number of extinct, endemic microfossil species,presenting a challenge to interpreting environmentalhistory.

Paleoecology

Sediment deposited during the last 24 000 years repre-sents the current interglacial and Sartan glacial periods(Isayeava, 1984). Peck et al. (1994) used rock magneticevidence and Colman et al. (1995) used biogenic silicaevidence to separate the glacial and interglacial periodsfor the last 250 000 years. Peck identified 13 550 BP(13 000 BP in Carter and Colman (1994)), as the dateseparating the last glacial and present interglacial peri-od. This is in agreement with our stratigraphic andradiocarbon dates. Diatoms occurred within Peak 1;below this peak low microfossil abundance occurredin clay-like sediment. Peak 1 was the only high micro-fossil abundance interval occurring in this section ofthe core (Figure 3). Dominant microfossil assemblagesoccurring in this peak are all represented in modernLake Baikal plankton. The early microfossil assem-blages were identical to Zone V identified by Bradburyet al. (1994). Microfossils consisted almost entirely ofAulacoseira baicalensis morphotypes and Aulacoseiraskvortzowii (Figure 5).

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Figure 5. Abundance (valves� 1010 g�1 dry wt sediment) of Aulacoseira baicalensis morphotypes and Aulacoseira skvortzowii in Lake Baikalsediment.

Modern Aulacoseira baicalensis populations arecommon in the spring flora. The taxon has a tempera-ture optimum of 4–6 �C (Bondarenko et al., 1993),and populations begin development under clear iceduring February and March, reach maximum abun-dance in April, and continue through May and June(Edlund et al., 1995). Population numbers vary dra-matically from year to year. Blooms occur every 3–4years and dominate the plankton assemblage (Kozhov,1963). Valve morphology expression is silica-related(Likoshway et al., 1993). The ‘squarosa’ morphotypeis a lightly silicified form and the ‘compacta’ morpho-type is a heavily silicified form. During periods of envi-ronmental stress, including sedimentation and darkperiods, individuals are able to produce physiologicalresting cells (Bondarenko et al., 1993). Aulacoseiraskvortzowii has an ecology similar to A. baicalensis.

It has a temperature optimum of 4–6 �C (Bondarenkoet al., 1993). It is most abundant March through Mayand can bloom under ice (Kozhova et al., 1982; Edlundet al., 1995). In times of environmental stress individ-uals can form true resting spores (Bonderenko et al.,1993).

Aulacoseira species are adapted to specific envi-ronmental conditions, generally spending a portionof their life in lake sediments, allowing reinocula-tion into the water column, and a portion in pelagicassemblages (Lund, 1954, 1955; Sicko-Goad et al.,1986, 1989). The presence of Aulacoseira species inthe pelagic zone is controlled, to a great extent, by cir-culation in lakes. Peak numbers of Aulacoseira speciesare typically found during periods of lake mixing andturnover. This generalization holds for Aulacoseiraspecies found in Lake Baikal, except that these species

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also live beneath the ice. Mixing conditions are notgenerally associated with ice- covered lakes, but otherresearchers have identified mixing conditions occur-ring in ice-covered lakes (Likens & Ragotzkie, 1965,1966; Hawes, 1983) including Lake Baikal (Sokol-nikov, 1960). The source of this mixing is often identi-fied as convection caused by solar radiation under theice surface (Farmer, 1975; Petrov, 1984) and computermodels have been developed to analyze the mechanicsof such mixing (Matthews & Heaney, 1987). Smallnumbers of Aulacoseira species could be sustained bythese mixing conditions, explaining the presence ofblooms under the ice, and eventually become support-ed by spring turnover in the lake.

The narrow temperature optimum of the Aulaco-seira species, in addition to their ability to grow underthe ice, may explain the dominance of these taxa atthe beginning of an interglacial event. Both of theseenvironmental factors would be expected to occur atthe beginning of deglaciation. Other factors that mayexplain the high numbers of these taxa include theirproduction of resting cells. Meltwater runoff shouldhave flowed into Lake Baikal during the Sartan Glacia-tion, increasing turbidity. Previous studies have shownAulacoseira species are able to bloom quickly in theseconditions due to the suspension of resting cells infavorable environments (Sicko-Goad et al., 1986; Car-rick et al., 1993). A mechanism similar to this mayhave occurred in Lake Baikal.

The second microfossil assemblage found in Peak 1was similar to Zone VI identified by Bradbury et al.(1994). Dominant microfossils included both Aulaco-seira species, Cyclotella minuta, and Synedra acus.Cyclotella minuta, C. baicalensis, and C. ornataare characteristic of mid-summer to late fall popula-tions (Edlund et al., 1995). Synedra acus is becom-ing increasingly important in modern plankton assem-blages. It has increased significantly since the 1950’s(Popovskaya, 1993), and is found in abundance April–June. Bradbury et al. (1994) suggested it is associatedwith more nutrient inputs and warmer climates. Thiscorresponds to the abundance of Synedra acus (Fig-ure 9) found in surficial samples of Lake Baikal’ssouthern basin where greatest anthropogenic influ-ences have occurred (Stoermer et al., 1995).

The environmental parameters of these modernpopulations seem consistent with the probable envi-ronmental parameters experienced by taxa during thissediment interval. Bespalyy (1984) identified 5000–7000 BP as the climate optimum for this interglacialperiod. Samples from just before this sediment interval

were dated to 5500 BP (Table 4, 84 cm). It seems rea-sonable to suggest that this assemblage was depositedwithin the climate optimum. Increased warming shouldhave preceeded this optimum, causing glacial melt andincreasing nutrient loading into the lake through runoff.This would have created ideal growth conditions forCyclotella minuta and Synedra acus.

The Karginsky Interstade was a relatively warminterval preceding the Sartan Glaciation. This intersta-dial period has been subdivided into 3 warming events(early 50 000–44 000 BP, middle 42 000-33 000 BP,and late 30 000–24 000 BP) and 2 cooling events(Arkhipov, 1984). External factors (glacial runoff, sed-iment dumping, and tectonic events) may have createdsediment anomalies which make interpretation of theseenvironmental events difficult.

Peak 2 (Figure 3) was the first interval of highmicrofossil abundance occurring before the SartanGlaciation. The microfossil assemblage within thispeak was identified as Zone VII by Bradbury et al.(1994). The assemblage was characterized by the pres-ence of Cyclotella gracilis (Figure 6), a taxon onlyfound in this section of the sediment core. Radiocarbonages below Zone VII as young as 22 100 BP suggest-ed by Bradbury et al. (1994) as potentially unreliableare also inconsistent with our age for sediments abovePeak 2 of 29 600 BP (Table 2, 515 cm). Peak 2 clearlyoccurs before the last glacial maximum and is probablycorrelative with oxygen isotope stage 3. Radiocarbonanalyses suggesting ages less than 30 000 BP appearto be unreliable, probably due to contamination (Cole-man et al., 1993).

Microfossil assemblages identified by Bradburyet al. (1994) above and below this interval correspond-ed well with those of this study. Zone V was estimatedto 11 000–14 000 BP in Bradbury et al. (1994); ourresults estimated sediment near the bottom of Peak 1(204 cm) as between 11 000 and 12 000 BP. The assem-blage below Zone VII was estimated to be older than29 800 BP in Bradbury et al. (1994), and we esti-mate Peak 2 to be between 36 000 BP (radiocarbonrate extrapolation) and 50 000 BP (by correlation withoxygen isotope stratigraphy; Martinson et al., 1987).

Cyclotella gracilis, the dominant species of theassemblage in Peak 2, bears no morphological resem-blance to endemic Baikalian Cyclotella spp. (Niki-teevka & Likoshway, 1994). It is unlikely that C. gra-cilis and endemic Cyclotella species share a recentevolutionary link. Cyclotella minuta, C. ornata, andC. baicalensis are all very similar and are almostcertainly sister taxa (Flower, 1993). Modern plank-

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Figure 6. Abundance (valves� 1010 g�1 dry wt sediment) of Cyclostephanos species #1, Cyclotella gracilis, Cyclotella minuta, and Cyclotellaornata in Lake Baikal sediment.

ton studies have encountered Cyclotella species oth-er than the endemic Cyclotella spp., but these taxaare rare, some probably associated with river load-ings, and abundance is low when they are observed(Bondarenko et al., 1993; Popovskaya et al., 1993).Cyclotella gracilis may represent an extinct taxon thatwas similar to these rare taxa, triggered into bloomby increased nutrient loading during Peak 2. It mighthave also been an exotic taxon deposited into a favor-able, high-nutrient environment. Waterfowl have beencredited with the transportation of algal species fromone system to another (Proctor, 1966), and a mecha-nism similar to this may be responsible for the pres-ence of the morphologically incongruent C. gracilis.Despite the reason for its presence, the range of C. gra-cilis appears to be an excellent stratigraphic indicator

for this sediment interval equivalent to oxygen isotopestage 3.

The development of Aulacoseira baicalensis (Fig-ure 5) in this period also suggests the end of a glacialperiod with this population’s development potentiallymimicking the previously described late winter-earlyspring cycle. Small easily separated colonial specieslike Aulacoseira skvortzowii can remain suspendedand are more easily resuspended into pelagic envi-ronments than longer heavy colonies. Strong evidencethen seems to exist suggesting a warming climate, giv-ing rise to the appearance of Aulacoseira baicalen-sis, which did not exist in the lake prior to this time.Warmer climate periods would create thinner ice coveron the lake allowing extended light penetration underthe ice. Currents created by this thermal energy inputwould be stronger than those experienced in glacial

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Figure 7. Abundance (valves� 1010 g�1 dry wt sediment) of Cyclotella ornata var. #1, Stephanodiscus carconensis, Stephanodiscus carconensisvar. minor, and Stephanodiscus carconensis var. pusilla in Lake Baikal sediment.

periods with thicker ice cover. These stronger currentswould be more likely to support the heavy Aulacoseirabaicalensis. Once introduced into the plankton Aula-coseira baicalensis could outcompete other diatoms,becoming the dominant taxa in microfossil assem-blages as it is found today.

Other dominant taxa in Peak 2 included Cyclotel-la ornata, C. minuta, and Stephanodiscus flabellatus.Stephanodiscus flabellatus is an endemic fossil tax-on which occurred in small numbers above this peak.Status as an endemic, fossil taxon allows little specula-tion about its ecology. Occurrence with other moderntaxa (Cyclotella ornata, C. minuta, and Aulacoseirabaicalensis) indicated a climate and environmental set-ting similar to modern conditions. This was consistent

with the correlation of this microfossil assemblage withthe Kharginsky Interstade.

Peaks 3 and 4 were the next intervals of highmicrofossil abundance. The flora in this assemblagewas identical to Zone VIII identified by Bradburyet al. (1994), the last microfossil zone they identi-fied. We equate this interval to oxygen isotope stage 5(73 190–129840 BP; Martinson et al., 1987). Aulaco-seira baicalensis, Cyclotella minuta, Stephanodiscusgrandis, and S. carconensis were present in abundancein this interval. Bradbury et al. (1994) identified S. car-coneiformis and S. bellus as biostratigraphic indicatorsof their Zone VIII. These taxa are present in our sam-ples as well, but occurred in numbers below 5% rela-tive abundance and are not included in our discussionof major floral components. Bradbury et al. (1994)

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Figure 8. Abundance (valves� 1010 g�1 dry wt sediment) of Stephanodiscus flabellatus, Stephanodiscus grandis, Stephanodiscus species #6,and Stephanodiscus species #7 in Lake Baikal sediment.

may have ‘lumped’ our S. carconensis with their mor-phological concept of S. carconeiformis and S. bellus.This would explain the absence of S. carconensis intheir stratigraphies. Both S. grandis and S. carconen-sis are extinct taxa; thus, knowledge of their ecologyis speculative. Bradbury et al. (1994) suggested theselarge, extinct Stephanodiscus spp. might have aute-cology similar to extant large Stephanodiscus. Theseorganisms live in large lakes and are able to survivein low light conditions growing slowly in deep wateruntil favorable environmental growth conditions occur,usually blooming in late fall to early winter or springduring periods of wind induced lake mixing. BothS. grandis and S. carconensis are found exclusivelyin large ancient lake systems. Stephanodiscus grandisis endemic to Lake Baikal (Loginova et al., 1990), and

S. carconensis is found in other ancient lake systems(Gasse, 1980). The environmental preferences sug-gested for the two taxa in these studies are very similarto environmental parameters expected during glacialand the beginning of interglacial periods. Ice on thelake would periodically be covered with snow creat-ing low light conditions. Removal of this snow coverand ice would allow extensive mixing of lake water,producing favorable environmental conditions for celldivision. Organisms able to last through harsh low lightperiods would be favored in these conditions. Whatev-er the ecology of these taxa, they are characteristic ofcold climates and appear in abundance (Figures 7–8)throughout the sediment record during and precedingwhat we interpret to be interglacial isotope stage 5.

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Figure 9. Abundance (valves� 1010 g�1 dry wt sediment) of Synedra acus, Cyst #1 ,and Cyst #18 in Lake Baikal sediment.

Peaks 5 and 6 were the next area of high microfos-sil abundance. The dominant microfossil taxa includ-ed Aulacoseira skvortzowii, Cyclotella ornata var. #1,Stephanodiscus grandis, S. carconensis, and Cysts #1and #18. Cyclotella ornata var. #1 appeared morpho-logically similar to the nominate variety of Cyclotellaornata. Differences in chamber structure appeared tooccur between the two organisms, but may be a prod-uct of silica dissolution in the highly eroded samplescontaining C. ornata var. #1. It is also reasonable tospeculate C. ornata var. #1 is the common ancestorof the endemic Baikal Cyclotella flora. The morpho-logical similarity of C. ornata var. #1 also provided abasis to suggest ecological similarity with C. ornata.Assumption of this environmental predisposition fitswith expected environmental conditions. The earlierportions of peaks 5 and 6 are dominated by Cyclotella

ornata var. #1 (Figure 6). The dominance of this taxonsuggests a climate with extended late spring to midsummer growth favored by endemic Cyclotella spp.The latter portion of the peak lacks C. ornata var. #1.The dominant assemblage (Aulacoseira skvortzowii,Stephanodiscus grandis, and S. carconensis) was char-acteristic of a cooler environment with an extend-ed period of lake mixing. The presence of abundantchrysophyte cysts in this latter period also agrees withthe occurence of cooler environments, with cyst pro-duction generally occurring in times of environmentalstress. This interval could represent interstadials (6.3and 6.41) within oxygen isotope stage 6, the earliermicrofossil assemblage constituting a warmer period(6.5), and the latter assemblage (6.3) representing thetransition to a cooling period.

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Peak 7 was similar to the earlier portions ofpeaks 5–6. The microfossil assemblage was dominat-ed by C. ornata var. #1 (Figure 6), suggesting a warmclimate. This peak potentially represents the 6.5 inter-stadial. Transitional assemblages were not representedin sediment immediately above or below this peak, butthis may result from our widely-spaced sampling inter-vals failing to obtain these intermediate assemblages.

Peaks 8–11 represented intervals of high produc-tivity; interpeak intervals are low productivity areas,possibly caused by snow and ice cover on the lake.Wolfe and Teller (1993) found sediment intervals withlarger grain sizes indicative of occasional high ener-gy events in winter climates. Each peak in biogenicsilica in this section of the core was associated withan increase in grain size, supporting the presence ofhigh microfossil productivity in these peaks. In eachinterval the microfossil assemblages were dominat-ed by Aulacoseira skvortzowii, Stephanodiscus gran-dis, S. carconensis and its varieties. Although basedon our extremely coarse sample intervals, our obser-vastions find microfossil assemblages in these peaksappeared to begin with an Aulacoseira skvortzowiibloom gradually developing into a Stephanodiscus spp.assemblages. Increased winds increase the depth ofdensity-driven circulation in Lake Baikal to as muchas 500 m (Shimarev et al., 1992). This increase in windspeed and mixing depths increase nutrient regenerationthrough deep ventilation of bottom waters (Weiss et al.,1991). Aulacoseira skvortzowii probably blooms ear-ly under ice cleared of snow cover, and as ice melts,wind circulation would help create favorable environ-ments for Stephanodiscus spp. In extreme incidencesof extended warming, Cyclotella ornata var. #1 popu-lations may develop as in Peak 11.

Peak 12 represented the oldest microfossil assem-blage in the core. Taxa representative of microfossilpeaks occurring above this sediment interval were notpresent, except Aulacoseira skvortzowii. Two unnamedspecies of Stephanodiscus (Figure 8) dominated themicrofossil assemblage in this interval. Environmentalreconstruction cannot be made because of the endemic,fossil status of the species characterizing this interval.Speculation concerning preferred environmental con-ditions is also impossible because of the absence oftaxa from younger intervals.

Sediment age and microfossil evolution

Bradbury et al. (1994) questioned the proposedPliocene age of the Stephanodiscus grandis flora (Logi-nova et al., 1990), based upon pollen analysis. Thisage is also questioned in this study. The S. grandisflora (Figure 8) is subtended by the assemblage con-taining Stephanodiscus sp. #6 and Stephanodiscus sp.#7. Although the assemblage containing Stephanodis-cus sp. #6 and Stephanodiscus sp. #7 may be reworkedfrom Pliocene deposits, it seems unlikely that sedi-ments containing S. grandis are also from this epoch.Stephanodiscus grandis abundance (Figure 8) fluctu-ations correlate with the interval of oxygen isotopestages 5 and 7.

Previous studies have noted the lack of diversityin recent Lake Baikal plankton assemblages (Edlundet al., 1995; Stoermer et al., 1995). This pecu-liarity is preserved throughout the lake’s sedimentrecord. Despite major changes, including extinctions,in microfossil assemblages, very few taxa dominateeach sediment interval. Competitive exclusion, asobserved by Stoermer et al. (1995), appears to haveoccurred at multiple times in Lake Baikal. The paradoxof the plankton (Hutchinson, 1961) does not appear tooccur at any time in the lake’s history. Diatom taxa nor-mally fill niches opened by new environmental condi-tions quickly, suggesting rapid evolution (on the scaleof centuries to millennia). This adaptive speed is sup-ported by other studies of diatoms (Theriot,1992). Lowdiatom species abundance is not common in aquaticsystems. Extremely simplified planktonic floras appearto be a unique character of ancient (Pliocene) lake sys-tems. This lack of diversity in ancient lake systems maybe attributed to their great age. Ancient lake systemsmay be the only freshwater habitats old enough forcompetitive exclusion to have been completely playedout.

Conclusions

Paleolimnological investigation of Lake Baikal sedi-ments revealed signals of change within the lake basinswhich are primarily controlled by climatic forces. Thisstudy traces the impact of major climate alterationsover approximately the last 500 000 years within theLake Baikal ecosystem.

The entire Pleistocene of Lake Baikal has been analternation between glacial and interglacial periods,but the diatom flora is only preserved from interglacial

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and interstadial intervals. Environmental conditions formost of the period sampled have no analogue withinthe modern human experience. This is reflected bythe dominance of extinct microfossil taxa within thistime period, and the presence of very few membersof the flora in modern lake assemblages. With eachwarming event the biological system appears to haverestarted, often with dramatic differences in the speciescomposition.

This study demonstrates the influence of climateon the Lake Baikal ecosystem, but provides only arough outline of these changes because of wide sam-pling intervals. A more detailed analysis is under way,in an effort to provide more resolution between indi-vidual warm intervals. Detail on the degree of warmingand floral development, and information on the rapidevolution and exclusion of taxa within the interglacialevents may be obtained. This information would beof great benefit to global change, limnological, andsystematic studies.

Acknowledgments

We wish to thank D. Francis for help in preparingillustrations. Contribution number 584 from the Centerfor Great Lakes and Aquatic Sciences was supportedin part by NSF Grant EAR-9318683.

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