to 3.49 1011 microfossils g 1 sediment. Fluctuations in ... up to 7.5 km of sediment in the lake (Hutchinson et al., 1992). ... and Central Basins, with a total thickness of over 7 km in the South and 7.5 ...... Earth and Planetary Science. Letters 122: ...
189
Journal of Paleolimnology 18: 189–206, 1997. c 1997 Kluwer Academic Publishers. Printed in Belgium.
A preliminary investigation of siliceous microfossil succession in late Quaternary sediments from Lake Baikal, Siberia M. L. Julius1, E. F. Stoermer1 , S. M. Colman2 & T. C. Moore1 1
Center for Great Lakes and Aquatic Sciences, University of Michigan, 2200 Bonisteel Blvd., Ann Arbor, MI 48109-2099, USA 2 U.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 varied greatly within the intervals sampled, ranging from samples devoid of siliceous microfossils to samples with up to 3.49 1011 microfossils g 1 sediment. Fluctuations in abundance appear to reflect trends in the marine 18 O record, with peak microfossil levels generally representing climate optima. Microfossil taxa present in sampled intervals changed considerably with core depth. Within each sample a small number of endemic diatom species dominated the assemblage. Changes in dominant endemic taxa between sampled intervals ranged from extirpation of some taxa, to shifts in quantitative abundance. Differences in microfossil composition and the association of variations in abundance with climate fluctuations suggest rapid speciation in response 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 also perhaps the world’s oldest continuously existing lake. Lake Baikal lies within the central third of the Baikal Rift Zone. Extensional forces in the rift zone formed, and continue to alter, the lake’s basins. The lake consists of three basins (North, Central, and South). The South Basin has a maximum depth of approximately 1400 m and is divided from the deeper (ca. 1600 m) Central Basin by an underwater ridge which expands northeasterly across the lake in the Selenga Delta region. The Central Basin is separated from the shallower (ca. 800 m) North Basin by the Academic Ridge, a structural and bathymetric high, which extends northeasterly from Ol’khon Island. Sediments in Lake Baikal have recorded much of the lake’s history and surrounding environmental conditions. Recent multichannel seismic profiling revealed up to 7.5 km of sediment in the lake (Hutchinson
*126968
et al., 1992). Three tectono-stratigraphic units have been identified in the sediments of the lake’s South and Central Basins, with a total thickness of over 7 km in the South and 7.5 km in the Central Basin. The North Basin appears to have simpler sedimentary structure, containing only two distinct units with a total thickness of 4.4 km. Current sedimentation rates in the South and Central Basins are estimated at 0.1–1.0 mm yr 1 based upon 137 Cs and 210 Pb dating (Edgington et al., 1991). Sedimentation rates are estimated between 0.03 mm yr 1 for Academic Ridge and 0.3 mm yr 1 for the Central Basin, based upon accelerator-mass spectrometer (AMS) analyses radiocarbon dating (Colman et al., 1993). These relatively low sedimentation rates and the high volume of deposited sediment suggest the potential for reconstructing the entire history of the lake and its surrounding environments. Investigations of modern, large Pleistocene-age lakes have shown changes due to anthropogenic effects (Stoermer et al., 1990). These changes included major fluctuations 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-South Basin, and X-site of core. Bathymetric contour interval 500 m.
diatom taxa. Earlier paleolimnological investigations of Lake Baikal demonstrated great changes in diatom populations over time. Chernyeava (1970) recorded diatom populations in the upper meter of sediment from seven transect cores in Baikal’s North Basin. She found changes in diatom assemblages ranging from variations in abundance to extirpation of some taxa. The floristic changes revealed by her study could not be attributed directly to anthropogenic influences. High resolution sampling techniques were not employed and the near surface sediment was not sampled. Large, within-system variations occur in large lakes subjected to different degrees and histories of perturbations (Stoermer et al., 1993). Because of this, knowledge of assemblages being deposited under present conditions is needed to aid in the interpretation of assemblages in older lake sediments. This is particularly true for Lake Baikal because of its many endemic species. Phytoplankton assemblages contain Baikalian species which are not known to exist in other lakes, making knowledge of the components and influences on the surficial sediment record crucial for paleolimnological investigations. Modern records of Lake Baikal plankton assemblages are incomplete, making plankton ecology difficult to interpret. Stoermer et al. (1995) investigated
a number of surficial samples from stations in each of Lake Baikal’s basins. These results can serve as a within-lake calibration set and show general north to south decrease in weight abundance of diatoms. This agrees with earlier work by Popovskaya (1991), who indicated regional differences in phytoplankton abundance. Endemic diatoms (Cyclotella baicalensis Skv., C. ornata (Skv.) Flower, C. minuta (Skv.) Antipova, Aulacoseira baicalensis C. Meyer, and Aulacoseira skortzowii Edlund, Stoermer, and Taylor) were dominant in all basins. The interaction of these taxa is similar to that of Cyclotella and Aulacoseira species found in other large northern lakes (Stoermer et al., 1981). Cyclotella species dominate the summer flora and certain Aulacoseira species are more abundant in colder portions of the year. Non-endemic diatom taxa (Synedra acus K¨utz., Stephanodiscus binderanus (K¨utz.) Krieg., and Cyclostephanos dubius (Fricke) Round) were more abundant in the South Basin than in the North Basin. The southern end of Lake Baikal has been the site of regional urbanization and industrialization (Galazii, 1991; Belt, 1992), and the presence of these taxa has been attributed to anthropogenic influences (Stoermer et al., 1995). Investigations of longer sediment cores utilized information from the surficial sediments to recon-
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191 struct pre-modern lake environments. Edlund et al. (1995) analyzed diatom succession in two short cores. One core from the South Basin recorded influences of anthropogenic activity and showed two zones of high anthropogenic influence: one estimated from 1950 to 1991 and the other occurring during the late 1700’s. These periods of perturbation are in agreement with the history of human activity in the area. The 1950’s were a period of rapid industrialization in the area, including the construction of a dam at Irkutsk which controlled and significantly changed the water levels in the lake (Stewart, 1990). The earlier activity corresponds to a shift in the lifestyle of humans in the area. Prior to this period the native Buryat people were primarily nomadic. The late 1700’s marked the peak influx of European peoples in the region, who gradually influenced the native people to shift to a more agrarian economy, including slash and burn agricultural techniques (Humphrey, 1983). The second core reported by Edlund et al. (1995) was retrieved from the North Basin. Microfossils in these sediments were less influenced by anthropogenic activity and appear to reflect fluctuations in past climate. Major changes were present in the abundance of dominant diatoms deposited during a period known as the Little Ice Age (ca. 1600–1850), which was characterized by cooler climate conditions in Siberia (Khotinskiy, 1984). Lake Baikal’s sediment record is well suited for paleolimnological climate reconstruction on a longer time scale. Glaciations have occurred in the lake’s drainage basin, but glacial scouring has been limited to the surrounding mountain ranges rather than the lake bed (Peck et al., 1994), suggesting sediment records of glacial activity should be well preserved. Two major glacial events are recorded in land sections in Siberia during, approximately, the last 100 000 years. These are 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 periods were interrupted by a warm period known as the Karginsky Interstade (ca. 50 000–26 000 BP). Baikal’s high latitude also provides good potential to record global variations in climate, because the lake is sensitive to long-term changes in insolation patterns reflecting changes in the Earth’s orbital parameters (Kuzmin et al., 1993; Peck et al., 1994; Colman et al., 1995). Previous paleolimnological investigations documented climate fluctuations recorded in Baikal’s sediment record. Cores taken throughout Baikal’s basins suggested that changes in taxonomic composition of diatom microfossils reflected climatic shifts between
cold-dry and warm-moist periods (Bradbury et al., 1994). Unfortunately, available cores were a maximum of 11 m in length, limiting the time interval that could be examined. Peck et al. (1994) found evidence of glacial cycles in the magnetic properties record of Lake Baikal sediments. This study examined numerous cores from Lake Baikal with lengths up to 10 m. Influences of the most recent glacial event, the Sartan Glaciation, were evident in the concentration, grainsize, and minerology of the magnetic sediment components. Glacial intervals were characterized by high concentrations of magnetite and relatively large proportions of high coercivity minerals. Interglacial events showed the reverse, with increased diatom concentration diluting the magnetic minerals. Recent studies of the cores from Academic Ridge of Lake Baikal (Colman et al., 1995) not only indicate that higher levels of biogenic silica are associated with interglacial and interstadial intervals, but also that the detailed character of the variations in the biogenic silica content closely match the marine oxygen isotope record of climatic and global ice volume change. The spectral character of these variations is also very similar to that of the marine oxygen isotope record. This lends support to the conclusion that variations in the Earth’s orbit have driven glacial to interglacial climatic change in this interior continental area in a way that is nearly identical to the fluctuations seen in the marine realm (Colman et al., 1995). Based on these new findings, we relate the classical Sartan Glaciation to oxygen isotope stage 2 (12 050–24 110 BP; Martinson et al., 1987), the Karginsky Interstade to oxygen isotope stage 3 (24 110–58 960 BP; Martinson et al., 1987), and the Zyryanka Glaciation to oxygen isotope stage 4 (58 960–73 910 BP; Martinson et al., 1987). In this paper, siliceous microfossil composition is examined in a preliminary study of core material deposited during several glacial-interglacial cycles. As part of the cooperative Baikal Drilling Project, a 100 m sediment core was recovered near the village of Buguldeika on the southwestern side of Lake Baikal. Biogenic silica composition, and siliceous microfossil enumeration were performed on the first, relatively widely-spaced set of samples. Some age control was supplied by AMS dating. These data were compiled to create an initial record of past environments, focusing particularly on the major glacial-interglacial cycles of the last 500 000 years.
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192 Table 1. Taxa occurring at greater than 5% in any sample
1. Aulacoseira baicalensis (C. Meyer) Simonsen 2. Aulacoseira skvortzowii Edlund, Taylor, and Stoermer 3. Cyclostephanos species #1 4. Cyclotella gracilis Nikiteeva et Likhoshway 5. Cyclotella minuta (Skv.) Antipova 6. Cyclotella ornata (Skv.) Flower 7. Cyclotella ornata var. #1 8. 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 K¨utzing 16. Cyst #1 17. Cyst #18
Materials and methods Core recovery Two cores, each approximately 100 in length, were recovered on the southwestern side of Lake Baikal near the village of Buguldeika (Figure 1) (Kuzmin et al., 1993). Most of the core was obtained using an advanced hydraulic piston core system with a 2-m core barrel; some of the lower sections were obtained with a rotary action tool with a lined core receiver. Drilling was performed in a water depth of 365 m utilizing the coring rig mounted on a barge frozen in the lake. The coring operation proceeded in the following manner. The drill string was lowered with the bottom plate and buoyant collar attached. The bottom plate was deployed upon reaching bottom, indicated by a reduction in weight on the drill string. The core receiver was employed utilizing a pressure differential between the drill string and core barrel. The core barrel was raised, and the drill string was lowered with rotation allowing the flushing of the cavity created by sediment removal. The core was extracted using a wire-line system and the above process was repeated for a total of 31 individual core drives. Core recovery was approximately 75% in the first drill hole. Two samples were taken from each core drive, for a total of 62 samples. Splits were distributed to various members of the Baikal Drilling Project for analysis.
Table 2. AMS radiocarbon ages for Lake Baikal sediments Depth (cm)
Material
Date ( 1000)
Error ( 1000)
Sed. rate (mm/year)
84.5 188.5 264.5 392 450 515 1162 1490 1567
TOC TOC TOC TOC TOC TOC TOC TOC TOC
5.02 11.39 16.07 21.74 22.43 29.6 35.2 30.7 33.1
0.09 0.15 0.24 0.58 0.63 1.6 3.2 1.8 0
0.16 0.16 0.22 0.84 0.091 1.15 ? 3.00 0.32
Sedimentation rate calculated from 515 cm sample. Chemical analysis Biogenic silica measurements were made using inductively coupled plasma (ICP) analysis and extraction methods of weight percent biogenic silica following the procedures of Mortloch and Froelich (1989). All analyses were corrected for clay dissolution using aluminum concentration (Carter & Colman, 1994). Radiocarbon ages are based on AMS analysis of total organic carbon, as described by Colman et al. (1993). Most of the organic carbon is autochthonous (i.e. algal in origin (Colman et al., 1993)). AMS dates were used to calculate bulk sedimentation rates for the sediment intervals between dated samples. Siliceous microfossil analysis For siliceous microfossil analysis, a portion of each section was freeze-dried to reduce microfossil breakage associated with other drying methods. A dry, weighed subsample of each section was boiled for 30 min in 30% H2 O2 at 110 C. Twenty-five ml of concentrated HNO3 was added to the peroxide-sediment suspension, resulting in a rapid exothermic reaction within 5 min. The solution was then heated at 120 C for an hour. Samples were then rinsed six times with distilled water to remove oxidation byproducts. The entire portion of each sample was settled upon 18 mm circular cover slips in Battarbee chambers (Battarbee, 1973). Replicate slides were prepared from each sample using HyraxTM mounting medium. At least 500 microfossils or four 9 mm transects were enumerated, using brightfield oil immersion optics (N.A.>1.32) capable of 1200 magnification. All recognizable algal remains were counted and recorded according to
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193 their size relative to a whole specimen of their species. These functional categories were ‘reconstituted’ by addition and reported in terms of the base morphological unit, one valve of the species for diatoms and one cyst for chrysophyte cyst morphotypes. Counts were converted to absolute abundance and reported as diatom valves or microfossils g 1 dry sediment. Counts were also converted to percent abundance and reported relative to total microfossil abundance. Discussion of sedimentary distribution of taxa will be limited to the most abundant taxa. These are defined as those occurring at >5% relative abundance in at least one sample (Table 1).
Results Stratigraphy and age model Biogenic silica analysis identified 13 silica peaks in the sediment core (Figure 2). Biogenic silica levels (corrected for clay dissolution) ranged from peak values of 4.91–25.41%, to interpeak values of as low as 0%. Microfossil (g 1 sediment) data produced peaks almost identical to biogenic silica data (Figure 3). Differences between the two data sets included the absence of a microfossil peak corresponding to silica Peak 13 and the merger of silica Peaks 5 and 6 in the microfossil data. Peak microfossil values range from 2.71 108 –3.49 1011 (g 1 sediment), and interpeak values included intervals lacking microfossils. AMS dating results (Table 2) are comparable to others performed on Lake Baikal (Toyoda et al., 1993; Bradbury et al., 1994). Based on the earlier studies of cores from Lake Baikal (see introduction) and on the AMS radiocarbon ages from this drill core (Table 2), we take these intervals of high biogenic silica to represent interglacial and interstadial intervals. AMS radiocarbon ages were determined on total organic carbon and are thus subject to errors associated with redeposition and contamination. Because of these difficulties, we do not trust the ages older than approximately 30 000 BP, and believe that errors in the younger age determinations may be considerably more than the analytical precision given in Table 2. The youngest six 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 accumulation rate of about 19 cm ky 1 . This would place the barren sample at 290 cm (Figure 2) within oxygen isotope stage 2 and the sample at 454 cm toward the
Figure 2. Profile of dry weight percent biogenic silica vs depth in Lake Baikal sediment. Numbers identify peak biogenic silica levels.
end of the earlier interstadial associated with oxygen isotope stage 3. Extrapolating sedimentation rates is a dangerous exercise, especially in environments that are likely to undergo substantial changes in sediment accumulation driven by changes in depostional processes and patterns as well as by substantial changes in climate. If we take the biogenic silica peak 2 as representing the acme of oxygen isotope stage 3 (stage 3.3 of Martinson et al., 1987) and biogenic silica peaks 3 and 4 as being associated with oxygen isotope stage 5, then we can develop an age model for this core that reaches to at least 16 m (Figure 4). This model suggests that accumulation rates diminished somewhat during the last interstadal (stage 3). The widely spaced peaks in biogenic silica in the deeper part of the core suggest increased accumulation rates; however, the pattern is not sufficiently distinctive to confidently match the peaks in biogenic silica with specific interglacial and interstadial intervals of the oxygen isotope record; thus, we cannot extend an age model to the base of the recovered core section. We can only suggest that, based on the age model for the upper part of the core and on the number of silica peaks in
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194 Table 3. Dominant microfossil assemblage taxa in peak intervals in Lake Baikal sediments with correlation to zones identified by Bradbury et al. (1994) This study
Bradbury et. al., (1994)
Peak 1 assemblage 1
Stage 1
Zone V
Zone VI
assemblage 2
18 O Period
Peak 2
Zone VII
Stage 2
Peak 3
Zone VIII
Stage 5
Peak 4
Stage 5
Peak 5–6 assemblage 1
assemblage 2 Peak 7 Peak 8
Peak 9 assemblage 1
assemblage 2
Peak 10
Peak 11
Peak 12
Aulacoseira baicalensis Aulacoseira skvortzowii Cyclotella minuta Synedra acus Aulacoseira baicalensis Aulacoseira skvortzowii Stephanodiscus flabellatus Aulacoseira baicalensis Cyclotella gracilis C. minuta C. ornata Stephanodiscus flabellatus Aulacoseira baicalensis Cyclotella minuta Stephanodiscus carconensis S. grandis Aulacoseira baicalensis Cyclotella minuta C. minuta Stephanodiscus carconensis S. grandis Aulacoseira skvortzowii Stephanodiscus carconensis S. grandis Cyst #1 Cyst #18 Cyclotella ornata var. #1 Stephanodiscus grandis Cyclotella ornata var. #1 Stephanodiscus grandis Aulacoseira skvortzowii Stephanodiscus carconensis S. carconensis var. minor S. carconensis var. pusilla S. grandis Stephanodiscus carconensis S. carconensis var. minor S. grandis Aulacoseira skvortzowii Stephanodiscus carconensis S. grandis Cyst #1 Cyst #18 Stephanodiscus carconensis S. carconensis var. minor S. carconensis var.pusilla S. grandis Aulacoseira skvortzowii Cyclotella ornata var. #1 Stephanodiscus carconensis S. carconensis var. minor S. grandis Aulacoseira skvortzowii Stephanodiscus sp. #6 Stephanodiscus sp. #7
One of multiple assemblages within a single silica peak.
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195
Figure 3. Abundance (valves and cysts 1010 g 1 dry wt sediment) in Lake Baikal sediment. Numbers identify peak biogenic silica levels.
the remainder of the record, the base of the section is near 400 000 to 500 000 BP. The Brunhes-Matuyama magnetic reversal boundary was not encounterd in the core (BDP93 Baikal Drilling Project Members, 1996); therefore, the base of the core is certainly younger than about 800 000 BP. Microfossil assemblages Three types of microfossils were encountered in the sediment core: diatoms, chrysophyte cysts, and sponge spicules. Of these, the diatoms were most abundant. Planktonic diatom taxa comprise 66–93% of microfossil assemblages. Benthic diatom taxa occurred less frequently, comprising 0.6–5% of microfossil assemblages. Chrysophyte cysts are also common in Lake Baikal sediments, comprising 2–31% of microfossil assemblages. Chrysophyte cyst morphotypes were very diverse with over 100 different morphotypes identified. Siliceous components of chrysophytes other than cysts were not encountered in microfossil enumerations. Sponge spicules were rare and did not comprise a significant portion of the microfossil assem-
blages, but because of their size and solid construction, sponge spicules may constitute a significant portion of biogenic silica (Conley & Schelske, 1993). Of the 62 sediment samples analyzed for siliceous microfossils 18 contained significant quantities of these remains. This is reflected in the peaks produced in the microfossils g 1 sediment profile (Figure 3). Assemblages in each of the 18 samples were dominated by a few planktonic taxa endemic to Lake Baikal (Figures 5–9). Endemic taxa comprised over 45% of the microfossil assemblages in each sample. Endemic taxa varied in abundance within each of the samples and some species were extirpated. Persistent replacement of dominant taxa was the most striking feature of the sediment record. A variety of nomenclatural schemes have been applied to endemic Baikalian taxa, and there are differences in opinion concerning what encompasses a given species and the proper specific epithets. Because of this, an explanation of our concept is presented here. For chrysophyte cyst morphotypes an arbitrary numerical designator is used, because we cannot, at this point, correlate these structures with known species. In most cases, the most recent taxonomic interpretation of diatom systematics is used in naming diatom taxa. This included Nikiteeva and Likhoshway’s (1994) description of Cyclotella gracilis, which was previously identified as an unnamed species (Bradbury et al., 1994). Cyclotella ornata was separated from C. baicalensis and C. minuta. Observations in this study fully supported Flower’s (1993) conclusion that these are all individual species. We also separated Cyclotella ornata var. #1 from this group. This taxon appeared to differ morphologically from Cyclotella ornata, but specimens found are very eroded, making definitive taxonomic separation tenuous. All three morphotypes of Aulacoseira baicalensis (nominate, squarosa, compacta) are reported individually. These morphotypes were considered formae by Skvortzow (1937), but the opinion of Stoermer et al. (1995) was followed here. Stoermer et al. considered morphological differences ecophenotypic and similar to differences in A. islandica morphology in the Laurentian Great Lakes (Stoermer et al., 1981, 1985). The taxon reported here as Aulacoseira skvortzowii has been considered a sporangial frustule of A. baicalensis (Skvortzow, 1937; p. 303 lns 4–6), and a separate morphotype of A. islandica (Kozhova et al., 1982; Genkal & Popovskaya, 1991). This entity has been described as a species (Edlund et al., 1996) and we also treat it as a distinct species. Deep core taxa identified as Stephanodiscus carco-
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196
Figure 4. Proposed age model for first 16 m of Lake Baikal sediments.
nensis Grunow, S. carconensis var. minor Grun., and S. carconensis var. pusilla Grun. follow the interpretations of Gasse (1980). Although superficially similar to these taxa, small variations in morphology are noticed in light microscope observations of the Lake Baikal forms and more detailed ultrastructural investigation may prove them to be separate entities. Descriptions of microfossil assemblages will be given from bottom to top to simplify presentation. A summary of these descriptions with correlation to Bradbury et al. (1994) is given in Table 3. Diatoms were found first in samples from 6800–7100 cm. This corresponded to Peak 12 on the microfossil abundance profile (Figure 3). Sediment samples below this depth were devoid of siliceous microfossil remains. Two undescribed Stephanodiscus species (Stephanodiscus sp. #6, Stephanodiscus sp. #7) dominated the flora in this sediment interval. Each of these taxa share a similar ‘Bauplan’ and are not represented in any other sediment interval. Microfossils were not present in sediment samples from 5700–6800 cm. Microfossil remains appeared again between 5300–5700 cm, corresponding to Peak 11 on the microfossil abundance profile (Figure 3). A variety of Baikal diatoms were present in this interval. Stephanodiscus grandis Khursevich et Loginova, S. carconensis, S. carconensis var. minor, Aulacoseira skvortzowii, and Cyclotella ornata var. #1 were all major components of the diatom flora in this sediment interval. Chrysophyte cysts #1 and #18 were also major components of the microfossil flora. Each of these microfossil taxa was represented in other sediment intervals and formed significant portions of the population in each of these intervals. Stephanodiscus grandis, in particular, dominated in most of the sam-
ples in which it is represented, comprising 20–59% of the population. Because of the large size of Stephanodiscus grandis, it probably comprised an even greater portion of the phytoplankton biomass. Microfossils occurred infrequently in samples between 4950–5300 cm and reappeared in abundance between 4850–4950 cm. This represents Peak 10 on the microfossil abundance profile (Figure 3). Stephanodiscus grandis, S. carconensis, S. carconensis var. minor, S. carconensis var. pusilla were the major taxa in the microfossil flora. Stephanodiscus carconensis var. pusilla occurred in other sediment intervals and frequently comprised a significant portion of the population. Siliceous remnants became rare in samples between 4000–4950 cm. Stephanodiscus grandis, S. carconensis, S. carconensis var. minor, Aulacoseira skvortzowii, and chrysophyte cysts #1 and #18 appeared in abundance between 3550–4000 cm. This is represented as Peak 9 on the microfossil abundance profile (Figure 3). Aulacoseira skvortzowii levels fluctuated greatly in this interval, constituting >56% of the population in one sample and
