Palynological and Radiocarbon Evidence for Deglaciation Events in the Green Bay Lobe, Wisconsin

QUATERNARY RESEARCH ARTICLE NO.

46, 251–259 (1996)

0064

Palynological and Radiocarbon Evidence for Deglaciation Events in the Green Bay Lobe, Wisconsin LOUIS J. MAHER, JR.

DAVID M. MICKELSON

AND

Department of Geology and Geophysics, University of Wisconsin—Madison, 1215 West Dayton St., Madison, Wisconsin 53706 Received July 19, 1995

A new and significant site of organic silty sand has been found beneath the Valders till at Valders Quarry in northeastern Wisconsin. This is now the earliest known late-glacial site associated with red till ice advances in the western Great Lakes area. Leaves of terrestrial plants washed into a small depression provide a date of 12,965 { 200 yr B.P. (WIS-2293), which is significantly older than the Two Creeks Forest Bed (ca. 11,800 yr B.P.). Percentage and concentration pollen diagrams suggest that the site was open and distant from a closed Picea forest. No wood or Picea needles have been found. This date is statistically indistinguishable from 12,550 { 233 yr B.P., the mean of three dates for the end of inorganic varve sedimentation at Devils Lake, 160 km southwest at the terminus of the Green Bay Lobe. Assuming that the Green Bay lobe vacated its outermost moraine in the interval from 13,000 to 12,500 yr B.P., only a short time was available for retreat of the ice margin over 350 km, drainage of red sediment from Lake Superior into the Lake Michigan basin, readvance of over 250 km, retreat of at least 80 km, and advance to this site. The time for these events appears to have been too short to resolve by current radiocarbon technique. This extremely rapid collapse of the Green Bay lobe has a calibrated age of about 15,000 cal yr B.P., about that of the dramatic warming seen in the Greenland ice cores. q 1996 University of Washington.

INTRODUCTION

Until the early 1970s all of the reddish-brown till that covers most of eastern Wisconsin was thought to postdate the Two Creeks Forest Bed and represent a major late Wisconsin glacial event in the Great Lakes area. The till was called the Valders till and, at least locally, the time of presumed readvance had substage significance (Valderan Substage) (Frye and Willman, 1970). Evenson (1973) argued that the red till in northeast Wisconsin and western Michigan was deposited by several glacier advances, some of which predate the forest bed and others of which postdate the forest bed. Figure 1 shows the extent of red drift in the Lake Michigan region. Figure 2 shows the lithostratigraphic (till) units in an area covered by the Green Bay and Lake Michigan lobes. All of these were formally defined as members of the Kewaunee Formation by Mickelson et al. (1984). The limits of the Two Rivers and Glenmore members mark the

maximum readvance of post-Two Creeks ice as described by Evenson (1973) near Two Rivers and later by Evenson and Mickelson (1974; Mickelson and Evenson, 1975; McCartney and Mickelson, 1982; and Acomb et al., 1982). Evenson’s original arguments that there were pre- and post-Twocreekan tills were based mainly on geomorphology. In particular, he noted that the Glenwood shoreline of Lake Michigan (Glacial Lake Chicago) was cut into the red (Valders) till surface south of Manitowoc (Fig. 2), but not into red tills north of Manitowoc. Based on radiocarbon age determinations, the Glenwood shoreline is older than the Two Creeks Forest Bed (Eschman and Farrand, 1970; Hansel and Mickelson, 1988). Thus, the Valders till, the type section of which is at the Valders Quarry (Fig. 2), was argued (Evenson, 1973; Evenson, et al., 1976) to be pre-Two Creeks in age. Black (1980, 1983), in a series of papers and rebuttals, disagreed with this interpretation, preferring instead the views of Bretz (1951) and earlier workers who had argued that the till at Valders Quarry was equivalent in age to that above the Two Creeks Forest Bed at the Two Creeks type locality. Acomb et al. (1982) and McCartney and Mickelson (1982) studied the stratigraphy of the red till units in the Lake Michigan and Green Bay lobes. They argued that stratigraphic interpretation supported Evenson’s ideas about the relative age of the Valders till and the Two Creeks Forest Bed in spite of the fact that the authors were unable to find any site where the Valders till directly underlies the Two Creeks Forest Bed. Although the presumed equivalent in the Green Bay lobe (Chilton Member) (Fig. 2) is found beneath the forest bed, the Valders Member is missing at the Two Creeks type locality, where the forest bed overlies lake sediment and the older Haven Member. To our knowledge there is no documented site where the Valders Member can actually be shown to underlie the forest bed. In 1990, routine operations at the Valders Quarry, the type section of the Valders Member, exposed water-laid sediments beneath the Valders till and above sandy till of the Horicon Formation of the Green Bay lobe. The pond sediments below the Valders till do not contain macroscopic wood fragments that might be carbon dated and otherwise compared to Two Creeks wood. However, the silty sand

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does contain a sparse but well preserved suite of pollen. The pollen assemblage is very different from that at the Two Creeks sites. Instead, it represents an open-landscape tundra assemblage with a wide variety of pollen types that suggest long-distance transport. In 1992, an interval of the sand a meter below the Valders till was found to contain abundant small leaves of terrestrial plants that have proved suitable for carbon dating. GEOLOGY AT THE SITE

The Valders Lime and Stone Company Quarry is developed in Silurian dolomite about 20 km east of the Niagaran Escarpment (Fig. 2, site V). It is situated on a high spot on the landscape, and most of the section being mined is in Silurian reef carbonates. Striations on the dolomite surface have been recognized at the quarry since the early part of this century. The following is the traditional interpretation at the site (Black, 1980; Thwaites and Bertrand, 1957; Evenson and Mickelson, 1974). In parts of the quarry where older sandy, yellowishbrown (7.5 YR 4/6) till of the Horicon Formation rests on

FIG. 2. Locations and lithostratigraphic units in eastern Wisconsin. See index map at upper right. M, Manitowoc; S, Sheboygan; TC, Two Creeks; TR, Two Rivers; V, Valders.

FIG. 1. Map of the Lake Michigan and Green Bay lobes showing the terminal moraine and the area of red drift. C, Cheboygan Bryophyte Bed; DL, Devils Lake; V, Valders.

the bedrock surface, north–south striations are well-developed on the dolomite and others are absent. In the eastern part of the quarry the reddish-brown clayey Valders till (2.5 YR 4/6 to 5 YR 5/3) lies directly on the dolomite. Here the north–south set of striations is present, but is cut by a younger east–west set as well. Thus, the east–west set was clearly associated with the Valders till. More recently, striations with orientations between the north–south and east– west sets have been found beneath the Horicon Formation till. Thus, several generations of striations are present. It still appears, however, that the north–south striations are related to the Horicon advance and that later east–west ones are related to the Valders. North–south striations beneath till of the Horicon Formation at the quarry agree with flow directions observed from drumlin orientations in the vicinity. Although the direction of flow that produced the latest east–west striations was questioned by Evenson and Mickelson (1974), they now

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DEGLACIATION OF GREEN BAY LOBE, LAURENTIDE ICE SHEET

accept that the Valders till was deposited by ice advancing out of the Lake Michigan basin. Black agreed with that view, stating it again in his 1983 paper. Grain-size distribution and clay mineral content of the Valders till and other units in the area are discussed by Acomb et al. (1982). The Valders till at the quarry is clearly distinguishable from the Horicon Formation till based on its reddish-brown color and clayey texture. Along what was then the north face of the quarry [NE Sec 32 T19N R22E, 44704.6*N, 87753.3*W, elevation 275 m (900 feet)], stripping of overburden during the winter of 1989–1990 exposed a section with clay and silt, rich in organic matter, that grades upward to fine and medium sand (Fig. 3). These deposits directly overlie Horicon Formation diamicton that is reduced and gabbro-rich in its upper 0.5 m. The sand is directly overlain and somewhat deformed by reddish-brown clayey till of the Valders Member. The lateral extent of this sand and silt unit is not known, but the exposed section is about 60 m long, and it seems unlikely that the extent in the direction perpendicular to the exposed section is more than 30 or 40 m. A layer of reddish-brown clay, broken and deformed by sand diapirs, is present in the sand within 30 cm of the Valders till contact (Fig. 4). The interbedded clay has the same color as the overlying till, but none of the till’s pebbles

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FIG. 4. Sand diapir below contact of sand with Valders till. Till and clay are reddish-brown (5YR 5/3); sand is yellowish-gray (2.5 Y 5/2). Figure traced from color slide Maher-414-10.

and cobbles. The clay must have washed into the accumulating sand deposit close to the Valders ice margin, implying that sand deposition was associated with the Valders advance, not the earlier retreat of the Horicon ice. Sand deformation may be normal soft-sediment deformation or it could be associated with permafrost conditions that developed during and after late phases of sand deposition. Small ice wedge casts (Fig. 3) penetrate the upper part of the sand. After the winter of 1991–1992, abundant small fossil leaves were found exposed on a dry, wind-scoured sand surface about a meter below the base of the Valders till. The leaves are nearly invisible on a freshly cut surface, but had been etched out during winter storms. The leaves are abundant only in a restricted area in loose silt and fine sand; they represent terrestrial surface litter that was washed into the pond sediment. The leaves are concentrated easily by placing lumps of sand on a sieve and washing with a jet of distilled water. N. G. Miller (New York State Museum) and R. G. Baker (University of Iowa) are studying the macroflora. A portion of the organic debris was dried; 12 g of leaf material yielded an uncalibrated age of 12,965 { 200, yr B.P. d13C Å 025.8 (WIS-2293), about 1000 yr older than the Two Creeks Buried Forest. DESCRIPTION OF POLLEN

FIG. 3. Site stratigraphy of Valders Quarry ponded sediments.

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Samples for palynology were collected from the outcrop. An arbitrary zero datum was established about 1.2 m below the Valders till, at a place where the massive sand at the top of the unit was more obviously laminated. Fourteen samples were taken at 10-cm intervals through a depth of 130 cm (Fig. 3). The five samples from 0 to 45 cm were from a unit of laminated sand with beds up to 1 cm thick. Five samples from 45 to 92 cm were from an interval of silty sand with thinner bedding. Two samples were taken from a sand unit with thicker layers that extended from 92 to 110 cm. The

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FIG. 5. Pollen concentration diagram of the Valders Quarry sediments (grains/g(dry)). See Figure 3 for diagram’s stratigraphic position.

two lowermost samples were from the underlying diamicton, which is crudely laminated and contains abundant far-traveled lithologies. Three grams of moist (2.4 g dry) sediment were selected for each palynology sample. To calculate the concentration (grains/g) of the pollen taxa, Lycopodium spores were introduced to serve as a marker standard by adding to each sediment sample, one tablet of Stockmarr’s (1973) batch 212,761, whose mean and standard deviation is 12,490 { 490 spores. Following Faegri et al. (1964), the samples were treated sequentially in 10% HCl, 10% NaOH, 48% HF, and acetolysis solution. Because a large quantity of silica gel remained after processing, the residue was transferred to a Nitex filter (Cwynar et al., 1979), which passes the gel and clay-sized particles while retaining objects larger than about 7 mm. The material trapped by the filter consisted mostly of pollen and spores as well as certain resistant mineral crystals. This residue was transferred to an aqueous heavy liquid (ZnBr, density 2 g/ml) and centrifuged in a folded length of plastic tubing (Maher, 1987). When the ‘‘float’’ was mounted in glycerin and examined optically, it contained a very wellpreserved suite of palynomorphs. Figure 5 shows the pollen concentration (100’s of grains/ g of dry sediment) in the sediments determined by the exotic spike technique. For these 19 taxa, the total pollen concentration in the 12 samples from sandy silt ranges from 1100 to 5100 grains/g with a mean value of 2800 grains/g. The largest pollen concentrations occur in the stratigraphic inter-

val from 40 to 100 cm. Of the two samples from the underlying diamicton, that from 120 cm had 200 grains/g, whereas the sample from 130 cm (not shown) contained only the Lycopodium spike. The time taken to deposit this silty sand is not known, but it probably represents a few years or a few decades. The Valders Quarry sediments contain a very diverse pollen assemblage. Picea (spruce) pollen tends to dominate, as it does in most Midwestern late-glacial sites. It is significant, however, that there is no macrofossil evidence for the presence of Picea. When pollen samples are processed through the NaOH step, the material routinely is put through a finemesh copper screen in order to separate out conifer needles, wood fragments, seeds, and other large objects. Artemisia (sage or wormwood) is well represented throughout, and it makes the ratio of arboreal to nonarboreal pollen lower than at other comparable sites. The relative abundance of Pinus (pine) is greater than normally found in midwestern Picea-dominated zones. These characteristics suggest that the Valders sediments formed in an open environment far from the closed forest. When the local pollen production is low, grains arriving from long distances are more likely to be found, such as those of Pinus, Carya (hickory), Juglans (walnut), Fagus (beech), and Tsuga (hemlock). The pollen assemblage suggests that the site was treeless, but it does not specify its temperature. The absence of Picea needles and wood in the sediment, coupled with the presence of ice-wedge casts, suggests a locally cold, open tundra environment.

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FIG. 6. Comparison of three partially contemporary percentage pollen sequences. Devils Lake core samples older than 10,000 yr B.P. (spacing proportional to depth interval 519–633 cm in core). The individual wood-bearing Two Creeks sites (Fig. 2) all formed within an interval of about 250 yr. For purposes of this diagram the sites are ordered according to their present elevation. Wiese (1979) noted a statistically significant inverse correlation between elevation and the abundance of Cyperaceae; lower sites tend to be wetter which favors growth of sedge. See Figures 3 and 5 for the location and pollen concentration of the Valders Quarry sediment.

Figure 6 compares the pollen spectra of the Valders sediments with certain other pollen sites in eastern Wisconsin, all based on the sum of the 19 common terrestrial taxa. The Two Creeks Forest Bed is exposed in several outcrops along Lake Michigan and in the Green Bay lowland to the west (Fig. 2). Broecker and Farrand (1963) compiled many 14 C dates from wood attributed to Two Creeks time, concluding that the Two Creeks forest was killed about 11,850 yr B.P. That date has stood up well; Kaiser (1994, p. 295) reports 10 AMS 14C dates from the type region that range from 11,760 to 12,035 yr B.P. Kaiser, working with treerings from 21 wood samples from the type Two Creeks site and four other equivalent sites in the Green Bay Lowland, produced a cross-dated record totaling 252 yr. The individual records ranged from 30 to about 230 rings. Many of the outermost rings are very thin, suggesting that glacier advance and rising lake level drastically affected the trees. Wiese (1979) collected samples from a number of Two Creeks sites for pollen analysis, and he combined his counts with data from Schweger (1966). Spectra for 16 samples are shown on Figure 6, plotted according to the present elevation of each site. One of the frustrating aspects of Two Creeks palynology is that although all sites contain abundant macrofossils, the pollen diversity is very low. Picea and Cypera-

ceae (sedge family) always dominate; for reasonable count sizes, the other taxa are too scarce to yield continuous curves. Beetles at Two Creeks (Morgan and Morgan, 1979) suggest mild conditions (moisture from dry to wet and mean July temperatures of 147 to 167C), which apparently favored rapid colonization of the drier areas by Picea, with Cyperaceae in wetter areas. In such an environment, small openings in the forest would be overwhelmed with local spruce and sedge pollen—especially when one recalls that most Two Creeks sites are forest sites, found only because of the interest that buried wood arouses in the casual observer. The pollen spectra from the sediments under the Valders till at the Valders Quarry are shown at the bottom of Figure 6. The two samples from the underlying Horicon Formation diamicton had too little pollen to calculate valid percentages; the 12 levels are all from the stratified sediment above. The Valders Quarry sediments contain a much more diverse pollen assemblage than do the Two Creeks samples. The arboreal/nonarboreal pollen ratio at Valders is consistently smaller than at Devils Lake, 160 km to the southwest of Valders, where there is a contemporaneous sediment sequence (Fig. 1). Artemisia and Pinus percentages are much greater in the Valders sediments than in the early record at Devils Lake. The mean value of pollen concentration (Fig. 5) in the 12 samples from sandy silt is 2800 grains/g (range: 1100–5100 grains/g). For comparison, the mean value of

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COMPARISON WITH OTHER SITES

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total pollen concentration for the same 19 taxa in the Devils Lake sediments older than 10,000 yr B.P. is 406,000 grains/ g (range: 39,000–821,000 grains/g). The mean total pollen concentration at Valders is less than one percent that at Devils Lake. All these characteristics suggest that the Valders sediments formed in an open environment far from any closed forest. A goal of this study was to determine the relative age of the Valders Member with respect to the Two Creeks Forest Bed. If the type Valders till correlates with the red till over the forest bed (Two Rivers and Glenmore members), then the pond sediments deposited during advance of the Valders ice should correlate with the Two Creeks Forest Bed. We have demonstrated a marked difference in their pollen assemblages. The evidence in the pre-Valders sediment for open conditions, the absence of wood, the presence of icewedge casts, and the 14C date of 12,965 { 200 yr B.P., show that they were deposited in an earlier and much more severe environment than the Two Creeks. If our association of the deposition of the pond sediment with the Valders advance is correct, then the type Valders till predates the Two Creeks Forest Bed. The Cheboygan Bryophyte Bed in northern lower Michigan (Fig. 1), long thought to be a key site indicating that the Straits of Mackinac were free of ice about 13,000 yr B.P., has recently been redated at about 11,800 yr B.P. and reevaluated as a Two Creeks site (Larson et al., 1994). Thus, except for postglacial bottom-of-core sediment dates, the Valders site appears to be the oldest radiocarbon dated site in the western Great Lakes area associated with glacial events between 15,000 and 12,000 yr ago.

western Canada; the Medford till of northwest Wisconsin is thought to have such a source. An unoxidized sample of Medford till was processed for study, but no pre-Quaternary pollen was noted. Its major pollen taxa are Pinus, Cyperaceae, and Poaceae (grass family). Cretaceous sediments also occur in Canada along the Missinaibi and Abitibi rivers southwest of James Bay. This source is over 800 km northeast of Valders, directly along a likely glacier flowline. Cretaceous sediments entrained by the ice from this source may have contributed well-preserved Aquilapollenites pollen to the outwash silt and clay in Valders Quarry. RADIOCARBON DATES AND THE RESOLUTION OF THE DEGLACIATION CHRONOLOGY IN EASTERN WISCONSIN

A number of pre-Quaternary palynomorphs were also recovered from the Valders sediments. Five well-preserved grains of Aquilapollenites, a Cretaceous and Paleocene taxon, were recorded, along with a number of grains resembling the Myrtaceae, a dozen ‘‘hystrichosphaerid’’ acritarch tests, and some others. Half the samples contained specimens rather closely resembling Veryhachium, a three-armed Silurian acritarch (Traverse, 1988, Fig. 6.6(f), p. 125). The acritarchs probably come from the local Silurian limestone entrained by the glacier. Because the sediments lacked datable wood, before the terrestrial leaf fragments were discovered, we originally considered concentrating enough pollen for an accelerator date. Although the redeposited pollen makes up less than 10% of the total pollen, its presence would confuse any measurement, and the idea was dropped (Maher et al., 1993). The presence of Aquilapollenites pollen may bear on the source of the sand and silt. There are no known local Cretaceous strata. Some of the pre-Quaternary pollen in clay till in the Midwest may come from Cretaceous sediments in

The Green Bay lobe’s retreat history has been the subject of much speculation on the basis of little hard evidence from the region itself. The area is underlain by Paleozoic carbonate rocks, the lakes are usually hard-water lakes, and dates derived from their sediment—especially the lower part—may have little to do with the age of the sample. Estimates of the time of retreat from the Johnstown moraine at the maximum limit range from as little as 12,500 yr B.P. (Black and Rubin, 1967–1968) to as great as 15,560 yr B.P. (WIS-442; Disterhaft Farm Bog, Bender et al., 1971, p. 479) and 15,940 yr B.P. (WIS-1519; Hook Lake, Steventon and Kutzbach, 1985, p. 463). Dating the Green Bay lobe’s retreat based on tenuous correlation of ice front positions and moraines from farther west in Wisconsin and in the southern part of the Lake Michigan basin is also suspect because of the long distance of correlation and the difficulty of correlating across lobe boundaries. Maher (1982, p. 120) has argued that dates of basal organic sediment from Devils Lake provide better control on the time of ice retreat. The basin of Devils Lake formed when the Green Bay lobe was at its maximum, and its sediment should date back to the time the Johnstown moraine was built. A reasonable interpretation is that this time is represented by banded bluish clay encountered at a depth of about 6 m in the lake’s sediment. Three meters of this laminated clay was penetrated during coring operations (Maher, 1982); based on later subbottom echo-profiling, it may extend much deeper. The clay is very reduced, with occasional crystals of vivianite and tiny raspberry-shaped aggregates (‘‘framboids’’) of pyrite (E. D. Glover, personal communication, 1982). The bands average about 1-mm thick and, if they are annual layers, the sampled laminated interval represents several thousand years of glacier presence. Rhythmically laminated inorganic sediment is common in sediment of proglacial lakes along the Johnstown moraine, and rare in postglacial deposits. If this laminated sediment indicates the local presence of glacier ice, then Devils Lake, situated in a quartzite terrain, is an excellent place to date

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PRE-QUATERNARY FOSSILS

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TABLE 1 C Dates Calibrated by CALIB, v. 3.0.3c (Stuiver and Reimer, 1993 and Bard et al., 1993)

14

14

C AGE

Site and sample number(s)

yr B.P.

Cal yr B.P.

Valders Quarry (WIS-2293) Devils Lake (average of 3 dates: W-1004, W-1073, W-1075)

12,965 { 200 12,550 { 235

15,380 14,725

Two Creeks (Traditional values)

11,850 { 200

13,815

(Method B) (Probability distribution) (1s: (2s: (1s: (2s: (1s: (2s:

15,045–15,705) 14,700–15,990) 14,385–15,105) 14,075–15,475) 13,565–14,075) 13,345–14,355)

this retreat event. This softwater lake is perched above the surrounding lowlands. Its sediment does not have the carbonate problems of other lakes in the region, and its dozen radiocarbon dates (Bender et al., 1980, p. 121–122) can be modeled with a second-order function (r2 Å 0.998) that converges to a modern date at the sediment surface. Three 14 C dates were taken at the interface (603 cm) between the normal lake sediment and the underlying laminated sediment. Two dates are from the interval 599–603 cm [12,260 { 115 yr B.P. (WIS-1073) and 12,880 { 125 yr B.P. (WIS1004)], and the third from 603–611 cm [12,520 { 160 yr B.P. (WIS-1075)]. The mean of the three dates is 12,550 { 233 yr B.P. (geometric mean of the dates { the square root of the sum of the squares of the dates’ standard errors). If both Valders and Devils Lake date an ice margin position, then these dates of 12,550 { 233 yr B.P. at the outermost limit (Johnstown moraine) of the Horicon Formation, and the date of 12,965 { 200 yr B.P. above Horicon till 160 km inside the Johnstown moraine, are apparently in conflict. However, about 18% of the joint-probability density implied by their means and standard errors is shared by the two. There is no way to reject the null hypothesis that the two dates are simply random samples drawn from the same population. If these ice-margin positions are of similar age within the statistical uncertainty of radiocarbon dating, this means that the wastage of the Green Bay lobe from its maximum position was very rapid, even though conditions evidently were cold enough at times to allow formation of ice wedge casts and, at least in the Valders area, tundra vegetation. In addition, the retreating ice produced several end moraines and numerous minor moraines (Lee Clayton and Patrick Colgan, personal communication, 1995) that have been interpreted as indicators of slow, active retreat. If the interpretation of the radiocarbon ages is correct, the ice retreated at least 350 km within a few hundred years, then readvanced at least 250 km to Lake Winnebago while depositing the red Branch River Member, retreated at least 80 km, and then readvanced to this site. The initial collapse of the ice lobe may have been enabled by a rapid retreat of a calving margin in the Lake Michigan basin, which could have completely cut off a source of ice for the Green Bay lobe. A major retreat of

the Lake Michigan lobe is usually assumed during late-glacial time; red sediment introduced to its basin then formed the red drift deposited by later ice advances. But did the fluctuations occur within a relatively short interval of time— a time too short to resolve by radiocarbon dating? In order to test whether this event is correlative with known times of climate change, and in order to test the hypothesis that these are significantly overlapping dates after conversion to calendar years, we have calibrated the Valders Quarry, Devils Lake, and Two Creeks dates using the CALIB, v. 3.0.3c program (Stuiver and Reimer, 1993, Table 1). The Valders Quarry and Devils Lake dates are in the range calibrated by a spline fit to coral data; thus, the calibration curve is much smoother and more generalized than in the more recent part of the record that is based on tree-ring data. The bars of Figure 7 show the means {1s (black) and {2s (black / white). The curves show the probability distribution for the calibrated dates of Devils Lake and Valders Quarry; the area under each curve equals 1.0. These distributions can be modeled assuming normal distributions

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FIG. 7. Probability distributions of the calibrated ages of terrestrial leaves from the Valders Quarry sediment and the end of varve formation at Devils Lake. The bars show the mean {1s (black) and {2s (black / white). See text for discussion.

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FIG. 8. Oxygen isotope data for the last 40,000 calendar years from the GISP2 ice core of central Greenland (after Cuffey et al., 1995, Fig. 1, p. 455.). The ‘‘thermal filter’’ curve indicates an attempt to calibrate the isotope history with borehole thermometry; its time resolution is inherently lower than that for the isotope data. Alley et al. (1993) provide an assessment of the time resolution in different parts of the core. The heavy black bars delimit the interval 15,000 { 500 calendar years, during which time the change of d18O suggests a pronounced warming in Greenland.

with means of 14,745 { 360 cal yr B.P. (Devils Lake) and 15,375 { 330 cal yr B.P. (Valders Quarry). Almost 40% of the area under each normal curve overlaps with the other; 20% of their joint-probability density is shared by the two distributions. Thus, using calibrated dates, there is even less reason to reject the null hypothesis that the two dates were drawn from the same population than with the normal 14C dates. On the other hand, the Two Creeks calibrated date (13,815 cal yr B.P.) differs significantly from either the Valders date or the Devils Lake date. The null hypothesis that either could be drawn from the same population as the Two Creeks date can be rejected at the 95% confidence level (p õ 0.05n), supporting arguments based on the vegetation record and the stratigraphy that they are significantly different in age. After calibrating the dates, we are left with the conclusion that if the interpretation of the significance of the Devils Lake dates is correct, the Green Bay lobe suffered drastic retreat about 15,000 cal yr B.P.—at about the same time that the Greenland ice cores (Cuffey et al., 1995) show a dramatic warming (Fig. 8). Because of the error associated with the dates we cannot say which event was first or if there is any cause and effect relationship without further dating of critical localities.

in radiocarbon age. These pond sediments formed in a treeless terrain that was very different from the environmental conditions during the Two Creeks interval. The Green Bay lobe appears to have vacated the Johnstown moraine between 13,000 and 12,500 14C yr ago (15,000 cal yr, { Ç350 yr). During the initial retreat there was a general absence of trees, and there were periods of cold severe enough for ice-wedge formation and several glacier readvances. The time from the initial retreat until the Valders Quarry sediments were deposited appears to have been too short to resolve by current radiocarbon dating techniques. This time is coincident with abrupt warming in the Greenland ice cores. For about 1000 yr after the Valders Quarry sediments were deposited, tundra, then spruce trees, occupied the landscape during what may be called the Two Creeks interval. The Two Creeks forest was drowned and overridden by ice about 11,850 yr ago (13,800 cal yr) during a subsequent glacier advance down the Green Bay and Lake Michigan basins. ACKNOWLEDGMENTS

The Valders Quarry sediments are significantly older than Two Creeks, both palynologically and stratigraphically and

We thank E. B. Evenson, Thomas Lowell, and Patrick Colgan for helpful comments and criticism of the manuscript. The Wisconsin Geological and Natural History Survey provided funds for several of the radiocarbon dates used in this research. The radiocarbon dates from the Radiocarbon Laboratory at the Center for Climatic Research, University of Wisconsin-Madison, were supported by the Climate Dynamics Program, National Science Foundation, under Grant ATM89-02849.

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CONCLUSIONS

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Evenson, E. B., Farrand, W. R., Eschman, D. F., Mickelson, D. M., and

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