Vegetation and climate of the last glacial-interglacial cycle in Southern Illinois, USA

Journal of Paleolimnology 14: 337-354, 1995. @ 1995 KluwerAcademic Publishers. Printed in Belgium.

Vegetation and cl imate of the last glacial-interglacial cycle in Southern Illinois, USA *

Hong Zhu & Richard G. Baker Department of Geology, University of Iowa, Iowa City, IA 52242, USA

Received 5 October 1994; accepted 5 March 1995

Key words: pollen, palaeoecology, paleoclimate, glacial, interglacial, DCA, North America

Abstract

Raymond Basin and Bald Knob Basin, Montgomery County, Illinois, formed as kettles during Illinoian time. Fossil pollen from these basins provides information on vegetation and climate during the last glacial-interglacial cycle. The pollen profile at Raymond Basin contains an expanded Sangamonian section and an early Wisconsinan section, but both are missing the late Wisconsinan. The ages for the following pollen zones are estimated by correlation with the deep-sea t5180 record.

In the late Illinoian, Picea-Pinus pollen zone 1, dating from about 150-130 ka, represents an apparently closed boreal coniferous forest indicating a cold late-glacial climate. The Sangamonian includes three major pollen zones ranging from about 130-75 ka. The early Sangamonian is represented by Quercus-Ulmus-Carya-Fagus dominance in zone 2, indicating vegetation comparable to the modern deciduous forest and climate that was warm and moist. The middle Sangamonian in zone 3, which is characterized by Ambrosia-Poaceae-Cupressaceae-Quercus pollen, suggests a savanna vegetation and a warm, dry climate. The late Sangamonian is subdivided into a Quercus-Ulmus- Carya subzone (4a) that indicates a mesic forest and greater precipitation; a Quercus-Ambrosia subzone (4b) that suggests drier climate and savanna conditions; and a Quercus-Liquidambar-Carya subzone (4c), containing the 'southeastern forest' element, Liquidambar, suggesting the peak in warmth and moisture. The early Wisconsinan is represented by a transitional Picea-Chenopodiineae zone (5). This unusual assemblage suggests a cool prairie-like vegetation, perhaps with scattered Picea trees at the end of that depositional interval. The Middle Wisconsinan is marked by the return of high percentages of Picea and Pinus pollen in zone 6. The latest pollen zone (7) is dominated by Chenopodiineae pollen and is absent at Raymond Basin. It is most likely Holocene in age, and probably represents prairie conditions and warm, dry climate.

Apparent surface-temperature and apparent effective-moisture curves were derived from the first detrended correspondence analysis (DCA) axis of the pollen data. The first axis correlates well with the normalized deep-sea ~t80 curve of sea surface temperature, and the second is controlled mainly by precipitation. The interglacial vegetation differs from that predicted by models driven by orbitally-induced insolation curves.

Introduct ion

Ice cores, deep-ocean cores, and Chinese loess sequences commonly contain records of one or more glacial-interglacial cycles. Quaternary pollen records

* This is the 9th in a series of papers published in this special AMQUA issue. These papers were presented at the 1994 meeting of the American Quaternary Association held 19-22 June, 1994; at the University of Minnesota, Minneapolis, Minnesota, USA. Dr Linda C. K. Shane served as guest editor for these papers.

are the most widely used proxy of continental climatic change, but only a handful cover even one such cycle. One of the few examples is a continuous pollen record for the last 140000 years is the Grande Pile in east France (Woillard, 1978). Another is the sequence from Clear Lake, California (Adam et at., 1981; Sim et aL, 1981). Both these records of environmental change match the ocean record very well. The only one in the Midwest section of North America is the controversial Pittsburg Basin site in Illinois (Grtiger, 1972a). A num-

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ber of other paleobotanical sites formerly described as Illinoian or Sangamonian in age have been shown to be Wisconsinan (Leighton, 1960; Kapp & Gooding, 1964).

Pittsburg Basin is the site of the first palynological study in Southern Illinois (Grtiger, 1972a). It covered the late Illinoian, Sangamonian, the entire Wiscon- sinan and Holocene. Since then other lake beds in southern Illinois have been located on an Illinoian-age kettle complex, and they also contain long records back through the last interglacial.

Raymond and Bald Knob Basins are only about 32 km away from Pittsburg Basin (Fig. 1). They contain similar long records, beginning with sediments deposited as late-lllinoian ice retreated northward. B ar- ring unconformities, these basins were recognized by Griiger as having great potential for long and continuous pollen records from Late Illinoian, Sangamonian, Wisconsinan and Holocene time.

The purpose of this study is to document the history of the vegetation since Illinoian time by providing long and detailed pollen records from Raymond and Bald Knob Basins. Research on ostracodes from the same cores provide additional information on environments in the lakes (Curry, 1995) and will be published separately. This study also establishes regional pollen zones to provide an overview of long-term dynamics of vegetational succession in the mid-continent through a glacial-interglacial cycle using multivariate statisti- cal procedures and climatic correlation with deep-sea records.

Geological setting

Illinois is in the Central Interior Lowlands Province (Fenneman, 1938). Most of the state was glaciated and generally lacks high relief, except for the non- glaciated southern part and northwestern parts (Fig. 1), where Paleozoic rocks are at the surface. During the Illinoian glaciation the state was almost completely covered with ice. The ice sheet of the last glaciation (Wisconsinan) covered only about the northern third of the state (Fig. 1) and did not reach the study area.

Natural lakes are rare in south-central Illinois, and most of the few that were present were drained in the 1920's. These few originated as stagnant-ice depressions of the Illinoian glaciation (Jacobs, 1970) and are located in an area of numerous fiat-topped ridges. The ridges are underlain by well-drained sand and gravel (Jacobs & Lineback, 1969). In contrast, the generally closed, undrained depressions between them are below

the water table. The surrounding topography in Mont- gomery County is generally fiat with some hills and mounds.

Raymond Basin (RB) is a kettle basin located on the boundary between the hilly 'ridged drift' of the Illi- noian till plain, a fiat, undissected till plain, about 6 km north of Butler, Montgomery County (39 ° 14145" N. Lat.; 89 °3 lt15~ W. Long.; NW, NW, NW, NE Sec. 10, T 9 N, R 4 W). The morphology of the basin is eir-

-cular and shallow, and it is about 1.6 km in diameter. The coring site is in the middle of the Basin, which is now cultivated farmland. According to a landowner, the basin was used as a cattle pond in the early 1900's, and then drained in the early 1910's (B. B. Curry, written communication, Feb. 17, 1992). It is closer to the Wisconsinan Glacial Limit than Pittsburg Basin about 32 km to the south-east (Fig. 1).

Bald Knob Basin (BK; 39 °13~ N. Lat.; 89 ° 1T30" W. Long.; SE, NE, NE, NE Sec. 22, T 9 N, R 2 W) is north of Hopwood Farm (Sec. 23, T 8 N, R 2 W), closer to the Wisconsinan Ice front than Ray- mond Basin. The basin is round and shallow, and it is drained by Hurricane Creek. The present land owner says that the basin was never formally incorporated into a drainage district, but the land was tiled and drained prior to 1930 (B. B. Curry, written communication, 1992). The topography suggests to Curry that the basin was naturally breached, and the stratigraphy of the core suggests that breaching occurred in the early Wisconsinan or Sangamonian.

Modern environments

Illinois climate is continental and is characterized by strong seasonal variations. Warm, moist Gulf air masses move northward into the state and dominate the summer weather. Cold, dry Canadian air from the north and northwest flows into the state during the winter months. During the fall, Pacific air masses reach Illi- nois and provide mild, dry weather (Bryson, 1966; Bryson & Wendland, 1967; Waite, 1974).

The mean annual temperature in Illinois is about 13 °C, with extremes of +45 and - 3 0 °C. Average precipitation is about 1000 mm per year (Wendland et al., 1992), more than half of which occurs during the growing season of about 185 days (Norton et al., 1932; Fehrenbacher et al., 1967), but periods of summer drought are fairly frequent. Natural drainage is poor in the vast, almost level lowland areas (Ross & Case, 1956).

339

/ Pre-llllinoian

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0 50 mi. I I I I I

0 80 kin. -N

Fig. 1. Location map and glacial setting of study area RB = Raymond Basin, BK = Bald Knob Basin, HF = Hopwood Farm, and PB = Pittsburg Basin.

The pre-settlement vegetation (Fig. 2) of south- central Illinois is within the 'Prairie Peninsula', where a mosaic of Quercus-Carya forests and prairies were present (Ktichler, 1964). This mosaic was completely destroyed by intensive agriculture in the early 19th cen- tury and replaced by today's farmland. In Montgomery County, the major vegetation consisted of timber and prairie. Prairie occupied about two-thirds of the area of the county and was concentrated in the north; the

southern part was forested. Near Hillsboro the growth was principally black oak with some white oak, hickory, sassafras and hazel (Broadhead, 1875).

According to Worthen (1873), forests were common south of Raymond Basin on the ridged drift, and prairies common north, east and west of the basin on the till plain. The archival documents described Ray- mond Basin as a 'wet prairie with thin soil' and 'level prairie with 2nd rate soil' (B. B. Curry, personal corn-

340

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DECIDUOUS

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Fig. 2. The pre-settlement vegetation map in Illinois and surrounding area.

munication, 1992); in 1853 it was still an uncultivated swamp and had meandering streams flowing across the basin in 1875. The basin was drained and tiled in 1921. Bald Knob Basin was described as a marsh in 1817 (B. B. Curry, personal communication, 1992).

Materials and methods

Raymond Basin and Bald Knob Basin were cored in 1990 and 1991 respectively by Brandon Curry, Illinois State Geological Survey for his Ph.D. dissertation on ostracode paleoecology. A 16.6 m core was obtained from Raymond basin and a 9.51 m core from Bald Knob Basin. Both reached the underlying Illinoian till or diamicton. The cores were wrapped in Saran Wrap and aluminum foil to keep the sediments moist and to

avoid contamination. Lithology was described in the field and again in the lab by Curry. He split the core lon- gitudinally and sent us half. We used modified syringes to take volumetric pollen samples at 5 cm intervals in Raymond Basin and 10 cm intervals in Bald Knob Basin. Sample volume varied from less than 1 ml to 5.2 ml. For Raymond Basin, 218 samples were processed and 174 contained abundant pollen. Samples from the top 267 cm and bottom (1144-1665 cm) lack pollen. For Bald Knob Basin, 82 samples were processed and 66 samples had pollen. Sediment between 83 to 274 cm contained no pollen, and the basal 57 cm (899-956 cm), composed of sand and gravel, was not sampled.

Pollen was processed using a technique modified from Faegri & Iversen (1975). Each sample was immersed in water and its volume was measured in graduated 15-ml centrifuge tubes by displacement of water. The sample was placed in 10% hydrochloric acid and one marker tablet of Eucalyptus pollen grains was added to the sample (numbers were adjusted to weight of tablet; range: 0.085-0.1015 gram) to allow the cal- culation of pollen concentration (Maher, 1972). Sam- ples were then treated with 10% KOH, sieved through 125 micrometer mesh, and immersed in 48% hydroflu- oric acid for 24-48 hours. After acetolysis treatment the samples were suspended in 10% sodium pyrophos- phate and sieved through 7 micrometer nylon mesh to remove clay (Cwynar et al., 1979). The residues were washed with tertiary butyl alcohol, stained with alco- holic safranin, and mounted in silicone fluid (viscosity 4000 centistokes).

Pollen counts were done at 400 x; difficult grains were examined at lO00times under oil immersion. Identifications were based on keys in McAndrews et aL (1973), Kapp (1969), and Moore & Webb (1978), and on comparisons with the pollen reference col- lection in the Geology Department at the Universi- ty of Iowa. Pollen diagrams of both percentages and concentrations were subdivided into zones using the stratigraphically-constrained cluster analysis on the TILIA program (Grimm, 1987), and ordination was used to correlate between sites.

Results

Radiocarbon ages

Radiocarbon ages from Raymond Basin and Bald Knob Basin were obtained on bulk sediments and are

341

given in Table 1. Two ages from Raymond Basin are infinite dates and do not correlate with the three ages from BK-1 of Bald Knob Basin. The youngest age of 26 090 RCYBP is from BK-4 at Bald Knob Basin at the depth of 198-209 cm, which is in the barren zone above 272 cm of BK-1. The cause of these discrepan- cies is unknown.

The major part of the pollen analysis in this study deals with the geological time span older than 40 000 RCYBP. Geological evidence (till plains and core stratigraphy) and the available radiocarbon dates nar- row the possible time span to the period from late Illinoian (~ 150 ka) through Wisconsinan. The only alternative for dating these older assemblages is by correlating changes of vegetation - and the climatic changes inferred from them- with oceanic records.

As Bradley (1985, p. 412) states 'accurate dating remains the nemesis of most paleoclimatic studies dealing with events beyond 30 000 RCYBP, and many important questions remain unanswered due to the inadequate dating control'. In this study we interpret the changes in the pollen record in terms of temperature and precipitation, and correlate those paleoclimatic events with oceanic records.

Raymond basin pollen sequence

Raymond Basin pollen percentages are shown on Fig. 3 along with the clustering dendrogram depicting the results of the stratigraphically constrained cluster analysis (Grimm, 1987). The Raymond Basin sequence was arbitrarily divided into the six pollen assemblage zones that had formed at the total sum of squares value of 20 (Fig. 3). Zone boundaries are placed at the mid- point between the two samples of adjacent zones (Zhu, 1994).

Zone 1 (Picea-Pinus-Abies, 1144-1040 cm) is dominated by conifer pollen which comprises 90% of the AP (arboreal pollen) percentage. The rest of the AP is mainly contributed by Quercus (7%) and other deciduous trees. Among coniferous pollen, Picea (up to 60%) and Pinus (up to 50%) are the two big contributors. Abies (up to 10%) and Larix (up to 5%) reach their highest values in the entire sequence. In the upper part of zone 1, pollen of deciduous taxa tends to increase. For example, Alnus reaches 2%, Betula 3%, and Quer- cus 25%. Only 5% NAP (non-arboreal pollen) pollen is present continuously and includes Poaceae, Cyper- aceae, Artemisia, Ambrosia and Chenopodiineae. Total pollen concentrations of all taxa in the pollen counts range from 5 x 103 to 70 x 103 grains m1-1.

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Table 1. Radiocarbon dates from Raymond and Bald Knob Basins (Curry, personal communication, 1994)

Core Depth (cm) Material Age (RCYBP) Lab No. 613C

RB-2 271-281 gastropods >39,410+1a Beta-53295 - 6.5 RB-2 491-502 organic silt >41,100 ISGS-2147 -20.5 BK-4 198-209 ? 26,090-t-710 ISGS-2294 -26.9 BK-4 287-305 ? >35,300 ISGS-2293 -28.9 BK-1 320-326 organic silt 35,4804-800 ISGS-2291 -27.6 BK-1 393-399 organic silt 35,8604-850 ISGS-2292 -26.8 BK-1 427-430 ? 46,2004-2100 ISGS-2290 -26.4

In zone 2 (Quercus-Ulmus-Carya-Fagus, 1040- 988 cm) percentages of conifer pollen decrease abruptly to 5% and are replaced by high percentages of pollen of deciduous taxa. The most important taxa are Quer- cus (50%), Ulmus (15%), Carya (16%) and Fagus (10%). Quercus and Ulmus reach their highest values in the entire section. NAP remains low except for Ambrosia pollen, which increases to 10% at the top of the zone. Total pollen concentrations range from 25 x 103 to 50 x 103 grains/ml.

In zone 3 (Ambrosia-Poaceae-Cupressaceae- Quercus, 988-675 cm) the ratio of AP to NAP changes dramatically. NAP comprises 60%-65% of the total pollen grains in zone 3, and AP falls to 35%-40%. Ambrosia pollen dominates the NAP ranging from 25 % to 57%. The rest of NAP are contributed by Poaceae (7%), Chenopodiineae (4%) and Tubuliflorae (2%). The percentage of deciduous-tree pollen declines with increase of NAR Quercus pollen decreases from 40% to 20% (as low as 10%) in the lower part of zone 3, and then starts to increase to 25% at the very top of the zone. Other decreasing taxa include Ulmus (to 3%), Carya (to 4%) and Fagus (to 1%). Fraxinus nigra type (3%) and Fraxinuspennsylvanica (4%) remain unchanged in this zone. Liquidambar pollen first appears here and is sporadically present at up to 1%. Cupressaceae pollen (probably juniper) reaches its peak in the middle part of the zone at 15%. Total pollen concentrations range from 15 x 103 to 30 x 103 grains m1-1.

Zone 4, (Quercus-Liquidambar-Ambrosia, 676- 526 cm) is generally dominated by deciduous forest elements. AP percentages reach almost 98%, especially for the upper part. This zone is further divided into three subzones 4a, 4b and 4c.

In subzone 4a (Quercus-Carya-Ulmus, 675- 650 cm) AP percentage varies between 80%-90%.

Important deciduous trees and their percentage maxima for this zone are Quercus (32%), Carya (30%), Ulmus (15%), Fraxinus nigra type (5%) and Fraxi- nus pennsylvanica type (5 %). NAP percentages remain low at 5% to 10%, mainly represented by Ambrosia. Cupressaceae pollen is absent. Total pollen concentrations reach 60 x 103 grains/ml.

In subzone 4b (Ambrosia-Poaceae-Quercus, 650- 600 cm) AP percentages drop to a low of 52%, as pollen of most deciduous trees decreases in percentage. Ambrosia pollen rebounds to reach 35%, and Poaceae to 8%. Total pollen concentrations drop back to about 25 x 103 grains/ml, with Ambrosia increasing to 8 x 103 grains m1-1.

Subzone 4c (Quercus-Liquidambar-Carya, 600- 525 cm) is characterized by the highest AP percentages in the whole section at 98%. Deciduous-tree pollen types are the dominant contributors with more diversity than elsewhere in the sequence. While Quer- cus (40%), Carya (15%), Ulmus (10%) and Fraxinus (10%) rise to about the same percentages as in subzone 4a, Liquidambar pollen rises to its maximum of about 15%. In addition, Fagus pollen increases to 11%, and Ostrya-Carpinus, Juglans cinerea and Cupressaceae reach low peaks. Total pollen concentrations range from 52 x l03 to 150 z 103 grains ml - l , the highest total pollen concentrations of the whole sequence, and pollen concentrations of all the taxa of deciduous trees reach their maxima here.

Zone 5 (Chenopodiineae-Ambrosia, 525--415 cm) is dominated by NAP, which increases from 30% to 90% while AP percentages decline from 60% to 10%. Chenopodiineae, the major component of NAP reaches 80%, the highest value for this taxon in the core. Ambrosia pollen percentages fluctuate between 10% and 45%. Other NAP components appear in low per-

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centages between 1% to 5% such as Poaceae, Cyper- aceae, Artemisia and Tubuliflorae. This zone has significant transitional characteristics. All the deciduous taxa are present only in the lower part and disappear upward. Picea and Pinus come back in moderate percentages between 5% to 30%. Total pollen concentrations drop significantly and vary between 5 x 103 to 27 x 103 grains m1-1.

In zone 6 (Picea-Pinus-Chenopodiineae, 415- 271 cm) AP percentages increase to 75%. Dominant elements of AP are Picea (<5% to 60%) and Pinus (5%-60%). Quercus (15%) and Carya (5%) co-exist with the coniferous taxa. NAP percentages decrease to 25%, and include Chenopodiineae (10%-30%), Cyperaceae (0%-20%), Ambrosia (5%-10%). Cyper- aceae reaches its peak in this zone. Total pollen concentrations are the lowest in the section, ranging from 7 × 103 to 15 × 103 grains m1-1.

Bald Knob Basin sequence

Bald Knob Basin pollen percentages are plotted on Fig. 4 with the zonation dendrogram. The same stratigraphically-constrained cluster analysis is used and results are similar to Raymond Basin. Pollen concentrations are generally consistent with their pollen percentages (Zhu, 1994) and help to divide zone 4 into two subzones 4a and 4c.

Zone 1 (Picea-Pinus, 899-828 cm) is dominated by conifer pollen. The AP percentage is 90%, over two-thirds of which is conifer pollen. At the base of the zone, Picea pollen reaches 60%, the highest value for this taxon in the entire core. The other prominent coniferous tree is Pinus (25%). Both taxa nearly disappear at the top of the zone. Betula pollen is present throughout the whole zone but in very low percentages (2%). Fraxinus nigra type pollen reaches its maximum at 18%. Pollen from other deciduous tree taxa (Quercus, Ulmus, Fraxinus pennsylvanica type, Ostrya-Carpinus) begin to increase at the top of this zone as Picea and Pinus decrease. Quercus percentages range from 15% to 30% in the upper part of the zone. Low but continuously occurring NAP includes Ambrosia, Artemisia, Poaceae and Cyperaceae Total pollen concentrations range from 17 x 103 to 62 x 103 grains ml - 1.

In zone 2 (Quercus-Ostrya/Carpinus, 828-710 cm) pollen percentages of coniferous trees like Picea, Pinus, Abies, and Larix (not shown) decrease abruptly, and are nearly absent except for Cupressaceae, which rises and remains stable at about 5%. AP is still domi-

nant in this zone but conifer pollen is replaced by pollen of deciduous trees. Quercus pollen increases to a high of 43%, and other deciduous taxa reach primary or sec- ondary peaks (Ostrya/Carpinus (16%), Carya (12%), Ulmus (10%), Fraxinus nigra type (9%), Fraxinus pennsylvanica type (5%) and Fagus (5%). Fagus, Tilia and Ostrya/Carpinus pollen are at their highest levels attained in the entire pollen sequence. Poaceae pollen is consistently present (7%) and Ambrosia percentages increase to 10% at the top of the zone. Total pollen concentrations fluctuate markedly in zone 2 ranging between 15 x 103 and 90 x 103 grains/ml. Total pollen concentrations reach an early peak in this zone, which corresponds to the interval of dominantly deciduous taxa.

Zone 3 (Ambrosia-Chenopodiineae-Quercus, 710- 553 cm) total NAP percentages increase gradually from 15% at the bottom to 60% near the top, while the total AP percentage decreases from 90% to 30%. The increase of Ambrosia pollen from 10% to 47% and Chenopodiineae from 4% to 21% and the corresponding decrease in Quercus (to 20%) and Ulmus (to 4%) mark the shift from AP to NAP dominance. Percent- ages of all other deciduous tree taxa are highest in the lower part of the zone and decrease upward in the core. Pollen of Artemisia (2%), Poaceae (6%) and Cyper- aceae (2%) remain consistent as before, while Iva ciliata pollen increases slightly from 2% to 3%. Cupres- saceae percentages reach their peak (6% to 7%) in this zone. Total pollen concentrations in zone 3 decrease from 45 x 103 to 15 x 103 grains m1-1.

Zone 4 (Quercus- Ulmus-Liquidambar553-432 cm) as at Raymond Basin, is generally dominated by deciduous tree pollen. Total AP percentage rises to its maximum value (95%). In subzone 4a (Quercus-Carya- Ambrosia, 553-520 cm) AP percentages increase from 60% to 95% The average percentages of important deciduous trees in this subzone are Quercus (35%), Carya (12%), Fraxinus nigra type (8%), and Fraxinus pennsylvanica type (5%). Ulmus percentages increase from 2% to 13%. Ambrosia is the primary NAP pollen, and its percentages decrease upward in the core from 30% to 8%. Cupressaceae pollen remains steady at around 5%. Total pollen concentrations increase from 15 × 103 to 30 × 103 grains/ml. Pollen concentrations are very low in the subzone.

Subzone 4b is missing at Bald Knob Basin. Sub- zone 4c (Quercus-Liquidambar-Carya-Ulmus, 520- 432 cm) is marked by a peak of Liquidambar pollen similar to that in subzone 4c at Raymond Basin. Quer- cus is the dominant pollen (40%), followed by Carya

346

(17%), Ulmus (14%), Liquidambar (9%) and Juglans cinerea (2%). All reach their maxima for the entire pollen sequence. Total pollen concentration in subzone 4c rises from 30 x 103 at the bottom to 90 x 103 grains m1-1 at the top.

Zone 5 (Chenopodiineae-Poaceae-Ambrosia, 432- 355 cm) begins with an abrupt increase to 50% in Chenopodiineae, followed by maximum values of 47% in Ambrosia, and 58 % of Poaceae pollen. NAP replaces AP and becomes dominant. Cyperaceae pollen percentages are also highest at around 30%. Pollen percentages of deciduous trees decrease gradually at the beginning and then disappear. Quercus pollen grains decrease from 15% to 1%, and start to increase again later along with Pinus pollen, which increases to 10%. Total pollen concentrations drop and vary from 16 x 103 to 40 x 103 grains m1-1.

Zone 6 (Pinus-NAP, 355-300 cm) has transitional characteristics. Pollen percentages of all NAP decrease as Pinus pollen increases from 10% to a maximum 37%. Other taxa with small increases in percentages include Picea (2%), Abies (5%) and Betula (4%). Corresponding to the Pinus increase at the top of the zone, Quercus percentage decreases from 20% to 6%. AP grains once again are dominated by coniferous taxa, but only contribute 50% to the pollen sum. The 50% NAP grains are accounted for by Ambrosia (18%), Chenopodiineae (30%), Poaceae (10%), and Artemisia (10%). Total pollen concentration declines from 30 x 103 to 12 x 103 grains m1-1.

Zone 7 (Chenopodiineae-Ambrosia, 300-0 cm) is characterized by the highest percentages of NAP, between 76% to 89%, and moderate percentages of Quercus (10% to 15%). Pinus pollen, which was pre- viously the dominant element, is nearly absent in this zone. NAP is mainly Chenopodiineae pollen (60% to 77%), which reaches its maximum of the entire section. Ambrosia, a lesser contributor to NAP, has percentages that vary from 7% to 20%. Total pollen concentrations range from 2 x 10 3 tO 30 × 10 3 grains/ml, and correspond very well with pollen percentages.

Pollen is not preserved between 84 cm and 274 cm in zone 7. The pollen composition above and below this barren interval remains almost unchanged. This stratigraphic unit consists of oxidized and leached silty clay loam to clay (Curry, 1995).

Discussion

Ordination of the Raymond Basin Pollen sequence

We used detrended correspondence analysis (DCA) on 23 pollen taxa in the numerical analysis of pollen data. The eigenvalues for the first two axes (DECORANA calculates only the first four axes) are 0.54389 and 0.25572. According to Hill (1979), axes with eigenvalues much lower than the highest eigenvalues are probably not significant. Therefore, the first two axes with the highest eigenvalues can summarize the major features of pollen data.

The DCA ordination provides a view of the Ray- mond Basin pollen data in a reduced dimensional space. Because DCA corrects for the 'arch effect' (caused by a functional dependence of the second axis on the first) and for compression between sample distances toward the ends of the axes (Hill & Gauch, 1980; Gauch, 1982), equal distances anywhere in the ordination are thought to represent equal ecological distances.

The first axis of ordination separates northern trees, mostly conifers, from temperate taxa (Fig. 5). Larix, Abies, Picea and Pinus, which have the most northern centers of distribution, have the highest scores on the axis. Betula, a deciduous tree with northern distribution, occurs closer to the conifers than to the more southern deciduous trees. Deciduous trees have the lowest scores on the first axis and are not well separated from modern prairie elements, which have the medium scores between northern and southern forests. The first axis appears to reflect a temperature gradient.

The second axis separates arboreal from non- arboreal taxa. Fagus has the highest value of 4.32. Liquidambar, Juglans, Ulmus, Carya and Ostrya- Carpinus types of pollen also have relatively high scores (above 2). These taxa are all major elements of deciduous forest. Ambrosia, Chenopodiineae, Aster- aceae, Cyperaceae and Artemisia, all non-arboreal pollen taxa, have the lowest values, less than 1. Cupres- saceae pollen, probably representing Juniperus, also lies within the dry grassland range. The values of conifers are between deciduous trees and non-arboreal taxa. The significance of axis 2 appears to reflect effective moisture.

The scatter plot of the scores of the first two axes shows the clear separation among four major different vegetation types (Fig. 5). The boreal taxa group together, and indicate a climate with low tempera-

4.5

4

3.5

3

2.5

2

1.5

1

0.5

0

-0.5

• Fagus Raymond Basin

I Liquidambar

I Juglans nigra

• Ulmus • Juglans cinerea • Carya

• Ostrya-Carpinus

lIQuercus

1 Fraxinus pennsylvanica type ICel t is

1Fraxinus nigra type

• Cupressaceae

• Poaceae • Ambrosia

• Betula

Larix

Abies Pinus l l

• Artemisia Picea • Cyperaceae

• Tubuiiflorae

• Chenopodiineae

DCA Axis 1

z:s

Fig. 5. Raymond Basin DCA ordination of taxa.

ture and medium precipitation. Among the prairie taxa group, Ambrosia is surprisingly separated from the other prairie taxa. Instead, it is closer to the group composed of Fraxinus pennsylvanica type and Fraxi- nus nigra type, Celtis and Cupressaceae pollen. This association suggests that Fraxinus here is E quadran- gulata, not E nigra (Preston, 1980; McAndrews et aL, 1973; Burns & Honkala, 1990). This group is widely distributed, but the distributions of the most probable species are centered in the central Midwest, suggesting intermediate temperature and moisture regions. Pollen types of major trees in the southern and eastern deciduous forests are separated from others, and therefore indicate a warm and moist climate.

The larger the eigenvalue, the more important it is statistically (Hill, 1979). The first axis with higher eigenvalue (0.54) plays a more crucial role than the second axis (0.26) in the Raymond Basin pollen record. In other words, temperature appears to explain more of the variance than effective moisture in this data set. Today temperature controls the distribution of major vegetation types. Nevertheless, it is clear that both temperature and effective moisture are important in controlling vegetation.

The ordination also suggests that the number of taxa is closely related to temperature and moisture availabil-

2.5

~4 .to

< £3 173

0 .5

0

0

347

Raymond Basin

Zone 6

~7 Zone S

• Zone 4

O Zone 3

• Zone 2

O Zone 1 i . \

0.5 1 1.5 2 2.5 DCA Axis 1

Fig. 6. Raymond Basin DCA ordination of samples. B = basal sample in section, T = top sample in section. Arrows connect top of one zone with bottom of subsequent zone.

ity (Fig. 5). At low temperature, the range of available moisture appears to be narrower, as is the diversity of taxa; at higher temperature, the moisture range is broader and number of taxa is greater.

The ordination of RB samples (Fig. 6) is direct- ly related to the ordination of pollen types; that is, samples having high scores on an axis are dominated by pollen types having high scores on that axis. The distance between points is a measure of their sim- ilarity. The samples of each pollen zone should be distributed close to the vegetation type they represent in the ordination of taxa, but they will not be exactly superimposed unless they have nearly identical pollen composition.

The ordination of pollen samples traces the long term vegetation history at Raymond Basin through time. In Fig. 6, samples of each pollen zone are labeled by different symbols and connected in their stratigraphic order. The interpretation of Raymond Basin pollen stratigraphy in terms of both vegetation and climate shows a complete glacial-interglacial cycle from cold (boreal conifers) to warm and moist (deciduous forest), to warm and dry (grassland) and back to cold (conifers). The traces of vegetation changes in the plot

348

(Fig. 6) suggest that the change from cold to warm is abrupt while the change from warm to cold is slower and more gradual (assuming relatively constant sedi- mentation rate). There are several subcycles of changes within the interval of warm temperature suggesting changes between forest and grassland. This is evidence for substantial fluctuations in effective moisture during that period.

The ordination of pollen samples also provides a check on the pollen zonation by cluster analysis. It is useful to divide the major clusters into subzones which are not as apparent in the pollen percentages (Fig. 3). For example, the cluster analysis only suggested six major pollen zones; ordination confirms those clustering results, but it reveals significant differences within pollen zone 4, which is further divided into 3 subzones.

Ordination of the Bald Knob Basin pollen sequence

The DCA for Bald Knob Basin pollen data was per- formed on 22 pollen types. The first and second axis eigenvalues are 0.48805 and 0.24412, slightly small- er than those of Raymond Basin DCA with 23 pollen types. The first axis indicates the high positive values ofPicea, Pinus, Betula, Abies and NAP (Fig. 7). Low values are represented by prairie elements of NAP and widespread deciduous trees. The first DCA axis from B aid Knob Basin pollen data also seems to reflect a cold to warm temperature gradient. The second axis separates NAP from AP of deciduous trees, and NAP over- laps with boreal conifers in the middle. As discussed for Raymond Basin, the second axis seems primarily to indicate an effective moisture gradient for those taxa. The difference is the switch between the positive and negative values vs. moist and dry conditions in the two basins.

The ordination of taxa (Fig. 7) separates the taxa into three groups. The boreal forest taxa occur together along with Artemisia, which generally occurs with prairie forbs (Bartlein et al., 1984). In modern pollen rain studies of the Midwest, the 5% to 10% isopols of Artemisia pollen overlap the southern mixed area of boreal forest in Southern Canada (Webb & McAn- drews, 1976). Deciduous trees form a second group, and prairie taxa compose the third group. The ordination puts Cyperaceae and Tubuliflorae closer to the boreal forest taxa and leaves the Poaceae pollen in the driest environment. The ordination of samples for Bald Knob Basin pollen data (Fig. 8) groups seven

-1.5

- I

-0.5

0

0.5.

o~

"~ 1 <

Bald Knob Basin • Liqu idambar

1.5

2.

Z.5.

3"

Fagus • T i l ia • • Ulmus

Fraxinus penn. type • - - • •Ostrya-Carp inus

Carya Juglan • s Quercus • Fraxinus nigra type

cinerea

• Cupressaceae • Betula

Iva ciliata • Abies

• A r temis ia • • Pinus • Ambrosia

• Chenopodiineae • Tubuliflorae

• Cyperaceae

• Poaceae

Picea

3.s o ols ~ ~is i zls ~ fs DCA Axis 1

Fig. 7. Bald Knob Basin D C A ordinat ion o f t a x a .

major pollen zones in stratigraphic order. The ordination generally reflects the same climatic events as Raymond Basin. Although the rapid-warming event from zone 1 to 2 is not so sharply defined as in the Raymond Basin ordination, the rest of the climatic events correlate fairly well: 1) Zones 1 and 6 in each are cool with boreal taxa. Zones 2 and 4 are warm and moist with deciduous forest taxa, and zones 3 and 5 are warm and dry, and presumably represent prairie. 2) The transition in zone 5 from warm-humid to cool-dry conditions within zone 5 is gradual. 3) Both sequences suggest that the second episode of cold climate was not so severe as the first one. However, samples of zone 7 are not seen at Raymond Basin. They seem to represent the warmest-dry period in the region, possi- bly the Holocene.

Vegetation history

Zone 1. The vegetation during deposition of zone 1 is interpreted as a closed northern conifer forest composed primarily of Picea, Pinus, Abies and Lar- ix. Northern deciduous shrubs and trees (Alnus and Betula) were also present, suggesting a cold climate. The constant low percentages of Poaceae, Cyperaceae, Ambrosia, Chenopodiineae and Asteraceae suggest

0.5 ¸

c,J e~

~:l.S

c~

2.5

Bald Knob Basin

\ \

\

• Zone 7

~r Zone 6

V Zone S

• Zone 4

® Zone 3

[ ] Zone 2

O Zone 1

3 ~ 0 0.5 1 1.5 2 2.5 3 3.5

DCA AxEs 1

Fig. & Bald Knob Basin DCA ordination of samples. B =basal sample in section, T = top sample in section. Arrows connect top of one zone with bottom of subsequent zone.

either that the closed forest still had some openings to permit growth of those shade-intolerant plants, or they grew along the shore of the lake. The vegetation interpreted from the pollen diagram and DCA ordination is supported by the presence of Picea and Larix needles in the lower part of the zone.

Sands and diamicton underlie the basal sediments at Raymond Basin (Curry, 1995), indicating that the formation of the kettle basin and the accumulation of zone 1 sediments must have begun with the retreat of Illinoian ice. A late Illinoian age can therefore be assigned to zone 1. The gradual increase of percentages of deciduous trees (Quercus, Ulmus, Fraxinus) in the upper part of zone 1 suggests that Illinoian ice retreated from the region as climate ameliorated.

Zone 2. The vegetation represented by zone 2 is a closed and rich deciduous forest accompanied by low percentages of herbs. The primary forest components, Quercus, Ulmus, Carya and Fagus are the most common genera of upland forest in Holocene pollen diagrams in the eastern Midwest (Holloway & Bryant, 1985). Today, oak and hickory trees are still the major components of the upland forest in South-Central Illi- nois, and elm occurs primarily on flood plains and valley sides (Fowells, 1965). The high percentages of

349

Ulmus pollen in zone 2 make it difficult to assess its importance to the vegetation, but similar high percentages occur widely in the early Holocene (Webb et al., 1983). Fagus, an upland taxon, is not found in the modern local forests, but it occurs only 100 km to the east of Raymond Basin. It may imply that the vegetation of zone 2 is analogous to the deciduous forests presently found in Indiana and Ohio. All the northern conifers disappeared completely from the vegetation.

Zone 3. The vegetation of zone 3 is difficult to interpret. The high and consistent percentages of Ambrosia pollen are dominant following the sudden decrease in arboreal pollen. The DCA ordination of Raymond Basin pollen samples indicates that the change is primarily caused by lower effective moisture (Fig. 6). This effect may have resulted from lower precipitation, higher temperature, or most likely, a combination of the two. One hypothesis is that weedy annual species like A. trifida or A. artemisiifolia are locally very abundant on dry mud fiats exposed semi-regularly by fluctuat- ing lake levels during dry climatic episodes. Another hypothesis is that a perennial species of ragweed like A. psilostachya, a dry prairie perennial, was extremely abundant on upland prairies in the area. Both probably indicate grassland or prairie. Ambrosia is abundant as a prairie indicator in many Holocene diagrams.

The 'Prairie Peninsula' extended eastward through most of north central-Illinois in the Holocene prior to European settlement (Fig. 2). Evidence for the frequent oscillation of the Holocene prairie-forest boundary in the Midwest has been reported by many investigators for the entire Holocene interval (Bartlein et al., 1984; Davis, 1977; Kim, 1986; Maher, 1982: McAndrews, 1966; Van Zant, 1979; Webb et al., 1983; Winkler et al., 1986; Wright 1976; Chumbley, 1989; Chum- bley et al., 1990; Baker et al., 1992) The northern border moved from west to east across southern Min- nesota and Iowa in the Holocene, but probably never reached far into southern Wisconsin (Chumbley, 1989; Chumbley et al., 1990; Baker et al., 1992). We pos- tulate that an earlier version of this 'Prairie Peninsula' expanded and retreated across southern Illinois during the last interglacial, as represented by the fluctuations in AP-NAP in Figs 3 and 4 and on the DCA plots (Figs 6 and 8).

Peaks of Cupressaceae pollen exist consistently in the interval of high NAP between 880-680 cm in zone 3 at Raymond Basin. At Bald Knob Basin, this pollen type is sparsely present throughout much of the section, with low peaks where NAP is high. At Pitts-

350

burg Basin however, prominent peaks of this pollen type were interpreted as Taxodium (Griager, 1972) because it was found with other elements of the southern mesic deciduous forest in zones correlative with zone 2 and 4 at Raymond Basin and Bald Knob Basin. This interpretation has been controversial, and we have found no peaks of cupressaceous pollen in zone 4. Because our Cupressaceae peak occurs in this NAP- rich interval interpreted as prairie, we believe that the most likely taxon contributing the pollen would be Juniperus virginiana, a tree that presently grows in warm, dry habitats along the prairie edge.

Zone 4. This zone is primarily indicative of deciduous forest except for a short period of Ambrosia dominance in the middle. The vegetation of subzone 4a looks similar to the deciduous forest represented by pollen zone 2 and indicates warm, moist climate as well.

Subzone 4b is absent at Bald Knob Basin. The vegetation of this subzone signals the return of ragweed dominance although not as great as in zone 3; more tree pollen is present in subzone 4b. Those trees may suggest a savanna, or perhaps they are limited to mesic habitats near the lake. The composition of this pollen subzone implies a drier climate than subzones 4a and 4c, but not so dry as the one in zone 3.

The vegetation of subzone 4c is interpreted as analogous to a more southern deciduous forest than subzone 4a. Quercus Carya and Ulmus although present in the region throughout the whole period of zone 4, reach their maxima in this pollen subzone. The maxima of pollen concentrations (Zhu, 1994) supports the idea that forest cover was at its maximum. The maxima of Liquidambar and Fagus pollen suggest that subzone 4c is a very warm interval where effective moisture was highest. The modern northern limit of sweetgum is south of Raymond Basin, and its major distribution area is within the southeastern U.S. Fagus is now distributed mainly in the eastern U.S. (Burns & Honkala, 1990). Warm, moist air from the Gulf of Mexico presumably caused the expansion of the southern and eastern taxa into the Midwest.

Zone 5. The vegetation inferred from transitional zone 5 is complicated. The dominant upland vegetation is interpreted as savanna at the base of the zone, grading to prairie in the middle. At the start of the zone, remnants of the deciduous forest remained, but increas- ingly dry conditions caused the forest to be replaced by prairie. The high values of Chenopodiineae pollen suggest that the climate started to become drier, but the

dominance of this single NAP taxon presents similar problems in interpretation as Ambrosia did in zone 3. Both indicate open, dry conditions, but why is the Ambrosia of zone 3 replaced by the Chenopodiineae in zone 5? The DCA ordinations of pollen taxa provide one possible explanation. Axis 1 (Fig. 5) reflects mainly temperature, suggesting that the temperature requirements for the species of Ambrosia and those in the Chenopodiineae are different. The species of Ambrosia were apparently favored by warmer conditions, and those representing the Chenopodiineae by cooler conditions. The cooling conditions are supported by the increases of the conifer pollen in the top part of zone 5. This transition from warm back to cool and cold was slow and continuous (Fig. 6). The climate inferred for zone 5 from the ordination reaches the maximum dry conditions in the whole section. The presence ofbothAmbrosia and Chenopodiineae at Bald Knob Basin suggests that local hydrological conditions may be equally important in controlling the dominance of pollen taxa in the dry intervals. If these local conditions are a controlling factor, then the interpretation suggested by DCA ordinations would be invalid.

Zone 6. The latest vegetation recorded at Raymond Basin is that in zone 6, which is dominated by conifer pollen and less extensive deciduous tree pollen and Chenopodiineae pollen. The pollen assemblage differs from typical modern boreal forest pollen spectra as exemplified by zone 1; rather it seems to reflect a mixed vegetation consisting of elements of boreal forest, deciduous forest and prairie. In the DCA ordination (Fig. 6) the samples lie between boreal forest (zone 1) and dry chenopod-dominated prairie (zone 5). Oak and hickory trees, although present in the region throughout the whole period of zone 6, must not have formed extensive forests. The modern boundary between conifer-hardwood forest, boreal forest, and prairie in northwestern Minnesota may be the closest modern analog to this pollen assemblage. The pollen concentration decreases upward, reflecting a less pro- ductive vegetation with more open ground. The low concentrations of Picea and Pinus pollen suggest that these trees were not as dominant in the vegetation as they were in zone 1. Modern pollen rain studies in northwestern Minnesota indicate that the highest percentages of Pinus pollen also occur in the pollen rain of the northern edge of prairie (Webb & McAndrews, 1976). Together, the increase in Pinus and Picea in the upper part of the zone signals the return to cold climate. Zone 6 probably represents the early part of

spruce and pine dominance in the Midwest Wisconsi- nan succession, where Pinus pollen disappeared during the full-glacial Wisconsinan interval (Baker et aL, 1989, 1991; King, 1973, 1981; Watts, 1983; Wright, 1989).

Zone 7. Almost 2 m of sediment in the middle of this zone, which is present only at Bald Knob Basin, is barren of pollen. Sediment above zone 6 at Raymond basin is also barren. Curry (1995) suggests that the silty sediment in this interval represents regional loess fall correlating with late Wisconsinan Roxana Loess and Peoria Loess, and later deposition of Holocene alluvium. This evidence suggests that zone 7 represents at least part of the Holocene, and that the vegetation may represent oak savanna.

Vegetation and climate

When cold climate prevailed in the area, northern conifers flourished near the Illinoian glacial margin (zone 1). As the southern margin of Picea or Picea- Pinus forest shifted northward, the temperate vegetation quickly expanded into the area from the south and the east in response to increasing temperatures from warm, moist air masses from the east coast and the Gulf of Mexico (upper part of zone 1). A temperate deciduous forest with an eastern center of distribution, and a prairie originating from the west, alternative- ly occupied South-Central Illinois during the oscillat- ing humid and dry cycles within the long warm phases (zones 2 to 4). When moderate temperatures with abundant evenly spread precipitation prevailed, forests covered the region (zones 2 and 4); when the climate became drier, probably from expansion of Pacific air mass dominance, forests were replaced by prairie or savanna vegetation (zones 3, 5, and 7).

Both the ordinations (Figs 5, 6, 7 and 8) and the pollen sequences (Figs 3 and 4) indicate that the long- term dynamics of vegetation record a regular glacial- interglacial cold-warm-cold cycle, and within the interglacial, a humid-dry-humid cycle. Both also reveal the different transition patterns during cold-to-warm and warm-to-cold periods. The abrupt transition from cold to warm reflected by vegetation (zones 1 and 2) probably is partly related to rapid melting of the ice sheet and to the faster northward expansion rate of species when climate ameliorated; the relatively gradual transition from warm to cold is probably related to the slower glacial advance, slower southward migration rate, or

351

high competition rates among those species when their previous living space is much compressed because of climate deterioration.

Global climate and chronological correlation

To quantify the climatic signal from Raymond basin, we plot values of DCA axes 1 and 2 in stratigraphic order to interpret pollen stratigraphy in terms of rela- tive temperature and effective moisture. The resulting temperature and effective moisture curves correlate fairly well with the widely accepted deep-sea ~180 temperature profile (Fig. 9). The sea surface temperature profiles, based on oxygen-isotopic analysis of fossil Foraminifera and on faunal analysis, have similar patterns from all the world's oceans. The normalized paleo-temperature curve, derived from ~180 from shell carbonate (Martinson et al., 1987), shows three main warm phases within oceanic event 5 (between 130 and 70 ka), the oldest of which is the warmest. The apparent temperature curve from Raymond Basin (Fig. 9) is very subdued. The four periods of higher normalized 180 values appear to correlate with four intervals of very slightly warmer apparent temperature at Raymond Basin. Fluctuations in apparent effective moisture at Raymond Basin are more prominent; three dry (and still warm) periods may correlate best with the oceanic events 5a, 5c, and 5e, and it may be that these represent periods of maximum oceanic warmth.

An alternative correlation could be with event 5e, the GRIP ice core record (GRIP Members, 1993), which shows considerable variability in oxygen iso- topes during oceanic event 5e. At this point, we prefer the correlation with all of event 5, because other evidence appears to indicate a better fit with all of event 5 rather than with event 5e (Grootes et al., 1993; Keig- win et al., t994; McManus et al., 1994).

Recent models suggest that early peaks in obliqui- ty of the earth's orbit led to substantial warming in the North American mid-continent early in the last interglacial (Crowley & Kim, 1994; Harrisdon etal., 1995). Both models predict that summer temperatures would have been 4-8 °C warmer than at present and the latter predicts a slight decrease in soil moisture, all caused by orbitally-induced changes. The Harrison et aL (1995) models also predict that both present and last interglacial vegetation in Illinois and Iowa would be mesic deciduous forest. Our work indicates that prairie was dominant during much of the last interglacial, and a large body of literature indicates that prairie was also

352

age (ka) deph (cm) depth (cm)

o

~ ~ Holocene

2OO

onsinan 4oo.

t J. ~ o.g 1" 1.g ~ zd ~ a.g 12oo . . . . . . .

2~-1 0 1 0.5 1 a.5 2 2.5 3 3.5 cold warm dry wet

~5180 normalized curve (Martison eta/., 1987) RB apparent temperatures RB apparent effective moisture

Fig. 9. Correlation of the derived curves for apparent temperature and apparent effective moisture for Raymond Basin (RB) with the (6180 normalized curve of deep-sea record.

present in the Holocene of the Midwest. The location of the prairie-forest border may be among the most difficult to predict from models (Baker et al., 1992).

The major cold and warm events correlate fairly well throughout the last glacial-interglacial time. The rapid warming and slow cooling phenomena appear to be large-scale atmospheric events both in the sea and on the continent.

Conclusions

1. The pollen sequences at Raymond Basin and Bald Knob Basin record the late Illinoian through early Wiseonsinan vegetation in southern Illinois.

2. These sequences indicate that the vegetation imme- diately before and after glaciation was dominated by spruce-pine forests. Interglacial vegetation alternated between mesie deciduous forest and prairie.

3. The major controls of glacial-interglacial vegetation seem to be temperature; fluctuations with the

last interglacial are more related to available moisture.

4. The pollen stratigraphy in Southern Illinois can be translated into climatic terms, and these major oscillations correlate well with the deep-sea record (Fig. 9).

5. The transitions from cold to warm are rapid in both the oceanic and continental records; warm to cold changes are more gradual.

6. The late Full Wisconsinan is probably not recorded by pollen-bearing sediments in this region.

Acknowledgments

The authors thank Dr Brandon Curry for supplying us with the cores and for many helpful ideas. Critical comments by an anonymous reviewer and especially by L. J. Maher greatly improved the manuscript.

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Documents

Vegetation and climate of the last glacial-interglacial cycle in Southern Illinois, USA